Performance Evolution of Polyelectrolyte Hollow Fiber Nanofiltration Membranes under Chlorine Exposure: Mechanisms and Insights for Application

Polyelectrolyte multilayer hollow fiber (PEM HF) nanofiltration membranes have emerged as promising candidates for drinking water treatment due to their favorable antifouling properties and feasibility of backwashing. However, their multiscale degradation under sodium hypochlorite (NaClO) exposure remains insufficiently explored, constraining material improvement and operational innovation. This study systematically elucidates the structural and performance evolution of PEM HF membranes under varying concentrations and durations of NaClO exposure. It was revealed that NaClO preferentially targeted the cationic poly(allylamine hydrochloride) layer, inducing <i>N</i>-chlorination, hydrolysis, and chain scission, ultimately resulting in its detachment and progressive exposure of the anionic poly(styrenesulfonate) layer. These changes endowed the membrane with increased surface roughness, an enlarged pore size, and more negative surface charges, which enhanced water permeability and particularly impaired the rejections of small-molecule organics and divalent cations. The membrane degradation followed pseudo-first-order kinetics, with more rapid functional deterioration during the early-stage exposure of NaClO, and was more facilitated by the chlorine concentration than the exposure duration. Additionally, the polycations bearing quaternary ammonium groups, instead of amine-rich ones, displayed higher chlorine resistance and thus were recommended for the development of chemically robust PEM membranes. This study deepens the mechanistic understanding of the structure-performance relationships of PEM membranes under oxidative stress and provides insights for potentially combining nanofiltration processes with chlorination.