Direct determination of the chemical form of trace metals in soils still remains a challenge for instrumental analytical techniques. This paper examines the potential of EXAFS spectroscopy to speciate and quantify the form of trace metals in the solid fraction of soil materials using lead as a case study. Three soils contaminated by different sorts of industrial activities, including the synthesis of lead organometallics for gasoline antiknocks, Pb-Zn smelting, and recycling of lead acid battery, were investigated. In soil contaminated by alkyl-tetravalent lead compounds, lead was found to be divalent and complexed to salicylate and catechol-type functional groups of humic substances. Lead sulfate and silica-bound lead are the predominant forms in the vicinity of the battery reclamation area. Near the smelter, lead was found to be divalent and coordinated to O,OH ligands. It is present in several chemical forms, which prevented them from being identified individually. The multiplicity of lead species in soils contaminated by smelting activities is thought to be due to long-term atmospheric emissions and to the variety of lead-containing phases simultaneously, and successively, emitted in the atmosphere. EXAFS can be applied to a wide variety of matrices including sediments, solid and liquid wastes, and fly ash particles.

(M.A.M.) The chemical forms of zinc (Zn) in the Zn-tolerant and hyperaccumulator Arabidopsis halleri and in the non-tolerant and nonaccumulator Arabidopsis lyrata subsp. petraea were determined at the molecular level by combining chemical analyses, extended x-ray absorption spectroscopy (EXAFS), synchrotron-based x-ray microfluorescence, and EXAFS. Plants were grown in hydroponics with various Zn concentrations, and A. halleri specimens growing naturally in a contaminated site were also collected. Zn speciation in A. halleri was independent of the origin of the plants (contaminated or noncontaminated) and Zn exposure. In aerial parts, Zn was predominantly octahedrally coordinated and complexed to malate. A secondary organic species was identified in the bases of the trichomes, which contained elevated Zn concentrations, and in which Zn was tetrahedrally coordinated and complexed to carboxyl and/or hydroxyl functional groups. This species was detected thanks to the good resolution and sensitivity of synchrotron-based x-ray microfluorescence and EXAFS. In the roots of A. halleri grown in hydroponics, Zn phosphate was the only species detected, and is believed to result from chemical precipitation on the root surface. In the roots of A. halleri grown on the contaminated soil, Zn was distributed in Zn malate, Zn citrate, and Zn phosphate. Zn phosphate was present in both the roots and aerial part of A. lyrata subsp. petraea. This study illustrates the complementarity of bulk and spatially resolved techniques, allowing the identification of: (a) the predominant chemical forms of the metal, and (b) the minor forms present in particular cells, both types of information being essential for a better understanding of the bioaccumulation processes.Metal tolerant plants have the ability to survive and reproduce on soils containing high concentrations of metals in forms that are toxic or inimical to other plants (Macnair and Baker, 1994). Metalhyperaccumulating plants have the additional property of storing large amounts of metals in their aerial parts, more than typically 10,000 g g Ϫ1 dry weight for zinc (Zn;Baker and Walker, 1990). This characteristic makes hyperaccumulators highly suitable for phytoremediation, a soft method in which plants are used for the cleanup of metal-polluted soils (Brooks, 1998;Baker et al., 2000). The genetics and the biochemical processes involved in metal uptake, transport, and storage by hyperaccumulating plants are still poorly understood, although this basic information is fundamental for the improvement of the technique (Van Der Lelie et al., 2001). Zn is one of the most important metal contaminant in industrialized countries (Nriagu and Pacyna, 1988), and numerous studies have been conducted on the species Thalspi caerulescens (Vazquez et al., 1992(Vazquez et al., , 1994Pollard and Baker, 1996;Lasat et al., 1998Lasat et al., , 2000Kü pper et al., 1999;Salt et al., 1999;Frey et al., 2000;Assunçaõ et al., 2001) and, to a lesser extent, on Arabidopsis halleri (Macnair et al., 1999;Bert e...

The influence of aqueous silica on the hydrolysis of iron(III) nitrate and chloride salts in dilute aqueous solutions (m Fe ϳ 0.01 mol/kg) was studied at ambient temperature using X-ray absorption fine structure (XAFS) spectroscopy at the Fe K-edge. Results show that in Si-free iron nitrate and chloride solutions at acid pH (pH Ͻ 2.5), Fe is hexa-coordinated with 6 oxygens of H 2 O-and/or OH-groups in the first coordination sphere of the metal, at an Fe-O distance of 2.00 Ϯ 0.01 Å. With increasing pH (2.7 Ͻ pH Ͻ 13), these groups are rapidly replaced by bridging hydroxyls (-OH-) or oxygens (-O-), and polymerized Fe hydroxide complexes form via Fe-(O/OH)-Fe bonds. In these polymers, the first atomic shell of iron represents a distorted octahedron with six O/OH groups and Fe-O distances ranging from 1.92 to 2.07 Å. The Fe octahedra are linked together by their edges (Fe-Fe distance 2.92-3.12 Å) and corners (Fe-Fe distance ϳ3.47 Ϯ 0.03 Å). The Fe-Fe coordination numbers (N edge ϭ 1-2; N corner ϭ 0.5-0.7) are consistent with the dominant presence of iron dimers, trimers and tetramers at pH 2.5 to 2.9, and of higher-polymerized species at pH Ͼ 3. At pH Ͼ 2.5 in the presence of aqueous silica, important changes in Fe(III) hydrolysis are detected. In 0.05-m Si solutions (pH ϳ 2.7-3.0), the corner linkages between Fe octahedra in the polymeric complexes disappear, and the Fe-Fe distances corresponding to the edge linkages slightly increase (Fe-Fe edge ϳ 3.12-3.14 Å). The presence of 1 to 2 silicons at 3.18 Ϯ 0.03 Å is detected in the second atomic shell around iron. At basic pH (ϳ12.7), similar structural changes are observed for the iron second shell. The Fe-Si and Fe-Fe distances and coordination numbers derived in this study are consistent with (1) Fe-Si complex stoichiometries Fe 2 Si 1-2 and Fe 3 Si 2-3 at pH Ͻ 3; (2) structures composed of Fe-Fe dimers and trimers sharing one or two edges of FeO 6-octahedra; and (3) silicon tetrahedra linked to two neighboring Fe octahedra via corners. At higher Si concentration (0.16 m, polymerized silica solution) and pH ϳ 3, the signal of the Fe second shell vanishes indicating the destruction of the Fe-Fe bonds and the formation of different Fe-Si linkages. Moreover, ϳ20 mol.% of Fe is found to be tetrahedrally coordinated with oxygens in the first coordination shell (R Fe-O ϭ 1.84 Å). This new finding implies that Fe may partially substitute for Si in the tetrahedral network of the silica polymers in Si-rich solutions. The results of this study demonstrate that aqueous silica can significantly inhibit iron polymerization and solid-phase formation, and thus increase the stability and mobility of Fe(III) in natural waters. The silica "poisoning" of the free corner sites of iron-hydroxide colloids should reduce the adsorption and incorporation of trace elements by these colloids in Si-rich natural waters.