The Transformation of Cr(III) in Different Redox Environments

[Objective] How Fe(II)-containing clay minerals affect the adsorption-desorption and valence transformation of Cr(III) when the environment is in redox fluctuation deserves in-depth study. [Methods] The adsorption experiments of Cr(III) were carried out by preparing montmorillonite in three different states: oxidized, reduced, and reduced-re-oxidized, to determine the adsorption value, Fe(II)/total Fe, hydroxyl radical concentration, and valence changes, and the montmorillonite solid materials before and after adsorption were characterized by XPS, XRD, and FTIR. [Results] The results showed that the adsorption rate of Cr(III) by oxidized and reduced montmorillonite increased with increasing pH, and reduced montmorillonite had the highest adsorption rate because of the highest Fe content, and no valence change occurred in these two states. The adsorption rate of reduced and reoxidized state montmorillonite was the smallest, which was due to the rapid activation of oxygen by Fe(II) to produce hydroxyl radicals, Cr(III) was rapidly oxidized to Cr(VI), and the lower the pH was, the higher the oxidation rate was, and it was measured at 8h that Cr(VI) was again reduced to Cr(III) again by the reducing substance Fe(II). The high correlation between the consumption rate of Fe(II) and the generation rate of hydroxyl radicals was verified by the change of Fe(II)/total Fe value and hydroxyl radical concentration, which changed the redox environment. [Conclusion] It can be found through the experiment that hydroxyl radicals are generated after reduction and re-oxidation, which affects the adsorption and desorption of Cr(III), and the strong oxidizing hydroxyl radicals can oxidize Cr(III) to toxic Cr(VI), which verifies the assumption that chromium appears to return to the yellowish color after redox fluctuation in the underground environment.