Nanofiltration membrane comprising structural regulator Cyclen for efficient Li+/Mg2+ separation

Selective lithium recovery from salt lake brine using emerging nanofiltration membranes is expected to solve the insufficient supply of lithium resources. In this work, 1,4,7,10-tetraazacyclododecane (Cyclen) was employed as a novel structural regulator through interfacial polymerization of polyethyleneimine (PEI) and trimesoyl chloride (TMC) to develop polyamide NF membrane with efficient Li+/Mg2+ separation performance. After the introduction of Cyclen to aqueous phase, the PEI/Cyclen-TMC selective layer was confirmed to possess a higher crosslinking degree and larger d-spacing than PEI-TMC layer, endowing a dedicated channel for Li+ transport. The water permeance of PEI/Cyclen-TMC composite membrane reached 14.0 L·m−2·h−1·bar−1, about 3.8 times higher than PEI-TMC composite membrane, and meanwhile the separation factor (SLi,Mg) could be stabilized as 8.7 to realize a considerable reduction of Mg2+/Li+ mass ratio from 20 to 2.3. Additionally, PEI/Cyclen-TMC composite membrane was utilized to operate a three-stage nanofiltration treatment procedure of a simulated salt lake brine (Mg2+/Li+ mass ratio: 40). The results indicated that Li+ could be efficiently retracted to obtain a solution with 19.5 mg·L−1 Li+ and Mg2+/Li+ mass ratio of 0.3. After the subsequent precipitation and filtration, the obtained Li2CO3 purity could reach 93 %, indicating the potential application of nanofiltration for lithium recovery.