where D0 is in µm2/s, X is mole fraction of H2Ot on a single oxygen basis, T is temperature in K, and P is pressure in GPa.H2Ot diffusivities (DH2Ot, in µm2/s) can be calculated from H2Om diffusivity, or directly from the following expression:
At low H2Ot content (up to 2 wt.% if an error of a factor of 2 is allowed), H2Ot diffusivity is approximately proportional to H2Ot content:
where C is H2Ot content in wt.% and C0 is 1 wt.%. The new expressions for H2O diffusion not only reproduce our own data, but also match data in literature from different laboratories and using different methods, indicating good inter-laboratory and multi-method consistency. The new expressions cover a wide range of geological conditions, and can be applied to H2O diffusion in rhyolitic melts in various volcanic and magmatic processes.  相似文献   

8.
Stable boron isotope fractionation between dissolved B(OH)3 and B(OH)4     
Richard E. Zeebe 《Geochimica et cosmochimica acta》2005,69(11):2753-2766
The stable boron isotope ratio (11B/10B) in marine carbonates is used as a paleo-pH recorder and is one of the most promising paleo-carbonate chemistry proxies. Understanding the thermodynamic basis of the proxy is of fundamental importance, including knowledge on the equilibrium fractionation factor between dissolved boric acid, B(OH)3, and borate ion, B(OH)4− (, hereafter α(B3-B4)). However, this factor has hitherto not been determined experimentally and a theoretically calculated value (Kakihana and Kotaka, 1977, hereafter KK77) has therefore been widely used. I examine the calculations underlying this value. Using the same spectroscopic data and methods as KK77, I calculate the same α(B3−B4) = 1.0193 at 300 K. Unfortunately, it turns out that in general the result is sensitive to the experimentally determined vibrational frequencies and the theoretical methods used to calculate the molecular forces. Using analytical techniques and ab initio molecular orbital theory, the outcome for α(B3-B4) varies between ∼1.020 and ∼1.050 at 300 K. However, several arguments suggest that α(B3-B4) ? 1.030. Measured isotopic shifts in various 10B-, 2D-, and 18O-labeled isotopomers do not provide a constraint on stable boron isotope fractionation. I conclude that in order to anchor the fundamentals of the boron pH proxy, experimental work is required. The critics of the boron pH proxy should note, however, that uncertainties in α(B3-B4) do not bias pH reconstructions provided that organism-specific calibrations are used.  相似文献   

9.
A simple predictive model of quartz solubility in water-salt-CO2 systems at temperatures up to 1000 °C and pressures up to 1000 MPa     
Nikolay N. Akinfiev 《Geochimica et cosmochimica acta》2009,73(6):1597-1608
Knowledge of the solubility of quartz over a broad spectrum of aqueous fluid compositions and T-P conditions is essential to our understanding of water-rock interaction in the Earth’s crust. We propose an equation to compute the molality of aqueous silica, mSiO2(aq), mol·(kg H2O)−1, in equilibrium with quartz and water-salt-CO2 fluids, as follows:
  相似文献   

10.
Uranophane dissolution and growth in CaCl2-SiO2(aq) test solutions     
James D. Prikryl 《Geochimica et cosmochimica acta》2008,72(18):4508-4520
The dissolution and growth of uranophane [Ca(UO2)2(SiO3OH)2·5H2O] have been examined in Ca- and Si-rich test solutions at low temperatures (20.5 ± 2.0 °C) and near-neutral pH (∼6.0). Uranium-bearing experimental solutions undersaturated and supersaturated with uranophane were prepared in matrices of ∼10−2 M CaCl2 and ∼10−3 M SiO2(aq). The experimental solutions were reacted with synthetic uranophane and analyzed periodically over 10 weeks. Interpretation of the aqueous solution data permitted extraction of a solubility constant for the uranophane dissolution reaction and standard state Gibbs free energy of formation for uranophane ( kJ mol−1).  相似文献   

