Chromium, element 24 on the periodic table, is widely distributed in the Earth’s crust. Once it’s mined, the ores are refined to the metal. FeCr2O4, called chromite, is the major ore. Chromium and its salts have numerous commercial applications including: an essential component of stainless steel, pigments and colorants in glass and ceramic glazes, catalysts, paints, pyrotechnic displays, fungicides, photography, chrome plating and corrosion control.
Chromium can exist in oxidation states between +2 and +6, but the two most common and important forms are chromite (+3) and chromate (+6). The environmental forms are both natural and from anthropogenic activity. The two oxidation state forms are frequently described as chromium (III) and chromium (VI). Chromium (III) is the reduced, more stable form and exists as cationic salts (CrCl3), as well as green Cr2O3. Chromium (VI) is the highest oxidation state and is always present as an oxy compound (e.g., CrO3and Cr2O7-2), and it is a strong oxidizing agent especially under acidic conditions. Cr2O3 and several other chromium (III) compounds are slightly soluble or insoluble in water, but chromium (VI) compounds/ions are generally very soluble. Food is the likely primary source of daily exposure.
Somewhat surprisingly, some groundwaters contain chromium (VI) of natural origin. Chromium (VI) can be produced in drinking water from chlorine oxidation of chromium (III), which is usually present in very small amounts. The U.S. Environmental Protection Agency (EPA) Unregulated Contaminants Monitoring Survey (2013–2015) of about 4,000 large and about 800 small water supplies measured total chromium and chromium (VI) several times. The concentrations typically ranged from 0.057 to 7.51 µg/L. One result out of about 63,000 water samples exceeded the reference maximum contaminant level (MCL) of 0.1 mg/L (100 µg/L). There were no statistical differences between treated entry and distribution system water concentrations in the 1 to 2 µg/L concentration range. There were several anomalous results at low concentrations where chromium (VI) concentrations exceeded total chromium levels in the same samples, so caution is appropriate when interpreting low values.
There are numerous stainless steel compositions that are geared to particular applications. The common 300 series is resistant to corrosion and contains about 18% chromium and 8% nickel, plus small amounts of manganese, silicon, carbon, phosphorus and sulfur. Other types have different formulations for more or less extreme applications and can be in the range of 16% to 26% chromium and 6% to 22% nickel, plus other components.
Pigments and colorants
Pigments are used for coloration of numerous materials such as glass and plastics, including paints for art. Several chrome-based pigments are in use. Chrome yellow is lead chromate. Chrome orange is lead chromate and lead oxide. Chrome green is chromic oxide (Cr2O3). Viridian is a dark green pigment of hydrated chromic oxide. The lead components limit applications where human exposure is possible.
Chrome plating on metal surfaces provides resistance to corrosionand improved decorative appearance. The process generally involves cleaning, pretreatments and immersion in an electroplating bath of a chromium chemical in sulfuric acid. Both trivalent and hexavalent chromium processes are used — each has advantages and disadvantages. Disposal of the electroplating bath solutions presents problems and is regulated.
Corrosion management in cooling systems
Chromate and dichromate additives have been used for at least 90 years to reduce corrosion of metal surfaces in water cooling systems and to avoid deterioration and reduction of heat transfer efficiency. Aluminum, galvanized iron, copper and zinc surfaces are among surfaces that are protected with chromium-based additives. Closed cooling systems, like automobile radiators containing methanol or glycols have been commonly blended with dichromate to prevent decomposition of the metal surfaces and solders.
Chromate and molybdate are among the widely used corrosion inhibitors for closed cooling water systems. Chromate treatments in the range of 500 to 1,000 parts per million (ppm) are satisfactory for single metal surfaces. Bimetallics can form electronic couples that require chromate treatment concentrations to exceed 2,000 ppm. Improved effectiveness requires pH control in the range of 7.5 and 9.5. Limits on chromate corrosion inhibitors concentrations include graphite mechanical seals and in applications with a very high heat transfer rate.
The toxicity of high-chromate concentrations restricts their use, particularly when a system must be drained frequently. Chromium (VI) chemicals are considered to be carcinogenic when inhaled, so human contact should be avoided especially when inhalation of aerosols is likely. Substitutes such as molybdate-based products are employed more frequently in some applications.
