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1.
The investigation of hydrous boro(alumino)silicate melts and glasses with near infrared (NIR) spectroscopy revealed an important effect of boron on the water speciation. In the NIR spectra of B-bearing glasses new hydroxyl-related bands develop at the high frequency side of the 4500 cm−1 peak. In NaAlSi3O8 + B2O3 glasses this new peak is present as a shoulder at 4650 cm−1, and in NaAlSi3O8-NaBSi3O8 (Ab-Rd) glasses it appears as a resolved peak at 4710 cm−1. These bands increase with increasing boron concentration, suggesting that they are due to B-OH complexes. Furthermore, the variations in the NIR spectra indicate that with increasing B-content, but constant total water concentration, the amount of structurally bonded hydroxyl groups increases at the expense of molecular H2O. For example, at a total water concentration of 4 wt.%, pure Rd-glass contains ∼50% more water as hydroxyl groups than pure Ab-glass.In-situ NIR spectroscopy at high P and T using a hydrothermal diamond-anvil cell was used to gain information about the temperature dependence of the water speciation in NaBSi3O8 melts. The data demonstrate the conversion of molecular H2O to hydroxyl groups with increasing temperature. However, a fully quantitative evaluation of the high T spectra was hampered by problems with defining the correct baseline in the spectra. As an alternative approach annealing experiments on a Rd-glass containing 2.8 wt.% water were performed. The results confirm the conversion of H2O to OH groups with increasing T, but also suggest that the OH groups represented by the 4710 cm−1 peak (B-OH) participate much less in the conversion reaction compared to X-OH, represented by the 4500 cm−1 peak.  相似文献   

2.
The development of an accurate analytical procedure for determination of dissolved water in complex alumino-silicate glasses via micro-Raman analysis requires the assessment of the spectra topology dependence on glass composition. We report here a detailed study of the respective influence of bulk composition, iron oxidation state and total water content on the absolute and relative intensities of the main Raman bands related to glass network vibrations (LF: ∼490 cm−1; HF: ∼960 cm−1) and total water stretching (H2OT: ∼3550 cm−1) in natural glasses. The evolution of spectra topology was examined in (i) 33 anhydrous glasses produced by the re-melting of natural rock samples, which span a very large range of polymerisation degree (NBO/T from 0.00 to 1.16), (ii) 2 sets of synthetic anhydrous basaltic glasses with variable iron oxidation state (Fe3+/FeT from 0.05 to 0.87), and (iii) 6 sets of natural hydrous glasses (CH2OT from 0.4 to 7.0 wt%) with NBO/T varying from 0.01 to 0.76.In the explored domain of water concentration, external calibration procedure based on the H2OT band height is matrix-independent but its accuracy relies on precise control of the focusing depth and beam energy on the sample. Matrix-dependence strongly affects the internal calibrations based on H2OT height scaled to that of LF or HF bands but its effect decreases from acid (low NBO/T, SM) to basic (high NBO/T, SM) glasses. Structural parameters such as NBO/T (non-bridging oxygen per tetrahedron) and SM (sum of structural modifiers) describe the matrix-dependence better than simple compositional parameters (e.g. SiO2, Na2O + K2O). Iron oxidation state has only a minor influence on band topology in basalts and is thus not expected to significantly affect the Raman determinations of water in mafic (e.g. low SiO2, iron-rich) glasses. Modelling the evolution of the relative band height with polymerisation degree allows us to propose a general equation to predict the dissolved water content in natural glasses:
  相似文献   

3.
An experimental study of Ca-(Fe,Mg) interdiffusion in silicate garnets   总被引：1，自引：0，他引：1  
Ca-(Fe,Mg) interdiffusion experiments between natural single crystals of grossular (Ca2.74Mg0.15 Fe0.23Al1.76Cr0.04Si3.05O12) and almandine (Ca0.21Mg0.40 Fe2.23Mn0.13Al2.00Cr0.08Si2.99O12 or Ca0.43Mg0.36Fe2.11 Al1.95Si3.04O12), were undertaken at 900–1100 °C and 30 kbar, and pressures of 15.0–32.5 kbar at 1000 °C. Samples were buffered by Fe/FeO in most cases. Diffusion profiles were determined by electron microprobe. Across the experimental couples the interdiffusion coefficients (D˜) were almost independent of composition. The diffusion rates in an unbuffered sample were significantly faster than in buffered samples. The temperature dependence of the D˜ (Ca-Fe,Mg) interdiffusion coefficients may be described by
at 30 kbar and 900–1100 °C. This activation energy is marginally higher than previous experimental studies involving Ca-free garnets; the interdiffusion coefficients are higher than previous studies for Fe-Mg and Fe-Mn exchange in garnet. The pressure dependence of D˜ (Ca-Fe,Mg) at 1000 °C yielded an activation volume of 11.2 cm3 mol−1, which is higher than previous results from studies involving garnet and olivine. Comparison with simulation studies suggests a vacancy mechanism for divalent ion migration in garnet, with extrinsic processes being dominant up to very high temperatures. Received: 15 December 1996 / Accepted: 3 November 1998  相似文献   

