The effect of increasing water content on transition metal speciation in deep eutectic solvents

A move towards the use of green solvents in metal recovery processes is going to be necessary for the development of a sustainable circular economy. This paper determines the effect of adding water to choline chloride based deep eutectic solvents on transition metal speciation, a common method for reducing their viscosity, using UV/Vis absorption spectroscopy. Even small amounts of water (< 5 %) were seen to cause a hypsochromic shift in UV/Vis spectra, indicating a change in metal speciation. In the choline chloride – urea solvent, the hydrolysis of the studied metals (V3+, Fe3+, Ni2+, Co2+, Cu2+) was observed. In the ethylene glycol and malonic acid systems precipitation was not observed, and Mx+ − H2O interactions were generally seen to dominate above ca. 30 % H2O. The presented results will impact metal recovery process design, where water additions to deep eutectic solvents to lower viscosity must be balanced with changes in metal speciation. This will allow for optimization of pumping systems whilst maintaining an effective process.