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Hexavalent chromium

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"Hex chrome" redirects here. For the color printing process, see Hexachrome.
An example of a chromium(VI) compound: chromium trioxide

Hexavalent chromium (chromium VI) refers to chemical compounds that contain the element chromium in the +6 oxidation state. Virtually all chromium ore is processed via hexavalent chromium, specifically the salt sodium dichromate. Approximately 136,000,000 kilograms (300,000,000 lb) of hexavalent chromium were produced in 1985.[1] Other hexavalent chromium compounds are chromium trioxide and various salts of chromate and dichromate. Hexavalent chromium is used for the production of stainless steel, textile dyes, wood preservation, leather tanning, and as anti-corrosion and conversion coatings as well as a variety of niche uses.

Hexavalent chromium is recognized as a human carcinogen via inhalation.[2] Workers in many different occupations are exposed to hexavalent chromium. Problematic exposure is known to occur among workers who handle chromate-containing products as well as those who arc weld stainless steel.[2] Within the European Union, the use of hexavalent chromium in electronic equipment is largely prohibited by the Restriction of Hazardous Substances Directive.

Toxicity

Hexavalent chromium is transported into cells via the sulfate transport mechanisms, taking advantage of the similarity of sulfate and chromate with respect to their structure and charge. Trivalent chromium, which is the more common variety of chromium compounds, is not transported into cells.

Inside the cell, Cr(VI) is reduced first to metastable pentavalent chromium (Cr(V)), then to trivalent chromium (Cr(III)). Chromate-dyed textiles or chromate-tanned leather shoes can cause or exacerbate contact dermatitis. Vitamin C and other reducing agents combine with chromate to give Cr(III) products inside the cell.[3]

Hexavalent chromium compounds are genotoxic carcinogens. Chronic inhalation of hexavalent chromium compounds increases risk of lung cancer (lungs are especially vulnerable, followed by fine capillaries in kidneys and intestine). According to some researchers, the damage is caused by hydroxyl radicals, produced during reoxidation of pentavalent chromium by hydrogen peroxide molecules present in the cell. Strontium chromate is the strongest carcinogen of the chromates used in industry. Soluble compounds, like chromic acid, are much weaker carcinogens.[3]

In the U.S., the OSHA PEL for airborne exposures to hexavalent chromium is 5 µg/m3 (0.005 mg/m3).[4][5]

Air and water pollution

United States

Major cities

In 2010, the Environmental Working Group studied the drinking water in 35 American cities known to have high Chromium levels in their water supply. The study was the first nationwide analysis measuring the presence of the chemical in U.S. water systems. The study found measurable hexavalent chromium in the tap water of 31 of the cities sampled, with Norman, Oklahoma, at the top of list; 25 cities had levels that exceeded California's proposed limit.[6]

Hinkley, California

Main article: Hinkley groundwater contamination

Hexavalent chromium was found in drinking water in the southern Californian town of Hinkley and was brought to popular attention by the involvement of Erin Brockovich. The 0.58ppm Chromium VI in the groundwater in Hinkley exceeded the Maximum Contaminant Level (MCL) of 0.10 ppm for total Chromium currently set by the United States Environmental Protection Agency(EPA).[7] There is currently no drinking water MCL established by the EPA for Chromium VI. The U.S. state of California has proposed, but not enacted, an MCL of 0.1 parts per billion.

A controversial studied claimed that from 1996 to 2008, 196 cancers were identified among residents of the census tract that includes Hinkley — a slightly lower number than the 224 cancers that would have been expected given its demographic characteristics.[8][9] 196 cases over 12 years for a population of 1915 equates to roughly 853 cases per 100k population per year. By comparison San Bernadino County averaged 359/100k/yr over the same period.[10]

Average Cr(VI) levels in Hinkley were recorded as 1.19 ppb with a peak of 3.09 ppb. The PG&E Topock Compressor Station averaged 7.8ppb and peaked at 31.8ppb.[11] Compare to the California proposed health goal of 0.06ppb. The same day the study came out, the plume of contaminated water was reported to be spreading.[12] Ongoing cleanup documentation is maintained at California EPA's page. Cr(VI) contaminated water supply is apparently a widespread problem and not isolated to Hinkley.[13]

Midland, Texas

In June 2009, the ground water in Midland, Texas (US), was found to be contaminated with chromium. The Midland groundwater reached higher levels of contamination than in Hinkley with 5250 ppb or 5.25 ppm.[14]

Davenport, California

The Unified Air Pollution Control District reported high airborne levels of chromium(VI) at an elementary school and fire department in Davenport, California. The substance apparently originated from a local Cemex cement plant. The levels of chromium(VI) were eight times the air district's acceptable level at Pacific Elementary School and ten times at the Davenport Fire Department.[15] The levels detected did not exceed EPA limits. However, the air samples taken by the air district from June to August at the elementary school and fire department in Davenport registered measurements of hexavalent chromium that were up to ten times higher than allowed by Californian environmental standards.[16] The case highlights the previously unrecognized possible release of chromium(VI) from cement-making.[16][17]

