Tuesday, September 22, 2009

Hexavalent Chromium CrVI - Team Beta

Hexavalent Chromium (CrVI) – Team Beta

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.
CrVI Facts and Figures -
The 2007 CERCLA NPL (National Priority List) of Hazardous Substances lists Hexavalent Chromium as number 18 of 275 hazardous materials. The levels of contamination may also violate the Clean Water Act of 1972 and the California Drinking Water Standards of 50 ppb total chromium. The California Air Resources Board has identified CrVI as a Toxic Air Contaminant, CAS 18540-29-9.
The Department of Health and Human Services’ Agency for Toxic Substances & Disease Registry estimates that 2,700–2,900 tons of chromium is emitted into the air annually by anthropogenic sources. 35% of these emissions are expected to be hexavalent chromium. (ATSDR “5. Potential for Human Exposure”, retrieved September 15, 2009). The ATSDR also calculates the mean daily dietary intake of chromium from air <0.2-0.4μg, from water 2.0μg, and food 60μg.
Sources of hexavalent chromium (CrVI) –
Chromium is a chemical element that occurs naturally in minerals and soils. It is also found in volcanic dust and gases. The most predominant industrial sources of chromium are from the metallurgical, refractory and chemicals industries. Metallurgical industry use includes mining/refining of chromite ore, and manufacture of ferrochromiums, such as stainless steel and other ferrous and non-ferrous alloys. Chromium is contained in refractory bricks and other refractory materials used for linings of high temperature industrial furnaces. Predominant chemical industry uses of chromium are in the manufacture of pigments, metal finishing (such as chrome plating), leather tanning, and wood preservatives. (ATSDR, 2008)
CrVI Chemistry -
Chromium exists primarily in trivalent (Cr(III)) or hexavalent (Cr(VI)) oxidation states. Cr(VI) is a notorious environmental pollutant because it is a strong oxidant and much more toxic than Cr(III) and carcinogenic. Cr(VI) exists as the chromate ion in basic solutions and as dichromate in acidic solutions.
Hexavalent chromium is recognized as a human carcinogen via inhalation. 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. Within the European Union, the use of hexavalent chromium in electronic equipment is largely prohibited by the Restriction of Hazardous Substances Directive

Chemical name: Ammonium Dichromate Chromium[VI]

Synonyms: Chromic acid, diamonium salt

Chemical Formula: (NH4)2Cr2O7
CAS registry: 7789-09-05
Chromium VI is a hazardous chemical due to its toxicity. Chromium exposure can be by breathing, contaminated air, or drinking water, or eating tainted food. Contaminated well water presents the most potential for ingestion, since chromium will stay longer at it´s hexavalent state. The potential impact is strictly to human health, since hexavalent chromium has been shown to cause cancer in lab animals. Toxic effects may include liver and heart failure, reproductive and respiratory failure. (ATSDR, 2009)

CrVI Health Effects -

Inhalation risks are assessed in different ways (LC 50, TLV, etc). The main reason is that CrVI is a carcinogen. Cancer risk or the incremental cancer risk caused by an emission is better assessed when concentrations are compared to OEHHA’s (Office of Environmental Health and Hazard Assessment) reference level of 0.002 μg Cr(VI)/m3 (OEHHA, 2009).

“The respiratory tract is the major target organ for chromium (VI) toxicity, for acute (short-term) and chronic (long-term) inhalation exposures. Shortness of breath, coughing, and wheezing were reported from a case of acute exposure to chromium (VI), while perforations and ulcerations of the septum, bronchitis, decreased pulmonary function, pneumonia, and other respiratory effects have been noted from chronic exposure. Human studies have clearly established that inhaled chromium (VI) is a human carcinogen, resulting in an increased risk of lung cancer. Animal studies have shown chromium (VI) to cause lung tumors via inhalation exposure. “ (EPA, 2000)

Acute Effects of Cr VI
- Shortness of breath, coughing, wheezing
- Gastrointestinal and neurological effects
- Skin burns

Chronic (Noncancer) Effects of Cr VI
- Perforations and ulcerations of the septum, bronchitis, decreased pulmonary function, pneumonia and asthma
- Effects on the liver, kidney, gastrointestinal and immune systems and possibly the blood
- Particulates effects include upper respiratory tract, reproductive and renal effects

Reproductive and Developmental Effects of Cr VI
- Possible complications during pregnancy and childbirth

Cancer Risks from Cr VI
- Inhaled chromium is a human carcinogen, causing an increased risk of lung cancer

(From EPA website http://www.epa.gov/ttn/atw/hlthef/chromium.html)

