Bauhinia racemosa sawdust as an novel biosorbent for hexavalent chromium removal with optimization and mechanistic insight

Present study evaluates Bauhinia racemosa sawdust powder (BRSP) an underutilized agro waste as a sustainable, low-cost biosorbent for hexavalent chromium [Cr(VI)] removal from aqueous solutions. Systematic batch adsorption experiments revealed a peak Cr(VI) removal of 99.78% and an adsorption capacity of 106.86 mg g⁻¹ under optimized conditions (pH 4.0, 0.5 g/100 mL dosage, particle size 75 μm, 100 mg L⁻¹ initial concentration, and 60 min contact time). Comprehensive physiochemical characterization confirmed its suitability: BET analysis revealed a specific surface area of 212.2 m²g⁻¹, pore volume of 0.323 cm³g⁻¹, and mesoporous structure (average pore diameter of 6.4 nm). Boehm titration and pHPZC (2.7) showed abundant acidic groups while FTIR spectroscopy confirmed the involvement of hydroxyl, carboxyl, and amine groups. SEM-EDX analyses revealed morphological changes and chromium deposition (~ 0.6 wt%) post adsorption. Adsorption equilibrium data fitted best to the Langmuir model (qmax = 106.86 mg g⁻¹, R² = 0.991), with kinetics following a pseudo-second-order model (R² = 0.999), and thermodynamic analysis (ΔG° = − 8.1 to − 10.5 kJ mol⁻¹; ΔH° = +22.6 kJ mol⁻¹) indicating a spontaneous, endothermic process. Bauhinia racemosa sawdust, an underexplored lignocellulosic biomass, exhibited superior performance compared to previously reported agro-waste biosorbents, highlighting its novelty and potential. This study positions BRSP as a promising and sustainable material for efficient Cr(VI) remediation contributing to the progression of green technologies in wastewater treatment.