Improvement the performance of TFC membranes by deposition a cationic/anionic polyelectrolyte onto their partially hypochlorite-degraded surfaces

Abstract

Desalination using thin-film composite reverse osmosis (TFC-RO) polyamide (PA) membranes has been predominating for decades to reduce rising water demand. Organic fouling, which causes blocking of the membrane pores and hence a decline in permeability, is one of the main drawbacks of these membranes. Membrane surface modification has been a promising technique for enhancing flux permeability and antifouling characteristics. In this study, a novel coating process to modify the TFC membrane surface was developed. The coating is based on the deposition of supramolecular biodegradable cationic (chitosan, CS), anionic (sodium alginate, SA), and/or chitosan-alginate polyelectrolyte complexes onto the membranes surface using a layer-by-layer technique. To favor the deposition of the first layer of CS on the surface, the membranes first underwent a chlorine treatment to obtain more negative charges on the PA active layer. Different parameters of chlorination time and concentration of sodium hypochlorite, and concentration and time of the supramolecular deposition onto the membrane surface were studied. The appropriate conditions for the modification process were found to be as follows; NaOCl concentration of 5000 ppm, chlorination time of one hour, and polymer ratio of 1:1 (1000 ppm each). The chemical composition was confirmed using XPS and ATR-FTIR, and surface features were observed using SEM, zeta potential, and contact angle. The modified membrane was observed to be more hydrophilic with a twice increment of the water flux, 48 L/m2.h compared to 24.2 L/m2.h of the pristine membrane, and an insignificant change of salt rejection when treating a 5000 mg/L NaCl solution. The results showed that the coated membrane exhibited superior performance compared to the pristine membrane. In terms of antifouling properties, the coated membrane exhibited superior performance compared to the pristine membrane. Because the process is simple and all polymers are biodegradable, the proposed surface treatment could be a promising technique for membranes modification.