pH-dependent effects of chlorine dioxide on polyamide membranes: Performance, surface characterization, and degradation mechanisms

Chlorine dioxide (ClO₂) is known to degrade polyamide (PA) membranes, but the extent and mechanism of this degradation under varying pH conditions remain poorly understood. In this study, we systematically evaluated the impact of ClO₂ exposure (100 ppm) across different pH values (4, 10, and 12) on the performance and physicochemical properties of two classes of PA membranes: m-phenylenediamine (MPD)-based membranes (NF90 and BW30) and a piperazine-based membrane (NF270). Under acidic conditions (pH 4), minimal changes in pure water flux were observed for all membranes. Cross-flow filtration tests indicated a moderate decrease in flux (~3 LMH) and an increase in salt rejection (from 84 % to 92 %) for NF90 membranes, reflecting structural rearrangement from N-chlorination reactions induced by chlorine species formed under acidic conditions. In contrast, alkaline conditions (pH 12) resulted in extensive degradation, evidenced by significant flux increases (approximately 10-fold for NF90 and 24-fold for BW30) and drastic reductions in salt rejection (from 81 % to 0 % for NF90). This degradation was due to direct electrophilic attacks by ClO₂ molecules on deprotonated amide nitrogens, causing substantial removal of the PA layer. The piperazine-based NF270 membrane demonstrated excellent resistance to ClO₂ degradation across all pH conditions, exhibiting only minimal performance changes. These results highlight the critical role of membrane chemical structure in determining resilience to ClO₂ treatment and provide a quantitative basis for selecting appropriate membranes and operating conditions in industrial desalination processes.