Designed strategies of nanofiltration technology for Mg2+/Li+ separation from salt-lake brine: A comprehensive review

The worldwide demand for lithium has greatly increased with the rapid development of industries especially for lithium-ion battery. As the global potential resource, salt-lake brine has extensive value for Li exploitation. However, the actual salt-lake brine with high mass ratio of Mg2+ and Li+ (MLR) further restricts Li extraction via the conventional technologies. Nanofiltration (NF) exhibits the outstanding potential for Mg2+/Li+ separation, owing to its selective separation mechanism. Extensive investigations and industrial applications involve negatively charged commercial NF membrane in lithium extraction. Recently, novel positively charged NF membranes have been proposed to specifically separate Mg2+ and Li+ ions with the similar hydrate ion radius. The present work reviews the current Li extraction process based on commercial NF membrane, and highlights positively charged NF membranes for Mg2+/Li+ separation inspired by the separation mechanism of Donnan effect. The strategies of introducing functional groups, specific nanopores and nanochannels into membrane to enhance the separation performance are presented. Finally, the main operating parameters associated with Mg2+/Li+ separation process are also discussed. This review is expected to provide a new insight in prospective Mg2+/Li+ separation from salt-lake brine with high MLR.