Showing papers by "Bruno Lanson published in 2000"

Quantitative zn speciation in smelter-contaminated soils by exafs spectroscopy

Abstract: 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-hyperaccumulator 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 remedia- tion technologies. In this study, X-ray diffraction (XRD), X-ray texture goniom- etry, 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 (SXRF) and mi- cro-EXAFS (EXAFS) were also used for one of these soils. Of these techniques, the application of P-EXAFS and 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 ·H 2O), and Zn-containing magnetite ((Fe,Zn)Fe2O4) were identi- fied in dense soil fractions by XRD and powder EXAFS. These primary minerals originate from atmospheric fallout of Zn dusts emitted during the pyrometallurgi- cal 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 (oxyhydr) oxides. Zn-containing phyllosilicates were identi- fied 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 trioctahe- dral. The phyllomanganate Zn-sorbed birnessite and Zn-containing Fe grains having a FeOOH-like local structure were unambiguously identified bySXRF— 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 investiga- tion, 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 phyllosili- cates is consistent with calculated thermodynamic solubilities. For the range of measured Zn 2 (D10 ppm), Si(OH)4 (10-20 ppm), and H (5.6 F pH F 7.5) concen- trations, soil solutions are supersaturated (pH G 6) or near saturation (pH F 6) with respect to the trioctahedral Zn phyllosilicate, Zn-kerolite. Finally, the plausi- bility of the formation of (Zn,Al) hydrotalcite-like species contemplated by Julliot (1999) is critically assessed.

Structure of H-exchanged hexagonal birnessite and its mechanism of formation from Na-rich monoclinic buserite at low pH.

Abstract: The structural transformation of high pH Na-rich buserite (NaBu) to H-exchanged hexagonal birnessite (HBi) at low pH was studied by simulation of experimental X-ray diffraction patterns. Four HBi samples were prepared by equilibration of NaBu at constant pH in the range pH 5-2. The samples differ from each other by the presence of one (at pH 2 and 3) or two (at pH 4 and 5) phases, and by the structural heterogeneity of these phases which decreases with decreasing pH. The sample obtained at pH 5 is a 4:1 physical mixture of a 1H phase ( a = 4.940 A, b = a /√3 = 2.852 A, c = 7.235 A, β = 90°, γ = 90°) and of a 1M phase ( a = 4.940 A, b = a /√3 = 2.852 A, c = 7.235 A, β = 119.2°, γ = 90°) in which successive layers are shifted with respect to each other by + a /3 along the a axis as in chalcophanite. Both the 1H and 1M phases contain very few well-defined stacking faults at pH 5. At pH 4, the sample is a 8:5 physical mixture of a 1H phase containing 15% of monoclinic layer pairs and of a 1M phase containing 40% of orthogonal layer pairs. Any further decrease of the pH leads to the formation of a single defective 1H phase. This 1H phase contains 20% and 5% of monoclinic layer pairs at pH 3 and 2, respectively. Independent of pH, all phases contain 0.833 Mnlayer cations, 0.167 vacant layer sites, and 0.167 interlayer Mn cations located either above or below layer vacancies per octahedron. A structural formula is established at each pH. The origin of the observed phase and structural heterogeneities has been analyzed. 1H and 1M phases are assumed to inherit their specific structural and crystal chemical features from the two distinct NaBu modifications. NaBu type I, with a high proportion of Mn4+layer cations, is thought to be responsible for the monoclinic layer stacking because this configuration allows Mn cations from adjacent layers to be as far as possible from each other, thus minimizing the electrostatic repulsion between these high charge cations. In contrast, NaBu type II has a high interlayer charge induced by Mn3+layer for Mn4+layer substitutions. Consequently, the 1H phase has a high amount of interlayer protons and achieves compensation of the unfavorable overlap of layer and interlayer Mn cations, in projection on the ab -plane, by the presence of strong hydrogen bondings between layers. The higher proportion of defined stacking faults in both 1H and 1M phases at pH 4 compared to pH 5 can be attributed to the increase in reaction rate with decreasing pH. At lower pH (3 and 2) the formation of strong hydrogen bonds between adjacent layers controls the layer stacking mode and leads to the formation of a unique 1H phase. The proportion of well-defined stacking faults in this phase decreases from pH 3 to 2.

Oxidation-reduction mechanism of iron in dioctahedral smectites: I. Crystal chemistry of oxidized reference nontronites

