Salt permeability in polyamide reverse osmosis membranes

Abstract: Salt permeability of polyamide reverse osmosis membranes has been shown to increase with increasing feed salt concentration. This relationship is believed to be connected to how salt distribution varies with feed salt concentration. However, contrary to expectations, investigations utilizing different analytical techniques have revealed that the salt (total ion) partitioning coefficient actually decreases as the salt concentration increases, despite the observed increase in salt permeability. In this study, we conduct a comprehensive investigation into the influence of salt concentration and solution pH on the partitioning coefficients of total ions and co-ions. We measure salt partitioning using a quartz crystal microbalance (QCM) and calculate co-ion partitioning through a modified Donnan theory based on the measured salt partitioning. Our findings show a stark contrast in the behavior of co-ion and total ion partitioning with increasing salt concentrations. Specifically, co-ion partitioning increases while total ion partitioning decreases as the salt concentration (NaCl) increases. This phenomenon is observed in pristine and leaky reverse osmosis membranes. This increase in co-ion partitioning aligns with the rising trend of salt permeability observed in filtration experiments. Moreover, we demonstrate that the dependency of salt and co-ion partitioning on salt concentration becomes more pronounced at higher solution pH. This study offers a profound understanding of ion partitioning in polyamide RO membranes and its correlation with salt transport.