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Open Access

Peer-reviewed

Research Article

Water Filtration Using Plant Xylem

Contributed equally to this work with: Michael S. H. Boutilier, Jongho Lee

Affiliation Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States of America

* E-mail: [email protected]

• Michael S. H. Boutilier, 
• Jongho Lee, 
• Valerie Chambers, 
• Varsha Venkatesh, 
• Rohit Karnik

• Published: February 26, 2014
• https://doi.org/10.1371/journal.pone.0089934
• Reader Comments

Effective point-of-use devices for providing safe drinking water are urgently needed to reduce the global burden of waterborne disease. Here we show that plant xylem from the sapwood of coniferous trees – a readily available, inexpensive, biodegradable, and disposable material – can remove bacteria from water by simple pressure-driven filtration. Approximately 3 cm 3 of sapwood can filter water at the rate of several liters per day, sufficient to meet the clean drinking water needs of one person. The results demonstrate the potential of plant xylem to address the need for pathogen-free drinking water in developing countries and resource-limited settings.

Citation: Boutilier MSH, Lee J, Chambers V, Venkatesh V, Karnik R (2014) Water Filtration Using Plant Xylem. PLoS ONE 9(2): e89934. https://doi.org/10.1371/journal.pone.0089934

Editor: Zhi Zhou, National University of Singapore, Singapore

Received: October 17, 2013; Accepted: January 23, 2014; Published: February 26, 2014

Copyright: © 2014 Boutilier et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Funding: This work was supported by the James H. Ferry, Jr. Fund for Innovation in Research Education award to R.K. administered by the Massachusetts Institute of Technology. SEM imaging was performed at the Harvard Center for Nanoscale Systems, a member of the National Nanotechnology Infrastructure Network (NNIN), which is supported by the National Science Foundation under NSF award no. ECS-0335765. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: The authors have declared that no competing interests exist.

Introduction

The scarcity of clean and safe drinking water is one of the major causes of human mortality in the developing world. Potable or drinking water is defined as having acceptable quality in terms of its physical, chemical, and bacteriological parameters so that it can be safely used for drinking and cooking [1] . Among the water pollutants, the most deadly ones are of biological origin: infectious diseases caused by pathogenic bacteria, viruses, protozoa, or parasites are the most common and widespread health risk associated with drinking water [1] , [2] . The most common water-borne pathogens are bacteria (e.g. Escherichia coli , Salmonella typhi , Vibrio cholerae ), viruses (e.g. adenoviruses, enteroviruses, hepatitis, rotavirus), and protozoa (e.g. giardia) [1] . These pathogens cause child mortality and also contribute to malnutrition and stunted growth of children. The World Health Organization reports [3] that 1.6 million people die every year from diarrheal diseases attributable to lack of access to safe drinking water and basic sanitation. 90% of these are children under the age of 5, mostly in developing countries. Multiple barriers including prevention of contamination, sanitation, and disinfection are necessary to effectively prevent the spread of waterborne diseases [1] . However, if only one barrier is possible, it has to be disinfection unless evidence exists that chemical contaminants are more harmful than the risk from ingestion of microbial pathogens [1] . Furthermore, controlling water quality at the point-of-use is often most effective due to the issues of microbial regrowth, byproducts of disinfectants, pipeline corrosion, and contamination in the distribution system [2] , [4] .

Common technologies for water disinfection include chlorination, filtration, UV-disinfection, pasteurization or boiling, and ozone treatment [1] , [2] , [5] . Chlorine treatment is effective on a large scale, but becomes expensive for smaller towns and villages. Boiling is an effective method to disinfect water; however, the amount of fuel required to disinfect water by boiling is several times more than what a typical family will use for cooking [1] . UV-disinfection is a promising point-of-use technology available [1] , yet it does require access to electricity and some maintenance of the UV lamp, or sufficient sunlight. While small and inexpensive filtration devices can potentially address the issue of point-of-use disinfection, an ideal technology does not currently exist. Inexpensive household carbon-based filters are not effective at removing pathogens and can be used only when the water is already biologically safe [1] . Sand filters that can remove pathogens require large area and knowledge of how to maintain them [1] , while membrane filters capable of removing pathogens [2] , [4] suffer from high costs, fouling, and often require pumping power due to low flow rates [6] that prevents their wide implementation in developing countries. In this context, new approaches that can improve upon current technologies are urgently needed. Specifically, membrane materials that are inexpensive, readily available, disposable, and effective at pathogen removal could greatly impact our ability to provide safe drinking water to the global population.

If we look to nature for inspiration, we find that a potential solution exists in the form of plant xylem – a porous material that conducts fluid in plants [7] . Plants have evolved specialized xylem tissues to conduct sap from their roots to their shoots. Xylem has evolved under the competing pressures of offering minimal resistance to the ascent of sap while maintaining small nanoscale pores to prevent cavitation. The size distribution of these pores – typically a few nanometers to a maximum of around 500 nm, depending on the plant species [8] – also happens to be ideal for filtering out pathogens, which raises the interesting question of whether plant xylem can be used to make inexpensive water filtration devices. Although scientists have extensively studied plant xylem and the ascent of sap, use of plant xylem in the context of water filtration remains to be explored. Measurements of sap flow in plants suggest that flow rates in the range of several liters per hour may be feasible with less than 10 cm-sized filters, using only gravitational pressure to drive the flow [7] .

We therefore investigated whether plant xylem could be used to create water filtration devices. First, we reason which type of plant xylem tissue is most suitable for filtration. We then construct a simple water filter from plant xylem and study the resulting flow rates and filtration characteristics. Finally, we show that the xylem filter can effectively remove bacteria from water and discuss directions for further development of these filters.

Materials and Methods

Branches were excised from white pine growing on private property in Massachusetts, USA, with permission of the owner and placed in water. The pine was identified as pinus strobus based on the 5-fold grouping of its needles, the average needle length of 4.5 inches, and the cone shape. Deionized water (Millipore) was used throughout the experiments unless specified otherwise. Red pigment (pigment-based carmine drawing ink, Higgins Inks) was dissolved in deionized water. Nile-red coated 20 nm fluorescent polystyrene nanospheres were obtained from Life Technologies. Inactivated, Alexa 488 fluorescent dye labeled Escherichia coli were obtained from Life Technologies. Wood sections were inserted into the end of 3/8 inch internal diameter PVC tubing, sealed with 5 Minute Epoxy, secured with hose clamps, and allowed to cure for ten minutes before conducting flow rate experiments.

Construction of the Xylem Filter

1 inch-long sections were cut from a branch with approximately 1 cm diameter. The bark and cambium were peeled off, and the piece was mounted at the end of a tube and sealed with epoxy. The filters were flushed with 10 mL of deionized water before experiments. Care was taken to avoid drying of the filter.

Filtration and Flow Rate Experiments

Approximately 5 mL of deionized water or solution was placed in the tube. Pressure was supplied using a nitrogen tank with a pressure regulator. For filtration experiments, 5 psi (34.5 kPa) pressure was used. The filtrate was collected in glass vials. For dye filtration, size distribution of the pigments was measured for the input solution and the filtrate. Higgins pigment-based carmine drawing ink, diluted ∼1000× in deionized water, was used as the input dye solution. For bacteria filtration, the feed solution was prepared by mixing 0.08 mg of inactivated Escherichia coli in 20 mL of deionized water (∼1.6×10 7 mL −1 ) after which the solution was sonicated for 1 min. The concentration of bacteria was measured in the feed solution and filtrate by enumeration with a hemacytometer (inCyto C-chip) mounted on a Nikon TE2000-U inverted epifluorescence microscope. Before measurement of concentration and filtration experiments, the feed solution was sonicated for 1 min and vigorously mixed.

Xylem structure was visualized in a scanning electron microscope (SEM, Zeiss Supra55VP). Samples were coated with gold of 5 nm thickness before imaging. To visualize bacteria filtration, 5 mL of solution at a bacteria concentration of ∼1.6×10 7 mL −1 was flowed into the filter. The filter was then cut longitudinally with a sharp blade. One side of the sample was imaged using a Nikon TE2000-U inverted epifluorescence microscope and the other was coated with gold and imaged with the SEM. Optical images were acquired of the cross section of a filter following filtration of 5 mL of the dye at a dilution of ∼100×.

Particle Sizing

Dynamic light scattering measurements of particle size distributions were performed using a Malvern Zetasizer Nano-ZS.

Xylem Structure and Rationale for use of Conifer Xylem

The flow of sap in plants is driven primarily by transpiration from the leaves to the atmosphere, which creates negative pressure in the xylem. Therefore, xylem evolution has occurred under competing pressures of providing minimal resistance to the flow of sap, while protecting against cavitation (i.e. nucleation) and growth of bubbles that could stop the flow of sap and kill the plant, and to do this while maintaining mechanical strength [7] . The xylem structure comprises many small conduits that work in parallel and operate in a manner that is robust to cavitation [7] , [8] ( Figure 1 ). In woody plants, the xylem tissue is called the sapwood, which often surrounds the heartwood (i.e. inactive, non-conducting lignified tissue found in older branches and trunks) and is in turn surrounded by the bark ( Figure 1b,c ). The xylem conduits in gymnosperms (conifers) are formed from single dead cells and are called tracheids ( Figure 1c ), with the largest tracheids reaching diameters up to 80 µm and lengths up to 10 mm [7] . Angiosperms (flowering plants) have xylem conduits called vessels that are derived from several cells arranged in a single file, having diameters up to 0.5 mm and lengths ranging from a few millimeters to several meters [7] . These parallel conduits have closed ends and are connected to adjacent conduits via “pits” [8] ( Figure 1d,e ). The pits have membranes with nanoscale pores that perform the critical function of preventing bubbles from crossing over from one conduit to another. Pits occur in a variety of configurations; Figure 1d,e shows torus-margo pit membranes that consist of a highly porous part shaped like a donut (margo) and an impermeable part in the center called torus, occurring in conifers [8] . The porosity of the pit membranes ranges in size from a few nanometers to a few hundred nanometers, with pore sizes in the case of angiosperms tending to be smaller than those in gymnosperms [8] , [9] . Pit membrane pore sizes have been estimated by examining whether gold colloids or particles of different sizes can flow through [8] , [10] . Remarkably, it was observed that 20 nm gold colloids could not pass through inter-vessel pit membranes of some deciduous tree species [10] , indicating an adequate size rejection to remove viruses from water. Furthermore, inter-tracheid pit membranes were found to exclude particles in the 200 nm range [8] , as required for removal of bacteria and protozoa.

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a) Structure of xylem vessels in flowering plants and tracheids in conifers. Longer length of the vessels can provide pathways that can bypass filtration through pit membranes that decorate their circumference. b) Photograph of ∼1 cm diameter pine ( pinus strobus ) branch used in the present study. c) Scanning electron microscope (SEM) image of cut section showing tracheid cross section and lengthwise profile. Scale bar is 40 µm. d) SEM image showing pits and pit membranes. Scale bar is 20 µm. e) Pit membrane with inset showing a cartoon of the pit cross-section. The pit cover has been sliced away to reveal the permeable margo surrounding the impermeable torus. Arrow indicates observed hole-like structures that may be defects. The margo comprises radial spoke-like structures that suspend the torus, which are only barely visible overlaying the cell wall in the background. Scale bar is 1 µm. f) Dependence of area amplification, defined as the pit membrane area divided by the nominal filter area, on the tracheid aspect ratio L / D and fractional area α occupied by pit membranes.

https://doi.org/10.1371/journal.pone.0089934.g001

Since angiosperms (flowering plants, including hardwood trees) have larger xylem vessels that are more effective at conducting sap, xylem tissue constitutes a smaller fraction of the cross-section area of their trunks or branches, which is not ideal in the context of filtration. The long length of their xylem vessels also implies that a large thickness (centimeters to meters) of xylem tissue will be required to achieve any filtration effect at all – filters that are thinner than the average vessel length will just allow water to flow through the vessels without filtering it through pit membranes ( Figure 1a ). In contrast, gymnosperms (conifers, including softwood trees) have short tracheids that would force water to flow through pit membranes even for small thicknesses (<1 cm) of xylem tissue ( Figure 1a ). Since tracheids have smaller diameters and are shorter, they offer higher resistance to flow, but typically a greater fraction of the stem cross-section area is devoted to conducting xylem tissue. For example, in the pine branch shown in Figure 1b used in this study, fluid-conducting xylem constitutes the majority of the cross-section. This reasoning leads us to the conclusion that in general the xylem tissue of coniferous trees – i.e. the sapwood – is likely to be the most suitable xylem tissue for construction of a water filtration device, at least for filtration of bacteria, protozoa, and other pathogens on the micron or larger scale.

The resistance to fluid flow is an important consideration for filtration. Pits can contribute a significant fraction (as much as 30–80%) [7] , [8] of the resistance to sap flow, but this is remarkably small considering that pit membrane pore sizes are several orders of magnitude smaller than the tracheid or vessel diameter. The pits and pit membranes form a hierarchical structure where the thin, highly-permeable pit membranes are supported across the microscale pits that are arranged around the circumference of the tracheids ( Figure 1a ). This arrangement permits the pit membranes to be thin, offering low resistance to fluid flow. Furthermore, the parallel arrangement of tracheids with pits around their circumference provides a high packing density for the pit membranes. For a given tracheid with diameter D and length L , where pit membranes occupy a fraction α of the tracheid wall area, each tracheid effectively contributes a pit membrane area of πDLα /2, where the factor of 2 arises as each membrane is shared by two tracheids. However, the nominal area of the tracheid is only πD 2 /4, and therefore, the structure effectively amplifies the nominal filter area by a factor of 2 α ( L / D ) ( Figure 1f ). The images in Figure 1c indicate that typical D ∼ 10–15 µm and α ∼ 0.2 yield an effective area amplification of ∼20 for tracheid lengths of 1–2 mm. Therefore, for a filter made by cutting a slice of thickness ∼ L of the xylem, the actual membrane area is greater by a large factor due to vertical packing of the pit membranes. Larger filter thicknesses further increase the total membrane area, but the additional area of the membrane is positioned in series rather than in parallel and therefore reduces the flow rate, but potentially improves the rejection performance of the filter due to multiple filtration steps as shown in Figure 1a .

Construction of the Xylem Filter and Measurement of Flow Rate

a) Construction of xylem filter. b) Effect of applied pressure on the water flux through the xylem filter. c) Hydrodynamic conductivity of the filter extracted at each measured pressure using the total filter cross-section area and thickness as defined by Equation 1 . Error bars indicate ±S.D. for measurements on three different xylem filters.

https://doi.org/10.1371/journal.pone.0089934.g002

Biologists have performed similar flow rate measurements by cutting a section of a plant stem under water, flushing to remove any bubbles, and applying a pressure difference to measure the flow rate [11] , [12] . Xylem conductivities of conifers [7] typically range from 1–4 kg s −1 m −1 MPa −1 , which compares very well with the conductivities measured in our experiments. Lower conductivities can easily result from introduction of bubbles [11] or the presence of some non-conducting heartwood. We can therefore conclude that the flow rate measurements in our devices are consistent with those expected from prior literature on conductivity of conifer xylem.

Filtration of Pigment Dye

After construction of the filter, we tested its ability to filter a pigment dye with a broad particle size distribution. The red color of the feed solution disappeared upon filtration ( Figure 3a ) indicating that the xylem filter could effectively filter out the dye.

a) Feed solution of a pigment dye before filtration (left), compared to the filtrate (right). b) Size distribution of the pigment particles in the feed and filtrate solutions measured by dynamic light scattering. c) Dependence of the rejection on the particle size estimated from the data in (b). d) Cross-section of the xylem filter after filtration. Scale is in centimeters and inches.

https://doi.org/10.1371/journal.pone.0089934.g003

Since the dye had a broad pigment size distribution, we investigated the size-dependence of filtration by quantifying the pigment size distribution before and after filtration using dynamic light scattering. We found that the feed solution comprised particles ranging in size from ∼70 nm to ∼500 nm, with some larger aggregates ( Figure 3b ). In contrast, the filtrate particle size distribution peaked at ∼80 nm, indicating that larger particles were filtered out. In a separate experiment, we observed that 20 nm fluorescent polystyrene nanoparticles could not be filtered by the xylem filter, confirming this size dependence of filtration. Assuming that pigment particles 70 nm or less in size were not rejected, the size distributions before and after filtration enable calculation of the rejection performance of the xylem filter as a function of particle size ( Figure 3c ). We find that the xylem filter exhibits excellent rejection for particles with diameters exceeding 100 nm, with the estimated rejection exceeding 99% for particles over 150 nm. Smaller particles are expected to pass through the larger porosity of the pit membrane: SEM images in Figure 1e indicate sub-micron spacing between the radial spoke-like margo membrane through which the pigment particles can pass, although the porosity is difficult to resolve in the SEM image.

After filtration, we cut the xylem filter parallel to the direction of flow to investigate the distribution of dye in the filter. We observed that the dye was confined to the top 2–3 millimeters of the xylem filter ( Figure 3d ), which compares well with the tracheid lengths on the millimeter scale expected for coniferous trees [7] . These results show that the majority of the filtration occurred within this length scale, and suggests that the thickness of the xylem filter may be reduced to below 1 cm while still rejecting the majority of the dye.

