From extraction to closed-loop recovery: recent advances in ionic-liquid-based recycling of platinum-group metals

Platinum-group metals (PGMs), consisting of Pt, Pd, Rh, Ru, Ir, and Os, are strategically important materials because of their scarcity, high economic value, and essential roles in automotive catalysts, fuel cells, electronics, chemical processing, and emerging energy technologies. However, their increasing accumulation in spent catalysts and other secondary resources has spurred the development of more selective and resource-efficient recovery processes. Conventional hydrometallurgical processes can reach high recoveries but often require aggressive leaching media, multiple separation steps, and large reagent consumption. Therefore, ionic liquids (ILs) have been widely considered as tunable media for PGM recovery because the polarity, hydrophobicity, coordinating ability, and ion-exchange behavior can be tuned by the selection of the appropriate constituent ions. This review critically discusses IL-based recovery of the six PGMs, with special emphasis on advancements beyond that of classical liquid-liquid extraction. Recent developments include supported and immobilized IL materials, polymer inclusion membranes, aqueous biphasic and multi-phase systems, direct oxidative leaching with reactive ILs, and integrated extraction-recovery processes involving electrodeposition, reduction, selective stripping, and solvent regeneration. Pd and Pt remain the most investigated metals, while Rh, Ru, and Ir have been less studied. The IL-based recovery of Os has been scarcely investigated. Special focus is paid to selectivity in complex matrices, metal speciation, stripping and final metal recovery, IL consumption and stability, recyclability, and the distinction between laboratory-scale extraction efficiency and a feasible closed-loop operation. The future potential of ILs in PGM recycling is expected to rely less on their use as mere replacements for conventional solvents and more on their ability to integrate leaching, separation, metal recovery, and solvent regeneration into simplified, recyclable process flowsheets.