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1.
《Geochimica et cosmochimica acta》1999,63(19-20):3417-3427
In order to verify Fe control by solution - mineral equilibria, soil solutions were sampled in hydromorphic soils on granites and shales, where the occurrence of Green Rusts had been demonstrated by Mössbauer and Raman spectroscopies. Eh and pH were measured in situ, and Fe(II) analyzed by colorimetry. Ionic Activity Products were computed from aqueous Fe(II) rather than total Fe in an attempt to avoid overestimation by including colloidal particles. Solid phases considered are Fe(II) and Fe(III) hydroxides and oxides, and the Green Rusts whose general formula is [FeII1−xFeIIIx(OH)2]+x· [x/z A−z]−x, where compensating interlayer anions, A−, can be Cl−, SO42−, CO32− or OH−, and where x ranges a priori from 0 to 1. In large ranges of variation of pH, pe and Fe(II) concentration, soil solutions are (i) oversaturated with respect to Fe(III) oxides; (ii) undersaturated with respect to Fe(II) oxides, chloride-, sulphate- and carbonate-Green Rusts; (iii) in equilibrium with hydroxy-Green Rusts, i.e., Fe(II)-Fe(III) mixed hydroxides. The ratios, x = Fe(III)/Fet, derived from the best fits for equilibrium between minerals and soil solutions are 1/3, 1/2 and 2/3, depending on the sampling site, and are in every case identical to the same ratios directly measured by Mössbauer spectroscopy. This implies reversible equilibrium between Green Rust and solution. Solubility products are proposed for the various hydroxy-Green Rusts as follows: log Ksp = 28.2 ± 0.8 for the reaction Fe3(OH)7 + e− + 7 H+ = 3 Fe2+ + 7 H2O; log Ksp = 25.4 ± 0.7 for the reaction Fe2(OH)5 + e− + 5 H+ = 2 Fe2+ + 5 H2O; log Ksp = 45.8 ± 0.9 for the reaction Fe3(OH)8 + 2e− + 8 H+ = 3 Fe2+ + 8 H2O at an average temperature of 9 ± 1°C, and 1 atm. pressure. Tentative values for the Gibbs free energies of formation of hydroxy-Green Rusts obtained are: ΔfG° (Fe3(OH)7, cr, 282.15 K) = −1799.7 ± 6 kJ mol−1, ΔfG° (Fe2(OH)5, cr, 282.15 K) = −1244.1 ± 6 kJ mol−1 and ΔfG° (Fe3(OH)8, cr, 282.15 K) = −1944.3 ± 6 kJ mol−1.  相似文献   

2.
《Applied Geochemistry》2001,16(5):559-570
Fe(II)–Fe(III) layered double hydroxysalt green rusts, GRs, are very reactive compounds with the general formula, [FeII(1−x) FeIIIx (OH)2]x+·[(x/n) An−·(m/n) H2O]x−, where x is the ratio FeIII/Fetot, and reflects the structure in which brucite-like layers alternate with interlayers of anions An− and water molecules. Two types of crystal structure for GRs, GR1 and GR2, represented by the hydroxychloride GR1(Cl−) and the hydroxysulphate GR2(SO42−) are distinguished by X-ray diffraction due to different stacking. By analogy with GR1(Cl−) the structure of the fougerite GR mineral, [FeII(1−x) FeIIIx (OH)2]x+·[x OH−·(1−x) H2O]x-  Fe(OH)(2+x)·(1−x) H2O, is proposed displaying interlayers made of OH− ions and water molecules (in situ deprotonation of water molecules is necessary for explaining the flexibility of its composition). The space group of mineral GR1(OH−) would be R3̄m, with lattice parameters a≅0.32 and c≅2.25 nm. Stability conditions and the Eh-pH diagram of Fe(OH)(2+x) (the water molecules are omitted) are determined from hydromorphic soil solution equilibria with GR mineral in Brittany (France). Computed Gibbs free energies of formation from soil solution/mineral equilibrium fit well with a regular solid solution model: μ°[Fe(OH)(2+x)]=(1−x) μ°[Fe(OH)2]+x μ°[Fe(OH)3]+RT [(1−x) ln (1−x)+x ln x]+A0 x (1−x), where μ°[Fe(OH)2]=−492.5 kJ mol−1, μ°[Fe(OH)3]=−641 kJ mol−1 and A0=−243.9 kJ mol−1 at the average temperature of 9±1°C. The upper limit of occurrence of GR mineral at x=2/3, i.e. Fe3(OH)8, is explained by its unstability vs. α-FeOOH and/or magnetite; Fe(OH)3 is thus a hypothetical compound with a GR structure which cannot be observed. These thermodynamic data and Eh-pH diagrams of Fe(OH)(2+x) can be used most importantly to predict the possibility that GR minerals reduce some anions in contaminated soils. The cases of NO3−, Se(VI) or Cr(VI) are fully illustrated.  相似文献   

