Chromium Eh-pH and pe-pH diagrams, solubility, remediation, speciation : Guide for Chemical Modeling

Chromium is one of the RCRA elements and considered teratogenic, mutagenic and carcinogenic. In this article, we will explore the basic geochemical properties of chromium. If you are involved in any environmental projects involving dissolved chromium, the Eh-pH diagrams may help. This is my own version of CHROMIUM 101! Article by Ankan Basu, P.G (Contact – admin@coalgeology.com). This article is an advanced form of my earlier example of chromium geochemistry with REAL sample (http://coalgeology.com/chromium-geochemistry-eh-ph-diagram/5642/).

What is the national primary drinking water standard for chromium?

USEPA standard for total chromium is 0.1 mg/L (note, no MCL is currently developed for different species of chromium)

What is Hexavalent Chromium?

Hexavalent chromium, or chromium (VI), is one the three most common forms of chromium. Hexavalent form of chromium is carcinogenic with the other forms are not.

What is the crustal abundance of chromium?

  • Crustal abundance – 100 mg/Kg
  • In granites – 20 mg/Kg
  • In ultramafics – 2000 mg/Kg
  • Black Shale – 20 to 3000 mg/Kg
  • Soil (worldwide): ~200 mg/Kg
  • US soil: 1.0 to 2000 mg/Kg (average 54mg/Kg)
  • Rainwater: 0.2 to 1.9 microgram/L
  • Groundwater: generally less than 0.1 microgram/L
  • Sea water: average 0.3 microgram/L

 

What is the average concentration of chromium in Coals?

Average concentration of chromium concentration is US coals is 15 mg/Kg.

What are some of the common use of chromium?

The greatest use of chromium is in metal alloys (cast iron, stainless steel; protective coatings) and pigments for paints, cement, paper, rubber, composition floor covering and other materials.

Learn about chromium from Q&A:

  • Name a mineral with Cr (VI): Crocoite – PbCrO4. Generally Cr(VI) minerals are rare in nature. Most of the deep earth minerals with chromium have +3 oxidation states.
  • What are the oxidation states of chromium?  0, +3 and +6 oxidation states found in nature.
  • What form of chromium is dominant in oxygen rich environment? In pH condition greater than 2 and under oxidizing conditions, Cr(III) is thermodynamically unstable and converts to Cr(VI).

Example of chromium compounds:

  • Cr (0): Native chromium, rare in nature.
  • Cr (III): chromium oxide (Cr2O3), chromium hydroxide [Cr(OH)3], Cr(OH)++, Cr(OH)2+
  • Cr(VI): Chromate (CrO4–) and dichromate (Cr2O7-2)

Did you know?

  • Evaporites and chromium: In the Atacama Desert of South America, many Cr(VI) minerals have been found.  Na, K, Ca and Ba are found to form minerals with both chromate (CrO4–) and dichromate (Cr2O7–).
  • Chromium and Paradise Valley, Arizona: 100s to 1000s microgram/L of chromium (Cr+6) is naturally present in the groundwater at the Paradise Valley of Arizona.
  • Chromium has 26 known isotopes; four of them are stable and naturally occurring.
  • Earth’s mantle has chromium concentration between .41-.55 percent.
  • Chromite ore is NOT actively mined in the United States, Canada or Mexico. US stopped mining for chromium in 1961.
  • Primary chromite deposits are ONLY associated with ultra-mafic rocks either as stratiform or podiform deposits.
  • Precambrian Stillwater complex in Montana (sill) is a basic layered intrusion, known for high chromium reserve. Bushveld Complex of South Africa is another example of chromium complex.

Grades of chromium grade:

  • Chemical grade: 28.6% average chromium
  • Metallurgical grade: 28.6% average chromium
  • Refractory grade: average 23.9% chromium

 

What are the chemical conditions that favor Cr(VI) stay dissolved in ground water?

  • Oxygen rich groundwater
  • Neutral to alkaline pH
  • Moderate to high concentration of other anions such as sulfate

General geochemistry of chromium:

  • Speciation of chromium in natural water depends on concentration of chromium and pH.
  • CrO4-2 dominates above pH=6.
  • HCrO4- dominates between 0 to 6 pH with low Cr(VI) activity.
  • Cr2O7-2 dominates between 0 to 6 pH with high Cr(VI) activity.
  • H2CrO4 dominates when pH<0
  • Cr(III) speciation: As pH increases, the dominant species changes through Cr+3, CrOH++, Cr(OH)+, Cr(OH)3aq, Cr(OH)4-.
  • Cr(VI) Speciation: At higher oxidizing condition, the changes are HCrO4-, CrO4-2, CrO4-3 (See Eh-pH diagram)
  • Cr is known to form complexes with organic ligands.
  • Cr+6 form soluble compounds with alkali and alkali earth metals.
  • In presence of high Mg, chromium forms magnetiochromite and precipitates (see Eh-pH diagram)
Chromiun Eh-pH diagram with Cr activity= 1e-4

Chromiun Eh-pH diagram with Cr activity= 1e-4

Chromium Eh-pH diagram with Cr activity=1e-10

Chromium Eh-pH diagram with Cr activity=1e-10

Chromium pe-pH diagram with Cr activity=1e-10

Chromium pe-pH diagram with Cr activity=1e-10

 

Adsorption of Chromium:

  • Chromium adsorbs to mineral surfaces better as pH decreases. (Lead in the other hand adsorbs better as pH increases)
  • Adsorption of Cr(VI) on soils and sediments depends of the composition of the soil/sediment.

Natural Attenuation of Chromium:

  • Cr+6 is carcinogenic. Cr+3 is not. The reduction of hexavalent chromium to trivalent chromium is important in natural attenuation.
  • Low pH and high dissolved organic carbon promotes the reduction of chromium from +6 to +3 state.
  • Fe+2 could reduce Cr+3 to Cr+3 over wide range of pH

Colloidal transport of chromium:

  • Just like lead, chromium could also transport as part of the colloid matter in ground and surface water.

Site characterization for chromium remediation:

  • Oxidation – reduction of chromium is characterized by dis equilibrium. Eh-pH diagram may not properly characterize the natural condition.
  • Identify dissolved species of chromium in the aquifer. (Cr+6 is most mobile).

 

This article is based on http://www.epa.gov/nrmrl/pubs/600R07140/600R07140.pdf. Along with other sources and speciation diagrams generated using Geochemist’s Workbench.

 

 

References

·         Chromium-6 in Drinking Water Sources:  Sampling Results: http://www.cdph.ca.gov/certlic/drinkingwater/pages/chromium6sampling.aspx

·         Drinking Water Contaminants- Chromium: http://www.freedrinkingwater.com/water-contamination/chromium-contaminants-removal-water.htm

·         Low-Pressure Cr Speciation in Drinking Water using the SC-DX chromFAST System with ICPMS Detection http://www.icpms.com/products/chromfast-cr-speciation.php

Analytical labs for chromium analysis in USA (only few listed)

Consulting firm for chromium:

Marshall Miller and Associates; http://www.mma1.com, contact Ankan Basu, P.G at admin@coalgeology.com

 

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