Hexavalent chromium (chromium(VI), Cr(VI), chromium 6) is any chemical compound that contains the element chromium in the +6 oxidation state (thus hexavalent). It has been identified as carcinogenic, which is of concern since approximately of hexavalent chromium were produced in 1985. Hexavalent chromium compounds can be carcinogens (IARC Group 1), especially if airborne and inhaled where they can cause lung cancer.
Occurrence and uses Hexavalent chromium occurs only rarely in nature, an exception being crocoite (PbCrO4). It is, however, produced on a large scale industrially. Virtually all chromium ore is processed via the formation of hexavalent chromium, specifically the salt sodium dichromate. Sodium chromate is converted into other hexavalent chromium compounds such as chromium trioxide and various salts of chromate and dichromate.
Industrial uses of hexavalent chromium compounds include chromate pigments in dyes, paints, inks, and plastics; chromates added as anticorrosive agents to paints, primers, and other surface coatings; and chromic acid electroplated onto metal parts to provide a decorative or protective coating.
Hexavalent chromium indeed is one of the more widely used heavy metals in various sectors and industries (metallurgy, chemicals, textiles, etc.) with particular involvement in the metal coating sector, especially when subjected to plating or coating processes involving hexavalent chromium.
Hexavalent chromium can be formed when performing "hot work" such as welding on stainless steel or melting chromium metal. In these situations the chromium is not originally hexavalent, but the high temperatures involved in the process result in oxidation that converts the chromium to a hexavalent state.29 CFR OSHA General Industry 1910 Hexavalent chromium can also be found in drinking water and public water systems.
Toxicity Many hexavalent chromium compounds can be carcinogens (IARC Group 1), especially if airborne and inhaled where they can cause lung cancer. Positive associations have also been observed between exposure to chromium(VI) compounds and cancer of the nose and nasal sinuses. Workers in many occupations are exposed to hexavalent chromium. Problematic exposure is known to occur among workers who handle chromate-containing products and those who grind or weld stainless steel. Workers who are exposed to hexavalent chromium are at increased risk of developing lung cancer, asthma, or damage to the nasal epithelia and skin. Within the European Union, the use of hexavalent chromium in electronic equipment is largely prohibited by the Restriction of Hazardous Substances Directive and the European Union regulation on Registration, Evaluation, Authorisation and Restriction of Chemicals.Commission Regulation (EU) No 301/2014 of 25 March 2014 amending Annex XVII to Regulation (EC) No 1907/2006 of the European Parliament and of the Council on the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) as regards chromium VI compounds, Retrieved September 21, 2022.
Hexavalent chromium compounds can be 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.
The LD50 of lead chromate is 5 g/kg (oral, rats). This low toxicity is attributed to its extremely low solubility. Consequently lead chromate remains a common, even preferred, pigment.
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.
Of 2,345 unsafe products in 2015 listed by the EU Commission for Justice, Consumers and Gender Equality some 64% came from China, and 23% were clothing articles, including leather goods (and shoes) contaminated with hexavalent chromium. Chromate-dyed textiles or chromate-tanned leather shoes can cause skin sensitivity.
In the U.S., the OSHA PEL for airborne exposures to hexavalent chromium is 5 μg/m3. The U.S. National Institute for Occupational Safety and Health proposed a REL of 0.2 μg/m3 for airborne exposures to hexavalent chromium.
Based on the findings of the National Toxicology Program (NTP)—which is headquartered in the National Institute of Environmental Health Sciences (NIEHS)—in 2014, California established a state-wide drinking water standard of 10 parts per billion (ppb)—micrograms per liter (MCL) of 10 ppb—"specifically for hexavalent chromium, not total chromium."
For drinking water, the United States Environmental Protection Agency (EPA) does not have a Maximum Contaminant Level (MCL) for hexavalent chromium.
Remediation of hexavalent chromium in water The removal of hexavalent chromium from aqueous solutions can be achieved in several ways. The more practical methods focus on reduction of Cr(VI) to Cr(III). Biological agents have also been investigated to see if they are capable of reducing Cr(VI). Experiments with activated sludge have also shown its ability to reduce Cr(VI) to Cr(III).
