Lithium Extraction from Dilute Simulated Brines by Membrane-Assisted Liquid-Liquid Extraction Using [P6,6,6,14][PHOS]: Li+/Mg2+ Selectivity in an NF90 Nanofiltration Contactor

Selective lithium recovery from dilute brines remains challenging because Li+ must be separated from competing ions, particularly Mg2+. In this work, a commercial NF90 nanofiltration membrane was used as a non-dispersive interface between a simulated brine and the hydrophobic ionic liquid [P6,6,6,14][PHOS]. Individual-salt experiments yielded apparent overall mass transfer coefficients of 638 ± 273, 100.5 ± 19.8, and 10.1 ± 0.7 L m−2 h−1 for Na+, Li+, and Mg2+, respectively. Replacing water with 10% (v/v) [P6,6,6,14][PHOS] in hexane as the receiving phase increased Li extraction from 18.6 ± 3.0% to 40.9 ± 2.0%. Under direct Li+/Mg2+ competition, selectivities of 2.19 ± 1.10 and 3.83 ± 0.56 were obtained with the diluted and neat ionic liquid receiving phases, respectively. With neat [P6,6,6,14][PHOS], Li extraction reached 40.3%, and 6.0% of the extracted Li was subsequently recovered in pure water under non-optimized conditions. CHNS analysis showed no detectable transfer of organic material derived from the ionic liquid to the aqueous feed. Overall, NF90 was the main source of Li+/Mg2+ transport discrimination, whereas [P6,6,6,14][PHOS] mainly increased Li uptake.