Research Interests - Environmental
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Introduction
Heavy elements such as arsenic and selenium enter the environment through natural and manmade processes and then become available to living things. Selenium, and possibly also arsenic, is an essential trace element for mammals, but both are also well known for their toxicity. The molecular form of any element critically affects its transport, bioavailability and toxicology, in other words how it interacts with the environment and ultimately with man. Knowledge of molecular form is also required in order to propose effective remediation strategies. X-ray absorption spectroscopy (XAS) can investigate the chemical form of an element essentially without pretreatment of the sample, and hence can yield in situ information about chemical form. XAS imaging is a powerful microprobe technique in which the sample is raster scanned in a small beam at energies sensitive to the different chemical forms in order to generate maps of those species. Below we highlight a couple of milestones from a fairly extensive list of publications.
The Mechanisms of Hyperaccumulating Plants
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J. Pickering, R. C. Prince, D. E. Salt and G. N. George,
"Quantitative, Chemically Specific Imaging of Selenium Transformation in
Plants",
Proceedings of the
Hyperaccumulators are plants that specifically take up and store particular metals or metalloids from the environment, possibly as a deterrent to herbivores. The mechanisms of tolerance, metabolism, and partitioning of metals and metalloids in these species are of particular importance as they could provide the foundation for a phytoremediation strategy – the use of plants to clean up contaminated water or soil. The two grooved milk vetch Astragalus bisulcatus, a plant found in the western plains of North America, can accumulate up to 0.6 % dry weight Se in its shoots. We have studied the Se biochemistry of A. bisulcatus using plants hydroponically cultured in low levels (5 µM) of selenate. We developed techniques to determine in vivo concentrations of Se species from XAS imaging data, and showed that the Se chemistry changes as the plant grows. In young leaves mostly organic forms are present, and with increasing age the leaves have progressively less organic and total Se but more selenate.
Speciation of Selenium in Environmental Samples
I. J. Pickering, G. E. Brown, Jr. and T. K. Tokunaga,
"Quantitative speciation of selenium in soils using X-ray absorption spectroscopy",
Environmental Science and Technology, 29(9), 2456-2459 (1995)
Our group pioneered the use of the Se near-edge spectrum to quantitatively determine the type of selenium present in soils from the Kesterson reservoir in California, U.S.A. The Se K near-edge spectrum is particularly sensitive to the oxidation state and coordination, and the method has become routine for the study of selenium and other elements in the environmental sciences. Selenium in Kesterson soils was found to be predominantly in the red, a-form of elemental selenium. Laboratory simulations showed the reduction to occur on the timescale of a few days.
Funding and Future Projects
Current work is funded by NSERC and by the Province of Saskatchewan. X-ray absorption spectroscopy was carried out at the Stanford Synchrotron Radiation Laboratory, which is supported by the U.S. Department of Energy, Office of Biological and Environmental Research and by the U.S. National Institutes of Health, National Center for Research Resources, Biomedical Technology Program. Current and future work in this area includes continuation of investigations of hyperaccumulating plants, and assessing the biotransformation and accumulation of elements in insect ecosystems.
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Last Modified April 23, 2004