3.
Given the direction cosines a i ′ = (a 1 i , a 2 i , a 3 i )corresponding to a set of pspherically projected fabric poles, an initial estimate x′ = (x1, x2, x3, x4)for the angular radius x4,and direction cosines of the center of the least-squares small circle which minimizes the sum of the squares of the angular residuals $$r = \sum\limits_p {\left[ {x_4 - \cos ^{ - 1} \left( {a_1^i x_1 + a_2^i x_2 + a_3^i x_3 } \right)} \right]} ^2 $$ can be iteratively improved by taking xj+1 = xj + Δxwhere xj is the value of xat the jth iteration and $$\Delta x = - H_j^{ - 1} \left[ {q_j + x_j \left( {x'_j H_j^{ - 1} x_j } \right)\left( {q_j - x'_j H_j^{ - 1} q_j } \right)} \right],$$ where As an initial approximation for xwe have found it convenient to ignore the fact that the data are constrained to lie on the surface of the reference sphere and to use the parameters of a least-squares plane through the given poles. Generalization of this approach to fitting variously constrained great and small circles is easily made. The relative merits of differently constrained fits to the same data can be tested approximately if it is assumed that the errors in the location of the poles are isotropic and normally distributed. It is thus possible to statistically assess the relative significance of conflicting structural models which predict different geometrical patterns of fabric elements.  相似文献   

4.
Solubilities of silver chloride in aqueous hydrochloric acid solutions have been determined from 100 up to 350°C. From these measurements, the ionisation constant of HC1 has been evaluated up to 225°C. Evidence is presented to show that a protonated silver species, HAgCl20, exists at 275°C and above. Available experimental data up to 200°C have been firted to Pitzer's equation to generate an algorithm to calculate stoichiometric activity and osmotic coefficients of HCl up to 350°C and concentrations up to at least 3.0 m. Using the present results and those of Wrightet al. (1961), Pearsonet al. (1963) and Lukashowet al. (1976), the dissociation constant (Kd) of HCl as a function of temperature is described by the equation log10K = 2136.898 + 1.020349T−4.5045 × 10−4T2−50396.40/T−901.770 10g10T (Tin °K) which is valid in the range 25–350°C. Calculated enthalpy (ΔH0), entropy (ΔS0) and heat capacity change (ΔCp0) functions for HCl dissociation have been rationalized in terms of changing solute and solvent characteristics as temperature is raised.  相似文献   

5.
《Applied Geochemistry》2004,19(6):835-841
Experiments on dissolution kinetics of galena were performed in 1 mol l−1 NaCl solutions at pH 0.43–2.45 and 25–75 °C. When the dissolution reaction is far from equilibrium, a linear relation exits between the dissolution rate, r, and the H+ ion activity, [H+]. The rate law for galena dissolution is given by the following equation: r=k[H+]. With respect to H+, the dissolution reaction is in the first order. The apparent rate constant, k, has values of 2.34×10−7 mol m−2 s−1 at 25 °C, 1.38×10−6 mol m−2 s−1 at 50 °C, and 7.08×10−6 mol m−2 s−1 at 75 °C. The activation energy of dissolution reaction is 43.54 kJ mol−1. The mechanism of dissolution is suggested to be surface chemical reaction, and the rate determining step is the dissociation of the Pb–S bond of the surface complex, which releases Pb2+ into the solution.  相似文献   

