Latent Heat Recovery Dual-Stage Solar Evaporator with a Photoactive and Photothermal Hydrogel-Coated Sponge for Treating Contaminated Saline Water.

Abstract

The discharge of saline wastewater with high-concentration organic pollutants from industries such as intensive seawater aquaculture poses a severe threat to aquatic ecosystems, calling for green and energy-efficient treatment technologies. Here, this study reports a rationally designed photothermal-photocatalytic multifunctional hydrogel-coated sponge (TiO2@PAM@PPy@PDA@MS, TPpPM) for simultaneous solar-driven water evaporation and in situ degradation of organic contaminants in saline wastewater. The composite is fabricated by sequentially coating polyacrylamide (PDA) and polypyrrole (PPy) onto a three-dimensional melamine sponge (MS) skeleton, followed by in situ polymerization of a polyacrylamide (PAM) hydrogel network loaded with TiO2 nanoparticles. The optimized TPpPM exhibits nearly 100% solar absorption across the full spectrum (250-2500 nm), achieves a surface temperature rise of 15 °C under one sun irradiation during solar-driven water evaporation, and delivers an evaporation rate of 1.16 kg m-2 h-1. The integrated PAM hydrogel provides superhydrophilicity and abundant anchoring sites for the low-cost but effective photocatalyst TiO2, enabling efficient photocatalytic degradation of 90% organic contaminants within 80 min. More importantly, a gravity-driven continuous-flow device incorporating TPpPM demonstrates excellent antiscaling performance and achieves a clean water yield of 400 mL m-2 h-1 with a salinity of 0.015% while simultaneously reducing total organic carbon (TOC) by 12.8% and total nitrogen (TN) by 38.2% from real shrimp aquaculture effluent. COMSOL Multiphysics simulations validate the coupled transport processes of interfacial evaporation transmembrane vapor diffusion and latent heat recovery. This work provides a low-cost and sole solar-driven strategy for treating complex saline wastewater, with promising potential for large-scale deployment under carbon-neutral scenarios.