Salt rejection and mitigation strategies in solar-driven photothermal membrane distillation

This chapter discusses advanced approaches to prevent and manage salt accumulation in solar-driven desalination, spanning passive interfacial evaporators and active membrane distillation (MD). It begins by detailing the transport and crystallization mechanisms behind scaling, temperature polarization, fouling, and wetting, and then assesses mitigation strategies that shift from mere resistance to active regulation of salt at the evaporation front. Passive systems that rely on solar interfacial desalination, with bulk-water pumping driven by capillary forces, focus on self-regulating strategies, including ionic backflow, surface-functionalized membranes, Marangoni-induced convection, and hydrophobic/hydrophilic structures. Emerging approaches leverage noncapillary-driven mechanisms such as ionic hydrogels and siphon-based systems, promoting operational stability and scalability under high-salinity conditions. In active systems, particularly solar-driven MD, progress centers on interfacial chemistry and hydrodynamic control: self-cleaning, surface-functionalized membranes; additively manufactured spacers that thin boundary layers; and module-level flow/thermal management that curbs polarization and stabilizes long-term operation. Across both domains, the most promising pathway integrates surface chemistry, hierarchical architecture, and flow-field tailoring to couple high flux, brine tolerance, and controlled salt harvesting within modular, robust platforms. Remaining barriers include durability under intermittent solar input, reproducibility across complex feeds, scalable fabrication, and gaps in techno-economic and life-cycle evidence, which are priorities for extended field trials and multiscale modeling that connect molecular wetting/crystallization to device performance.