Abstract: The crystal chemistry of Fe in four nontronites (Garfield, Panamint Valley, SWa-1, and NG-1) was investigated by chemical analysis, X-ray goniometry, X-ray absorption pre-edge spectroscopy, powder and polarized extended X-ray absorption fine structure (EXAFS, P-EXAFS) spectroscopy, and X-ray diffraction. The four reference nontronites have Fe/(Fe + Al + Mg) ratios ranging from 0.58 to 0.78, and are therefore representative of the different chemical compositions of dioctahedral ferruginous smectites. Pre-edge and powder EXAFS spectroscopy indicate that NG-1 contains 14 to 20% of tetrahedrally coordinated Fe 3+ , whereas the other three samples have no detectable IV Fe 3+ . The partitioning of VI Fe 3+ between cis (M2) and trans (M1) sites within the octahedral sheet was determined from the simulation of X-ray diffraction patterns for turbostratic nontronite crystallites by varying the site occupancy of Fe. Based on this analysis, the four nontronite samples are shown to be trans-vacant within the detection limit of 5% of total iron. The in-plane and out-of-plane local structure around Fe atoms was probed by angular P-EXAFS measurements performed on highly oriented, self-supporting films of each nontronite. The degree of parallel orientation of the clay layers in these films was determined by texture goniometry, in which the half width at half maxi- mum of the deviation of the c* axis of individual crystallites from the film plane normal, was found to be 9.9° for Garfield and 19° for SWa-1. These narrow distributions of orientation allowed us to treat the self-supporting films as single crystals during the quantitative analysis of polarized EXAFS spectra. The results from P-EXAFS, and from infrared spectroscopy (Madejova et al. 1994), were used to build a two-dimensional model for the distribution of Fe, and (Al,Mg) in sample SWa-l. In this nontronite, Fe, Al, and Mg atoms are statistically distributed within the octahedral sheet, but they exhibit some tendency toward local ordering. Fe-Fe and (Al, Mg)-(Al,Mg) pairs are preferen- tially aligned along the (010) direction and Fe-(Al,Mg) pairs along the (31 - 0), and (3 - 1 0) directions. This distribution is compatible with the existence of small Fe domains separated by (Al,Mg), and empty octahedra, which segregation may account for the lack of magnetic ordering observed for this sample at low temperature (5 K) (Lear and Stucki 1990).

Oxidation-reduction mechanism of iron in dioctahedral smectites: II. Crystal chemistry of reduced Garfield nontronite

Abstract: The crystallochemical structure of reduced Garfield nontronite was studied by X-ray absorption pre-edge and infrared (IR) spectroscopy, powder X-ray diffraction, polarized extended X-ray absorption fine structure (P-EXAFS) spectroscopy, and texture goniometry. Untreated and highly reduced (>99% of total Fe as Fe 2+ ) nontronite samples were analyzed to determine the coordination number and the crystallographic site occupation of Fe 2+ , changes in in-plane and out-of-plane layer structure and mid-range order between Fe centers, and to monitor the changes in structural and adsorbed OH/H 2 O groups in the structure of reduced nontronite. Contrary to earlier models predicting the formation of fivefold coordinated Fe in the structure of nontronites upon reduction, these new results revealed that Fe maintains sixfold coordination after complete reduction. In-plane P-EXAFS evidence indicates that some of the Fe atoms occupy trans-sites in the reduced state, forming small trioctahedral domains within the structure of reduced nontronite. Migration of Fe from cis- to trans sites during the reduction process was corroborated by simulations of X-ray diffraction patterns which revealed that about 28% of Fe 2+ cations exist in trans sites of the reduced nontronite, rather than fully cis occupied, as in oxidized nontronite. Out-of-plane P-EXAFS results indicated that the reduction of Fe suppressed basal oxygen corrugation typical of dioctahedral smectites, and resulted in a flat basal surface which is characteristic of trioctahedral layer silicates. IR spectra of reduced nontronite revealed that the dioctahedral nature of the nontronite was lost and a band near 3623 cm −1 formed, which is thought to be associated with trioctahedral [Fe 2+ ] 3 OH stretching vibrations. On the basis of these results, a structural model for the reduction mechanism of Fe 3+ to Fe 2+ in Garfield nontronite is proposed that satisfies all structural data currently available. The migration of reduced Fe ions from cis-octahedra to adjacent trans-octahedra is accompanied by a dehydroxylation reaction due to the protonation of OH groups initially coordinated to Fe. This structural modification results in the formation of trioctahedral Fe 2+ clusters separated by clusters of vacancies in which the oxygen ligands residing at the boundary between trioctahedral and vacancy domains are greatly coordination undersaturated. The charge of these O atoms is compensated by the incorporation of protons, and by the displacement of Fe 2+ atoms from their ideal octahedral position toward the edges of trioctahedral clusters, thus accounting for the incoherency of the Fe-Fe1 and Fe-Fe2 distances. From these results, the ideal structural formula of reduced Garfield nontronite is Na 1.30 [Si 7.22 Al 0.78 ] [Fe 2+ 3.65 Al 0.32 Mg 0.04 ]O 17.93 (OH) 5 in which the increased layer charge due to reduction of Fe 3+ to Fe 2+ is satisfied by the incorporation of protons and interlayer Na.

Vermiculitization of smectite interfaces and illite layer growth as a possible dual model for illite-smectite illitization in diagenetic environments : a synthesis

Abstract: A structural model is proposed for illite-smectite (I-S) from diagenetic environments which accounts for the presence of three different layer types which are defined as follows: montmorillonite (low-charge, octahedrally substituted, fully expandable), vermiculite (high-charge, octa- and tetrahedrally substituted, only partly expandable) and illite (K 0.9 Si 3.3 Al 0.7 R 3+ 1.8 R 2+ 0.2 O 10 (OH) 2 ). All three layers may be found within the MacEwan crystallites, whereas external edges of the crystallites are only vermiculitic during the illitization process. In the proposed model, a layer is defined symmetrically on each side of the interlayer space, leading to the existence of polar 2:1 units. It is proposed that the I-S growth is a three step mechanism: (1) formation, from sediments of variable composition, of montmorillonite crystallites; (2) vermiculitization of the montmorillonite crystallite interfaces and of inner montmorillonite layers; and (3) precipitation of illite of fixed chemical composition. The I-S crystal grows by addition of illite layers linked by K + or NH + 4 ions saturating the vermiculitic interfaces.