6.
A newly developed mixed flow reactor was used to measure the rate of hydrolysis of wollastonite over the pH range of 3 to 8. This design avoids abrasion of the solid sample by confining it within a nylon mesh while the reacting solution is circulated over it by a stirrer. The rate of reaction was determined from the difference of the compositions of the input and output solutions following the methods used by chemical engineers for the analysis of mixed flow reactors, also called continuously stirred tank reactors (CSTR). This apparatus, constructed from easily obtainable parts, avoids many of the problems inherent in studying mineral/solution reaction kinetics in batch reactors.The hydrolysis of wollastonite CaSiO3 + 2H+ + H2O = Ca2+ + H4SiO4 can be fit to a rate law of the form: dnH+/dt = kadKH+mH+/(1.0 + KH+mH+) where kad = 9.80 × 10−8molm−2sec−1 and KH+ = 2.08 × 105. Over the pH range of 4 to 7, the data also may fit a simple linear form: dnH+/dt = −Ak+(aH+)0.40 where k+ = 3.80 × 10−6 sec−1 at 25°C. The presence of calcium ion in the solution at concentrations up to 1.0 mol kg−1 produces only a minor reduction of the reaction rate. The activation energy for this reaction is 79.2 kJ mol−1. Examination of the surfaces of the reacted grains showed no evidence of incongruent reaction leading to a product layer but did show the extensive development of etch pits leading to a rapid increase in the specific surface area. At large extents of reaction at low pH, diffusion of ions into or from these deep etch pits may limit the reaction rate.  相似文献   

7.
A hypothesis is presented that the dissolution of albite includes the exchange of sodium for hydrogen ion in a surface layer of the mineral and the structural collapse of the residual anionic lattice of the layer. The ion exchange is described by the first law of diffusion (D25°C = 3 × 10?22 and 1.5 × 10?20 cm2sec?1 at PCO2 = 0 and 26.2 atm, respectively). The surface residual layer reaches a steady-state thickness ranging from n × 10?8 to n × 10?5 cm according to the temperature and PCO2. The increase in aqueous sodium with time in a continuous ground-water system is described by a simple exponential equation. The equation is used to estimate the percolation time of ground water from the data on the chemical composition of a water sample. The probable times range from 14 to 3840 days for various ground-water systems and are compared to the times of percolation calculated from the geothermal and hydraulic data. Both estimates are found to be in general agreement. The concentrations of Al and Si in cold water from granitic rocks are shown to be controlled by the chemical equilibrium with respect to an aged aluminosilicate. The aluminosilicate precipitates from ground water as an amorphous isoelectric solid. Its chemical composition is represented by a simplified stoichiometric formula [Al(OH)3](1?x)[SiO2]x and varies linearly with pH of the solution. The atoms of Al, O and H tend to occupy a fixed position in the solid given by the gibbsite structure upon aging in the field. The solubility product of the solid is estimated from the published data on experimental and field research into the dissolution of feldspars: logK = (1 ? x) × log [Al3+] + xlog [H4SiO4] ? (3 ? 3x) log [H+] = 8.56 ? 11.26x, where x is the molar fraction of silica in the aluminosilicate.  相似文献   

8.
《Geochimica et cosmochimica acta》1999,63(19-20):3407-3416
The apparent solubilities of schwertmannite and ferrihydrite were estimated from the H+, OH−, Fe3+, and SO42− activities of the natural stream waters in Korea and mine drainage in Ohio, USA. Both chemical composition of the stream waters and the mineralogy of the precipitates were determined for samples from two streams polluted by coal mine drainage. This study combines these new results with previous data from Ohio, USA to redetermine solubilities. The activities of the dissolved species necessary for the solubility determinations were calculated from the chemical compositions of the waters with the WATEQ4F computer code.Laboratory analyses of precipitates indicated that the main minerals present in Imgok and Osheep creek were schwertmannite and ferrihydrite, respectively. The schwertmannite from Imgok creek had a variable chemical formula of Fe8O8(OH)8−2x(SO4)x· nH2O, where 1.74 ≤ x ≤ 1.86 and 8.17 ≤ n ≤ 8.62. The chemical formula of ferrihydrite was Fe2O3· 1.6H2O. With known mineralogy of the precipitates from each stream, the activities of H+, OH−, Fe3+, and SO42− in the waters were plotted on logarithmic activity-activity diagrams to determine apparent solubilities of schwertmannite and ferrihydrite. The best estimate for the logarithm of the solubility product of schwertmannite, logKs, was 10.5 ± 2.5 around 15°C. This value of logKs constrains the logarithm of the solubility product of ferrihydrite, logKf, to be 4.3 ± 0.5 to maintain the stability boundary with schwertmannite observed in natural waters.  相似文献   

