A cotton straw-based modified lignin-bentonite composite for efficient adsorption of methylene blue dye

The removal of toxic organic dyes such as methylene blue (MB) poses a significant challenge in wastewater recycling, creating demand for efficient and low-cost adsorbents. In this study, three lignin-bentonite composites—LT/B, BC/L, and LT/BC—were prepared using cotton straw lignin and/or quaternary ammonium lignin in combination with bentonite or CTAB-modified bentonite. The obtained materials were characterized by SEM, XRD, FTIR, XPS, TG, and zeta potential analysis. Their adsorption performance was systematically evaluated through kinetic, isotherm, and thermodynamic studies, with selectivity tested using Congo Red dye. Among the three composites, LT/BC exhibited outstanding performance, achieving a maximum adsorption capacity of 151.20 mg·g^-1 and a removal rate of 94.5% for MB under optimal conditions (308 K, pH 8, initial MB concentration 100 mg·L^-1, and contact time 180 min). Adsorption follows the pseudo-second-order kinetic model and the Langmuir isotherm, indicating that the adsorption process is a mono-layer adsorption controlled by chemisorption. Thermodynamic analysis confirms that this is a spontaneous and endothermic process. The composite showed high selectivity for cationic MB over anionic dyes, driven primarily by ion exchange, with additional contributions from hydrogen bonding and π-π interactions. Zeta potential analysis further confirmed that electrostatic attraction was not the sole mechanism. This work provides a sustainable strategy for wastewater treatment and can be extended to other lignin-rich biowastes, such as black liquor, for value-added applications.