Hydrogen bonding and coordination in normal and supercritical water from X-ray inelastic scattering

A direct measure of hydrogen bonding in water under conditions ranging from the normal state to the supercritical regime is derived from the Compton scattering of inelastically-scattered X-rays. First, we show that a measure of the number of electrons $n_e$ involved in hydrogen bonding at varying thermodynamic conditions can be directly obtained from Compton profile differences. Then, we use first-principles simulations to provide a connection between $n_e$ and the number of hydrogen bonds $n_{HB}$. Our study shows that over the broad range studied the relationship between $n_e$ and $n_{HB}$ is linear, allowing for a direct experimental measure of bonding and coordination in water. In particular, the transition to supercritical state is characterized by a sharp increase in the number of water monomers, but also displays a significant number of residual dimers and trimers.Comment: 14 pages, 5 figures, 1 tabl

The influence of high hydrostatic pressure on bacterial dissimilatory iron reduction

International audienceThe impact of deep-subsurface pressure conditions on microbial activity is still poorly constrained. In particular it is unknown how pressure of deep environments affects microbial transformations of iron. We investigated the effects of high hydrostatic pressure (HHP) on the rate and the extent of bacterial dissimilatory iron reduction (DIR). We employed a novel experimental setup that enables in situ monitoring of Fe oxidation state and speciation in bacterial cultures in an optimized HHP incubation system using X-ray Absorption Near-Edge Structure (XANES) spectroscopy. The iron-reducing bacterium Shewanella oneidensis MR-1 was incubated at 30 degrees C with Fe(III) citrate and tryptone at pressures between 0.1 and 100 MPa. For pressures up to 70 MPa strain MR-1 (10(8) cells ml(-1)) was able to reduce all 5 mM Fe(III) provided. Above 70 MPa, the final amount of Fe(III) that MR-1 could reduce decreased linearly and DIR was estimated to stop at 109 +/- 7 MPa. The decrease in the reduction yield was correlated with the dramatic decrease in survival (as determined by CFU counts) above 70 MPa. The initial rate of DIR increased with pressure up to 40 MPa, then decreased to reach zero at about 110 MPa. Increased rates of DIR activity and relatively high growth rates for pressures below 40 MPa would potentially ensure the maintenance of MR-1 in most of deep subsurface environments where moderate pressures occur, i.e. deep-sea environments. This study not only provides the first in situ quantitative results for microbial iron metabolism under HHP conditions but also sets the stage for future investigations of deep-sea pressure-adapted iron reducers. Moreover it demonstrates for the first time that XANES at the Fe K-edge is a powerful probe for in vivo monitoring of iron transformations in living microbial cultures. (C) 2012 Elsevier Ltd. All rights reserved

Pressure-induced amorphization mechanism in Eu(2)(MoO(4))(3)

International audienceWe identify a mechanism responsible for amorphization of rare-earth molybdate isostructural compounds by means of x-ray absorption spectroscopy (XAS) and Raman spectroscopy as well as first-principles calculations. Our present Raman spectra show that both beta' and alpha-Eu(2)(MoO(4))(3) undergo a pressure-induced amorphization. Both first-principles calculations and XAS measurements explain the process of amorphization by a spatial self-reorganization of the oxygen clouds around the Mo and Eu subnetworks as the pressure is increased inducing a change in the coordination number of the Mo atoms. This latter transforms the eg crystal field of the tetrahedra into a series of stabilization-degenerated peaks, which energetically favors a disordered overlapping of the former MoO(4) tetrahedra instead of a mere ordered compression and rotation of these fragment

Dissolution of magnetite in hydrothermal solutions: Kinetics and speciation by in situ X-ray absorption spectroscopy

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The solubility of nantokite (CuCl(s)) and Cu speciation in low-density fluids near the critical isochore: An in-situ XAS study

