Biguanide-functionalized nanofiltration membranes for high-efficient Mg2+/Li+ separation

Nanofiltration (NF) is promising for the efficient Mg 2+ /Li + separation from salt-lake brines, which was expected to solve the insufficient supply of lithium resources. However, the conventional NF membranes fabricated by piperazine (PIP) and trimesoyl chloride (TMC) usually exhibit negative charge surface with low Mg 2+ /Li + selectivity. To improve the Mg 2+ /Li + separation, NF membranes with enhanced surface positive charge were fabricated by introducing polyaminopropylbiguanide (PAPB) into the polyamide (PA) active layer. Compared with the pristine membrane, the PAPB modified membrane exhibited both enhanced water flux from 7.50 to 10.50 L/m 2 h•bar and obviously improved MgCl 2 retention from 65.51 % to 96.52 %. High separation factor (S Li/Mg ) of 30.81 was obtained with the Mg 2+ /Li + mass ratios (MLR) of 50:1, attributing to the strong protonation capacity of guanidine groups in a wide pH range. Moreover, the PAPB modified membranes demonstrated a superior anti-fouling performance during NF test. This work presents a novel and facile strategy for the preparation of high-performance NF membranes and provides new insights into the underlying mechanism for the enhancement of Mg 2+ /Li + separation by enhancing positive charge and Donnan exclusion of NF membranes.