Degradation of secondary polyamide reverse osmosis membrane by hypochlorite in the presence of calcium ions

Abstract

Certain metal ions accelerate deterioration of secondary polyamide reverse osmosis membranes during hypochlorite treatment. However, the mechanisms underlying this metal ion–mediated membrane deterioration have not been clarified. Here, we examined the deterioration of a secondary polyamide reverse osmosis membrane during hypochlorite treatment at various pHs by Ca2+, one of the most abundant ions in surface water. In the presence of Ca2+, accelerated membrane deterioration was observed after chlorination and hydrolysis treatment. In addition, the greater membrane deterioration was observed under neutral and alkaline conditions than under acidic conditions, suggesting that Ca2+ might accelerate hydrolysis rather than chlorination. X-ray photoelectron spectroscopy analysis confirmed that the Cl atomic concentration in the active layer was increased by hypochlorite treatment; however, the presence of Ca2+ did not enhance chlorination of the membrane. In addition, measurement of the water contact angle of the active layer of membranes treated by hypochlorite in the presence of Ca2+ showed a significant increase of wettability. Together, these results provided the evidence that Ca2+ increased hydrolysis rather than chlorination. Solid-state 13C nuclear magnetic resonance spectra of the membranes after hypochlorite treatment showed a signal attributed to the -C*-NH2 of phenylenediamine structures in the active layer, the intensity of which was increased in the presence of Ca2+, indicating that the presence of Ca2+ in hypochlorite treatment resulted in increased hydrolysis of the polyamide structure. In addition, evolved gas analysis–mass spectrometry analysis provided further evidence supporting the production of ions with an m/z of 108, which corresponds to phenylenediamine. Together, the present results showed that deterioration of polyamide membrane by hypochlorite is increased in the presence of Ca2+ via acceleration of hydrolysis but not of chlorination. Based on these findings, we proposed a mechanism for the accelerated degradation of polyamide membrane by Ca2+.