Adsorption Mechanism of Hexavalent Chromium on Biochar: Kinetic, Thermodynamic, and Characterization Studies

The adsorption mechanism of Cr⁶⁺ on biochar prepared from corn stalks (raw carbon) was studied by extracting the organic components (OC) and inorganic components (IC). Scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy were used to characterize the properties of three kinds of carbon. Kinetic and thermodynamic experiments were performed. The results showed that the experimental data were fitted well by the Freundlich model and the pseudo-second-order kinetic model, and the adsorptions on the three kinds of carbon were all spontaneous, endothermic processes. The adsorption of Cr⁶⁺ by biochar was in accordance with a chemisorption process. The adsorption contribution rate of the OC was 97%, which was much higher than that of the IC. Electrostatic attraction and redox reaction were the main mechanisms of adsorption, and among them, the contribution rate of the redox reaction accounted for 61.49%. The reduced Cr³⁺ could both exchange ions with K⁺ and dissociate into solution by electrostatic repulsion; the amount of Cr³⁺ released into the solution was approximately 17.07 mg/g, and the amount of Cr³⁺ ions exchanged with K⁺ was 0.29 mg/g. These results further elucidate the adsorption mechanism of Cr⁶⁺ by biochar.