Microstructural analyses of Cr(VI) speciation in chromite ore processing residue (COPR)

Area of Science:

• Environmental Science
• Geochemistry
• Materials Science

Background:

• Chromite ore processing residue (COPR) is a significant industrial byproduct.
• Understanding chromium(VI) [Cr(VI)] speciation is crucial for environmental risk assessment and remediation.
• Two distinct COPR types, gray-black (GB) and hard brown (HB), exist at US deposition sites.

Purpose of the Study:

• To investigate the solid-phase speciation and distribution of Cr(VI) in GB and HB COPR.
• To characterize the mineralogy and chemistry of these COPR types.
• To identify Cr(VI) binding phases and assess their accessibility for potential treatment strategies.

Main Methods:

• Micro-X-ray absorption spectroscopy (μ-XAS) for Cr(VI) speciation.
• Micro-X-ray diffraction (μ-XRD) for mineralogical analysis.
• Complementary laboratory analyses for chemical characterization.

Main Results:

• GB COPR contains ~30% of total Cr(VI) as large, previously unreported Na-rich calcium aluminum chromate hydrate crystals, potentially vulnerable to treatment.
• Over 50% of Cr(VI) in GB COPR is within nodules, bound to stable phases like hydrogarnet and hydrotalcite, making it less accessible.
• HB COPR has all Cr(VI) diffusely distributed within nodules, primarily bound to hydrogarnet and hydrotalcite, presenting significant treatment challenges due to inaccessibility and stability.

Conclusions:

• Cr(VI) speciation and distribution vary significantly between GB and HB COPR types.
• The presence of accessible Cr(VI) phases in GB COPR offers potential treatment pathways.
• The inaccessible and stable Cr(VI) phases in both COPR types, particularly in HB COPR, necessitate advanced remediation strategies.