Preparation of a novel environmentally friendly hydrophobic deep eutectic solvent ChCl-THY and its application in removal of hexavalent chromium from aqueous solution

A liquid-liquid extraction methodology was developed for the removal of Cr(VI) from contaminated water using a novel green hydrophobic deep eutectic solvent (DES) as an efficient sole extracting agent. The hydrophobic DES was obtained by mixing choline chloride and thymol in 1:4 molar ratio at 70 degrees C for 10 min and was denoted as ChCl-THY(1:4). The ChCl-THY(1:4) works efficiently for removal of high (20 mg/L) and low (500 mu g/L) concentration of Cr(VI) from artificially contaminated natural water with >95% extraction efficiency (E%) at optimized reaction conditions (pH 2-6, 40 degrees C). The DES was characterized by H-1 NMR and FTIR spectroscopy, and the data suggest that interaction occurs between Cl- ion of choline chloride and H atoms of thymol molecules. Physicochemical properties such as density, melting point, moisture, and solubility were studied and discussed. Herein, no sharp melting point was observed for ChCl-THY(1:4) in DSC curve. DES was regenerated using 0.1 M NaOH as stripping agent, and 50%-60% extraction efficiency could be attained in the next cycle. A plausible mechanism of interaction between Cr(VI) species and DES was also explored with the help of FTIR spectroscopy. Practitioner Points A novel hydrophobic DES (ChCl-THY) is prepared by mixing choline chloride and thymol at 1:4 molar ratio. ChCl-THY(1:4) is employed for the first time as sole extracting agent to remove the Cr(VI) from contaminated aqueous solution. >95% extraction efficiency was achieved by ChCl-THY(1:4) in natural water conditions at mu g/L and mg/L level of contamination. Both the component used to prepare the DES are naturally abundant; hence, DES is not toxic for biota. The element present in natural water did not show any interference with extraction of Cr(VI).

Automated summary

A liquid-liquid extraction methodology using a novel green hydrophobic deep eutectic solvent (DES) ChCl-THY(1:4) was developed for efficient removal of Cr(VI) from contaminated water. The ChCl-THY(1:4) showed >95% extraction efficiency for high and low concentration of Cr(VI) under optimized conditions, and could be regenerated with 0.1 M NaOH for further extraction cycles. Characterization of the DES was done using H-1 NMR and FTIR spectroscopy, revealing potential interaction mechanisms between components.