Michel L. Schlegel

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.

Structures of quartz (100)- and (101)-water interfaces determined by x-ray reflectivity and atomic force microscopy of natural growth surfaces

Abstract The structures of prismatic (100) and pyramidal (101) growth faces of natural quartz crystals, and their modification upon annealing at T ≤ 400°C were investigated ex situ by atomic force microscopy (AFM) and in water by high-resolution X-ray reflectivity. AFM images revealed the presence of ∼ 0.1 to 1 μm-wide flat terraces delimited by steps of one to several unit cells in height. These steps follow approximately directions given by the intersection of growth faces. Modeling of X-ray reflectivity data indicates that surface silica groups on flat terraces have only one free Si-O bond each (presumably hydroxylated), except for some having two free Si-O bonds observed on a single (100) surface. Vertical relaxation of atomic positions (

Structural evidence for the sorption of Ni(II) atoms on the edges of montmorillonite clay minerals: A polarized X-ray absorption fine structure study

The nature of surface complexes formed on Ni uptake onto montmorillonite (a dioctahedral smectite) has been investigated over an extended time period by polarized extended X-ray absorption fine structure (P-EXAFS) spectroscopy. Self-supporting films of Ni-sorbed montmorillonite were prepared by contacting Ni and montmorillonite at pH 7.2, high ionic strength (0.3 M NaClO4), and low Ni concentration ([Ni]initial = 19.9 μM) for 14- and 360-d reaction time. The resulting Ni concentration on the clay varied from 4 to 7 μmol/g. Quantitative texture analysis indicates that the montmorillonite particles were well orientated with respect to the plane of the film. The full width at half maximum of the orientation distribution of the c* axes of individual clay platelets about the normal to the film plane was 44.3° (14-d reaction time) and 47.1° (360-d reaction time). These values were used to correct the coordination numbers determined by P-EXAFS for texture effects. Ni K-edge P-EXAFS spectra were recorded at angles between the incident beam and the film normal equal to 10, 35, 55, and 80°. Spectral analysis led to the identification of three nearest cationic subshells containing 2.0 ± 0.5 Al at 3.0 A and 2.0 ± 0.5 Si at 3.12 A and 4.0 ± 0.5 Si at 3.26 A. These distances are characteristic of edge-sharing linkages between Al and Ni octahedra and of corner-sharing linkages between Ni octahedra and Si tetrahedra, as in clay structures. The angular dependence of the Ni-Al and Ni-Si contributions indicates that Ni-Al pairs are oriented parallel to the film plane, whereas Ni-Si pairs are not. The study reveals the formation of Ni inner-sphere mononuclear surface complexes located at the edges of montmorillonite platelets and thus that heavy metals binding to edge sites is a possible sorption mechanism for dioctahedral smectites. Data analysis further suggests that either the number of neighboring Al atoms slightly increases from 1.6 to 2 or that the structural order of the observed surface complexes increases from 0.01 A2 to 0.005 A2 with increasing reaction time. On the basis of the low Ni-Al coordination numbers, it appears that over an extended reaction time period of 1 yr the diffusion of Ni atoms in the octahedral layer is not the major uptake mechanism of Ni onto montmorillonite.

Sorption of metal ions on clay minerals. III. Nucleation and epitaxial growth of Zn phyllosilicate on the edges of hectorite

