Waste-to-resource conversion of sugar beet pulp into sulfonated biochar for efficient dye adsorption

Abstract

This study presents a sustainable “waste-to-resource” approach by converting sugar beet pulp into a high-performance sulfonated biochar (CSBP) through a streamlined chemical carbonization route. By utilizing concentrated sulfuric acid, simultaneous carbonization and surface functionalization were achieved under mild conditions, eliminating the need for high-temperature pyrolysis. Comprehensive physicochemical characterization revealed a highly porous carbonaceous framework enriched with sulfonic acid groups, yielding a fine-grained adsorbent (<20 μm) with high thermal stability and a carbon content of approximately 85 wt%. Batch adsorption experiments were systematically performed to evaluate the effects of adsorbent dosage, solution pH, initial dye concentration, contact time, and temperature. The adsorption process exhibited high removal efficiency (>90%) at the natural pH of the dye solution, eliminating the need for pH adjustment. Kinetic analysis demonstrated that the adsorption behavior followed a pseudo-second-order model, indicating that chemisorption governs the rate-controlling step, with an activation energy of 9.01 kJ/mol. Equilibrium data were best described by the Langmuir isotherm, confirming monolayer adsorption on a homogeneous surface, with a maximum adsorption capacity of 2.70 mmol/g at 55°C. Thermodynamic parameters (ΔH° = +9.301 kJ/mol and negative ΔG° values) indicated that the adsorption process is endothermic and spontaneous. The adsorption mechanism was attributed to the synergistic contribution of electrostatic interactions, π–π stacking, surface complexation involving sulfonic and oxygen-containing functional groups, and pore-filling effects. Overall, this study demonstrates that sulfuric acid-carbonized sugar beet pulp is a highly effective and environmentally benign adsorbent for cationic dye removal.