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Studying speciation of heavy metals instead of their total concentration in a complex matrix such as soil is a scientific challenge that requires a combination of different analytical techniques. We compared the speciation of Cr within an andosol (island of Réunion) by using X-ray absorption spectroscopy (XAS) and sequential extraction. Contradictory results were obtained since the fraction of Cr bound to organic matter was detected only with the latter method. As bulk soil is rather complex, its fractionation by a densimetric method decreased its heterogeneity. We found that 60% Cr was within chromite-type primary minerals. Weathering of these phases led to Fe and Cr release, eventually resulting in either coprecipitation as mixed FeCr oxyhydroxide (16% total Cr) or precipitation of a Cr oxyhydroxide (5% total Cr). Our results also revealed that 13% Cr was bound to organic matter. The organic matter was complexed with mineral phases to form organomineral complexes with a density ranging from 1.9 to 2.6. The use of an original density fractionation-based sample preparation allowed identification of the role of organic matter in chromium speciation within an andosol and to overcome the difficulties of EXAFS to detect light elements in the vicinity of heavy elements.

The geometrical and electronic structure of the arsenious acid molecule As(OH)3 in aqueous solutions has been investigated by x-ray absorption spectroscopy (XAS) within extended x-ray absorption spectroscopy (EXAFS) and x-ray absorption near edge structure (XANES), using realistic first-principle calculations in the latter case. This investigation was performed on aqueous solutions of arsenious acid from ambient to supercritical conditions (P=250 and 600 bars, T≤500 °C) using a new optical cell. The analysis of the XAS spectra is consistent with (1) a constant As-O distance, (2) an opening of the O-As-O angles within the C3V pyramidal structure in the range 30–200 °C. This structural evolution comes along with a small decrease of the partial charges of the atoms in the As(OH)3 molecule. The explanation invoked for both structural and electronic modifications observed is the weakening of the interactions, through hydrogen bonds, between the As(OH)3 complex and water molecules. This is a fingerprint of the similar weakening of hydrogen bonding interactions in the solvent itself. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A bent-crystal spectrometer based on the Rowland circle geometry has been installed and tested on the BM30b/FAME beamline at the European Synchrotron Radiation Facility to improve its performances. The energy resolution of the spectrometer allows different kinds of measurements to be performed, including X-ray absorption spectroscopy, resonant inelastic X-ray scattering and X-ray Raman scattering experiments. The simplicity of the experimental device makes it easily implemented on a classical X-ray absorption beamline. This improvement in the fluorescence detection is of particular importance when the probed element is embedded in a complex and/or heavy matrix, for example in environmental sciences. [ABSTRACT FROM AUTHOR]

The oxidation state of chromium in glasses melted in an air atmosphere with and without refining agents was investigated by Cr K-edge X-ray absorption near-edge structure (XANES) and optical absorption spectroscopy. A good agreement in the relative proportion of Cr(III) and Cr(VI) is obtained between both methods. We show that the chemical dependence of the absorption coefficient of Cr(III) is less important in XANES than in optical absorption spectroscopy. The comparison of optical absorption and XANES spectra of glasses melted under different conditions provides an indirect assessment of the molar extinction coefficient of Cr(VI) in glasses. [ABSTRACT FROM AUTHOR]

A new set-up is reported of an indirect cryogenic cooling system for a double-crystal monochromator which runs on the BM30b/FAME beamline at the ESRF (Grenoble, France). This device has been conceived to limit the vibrations on the first diffracting crystal and to maintain it at a constant temperature. These points are crucial for maximizing the beamline stability. Moreover, the relative angular position of the second crystal can be dynamically adjusted by a piezoelectric transducer coupled with a feedback system in order to always be at the maximum of the rocking curve during an X-ray absorption spectroscopy scan. The temperature is stabilized to an accuracy of 0.01°, with two principal consequences. The energy resolution is close to the theoretical value [Δ E/ E = 5.6 × 10−6 for Si(220)] and the precision of the energy positioning is extremely good even if the power load changes. A feedback mechanism allows a permanent and automatic optimization of the angle between the two crystals of the monochromator. The intensity of the monochromatic beam remains optimized (i) when the intensity of the electron beam decreases in the storage ring and (ii) during an energy scan. [ABSTRACT FROM AUTHOR]

