Study on the Adsorption Mechanism of Cr(Ⅵ) by Two New Composite Modified Biochars

Cr(Ⅵ) has been identified as a significant contaminant in the environment, exhibiting properties of high toxicity, easy migration and strong carcinogenicity. These characteristics render it a substantial hazard to both water environment safety and human health. The efficient removal of Cr(Ⅵ) has consistently been a focal point in the field of environmental remediation. Biochar has been demonstrated to possess a good ability to adsorb heavy metal ions. The preparation and modification of biochar from agricultural waste is not only an effective approach to realize the resource utilization of agricultural waste, but also can effectively enhance the removal efficiency of Cr(Ⅵ) by biochar. In this study, peanut shell and sesame straw were utilised as raw materials to prepare original biochar, and consequently SF-HBC and PF-ZBC were prepared by composite modification methods of “ferric chloride+sodium alginate” and “ferric chloride+chloroapatite” , respectively. Then, the experiments were conducted to investigate the impacts of biochar type, pH value and biochar dosage on the adsorption efficiency of Cr(Ⅵ) in water. Combined with the theories of adsorption kinetics, adsorption isotherms and adsorption thermodynamics, the adsorption processes of Cr(Ⅵ) by two kinds of composite-modified biochars (SF-HBC, PF-ZBC) were analyzed. At the same time, by comprehensively applying characterization methods such as SEM, FTIR, XRD and XPS, the adsorption mechanisms and differences of Cr(Ⅵ) in water by SF-HBC and PF-ZBC were analyzed. The research results showed that acidic conditions were more conducive to the adsorption of Cr(Ⅵ) in water by SF-HBC and PF-ZBC. In comparison with PF-ZBC, SF-HBC exhibited a broader pH application range. The removal efficiency of Cr(Ⅵ) by SF-HBC in water maintained at 63% at pH=8, which was significantly higher than that of PF-ZBC(17.5%). The adsorption processes of Cr(Ⅵ) by SF-HBC and PF-ZBC both conformed to the pseudo-second-order kinetic model and the Langmuir isothermal adsorption model, indicating that the adsorption processes of Cr(Ⅵ) by the two were mainly chemical adsorption of monolayer and were spontaneous endothermic processes. A comparison was made for the pseudo-second-order kinetic parameter k 2 and the Langmuir isothermal adsorption parameter K L. It was determined that SF-HBC exhibited a faster adsorption rate, while PF-ZBC demonstrated a stronger binding force with Cr(Ⅵ), indicating that there were significant differences in the adsorption behaviors between the two.The mechanism study demonstrated that the adsorption mechanism of Cr(Ⅵ) by SF-HBC was a multifaceted process involving physical adsorption, redox and complexation. The adsorption mechanism of Cr(Ⅵ) by PF-ZBC was found to be physical adsorption, redox, complexation and co-precipitation. It was determined that the adsorption of Cr(Ⅵ) by SF-HBC was predominantly governed by the complexation of organic functional groups and redox reactions, while PF-ZBC mainly exhibited a more efficacious removal of Cr(Ⅵ) through the combination of redox, complexation, and phosphate co-precipitation mechanisms. A significant disparity was observed in the adsorption mechanism of Cr(Ⅵ) between SF-HBC and PF-ZBC.The study findings can provide theoretical basis and technical reference for the efficient resource utilisation of agricultural waste and the efficient treatment of Cr(Ⅵ), which is of positive significance for promoting the treatment of heavy metal pollution and the development of green environmental functional materials.