(625e) Changes in Physiochemical Properties and Performance of a Polyamide-Based Nanofiltration Membrane Due to Chlorine Exposure

Owing to their high selectivity and water permeability, polyamide-based (PA) thin film composite (TFC) membranes are widely employed in water and wastewater treatment using nanofiltration (NF) and reverse osmosis (RO) processes. However, the performance and life -span of these membranes are susceptible to attack of chlorine, which is widely used to control membrane bio-fouling. In this study, the effects of chlorine exposure conditions on the performance of a commercial PA-TFC membrane, namely NF90 were investigated and the conditions that resulted in an optimum membrane performance were identified. The membrane samples were exposed to sodium hypochlorite (NaOCl) of different concentrations: 10, 100, 1000 ppm Cl at pH 5, 7 and 9 for 100 h. The membrane performance was evaluated based on the water permeability values and the rejection of NaCl and inorganic contaminants of significant environmental and health concern (boron (H₃BO₃) and arsenic (H₂AsO₄^–)). At pH 5, the water permeability of the chlorinated membranes decreased when NaOCl concentrations increased. However, at the pH 7 and 9, the water permeability of the chlorinated membranes increased with NaOCl concentrations. The rejection of charged NaCl and arsenic improved in most chlorinating conditions except for 1000 ppm Cl at pH 7 and 9. Meanwhile, the rejection of neutral boric acid was lower in most cases, except for 10 ppm Cl and pH 5. The disparate changes in membrane performance due to different chlorine treatments were explained in terms of competing mechanisms: membrane tightening, bond cleavage by N-chlorination and chlorination promoted polyamide hydrolysis. X-ray Photoelectron Spectroscopy (XPS), attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy, zeta potential and contact angle measurement were the analytical methods to provide evidences for the mechanisms.