High performance Li+/Mg2+ separation membrane by grafted short chain amino-rich monomers

Constructing a suitable charged surface layer of nanofiltration (NF) membrane is one of the key challenges to achieve efficient Li+/Mg2+ separation performance with high Mg2+/Li+ mass ratio (WMg2+/Li+). Herein, we proposed a facile route, i.e., grafting the short chain amino-rich monomers (SCA, H(NHCH2CH2)1-4-NH2) instead of conventional amino-rich macro-monomers (ARM), to construct a separation layer with uniform positive charge on the surface of pristine polyamide (PA) NF membrane to achieve effective Li+/Mg2+ separation. The SCA grafted NF membrane (PA-g-SCA) retained a more uniform distribution of positively charged groups on the PA layer compared with ARM grafted NF membrane. The SCA grafted NF membrane exhibited a high rejection of Mg2+ and high permeation of Li + at the same time, leading to an extremely high separation factor for Li+/Mg2+ of 130 even in the WMg2+/Li+ of 200:1 in the feed, which greatly surpassed those reported. The pure water flux was 1.8 folds than that of the pristine NF membrane due to the hydrophilicity improvement. The results suggested that the distribution uniformity of positive charges could be the key to efficient Li+/Mg2+ separation of the NF membrane. Our study provided a facile and robust route to design and construct high Li+/Mg2+ separation performance NF membranes.