Quantitative Zn speciation in smelter-contaminated soils by EXAFS spectroscopy

More than a century of non-ferrous metallurgical activities have had a severe impact on the natural environment leading in most heavily contaminated sites, to a complete loss of the vegetation cover (that is, desert-like areas) or to the selection of metal-hyperaccumator plant species. Identifying the chemical forms of toxic metals is of vital importance for a realistic assessment of the chemical risk posed by their presence in soils and selecting effective remediation technologies. In this study, X-ray diffraction (XRD), X-ray texture goniometry, and powder and polarized extended X-ray absorption fine structure (EXAFS, P-EXAFS) have been used to investigate quantitatively the speciation of Zn in soils contaminated by three smelters from northern France and Belgium, and coupled synchrotron-based micro-X-ray radiation fluorescence (mu SXRF) and micro-EXAFS (mu EXAFS) were also used fbr one of these soils. Of these techniques, the application of P-EXAFS and mu EXAFS to molecular environmental science was unprecedented, and we show that their complementarity greatly improves the sensitivity of powder EXAFS to identify the nature of metal-containing minerals in soils. Franklinite (ZnFe2O4), willemite (Zn2SiO4), hemimorphite (Zn4Si2O7(OH)(2). H2O), and Zn-containing magnetite ([Fe,Zn]Fe2O4) were identified in dense soil fractions by XRD and powder EXAFS. These primary minerals originate from atmospheric fallout of Zn dusts emitted during the pyrometallurgical smelting process, and they act as the main source of Zn in contaminated soils. In all soil samples, Zn released in solution during the weathering of these high-temperature minerals is taken up partly by phyllosilicates and, to a lesser extent, by Mn and Fe (oxyhdr) oxides. Zn-containing phyllosilicates were identified by comparing powder EXAFS spectra to a library of model compounds and from the noteworthy angular dependence of EXAFS spectra collected on self-supporting films of clay soil fractions. Analysis of higher correlations in EXAFS spectra suggests that the local structure around Zn in phyllosilicates is trioctahedral. The phyllomanganate Zn-sorbed birnessite and Zn-containing Fe grains having a delta FeOOH-like local structure were unambiguously identified by mu SXRF-mu EXAFS. In birnessite Zn is sorbed in the interlayer space above/below vacant sites and can be either 4-fold or 6-fold coordinated depending, presumably, on the anionic stacking of birnessite layers. Based on this micro-mineralogical investigation, a satisfactory fit of the three identified Zn species (that is, phyllosilicate, Mn, and Fe (oxyhydr)oxides) to experimental powder EXAFS spectra of all clay soil fractions was obtained. The significance, origin, and stability of Zn-phyllosilicates are discussed. Specifically, we show that the formation of Zn-containing phyllosilicates is consistent with calculated thermodynamic solubilities. For the range of measured Zn2+ (similar to 10 ppm), Si(OH)(4) (10-20 ppm), and H+ (5.6 < pH < 7.5) concentrations, soil solutions are supersaturated (pH > 6) or near saturation (pH < 6) with respect to the trioctahedral Zn phyllosilicate, Zn-kerolite. Finally, the plausibility of the formation of (Zn,Al) hydrotalcite-like species contemplated by Julliot (1999) is critically assessed.