Abstract The impact of dissolved Si ([Si]aq) on Zn uptake in dilute suspensions (0.65 g/L) of hectorite was investigated at pH 7.30, a total Zn concentration (TotZn) of 520 μM, and ionic strength of 0.3 M (NaNO3 salt) by kinetics experiments and polarized extended X-ray absorption fine structure (P-EXAFS) spectroscopy. At low [Si]aq (∼30 to 60 μM), 5.8% of TotZn was adsorbed within the first 3 h of reaction. The sorption rate was lower afterwards, and Zn uptake amounted to 14.6% of TotZn after 168 h of reaction. These rates are consistent with Zn sorption on pH-dependent edge sites of hectorite platelets. At high [Si]aq (∼530 μM), a higher initial sorption rate was observed, the fraction of Zn removed amounting to 15.2% of TotZn at t = 3 h and 90.7% at t = 120 h. After 9 h of reaction time, Si uptake also occurred; the Si/Zn uptake ratio (1.09 ± 0.08) was between those of TO (∼0.67) and TOT (∼1.33) trioctahedral phyllosilicates, which suggests the neoformation of a Zn phyllosilicate. In the absence of hectorite, neither Zn nor Si were removed from solution, even at high [Si]aq, indicating that Zn uptake occurred by sorption on hectorite surface. Comparison of spectra for sorption samples and Zn references indicated that sorbed Zn was located in a clay-like structural environment. The angular dependence observed for all P-EXAFS spectra demonstrated that Zn cations are structurally attached to the edges of hectorite platelets. The size and structure of these Zn surface complexes varied with [Si]aq and reaction time. At low [Si]aq and after a long reaction time (t = 96 h), Zn was surrounded by in-plane 1.7 ± 0.6 Zn and 1.4 ± 0.3 Mg at 3.08 A, and by out-of-plane 0.6 ± 1.1 Si at 3.28 A. These results point to predominant formation of small polymers containing on average two to three Zn cations and located in structural continuity with the hectorite octahedral sheet. At high [Si]aq, higher numbers of Zn and Si and lower numbers of Mg neighbors were detected at t = 9 h; at t = 120 h, Zn was surrounded by in-plane 6.0 ± 0.4 Zn at 3.10 A and by out-of-plane 3.6 ± 0.4 Si at 3.27 A as in a Zn phyllosilicate. These results document for the first time the nucleation and epitaxial growth at ambient temperature of Zn phyllosilicate at the edges of smectite minerals under controlled laboratory conditions.

Neoformation of Ni phyllosilicate upon Ni uptake on montmorillonite: A kinetics study by powder and polarized extended X-ray absorption fine structure spectroscopy

Abstract Wet chemistry kinetics and powder and polarized extended X-ray absorption fine structure (EXAFS and P-EXAFS) spectroscopy were combined to investigate the mechanism of Ni uptake on montmorillonite, at pH 8, high ionic strength (0.2 M Ca(NO3)2), initial Ni concentration of 660 μM, and solid concentration of 5.3 g/L. Approximately 20% of Ni sorbed within the first 24 h; thereafter, the Ni uptake rate slowed, and 12% of the initial Ni concentration remained in solution after 206 d of reaction time. Powder EXAFS spectra collected on wet pastes at 1, 14, 90, and 206 d showed the presence of Ni-Ni pairs at ∼3.08 A in an amount that gradually increased with time. Results were interpreted by the nucleation of a Ni phase having either an α-Ni-hydroxide– or a Ni-phyllosilicate–like local structure. The latter possibility was confirmed by recording P-EXAFS spectra of a highly textured, self-supporting montmorillonite film prepared in the same conditions as the wet samples and equilibrated for 14 d. The orientation distribution of the c*-axes of individual clay particles off the film plane, as measured by quantitative texture analysis, was 32.8° full width at half maximum, and this value was used to correct from texture effect the effective numbers of Ni and Si nearest neighbors determined by P-EXAFS. Ni atoms were found to be surrounded by 2.6 ± 0.5 Ni atoms at 3.08 A in the in-plane direction and by 4.2 ± 0.5 Si atoms at 3.26 A in the out-of-plane direction. These structural parameters, but also the orientation and angular dependence of the Ni and Si shells, strongly support the formation of a Ni phyllosilicate having its layers parallel to the montmorillonite layers. The neoformation of a phyllosilicate on metal uptake on montmorillonite, documented herein for the first time, has important geochemical implications because this dioctahedral smectite is overwhelmingly present in the environment. The resulting sequestration of sorbed trace metals in sparingly soluble phyllosilicate structure may durably decrease their migration and bioavailability at the Earth’s surface and near surface.

Anodic Activation of Iron Corrosion in Clay Media under Water-Saturated Conditions at 90 °C: Characterization of the Corrosion Interface

To understand the process governing iron corrosion in clay over centuries, the chemical and mineralogical properties of solids formed by free or anodically activated corrosion of iron in water-saturated clay at 90 degrees C over 4 months were probed using microscopic and spectroscopic techniques. Free corrosion led to the formation of an internal discontinuous thin (<3 microm thick) magnetite layer, an external layer of Fe-rich phyllosilicate, and a clay transformation layer containing Ca-doped siderite (Ca(0.2)Fe(0.8)CO(3)). The thickness of corroded iron equaled approximately 5-7 microm, consistent with previous studies. Anodic polarization resulted in unequally distributed corrosion, with some areas corrosion-free and others heavily corroded. Activated corrosion led to the formation of an inner magnetite layer, an intermediate Fe(2)CO(3)(OH)(2) (chukanovite) layer, an outer layer of Fe-rich 7 A-phyllosilicate, and a transformed matrix layer containing siderite (FeCO(3)). The corroded thickness was estimated to 85 microm, less than 30% of the value expected from the supplied anodic charge. The difference was accounted for by reoxidation at the anodically polarized surface of cathodically produced H(2)(g). Thus, free or anodically activated corroding conditions led to structurally similar interfaces, indicating that anodic polarization can be used to probe the long-term corrosion of iron in clay. Finally, corrosion products retained only half of Fe oxidized by anodic activation. Missing Fe probably migrated in the clay, where it could interact with radionuclides released by alteration of nuclear glass.