4.
The determination of total water content (H2OT: 0.1-10 wt%) and water speciation (H2Omolecular/OH) in volcanic products by confocal microRaman spectrometry are discussed for alkaline (phonolite) and calcalkaline (dacite and rhyolite) silicic glasses. Shape and spectral distribution of the total water band (H2OT) at ∼3550 cm−1 show systematic evolution with glass H2OT, water speciation and NBO/T. In the studied set of silicic samples, calibrations based on internal normalization of the H2OT band to a band related to vibration of aluminosilicate network (TOT) at ∼490 cm−1 vary with glass peraluminosity. An external calibration procedure using well-characterized glass standards is less composition-dependent and provides excellent linear correlation between total dissolved water content and height or area of the H2OT Raman band. Accuracy of deconvolution procedure of the H2OT band to quantify water speciation in water-rich and depolymerized glasses depends on the strength of OH hydrogen bonding. System confocal performance, scattering from embedding medium and glass microcrystallinity have a crucial influence on accuracy of Raman analyses of water content in glass-bearing rocks and melt inclusions in crystals.  相似文献   

5.
Equilibrium and kinetic Fe isotope fractionation between aqueous ferrous and ferric species measured over a range of chloride concentrations (0, 11, 110 mM Cl−) and at two temperatures (0 and 22°C) indicate that Fe isotope fractionation is a function of temperature, but independent of chloride contents over the range studied. Using 57Fe-enriched tracer experiments the kinetics of isotopic exchange can be fit by a second-order rate equation, or a first-order equation with respect to both ferrous and ferric iron. The exchange is rapid at 22°C, ∼60-80% complete within 5 seconds, whereas at 0°C, exchange rates are about an order of magnitude slower. Isotopic exchange rates vary with chloride contents, where ferrous-ferric isotope exchange rates were ∼25 to 40% slower in the 11 mM HCl solution compared to the 0 mM Cl− (∼10 mM HNO3) solutions; isotope exchange rates are comparable in the 0 and 110 mM Cl− solutions.The average measured equilibrium isotope fractionations, ΔFe(III)-Fe(II), in 0, 11, and 111 mM Cl− solutions at 22°C are identical within experimental error at +2.76±0.09, +2.87±0.22, and +2.76±0.06 ‰, respectively. This is very similar to the value measured by Johnson et al. (2002a) in dilute HCl solutions. At 0°C, the average measured ΔFe(III)-Fe(II) fractionations are +3.25±0.38, +3.51±0.14 and +3.56±0.16 ‰ for 0, 11, and 111 mM Cl− solutions. Assessment of the effects of partial re-equilibration on isotope fractionation during species separation suggests that the measured isotope fractionations are on average too low by ∼0.20 ‰ and ∼0.13 ‰ for the 22°C and 0°C experiments, respectively. Using corrected fractionation factors, we can define the temperature dependence of the isotope fractionation from 0°C to 22°C as: where the isotopic fractionation is independent of Cl− contents over the range used in these experiments. These results confirm that the Fe(III)-Fe(II) fractionation is approximately half that predicted from spectroscopic data, and suggests that, at least in moderate Cl− contents, the isotopic fractionation is relatively insensitive to Fe-Cl speciation.  相似文献   

