A fully biomass hydrogel made from coffee grounds and seaweed polymer turns black when wet, capturing 94% of sunlight to desalinate seawater without synthetic additives.
(Nanowerk Spotlight) Spent coffee grounds darken noticeably when wet. It is an everyday observation, easy to overlook, but it reflects a real change in how the material interacts with light. Water filling the pores of a dark, granular substance reduces internal reflections and allows more light to be absorbed rather than scattered. The same principle applies at larger scales: any porous, pigment-rich material that darkens significantly upon wetting becomes, in effect, a better solar absorber.
That property is directly relevant to an emerging class of water purification technology known as solar-driven interfacial evaporation. The technique concentrates sunlight at a thin material layer floating at the water surface, heating just that interface to accelerate evaporation while keeping the bulk water cool. The vapor is then collected and condensed as freshwater.
Because the energy source is free and the device structure simple, it offers a compelling alternative to conventional desalination methods such as reverse osmosis, which demand substantial electrical energy and expensive membrane infrastructure.
But nearly all high-performing solar evaporators rely on engineered photothermal materials, such as carbon nanotubes, MXene nanosheets, or conductive polymers, to capture sunlight efficiently. These additives are often costly, require complex fabrication, and can degrade under prolonged ultraviolet exposure or high salinity, sometimes releasing microplastics or metal contaminants. Even recent evaporators built from biomass feedstocks typically still incorporate external light-absorbing agents.
The researchers combined spent coffee grounds with sodium alginate, a gel-forming polymer derived from brown seaweed, to build a fully biomass-based evaporator that exploits wetting-induced darkening as its primary light-harvesting mechanism. With global coffee ground waste exceeding 9 million tons per year and sodium alginate both inexpensive and widely available, the raw materials are abundant and cheap.
a) Fabrication process of the porous hydrogel-based evaporator. b) Digital photograph and SEM image of the SCH surface. c-d) Magnified SEM images of the SCH surface. e) SEM image of the side view of SCH. f,g) Cross-sectional SEM image and corresponding EDS mapping of SCH. h) Schematic illustration of the evaporator structure. (Image: Reproduced with permission from Wiley-VCH Verlag) (click on image to enlarge)
In its dry state, the porous hydrogel appears brown and absorbs an average of about 72% of incoming solar radiation. When water fills the pores, the material darkens to near-black, and absorbance rises to roughly 94%. Dry pores contain air, creating optical contrasts at internal surfaces that scatter light back out. Water replaces that air, smooths out the contrasts, and suppresses reflection. Light penetrates deeper, where the dark pigments naturally present in roasted coffee grounds absorb it efficiently. The transformation reverses completely upon drying, confirming that no permanent chemical change is involved.
The internal architecture plays an equally important role. The researchers used radial directional freeze-drying to create aligned lamellar channels radiating from the center of the cylindrical device to its edges. Ice crystals growing outward during freezing template sheet-like structures with a built-in pore-size gradient, smaller near the center and larger at the periphery. This gradient generates capillary forces that draw water preferentially inward from the sides.
Regulating the speed of water delivery proved critical. Excessive water flow cools the evaporation surface and wastes energy heating liquid rather than producing vapor. Insufficient flow allows dissolved salt to crystallize and block pores. The researchers tuned surface wettability by exploiting a natural coordination chemistry: tannic acid present in the coffee grounds bonds with iron(III) ions during fabrication, masking water-attracting groups on the surface and exposing water-repelling ones.
The result is a surface that initially resists wetting but gradually absorbs water over about 90 seconds. This controlled delay keeps the liquid film thin and the surface temperature elevated.
Together, high wet-state absorbance, reduced evaporation enthalpy, and effective thermal management yielded an evaporation rate of 2.2 kg m⁻² h⁻¹ under standard 1.0 sun irradiation, with a solar-to-vapor conversion efficiency of 91.2%. The hydrogel matrix holds water in a loosely bound intermediate state, weakening the intermolecular forces that normally resist evaporation. As a result, the energy required to vaporize water from the device is about 29% lower than for pure water.
Salt fouling posed a remaining challenge. In 10 wt% saline solution over 10 hours, the flat-surfaced evaporator’s rate fell to 1.32 kg m⁻² h⁻¹ as crystals accumulated at the interface. To counter this, the team patterned the evaporator surface with a trapezoidal microstructure.
The geometry creates temperature and concentration gradients across each trapezoid. The top, where evaporation is fastest, becomes cooler and saltier than the base. Because surface tension rises with decreasing temperature and increasing salinity, a gradient develops that drives liquid upward along the trapezoidal walls. This Marangoni convection continuously flushes salt away from crystallization-prone zones. With the patterned surface, the evaporator held a stable rate of 2.39 kg m⁻² h⁻¹ in 10 wt% saline for 10 hours with no visible salt deposits.
The trapezoidal pattern also improved performance under oblique sunlight, a condition that dominates real-world operation during morning and late afternoon hours. At low solar angles, the patterned device exceeded its flat counterpart by up to 28% under full sun and nearly 35% under half-intensity illumination, as the angled surfaces trapped more light through multiple internal reflections.
A field test in Luoyang, China, confirmed practical viability. Over 12 hours of natural sunlight, the patterned evaporator produced 5.01 kg m⁻² of freshwater from 3.5 wt% simulated seawater. Ion concentrations in the collected water fell well below World Health Organization drinking water guidelines. The device also resisted microbial colonization over 10 days in natural water, an effect attributed to antimicrobial copper ions incorporated in the hydrogel matrix during fabrication.
With raw material costs below 6 USD per kilogram and full biodegradability at end of life, the evaporator addresses cost and environmental concerns that have limited earlier systems. More broadly, the work demonstrates that careful structural and surface engineering of inexpensive waste-stream materials can match or exceed the performance of systems built with engineered nanomaterials.
For decentralized water purification in resource-limited settings, where simplicity, cost, and environmental safety matter as much as peak efficiency, that principle could prove especially valuable.
For authors and communications departmentsclick to open
Lay summary
Prefilled posts
ORCID information
Chang Lu (Henan University of Science and Technology)
, 0000-0002-3133-3196 corresponding author
Xinchang Pang (Henan University of Science and Technology)
, 0000-0003-2445-5221 corresponding author
Nanowerk Newsletter
Get our Nanotechnology Spotlight updates to your inbox!
Thank you!
You have successfully joined our subscriber list.
Become a Spotlight guest author! Join our large and growing group of guest contributors. Have you just published a scientific paper or have other exciting developments to share with the nanotechnology community? Here is how to publish on nanowerk.com.
