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David M. Sherman 《Geochimica et cosmochimica acta》2007,71(3):714-722
Chloride complexation of Cu+ controls the solubility of copper(I) oxide and sulfide ore minerals in hydrothermal and diagenetic fluids. Solubility measurements and optical spectra of high temperature CuCl solutions have been interpreted as indicating the formation of CuCl, , and complexes. However, no other monovalent cation forms tri- and tetrachloro complexes. EXAFS spectra of high temperature Cu-Cl solutions, moreover, appear to show only CuCl and complexes at T > 100 °C. To reconcile these results, I investigated the nature and stability of Cu-Cl complexes using ab initio cluster calculations and ab initio (Car-Parrinello) molecular dynamics simulations for CuCl-NaCl-H2O systems at 25 to 450 °C. Ab initio molecular dynamic simulations of 1 m CuCl in a 4 m Cl solution give a stable complex at 25 °C over 4 ps but show that the third Cl− is weakly bound. When the temperature is increased along the liquid-vapour saturation curve to 125 °C, the complex dissociates into and Cl−; only forms at 325 °C and 1 kbar. Even in a 15.6 m Cl brine at 450 °C, only the complex forms over a 4 ps simulation run.Cluster calculations with a static dielectric continuum solvation field (COSMO) were used in an attempt directly estimate free energies of complex formation in aqueous solution. Consistent with the MD simulations, the complex is slightly stable at 25 °C but decreases in stability with decreasing dielectric constant (ε). The complex is predicted to be unstable at 25 °C and becomes increasingly unstable with decreasing dielectric constant. In hydrothermal fluids (ε < 30) both the and complexes are unstable to dissociation into and Cl−.The results obtained here are at odds with recent equations of state that predict and complexes are the predominant species in hydrothermal brines. In contrast, I predict that only complexes will be significant at T > 125 °C, even in NaCl-saturated brines. The high-temperature (T > 125 °C) optical spectra of CuCl solutions and solubility measurements of Cu minerals in Cl-brines need to be reinterpreted in terms of only the CuCl and complexes. 相似文献
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The influence of solution complexation on the sorption of yttrium and the rare earth elements (YREEs) by amorphous ferric hydroxide was investigated at 25 °C over a range of pH (4.0-7.1) and carbonate concentrations . Distribution coefficients, defined as , where [MSi]T is the total concentration of sorbed YREE, MT is the total YREE concentration in solution, and [Si] is the concentration of amorphous ferric hydroxide, initially increased in magnitude with increasing carbonate concentration, and then decreased. The initial increase of is due to sorption of YREE carbonate complexes , in addition to sorption of free YREE ions (M3+). The subsequent decrease of , which is more extensive for the heavy REEs, is due to the increasing intensity of YREE solution complexation by carbonate ions. The competition for YREEs between solution complexation and surface complexation was modeled via the equation:
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Jacob J. Hanley Thomas Pettke Edward T.C. Spooner 《Geochimica et cosmochimica acta》2005,69(10):2593-2611
The concentration and distribution of Pt and Au in a fluid-melt system has been investigated by reacting the metals with S-free, single-phase aqueous brines (20, 50, 70 wt% eq. NaCl) ± peraluminous melt at a confining pressure of 1.5 kbar and temperatures of 600 to 800 °C, trapping the fluid in synthetic fluid inclusions (quartz-hosted) and vesicles (silicate melt-hosted), and quantifying the metal content of the trapped fluid and glass by laser ablation ICP-MS. HCl concentration was buffered using the assemblage albite-andalusite-quartz and fO2 was buffered using the assemblage Ni-NiO. Over the range of experimental conditions, measured concentrations of Pt and Au in the brines (, ) are on on the order of 1-103 ppm. Concentrations of Pt and Au in the melt (, ) are ∼35-100 ppb and ∼400-1200 ppb, respectively. Nernst partition coefficients (, ) are on the order of 102-103 and vary as a function of (non-Henry’s Law behavior). Trapped fluids show a significant range of metal concentrations within populations of inclusions