6.
Interpretation of the origins of iron-bearing minerals preserved in modern and ancient rocks based on measured iron isotope ratios depends on our ability to distinguish between biological and non-biological iron isotope fractionation processes. In this study, we compared 56Fe/54Fe ratios of coexisting aqueous iron (Fe(II)aq, Fe(III)aq) and iron oxyhydroxide precipitates (Fe(III)ppt) resulting from the oxidation of ferrous iron under experimental conditions at low pH (<3). Experiments were carried out using both pure cultures of Acidothiobacillus ferrooxidans and sterile controls to assess possible biological overprinting of non-biological fractionation, and both SO42− and Cl− salts as Fe(II) sources to determine possible ionic/speciation effects that may be associated with oxidation/precipitation reactions. In addition, a series of ferric iron precipitation experiments were performed at pH ranging from 1.9 to 3.5 to determine if different precipitation rates cause differences in the isotopic composition of the iron oxyhydroxides. During microbially stimulated Fe(II) oxidation in both the sulfate and chloride systems, 56Fe/54Fe ratios of residual Fe(II)aq sampled in a time series evolved along an apparent Rayleigh trend characterized by a fractionation factor αFe(III)aq-Fe(II)aq ∼ 1.0022. This fractionation factor was significantly less than that measured in our sterile control experiments (∼1.0034) and that predicted for isotopic equilibrium between Fe(II)aq and Fe(III)aq (∼1.0029), and thus might be interpreted to reflect a biological isotope effect. However, in our biological experiments the measured difference in 56Fe/54Fe ratios between Fe(III)aq, isolated as a solid by the addition of NaOH to the final solution at each time point under N2-atmosphere, and Fe(II)aq was in most cases and on average close to 2.9‰ (αFe(III)aq-Fe(II)aq ∼ 1.0029), consistent with isotopic equilibrium between Fe(II)aq and Fe(III)aq. The ferric iron precipitation experiments revealed that 56Fe/54Fe ratios of Fe(III)aq were generally equal to or greater than those of Fe(III)ppt, and isotopic fractionation between these phases decreased with increasing precipitation rate and decreasing grain size. Considered together, the data confirm that the iron isotope variations observed in our microbial experiments are primarily controlled by non-biological equilibrium and kinetic factors, a result that aids our ability to interpret present-day iron cycling processes but further complicates our ability to use iron isotopes alone to identify biological processing in the rock record.  相似文献   

7.
The results from batch sorption experiments on montmorillonite systems have demonstrated that bivalent transition metals compete with one another for sorption sites. For safety analysis studies of high level radioactive waste repositories with compacted bentonite near fields, the importance of competitive sorption on the migration of radionuclides needs to be evaluated. Under reducing conditions, the bentonite porewater chosen has a Fe(II) concentration of ∼5.3 × 10−5 M through saturation with siderite. The purpose of this paper is to assess the influence of such high Fe(II) concentrations on the transport of Ni(II) through compacted bentonite, Ni(II) was chosen as an example of a bivalent transition metal. The one-dimensional calculations were carried out at different Ni(II) equilibrium concentrations at the boundary (Ni(II)EQBM) with the reactive transport code MCOTAC incorporating the two site protolysis non electrostatic surface complexation/cation exchange sorption model, MCOTAC-sorb. At a Ni(II)EQBM level of 10−7 M without Fe(II) competition, the reactive transport calculations using a constant Kd approach and the MCOTAC-sorb calculation yielded the same breakthrough curves. At higher Ni(II)EQBM (10−5 M), the model calculations with MCOTAC-sorb indicated a breakthrough which was shifted to later times by a factor of ∼5 compared with the use of the constant Kd approach.When sorption competition was included in the calculations, the magnitude of the influence depended on the sorption characteristics of the two competing sorbates and their respective concentrations. At background Fe(II) concentrations of 5.3 × 10−5 M, and a Ni(II)EQBM level of 10−7 M, the Ni(II) breakthrough time was ∼15 times earlier than in the absence of competition. At such Fe(II) concentrations the Ni(II) breakthrough curves at all source concentrations less than 3.5 × 10−5 M (fixed by the NiCO3,S solubility limit) are the same i.e. Ni(II) exhibits linear (low) sorption.Competitive sorption effects can have significant influences on the transport of radionuclides through compacted bentonite i.e. reduce the migration rates. Since, for the case considered here, the Fe(II) concentration in the near field of a high-level radioactive waste repository may change in time and space, the transport of bivalent transition metal radionuclides can only be properly modelled using a multi-species reactive transport code which includes a sorption model.  相似文献   

