This is the first report on metallic species transfer from aqueous hydrochloric and oxalic acid media into hydrophobic deep eutectic and low-transition-temperature mixtures composed of quaternary ammonium salts or menthol and carboxylic acids.

Deep eutectic solvents (DESs) appeared recently as a new class of green designer solvents. The recovery of metals using hydrometallurgy is of major importance with the growth in metal demand. Several authors used these solvents for the hydrometallurgical recovery of metals from primary and secondary resources, and these studies are reviewed in the present work. Hydrophilic DESs can be used for the leaching of metals and have great potential to replace mineral acids, and even to reduce water consumption. Efficient and selective leaching of metals from minerals or wastes is feasible by using DESs. However, the kinetics of leaching as well as the physicochemical properties of DESs are still limiting their large-scale application. Electrochemical recovery from DES is also possible but deserves further investigation. Finally, the recovery of metals from aqueous solutions using hydrophobic DESs was studied in several works. For the solvent extraction of metals, hydrophobic DESs constitute credible alternative ionic liquids.

Previous studies have shown that the presence of nitric acid, the principal solute in various hydrometallurgical processes, and perchloric acid in the aqueous phase is an important factor for increased aqueous solubility of hydrophobic ionic liquids. In this study, the effect of hydrochloric acid in the aqueous phase on the mutual solubility of water and hydrophobic 1,3-dialkylimidazolium- and N,N-dialkylpyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquids, [C_nmim][Tf₂N] (n = 2, 4, 6, and 8) and [C₃C₁pyrr][Tf₂N], is examined at room temperature and atmospheric pressure. Hydrochloric acid caused a considerable increase in the aqueous solubility of all the studied ionic liquids. The amount of water transferred into the organic phase increases with increasing hydrochloric acid concentration for short alkyl chain ILs, and the opposite trend was observed for long alkyl chain ILs. The effect of the N–H acid bis(trifluoromethylsulfonyl)imide, H[Tf₂N], and the salts lithium bis(trifluoromethylsulfonyl)imide, Li[Tf₂N], and 1-butyl-3-methylimidazolium chloride, [C₄mim]Cl, dissolved in hydrochloric acid solutions on the mutual solubility of water and the [C₄mim][Tf₂N] ionic liquid were also investigated. The salting-out effect is observed and it was shown to be dependent on the nature of the salt, its concentration and the hydrochloric acid concentration in the aqueous phase. A mathematical model has been developed to describe the dependence of the ionic liquid cation and anion concentration on the common ion salt concentration in the aqueous hydrochloric acid phase. This model describes the basic character of ionic liquid dissolution in the aqueous phase and allows for estimation of solubility product values.