The Effect of Water Content on Metal Oxide Dissolution by Deep Eutectic Solvent

The dissolution behaviour of individual valuable metal oxides in deep eutectic solvent (DES) and determination of water effects on coordination and solubility is necessary for understanding the underlying leaching process and potential industrial application. Choline chloride (ChCl) / ethylene glycol (EG) DES, DES-water mixture and pure water solutions were utilized to dissolve Li 2 O, Co 3 O 4 , MnO 2 , NiO and Fe 3 O 4 at various temperatures. It was found that the solubility of all metal oxides increased with increasing dissolution temperature from 298.15 to 413.15 K. Moreover, Li 2 O can be easily dissolved in water-based solvents, while transition metal oxides are less likely to be dissolved with increased water content due to the coordination with ligands of [H 2 O] and [Cl - ]. Density functional theory (DFT) was applied to generate dissolution mechanisms for each oxide. As a result, in DES-rich systems, transition metals were shown to form stable tetrahedral [MCl 4 ] 2- complexes while in water-rich systems, they have a tendency to form octahedral [M(H 2 O) 6 ] 2+ . In addition, in both DES-rich and water-rich systems, the affinity between metal ions with [Cl - ], and metal ions with [H 2 O] followed the order: Fe > Ni > Co > Mn.