from single experiments (∼ 1 log unit variability for Au; ∼2-3 log unit variability for Pt). Variability in metal concentration within single inclusion groups is attributed to premature brine entrapment (prior to metal-fluid-melt equilibrium being reached); this allows us to make only minimum estimates of metal solubility using metal concentrations from primary inclusions. The data show two trends: (i) maximum and average values of and in inclusions decrease ∼2 orders of magnitude as fluid salinity () increases from ∼4 to 40 molal (20 to 70 wt % eq. NaCl) at a constant temperature; (ii) maximum and average values of increase approximately 1 order of magnitude for every 100°C increase temperature at a fixed . The observed behavior may be described by the general expression:
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Dawsonite synthesis and reevaluation of its thermodynamic properties from solubility measurements: Implications for mineral trapping of CO2 总被引：1，自引：0，他引：1
Pascale Bénézeth Donald A. Palmer Juske Horita 《Geochimica et cosmochimica acta》2007,71(18):4438-4455
Over the last decade, a significant research effort has focused on determining the feasibility of sequestering large amounts of CO2 in deep, permeable geologic formations to reduce carbon dioxide emissions to the atmosphere. Most models indicate that injection of CO2 into deep sedimentary formations will lead to the formation of various carbonate minerals, including the common phases calcite (CaCO3), dolomite (CaMg(CO3)2), magnesite (MgCO3), siderite (FeCO3), as well as the far less common mineral, dawsonite (NaAlCO3(OH)2). Nevertheless, the equilibrium and kinetics that control the precipitation of stable carbonate minerals are poorly understood and few experiments have been performed to validate computer codes that model CO2 sequestration.In order to reduce this uncertainty we measured the solubility of synthetic dawsonite according to the equilibrium: , from under- and oversaturated solutions at 50-200 °C in basic media at 1.0 mol · kg−1 NaCl. The solubility products (Qs) obtained were extrapolated to infinite dilution to obtain the solubility constants (. Combining the fit of these values and fixing at 25 °C, which was derived from the calorimetric data of Ferrante et al. [Ferrante, M.J., Stuve, J.M., and Richardson, D.W., 1976. Thermodynamic data for synthetic dawsonite. U.S. Bureau of Mines Report Investigation, 8129, Washington, D.C., 13p.], the following thermodynamic parameters for the dissolution of dawsonite were calculated at 25 °C: , and . Subsequently, we were able to derive values for the Gibbs energy of formation (, enthalpy of formation ( and entropy ( of dawsonite. These results are within the combined experimental uncertainties of the values reported by Ferrante et al. (1976). Predominance diagrams are presented for the dawsonite/boehmite and dawsonite/bayerite equilibria at 100 °C in the presence of a saline solution with and without silica-containing minerals. 相似文献
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Asfaw Zegeye Cédric Carteret Martine Mullet Frédéric Jorand 《Geochimica et cosmochimica acta》2007,71(22):5450-5462
Microbiological reduction of a biogenic sulfated green rust , was examined using a sulfate reducing bacterium (Desulfovibrio alaskensis). Experiments investigated whether could serve as a sulfate source for D. alaskensis anaerobic respiration by analyzing mineral transformation. Batch experiments were conducted using lactate as the electron donor and biogenic as the electron acceptor, at circumneutral pH in unbuffered medium. transformation was monitored with time by X-ray diffraction (XRD), Transmission Mössbauer Spectroscopy (TMS), Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), Transmission Electron Microscopy (TEM) and X-ray Photoelectron Spectroscopy (XPS). The reduction of sulfate anions and the formation of iron sulfur mineral were clearly identified by XPS analyses. TMS showed the formation of additional mineral as green rust (GR) and vivianite. XRD analyses discriminated the type of the newly formed GR as GR1. The formed GR1 was as indicated by DRIFTS analysis. Thus, the results presented in this study indicate that D. alaskensis cells were able to use as an electron acceptor. , vivianite and an iron sulfur compound were formed as a result of reduction by D. alaskensis. Hence, in environments where geochemical conditions promote biogenic formation, this mineral could stimulate the anaerobic respiration of sulfate reducing bacteria. 相似文献
