Improving Mg2+/Li+ separation performance of polyamide nanofiltration membrane by swelling-embedding-shrinking strategy

Extraction of lithium from salt lakes is an important route for lithium sources. Due to the unique structure and separation mechanism, i.e., size exclusion and Donnan effect, nanofiltration (NF) is considered as a promising way to separate magnesium (Mg2+) and lithium (Li+) ions. However, the Mg2+/Li+ separation factor of NF membrane fabricated by traditional interfacial polymerization (IP) is usually dissatisfactory. Here, a swelling-embedding-shrinking strategy was proposed to modify polyethyleneimine (PEI)/trimesoyl chloride (TMC) NF membranes using diethylenetriamine (DETA) to improve the performance in Mg2+/Li+ separation. The pristine PEI/TMC NF membrane was swollen with methanol to increase the polyamide chain spacing followed by embedding electropositive DETA as a modifier. The charge properties and pore structure of the active layer could be regulated by optimizing the modification conditions. Within comparison to the pristine membrane (TFC-0), the pure water flux of modified NF membrane (TFC-3) increased by 63.6% because of the rougher surface and the larger pore size. Meanwhile, the embedding of electropositive DETA reduced the electronegative property of membrane surface. In consequence, TFC-3 membrane exhibited higher rejection of MgCl2 (93.9% against 88.4%) and lower rejection of LiCl (31.0% against 32.1%), thus showing a twofold improvement in Mg2+/Li+ separation factor (SLi, Mg = 11.38 against 5.87). The swelling-embedding-shrinking strategy in this article provided a good reference for the modification of NF membranes in the separation of multivalent and monovalent ions.