Polyethyleneimine modified polyamide composite nanofiltration membrane for separation of lithium and magnesium

The application of nanofiltration (NF) membrane with positive charge on the surface in the separation of lithium‑magnesium is a convenient, energy-saving and sustainable strategy. In this work, polyethyleneimine (PEI) was introduced into the composite NF membrane through two methods to investigate and make a comparation in surface morphology and properties, application in the separation of membrane to Li+ and Mg2+, respectively. Both PEI and PIP were involved in the preparation of membranes as aqueous monomers, which can reduce the cross-linking degree of active layer but reduce the electronegativity of membrane surface. The rejection of Mg2+ reached 98.1 % and the purity of Li+, which is the mass fraction of Li+ in cations of permeate, reached 62 %, respectively, the separation factor SLi,Mg reached 32.6. By contrast, PEI with different molecular weight and concentration was grafted on the PA layer which improved the density of membrane but reduced the electronegativity of the membrane surface. The rejection of Mg2+ reached 97.2 % and the purity of Li+ reached 63 %, respectively, the SLi,Mg reached 33.6 with an acceptable flux. Both of two modification methods had a preferable performance on separation of lithium‑magnesium with a higher SLi,Mg. Therefore, it is an efficient strategy to add PEI through different methods into the preparation of NF membrane to separate lithium‑magnesium and extract the Li+ from salt lake brine with high Mg/Li mass ratio.