Salts retention by nanofiltration membranes: Physicochemical and hydrodynamic approaches and modeling

The nanofiltration membrane process is a technique that is increasingly used today for water treatment. This process is situated between ultrafiltration and reverse osmosis whose transfer mechanisms are very different. Thus, in nanofiltration, two additives and independent transfer mechanisms can be observed: the first one is the transfer by convection, similar to that observed in ultrafiltration and the other one is the transfer by diffusion, similar to that observed in reverse osmosis. In ultrafiltration, the selectivity is of physical nature, whereas in reverse osmosis the selectivity is of chemical nature. The aim of this study was to study separately these two mechanisms (using NF200 and NF270 membranes) and evaluate the influence of each of them by predicting the selectivity towards three sodium salts (Na2SO4, NaCl, and NaNO3). For that, Kedem, Katchalsky and Spiegler (KKS) models were used. The chloride ion transfer was mainly occurred by diffusion, whereas the sulfate ions were removed by convection. In the case of nitrate ion, the two mechanisms (convection and diffusion) occurred simultaneously. The retention was higher for divalent ions (SO42−) with over 90%, whereas it was lower for monovalent ions, such as chloride and nitrate ions (below 50%).

Research highlights
► Membranes transfer mechanisms were characterized by Kedem and Spiegler model.
► The estimated model perfectly explains the phenomenon of 3 sodium salts retention.
► Salt retention is higher for divalent ions (90%) than monovalent ions (50%).
► NF is more competitive than RO for selective desalination of brackish waters.
► This study allows to choose the most appropriate membrane for specific application.