Accelerated carbonation of fresh and aged chromite ore processing residues (COPR): Enhanced Cr(VI) release and impact on layered double hydroxides (LDH)

Chromite ore processing residue (COPR) has caused large-scale Cr(VI) contamination of soil and groundwater worldwide. In Kanpur, India, vast quantities are generated during the production of Cr-based leather tanning salts and often deposited on open landfills. We previously hypothesized that under such conditions COPR naturally sequester CO 2 , which might promote additional Cr(VI) release. Here, we tested this in accelerated carbonation experiments on fresh and aged COPR. After quantifying the CO 2 uptakes, native and carbonated COPR were tested for Cr(VI) mobility by aqueous batch leaching. To unravel the effects of CO 2 uptake on COPR, we further conducted pH-adjusted and Na 2 CO 3 -spiked leaching. Mineralogical changes were assessed by thermogravimetry-mass spectrometry (TG-MS). Unlike the aged, hence naturally carbonated COPR, the fresh COPR showed high CO 2 uptakes of up to 5.6 wt.-%, causing a pH drop and increasing Cr(VI) leachability by up to 450 mg ∙ L −1 . Modified leaching revealed CO 3 2− had a much stronger effect on Cr(VI) release than pH drop. TG-MS indicated that upon carbonation, calcite and hydrotalcite precipitated by consuming hydrocalumite and katoite, thus liberating their hosted Cr(VI) and shifting the mineralogy of the fresh COPR towards the aged material. Ongoing carbonation likely exacerbates groundwater pollution at COPR dumpsites, precluding natural attenuation. • CO 2 uptake by COPR as precipitated calcium carbonate was confirmed experimentally • It lowers the material’s pH and significantly enhances the leachability of Cr(VI) • CO 2 exposure shifts layered double hydroxide assemblage, liberating hosted Cr(VI) • This exacerbates groundwater pollution at COPR sites calling for urgent remediation • CO 3 2− -assisted soil washing may be viable as a COPR remediation process