Influence of pH and NaCl concentration on boron rejection during nanofiltration

Abstract Nanofiltration (NF) is promising for replacing reverse osmosis for boron removal more effectively and at a higher energy efficiency. However, the interplay of steric and charge effects underlying rejection behaviors of solutes smaller than the membrane pore size remained incompletely understood, which impedes implementation. This study investigated the influence of pH and NaCl concentration on boric acid/borate rejection by the Desal 5DK NF membrane, with acetic acid/acetate (comparable size and charge nature) and glucose (larger and neutral) serving as benchmarks. Filtration experiments demonstrate that varying pH in the range of 3–11 and NaCl concentration up to 100 mM had little effect on glucose rejection (consistently greater than 95%), but affected the two smaller solutes significantly. Increasing pH from 7 to 11 led to 15% higher rejection of charged acetate because of the increased charges of both the membrane and solute, while increasing pH from 3 to 7 gave 30% higher rejection of neutral boric acid due to steric effects stemming from pore-size reduction. The rejections of the neutral boric acid and acetic acid were negligibly affected by NaCl concentration, but rejection decreased by 15% and 50% for the charged borate and acetate, respectively, due to salt-induced pore-swelling.