The input of heavy metals in the environment, and specifically in soils and industrial or domestic urban wastes, endangers living organisms. Assessing the risk associated with their presence is a prerequesite for designing recovering techniques of contaminated sites and to prevent future contaminations in the case of landfills. In either case, the risk associated with the presence of heavy metals depends primarily on their speciation. The notion of speciation is taken in its broadest sense and includes metal characteristics, such as electronic structure (oxidation state, nature of chemical bonds) and chemical state: its association with the organic or inorganic fraction, the nature of functional groups the metal is bonded to, fixation by minerals (clays, oxides...) and the crystal chemical mechanism of this fixation (adsorption vs. lattice substitution), or, more simply, precipitation of the pollutant as a salt (sulfate, carbonate, phosphate...), an oxide or a silicate. This chemical state determines the intrinsic toxicity of the host matrix (soil or waste), as well as the mobility of metals in the environment to either surface and ground waters, living organisms (plants, microorganisms, mesofauna) or the atmosphere.
Determining the speciation of heavy metals is a difficult task because of their relatively high dilution and the structural and chemical complexity of host materials. Classical approaches are based on the "selective" or the "sequential" dissolution of soils or waste components and can be classified as "indirect methods" since they are perturbative. The goal of this contribution is to report on new possibilities offered by EXAFS spectroscopy for determining the speciation of metals. In contrast to previous methods, EXAFS is a "direct method" since it can be applied to non-disturbed, polymetallic and polyphasic materials. The potential of the EXAFS method will be illustrated with two case studies. The first example concerns the speciation of lead in a soil contaminated by emission of tetraethyl and tetramethyl lead in the vicinity of an industrial facility. Four-valent lead was shown to be reduced to divalent lead and to be complexed by salicylate and, to a lesser extent, by carboxylate functional groups of soil humic substances. The strong covalent binding of Pb(II) by organics accounts for its low mobility and high residence time of 150-200 years once deposited. A second example deals with the speciation of lead in soils surrounding a former recycling industry of lead-acid batteries. The speciation of lead is not unique and was found to be predominantly present as PbO and PbSO4. The relative proportion of these two components varies from one sampling location to an other. EXAFS also provides evidence for the existence of a third Pb-containing phase, but its nature has not yet been determined. These first results show that the speciation of lead in these soils varies with the source of contamination. These two examples will be complemented by others presently under study that relate
to the speciation of various metals (Pb, Zn, Cd...) in sites contaminated by other wastes (pigments, sewage sludge...).
Manceau, A. et al. Env. Sci. Tech., in press (1996).