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1.
Initial dissolution kinetics at orthoclase (001) and (010) cleavage surfaces were measured for ∼2 to 7 monolayers as a function of temperature using in situ X-ray reflectivity. The sensitivity of X-ray reflectivity to probe mineral dissolution is discussed, including the applicability of this approach for different dissolution processes and the range of dissolution rates (∼10−12 to 10−6 mol/m2/sec) that can be measured. Measurements were performed at pH 12.9 for the (001) surface and at pH 1.1 for the (001) and (010) surfaces at temperatures between 46 and 83°C. Dissolution at pH 12.9 showed a temperature-invariant process with an apparent activation energy of 65 ± 7 kJ/mol for the (001) cleavage surface consistent with previous powder dissolution results. Dissolution at pH 1.1 of the (001) and (010) surfaces revealed a similar process for both surfaces, with apparent activation energies of 87 ± 7 and 41 ± 7 kJ/mol, respectively, but with systematic differences in the dissolution process as a function of temperature. Longer-term measurements (five monolayers) show that the initial rates reported here at acidic pH are greater than steady-state rates by a factor of 2. Apparent activation energies at acidic pH differ substantially from powder dissolution results for K-feldspar; the present results bracket the value derived from powder dissolution measurements. The difference in apparent activation energies for the (001) and (010) faces at pH 1.1 reveals an anisotropy in dissolution kinetics that depends strongly on temperature. Our results imply a projected ∼25-fold change in the ratio of dissolution rates for the (001) and (010) surfaces between 25 and 90°C. The dissolution rate of the (001) surface is higher than that of the (010) surface above 51°C and is projected to be lower below this temperature. These results indicate clearly that the kinetics and energetics of orthoclase dissolution at acidic pH depend on crystal orientation. This dependence may reflect the different manifestation of the Al-Si ordering between the T1 and T2 tetrahedral sites at these two crystal faces and can be rationalized in terms of recent theoretical models of mineral dissolution.  相似文献   

2.
Atomic force microscopy (AFM) was used to study the rates of migration of the (10¯1 4) plane of a single-crystal of calcite dissolving in 0.1 M NaCl aqueous solutions at room temperature. The solution pH and PCO 2 controlled in the ranges 4.4 < pH < 12.2 and 0 < PCO 2 < 10-3.5 atm (ambient), respectively. Measured step velocities were compared with the mineral dissolution rates determined from the calcium fluxes. The step velocity is defined as the average of the velocities of the obtuse and acute steps. Rates of step motion increased gradually from 1.4(±0.2) at pH 5.3 to 2.4(±0.3) nm s-1 at pH 8.2, whereas the rates inverted and decreased to the minimum value of 0.69(±0.18) nm s-1 at pH 10.8. For pH > 10.8, only the velocity of the obtuse steps increased as pH increased, whereas that of acute steps gradually decreased.The dissolution rate of the mineral can be calculated from the measured step velocities and average slope, which is proportional to the concentration of exposed monomolecular steps on the surface. The average slope of the dissolving mineral, measured at pH 5.6 and 9.7, was 0.026 (±0.015). Using this slope, we calculate bulk dissolution rates for 5.3 < pH < 12.2 of 4.9(±3.0) × 10-11 to 1.8(±1.0) × 10-10 mol cm-2 s-1. The obtained dissolution rate can be expressed by the following empirical equation:Rdss = 10-4.66(±0.13)[H+] + 10-3.87(±0.06)[HCO3 -] + 10-7.99(plusmn; 0.08)[OH-]We propose that calcite dissolution in these solutions is controlled by elementary reactions that are similar to those that control the dissolution of other amphoteric solids, such as oxides. The mechanisms include the proton-enhanced hydration and detachment of calcium-carbonate ion pairs. The detachments are enhanced by the presence of adsorbed nucleophiles, such as hydroxyl and bicarbonate ions, and by protons adsorbed to key oxygens. A molecular model is proposed that illustrates these processes.  相似文献   