Filtration of Bacteria from Water

Finally, we investigated the ability of the xylem filter to remove bacteria from water. As a model bacterium, we used fluorescently labeled and inactivated Escherichia coli bacteria that are cylindrical in shape with a diameter of ∼1 µm. Use of fluorescently labeled bacteria enabled easy enumeration of their concentrations, and also allowed us to track the location in the xylem filter where they were trapped. Since filtration is dominated by size-exclusion at this length scale, we do not expect modification with the dye to significantly affect filtration characteristics. Filtration using three different xylem filters showed nearly complete rejection of the bacteria ( Figure 4a ). Using a hemacytometer to count the bacteria, we estimate that the rejection was at least 99.9%.

a) Concentrations of bacteria in the feed and filtrate solutions. Inset shows fluorescence images of the two solutions. Scale bar is 200 µm. Error bars indicate ±S.D. for experiments performed on three different xylem filters. b) Fluorescence image of xylem filter cross-section showing accumulation of bacteria over the margo pit membranes. Scale bar is 20 µm. c) Low-magnification fluorescence image shows that bacteria are trapped at the bottoms of tracheids within the first few millimeters of the top surface. Scale bar is 400 µm. Arrow indicates top surface of the xylem filter and also the direction of flow during filtration. Autofluorescence of the xylem tissue also contributes to the fluorescence signal in (b) and (c). d), e) SEM images showing bacteria accumulated on the margo pit membranes after filtration. Scale bars are 10 µm and 2 µm, respectively.

https://doi.org/10.1371/journal.pone.0089934.g004

To investigate the mechanism of filtration, the xylem filter was cut parallel to the direction of flow after filtration. When examined under a fluorescence microscope, we observed that the bacteria accumulated over the donut-shaped margo pit membranes ( Figure 4b ). This distribution is consistent with the expectation that the bacteria are filtered by the porous margo of the pit membranes. The distribution of trapped bacteria was not uniform across the cross section of the filter. Similar to the case of the dye, bacteria were observed only within the first few millimeters from the end through which the solution was infused (indicated by the white arrow in Figure 4c ). In addition, the low-magnification fluorescence image shows that the bacteria had accumulated primarily over pit membranes at the bottom of the tracheids, which is again not unexpected. Further investigation by SEM clearly showed individual bacterial cells accumulated on the pit membranes over the porous margo ( Figure 4d,e ). These results confirm the pit membranes as the functional units that provide the filtration effect in the xylem filter.

Wood has been investigated recently as a potential filtration material [13] , showing moderate improvement of turbidity. While we showed filtration using freshly cut xylem, we found that the flow rate dropped irreversibly by over a factor of 100 if the xylem was dried, even when the xylem was flushed with water before drying. We also examined flow through commercially available kiln-dried wood samples cut to similar dimensions. Wood samples that exhibited filtration showed two orders of magnitude smaller flow rates than in the fresh xylem filter, while those that had high flow rates did not exhibit much filtration effect and seemed to have ruptured tracheids and membranes when observed under SEM. Wetting with ethanol or vacuuming to remove air did not significantly increase the flow rate in the wood samples that exhibited the filtration effect, suggesting that the pit membranes may have a tendency to become clogged during drying. These results are consistent with literature showing that the pit membranes can become irreversibly aspirated against the cell wall, blocking the flow [14] . In fact, the pit membranes in the SEM images ( Figure 1d,e and Figure 4d,e ), which were acquired after drying the samples, appear to be stuck to the walls. Regardless, our results demonstrate that excellent rejection (>99.9%) of bacteria is possible using the pit membranes of fresh plant xylem, and also provide insight into the mechanism of filtration as well as guidelines for selection of the xylem material.

Peter-Varbanets et al. [2] have outlined the key requirements for point-of-use devices for water disinfection: a) performance (ability to effectively remove pathogens), b) ease of use (no time-consuming maintenance or operation steps), c) sustainability (produced locally with limited use of chemicals and non-renewable energy), and d) social acceptability. Meeting all of these requirements has proved to be challenging, but point-of-use methods that have been successfully used for low-cost water treatment in developing countries include free-chlorine/solar disinfections, combined coagulant-chlorine disinfection, and biosand/ceramic filtrations [5] . While chlorine is a very effective biocide, its reaction with organic matter can produce carcinogenic by-products [15] and some waterborne pathogens such as Cryptosporidium parvum and Mycobacterium avium are resistant to the chlorine [16] . Solar disinfection based on ultraviolet irradiation can effectively inactivate C. parvum , but this requires low turbidity of source water [17] and is not effective for control of viruses [16] . Filtration based on biosand and ceramic filters is also effective at removing pathogens, but the effectiveness against viruses is low or unknown [18] . Coagulation combined with chlorine disinfection removes or inactivates viruses and pathogens effectively. However, necessity of an additional filtration step and relatively high cost are potential barriers for practical use [18] . Among these methods, a review on field studies by Sobsey et al. [5] suggested that biosand and ceramic filtration are the most effective methods in practice, because once the apparatus is installed, the effort for use and dosage is significantly reduced and therefore promotes persistent use compared to disinfection approaches. Although membrane-based filtration is the most widely used for water treatment in industrialized nations and the cost of membranes has significantly decreased, membranes are still unaffordable to poor communities in the developing world [2] . Ultrafiltration systems run by hydrostatic pressure [19] and some recently invented point-of-use devices using ultrafiltration membranes may provide water to developing regions at reasonable cost [2] . However, membranes still require specialized chemicals and processes for manufacture, and need cleaning or replacement.

Xylem filter technology could be an attractive option for low-cost and highly efficient point-of-use water treatment by filtration, overcoming some of the challenges associated with conventional membranes. Xylem filters could provide the advantage of reduced human effort compared to existing point-of-use water treatment options, requiring only simple periodic filter replacement. In addition, the pressures of 1–5 psi used here are easily achievable using a gravitational pressure head of 0.7–3.5 m, implying that no pumps are necessary for filtration. The measured flow rates of about 0.05 mL/s using only ∼1 cm 2 filter area correspond to a flow rate of over 4 L/d, sufficient to meet the drinking water requirements of one person [20] . This is comparable to chlorination and biosand filtration, which have the highest production rates of prevalent point-of-use water treatment methods, and far exceeds typical production rates for solar disinfection. Xylem filters could potentially be produced locally and inexpensively, and disposed of easily owing to their biodegradability. The high flow rates and low cost would certainly help address the issues of maintenance and replacement. For example, 200 filters of 10 cm 2 area and 0.5 cm thickness could be packaged into a volume of about 1 L, which will be inexpensive and last a few years even with weekly replacement. Furthermore, as suggested by the dye filtration experiment, xylem filters should be able to significantly reduce water turbidity, enhancing the aesthetic qualities of the drinking water, which is hardly achieved by chlorination and solar disinfection.

Wood is an easily available material. While use of fresh xylem does not preclude its use as a filter material, dried membranes have definite practical advantages. Therefore, the process of wood drying and its influence on xylem conductivity needs further study. In particular, processes that yield intact yet permeable xylem tissues that can be stored long-term will be helpful for improving the supply chain if these filters are to be widely adopted. In addition, flow through xylem of different plants needs to be studied to identify locally available sources of xylem, which will truly enable construction of filters from locally available materials. In the present study, we report results using xylem derived from only one species. These xylem filters could not filter out 20 nm nanoparticles, which is a size comparable to that of the smallest viruses. It will be interesting to explore whether there exist any coniferous species that have pit membranes with smaller pore sizes that can filter out viruses, or whether conifer xylem can be impregnated with particles such as carbon black to improve rejection of viruses. In their absence, angiosperms with short vessels that yield practical filter lengths may be the best alternative due to their smaller pit membrane pore sizes [8] . Further exploration of xylem tissues from different plants with an engineering perspective is needed to construct xylem filters that can effectively reject viruses, exhibit improved flow rates, or that are amenable to easy storage. It is also conceivable that plants could be selected or developed for enhanced filtration characteristics, as has been the norm in agriculture for enhancement of many desirable characteristics including resistance to pests, flavor, or productivity.

Conclusions

Plant xylem is a porous material with membranes comprising nanoscale pores. We have reasoned that xylem from the sapwood of coniferous trees is suitable for disinfection by filtration of water. The hierarchical arrangement of the membranes in the xylem tissue effectively amplifies the available membrane area for filtration, providing high flow rates. Xylem filters were prepared by simply removing the bark of pine tree branches and inserting the xylem tissue into a tube. Pigment filtration experiments revealed a size cutoff of about 100 nm, with most of the filtration occurring within the first 2–3 mm of the xylem filter. The xylem filter could effectively filter out bacteria from water with rejection exceeding 99.9%. Pit membranes were identified as the functional unit where actual filtration of the bacteria occurred. Flow rates of about 4 L/d were obtained through ∼1 cm 2 filter areas at applied pressures of about 5 psi, which is sufficient to meet the drinking water needs of one person. The simple construction of xylem filters, combined with their fabrication from an inexpensive, biodegradable, and disposable material suggests that further research and development of xylem filters could potentially lead to their widespread use and greatly reduce the incidence of waterborne infectious disease in the world.

Acknowledgments

The authors thank Yukiko Oka for assistance with preparation of illustrations and Sunandini Chopra for help with dynamic light scattering measurements.

Author Contributions

Conceived and designed the experiments: MSHB JL VV VC RK. Performed the experiments: MSHB JL VV VC. Analyzed the data: MSHB JL RK. Contributed reagents/materials/analysis tools: VC. Wrote the paper: MSHB JL RK.

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Existing Filtration Treatment on Drinking Water Process and Concerns Issues

Mashitah che razali.

1 Faculty of Electrical Engineering, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, Durian Tunggal, Melaka 76100, Malaysia

2 Faculty of Electrical Engineering, Universiti Teknologi Malaysia, Johor Bahru 81310, Malaysia

Norhaliza Abdul Wahab

Noorhazirah sunar, nur hazahsha shamsudin, associated data.

Data sharing not applicable.

Water is one of the main sources of life’s survival. It is mandatory to have good-quality water, especially for drinking. Many types of available filtration treatment can produce high-quality drinking water. As a result, it is intriguing to determine which treatment is the best. This paper provides a review of available filtration technology specifically for drinking water treatment, including both conventional and advanced treatments, while focusing on membrane filtration treatment. This review covers the concerns that usually exist in membrane filtration treatment, namely membrane fouling. Here, the parameters that influence fouling are identified. This paper also discusses the different ways to handle fouling, either based on prevention, prediction, or control automation. According to the findings, the most common treatment for fouling was prevention. However, this treatment required the use of chemical agents, which will eventually affect human health. The prediction process was usually used to circumvent the process of fouling development. Based on our reviews up to now, there are a limited number of researchers who study membrane fouling control based on automation. Frequently, the treatment method and control strategy are determined individually.

1. Introduction

The quality of drinking water resources is being enthusiastically addressed around the world since it is essential to health and development issues. Due to uncontrolled industrial waste and low public awareness, water pollutants can be discharged either directly or indirectly to water resources such as lakes, ponds, rivers, seawater, and groundwater, which later become contaminated. The contaminated or poor quality of drinking water can cause various infectious diseases and negatively impact our overall health [ 1 ]. According to the World Health Organization (WHO), contaminated drinking water can cause serious diseases such as diarrhea, cholera, dysentery, hepatitis A, typhoid, and polio [ 2 ]. It is estimated that around 502,000 people die each year from diarrhea due to unsafe drinking water. The quality of water resources has been gradually depreciating due to industrialization and urbanization [ 3 ]. It has become a crucial problem due to the difficulty of meeting effluent quality standards with conventional treatment processes [ 4 , 5 , 6 ]. Good-quality drinking water helps people achieve maximum body health and well-being.

To obtain high-quality drinking water, a good and reliable water treatment process is desirable. Traditional drinking water treatment includes five common units such as coagulation, flocculation, sedimentation, filtration, and disinfection [ 7 , 8 , 9 ]. More than ten decades ago, the only treatment processes used in municipal and industrial water treatment were conventional filtration, such as clarification and granular media filtration, and chlorination methods. However, in the past twenty years, industrial water has shown high interest in the implementation of advanced water treatment technologies, particularly for water purification technologies such as membrane filtration, ultraviolet irradiation, the advanced oxidation process (AOP), ion exchange, and biological filtration for the removal of water contaminants in drinking water [ 10 ]. As of today, the latest water purification technologies are nanotechnology, acoustic nanotube, photocatalytic water purification, aquaporin inside TM , and automatic variable filtration. The use of technologies in water treatment is mainly due to three main reasons: a new standard for water quality, an increase in water contamination, and cost. Certainly, the new technology to be introduced should provide more advantages over the conventional treatment processes, such as lower operation and maintenance costs, being more efficient and simple to operate, having higher effluent quality and a high degree of reliability, having lower waste production, and most importantly, meeting regulatory requirements.

This paper identifies and reviews some of the available technologies ‘often’ used in drinking water treatment. Many of them are certainly not new to the water industry, but their application has been limited due to many circumstances, which are highlighted in this paper. This review focuses on membrane filtration technology and its application to municipal and industrial water systems. This review was motivated to establish an understanding of the related issues that come up in the drinking water treatment process.

We gather information about the available filtration systems, with a focus on the differences between conventional and membrane filtration, from these studies. Membrane filtration problems such as fouling phenomena, membrane cleaning, fouling prediction, and fouling prevention are discussed thoroughly. We also discussed the consequences of this review for the selection of a control strategy to overcome the problem in drinking water treatment, particularly due to the fouling phenomena. The goal is to organize and summarize most of the work and to identify the research focus and the trends in the literature on filtration treatment methods for drinking water processes.

2. Available Drinking Water Treatment Technologies

In general, the treatment technologies for treating water depend on the type of raw intake water that comes from various water sources, such as surface water and groundwater. The existing filtration treatments that are covered in this section are divided into conventional and advanced methods. Some of the available drinking water filtration treatment technologies, both conventional and advanced, as well as their concerns, are described.

2.1. Conventional Treatment

Conventional treatment is one of the popular approaches that has been used for water and wastewater treatment systems, where it involves several processes, including bar screening, grit removal, pre-oxidation, coagulation, flocculation, sedimentation, rapid/slow sand, granular active carbon filtration, and/or disinfection [ 11 ]. These processes can remove various solid sizes and organic matter from the liquid phase. It is also able to contribute to the reduction of microorganisms that cause concern for public health. There are several types of conventional filtration treatments, such as simple screen filters, slow and fast sand filters, diatom filters, and charcoal filters. The effect of filter media on the filtration process needs to be considered when designing the filtration unit. Additionally, the design of the backwash filter needs to be taken into account when high turbidity in effluent water increases head losses and requires long filtration operations [ 12 , 13 ].

Many studies have been performed to investigate the effectiveness of conventional filtration in treating drinking water. The previous study of the removal of diclofenac from drinking water is reported by Rigobello et al. [ 14 ], where the conventional sand filter is compared with granular activated carbon (GAC) filtration. The results showed that a sand filter could not effectively remove diclofenac, whereas a combination of a sand filter and GAC filtration could remove diclofenac with ≥99.7% efficiency. A slow sand filter and charcoal filter have been used in the study by Murugan and Ram [ 15 ]. The application of a slow sand filter can help in the reduction of water turbidity and prevent fouling at the reactor tubes. The charcoal filter is used to help in the absorption of heavy metals that are present in the water. In this work, slow sand filters require periodic removal of the microbial layer, while charcoal must be replaced in the filter every month as there are no indications that the charcoal has reached its breakthrough.

Zheng et al. [ 16 ] investigate the use of a slow sand filter as a pre-treatment for the removal of organic foulants in secondary effluent. The investigation was conducted with different filtration rates and showed that the proposed pre-treatment can effectively control the fouling rate at low filtration rates with respect to biopolymer removal and cycle time. Another study on the effect of a flow configuration based on a slow sand filter was performed by Sabogal-Paz et al. [ 17 ], where a comparison study was performed for the household system between intermittent and continuous flows. The authors observe that the flow configuration of a slow sand filter cannot be applied as a single treatment because it is not able to remove the organic foulants effectively. The work proposed by Ahammed and Darva [ 18 ] investigates the effect of a modified slow sand filter by introducing a thin layer of iron oxide-coated sand. The performance of the proposed method is measured based on its capability to remove bacteria and turbidity. Results showed that the modified slow sand filter was able to increase the removal rate of bacteria, but there was no significant reduction in turbidity. Work by Mizuta et al. [ 19 ] presents bamboo powder charcoal and activated carbon filtration in the removal of nitrate and nitrogen from drinking water. The results showed that bamboo powder charcoal filtration was able to provide higher adsorption and less influence on temperature compared to activated carbon filtration. Bamboo charcoal filtration was studied by Zhang et al. [ 20 ] to remove microcystin-LR from drinking water. In this study, bamboo charcoal filtration was modified with chitosan, and the results indicate that the applied treatment was able to effectively remove the microcystin-LR, especially when the amount of bamboo charcoal was increased.

Based on previous studies of conventional treatment methods, it is clear that the method is incapable of producing satisfactory effluent quality. Most of the treatments require either modification or combination with other methods, which is costly due to frequent maintenance. Moreover, this treatment is considered economically unbeneficial for developing countries [ 21 ], where the treatments require a long operating period and a large footprint [ 22 ]. Due to the importance of having safe and healthy water, water utilities have started to consider alternative treatment technologies to traditional drinking water treatment.

