Influence of Temperature and Hydrogen Peroxide Treatment on the Physiochemical and Adsorptive Characteristics of Rice Stalk Biochar for Cadmium Remediation: A Sustainable Solution

Cadmium contamination of water poses serious environmental and health risks, necessitating low-cost, sustainable adsorbents. This study evaluated the combined effects of pyrolysis temperature and hydrogen peroxide oxidation on the physicochemical properties and cadmium adsorption performance of rice stalk biochar. Unlike previous studies that mainly examined single-temperature biochars or conventionally modified biochars, this study systematically links pyrolysis temperature, H₂O₂ oxidation, surface oxygen enrichment, pore development, and Cd adsorption mechanisms in rice stalk biochar within one comparative framework. Biochars were produced at 500, 600, and 700 °C and modified using 30% H₂O₂ for 32 h. The modified biochars showed improved surface properties, with specific surface area increasing by 73.66 to 74.21%. Elemental analysis confirmed a reduction in carbon content by 2.83 to 3.25% and an increase in oxygen content by 4.10 to 10.25% after oxidation, indicating enrichment of oxygen-containing functional groups. FTIR and XPS analyses further confirmed the formation of hydroxyl, carboxyl, and carbonyl groups, which enhanced Cd binding. Adsorption performance increased with pyrolysis temperature and chemical modification, following the order BC500 < BC600 < BC700 < MBC500 < MBC600 < MBC700. The highest maximum adsorption capacity was obtained for MBC700, reaching 252.37 mg g⁻¹ at 30 °C. The Langmuir model fitted the adsorption data better than the Freundlich model, indicating dominant monolayer adsorption on specific active sites. Kinetic data followed the pseudo-second-order model, suggesting that chemisorption contributed strongly to Cd uptake. The main adsorption mechanisms included surface complexation, ion exchange, electrostatic attraction, pore filling, and mineral-associated precipitation. Overall, H₂O₂-modified rice stalk biochar, especially MBC700, showed strong potential as an efficient and sustainable adsorbent for Cd removal from aqueous solutions.