Hexavalent chromium can irritate the nose, throat, and lungs. Repeated or prolonged exposure can damage the mucous membranes of the nasal passages and result in ulcers. In severe cases, exposure causes perforation of the septum (the wall separating the nasal passages).
All hexavalent chromium compounds are toxic (due to their oxidizing power) as well as carcinogenic (IARC Group 1). Especially if airborne and inhaled where they cause lung cancer. Positive associations have also been observed between exposure to chromium (VI) compounds and cancer of the nose and nasal sinuses.
Hexavalent chromium is a form of the metallic element chromium. Chromium is a naturally occurring element found in rocks, animals, plants, soil, and volcanic dust and gases. Chromium in stainless steel is in the metallic state (zero valences) and stainless steel does not contain hexavalent chromium. Welding and flame-cutting fumes may contain hexavalent chromium compounds. Studies have shown that some hexavalent chromium compounds can cause cancer.
What is Hexavalent Chromium (or Chromium-6)?
Many people know hexavalent chromium as a silent antagonist in the biopic Erin Brockovich (2000), which starred American actress Julia Roberts as a legal assistant taking on a company accused of polluting the water of rural Hinkley, California, which resulted in elevated rates of cancer and death among the town’s residents. But what is hexavalent chromium, and what is it used for?
Industry adds chromium (Cr) to iron and nickel to make metal alloys especially characterized by their high resistance to corrosion and oxidation. Used in small amounts, chromium hardens steel. Stainless steels are alloys of chromium and iron in which the chromium content varies from 10 to 26 percent.
Chromium alloys are used to make products such as oil tubing, automobile trim, and cutlery. Chromite is used as a refractory (a substance that is resistant to heat) and as a raw material for the production of chromium chemicals. The most common forms of chromium are +2, +3, and +6 oxidation states.
In very small amounts
In very small amounts, trivalent chromium (Cr+3) is useful in animal metabolism, helping to regulate glucose levels and increase the effectiveness of insulin in the body. This form of chromium appears in a number of foods, including broccoli, potatoes, and garlic. The U.S. Environmental Protection Agency (EPA) notes that, on average, people take in less than 0.2 to 0.4 microgram from the air, less than 2 micrograms from the water, and less than 60 micrograms from food each day.
Hexavalent chromium (Cr+6), in contrast, is toxic. The chemical is used in a number of industrial processes as well as for leather tanning, chromium plating, colored glass making, and in paint pigments and inks that color plastics and fabrics and serve as corrosion-resistant coatings. It is hazardous when breathed in, ingested, or touched. It carries a 50 percent lethality (LD50) when ingested to the amount of 50 milligrams per liter of body weight.
According to the Occupational Safety and Health Administration (OSHA), in any 8-hour work shift. The average exposure to hexavalent chromium should not go beyond a concentration of 5 micrograms per cubic meter. Repeated exposure to hexavalent chromium can cause a number of respiratory conditions. Including asthma, bronchitis, itching, physical trauma to the respiratory tract, and lung cancer.
Real-life Hinkley has been all but abandoned since Erin Brockovich was in theaters. Although some remediation. And cleanup have been done, the worry of contaminated water continues to hover over the town. The United States Geologic Survey (USGS) determined that even as late as 2007, concentrations of hexavalent chromium in Hinkley averaged about 1.2 micrograms per liter (about 1.2 parts per billion [ppb]), peaking at about 3.1 micrograms per liter.
In the wake of the Hinkley event, the state of California set a public health goal of 0.02 ppb for drinking water in 2011, which is far less than the current EPA standard of 100 ppb for all forms of chromium.
Remediation of hexavalent chromium in groundwater and drinking water
There are mainly three types of methods to remediate hexavalent chromium in groundwater and drinking water:
- Reduction of toxicity;
- Removal technologies; and;
- Containment technologies
Reduction of toxicity of hexavalent chromium involves methods using chemicals, microbes, and plants. Some removal technologies include transporting contaminated soil offsite to a landfill, using ion exchange resins to reduce chromium(VI) concentrations to less than the detectable limit, and granular activated carbon (GAC) filter. Containment technologies can be employed with the use of physical barriers such as grouts, slurries, or sheet piling.
Attempts have been made to test the removal or reduction of hexavalent chromium from aqueous solutions and the environment. For example, a research study conducted by the School of Industrial Technology, University Sains Malaysia in 2010 found that chitosan-coated with poly 3-methyl thiophene can be effectively employed for the removal of hexavalent chromium ions from aqueous solutions.
Chitosan is a very cheap
Chitosan is a very cheap, economical, and environmentally-friendly substrate for coating this polymer. Adsorption of chromium(VI) is found to be effective in the lower pH range and at higher temperatures and subsequent desorption is readily achieved upon alkaline treatment of the adsorbent.
Another study done by the American Industrial Hygiene Association indicates that the airborne hexavalent chromium in acidic mists of an electroplating tank collected on PVC filters was reduced over time after mist generation. A number of other emerging technologies for removing chromium from water are also currently under research, including the use of cationic metal-organic frameworks to selectively adsorb chromium oxyanions.
Hexavalent chromium compounds are genotoxic carcinogens. Due to its structural similarity to sulfate, chromate (a typical form of chromium(VI) at neutral pH) is transported into cells via sulfate channels. Inside the cell, hexavalent chromium(VI) is reduced first to pentavalent chromium(V) then to trivalent chromium(III) without the aid of any enzymes. The reduction occurs via direct electron transfer primarily from ascorbate and some nonprotein thiols. Vitamin C and other reducing agents combine with chromate to give chromium(III) products inside the cell.
The resultant chromium(III) forms stable complexes with nucleic acids and proteins. This causes strand breaks and Cr–DNA adducts which are responsible for mutagenic damage. According to Shi et al., the DNA can also be damaged by hydroxyl radicals produced during reoxidation of pentavalent chromium by hydrogen peroxide molecules present in the cell, which can cause double-strand breakage.
Both insoluble salts of lead and barium chromates, as well as soluble chromates, were negative in the implantation model of lung carcinogenesis. Yet, soluble chromates are a confirmed carcinogen so it would be prudent to consider all chromates carcinogenic.
Chronic inhalation from occupational exposures increases the risk of respiratory cancers. The most common form of lung malignancies in chromate workers is squamous cell carcinoma. Ingestion of chromium(VI) through drinking water has been found to cause cancer in the oral cavity and small intestine.
It can also cause irritation or ulcers in the stomach and intestines, and toxicity in the liver. Liver toxicity shows the body’s apparent inability to detoxify chromium(VI) in the GI tract where it can then enter the circulatory system.
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