Sustainable and fast adsorption of Safranin O dye via deep eutectic solvent-functionalized coconut waste-based...

Safranin O (SO) is a commonly used industrial dye, and its presence in wastewater poses significant risks to both ecosystems and human health. In this study, thymol-menthol-based and thymol-phenol-based deep eutectic solvents (DES)-functionalized activated carbons derived from desiccated coconut waste, namely T:M-DES@AC-DCW and T:P-DES@AC-DCW were synthesized and evaluated as effective adsorbents for SO dye removal in aqueous media. Comprehensive physicochemical characterization revealed that both materials possess highly hydrophobic, thermally stable, porous, and crystalline structures, with functional groups such as O-H, C-H, C = O, C-O, and CH₂ contributing to their adsorption capabilities. Among the two, T:P-DES@AC-DCW exhibited superior adsorption performance, achieving a maximum SO dye removal efficiency of 94.73% under optimized conditions (adsorbent dosage: 100 mg, contact time: 10 min, initial dye concentration: 15 ppm, temperature: 30 °C, and pH: 8). Kinetic modeling indicated that the adsorption process follows a pseudo-second-order model (R² = 0.999), while isotherm analysis showed better alignment with the Freundlich model (R² = 0.994), suggesting multilayer adsorption on a heterogeneous surface. Thermodynamic parameters confirmed that the process is spontaneous (ΔG° = 1.472-1.596 kJ mol^-1), endothermic (ΔH° = 0.380 kJ mol^-1), and associated with increased randomness (ΔS° = 6.213 J mol^-1.K) at the solid-liquid interface. Additionally, T:P-DES@AC-DCW demonstrated excellent reusability over five consecutive cycles with minimal loss in efficiency and showed promising performance for the adsorption of various textile dyes and real wastewater samples. An EcoScale score of 78 out of 100 further confirms the sustainability of the proposed method for dye removal. These findings highlight the potential of T:P-DES@AC-DCW as a low-cost, eco-friendly, and highly efficient adsorbent for the treatment of dye-contaminated industrial wastewater.