2.2. Advanced Treatment

Here, several advanced treatments of water technologies, particularly for water purification technologies such as membrane filtration, ultraviolet irradiation, the advanced oxidation process, ion exchange, and biological filtration, are discussed. Recently, membrane filtration is increasingly being accepted and implemented in drinking water treatment plants [ 23 ]. Membrane technology is widely used in filtration systems, particularly for the removal of particulate matter in solid-liquid separation processes [ 24 , 25 ]. Moreover, the combination of membrane technology with a bioreactor is called a membrane bioreactor, and this technology has proven its high capacity for the removal of pollutants in water and wastewater treatment processes [ 26 , 27 ]. The main issue in membrane filtration is the fouling phenomenon, which, if not prevented, will affect the overall filtration performance in the long run.

Another advanced technology that is primarily used in drinking water is ultraviolet (UV) irradiation technology [ 28 ]. UV irradiation is used as a disinfection process and is commonly designed with a series of UV lamps so that the microorganisms in the water will be inactivated when exposed to UV light [ 29 ]. Although UV irradiation is a promising disinfection technology due to its compactness and low cost, it faces a challenge due to its reliance on electrical component sensitivity [ 30 ], which can result in high failure rates.

The advanced oxidation process (AOP) is another technology generally applied in water treatment. The AOP includes several processes that produce hydroxyl radicals for the oxidation of organic and inorganic water impurities [ 31 ]. Among the three main AOP processes are ozone, ozone with hydrogen peroxide addition, and UV irradiation with hydrogen peroxide addition. Each of the processes has its challenges and will not be discussed in detail here. To summarize, AOP can provide multiple uses in water treatment, such as color, oxidation of synthetic organic chemicals, taste and odor, and many more. However, the complexity of AOPs in terms of chemical reactions between processes makes it hard to achieve an optimum treatment system design [ 32 ]. The next advanced water treatment is ion exchange (IX) technology. This technology was previously limited to only softening water for use in water treatment plants. However, the limits are now also being set on several inorganic chemicals, making the IX a more interesting technology to explore in water treatment applications. Lastly, biological filtration is another type of advanced treatment in water technology. The filtration is based on biological processes, which are different from the previously mentioned technologies that are based on physical and/or chemical processes. Works by Wang et al. [ 33 ] claim this biological filtration is the most effective process to produce biologically stable water. However, there are still unanswered issues regarding the proper design and implementation of biological filtration, particularly in terms of the size and type of filter media to be used. Figure 1 summarizes the conventional and advanced filtration methods for drinking water treatment.

Available treatment for drinking water [ 5 , 11 , 15 , 21 , 34 , 35 , 36 ].

2.3. Hybrid Treatment

In general, most industrial drinking water treatments still involve conventional and advanced treatment processes [ 8 ]. Figure 2 shows an example of industry-standard potable reuse water plants that involve conventional and advanced treatment processes [ 8 ]. In the primary treatment, the sedimentation of solid waste is performed. Water from secondary and tertiary treatment can be used for potable and non-potable reuse applications. The secondary treatment involves biological processes (e.g., the activated sludge process), and the tertiary treatment involves physical and/or chemical processes. For the disinfection process, chlorine is used to disinfect water to kill bacteria, parasites, and viruses in drinking water [ 37 ]. Alternatively, disinfectants such as chlorine dioxide, ozone, and ultraviolet radiation are also used. In advanced treatment, the integrated membrane system (IMS) and full advanced treatment (FAT) are implemented. The IMS uses a low-pressure membrane filtration process either microfiltration (MF) or ultrafiltration (UF). Meanwhile, FAT applies called either nanofiltration (NF) or reverse osmosis (RO), which are high-pressure membrane filtration processes. The application of IMS can provide high efficacy in the removal rate of particulate matter, microbial pathogens, and natural organic matter, whereas FAT is capable of removing magnificently organic–inorganic dissolved constituents such as salts and organic chemicals that are impossible to be removed by IMS. Ultraviolet and advanced oxidation processes act as post-treatment disinfection. In this stage, it will break down small neutral organic compounds that pass-through FAT. The final stage is known as degassing and lime dosing, which act as a water stabilizer and increase the pH and alkalinity of the water. The industry standard potable reuse water plant shown in Figure 2 can meet the specification for drinking water quality, but there are several drawbacks, including a large footprint, high capital cost, and high energy consumption, which make it essential to discover another technology that can overcome the drawbacks [ 38 ].

Industry-standard potable reuse plant.

The conventional design of the drinking water treatment process includes five common units, and four of them (coagulation, flocculation, sedimentation, and filtration) are the lines that remove suspended particles from surface water treatment plants. Filtration is the final step in the removal of suspended particles, and without it, the plants are considered untreatable. Therefore, proper control, design, and implementation of the filtration operation unit are crucial to improving the effluent quality and reducing the risk of waterborne diseases. The next section then focuses on a review of numerous types of membrane filtration technologies. The advantages and disadvantages of each type of filtration are also discussed, and this will provide some hints for researchers on how to choose the most suitable membrane filtration for their applications.

3. Membrane Filtration Technology

Membrane filtration is an advanced drinking water treatment that is widely used nowadays in water treatment processes, mainly for drinking water. Examples of types of membranes include microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), electrodialysis (ED), forward osmosis (FO), and membrane distillation (MD). Each method has its own specific range of membrane pore sizes, surface charge, and hydrophobicity that is produced from different materials [ 39 ]. Table 1 shows the pore size ranges of various membrane filtration systems as compared to the size of common water contaminants.

Contaminant with respective membrane filtration type.

The application of membrane filtration technology to drinking water treatment on a large-scale [ 40 ] has received attention due to its advantages, including excellent effluent quality [ 41 ], simple process management [ 42 ], and strict solid-liquid separation with a small footprint requirement [ 43 , 44 ]. The technology is also easy to adapt to the existing treatment facilities [ 45 ], provides low energy consumption [ 11 ], and removes various contaminants [ 46 ]. The removal rate of contaminants depends on the characteristics of the membrane and the properties of the contaminant [ 36 ]. Aside from these benefits, the main disadvantage of this technology is the cost of the membrane itself, which can be reduced or eliminated if the membrane filtration process is handled properly. Figure 3 shows the advantages and disadvantages of each membrane filtration treatment applied to drinking water treatment.

In general, membrane filtration can be classified into two categories: low-pressure membrane (10 to 30 psi) and high-pressure membrane (75 to 250 psi). The low-pressure membrane system includes MF and UF, while NF and RO are categorized as high-pressure membrane systems.

The low-pressure MF and UF membranes for the application of municipal surface water treatment have been studied and implemented since the 1980s. In these studies, the MF (nominal pore size of 0.2 mm) and UF (nominal pore size of 0.01 mm) have proven their high capabilities for the removal of particulate matter (turbidity) and microorganisms [ 47 , 48 ]. MF and UF membranes were proven to provide a barrier to microorganisms such as Giardia cysts and Cryptosporidium oocysts, while the UF was proven to be an absolute barrier to viruses due to its smaller pore size of 0.01 mm [ 49 , 50 ]. Previous studies [ 51 , 52 ] also demonstrated that low-pressure membranes were able to treat turbidity efficiently using pilot and full-scale plants. The low-pressure MF and UF membrane systems provide high performance for the removal of contaminants from surface water, and other advantages include a smaller footprint, low chemical usage, and more automation. However, the limitation of membrane technology, including MF and UF, is the high cost of membrane replacement and the lower effectiveness in removing dissolved organic matter in the treated water. The study of modified MF membrane technology is reported by Sinclair et al. [ 53 ], and it showed an improvement in reducing cost as they do not require any external driving force. Unfortunately, the modification resulted in an approximately 22% loss of membrane permeability.

Meanwhile, He et al. [ 54 ] published a study on improved UF technology in which they combined heterogeneous catalytic ozonation and a UF membrane filtration technique for the long-term degradation of bisphenol A (BPA) and humid acid (HA). Results have shown improvements in removal efficiency, reduction of membrane resistance, and mitigation of membrane fouling. Another study concerning UF was reported by Chew et al. [ 55 ], which compared and evaluated industrial-scale UF with conventional drinking water treatment systems. The study showed that UF systems can provide reliable filtrate quality even with the existence of fluctuation in the raw water quality. In addition, the UF system offers promising sustainability, with no coagulant required for high-quality filtrate and non-toxic sludge discharge.

High-pressure NF and RO membranes can provide an alternative method for removing organic and inorganic matter. The NF process is already known for its capabilities in the removal of total organic carbon (TOC) in surface water treatment [ 56 ]. This process has been implemented in several drinking water industries [ 57 , 58 , 59 ]. In an experiment conducted using pre-ozonation as a pre-treatment process for NF membranes proposed by Vatankhah et al. [ 60 ], it was found that pre-ozonation with a low specific ozone dose could effectively mitigate a significant portion of fouling. However, the removal performance of dissolved organic carbon (DOC) of the NF membrane did not show a substantial change, which may be due to the relatively low applied ozone dose. The RO process is applied for drinking water treatment, whether the source water comes from seawater, brackish water, or groundwater [ 21 ]. However, RO has a problem with the ability of suspended solids, colloidal material, and dissolved ions in raw water to foul the system [ 61 ]. A study conducted by Touati et al. [ 62 ] combined UF, NF, and RO processes for isotonic and drinking water treatment. Results showed that the UF process used as pre-treatment was able to eliminate natural organic matter (NOM), while the NF process was able to characterize the fouling mechanism. The overall performance’s energy consumption is determined by salt rejection during the NF process.

Apart from RO, ED is another process that can be used to treat brackish water with high performance and energy efficiency [ 63 , 64 ]. The process involved the transfer of electrolytes or ions through a solution and membranes based on an applied electric field as the driving force [ 65 ]. Walha et al. [ 66 ] investigated the use of the NF, RO, and ED processes in producing drinking water from a brackish water source. The results showed the treatment based on RO and ED processes is more efficient, as shown by the high rejection of inorganic matters present in the feed waters. The concentration of ions in the permeate flux can achieve World Health Organization (WHO) standards, and it is more economical than the NF process.

Forward osmosis (FO) and membrane distillation (MD) processes are driven by heat, which is different from the pressure-driven process usually used for potable water reuse [ 67 ]. FO processing operates at low or no hydraulic pressure, which may reduce irreversible fouling and achieve high rejection of contaminants [ 68 ]. However, Li et al. [ 69 ] reported that the water flux produced by the FO process was still inadequate compared to the RO process under a similar applied pressure. FO processes involve a permeable membrane and two solutions, known as feed and draw solutions. The feed and draw solution consists of different concentrations that produce the osmotic pressure gradient that acts as the driving force for water permeation across a semi-permeable membrane [ 70 ]. An experiment conducted by Tow et al. [ 71 ] studied the fouling propensity between RO, FO, and MD. The experiment was conducted using a single membrane module and showed that both FO and MD exhibit a significant advantage in fouling resistance but neither of them performed well with both organic and inorganic foulants.

Available membrane filtration treatment for drinking water [ 34 , 61 , 68 , 72 , 73 , 74 , 75 , 76 , 77 ].

Membrane Filtration and Fouling Issue

In membrane filtration, fouling is still the main reason for flux decline, and it needs to be reduced appropriately. Fouling is formed during the membrane filtration process. It is a very complex phenomenon that developed based on a combination of physical, chemical, and biological aspects. Membrane fouling can cause a reduction in permeate flux [ 78 ], an increment of trans-membrane pressure (TMP) [ 79 ], a shorter membrane life span [ 80 ], and consequently, cause a reduction of water quality [ 81 ]. Another work in [ 82 ] also claimed that membrane filtration is fraught with disadvantages regarding the amount of permeate flux and fouling tendency. Membrane filtration treatment is also struggling with a downside where it requires high operation and maintenance costs including labor, chemicals, membrane replacement, energy, and sludge disposal [ 83 , 84 ], when irreversible fouling on the membrane surface is not properly controlled.

Fouling takes place in the membrane filtration process based on four types of foulants: particulates/colloidal, organics, inorganics, and micro-biological organisms [ 85 ]. Table 2 illustrates each foulant type and its associated membrane fouling mode. Particulates or colloids with a similar or close diameter to the membrane pores can cause the membrane pores to become clogged, whereas larger particulates that are unable to pass through the membrane pores can cause the formation of a cake layer on the upstream face of a membrane. Organic and inorganic foulants tend to adsorb and precipitate in the membrane pores and consequently cause blockage of the membrane pores, whereas the accumulation of microorganisms on the membrane surface will cause the development of biofouling. Pore blocking, cake layering, adsorption and precipitation of organic-inorganic fouling, and biofouling occurrences cause a reduction in the rate of permeate production and escalate the complexity of the filtration process.

Types of foulant.

In general, there are two categories of fouling: reversible and irreversible fouling. The reversible fouling which is back washable and non-back washable occurs when organic or inorganic materials accumulate on either side of the membrane surface as operating time increases [ 86 , 87 ]. Back washable reversible fouling can be restored based on physical and hydrodynamic methods, while non-back washable reversible fouling can only be removed based on chemical cleaning. The irreversible fouling is usually occurring after quite a long run of filtration process where the particles formed a matrix that strongly attached to the membrane surface like pore blocking, clogging, biofilm, and cake gel [ 43 , 88 ]. Arise of irreversible fouling caused a loss in transmembrane flux. In this case, the membranes can only be fixed by extensive chemical cleaning. In the worst case, the membrane needs to be replaced.

The occurrence of membrane fouling in membrane filtration processes is due to many factors [ 89 ]. It is due to the characteristics of the feed water, membrane properties, and configuration of the filtration system itself [ 90 ]. Several studies on the factors that influence fouling have been conducted to control and mitigate its development. For example, Mozia et al. [ 91 ] showed the effect of process parameters which are feed cross-flow velocity and TMP, on the fouling behavior of the MF/UF system, whereas Kola et al. [ 92 ] discovered the fouling behavior for different feed water types and different membrane pore sizes. Results indicated that the parameters involved in both studies closely influence the fouling growth rate. In the work of Zhao et al. [ 93 ], the fouling was mitigated by controlling the membrane surface shear rate. The authors observed that by providing a high shear rate, the filtration process was able to achieve high critical flux. High shear rates cause algae to foul the membrane. This claim can be supported by similar research done by Jaffrin [ 94 ]. Table 3 tabularizes the parameters that influence membrane fouling during the filtration process. From the table, systematic approaches can be strategized to provide high-quality drinking water.

Parameter that influences the fouling growth rate.

4. Current Solutions and Way Forward

Numerous fouling reduction techniques have been studied by many researchers to ensure the successful application of membrane filtration systems. In this review, the fouling reduction methods proposed by the previous researchers can be classified into three main categories: chemical cleaning, physical cleaning, and hydrodynamic cleaning [ 111 ], as summarized in Table 4 . Chemical cleaning is a process that is usually used as a pre-treatment method. The process is recognized as a prevention method. It involved chemical agents as a tool to reduce or eliminate the deposition of fouling. Reversible fouling based on the natural organic matter can be partially or fully restored by chemical cleaning. Reversible fouling can also be removed physically. Irreversible fouling can only be removed by chemical cleaning. In general, chemical cleaning is executed when physical cleaning no longer provides effective cleaning performance and the flux cannot restore the environment sufficiently. However, the cleaning method for each filtration process is dependent on many factors. Still, trial-and-error practice is the most suitable method to get the best strategy for any process.

In this review, different views and perspectives on fouling reduction methods are discussed as a way forward to solving the issue, which are prevention, prediction, or control automation. Figure 4 summarizes the main strategies that were used for membrane fouling control. The prevention method is usually related to chemical cleaning, while the prediction and control automation methods are related to physical cleaning. The hydrodynamic technique involves modification of module design and arrangement of flow such as for feed and permeate. The hydrodynamic technique has been studied by Lee et al. [ 112 ] to control the fouling during the forward osmosis-reverse osmosis (FO-RO) hybrid process. The study evaluated the influence of feed flow rate, draw flow rate, and hydraulic pressure difference. The results showed that the high feed flow rate was able to effectively mitigate the fouling. The high draw flow rate, on the other hand, causes an increase in the fouling growth rate. In addition, increasing hydraulic pressure does not affect reducing the fouling growth rate.

Membrane fouling control.

Fouling reduction techniques.

There are also efforts to improve methods by optimizing operational conditions [ 123 , 124 ], both in chemical and physical cleaning operations. To optimize operating conditions, it is important to understand the characteristics of accumulated irreversible fouling. The irreversible fouling is a very complicated phenomenon in which membrane characteristics (membrane materials, pore size, configuration, hydrophobicity, charge), process operating conditions (TMP, temperature, permeate flux), and influent physicochemical properties (particle size distribution, inorganic or organic mattes) closely influence each other. Soft computing optimization is one of the best solutions to handle complex and nonlinear processes.

4.1. Fouling Prevention

Fouling prevention is important in order to prevent fouling (reversible or irreversible) from occurring or arising. In the prevention step, the foulants that can cause fouling are eliminated before the feed water enters and passes through the membrane [ 125 ]. In practice, an irreversible type of fouling is removed using chemical cleaning methods [ 126 ]. For safer operation, acids, bases, and oxidants are usually used in chemical cleaning [ 127 , 128 ]. The quantity of chemical cleaning is monitored to avoid excessive chemical use that can damage the membrane surface and increase the cost of operations [ 129 ]. Therefore, it is important to optimize the operating conditions of chemical cleaning, which involve cleaning intervals, cleaning duration time, chemical type, and chemical concentration. Yoo et al. [ 3 ] showed that proper optimization of operating conditions for chemical cleaning was able to reduce energy consumption, chemical use, and sludge production. However, the process causes an increase in the membrane replacement cycle.

