BASIC CHEMISTRY OF CHROMIUM
Chromium is an odorless, tasteless element occurring both naturally and as a product of industrial processes. The Chemical Abstract Services identification number for elemental chromium is 7440-47-3. It exists in the environment primarily as Cr (III), but also exists as Cr (VI) and Cr (0), the latter two resulting from industry. In the periodic table of elements, chromium is a primordial transition metal with the atomic number 24. The term ‘primordial’ refers to elements that have been found on earth and existed in their current state prior to the existence of the earth, based on the best accepted theories of geochemistry. The half-life of these atoms is approximately greater than 108 years. Cr (III) is a naturally occurring element found in rocks, animals, plants, soils, volcanic gases, and it is an essential nutrient. There are reactions, as in the case of in-situ oxidation with permanganate, producing MnO2, which can convert Cr (III) to Cr (VI), the more toxic form found in 1,127 of the 1,699 current or former National Priorities List sites.
The densities of the most common compounds containing chromium vary from 2-3 times that of water (with some variances in temperature). Chromium compounds are primarily soluble in water. The densities of compounds that are greater than 3 times the density of water are slightly soluble to insoluble. Oxidation states range from -2 to +6, with the most common being +2, +3, and +6. The +2 can be readily oxidized to +3 due to its relative instability. The pH of water causes the solubility of Cr (VI) to be very high while Cr (III) compounds tend not to be very soluble.
SOURCES AND TYPES OF CONTAMINATION
People love to chrome-plate anything and everything. Take a look at this beauty:
It is also what makes rubies RED, especially the lab-grown synthetic ones!
But let’s face it – despite the (usually truthful) claims of increased durability and decreased corrosion, we like it for one reason: it is *shiny*. We can see our own reflection in it and it is a bling-ready status symbol. Rarely do we take a minute to think about what went into the chrome-plating, and how it affects the environment.
In The Environment
Chromium can be found in the environment as trivalent chromium (Cr III) and as hexavalent chromium (Cr VI); metal chromium (Cr 0) is rarely found in the environment. Trivalent chromium is an essential human nutrient, found in contaminated and non-contaminated (natural) sites. Hexavalent chromium is the dangerous form, with known toxicity and carcinogenicity issues.Controllable sources of this contaminant include the following:
• Airborne emissions (chemical plants, incinerators, stationary power plant)
• Contaminated landfills
• Water-borne emissions (chemical plants)
• Cement production and dust
• Chrome alloy production and use
• Electroplating
• Welding
• Acid mine drainage
• Combustion activities at utilities
• Fugitive emissions from road dust, and former (pre 1993) industrial cooling towers
• Drilling wells
Industrial / Occupational Sources
There are 3 industries that use/discard the majority of chromium. It is used in metallurgy as a vital component of stainless steel, as well as other metal alloys. The chemical industry uses it for chrome-plating, leather tanning, paint pigmentation, corrosion inhibition, magnetic tape, catalytic manufacture, electroplating cleaning agents, wood preservation, and water treatment. The refractory industry uses it in heat-resistant materials like firebrick for furnaces.
Air Contamination
Chromium released into the air only accounts for 2.2% of environmental releases. These emissions are mostly from stationary fuel combustion and the metal industry. These emissions are mostly Cr (III), not the more dangerous Cr (VI) form. The chrome-plating industry, however, it estimated to release ONLY Cr (VI) into the atmosphere.
Water Contamination
Chromium released into the water accounts for just 0.3% of the contamination into the environment. These releases are mostly from electroplating and leather tanning.
Soil Contamination
Chromium released into the soil accounts for a whopping 94.1% of the contamination. Chromium waste slag found a secondary use as fill material in construction sites (residential, commercial, recreational, and industrial). When the soil is contaminated, it affects the air and water as well, via wind erosion and rainwater leaching into aquifers/surface waters.Hexavalent chromium is very soluble in soils and groundwater, making it a very mobile pollutant. In contrast, trivalent chromium is immobile, forming tight complexes with the soil minerals.
