Sustainable Extraction of Hydrophobic Azo Dyes Using Recyclable Fatty Acid–Based Deep Eutectic Solvents for Industrial Wastewater Remediation

Abstract The design of sustainable solvents for removing hydrophobic pollutants from aqueous environments remains a major challenge in environmental chemistry. In this study, four hydrophobic deep eutectic solvents (HDESs) were synthesized by combining benzyl triethylammonium chloride (BTEAC) with medium-chain fatty acids: n-butyric (DES-1), valeric (DES-2), heptanoic (DES-3), and caprylic acid (DES-4), and evaluated for extracting Sudan III and Sudan IV dyes. The synthesized DESs were characterized using Fourier-transform infrared spectroscopy (FT-IR), 1H NMR, rheological behavior, solubility, and conductivity measurements, confirming hydrogen-bond network formation and tunable hydrophobicity. Partition coefficient (log Kow) analysis revealed a hydrophobicity gradient across the DES series, correlating with extraction performance. Extraction parameters, including pH, DES volume, dye concentration, sample volume, and centrifugation time, were optimized using response surface methodology based on a central composite design. Under optimal conditions, DES-4 achieved extraction efficiencies of 92.66% for Sudan III and 94.84% for Sudan IV. The extraction followed the Freundlich isotherm and pseudo-first-order kinetics. Negative ΔG values (−35 to −58 kJ mol–1) and ΔH confirmed a spontaneous, exothermic process dominated by hydrophobic and van der Waals interactions. The DESs retained more than 65% efficiency after three reuse cycles. Greenness evaluation using AGREE (0.71), GAPI, and complex GAPI tools confirmed superior environmental performance.