A biodegradable nanocellulose and carbon aerogel uses sunlight to purify seawater efficiently, achieving high evaporation rates and strong salt resistance for sustainable, low-cost desalination systems.
(Nanowerk Spotlight) On a clear afternoon, a square meter of sunlight carries enough energy to boil several liters of water. Yet much of the world still struggles with drought, pollution, and rising salinity. Turning sunlight into clean water should be possible, but the physics and chemistry of doing it efficiently remain unresolved. The challenge is not the supply of energy but the means of capturing it precisely at the surface where water becomes vapor.
Current desalination systems take a different route. They use high pressure and heat to force saltwater through membranes or drive evaporation in large distillation plants. These methods are effective but demand heavy energy and expensive infrastructure. For small or remote communities, such systems are often beyond reach.
The idea of using sunlight directly has therefore gained momentum. If only the top layer of water could be heated instead of the entire volume, evaporation could proceed quickly with very little loss. This approach, known as solar interfacial evaporation, uses engineered surfaces that draw water upward, trap sunlight, and release vapor that can be condensed as fresh water.
The concept is simple in outline and difficult in execution. A material must pull water efficiently, absorb nearly all light, keep heat concentrated at the interface, and resist salt accumulation. Carbon absorbs light effectively but deteriorates under repeated use. Polymers transport water but persist in the environment. Metallic particles can intensify heating but are costly and unstable. No single material has yet achieved durability, efficiency, and sustainability in one design.
The structure forms a porous, two-layer matrix that channels water upward and converts sunlight into concentrated heat. The device is biodegradable and maintains performance even in salty conditions, pointing toward compact solar desalination systems that could operate wherever sunlight and seawater are available.
Schematic illustration of the bilayer solar evaporator fabricated through sequential ice templating. The bottom layer, made of pure cellulose nanofibers (CNF)-based aerogel, contains vertically aligned water channels that facilitate efficient water transport. The top layer, a hybrid gold nanoparticle loaded ZIF-8 MOF (AZC)-embedded CNF photothermal layer (AZC–CA), provides effective solar-to-heat conversion. (Image: Reprinted from DOI:10.1002/advs.202516158, CC BY) (click on image to enlarge)
The design is built through a process known as sequential ice templating. During controlled freezing, ice crystals grow vertically and leave aligned channels when the ice is removed. These channels allow water to rise through capillary action and let vapor escape efficiently.
The lower layer is composed of pure nanocellulose fibers that transport water. The upper layer combines the same fibers with porous carbon derived from a zinc-based metal organic framework called ZIF-8. When heated under nitrogen, ZIF-8 forms a carbon skeleton filled with micro and mesopores. Gold nanoparticles are then created directly on the carbon surface through a gentle water-based chemical reaction. The two layers bond naturally during freezing, producing a strong and seamless interface without synthetic binders.
Gold nanoparticles exhibit what physicists call a plasmonic effect. When exposed to light, their electrons oscillate in unison, producing localized heating. The porous carbon absorbs light across ultraviolet, visible, and near-infrared wavelengths while also providing structural strength. Together they capture nearly all incoming sunlight and scatter it within the structure to minimize reflection.
Microscopy shows that the ZIF-8 crystals shrink and roughen after carbonization and that gold particles between two and eight nanometers distribute evenly across the surface. Although total surface area decreases from about 793 to 371 square meters per gram after the gold is added, the optical performance improves because the nanoparticles enhance local heating.
The cellulose framework underneath provides both strength and water transport. Its nanofibers, derived from plant pulp, contain hydroxyl groups that attract water molecules. Cross linking with glutaraldehyde strengthens the network and helps it maintain shape when wet.
Mechanical testing shows compressive strength of about 56 kilopascals when dry and 17 kilopascals when wet at 40 percent strain. The aerogel has low thermal conductivity, about 0.045 watts per meter kelvin, which helps keep heat near the surface. The composite layer containing gold and carbon remains similar at around 0.053 watts per meter kelvin.
