Probing the Speciation of Metals at the Soil-Plant Interface Using Micrometer-Scale X-Ray Fluorescence, Diffraction and Absorption Techniques.
Alain Manceau, CNRS, Maison des Geosciences - Univ. J. Fourier, BP 53, Grenoble cedex 9, 38041, France
Understanding the mobility and speciation of toxic elements in the rhizosphere is critical for Earth sustainability issues of the future, and in the short-term, for formulating effective spin-off phytoremediation technologies. This goal cannot be met without first having in hand a fundamental understanding of the spatial distribution and chemical form of trace elements in the non-rhizospheric and rhizospheric soil and in plants. Since natural systems are heterogeneous over distances of microns, probes with millimeter spatial resolution will lose information. On the other hand, as powerful and promising are nanometer-scale X-ray probes, at present they essentially provide chemical, but not structural information, and on relatively concentrated elements. Such probes therefore do not allow one to identify either the nature of the host species or the incorporation mechanism of trace metals at the molecular scale. The novel combination of hard X-ray fluorescence, diffraction, and EXAFS spectroscopy, when all performed to micrometer-scale resolution on the same spot, provides the microanalytical tool needed to identify individual trace metal constituents and their structural forms and relationships with biota in heterogeneous matrices, such as the soil-plant interface. Micro-fluorescence (SRXF) is used first to map trace element distribution and to identify elemental associations. Then, the nature of minerals hosting a particular trace element is deduced from micro-diffraction (XRD). Finally, with micro-EXAFS spectroscopy, the structural relationship between metal and mineral host is examined. This new three-pronged approach to the speciation of metal contaminants in heterogeneous bioinorganic systems will be illustrated with home-built examples that are designed to explicate its potential for advancing frontiers in soil and plant research, and to formulating effective metal containment biogeo-technologies.