Hydrophobic deep eutectic solvents as the green media for highly efficient extraction of Cr(VI) over a broad pH range and low oil-water ratio

Abstract

The occurrence of Cr(VI) ions in aquatic ecosystems poses serious public health risks, making the development of effective removal technology for Cr(VI) ions urgent. This study presents a green and effective method for extracting Cr(VI) ions using hydrophobic deep eutectic solvents (HDESs) composed of tetran-octyl ammonium bromide and carboxylic acids. The results show that the extraction efficiency of [N8888][Br]-C6OOH(1:2) for Cr(VI) is as high as 98.2 % for high concentration of Cr(VI) (500 mg⋅g−1) with a low oil–water ratio of 1:66. Surprisingly, these HDESs demonstrate remarkable efficacy in extracting Cr(VI) ions over a broad pH range and at a low oil–water ratio, even in the presence of multiple competing ions. A serious of kinetic, thermodynamic, and spectroscopic experiments suggested that the extraction mechanism of Cr(VI) ions is a synergistic dissolution diffusion and ion exchange. As a proof of concept, we have successfully demonstrated the application of prepared HDESs for extracting trace amounts of Cr(VI) ions from H3PO4 solutions with varying mass fractions, and the extraction process resulted in a Cr(VI) removal rate that achieved 99.7 %. Furthermore, the extraction performance HDESs for Cr(VI) ions remained nearly unchanged after eight cycles of continuous extraction and regeneration. Impressively, a precipitation method was also employed to convert the isolated Cr(VI) ions into valuable BaCrO4. The research findings provide valuable insights for the development of highly efficient Cr(VI) extractants.