Chlorination-properties-performance of polyamide nanofiltration membranes for the rejection of emerging contaminants

Polyamide nanofiltration (NF) membranes play important roles in water decontamination, especially in the context of widespread occurrence of emerging contaminants (ECs) with severe environmental and health concerns. A common belief is that polyamide is vulnerable to chlorination, leading to the compromised separation performance. Nevertheless, how chlorination affects membrane separation capability of ECs such as per- and poly-fluoroalkyl substances (PFASs) and antibiotics remains unclear. Herein, we systematically investigated the impacts of chlorination on the physicochemical properties and separation performance of two NF membranes with different polyamide chemistry. Compared with the virgin NF90, the chlorinated NF90 under a chlorination intensity of 1000 ppm × h at pH 9 showed a more negatively charged membrane surface, a higher O/N ratio and a narrower membrane pore size distribution. Consequently, it exhibited enhanced rejection of negatively charged PFASs due to stronger electrostatic repulsion and size exclusion. Despite the reduced water permeance, the chlorinated NF90 membrane had enhanced water-PFASs selectivity due to the greater reduction in PFASs permeability coefficient. Meanwhile, the chlorinated membrane also maintained stable rejection and selectivity of antibiotics. Further increasing chlorination intensity led to significantly reduced selectivity against antibiotics and most PFASs because of damaged pore structure. Moreover, we analyzed the correlation of chlorination-properties-performance for the membranes. The result revealed the critical influence of chlorination induced pore size change on the rejection and selectivity of ECs for NF90. In comparison, NF270 membrane maintained a relatively stable rejection and selectivity of ECs even under a chlorination intensity of 100000 ppm × h, thanks to the chlorine-resistant property of tertiary amide in its semi-aromatic polyamide. These findings can promote the fundamental understating on chlorination impacts for NF membrane properties and performance, which may further guide the membrane selection and optimization for chlorine involved water treatment.