Both layers absorb water rapidly, swelling to about fifteen times their dry weight. This continuous water supply is essential for stable evaporation under sunlight.
Optical and evaporation tests confirm strong light harvesting. The composite layer absorbs 95.9 percent of incoming sunlight. Under standard one-sun illumination, the surface temperature rises faster and higher than that of bulk water or a control made from pure carbon. The evaporation rate reaches 2.36 kilograms of water per square meter per hour at the optimal composition.
This value is among the highest reported for similar systems. The apparent solar-to-vapor efficiency is calculated at 119 percent. This figure is physically consistent because the cellulose confines water in very small pores where it is less tightly bound, which lowers the energy needed to vaporize it. As a result, the system produces more vapor from the same solar input without violating energy conservation.
The fabrication process itself determines much of the performance. When the same gold-carbon mixture is applied as a surface coating rather than formed by sequential freezing, the vertical channels collapse and light absorption becomes uneven. Infrared imaging shows cooler surfaces for these coated samples, and their evaporation rates drop to about half the optimal value. The layered freezing method produces continuous channels, stronger adhesion, and more uniform heat distribution, all of which raise efficiency.
Salt handling is a crucial test for desalination. The bilayer aerogel maintains steady evaporation even in concentrated brine with up to 20 percent sodium chloride. During an eight-hour run in seawater-level salinity, salt begins to crystallize on the surface after about ninety minutes but dissolves again when the system rests in the dark. After ten cycles of operation and rest, the device retains about 86 percent of its original rate.
Microscopy after cycling shows that the channels remain clear. Chemical analysis of the condensed water reveals sodium concentrations below 30 parts per million compared with about 10,000 in the feed. Levels of potassium, magnesium, and calcium also fall below the limits for drinking water set by the World Health Organization.
Field tests confirm performance outside the laboratory. When placed on a rooftop under natural sunlight, the aerogel begins producing vapor within minutes and continues steadily from morning to evening. The evaporation rate follows the solar intensity throughout the day.
Scaling the material to panels 15 centimeters wide and 2 centimeters thick causes no cracking or delamination. Samples buried in soil degrade visibly within thirty days, showing that the material is biodegradable. Since all fabrication steps use water-based chemistry and renewable components, disposal poses little environmental concern.
Compared with other cellulose-based or plasmonic evaporators, this aerogel achieves higher performance and better durability. Typical devices in the same class reach 1.5 to 2.3 kilograms of vapor per square meter per hour. The reported 2.36 kilograms per square meter per hour puts this system near the top of that range.
More importantly, it reaches this level using renewable materials and a simple fabrication process. The porous carbon framework holds the gold nanoparticles securely, preventing them from aggregating or detaching. The aligned cellulose channels provide efficient water transport and minimal heat loss, and the structure cleans itself during use by dissolving surface salt.
Some practical limits remain. Gold improves performance but raises cost, so future work may explore less expensive metals or smaller loadings. The tests cover ten cycles and one day outdoors, while real operation would require longer runs under conditions that include biofouling and suspended solids.
Scaling the process to large systems will also require efficient condensers and strategies for heat recovery. Despite these open questions, the results outline a realistic path for solar-powered desalination that avoids synthetic plastics and chemical waste.
This study shows how nanocellulose and carbon from metal-organic frameworks can be combined into a stable and efficient platform for solar-driven water purification. The sequential ice templating process produces aligned channels that control water flow and enhance evaporation. Gold nanoparticles strengthen light absorption and local heating without weakening the structure.
The measured performance, salt tolerance, and biodegradability place this device among the most effective bio-based evaporators yet developed. With further refinement, such materials could support portable desalination units, off-grid purification systems, or small wastewater treatment devices.
The underlying design principles—efficient vertical transport of water and vapor, heat confinement through low-conductivity scaffolds, and hybrid absorbers anchored in porous frameworks—offer a practical foundation for the next generation of solar evaporation technologies.
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