Application of the Rietveld method to assess chromium(VI) speciation in chromite ore processing residue

The Rietveld method allows the quantification of crystalline phases and amorphous material identified by X-ray powder diffraction (XRPD) and other diffraction methods. The method assists in determining the speciation of contaminants in solid matrices both qualitatively and quantitatively in a statistically defensible approach, as it does not focus on a microscale. Rietveld was applied to chromite ore processing residue (COPR), a cementitious waste containing hexavalent chromium. Calcium aluminum chromium oxide hydrates (CACs) were the crystalline phases identified by XRPD that bind Cr6+ in COPR according to their chemical formula. Rietveld quantification, combined with mass balances on Cr6+, showed that CACs may bind Cr6+ in variable percentages, ranging from 25% to 85%. Analysis of duplicate samples showed that material variability is the predominant factor of uncertainty in evaluating the role of CACs in Cr6+ speciation, provided that a consistent quantification strategy is pursued. The choice of strategy was performed on the basis of the pertinent literature, preliminary analyses of the equipment and the software settings, and mass balances. The correlation between the average CAC-bound Cr6+ concentration and the total Cr6+ for five samples (R2 = 0.94), extracted from different zones and soil borings, suggests that CACs are a primary sink for Cr6+ in COPR.