Hexavalent Chromium

Where hexavalent chromium comes from and why it moves

Hexavalent chromium (Cr(VI), Cr⁶⁺) is the chromium of plating shops, leather tanneries, chromated copper arsenate wood treatment, chromate corrosion inhibitors in cooling towers, pigment and alloy manufacturing, and chromite ore processing residue (COPR) fill. In water it exists as the chromate (CrO₄²⁻) and bichromate (HCrO₄⁻) anions, which are highly soluble and poorly retained by most soils, so a Cr(VI) release produces a plume that travels nearly as fast as the groundwater itself. Cr(VI) compounds are classified as human carcinogens. The federal drinking water standard for total chromium is 100 µg/L; California adopted a separate 10 µg/L limit for hexavalent chromium in 2024, and other states apply site-specific cleanup goals well below the federal number.

Trivalent chromium (Cr(III), Cr³⁺) behaves in the opposite way. In neutral to alkaline water it precipitates as chromium hydroxide, Cr(OH)₃, or as a mixed iron-chromium hydroxide, both of which have very low solubility, and it is far less toxic. That difference is the whole basis of Cr(VI) remediation: reduce it, and the problem precipitates.

The three reductants, and the stoichiometry behind each

Every effective in situ treatment supplies electrons. Cr(VI) needs three of them to become Cr(III). Which reductant to use depends on whether you are injecting into an aquifer, blending excavated soil or building a barrier, and on the geochemistry of the site.

Remotox calcium polysulfide

Remotox is Hepure’s calcium polysulfide, supplied as a 29% CaS₅ solution. In water it dissociates into bisulfide, hydrogen sulfide and other reduced sulfur species, and in the subsurface it also forms thiosulfate; all of these reduce Cr(VI). The chromium then precipitates as Cr(OH)₃, because chromium sulfide is unstable in water. The simplified reaction is CrO₄²⁻ + CaSₓ → Cr(OH)₃ + CaSO₄; the commonly cited overall reaction, 2 CrO₄²⁻ + 3 CaS₅ + 10 H⁺ → 2 Cr(OH)₃ + 15 S + 3 Ca²⁺ + 2 H₂O, works out to 1.5 moles of CaS₅ per mole of Cr(VI), or roughly 6 lb of CaS₅ (about 20 lb of 29% Remotox) per pound of Cr(VI). Real doses run higher because soil organic matter, oxygen, nitrate and ferric iron also consume reductant, which is why we ask for bench results before sizing an order. Remotox works across a wide pH range, with the best results between pH 7 and 10, and the excess polysulfide leaves a reducing buffer zone that keeps treating chromium that migrates into it later. It is the usual choice for direct-push injection, infiltration, excavation backfill and COPR sites. One field note that belongs in every health and safety plan: the reaction releases hydrogen sulfide, so excavation work needs ventilation and monitoring.

Ferrous sulfate

Ferrous sulfate supplies ferrous iron, Fe(II), which reduces Cr(VI) directly in solution: Cr(VI) + 3 Fe(II) → Cr(III) + 3 Fe(III). Three moles of iron per mole of chromium works out to 3.2 lb of Fe(II), or about 16 lb of ferrous sulfate heptahydrate, per pound of Cr(VI), before any competing demand. The chromium and the ferric iron then co-precipitate as a mixed hydroxide. Because ferrous iron is itself oxidized by dissolved oxygen, the dose is applied in excess, and where the water is acidic the pH is raised with sodium hydroxide into the range where Cr(OH)₃ is least soluble. Ferrous sulfate ships as a dry solid in 50-lb bags with a certificate of analysis, which makes it the common choice for ex situ soil blending and for tank-based water treatment, and it is the same ferrous iron that polysulfide generates indirectly when it reduces the ferric iron already in the soil.

