Chitosan-based hydrogel composite membrane for stable and enhanced freshwater production in photothermal membrane distillation system

Traditional photothermal membrane distillation (PMD) for seawater desalination suffers from low photothermal conversion efficiency and poor operational stability under high-salinity brine conditions. In response to these limitations, this study reports a hydrogel composite membrane (HCM) mediated photothermal membrane distillation device. The HCM consists of a chitosan/agarose hydrogel (CAH) layer, carbon nanotube (CNT) coating, and polytetrafluoroethylene (PTFE) hydrophobic membrane. The CNT layer significantly improves the photothermal conversion efficiency, achieving 98.5% solar absorption under standard solar irradiation. The CAH modulates intermediate water states by tuning the chitosan-to-agarose ratio, thereby reducing water evaporation enthalpy and enabling stable freshwater production. Under 1 kW m −2 solar intensity (i.e. 1 sun) for 1.5 h, the water flux of the device increased by 40.7% than pure CNT/PTFE membrane. Simultaneously, CAH employs its three-dimensional porous structure and abundant amino (–NH 2 ) and hydroxyl (–OH) groups to achieve synergistic salt rejection through electrostatic repulsion. The device achieved continuous and stable freshwater production in a 7 wt% NaCl solution, demonstrating outstanding salt-rejection properties. This study developed hydrogel composite membrane to enhance water flux and system stability in PMD, proposing novel concepts for performance enhancement of PMD.