3.
An established engineering model is used to identify conditions where diffusion controls the dissolution of quartz and forsterite in packed beds. The model shows that diffusion control is favored at low advection flux, large grain size, high temperature, and high pH (if the reaction consumes H+). Quartz dissolution is chemical reaction controlled for most geochemically reasonable combinations of temperature, grain size, and flow rate. On the other hand, forsterite dissolution rates can be diffusion controlled for typical advection fluxes, grain sizes, temperatures, and pH’s. The apparent activation energy for diffusion-controlled reactions in a packed bed is much higher than the <∼20 kJ/mol value that is often used to identify diffusion controlled reactions. The models are quite general and can be adapted to deal with other mineral dissolution and precipitation reactions.  相似文献   

4.
Direct measurements of calcite faces were performed using in situ atomic force microscopy (AFM) to reveal the dissolution processes as a function of solution saturation state and temperature. Time-sequential AFM images demonstrated that step velocities at constant temperature increased with increasing undersaturation. The anisotropy of obtuse and acute step velocities appeared to become more significant as solutions approached equilibrium and temperature increased. At saturation state Ω > 0.02, a curvilinear boundary was formed at the intersection of two acute steps and the initially rhombohedral etch pit exhibited a nearly triangular shape. This suggests that the and steps may not belong to the calcite-aqueous solution equilibrium system. Further increase in the saturation state (Ω ? 0.3) led to a lack of etch pit formation and dissolution primarily occurred at existing steps, in accordance with Teng (2004). Analysis of step kinetics at different temperatures yielded activation energies of 25 ± 6 kJ/mol and 14 ± 13 kJ/mol for obtuse and acute steps, respectively. The inconsistencies in etch pit morphology, step anisotropy, and step activation energies from the present study with those of studies far-from-equilibrium can be explained by increased influence of the backward reaction, or growth, near-equilibrium. We propose that the backward reaction occurs preferentially at the acute-acute kink sites. The kinetics and effective activation energies of near-equilibrium calcite dissolution presented in this work provide accurate experimental data under likely CO2 sequestration conditions, and thus are crucial to the development of robust geochemical models that predict the long-term performance of mineral-trapped CO2.  相似文献   

5.
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6.
The main goal of this paper is to propose a new rate law describing the combined effect of pH (1 to 4.5) and temperature (25 to 70 °C) on smectite dissolution rate, under far from equilibrium conditions, as a step towards establishing the full rate law of smectite dissolution under acidic conditions. Dissolution experiments were carried out using non-stirred flow-through reactors fully immersed in a thermostatic water bath held at a constant temperature of 25.0°C, 50.0°C or 70.0°C ± 0.1°C. Smectite dissolution rates were obtained based on the release of silicon and aluminum at steady state. The results show good agreement between these two estimates of smectite dissolution rate. Low Al/Si ratios were obtained in experiments that were conducted at pH ≥4. These low Al/Si ratios are explained by precipitation of gibbsite and/or diaspore.Dissolution rate increases with temperature and decreases with increasing pH. Dissolution rates of experiments in which ΔGr ≤ −21 kcal mol −1, are not affected by deviation from equilibrium. Dissolution rates in most experiments are not affected by the addition of up to 0.3 M NaNO3 to the input solution.A simple model is used to describe the combined effect of pH and temperature on smectite dissolution rate. According to this model, dissolution rate is linearly proportional to the concentration of adsorbed protons on the mineral surface, and proton adsorption is described using a Langmuir adsorption isotherm. All experimental results at pH <4 were fitted to the model using a multiple non-linear regression. The resulting rate law is:
(A1)  相似文献   