Previous researchers applied numerous techniques as a pre-treatment method of preventing fouling from occurring, such as coagulation [ 130 ], oxidation, ozonation, and adsorption methods [ 73 ]. The coagulation process disperses and suspends contaminants and is suitable for natural organic matter (NOM) with a high molecular weight [ 131 , 132 ]. The process commonly uses pre-hydrolyzed salts such as polyaluminium and sulfates as coagulant agents. The process required low cost due to the simple operation, conversely it produces high sludge formation. Unlike coagulation, the oxidation process produces a lesser amount of sludge formation. The process is useful for the removal of dissolved organic contaminants such as arsenic and humic acid [ 133 ], the same as the ozonation process [ 60 , 134 ]. The main advantage of ozonation treatment is that it does not produce any sludge, but it can cause the degradation of biopolymers and high energy consumption. The adsorption process is frequently used to remove organic and inorganic micro-contaminants from pharmaceuticals and personal care products, such as pesticides, antibiotics, detergents, soaps, and oils [ 135 ]. The process is drive by the electrostatic interaction of negative and positive charged that produce between influent and adsorbent [ 136 ]. The influent, which is a liquid/water or gaseous contaminant, will change into solid formation [ 137 , 138 ]. The absorbent can be restored and reused. The process is capable of removing micro-contaminants even when they are present in trace amounts in water. Adsorption process is more efficient when ozonation is used as a pre-treatment.

Oloibiri et al. [ 139 ] show that discovered that combining pre-treatment methods yields better results in reducing fouling tendency than a single pre-treatment method [ 140 ]. Yu et al. [ 125 ] studied the effect of the conventional coagulation technique and hydrogen peroxide (H 2 O 2 ) addition at three doses level during the backwash process. The performance of the technique was measured based on the rate of TMP. The authors found that the addition of H 2 O 2 at all doses was able to prevent any measurable increase in TMP, which represents the success of the proposed technique to prevent the development of membrane fouling. Wang et al. [ 141 ] used H 2 O 2 in the pre-oxidation process before executing the coagulation process. In this study, the TMP value, microorganism development, and cake layer rate were monitored to observe the effect of biofouling in the presence of H 2 O 2 . Results showed that the proposed technique was able to decelerate the microorganisms’ growth rate and reduce the cake layer, hence decreasing the TMP value, which indicated a reduced membrane fouling tendency.

Park et al. [ 142 ], investigated a pre-ozonation technique based on two doses for the NF process’s surface water brine. The technique was mainly applied to reduce or control membrane fouling, where the doses are determined based on the residual ozone dose. A precise ozone dose is required to avoid membrane damage and an increase in operating costs. The authors observe that the pre-ozonation technique was able to reduce a significant amount of organic fouling potential with relatively low ozone doses. Results also indicated that the applied technique was able to act as a barrier for the removal of trace organic compounds which are important for water treatment.

Another study on the pre-ozonation technique as a membrane fouling prevention method was reported by Wang et al. [ 143 ]. In this work, the effects of pre-ozonation as a pre-treatment for the UF process on secondary wastewater effluents are investigated. The research is based on two types of UF membranes: hydrophilic regenerated cellulose membranes and hydrophobic polyethersulfone membranes. The result showed that high fouling reduction was attained for the hydrophobic membrane at high ozone doses. Table 5 presents the settings for membrane fouling prevention from previous researchers. The results from the previous studies cannot be generalized because the result and consequence of each pre-treatment method are expected to diverge according to the feed water, filtration technology, and pre-treatment material, such as the types of absorbent and oxidation agent.

Setting of membrane fouling prevention in drinking water treatment.

4.2. Fouling Prediction

Many researchers are interested in foul prediction. The prediction will be able to help the researchers forecast the best operating conditions for a particular process and determine the parameter that can trigger the fouling. It is also useful to circumvent or slow down the process of fouling developments. The prediction process is a part of modeling and controlling development. Some researchers used prediction terms to describe the modeling process, which can be categorized as mathematical and empirical processes.

As mentioned previously, the prediction or modeling of physical cleaning operations has been widely utilized by researchers to understand fouling behavior. Physical cleaning operations include air scouring, backwashing, and relaxation operations. Air scouring, also known as aeration or air bubble control, is a widely used method of membrane cleaning. The method boosts the saturation of oxygen by applying air bubbles that exhibit cross-flow velocity and can eliminate reversible fouling [ 150 ]. The backwashing process involves the pumping of permeate or water backward through the filtration module (membrane) in order to remove the particles attached to the membrane surface. Other than permeate, the backwash can be implemented using either chemicals, clean water, or air. Finally, relaxation is a process where the permeating or filtration process is temporarily idle, but with the air bubbles scouring continuously working to relieve the membrane from the generated pressure [ 151 , 152 ]. A comparative study of physical cleaning involving air scouring, backwashing, and relaxation techniques to control the fouling in drinking water treatment was conducted by De Souza and Basu [ 153 ]. In this study, it was shown that in some cases, backwashing and relaxation durations have integrated results for the reduction of fouling, while air scouring can reduce fouling at the highest level with the highest air scour rate. Overall, the result indicated that the combination of the three techniques outperformed air-assisted backwashing alone in terms of fouling reduction. It is crucial to understand the effectiveness of each operation (air scouring, backwashing, and relaxation) when controlling membrane fouling in order to properly strategize the coordination of the operations. Fouling may also be controlled by operating UF under its critical flux [ 26 ]. When UF is operated under its critical flux, foulant deposition on the membrane surface can be avoided. Thus, membranes can be operated with a stable flux. Vigneswaran et al. [ 154 ] also mentioned that the performance of the membrane combined with the adsorption process is influenced by the reactor configuration, mode of operation, carbon dosage, adsorption, and influent characteristics.

Work by Kovacs et al. [ 155 ] proposes a mathematical framework for batch and semi-batch modeling techniques for membrane filtration processes. The proposed method uses feed concentrations as the basis for calculation and can be applied to all pressure-driven membrane filtration processes. The main advantage of the proposed method is that it can capture the dynamic behavior of all types of batches and semi-batch configurations without changing the general mathematical framework. However, it required challenging mathematical problem-solving to obtain the general framework, whereas Ghandehari et al. [ 156 ] proposed a semi-empirical and artificial neural network (ANN) modeling technique to predict the characteristics of microfiltration systems based on permeate flux decline and membrane rejection. Results showed that the semi-empirical method was able to predict the flux only for a specific time, unlike the ANN method. The ANN method can model the membrane filtration system over the entire filtration time for all tested operating conditions.

Ling et al. [ 157 ] proposed a tent sparrow search algorithm back propagation network (Tent-SSA-BP) technique for predicting membrane flux in a membrane bioreactor (MBR) fouling model. They utilized the principal component analysis (PCA) algorithm to reduce the initial auxiliary variables. A study was conducted to compare the genetic algorithm back propagation (GA-BP), particle swarm optimization back propagation (PSO-BP), sparrow search algorithm extreme learning machine (SSA-ELM), sparrow search algorithm back propagation (SSA-BP), and tent particle swarm optimization back propagation (Tent-PSO-BP) networks. The results indicated that the Tent-SSA-BP technique provided the best performance in terms of training speed and prediction accuracy. The Tent-SSA-BP technique predicts with 97.4% accuracy, whereas BP predicts with only 48.52% accuracy. A model for MBR prediction has also been studied by Kovacs et al. [ 158 ], where it predicts transmembrane pressure (TMP) at various stages of the MBR production cycle. The prediction was performed based on a data-driven machine learning technique involving a random forest (RF), artificial neural network (ANN), and long-short-term memory (LSTM) network. Among the proposed methods, RF models provide the best statistical measures. The obtained prediction models produce promising results, but their ability to predict the data is limited at this time. Yao et al. [ 159 ] predict the variation of the TMP in the constant flux mode by proposing a novel method based on the loss of effective filtration area. The result showed a high correlation coefficient, which indicates a good model prediction.

Another study on fouling prediction was conducted by Chew et al. [ 160 ], where the first principle equation of Darcy’s law on cake filtration and ANN were combined to predict the models that represent the dead-end ultrafiltration process. In this study, the turbidity of the feed water, filtration time, and TMP were used as the input parameters. The sensitivity analysis showed that there was a strong linear correlation between specific cake resistance and turbidity. The proposed models can predict the specific cake resistance and total suspended solids (TSS) of feed water with high accuracy, which provide an early indication of fouling development.

Another study on fouling prediction was reported by Lie et al. [ 161 ]. In this work, experiments were conducted on a constant flow microfiltration membrane system at critical flux and supra-critical flux conditions with various permeate fluxes and feed water qualities. In this study, five input variables of the ANN model, including permeate flux, turbidity, UV 254 , time, and backwash frequency were used for the prediction of TMP. The results show that the ANN model with five input parameters can predict TMP behaviors, where the TMP value is used to indicate fouling propensity. A similar study using ANN models was done by Hazrati et al. [ 162 ], where back propagation algorithms were used to predict the effluent chemical oxygen demand (COD) and TMP. The research indicated that the ANN model can easily be used to predict the concentration of COD and TMP in effluent. The study also investigated the specifications of the cake layer at different hydraulic retention times (HRTs) in order to control membrane fouling. Results indicated a linear relationship between the reduction in HRT and the particle size of the cake layer.

An ANN technique was also found by Abbas and Al-Bastaki [ 163 ], where an experiment was conducted using a spiral wound reverse osmosis membrane system with three operating conditions of inputs were studied. The first one was trained using a total of sixty-three data points from different operating temperatures for training purposes. The second one is trained only using forty-two data points corresponding to the operating temperatures of 10 °C and 30 °C; another twenty-one data points corresponding to the operating temperature of 20 °C were employed for testing purposes. The third condition was trained using the data corresponding to the operating temperatures of 10 °C and 20 °C, whereas another 21 data points corresponding to the operating temperature of 30 °C were employed for testing purposes. It was found that ANN was able to interpolate the data with good accuracy but was unable to produce acceptable results for data extrapolation, whereby works by Chen and Kim [ 164 ] used 17% of experimental data for training purposes and 83% for verification. The authors studied the capability of a radial basis function neural network (RBFNN) and a multilayer feed-forward backpropagation neural network (BPNN) to predict the permeate flux in cross-flow membrane filtration. The predictions are based on five input parameters, which are particle size, ionic strength, pH, TMP, and elapsed time. The result shows that a single RBFNN is able to predict the permeate flux and provide better predictability than a BPNN.

Based on the review conducted, it was found that fouling prediction has been broadly applied for the mitigation of membrane fouling. Various techniques have been implemented, but most of them apply ANN as a primary strategy. Due to the difficulty of solving tricky mathematical problems, only a few studies use mathematical frameworks in prediction. Many of them also combine the ANN technique with other methods. The ability of ANN to solve highly complex and nonlinear problems makes it extremely useful in the treatment of drinking water. ANN is capable of providing good predictions even without detailed information about the physical parameters of the system, relying solely on input-output data. Even so, the process of determining appropriate input-output parameters is crucial and plays a significant role. Without a good relationship between the selected input and output parameters, acceptable prediction cannot be achieved. Therefore, it is important to decide the respective input-output parameter before proceeding with the ANN architecture. Every process comes with differences and complex characteristics that are likely due to the system itself. Certainly, changing the concentration of the feed water will change the entire process. As a result, understanding the process in terms of which parameter caused an effect on which parameter is critical in the ANN technique. Table 6 shows the various ANN settings for the prediction of membrane fouling in drinking water treatment. Based on Table 6 , it is clearly shown that each system with different feed water characteristics involves different input and output parameters. Hence, this will affect the ANN architecture.

ANN setting for the prediction of membrane fouling in drinking water treatment.

4.3. Fouling Control and Automation

To overwhelm the problem that fouling causes, it is essential to equip the membrane filtration process with an effective controller. The effective design of the controller will be able to improve the overall efficiency, increase the membrane’s lifespan, and reduce the total operating costs. However, the design of the controller is not an easy task due to the many impediments, such as the dynamic processes of the system itself, the difficulty of modeling the system, variations in feed water quality, system faults, membrane fouling, and the requirement of continuous monitoring for membrane cleaning.

As the system becomes more complex, the control strategies make it easier to handle the membrane filtration process by estimating the uncertainties and making control systems that are robust and reliable. However, based on the literature, there is still a lack of research that applies control automation to the membrane fouling problem [ 171 ]. Most of the previous research focused on open-loop control, membrane modifications, physical cleaning, and pre-treatment methods.

In the previous study on control automation, Azman et al. [ 172 ] applied a proportional-integral-derivative (PID) controller with the Ziegler Nichols (ZN) and Cohen-Coon (CC) tuning methods for the coagulation and flocculation filtration processes. The robustness of the controller’s performance was measured based on the step test, set point change, and load disturbance test. At the end of the study, it was shown that the PID controller with the ZN tuning method exhibits better performance than the PID controller with the CC.

The design of model predictive control (MPC) based on a support vector machine (SVM) model for the ozone dosing process is reported by Dongsheng et al. [ 173 ]. The results have shown an improvement in maintaining a constant ozone exposure compared to the use of the proportional-integral (PI) controller. However, the controller design was only tested for a plant-scale experiment. The design of MPC was also found in the works by Bartman et al. [ 174 ], where the purpose was to determine and control the optimal switching path of flow operating conditions, thereby reducing the fouling problem for a RO desalination process. Results showed that the proposed controller was able to reduce the variation of system pressure, and hence, provide smaller pressure fluctuations with a shorter transition time. The designed MPC can control and prevail over the disturbance that comes through the system and reduce the percentage error between the actual and the desired final steady-state value.

Multiple model predictive control (MMPC) was used in the simulation works of Bello et al. [ 175 ] to control and optimize the amount of chemicals used in the coagulation process of water treatment plants. They applied switching mechanisms to deal with the control input constraints explicitly. Simulation results show that the proposed MMPC provides better performance than conventional control. However, the work is only conducted based on the linear model; future work may use the nonlinear model, which represents the real system. Rivas-Perez et al. [ 176 ] designed an expert model of predictive control (EMPC) to control the critical variables of the pilot scale RO desalination plant. Based on known information, an expert system was created that can lead to decision-making strategies. The robustness of the proposed controller was evaluated based on two real-time cases. In the first case, the performance of EMPC and the ability to ignore disturbances were tested. In the second case, the performance of the proposed EMPC was compared to the performance of the standard MPC. The results showed that the control plant with EMPC provided higher accuracy and robustness than standard MPC, especially for time-varying parameter rejection. Table 7 shows the modeling and control strategy that has been reported based on several techniques by the previous researcher to maximize and control the quality of drinking water treatment.

Setting of control strategy in drinking water treatment.

5. Conclusions

This paper summarizes the available filtration treatments for treating drinking water. Filtration treatment can be categorized into two main types, i.e., conventional and advanced treatment. As discussed in the relevant section, conventional treatment entailed additional costs due to the need for additional treatment and a large footprint, whereas advanced treatment, specifically membrane filtration treatment, is now well-established in the industry because it is capable of overcoming the disadvantages caused by conventional treatment. Membrane filtration treatment achieves satisfactory results in the elimination of different kinds of contaminants from effluent. As a result, the rate of permeate flux (effluent) production will increase. However, membrane filtration is facing problems with membrane fouling as the operating time increases.

Until now, many researchers’ studies on the parameter that causes fouling have resulted in the development of a model for prediction and prevention. Membrane fouling is affected by many factors, including feed water type, feed water and membrane properties, membrane material, filtration strategy, and process operating conditions such as transmembrane pressure and sludge retention time. Previous studies showed that membrane fouling in processes can be very diverse, and it is mainly due to the feed water type and the process treatment itself. In this case, understanding the composition of the feed water and the characteristics of the process treatment are crucial. Fouling mitigation is typically based on prevention, prediction, and control automation process. The prevention method has been utilized broadly and presents promising results for water treatment. The procedure involving the use of chemicals as an agent to mitigate fouling is the method’s main shortcoming. Since the process discussed drinking water, which is closely related to human health, it is remarkable to prevent any approach that could cause undesirable consequences. For the prediction method, a former researcher mostly applied ANN as a tool to predict the development of fouling. Conversely, the study did not discuss in detail the technique to reduce fouling but instead focused only on prediction purposes. Nevertheless, there is not much information presented on control automation strategies. The majority of researchers control membrane fouling through pre-treatment or modification of membrane characteristics, both of which required the use of chemicals. It is critical for ecologically mitigating membrane fouling. Future research is needed to add value to the control automation method via the application of control strategies such as controllers (proportional-integral-derivative controllers, model predictive controllers, etc.). A study on membrane automation is necessary to control the occurrence of fouling without the use of chemical agents. It is thought that this will lead to more exciting discoveries, directly encounter fouling, and produce high-quality drinking water. In the years ahead, it might be switched to fresh strategies and technologies.

Acknowledgments

This work was supported financially by the Faculty of Electrical Engineering and the Centre for Research and Innovation Management (CRIM) from the Universiti Teknikal Malaysia Melaka (UTeM) and the Universiti Teknologi Malaysia High Impact University Grant (UTMHI) vote Q.J130000.2451.08G74. The first author wants to thank the UTeM and the Ministry of Higher Education (MOHE) for the ‘Skim Latihan Akademik Bumiputera’ (SLAB) scholarship.

