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1.
The process of pressure solution is defined as the dissolution of materials under high stress at grain-to-grain contacts and precipitation at interfaces under low stress. The kinetics of this process are still poorly understood mainly because of the large timescales involved. In this research, the Surface Forces Apparatus (SFA) technique was coupled with an optical interference technique for in situ visualization of the nanoscale deformations and thickness changes. The SFA was used to measure the forces (or pressures) and distances between two solid surfaces pressed together with a thin film between them. Using the SFA, combined with geological observations, we are studying the short-range colloidal forces between surfaces of mica and silica at the nanoscale such as van der Waals, electrostatic, and hydration forces.

This study involves two cases, the symmetric case of mica in contact with mica and the asymmetric case of a quartz surface in contact with mica. Our results reveal highly subtle effects depending on the nature and concentration of the counterions present in the solution either of Na+, Ca2+, or mixtures of these ions, as well as on the pH. For the symmetric case, the equilibrium interactions of force F or pressure P versus fluid film thickness T have been measured between the mica surfaces across aqueous films in the thickness range from T = 25 Å down to contact separations around T = 0 Å, and depend on the solution conditions and applied lithostatic pressure. Measurements have also been made of the rates of diffusion of ions through such ultra-thin films and on the precipitation and growth of ionic crystallite layers on the surfaces. Our results show that the diffusion coefficient of hydrated sodium is two orders of magnitude lower than the diffusion of water into mica–mica cleavage and a factor of 40 lower than the coefficient of sodium ions in bulk water.

For the asymmetric case, the dissolution of the quartz surface was observed to be dependent on the interfacial fluid composition and pH, the externally applied ‘lithostatic’ pressure, and the type of crystalline structure exposed to the mica surface. Our experiments also show that there is an initial stage after fresh solution is added in which the spacing between the surfaces increases, however, the thickness started decreasing steadily after approximately 4 h of exposure independently of the crystallinity of the quartz surface. For a particular set of conditions, the process eventually slows down and reaches equilibrium after some time, but a further increase in pressure restarts the dissolution process. This is also true for the addition of fresh interfacial solution during the experiment after a period of thickness fluctuation. These results are consistent with the observation that pressure solution of quartz is greatly enhanced with the presence of mica.  相似文献   


2.
We present rock mechanical test results and analytical calculations which demonstrate that a negative surface charge, resulting from sulfate adsorption from the pore water, impacts the rock mechanical behavior of high-porosity chalk. Na2SO4 brine flooded into chalk cores at 130 °C results in significantly reduced bulk modulus and yield point compared with that of NaCl brine at the same conditions. The experimental results have been interpreted using a surface complexation model combined with the Gouy-Chapman theory to describe the double layer. The calculated sulfate adsorption agrees well with the measured data. A sulfate adsorption of about 0.3 μmol/m2 and 0.7–1 μmol/m2 was measured at 50 and 130 °C, respectively. Relative to a total sites of 5 sites/nm2 these values correspond to an occupation of 4 % and 8–13 % which sufficiently explains the negative charging of the calcite surfaces. The interaction between charged surfaces specifically in the weak overlaps of electrical double layer gives rise to the total disjoining pressure in granular contacts. The net repulsive forces act as normal forces in the grains vicinity, counteracting the cohesive forces and enhance pore collapse failure during isotropic loading, which we argue to account for the reduced yield and bulk modulus of chalk cores. The effect of disjoining pressure is also assessed at different sulfate concentrations in aqueous solution, temperatures, as well as ionic strength of solution; all together remarkably reproduce similar trends as observed in the mechanical properties.  相似文献   

3.
High-resolution in situ X-ray specular reflectivity was used to measure the structures of orthoclase (001) and (010) cleavage surfaces in contact with deionized water at 25°C. X-ray reflectivity data demonstrate a high degree of structural similarity between these two orthoclase-water interfaces. Both interfacial structures include cleavage along the plane of minimal bond breakage resulting in surfaces terminated by non-bridging oxygens; structured water within 5 Å of the orthoclase surface (consisting of adsorbed species at the surface and layered water above the surface), with a featureless water profile beyond 5 Å; substitution of outermost K+ ions by an oxygen containing species (presumably H3O+); and small structural displacements of the near surface atoms. The interfacial water structure, in comparison with recent results for other mineral-water interfaces, is intermediate between the minimal structure found at calcite-, barite-, and quartz-water interfaces and the more extensive structure found at the muscovite-water interface.  相似文献   