CHROMIUM TOXICOLOGY AND HEALTH EFFECTS
The three main forms of chromium include elemental or metallic chromium, trivalent chromium (Cr3+) and hexavalent chromium (Cr6+). The trivalent and hexavalent forms of chromium however, are the most prevalent forms found in soil.
Trivalent chromium is of lesser concern than hexavalent chromium and is actually an essential nutrient. Trivalent chromium is also found to be less mobile and reactive in soil, therefore it is generally not found in ground water concentrations above water quality standards.
Hexavalent chromium is found to cause a wide range of health problems once ingested. These health problems include ulcers in the stomach and small intestine, as well as anemia and cancer. Once ingested, chromium rapidly distributes itself to almost all tissues. The highest concentrations of chromium, however, will be found in the kidneys and liver.
A person can experience what is called allergic contact dermatitis when coming into contact with hexavalent chromium. The symptoms are an itchy red rash with swelling. Also, an individual can experience what is called chrome ulcers which are small crusted sores with round borders.
Respiratory exposure is the most dangerous way to be exposed to hexavalent chromium. Symptoms of exposure may include irritation in the nose, throat and lungs. If an individual is repeatedly exposed, damage to the mucous membranes may result. Under severe cases, exposure can cause perforations of the septum (the wall separating the nasal passages).
The Occupational Safety and Health Administration (OSHA) has established an 8-hour time-weighted average (TWA) exposure limit of 5 micrograms of Cr(VI) per cubic meter of air (5 µg/m³). This is a considerable reduction from the previous permissible exposure limit (PEL) of 52 µg/m³.
For Cr(II) and Cr(III) compounds, the PEL is an 8-hour TWA of 500 µg Cr/m³. For chromium metal and for insoluble compounds, the PEL is 1,000 µg Cr/m³.
Since hexavalent chromium is unstable in the body, it is reduced within the liver to form the stable trivalent chromium species. Although the reduction of hexavalent chromium to trivalent chromium can produce a number of reactive intermediates, it is actually the reduction to trivalent chromium that may cause cancer in individuals. Once formed during metabolism, trivalent chromium is inserted within a cell nucleus where it can cross link DNA to the protein actin.
REMEDIATION
Chromium poses a significant threat to both the environment and human health. The main goal of chromium remediation is to take the more harmful form (hexavalent chromium) and reduce it to the less harmful form. That being trivalent chromium. There are numerous methods that can be used to remediate a site contaminated with chromium, though they can be broken up in to either above ground treatment or treatment of the subsurface (groundwater).
The two methods for above ground treatment include excavation of the contaminated soil, as well as pumping contaminated groundwater to the surface where it can be treated and re-injected back into the aquifer.
Subsurface treatment may include pumping different chemicals into the groundwater that can reduce the hexavalent chromium to the trivalent form. Sulfur compounds are generally used. A permeable barrier can be placed in the path of a chromium plume. When chromium contacts this barrier, it is essentially immobilized. Another possible chromium remediation method includes “inoculating” groundwater with nutrients suitable for microbial growth. The microbial growth decreases oxygen levels in the groundwater which in turn can reduce hexavalent chromium to trivalent chromium. Finally, phytoremediation may be an option, albeit a slower one. With phyotoremediation, certain plant species are used to take up the chromium both in the soil as well as in shallow aquifers. The plants are chosen based on how favorably they can take up the chromium and store it.
Several technologies exist when searching for ways to remediate water or wastewater contaminated with chromium. (Mohen and Pittman Jr., 2006) Although found naturally in the environment in several different forms, Chromium, produced by industries, can be harmful to the environment. (ASTDR 2008) Among various chromium species, trivalent Cr(III) and hexavalent chromium Cr(VI), are the most stable in the environment. Cr(VI) is more toxic than Cr(III). (Zhao et al, 2006) Chemical Precipitation, among several precipitation removal processes, is the most commonly used method to remediate chromium contaminated waters, despite sludge production. (Mohen and Pittman Jr., 2006)
References:
Basic Chemistry of Chromium:
Agency for Toxic Substances and Disease Registry. (ATSDR, 2000). Public Health Statement: Chromium. Retrieved September 17, 2009 from http://www.atsdr.cdc.gov/toxprofiles/tp7-c1-b.pdf.
