Harvesting lithium from brines via high-selectivity nanofiltration integrated with membrane distillation

Effective separation of Li + from Mg 2+ in salt-lake brines is critical to meeting the growing demand for lithium. Nanofiltration (NF) membranes are particularly attractive due to their high separation efficiency and low energy consumption, but pristine polyamide NF membranes exhibit intrinsically low Li + /Mg 2+ selectivity. To overcome this limitation, we modified the membrane surface with star -shaped block copolymers (SPs) comprising a hydrophobic polystyrene (PS) core and multiple positively charged hydrophilic poly( N , N -dimethylaminoethyl methacrylate) (PDMAEMA) arms. The SPs self-assembled onto the polyamide surface via combined hydrophobic and ionic interactions, resembling brisingid starfish anchoring to rocks. The compact architecture and high charge density of the SPs substantially increased the membrane's positive surface charge, thereby enhancing Li + /Mg 2+ selectivity through the Donnan exclusion principle. The SP-modified membrane (M200) demonstrated a remarkable Li + /Mg 2+ selectivity of 95.4, with 97.2% Mg 2+ rejection, −167.0% Li + rejection, and a water flux of 35.5 Lm −2 h −1 at 70 psi, representing a substantial improvement over the unmodified thin-film composite (TFC) membrane (selectivity 14.3). To further enhance lithium recovery, a two-stage NF process reduced Mg 2+ concentration to 0.33 ppm while enriching Li + to ~40 ppm. Since this concentration was insufficient for direct precipitation, the Mg-depleted solution was concentrated using a hydrophobic poly(vinylidene fluoride) (PVDF) nanocomposite membrane in a membrane distillation (MD) process to a volume concentration factor (VCF) of 240. This process yielded a Li + concentration of 10 g/L with >99.8% salt rejection and 99.78% Li + retention. Finally, precipitation with Na 2 CO 3 at 80 °C yielded Li 2 CO 3 , with a recovery efficiency of 94.4% of the theoretical maximum and a purity of 97.85%. By integrating NF with MD and precipitation steps, we demonstrated a practical workflow for lithium harvesting from brines.