4.
The structure and dynamics of water on muscovite mica (0 0 1) surfaces have been investigated by molecular dynamics simulations. X-ray reflectivity profiles highly reflecting the interfacial structure are directly calculated and compared with those of experiments. The direct comparison has validated the usefulness of MD simulations to understand the real interfacial structure of the mica−water system. We observed five distinguished peaks in the density profile of oxygen present in water, and these peaks are attributable to the water molecules directly adsorbed on mica, hydrated to the K+ ions on the mica surface, and ordered due to hydrogen bonds between hydrated K+ ions. The hydrated K+ ions make an inner-sphere complex and have an explicit first hydration shell with a radius of 3.6 Å and a hydration number of 2.9. The change of the viscosity of water located above 1 nm apart from the mica surface was not observed. This feature is in good agreement with a recent experimental study in which the shear measurement was conducted using a surface forces apparatus. The increase of the viscosity by a factor of ca. 2-3 relative to that of the bulk water was observed at water located within 1 nm from the isolated mica surface.  相似文献   

5.
Dipeptide hydrolysis and amino acid decomposition appear to follow a first-order rate law. The hydrolysis rate increases exponentially with increasing temperature in aqueous solution at both 265 atm and water steam pressures over the temperature range of 100 to 220 degrees C. Dipeptide hydrolysis has a lower apparent activation energy at 265 atm (44.1 KJ/mol) than at water steam pressure (98.9 KJ/mol). At lower temperatures (<200-220 degrees C), the rate of peptide bond hydrolysis is faster at 265 atm than at water steam pressure. At higher temperatures (>200-220 degrees C), however, peptide bond hydrolysis is slower at 265 atm than at water steam pressure. In aqueous solution, amino acid decomposition rates also increase exponentially with increasing temperature. Amino acid decomposition rates are much higher at 265 atm than at water steam pressure over the entire temperature range investigated.  相似文献   

6.
Two different dry-ground muscovites were placed separately in deionized water saturated with 1 atm partial pressure of carbon dioxide at room temperature and pressure for about 1000 hr. The carbonic acid formed buffered the bulk solution between 4.6 and 5.1 during the whole time of the experiment. K and Na were rapidly released by a reaction with hydrogen ions in the solution, which is usually attributed to a surface exchange reaction. The cation exchange capacity is close to the amount of K and Na released to the solution. However, measurement of surface areas of the micas both before and after the rapid release of K and Na, indicate that only one third of the cation exchange capacity can be attributed to release from the outer surfaces, whereas two thirds must be attributed to removal from within deeper parts of the structure.Approximately 2.5% of total K and Na from the interlayer regions, 1.5% of Mg from the octahedral sheets, and 0.5% of Si from the tetrahedral sheets were released by the end of the experiment, indicating that dissolution was incongruent in both cases. Si showed the slowest total release rate of all the ions. In the later stages (after 400 hr), the release rate of Si becomes a linear function of time. The linear release rate of Si was found to be 2.4 × 10?17 mol/cm2/sec for the ruby mica, and 2.7 × 10?17 mol/cm2/sec for the green mica. The rate-controlling mechanism in the dissolution of muscovite in the later stages is probably related to the rate of destruction of the tetrahedral sheets (Si-O bonds) which in turn is reflected by the rate of release of Si into the solution.  相似文献   

