Kinetic, thermodynamic, and molecular interaction analysis of dye removal using deep eutectic solvents: A green separation approach

This study reports the first application of a Thymol: Phenol deep eutectic solvent (T:P DES) for the liquid–liquid extraction (LLE) of Rhodamine B (RhB). The optimized 800 µL of T:P 1:4 achieved a 95% removal efficiency at an initial RhB concentration of 10 ppm and pH 4. This high extraction efficiency was attributed to nonpolar interactions, including van der Waals, hydrogen-bonding, hydrophobic, and π-based interactions, as supported by NMR, FTIR, COSMO-RS, and molecular docking. Furthermore, the extraction kinetics showed that the Pseudo Second Order (PSO) model was the best fit, with a high regression value (R2 = 0.99971) and low RMSE value (0.0015). In addition, the close agreement between the third-order and linear Van’t Hoff models indicated that the third-order Van’t Hoff model provides thermodynamic parameters consistent with those obtained using the conventional linear Van’t Hoff model, with only a 1.95% difference in ΔH°. Green Analytical Chemistry Assessment using AGREE, ComplexGAPI, and EcoScale confirmed the method’s environmental friendliness, yielding an AGREE score of 0.75, an EcoScale score of 97, and predominantly green ComplexGAPI pictograms. The proposed T:P 1:4 therefore represents a promising laboratory-scale extraction system for RhB removal and provides a basis for future studies on solvent regeneration, process optimization, and scale-up for practical wastewater treatment.