Removal of methylene blue from aqueous solution using wine-processing waste sludge

Dye wastewaters usually contain toxins and high chroma, making them difficult to treat with biological methods. The adsorption process plays an important role in removing dyes from wastewaters. This study aimed to explore the methylene blue (MB) adsorption mechanism by wine-processing waste sludge (WPWS). The WPWS contains a high cation-exchange capacity (64.2 cmol_c kg⁻¹) and organic matter (52.8%). The parameters affecting MB adsorption included pH, initial concentration of MB, reaction temperature, particle size and dosage of WPWS. The WPWS adsorption isotherms of MB were only well described by Langmuir adsorption isotherm. The maximum adsorption capacity (Q_m) of MB was 285.7 mg g⁻¹ at 25 °C. The activation energy determined by Arrhenius equation is 29.995 kJ mol⁻¹. Under steady-state reaction conditions, the Gibb free energy (ΔG°) ranged from −24.607 to −27.092 kJ mol⁻¹ and ΔH° was −8.926 kJ mol⁻¹, indicating that lower reaction temperature would favor MB adsorption. Therefore, MB adsorption by WPWS was a spontaneous, exothermic and physisorption reaction.