7.
《Applied Geochemistry》1993,8(3):207-221
The gases dissolved in Lake Nyos, Cameroon, were quantified recently (December 1989 and September 1990) by two independent techniques: in-situ measurements using a newly designed probe and laboratory analyses of samples collected in pre-evacuated stainless steel cylinders. The highest concentrations of CO2 and CH4 were 0.30 mol/kg and 1.7 mmol/kg, respectively, measured in cylinders collected 1 m above lake bottom. Probe measurements of in-situ gas pressure at three different stations showed that horizontal variations in total dissolved gas were negligible. Total dissolved-gas pressure near the lake bottom is 1.06 MPa (10.5 atm), 50% as high as the hydrostatic pressure of 2.1 MPa (21 atm). Comparing the CO2 profile constructed from the 1990 data to one obtained in May 1987 shows that CO2 concentrations have increased at depths to below 150 m. Based on these profiles, the average rate of CO2 input to bottom waters was 2.6 × 108 mol/a. Increased deep-water temperatures require an average heat flow of 0.32 MW into the hypolimnion over the same time period. The transport rates of CO2, heat, and major ions into the hypolimnion suggest that a low-temperature reservoir of free CO2 exists a short distance below lake bottom and that convective cycling of lake water through the sediments is involved in transporting the CO2 into the lake from the underlying diatreme. Increased CH4 concentrations at all depths below the oxycline and a high14C content (41% modern) in the CH4 4 m above lake bottom show that much of the CH4 is biologically produced within the lake. The CH4 production rate may vary with time, but if the CO2 recharge rate remains constant, CO2 saturation of the entire hypolimnion below 50 m depth would require ∼140a, given present-day concentrations.  相似文献   

8.
The influence of background electrolytes on the mechanism and kinetics of calcite dissolution was investigated using in situ Atomic Force Microscopy (AFM). Experiments were carried out far from equilibrium by passing alkali halide salt (NaCl, NaF, NaI, KCl and LiCl) solutions over calcite cleavage surfaces. This AFM study shows that all the electrolytes tested enhance the calcite dissolution rate. The effect and its magnitude is determined by the nature and concentration of the electrolyte solution. Changes in morphology of dissolution etch pits and dissolution rates are interpreted in terms of modification in water structure dynamics (i.e. in the activation energy barrier of breaking water-water interactions), as well as solute and surface hydration induced by the presence of different ions in solution. At low ionic strength, stabilization of water hydration shells of calcium ions by non-paired electrolytes leads to a reduction in the calcite dissolution rate compared to pure water. At high ionic strength, salts with a common anion yield similar dissolution rates, increasing in the order Cl− < I− < F− for salts with a common cation due to an increasing mobility of water around the calcium ion. Changes in etch pit morphology observed in the presence of F− and Li+ are explained by stabilization of etch pit edges bonded by like-charged ions and ion incorporation, respectively. As previously reported and confirmed here for the case of F−, highly hydrated ions increased the etch pit nucleation density on calcite surfaces compared to pure water. This may be related to a reduction in the energy barrier for etch pit nucleation due to disruption of the surface hydration layer.  相似文献   

9.
Shallow fresh water bodies in peat areas could be an important contributor to greenhouse gases in the atmosphere.In this study,the partial pressure of CO2 in the surface water of the Dianshan Lake was investigated insitu in August 2011.The average pCO2 in the study area was 2300μatm and fluctuated within the range of 989–5000μatm.pCO2 showed a reverse trend to the variations of pH and DO in the surface water of the Dianshan Lake.The water to air diffusion flux of CO2 of the upstream,middle lake and downstream were respectively 63,33 and 14mmol/m2/d.On average,the diffusion flux of CO2 of the whole lake was 31 mmol/m2/d.Consequently,our results show that during the sampling season,the Dianshan Lake appears to be a great source of CO2.It is also demonstrated that respiration could be the dominant biochemical reaction in the Dianshan Lake in summer.  相似文献   

10.
The diffusion coefficients of aquo metal ions (Mz+) and their EDTA complexes (M-EDTA(z−4)+) were compared to understand the effect of EDTA complexation on the diffusion of metal ions by the diffusion cell method for Co2+, Ga3+, Rb+, Sr2+, Ag+, Cd2+, Cs+, Th4+, , and trivalent lanthanides. Most studies about ionic diffusion in water have dealt with free ion (hydrated ion). In many cases, however, polyvalent ions are dissolved as complexed species in natural waters. Hence, we need to study the diffusion behavior of complexes in order to understand the diffusion phenomenon in natural aquifer and to measure speciation by diffusive gradient in thin films (DGT), which requires the diffusion coefficients of the species examined. For many ions, the diffusion coefficients of M-EDTA(z−4)+ are smaller than those of hydrated ions, but the diffusion coefficients of M-EDTA(z−4)+ are larger than those of hydrated ions for ions with high ionic potentials (Ga3+ and Th4+). As a result, the diffusion coefficients of EDTA complexes are similar among various metal ions. In other words, the diffusion of each ion loses its characteristics by the complexation with EDTA. Although the difference is subtle, it was also found that the diffusion coefficients of EDTA complexes increase as the ionic potential increases, which can be explained by the size of the EDTA complex of each metal ion.  相似文献   