Oxidation of FeS by oxygen-bearing acidic solutions

Oxidation of FeS in oxygen-bearing acidic solutions was investigated at different temperatures (25 to 45 degrees C) and pH (2.75 to 3.45). The rate of the oxidative dissolution of FeS is strongly dependent on pH. The reaction order with respect to hydrogen ions has been found to be 1.03+/-0.02 at 25 degrees C, and the apparent activation energy (E(a)) is 41.6 +/- 10.7 kJ mol(-1) at initial pH 3.00, suggesting that the FeS oxidative dissolution is controlled by the diffusion of oxidant species across a sulfur-rich layer (SRL) that undergoes chemical transformations leading to an increase in the mean number of sulfur atoms in polysulfide chains and the rearrangement of these chains. Fourier transform infrared spectroscopy and X-ray diffraction results obtained for the FeS samples reacted for 72 h at 25 degrees C and pH between 2.75 and 3.45 indicate the formation of goethite, of lepidocrocite, and of poorly ordered solid phases (assigned as SRL) on initial surfaces. The experimental data suggest a mechanism based on the protonation of FeS surfaces followed by oxidation of FeS by dissolved oxygen to produce Fe(2+), S(0), and S(2-)(n). Fe(2+) is unstable under oxidative conditions and transforms into Fe(OH)(3(s)), goethite and lepidocrocite.

Application of Polarized Exafs to Fine-Grained Layered Minerals

CONTENTS Introduction 69 Limitations of Powder EXAFS Spectroscopy 7 0 Overlap of Atomic Shell Contributions 70 Differentiation Between Dioctahedral and Trioctahedral Frameworks 72 Principles of P-EXAFS 74 Methodology 81 Obtaining the Out-of-plane EXAFS Spectrum 81 Texture of Self-supporting Films 82 Applications 87 Dioctahedral Clay Structures 87 Trioctahedral Clay Structures 93 Phyllomanganates 97 CoOOH 100 Conclusions 110 Acknowledgments 110 References 110

Sites of Lu(III) sorbed to and coprecipitated with hectorite.

The Lu(III) binding mechanisms by trioctahedral smectite hectorite in aqueous systems were investigated by extended X-ray absorption fine structure (EXAFS) spectroscopy. Coprecipitated hectorite (Lu755Hec), its precursor phase (Lu/Brucite), and the surface sorbed hectorite (Lu/SHCa1) were prepared as oriented samples to collect polarized EXAFS (P-EXAFS) data. EXAFS analysis indicated that Lu(III) is 6-fold coordinated by oxygen in Lu/Brucite and in Lu755Hec, and surrounded by Mg/Si shells. The angular dependence of the O and Mg coordination numbers for Lu/Brucite hinted an Lu(III) incorporation in brucite layers. Mg and Si cationic shells were detected at distances suggesting a clay-like octahedral environment in Lu755Hec. EXAFS data for Lu/SHCa1 were consistent with Lu(III) forming inner-sphere surface complexes at hectorite platelets edges, but slightly above/below the octahedral plane. Finally, Lu(III) polyhedra share edge(s) and corner(s) with Si tetrahedra upon sorption to silica (Lu/Silica). Lu(III) binding to silicate oligomers or to silicate groups of the clay basal planes and formation of Lu(III) surface complexes during the coprecipitation experiment are marginal.

Ni clay neoformation on montmorillonite surface.

Polarized extended X-ray absorption fine structure spectroscopy (P-EXAFS) was used to study the sorption mechanism of Ni on the aluminous hydrous silicate montmorillonite at high ionic strength (0.3 M NaClO4), pH 8 and a Ni concentration of 0.66 mM. Highly textured self-supporting clay films were obtained by slowly filtrating a clay suspension after a reaction time of 14 days. P-EXAFS results indicate that sorbed Ni has a Ni clay-like structural environment with the same crystallographic orientation as montmorillonite layers.