Toxicity and Metabolism of CrVI –

Hexavalent chromium is absorbed to a greater extent than the trivalent Chromium and readily enters cells through nonspecific anoin carriers. In significant amounts of hexavalent chromium are likely to be consumed orally because chromium(+6) is reduced in the environment to Chromium (+3) , the more stable oxidation state. chromium(+6) consumed is totally or partly reduced to Chromium (+3) in the acid environment of the stomach. Hexavalent chromium also doesn’t generally occur naturally and is produced almost totally from human activities as discussed above.
Chromium is transported in the blood primarily bound to transferring. Plasma or serum chromium conc. In humans are less than 0.30ug/L. Blood chromium concentrations are not in equilibrium with tissue chromium concentrations and do not reflect body stores. Tissue concentrations are low with kidney, liver, spleen and bone containing the highest concentrations.
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. 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.
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 forms of Chromium (+6) are several fold more toxic than soluble forms of Chromium (+3)

Ground Water Field Sampling –

The USGS (United States Geological Survey) developed a new field method for collecting and analyzing ground water for the presence of chromium VI, widely used since 2003. Hexavalent chromium (Cr VI) is very toxic and a carcinogen. The new method, developed by USGS, enables the field distinguishment between Cr VI and its less toxic form, chromium III. The advantages of the new field method include lower detection limits, down to 0.05 micrograms per liter; a small, disposable cation exchange cartridge that allows Cr VI to be separated and stabilized in the field; storage of field samples for up to several weeks; as well as the use of common lab equipment to reduce analytical cost. Time stability of preserved samples is a great advantage over the 24 hour time constraint specified for EPA method 218.6. Prior to 2003, Cr VI field analysis was not very reliable and very expensive. The newer method is comparable with standard laboratory based methods. Since the toxicity of Cr VI is widely known, quicker more accurate field sampling is of great benefit to communities. (Ball, James. 2003. A New Cation-Exchange Method for Accurate Field Speciation of Hexavalent Chromium).

One of the traditional methods for determining Cr(VI) uses diphenylcarbohydrazide (DPC) to form an intensely colored complex with Cr(VI). The complex is measured quantitatively by its visible absorption at 520 nm. However, as in any colorimetric analysis, this test is subject to positive interferences from other colored materials in the sample as well as from other elements that form colored complexes with DPC.

Cr VI Case Study 1 –

Erin Brockovich, who was depicted by Julia Roberts in a Hollywood motion picture, has played a large roll in the world of Hexavalent Chromium in drinking water. Her first major case was in Henkley, California where she won over 330 million dollars for a town whose drinking water was nearly 6 times the Maximum Contaminant Level (0.10 PPM) set by the EPA (Brockovich, Famous Trials, Chapter 1 Preface." Enemy at the Gates, Thirteen Days, Erin Brockovich, Stories Behind The Movies. 15 Sep. 2009 ).

Later, in 2007, at a site in Oinofyta, Greece, Brockovich became involved at a similar case involving the Asopos River.

Most recently Brockovich has been invovled in a major drinking water case in Midland, Texas where well water has been reported to have hexavalent chromium levels 10 times higher than those at the Henkley California site

Case Study 3 -

A relatively recent industrial source of concern for CrVI is Portland Cement. Concrete, a very common building material is a mixture of portland cement, aggregate (sand and rock), and water. Raw materials used in the manufacture of portland cement, such as limestone, clay, and silica contain naturally occurring sources of chromium. During the cement manufacturing process, raw materials, are mixed and heated in a large kiln to produce an intermediate pebble-sized material called clinker. Fugitive dust emissions from stored piles of clinker are alleged to be a significant source of CrVI contamination to air, soil, and potentially groundwater in the area surrounding a cement plant operated by TXI International near Riverside, California. (Insurance Journal, 2008)

CrVI Case Study 2 -

An example of an airborne Cr VI case study comes from Davenport California where a local Cemex cement plant was the source of levels of contamination above those set by local standards, however the recorded levels were below those set by the EPA. The significance of this case is that it bring to light CrVI as a harmful by product possible in the production of cement

Case Study General –

A general case study are Cr(VI) in discharge from tanneries, this is regulated and can be treated at the source. However in India, reguation hasn't happened until recently and studies have conducted to remediate contamination sites.

As part of the tanning process hexavalent chromium compounds are often a byproduct of the chemicals used to alter the rawhide material into leather. In areas with little to no regulation hexavalent chromium compounds are discharged in effluent into the environment (Sirnath, et al 2002). Tanneries in India discharge effluent containing hexavalent chromium compounds into the environment which in turn contaminates agricultural land and ground water (ACIAR, 2003). Effluent discharge from tanneries can be treated to reduce hexavalent chromium compounds; however these processes may is seen as not economically feasible (Sirnath et al, 2002). Studies, using microbial remediation and phytoremediation at contaminated sites in India have proven insightful. Microbes that can bioaccumulate hexavalent chromium compounds can be added to tannery sludge which will in turn reduce chromium compounds in sludge to acceptable levels (Sirnath et al, 2002). Other techniques are being used to remove chromium compounds from tannery sludge, by planting crops that are more tolerant to chromium or by planting crops that will absorb chromium and reduce it from contaminate sites (ACIAR, 2003). To curb the environmental contamination from tannery discharge India has enacted regulations (ACIAR, 2003). These remediation techniques will help to met these regulations in addition to better treatment at discharge source.


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