Efficient and regenerable thymol‐based hydrophobic deep eutectic solvents for extraction of cationic dyes from wastewater

Methylene blue (MB) and rhodamine B (RhB) are two representative cationic dyes that are commonly encountered in printing and dyeing wastewater. In this study, two thymol‐based hydrophobic deep eutectic solvents (HDESs) were designed and applied for removing MB and RhB from wastewater. The two HDESs were prepared at an equimolar ratio, where thymol (Thy) acts as the hydrogen bond acceptor (HBA), octanoic acid (OA) and benzyl alcohol (BA) as the hydrogen bond donors (HBDs). The experimental results identified the HDES Thy‐OA as the superior extracting agent. The extraction efficiency of 98% and 99.8% was achieved for MB and RhB with initial concentration of 200 ppm for MB and RhB, mass ratio of the HDESs to dye aqueous solution of 1:300, pH = 4, and extraction temperature of 298.15 K. The HDESs maintained an extraction efficiency of 93% after five regeneration cycles. The results of the COSMO‐SAC model and quantum chemical calculations revealed that the carboxyl group of OA formed strong hydrogen bonds with Cl − of MB and the carboxyl group of RhB. The findings provide the theoretical and experimental insights for the rational design of efficient extractants for extraction of the cationic dyes from wastewater.