On the salt rejection properties of nanofiltration polyamide membranes formed by interfacial polymerization

Abstract

Nanofiltration polyamide membranes obtained by interfacial polymerization possess skin layers containing amine and carboxylic acid groups that are distributed in an inhomogeneous fashion, leading to a bipolar fixed charge distribution (i.e. the sign of the surface charge density changes over the skin layer thickness). In this work we have modified the standard NF model (based on steric/Donnan exclusion and the extended Nernst–Planck equation) so as to account for the spatial variations of the fixed charge inside pores. The retention performances of NF membranes having bipolar charge distributions that capture the main electrostatic features of polyamide membranes have been investigated by considering 1-2, 2-1 and 1-1 electrolytes. In the range of volume fluxes usually obtained with NF polymer membranes, calculations show that the theoretical retention sequence of polyamide membranes for the above-mentioned electrolytes is 1-2 > 2-1 > 1-1, in agreement with experimental data available in the literature. This retention sequence has been shown to be specific of membranes with bipolar charge distributions since both homogeneously charged membranes and membranes with unipolar charge distributions (i.e. the concentration of charged surface groups can vary over the skin layer thickness but the sign of surface groups remains the same) would be more permeable to asymmetric electrolytes having divalent counterions than to symmetric mono-monovalent electrolytes. Conclusions drawn in this work are also likely to benefit the design of new NF membranes with targeted distribution of ionizable surface groups for desalination and water treatment.