Elucidating the mechanisms underlying the difference between chloride and nitrate rejection in nanofiltration

Abstract

Despite the strong similarity between chloride (Cl-) and nitrate (NO3-) anions in terms of their hydrated radius and charge, Cl- is rejected more favorably than NO3- by nanofiltration (NF) membranes. The main goal of this study is to provide a better understanding of the removal mechanisms favoring the higher rejection of Cl- over NO3- in NF. A series of experiments with polyamide (NF270) and cellulose acetate (CK) NF membranes at different pH values, followed by calculation of the activation energies for Cl- and NO3- passage through the membranes, showed that the higher Cl- than NO3- rejection is attributed to both size-exclusion and Donnan (charge)-exclusion mechanisms. At a neutral membrane charge, a size-exclusion mechanism dominates the rejection of both anions. In this case, we observe higher rejection of Cl- over NO3- due to the lower hydration energy of NO3-, which corresponds to higher degree of dehydration and thus higher rate of passage through the NF membrane pores. At a negative membrane charge, the smaller volume of Cl- compared to NO3-, corresponding to higher ionic charge density, results in a stronger electrostatic repulsion of Cl- by the negatively charged membrane and therefore higher Cl- rejection than NO3-. The coupling of size- and Donnan-exclusion mechanisms with the NF270 membrane results in a maximum Cl- to NO3- rejection ratio at near the isoelectric pH where the membrane is slightly negatively charged. At a positive membrane charge, the sodium (Na+) counter ions dictate salt rejection independently of the anion type, resulting in almost similar rejections of Cl- and NO3-. Based on the insight gained from these experiments, a layer-by-layer (LbL) polyelectrolyte modification was applied to the NF270 membrane to control its surface charge. This modification showed that shifting the isoelectric point of the NF270 membrane from its original value (pH 4–5) to higher values (pH 6–9) increased the Cl- to NO3- rejection ratio at near neutral pH conditions, thus providing further support for our proposed mechanism underlying the difference between Cl- and NO3- rejection by NF membranes.