Model allows better understanding of groundwater contamination

April 26, 2002

How easily metals such as cobalt, copper and lead travel through the environment depends on the compounds they form in the soil and how these dissolve in water. New work by researchers at the University of California, Davis, and Olin College shows a simple way to work out the stability and solubility of a whole class of clay-like compounds called hydrotalcites. The finding should help geochemists make much better estimates of soil contamination.

Hydrotalcites are layered compounds that form when metals combine with aluminum oxides and hydroxides in the soil. By understanding how easily hydrotalcites form from various metals, scientists can predict how much of the metal stays dissolved in groundwater and how fast it can spread.

Hydrotalctites are "garbage bags" with a layered structure that can take up many metals, nitrates and other chemicals, said William Casey, a professor of land, air and water resources at UC Davis and one of the authors of the study.

Rama Allada, a graduate student in the Nanophases in the Environment, Agriculture and Technology (NEAT) initiative at UC Davis, measured the energy needed to form three cobalt-aluminum hydrotalcites. The team then developed a simple model to predict the corresponding results for other hydrotalcites containing different metals.

With some further measurements, the model could be used to make predictions about the solubility of a wide range of environmental contaminants, such as chromium, said UC Davis professor Alexandra Navrotsky, Allada's thesis supervisor and an author on the paper. The same methods could also be used to study contamination with radioactive wastes such as carbon, iodine and technetium isotopes, Casey said.

Hillary Berbeco, an assistant professor at Olin College in Needham, Mass., conducted the difficult synthesis of the hydrotalcites used to establish the model.
The paper is published in the April 26 issue of the journal Science.

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University of California - Davis

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