Relating Solute–Membrane Electrostatic Interactions to Solute Permeability in Reverse Osmosis Membranes

Despite the widespread use of reverse osmosis (RO) membranes in water desalination, the role of solute-membrane interactions in solute transport remains complex and relatively not well understood. This study elucidates the relationship between solute-membrane electrostatic interactions and solute permeability in RO membranes. The transport of salt and neutral molecules through charged polyamide (PA) and uncharged cellulose triacetate (CTA) RO membranes was examined. Results show that salt rejection and salt permeability in the PA membrane are highly dependent on the solution pH due to the variations of membrane charge density and the Donnan potential at the membrane-solution interface. Specifically, a higher salt rejection (and hence lower salt permeability) of the PA membrane is observed under alkaline conditions compared to acidic conditions. This observation is attributed to the enhanced Donnan potential at higher solution pH, which hinders co-ion partitioning into the membrane. In contrast, for salt transport through the CTA membrane and neutral solute transport through both membranes, solute permeability is independent of the solution pH and solute concentration due to the negligible Donnan effect. Overall, our results demonstrate the important role of solute-membrane electrostatic interactions, combined with steric exclusion, in regulating solute permeability in RO membranes.