A review of hydrogel membranes for sustainable separations: from material design and fabrication to structural regulation, mechanisms, and applications

Hydrogel membranes have emerged as a promising platform for sustainable separation due to their intrinsic hydrophilicity, tunable pore structures, and multifunctional properties. Compared with conventional membranes, which are often limited by the trade-off between permeability and selectivity, hydrogel membranes provide new opportunities for efficient purification and separation. This review summarizes the material basis, design principles, fabrication strategies, and representative applications of hydrogel membranes. The discussion begins with the classification of hydrogels, the materials used for membrane formation, and the main separation mechanisms, including size sieving, chemical adsorption, and the Donnan effect. Recent progress in membrane fabrication and structural design is then discussed, with emphasis on direct molding, surface coating, sandwich architectures, embedded structures, and nanomaterial composite systems, as well as reinforcement mechanisms related to antibacterial performance, fouling resistance, permeability and selectivity regulation, and photothermal interfacial heating. Recent advances in wastewater treatment, resource recovery, and energy conversion are also reviewed. Finally, current limitations and future research directions are outlined. This review provides a useful reference for the continued development of hydrogel membranes in addressing global water and energy challenges.