Sustainable transformation of cotton stalk waste as a biochar-based copper-silicon composite for chromium (VI) detoxification

This study investigates the adsorption efficiency of a biochar composite derived from cotton stalks with copper and silicon (CSB-Si-Cu) for chromium (VI) removal from water. Characterization techniques, including XRD, FTIR, EDX, SEM, TEM, XPS, and PZC analysis, confirmed the material's structural and chemical properties. Peaks at 153 eV (Si2p) and 932.1 eV (Cu 2p) validated the presence of silicon and copper, while a peak at 529.9 eV indicated Cr₂O₃/Cr(OH)₃ formation after adsorption. The adsorption followed the pseudo-second-order and Elovich models, with an equilibrium adsorption capacity of 39.63 mol g⁻¹ at 290 minutes. Chromium (VI) removal increased at lower pH and higher temperatures, reaching a maximum of 61.10 × 10⁵ mol g⁻¹ at pH 2 and 41.30 × 10⁵ mol g⁻¹ at 318 K. Adsorption data aligned with Langmuir and Freundlich models, indicating a complex mechanism, while the Temkin isotherm highlighted the role of thermal energy. Thermodynamic analysis confirmed the process as endothermic, spontaneous, and chemisorption-driven. Electrostatic interactions and reduction by oxygen-containing functional groups facilitated chromium removal. CSB-Si-Cu exhibited excellent reusability, maintaining 97.53 %–42.92 % adsorption efficiency over multiple cycles, demonstrating its potential for wastewater treatment and pollutants remediation.