Negative rejection phenomenon in the mixed salt nanofiltration: Law and mechanism

The negative rejection of certain solutes is an interesting phenomenon often observed in nanofiltration (NF) process, which presents an ideal solute separation effect. In this study, the NF membrane pore size, electric charge property and influent ions composition were controlled to investigate the rule and mechanism of negative rejection phenomenon during SO42−/Cl− mixed salt NF. The results show that the changing of the membrane pore diameter and charge will significantly affect the NF selectivity to SO42−/Cl−, then affect the negative rejection phenomenon. Selective efficiency of Cl−/SO42− can be up to 156.04–283.20 for the negative charged NF membrane with an average pore diameter of 0.62–0.68 nm, and the Cl− was negatively rejected under many influent conditions with the lowest rejection rate of −252.36 %. The substitution of Na+ for Mg2+ and the increase of SO42−/Cl− ratio are beneficial to improve selectivity. The Donnan Steric Pore Model with dielectric exclusion model was used to calculate the rejection curve which is consistent with the experimental results. The analysis of NF mechanism shows that the negative rejection of Cl− is caused by the extra transfer power derived from the NF selectivity to Cl−, which is a compensation for the higher rejection rate of SO42−.