(169az) Examining Ion Dehydration Mechanisms at the High Pressures and Concentrations Required for Desalination

High-salinity brine treatment is critical to increase water supplies and minimize waste discharge. Membrane-based desalination processes such as reverse osmosis (RO) and nanofiltration (NF) are being increasingly considered for brine treatment due to their high energy efficiency compared to thermal processes. However, brine treatment exposes membranes to extremely high pressures and salinities that alter membrane transport properties. The underlying mechanisms by which pressure and concentration affect the rejection of ions have not been fully determined. In this work, we study the effect of high pressures and salinities on the ion dehydration mechanism using atomistic molecular simulations of ions in aqueous solution and within an RO membrane. We explore these trends for ions of different size, valency, and charge. We quantify the energy barriers to remove one or more waters from the hydration shell for many relevant salts and operating conditions. We show that system pressure in the operating range of RO and NF membranes does not change the free energy of dehydration in solution. However, at high concentration, the free energy to strip waters is reduced. Stripped waters can more easily coordinate with other ions than disrupt the second hydration shell or bulk water structure. We observe the geometry of dehydration within an RO membrane in order to determine how confinement and membrane groups influence the hydration shell. Our results will provide a valuable reference for identifying the primary mechanisms for ion transport in industry-standard membrane materials.