Electrochemical oxidation as alternative for dissolution of metal oxides in deep eutectic solvents

Metal oxide dissolution is central to most metal processing and is generally carried out using mineral acids. An alternative approach has been demonstrated for metal sulfides using electrochemical oxidation. Here the so-called paint casting method was employed to investigate the electrochemical dissolution and recovery of selected metal oxides. Cyclic voltammetry showed that all of the metal oxides were electrochemically active in deep eutectic solvent media, and the majority displayed a redox couple that could be potentially assigned to the redox behaviour of the oxide moiety. Bulk anodic dissolution was carried out on the metal oxides, and it was shown that dissolution was enhanced to up to >10 000 times in pH neutral solutions. The rate of electrochemical dissolution was shown to be influenced by the band gap of the compounds. It is proposed that oxidation of the oxide moiety, potentially to the superoxide, enables the solubilisation of the metal ions. The metal speciation appears to remain the same as for chemical dissolution. Anodic dissolution increases metal ion content in DES, with oxide being oxidised to form semi-stable superoxide species.