Nanofiltration membranes with sandwich-like mixed charge layers for high-efficiency Mg2+/Li+ separation

Efficient separation of monovalent/multivalent cations and anions is essential for optimizing lithium extraction from Salt-Lake to simplify production processes and reduces costs. Nevertheless, the conventional nanofiltration membranes with specific charges can only separate either cations or anions. To overcome this limitation, a novel strategy is proposed for fabricating nanofiltration membranes with “negative-positive-negative” sandwich-like mixed charge configuration. Herein, two aqueous monomers (phenylbiguanide and polyethyleneimine) with significantly different diffusion rates are employed to react with trimesoyl chloride to form a nascent polyamide layer via interfacial polymerization. Subsequently, phenylbiguanide that preferentially diffuses to top surface of nascent membrane and unreacted amine groups from polyethyleneimine are used to react with m-phenylenedisulfonyl chloride to form a polysulfonamide/polyamide functional layer. The unreacted sulfonyl chloride groups and trimesoyl chloride near aqueous phase could hydrolyzes more easily, producing negatively charged sulfonic acid and carboxyl groups on the top and bottom. Meanwhile, the main part of polyamide layer is positively charged attributed to numerous unreacted amine groups. The “negative-positive-negative” sandwich-like configuration was proven using TOF-SIMS. The fabricated membrane exhibited a selectivity of 57.22 for MgCl₂/LiCl and 30.61 for Na₂SO₄/NaCl. Furthermore, Mg²⁺/Li⁺ selectivity of 57.34 was achived for the mixed salt solution (Mg²⁺/Li⁺ = 45), showing good application potential in lithium extraction.