Enhanced solar-driven evaporation process via f-MWCNTs/PVDF photothermal membrane for forward osmosis draw solution recovery

Product water recovery and draw solution (DS) reuse is the most energy-intensive stage in forward osmosis (FO) technology. Sucrose solution is the most suitable DS for FO application in food and beverages. However, sucrose DS recovery by conventional pressure-driven or thermal-driven concentration techniques consumes high energy. Herein, we developed a spontaneous and sustainable solar-driven evaporation process based on a photothermal membrane for the concentration and recovery of sucrose solution. The photothermal membrane composed of multi-walled carbon nanotubes (f-MWCNTs) phtotothermal layer on a hydrophilic polyvinylidene fluoride (PVDF) substrate. The f-MWCNTs photothermal layer with rough surface and interconnected network structures not only improves the light harvesting and light-to-heat conversion performance, but also facilitates the transport of water molecules. The hydrophilic PVDF substrate can promote the rapid transport of water for adequate water supply to photothermal layer. As a result, the optimized f-MWCNTs/PVDF photothermal membrane exhibits an excellent light absorption of 95%, and a high surface temperature of 74 °C at 1 kW m^-2. Besides, it realizes an evaporation rate of 1.17 kg m^-2 h^-1 for 5% (w/v) of sucrose solution, which is about 5 times higher than that of the natural evaporation. The designed photothermal evaporation process is capable of concentrating sucrose solution efficiently from 5% to 75% (w/v), which has great potential in FO process and juice concentration.