Combining the bridging effect of tannic acid and solution activation for improving the lithium/magnesium separation performance of nanofiltration membrane

Abstract

As a new type of energy storage material, lithium battery is widely used in smart electronic products and new energy vehicles. The polyamide nanofiltration (NF) membrane technique has become a crucial tool for separating lithium and magnesium from natural salt-lake brine because of the unique charged characteristics and nanopore of polyamide membrane. However, the lithium/ magnesium separation factor (S_{Li, Mg}) of NF membrane prepared by conventional interfacial polymerization is usually unsatisfactory. In this study, we proposed a novel strategy combining the bridging effect of tannic acid (TA) and the swelling-embedding effect of solution activation to introduce more short-chain amines (DETA) into NF membrane for improving lithium/magnesium separation performance of membrane. It was found that this combination is superior to either of the two alone. The characterization and performance testing results showed that this strategy can introduce more DETA into the membrane because activating solution can swell polyamide layer or dissolve lower molecular weight polyamide fragments to provide spaces for more DETA to be embedded in the polyamide layer, and TA can bridge DETA to membrane due to the interactions between TA and them. The introduction of more DETA improved the surface positive charge and simultaneously increased the cross-linking reactions between amino groups of DETA and acyl chlorides of 1,3,5-benzenetricarbonyl trichloride to optimize the pore structure (decreasing the pore size and narrowing the pore size distribution), resulting in a better lithium/magnesium separation performance. Compared with the original membrane (PIP membrane), the Mg²⁺ rejection of the optimal membrane (PIP + DETA0.2/TA0.04-MeOH membrane) increased from 74.41 % to 95.1 %, and the corresponding S_Li,Mg increased from 2.98 to 13.06 (by 338 %). This work may provide a novel avenue for the preparation of NF membranes with high lithium/magnesium separation performance.