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An example of a chromium(VI) compound: chromium trioxide

Hexavalent chromium refers to chemical compounds that contain the element chromium in the +6 oxidation state. Virtually all chromium ore is processed via conversion to 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.

Contents

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)). Trivalent chromium binds to proteins and creates haptens that trigger immune response. Once developed, chrome sensitivity can be persistent. In such cases, contact with chromate-dyed textiles or wearing of 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). It appears that the mechanism of genotoxicity relies on pentavalent or trivalent chromium. 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]

Case studies of chromium (VI) in the environment

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Hinkley drinking water and other cases pursued by Erin Brockovich

Hexavalent chromium was found in drinking water in the Southern California town of Hinkley and was brought to wide popular attention by the involvement of Erin Brockovich. The 0.58 ppm in the groundwater in Hinkley exceeded the Maximum Contaminant Level of 0.10 ppm currently set by the United States Environmental Protection Agency.[6] A similar case occurred in 2007 in Asopos River, near Oinofyta, Greece and Brockovich was again involved.[7] In June 2009, the ground water in Midland, Texas (US) was found to be contaminated with chromium. Erin Brockovich was involved again. The Midland groundwater reached higher levels of contamination than in Hinkley with 5250 ppb or 5.250 ppm.[8]

Davenport airborne chromium case

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.[9] 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 California environmental standards. [10] The case highlights the possible release of chromium(VI) as a previously unrecognized byproduct of the cement-making.[11] [12]

KBR cases

In 2008, the 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.[13] Later, 433 members of the Oregon National Guard's 162nd Infantry Battalion were informed of possible exposure to hexavalent chromium while escorting KBR contractors.[14] 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.[15]

Hexavalent chromium and other toxic elements in natural waters in the Thiva – Tanagra – Malakasa Basin, Greece

Trivalent chromium (Cr(III)) is considered to be essential for the human metabolism, whilst hexavalent (Cr(VI)) is very toxic and soluble in water and can be migrated into the direction of the groundwater. Using the GFAAS and ICP-MS for total chromium, diphenylcarbazide-Cr(VI)-complex colorimetric method for hexavalent chromium, and flame-AAS and ICP-MS for other elements, their concentrations were determined in 63 surface and groundwater samples from the Thiva – Tanagra – Malakasa basin, Eastern Sterea Hellas, Greece. [1]. In that area, which is notorious for the industrial activities during the last 40 years, significant chromium concentrations were determined in the urban water supply of Oropos (up to 80 μg/L Cr(VI)) and Inofyta (up to 53 μg/L Cr(VI)). High concentrations of Cr(VI), ranging from 5 to 33 μg/L Cr(VI), were also found in the groundwater that is used for the urban water supply of the town of Thiva (NW). High As content (up to 34 μg/L As) among with Cr(VI) (up to 40 μg/L) were detected in the urban water supply of Schimatari. In the Asopos River, although total chromium values are up to 13 μg/L, hexavalent chromium less than 5 mg/L and other toxic elements were relatively low, during our research, their values suggest a connection with the industrial activity in the area. The pollution of groundwater by Cr(VI) in the majority of water wells in the Thiva – Tanagra – Malakasa basin, is related to the widespread industrial activity, the usage of hexavalent chromium in various processes and the discharges of Cr-bearing wastes[2].

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.
  2. ^ a b IARC (1999-11-05) [1990] (PDF). Volume 49: Chromium, Nickel, and Welding. Lyon: International Agency for Research on Cancer. ISBN 92-832-1249-5. http://monographs.iarc.fr/ENG/Monographs/vol49/volume49.pdf. 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
  4. ^ OSHA: Small Entity Compliance Guide for the Hexavalent Chromium Standards
  5. ^ David Blowes (2002). "Tracking Hexavalent Cr in Groundwater". Science 295: 2024–25. doi:10.1126/science.1070031. PMID 11896259. 
  6. ^ U.S. Environmental Protection Agency, Consumer Factsheet on: Chromium.
  7. ^ Pollution Flows in Asopos, by Erin Brockovich, published 25 August 2007 in The Brockovich Report, a blog.
  8. ^ [http://therealnews.com/t/index.php?option=com_seyret&Itemid=91&task=videodirectlink&id=2109 Environmental crusader Erin Brockovich is investigating contaminated well water in Midland, Texas.
  9. ^ Carcinogenic dust found in Davenport Santa Cruz Sentinel 4 Oct 2008
  10. ^ Chromium 6 testing continues in Davenport, Cemex changes business practices Santa Cruz Sentinel 9 Oct 2008
  11. ^ Cemex plant could have been unwittingly releasing chromium 6 MetroSantaCruz.com 15 October 2008
  12. ^ Chromium 6 testing continues in Davenport, Cemex changes business practices Santa Cruz Sentinel 9 Oct 2008
  13. ^ Indiana Guardsmen Sue KBR Over Chemical from Democracy Now!, December 4, 2008
  14. ^ Associated Press (February 12, 2009). "Oregon: Possible Chemical Exposure". New York Times. Retrieved on February 12, 2009.
  15. ^ Associated Press (June 27, 2009). "Did toxic chemical in Iraq cause GIs' illnesses?". Associated Press. Retrieved on June 27, 2009.

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