8.
Natural attenuation of arsenic by simple adsorption on oxyhydroxides may be limited due to competing oxyanions, but uptake by coprecipitation may locally sequester arsenic. We have systematically investigated the mechanism and mode (adsorption versus coprecipitation) of arsenic uptake in the presence of carbonate and phosphate, from solutions of inorganic composition similar to many groundwaters. Efficient arsenic removal, >95% As(V) and ∼55% in initial As(III) systems, occurred over 24 h at pHs 5.5-6.5 when Fe(II) and hydroxylapatite (Ca5(PO4)3OH, HAP) “seed” crystals were added to solutions that had been previously reacted with HAP, atmospheric CO2(g) and O2(g). Arsenic adsorption was insignificant (<10%) on HAP without Fe(II). Greater uptake in the As(III) system in the presence of Fe(II) was interpreted as due to faster As(III) to As(V) oxidation by molecular oxygen in a putative pathway involving Fe(IV) and As(IV) intermediate species. HAP acts as a pH buffer that allows faster Fe(II) oxidation. Solution analyses coupled with high-resolution transmission electron microscopy (HRTEM), X-ray Energy-Dispersive Spectroscopy (EDS), and X-Ray Absorption Spectroscopy (XAS) indicated the precipitation of sub-spherical particles of an amorphous, chemically-mixed, nanophase, FeIII[(OH)3(PO4)(AsVO4)]·nH2O or FeIII[(OH)3( PO4)(AsVO4)(AsIIIO3)minor]·nH2O, where AsIIIO3 is a minor component.The mode of As uptake was further investigated in binary coprecipitation (Fe(II) + As(III) or P), and ternary coprecipitation and adsorption experiments (Fe(II) + As(III) + P) at variable As/Fe, P/Fe and As/P/Fe ratios. Foil-like, poorly crystalline, nanoparticles of FeIII(OH)3 and sub-spherical, amorphous, chemically-mixed, metastable nanoparticles of FeIII[(OH)3, PO4]·nH2O coexisted at lower P/Fe ratios than predicted by bulk solubilities of strengite (FePO4·2H2O) and goethite (FeOOH). Uptake of As and P in these systems decreased as binary coprecipitation > ternary coprecipitation > ternary adsorption.Significantly, the chemically-mixed, ferric oxyhydroxide-phosphate-arsenate nanophases found here are very similar to those found in the natural environment at slightly acidic to circum-neutral pHs in sub-oxic to oxic systems, such phases may naturally attenuate As mobility in the environment, but it is important to recognize that our system and the natural environment are kinetically evolving, and the ultimate environmental fate of As will depend on the long-term stability and potential phase transformations of these mixed nanophases. Our results also underscore the importance of using sufficiently complex, yet systematically designed, model systems to accurately represent the natural environment.  相似文献   

9.
The oxidation of Fe(II) with H2O2 at nanomolar levels in seawater have been studied using an UV-Vis spectrophotometric system equipped with a long liquid waveguide capillary flow cell. The effect of pH (6.5 to 8.2), H2O2 (7.2 × 10−8 M to 5.2 × 10−7 M), HCO3− (2.05 mM to 4.05 mM) and Fe(II) (5 nM to 500 nM) as a function of temperature (3 to 35 °C) on the oxidation of Fe(II) are presented. The oxidation rate is linearly related to the pH with a slope of 0.89 ± 0.01 independent of the concentration of HCO3−. A kinetic model for the reaction has been developed to consider the interactions of Fe(II) with the major ions in seawater. The model has been used to examine the effect of pH, concentrations of Fe(II), H2O2 and HCO3− as a function of temperature. FeOH+ is the most important contributing species to the overall rate of oxidation from pH 6 to pH 8. At a pH higher than 8, the Fe(OH)2 and Fe(CO3)22− species contribute over 20% to the rates. Model results show that when the concentration of O2 is two orders of magnitude higher than the concentration of H2O2, the oxidation with O2 also needs to be considered. The rate constants for the five most kinetically active species (Fe2+, FeOH+, Fe(OH)2, FeCO3, Fe(CO3)22−) in seawater as a function of temperature have been determined. The kinetic model is also valid in pure water with different concentrations of HCO3− and the conditions found in fresh waters.  相似文献   

10.
The influence of ferrous and ferric iron on the low-temperature heat capacity and vibrational entropy of silicate glasses has been determined by adiabatic calorimetry. Two pairs of samples based on sodium disilicate and calcium Tschermak molecule compositions have been studied. Along with previous data for another Fe-bearing glass, these results have been used to complement the available set of composition independent partial molar relative entropies of oxides in silicate glasses with S298 − S0 values of 56.7 and 116 J/mol for FeO and Fe2O3, respectively. The calorimetric data indicate that the fraction of fivefold coordinated Al is significant in the CaO-“FeO”-Al2O3-SiO2 system and that association of Ca2+ and Na+ with Fe3+ in tetrahedral coordination for charge compensation does not entail significant changes in coordination for these two cations. At very low temperatures, however, the heat capacity is no longer an additive function of composition because of unexpectedly high positive deviations from Debye laws. These anomalies are stronger for the reduced than the oxidized glasses and considerably larger than for iron-free glasses, but their origin cannot be established from the present measurements.  相似文献   

