Preparation of hollow fiber nanofiltration membranes with intensified charge density for Li+/Mg2+ separation from brines with high Mg2+/Li+ ratios

Abstract

To achieve efficient separation of Li⁺/Mg²⁺, nanofiltration membranes (NFMs) with intensified positive charge density were fabricated by surface grafting of guanazole (GAZ) on the surface of piperazine (PIP)/trimesoyl chloride (TMC) NFMs. GAZ is rich in amino groups with smaller molecular size, could contact and react more sufficiently with reaction sites on membrane surface, and give less potential blockage of surface pores, compared to traditional surface grafting molecules. The influence of the chargeability and pore size of pristine PIP/TMC NFMs and surface grafting reaction on the structure and performance of GAZ grafted NFMs was investigated. Simulated brine with Mg²⁺/Li⁺ ratio of 150 was utilized to evaluate the feasibility of GAZ grafted NFMs in treating salt-lake brine with high Mg²⁺/Li ⁺ ratios. The results showed that the positive charge density on the NFMs surface was improved and the percentage of larger pores was reduced with the successful grafting of GAZ. The optimal GAZ grafted NFMs exhibited higher MgCl₂ rejection (94.2 %) than that of PIP/TMC NFMs (64.2 %) and maintained a stable pure water permeance. It was worth noting that the Li⁺/Mg²⁺ separation factor for simulated brine of 2000 mg L⁻¹ (Mg²⁺/Li⁺ ratio of 150) reached 25.7, which was potentially applicable for treating salt-lake brine with high Mg²⁺/Li⁺ ratios.