Assessing environmental fate of hexavalent chromium as influenced by fractionation of ferrihydrite with dissolved organic matter.

The dynamic interactions among iron (Fe) oxides, dissolved organic matter (DOM) and toxic trace metals play crucial roles in risk assessment and environmental remediation. Although the inhibitory effects of DOM on the iron oxides transformation process have been studied previously, there is still a lack of mechanistic and quantitative understanding on the kinetics of Cr(VI) and ferrihydrite transformation in the present of DOM. In this study, we investigated the fractionation process of DOM on ferrihydrite and its influence on the fate of Cr(VI) and transformation of ferrihydrite. The result of three-dimension excitation emission matrix (3D-EEM), Q-Exactive LC-MS/MS, X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM) indicated that fulvic acid-like compounds of DOM were the mainly fractionated compounds on the surface of ferrihydrite, which further inhibited the transformation of ferrihydrite. Besides, bracewellite (CrO(OH)) generated as an accompanied mineral during the transformation of ferrihydrite in the present of Cr(VI). Based on the DFT theoretical calculation, we concluded that Cr(VI) mainly in the form of HCr O4- was more inclined to be adsorbed on iron-oxide tetrahedron by inner-sphere monodentate mononuclear configurations. The findings on the dynamic coupling among Fe oxide transformation and Cr(VI) sequestration under the effect of DOM provided the basis for accurately predicting the fate of trace elements and iron mineral.