Enhanced removal of methylene blue using KMnO4-modified kitchen waste-derived lignin†

Adsorption is considered a simple and efficient method for treating dye wastewater. However, poor adsorption capacity, high cost, and complex production processes significantly limit the widespread use of adsorption. Kitchen waste-derived lignin, an inert component, is challenging to treat and underutilized. Nevertheless, the presence of various functional groups in lignin makes it a potential adsorbent for organic pollutants. Therefore, kitchen waste-derived lignin adsorption enables efficient waste management and cost-effective pollutant removal. Herein, potassium permanganate-modified lignin powder (KMnO₄-LP) was synthesized using methylene blue (MB) as a target pollutant and kitchen waste-derived lignin as a raw material. The effects of factors such as dosage, pH, contact time, and initial MB concentration on the adsorption capacity of KMnO₄-LP were investigated. Batch experiments revealed that KMnO₄-LP achieved an MB removal rate of 95.94% under optimal conditions (MB concentration of 50 mg L⁻¹, KMnO₄-LP dosage of 3 g L⁻¹, and pH 13). The adsorption process was well described by the quasi-secondary kinetic and Langmuir isotherm models. KMnO₄-LP exhibited a significantly higher maximum adsorption capacity (48.19 mg g⁻¹) than the unmodified lignin (16.18 mg g⁻¹). Thermodynamic analysis indicated that the negative ΔG values (−0.43, −0.87, and −6.33 kJ mol⁻¹) and the positive ΔH value (86.66 kJ mol⁻¹) indicated that the adsorption process was both spontaneous and endothermic. The adsorption site energy distribution highlighted that KMnO₄-LP had a strong affinity for MB. After five regeneration cycles using NaOH solution as the desorbent, the removal efficiency of KMnO₄-LP for MB decreased from 93.1% to 86.3%. This study indicates that KMnO₄-lignin is an effective adsorbent for MB removal from water and provides novel insights into the coupling of MB removal with the utilization of kitchen waste-derived lignin.