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

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 alpha-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.

Automated summary

The vulnerability to active chlorine poses a significant challenge for PA-TFC RO membranes during water desalination. This study successfully tailored a chlorine-resistant RO membrane with a sacrificial LA unit on the surface, demonstrating improved chlorine-resistance and minimal flux loss after exposure to high chlorine levels. The bioinspired work provides a novel and effective anti-chlorine strategy for maintaining the stability of PA-TFC RO membrane performance.