(383ae) Effect of Surface Charges and Solution pH on Li+/Mg2+ Separation Performance of Modified-NF270 Membrane

Lithium is vital in energy storage applications, and its recovery from salt brines (containing mainly Mg²⁺) using membranes has emerged as an important industrial process. Commercial polyamide-based nanofiltration (NF) membranes have been extensively investigated for Li⁺/Mg²⁺ separations. However, these membranes often show low Li⁺/Mg²⁺ selectivity due to their similar hydration radius, and most studies focus on solutions with pH=7. This study shows that the Li⁺/Mg²⁺ selectivity of commercial NF270 membranes can be improved by manipulating the membrane surface charge, including the solution pH and surface modification. First, the solution pH values are varied from 1.0 to 8.0 by adding HCl or NaOH. Interestingly, the pristine NF270 shows a peak of Li⁺/Mg²⁺ selectivity of >50 at pH values of ~3.5. For instance, decreasing pH from 8 to 4 in the solutions increases MgCl₂ rejection from 40% to 99% and Li⁺/Mg²⁺ selectivity from 4.0 to 50 because of the reduced charge negativity and improved repelling force with Mg²⁺. Second, the membrane surface can be deposited with dopamine and 2-(methacryloyloxy)ethyltrimethylammonium chloride (APA), which form a thin layer of polydopamine (PDA) and polyAPA, making the surface less negative. All membranes still exhibit a peak of Li⁺/Mg²⁺ selectivity. The PDA modification reduces the surface negativity, increases the Li⁺/Mg²⁺ selectivity at pH=7 compared with the pristine NF270, and moves the optimal pH from 3.4 to 4.0. The membranes are also challenged with mixed salts (LiCl and MgCl₂) at various compositions, and their Li⁺/Mg²⁺ selectivity is similar to those from the single salt tests. We will also present the modeling of the effect of the surface charges on the selectivity to derive structure/property relationships for this emerging membrane application.