Identification of the key host phases of Cr in fresh chromite ore processing residue (COPR)

[Display omitted] •Multiple quantification methods were used to study the Cr speciation in fresh COPR.•Quantification of all the Cr speciation in fresh COPR was achieved.•Brownmillerite, hydrogarnet and amorphous phase are the key host phases of Cr(VI).•Spinel and amorphous phase are the key host phases of Cr(III). Chromite ore processing residue (COPR) poses a serious Cr(VI) pollution to the environment, and ascertaining the Cr speciation in COPR is significant for guiding remediation. In this study, a systematic investigation on the Cr speciation in fresh COPR was carried out by multiple quantification methods as follows: i) via X-ray absorption spectroscopy (XAS), it was determined that 35% of the total Cr is Cr(VI); ii) the host phases of Cr(VI) and Cr(III) were identified and their Cr content were analyzed by X-ray diffraction (XRD), scanning electron microscopy combined with energy dispersive spectroscopy (SEM-EDS) and leaching tests; iii) the weight percent of each Cr host phase was determined by EDS-assisted quantitative phase analysis; iv) the Cr occupancy percentage of each Cr host phase was determined by integrative calculation based on the above analysis. Results indicate that brownmillerite, hydrogarnet and amorphous phase are the key host phases of Cr(VI), which hold 24.2%, 19.6% and nearly 50% of the total Cr(VI), respectively; spinel and amorphous phase are the key host phases of Cr(III), which hold 25.4% and 71.9% of the total Cr(III), respectively. This study has improved the understanding of Cr speciation in COPR, which is significant for developing effective and practical remediation technology. The quantification methods employed in this study can be extended to research on the speciation of Cr or other metals in other solid wastes.