11.
 Grain boundary diffusion rates of oxygen, potassium and calcium in fine-grained feldspar aggregates were determined experimentally. The starting materials were a natural albite rock from the Tanco pegmatite and aggregates hot-pressed from fragments of Amelia albite or Ab, Or and An composition glasses. The technique employed isotopic tracers (18O, 41K, 42Ca) either evaporated onto the surface or in an aqueous solution surrounding the sample, and depth profiling using an ion microprobe (SIMS). From the depth profiles, the product of the grain boundary diffusion coefficient (D′) and effective boundary width (δ) was calculated using numerical solutions to the appropriate diffusion equation. The experimental reproducibility of D′δ is a factor of 3. A separate determination of D′ independent of δ yields an effective grain boundary width of ∼3 nm, consistent with high resolution TEM observations of a physical grain boundary width <5 nm. Oxygen (as molecular water) grain boundary diffusion rates were determined in the Ab and Or aggregates at 450°–800° C and 100 MPa (hydrothermal), potassium rates in Or aggregates at 450°–700° C both at 0.1 MPa (in air) and at 100 MPa (hydrothermal), and calcium rates in An aggregates at 700°–1100° C and 0.1 MPa (in air). Oxygen grain boundary diffusion rates are similar in all three of the Ab aggregates and in the Or aggregate. Potassium and oxygen depth profiles measured in the same samples yield different D′δ values, confirming a diffusional transport mechanism. Potassium diffusion in the Or aggregate has a greater activation energy (216 vs 78 kJ/mol) than oxygen, and the Arrhenius relations cross at ∼625° C. Potassium D′δ values in Or aggregates are about a factor of five greater in hydrothermal experiments at 100 MPa than in experiments at 0.1 MPa in air. Calcium grain boundary diffusion rates in An aggregates are 4 to 5 orders of magnitude slower than potassium in Or and have a greater (291 kJ/mol) activation energy. This suggests that differences in formal charge and/or size of diffusing species may play an important role in their relative grain boundary diffusion rates. Received: 24 December 1993 / Accepted: 16 June 1994  相似文献   

12.
 Calcium self-diffusion rates in natural calcite single crystals were experimentally determined at 700 to 900° C and 0.1 MPa in a stream of CO2. Diffusion coefficients (D) were determined from 42Ca concentration profiles measured with an ion microprobe. The Arrhenius parameters yield an activation energy (Q)=382±37 kJ/mol and pre-exponential factor (D0)=0.13 m2/s, and there is no measurable anisotropy. Calcium grain boundary diffusion rates were experimentally determined in natural (Solnhofen) limestone and hot-pressed calcite aggregates at 650° to 850° C and 0.1 to 100 MPa pressure. The Solnhofen limestone was first pre-annealed for 24 h at 700° C and 100 MPa confining pressure under anhydrous conditions to produce an equilibrium microstructure for the diffusion experiments. Values for the product of the grain boundary diffusion coefficient (D′) and the effective grain boundary diffusion width (δ) were determined from 42Ca concentration profiles measured with an ion microprobe. The results show that there is no measurable difference between D′δ values obtained for pre-annealed Solnhofen samples at 0.1 and 100 MPa or between hot-pressed calcite aggregates and pre-annealed Solnhofen samples. The temperature dependence for calcium grain boundary diffusion in Solnhofen samples annealed at 0.1 MPa is described by the Arrhenius parameters D′ 0δ=1.5×10−9 m3/s and Q=267±47 kJ/mol. Comparison of the results of this study with previously published data show that calcium is the slowest volume diffusing species in calcite. The calcium diffusivities measured in this study place constraints on several geological processes that involve diffusive mass transfer including diffusion-accommodated mechanisms in the deformation of calcite rocks. Received: 19 December 1994/Accepted: 30 June 1995  相似文献   

