Environmentally benign hydrophobic deep eutectic solvents for palladium(II) extraction from hydrochloric acid solution

Abstract

Hydrophobic deep eutectic solvents (DESs) are considered to be clean and green alternatives to traditional organic solvents for metal ion extraction. Herein, a series of novel hydrophobic DESs were synthesized via the combination of methyltrioctylammonium chloride ([N8881]Cl) as hydrogen bond acceptors (HBAs) and saturated fatty acids or saturated fatty alcohols as hydrogen bond donors (HBDs). Then, the DESs were used for palladium(II) recovery from acid solution. The factors that affect palladium(II) extraction (including the DES species, contact time, DES dose, solution pH, and temperature) were extensively investigated. The DESs exhibit excellent performance for extracting palladium(II) from hydrochloric acid media, and more than 99% of palladium(II) was extracted under optimal conditions. UV–vis, 1H NMR, and FTIR spectral characterizations confirm that the extraction mechanism of palladium(II) involves an anion exchange reaction whereby PdCl42− anions in the aqueous phase replace Cl− anions in DES to form ion-pairs with [N8881+] via electrostatic interactions. Meanwhile, N8881Cl:1-hexanol (1:1) and N8881Cl:hexanoic acid (1:2) exhibit excellent selectivity for palladium(II) over other impurity metal ions. Also, palladium(II) in the loaded DES phase was efficiently recovered as elemental palladium via the hydrazine hydrate reduction method; thus, the DES could be reused for at least 5 cycles.