Allowable discharge limits have been established, and spills of chromate-based products must be reported. 40 CFR 749.68 under the Toxic Substances Control Act regulates use of chromium (VI)-based corrosion water treatment chemicals in certain cooling systems. The intent is to prevent unreasonable risks from human exposures to air emissions of chromium (VI) from comfort cooling towers. They are defined as an integral part of heating, ventilation and air conditioning or refrigeration systems. Distribution in commerce and use in industrial cooling towers and closed cooling water systems are not prohibited by that regulation.
Ingestion health issues
Chromium became an issue again in the l990s resulting from local groundwater contamination caused by uncontrolled discharges of cooling water containing chromium (VI) from an electrical generating facility in California. Total chromium, and chromium (VI) have been regulated in drinking water since 1946 in the U.S. Public Health Service (PHS) standards at 50 µg/L (50 ppb). Chromium (III) is considered to be innocuous and a possible essential nutrient. The PHS concluded that although there was agreement that ingested chromium VI was probably not carcinogenic, the regulations assumed that the standard and water concentration could even be all chromium (VI).
Subsequently, the EPA produced an Interim National Drinking Water Maximum Contaminant Level of 50 µg/L for total chromium in 1976. The revised MCL was raised to 100 µg/L in 1991. Again, it was concluded that it was not carcinogenic by ingestion in water, and the drinking water concentration could be all chromium (VI). The World Health Organization (WHO) guideline is 50 µg/L. As the result of a National Toxicology Program (NTP) bioassay where rats and mice were dosed at very high levels in drinking water, there were cancers produced at the highest doses, and NTP concluded that chromium (VI) was carcinogenic “under the conditions of the test.” California used that result as the basis to compute a Public Health Goal (PHG) hypothetical human cancer risk of 1/1,000,000 using linear nonthreshold modeling for lifetime consumption of drinking water at 0.02 µg/L (0.02 ppb), 2,500 to 5,000 times lower than all other drinking water regulations and guidelines, and it published an MCL at 10 µg/L. The MCL has since been remanded by a court and withdrawn for legal technical reasons, so its current MCL has reverted to 50 µg/L, pending a review and reregulation.
However, chromium (VI) is well-known to be rapidly reduced to nontoxic chromium (III) upon ingestion. An extensive Mode of Action (MOA) study later showed that there was no cancer risk at even unusually high drinking water concentrations, because of the rapid conversion to chromium (III). Canada recently reviewed its drinking water guideline in light of the mechanistic MOA studies and concluded that a guideline of 50 µg/L was safe and did not cause a cancer risk. Its basis was that linear nonthreshold cancer risk modeling was not appropriate, so there was a safe level below which there was no cancer risk, and its guideline value was conservative and well below that threshold.
WHO is reviewing its drinking water guideline based on the new mechanistic MOA studies and will reach a conclusion shortly. It is not known whether California will reexamine its 0.02 µg/L PHG and issue a new PHG and regulation consistent with the latest scientific information.
Products containing chromium have been widely used for many years in many applications from production of stainless steel to pigments and corrosion inhibitors in cooling systems. Because of cancer risk concerns from inhalation of chromium (VI), some of their uses have been reduced, although they are still widely used. Environmental presence of chromium (VI) is mostly due to waste discharges; however, there are locations with naturally occurring chromium (VI).
U.S. drinking water has been regulated since 1946 and with later EPA MCLs that assumed chromium (VI) is not carcinogenic by ingestion well above the regulated values. That was put into question by a very high dose NTP animal bioassay, which produced cancers in rats and mice under the test conditions that were not relevant to normal human ingestion and gastrointestinal chemical reduction conditions. Using the latest MOA information, Canada has produced a revised drinking water guideline reaffirming its original guideline and concluded that chromium was not a cancer risk at concentrations even above known drinking water levels. So, the 1946 PHS judgment is still valid. It is unknown how California will use the MOA information and include gastrointestinal reduction to innocuous chromium (III) in a revision of its questionable PHG and drinking water regulation.
Joseph Cotruvo, Ph.D., BCES, is president of Joseph Cotruvo and Associates LLC — water, environment and public health consultants — and he is a technical editor of Water Technology. He is a former director of both the EPA Drinking Water Standards and the Risk Assessment Divisions.