A Full-Cycle Recycling Strategy for End-of-Life Nanofiltration Membranes toward Lithium Extraction, Implemented by Membrane Cleaning and Targeted Restoration

Abstract

Nanofiltration (NF) membranes have garnered extensive implementation for lithium extraction from salt lakes, yet the disposal and management of end-of-life (EOL) membranes after an extended service duration still represent a persistent and intractable challenge, particularly given the distinctive and fragile ecosystems in regions where these membranes are routinely deployed. Herein, we propose a full-cycle recycling protocol, including membrane cleaning and targeted restoration, for the valorization of authentic, decommissioned nanofiltration membranes for Li+/Mg2+ separation from brine. Surfactant-aided alkaline cleaning was adopted according to the compositional characteristics of the fouling deposit, coupled with membrane structure modeling and real-time monitoring via electrochemical impedance spectroscopy (EIS), thereby enabling efficient removal of the membrane surface fouling layer. Considering the abundant membrane surface carboxyl moieties induced by amide bond cleavage during long-term operation, PEI was further applied for the targeted in situ restoration of defective regions through electrostatic deposition or covalent immobilization after membrane cleaning; both approaches demonstrated a water permeance increase from below 1 to over 3 L m–2 h–1 bar–1 and nearly a 20-fold improvement in Li+/Mg2+ separation factor, exceeding that of practical nanofiltration products in a simulated brine water system, with stable performance maintained during 288 h of long-term operation. Intriguingly, despite comparable overall performance of the two strategies, the fundamentally disparate restoration mechanisms conferred on them versatility in operational simplicity and restoration stability across multiple application scenarios, presenting an in situ restoration platform integrated into regular maintenance protocols and suggesting diverse perspectives and customized solutions for the sustainable recycling of EOL membrane resources.