Salt-Resistant Self-Floating Composite Hydrogel Foam Evaporator Enabled by Donnan Effect for Efficient Solar Interfacial Evaporation and Water Purification

Solar interfacial evaporation is a promising approach for desalination and wastewater treatment, yet practical evaporators remain challenged by limited water transport, interfacial heat loss, and salt deposition in high-salinity brines. Herein, a self-floating, salt-resistant composite hydrogel foam evaporator, PTF@PGAS-Foam, was developed by integrating an electrospun tannic acid-Fe3+ photothermal membrane with a polyelectrolyte double-network hydrogel foam. The upper photothermal membrane enables strong solar absorption and thermal localization, while the lower foam layer provides buoyancy, low thermal conductivity, and continuous three-dimensional water-transport pathways. Fixed sulfonate groups in the hydrogel induce the Donnan effect, suppressing salt-ion migration toward the evaporation interface and alleviating salt crystallization during continuous evaporation. Under 1 sun irradiation, PTF@PGAS-Foam achieved an evaporation rate of 2.61 kg·m-2·h-1 and an efficiency of 92% in a 20 wt % NaCl solution, with stable performance during continuous and cyclic tests. The evaporator also adsorbed heavy metal ions, including Pb2+ and Cd2+, and cationic dyes. In outdoor scaled-up tests, a 15 cm diameter evaporator produced 20.35 kg·m-2 of water per day. This multifunctional evaporator integrates solar absorption, heat localization, self-floating behavior, salt resistance, and adsorption, showing potential for solar desalination and wastewater purification.