Catalytic effect of iron on the tolerance of thin-film composite polyamide reverse osmosis membranes to hydrogen peroxide

Hydrogen peroxide (H2O2) is potentially an attractive alternative to chlorine-based (hypochlorous acid and monochloramine) antifouling agents in reverse osmosis (RO) because H2O2 does not form toxic disinfection byproducts and is tolerated by polyamide (PA) membranes up to high concentrations. However, aqueous H2O2 solutions are corrosive and iron corrosion products activate H2O2 to reactive oxygen species (ROS) that can degrade the PA separation layer. The impact of iron oxides on membrane stability was studied in the presence of H2O2 in two different systems: a corrosion-resistant system (constructed with plastic components) and an all-steel system. Tests in the all-steel system were conducted under enhanced corroding conditions and in deoionized (DI) water. H2O2 concentrations were 2.0 mM (68 mg/L) or 10 mM (340 mg/L). Corrosion was enhanced by adding 10 mM Cl- or suppressed by adding phosphate buffer. Membrane performance was evaluated by determining salt rejection and the water flux. Under corrosion-suppressed conditions, membranes were stable during the 8-d test. In the all-steel testing system containing 10 mM Cl- ion as corrosion promoter, the membrane tolerance was significantly diminished. In DI water, corrosion was relatively slow but degradation of the membranes was noticeable. Kinetic data of pCBA degradation indicated that membrane damage was caused by center dot OH radicals. Quenching of the center dot OH radical by methanol, and X-ray photoelectron spectroscopy (XPS) and Scanning Electron Microscopy (SEM) data are consistent with the hypothesis that Fenton reactions caused cleavage of the polyamide cross-linkages.