A solar-powered electrokinetic filter removes nanoplastics and bacteria using electric fields instead of pressure, offering efficient, low-energy water purification with simple, scalable materials.
(Nanowerk Spotlight) Water treatment technology faces a persistent tradeoff. Filters that trap very small contaminants tend to clog or require high pressure and energy, while those that allow faster flow often fail to remove nanoscale particles. Engineers have tried to balance these factors with smaller pores, surface coatings, and microfluidic devices that use electric fields to steer charged contaminants. These approaches perform well in laboratories but remain difficult to scale into durable, low-cost systems that operate without pumps or reliable power.
Recent progress in porous materials, control of electrokinetic effects, and inexpensive solar hardware is changing that equation. These developments now make it feasible to build filtration systems that use electric forces, not pressure, to remove ultrafine particles, organic molecules, and microbes from freshwater.
The technique relies on ion concentration polarization. When an electric field passes through a surface that lets one type of ion move more freely than another, a low-salt region forms near the interface. The local field strengthens there, repelling charged particles such as dyes, nanoplastics, and bacteria. This effect is well known in small microfluidic channels but rarely extended to large-area membranes that process useful volumes of water.
Schematic representations of (a) conventional membrane-based filtration, b) the nanoelectrokinetic system, and c) the proposed scalable nanoelectrokinetic filtration platform using a washable, microstructured cellulose-cotton composite coated with an ion-exchangeable porous sponge. The applied electric field forms an ion depletion region that amplifies electrophoretic forces to repel charged nanoparticles against drag force. d) The system operates using solar energy and utilizes gravity-driven flow (<1 kPa), allowing for off-grid operation without the need for a pump. e) Comparison of filtrate flow rates from a conventional microfluidic systems, a previous electrokinetic system, and this study highlights the scalability achieved by the hierarchical porous membrane design. (Image: Reprinted from DOI:10.1002/advs.202515435, CC BY) (click on image to enlarge)
The researchers built a layered membrane that creates both rapid flow and stable ion control. The first layer is a cotton cloth with pores roughly ten to one hundred micrometers wide. It distributes flow evenly. The second is a cellulose filter paper with smaller pores, around four to ten micrometers, which stabilizes the electric field. The third is a porous sponge coated with Nafion, a polymer that conducts positive ions better than negative ones. The coating forms networks of nanochannels five to fifteen nanometers wide within the larger sponge pores. When voltage is applied, these channels generate the depletion zone that blocks negatively charged species.
The layers are assembled into a small housing with separate paths for clean and waste streams. Water moves under gravity, and the relative heights of the inlet and outlets control the split between filtrate and concentrate. No pump is needed. Tests confirmed that the flow obeys predictable fluid behavior, which means the process can be scaled by simple adjustment of membrane area and reservoir height.
Electrical measurements revealed the expected pattern for ion concentration polarization. At low voltage, current increases linearly; as the depletion zone forms, the slope decreases; and at higher voltage, current rises again as new transport modes appear. The shape of this curve confirms stable ion selectivity under typical freshwater conditions.
Performance tests show high removal efficiency. Two negatively charged dyes—Amaranth Red and fluorescein sodium salt—were almost completely blocked when the voltage reached about sixty volts. Negatively charged nanoplastics of polystyrene, polyethylene, and polypropylene were removed with more than 99 percent efficiency, even for particles around 47 nanometers in diameter.
The separation mechanism depends on electric force, not pore size. Each charged particle experiences an electrophoretic force in the electric field. If that force exceeds the drag from the flowing water, the particle cannot cross the membrane. Because this relies on charge rather than physical sieving, the membrane avoids clogging. The same principle excluded gold nanoparticles as small as five nanometers, which would normally pass through size-based filters.
Durability tests showed consistent performance. The membrane was rinsed and reused twenty times without loss of removal efficiency. The electric field prevented particles from adhering, keeping the surface clear.
The system also removed Escherichia coli, which carries a negative surface charge in freshwater. Under a sixty-volt field, filtered samples contained no detectable bacteria. Humic acid, a complex natural organic compound, was only partly removed, suggesting that supplementary treatment would be required for waters rich in organic matter.
Energy consumption remained modest. Because the system relies on gravity, most of the energy maintains the electric field. In freshwater with low salt content, energy use was about 6.5 watt-hours per liter. In tap water it rose to about ten watt-hours per liter, and in higher-salt water to about 17 watt-hours per liter. The operating power was roughly 2.4 watts at sixty volts. A small 50-watt solar panel charging a 12-volt battery could provide enough energy in four hours of sunlight to purify about 11 liters of water.
Scaling tests doubled the membrane area and produced roughly double the output flow while keeping rejection above 99.9 percent. The electrical behavior remained steady, showing that larger modules built from repeated units should perform predictably without pumps or complex control systems.
The results place this platform in a useful niche. It achieves more than 99.9 percent rejection of nanoplastics and bacteria at fluxes around 400 liters per square meter per hour, using a gravity head below one kilopascal. The membrane materials are inexpensive and mostly biodegradable, apart from the Nafion coating. The paper notes that developing fluorine-free alternatives would improve environmental sustainability.
This electrokinetic system is not meant to replace nanofiltration or reverse osmosis, which are still necessary to remove dissolved salts and many small organic compounds. Instead, it offers a complementary step that targets suspended ultrafine particles and microorganisms in low-salinity water. It could also serve as a pretreatment stage to protect other filters from fouling.
The paper identifies practical tasks ahead: improving ion-selective materials, reducing energy losses, and enhancing removal of natural organic matter. These are engineering refinements rather than unresolved scientific barriers.
This study shows that a simple layered membrane, driven by gravity and powered by sunlight, can use electrokinetic forces to purify water efficiently. By steering charged contaminants away instead of forcing water through ultrafine pores, it provides a low-pressure, low-energy route to cleaner water for settings where electricity and maintenance are limited.
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Geunbae Lim (Pohang University of Science and Technology)
, 0000-0002-7062-3575 corresponding author
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