Cotton-straw-derived sulfonated lignin/bentonite composites: Preparation and methylene blue adsorption performance

Abstract Cotton straw is an abundant lignocellulose-rich agricultural residue that provides a sustainable and low-cost source of lignin. In this study, lignin was first extracted from cotton straw using a lactic acid-based deep eutectic solvent and then sulfonated to prepare two lignin/bentonite composites—LS/B (raw bentonite) and LS/CB (calcined bentonite)—as functional adsorbents. The composites were systematically characterized for their physicochemical properties, and their adsorption performance toward methylene blue (MB) was evaluated. Under optimized conditions, LS/B and LS/CB delivered remarkable MB adsorption capacities of 238.61 and 277.56 mg·g − 1 , respectively. The adsorption data fitted well to the pseudo-second-order kinetic and Langmuir isotherm models, indicative of monolayer chemisorption. Thermodynamic parameters confirmed the spontaneous and endothermic nature of the adsorption process. Mechanistically, adsorption was predominantly governed by electrostatic interactions, with synergistic contributions from π–π stacking and hydrogen bonding. Notably, LS/CB exhibited favorable recyclability, retaining 71.41% of its initial removal efficiency (86.37%) after five consecutive adsorption–desorption cycles. These findings demonstrate the valorization of cotton straw waste into efficient adsorbents for dye wastewater treatment, offering a scalable strategy applicable to other lignin-rich biowastes including agricultural waste and black liquor.