11.
The Fe(II) adsorption by non-ferric and ferric (hydr)oxides has been analyzed with surface complexation modeling. The CD model has been used to derive the interfacial distribution of charge. The fitted CD coefficients have been linked to the mechanism of adsorption. The Fe(II) adsorption is discussed for TiO2, γ-AlOOH (boehmite), γ-FeOOH (lepidocrocite), α-FeOOH (goethite) and HFO (ferrihydrite) in relation to the surface structure and surface sites. One type of surface complex is formed at TiO2 and γ-AlOOH, i.e. a surface-coordinated Fe2+ ion. At the TiO2 (Degussa) surface, the Fe2+ ion is probably bound as a quattro-dentate surface complex. The CD value of Fe2+ adsorbed to γ-AlOOH points to the formation of a tridentate complex, which might be a double edge surface complex. The adsorption of Fe(II) to ferric (hydr)oxides differs. The charge distribution points to the transfer of electron charge from the adsorbed Fe(II) to the solid and the subsequent hydrolysis of the ligands that coordinate to the adsorbed ion, formerly present as Fe(II). Analysis shows that the hydrolysis corresponds to the hydrolysis of adsorbed Al(III) for γ-FeOOH and α-FeOOH. In both cases, an adsorbed M(III) is found in agreement with structural considerations. For lepidocrocite, the experimental data point to a process with a complete surface oxidation while for goethite and also HFO, data can be explained assuming a combination of Fe(II) adsorption with and without electron transfer. Surface oxidation (electron transfer), leading to adsorbed Fe(III)(OH)2, is favored at high pH (pH > ∼7.5) promoting the deprotonation of two FeIII-OH2 ligands. For goethite, the interaction of Fe(II) with As(III) and vice versa has been modeled too. To explain Fe(II)-As(III) dual-sorbate systems, formation of a ternary type of surface complex is included, which is supposed to be a monodentate As(III) surface complex that interacts with an Fe(II) ion, resulting in a binuclear bidentate As(III) surface complex.  相似文献   

12.
Experimental investigations have been performed at T = 1200°C, P = 200 MPa and fH2 corresponding to H2O-MnO-Mn3O4 and H2O-QFM redox buffers to study the effect of H2O activity on the oxidation and structural state of Fe in an iron-rich basaltic melt. The analysis of Mössbauer and Fe K-edge X-ray absorption nearedge structure (XANES) spectra of the quenched hydrous ferrobasaltic glasses shows that the Fe3+/ΣFe ratio of the glass is directly related to aH2O in a H2-buffered system and, consequently, to the prevailing oxygen fugacity (through the reaction of water dissociation H2O ↔ H2 + 1/2 O2). However, water as a chemical component of the silicate melt has an indistinguishable effect on the redox state of iron at studied conditions. The experimentally obtained relationship between fO2 and Fe3+/Fe2+ in the hydrous ferrobasaltic melt can be adequately predicted in the investigated range by the existing empiric and thermodynamic models. The ratio of ferric and ferrous Fe is proportional to the oxygen fugacity to the power of ∼0.25 which agrees with the theoretical value from the stoichiometry of the Fe redox reaction (FeO + ¼ O2 = FeO1.5). The mean centre shifts for Fe2+ and Fe3+ absorption doublets in Mössbauer spectra show little change with increasing Fe3+/ΣFe, suggesting no significant change in the type of iron coordination. Similarly, XANES preedge spectra indicate a mixed (C3h, Td, and Oh, i.e., 5-, 4-, and sixfold) coordination of Fe in hydrous basaltic glasses.  相似文献   

13.
The diffusion coefficients of Fe2+ and Mg in aluminous spinel at ∼20 kb, 950 to 1325°C, and at 30 kb, 1125°C have been determined via diffusion couple experiments and numerical modeling of the induced diffusion profiles. The oxygen fugacity, fO2, was constrained by graphite encapsulating materials. The retrieved self-diffusion coefficients of Fe2+ and Mg at ∼20 kb, 950 to 1325°C, fit well the Arrhenian relation, D = D0exp(−Q/RT), where Q is the activation energy, with D0(Fe) = 1.8 (±2.8) × 10−5, D0(Mg) = 1.9 (±1.4) × 10−5 cm2/s, Q(Fe) = 198 ± 19, and Q(Mg) = 202 ± 8 kJ/mol. Comparison with the data at 30 kb suggests an activation volume of ∼5 cm3/mol. From analysis of compositional zoning in natural olivine-spinel assemblages in ultramafic rocks, previous reports concluded that D(Fe-Mg) in spinel with Cr/(Cr + Al) ≤0.5 is ∼10 times that in olivine. The diffusion data in spinel and olivine have been applied to the problems of preservation of Mg isotopic inhomogeneity in spinel within the plagioclase-olivine inclusions in Allende meteorite and cooling rates of terrestrial ultramafic rocks.  相似文献   

