Treatment of reverse osmosis membrane by sodium hypochlorite and alcohols for enhanced performance using the swelling-fastening effect

Chemicals soaking is generally acknowledged as a convenient and efficient method to improve the performance of reverse osmosis (RO) membranes. The conventional soaking of RO membranes in alkaline sodium hypochlorite (NaClO) usually promotes extensive hydrolysis and cleavage amide bonds, resulting in improved water flux but declined salt rejection. Here, alcohols were added into the NaClO solution to regulate the chlorination processes using their "swelling-fastening" effect. The alcohols could interact with polyamide chains, and thus swell the polyamide network. Due to this interaction, the NaClO has less probability of attacking the polyamide chains. Hence, the chlorine-promoted hydrolysis was partly eased, which could weaken the decrease of salt rejection. Besides, after removing alcohols as well as the dissolved small oligomers and fragments of polyamide, the network was compacted and the loosened sites were healed, which is also beneficial to increase the difficulty of salt penetration. The treatment of RO membrane by the NaClO and alcohols could produce a hydrophilic surface with increased water flux and high salt rejection. The membrane chloridized at 2000 ppm NaClO exhibited a water flux improvement of 20.28% and a salt rejection declination of 0.95%. When treated with 2000 ppm NaClO associated with 5% methanol, the water flux improved 20.13% with little declination in salt rejection.