Funding Statement

This research was funded by the Faculty of Electrical Engineering and the Centre for Research and Innovation Management (CRIM) from the Universiti Teknikal Malaysia Melaka (UTeM) and the Universiti Teknologi Malaysia High Impact University Grant (UTMHI) vote Q.J130000.2451.08G74.

Author Contributions

Conceptualization, M.C.R. and N.A.W.; formal analysis, M.C.R.; investigation, M.C.R.; resources, M.C.R.; data curation, M.C.R. and N.A.W.; writing—original draft preparation, M.C.R.; writing—review and editing, N.A.W. and N.S.; supervision, N.A.W. and N.S.; project administration, N.H.S.; funding acquisition, M.C.R., N.A.W., N.S. and N.H.S. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Data availability statement, conflicts of interest.

The authors declare no conflict of interest.

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• Published: 05 September 2024

Phosphate ester-linked carbonized polymer nanosheets to limit microbiological contamination in aquaculture water

• Anisha Anand 1   na1 ,
• Binesh Unnikrishnan 2   na1 ,
• Chen-Yow Wang 2 ,
• Jui-Yang Lai 1 , 3 , 4 , 5 ,
• Han-Jia Lin 2 , 6 &
• Chih-Ching Huang   ORCID: orcid.org/0000-0002-0363-1129 2 , 6 , 7  

npj Clean Water volume  7 , Article number:  84 ( 2024 ) Cite this article

Metrics details

• Nanoscale materials
• Pollution remediation
• Structural materials

In this study, we developed a simple, low-temperature method to synthesize carbonized polymer nanosheets (CPNSs) using sodium alginate, a biopolymer derived from algae, and diammonium hydrogen phosphate. These nanosheets are produced through a solid-state pyrolysis at 180 °C, involving dehydration, cross-linking through phosphate ester bonds, and subsequent carbonization, forming 2D structured CPNSs. These synthesized CPNSs exhibit excellent bacterial adsorption capabilities, particularly against V. parahaemolyticus and S. aureus . When applied to ordinary filter paper, the CPNS-modified paper efficiently filters bacteria from aquaculture water, removing over 98% of V. parahaemolyticus within two hours and maintaining effectiveness after 24 h. In contrast, control filter paper showed significantly reduced efficiency over the same period. Our filtration tests demonstrated enhanced survival rates for shrimp in aquaculture systems, highlighting the potential of CPNSs-modified filter paper as a suitable treatment to reduce the microbiological contamination levels in recirculating aquaculture systems in the event of a disease outbreak.

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Introduction.

The properties and ultimate applications of carbon-based nanomaterials (CNMs) are defined by their structure and surface functionalization, hence, research endeavors are directed toward fine-tuning these characteristics 1 . 2D nanosheets (NSs) are remarkable for their nanoscale thickness and high surface-to-volume ratio, attributes that render them suitable for diverse applications, such as energy storage devices, optoelectronics, sensors, composite materials, water purification and filtration, and even biomedical applications 2 , 3 , 4 , 5 , 6 , 7 , 8 . Particularly, carbon nanosheets (CNSs), including graphene, graphene oxide (GO), and reduced graphene oxide (rGO), have demonstrated efficacy in areas like energy storage and batteries, fuel cells and solar cells, gas separation, and biomedical applications 4 , 9 , 10 , 11 , 12 . These benefits can be attributed to their unique honeycomb structure and the ease of surface functionalization, which enable tailoring for specific applications 1 , 4 . Besides the sheet-like structures, there has been a rise in reports on CNMs with different morphologies like carbon microtrees, flower-like carbon, carbon cones, carbon cubes, and carbon nanocontainers in the past years 13 , 14 , 15 , 16 , 17 . Despite these varied structures, 2D nanomaterials are favored for creating stable thin films and membranes due to their large surface area, customizable porosity, high chemical stability, and potential for layered arrangements 18 . Hybrid nanosheet membranes have been reported as superior candidates for molecular separation processes when compared to stand-alone membranes 19 , 20 . CNSs have displayed potential for the adsorption of organic pollutants from water, including pesticides and pharmaceuticals 21 , 22 . Furthermore, the CNS-based nanocomposites can also function as photocatalysts, initiating the breakdown of these pollutants 23 . Additionally, CNSs have exhibited antimicrobial or bacteria adsorption properties, offering an effective method for deactivating or eliminating harmful microbes when used in water purification processes 24 , 25 , 26 , 27 .

Various methods for synthesizing CNSs have been adopted, including chemical vapor deposition (CVD), mechanical/ultrasonic/chemical/electrochemical exfoliation, epitaxial growth, solvothermal synthesis, and thermal decomposition 2 , 28 , 29 , 30 . Each of these methods possesses unique requirements suited to its specific synthesis process. For instance, CVD requires hydrocarbon gases as precursors, exfoliation is most effective with raw materials featuring a multi-layered structure and weak inter-layer interactions, and solvothermal synthesis typically employs small molecules 2 . Control over the sheet size has been achieved through different parameters and approaches, such as the utilization of oxidants, density gradient ultracentrifugation, pH-assisted sedimentation, and the application of sonication. However, these methods often require harsh reaction conditions, resulting in high costs, and lower yield 2 , 31 , 32 . Furthermore, CNSs tend to restack, compromising the unique properties of single-layered structure. The introduction of functional groups on CNS mitigates this issue 1 . Furthermore, many existing methods for synthesizing CNSs are typically constrained to small-scale laboratory processes. Scaling up these synthesis techniques while preserving the desired properties and quality of the materials remains a significant challenge. Consequently, the current research is centered on devising efficient, scalable, and economically viable methods for synthesizing stable CNSs using cost-effective precursors, such as biomass 30 , 33 , 34 .

While existing methods for the synthesis of CNSs often involve high temperatures, solvents, or sophisticated instruments, our approach focuses on a simple low-temperature synthesis procedure, and the use of marine polysaccharides. In this study, we developed carbonized polymer nanosheets (CPNSs) from the polysaccharide, sodium alginate (Alg) and diammonium hydrogen phosphate (DAHP) through low-temperature synthesis in solid-state (Fig. 1 ). We proposed the formation mechanism of CPNSs could be through the cross-linking of Alg units via phosphate diester linkages. Though previous reports indicate that CNSs exhibit antibacterial activity toward both Gram-negative and Gram-positive bacterial species 35 , 36 , 37 , 38 , our CPNSs do not possess inherent antibacterial activity. Nevertheless, when adsorbed onto a filter paper support, they effectively remove bacteria via specific adsorption, and no leakage of bacteria was observed from the membrane even after 24 h. The recirculating aquaculture system (RAS) has demonstrated its eco-friendly nature, water efficiency, and exceptional productivity in farming 39 . However, if pathogenic bacteria are introduced into the RAS, they may survive and recirculate in the system which poses a potential risk to the aquatic species in the system 40 . In this work, we employed these CPNSs-modified filter papers to reduce artificial Vibrio parahaemolyticus ( V. parahaemolyticus ) contamination in aquaculture water.

Heating of Alg/DAHP mixture in solid-state at 180 °C leads to phosphorylation at the equatorial hydroxyl groups of the mannuronic units in the Alg chain resulting in cross-linking and 2D polymer sheet formation via phosphate ester linkage, followed by carbonization to form CPNSs.

Results and discussion

Dahp plays a crucial role in the formation of cpnss.

CPNSs obtained by heating a solid mixture of Alg and DAHP in 1:5 mass ratio at 120, 150, 180, 210, and 240 °C for 3 h were denoted as CPNSs-120, CPNSs-150, CPNSs-180, CPNSs-210, and CPNSs-240, respectively. The Alg/DAHP mixture was colorless and showed a mild color change to off-white at 120 °C. The mixture experienced mild dehydration at 150 °C, resulting in light brown hue, and showed higher degree of carbonization at 180 °C and above displaying brown or black color (Fig. 2A ). The transmission electron microscopic (TEM) images in Fig. 2B show that the Alg/DAHP mixture without heating had a gel-like structure, and the morphology changed to polymeric form at 120 °C. From 150 °C and above, large sheet-like formation can be observed. At 180 °C, the mixture forms 2D layered CPNSs. The resulting CPNSs-180 have sizes ranging between 200 and 500 nm, with a thickness of approximately 1.43 ± 0.25 nm and surface roughness was calculated to be 0.23 nm, as determined by atomic force microscopy (AFM) (Fig. 2C ). This thickness is considerably greater than that of single-layered graphene, which ranges from 0.4 to 1.0 nm, and GO, with a range of 0.7−1.2 nm 41 . The surface roughness (0.23 nm) is higher than that reported (0.2 nm) for single-layer free-standing chemically modified GO 42 , which suggests the presence of polymeric alginate fragments on the CPNSs. At higher carbonization temperatures of 210 and 240 °C, sheet-like structures were formed; however, along with other carbonized products with different morphology and their aggregated forms were adsorbed onto the sheets (TEM images in Fig. 2B ).

Dry heating of a mixture of Alg and DAHP in 1:5 ratio ( A ) photographs and ( B ) TEM images of products obtained by heating at different temperatures. C AFM image of CPNSs-180. The horizontal line and the rectangle box indicate the thickness mapping and roughness, respectively, calculated using NanoScope Analysis 2.0 software. D Raman spectra of GO, Alg, and CPNSs prepared at different temperatures.

Alg suspended in sodium phosphate buffer (5 mM, pH 7.4) exhibits a negative charge, characterized by a zeta potential of ca. −50 mV (Supplementary Table 1 ). The zeta potential slightly changed as the synthesis temperature increased during the preparation of CPNSs to ca. −46 mV for 180 °C. However, with subsequent temperature increments, the zeta potential decreased significantly to ca. −11 mV, probably due to a higher degree of carbonization resulting in elimination of carboxylate functional groups. As the synthesis temperature increased, the hydrodynamic size of the CPNSs increased significantly for 210 and 240 °C. The thermal-driven dehydration and cross-linking of Alg lead to the formation of sheet-like structures. The significant carbonization at higher temperatures results in the stacking of CPNSs and other carbonized particles to form larger aggregates.

The UV–vis absorption spectra of the CPNSs prepared at different temperatures are presented in Supplementary Fig. 1 . CPNSs obtained at 180 °C and above showed a band at ca. 285 nm and extending to the visible region of the spectra attributed to the π → π * electronic transition of the aromatic sp 2 domains of the C=C and n  →  π * transition of C=O and N-containing functional groups, respectively 43 . The baseline of the spectra in the entire wavelength region increased with the increase in the synthesis temperature due to the formation of larger nanosheet structures. Alg exhibited an X-ray diffraction (XRD) pattern with peaks at 2 θ of 13.7°, 21.6°, and a broad band around 39.0° corresponding to the (110) plane of polyguluronate unit (G), (200) plane of polymannuronate (M), and amorphous halo, respectively (Supplementary Fig. 2 ) 44 . The crystallinity of Alg is due to inter and intramolecular hydrogen bonding. When heated at 150 °C and higher, the XRD peak corresponding to guluronate and mannuronate structure disappeared. Instead, a broad peak emerged, indicating the presence of carbonized nanostructures with disordered carbon phases 45 . The Raman spectra of Alg and CPNSs synthesized at different temperatures were compared with that of GO (Fig. 2D ). Alg did not show D and G bands, whereas, CPNSs prepared at temperatures 180 °C and above showed the D band around 1350 cm −1 and G band around 1600 cm −1 , and their intensity and shape gradually increased and sharpened, respectively, with synthesis temperature. Nevertheless, they are still not well defined as that of GO and therefore, do not reflect structures identical to GO, due to a low degree of graphitization and ultrasmall size of the graphene-like domains 46 . Heating the precursors such as sodium alginate and sucrose at high temperatures (>500 °C) and inert atmosphere (e.g., N 2 and Ar) produces carbon with well-defined D and G bands 47 , 48 . However, in this work, no high temperature or inert atmosphere was used. The G band is due to the formation of in-plane stretching of carbon-carbon bonds in the aromatic rings of graphene-like structures 49 , revealing that DAHP assists in the alignment of polymer chains and carbonization of Alg to form CPNSs; whereas, the D band indicates the amorphous and disordered nature of graphene-based materials 50 . The oxygen (O), nitrogen (N), and phosphorous (P) contained functional groups and dopped in the CPNSs disrupt the periodicity and long-range order of the graphene lattice, leading to a loss of crystallinity. Therefore, the CPNSs must be carbonized alginate having carbon-based structures with distinctive polymeric characteristics.

To verify whether Alg could form nanosheets in the presence of other ammonium-, phosphate- or sulfate-containing compounds upon heating at 180 °C, we carbonized Alg in the presence of ammonium dihydrogen phosphate ((NH 4 )H 2 PO 4 ), phosphoric acid (H 3 PO 4 ), ammonium hydroxide (NH 4 OH), NH 4 OH/H 3 PO 4 mixture, and disodium hydrogen phosphate (Na 2 HPO 4 ) (Supplementary Fig. 3 ). It is noteworthy to mention that CPNSs were obtained only with (NH 4 )H 2 PO 4 . Alg did not form nanosheet structures in the presence of H 3 PO 4 , NH 4 OH, H 3 PO 4 and NH 4 OH mixture, or Na 2 HPO 4 . Also, CPNSs were not formed with Alg in the presence of other sulfates, sulfite, and ammonium-related salts, such as ammonium sulfite ((NH 4 ) 2 SO 3 ), ammonium sulfate ((NH 4 ) 2 SO 4 ), ammonium chloride (NH 4 Cl), and sodium sulfite (Na 2 SO 3 ) in the same mass ratio (Supplementary Fig. 4 ). Thus, we conclude that solid-state heating at 180 °C and DAHP have a crucial role in the formation of perfect CPNS structures.

Phosphate diester linkages mediate the formation of CPNSs

In order to investigate the process of CPNSs formation, we performed a time-course analysis of Alg/DAHP mixture that was heated at 180 °C for a duration of 3 h (Fig. 3 ). As the heating progressed, notable changes occurred. Within 5 min, the color of the mixture shifted to a pale brown hue, followed by a transition to dark brown at 15 min (Fig. 3A ). Eventually, within 30 min, the mixture turned black due to carbonization. Concurrently, the time-course TEM analysis demonstrated the formation of supramolecular structures by Alg within the initial 5-min heating period (Fig. 3B ). At 15 min, it tended to form thick and large sheet-like structures, which subsequently became thin at 30 min. After 3 h of heating thin-layered clean sheets of varying sizes were formed, likely due to fragmentation during the later stage of the thermal process. The time-course XRD pattern of the products and Alg are presented in Fig. 3C . Over time, a noticeable alteration in the crystallinity of Alg becomes apparent. The crystallinity in Alg arises from the arrangement of G (2 θ  = 13.7°) and M (2 θ  = 21.6°) units and the amorphous halo (broad peak centered at 2 θ  = 39°), which are disrupted and realigned during the heating. The amorphous halo completely disappeared after 15 min. The appearance of a broad peak centered at 2 θ of 26.2° indicates very small graphene domains in the carbonized products with highly disordered carbon 45 , 46 . This transformation, denoting carbonization, can be observed from 30 min onwards. The hydrodynamic diameter of the CPNSs by heating for different time intervals shows an increase in size to as high as ca. 2200 nm at 30 min, which decreases to ca. 440 nm after 3 h due to fragmentation of the polymer sheets during carbonization (Supplementary Table 2 ). The TEM-energy-dispersive X-ray spectroscopy (EDS) mapping of CPNSs-180 displayed in Supplementary Fig. 5A confirms the presence of nitrogen and phosphorus, and the HRTEM image and the selective area electron diffraction (SAED) pattern suggest the low crystalline nature of the CPNSs (Supplementary Fig. 5B ), in agreement with the XRD pattern. If the 2D structures are not carbon nanosheets, they might instead be black phosphorous nanosheets. It is a thermodynamically stable allotrope of phosphorus with 2D structure of atomic arrangement very similar to that of graphite 51 . Black phosphorus is highly crystalline and has orthorhombic structure 52 . However, the XRD patterns in Fig. 3C and Supplementary Fig. 2 and SAED pattern in Supplementary Fig. 5B did not show any crystalline properties corresponding to black phosphorus. Furthermore, black phosphorus is formed only at very high temperatures and pressures 51 . Thus, the possibility of 2D phosphorus allotropes can be ruled out, and we believe the 2D nanostructures obtained by heating a mixture of Alg and DAHP as shown in Fig. 2B must be CPNSs.

A photographs, B TEM images, C XRD patterns, and D 31 P NMR spectra.

