Active biochar-supported iron oxides for Cr(VI) removal from groundwater: Kinetics, stability and the key role of FeO in electron-transfer mechanism

Chromium (Cr), especially in forms of hexavalent chromium (Cr(VI)) remains a serious threat to public health and environmental safety for its high toxicity. Herein, two types of iron-modification methods adopting co-pyrolysis and surface-deposition respectively were carried out to prepare active Fe-biochar composites (FeBC) for Cr(VI) removal in the simulated groundwater environment. The systematic characterization demonstrated that larger BET surface area and diversified iron oxides of FeBC-1 obtained from the co-pyrolysis method contributed to higher adsorption and reduction activity towards Cr(VI) degradation in comparison with FeBC-2 produced from surface-deposition method. Further, FeO was evidenced to be a main active component for transforming Cr(VI) to lower-toxicity Cr(III) uniting XRD and XPS analysis. Also, the designed batch experiments aiming at deeper clarifying FeBC-1 revealed that the pseudo-second-order kinetic and intra-particle diffusion model could well describe the Cr(VI) sorption behaviors, suggesting that a single-layer, chemical adsorption process as well as internal particle diffusion both controlled the removal process of Cr(VI) using FeBC-1. Finally, the stability experiments stated that FeBC-1 was basically stable at acidic and neutral conditions. Thus, it was found that co-pyrolysis of FeBC-1 is a potential technology for Cr(VI) remediation.