11.
Coupled phase and aqueous species equilibrium of the H2O-CO2-NaCl-CaCO3 system from 0 to 250 °C, 1 to 1000 bar with NaCl concentrations up to saturation of halite     
Zhenhao Duan  Dedong Li 《Geochimica et cosmochimica acta》2008,72(20):5128-5145
A model is developed for the calculation of coupled phase and aqueous species equilibrium in the H2O-CO2-NaCl-CaCO3 system from 0 to 250 °C, 1 to 1000 bar with NaCl concentrations up to saturation of halite. The vapor-liquid-solid (calcite, halite) equilibrium together with the chemical equilibrium of H+, Na+, Ca2+, , Ca(OH)+, OH−, Cl−, , , CO2(aq) and CaCO3(aq) in the aqueous liquid phase as a function of temperature, pressure, NaCl concentrations, CO2(aq) concentrations can be calculated, with accuracy close to those of experiments in the stated T-P-m range, hence calcite solubility, CO2 gas solubility, alkalinity and pH values can be accurately calculated. The merit and advantage of this model is its predictability, the model was generally not constructed by fitting experimental data.One of the focuses of this study is to predict calcite solubility, with accuracy consistent with the works in previous experimental studies. The resulted model reproduces the following: (1) as temperature increases, the calcite solubility decreases. For example, when temperature increases from 273 to 373 K, calcite solubility decreases by about 50%; (2) with the increase of pressure, calcite solubility increases. For example, at 373 K changing pressure from 10 to 500 bar may increase calcite solubility by as much as 30%; (3) dissolved CO2 can increase calcite solubility substantially; (4) increasing concentration of NaCl up to 2 m will increase calcite solubility, but further increasing NaCl solubility beyond 2 m will decrease its solubility.The functionality of pH value, alkalinity, CO2 gas solubility, and the concentrations of many aqueous species with temperature, pressure and NaCl(aq) concentrations can be found from the application of this model. Online calculation is made available on www.geochem-model.org/models/h2o_co2_nacl_caco3/calc.php.  相似文献   

12.
Interdiffusion of H2O and CO2 in metamorphic fluids at ∼490 to 690°C and 1 GPa     
David A. Wark  E.Bruce Watson 《Geochimica et cosmochimica acta》2004,68(12):2693-2698
Interdiffusion coefficients have been determined for H2O-CO2 mixtures by quantifying the flux of CO2 between two fluid-filled chambers in a specially designed piston-cylinder cell. The two chambers, which are maintained at 1.0 GPa and at temperatures differing by ∼100°C, each contain the XCO2-buffering assemblage calcite + quartz + wollastonite, in H2O. The positive dependence of XCO2 on temperature results in a down-temperature, steady-state flux of CO2 through a capillary tube that connects the two chambers. This flux drives the wollastonite = calcite + quartz equilibrium to the right in the cooler chamber, producing a measurable amount of calcite that is directly related to CO2-H2O interdiffusion rates. Diffusivities calculated from seven experiments range from 1.0 × 10−8 to 6.1 × 10−8 m2/s for mean capillary temperatures between ∼490 and 690°C. The data set can be approximated by an Arrhenius-type relation:
  相似文献   

13.
The rates of Sb(III) oxidation by O2 and H2O2 were determined in homogeneous aqueous solutions. Above pH 10, the oxidation reaction of Sb(III) with O2 was first order with respect to the Sb(III) concentration and inversely proportional to the H+ concentrations at a constant O2 content of 0.22 × 10−3 M. Pseudo-first-order rate coefficients, kobs, ranged from 3.5 × 10−8 s−1 to 2.5 × 10−6 s−1 at pH values between 10.9 and 12.9. The relationship between kobs and pH was:
  相似文献   

14.
A thermodynamic model is developed for the calculation of both phase and speciation equilibrium in the H2O-CO2-NaCl-CaCO3-CaSO4 system from 0 to 250 °C, and from 1 to 1000 bar with NaCl concentrations up to the saturation of halite. The vapor-liquid-solid (calcite, gypsum, anhydrite and halite) equilibrium together with the chemical equilibrium of H+,Na+,Ca2+, , , and CaSO4(aq) in the aqueous liquid phase as a function of temperature, pressure and salt concentrations can be calculated with accuracy close to the experimental results.Based on this model validated from experimental data, it can be seen that temperature, pressure and salinity all have significant effects on pH, alkalinity and speciations of aqueous solutions and on the solubility of calcite, halite, anhydrite and gypsum. The solubility of anhydrite and gypsum will decrease as temperature increases (e.g. the solubility will decrease by 90% from 360 K to 460 K). The increase of pressure may increase the solubility of sulphate minerals (e.g. gypsum solubility increases by about 20-40% from vapor pressure to 600 bar). Addition of NaCl to the solution may increase mineral solubility up to about 3 molality of NaCl, adding more NaCl beyond that may slightly decrease its solubility. Dissolved CO2 in solution may decrease the solubility of minerals. The influence of dissolved calcite on the solubility of gypsum and anhydrite can be ignored, but dissolved gypsum or anhydrite has a big influence on the calcite solubility. Online calculation is made available on www.geochem-model.org/model.  相似文献   