The molecular structural changes occurring during the heating were further studied by Fourier-transform infrared spectroscopy (FTIR) (Supplementary Fig. 6 ). The FTIR spectrum of Alg exhibited specific vibrational modes, such as –OH stretching at 3200−3400 cm –1 , asymmetric stretching of –COO – at 1610 cm –1 , the symmetric stretching of –COO – at 1412 cm –1 , and the C–O–C (ring) vibrational modes at 1081 cm –1 of the pyranose rings 53 . The C–O(H) symmetric vibration peak appeared at 1306 cm –1 , and stretching vibration of the C–O–C glycosidic linkage in alginate polymer appeared at 1036 cm –1 . FTIR spectra of Alg show significant changes in peaks at lower wavenumber region, 500–1700 cm –1 for different durations of heating with DAHP. Notably, the C–O(H) peak at 1306 cm – 1 decreased significantly with heating time, probably due to the dehydration process. The –C–O(H) symmetric vibration peak at 1306 cm –1 began to disappear within 5 min, meanwhile a new P=O asymmetric stretching peak emerged at 1246 cm –1 and then started disappearing after 30 min. A peak at 1739 cm –1 appeared after 5 min and disappeared after 30 min, indicating some new carbonyl groups of esters are formed and then degraded with time 54 . After 15 min of heating, new peaks emerged at 1054 cm –1 , corresponding to P–O–C stretching in the phosphate ester bond. The Alg after reaction with DAHP and without heating (i.e., 0 min) showed an O–P–O bending vibration peak of the phosphate ester at 546 cm –1 , which slightly shifted to 515 cm –1 after 5 min onwards and decreased significantly after 1 h. A new peak emerged at 1054 cm –1 , corresponding to P–O–C stretching in the phosphate ester bond. It can be inferred that Alg polymer chains are cross-linked via phosphate diester bonds. During the heating, (NH 4 ) 2 HPO 4 undergoes thermal decomposition to form various chemical species, such as NH 3 ( g ) , (NH 4 )H 2 PO 4 ( s ) , and H 3 PO 4 ( l ) 55 . The phosphoric acid reacts with Alg to form esters, which is in agreement with a similar work reported by Marcilla et al., in which the various acid species react with different compounds in tobacco to form esters 55 . Meanwhile, a portion of the NH 3 formed by the thermal degradation of (NH 4 ) 2 HPO 4 could form quaternary ammonium salts of the carboxylic acid group 55 .

We conducted the 31 P nuclear magnetic resonance (NMR) spectroscopy analysis of the time course formation of CPNSs-180 (Fig. 3D ). The 31 P NMR peak of phosphate group appeared at a chemical shift of 0.87 ppm for the purified, non-heated Alg/DAHP mixture, indicating the adsorption of phosphate on the Alg. Upon heating for 1 min at 180 °C, the peak shifts a little downfield (to 1.21 ppm), probably due to the formation of monoesters 56 , 57 . With a further increase in heating time to 7.5 min, peak resonances toward an upfield of the central peak were observed (–5.39, –8.50, and –9.26 ppm), depicting the formation of phosphate diesters and pyrophosphate structures. After 15 min of heating, the peaks for pyrophosphate (–8.50 and –9.26 ppm) disappeared, and with further heating the peaks at 1.21 and –5.39 ppm depicting phosphate esters, including monoesters and diesters remained.

The plausible mechanism of the formation of CPNSs is illustrated in Fig. 1 . The formation of diester in solid-state will form bridges between the alginate polymer chains to form 2D polymer sheets. A previous report reveals that dry phosphorylation of starch using orthophosphate occurs through the reactive hydroxyl groups of the starch molecules to form brown-colored products at temperatures of 170 °C and higher 58 . Investigation on the phosphorylation of Alg using urea/phosphate system using various NMR spectroscopy techniques revealed that the most probable site for phosphorylation is the equatorial hydroxyl group of mannuronic acid units in the polymeric chain 53 . Therefore, it is evident that heating of Alg with DAHP in a solid-state initially leads to the formation of sheet-like structures formed by the cross-linking of Alg polymer chains via phosphate diester linkages. Since this reaction system contains a mixture of monoesters, diesters, and unreacted alginate chains, the carbonized product contains CPNSs along with polymeric Alg featuring nonspecific shapes. The molecular arrangement in pure Alg is mainly due to the solid-state intra- and inter-molecular hydrogen bonding, which are disrupted upon heating above 170 °C and produce carbonized products without specific shape 59 , 60 . However, the formation of cross-linking among the alginate polymer chains by phosphate ester bonds dominates the formation of a stable 2D structure, resulting in carbonized polymeric nanosheets.

The elemental composition of Alg and the products obtained at various time intervals is presented in Supplementary Table 3 . The carbon content (weight percentage) of pure Alg has been found to be 29.16%, which is close to the values reported by previous studies, and some report reveals that it varies with the harvesting season of the algae 60 , 61 , 62 . The carbon and oxygen contents of Alg after reacting with DAHP (i.e., CPNSs-180 0 min) is determined to be 19.94% and 43.34%, respectively. The carbon content increased up to 38.73% and the oxygen content decreased to 33.93% after being heated at 180 °C for 3 h, indicating carbonization to form slightly carbonaceous nanomaterials (i.e., CPNSs). A previous report also suggests that P/N-doped carbon dots synthesized at low temperature (90 °C) possess a low degree of carbon content with a weight percentage of 8.62 63 . Carbonization of precursors in semi-closed atmosphere and temperatures as high as 900 °C has been reported to yield carbonized products with higher carbon content (as high as ~69%) and low oxygen content (10%) 64 . The carbonized polymer products have nitrogen doping, and the final CPNSs obtained after 3 h possess 6.8% nitrogen by weight. Alginate polymer has high affinity toward phosphate ions 65 and it forms phosphate ester after reacting with the DAHP during the heating process, resulting in high phosphorous content (12.45%) in the CPNSs as determined by inductively coupled plasma optical emission spectroscopy (ICP-OES). The initial heating of Alg/DAHP mixture at 180 °C leads to the crosslinking of the hydroxyl groups of the DAHP with the side chain hydroxyl group of the Alg to form phosphate ester bond 66 . With an increase in the reaction time, carbonization progressed, resulting in a decrease in P content. At 3 h of heating intense phosphorylation and phosphorus doping occurred in the CPNSs due to the presence of phosphoric acid and P 2 O 5 in the system thereby increasing the P content to 12.45%. Thus, the CPNSs obtained from Alg by carbonization in the presence of DAHP are N and P co-doped. The C1s, O1s, N1s, and P2p XPS spectra of the CPNSs synthesized at 180 °C for 3 h are presented in Supplementary Fig. 7 . The CPNSs have oxygen-containing functional groups and N-doping in the form of pyridinic (399.1 eV), graphitic (399.85 eV), and pyrrolic (400.66 eV) nitrogen. The deconvoluted P2p spectra of the CPNSs show peaks at 132.49, 133.01, 133.63, and 134.42 eV corresponding to the presence of P–C, P–O, P–O–C, and P=O bonding, respectively, which confirm phosphorous is incorporated in the CPNSs 67 .

CPNSs-modified filter paper for efficient removal of bacteria

The CPNSs were tested for their antibacterial activity toward Gram-negative Escherichia coli ( E. coli ) and Gram-positive Staphylococcus aureus ( S. aureus ) bacteria and toward V. parahaemolyticus , a Gram-negative bacteria that poses a significant risk in aquaculture. V. parahaemolyticus can rapidly multiply and infect cultured seafood species, which not only damages the health of these species but also increases the risk of foodborne illnesses when these products are consumed 68 . Different from the GO nanosheets which exhibit antibacterial activity through different mechanisms such as direct interaction with the bacteria through their sharp edges or by wrapping on the bacterial cell 69 , 70 , the CPNSs having 2D structure do not show antibacterial activity (Supplementary Fig. 8 ). The difference in the antibacterial behavior of the CPNSs may be ascribed to only adsorbing bacteria by the polymeric structures on the CPNSs’ surfaces but could not further disrupt the bacterial membranes. It has been reported that the functional groups on GO play a crucial role in its antimicrobial activities 71 . However, these CPNSs, when modified on filter paper could effectively remove bacteria from contaminated water. Incubating the filter paper with CPNSs resulted in the effective coating of the nanosheets on the fibers, as evident from the scanning electron microscopic (SEM) images in Fig. 4A , however, it did not form a separate layer above the filter paper. The AFM images of different CPNSs-modified filter papers show the CPNS coatings on the filter paper fibers, highlighting noticeable changes in their surface morphology (Supplementary Fig. 9A ). The roughness values are 196 nm for the unmodified filter paper, and 111, 300, 220, and 465 nm for the CPNSs-150-, CPNSs-180-, CPNSs-210-, and CPNSs-240-modified filter papers, respectively. CPNSs-150 is mostly alginate polymer with very less degree of carbonization, which provides a smooth surface finish to the filter paper similar to that of the role of starch providing smooth finish to paper fibers. Meanwhile, the coating of CPNSs-180-, CPNSs-210, and CPNSs-240 enhanced the surface roughness of the filter papers. Kelvin probe force microscopy (KPFM) analysis on both the unmodified and CPNSs-modified filter papers is presented in Supplementary Fig. 9B . The KPFM results provide insights into the surface potential variations, which serve as an indirect measure of the zeta potential of the membranes. This analysis helps us understand how the surface properties of the filter papers are altered following modification with CPNSs. The constant potential difference (CPD) of the probe ( V CPD  = 5.5 V) was determined using the work function ( V probe  = 5.2 eV) of gold film as standard, with its CPD (−300 mV) obtained from KPFM potential mapping. The surface potential of the sample ( V sample ) was calculated using the relation ( V sample = V probe − V CPD ). The surface potentials for unmodified filter paper and CPNSs-modified filter papers (CPNSs-150, CPNSs-180, CPNSs-210, and CPNSs-240) were found to be 4.67 V, 4.95 V, 4.82 V, 4.92 V, and 4.95 V, respectively. The surface chemistry of the CPNSs-modified filter paper, with plenty of surface functional groups and alginate-derived polymer fragments, is illustrated in Supplementary Fig. 10 . Modification with CPNSs alters the surface properties of the filter papers demonstrated by KPFM (Supplementary Fig. 9B ), further confirming that CPNSs have been successfully modified on the filter paper. The higher and more uniform surface potentials of CPNSs-modified filter papers suggest an increased surface charge density with potential gradients, enhancing the filter’s ability to adsorb and trap bacteria through electrostatic and polar interactions, as well as interactions between bacterial fibrillar adhesins and the alginate on the CPNSs’ surface. The CPNS-modified filter paper was effective in removing V. parahaemolyticus (10 5 CFU mL −1 ) from contaminated seawater samples, with CPNSs-180 and CPNSs-210 showing superior effects (>90%) (Fig. 4B ). Notably, neither the bacteria’s morphology nor the bacterial membrane was disrupted after passing through the CPNS-modified membrane (Fig. 4C ). Adsorbing bacteria without membrane disruption is advantageous, preventing toxin release into the filtrate. For instance, disruption of V. parahaemolyticus ’ cell membrane releases toxins like PirA and PirB proteins, which induce necrosis and functional loss in the hepatopancreas of shrimp 72 .

A Photographs, SEM images, and agar plates showing bacterial removal efficiency of CPNSs-modified filter paper (0.2 mg cm −2 ) as compared with that of control. B Relative viability of V. parahaemolyticus after passing through paper membrane coated with CPNSs (0.2 mg cm −2 ) at a water flux of 400 mL min −1 m −2 . Error bars in B represent the standard deviations from triplicate experiments. C V. parahaemolyticus (a) before and (b) after passing through CPNSs-180-modified filter paper. D SEM images of CPNSs-modified filter paper after passing 10 mL 10 7 CFU mL −1 solution of V. parahaemolyticus . Magnification: (a) ×1.0k and (b) ×5.0k. The red circles indicate the bacteria adsorbed on the CPNSs-coated filter.

We further evaluated CPNSs-180-modified filter paper for the removal of E. coli and S. aureus . The removal efficiency for E. coli was less than 30%, and that for S. aureus was around 80% (Fig. 5 ). The difference in the bacterial removal efficiency of CPNSs-modified filter paper may be attributed to the different bacterial shapes and membrane structures 24 , 27 , 73 . The SEM image of the CPNS-modified filter paper after passing the V. parahaemolyticus bacteria solution clearly shows bacteria trapped on the membrane (Fig. 4D ). In contrast to our previous work, graphene oxide@carbon nanogels (GO@CNGs)-modified membrane reported for the removal of bacteria from contaminated water, where the efficiency of the membrane decreases with an increase in water flux 24 , the efficiency of the CPNS-modified membrane was not affected by the increase in water flux (Supplementary Fig. 11 ). Supplementary Fig. 12A shows post-filtration SEM images of control filter paper (a) and CPNSs-180-modified filter paper (b) after passing V. parahaemolyticus solution, at 0.5k magnification, and (c) and (d) show the same at 5k magnification. The control filter showed no trapped bacteria; instead, salt crystals from the sodium phosphate buffer containing 3% NaCl were evident. In contrast, the CPNSs-180-modified filter paper displayed adsorbed V. parahaemolyticus on its surface. Notably, changes in the morphology of the filter paper due to the filtration process were minimal and difficult to discern. Supplementary Fig. 12B shows the SEM images of control filter paper (a) and CNPSs-180-modified filter paper (b) after circulation of sodium phosphate buffer at a flow rate of 1150 L min −1 m −2 for 24 h. These images confirm that the structural integrity of the filter papers remained intact, with no significant damage observed.

A , B The bacterial removal efficiency of the CPNS-modified filter papers (0.2 mg cm −2 ) toward 10 5 CFU mL −1 E. coli and S. aureus at a water flux of 400 mL min −1 m −2 . C TEM images of E. coli and S. aureus before and after passing through CPNS-180-modified filter paper. Error bars in B represent the standard deviations from triplicate experiments.

Live/dead staining images of bacteria samples (10 7 CFU mL −1 ), before and after passing through the membrane and from the samples collected from the membrane surface are presented Supplementary Fig. 13 . The fluorescence microscopic images showed significant amount of bacteria in the filtrate and a few numbers on the membrane surface of control filter paper. In contrast, no viable bacteria was observed in the filtrate of CPNSs-180-modified filter paper and a significant amount of live bacteria was seen on the membrane surface. Furthermore, we performed the flow cytometry of the bacteria samples before and after filtration, and those trapped on the filter paper, after stained with live/dead stain [LIVE/DEAD BacLight Bacterial Viability Kit containing green-fluorescent SYTO™ 9 dye and red-fluorescent propidium iodide]. The flow cytometry results showed that the gating strategy and staining were effective in distinguishing live and dead cells (Supplementary Fig. 14 ). 84.9% viable bacterial population was observed in the control group, with a smaller portion being injured (15.1%), and negligible amounts of debris or unstained cells were noted. The filtrate from the control filter paper showed around 75.9% bacteria, with a very small amount observed on the membrane surface. Notably, the amount of bacteria observed in the filtrate of CPNSs-180-modified filter paper was negligible compared to the control, and a very high amount of viable bacteria was observed on the membrane surface. Additionally, a negligible amount of debris was observed.

While most of the reported membranes used for the removal of bacteria are based on the modification of polymer membrane with antibacterial material such as GO 5 , 27 , this study employed filter paper to modify the surface with CPNSs. Modifying the commercial membrane filters using GO-derived materials improves the antibacterial properties of the membrane filters due to the combined effect of bacterial retention by the membrane filters and the antibacterial properties imparted by the GO 27 . However, the removal efficacy of the CPNSs-modified filter paper is solely based on the adsorption property, rather than bacterial inactivation and retention. Filter papers are rarely reported for filtering out bacteria samples due to their large pore size. A report by Ottenhall et al. demonstrates the use of a cellulose-based filter modified with cationic polyelectrolyte polymer and anionic polyelectrolyte polyacrylic acid in multilayers to remove bacteria from contaminated water 74 . The filter traps the bacteria through electrostatic interactions and the removal efficacy increased with the increase in the number of filter layers. Another report by Mansur-Azzam et al. shows the modification of the filter paper with cationic binder polyacrylamide followed by coating with triclosan-loaded micelle to remove bacteria and the efficacy depends on the time of interaction of bacteria with the micelle and the thickness of the membrane 75 . The CPNSs feature a sheet-like morphology, similar to GO, with copious polymeric alginate fragments on their surfaces. This structure enables effective bacterial removal from contaminated water. Notably, polysaccharide-modified GO has demonstrated a significant capacity to interact with bacteria 76 , which aligns with the properties of our CPNSs-modified filter papers. V. parahaemolyticus secretes a variety of adhesin proteins that facilitate attachment to host cells, tissues, and non-biological surfaces 77 . We hypothesize that the interaction between these fibrillar adhesins and the alginate on the CPNSs’ surface is key to the adsorption process, effectively capturing the bacteria on the membrane 77 , 78 .

Combating V. parahaemolyticus

The efficiency for the removal of V. parahaemolyticus at a higher concentration (10 7 CFU mL −1 ) was further evaluated in aquarium condition (2 L water in the aquarium tank) using the CPNSs-modified filter paper with a larger surface area (17.34 cm 2 ). The CPNSs-modified filter paper was effective in eliminating >98% V. parahaemolyticus within 2 h of circulation (Fig. 6A ), and no leakage of bacteria was observed from the membrane even after 24 h. The uncoated filter paper (Ctrl) showed removal of ca. 70% within 1 h; however, it decreased with time and down to ~17% after 24 h, which shows that though the filter paper can adsorb bacteria, upon continuous passage of water, the bacteria are washed from the membrane back to the solution, due to the large pore size of the membrane and weak affinity toward V. parahaemolyticus . It is noteworthy that the bacterial removal efficiency for the GO-coated filter paper was ~74% after 4 h and remained stagnant beyond that time, probably due to the clogging of pores due to fouling of the membrane 79 ; which shows the superior efficacy of our CPNSs-modified filter paper. The decrease in water flux and removal efficiency of the membrane due to the clogging of pores of the membrane is a major drawback in membrane-based filtration systems.

A Relative removal of V. parahaemolyticus (10 7 CFU mL −1 ) from an aquarium tank at different time periods upon passing through control membrane (uncoated filter paper), CPNSs-180- and GO-modified filter papers, at a flow rate of 1150 L min −1 m −2 . B The survival of shrimp infected with V. parahaemolyticus (10 6 CFU mL −1 ) and circulating water through control membrane (uncoated filter paper) and CPNSs-180-coated filter paper. Error bars represent the standard deviation of triplicate experiments.