9.
The β-factors of corundum were estimated on the basis of DFT calculations of vibrational frequency changes due to 16O–18O isotope substitution in a harmonic approximation using an all-electron Gaussian-type basis set and the B3LYP hybrid functional (the CRYSTAL program). Calculations were performed accounting for eight phonon wave vectors within the first Brillouin zone. The results are approximated by the relation 1000ln β crn = 9.19874x–0.12326x 2 + 0.00213x 3 (x = 106/T(K)2, 400 < T(K) < 1300), which can be used in isotope geochemical studies in combination with the known temperature effects on the β-factors of other phases.  相似文献   

10.
《Geochimica et cosmochimica acta》1999,63(19-20):3261-3275
Studies on the dissolution kinetics of kaolinite were performed using batch reactors at 25°C and in the pH range from 1 to 13. A rapid initial dissolution step was first observed, followed by a linear kinetic stage reached after approximately 600 hr of reaction during which the kaolinite dissolves congruently at pH < 4 and pH > 11. The apparent incongruency between pH 5 and 10 was due to the precipitation of an Al–hydroxide phase. The true dissolution rates were computed from the amount of Si released into solution. The rate dependence on pH can be described by: r = 10−12.19aH+0.55 + 10−14.36 + 10−10.71aOH−0.75Between pH 5 and 10, the rate is approximately constant, although a smooth minimum was observed at pH close to 9. mAn attempt was made to obtain a general rate law based on the coordination theory, which was first applied to the mineral dissolution studies by Stumm and co-workers. The kinetic data were combined with the results obtained for the surface speciation by Huertas et al. (1998). It is possible to express the linear dissolution rate as a simple power function of the concentration of the surface sites active in various pH ranges: r = 10−8.25 [>Al2OH2+] + 10−10.82 [>AlOH2+]0.5 + 10−9.1 [>Al2OH + >AlOH + >SiOH] + 103.78 [>Al2O− + >AlO−]3This equation assumes that the dissolution mechanism is mainly controlled by the two Al surface sites (external and internal structural hydroxyls, and aluminol at the crystal edges) under both acidic and alkaline conditions. The model reflects well the important contribution of the crystal basal planes to the dissolution of kaolinite.  相似文献   

11.
Solution of Laplace’s equation can be done by iteration methods likes Jacobi, Gauss–Seidel, and successive over-relaxation (SOR). There is no new knowledge about the relaxation coefficient (ω) in SOR method. In this paper, we used SOR for solving Laplace’s differential equation with emphasis to obtaining the optimum (minimum) number of iterations with variations of the relaxation coefficient (ω). For this purpose, a code in FORTRAN language has been written to show the solution of a set of equations and its number of iterations. The results demonstrate that the optimum value of ω with minimum iterations is achieved between 1.7 and 1.9. Also, with increasing β?=??x/?y from 0.25 to 10, the number of iterations reduced and the optimum value is obtained for β?=?2.  相似文献   

12.
《Applied Geochemistry》2000,15(8):1203-1218
Ca6[Al(OH)6]2(CrO4)3·26H2O, the chromate analog of the sulfate mineral ettringite, was synthesized and characterized by X-ray diffraction, Fourier transform infra-red spectroscopy, thermogravimetric analyses, energy dispersive X-ray spectrometry, and bulk chemical analyses. The solubility of the synthesized solid was measured in a series of dissolution and precipitation experiments conducted at 5–75°C and at initial pH values between 10.5 and 12.5. The ion activity product (IAP) for the reaction Ca6[Al(OH)6]2(CrO4)3·26H2O⇌6Ca2++2Al(OH)−4+3CrO2−4+4OH−+26H2O varies with pH unless a CaCrO4(aq) complex is included in the speciation model. The log K for the formation of this complex by the reaction Ca2++CrO2−4=CaCrO4(aq) was obtained by minimizing the variance in the IAP for Ca6[Al(OH)6]2(CrO4)3·26H2O. There is no significant trend in the formation constant with temperature and the average log K is 2.77±0.16 over the temperature range 5–75°C. The log solubility product (log KSP) of Ca6[Al(OH)6]2(CrO4)3·26H2O at 25°C is −41.46±0.30. The temperature dependence of the log KSP is log KSP=A−B/T+D log(T) where A=498.94±48.99, B=27,499±2257, and D=−181.11±16.74. The values of ΔG0r,298 and ΔH0r,298 for the dissolution reaction are 236.6±3.9 and 77.5±2.4 kJ mol−1. the values of ΔC0P,r,298 and ΔS0r,298 are −1506±140 and −534±83 J mol−1 K−1. Using these values and published standard state partial molal quantities for constituent ions, ΔG0f,298=−15,131±19 kJ mol−1, ΔH0f,298=−17,330±8.6 kJ mol−1, ΔS0298=2.19±0.10 kJ mol−1 K−1, and ΔC0Pf,298=2.12±0.53 kJ mol−1 K−1, were calculated.  相似文献   