The solubility of nantokite (CuCl(s)) and the structure of the predominant copper species in supercritical water (290-400 bar at 420 °C; 350-450 °C at 290 bar; 500 °C at 350 bar; density = 0.14-0.65 g/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. © 2008 Elsevier Ltd. All rights reserved.Weihua Liu, Joël Brugger, Barbara Etschmann, Denis Testemale, Jean-Louis Hazeman

Formation of As(II)-pyrite during experimental replacement of magnetite under hydrothermal conditions

A 'new' type of arsenian pyrite was formed during experimental replacement of magnetite under hydrothermal conditions (T=125 and 220°C; Psat) and in the presence of S(-II) and various As-containing species. The amount of As in pyrite depended on the As-source, with sources containing cationic As (As(II), As(III) and As(V)) resulting in considerably higher amounts of As in the product arsenian pyrite than anionic sources. The highest As content was 23.83±0.20wt%, corresponding to a S:Fe:As molar ratio of 2:0.58:0.42. Electron probe micro-analyses revealed an inverse correlation between the Fe and As contents in the arsenian pyrite, indicating that As is substituting for Fe. Arsenic concentrations were highly inhomogeneous within the pyrite rim; in general, higher As contents were found within solid pyrite growing on the outer rim, compared to the highly porous and texturally complex pyrite found close to the reaction boundary. This likely reflects different uptake mechanisms for As during the pyrite nucleation and growth stages. X-ray Absorption Near Edge Structure (XANES) analyses showed that the As in the arsenian pyrite was predominantly in the form of As(II). Cross-sectional X-ray photoelectron spectroscopy (XPS) analysis of the arsenian pyrite confirmed the presence of As(II), but also showed evidence for more oxidized species (As(III) and As(V) oxides), as well as small amounts of polymeric As-As bonding. This indicates a large difference between As in the bulk (XANES measurements) and at the pyrite surface (XPS). Ab initio XANES calculations are consistent with As replacing Fe in pyrite in the form of As(II). Our experimental study suggests that the formal oxidation state of As in this type of arsenian pyrite is close to +2, and that in addition to fluid composition and oxidation state, the reaction path leading to pyrite formation plays a significant role in controlling the chemistry of arsenian pyrite. © 2012 Elsevier Ltd.Gujie Qian, Joël Brugger, Denis Testemale, William Skinner, Allan Prin

In-situ X-ray absorption study of Iron(II) speciation in brines up to supercritical conditions

X-ray absorption spectroscopy (XAS) measurements were used to determine the coordination structure and to derive the speciation of aqueous ferrous chloride complexes in acidic chloride brines over a wide range of conditions (25-450 °C, 500 bar, 0.5-12 m chloride molality), covering the range from sedimentary brines to magmatic hydrothermal fluids. EXAFS analysis coupled with ab initio free potential XANES calculations confirmed the octahedral geometry of the different Fe chlorocomplexes at low temperature ( 300 °C) and high (> 2 m) chloride molality ([FeCly]2 - y; y = 4 or y = 3; Fe-Cl distance = 2.31 ± 0.01 Å). These spectroscopic results contrast with the interpretation of most recent high-temperature studies of Fe(II) speciation in brines, which assumed that [FeCl2]0 is the predominant species in brines at high temperature. A reinterpretation of the experimental Fe solubilities measured by Fein et al. [Fein, J.B., Hemley, J.J., D'Angelo, W.M., Komninou, A., Sverjensky, D.A., 1992. Experimental study of iron-chloride complexing in hydrothermal fluids. Geochim. Cosmochim. Acta 56, 3179-3190.] for the magnetite-pyrite-pyrrhotite-quartz-muscovite-K-feldspar assemblage in KCl solutions at 300 °C/500 bar and 400 °C/500 bar shows that these solubility data can be explained using the high-order [FeCl4]2- complex. This study illustrates the complementarity between solubility and spectroscopic studies, and provides further evidence of the importance of high-order chlorocomplexes for the transport of transition metals (e.g., Zn, Ni) in high-temperature and/or supercritical fluids. © 2009 Elsevier B.V.Denis Testemale, Joël Brugger, Barbara Etschmann, Jean-Louis Hazeman