7.
The dissolution of siderite (FeCO3) and rhodochrosite (MnCO3) under oxic and anoxic conditions is investigated at 298 K. The anoxic dissolution rate of siderite is 10−8.65 mol m−2 s−1 for 5.5 < pH < 12 and increases as [H+]0.75 for pH < 5.5. The pH dependence is consistent with parallel proton-promoted and water hydrolysis dissolution pathways. Atomic force microscopy (AFM) reveals a change in pit morphology from rhombohedral pits for pH > 4 to pits elongated at one vertex for pH < 4. Under oxic conditions the dissolution rate decreases to below the detection limit of 10−10 mol m−2 s−1 for 6.0 < pH < 10.3, and hillock precipitation preferential to steps is observed in concurrent AFM micrographs. X-ray photoelectron spectroscopy (XPS) and thermodynamic analysis identify the precipitate as ferrihydrite. At pH > 10.3, the oxic dissolution rate is as high as 10−7.5 mol m−2 s−1, which is greater than under the corresponding anoxic conditions. A fast electron transfer reaction between solution O2 or [Fe3+(OH)4]− species and surficial >FeII hydroxyl groups is hypothesized to explain the dissolution kinetics. AFM micrographs do not show precipitation under these conditions. Anoxic dissolution of rhodochrosite is physically observed as rhombohedral pit expansion for 3.7 < pH < 10.3 and is chemically explained by parallel proton- and water-promoted pathways. The dissolution rate law is 10−4.93[H+] + 10−8.45 mol m−2 s−1. For 5.8 < pH < 7.7 under oxic conditions, the AFM micrographs show a tabular precipitate growing by preferential expansion along the a-axis, though the macroscopic dissolution rate is apparently unaffected. For pH > 7.7 under oxic conditions, the dissolution rate decreases from 10−8.45 to 10−9.0 mol m−2 s−1. Flattened hillock precipitates grow across the entire surface without apparent morphological influence by the underlying rhodochrosite surface. XPS spectra and thermodynamic calculations implicate the precipitate as bixbyite for 5.8 < pH < 7.7 and MnOOH (possibly feitnkechtite) for pH >7.7.  相似文献   

8.
Magnesium inhibition of calcite dissolution kinetics   总被引：1，自引：0，他引：1  
We present evidence of inhibition of calcite dissolution by dissolved magnesium through direct observations of the (104) surface using atomic force microscopy (AFM) and vertical scanning interferometry (VSI). Far from equilibrium, the pattern of magnesium inhibition is dependent on solution composition and specific to surface step geometry. In CO2-free solutions (pH 8.8), dissolved magnesium brings about little inhibition even at concentrations of 0.8 × 10−3 molal. At the same pH, magnesium concentrations of less than 0.05 × 10−3 molal in carbonate-buffered solutions generate significant inhibition, although no changes in surface and etch pit morphology are observed. As concentrations exceed magnesite saturation, the dissolution rate shows little additional decrease; however, selective pinning of step edges results in unique etch pit profiles, seen in both AFM and VSI datasets. Despite the decreases in step velocity, magnesium addition in carbonated solutions also appears to activate the surface by increasing the nucleation rate of new defects. These relationships suggest that the modest depression of the bulk rate measured by VSI reflects a balance between competing reaction mechanisms that simultaneously depress the rate through selective inhibition of step movement, but also enhance reactivity on terraces by lowering the energy barrier to new etch pit formation.  相似文献   

9.
A comparison of published calcite dissolution rates measured far from equilibrium at a pH of ∼ 6 and above shows well over an order of magnitude in variation. Recently published AFM step velocities extend this range further still. In an effort to understand the source of this variation, and to provide additional constraint from a new analytical approach, we have measured dissolution rates by vertical scanning interferometry. In areas of the calcite cleavage surface dominated by etch pits, our measured dissolution rate is 10−10.95 mol/cm2/s (PCO2 10−3.41 atm, pH 8.82), 5 to ∼100 times slower than published rates derived from bulk powder experiments, although similar to rates derived from AFM step velocities. On cleavage surfaces free of local etch pit development, dissolution is limited by a slow, “global” rate (10−11.68 mol/cm2/s). Although these differences confirm the importance of etch pit (defect) distribution as a controlling mechanism in calcite dissolution, they also suggest that “bulk” calcite dissolution rates observed in powder experiments may derive substantial enhancement from grain boundaries having high step and kink density. We also observed significant rate inhibition by introduction of dissolved manganese. At 2.0 μM Mn, the rate diminished to 10−12.4 mol/cm2/s, and the well formed rhombic etch pits that characterized dissolution in pure solution were absent. These results are in good agreement with the pattern of manganese inhibition in published AFM step velocities, assuming a step density on smooth terraces of ∼9 μm−1.  相似文献   

