Reactivity of various brominating agents toward polyamide nanofiltration membranes

Abstract

Polyamide (PA) membranes, widely used for reverse osmosis and nanofiltration, are prone to bromination under chlorinated bromide-containing water conditions. Conventional wisdom generally assumes HOBr as the only active brominating agent responsible for PA membrane degradation, while some more reactive but less abundant brominating agents (including Br2O, BrOCl, BrCl and Br-2) under these conditions are often overlooked. The current study addresses this critical literature gap by systematically evaluating membrane degradation under various [Br-], [Cl-] and [HOCl] conditions. The observed pseudo-first-order rate constant of membrane degradation (k(m)(obs), using change in water flux as a surrogate indicator) was found to be well correlated to [Br-], [Cl-] and [HOCl] (R-2 > 0.90). The gradual increase of [Cl-] and [Br-] transforms the predominant brominating agent from HOBr to BrCl and Br-2, respectively, under excessive [Br-] conditions. The species-specific second -order reaction rate constants followed a decreasing order of k(BrCl)(m)(2.6 x 10(4) M-1.s(-1)) >k(BrOCl)(m) (2.0 x 10(3) M-1.s(-1)) >k(Br2O)(m)(9.6 x 10(2) M-1.s(-1)) >k(Br2)(m)(1.5 x 10(1) M-1.s(-1)) > k(HOBr)(m)(5.4 x 10(-1) M-1.s(-1)). Additional decay tests using benzanilide (BA) as a surrogate monomer compound confirmed BrCl as the most reactive species. Under typical seawater conditions (pH 8.0), the more reactive but less abundant BrCl had significantly greater contribution to membrane degradation (85 %) than HOBr (3 %). Under typical neutral wastewater conditions, both BrCl and HOBr contributed equally. The current study developed a novel characterization technique to assess membrane degradation by determining the kinetics of the oxidant-PA reactions.