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Frank Millero Fen Huang Taylor Graham Denis Pierrot 《Geochimica et cosmochimica acta》2007,71(1):46-55
Potentiometric measurements of the stoichiometric constants for the dissociation of carbonic acid in NaCl solutions ( and ) have been made as a function of molality (0-6 m) and temperature (0-50 °C). The results have been fitted to the equations
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Four or five sets of ab initio models, including Unrestricted Hartree Fock (UHF) and hybrid Density Functional Theory (DFT) are calculated for each species in a series of aqueous ferric aquo-chloro complexes: , , , FeCl3(H2O)3, FeCl3(H2O)2, , FeCl5H2O2−, , ) in order to determine the relative isotopic fractionation among the complexes, to compare the results of different models for the same complexes, to examine factors that influence the magnitude of the isotopic fractionation, and to compare bond-partner-driven fractionation with redox-driven fractionation.Relative to , all models show a nearly linear decrease in 56Fe/54Fe as the number of Cl− ions per Fe3+ ion increases, with slopes of −0.8‰ to −1.0‰ per Cl− at 20 °C. At 20 °C, 1000 ln β (β = 56Fe/54Fe reduced partition function ratio relative to a dissociated Fe atom) values range from 8.93‰ to 9.73‰ for , 8.04-9.12‰ for , 7.61-8.73‰ for , 7.14-8.25‰ for , and 3.09-4.41‰ for . The fractionation between and ranges from 1.5‰ to 2.6‰, depending on the model; this is comparable in magnitude to fractionation effects due to Fe3+/Fe2+ redox reactions. β values from the UHF models are consistently higher than those from the hybrid DFT models.Isotopic fractionation is shown to be sensitive to differences in ligand bond stiffness (above), coordination number, bond length, and the frequency of the asymmetric Fe-X stretching vibrational mode, as predicted by previous theoretical studies. Complexes with smaller coordination numbers have higher 1000 ln β (7.46‰, 5.25‰, and 3.48‰ for , ,, respectively, from the B3LYP/6-31G(d) model). Species with the same number of chlorides but fewer waters also show the effect of coordination number on 1000 ln β: (7.46‰ vs. 7.05‰ for FeCl3(H2O)2 vs. FeCl3(H2O)3 and 5.25‰ vs. 4.94‰ for vs. FeCl5H2O2− with the B3LYP/6-31G(d) model). As more Fe-Cl bonds substitute for Fe-OH2 bonds (with a resulting decrease in β), the lengths of the Fe-Cl bonds and the Fe-O bonds increase.Preliminary modeling of shows an Fe3+/Fe2+ fractionation of 3.2‰ for the B3LYP/6-31G(d) model, in agreement with previous studies. The addition of an explicit outer hydration sphere of 12 H2O molecules to models of improves agreement with measured vibrational frequencies and bond lengths; 1000 ln β increases by 0.8-1.0‰. An additional hydration sphere around increases 1000 ln β by only 0.1‰.Isotopic fractionations predicted for this simple system imply that ligands present in an aqueous iron environment are potentially important drivers of fractionation, and suggest that significant fractionation effects are likely in other aqueous systems containing sulfides or organic ligands. Fractionation effects due to both speciation and redox must be considered when interpreting iron isotope fractionations in the geological record. 相似文献
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Jung-Hyun Kim Jaap van der Meer Peer Helmke Francesca Sangiorgi Ellen C. Hopmans 《Geochimica et cosmochimica acta》2010,74(16):4639-4654
Several studies have shown that there is a strong relationship between the distribution of crenarchaeotal isoprenoid glycerol dibiphytanyl glycerol tetraethers (GDGTs) and sea surface temperature (SST). Based on this, a ratio of certain GDGTs, called TEX86 (TetraEther indeX of tetraethers consisting of 86 carbon atoms), was developed as a SST proxy. In this study, we determined the distribution of crenarchaeotal isoprenoid GDGTs in 116 core-top sediments mostly from (sub)polar oceans and combined these data with previously published core-top data. Using this extended global core-top dataset (n = 426), we re-assessed the relationship of crenarchaeal isoprenoid GDGTs with SST. We excluded data from the Red Sea from the