Chromium VI

Elemental chromium (CAS 7440-47-3) is a transition group metal that can have oxidation states ranging from -2 to +6. The most commonly occurring states in chromium compounds are +2, +3, and +6 with the +2 being unstable and readily oxidized to +3. Cr(III) compounds are the most stable form. Chromium metal is not naturally occurring and the presence of Cr(VI) can generally be attributed to industrial activity. Most Cr(III) compounds are only sparingly soluble at the normal range of groundwater pH (5.5-8.0) while some Cr(VI) compounds can be very soluble. Table 1 lists some of the more common Cr(VI) compounds and their properties.

Table 1. Common Chromium IV Compounds

Compound	Form	Density (gm/cm3)	Solubility in Water
Calcium Chromate CaCrO4	Yellow Crystal or Powder	2.89	Slightly Soluble
Chromium Trioxide CrO3	Dark Red or Brown Crystal, Flake, or Powder	2.7 (25ºC)	Soluble
Sodium Chromate Na2CrO4	Yellow Crystal	2.71-2.74	Soluble
Sodium Dichromate Na2Cr2O7	Orange Red Crystal	2.52 (13ºC)	Soluble
Potassium Chromate K2CrO4	Yellow Crystal	2.73 (18ºC)	Soluble
Potassium Dichromate K2Cr2O7	Orange Red Crystal	2.68 (25ºC)	Soluble
Lead Chromate PbCrO4	Yellow Orange or Red Crystal or Powder	6.3	Insoluble
Strontium Chromate SrCrO4	Yellow Monoclinic Crystal	3.9 (13ºC)	Slightly Soluble
Zinc Chromate ZnCrO4* 7H2O	Lemon Yellow Crystal	3.40	Insoluble

(Report on Carcinogens)

Cr(VI) generally exists in water in the monomeric state (HCrO₄^- and CrO₄^-2) or bimeric state (Cr₂O₇^-2). Monomeric species impart a yellow color to water while Cr₂O₇^-2 has an orange color. The relative concentrations of these species is both pH and concentration [Cr(VI)] dependent. Figure 1 displays an example of the relation to pH and Figure 2 to concentration (Palmer and Puls, 2004). Cr(VI) is a strong oxidant and is reduced in the presence of electron donors. These donors are generally found in a reduced subsurface environment where such ions as ferrous iron, reduced sulfur, and some organic materials occur. Dichromate has been shown to react with soil organic carbon to produce water, Cr(III), and CO₂ with Cr(III) likely to precipitate as a hydroxide (Palmer and Puls, 2004). As a precautionary note, insitu oxidation with permanganate produces MnO₂, which has been shown to convert Cr(III) to Cr(VI).

Figure 1. Distribution of Cr(VI) species as a function of pH

Figure 2. Fraction of HCrO₄^- and Cr₂O₇^-2 at pH 4 as a function of total Cr(VI) concentration.

For Further Information

Behavior of Metals in Soils
McLean, J.E. and B.E. Bledsoe.
EPA 540-S-92-018, 25 pp, 1992

This paper covers the fundamental processes that control the mobility of metals (lead, chromium, arsenic, cadmium, nickel, zinc, copper, mercury, silver, and selenium) in soil and describes laboratory methods used to evaluate the behavior of metals in soil.

Chromium Isotopes and the Fate of Hexavalent Chromium in the Environment
Andre S. Ellis, Thomas M. Johnson, and Thomas D. Bullen.
Science, 295(5562):2060-2062, 15 Mar 2002

Chromium(VI) Reduction Pathway Map
Jennifer Dommer, University of Minnesota.
The University of Minnesota Biocatalysis/Biodegradation Database

Evaluation of Chromium Mobility in an Electrokinetic Environment
C.M. Fetters, MS thesis, Mississippi State University.
University Microfilm, Ann Arbor, MI. ISBN: 0-496-26705-1, Publication AAT 1421964, 178 pp, 2004

When a synthetic soil matrix was tested in conjunction with four common ion pairs found in soils and ground water, the interaction of the indigenous ions was sufficient to inhibit the effectiveness of an electrokinetic remediation process, and the mobility of chromium through the soil was altered in the presence of high concentrations of the indigenous ions.

Geochemical Controls on Chromium Occurrence, Speciation, and Treatability
J. Hering and T. Harmon.
IWA Pub., London. AwwaRF Report 91043F, ISBN: 1843399253, 164 pp, July 2005 [Originally released to Awwa Research Foundation subscribers in 2004]

Though the occurrence of Cr(VI) in ground water is often attributed to industrial contamination, it can also derive from natural sources, specifically the weathering of Cr-containing aquifer minerals. This report describes research to assess the influence of oxidizing conditions on the release of Cr(VI) from Cr(III)-containing minerals, to predict the potential for Cr accumulation in recovered water, and to investigate redox-assisted coagulation with Fe(II) as a technology for Cr(VI) removal.