14.
 Thermodynamic analysis of Fe- and Mg-bearing plagioclase and silicate liquid was carried out based on reported element partitioning data between plagioclase and silicate liquid in reduced conditions, solution properties of ternary feldspar, standard state properties of plagioclase endmembers and solution properties of multicomponent silicate liquid. Derived mixing properties of Fe- and Mg-bearing plagioclase are in harmony with estimated results from synthetic experiments in the systems CaAl2Si2O8-CaFeSi3O8 and CaAl2Si2O8-CaMgSi3O8. Based on the determined solution properties of the plagioclase, a computer program to calculate the element partition relationships between Fe- and Mg-bearing plagioclase and multicomponent silicate liquid was developed. The FeO, MgO and MgO/(MgO + FeO) in plagioclase predicted from known liquid compositions and pressure are in agreement with measurements within 0.2 wt%, 0.1 wt% and 0.1 (mol ratio), respectively. The Fe3+ content in plagioclase crystallized at high oxygen fugacity can be estimated with this program. The Fe3+/total Fe ratio in plagioclase crystallized near the quartz-fayalite-magnetite buffer ranges from 0 to 0.5, which is consistent with previous study on natural plagioclase in submarine basalt. Derived solution properties of the Fe- and Mg-bearing plagioclase are also used to calculate equilibrium composition relationship between olivine and plagioclase. Change of X Fo in olivine coexisting with plagioclase affects MgO and FeO contents in plagioclase greatly. The present model predicts X Fo of coexisting olivine from the chemical composition of plagioclase to ±0.1 accuracy at given pressure and temperature. Received: 27 March 1998 / Accepted: 30 September 1999  相似文献   

15.
Schwertmannite (Fe8O8(OH)6SO4) is a common Fe(III)-oxyhydroxysulfate mineral in acid-sulfate systems, where its formation and fate strongly influence water quality. The present study examines transformation of schwertmannite to goethite (FeOOH), as catalyzed by interactions with Fe(II) in anoxic aquatic environments. This study also evaluates the role of the Fe(II) pathway in influencing the formation of iron-sulfide minerals in such environments. At pH > 5, the rates of Fe(II)-catalyzed schwertmannite transformation were several orders of magnitude faster than transformation in the absence of Fe(II). Complete transformation of schwertmannite occurred within only 3-5 h at pH > 6 and Fe(II)(aq) ? 5 mmol L−1. Model calculations indicate that the Fe(II)-catalyzed transformation of schwertmannite to goethite greatly decreases the reactivity of the Fe(III) pool, thereby favoring SO4-reduction and facilitating the formation of iron-sulfide minerals (particularly mackinawite, tetragonal FeS). Examination of in situ sediment geochemistry in an acid-sulfate system revealed that the rapid Fe(II)-catalyzed transformation was consistent with an abrupt shift from an acidic Fe(III)-reducing regime with abundant schwertmannite near the sediment surface, to a near-neutral mackinawite-forming regime where goethite was dominant. This study demonstrates that the Fe(II) pathway exerts a major influence on schwertmannite transformation and iron-sulfide formation in anoxic acid-sulfate systems. These findings have important implications for understanding acidity dynamics and trace element mobility in such systems.  相似文献   

