Vertical migration in the soil of Cr(VI) and chromite ore processing residue: Field sampling and benchtop simulation

Chromite ore processing residue (COPR) keeps releasing Cr(VI) over time, and the mixing of residual COPR into soil makes the remediation of COPR-contaminated sites challenging. In this study, a sample of COPR and two soil profiles were collected from a typical historical COPR-contaminated site, and the vertical migration of Cr(VI) and COPR particles in contaminated soil was simulated in the laboratory. Cr(VI) was detected in the upper layer of the field samples at thousands of milligrams per kilogram even after decades of aging, and it can be leached out and migrate vertically deep into the surrounding soil and groundwater. In the COPR-containing soil, more diverse hydrated minerals of brownmillerite were produced than the COPR in the open air on the site. Minerals with high Cr content in COPR-containing soils have a relatively high proportion of particles smaller than 10 µm. COPR particles smaller than 5 µm were found to have migrated downward into the deep soil. During simulated one-year of precipitation, 578.9 mg Cr(VI)/kg was leached from COPR, while 35.5% of the COPR particles smaller than 5 µm had the potential to migrate vertically. The management of COPR particles should be emphasized during risk management or remediation of COPR-contaminated sites.