Effects of thiol substitution in low-transition-temperature mixtures (LTTMs) as solvents for metal oxides

This study deals with an investigation of how substitution of an alcohol group by a thiol group in mixtures of choline chloride with a series of bio-sourceable molecules affects the physico-chemical properties of the mixtures and their ability to dissolve metal oxides. All of the thiol mixtures studied showed a higher affinity and selectivity for late transition metals and the physical properties of the mixtures were improved compared to their alcohol analogues (<em>i.e.</em> lower viscosity, wider liquid range). The metal solubility was assessed<em> via </em>determinination of the final concentration of the metal oxides dissolved in thiol mixtures<em> via </em>inductively coupled plasma optical emission spectroscopy (ICP-OES). The thiol function selectively improved the solubilities of the late transition metal oxides (<em>i.e.</em> copper and zinc), which are valuable metals often present as residue in industrial waste. The solubility of iron oxides was much lower than that of the valuable metals, which is a significant benefit in industrial applications. The different solubilization behaviour of metal oxides in the thiol and alcohol mixtures was further investigated<em> via </em>UV-vis absorption and infrared spectroscopy. This study allowed to assess the potential of these low-temperature-transition mixtures for the selective recovery of metals.