(400bd) Affinity Nanofiltration Membranes Containing Complexing Agents for Enhancing Li+/Mg2+ Separation

Lithium (Li) is highly desired for clean technology. Its production from subsurface brine often involves the separation of lithium chloride (LiCl) from mixtures containing other salts, such as magnesium chloride (MgCl₂). Nanofiltration (NF) membranes present an exciting, alternate method to extract lithium, being cost-effective and environmentally friendly. NF membranes are commonly surface-modified to enhance properties. Chelating agents, such as ethylenediaminetetraacetic acid (EDTA), can form strong complexes with Mg²⁺ and weak complexes with Li⁺. Herein, we demonstrate that NF membranes can be surface-modified with EDTA to achieve superior Li⁺/Mg²⁺ selectivity (SF_Li/Mg), by forming strong complexes with divalent Mg²⁺ and enhancing its rejection, without a significant compromise to permeance. Specifically, commercial NF270 was first primed using bio-adhesive dopamine, followed by EDTA grafting via its reaction with polydopamine (PDA). Binding affinity, stoichiometry coefficients, and the molecular structure of EDTA with LiCl or MgCl₂ were analyzed with isothermal titration calorimetry (ITC) and proton nuclear magnetic resonance (1H-NMR). Membrane surfaces were characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), water contact angle (WCA), thin film analyzer, X-ray photoelectron spectroscopy (XPS), and zeta potential. The effect of EDTA concentration and feed solution pH on Li⁺/Mg²⁺ separation properties were thoroughly investigated. EDTA grafting increases the SF_Li/Mg. For example, the modification using PDA-EDTA (2 g/L) slightly increases the Mg²⁺ rejection to 99.99% but dramatically increases the SF_Li/Mg from 15 to 742 at pH 4. Both single-salt and mixed-salt tests were completed and showed consistent trends. Overall, this study showcases chelating agents’ affinity with ions to improve Li⁺/Mg²⁺ selectivity in membranes.