A Truly Bad Actor

Hexavalent chromium is known for its negative health and environmental impacts and its extreme toxicity. U.S. EPA rulemaking for this metal is under way.
A Truly Bad Actor

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The U.S. EPA is studying the effects of low concentrations of chromium (VI) — hexavalent chromium — in drinking water.

Because of the study, a small town approached Florida Gateway College to find out what steps it could take to remove hexavalent chromium from raw water supplies, if that were to become the rule. The community system serves more than 10,000 customers and was concerned about how a new standard might affect the design of its water treatment facility.

Safe drinking water for all Americans is a top priority for the EPA and water treatment plant operators. In 1991, the EPA set an enforceable drinking water standard of 0.1 mg/L for total chromium, which includes hexavalent chromium and chromium (III)—trivalent chromium. The standard was based on the best available science at the time. At present, there is no separate limit for hexavalent chromium.

The EPA regularly re-evaluates drinking water standards and, based on new science, began a review of the health effects of hexavalent chromium in 2008. In September 2010, the EPA released a draft of its findings for public comment. When the assessment is finalized, the agency will review the conclusions and consider whether a new standard for hexavalent chromium or a revision to the current total chromium standard is warranted.

Chromium toxicity

Chromium is a dietary requirement for a number of organisms, but that is only true of the trivalent form. Hexavalent chromium is highly toxic to the environment and to humans. Discharge of chromium-polluted wastewater into rivers has caused environmental disasters. An investigation into a hexavalent chromium release into drinking water was the basis of the plot of the motion picture, "Erin Brockovich."

Hexavalent chromium is 1,000 times as toxic as trivalent chromium. Health effects related to exposure include allergic and asthmatic reactions, diarrhea, stomach and intestinal bleeding, cramps, and liver and kidney damage. Several studies have demonstrated that hexavalent chromium compounds increase the risk of lung cancer. The World Health Organization (WHO) and the U.S. Department of Health and Human Services (DHHS) have found that these compounds cause cancer in humans.

Hexavalent chromium is also mutagenic, and its effects may be passed on to children through the mother's placenta. The lethal dose is about one or two grams, and most countries have a legal limit of 0.05 mg/L, or 50 parts per billion (ppb), in drinking water. The World Health Organization recommends that limit. In addition, use of hexavalent chromium is covered by the European Restriction of Hazardous Substances Directive.

Sources of contamination

Since chromium compounds are used in dyes and paints and in leather tanning, they are often found in soil and groundwater at abandoned industrial sites. In addition, primer paint containing hexavalent chromium is still used in aerospace, automobile, and refinishing applications. Hexavalent chromium is used in production of stainless steel, textile dyes, wood preservatives, anti-corrosion and conversion coatings, and for other niche purposes.

The EPA began a toxicology study of hexavalent chromium after a report by the National Toxicology Program. Rulemaking was expected to begin in 2012, based on a final assessment. In 2010, California established a final public health goal of 0.02 ppb of hexavalent chromium in drinking water. In 25 cities where it was detected by testing, hexavalent chromium was present in concentrations exceeding California's proposed maximum. At least 74 million Americans in 42 states drink tap water containing chromium, much of it likely in the hexavalent form.

The community concerned over new chromium limits is home to a metal manufacturer, and its raw groundwater contains an elevated level of chromium, although the water treatment plant meets the EPA maximum contaminant level (MCL) of 0.1 mg/L for total chromium.

Removal of chromium

MCLs are set as close to health goals as possible after considering cost, benefits, and the ability of public water systems to detect and remove contaminants using suitable treatment technologies. We explained to the community leaders that the EPA has listed as the best available technologies for removal of total chromium to below the MCL as being:

Coagulation, sedimentation and filtrationIon exchangeLime softeningReverse osmosis membrane processes

These technologies are generally applied at the point of treatment, but there are point-of-use units with ion exchange and reverse osmosis processes that can remove chromium to below the MCL. Increased urbanization and drinking water demand in areas of industrial activity have increased the frequency of problem metals in drinking water, and there have been several incidents in which hexavalent chromium has been the source of human health concerns.

Reliable methods

Ion exchange is the most frequently used treatment technology for chromium removal. It removes chromium ions from the aqueous phase by replacing them with the anion present in an ion exchange resin. As contaminated water is passed through the resin, contaminant ions are exchanged for other ions, such as hydroxides or chlorides, in the resin. Ion exchange is often preceded by treatments such as filtration to remove organics, suspended solids, and other contaminants that can foul the resin and reduce its effectiveness.

Coagulation and filtration is an alternative treatment for chrome removal. This technology consists of lowering the pH to 4 or 5 and increasing the feed rate of a chemical coagulant, combined with mechanical flocculation to allow fine suspended and dissolved solids to clump together as floc.

A variety of coagulants are available, such as aluminum sulfate, ferric chloride, ferric sulfate, and poly aluminum chloride. The choice depends on water quality, contaminant removal requirements, and cost. The majority of the floc and other suspended solids are removed by settling. The remaining suspended particles are removed by filtration using multimedia filters. Filter backwash wastewater and spent filtration media require special handling and disposal because of their high concentrations of chromium.

We explained to the community leaders that the goal would be to remove all of the hexavalent chromium, because there is a risk at any level of concentration when a contaminate is carcinogenic. We also noted that the available technologies are adequate for the removal of hexavalent chromium, and that any new EPA rule should not affect the water treatment facility's ability to comply with a new MCL for hexavalent chromium or total chromium.

ABOUT THE AUTHOR

John Rowe, Ph.D., is a professor of Water Resources at Florida Gateway College in Lake City, Fla.



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