Chromium Cleanup

THE PRIZED RED COLOR that trivalent chromium brings to ruby gems belies the toxicity of its hexavalent counterpart in the environment.

The widespread industrial use and health hazards of chromium motivated a daylong symposium sponsored by the Division of Environmental Chemistry at the American Chemical Society national meeting in Chicago last month, according to organizer Jorge L. Gardea-Torresdey, a chemist at the University of Texas, El Paso. The session cataloged new remediation strategies for tackling this metal.

Chromium's dark side is perhaps best publicized in the blockbuster film "Erin Brockovich," which was based on the true story of a legal assistant who helped sue a power company for hexavalent chromium contamination in California.

Hexavalent chromium increases the risk of some cancers, can cause skin ulcers, and has potentially harmful impacts on kidney and liver. The trivalent form of chromium is an essential nutrient and is much more benign than its hexavalent counterpart, although some studies suggest that high exposure to Cr(III) might interfere with essential iron uptake.

Chromium is also pivotal to a wide variety of chemical processes, including the manufacturing of textile dyes and nuclear weapons, leather and wood preservation, and chrome electroplating of hubcaps, faucets, and everything in between. Both Cr(VI) and Cr(III) are present in the wastewater of most of these industries.

But establishing the precise extent of current chromium contamination in the U.S. is actually hard to do, because government agencies give radically different estimates about how widespread the environmental contamination is.

According to the Centers for Disease Control & Prevention's Agency for Toxic Substances & Disease Registry (ATSDR), chromium was present in about two-thirds of the 1,591 sites that, as of 2001, were on a national cleanup priority list of contaminated sites. Meanwhile, the Environmental Protection Agency says the metal has been found in at least 120 of these sites. To explain the dichotomy, EPA spokeswoman Roxanne Smith says ATSDR uses its own data as well as EPA's to reach its assessment.

Although the majority of hexavalent chromium present in the environment is thought to be anthropogenic, natural sources also exist. For example, researchers from Stanford University reported this month that the manganese mineral birnessite can trigger the abiotic formation of Cr(VI) from Cr(III) in nature (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.0701085104). This process leads to some natural leaching of hexavalent chromium into surface and groundwaters, they noted.

A variety of soil bacteria and fungi can make this harmful conversion, but microbes that can catalyze its reversal exist, too.

A major challenge in remediating Cr(VI) lies with the ion's incredible solubility, making it nearly impossible to precipitate out of wastewater or aquifers. The solubility also means that Cr(VI) travels easily through wet soil to aquifers or into biological tissues, where it can wreak havoc until reduced to the less harmful and less soluble Cr(III) state.

As such, a major strategy in chromium remediation "is to make the conversion to trivalent chromium before the metal" enters biological tissues, said A. Paul Schwab, a soil chemist at Purdue University.

Everything from manure to microbes is being tested as potential reducing agents that can be placed directly, and ideally inexpensively, in contaminated soil. Even simple sugars can reduce hexavalent chromium in soil, said Bryan Bilyeu, an engineer at Xavier University, New Orleans.

To reduce chromium to its trivalent state in contaminated water, several groups are evaluating zero-valent iron nanoparticles. These approximately 50-nm spheres have previously been shown to reduce chlorinated solvents such as trichloroethylene and vinyl chloride to less harmful species, said Jiasheng Cao, a postdoc with chemist Wei-xian Zhang at Lehigh University. They are working with EPA to study the ability of iron nanoparticles to knock hexavalent chromium into a less toxic state at a few contaminated water sites across the U.S.

PLANTS TAKE reduction of Cr(VI) a step further. Absorption of the metal into plant tissue inevitably results in conversion to Cr(III), but it also removes the metal from the ground, where changes in the soil chemistry could create oxidizing conditions that revert Cr(III) back to Cr(VI).

Plants as diverse as tumbleweed and alfalfa are being assessed for their ability to stockpile Cr(III) so that harvesters can incinerate the toxic crop. Ideal remediation species are hardy, can grow in a variety of climates, readily accumulate large quantities of chromium, and produce a lot of biomass, so that more chromium can be removed.

Tag-teaming plant chromium remediation with biofuel production is an approach taken by Guadalupe de la Rosa, a chemist at the University of Guanajuato, in Mexico. She's evaluating the remediation potential of sunflowers and hopes the seeds can be used as an energy source if they don't harbor too much chromium.

In fact, figuring out where the chromium ends up in a plant is important not only for choosing remediation species but also for understanding the biology of chromium uptake. Many researchers use X-ray absorption spectroscopy to probe pulverized components of plants to find out where the chromium is located. Others are developing strategies to look for the chromium in intact plant tissues, explained Kenneth M. Dokken, a postdoc working with Gardea-Torresdey.

Dokken is developing an infrared microspectroscopy method to determine what plant microstructures are affected when plants are exposed to Cr(VI). By tuning the detector to measure IR absorption of different biomolecules in thin sections of plant tissues, "we can see if the chromium is interacting with sugars or proteins in the plant and localize the interaction within the plant's anatomy," Dokken explained.

ALTHOUGH THE MECHANISM of chromium's action in plant cells is still being unraveled, the carcinogenic properties of Cr(VI) have long been thought to occur via DNA alkylation. At the Chicago meeting, Stanford microbiologist A. C. Matin presented a novel way that Cr(VI) may damage biological cells, results he obtained by observing the impact of Cr(VI) on bacteria.

Matin proposes that in biological cells, single-electron reduction of Cr(VI) to Cr(V) occurs quickly via cofactors like nicotinamide adenine dinucleotide phosphate (NADPH), and then Cr(V) is quickly oxidized back to Cr(VI) by O₂. This redox cycle not only fails to eliminate Cr(VI) but also creates potentially harmful reactive oxygen species.

Both of these end points could disrupt fundamental biochemical pathways like photosynthesis or sugar metabolism that rely on NADPH and O₂. Matin has identified a protein that can catalyze two-electron reductions of Cr(VI), thereby stopping the redox cycle. He proposes that engineering bacteria or plants to possess such a two-electron reducing protein might lead to better chromium remediators.

But if any strategy to remove chromium from the environment, from plants to sugar, is to be assessed, keeping a careful eye on chromium levels and its oxidation state over time is essential. One way to keep tabs on the relative amounts of Cr(III) and Cr(VI) in contaminated sites is by looking at isotopic ratios of ⁵³Cr and ⁵²Cr as determined by mass spectrometry.

Andre S. Ellis, who is a geochemist at the University of Texas, El Paso, told C&EN that lighter isotopes are more reactive because they form weaker bonds. Successful remediation of Cr(VI) to Cr(III) would therefore be marked by less ⁵²Cr(VI).

Of course, chromium is not the only metal in need of remediation, and many contaminated sites host a cocktail of toxic species. Currently, EPA uses a broad spectrum of technologies for contamination cleanup. These technologies range from plant and bacterial remediation to soil washing, where soil is pulled out of the ground, "washed" by using a variety of chemical strategies to precipitate and filter out contaminants, and then returned to the ground.

As each site is individually assessed and a cleanup strategy conceived, novel cleanup approaches—from doubly-reducing proteins to contaminant-gobbling tumbleweed—may one day end up as tools on EPA's chromium remediation belt.