13.
An experimental arrangement suitable for application of high temperature calorimetry to liquid sulfide systems has been developed. Using this approach, we have measured the integral enthalpies of mixing of Ni + NiS at 1100 K to form liquid alloys with compositions from XNis = 0.576 to XNis = 0.754. Partial enthalpies of the two components also were measured. After correcting for the enthalpies of fusion of Ni and NiS at 1100 K, the results of all measurements can be represented by an analytical expression which reflects subregular behavior of the mixing enthalpies ΔHmixl−1 = XNis2XNiA + XNisXNiS2B with A = −97.712 kJ mol−1 and B = −4.772 kJ mol−1.The standard enthalpies of formation of the high and low temperature forms of NiS were evaluated from the calorimetrically measured enthalpy change associated with the reaction between nickel and sulfur at 1021 K. The standard enthalpies of formation of Ni3S2 (heazlewoodite), Ni7S6 and Ni0.958S were determined from the enthalpy changes of reactions in which the compounds were formed from NiS and Ni at 873 K and 833 K. The standard enthalpy of formation of NiS2(vaesite) was obtained from the enthalpy change of the reaction of NiS2 with Ni to give NiS at 873 K. The following values are reported for the standard enthalpies of formation of the phases studied (in kJ mol−1): ΔHf,NiS(HT)0 = −88.1 ± 1.0 ΔHf, Ni0.958S0 = −93.2 ± 0.7ΔHf,Ni7S60 = −582.8 ± 5.7 ΔHf,NiS(LT)0 = −91.0 ± 1.0ΔHf,Ni3S2(LT)0 = −217.2 ± 1.6 ΔHf,NiS20 = −124.9 ± 1.0.  相似文献   

14.
《Geochimica et cosmochimica acta》1999,63(13-14):1969-1980
The solubility of ettringite (Ca6[Al(OH)6]2(SO4)3 · 26H2O) was measured in a series of dissolution and precipitation experiments at 5–75°C and at pH between 10.5 and 13.0 using synthesized material. Equilibrium was established within 4 to 6 days, with samples collected between 10 and 36 days. The log KSP for the reaction Ca6[Al(OH)6]2(SO4)3 · 26H2O ⇌ 6Ca2+ + 2Al(OH)4− + 3SO42− + 4OH− + 26H2O at 25°C calculated for dissolution experiments (−45.0 ± 0.2) is not significantly different from the log KSP calculated for precipitation experiments (−44.8 ± 0.4) at the 95% confidence level. There is no apparent trend in log KSP with pH and the mean log KSP,298 is −44.9 ± 0.3. The solubility product decreased linearly with the inverse of temperature indicating a constant enthalpy of reaction from 5 to 75°C. The enthalpy and entropy of reaction ΔH°r and ΔS°r, were determined from the linear regression to be 204.6 ± 0.6 kJ mol−1 and 170 ± 38 J mol−1 K−1. Using our values for log KSP, ΔH°r, and ΔS°r and published partial molal quantities for the constituent ions, we calculated the free energy of formation ΔG°f,298, the enthalpy of formation ΔH°f,298, and the entropy of formation ΔS°f,298 to be −15211 ± 20, −17550 ± 16 kJ mol−1, and 1867 ± 59 J mol−1 K−1. Assuming ΔCP,r is zero, the heat capacity of ettringite is 590 ± 140 J mol−1 K−1.  相似文献   

