The +6 oxidation state of chromium, a Class 1 human carcinogen by inhalation and ingestion, more mobile in groundwater than the less-toxic trivalent form. Historical sources include chrome plating, leather tanning, wood preservation, and cooling-tower corrosion inhibitors.
Hexavalent chromium is highly soluble and mobile in groundwater, forms long dissolved plumes, and drives drinking-water standards measured in micrograms per litre (U.S. EPA total chromium MCL 100 µg/L; California Cr(VI)-specific MCL 10 µg/L; Health Canada total chromium guideline 50 µg/L). Inhalation of Cr(VI)-bearing dusts and aerosols is the dominant occupational hazard at plating and tanning operations.
Remediation typically targets reduction of Cr(VI) to the less mobile, less toxic Cr(III) form via in-situ chemical reduction with ferrous iron, zero-valent iron, calcium polysulfide, or sodium dithionite. Stabilization/solidification with reducing amendments is the dominant ex-situ remedy. Re-oxidation under naturally oxic conditions is a documented long-term failure mode for some sites, which is why monitoring continues for years after treatment.
Also known as: hexavalent chromium, Cr(VI), chromium VI, chromate.
Hexavalent Chromium (Cr(VI)): treatment approach
The oxidized, mobile, IARC Group 1 carcinogenic form of chromium — regulated at 10 μg/L in California drinking water — whose remediation is defined by in-situ chemical reduction to immobile, non-toxic Cr(III).
Per- and polyfluoroalkyl substances: a class of more than 12,000 synthetic compounds built around exceptionally stable carbon-fluorine bonds. PFAS is the family name, not a single chemical. PFOA and PFOS are two specific members of this class.
Perfluorooctanoic acid: a specific eight-carbon long-chain PFAS compound, historically used in non-stick coatings and AFFF firefighting foam. Classified by IARC in 2023 as Group 1, carcinogenic to humans.
Perfluorooctane sulfonic acid: a specific eight-carbon long-chain PFAS compound, historically the active surfactant in 3M Scotchgard and AFFF firefighting foam. Classified by IARC in 2023 as Group 2B, possibly carcinogenic to humans.
Polychlorinated dibenzo-p-dioxins and related furans: highly persistent, fat-soluble chlorinated compounds formed as unintended byproducts of combustion, chlorine-based manufacturing, and herbicide production, with TCDD among the most toxic substances ever characterized.