Iraq

In 2008, defense contractor KBR was alleged to have exposed 16 members of the Indiana National Guard, as well as its own workers, to hexavalent chromium in Iraq in 2003.[18] Later, 433 members of the Oregon National Guard's 162nd Infantry Battalion were informed of possible exposure to hexavalent chromium while escorting KBR contractors.[19] One of the National Guard soldiers, David Moore, died in February 2008. The cause was lung disease at age 42. His death was ruled service-related. His brother believes it was hexavalent chromium.[20]

Greece

Thiva – Tanagra – Malakasa Basin

In the Thiva – Tanagra – Malakasa basin, Eastern Sterea Hellas, Greece,[21] which supports many industrial activities, concentrations of chromium (up to up to 80 μg/L Cr(VI)) and inofyta (up to 53 μg/L Cr(VI) were found in the urban water supply of Oropos. Cr(VI) concentrations ranging from 5 to 33 μg/L Cr(VI) were found in groundwater that is used for Thiva's water supply. Arsenic concentrations up to 34 μg/L along with Cr(VI) levels up to 40 μg/L were detected in Schimatari's water supply.

Asopos

In the Asopos River, although total chromium values are up to 13 μg/L, hexavalent chromium was less than 5 mg/L and other toxic elements were relatively low. Their values suggest a connection with the industrial activity in the area.[22][23]

Eastern Sterea Hellas

The chemistry of the groundwater in Eastern Sterea Hellas (central Euboea and Asopos valley), central Greece, revealed high concentrations of hexavalent chromium in groundwater systems sometimes exceeding the Greek and the EU drinking water maximum acceptable level for total chromium.[24] Hexavalent chromium pollution here is associated with industrial waste.

By using the GFAAS for total chromium, diphenylcarbazide-Cr(VI) complex colorimetric method for hexavalent chromium, and flame-AAS and ICP-MS for other toxic elements, their concentrations were investigated in several groundwater samples. The contamination of water by hexavalent chromium in central Euboea is mainly linked to natural processes, but there are anthropogenic cases.[25]

References

  1. ^ Gerd Anger, Jost Halstenberg, Klaus Hochgeschwender, Christoph Scherhag, Ulrich Korallus, Herbert Knopf, Peter Schmidt, Manfred Ohlinger, "Chromium Compounds" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005.doi:10.1002/14356007.a07_067
  2. ^ a b IARC (1999-11-05) [1990]. Volume 49: Chromium, Nickel, and Welding (PDF). Lyon: International Agency for Research on Cancer. ISBN 92-832-1249-5. Retrieved 2006-07-16. There is sufficient evidence in humans for the carcinogenicity of chromium[VI] compounds as encountered in the chromate production, chromate pigment production and chromium plating industries.
  3. ^ a b Salnikow, K. and Zhitkovich, A., "Genetic and Epigenetic Mechanisms in Metal Carcinogenesis and Cocarcinogenesis: Nickel, Arsenic, and Chromium", Chem. Res. Toxicol., 2008, 21, 28-44. doi:10.1021/tx700198a PMID 17970581
  4. ^ OSHA: Small Entity Compliance Guide for the Hexavalent Chromium Standards
  5. ^ David Blowes (2002). "Tracking Hexavalent Cr in Groundwater". Science. 295 (5562): 2024–25. doi:10.1126/science.1070031. PMID 11896259.
  6. ^ "US water has large amounts of likely carcinogen: study". Yahoo News. 2010-12-19. Retrieved 2010-12-19.
  7. ^ U.S. Environmental Protection Agency, Consumer Factsheet on: Chromium.
  8. ^ SCHWARTZ, NOAKI (Monday, December 13, 2010). "Survey shows unremarkable cancer rate in CA town". The Associated Press. Retrieved December, 2010. {{cite news}}: Check date values in: |accessdate= and |date= (help)
  9. ^ California Cancer Registry
  10. ^ California Cancer Rates
  11. ^ PG&E Background Study
  12. ^ Carrie Kahn (2010-12-13). "Erin Brockovich II? Activist Returns To Aid Town". NPR.
  13. ^ Environmental Working Group report}
  14. ^ Environmental crusader Erin Brockovich is investigating contaminated well water in Midland, Texas.
  15. ^ Carcinogenic dust found in Davenport Santa Cruz Sentinel 4 Oct 2008
  16. ^ a b Chromium 6 testing continues in Davenport, Cemex changes business practices Santa Cruz Sentinel 9 Oct 2008
  17. ^ Cemex plant could have been unwittingly releasing chromium 6 MetroSantaCruz.com 15 October 2008
  18. ^ Indiana Guardsmen Sue KBR Over Chemical from Democracy Now!, December 4, 2008
  19. ^ Associated Press (February 12, 2009). "Oregon: Possible Chemical Exposure". New York Times. Retrieved on February 12, 2009.
  20. ^ Associated Press (June 27, 2009). "Did toxic chemical in Iraq cause GIs' illnesses?". Associated Press. Retrieved on June 27, 2009.
  21. ^ [1]
  22. ^ [2]
  23. ^ Pollution Flows in Asopos, by Erin Brockovich, published 25 August 2007 in The Brockovich Report, a blog.
  24. ^ [3]
  25. ^ [4]
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