10.
Probertite (NaCaB5O9·5H2O) and ulexite (NaCaB5O9·8H2O) posses identical chemical formula except for their water content. In this study, the dissolution of probertite in boric acid solution was investigated as a function of temperature and time. As the boric acid concentration increased, the dissolution of probertite also increased. However, the boric acid concentrations above 5 wt.% at 60 °C and 80 °C did not significantly affect the dissolution of probertite. The stirring speed had almost no effect on the dissolution of probertite. The dissolution kinetics of probertite in boric acid solution was controlled by first order pseudo homogeneous reaction. The activation energies for different probertite particle sizes varied from 25.25 kJ/mol K to 28.25 kJ/mol K, indicating that particle size had minor effect on the dissolution of probertite.  相似文献   

11.
We used titration calorimetry to measure the bulk heats of proton and Cd adsorption onto a common Gram positive soil bacterium Bacillus subtilis at 25.0 °C. Using the 4-site non-electrostatic model of Fein et al. [Fein, J.B., Boily, J.-F., Yee, N., Gorman-Lewis, D., Turner, B.F., 2005. Potentiometric titrations of Bacillus subtilis cells to low pH and a comparison of modeling approaches. Geochim. Cosmochim. Acta69 (5), 1123-1132.] to describe the bacterial surface reactivity to protons, our bulk enthalpy measurements can be used to determine the following site-specific enthalpies of proton adsorption for Sites 1-4, respectively: −3.5 ± 0.2, −4.2 ± 0.2, −15.4 ± 0.9, and −35 ± 2 kJ/mol, and these values yield the following third law entropies of proton adsorption onto Sites 1-4, respectively: +51 ± 4, +78 ± 4, +79 ± 5, and +60 ± 20 J/mol K. An alternative data analysis using a 2-site Langmuir-Freundlich model to describe proton binding to the bacterial surface (Fein et al., 2005) resulted in the following site-specific enthalpies of proton adsorption for Sites 1 and 2, respectively: −3.6 ± 0.2 and −35.1 ± 0.3 kJ/mol. The thermodynamic values for Sites 1-3 for the non-electrostatic model and Site 1 of the Langmuir-Freundlich model of proton adsorption onto the bacterial surface are similar to those associated with multifunctional organic acid anions, such as citrate, suggesting that the protonation state of a bacterial surface site can influence the energetics of protonation of neighboring sites. Our bulk Cd enthalpy data, interpreted using the 2-site non-electrostatic Cd adsorption model of Borrok et al. [Borrok, D., Fein, J.B., Tischler, M., O’Loughlin, E., Meyer, H., Liss, M., Kemner, K.M., 2004b. The effect of acidic solutions and growth conditions on the adsorptive properties of bacterial surfaces. Chem. Geol.209 (1-2), 107-119.] to account for Cd adsorption onto B. subtilis, yield the following site-specific enthalpies of Cd adsorption onto bacterial surface Sites 2 and 3, respectively: −0.2 ± 0.4 and +14.4 ± 0.9 kJ/mol, and the following third law entropies of Cd adsorption onto Sites 2 and 3, respectively: +57 ± 4 and +128 ± 5 J/mol K. The calculated enthalpies of Cd adsorption are typical of those associated with Cd complexation with anionic oxygen ligands, and the entropies are indicative of inner sphere complexation by multiple ligands. The experimental approach described in this study not only yields constraints on the molecular-scale mechanisms involved in proton and Cd adsorption reactions, but also provides new thermodynamic data that enable quantitative estimates of the temperature dependence of proton and Cd adsorption reactions.  相似文献   

