Investigating the effect of inhomogeneous fixed charge distribution on dielectric exclusion in nanofiltration membranes

Nanofiltration has been applied as a promising way in water treatment because of its special separation mechanisms. Most thin-film polyamide nanofiltration membranes obtained by inter facial polymerization possess active layer containing a mine and carboxylic acid groups that are distributed in a highly non-uniform fashion, leading to an inhomogeneous fixed charge distribution. This work is a theoretical research to investigate the impact of inhomogeneous fixed charge distribution on dielectric exclusion in negatively charged nanofiltration membranes. NaCl rejection rate has been computed as a function of different pore radius, feed flow concentration and dielectric constant inside the pore for various inhomogeneous charge distributions with an identical average volume charge density. It has been shown that the difference of rejection performance for fixed charge distributions changes with the variation of pore radius and feed concentration. This phenomenon is related with the electric field behavior determined by functional groups, which also affects the ion partitioning at the solution/membrane interface as well. The dielectric exclusion is therefore affected by the inhomogeneous fixed charge distribution at relatively low pore radius and low feed flow concentration in this work. Thus, the dielectric effect promotes the electrolyte rejection performance differently for fixed charge distributions studied in this work. Conclusions drawn in this work are also likely to benefit the comprehension of separation mechanism of polyamide membranes in nanofiltration process and is valuable for the development of membrane fabrication process on membrane structure and functional group distribution.