A solar-powered water filter removes contaminants smaller than viruses without clogging or needing pumps, exploiting electrokinetic repulsion instead of nanopores.
(Nanowerk Spotlight) Nanoplastics have infiltrated virtually every corner of the planet. These plastic fragments, smaller than 1 μm across, drift through oceans, settle into soils, and circulate in drinking water supplies. Research suggests they cross biological barriers, accumulate in human tissues, and trigger cellular damage. Removing them from water presents an extraordinary challenge.
Conventional filters capable of catching particles measuring tens of nanometers demand enormous pressure, consume substantial energy, and clog rapidly as contaminants accumulate on membrane surfaces. This fouling problem has plagued filtration technology for decades. Shrink the pores to catch smaller particles, and flow rates plummet while energy costs climb. The tradeoff seems inescapable.
Nanofiltration membranes can block viruses and the tiniest pollutants, but operating them requires infrastructure, grid power, and high-pressure pumps that remain inaccessible across much of the world. Roughly two billion people lack reliable access to safe drinking water. Projections suggest more than 3.5 billion will face water stress by 2025. Remote communities far from electrical grids and treatment plants bear the heaviest burden.
An alternative approach sidesteps pore-based sieving entirely. Electrokinetic separation exploits how charged particles behave in electric fields. Certain membranes selectively transport ions of one charge while blocking others. Apply voltage across such a membrane, and ions migrate through it unevenly, creating a zone of depleted ion concentration on one side. Within this depletion zone, the local electric field intensifies dramatically. Charged particles approaching the zone encounter strong repulsive forces that push them away from the membrane surface. Think of it as an invisible barrier: particles never touch the membrane, so they cannot clog it.
Particle rejection depends on surface charge rather than physical size. Micrometer-scale pores could theoretically block nanometer-scale contaminants. Previous electrokinetic systems, however, delivered flow rates measured in microliters per minute. They required complex microfabricated channels, external pumps, and grid power. Scaling beyond laboratory demonstrations remained out of reach.
A study published in Advanced Science (“Solar‐Powered Electrokinetic Filtration using Hierarchical Porous Membranes for the Off‐Grid Removal of Ultrafine Contaminants”) presents a filtration platform that overcomes these limitations. Researchers at Pohang University of Science and Technology in South Korea built a hierarchically structured membrane that extends electrokinetic phenomena to centimeter-scale areas while operating under gravity alone. Their system removes particles below 10 nm at flow rates exceeding 400 L m⁻² h⁻¹ with pressure drops under 1 kPa. It runs entirely on solar power and needs no pumps.
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: Reproduced from DOI:10.1002/advs.202515435, CC BY) (click on image to enlarge)
The membrane stacks three layers, each with a distinct role. The first is a biodegradable scaffold made from cellulose and cotton sponge cloth. Pores in this layer reach approximately 319 μm, with most volume concentrated between 10 μm and 100 μm. The material’s natural wettability draws water uniformly across the entire surface.
The second layer is commercial cellulose filter paper with pores ranging from 4 μm to 10 μm. It catches larger debris and provides geometric confinement that stabilizes the electrokinetic behavior. Without this constraint, fluid instabilities would disrupt the ion depletion zone.
The third layer provides ion selectivity. Researchers coated a sponge scaffold with Nafion, a polymer containing negatively charged sulfonate groups. This coating creates nanochannels measuring 5 nm to 15 nm within the larger porous structure. When voltage is applied, positively charged ions pass through while negatively charged species are excluded. The resulting imbalance generates the depletion zone where electric fields intensify and particle repulsion occurs.
Contaminants never lodge in pores. The amplified electric field deflects them toward a concentrate outlet while purified water passes through to the filtrate stream.
Testing validated the design across multiple contaminant types. The system achieved greater than 99% dye removal at fluxes below approximately 487 L m⁻² h⁻¹ in tap water with conductivity between 320 μS cm⁻¹ and 380 μS cm⁻¹. Polystyrene, polyethylene, and polypropylene microplastics showed rejection efficiencies exceeding 99.9% at fluxes near 400 L m⁻² h⁻¹.
Gold nanoparticles measuring 5 nm to 20 nm provided a critical benchmark. Commercial nanofiltration membranes with 50 nm and 100 nm pores failed to block these particles completely under comparable conditions. The electrokinetic system achieved full rejection, confirming that charge-based repulsion can succeed where size-based sieving fails.
The fouling resistance proved equally significant. Over 20 filtration cycles using a 300 ppm mixed nanoplastic suspension, removal efficiency held consistently above 99.9%. Cleaning required only a water rinse between runs.
The power system emphasizes practicality over complexity. A 50 W photovoltaic panel charges a 12 V battery through a charge controller. A DC-DC converter boosts output to a stable 60 V. A Mariotte bottle, a passive reservoir maintaining constant pressure, feeds water to the inlet without pumps. A 10 cm height difference between inlet and outlet drives flow at approximately 8 mL min⁻¹.
Energy consumption scales with feed water conductivity. At tap water conditions around 350 μS cm⁻¹, the system drew roughly 10 Wh L⁻¹, translating to about 2.4 W of steady-state power. Four hours of average sunlight would generate enough stored energy to purify approximately 11 L.
Bacterial contamination poses one of the gravest threats in untreated water sources. The researchers tested their system against Escherichia coli K-12, a standard indicator organism. After one hour of continuous operation at 60 V, the filtrate contained bacterial levels below the detection threshold of 1 CFU per 100 mL.
An enzymatic assay confirmed the result: concentrate samples turned violet, indicating bacterial presence, while purified output remained clear. The finding suggests the platform could address both chemical and microbial contamination simultaneously.
Scaling experiments expanded the effective membrane area from 2 cm² to 4 cm². Total flow rate increased proportionally while removal efficiency held above 99.9%. The ion depletion region remained stable for more than seven hours, indicating that further expansion is feasible.
Challenges remain before widespread deployment. More durable cation-exchange membranes would extend operational life. Fluorine-free alternatives to Nafion would address environmental concerns about fluoropolymers. Anion-exchange membranes could broaden the approach to positively charged pollutants. Natural organic matter in surface waters resists single-stage treatment and may require integrated strategies.
What the work demonstrates is a path around a longstanding impasse. Ultrafine particle removal has traditionally demanded ultrafine pores, with all the energy penalties and fouling problems they entail. By substituting electrostatic repulsion for physical sieving, the researchers achieved nanoscale filtration through micrometer-scale structures.
The combination of gravity-driven flow, solar compatibility, and inherent fouling resistance offers a practical architecture for water treatment where pumps and grid power are unavailable.
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Geunbae Lim (Pohang University of Science and Technology (POSTECH))
, 0000-0002-7062-3575 corresponding author
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