Hydrophobic deep eutectic solvents: Towards a greener and more efficient extraction process of Cr(VI) from aqueous solutions

Liquid-liquid extraction is a well-established technique for removing hexavalent chromium (Cr(VI)) from aqueous solutions. However, conventional extractants often face limitations such as low extraction efficiency, poor reversibility, and organic vapor contamination due to their volatility. In this work, we prepared five types of hydrophobic deep eutectic solvents (HDESs) with excellent thermal stability by employing methyltrioctylammonium chloride (N8881Cl) as the hydrogen bond acceptor (HBA). We thoroughly investigated their thermophysical properties and performance in Cr(VI) extraction. The kinetic and thermodynamic results of the Cr(VI) extraction process revealed that these HDESs can achieve extraction equilibrium within 5 min, with increasing efficiency at higher temperatures. Among the tested HDESs, N8881Cl-MPD(1:1) exhibited an extraction efficiency of 99.9 % with a Cr(VI) concentration of 500 mg center dot L-1 at a low oil-to-water ratio of 1:50 (pH=2). This performance surpasses traditional extractants by more than four times. Analysis of extraction mechanisms revealed that ion exchange and electrostatic interactions primarily drive the extraction process. This research presents a sustainable alternative to the conventional organic solvent extraction of Cr(VI), offering a more efficient and environmentally friendly approach.