Use of micro X-ray absorption spectroscopy and diffraction to delineate Cr(VI) speciation in COPR

The speciation of Cr(VI) in Cromite Ore Processing Residue was investigated by means of
bulk XRD, and a combination of micro-XRF, - XAS and –XRD at the Advanced Light Source
(ALS), Berkeley, CA, U.S.A.. Bulk XRD yielded one group of phases that contained explicitly
Cr(VI) in their structure, Calcium Aluminum Chromium Oxide Hydrates, accounting for 60% of
the total Cr(VI). Micro-analyses at ALS yielded complimentary information, confirming that
hydrogarnets and hydrotalcites, two mineral groups that can host Cr(VI) in their structure by
substitution, were indeed Cr(VI) sinks. Chromatite (CaCrO4) was also identified by micro-
XRD, which was not possible with bulk methods due to its low content. The acquisition of
micro-XRF elemental maps enabled not only the identification of Cr(VI)-binding phases, but
also the understanding of their location within the matrix. This information is invaluable when
designing Cr(VI) treatment, to optimize release and availability for reduction.