Coupling solar-driven interfacial evaporation with forward osmosis for continuous water treatment

Forward osmosis (FO) driven by osmotic pressure difference has great potential in water treatment. However, it remains a challenge to maintain a steady water flux at continuous operation. Herein, a FO and photothermal evaporation (PE) coupling system (FO-PE) based on high-performance polyamide FO membrane and photothermal polypyrrole nano-sponge (PPy/sponge) is developed for continuous FO separation with a steady water flux. The PE unit with a photothermal PPy/sponge floating on the surface of draw solution (DS) can continuously in situ concentrate DS by solar-driven interfacial water evaporation, which effectively offsets the dilution effect due to the injected water from FO unit. A good balance between the permeated water in FO and the evaporated water in PE can be established by coordinately regulating the initial concentration of DS and light intensity. As a consequence, the polyamide FO membrane exhibits a steady water flux of 11.7 L m^-2 h^-1 over time under FO coupling PE condition, effectively alleviating the decline in water flux under FO alone. Additionally, it shows a low reverse salt flux of 3 g m^-2 h^-1. The FO-PE coupling system utilizing clean and renewable solar energy to achieve a continuous FO separation is significantly meaningful for practical applications.