Tailored l-Arginine modified Poly(piperazine-amide) nanofiltration membrane with enhanced water permeability for efficient Li+/Mg2+ separation

In recent years, there has been a growing focus on lithium recovery from water resources using thin-film composite (TFC) nanofiltration (NF) membranes. In this study, a poly(piperazine-amide) NF membrane was fabricated via interfacial polymerization (IP) between piperazine (PIP) and trimesoyl chloride (TMC) to effectively separate Li⁺ from Mg²⁺. However, the trade-off between water permeability and Li⁺/Mg²⁺ selectivity presented a challenge, requiring additional surface modification to optimize performance. The surface of the poly(PIP-amide) membrane was modified with ARG amine-based hydrophilic monomer, followed by crosslinking with glutaraldehyde (GLA). The amine groups of ARG are expected to interact with the unreacted acyl chloride groups of TMC, increasing the positive surface charge and thereby improving Li⁺/Mg²⁺ selectivity. The membrane modified with ARG and GLA (A2-G0.3) exhibited a significantly improved Li⁺/Mg²⁺ selectivity of 17.11, compared to 5.14 for the unmodified membrane when tested in a solution containing 2000 ppm of salts (Li⁺/Mg²⁺ of 1:20). Notably, the A2-G0.3 membrane demonstrated a Li⁺ rejection of −45.4 and Mg²⁺ rejection of 91.5 %, with a water flux of 47.0 Lm⁻²h⁻¹ at 70 psi. When tested with a simulated brine with a total salt concentration exceeding 21,000 ppm, the membrane exhibited a Li⁺ rejection of 9.5 % and Mg²⁺ rejection of 90.1 %, along with a water flux of 18.6 Lm⁻²h⁻¹ at low pressure of 70 psi. The membrane maintained consistent performance over 200 h of simulated feed filtration, demonstrating its long-term stability. Moreover, the antifouling performance of the membrane was greatly improved by grafting the ARG-GLA layer onto its surface. These findings highlight the modified membrane's potential for effective lithium recovery in high-salinity environments.