New X-Ray Reveals Arsenic Reactions September 18, 2009
Univ. of Delaware scientists can now pinpoint, at the millisecond level, what happens as harmful environmental contaminants such as arsenic begin to react with soil and water under various conditions.
Quantifying the initial rates of such reactions is essential for modeling how contaminants are transported in the environment and predicting risks.
The research method, which uses quick-scanning X-ray absorption spectroscopy (Q-XAS), was developed by Donald Sparks, director of the Delaware Environmental Institute. The work is reported in the Proceedings of the National Academy of Sciences.
Postdoc Matthew Ginder-Vogel is the first author of the study, along with Gautier Landrot and Jason Fischel. The research method was developed at the National Synchrotron Light Source at Brookhaven National Lab.
"This method is a significant advance in elucidating mechanisms of important geochemical processes, and is the first application, at millisecond time scales, to determine in real-time, the molecular scale reactions at the mineral/water interface. It has applications to many environmental processes including sorption, redox, and precipitation," says Sparks.
"My group has been conducting kinetics studies on soils and soil minerals for 30 years," Sparks adds. "I've been hopeful that we could follow extremely rapid reaction processes and simultaneously collect mechanistic information."
X-ray spectroscopy was developed to illuminate structures and materials at the atomic level. The technique has been used by physicists, chemists, materials scientists, and engineers, but only recently by environmental scientists.
"In studying soil kinetics, we want to know how fast a contaminant begins to stick to a mineral," says Ginder-Vogel. "In general, these reactions are rapid--
90% of the reaction is over in the first 10 sec. Now we can measure the first few seconds of these reactions that couldn't be measured before. We can now look at things as they happen versus attempting to freeze time after the fact," he notes.
For their study, the UD researchers made millisecond measurements of the oxidation rate of arsenic by hydrous manganese oxide, a mineral that absorbs heavy metals and nutrients.
Contamination of drinking water supplies by arsenic is a serious health concern. The poisonous element occurs naturally in rocks and minerals and is used in a wide range of products, from wood preservatives and insecticides, to poultry feed.
The toxicity and availability of arsenic to living organisms depends on its oxidation state--the number of electrons lost or gained by an atom when it reacts with minerals and microbes. For example, arsenite [As(III)] is more mobile and toxic than its oxidized counterpart, arsenate [As(V)].
"Our technique is important for looking at groundwater flowing through minerals," Ginder-Vogel notes. "We look at it as a very early tool that can be incorporated into predictive modeling for the environment."
Source: Univ. of Delaware
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