Chemical Characterization of Terpene-Based Hydrophobic Eutectic Solvents (HDESs) and their Application as Greener Alternatives for Pb(II) Extraction Procedure

Deep eutectic solvents (DESs) are receiving increasing attention as environmentally friendly solvents in various analytical techniques. These solvents as new generation of ionic liquids (ILs), and based upon what they are derived from, can be the safer, cheaper, and more efficient alternative to conventional solvents in solvent extraction methods. Solvents prepared from natural terpenes (menthol and thymol), as H-bond acceptors, and a series of organic acids (chain lengths of 8, 10, and 14 C atoms), as H-bond donors, were characterized and tested as reaction media for liquid–liquid extraction purposes. Due to their high hydrophobicity, they seem to be promising alternatives to conventional (nonpolar and toxic) solvents, since they possess relatively less toxic, less volatile, and consequently, more environmentally friendly characteristics. Potential application of these solvents for the extraction of Pb(II) ions was investigated; for that purpose, several hydrophobic DESs (HDESs), with required chemical stability and desirable physicochemical and thermal properties, were selected as reaction media in the liquid–liquid extraction (LLE) procedure. Low viscosities and high hydrophobicities of prepared solvents were confirmed as desirable properties for their application. Extraction parameters were optimized, and chosen solvents were applied. The results showed satisfactory extraction efficiencies in simple and fast procedures, followed by low solvent consumption. Among tested solvents, the best results (98%) were obtained by the thymol-based solvent, thymol–decanoic acid (Thy-DecA) 1:1, followed by L-menthol-based solvents: menthol–octanoic acid (Men-OctA) 1:1 with 97% and menthol–decanoic acid (Men-DecA) 1:1 with 94.3% efficiency. The results also showed that unlike of conventional liquid-liquid extraction methods, counterions are not required to transfer the analyte to the hydrophobic phase. Furthermore, no ligands were required either: it was assumed that the equilibrium is established between solvent and analyte during a ligandless procedure, so it can be concluded that those nonpolar solvents can efficiently extract nonpolar analytes from the aqueous environment. Proposed HDES-based extraction procedure is simple, less expensive, and most importantly, more environmentally friendly, compared to conventional procedures.