Alain Manceau

The mechanism of anion adsorption on iron oxides: Evidence for the bonding of arsenate tetrahedra on free Fe(O, OH)6 edges

Parallel studies conducted recently on the mechanism of adsorption of arsenate and selenate oxyanions on iron oxyhydroxides have resulted in different structural models. On the basis of EXAFS-derived AsFe distances, Waychunas et al. (1993) have postulated the existence of two kinds of arsenate surface complexes, both of them being bonded to A-type surface functional groups. One surface complex involves the sharing of two adjacent A-type groups (2C[hkl](hk0) surface complex, d(AsFe) = 3.25 A) and the other the sharing of two adjacent A-type group (1V[hk0](hk0) surface complex, d(AsFe) = 3.60 A). While the former complex has also been identified in the case of selenate, single corner 1V linkage between arsenate tetrahedra and Fe(O, OH)6 octahedra is not observed in the case of selenate. The second FeSe distance has been instead attributed to an edge (E) linkage (1E[001](001) surface complex, d(SeFe) = 2.80 A; Manceau and Charlet, 1994). Comparison of the EXAFS results obtained at the As and Se K-edges leads to the conclusion that this difference of structural behavior is not supported by the experimental data. The reason for this discrepancy is shown to result from a mistaken determination by Waychunas et al. (1993) of the AsFe distance of 3.60 A due to a 2π phase shift of the theoretical As-Fe electronic wave constructed for least-square fitting of the experimental data. The sorption of arsenate on free Fe(O, OH)6 edges is shown to be critical to understanding structural changes of the ferrihydrite structure at increasing As loading and ageing times. Specifically, it provides a molecular-level explanation for the poisoning of particles growth in the direction of octahedral Fe chains and for the slowing down of the transformation of ferrihydrite to hematite.

Forms of Zinc Accumulated in the Hyperaccumulator Arabidopsis halleri

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 μEXAFS. 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 non-contaminated) 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 μEXAFS. 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.

X-ray absorption spectroscopic study of the sorption of Cr(III) at the oxide-water interface: II. Adsorption, coprecipitation, and surface precipitation on hydrous ferric oxide

Abstract The sorption of Cr(III) by hydrous Fe oxides involves adsorption, surface precipitation, and coprecipitation phenomena. These phenomena lead to different phases which are here compared with regard to both their Cr solubility and their local structure. The former is simulated by the “surface precipitation model” and the latter is derived from X-ray absorption fine-structure (EXAFS) spectroscopic data interpreted by the “polyhedral approach method.” The adsorption of a Cr(III) atom onto goethite or hydrous ferric oxide (HFO) occurs via the formation of inner-sphere surface complexes. In such complexes, Cr atoms are never isolated, but present as small surface hydroxy polymers. This polymerization has been catalyzed by the surface, and it occurs when Cr(III) bonds only 10% of the surface “active” sites (i.e., when it covers 1% of the BET surface area). In these surface polymers, Cr(III) atoms are surrounded by three metal (Fe or Cr) shells at 3.00–3.05, 3.40–3.46, and 3.94–4.03 A, as in a mixed α- and γ-MeOOH local structure (Me = Fe or Cr). These polymers act as nuclei for the precipitation of a surface hydrous Cr oxide, a precipitation that starts under conditions undersaturated with respect to the homogeneous precipitation of the same oxide, a phenomenon well described by the surface precipitation model. This surface precipitate and the pure hydrous chromium oxide have identical solubility products and the same local structure (γ-CrOOH). On the contrary, when Cr(III) is coprecipitated with Fe(III), only two cation shells are detected around Cr(III) at 2.99 and 3.40 A, indicating that Cr substitutes for Fe in an α-(Fe, Cr)OOH framework. A difference in local structure (α-CrOOH versus γ-CrOOH) therefore accounts for the difference in solubility products of the surface-precipitated and coprecipitated Cr hydroxides.