16.
Micro-Raman spectroscopy, even though a very promising technique, is not still routinely applied to analyse H2O in silicate glasses. The accuracy of Raman water determinations critically depends on the capability to predict and take into account both the matrix effects (bulk glass composition) and the analytical conditions on band intensities. On the other hand, micro-Fourier transform infrared spectroscopy is commonly used to measure the hydrous absorbing species (e.g., hydroxyl OH− and molecular H2O) in natural glasses, but requires critical assumptions for the study of crystal-hosted glasses. Here, we quantify for the first time the matrix effect of Raman external calibration procedures for the quantification of the total H2O content (H2OT = OH− + H2Om) in natural silicate glasses. The procedures are based on the calibration of either the absolute (external calibration) or scaled (parameterisation) intensity of the 3550 cm−1 band. A total of 67 mafic (basanite, basalt) and intermediate (andesite) glasses hosted in olivines, having between 0.2 and 4.8 wt% of H2O, was analysed. Our new dataset demonstrates, for given water content, the height (intensity) of Raman H2OT band depends on glass density, reflectance and water environment. Hence this matrix effect must be considered in the quantification of H2O by Raman spectroscopy irrespective of the procedure, whereas the parameterisation mainly helps to predict and verify the self-consistency of the Raman results. In addition, to validate the capability of the micro-Raman to accurately determine the H2O content of multicomponent aluminosilicate glasses, a subset of 23 glasses was analysed by both micro-Raman and micro-FTIR spectroscopy using the band at 3550 cm−1. We provide new FTIR absorptivity coefficients (ε3550) for basalt (62.80 ± 0.8 L mol−1 cm−1) and basanite (43.96 ± 0.6 L mol−1 cm−1). These values, together with an exhaustive review of literature data, confirm the non-linear decline of the FTIR absorptivity coefficient (ε3550) as the glass depolymerisation increases. We demonstrate the good agreement between micro-FTIR and micro-Raman determination of H2O in silicate glasses when the matrix effects are properly considered.  相似文献   

17.
The potential for reduction of 99TcO4−(aq) to poorly soluble 99TcO2 · nH2O(s) by biogenic sediment-associated Fe(II) was investigated with three Fe(III)-oxide containing subsurface materials and the dissimilatory metal-reducing subsurface bacterium Shewanella putrefaciens CN32. Two of the subsurface materials from the U.S. Department of Energy’s Hanford and Oak Ridge sites contained significant amounts of Mn(III,IV) oxides and net bioreduction of Fe(III) to Fe(II) was not observed until essentially all of the hydroxylamine HCl-extractable Mn was reduced. In anoxic, unreduced sediment or where Mn oxide bioreduction was incomplete, exogenous biogenic TcO2 · nH2O(s) was slowly oxidized over a period of weeks. Subsurface materials that were bioreduced to varying degrees and then pasteurized to eliminate biological activity, reduced TcO4−(aq) at rates that generally increased with increasing concentrations of 0.5 N HCl-extractable Fe(II). Two of the sediments showed a common relationship between extractable Fe(II) concentration (in mM) and the first-order reduction rate (in h−1), whereas the third demonstrated a markedly different trend. A combination of chemical extractions and 57Fe Mössbauer spectroscopy were used to characterize the Fe(III) and Fe(II) phases. There was little evidence of the formation of secondary Fe(II) biominerals as a result of bioreduction, suggesting that the reactive forms of Fe(II) were predominantly surface complexes of different forms. The reduction rates of Tc(VII)O4− were slowest in the sediment that contained plentiful layer silicates (illite, vermiculite, and smectite), suggesting that Fe(II) sorption complexes on these phases were least reactive toward pertechnetate. These results suggest that the in situ microbial reduction of sediment-associated Fe(III), either naturally or via redox manipulation, may be effective at immobilizing TcO4−(aq) associated with groundwater contaminant plumes.  相似文献   

18.
Iron-57 resonant absorption Mössbauer spectroscopy was used to describe the redox relations and structural roles of Fe3+ and Fe2+ in meta-aluminosilicate glasses. Melts were formed at 1500 °C in equilibrium with air and quenched to glass in liquid H2O with quenching rates exceeding 200 °C/s. The aluminosilicate compositions were NaAlSi2O6, Ca0.5AlSi2O6, and Mg0.5AlSi2O6. Iron oxide was added in the form of Fe2O3, NaFeO2, CaFe2O4, and MgFe2O4 with total iron oxide content in the range ∼0.9 to ∼5.6 mol% as Fe2O3. The Mössbauer spectra, which were deconvoluted by assuming Gaussian distributions of the hyperfine field, are consistent with one absorption doublet of Fe2+ and one of Fe3+. From the area ratios of the Fe2+ and Fe3+ absorption doublets, with corrections for differences in recoil-fractions of Fe3+ and Fe2+, the Fe3+/ΣFe is positively correlated with increasing total iron content and with decreasing ionization potential of the alkali and alkaline earth cation. There is a distribution of hyperfine parameters from the Mössbauer spectra of these glasses. The maximum in the isomer shift distribution function of Fe3+, δFe3+, ranges from about 0.25 to 0.49 mm/s (at 298 K relative to Fe metal) with the quadrupole splitting maximum, ΔFe3+, ranging from ∼1.2 to ∼1.6 mm/s. Both δFe3+ and δFe2+ are negatively correlated with total iron oxide content and Fe3+/ΣFe. The dominant oxygen coordination number Fe3+ changes from 4 to 6 with decreasing Fe3+/ΣFe. The distortion of the Fe3+-O polyhedra of the quenched melts (glasses) decreases as the Fe3+/ΣFe increases. These polyhedra do, however, coexist with lesser proportions of polyhedra with different oxygen coordination numbers. The δFe2+ and ΔFe2+ distribution maxima at 298 K range from ∼0.95 to 1.15 mm/s and 1.9 to 2.0 mm/s, respectively, and decrease with increasing Fe3+/ΣFe. We suggest that these hyperfine parameter values for the most part are more consistent with Fe2+ in a range of coordination states from 4- to 6-fold. The lower δFe2+-values for the most oxidized melts are consistent with a larger proportion of Fe2+ in 4-fold coordination compared with more reduced glasses and melts.  相似文献   

