Effect of highly concentrated salt on retention of organic solutes by nanofiltration polymeric membranes

During desalination of feed with highly concentrated salt by nanofiltration (NF), predictive modeling was difficult due to the effect of salt on retention of organic solutes. Consequently, a better understanding of salt effect on membrane and organic solutes was required. In this study, four well-known commercially available NF polymeric membranes, NF270, NF-, Desal-5 DL and Nanomax50, were analyzed by a model based on an extended Nernst–Planck equation, using highly concentrated glucose and sodium chloride (NaCl) solutions. The results showed that with increasing salt concentration, the solute-to-pore size ratio (λi) decreased while the ratio of effective membrane thickness to porosity (Δx/ɛ) increased, indicating that the effect of salt may include decreasing solutes size, increasing membrane pore size, and increasing effective membrane thickness. Moreover, such salt effect appeared to be independent of membrane and solute types, and the correction model could well predict the retention of charged solutes at high salt concentration because electrostatic repulsion effect between charged solutes and membranes was completely screened by the salt ions. Meanwhile, several hypotheses such as membrane swelling, hydration layer thinning and particle collision were provided to explain the change of model parameters by highly concentrated salt.