15.
Energetics for the condensation dimerization reaction of monosilicic acid:
2Si4(OH)⇒2SiO7H6+H2O  相似文献   

16.
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17.
Excess N from agriculture induces eutrophication in major river systems and hypoxia in coastal waters throughout the world. Much of this N is from headwaters far up the watersheds. In turn, much of the N in these headwaters is from ground-water discharge. Consequently, the concentrations and forms of N in groundwater are important factors affecting major aquatic ecosystems; despite this, few data exist for several species of N in groundwater and controls on speciation are ill-defined. Herein, we report N speciation for a spring and well that were selected to reflect agricultural impacts, and a spring and well that show little to no agricultural-N impact. Samples were characterized for NO3−, NO2−, N2O, NH4+, urea, particulate organic N(), and dissolved organic N(). These analytes were monitored in the agricultural spring for up to two years along with other analytes that we reported upon previously. For all samples, when oxidized N was present, the dominant species was NO3− (88-98% of total fixed N pool) followed by (<4-12%) and only trace fractions of the other N analytes. In the non-agriculturally impacted well sample, which had no quantifiable NO3− or dissolved O2, comprised the dominant fraction (68%) followed by NH4+ (32%), with only a trace balance comprised of other N analytes. Water drawn from the well, spring and a wetland situated in the agricultural watershed also were analyzed for dissolved N2 and found to have a fugacity in excess of that of the atmosphere. H2O2 was analyzed in the agricultural spring to evaluate the O2/H2O2 redox potential and compare it to other calculated potentials. The potential of the O2/H2O2 couple was close in value to the NO3−/NO2− couple suggesting the important role of H2O2 as an O2-reduction intermediate product and that O2 and NO3− are reduced concomitantly. The O2/H2O2 and NO3−/NO2− couples also were close in value to a cluster of other inorganic N and Fe couples indicating near partial equilibrium among these species. Urea mineralization to NO2− was found to approach equilibrium with the reduction of O2 to H2O2. By modeling as amide functional groups, as justified by recent analytical work, similar thermodynamic calculations support that mineralization to NO2− proceeds nearly to equilibrium with the reduction of O2 to H2O2 as well. This near equilibration of redox couples for urea- and -oxidation with O2-reduction places these two couples within the oxidized redox cluster that is shared among several other couples we have reported previously. In the monitored agricultural spring, [NO3−] was lower in the summer than at other times, whereas [N2O] was higher in the summer than at other times, perhaps reflecting a seasonal variation in the degree of denitrification reaction progress. No other N analytes were observed to vary seasonally in our study. In the well having no agricultural-N impact, Corg/Norg = 5.5, close to the typical value for natural aqueous systems of about 6.6. In the agricultural watershed Corg/Norg varied widely, from ∼1.2 to ?9.  相似文献   

18.
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19.
The osmotic coefficients of FeCl3 at 25 °C from 0.15 to 1.7 m [Rumyantsev et al., Z. Phys. Chem., 218, 1089-1127, 2004] have been used to determine the Pitzer parameters (β(0), β(1) and C?) for FeCl3. Since the differences in the Pitzer coefficients of rare earths in NaCl and NaClO4 are small, the values of Fe(ClO4)3 have been estimated using the differences between La(ClO4)3 and LaCl3. The Pitzer coefficients for FeCl3 combined with enthalpy and heat capacity data for the rare earths can be used to estimate the activity coefficients of Fe3+ in NaCl over a wide range of temperatures (0 to 50 °C) and ionic strength (0 to 6 m).The activity coefficients of Fe3+ in NaCl and NaClO4 solutions have been used to determine the activity coefficients of Fe(OH)2+ in these solutions from the measured first hydrolysis constants of Fe3+ [Byrne et al., Mar. Chem., 97, 34-48, 2005]. The activity coefficients of , Fe(OH)3 and from 0 to 50 °C have also been determined from the solubility measurements of Fe(III) in NaCl solutions [Liu and Millero, Geochim. Cosmochim Acta, 63, 3487-3497, 1999]. These activity coefficients have been fitted to the Pitzer equations. These results can be used to estimate the speciation of Fe(III) with OH− in natural waters with high concentrations of NaCl from 0 to 50 °C.  相似文献   

20.
We measured the adsorption of Cu(II) onto goethite (α-FeOOH), hematite (α-Fe2O3) and lepidocrocite (γ-FeOOH) from pH 2-7. EXAFS spectra show that Cu(II) adsorbs as (CuO4Hn)n−6 and binuclear (Cu2O6Hn)n−8 complexes. These form inner-sphere complexes with the iron (hydr)oxide surfaces by corner-sharing with two or three edge-sharing Fe(O,OH)6 polyhedra. Our interpretation of the EXAFS data is supported by ab initio (density functional theory) geometries of analogue Fe2(OH)2(H2O)8Cu(OH)4and Fe3(OH)4(H2O)10Cu2(OH)6 clusters. We find no evidence for surface complexes resulting from either monodentate corner-sharing or bidentate edge-sharing between (CuO4Hn)n−6 and Fe(O,OH)6 polyhedra. Sorption isotherms and EXAFS spectra show that surface precipitates have not formed even though we are supersaturated with respect to CuO and Cu(OH)2. Having identified the bidentate (FeOH)2Cu(OH)20 and tridentate (Fe3O(OH)2)Cu2(OH)30 surface complexes, we are able to fit the experimental copper(II) adsorption data to the reactions
  相似文献   