Structure of Mn and Fe oxides and oxyhydroxides: a topological approach by EXAFS

The structure of Mn and Fe oxides and oxyhydroxides has been probed by EXAFS. It is shown that EXAFS spectroscopy is sensitive to the nature of interpolyhedral linkages relying on metal-two nearest metal distances. Spectra recorded at 290 K and 30 K indicate that intercationic distances can be determined by EXAFS with a good accuracy (0.02 Å) assuming a purely Gaussian distribution function, even at room temperature. Although the accuracy on atomic numbers determination is fair for these disordered systems, EXAFS can differentiate structures with contrasted edge- over corner-sharing ratio like pyrolusite, ramsdellite, todorokite and lithiophorite or lepidocrocite and goethite. A direct application of this result has shown that the proportion of pyrolusite domains within the lattice of nsutite from Ghana is equal to 35±15 percent. The systematic study of Mn dioxides also put forward the sensitivity of EXAFS to the presence of corner-sharing octahedra, with a detection limit found to be less than 8 percent. In spite of their similar XRD patterns, the EXAFS study of todorokite and asbolane confirms that they possess a distinct structure; that is, a tunnel structure for the former and a layered structure for the second.Such a topological approach has been used to probe the structure of ferruginous vernadite; a highly disordered iron-bearing Mn oxide. Fe and Mn K-edges EXAFS spectra are very dissimilar, traducing a different short range order. The Mn phase is constituted by MnO2 layers. Its large local structural order contrasts with the short range disorder of the iron phase. This hydrous Fe oxyhydroxide is constituted by face-, edge- and corner-sharing octahedra. This iron phase possesses the same local order as feroxy-hyte, but is long range disordered. The presence of face-sharing Fe(O,OH)6 octahedra prevents its direct solid-state transformation into well crystallized oxyhydroxides, and explains the necessary dissolution-reprecipitation mechanism generally invoked for the hydrous ferric gel → goethite transformation.

Structural model for the biogenic Mn oxide produced by Pseudomonas putida

Abstract X-ray diffraction (XRD) and Mn K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy were combined to elaborate a structural model for phyllomanganates (layer-type Mn oxides) lacking 3D ordering (turbostratic stacking). These techniques were applied to a sample produced by a common soil and freshwater bacterium (Pseudomonas putida), and to two synthetic analogs, δ-MnO2 and acid birnessite, obtained by the reduction of potassium permanganate with MnCl2 and HCl, respectively. To interpret the diffraction and spectroscopic data, we applied an XRD simulation technique utilized previously for well-crystallized birnessite varieties, complementing this approach with single-scattering-path simulations of the Mn K-edge EXAFS spectra. Our structural analyses revealed that all three Mn oxides have an hexagonal layer symmetry with layers comprising edgesharing Mn4+O6 octahedra and cation vacancies, but no layer Mn3+O6 octahedra. The proportion of cation vacancies in the layers ranged from 6 to 17%, these vacancies being charge-compensated in the interlayer by protons, alkali metals, and Mn atoms, in amounts that vary with the phyllomanganate species and synthesis medium. Both vacancies and interlayer Mn were most abundant in the biogenic oxide. The diffracting crystallites contained three to six randomly stacked layers and have coherent scattering domains of 19.42 Å in the c* direction, and of 60.85 Å in the a-b plane. Thus, the Mn oxides investigated here are nanoparticles that bear significant permanent structural charge resulting from cation layer vacancies and variable surface charge from unsaturated O atoms at layer edges.

Beamline 10.3.2 at ALS: a hard X-ray microprobe for environmental and materials sciences

Beamline 10.3.2 at the ALS is a bend-magnet line designed mostly for work on environmental problems involving heavy-metal speciation and location. It offers a unique combination of X-ray fluorescence mapping, X-ray microspectroscopy and micro-X-ray diffraction. The optics allow the user to trade spot size for flux in a size range of 5-17 microm in an energy range of 3-17 keV. The focusing uses a Kirkpatrick-Baez mirror pair to image a variable-size virtual source onto the sample. Thus, the user can reduce the effective size of the source, thereby reducing the spot size on the sample, at the cost of flux. This decoupling from the actual source also allows for some independence from source motion. The X-ray fluorescence mapping is performed with a continuously scanning stage which avoids the time overhead incurred by step-and-repeat mapping schemes. The special features of this beamline are described, and some scientific results shown.

