Probing the size and charge effects on the rejection and selectivity of CaCl2 and Na2SO4 in NF membranes with progressive tailored polyamide characteristics

Nanofiltration (NF) technology has been extensively applied in diverse water treatment scenarios, with a growing emphasis on improving its selective separation of different salts. While existing research broadly linked pore size and surface charge to separation performance, the differential impacts of these two characteristics on specific salts remain unclear. Herein, this study systematically investigated specific effects of polyamide pore size and surface charge on salt rejection and selectivity of NF membranes. By decreasing piperazine concentration in the support-free polyamide formation, we fabricated a series of NF membranes with progressively increasing pore size and surface charge. CaCl 2 rejection exhibited the strongest correlation with surface charge ( R 2  = 0.86), while Na 2 SO 4 rejection showed highest dependence on pore size ( R 2  = 0.88). In addition, CaCl 2 /Na 2 SO 4 selectivity exhibited a two-stage dependence on pore size, peaking at a threshold around 0.6 nm. This suggested potential optimizations for CaCl 2 /Na 2 SO 4 selectivity, where polyamide pore size can be well-tuned to approach the threshold with maximizing surface charge. Moreover, the NF membrane with higher CaCl 2 /Na 2 SO 4 selectivity exhibited the better anti-scaling propensity, with a significant reduction in irreversible scaling ratio. Our findings provided insights into the regulation of polyamide pore size and surface charge for achieving the high salt/salt selectivity of NF membranes.