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1.
The solubility of Fe-ettringite (Ca6[Fe(OH)6]2(SO4)3 · 26H2O) was measured in a series of precipitation and dissolution experiments at 20 °C and at pH-values between 11.0 and 14.0 using synthesised material. A time-series study showed that equilibrium was reached within 180 days of ageing. After equilibrating, the solid phases were analysed by XRD and TGA while the aqueous solutions were analysed by ICP-OES (calcium, sulphur) and ICP-MS (iron). Fe-ettringite was found to be stable up to pH 13.0. At higher pH-values Fe-monosulphate (Ca4[Fe(OH)6]2(SO4) · 6H2O) and Fe-monocarbonate (Ca4[Fe(OH)6]2(CO3) · 6H2O) are formed. The solubilities of these hydrates at 25 °C are:   相似文献   

2.
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3.
In a recent study, sulphate-bearing green rust (GRSO4) was shown to incorporate Na+ in its structure (NaFeII6FeIII3(OH)18(SO4)2(s); GRNa,SO4). The compound was synthesised by aerial oxidation of Fe(OH)2(s) in the presence of NaOH. This paper reports on its free energy of formation .Freshly synthesised GRNa,SO4 was titrated with 0.5 M H2SO4 in an inert atmosphere at 25 °C, producing dissolved Fe2+ and magnetite or goethite. Solution concentrations, PHREEQC and the MINTEQ database were used to calculate reaction constants for the reactions:
  相似文献   

4.
The solubility of KFe(CrO4)2·2H2O, a precipitate recently identified in a Cr(VI)-contaminated soil, was studied in dissolution and precipitation experiments. Ten dissolution experiments were conducted at 4–75°C and initial pH values between 0.8 and 1.2 using synthetic KFe(CrO4)2·2H2O. Four precipitation experiments were conducted at 25°C with final pH values between 0.16 and 1.39. The log KSP for the reaction
相似文献   

5.
In this study, the solubility constant of magnesium chloride hydroxide hydrate, Mg3Cl(OH)5·4H2O, termed as phase 5, is determined from a series of solubility experiments in MgCl2-NaCl solutions. The solubility constant in logarithmic units at 25 °C for the following reaction,
Mg3Cl(OH)5·4H2O+5H+=3Mg2++9H2O(l)+Cl-  相似文献   

6.
The solubility of ZnS(cr) was measured at 100 °C, 150 bars in sulfide solutions as a function of sulfur concentration (m(Stotal) = 0.02-0.15) and acidity (pHt = 2-11). The experiments were conducted using a Ti flow-through hydrothermal reactor enabling the sampling of large volumes of solutions at experimental conditions, with the subsequent concentration and determination of trace quantities of Zn. Prior to the experiments, a long-term in situ conditioning of the solid phase was performed in order to attain the reproducible Zn concentrations (i.e. solubilities). The ZnS(cr) solubility product was monitored in the course of the experiment. The following species were found to account for Zn speciation in solution: Zn2+ (pHt < 3), (pHt 3-4.5), (pHt 5-8), and ZnS(HS)− (pHt > 8) (pHt predominance regions are given for m(Stotal) = 0.1). Solubility data collected in this study at pHt > 3 were combined with the ZnS(cr) solubility product determined at lower pH to yield the following equilibrium constants (t = 100 °C, P = 150 bars):
  相似文献   

7.
Huaiwei Ni  Youxue Zhang   《Chemical Geology》2008,250(1-4):68-78
Water diffusion in silicate melts is important for understanding bubble growth in magma, magma degassing and eruption dynamics of volcanos. Previous studies have made significant progress on water diffusion in silicate melts, especially rhyolitic melt. However, the pressure dependence of H2O diffusion is not constrained satisfactorily. We investigated H2O diffusion in rhyolitic melt at 0.95–1.9 GPa and 407–1629 °C, and 0.2–5.2 wt.% total water (H2Ot) content with the diffusion-couple method in a piston-cylinder apparatus. Compared to previous data at 0.1–500 MPa, H2O diffusivity is smaller at higher pressures, indicating a negative pressure effect. This pressure effect is more pronounced at low temperatures. Assuming H2O diffusion in rhyolitic melt is controlled by the mobility of molecular H2O (H2Om), the diffusivity of H2Om (DH2Om) at H2Ot ≤ 7.7 wt.%, 403–1629 °C, and ≤ 1.9 GPa is given by
DH2Om=D0exp(aX),
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