Highly efficient separation of Li+/Mg2+ via nanofiltration membranes with conductive substrates under an electric field

Abstract

Electro-nanofiltration (ENF) achieved efficient separation of Li⁺/Mg²⁺. The conductivity of an NF membrane had a notable impact on the ENF process. In this work, an NF membrane was prepared by interface polymerization on a conductive polyether sulfone (PES)/MXene@CNT membrane, which was used in an ENF process to enable the highly efficient and precise separation of Li⁺/Mg²⁺. It was found that the PES/MXene@CNT membranes had fairly good electrical conductivity (>0.5 S/m) and hydrophilicity after the addition of MXene and CNT into PES membranes. The ENF membrane with a conductive substrate exhibited extremely excellent Li⁺/Mg²⁺ separation performance even at a low voltage. It was because the conductive substrate of the NF membrane could be directly considered as a cathode to enhance electric field strength and current density during ENF process, which was more suitable for surface charge reconstruction and Li⁺ transport. When the voltage was 2.5 V, Mg²⁺ was nearly completely rejected; while Li⁺ rejection was up to −100.6 %. The Li⁺/Mg²⁺ separation factor (S_Li,Mg) was as high as 1164.6, which was far higher than that (S_Li,Mg = 25.7) based on the common NFmembrane with a non-conductive support membrane at 2.5 V. The ENF showed a stable separation performance at 2.5 V. The results suggested that the NF membrane a with conductive substrate possessed obvious advantages in the preparation and performance of ENF membrane compared with the NF membranes with non-conductive substrates. This work offered a viable way for the efficient separation of Li⁺/Mg²⁺ and possessed tremendous potential in extracting high-purity Li⁺ salts from salt lakes.