Purity metric for evaluating Li+/Mg2+ separation performance of nanofiltration membranes

Nanofiltration membranes with Li+/Mg2+ separation capability offers a highly efficient method for extraction of Li+ from brine. As the key performance metrics, the salt rejection and separation factor between Li+ and Mg2+ (S) have been frequently selected for fair comparison of basic characteristic of NF membrane. Two values have been commonly evaluated at a Zero-Recovery, which does not reflect the realistic performance of the membrane in a nanofiltration process. The correlation between ion rejection, S and the product LiCl purity at two different scenarios: Zero-Recovery, and Non-Zero-Recovery to simulate the realistic application. Poly(styrene sulfonate) and poly(allylamine hydrochloride) (PSS/PAH)2.5 layer-by-layer self-assembled membranes were utilized. The importance of the feed purity was identified in selecting suitable NF membranes. In case of sufficiently high feed purity, the high rejection of Mg2+ was not critical, but the permeance was important. The instant and collective ion rejection and S varied following the water recovery during concentration process. The collective ion rejection is more practical metric when considering the product purity. The LiCl purity, collective values of rejection and S were proposed as more realistic measures of the NF membranes. The study presents a new aspect for design of fit-to-application NF membranes for resource recovery.