Crystal chemistry of trace elements in natural and synthetic goethite

Abstract The crystal chemistry of Cr (0.73%), Mn (0.88%), Co (0.166%), Ni (0.898%), Cu (0.263%), and Zn (0.026%) in natural goethite (αFeOOH) from an earthy saprolite formation in West-Africa was investigated by fluorescence-yield extended X-ray absorption fine structure (EXAFS) spectroscopy. Synthetic goethites and phyllomanganates were used as model compounds for structural determinations. The measurement of EXAFS spectra at energies higher than the Fe K-edge (Co, Ni, Cu, and Zn) is notoriously difficult because the fluorescence signal from trace elements is typically two orders of magnitude less intense than the Fe fluorescence from the matrix. This difficulty was circumvented by measuring total intensity (signal + background) with high precision on the ID26 undulator EXAFS spectrometer of the European Synchrotron Radiation Facility (ESRF) with a fast, highly linear, and low-noise diode detector. Cr, Cu, and Zn have the same local structure in natural and synthetic goethites. With the exception of the Cu polyhedron, which is distorted by the Jahn-Teller effect, Me-O and Me-Fe distances are similar to Fe-O and Fe-Fe distances in goethite. No significant steric effect was observed around Cu and Zn in spite of the ∼14% increase in ionic radii compared to Fe3+. The compliance of the Fe site to these substitutional impurities probably is accomplished by displacement of nearest O and Fe shells (relaxation effect) and modification of interpolyhedral angles (compliance effect) owing to the corner-sharing topology of the goethite structure. X-ray absorption near-edge structure analysis reveals an average valence of ∼3.7 to 3.8 for Mn, and EXAFS indicates that manganese is present as a phyllomanganate impurity having a hexagonal layer symmetry like asbolane, lithiophorite, and hexagonal birnessite. Cobalt is trivalent and located in the phyllomanganate layer, and also possibly in the interlayer, substituting for Mn. Selective uptake of cobalt by the Mn oxide impurity results from the oxidation of soluble Co2+ to insoluble Co3+ by Mn3+/Mn4+, this mineral-surface redox reaction accounting for the well-known geochemical affinity of Co for Mn at the earth’s surface. No more than ∼20% of the amounts of Mn and Co in natural goethite substitute for Fe in the structure, if present at all. About 75% of total Ni is substituted for Fe in natural goethite and ∼25% is associated with the phyllomanganate impurity as a Ni(OH)2–MnO2 mixed-layer phase (Ni-asbolane). The Ni site in synthetic goethite is strongly relaxed with a contraction of the goethite structure in the plane of edge-sharing double chains (bc plane), and an expansion in the direction of corner linkages (approximately the ab plane). This anisotropic relaxation of the Ni site locally reduces the distortion of the goethite structure, and could be due to a clustering of Ni atoms.

Structure of synthetic monoclinic Na-rich birnessite and hexagonal birnessite: II. Results from chemical studies and EXAFS spectroscopy

Abstract Solution chemical techniques were used to study the conversion of synthetic Na-rich buserite (NaBu) to hexagonal (H+-exchanged) birnessite (HBi) at low pH. The low-pH reaction is broadly characterized by the exchange of structural Na+ with solution H+ and the partial loss of Mn2+ to the aqueous phase. The desorption of Na+ in two temporally distinct steps indicates the existence of two types of binding sites for this cation. Mn2+ appears to originate from a partial disproportionation of Mn31 in the NaBu layers, according to the sequence Mn3+layer + Mn3+layer → Mn4+layer + Mn2+layer → Mn4+layer + Vacancy + Mn2+aq. EXAFS measurements on Na-rich birnessite (NaBi) show that this mineral is primarily a layered structure formed by edge-sharing MnO6 octahedra, with no evidence for triplecorner (TC) sharing Mn. HBi is significantly different with strong evidence for TC-sharing Mn and therefore layer vacancies. The relative numbers of edge (E)-sharing and TC-sharing neighbors determined from EXAFS measurements on HBi is consistent with SAED results (Drits et al. 1997), which suggest that the layer vacancies are restricted to every third row of Mn cations, with 50% of the Mn sites along these rows vacant. The density of vacancies in the entire layer is therefore one in six of layer Mn sites. Polarized EXAFS measurements on orientated films of NaBi and HBi confirm the absence of TC-sharing Mn in NaBi and indicate that Mn adsorbed at layer vacancy sites in HBi at pH 4 is dominantly Mn3+. The intensity of the TC-sharing contribution to the Mn EXAFS spectra of HBi samples increases with increasing pH from pH 2 to 5, and supports a mechanism of formation involving both the direct migration of layer Mn3+ to interlayer TC-sharing positions and re-adsorption of Mn2+ from solution onto layer vacancy sites. The migration of Mn31 cations into the interlayer releases the steric strain associated with the Jahn-Teller distortion of these octahedra. This model of the NaBu-to-HBi conversion explains the transformation from orthogonal to hexagonal layer symmetry, respectively, as reported by Drits et al. (1997). Analysis of the Zn EXAFS spectrum of Zn21-exchanged birnessite shows that Zn2+ also occupies TC-sharing positions at layer vacancy sites. The results of this study strongly suggest that lattice cation vacancies are of critical importance in adsorption and electron transfer processes occurring at the surface of this mineral.

