Selective extraction of platinum and palladium using reusable carvacrol-based deep eutectic solvents: experimental and computational insights

Abstract

Pt and Pd, essential in diverse technologies yet limited in natural supply, necessitate sustainable recovery from secondary resources. Metallurgical approaches using deep eutectic solvents (DESs) offer greener and more selective routes for their extraction from acidic leachates. This study presents an integrated computational and experimental approach with new non-ionic DESs for the mutual separation of Pt(IV) and Pd(II) from model solutions via solvent extraction. New tri-n-octylphosphine oxide:carvacrol (TOCA) and lidocaine:carvacrol (LICA) DESs were developed. COSMO-RS predictions as well as DSC, and FT-IR characterisation confirmed the strong hydrogen-bonding interaction and non-ideal behaviour between the constituents of the mixtures. Extraction efficiency and selectivity in Pt and Pd separation were optimised under varying conditions (DES type and composition, and HCl concentration in the aqueous phase). TOCA demonstrated excellent selectivity for Pt(IV), with = 0.40, achieving 79.23% Pt and 18.74% Pd extraction efficiencies from 2 HCl, with a separation factor of 19.21. Conversely, LICA with = 0.50 selectively extracted Pd (nearly complete) from 0.25 HCl, with only 26.43% Pt co-extraction. FT-IR, UV-visible, and pH measurements suggested a neutral ion-pair association mechanism facilitated by proton co-extraction. Efficient back-extraction of metals using ethylenediaminetetraacetic acid for TOCA and acidified thiourea for LICA systems enabled solvent recyclability. TOCA maintained performance over five cycles, while LICA showed lidocaine leaching, indicating the need for rebalancing its composition to retain selectivity. These findings underscore the remarkable potential of carvacrol-based DESs as efficient media to close the loop of greener Pt and Pd separations.