13.
Fe-, Cr- and Al-spinels were synthesized and their unit cell sizes determined by means of X-rays. Differential thermal curves show that the magnetic inversion of Fe2O3 at 680° C accelerates the formation of the ferrites when the constituent oxides are heated together.A correlation can be made between ionic radii of cations and unit cell dimensions provided the effect of covalent forces in the lattice is taken into account. The values for ionic radii of cations as given byAhrens (1952) permit a better correlation than those ofGoldschmidt.A shrinkage of 0.01 Å in the unit cell size per 0.01 Å decrease in the ionic radius of the divalent cations was determined when spinels with the same cation arrangement in the same group were compared. A shrinkage of 0.027 Å in the unit cell size per 0.01 Å decrease in the ionic radius of the trivalent cations was determined in spinels having the same divalent cation and cation arrangement when the trivalent cations form the same type of bonds.The half-occupation of the 3d orbits in Mn2+ and Fe3+ causes abnormally high unit cell dimensions in spinels where these ions are incorporated in octahedral sites. This is attributed to the formation of electrovalent bonds by these ions. Variable forces of contraction in the lattice are revealed when the unit cell dimensions are correlated with the ionic radii of cations. The force of contraction can be satisfactorily explained as being due to covalent forces in the spinel structure. The magnitude of this force or the degree of covalence in the bonds increases in the following order of cations where these are situated in tetrahedral sites:The divalent transition element ions, Fe2+, Co2+ and Ni2+; the B-Sub-group element ions Cd2+ and Zn2+; Fe3+ in tetrahedral co-ordination.  相似文献   

14.
We have investigated grain boundary diffusion rates in enstatite by heating single crystals of quartz packed in powdered San Carlos olivine (Mg0.90Fe0.10)2SiO4 at controlled oxygen fugacities in the range 10?5.7 to 10?8.7?atm and temperatures from 1350° to 1450?°C for times from 5 to 100?h at 1?atm total pressure. Following the experiments, the thickness of the coherent polycrystalline reaction rim of pyroxene that had formed between the quartz and olivine was measured using backscatter scanning imaging in the electron microprobe. Quantitative microprobe analysis indicated that the composition of this reaction phase is (Mg0.92Fe0.08)2Si2O6. The rate of growth of the pyroxene increases with increasing temperature, is independent of the oxygen fugacity, and is consistent with a parabolic rate law, indicating that the growth rate is controlled by ionic diffusion through the pyroxene rim. Microstructural observations and platinum marker experiments suggest that the reaction phase is formed at the olivine-pyroxene interface, and is therefore controlled by the diffusion of silicon and oxygen. The parabolic rate constants determined from the experiments were analyzed in terms of the oxide activity gradient across the rim to yield mean effective diffusivities for the rate-limiting ionic species, assuming bulk transport through the pyroxene layer. These effective diffusivities are faster than the lattice diffusivities for the slowest species (silicon) calculated from creep experiments, but slower than measured lattice diffusivities for oxygen in enstatite. Thus, silicon grain boundary diffusion is most likely to be the rate-limiting process in the growth of the pyroxene rims. Also, as oxygen transport through the pyroxene rims must be faster than silicon transport, diffusion of oxygen along the grain boundaries must be faster than through the lattice. The grain boundary diffusivity for silicon in orthopyroxenite is then given by D¯gbSiδ=(3.3±3.0)×10?9f0.0O2e?400±65/RT?m3s?1, where the activation energy for diffusion is in kJ/mol, and δ is the grain boundary width in m. Calculated growth rates for enstatite under these conditions are significantly slower than predicted by an extrapolation from similar experiments performed at 1000?°C under high pressure (hydrous) conditions by Yund and Tullis (1992), perhaps due to water-enhancement of diffusion in their experiments.  相似文献   