Therefore, we further performed the shrimp challenge experiments with the CPNSs-modified filter (Fig. 6B ). After challenging white leg shrimp ( Litopenaeus vannamei , 10 no. in 2 L sea water) with V. parahaemolyticus (10 6 CFU mL −1 ), CPNSs-180-modified filter paper was loaded onto a filter holder, and the aquarium water was circulated through it, and the results were compared with that of the control filter (filter paper without CPNSs-180 coating) and control (without any filter paper or membranes). The shrimps in the CPNSs-modified filter paper group showed 100% survival even after 48 h, while that of the other two groups decreased to <50%. After 72 h, the survival rate of the shrimp decreased to 10% and 20% for the control and the control filter paper group, respectively; >50% survival rate was observed in CPNSs-modified filter paper group. Therefore, we hope that the CPNSs-modified filter may serve its use for filtering out even a very high concentration of bacteria contaminated in aquarium water. Notably, replacing the CPNSs-modified paper every 48 h after filtration could remove the bacteria completely without affecting the survival rate of the shrimp upon Vibrio infection (Supplementary Fig. 15 ). Though the control filter paper was also replaced every 48 h, only 10% survival was observed after 96 h.

In summary, this study introduces a low-temperature carbonization process of Alg with DAHP to fabricate robust 2D carbonized nanomaterials designed for pathogen filtration in aquaculture waters. This method offers significant advantages by avoiding the need for high-temperature conditions, sophisticated equipment, and hazardous chemicals, thereby enhancing sustainability and cost-effectiveness. The resulting CPNSs exhibit substantial bacterial adsorption capabilities, notably against V. parahaemolyticus , which could significantly improve aquaculture health and productivity. Our observations indicate that replacing CPNSs-modified filter papers every 48 h even under high bacterial loads ensures optimal performance, suggesting a practical solution for managing microbiological risks in recirculating aquaculture systems. While the study marks a promising step forward, continued research is necessary to fully assess the long-term benefits and any potential limitations of this approach. Ultimately, the findings underscore the potential of CPNSs to contribute significantly to sustainable aquaculture practices.

Sodium alginate (Alg, >98%) and ammonium chloride (NH 4 Cl) were purchased from ACROS (Geel, Belgium). Diammonium hydrogen phosphate ((NH 4 ) 2 HPO 4 ) and ammonium dihydrogen phosphate ((NH 4 )H 2 PO 4 ) were purchased from Showa Chemical (Tokyo, Japan). Ammonium sulfate ((NH 4 ) 2 SO 4 ), ammonium sulfite ((NH 4 ) 2 SO 3 ), and sodium sulfite (Na 2 SO 3 ) were obtained from Sigma-Aldrich (St. Louis, MO, USA). Hydrochloric acid and ammonium hydroxide were purchased from Honeywell (NC, USA). Qualitative filter paper-Advantec® 2, thickness 0.26 mm and pore size 5 µm was used as a substrate to prepare the CPNSs membrane for water filtration application. Ultrapure water (18.2 MΩ cm) from a Milli-Q ultrapure water system (Millipore, Billerica, MA, USA) was used for all experiments.

Synthesis of CPNSs

The CPNSs were prepared by heating a solid mixture of Alg and DAHP in 1:5 mass ratio in a muffle furnace (DH 300, Dengyng, New Taipei City, Taiwan). The mixture of Alg and DAHP was blended in a coffee grinder for 5 min. 1.0 g of the mixture was then placed in 50 mL glass vials and heated in two steps: first at 60 °C for 3 h followed by raising the temperature to 120, 150, 180, 210, or 240 °C and heated for 3 h. The carbonized residue thus obtained was allowed to cool to room temperature, and 50 mL of deionized (DI) water was added, mixed well, and centrifuged at a relative centrifugal force (RCF) of 15,000  g for 1 h. After three centrifugation/washing cycles, the pellets were dispersed again in DI water, and the pH of the solutions were adjusted to 9 by adding NaOH solution (0.5 M), and then sonicated (100 W) for 15 min. The sonicated dispersions were centrifuged at 500  g for 15 min to remove larger carbonized particles, and the supernatants containing the CPNSs were collected and quantified by freeze-drying. The CPNSs dispersion was stored at 4 °C when not in use.

Characterization of carbonized polymer nanosheets (CPNSs)

The CPNSs were characterized by transmission electron microscopy (TEM) using an HT-7700 system (Hitachi, USA), operated at 75 kV and high-resolution transmission electron microscopy (HRTEM, Tecnai-F20-G2 Philips/FEI, Hillsboro, OR, USA) at 200 kV. The samples for TEM were prepared using diluted solutions of the CPNSs, dropped onto carbon-coated copper grids followed by vacuum drying. The UV–vis absorption spectra of the CPNSs were recorded using Synergy TM 4 multi-mode microplate reader (Biotek Instruments, Winooski, VT, USA). The zeta potential and hydrodynamic size were measured using Zetasizer 3000HS analyzer (Malvern Instruments, Malvern, Worcestershire, UK). The XRD spectra of the samples were recorded using Powder X-ray diffractometer (Bruker, D2 Phaser). The atomic force microscopy (AFM) analysis of the materials and filter papers was performed using Bruker’s Dimension Icon® AFM and Kelvin probe force microscopy (KPFM) analysis of the CPNSs-modified filter papers was performed by Bruker’s Dimension Icon XR (Bruker Daltonics, Bremen, Germany). Scanning electron microscopy (SEM) measurements (HR-FESEM S-4800, Hitachi, Tokyo, Japan) of the CPNSs-coated membranes were carried out after sputtering the membrane with platinum to understand the morphology of the membrane surfaces. X-ray photoelectron spectroscopy (XPS) was carried out using ESCALAB 250 spectrometer (VG Scientific, East Grinstead, UK) with Al Kα X-ray radiation for excitation. Nuclear magnetic resonance (NMR) spectroscopy measurements were obtained using Bruker AVANCEIII 600 WB Solid State NMR Spectrometer-NMR005200 (Billerica, Massachusetts, USA). The samples for the NMR spectroscopy were purified by dialysis (MWCO = 0.5−1 kD) against deionized water with water replaced every hour for 6 h, and then after 12 h.

Preparation of CPNSs-modified filter paper and determination of bacteria removal efficiency

Qualitative filter paper (Advantec ® 2, pore size 5 µm, and thickness 0.26 mm) was modified with CPNSs for the bacterial removal from water. Briefly, the filter paper (25 mm in diameter) was incubated with CPNSs solution (0.5 mg mL −1 , 5 mL) for 2 h under shaking at 150 rpm to obtain a loading of ca. 0.2 mg cm −2 . The CPNSs-modified filter papers (area 4.91 cm 2 and active area 2.83 cm 2 ) were then placed in a syringe holder (diameter 25 mm) and washed with 50 mL of sodium phosphate buffer (5 mM, pH 7.4) to remove the unbound CPNSs (flow rate 5.8 L min −1 m −2 ). The CPNSs-modified filter paper was tested for the removal of E. coli , S. aureus , and V. parahaemolyticus . E. coli and S. aureus were cultured overnight in Lysogeny broth (LB) medium at 37 °C with shaking at 150 rpm, while V. parahaemolyticus was cultured in tryptic soy broth (TSB) containing 3% NaCl at 25 °C. The bacteria cells were washed twice with sodium phosphate buffer (5 mM, pH 7.4) for E. coli and S. aureus ; sodium phosphate buffer (5 mM, pH 7.4) containing 3% NaCl for V. parahaemolyticus after removing the medium by centrifugation at 3000  g for 5 min at 25 °C. The bacteria removal efficiency of the CPNSs-modified filter paper was determined using 10 5 CFU mL −1 bacteria solution by a dead-end mode filtration, using a syringe pump (KDS100, KD Scientific, Holliston, MA, USA), with a water flux of 400 mL min −1 m −2 . The filtrate was collected and diluted 100-fold, and then 100 µL of the diluted solution was spread on LB-agar plates and incubated for 12 h at 37 °C (TSB agar plates were used for V. parahaemolyticus and incubated at 25 °C). The liquid culture of the bacteria was also carried out by supplementing with the respective medium and incubating overnight, followed by measuring the absorbance at 600 nm (OD 600 ). Each experiment was performed in triplicate for each condition. Transmission electron microscopic (TEM; Tecnai 20 G2 S-Twin, Philips/FEI, Hillsboro, OR, USA) images were recorded to understand the morphology of the bacteria. The scanning electron microscopic (SEM; Hitachi S-4800, Hitachi High-Technologies, Tokyo, Japan) images of CPNSs-modified filter paper after passing 10 7 CFU mL −1 V. parahaemolyticus were taken to understand the bacteria removal mechanism.

The bacteria removal efficiency of the CPNSs-modified filter paper was verified by fluorescence microscopy and flow cytometry. The fluorescence microscopic analysis of the bacteria solution before and after filtration, and those trapped on the filter paper were assessed using LIVE/DEAD® BacLight™ Bacterial Viability Kit (Invitrogen). Flow cytometric measurements were performed on a Attune NxT Flow Cytometer (ThermoFischer Scientific, CA, USA), equipped with a 488 nm blue solid-state laser operating at 50 mW. Optical filters were configured to measure red fluorescence above 630 nm and green fluorescence at 520 nm, with the trigger set to the green fluorescence channel. Briefly, the bacteria samples were incubated with live/dead stain (LIVE/DEAD® BacLight Bacterial Viability Kit containing green-fluorescent SYTO™ 9 dye and red-fluorescent propidium iodide) at room temperature for 30 min, and transferred to flow cytometry tubes. After calibrating the flow cytometer and setting up the fluorescence channel (green fluorescence for live bacteria (SYTOX Green-A channel) and red fluorescence for dead bacteria), the samples were run and the data were analyzed by gating strategy.

Removal of V. parahaemolyticus and shrimp challenge experiments

To test the effectiveness of the CPNS-modified filter paper for removing V. parahaemolyticus under circulation conditions, we used 2 L of bacteria-contaminated aquarium water at a higher bacterial concentration of 10 7 CFU mL −1 in an aquarium tank. The CPNSs-180-modified filter papers (filtration area 17.34 cm 2 ) were placed in a syringe holder, and water was circulated at a flow rate of 1150 L min −1 m −2 by a micro diaphragm pump. Samples were taken out from the tank at regular intervals and screened for bacterial viability. Pristine filter paper was the control, and the results were compared with graphene oxide (GO)-modified filter paper. We further evaluated the effectiveness of the CPNSs-180-modified filter paper in the survival rate of shrimp infected with V. parahaemolyticus (10 6 CFU mL −1 ). The Litopenaeus vannamei shrimp (body-weight; 1.0 ± 0.2 g) were purchased from Taikong Corporation (Taipei, Taiwan). 2.5 L aquarium tanks with 10 shrimps in each tank were used for the study. The tank was aerated throughout the experiment and water was circulated through the CPNSs-180-modified filter paper as described above. The tank with water circulation without using any membrane and the one with filter paper without modification served as control group and control membrane group, respectively. All the experiments were performed in triplicates.

Data availability

All data generated or analyzed during this study are included in this published article and its supplementary information files.

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Acknowledgements

This work was supported by the National Science and Technology Council (NSTC) of Taiwan under Contract Nos. 110-2221-E-019-001, 110-2811-M-019-501, 113-2622-E-182-004, and 112-2811-B-182-022, Chang Gung Memorial Hospital, Linkou under Contract No. CMRPD2L0161, Chang Gung University under Contract No. OMRPD2N0011, and the Center of Excellence for the Oceans, National Taiwan Ocean University from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan.

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These authors contributed equally: Anisha Anand, Binesh Unnikrishnan.

Authors and Affiliations

Department of Biomedical Engineering, Chang Gung University, Taoyuan, 33302, Taiwan

Anisha Anand & Jui-Yang Lai

Department of Bioscience and Biotechnology, National Taiwan Ocean University, Keelung, 202301, Taiwan

Binesh Unnikrishnan, Chen-Yow Wang, Han-Jia Lin & Chih-Ching Huang

Department of Ophthalmology, Chang Gung Memorial Hospital, Linkou, Taoyuan, 33305, Taiwan

Jui-Yang Lai

Department of Materials Engineering, Ming Chi University of Technology, New Taipei City, 24301, Taiwan

Research Center for Chinese Herbal Medicine, College of Human Ecology, Chang Gung University of Science and Technology, Taoyuan, 33303, Taiwan

Center of Excellence for the Oceans, National Taiwan Ocean University, Keelung, 202301, Taiwan

Han-Jia Lin & Chih-Ching Huang

School of Pharmacy, College of Pharmacy, Kaohsiung Medical University, Kaohsiung, 80708, Taiwan

Chih-Ching Huang

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J.-Y.L. and C.-C.H conceived the original idea and supervised the project from the beginning to the end, and helped in the manuscript preparation. A.A. and B.U. carried out the experiments and prepared the manuscript. C.-Y.W. and H.-J.L. helped in the manuscript preparation. A.A. and B.U. contributed equally. All authors discussed the results and contributed to the manuscript.

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Anand, A., Unnikrishnan, B., Wang, CY. et al. Phosphate ester-linked carbonized polymer nanosheets to limit microbiological contamination in aquaculture water. npj Clean Water 7 , 84 (2024). https://doi.org/10.1038/s41545-024-00378-7

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DOI : https://doi.org/10.1038/s41545-024-00378-7

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Development and Evaluation of Surface-Enhanced Raman Spectroscopy (SERS) Filter Paper Substrates Coated with Antibacterial Silver Nanoparticles for the Identification of Trace Escherichia coli

• Original Article
• Published: 03 September 2024

Cite this article

• Safaa Mustafa Hameed 1 ,
• Naeema Hadi Ali 2 ,
• Akram Rostaminia 3 ,
• Sattar H. Abed 4 ,
• Hossein Khojasteh   ORCID: orcid.org/0000-0001-7483-3396 5 ,
• Shaymaa Awad Kadhim 6 ,
• Peyman Aspoukeh 5 &
• Vahid Eskandari 7  

In this work, a sensitive and reasonably priced surface-enhanced Raman spectroscopy (SERS)-based biosensor is developed for the quick identification of Escherichia coli ( E. coli ), a key marker of fecal contamination in food and water. A filter paper (FP) substrate coated with silver nanoparticles (AgNPs), which were produced by a straightforward chemical reduction method, was used for developing the biosensor. After the AgNPs were carefully examined, it was discovered that they produced active plasmonic sites on the FP substrate, which made it possible to detect the molecular vibrations of E. coli . The remarkable sensitivity of the SERS-based FP-AgNP biosensor was shown by its ability to detect very low concentrations of E. coli , as low as 10 colony-forming units (CFU)/mL. The AgNPs also shown antimicrobial properties. The substrates’ repeatability was verified by experimental Raman measurements, and the enhancement factor for identifying the molecular vibrations of E. coli was determined to be 2.197 × 10 5 based on empirical calculations and 4.587 × 10 5 based on numerical estimations. These findings demonstrate how well the proposed SERS-based biosensor works for the quick and accurate detection of E. coli , which is essential for guaranteeing the safety of food and water. The results of the research open the door to the development of sophisticated SERS-based monitoring and detection systems with the additional benefits of being inexpensive, straightforward, adaptable, and chemically stable for a range of uses in environmental protection and public health.

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Data availability.

The data related to the analyses are available from the corresponding author on reasonable request.

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Acknowledgements

We extend our sincere appreciation to Mr. Hossein Sahbafar (ORCID: 0009-0003-7806-9550) for his expertise in conducting the FDTD simulation, enriching the academic value of our research.

The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

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Department of Optics, College of Health and Medical Technology, Sawa University, Almuthana, Iraq

Safaa Mustafa Hameed

Department of Physics, Faculty of Science, University of Kufa, Najaf, Iraq

Naeema Hadi Ali

Department of Medical Biochemical Analysis, Cihan University-Erbil, Kurdistan Region, Iraq

Akram Rostaminia

College of Education for Pure Sciences, University of Al-Muthanna, Samawah, Iraq

Sattar H. Abed

Scientific Research Center, Soran University, Soran, Kurdistan Region, Iraq

Hossein Khojasteh & Peyman Aspoukeh

Physics Department, Faculty of Science, University of Kufa, Najaf, Iraq

Shaymaa Awad Kadhim

Nanoscience and Nanotechnology Research Center, University of Kashan, Kashan, Iran

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S.M.H., N.H.A., and K.H.: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Resources, Writing - Original Draft, Writing - Review & Editing; S.H.A.: Visualization; S.H.A., H.K., and S.A.K: Resources, Writing - Review & Editing; V.E. and K.H.: Supervision, Project administration, Writing - Original Draft, Writing - Review & Editing.

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Hameed, S.M., Ali, N.H., Rostaminia, A. et al. Development and Evaluation of Surface-Enhanced Raman Spectroscopy (SERS) Filter Paper Substrates Coated with Antibacterial Silver Nanoparticles for the Identification of Trace Escherichia coli . Chemistry Africa (2024). https://doi.org/10.1007/s42250-024-01064-4

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Environmental Science: Water Research & Technology

Production of birnessite-type manganese oxides by biofilms from oxygen-supplemented biological activated carbon (bac) filters.

