17 September 2009

New X-ray technique shines light on contaminants

A new analytical method 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 reactions is essential for modeling how contaminants go through the environment and predicting risks.

The research method, which uses an analytical technique known as quick-scanning X-ray absorption spectroscopy, came from a research team led by Donald Sparks, S. Hallock du Pont Chair of Plant and Soil Sciences and director of the Delaware Environmental Institute at the University of Delaware.

The research method came about by using beamline X18B at the National Synchrotron Light Source at Brookhaven National Laboratory in Upton, N.Y.

“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,” Sparks said. “It has tremendous applications to many important environmental processes including sorption, redox, and precipitation.”

“My group and I have been conducting kinetics studies on soils and soil minerals for 30 years,” Sparks said. “Since the beginning, I have been hopeful that someday we could follow extremely rapid reaction processes and simultaneously collect mechanistic information.”

X-ray spectroscopy illuminates structures and materials at the atomic level. Physicists, chemists, materials scientists, and engineers use the technique, but only recently environmental scientists have jumped on board.

“In studying soil kinetics, we want to know how fast a contaminant begins to stick to a mineral,” said postdoctoral researcher Matthew Ginder-Vogel, who is also first author of the study. “In general, these reactions are very rapid, 90% of the reaction is over in the first 10 seconds. 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.”

For their study, the Delaware researchers made millisecond measurements of the oxidation rate of arsenic by hydrous manganese oxide, which is a mineral that absorbs heavy metals and nutrients.

Contamination of drinking water supplies by arsenic is a serious health concern in the U.S. and abroad. The poisonous element occurs naturally in rocks and minerals and also sees use 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, in other words, the number of electrons lost or gained by an atom when it reacts with minerals and microbes. For example, arsenite is more mobile and toxic than its oxidized counterpart, arsenate.

“Our technique is important for looking at groundwater flowing through minerals,” Ginder-Vogel said. “We look at it as a very early tool that can be incorporated into predictive modeling for the environment.”

For related information, go to www.isa.org/manufacturing_automation.