global core-top dataset to define new indices and calibration models, as the Red Sea with its elevated salinity appeared to behave differently compared to other parts of the oceans. We tested our new indices and calibration models on three different paleo datasets, representing different temperature ranges. Our results indicate that the crenarchaeol regio-isomer plays a more important role for temperature adaptation in (sub)tropical oceans than in (sub)polar oceans, suggesting that there may be differences in membrane adaptation of the resident crenarchaeotal communities at different temperatures. We, therefore, suggest to apply two different calibration models. For the whole calibration temperature range (−3 to 30 °C), a modified version of TEX86 with a logarithmic function which does not include the crenarchaeol regio-isomer, called , is shown to correlate best with SST: (r2 = 0.86, n=396, p <0.0001). Application of on sediments from the subpolar Southern Ocean results in realistic absolute SST estimates and a similar SST trend compared to a diatom SST record from the same core. , which is defined as the logarithmic function of TEX86, yields the best correlation with SST, when the data from the (sub)polar oceans are removed: (r2 = 00.87, n = 255, p < 0.0001). Furthermore, gives the best correlation for mescosm data with temperatures ranging between 10 and 46 °C. For Quaternary sediments from the tropical Arabian Sea, both and yield similar trends and SST estimates. However, the extrapolation of calibration on a sediment record from a greenhouse world ocean predicts more reliable absolute SST estimates and relative SST changes in agreement with estimates based on the δ18O of planktonic foraminifera. Based on the comparison of and derived SSTs using the core top data, we recommend applying above 15 °C and below 15 °C. In cases where paleorecords encompass temperatures both below and above 15 °C, we suggest to use . 相似文献
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Flow and nutrient dynamics in a subterranean estuary (Waquoit Bay, MA, USA): Field data and reactive transport modeling 总被引：3，自引：0，他引：3
A two-dimensional (2D) reactive transport model is used to investigate the controls on nutrient (, , PO4) dynamics in a coastal aquifer. The model couples density-dependent flow to a reaction network which includes oxic degradation of organic matter, denitrification, iron oxide reduction, nitrification, Fe2+ oxidation and sorption of PO4 onto iron oxides. Porewater measurements from a well transect at Waquoit Bay, MA, USA indicate the presence of a reducing plume with high Fe2+, , DOC (dissolved organic carbon) and PO4 concentrations overlying a more oxidizing -rich plume. These two plumes travel nearly conservatively until they start to overlap in the intertidal coastal sediments prior to discharge into the bay. In this zone, the aeration of the surface beach sediments drives nitrification and allows the precipitation of iron oxide, which leads to the removal of PO4 through sorption. Model simulations suggest that removal of through denitrification is inhibited by the limited overlap between the two freshwater plumes, as well as by the refractory nature of terrestrial DOC. Submarine groundwater discharge is a significant source of to the bay. 相似文献
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Richard E. Zeebe 《Geochimica et cosmochimica acta》2010,74(3):818-7445
Although the stable oxygen isotope fractionation between dissolved sulfate ion and H2O (hereafter ) is of physico-chemical and biogeochemical significance, no experimental value has been established until present. The primary reason being that uncatalyzed oxygen exchange between and H2O is extremely slow, taking ∼105 years at room temperature. For lack of a better approach, values of 16‰ and 31‰ at 25 °C have been assumed in the past, based on theoretical ‘gas-phase’ calculations and extrapolation of laboratory results obtained at temperatures >75 °C that actually pertain to the bisulfate system. Here I use novel quantum-chemistry calculations, which take into account detailed solute-water interactions to establish a new value for of 23‰ at 25 °C. The results of the corresponding calculations for the bisulfate ion are in agreement with observations. The new theoretical values show that sediment -data, which reflect oxygen isotope equilibration between sulfate and ambient water during microbial sulfate reduction, are consistent with the abiotic equilibrium between and water. 相似文献
