Synchrotron x-ray absorption fine structure (XAFS) spectroscopy provides quantitative information on the speciation of Cr in complex oxide samples, including the Cr(VI):Cr(III) ratio, without any sample pretreatment. We discuss several XAFS methods that can be used for this purpose, including: (1) measurement of the height of the pre-edge peak of Cr K-edge spectra which is caused by ls→3d electronic transitions occurring predominately in the Cr(VI)O4 tetrahedron; (2) fitting observed XANES spectra using combinations of experimental spectra from Cr(III)- and Cr(VI)-containing model compounds; (3) fitting observed EXAFS spectra using combinations of experimental spectra from Cr(III)- and Cr(VI)-containing model compounds; and (4) ab-initio modeling of observed Cr K-EXAFS spectra using theoretical phase-shift and amplitude parameters for Cr(III)O6 and Cr(VI)O4 models. The first method is shown to be the simplest and most accurate. The samples examined using these methods include highly pure model compounds, Cr3+
2O3 and Na2Cr6+O4·4H42O, as well as more complex soil samples taken from Cr-contaminated industrial sites and Cr waste disposal pits. We present XAFS evidence for the reduction of Cr(VI) to Cr(III) after reaction with the Fe(Il)-bearing oxide magnetite, Fe3O4, and show that Cr associated with magnetite in soils is reduced, whereas that associated with non-Fe(II)-bearing silicate minerals is not. Also presented are ab-initio multiplescattering calculations of XANES spectra of an Cr(VI)O4 tetrahedron, which help explain the XANES features of this Cr species in solids and aqueous solutions.