Wednesday, November 7, 2007 - 11:00 AM
321-4
Multiscale View of Trace Metal Speciation in the Rhizosphere.
Alain Manceau1, Bruno Lanson1, Matthew Marcus2, and Kathryn Nagy3. (1) CNRS, L.G.I.T. - Maison des Géosciences, B.P. 53, 38041 Grenoble Cedex 9, France, (2) Advanced Light Source, Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, CA 94720, (3) Department of Earth and Environmental Sciences, University of Illinois at Chicago, 845 West Taylor Street MC-186, Chicago, IL 60607-7059
The solid-state speciation of trace metals (TMs) in the rhizosphere must be understood before mitigating the impact of heavy metals on the biosphere. The soil-root interface is by definition heterogeneous and its multiscale chemical and structural complexity requires the synergetic application of several analytical probes with high to low spatial resolution. In most cases, the forms of TMs in solid phases can be determined by using X-ray micro fluorescence (micro-SRXF), diffraction (micro-XRD), and spectroscopy (micro-EXAFS). We use micro-SRXF to map TMs in coexisting constituents, thus determining their distribution and relative abundance with good sensitivity. Micro-XRD is employed to identify nanocrystalline minerals and, more importantly, to determine the nature of defects (stacking faults, cationic and anionic vacancies and occupancies, site occupation of impurities, stoichiometry) through modeling of their scattering properties. From micro-EXAFS we infer uptake and complexation mechanisms of TMs at the atomic scale by individual constituents. Since the distribution of TMs is heterogeneous at nanometer to micrometer length scales and natural nanoparticles are generally aggregated, the micrometer length scale is optimal for identifying single metal species. However, probes at small scale may lead to unrepresentative sampling and are unable to quantify the proportions of all metal species in the bulk sample. This limitation is overcome by collecting the EXAFS spectrum of the average species with a broad X-ray beam and reconstructing the multi-component powder spectrum with the micro-EXAFS spectra from component species or references using principal component analysis. This approach to determining the speciation of TMs in the rhizosphere will be illustrated with examples that show its potential for expanding our knowledge in soil and plant science, for phytoremediating soils, and for purifying wastewaters.