19.
The reductive biotransformation of two Si-ferrihydrite coprecipitates (1 and 5 mole % Si) by Shewanella putrefaciens, strain CN32, was investigated in 1,4-piperazinediethanesulfonic acid-buffered media (pH ∼7) with lactate as the electron donor. Anthraquinone-2,6-disulfonate, an electron shuttle, was present in the media. Experiments were performed without and with PO43− (P) (1 to 20 mmol/L) in media containing 50 mmol/L Fe. Our objectives were to define the combined effects of SiO44− (Si) and P on the bioreducibility and biomineralization of ferrihydrites under anoxic conditions. Iron reduction was measured as a function of time, solids were characterized by powder X-ray diffraction and Mössbauer spectroscopy, and aqueous solutions were analyzed for Si, P, Cl− and inorganic carbon. Both of the ferrihydrites were rapidly reduced regardless of the Si and P content. Si concentration had no effect on the reduction rate or mineralization products. Magnetite was formed in the absence of P whereas carbonate green rust GR(CO32−) ([Fe(6−x)IIFeIIIx(OH)12]x+(CO32−)0.5x · yH2O) and vivianite [Fe3(PO4)2 · 8H2O], were formed when P was present. GR(CO32−) dominated as a mineral product in samples with <4 mmol/L P. The Fe(II)/Fe(III) ratio of GR(CO32−) varied with P concentration; the ratio was 2 in 1 mmol/L P and approached 1 with 4- and 10 mmol/L P. Green rust appeared to form by solid-state transformation of ferrihydrite. Media P and Si concentration dictated the mechanism of transformation. In the 1 mole % Si coprecipitate with 1 mmol/L P, an intermediate Fe(II)/Fe(III) phase with structural Fe(II) slowly transformed to GR with time. In contrast, when ferrihydrite contained more Si (5 mole %) and/or contained higher P (4 mmol/L), sorbed Fe(II) and residual ferrihydrite together transformed to GR. Despite similar chemistries, P was shown to have a profound effect on extent of ferrihydrite reduction and biotransformations while that of Si was minimal.  相似文献   

20.
Olivine/melt partitioning of ΣFe, Fe2+, Mg2+, Ca2+, Mn2+, Co2+, and Ni2+ has been determined in the systems CaO-MgO-FeO-Fe2O3-SiO2 (FD) and CaO-MgO-FeO-Fe2O3-Al2O3-SiO2 (FDA3) as a function of oxygen fugacity (fO2) at 0.1 MPa pressure. Total iron oxide content of the starting materials was ∼20 wt%. The fO2 was to used to control the Fe3+/ΣFe (ΣFe: total iron) of the melts. The Fe3+/ΣFe and structural roles of Fe2+ and Fe3+ were determined with 57Fe resonant absorption Mössbauer spectroscopy. Changes in melt polymerization, NBO/T, as a function of fO2 was estimated from the Mössbauer data and existing melt structure information. It varies by ∼100% in melts coexisting with olivine in the FDA3 system and by about 300% in the FD system in the Fe3+/ΣFe range of the experiments (0.805-0.092). The partition coefficients ( in olivine/wt% in melt) are systematic functions of fO2 and, therefore, NBO/T of the melt. There is a -minimum in the FDA3 system at NBO/T-values corresponding to intermediate Fe3+/ΣFe (0.34-0.44). In the Al-free system, FD, where the NBO/T values of melts range between ∼1 and ∼2.9, the partition coefficients are positively correlated with NBO/T (decreasing Fe3+/ΣFe). These relationships are explained by consideration of solution behavior in the melts governed by Qn-unit distribution and structural changes of the divalent cations in the melts (coordination number, complexing with Fe3+, and distortion of the polyhedra).  相似文献   

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