A high pressure/high temperature cell dedicated to x-ray absorption spectroscopy, small angle x-ray scattering, and inelastic x-ray scattering techniques is presented. The P and T parameters are controlled independently and their range allow the study of aqueous solutions (T<=500 °C and P<=2000 bar) and liquid metals and glasses (T<=1700 °C and P<=2000 bar). The autoclave technology is inspired from previous high pressure/high temperature equipments but great improvements are achieved. Original high pressure windows have been developed to ensure both pressure resistance and low absorbance combined with large angular aperture. Different configurations are available for the internal cell that contains the sample whether it is aqueous or not. As an example of the efficiency of the set-up, we present preliminary x-ray absorption results on 0.01 m FeCl3 aqueous solutions from ambient to supercritical conditions (375 °C and 300 bar). These low concentrations samples and low energy spectra (Fe K-edge is at 7112 eV) represent challenging experimental conditions. [ABSTRACT FROM AUTHOR]

Abstract: Dissolution of synthetic argutite (GeO2, tetragonal, rutile-like) in pure water, and local atomic structure around Ge(IV) in aqueous solution were characterized by in situ X-ray Absorption Fine Structure (XAFS) spectroscopy at the Ge K-edge using a new high temperature–high pressure cell. XAFS transmission and fluorescence spectra were collected from germanic acid aqueous solutions of 0.02–0.09 mol Ge/kg H2O at ambient pressure and temperature, and from a 0.04 mol Ge/kg H2O solution at 400 bar from 20 to 450 °C. Spectra at all temperatures and concentrations exhibit the 1st shell contribution from 4±0.3 oxygen atoms at 1.75±0.01 Å, consistent with a tetrahedral structure of the Ge(OH)4 complex which is a chemical analog of Si(OH)4 and is the dominant Ge aqueous species, in agreement with previous solubility and potentiometric measurements. The constancy of Ge(OH)4 structural parameters over the wide temperature and fluid-density range is consistent with the strong covalent character of the Ge–(OH) bonds. A dissolution experiment on argutite in pure H2O was carried out at 400 bar from 200 to 500 °C by monitoring the height of the Ge absorption edge of transmission spectra. The solubility of GeO2, derived from a classical X-ray absorption relation using the absorption height values and fluid density, increases with increasing temperature up to 400 °C but decreases at 450 and 500 °C, similar to that observed for quartz. The new solubility data were analyzed using both a density model and the revised HKF equation of state to better constrain Ge(OH)4 thermodynamic properties. Calculations using these models indicate that despite strong similarities in the structure and energetics of aqueous germanic acid and silica, differences in the thermodynamic properties of germanate and silicate dissolution reactions may account for the increase of the Ge/Si ratio observed in high-temperature natural fluids. [Copyright &y& Elsevier]

Cupriavidus metallidurans CH34 is a bacterium that is resistant to high metal concentrations in the environment. Increased copper resistance is associated with the cop cluster on the large plasmid pMOL30 that is composed of at least 21 genes. The copK gene encodes a 74 residue periplasmic protein whose expression is strongly upregulated in the presence of copper. CopK was previously shown to cooperatively bind Cu(I) and Cu(II) in distinct, specific sites. The solution structure of Cu(I)-CopK and the characterization of the Cu(I) site by X-ray absorption spectroscopy and NMR are reported here. EXAFS spectra are in agreement with a tetrathioether Cu(I) site, providing so far unique spectral information on a 4S-coordinated Cu(I) in a protein. The methionine residues forming the Cu(I) site, M28, M38, M44, and M54, are identified by NMR. We propose the chemical shift of the methionine ′1 as a new and sensitive probe for the detection of Cu(l) bound to thioether groups. The solution structure of Cu(I)-CopK demonstrates that Cu(I) binding induces a complete structural modification with the disruption of the second β-sheet and a rotation of the C-terminal part of nearly 180° around a hinge formed by asparagine 57. This conformational change is directly related to the loss of the dimer interface and most probably to the formation of the Cu(II) site involving histidine 70. The solution structure of Cu(I)-CopK therefore provides the molecular basis for the understanding of the Cu(I)/Cu(II) binding cooperativity. [ABSTRACT FROM AUTHOR]