15.
Hydrogen gas (H2) may be produced by the anoxic corrosion of steel components in underground structures, such as geological repositories for radioactive waste. In such environments, hydrogen was shown to serve as an electron donor for autotrophic bacteria. High gas overpressures are to be avoided in radioactive waste repositories and, thus, microbial consumption of H2 is generally viewed as beneficial. However, to fully consider this biological process in models of repository evolution over time, it is crucial to determine the in situ rates of microbial hydrogen oxidation and sulfate reduction. These rates were estimated through two distinct in situ experiments, using several measurement and calculation methods. Volumetric consumption rates were calculated to be between 1.13 and 1.93 μmol cm−3 day−1 for H2, and 0.14 and 0.20 μmol cm−3 day−1 for sulfate. Based on the stoichiometry of the reaction, there is an excess of H2 consumed, suggesting that it serves as an electron donor to reduce electron acceptors other than sulfate, and/or that some H2 is lost via diffusion. These rate estimates are critical to evaluate whether biological H2 consumption can negate H2 production in repositories, and to determine whether sulfate reduction can consume sulfate faster than it is replenished by diffusion, which could lead to methanogenic conditions.  相似文献   

16.
Rate laws have been determined for the aqueous oxidation of pyrite by ferric ion, dissolved oxygen and hydrogen peroxide at 30°C in dilute, acidic chloride solutions. Fresh, smooth pyrite grain surfaces were prepared by cleaning prior to experiments. Initial specific surface areas were measured by the multipoint BET technique. Surface textures before and after oxidation were examined by SEM. The initial rate method was used to derive rate laws.The specific initial rates of oxidation (moles pyrite cm−2 min−1) are given by the following rate laws (concentrations in molar units): rsp,Fe3+ = −10−9.74M0.5Fe3+M−0.5H+ (pH 1–2)rsp,o2 = −10−6.77M0.5O2 (pH 2–4)rsp,h2o2 = −10−1.43MH2O2 (pH 2−4)An activation energy of 56.9 ± 7.5 kJ mole−1 was determined for the oxidation of pyrite by dissolved oxygen from 20–40°C. HPLC analyses indicated that only minor amounts of polythionates are detectable as products of oxidation by oxygen below pH 4; the major sulfur product is sulfate. Ferric ion and sulfate are the only detectable products of pyrite oxidation by hydrogen peroxide. Hydrogen peroxide is consumed by catalytic decomposition nearly as fast as it is by pyrite oxidation.SEM photomicrographs of cleaned pyrite surfaces indicate that prior to oxidation, substantial intergranular variations in surface texture exist. Reactive surface area is substantially different than total surface area. Oxidation is centered on reactive sites of high excess surface energy such as grain edges and corners, defects, solid and fluid inclusion pits, cleavages and fractures. These reactive sites are both inherited from mineral growth history and applied by grain preparation techniques. The geometry and variation of reactive sites suggests that the common assumption of a first-order, reproducible dependence of oxidation rates on surface area needs to be tested.  相似文献   

17.
We have performed a series of interdiffusion experiments on magnesiowüstite samples at room pressure, temperatures from 1,320° to 1,400°C, and oxygen fugacities from 10?1.0 Pa to 10?4.3 Pa, using mixed CO/CO2 or H2/CO2 gases. The interdiffusion couples were composed of a single-crystal of MgO lightly pressed against a single-crystal of (Mg1-x Fe x )1-δO with 0.07<x<0.27. The interdiffusion coefficient was calculated using the Boltzmann–Matano analysis as a function of iron content, oxygen fugacity, temperature, and water fugacity. For the entire range of conditions tested and for compositions with 0.01<x<0.27, the interdiffusion coefficient varies as $$\tilde D\, =\,2.9\times10^{ - 6}\,f_{{\text{O}}_2 }^{0.19}\,x^{0.73}\,{\text{e}}^{ - (209,000\, -\,96,000\,x)/RT}\,\,{\text{m}}^{\text{2}} {\text{s}}^{ -1} $$ These dependencies on oxygen fugacity and composition are reasonably consistent with interdiffusion mediated by unassociated cation vacancies. For the limited range of water activity that could be investigated using mixed gases at room pressure, no effect of water on interdiffusion could be observed. The dependence of the interdiffusion coefficient on iron content decreased with increasing iron concentration at constant oxygen fugacity and temperature. There is a close agreement between our activation energy for interdiffusion extrapolated to zero iron content (x=0) and that of previous researchers who used electrical conductivity experiments to determine vacancy diffusivities in lightly doped MgO.  相似文献   