15.
In metabasites from the high‐grade metamorphic Kimi Complex of the Greek Rhodope, lamellae of quartz and amphibole were observed together with either rutile or titanite precipitates in the cores of clinopyroxene that underwent a prolonged exhumation history from eclogite via granulite and amphibolite facies conditions. Lamellae are crystallographically oriented along clinopyroxene cleavage planes (010) and interpreted as the result of precipitation during a two‐step intracrystalline compositional adjustment from an omphacite‐ to a Ca‐Tschermak‐rich clinopyroxene and finally to diopside. Even though retrograde diffusion profiles develop in clinopyroxene, the diffusion rates of highly charged ions like Si4+, Al3+and Ti4+are too slow, so precipitates rich in Si4+ (quartz), Al3+(amphibole) and Ti4+(rutile or titanite) form as viable kinetic alternatives. These precipitation reactions are not possible isochemically, and additional exchange of ions with the matrix is required for precipitation to occur. This exchange of mainly rapidly diffusing low‐charge ions is postulated to proceed by bulk diffusion (self‐diffusion) and does not require special pathways or fluid, even though these may play an additional role during metamorphism. This hitherto undescribed process of precipitation made possible by diffusional exchange with the matrix through an intact host crystal lattice is termed here as ‘open system precipitation’ (OSP). No prior Ca‐Eskola‐rich (ultrahigh‐pressure, UHP) composition of omphacite is required to explain the observed precipitates. It is proposed that many precipitation phenomena that were hard to explain in a closed‐system scenario can now be explained more readily by OSP. Calculation of an original solid solution composition, as in an UHP phase, from precipitate and host mineral analyses becomes imprecise to the degree that bulk diffusion and hence OSP have been important.  相似文献   

16.
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17.
The distribution and dynamics of water molecules and monovalent cations (Li+, Na+, K+, Cs+, and H3O+) on muscovite surfaces were investigated by molecular dynamics (MD) simulations. The direct comparison of calculated X-ray reflectivity profiles and electron density profiles with experiments revealed the precise structure at the aqueous monovalent electrolyte solutions/muscovite interface. To explain the experimentally observed electron density profiles for the CsCl solution-muscovite interface, the co-adsorption of Cs+ and Cl− ion pairs would be necessary. Two types of inner-sphere complexes and one type of outer-sphere complex were observed for hydrated Li+ ions near the muscovite surface. For Na+, K+, Cs+, and H3O+ ions, the inner-sphere complexes were stable on the muscovite surface. The density oscillation of water molecules was observed to approximately 1.5 nm from the muscovite surface. The number of peaks and the locations for the density of water oxygen atoms were almost similar among the water molecules coordinated to Li+, Na+, K+, and H3O+ ions adsorbed on the muscovite surfaces. The water molecules around Cs+ ions that were adsorbed to muscovite surfaces seemed to avoid coordinating with Cs+ ions on the surface, and the density of water oxygen near the muscovite surface decreased relative to that in a bulk state. There was no significant difference in self-diffusion, viscosity, retention time, and reorientation time of water molecules among different cations adsorbed to muscovite surfaces. These translational and rotational motions of water molecules located at less than 1 nm from the muscovite surfaces were slower than those in a bulk state. A significant difference was observed for the exchange times of water molecules around monovalent cations. The exchange time of water molecules was long around Li+ ions and decreased with an increase in the ionic radius.  相似文献   

18.
The diffusion of water in natural obsidian and model dacitic melts (Ab90Di8Wo2, mol %) has been studied at water vapor pressure up to 170 MPa, temperatures of 1200°C, H2O contents in melts up to ～6 wt % using a high gas pressure apparatus equipped with a unique internal device. The experiments were carried out by a new low-gradient technique with application of diffusion hydration of a melt from fluid phase. The water solubility in the melts and its concentration along $C_{H_2 O} $ diffusion profiles were determined using IR microspectrometry by application of the modified Bouguer-Beer-Lambert equation. A structural-chemical model was proposed to calculate and predict the concentration dependence of molar absorption coefficients of the hydroxyl groups (OH?) and water molecules (H2O) in acid polymerized glasses (quenched melts) in the obsidian-dacite series. The water diffusion coefficients $D_{H_2 O} $ were obtained by the mathematical analysis of concentration profiles and the analytical solution of the second Fick diffusion law using the Boltzman-Matano method. It was shown experimentally that $D_{H_2 O} $ exponentially increases with a growth of water concentration in the obsidian and dacitic melts within the entire range of diffusion profiles. Based on the established quantitative correlation between $D_{H_2 O} $ and viscosity of such melts, a new method was developed to predict and calculate the concentration, temperature, and pressure dependences of $D_{H_2 O} $ in acid magmatic melts (obsidian, rhyolite, albite, granite, dacite) at crustal T, P parameters and water concentrations up to 6 wt %.  相似文献   

