The influence of feedwater pH on membrane charge ionization and ion rejection by reverse osmosis: An experimental and theoretical study

Modeling mass transport of ions across the polyamide active layer of a reverse osmosis (RO) membrane requires a comprehensive understanding of membrane structure and chemistry. For instance, membrane charge ionization and thus salt transport greatly depend on feedwater pH and composition, but these relations are not yet well understood. To address this gap in understanding, a one dimensional model is developed that couples transport of all ions using the extended Donnan steric partitioning pore model. The model includes membrane charge ionization as well as interaction with H+ and OH−-ions. The dependence of ion rejection and permeate pH is described as function of feedwater pH. Finally, model predictions are quantitatively compared with experimental data by adjusting a few fitting parameters using the Nelder-Mead algorithm. Contrary to other RO studies, we show that the polyamide is only weakly charged, but this small charge still plays a key role to determine membrane performance. These findings reveal the key role of local pH in the ionization of membrane functional groups, and how local charge affects overall membrane rejection of ions as well as permeate pH. [Display omitted] •Membrane charge density is a function of local pH which varies across the membrane.•In reverse osmosis, feed pH significantly influences ion rejection and permeate pH.•Separate, detailed measurement of anion and cation rejection, and investigation of charge balance closure.•Membrane charge significantly influences ion rejection and permeate pH.