18.
FeII-III hydroxycarbonate green rust GR(CO32−), FeII4 FeIII2 (OH)12 CO3·3H2O, is oxidized in aqueous solutions with varying reaction kinetics. Rapid oxidation with either H2O2 or dissolved oxygen under neutral and alkaline conditions leads to the formation of ferric oxyhydroxycarbonate GR(CO32−)∗, FeIII6 O12 H8 CO3·3H2O, via a solid-state reaction. By decreasing the flow of oxygen bubbled in the solution, goethite α-FeOOH forms by dissolution-precipitation mechanism whereas a mixture of non-stoichiometric magnetite Fe(3−x)O4 and goethite is observed for lower oxidation rates. The intermediate FeII-III oxyhydroxycarbonate of formula FeII6(1−x) FeIII6x O12 H2(7−3x) CO3·3H2O, i.e. GR(x)∗ for which x ? [1/3, 1], is the synthetic compound that is homologous to the fougerite mineral present in hydromorphic gleysol; in situ oxidation accounts for the variation of ferric molar fraction x = [FeIII]/{[FeII]+[FeIII]} observed in the field as a function of depth and season but limited to the range [1/3, 2/3]. The domain of stability for partially oxidized green rust is observed in the Eh-pH Pourbaix diagrams if thermodynamic properties of GR(x)∗ is compared with those of lepidocrocite, γ-FeOOH, and goethite, α-FeOOH. Electrochemical equilibrium between GR(x)∗ and FeII in solution corresponds to Eh-pH conditions close to those measured in the field. Therefore, the reductive dissolution of GR(x)∗ can explain the relatively large concentration of FeII measured in aqueous medium of hydromorphic soils containing fougerite.  相似文献   

19.
Galvanic cells with oxygen-specific solid electrolytes made of calcia-stabilized zirconia have been used to make equilibrium measurements of the standard Gibbs free energy of formation, ΔfG0m,(T), for copper (I) oxide (Cu2O), nickel (II) oxide (NiO), cobalt (II) oxide (CoO), and wüstite (FexO) over the temperature range from 900–1400 K. The measured values of ΔfG0m at 1300 K are −73950, −123555, −142150, and −179459 J · mol−1 for Cu2O, NiO, CoO, and Fe0.947O, respectively. The precision of these measurements is ± 30–60 J · mol−1, and their absolute accuracy is estimated to be ± 100–200 J·mol−1. Using values of –76.557, −94.895, −79.551, and −71.291 J · K−1 · mol−1 for the entropies of formation, ΔfSm0, (298.15 K), the calculated enthalpies of formation, ΔfHm0, (298.15 K), are −170508, −240110, −237390, and −266458 J · mol−1 for Cu2O, NiO, CoO, and Fe0.947O, respectively. These values of ΔfSm0 (298.15 K) and ΔfHm0 (298.15 K) are in good agreement with the best available calorimetric measurements.  相似文献   

20.
Celestite solubility measurements have been conducted in pure water at temperatures from 10 to 90°C. Equilibrium was achieved with respect to a crystalline solid phase from both undersaturated and supersaturated solutions. The measurements show that the solubility undergoes a maximum near 20°C. LogK values for the solubility reaction are adequately described by the following expression over the temperature range 283.15 to 363.15 K: −logK= −35.3106+0.00422837T+318312/T2+14.99586 logT.The following thennodynamic values for the dissolution reaction of SrSO4(s), at 25°C have been derived: ΔGR0 = 37852 ± 30 Jmol−1ΔHR0 = −1668±920Jmol−1ΔSR0= −132.6±3.2JK−1mol−1Celestite solubility measurements were also determined in NaCl solutions up to 5 m concentration and from 10 to 40°C. These data are in good agreement with the work of StrÜbel (1966), who reports solubility measurements to temperatures of 100°C.The application of the Pitzer relations and the solubility constants determined in this study to calculate celestite solubility in NaCl solutions yields excellent agreement between predicted values and experimental measurements over the entire range of temperature and NaCl concentration conditions. For the limited number of solubility measurements in seawater-type solutions and mixed-salt brines, the agreement using the Pitzer relations is within three percent of the measured solubility.  相似文献   

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