Abstract: The solubility of nantokite (CuCl(s)) and the structure of the predominant copper species in supercritical water (290–400bar at 420°C; 350–450°C at 290bar; 500°C at 350bar; density=0.14–0.65g/cm3) were investigated concurrently using synchrotron X-ray absorption spectroscopy (XAS) techniques. These conditions were chosen as they represent single phase solutions near the critical isochore, where the fluid density is intermediate of typical values for vapour and brine and is highly sensitive to even small changes in pressure. X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption spectroscopy (EXAFS) analyses show that aqueous copper occurs in a slightly distorted linear coordination in the solutions studied, with an average of 1.35(±0.3) Cl and 0.65(±0.3) O neighbours. The solubility of CuCl(s) decreases exponentially with decreasing water density (i.e., decreasing pressure at constant temperature), in a manner similar to the solubility behaviour of salts such as NaCl in water vapour. Based on this similarity, an apparent equilibrium constant for the dissolution reaction of 0.5±0.4 was calculated from a regression of the data at 420°C, and it was determined that each Cu atom is solvated by approximately three water molecules. This indicates that under these conditions, copper solubility is controlled mainly by the structure of the second-shell hydration, which is essentially invisible to the XAS techniques used in this study. These results demonstrate that for a supercritical fluid near the critical isochore, decreasing pressure may initiate precipitation of copper even before boiling or phase separation. Such a process could be responsible for near-surface ore deposition in seafloor hydrothermal systems, where supercritical fluids experience rapid pressure changes during the transition between lithostatic and hydrostatic domains. [Copyright &y& Elsevier]

Studying speciation of heavy metals instead of their total concentration in a complex matrix such as soil is a scientific challenge that requires a combination of different analytical techniques. We compared the speciation of Cr within an andosol (island of Réunion) by using X-ray absorption spectroscopy (XAS) and sequential extraction. Contradictory results were obtained since the fraction of Cr bound to organic matter was detected only with the latter method. As bulk soil is rather complex, its fractionation by a densimetric method decreased its heterogeneity. We found that 60% Cr was within chromite-type primary minerals. Weathering of these phases led to Fe and Cr release, eventually resulting in either coprecipitation as mixed Fe-Cr oxyhydroxide (16% total Cr) or precipitation of a Cr oxyhydroxide (5% total Cr). Our results also revealed that 13% Cr was bound to organic matter. The organic matter was complexed with mineral phases to form organomineral complexes with a density ranging from 1.9 to 2.6. The use of an original density fractionation-based sample preparation allowed identification of the role of organic matter in chromium speciation within an andosol and to overcome the difficulties of EXAFS to detect light elements in the vicinity of heavy elements. [ABSTRACT FROM AUTHOR]

Abstract: Solubility of senarmontite (Sb2O3, cubic) in pure water and NaCl–HCl aqueous solutions, and local atomic structure around antimony in these fluids were characterized using in situ X-ray absorption fine structure (XAFS) spectroscopy at temperatures to 450°C and pressures to 600bars. These experiments were performed using a new X-ray cell which allows simultaneous measurement of the absolute concentration of the absorbing element in the fluid, and atomic environment around the absorber. Results show that aqueous Sb(III) speciation is dominated by the complex in pure water, mixed Sb-hydroxide-chloride complexes in acidic NaCl–HCl solutions (2m NaCl–0.1m HCl), and by Sb-chloride species in concentrated HCl solutions (3.5m HCl). Interatomic Sb–O and Sb–Cl distances in these complexes range from 1.96 to 1.97Å and from 2.37 to 2.47Å, respectively. These structural data, together with senarmontite solubility determined from XAFS spectra, were complemented by batch-reactor measurements of senarmontite and stibnite (Sb2S3, rhombic) solubilities over a wide range of HCl and NaCl concentrations from 300 to 400°C. Analysis of the whole dataset shows that Sb(III) speciation in high-temperature moderately acid (pH>2–3) Cl-rich fluids is dominated by mixed hydroxy-chloride species like Sb(OH)2Cl° and Sb(OH)3Cl−, but other species containing two or three Cl atoms appear at higher acidities and moderate temperatures (⩽300°C). Calculations using stability constants retrieved in this study indicate that mixed hydroxy-chloride complexes control antimony transport in saline high-temperature ore fluids at acidic conditions. Such species allow for a more effective Sb partitioning into the vapor phase during boiling and vapor–brine separation processes occurring in magmatic-hydrothermal systems. Antimony hydroxy-chloride complexes are however minor in the neutral low- to moderate-temperature solutions (⩽250–300°C) typical of Sb deposits formation; the antimony speciation in these systems is dominated by Sb(OH)3 and potentially Sb-sulfide species. [Copyright &y& Elsevier]