Exposure and safety issues
Australia
Kooragang Island, New South Wales Hexavalent chromium was released from the Newcastle Orica Kooragang Island ammonium nitrate plant on August 8, 2011. The incident occurred when the plant entered the 'start up' phase after the completion of a five-yearly maintenance overhaul. The process of converting the High Temperature Shift catalyst to its active (reduced) form began by passing steam through the catalyst bed to heat it, to be discharged from the SP8 vent stack. At this time lower temperatures in parts of the plant led some of the steam to condense, which caused chromium(VI) from the catalyst bed to dissolve into the liquid present. The amount of condensate overwhelmed the drainage arrangements, resulting in the emission of condensate through the SP8 vent stack. The leak went undetected for 30 minutes, releasing 200 kg of chromium(VI) into the atmosphere and exposing up to 20 workers at the plant and 70 nearby homes in Stockton.
The town was not notified of the exposure until three days later on the Wednesday morning. This sparked a major public controversy, with Orica criticized for playing down the extent and possible risks of the leak. The office of Environment and Heritage in Stockton collected 71 samples. Low levels of chromium were detected in 11 of them. These 11 samples were taken within six residential blocks close to the Orica plant, two of which were from water samples collected immediately south of the six block area.
The Select Committee on the Kooragang Island Orica Chemical Leak released their report on the incident in February 2012. They found Orica's approach to addressing the leak's impact was grossly inadequate. Orica failed to realize the potential impact that prevailing winds would have on an emission 60 meters high. Orica failed to inspect the area immediately downwind and notify the Office of Environment and Heritage until August 9, 2011. In Orica's initial report to the Office of Environment and Heritage they failed to disclose that the emissions had escaped off-site. In the initial report to WorkCover Orica did not disclose potential impacts on workers as well as that the substance emitted was chromium(VI). Orica's Emergency Response plan was not well understood by employees particularly about notification procedures. The original notification of residents in Stockton was only to households immediately downwind of the emission which failed to realize the potential for contamination of the surrounding area as well. The information presented at the original notification downplayed potential health risks and subsequently provided incomplete information and has led to a lack of trust between Stockton residents and Orica officials.
In 2014, Orica pleaded guilty to nine charges before the Land and Environment court and was fined $768,000. NSW Health findings ruled that it is very unlikely that anyone in Stockton would later develop cancer as a result of the incident.
Bangladesh Toxic poultry feed contaminated by chromium-based leather tanning waste products (as opposed to the non-toxic process of vegetable tanned leather) has been shown to have entered the food supply in Bangladesh through chicken meat, the most common source of protein in the country. Tanneries in Hazaribagh Thana, an industrial neighborhood of Dhaka, emit around of toxic waste each day, and generate as much as per day of scraps, trimmed raw hide, flesh and fat, which are processed into feed by neighborhood recycling plants and used in chicken and fish farms across the country. Chromium levels ranging from per kilogram were found in different organs of chickens which had been fed the tannery-scraps feed for two months, according to Abul Hossain, a chemistry professor at the University of Dhaka. The 2014 study estimated up to 25% of the chickens in Bangladesh contained harmful levels of chromium(VI).
Greece
Eastern Central Greece The chemistry of the groundwater in eastern Central Greece (central Euboea and the Asopos valley) revealed high concentrations of hexavalent chromium in groundwater systems sometimes exceeding the Greek and the EU drinking water maximum acceptable level for total chromium. Hexavalent chromium pollution in Greece 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.
Thebes–Tanagra–Malakasa (Asopos) basin In the Thebes–Tanagra–Malakasa basin of Eastern Central Greece, an area that supports many industrial activities, concentrations of chromium (up to Cr(VI)) and Inofyta (up to Cr(VI) were found in the urban water supply of Oropos). Chromium(VI) concentrations ranging from Cr(VI) were found in groundwater that is used for Thiva's water supply. Arsenic concentrations up to along with chromium(VI) levels up to were detected in Schimatari's water supply.
In the Asopos River, total chromium values were up to , hexavalent chromium was less than , with other toxic elements relatively low.
Iraq