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The abiotic, thermochemically controlled reduction of sulfate to hydrogen sulfide coupled with the oxidation of hydrocarbons, is termed thermochemical sulfate reduction (TSR), and is an important alteration process that affects petroleum accumulations in nature. Although TSR is commonly observed in high-temperature carbonate reservoirs, it has proven difficult to simulate in the laboratory under conditions resembling nature. The present study was designed to evaluate the relative reactivities of various sulfate species in order to provide greater insight into the mechanism of TSR and potentially to fill the gap between laboratory experimental data and geological observations. Accordingly, quantum mechanics density functional theory (DFT) was used to determine the activation energy required to reach a potential transition state for various aqueous systems involving simple hydrocarbons and different sulfate species. The entire reaction process that results in the reduction of sulfate to sulfide is far too complex to be modeled entirely; therefore, we examined what is believed to be the rate limiting step, namely, the reduction of sulfate S(VI) to sulfite S(IV). The results of the study show that water-solvated sulfate anions are very stable due to their symmetrical molecular structure and spherical electronic distributions. Consequently, in the absence of catalysis, the reactivity of is expected to be extremely low. However, both the protonation of sulfate to form bisulfate anions () and the formation of metal-sulfate contact ion-pairs could effectively destabilize the sulfate molecular structure, thereby making it more reactive.Previous reports of experimental simulations of TSR generally have involved the use of acidic solutions that contain elevated concentrations of relative to . However, in formation waters typically encountered in petroleum reservoirs, the concentration of is likely to be significantly lower than the levels used in the laboratory, with most of the dissolved sulfate occurring as , aqueous calcium sulfate ([CaSO4](aq)), and aqueous magnesium sulfate ([MgSO4](aq)). Our calculations indicate that TSR reactions that occur in natural environments are most likely to involve bisulfate ions () and/or magnesium sulfate contact ion-pairs ([MgSO4]CIP) rather than ‘free’ sulfate ions () or solvated sulfate ion-pairs, and that water chemistry likely plays a significant role in controlling the rate of TSR. 相似文献
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Batch experiments, combined with in situ spectroscopic methods, are used to examine the coprecipitation of Cr(VI) with calcite, including partitioning behavior, site-specific distribution of Cr on the surface of calcite single crystals, and local coordination of Cr(VI) in the calcite structure. It is found that the concentration of Cr incorporated in calcite increases with increasing Cr concentration in solution. The calculated apparent partition coefficient, , is highest at low Cr solution concentration, and decreases to a constant value with increasing Cr solution concentration. DIC images of the surface of calcite single crystals grown in the presence of exhibit well-defined growth hillocks composed of two pairs of symmetrically nonequivalent vicinal faces, denoted as + and −, which reflect the orientation of structurally nonequivalent growth steps. Micro-XRF mapping of the Cr distribution over a growth hillock shows preferential incorporation of Cr into the—steps, which is considered to result from differences in surface structure geometry. XANES spectra confirm that incorporated Cr is hexavalent, and no reduction of Cr(VI) in the X-ray beam was observed up to 2 days at room temperature. EXAFS fit results show the incorporated Cr(VI) has the expected first shell of 4 O at ∼1.64 ± 0.01 Å, consistent with . Best fit results show that the second shell is split with ∼2.5 Ca at ∼3.33 ± 0.05 and ∼2.2 Ca at ∼3.55 ± 0.05 Å, which confirms the incorporation of chromate into calcite. Consideration of possible local coordination indicates that significant distortion or disruption is required to accommodate in the calcite structure. 相似文献