Membrane Charge Weakly Affects Ion Transport in Reverse Osmosis

Polyamide reverse osmosis (RO) membranes have been a "golden standard"in water desalination, with an aromatic polyamide layer providing an excellent balance between salt rejection and water permeability. Yet, there still are gaps in understanding the salt transport in RO, closely related to the nature and formation of membrane charge and its effect on ion permeation. We report here a systematic investigation that examines the correlation between the nominal membrane charge, determined by ion-binding methods, and key ion permeation characteristics, salt permeability, and membrane conductivity, at different pH values. In the mid-pH range, the most important in the desalination practice, observed relations between the conductivity, salt permeability and nominal charge show a much weaker dependence than the Donnan model predicts. This indicates that fixed charged groups inside polyamide films are largely deactivated, thereby the membrane behaves as effectively neutral or weakly charged. However, a substantial charge may form at extreme acidic and basic pH via uptake of H+ and OH- ions, which increases ion uptake and conductivity, promoting polymer swelling and weakening salt rejection. Overall, the results strongly suggest that fixed charges weakly affect salt rejection in RO, and it is dominated by dielectric and steric mechanisms.