"Bridge" graphene oxide modified positive charged nanofiltration thin membrane with high efficiency for Mg2+/Li+ separation

To improve the efficiency during the process of Mg2+/Li+ separation, a novel nanofiltration (NF) membrane was optimized by doping graphene oxide (GO) additives into the ultrafiltration (UF) base membrane. The effects of GO doping content on the morphology, structure and surface properties of UF membrane and the final NF membrane were studied comprehensively. The hydrophilic GO acted as a "bridge" between UF membrane and polyamide layer due to the "anchor effect", which significantly enhanced the interaction between base membrane and polyamide layer. The results revealed that with ultra-low GO doping content of 0.05 wt%, the final NF005 membrane exhibited a high selective separation capacity for Mg2+ and Li+ (S-Mg,S-Li approximate to 0.062), and the flux increased by about 119% compared with the pure NF0 membrane. Additionally, due to the high stability of membrane, the excellent separation capacity of NF005 membrane only changed slightly after 7-day cycle filtration test. Importantly, a small amount of GO doping greatly improved the permeability of both UF and NF membranes, which correspondingly improved the separation efficiency and accelerated the filtration rate. This work provides a new direction for designing membrane with high efficiency for Mg2+/Li+ separation, which is potential in the field of lithium extraction.