Highly efficient extraction of rhodium(III) from hydrochloric acid solutions using neat ionic liquid trioctylammonium...

Rhodium is one of the most challenging platinum-group metals to recover by conventional solvent extraction owing to its kinetically inert coordination chemistry. In this study, the extraction of rhodium(III) from hydrochloric acid solutions into neat ionic liquids (ILs) was investigated, focusing on trioctylammonium chloride ([HTOA]Cl), a protic IL designed for anion extraction. To overcome the slow ligand substitution of rhodium(III), the effect of thermal pretreatment of the aqueous phase was examined. Heating a rhodium(III) solution in 8.0 mol dm^- 3 HCl at 100 °C for at least 2 h promoted chloro-complex formation. Subsequent dilution to 1.0 mol dm^- 3 HCl immediately before extraction minimized competitive inhibition by excess chloride ions, enabling efficient anion-exchange extraction. Under optimized conditions, a distribution ratio of 31.6 and an extraction percentage of 97% were achieved at an aqueous-to-IL volume ratio of 1:1. Among several ammonium- and imidazolium-based ILs examined, [HTOA]Cl exhibited the highest performance, demonstrating its suitability for rhodium(III) extraction from chloride media. ESI-MS analysis indicated the involvement of both mononuclear ([RhCl₆]^3-) and dinuclear ([Rh₂Cl₈]^2- and [Rh₂Cl₉]^3-) chloro complexes, depending on the rhodium(III) concentration. Nearly quantitative back extraction was achieved using 12 mol dm^- 3 HCl. This method enabled selective rhodium recovery from multicomponent hydrochloric acid solutions and was successfully applied to a spent automotive catalyst leachate without requiring additional thermal pretreatment beyond the initial leaching process.