Bioinspired chlorine-resistant tailoring for polyamide reverse osmosis membrane based on tandem oxidation of natural α-lipoic acid on the surface

Abstract

Active chlorine-vulnerability is the Achilles' heel for polyamide thin film composite (PA-TFC) reverse osmosis (RO) membranes during water desalination and/or purification. Chlorination disrupts the delicate PA skin layer on porous support, resulting in significantly deteriorative membrane permselectivity and rising maintenance cost. Inspired by the superior antioxidative capacity of biologically endogenous α-lipoic acid (LA), this work was dedicated to tailoring a chlorine-resistant RO membrane with LA sacrificial unit on the surface. It was evidenced that the characteristic dithiolane within LA preferentially reacted with active chlorine in feedwater, and the intimal PA selective layer was thus safeguarded from chlorine-triggered oxidation. The modifier was pre-assembled with LA and polyethylenimine (PEI) as the mediator, subsequently tethered onto membrane surface via one-step chemical coupling without any catalysts. A pretty balance between membrane permselectivity and grafted modifier amount was achieved via regulating the modifier concentration. The modified membrane exhibited significantly ameliorative chlorine-resistance relative to the pristine one, with normalized salt rejection still remained (99.3 ± 0.6)% and only (17 ± 5)% final flux loss after a rigorous static chlorine exposure of 8000 ppm·h under acidic condition (confidence level, P = 95%). The electronic energy differences of chlorination-involved chemical procedures were determined by density functional theory (DFT) calculation, and an energy-favored three-stage tandem oxidation mechanism of LA moiety was proposed. Accordingly, up to 5 equiv. of active chlorine can be captured by a single sacrificial unit. Such intrinsically high chlorine-consumption efficacy of LA-based sacrificial layer would retard the chlorination saturation that commonly invalidates sacrificial layers. This bioinspired work shed light on a novel, facile, and effective anti-chlorine strategy for PA-TFC RO membrane to maintain the stability of service performance.