Gold and palladium oxidation/complexation in water by a thioamide–iodine leaching system

Abstract

This paper concerns the oxidative dissolution ability of a thioamide/iodine leaching system in water towards gold, palladium and silver metals in powder in view of a potential application in the recovery process of these metals from electrical and electronic waste equipment (E-waste). The leaching mixture composed of equimolar amounts of thioamide 3-methyl-benzothiazole-2-thione and I₂ in water can oxidize/complex gold and palladium metals in powder in order to form the complexes [Au^I(mbtt)₂]I₃ and [Pd^II(mbtt)₃I]I₃. Considering the mild conditions under which the processes were performed ([I₂] = 1.14 × 10⁻³ M, T = 20 °C, pH = 5.0, leaching time = 24 h), the obtained oxidation yield of Au⁰ and Pd⁰ are to be considered satisfactory with values of 65% and 69%, respectively. No oxidation was observed for the silver powder. The oxidising/complexing properties of the leaching mixture mbtt + I₂ were interpreted considering the formation of the mbtt–I₂ adduct that features, as a consequence of the S-donor to I₂ interaction, a charge separation between the bridging and terminal iodine atoms in the fragment S–I_b–I_t. The feasibility of recovering gold from the complex [Au^I(mbtt)₂]I₃ was verified by performing a cementation with magnesium powder.