19.
Sandstone dissolution is a common water–rock reaction in the Earth’s crust, but a thorough understanding of this phenomenon is constrained by poorly determined kinetic data. To this end, kinetic data were determined for the dissolution of arkosic sandstone powders in deionised water (pH was about 7.0–7.3 and electrical conductivity was between 0.95 and 1.00 μS/cm). Release rates of dissolved elements were determined over the range 50–350 °C at 20, 15, and 10 MPa using a column flow-through pressure vessel reactor. The conductivity of the outlet solution, measured at room temperature, is dependent on the charge of major cations such as Na+, K+, Ca2+ and Mg2+ at these conditions. The conductivity of the outlet solution was used to determine the steady state of the dissolution of sandstone powders. The pH values of the outlet solutions at the steady state, measured ex situ at room temperature, were about 7.7, 8.3, 8.4, 8.4 and 7.6 at 75, 100, 150, 200 and 250 °C, respectively, at 10 MPa. Silicon, Na, K, Ca, Al and Mg are the major ions found in the solution at low temperatures, but Si is the only major ion retained at higher temperatures (>150 °C). Compared with static experiments, the flowing dissolution experiments occurred at conditions far from equilibrium. The relationship between temperature and dissolution rates of arkosic sandstone powders was described as log R = 0.005469t − 10.50 where R is the dissolution rates of sandstone powders in kg/(m2 s), t is temperature in °C which ranged from 100 to 350 °C at 20 and 15 MPa, and the dissolution rates of sandstone powders were measured only for the major dissolved elements without oxygen in the outlet solutions.  相似文献   

20.
The interfacial structures of the basal surface of muscovite mica in solutions containing (1) 5 × 10−3 m BaCl2, (2) 500 ppm Elliott Soil Fulvic Acid I (ESFA I), (3) 100 ppm Elliott Soil Fulvic Acid II (ESFA II), (4) 100 ppm Pahokee Peat Fulvic Acid I (PPFA), and (5) 5 × 10−3 m BaCl2 and 100 ppm ESFA II were obtained with high resolution in-situ X-ray reflectivity. The derived electron-density profile in BaCl2 shows two sharp peaks near the mica surface at 1.98(2) and 3.02(4) Å corresponding to the heights of a mixture of Ba2+ ions and water molecules adsorbed in ditrigonal cavities and water molecules coordinated to the Ba2+ ions, respectively. This pattern indicates that most Ba2+ ions are adsorbed on the mica surface as inner-sphere complexes in a partially hydrated form. The amount of Ba2+ ions in the ditrigonal cavities compensates more than 90% of the layer charge of the mica surface. The electron-density profiles of the fulvic acids (FAs) adsorbed on the mica surface, in the absence of Ba2+, had overall thicknesses of 4.9-10.8 Å and consisted of one broad taller peak near the surface (likely hydrophobic and positively-charged groups) followed by a broad humped pattern (possibly containing negatively-charged functional groups). The total interfacial electron density and thickness of the FA layer increased as the solution FA concentration increased. The sorbed peat FA which has higher ash content showed a higher average electron density than the sorbed soil FA. When the muscovite reacted with a pre-mixed BaCl2-ESFA II solution, the positions of the two peaks nearest the surface matched those in the BaCl2 solution. However, the occupancy of the second peak decreased by about 30% implying that the hydration shell of surface-adsorbed Ba2+ was partially substituted by FA. The two surface peaks were followed by a broad less electron-dense layer suggesting a sorption mechanism in which Ba2+ acts dominantly as a bridging cation between the mica surface and FA. When the muscovite reacted first with FA and subsequently with BaCl2, more Ba2+ could be adsorbed on the FA-coated mica surface. The peak closest to the mica included Ba2+ ions adsorbed directly on the mica in an amount similar to that in the BaCl2 solution but more broadly distributed. A second peak observed within the FA layer suggests that the FA coating provides additional sites for Ba2+ sorption. The results indicate that enhanced uptake of heavy metals can occur when an organic coating already exists on a mineral surface.  相似文献   

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