Geochemistry of an acidic chromium sulfate plume
L. Edmond Eary and Andy Davis
Applied Geochemistry Volume 22, Issue 2, February 2007, Pages 357-369

This article describes equilibrium modeling of a denser than water chromium sulfate plume.

Guidance for Developing Ecological Soil Screening Levels
U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response
OSWER Directive 9285.7-55

Provides background for developing ecological screening levels for a variety of chemicals including hexavalent chromium.

Handbook of Elemental Speciation, II: Species in the Environment, Food, Medicine and Occupational Health
R. Cornelis, J. Caruso, H. Crews, and K. Heumann, eds.
John Wiley & Sons, New York. ISBN 0-470-85598-3, 768 pp, 2005.

Covers the speciation of elements from aluminum to zinc, including arsenic, chromium, and mercury.

Influence of soil minerals on chromium(VI) reduction by sulfide under anoxic conditions
Yeqing Lan, Baolin Deng, Chulsung Kim, and Edward C Thornton
Geochem Trans, v.8; 2007

This article reports the effects of soil minerals on chromate reduction by sulfide in the pH range of 7.67 to 9.07 under anoxic conditions. The examined minerals included montmorillonite, illite , kaolinite, aluminum oxide, titanium oxide (primarily anatase), and silica.

Kinetics of Chromium (VI) Reduction by Ferrous Ion
Bill Batchelor, Mark Schlautman, Inseong Hwang, and Renjin Wang
Amarillo National Resource Center for Plutonium, 33 pp, 1998

This report discusses laboratory experiments that indicated that chromium was rapidly and stoichiometrically reduced by Fe(II) in solution. Also, slurry experiments showed that the aquifer solids removed Fe(II) from solution, but a portion of the iron removed remained available for reaction with Cr(VI), but at a slower rate. A model to predict different amounts of iron pseudo-components was developed, which allowed prediction of iron amounts required to reduce chromium under in situ conditions in the vadose zone.

Natural Attenuation of Hexavalent Chromium in Groundwater and Soils. EPA Ground Water Issue
Carl D. Palmer and Robert W. Puls.
EPA 540-5-94-505, 12 pp., 1994
Contact: Robert Puls, puls.robert@epa.gov

This paper explores what is known about the transformation of chromium in the subsurface and identifies conditions where it is most likely to occur.

Partition Coefficients for Metals in Surface Water, Soil, and Waste
Jerry D. Allison (HydroGeoLogic, Inc., Herndon, VA); Terry L. Allison (Allison Geoscience Consultants, Inc., Flowery Branch, GA).
Report No: EPA 600-R-05-074, 93 pp., July 2005

This report presents metal partition coefficients for the surface water pathway and for the source model used in the multimedia, multi-pathway, multi-receptor exposure and risk assessment (3MRA) technology under development by U.S. EPA. Literature searches, statistical methods, geochemical speciation modeling, and expert judgment were used to provide reasonable estimates of partition coefficients for antimony, arsenic, barium, beryllium, cadmium, chromium, cobalt, copper, lead, molybdenum, mercury, methylated mercury, nickel, selenium, silver, thallium, tin, vanadium, and zinc.

Retention and Transport of Hexavalent Chromium in Calcareous Karst Soils
Irfan Yolcubal and Nihat Hakan Akyol
Turkish Journal of Earth Sciences, Vol. 16, 2007, pp. 363-379.

Column studies that show that hexavalent chromium is very mobile in calcareous soils.

The Reduction Kinetics of Hexavalent Chromium by Soluble Fe(II) and Magnetite
Y. J. Jung and W. Lee
Geophysical Research Abstracts, Vol. 7, 04530, 2005

This study investigates the kinetics and chemical stoichiometry for the homogeneous and heterogeneous Cr(VI) reductions by soluble Fe(II) and Fe(II)-bearing soil mineral (magnetite, Fe₃O₄) in batch systems. The soluble Fe(II) in the homogeneous solution was more reactive than surface Fe(II) on magnetite in the heterogeneous suspension, when the Fe(II) content was the same in each system.

Understanding Variation in Partition Coefficient, K_d, Values, Volume II: Review of Geochemistry and Available K_d Values for Cadmium, Cesium, Chromium, Lead, Plutonium, Radon, Strontium, Thorium, Tritium (3H), and Uranium
U.S. Environmental Protection Agency, Office of Air and Radiation.
EPA 402-R-99-004B, 20 pp., 1994
Contact: Ronald G. Wilhelm, wilhelm.ron@epa.gov

For those cases when the partition coefficient parameter is not or cannot be measured, this volume provides the following assistance:

• a "thumb-nail sketch" of the key geochemical processes affecting the sorption of the selected contaminants;
• references to related key experimental and review articles for further reading;
• identification of the important aqueous- and solid-phase parameters controlling the sorption of these contaminants in the subsurface environment under oxidizing conditions; and
• where possible, minimum and maximum conservative partition coefficient values for each contaminant as a function of the key geochemical processes affecting their sorption.