Oxidation State, Local Structure, and Ab-Initio XAFS Modeling of Chromium in Contaminated Soils and Model Compounds

Abstract
We have used XAFS spectroscopy, combined with the ab-initio multiple scattering code FEFF 7, to model the structures of Cr compounds and Cr sorption complexes and precipitated phases on soils impacted by industrial Cr effluent. We have had success in ab-initio modeling of the near-edge structure of Cr model compounds, accurately simulating the Cr pre-edge and edge fine structure which are due to bound-state transitions and multiple scattering among the absorber Cr and its nearest shell of oxygen atoms. We have calibrated the pre-edge feature, a 1s to 3d electronic transition, and applied this to determining Cr(VI)/Cr(III) ratios in natural, Cr-contaminated soils. The pre-edge feature is particularly useful in following the electron transfer reaction between Cr(VI) and Fe(II) on oxide surfaces such as Fe₃O₄ (magnetite) which, as we have seen in both lab and field studies, results in the reduction of Cr(VI) to Cr(III).