Hydrogarnet: a host phase for Cr(VI) in chromite ore processing residue (COPR) and other high pH wastes

For understanding both the environmental behavior and developing remediation treatments for chromium ore processing residue (COPR) it is important to identify all the potentially soluble sources of Cr(VI). Hydrogarnet has been identified as a major phase in COPR and it has been previously speculated that it has a capacity to host Cr(VI). Here we provide direct evidence of this capacity by demonstrating the incorporation of Cr(VI) into laboratory synthesized hydrogarnet. Electron microscopy and energy dispersive X-ray microanalysis show incorporation of approximately 17000-22000 mg Cr(VI) kg(-1) hydrogarnet. X-ray powder diffraction data show that peak intensities are altered by chromium substitution and that chromium substituted hydrogarnets have a smaller unit cell than the pure Ca-Al end member. This is consistent with substitution of hydroxyl tetrahedra by smaller chromate tetrahedra. Electron energy loss spectroscopy confirms the tetrahedral coordination and hexavalent oxidation state of chromium in the hydrogarnets. The maximum amount of hexavalent chromium that can be introduced synthetically corresponds to a replacement of about one out of every eight hydroxyl tetrahedral per unit cell by a CrO4(2-) tetrahedra and tallies closely with the amount of chromium measured in hydrogarnets from COPR. Chromium-bearing hydrogarnet is the most abundant crystalline phase in millions of tons of COPR contaminating land around Glasgow, Scotland, and was recently identified in COPR from sites in North America. Calculations based on its abundance and its Cr(VI) content indicate that hydrogarnet can host as much as 50% of the Cr(VI) found in some COPR samples.