Chlorine-Induced Degradation of Polyamide Active Layers In Reverse Osmosis and Nanofiltration Membranes

Understanding the degradation of reverse osmosis (RO) and nanofiltration (NF) membranes by chlorine is imperative to develop chlorine-resistant membranes, an important component that provides the desired long-term durability of membrane-based water purification modules.  In this work, we study the degradation characteristics of thin polyamide active layers used in commercial RO and NF membranes (including both m-phenylenediamine-based fully aromatic and piperazine-based semi-aromatic polyamides) upon chlorine exposure.  We approach this study by quantifying the chlorine-induced changes in the mechanical properties of the active layers under various experimental conditions (such as chlorine concentration, exposure time, and pH) by means of a combined wrinkling-cracking technique.  We also conduct measurements of the water flux and salt rejection performance of the chlorine-treated membranes under various operation conditions using a dead-end flow cell geometry.  Our results show that the chlorine-induced oxidation mechanism of the active layers depends strongly on their initial structural nature.  This finding is further supported by chemical analysis using X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR).