Biochar–Cellulose–Sodium Alginate Composite Hydrogel (BCSA) for Methylene Blue Removal: Performance Evaluation and Mechanistic Investigation

To develop efficient and reusable adsorbents for cationic dye removal from dyeing wastewater, this study aimed to develop a novel composite hydrogel adsorbent and systematically investigate its adsorption performance, optimal preparation conditions, and multimechanistic adsorption behavior for methylene blue (MB). We developed BCSA, a composite hydrogel composed of biochar, cellulose, and sodium alginate, as an efficient adsorbent for MB removal. The adsorption performance and underlying mechanism of MB adsorption onto BCSA were systematically explored via batch adsorption experiments, kinetic/isotherm/thermodynamic model fitting, multiple characterization techniques (SEM, BET, FTIR, XPS), and interference/reusability assays. The cellulose content was found to exert a significant influence on the adsorption performance. The optimal adsorption properties were achieved when the mass ratio of biochar to sodium alginate to cellulose was fixed at 2:2:0–2 (based on 2 g: 2 g: 0–2 g of the three components) in 100 mL of deionized water. BCSA exhibited a high adsorption capacity (∼360 mg/g at 298.15 K and pH 11) and maintained an excellent performance over a broad pH range (4–11). Kinetic investigations demonstrated that the adsorption process conformed to the PFO model, and the equilibrium data were most appropriately characterized by the Temkin isotherm. Thermodynamic analysis confirmed the spontaneous and exothermic nature of the adsorption. Increased ionic strength (NaCl, Na2SO4) and the presence of a competing dye (RhB) reduced MB uptake, yet BCSA retained significant capacity under competitive conditions. Notably, BCSA exhibited outstanding reusability, retaining more than 85% of its original adsorption capacity following four consecutive adsorption–desorption cycles. Characterization and mechanistic studies indicated that MB removal involved multiple synergistic mechanisms including pore filling, electrostatic interactions, cation exchange, hydrogen bonding, and π–π interactions. These results highlighted BCSA as a highly efficient, cost-effective, and reusable adsorbent, exhibiting considerable potential for MB elimination in aqueous environments and providing a feasible and novel material design strategy for the development of high-performance composite hydrogel adsorbents for industrial cationic dye wastewater treatment.