Evaluation of Cr(III) Adsorption Using Activated Jatropha curcas Shell: Kinetics and Energy Recovery

This study investigates the potential of conditioned Jatropha curcas shell residues as sustainable and cost-effective bioadsorbents for the removal of Cr-(III) from aqueous solutions. Adsorption performance was evaluated under varying conditions of pH (3.05 and 3.89), temperature (30 and 50 °C), and particle size (250 and 420 μm). BET analysis revealed that smaller particles (250 μm) exhibited a higher surface area (2.175 m²/g) and pore volume (0.019 cm³/g) compared to larger ones. FTIR spectroscopy confirmed enhanced exposure of hydroxyl (-OH) and carboxylate (-COO^-) groups after mild conditioning, promoting effective Cr-(III) interactions. ANOVA results indicated a highly significant model (F = 1228.114, p < 0.0001), and Pearson correlation identified pH and particle size as the most influential variables. The Langmuir isotherm best described the equilibrium data, with a maximum adsorption capacity (Q _max) of 13.81 mg/g for 250 μm particles. The proposed adsorption mechanism involved electrostatic attraction, ion exchange, and surface complexation with a final pH increase to 5.96. Under optimal conditions, a maximum Cr-(III) removal efficiency of 98.75% was achieved. Importantly, this study introduces a dual-valorization strategy that transforms Jatropha curcas shells, an agroindustrial waste, into both an efficient bioadsorbent and a potential solid biofuel. Jatropha curcas shell analysis demonstrated that the spent shells retain a significantly higher heating value (15.4 MJ/kg), enabling their direct use in energy recovery applications without requiring conversion to biochar. This approach reduces processing steps while maintaining energetic value, enhancing the practicality and sustainability of the process. The proposed method offers a simple, nontoxic, and scalable solution to integrate heavy metal remediation with post-treatment energy recovery, contributing to zero-waste circular bioeconomy schemes and sustainable wastewater management.