Sorption of a species into a polymer structure is the first resistance encountered in liquid-filled membrane permeation. The sorption coefficient, K, relates the concentration of a species inside of the membrane polymer phase to the concentration in aqueous solution phase: K=cmcb where cm and cb are the concentrations inside the membrane and in the feed solution, respectively. A large sorption coefficient for a species indicates that little resistance is experienced when the species sorbs into the polymer phase (58). The sorption coefficient for water in a polymer is proportional to the volume fraction of dissolved water. The volume fraction of water can be increased by increasing the hydrophilicity of the membrane that is typically done by increasing the number of charged functional groups present within the membrane (57, 58).
Blue and red curves are concentration and temperature profiles. Yellow bars and gray bars with gaps represent dense polymer membranes and porous air-filled membranes, respectively. The left side of membrane represents the feed, and the right side represents the permeate (or draw solution for concentration-driven processes).
A key disadvantage of concentration-driven processes is the lower energy efficiency and more complicated operation than pressure-driven systems (16). Standalone concentration-driven processes cannot carry out desalination as the product water needs to be further separated from the diluted draw solution. This separation is inevitably more energy intensive than a standalone pressure-driven system (16). The use of thermolytic draw solutes, like ammonia-carbon dioxide, that allow regeneration of the draw solution with low-grade thermal energy provides a possible avenue to reduce electricity consumption (103). However, efforts to implement these systems have been stymied by loss of the draw solute via reverse salt flux and the need for energy-intensive draw solution recovery technologies. Concentration-driven processes also suffer from increased losses in their driving force due to polarization effects as compared to pressure-driven systems.
Pressure-driven desalination methods including PD and RO are inherently energy efficient as compared to concentration- and temperature-driven processes (7, 16, 94). However, the water output of pressure-driven processes is dependent on the salinity of the feed solution, making these processes less effective at treating high-salinity feed waters. Among the pressure-driven processes, RO is much more technically mature, but PD may have potential owing to the high removal of all nonvolatile compounds possible with air-filled membranes. Concentration-driven processes such as OD and FO are able to treat high-salinity brines but are more energy intensive than pressure-driven systems as draw solution regeneration inherently requires more energy than a direct pressure-driven separation. Temperature-driven MD systems are able to treat high-salinity brines with high selectivity but suffer from lower energy-efficiency than pressure- and concentration-driven systems. TO is not considered advantageous in any scenario currently because of its low driving force and severe flux decline brought on by boundary layer effects. 1e1e36bf2d