Sustainable and selective lithium recovery from oilfield produced water via solvent extraction: one-step lithium carbonate synthesis by carbonate-mediated stripping

Addressing the critical challenge of lithium recovery from hypersaline oilfield produced water, an integrated extraction-purification process for oilfield water with high calcium and sodium content was developed and the molecular mechanisms governing lithium selectivity was revealed. Through systematic screening and multi-parameter optimization, a synergistic extractant system comprising tributyl phosphate (TIBP), sodium tetraphenylborate (NaBPh 4 ), and 2-octanone achieved excellent performance under optimal conditions (70 vol% TIBP, n(NaBPh 4 ):n(Li + ) = 1.8:1, O/A = 1), delivering 51.4% single-stage Li + extraction efficiency (1.0 g/L Li + brine) with simultaneous high rejection of Ca 2+ and Na + . An innovative one-step stripping-precipitation approach using 2.0 mol/L NH 4 HCO 3 (O/A = 1:1) achieved >95% stripping efficiency, directly yielding battery-grade Li 2 CO 3 (>99.9% purity,) after two-stage cross-flow washing (O/A = 5:1, impurities <83 ppm). The organic phase retained stable performance over 11 cycles without replenishment, demonstrating exceptional stability and reusability. Mechanistic studies revealed that the P O group in TIBP coordinated with Li + via n → π* transitions, while BPh 4 − enhanced hydrophobicity through electrostatic interactions with [Li(TIBP) 2 (H₂O) 2 ] + . DFT calculations confirmed the thermodynamic preference for Li + binding ( ∆ G Θ  = −24.48 kJ/mol) over Na + (−6.08 kJ/mol) and Ca 2+ (−5.41 kJ/mol), with optimized coordination geometry and extraction sequence. This work established a molecular-to-process design paradigm for sustainable lithium recovery, offering a novel approach to unlock lithium resources from complex brines.