United States Environmental Protection Agency. (2008). CLU-IN Contaminant Focus: Chromium VI. Retrieved September 16, 2009 from http://www.clu-in.org/contaminantfocus/ default. focus/sec/chromium_VI/cat/Toxicology/.
Sources and Types of Chromium Contamination:
Agency for Toxic Substances & Disease Registry. (2008). Case Studies in Environmental Medicine (CSEM) Chromium Toxicity Where Is Chromium Found? Retrieved September 18, 2009, from http://www.atsdr.cdc.gov/csem/chromium/cr_where-found.html
United State of Environmental Protection Agency.(2009). Chromium VI Environmental Occurrence. Retrieved September 18, 2009, from http://www.clu-in.org/contaminantfocus/default.focus/sec/chromium_VI/cat/Environmental_Occurrence/United States Environmental Protection Agency (2009). Technology Transfer Network – Air Toxics Website. Chromium Compounds Hazard Summary. Retrieved September 17, 2009, from http://www.epa.gov/ttn/atw/hlthef/chromium.html
Hansel, C.M., Weilinga, B.W, Fendorf, S. (2003). Stanford Synchrotron Radiation Laboratory. Investigating Chromium-Contamination and Remediation. Abstract retrieved September 17, 2009, from http://ssrl.slac.stanford.edu/research/chromium_summary.html
Chromium Toxicology and Health Effects:
Pichtel, John. (2007). Fundamentals of Site Remediation, Second Edition. Lanham, Maryland: Government Institutes.
Agency for Toxic Substances & Disease Registry (2008). ToxFaqs for Chromium. Retrieved September 14, 2009 from http://www.atsdr.cdc.gov/tfacts7.html#bookmark05
Agency for Toxic Substances & Disease Registry. Health Effects of Chromium. Retrieved September 14, 2009 from http://www.atsdr.cdc.gov/toxprofiles/tp7-c3.pdf
Environmental Health Perspectives Supplements Volume 110, Number 5, October 2002. Metabolic Pathways of Carcinogenic Chromium. Retrieved September 14, 2009. http://www.ehponline.org/realfiles/members/2002/suppl-5/733-738gaggelli/gaggelli-full.html Agency for Toxic Substances & Disease Registry (2008). Case Studies in Environmental Medicine (CSEM) Chromium Toxicity. What are the Standards and Regulations for Chromium Exposure? Retrieved September 18, 2009. http://www.atsdr.cdc.gov/csem/chromium/cr_standards-regulations.html
Occupational Safety & Health Administration (2008). Hexavalent Chromium. Hazard Recognition. Retrieved September 18, 2009. http://www.osha.gov/SLTC/hexavalentchromium/recognition.html
Remediation:
In Situ Treatment of Soil and Groundwater Contaminated with Chromium, Environmental Protection Agency, October 2000, from: http://www.epa.gov/nrmrl/pubs/625r00005/625r00005.pdf
Mohan, Dinesh, and Charles U. Pittman Jr.. "Activated carbons and low cost adsorbents for remediation of tri- and hexavalent chromium from water ." Journal of Hazardous Materials 137.2 (2006): 762-811.
Zhao, Wei, Yu-Ming Zheng, Shuai-Wen Zou, Yen Peng Ting, and J. Paul Chen. "Effect of Hexavalent Chromium on Performance of Membrane Bioreactor in Wastewater Treatment."
JOURNAL OF ENVIRONMENTAL ENGINEERING 135.9 (2009): 796-805.
"Potential for Human Exposure." Agency for Toxic Substances and Disease Registry. 5 Oct. 2009
Photos:
Photo of chromium metal: http://www.goldbamboo.com/pictures-t6412.html
Photo of car: http://www.nu-chrome.com/
Figure of chromium speciation: http://www.hgcinc.com/summer99_news/chromium.htm
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