Wettability-Controlled Hydrophobic Deep Eutectic Solvents for Sustainable and Rapid Removal of Hexavalent Chromium

Abstract The removal of hexavalent chromium (Cr(VI)) from industrial effluents remains challenging for conventional solvent extraction systems due to sluggish mass transfer, limited extraction capacity, and the inherent toxicity of volatile organic compounds. Here, we report a wettability-modulated strategy to overcome these limitations using hydrophobic deep eutectic solvents (HDESs) based on quaternary ammonium salts and fatty acids. By correlating the contact angle, viscosity, and hydrogen-bonding network with extraction performance, we identify the optimized HDES, [N8881][Br]-OA(1:2), which exhibits a remarkably low viscosity (147.6 cP at 298.2 K) and a favorable contact angle (34.4°), enabling rapid interfacial anion exchange. This extractant achieves a single-stage Cr(VI) recovery exceeding 99% within only 10 min at an initial concentration of 500 mg·L–1 under an oil-to-water ratio of 1:50. Mechanistic investigations confirm a reversible Br–/HCrO4– anion-exchange pathway, while thermodynamic analysis reveals an endothermic and entropy-driven process. Notably, the HDES maintains over 95% extraction efficiency after 12 successive regeneration cycles without structural degradation and outperforms the benchmark solvent methyl isobutyl ketone (MIBK) by a factor of 12. This work demonstrates that tailoring the wettability of HDESs provides a green, sustainable, and industrially viable platform for the ultrafast decontamination of Cr(VI)-laden wastewater, with significant potential for scalable water treatment applications.