Progress in remediation and revegetation of the barren Jales gold mine spoil after in situ treatments

A series of single extractions and short-term plant tests were performed in order to test a variety of inexpensive mineral amendments for the in situ inactivation of trace elements on the fine-grained spoil of the former gold mine of Jales, Portugal. Based on the results of these tests, mesocosms (lysimeters) were constructed and a small-scale semi-field trial was carried out since 1998. The long-term effect of steelshots (SS, iron grit), beringite (B), and municipal compost (C) as spoil amendments was investigated. Vegetation establishment on the treated spoils was successful with Holcus lanatus L. in year 1 and Pinus pinaster Ait. in year 2. Therefore, a detailed monitoring program was implemented for determining the sustainability of trace elements in situ inactivation by C (5%), CB (5% compost combined with 5% beringite), CSS (5% compost combined with 1% steelshots), and CBSS (5% compost combined with 5% beringite and 1% steelshots) treatments (all amendments are expressed by soil dry weight) and of the revegetation. After 3 years, revegetation was excellent in the CSS treatment, and successful for the CBSS. Volunteer plant species became established in treated spoils during year 2. In contrast, the trees planted on the C treated spoil declined from year 2 and some died. In year 3, the trees on the CB treatment started to decline. Arsenic and zinc exposure are suggested to explain the negative effects on pine growth. In agreement with results of single extractions, compost addition in the spoil increased long-term arsenic percolation. Lead leaching was also enhanced. The CBSS and CSS treatments were the most effective for limiting water-soluble As and decreasing long-term metal leaching.

Structure of heavy metal sorbed birnessite. Part III: Results from powder and polarized extended X-ray absorption fine structure spectroscopy

The local structures of divalent Zn, Cu, and Pb sorbed on the phyllomanganate birnessite (Bi) have been studied by powder and polarized extended X-ray absorption fine structure (EXAFS) spectroscopy. Metal-sorbed birnessites (MeBi) were prepared at different surface coverages by equilibrating at pH 4 a Na-exchanged buserite (NaBu) suspension with the desired aqueous metal. Me/Mn atomic ratios were varied from 0.2% to 12.8% in ZnBi and 0.1 to 5.8% in PbBi. The ratio was equal to 15.6% in CuBi. All cations sorbed in interlayers on well-defined crystallographic sites, without evidence for sorption on layer edges or surface precipitation. Zn sorbed on the face of vacant layer octahedral sites (□), and shared three layer oxygens (Olayer) with three-layer Mn atoms (Mnlayer), thereby forming a tridentate corner-sharing (TC) interlayer complex (Zn-3Olayer-□-3Mnlayer). TCZn complexes replace interlayer Mn2+ (Mninter2+) and protons. TCZn and TCMninter3+ together balance the layer charge deficit originating from Mnlayer4+ vacancies, which amounts to 0.67 charge per total Mn according to the structural formula of hexagonal birnessite (HBi) at pH 4. At low surface coverage, zinc is tetrahedrally coordinated to three Olayer and one water molecule ([IV]TC complex: (H2O)-[IV]Zn-3Olayer). At high loading, zinc is predominantly octahedrally coordinated to three Olayer and to three interlayer water molecules ([VI]TC complex: 3(H2O)-[VI]Zn-3Olayer), as in chalcophanite ([VI]ZnMn34+O7·3H2O). Sorbed Zn induces the translation of octahedral layers from −a/3 to +a/3, and this new stacking mode allows strong H bonds to form between the [IV]Zn complex on one side of the interlayer and oxygen atoms of the next Mn layer (Onext): Onext…(H2O)-[IV]Zn-3Olayer. Empirical bond valence calculations show that Olayer and Onext are strongly undersaturated, and that [IV]Zn provides better local charge compensation than [VI]Zn. The strong undersaturation of Olayer and Onext results not only from Mnlayer4+ vacancies, but also from Mn3+ for Mn4+ layer substitutions amounting to 0.11 charge per total Mn in HBi. As a consequence, [IV]Zn,Mnlayer3+, and Mnnext3+ form three-dimensional (3D) domains, which coexist with chalcophanite-like particles detected by electron diffraction. Cu2+ forms a Jahn-Teller distorted [VI]TC interlayer complex formed of two oxygen atoms and two water molecules in the equatorial plane, and one oxygen and one water molecule in the axial direction. Sorbed Pb2+ is not oxidized to Pb4+ and forms predominantly [VI]TC interlayer complexes. EXAFS spectroscopy is also consistent with the formation of tridentate edge-sharing ([VI]TE) interlayer complexes (Pb-3Olayer-3Mn), as in quenselite (Pb2+Mn3+O2OH). Although metal cations mainly sorb to vacant sites in birnessite, similar to Zn in chalcophanite, EXAFS spectra of MeBi systematically have a noticeably reduced amplitude. This higher short-range structural disorder of interlayer Me species primarily originates from the presence of Mnlayer3+, which is responsible for the formation of less abundant interlayer complexes, such as [IV]Zn TC in ZnBi and [VI]Pb TE in PbBi.