Tag Archives: Chromium

03Mar 2012

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

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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

 

12Jan 2011

Enhanced Monitoring of Hexavalent Chromium in Drinking Water: EPA Guideline

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January 12, 2011, WASHINGTON (Coal Geology)– Several weeks ago, EPA Administrator Lisa P. Jackson committed to address hexavalent chromium (also known as chromium-6) in drinking water by issuing guidance to all water systems on how to assess the prevalence of the contaminant. Today, the agency is delivering on that promise and has issued guidance recommending how public water systems might enhance monitoring and sampling programs specifically for hexavalent chromium. The recommendations are in response to emerging scientific evidence that chromium-6 could pose health concerns if consumed over long periods of time.

[ReviewAZON asin="1583215778" display="inlinepost"]“Protecting public health is EPA’s top priority. As we continue to learn more about the potential risks of exposure to chromium-6, we will work closely with states and local officials to ensure the safety of America’s drinking water supply,” said Administrator Jackson. “This action is another step forward in understanding the problem and working towards a solution that is based on the best available science and the law.”

The enhanced monitoring guidance provides recommendations on where the systems should collect samples and how often they should be collected, along with analytical methods for laboratory testing. Systems that perform the enhanced monitoring will be able to better inform their consumers about any presence of chromium-6 in their drinking water, evaluate the degree to which other forms of chromium are transformed into chromium-6, and assess the degree to which existing treatment affects the levels of chromium-6 in drinking water.

EPA currently has a drinking water standard for total chromium, which includes chromium-6, and requires water systems to test for it. Testing is not required to distinguish what percentage of the total chromium is chromium-6 versus other forms such as chromium-3, so EPA’s regulation assumes that the sample is 100 percent chromium-6. This means the current chromium-6 standard has been as protective and precautionary as the science of that time allowed.

EPA LOGO
EPA LOGO

EPA’s latest data show that no public water systems are in violation of the standard. However, the science behind chromium-6 is evolving. The agency regularly re-evaluates drinking water standards and, based on new science on chromium-6, has already begun a rigorous and comprehensive review of its health effects. In September 2010, the agency released a draft of the scientific review for public comment. When the human health assessment is finalized in 2011, EPA will carefully review the conclusions and consider all relevant information to determine if a new standard needs to be set. While EPA conducts this important evaluation, the agency believes more information is needed on the presence of chromium-6 in drinking water. For that reason, EPA is providing guidance to all public water systems and encouraging them to consider how they may enhance their monitoring for chromium-6.

More information on the new guidance to drinking water systems: http://water.epa.gov/drink/info/chromium/guidance.cfm

More information on chromium:
http://water.epa.gov/drink/info/chromium/index.cfm

More information on the status of the ongoing risk assessment:
http://cfpub.epa.gov/ncea/iris_drafts/recordisplay.cfm?deid=221433

26Dec 2010

EPA to develop monitoring and sampling programs specifically for chromium-6

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December 26, 2010, WASHINGTON  (Coal Geology)  EPA Administrator Lisa P. Jackson met with Senators Richard Durbin (IL), Mark Kirk (IL), Debbie Stabenow (MI), Bob Casey (PA), Ben Nelson (NE), Bill Nelson (FL), Daniel Akaka (HI), Dianne Feinstein (CA), Jeff Bingaman (NM), and Jeff Merkley (OR) to brief them on the issue of chromium-6 in drinking water as it relates to this week’s Environmental Working Group (EWG) report.

EPA

EPA

The following is a statement from Administrator Lisa P. Jackson regarding that meeting:

“Yesterday, I briefed members of the Senate on chromium-6 in drinking water supplies as it relates to the recent Environmental Working Group report. EPA has already been working to review and incorporate the ground-breaking science referenced in this report. However, as a mother and the head of EPA, I am still concerned about the prevalence of chromium-6 in our drinking water.

Today, I am announcing a series of actions that the EPA will take over the coming days to address chromium-6 in our drinking water. It is clear that the first step is to understand the prevalence of this problem. While the EWG study was informative, it only provided a snapshot in time. EPA will work with local and state officials to get a better picture of exactly how widespread this problem is. In the meantime, EPA will issue guidance to all water systems in the country to help them develop monitoring and sampling programs specifically for chromium-6.  We will also offer significant technical assistance to the communities cited in the EWG report with the highest levels of chromium-6 to help ensure they quickly develop an effective chromium-6 specific monitoring program.

The science behind chromium-6 is evolving. EPA is already on a path toward identifying and addressing any potential health threats from excessive, long-term exposure with its new draft assessment released this past fall. This assessment still needs to be reviewed by independent scientists as an essential step toward tightening drinking water standards for chromium-6. Strong science and the law will continue to be the backbone of our decision-making at EPA. EPA takes this matter seriously and we will continue to do all that we can, using good science and the law, to protect people’s health and our environment.”

