Degradation of polyamide nanofiltration membranes by free chlorine and halide ions: kinetics, mechanisms, and implications

Abstract

The kinetics of polyamide membrane degradation by free chlorine and halide ions (Br⁻ and Cl⁻) were innovatively evaluated based on physicochemical properties and filtration performance, using water/solute permeability coefficient in addition to bromide incorporation as important indicators. The reaction rate constants for the reduced water and H₃BO₃ permeability coefficient were 1-2 orders of magnitude higher at 0–1 h than 1-10 h. N-bromination and bromination-promoted hydrolysis are dominant degradation mechanisms at 0−1 h (reflected by the breakage of hydrogen bond, the increased Ca binding content, and the increased charge density), and ring-bromination further occurs at 1−10 h (reflected by the disappearance or weakening of aromatic amide band and the nearly constant hydrogen bond). The more reactive but less abundant brominating agents (Br₂O, BrOCl, BrCl, and Br₂) played significant roles in membrane degradation, contradicting the conventional belief that HOBr is the only reactive species. BrCl at pH 4.0 and BrOCl and Br₂O at pH 7.0 made significantly higher contributions to membrane degradation than HOBr (>76% vs. <13%). The increased contribution of BrCl and Br₂ with the increased [Cl⁻] and [Br⁻]_ex (the excess bromide, defined as [Br⁻]_o − [HOCl]_o when [Br⁻]_o > [HOCl]_o), respectively, was responsible for the greater reduction of water permeability coefficient. The innovative and simple approach developed in this study provides important insights to evaluate and predict membrane degradation.