Biological oxidation of manganese (Mn) by bacteria results in the formation of biogenic Mn oxides (MnOx), which are known to be strong oxidants and effective catalysts. Manganese-oxidizing bacteria (MnOB) often develop in engineered systems for water treatment under oligotrophic conditions. In this study, we investigated the MnOB within biofilms sampled in two different seasons from full-scale oxygen-supplemented biological activated carbon (BAC) filters performing the complete removal of Mn from wastewater. By applying a novel batch enrichment approach ensuring continuous presence of soluble Mn, after 42 days the start-up microbial community grew into thick, floccular biofilms efficiently oxidizing Mn2+ into numerous black nodules. The amount of Mn oxidized was quantified using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). X-ray diffraction (XRD) analysis and Scanning electron microscopy (SEM) revealed that the MnOx formed was a birnessite-type (δ-MnO2) with a crystalline, nanoflower structure. Comparison of the microbial community composition before and after the enrichment by means of 16S rRNA gene amplicon sequencing showed increases of members of the orders Rhizobiales and Burkholderiales, and identified among the most abundant groups which have rarely or never been associated with Mn oxidation before (Rhodococcus, Ellin6067, Planctomycetota Pir4 lineage, Rhizobiales A0839 and Amb-16S-1323). This study unravels the potential of production of crystalline MnOx by mixed-microbial communities which uniquely generate in a man-made biofilter. The new insights provided implement the knowledge in the field, with the perspective to design innovative biotechnologies to remove recalcitrant compounds where MnOB find optimal growth conditions to produce catalytic forms of MnOx.

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A. Larasati, O. Bernadet, G. W. Euverink, H. P. J. van Veelen and M. C. Gagliano, Environ. Sci.: Water Res. Technol. , 2024, Accepted Manuscript , DOI: 10.1039/D4EW00208C

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Facility for Rare Isotope Beams

At michigan state university, ask the expert: how—and why—do you harvest isotopes, katharina domnanich is helping set up a lab at frib that will provide a bounty of isotopes useful for medicine, plant science, and more.

Katharina Domnanich joined Michigan State University in 2018, when the Facility for Rare Isotope Beams (FRIB) was entering its final phases of construction. 

The facility was closing in on becoming a world-class particle accelerator and user facility for the  U.S. Department of Energy Office of Science (DOE-SC), making short-lived isotopes  that couldn’t be made anywhere else .

 These exotic isotopes will help us better understand fundamental rules of nature and find answers about how the early universe was formed. But Domnanich wasn’t most interested in these fast-decaying nuclei that have likely never existed on Earth before. 

Rather, she was drawn to the more common isotopes that researchers already knew could be useful for a variety of applications that would be made in the background of FRIB’s isotope discovery. Domnanich came to FRIB to work as a postdoc with  Gregory Severin , associate professor of chemistry at FRIB and in the  MSU Department of Chemistry , who was building what’s called an  isotope harvesting laboratory . 

Severin’s team was assembling technology to extract or harvest those “by-product” isotopes and make them available to other researchers who could put them to work in fields like medicine, plant science and many more. The isotopes are harvested during routine operation for FRIB’s nuclear physics mission—without interfering with its primary users. The  U.S. Department of Energy Isotope R&D and Production Program (DOE Isotope Program) supports isotope harvesting at FRIB.

Today, FRIB is officially up and running and its isotope harvesting laboratory is nearing completion. Domnanich is now an assistant professor of chemistry at FRIB and in the  MSU Department of Chemistry  who has started her own team, won a  2023 FRIB Achievement Award for Early Career Researchers , and branched out into new research areas. 

In fact, she recently published about one of those in the journal  Applied Radiation and Isotopes   (“Preparation of stable and long-lived source samples for the stand-alone beam program at the Facility for Rare Isotope Beams”).

But she’s still a driving force on the team making isotope harvesting at FRIB a reality, working alongside Severin and their colleagues. 

The  College of Natural Science caught up with Domnanich to talk about the project, as well as what it was like launching her career at FRIB and what’s on the horizon for this rising star of nuclear science.

This conversation has been edited for length and clarity.

You joined FRIB to work on isotope harvesting, yet your new paper is about something else. It sounds like there’s no shortage of things to work on at FRIB.

Oh no. There are tons of things to do here.

My group is working on isotope harvesting, and I have newer projects working on what’s called mass separation. These all use radioactive isotopes and, to work safely with those, experiments have to be like a well-rehearsed show. 

To prepare students, they have to do an experiment at least 10 times with nonradioactive materials before using the radioactive stuff so they know how to handle everything.

So, in addition to all the work we have to do, students also have a lot of pre-experiments. If you ask them, they’ll tell you we’re always busy.

Why did you decide to stay at MSU and FRIB after completing your postdoctoral research?

I really enjoyed working together with Greg. It was incredible and a very productive time. I liked the dynamics in the group and how isotope harvesting at FRIB was being developed, so when there was an opening, it just seemed like a great opportunity.

You were recognized with an  FRIB Achievement Award for Early Career Researchers in 2023 —the year after you became a faculty member. What was that like?

It felt very encouraging. I was very positively surprised that I got that award and it was really nice. 

It also feels like there is a lot of interest in isotope harvesting and that people really want to use the isotopes. I know there are organic chemistry professors who are interested and some plant biologists. We also have a  Department of Radiology where people are interested. 

So, I think there will be tons of opportunities for collaboration when the isotope harvesting is running.

What sorts of things do researchers want to do with isotopes harvested from FRIB?

A lot of the isotopes are for nuclear medicine. For example, we’ve worked with scandium-47, which is being studied as a therapeutic for cancer treatment.

When I was a postdoc, I also looked into collecting zinc-62, which is interesting for nuclear medicine and also for plant sciences. Plants needs zinc, and I showed that plants could take up zinc-62, then scanned the plants to see where it goes. That could be quite a nice tool to study plant systems.

Then for materials science, you can use certain isotopes to visualize leaks and cracks in pipes and things like that.

We could also harvest isotopes to be used in nuclear batteries. The rovers we send to Mars use radioactive isotopes in their batteries, and some of those same isotopes will be produced at FRIB.

So, how do you harvest isotopes?

At FRIB, we accelerate ions into a primary beam that reacts with a target and produces secondary beams. It’s these exotic secondary beams that are used by nuclear physicists and nuclear scientists to study reactions that happen in stars, for example—or whatever they want to explore.

But maybe just 20 percent of the primary beam goes into producing secondary beams. That means you have 80 percent of the primary beam that you still need to do something with. It’s usually stopped by a solid metal block, just to have something that can absorb all its energy.

At FRIB, we want to instead stop it inside a rotating drum of water. By stopping the beam within water, you have tons of reactions happening between the beam particles and water molecules, producing many new isotopes in the process.

Those isotopes are then just floating in this water system. And FRIB’s water system will be huge, like 7,000 liters or almost 2,000 gallons as a ballpark number.

We can capture the isotopes on ion exchange resins, which function like a really fancy Brita filter. From there, we can extract them—kind of like remove them with a liquid—and purify them, thereby making them available for further experiments.

What’s the status of the isotope harvesting lab now?

We did some proof-of-principle tests when I was a postdoc in Greg’s group with a smaller type of water system. So that had about 50 liters of water instead of 7,000. But even that helped us develop the chemistry and learn what the  reaction rates are for certain isotopes. Still, scaling up is a huge task.

While you’re bringing the isotope harvesting lab online, you’re also exploring some new directions in your group, which led to your recent paper. Can you talk more about that?

My group is looking into something called mass separation to purify isotopes, and that’s the connection to the new paper.

What is mass separation?

Remember that I said we use something like fancy Brita filters in isotope harvesting? If you have a fancy setup, like us, you can separate the individual elements by their chemistry. You can separate the sodium from the magnesium from the calcium and so on.

But with a filter, you can’t separate different isotopes of the same element. Calcium, for example, has several stable isotopes, like calcium-40, calcium-42 and so on. That makes it almost impossible to separate isotopes using chemistry. 

But mass separation is feasible. These isotopes all have different masses and you can separate them by their mass using an incredibly strong magnet. 

And how does that fit in with your new work?

I was working with another group led by  Georg Bollen , director of the Experimental Systems Division at FRIB, and they have a setup to generate so-called offline beams. Before the main FRIB accelerator was running, but after FRIB’s predecessor, the National Superconducting Cyclotron Laboratory, was already switched off, researchers needed to use something else to be able to study rare isotopes. That’s why they made the Batch Mode Ion Source (BMIS). 

It’s a device that can make a lower energy beam than FRIB’s main beam, but it needs to have source samples that have specific chemical and physical properties. I collaborated with this BMIS group to prepare those source samples.

Actually, our last paper is about the preparation of source samples. And that fits into the mass separation project because I want to use this ion source and magnet as part of the mass separation setup to do my next mass separation experiments. 

So this was very good preparation for me, kind of like training, to figure out what is necessary to establish mass separation for further experiments.

Last question: Do you have a favorite isotope?

Yes. I love scandium-43. 

It’s a rare-earth-like element, which means it’s useful in electronics, but it also has cool applications in nuclear medicine. You can use different isotopes of scandium for therapy and diagnostics, so you can think about using the same medicine for diagnosing cancer and for treatment.

I worked with scandium a lot during my doctorate—I spent so many hours working with different scandium isotopes in the lab—and I still really like it.

Michigan State University operates the Facility for Rare Isotope Beams (FRIB) as a user facility for the U.S. Department of Energy Office of Science (DOE-SC), supporting the mission of the DOE-SC Office of Nuclear Physics. Hosting what is designed to be the most powerful heavy-ion accelerator, FRIB enables scientists to make discoveries about the properties of rare isotopes in order to better understand the physics of nuclei, nuclear astrophysics, fundamental interactions, and applications for society, including in medicine, homeland security, and industry.

The U.S. Department of Energy Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of today’s most pressing challenges. For more information, visit  energy.gov/science .

The  U.S. Department of Energy Isotope R&D and Production Program  (DOE Isotope Program) supports isotope harvesting at FRIB. MSU operates FRIB as a user facility for the  Office of Nuclear Physics  in the  U.S. Department of Energy Office of Science , supporting the mission of the DOE-SC Office of Nuclear Physics.

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Something’s Poisoning America’s Land. Farmers Fear ‘Forever’ Chemicals.

Fertilizer made from city sewage has been spread on millions of acres of farmland for decades. Scientists say it can contain high levels of the toxic substance.

Jordan Vonderhaar for The New York Times

Hiroko Tabuchi traveled to Texas and Michigan and interviewed ranchers, scientists, investigators and wastewater-treatment experts for this article.

Aug. 31, 2024

Supported by

For decades, farmers across America have been encouraged by the federal government to spread municipal sewage on millions of acres of farmland as fertilizer. It was rich in nutrients, and it helped keep the sludge out of landfills.

But a growing body of research shows that this black sludge, made from the sewage that flows from homes and factories, can contain heavy concentrations of chemicals thought to increase the risk of certain types of cancer and to cause birth defects and developmental delays in children.

Known as “forever chemicals” because of their longevity, these toxic contaminants are now being detected, sometimes at high levels, on farmland across the country , including in Texas, Maine, Michigan, New York and Tennessee. In some cases the chemicals are suspected of sickening or killing livestock and are turning up in produce. Farmers are beginning to fear for their own health.

The national scale of farmland contamination by these chemicals — which are used in everything from microwave popcorn bags and firefighting gear to nonstick pans and stain-resistant carpets — is only now starting to become apparent. There are now lawsuits against providers of the fertilizer, as well as against the Environmental Protection Agency, alleging that the agency failed to regulate the chemicals, known as PFAS.

In Michigan, among the first states to investigate the chemicals in sludge fertilizer, officials shut down one farm where tests found particularly high concentrations in the soil and in cattle that grazed on the land. This year, the state prohibited the property from ever again being used for agriculture. Michigan hasn’t conducted widespread testing at other farms, partly out of concern for the economic effects on its agriculture industry.

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Current research status and future trends of vibration energy harvesters.

1. Introduction

2. electromagnetic vibration energy harvester, 2.1. working principle and characteristics of the electromagnetic vibration energy harvester, 2.2. advances in electromagnetic vibration energy harvesters, 3. piezoelectric vibration energy harvester, 3.1. operating principle and characteristics of piezoelectric vibration energy harvester, 3.2. progress of research on piezoelectric vibration energy harvesters, 4. friction electric vibration energy harvester, 4.1. mechanism of operation and characteristics of the friction electric vibration energy harvesters, 4.1.1. friction nanogenerator, 4.1.2. principle of friction electric vibration energy harvester, 4.2. advances in friction electric vibration energy harvesters, 5. electrostatic vibration energy harvester, 5.1. the working principle of the electrostatic vibration energy harvester and its characteristics, 5.2. current research status of electrostatic vibration energy harvesters, 6. magnetostrictive vibration energy harvester, 6.1. operating principle and characteristics of magnetostrictive vibration energy harvesters, 6.2. current status of research on magnetostrictive vibration energy harvesters, 7. conclusions, author contributions, institutional review board statement, informed consent statement, data availability statement, conflicts of interest.

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Share and Cite

Qu, G.; Xia, H.; Liang, Q.; Liu, Y.; Ming, S.; Zhao, J.; Xia, Y.; Wu, J. Current Research Status and Future Trends of Vibration Energy Harvesters. Micromachines 2024 , 15 , 1109. https://doi.org/10.3390/mi15091109

Qu G, Xia H, Liang Q, Liu Y, Ming S, Zhao J, Xia Y, Wu J. Current Research Status and Future Trends of Vibration Energy Harvesters. Micromachines . 2024; 15(9):1109. https://doi.org/10.3390/mi15091109

Qu, Guohao, Hui Xia, Quanwei Liang, Yunping Liu, Shilin Ming, Junke Zhao, Yushu Xia, and Jianbo Wu. 2024. "Current Research Status and Future Trends of Vibration Energy Harvesters" Micromachines 15, no. 9: 1109. https://doi.org/10.3390/mi15091109

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Scientists invent filter that can recycle PFAS into renewable batteries

By Kenji Sato

ABC Radio Brisbane

Topic: Environmental Technology

Cheng Zhang says the filter is more effective at absorbing PFAS than anything else on the market. ( Supplied: University of Queensland )

Researchers have invented a filter that removes harmful PFAS chemicals from water and recycles them in renewable batteries. 

University of Queensland scientists say they believe the technology will be on the market in three years.

What's next?

The filters will be trialled at a Brisbane wastewater treatment plant before being expanded to other sites.

Scientists have invented a filter that can remove harmful "forever chemicals" from drinking water and use it in renewable batteries.

The University of Queensland's invention can extract polyfluroaklyl (PFAS) compounds, which are notoriously difficult to remove from the environment or human bodies.

Australian Institute for Bioengineering and Nanotechnology polymer chemist Cheng Zhang said the filter used their new sorbent solution and an ion-exchange technique.

He said this was over five times more effective than any existing technology on the market.

Researchers say this patented sorbent is several times more effective than existing technology. ( Supplied: University of Queensland )

'Confident' technology will be ready in three years

Dr Zheng said it was capable of reducing PFAS levels to "basically non-detectable" levels in drinking water, far below EPA safe drinking guidelines.

Additionally, he said the filter could treat contaminated landfill leachate — previously not possible with commercially available technology.

Dr Zheng said the institute had received its patent and was confident the technology would be ready for commercial production in three years.

Cheng Zhang says these renewable batteries were the first to harness filtered PFAS compounds. ( Supplied: University of Queensland )

"What we're trying to do now is either license the technology or create a start-up company," Dr Zheng said.

"The final goal for us is to further commercialise our technology and make it useful to solve real world problems and make a PFAS-free world.

"Not only does our filter technology remove harmful particles from water, those captured chemicals are available to be repurposed to help decarbonise the planet."

Testing at wastewater treatment plant

The technology will be tested at Brisbane's Luggage Point Wastewater Treatment Plant, one of the largest recycled water facilities in the world.

Dr Zheng said in the coming years they would expand to other trial sites, which were yet to be locked in.

The Luggage Point Wastewater Treatment Plant is one of the world's largest recycled water facilities. ( ABC News: Jessica Rendall )

Dr Zheng said they were looking at trialling landfill sites as well as working with companies that dealt with contaminated compost leachate.

New renewable battery technology

He said their renewable batteries were the first in the world to use PFAS compounds in this way.

"The increasing demand for high-performance rechargeable batteries means manufacturers are constantly searching for new materials that improve the energy density, safety and cycling stability of batteries," he said.

"Recycled PFAS has excellent properties for this purpose."

The PFAS filter pilot testing program has received $1 million in state grants from the Advance Queensland Industry Research Projects program.

Cheng Zhang says this PFAS is now being used to help the planet, instead of harming it. ( Supplied: University of Queensland )

PFAS detected in drinking water

A recent University of New South Wales study found PFAS was far more widespread than previously believed.

Civil and environmental engineering professor Denis O'Carroll said he was "surprised" to discover that a small part of the Sydney Water Catchment in the Blue Mountains had levels above safe drinking standards.

He said water providers such as Sydney Water did not routinely measure the broad range of PFAS compounds in drinking water.

Dr O'Carroll said much more research was needed to understand how widespread and damaging PFAS was on the environment.

"We need to look at the human health and ecosystem impacts of PFAS that we've put into the environment," Professor O'Carroll said.

"PFAS is one example, but there are a range of chemicals we put out into the environment every day so we need to have a broad consideration as a society."

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