Meeting Readout:

In yesterday’s meeting with the 10 U.S. senators, Administrator Jackson described EPA’s current chromium-6 risk assessment, which is a review EPA immediately started in response to new science in 2008 showing a link between chromium-6 ingestion and cancer. This risk assessment – which would be the first step to updating the drinking water regulations – will be finalized after an independent scientific peer review in 2011. Administrator Jackson told the senators that based on the draft risk assessment, EPA will likely revise drinking water regulations to account for this new science. These revisions would only take place after an independent science panel has verified the underlying science.

Administrator Jackson told the senators that EPA currently requires testing for total chromium which includes chromium-6. She noted that the testing does not distinguish what percentage of the total chromium is chromium-6 versus chromium-3, so EPA’s regulation assumes that the sample is 100% chromium-6. This means the current chromium-6 standard has been as protective and precautionary as the science of that time allowed.

Administrator Jackson told the senators that according to the most recent data, all public water facilities are in compliance with the existing total chromium standards, but she agrees that chromium-6 is a contaminant of concern. She also told the senators that people can have their water tested and install home treatment devices certified to remove chromium-6 if they are concerned about the levels of chromium-6 in their drinking water.

The administrator concluded the briefing by making the following points and commitments:

1)         While provocative, the EWG report is a self-described “snapshot” in time and does not provide a full, long-term picture of the prevalence of chromium-6 in our drinking water. EPA will work with state and local officials to better determine how wide-spread and prevalent this contaminant is.

2)         Meanwhile, EPA will issue guidance to all water systems on how to test for and sample drinking water specifically for chromium-6. This guidance will provide EPA-approved methods and other technical information.

3)         EPA will also offer technical expertise and assistance to the communities cited in the EWG study with the highest levels of chromium. This assistance will include providing technical experts to work with water system operators and engineers to ensure the latest testing and monitoring is being utilized.

4)         Once EPA’s chromium-6 risk assessment is finalized, EPA will work quickly to determine if new standards need to be set. Based on the current draft assessment, which has yet to undergo scientific peer review, it is likely that EPA will tighten drinking water standards to address the health risks posed by chromium-6.

More information on chromium:
http://water.epa.gov/drink/contaminants/basicinformation/chromium.cfm

To track the status of the ongoing risk assessment:
http://cfpub.epa.gov/ncea/iristrac/index.cfm?fuseaction=viewChemical.showChemical&sw_id=1107

16Aug 2010

Chromium Geochemistry: Eh-pH diagram-Toxicity-Stability Diagrams

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Chromium 0.1 ppm with 150 ppm HCO3-SO4-- Brezinaite Formation

Chromium is highly insoluble in water. Chromium can only be in solution at a very low pH below 3.5. In this exercise we will generate Eh-pH diagrams to study stable and  soluble phases. Cr(VI) is much more toxic and mobile than Cr(III). So, speciation of chromium is very important. Hexavalent chromium exists in alkaline, strongly oxidizing environments while Trivalent chromium exists in moderately oxidizing and reduced environments.

Most common oxidation states: 0, +3, +6

  • 0:  Elemental Chromium (Cr)
  • +3: Trivalent Chromium:     Species: Cr+3, Cr2O3
  • +6: Hexavalent Chromium:     Species: CrO42-, Cr2O7-

Drinking Water Standards:

  • MCLG: 0.1 ppm
  • MCL: 0.1 ppm

Chromium content in sea-water varies strongly, and is usually between 0.2 and 0.6 ppb. Rivers contain approximately 1 ppb of chromium. Many chromium compounds are relatively water insoluble. Chromium (III) compounds are water insoluble because these are largely bound to floating particles in water. Hexavalent chromium is very toxic to flora and fauna.

Chromium 0.1 ppm: in pure water
Figure 1: Chromium 0.1 ppm-in pure water

In natural surface and ground water Eh is about 0.5 V. So, at this Eh, chromium is insoluble between 3 to 7. At higher pH chromate and chromite ions would in the solution.

Now, lets add 150 bicarbonate in the solution above. Lime application is common in Acid Mine Drainages where raised alkalinity helps iron to precipitate. Higher pH also makes Manganese insoluble. Ler’s see how alkalinity may affect chromium.

Chromium 0.1 ppm with 150 ppm HCO3-Eh-pH
figure 2: Chromium 0.1 ppm with 150 ppm HCO3-Eh-pH

Figure 2 clearly shows that alkalinity has no effect on the solubility of chromium. As you can tell there are no changes in the solid and dissolved species in the diagram.

Now, let’s add 300 mg/l sulfate in the solution to check effect of SO4– ion.

Chromium 0.1 ppm with 150 ppm HCO3-SO4-- Brezinaite Formation
Figure 3: Chromium 0.1 ppm with 150 ppm HCO3-SO4– Brezinaite Formation

Figure 3 introduces Brezinaite in to the diagram as solid phase. So, under highly reducing condition Cr3S4 (Brezinaite) mineral would precipitate.

Remediation:
Chromium removal from water can be done by  ion exchangers and active carbon can be applied for this purpose.

EPA Fact Sheet on Chromium

Chromium was regarded with great interest because of its high corrosion  resistance and hardness. A major development was the discovery that steel could be made highly resistant to corrosion and discoloration by adding chromium to form stainless steel.Chromium is the 21st most abundant element in Earth’s crust with an average concentration of 100 ppm.