Lithium extraction from salt lake brines by HDBM-TBPO synergistic system: Molecular screening based on hammett constant, hydrophobicity and steric hindrance

First HDBM-TBPO synergistic system for Li + extraction from high Na/Li brine. • Under optimal conditions, Li + extraction reached 99.8% and β Li / Na =250. • The loaded organic phase stripped with 0.2 mol/L HCl and regenerated with 0.05 mol/L Na 2 CO 3 . As a core material of lithium-ion batteries, lithium is difficult to recover from salt lake brines because of complex procedures and excessive energy consumption. Herein, a variety of β-diketone extractants were screened by hydrophobicity (π) and Hammett substituent constant (σ) based on their electronic effects and Li + chelating strength. Subsequently, various neutral organophosphorus synergists were evaluated based on the steric hindrance around the P O group, as lower steric hindrance facilitates axial coordination with the Li-chelate complex. Dibenzoylmethane (HDBM) and tributylphosphine oxide (TBPO) were identified as the optimal extractant and synergist, respectively. Under the optimized conditions, the Li + extraction efficiency exceeded 99%, with a Li/Na separation factor of 250. The loaded organic phase was effectively stripped by 0.2 mol/L HCl and regenerated by 0.02 mol/L Na 2 CO 3 . The system retained 99% extraction efficiency after 11 cycles without emulsification, showing excellent reusability. Mechanistic investigations demonstrated that the extracted complex was Li•2HDBM•0.2TBPO, and the extraction was an enthalpy-driven process via chelation and coordination interactions. This synergistic system presents a promising and sustainable approach for lithium recovery from brines and aqueous resources.