Iron catalyzed degradation of an aromatic polyamide reverse osmosis membrane by free chlorine

The catalytic effect of iron on the degradation of a polyamide (PA) reverse osmosis (RO) membrane by free chlorine was evaluated by measuring the permeability, P, and salt rejection, R, at circumneutral pH, cumulative exposures (CT) of 6000 and 3000 ppm-h, varying free chlorine concentrations, and exposure times. The chemical transformations of the polyamide layer were characterized by X-ray photoelectron spectroscopy (XPS) and attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR). The role of free radicals was probed by studying the effect of the short-chained alcohols of increasing size (methanol (MeOH), ethanol (EtOH) and tertiary-butyl alcohol (tBuOH) on membrane performance and transformation. In the absence of iron, CT up to 6000 ppm-h caused N-chlorination (up to 4.9%), disrupted intra-molecular hydrogen bonding and caused membrane tightening. A one-time-addition of iron nitrate (Fe(NO₃)₃) (1 mg/L) to the recirculating feed water significantly accelerated oxidation and increased P and decreased R. The proposed membrane degradation mechanism involves Fe precipitates lodged on the membrane surface catalyzing the formation of hydroxyl radical by Fenton-like reactions. MeOH accelerated membrane degradation, apparently because the secondarily formed hydroxymethyl radical diffused into the separation layer. By contrast, tBuOH inhibited membrane degradation likely because tBuOH quenched hydroxyl radical and secondary radicals of tBuOH were prevented from diffusing into the separation layer due to their large size. In the initial loss of membrane permeability and the rejection after CT of 3000 ppm-h were proportional to the free chlorine concentration. Results highlight the potential significance of radical formation and quenching processes when evaluating membrane degradation by oxidants to under field conditions.