Highly efficient extraction/separation of Cr (VI) by a new family of hydrophobic deep eutectic solvents

Highly efficient and selective separation of Cr (VI) is great of importance in environmental pollution control. In this study, a novel family of hydrophobic DESs has been designed and synthesized by mixing parabens and quaternary ammonium, and then used to extract heavy metal Cr (VI) from water phase and the content of Cr (VI) is measured by 1,5-diphenylcarbazide spectrophotometry. It is found that the hydrophobic DESs exhibit excellent extraction performance towards Cr (VI). At the 1:1 M ratio of DES to Cr (VI), the extraction capacity is as high as 66.7 mg g^-1, the extraction efficiency is greater than 90% for ultra-trace Cr (VI) (0.51 μg L^-1), and the preconcentration factor is up to 700. When the concentration of coexisting metal ions (Cd²⁺, Cu²⁺) and organic compounds ((C₆H₁₀O₅)_n, C₆H₁₂O₆·H₂O) is 2000 times that of Cr (VI), the extraction efficiency is still above 90%. Compared with ionic liquid extractants, the preparation process of DES is very simple, the atomic utilization rate is 100%, and no waste is produced. These hydrophobic DESs have been used to separate and enrich Cr (VI) from actual water samples with satisfactory recoveries. The possible extraction mechanism is investigated by FT-IR and ¹H NMR.