Ferox zero valent iron

Ferox zero valent iron reduces Cr(VI) at the iron surface as the iron corrodes: Cr(VI) + Fe(0) → Cr(III) + Fe(III), one mole of iron per mole of chromium on paper, or about 1.1 lb of iron per pound of Cr(VI). In practice far more iron is placed than the stoichiometry asks for, because the iron also corrodes against water and oxygen and because the chromium-iron hydroxide that forms coats the particles. The payoff is longevity: granular Ferox PRB in a permeable reactive barrier treats a Cr(VI) plume passively for years, and the finer Ferox Flow and Ferox Target powders carry the same chemistry into slurry injection, with a rated longevity of 3 to 15 years. How particle size and delivery method are chosen is covered in our zero valent iron remediation guide.

Biological reduction

Where a long-lived reducing zone is wanted and time is available, an electron donor such as emulsified vegetable oil lets native bacteria drive Cr(VI) reduction, both directly and by producing the ferrous iron and sulfide that do the work chemically. It is slower than the chemical reductants and is usually paired with one of them rather than used alone.

Matching the reductant to the site

Situation	Usual choice	Why
Dissolved plume, direct-push injection	Remotox	Liquid, injects easily, wide pH range, leaves a reducing buffer
Excavated soil, backfill or disposal	Remotox or ferrous sulfate blended into the soil	Uniform contact; ferrous sulfate is the dry solid
Long-term passive plume control	Ferox PRB barrier	Years of treatment with no re-injection
Source zone with high concentrations	Ferox Target or Remotox	Fast reaction where mass is concentrated
COPR fill (pH 11 to 12)	Remotox, dose set on the bench	Above the optimal pH range; bench testing sets dose and any pH adjustment
Wastewater or mine water in a tank	Ferrous sulfate or Remotox, then sodium hydroxide	Reduce, raise pH, settle the hydroxide

What decides success at a chromium site

Reductant demand. Dissolved oxygen, nitrate, ferric iron and organic matter all consume reductant before chromium does. The theoretical doses above are floors, not budgets; bench testing on site soil and water turns them into a number you can defend.

pH. Cr(OH)₃ is least soluble between roughly pH 7 and 10. Below that it begins to redissolve, which is why ferrous sulfate treatment of acidic water is followed by caustic, and why polysulfide, which is alkaline as supplied (pH 11 to 11.5), usually needs no separate pH adjustment.

Reoxidation. Manganese oxides in soil can slowly oxidize Cr(III) back to Cr(VI). Leaving a reducing buffer behind, whether excess polysulfide, iron or a bioreduction zone, is the standard guard against rebound, and post-treatment monitoring should look for it.

Delivery. A reductant only treats what it touches. Injection spacing, soil mixing depth and barrier placement follow the same rules as any in situ amendment; see the soil remediation and groundwater remediation pages for the site factors that set them.

What Hepure provides

We supply the chemistry and help you size it: Remotox in pails, drums, totes and tankers; ferrous sulfate in 50-lb bags; Ferox iron from 1-lb and 5-lb samples through 100-lb bags, 1,000-lb barrels and 2,205-lb super sacks; sodium hydroxide as a 25% solution for pH control; and EVO for biological reduction. Send Pat or Bob the Cr(VI) concentration, the volume or mass to be treated, the pH, and whether the work is injection, mixing or a barrier, and you will get a product recommendation and a quantity estimate. We will also help design the bench test. We do not design the remedy or perform the injections; that stays with your engineer and your driller.

Common questions

Will the chromium come back? Cr(OH)₃ is stable under normal subsurface conditions. The exception is contact with manganese oxides, which is why a sustained reducing zone matters more than the initial dose.

How quickly does the reaction happen? Chemical reduction by polysulfide, ferrous iron or zero valent iron is fast, hours to days once the reductant reaches the chromium. The schedule is set by delivery and contact, not by reaction kinetics.

Can one product treat chromium together with other metals? Yes. Remotox precipitates lead, cadmium, copper and zinc as sulfides while it reduces chromium, and Ferox iron reduces or sorbs arsenic, nickel, selenium and others. See the heavy metals overview.

Do I need a bench test? We recommend one every time. Dose depends on reductant demand and geochemistry, and a bench test is cheaper than a second mobilization.