Quantitative Zn speciation in smelter-contaminated soils by EXAFS spectroscopy

More than a century of non-ferrous metallurgical activities have had a seve re impact on the natural environment leading in most heavily contaminated s ites, to a complete loss of the vegetation cover (that is, desert-like area s) or to the selection of metal-hyperaccumator plant species. Identifying t he chemical forms of toxic metals is of vital importance for a realistic as sessment of the chemical risk posed by their presence in soils and selectin g effective remediation technologies. In this study, X-ray diffraction (XRD ), X-ray texture goniometry, and powder and polarized extended X-ray absorp tion fine structure (EXAFS, P-EXAFS) have been used to investigate quantita tively the speciation of Zn in soils contaminated by three smelters from no rthern France and Belgium, and coupled synchrotron-based micro-X-ray radiat ion fluorescence (mu SXRF) and micro-EXAFS (mu EXAFS) were also used fbr on e of these soils. Of these techniques, the application of P-EXAFS and mu EX AFS to molecular environmental science was unprecedented, and we show that their complementarity greatly improves the sensitivity of powder EXAFS to i dentify the nature of metal-containing minerals in soils. Franklinite (ZnFe 2O4), willemite (Zn2SiO4), hemimorphite (Zn4Si2O7(OH)(2). H2O), and Zn-cont aining magnetite ([Fe,Zn]Fe2O4) were identified in dense soil fractions by XRD and powder EXAFS. These primary minerals originate from atmospheric fal lout of Zn dusts emitted during the pyrometallurgical smelting process, and they act as the main source of Zn in contaminated soils. In all soil sampl es, 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 b y comparing powder EXAFS spectra to a library of model compounds and from t he noteworthy angular dependence of EXAFS spectra collected on self-support ing films of clay soil fractions. Analysis of higher correlations in EXAFS spectra suggests that the local structure around Zn in phyllosilicates is t rioctahedral. The phyllomanganate Zn-sorbed birnessite and Zn-containing Fe grains having a delta FeOOH-like local structure were unambiguously identi fied by mu SXRF-mu EXAFS. In birnessite Zn is sorbed in the interlayer spac e above/below vacant sites and can be either 4-fold or 6-fold coordinated d epending, presumably, on the anionic stacking of birnessite layers. Based o n this micro-mineralogical investigation, a satisfactory fit of the three i dentified Zn species (that is, phyllosilicate, Mn, and Fe (oxyhydr)oxides) to experimental powder EXAFS spectra of all clay soil fractions was obtaine d. The significance, origin, and stability of Zn-phyllosilicates are discus sed. Specifically, we show that the formation of Zn-containing phyllosilica tes 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 ne ar saturation (pH < 6) with respect to the trioctahedral Zn phyllosilicate, Zn-kerolite. Finally, the plausibility of the formation of (Zn,Al) hydrota lcite-like species contemplated by Julliot (1999) is critically assessed.