The role of iron present in water environment in degradation of polyamide membranes by free chlorine

Chlorine exposure is one of the most commonly encountered challenges that cause polyamide (PA) membrane failure in use. The synergistic effect of iron, which is ubiquitous in water environment, with chlorination is however often overlooked. This study systematically investigated the performance of thin film composite (TFC) nanofiltration (NF) membranes under dynamic exposure of free chlorine and Fe2+ in filtration process. It was found that the 36%–70% reduction of water flux after chlorine-only exposure was mainly attributed to the competing effects of reduced hydrophilicity by N-chlorination over decreased crosslinking by chlorination-promoted hydrolysis, while the opposite trends of salt rejection for mild and severe chlorination were believed to be caused by the competing effects of decreased crosslinking and increased charged density. The presence of Fe2+ in the feed generally resulted in a higher (or equivalent) water flux and a lower salt rejection compared to chlorine-only conditions, owing to catalytic oxidation and iron deposition. Fe2+-induced catalytic oxidation by hydroxyl radicals (·OH) formed by the reaction between free chlorine and Fe2+ promoted C–N breakage and led to a much looser separation layer, which contributes more remarkably to performance variation (i.e., 2.5–6.5 time increase of water flux and near-zero salt rejection for 1000 mg/L Cl2) than chlorination. In addition, the much lower normalized salt rejection for 10 mg/L Fe2+ than 0.1–1 mg/L Fe2+ (10%–30% vs. 100%–120%) under 10–100 mg/L Cl2 at 24 h was attributed mainly to a much denser iron deposition layer, which neutralized the membrane surface charge and decreased the electrostatic repulsion between salts and membranes. Hence, the combination of chlorination, catalytic oxidation and iron deposition was proposed to be the mechanisms synergistically affecting the membrane behavior.