Tailorable choline chloride–methanesulfonic acid deep eutectic solvents: Linking structural evolution and superacidity to enhanced critical metal extraction from complex carbonate-clay lithium ores

Choline chloride–methanesulfonic acid (ChCl–MSA) deep eutectic solvents (DESs) with molar ratios ranging from 1:1 to 1:8 were tailored as superacidic media for the sustainable and selective extraction of lithium and associated valuable metals from a complex carbonate-clay lithium ore. FTIR and Raman spectroscopy revealed that increasing the MSA content transformed the DES structure from a hydrogen-bonded network to a highly ionic environment through strong interactions between chloride anions and sulfonic acid groups (–SO₃H). This structural evolution was confirmed by NMR spectroscopy, which showed a pronounced downfield shift of the integrated –SO₃H/OH resonance from 9.51 ppm (1:1) to 13.31 ppm (1:8), indicating enhanced Brønsted acidity and proton mobility. UV–Vis analysis using Hammett indicators further confirmed the superacidic nature of the DESs, with the Hammett acidity function (H₀) reaching −5.68 for the ChCl–MSA (1:8) system and remaining strongly acidic (H₀ = −4.178) even at 20% water content. The hydrated 80% ChCl–MSA (1:8) + 20% water formulation achieved complete extraction of Li, Na, and Sr from the complex carbonate-clay lithium ore. Although oxalic acid also achieved complete lithium extraction, the ChCl–MSA DES demonstrated superior multi-element extraction, recovering 72% Fe and 88% Mn, while significantly suppressing aluminium dissolution (22%), resulting in a purer leachate. XRD and FTIR analyses revealed that the enhanced extraction performance was associated with extensive disruption of the ore's crystalline structure and a shift of the primary silicate band to 1014.2 cm −1 . These findings demonstrate that tailoring the structure and superacidity of ChCl–MSA DESs provides a sustainable strategy for the selective recovery of lithium and associated valuable metals while suppressing impurity dissolution from complex carbonate-clay lithium ores, offering a promising platform for the valorisation of challenging sedimentary critical raw material resources.