Chemical Transformations of Chromium in Soils:
Relevance to Mobility, Bio-availability and Remediation
prepared by Dr. Bruce R. James, Ph.D., Professor of Soil Chemistry and Director of Environmental Science and Policy, University of Maryland, College Park, USA
Chromium mobility and bio-availability in soils and natural waters are linked to transfers of electrons between this metal and other constituents in these environments, where biological and chemical processes control oxidation and reduction reactions. Gains of electrons (reduction) by electron-poor, hexavalent chromium (Cr (VI)) convert this toxic, soluble anion [negatively-charged species e.g. CrO42-] to electron-rich, trivalent chromium (Cr (III)), a form that is non-toxic, essential for human health, cationic (positively-charged, e.g. Cr3+), and only sparingly-soluble in most natural environments. Losses of electrons (oxidation) by Cr (III) reverse this oxidation state change and increase the solubility of Cr, thereby increasing the potential movement of soil-borne Cr with percolating water. Purposeful reduction of Cr (VI) to Cr (III) can be used in "remediation-by-reduction" schemes in soils to lower the hazards associated with high levels of Cr (VI) without changing the total Cr content of a soil, and in so doing, the potentially-harmful ecological and human health effects of Cr (VI)-enriched soils are minimized.