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RESEARCH HIGHLIGHT
   Not Just for Grilling: Mesquite May Help Clean the Environment
BY KIRSTIE SALTSMAN, PH.D.

The researchers grew mesquite seedlings for 26 days in chromium-containing solutions. They found that the plants absorbed an astonishing 1 percent of their weight in chromium. The next step is to see if mesquite behaves as well when grown in soil.

Think mesquite is just a type of charcoal? Think again. It also may help remove toxic waste from the environment.

A research team led by Dr. Jorge Gardea-Torresdey, an MBRS-supported investigator at the University of Texas at El Paso, has shown that the mesquite tree-the wood of which is used for charcoal-may help remove chromium from industrial waste sites. Chromium is a known carcinogen.

Gardea-Torresdey's group found that mesquite seedlings take in huge amounts of chromium and convert the metal into a harmless form. They also found that this chemical transformation occurs primarily in the roots of the plant, suggesting that the toxic form of chromium is unlikely to harm animals that feed on the tree's leaves.

The researchers reported their findings in the May 1, 2003, issue of Environmental Science and Technology.

Chromium-contaminated water and soil result from a number of industries, such as stainless steel welding, chrome plating, chrome pigment manufacturing, tanning, and a variety of mining activities. The metal is a significant environmental problem and has been found in half of the Environmental Protection Agency's Superfund sites, which are chemical waste sites designated as being among the most hazardous in the country.

The biggest health risk comes from inhaling chromium-containing dust, which increases the risk of lung cancer. One study showed that nearly a quarter of workers employed at a chromate-production plant in Ohio during the 1930s died of lung cancer-a death rate that was more than 200 times the death rate from lung cancer in the general population.

The public health concerns are greatest in Southwestern states. Not only are mining activities common in these states, but the arid conditions make it likely that contaminated dust will be blown into populated areas.

Using green plants to remove chromium or other toxic waste, a practice known as phytoremediation, is an attractive alternative to conventional cleanup methods because it is comparatively inexpensive and environmentally friendly. Desert plants, in particular, are good candidates for phytoremediation. They are accustomed to harsh environmental conditions, so they are more likely to survive in a toxic waste site than their more frail cousins. Because the hardy mesquite tree grows in desert areas where chromium contamination is problematic, it is an ideal choice for this application.

Gardea-Torresdey's work has been praised by environmental groups and was selected as one of the best technological solutions of the year by the editors of Environmental Science and Technology. Gardea-Torresdey presented his work in July 2003 as a keynote speaker at the International Union of Pure and Applied Chemistry conference in Gaborone, Botswana. At this conference, African scientists exchanged information with one another and with scientists from around the world. Phytoremediation technologies hold much promise in Africa, where cost is often a limiting factor, says Gardea-Torresdey.

Gardea-Torresdey is now trying to identify the compounds within the mesquite plant that are responsible for absorbing and detoxifying chromium. A better understanding of the process involving these compounds may lead to genetically engineered plants that are more effective at cleaning the environment.

Reference: Aldrich MV, Gardea-Torresdey JL, Peralta-Videa JR, Parsons JG. Uptake and reduction of Cr(VI) to Cr(III) by mesquite (Prosopis ssp.): chromate-plant interaction in hydroponics and solid media studied using XAS. Environ Sci Technol 2003;37:1859-64.