Direct and highly efficient lithium extraction from real oilfield brine via a selective TOP-FeCl3 system

To address the growing demand for lithium amidst its limited supply, it is crucial to explore diverse and economically viable lithium resources to alleviate the pressure. Oilfield brine offers significant potential, particularly due to their reduced land and freshwater requirements for lithium extraction, which dramatically lowers costs and provides environmental benefits. However, the recovery of lithium from oilfield brine is relatively under-explored yet due to highly excess of coexisted magnesium and particularly calcium as well as extensive pretreatment process. To tackle this challenge, we reported a direct lithium extraction method using tris(2-ethylhexyl) phosphate (TOP)-FeCl 3 solvent extraction system in this work. The proposed system achieved superior Li + binding affinity with 77 % extraction in single stage and especially ultrahigh selectivity for Li + /Mg 2+ (separation factor: 5737) and Li + /Ca 2+ (separation factor: 96) from real oilfield brine containing 52.1 g‧L -1 Mg 2+ and 84.4 g‧L -1 Ca 2+ . The results significantly outperform traditional tri-n-butyl phosphate (TBP)-FeCl 3 system, where Li + /Ca 2+ selectivity is neglectable. The extraction mechanism was investigated using UV–visible, Raman spectroscopy, ESI-MS and slope ratio method, suggesting [Li‧2TOP·2H 2 O][FeCl 4 ] may be the active lithium complex species during extraction process in our system. The increased hydrophobicity and extended carbon chain compared to TBP molecule may contribute to the extraordinary selectivity and stability. This work presents a comprehensive extraction process and offers new insights into optimizing extractant structures to achieve enhanced selectivity. These results demonstrate that TOP-FeCl 3 extraction system is highly promising for the industrial recovery of Li + from real oilfield brine.