Transport and structural properties of osmotic membranes in high-salinity desalination using cascading osmotically mediated reverse osmosis

Management of high-salinity brines is a global environmental challenge. Recently, we proposed a novel cascading osmotically mediated reverse osmosis (COMRO) technology for energy-efficient hypersaline desalination. In this study, a transport model is established for COMRO. We investigate the impacts of hydraulic pressure and salinity on transport and structural properties of thin-film composite osmotic membranes in COMRO. Our results show that transport properties are not affected by transitory pressure changes on the order of hours. But on longer timescales, on the order of days, the membrane undergoes compaction/relaxation in response to pressurization/depressurization, with water and salt permeabilities declining/recovering accordingly. Importantly, the water and salt permeabilities change in the same proportion. We found that this is due to morphological changes of the active-support interlayer altering the effective membrane area. The membrane structural parameter is demonstrated to be consistent at different salinities. As salinity increases, both water and salt permeabilities increase, but salt permeability rises substantially more. Lastly, the presented transport model is validated by good agreement between experimental and predicted water fluxes. This study advances the understanding of membrane transport and structural properties in the emerging COMRO technology, and provides insights into water and salt transport for other osmotic membrane processes.