12.
A natural quartz sample free of mineral and fluid inclusions was irradiated with a 200 MeV proton beam to produce spallogenic 21Ne, 3He and 4He. Temperature-dependent diffusivities of these three nuclides were then determined simultaneously by high precision stepped-heating and noble gas mass spectrometry. The outward mobility of proton-induced nuclides reflects diffusion through the quartz lattice. In the studied range of 70 to 400°C the helium diffusion coefficients exceed those of neon by 5-7 orders of magnitude. The implied diffusion parameters Ea = 153.7 ± 1.5 (kJ/mol) and ln(Do/a2) = 15.9 ± 0.3 (ln(s−1)) and Ea = 84.5 ± 1.2 (kJ/mol) and ln(Do/a2) = 11.1 ± 0.3 (ln(s−1)) for proton-induced 21Ne and 3He, respectively, indicate that cosmogenic neon will be quantitatively retained in inclusion-free quartz at typical Earth surface temperatures whereas cosmogenic helium will not. However, the neon diffusion parameters also indicate that diffusive loss needs to be considered for small (<1 mm) quartz grains that have experienced elevated temperatures. Since natural quartz often contains fluid inclusions which may enhance noble gas retentivity, these parameters likely represent an end-member case of purely solid-state diffusion. The ∼70 kJ/mol higher activation energy for neon diffusion compared to helium diffusion likely represents an energy barrier related to its ∼13% greater diameter and provides a fundamental constraint with which to test theories of solid state diffusion. The diffusion parameters for proton-induced 4He are indistinguishable from those for 3He, providing no evidence for the commonly expected inverse square root of the mass diffusion relationship between isotopes. We also find preliminary indication that increased exposure to radiation may enhance neon and helium retentivity in quartz at low temperatures.  相似文献   

13.
Although pseudomorphic mineral replacement reactions are common in all geological environments, and have long been considered important to many geological processes such as metamorphism, metasomatism, diagenesis, and chemical weathering, their mechanisms are still not well known. We present a combined textural and kinetic study of the replacement of pentlandite, (Fe,Ni)9S8, by violarite (NiFe)3S4, and describe the mechanisms and kinetic behavior of this reaction by considering the role of the fluid phase, the causes of coupling between pentlandite dissolution and violarite precipitation, the rate-limiting steps controlling the kinetic behavior, and the origin of the length scale of the features preserved during pseudomorphism.The experiments were conducted under mild hydrothermal conditions (80-210 °C, vapor saturated pressures). Reaction kinetics shows a complex behavior depending on various physical and chemical parameters including temperature, pH, concentrations of various reaction species, solid-weight-to-fluid-volume-ratio and specific surface area. The rate of replacement (i) increases with temperature from 80 to 125 °C, then decreases as temperature further increases to 210 °C, (ii) first increases then decreases with decreasing pH from pH 6 to 1, (iii) increases with increasing concentration of oxidants such as O2(aq), H2O2, and KMnO4, but decreases with increasing concentration of Ni2+ and Fe3+, and with increasing solid-weight-to-fluid-volume ratio, (iv) increases linearly with the specific surface area. This kinetic behavior as well as the resulting textures revealed a coupled dissolution-reprecipitation reaction mechanism.Nanometer-scale pseudomorphic replacement, through which the crystallographic orientation of pentlandite is inherited by violarite, occurs only between 1 < pH < 6, and spatial coupling between dissolution and reprecipitation is controlled by the local solution chemistry as well as by epitaxial nucleation mediated by the pentlandite substrate. The kinetic results show that pentlandite dissolution is rate-limiting under mild acidic to neutral conditions (1 < pH < 6), while violarite precipitation is rate-limiting under strong acidic conditions (pH 1). The difference in rate-limiting steps influences the coupling mechanism and causes the different degrees of preservation (length scale of pseudomorphism) and different morphologies observed at high and low pHs: pentlandite dissolution being rate-limiting results in nanoscale coupling between dissolution and precipitation and thus nanoscale pseudomorphism (length scale <20 nm), in which the replacement precisely preserves the morphology and internal details, resembling remarkably the natural pentlandite/violarite assemblages. In contrast, violarite precipitation being rate-limiting results in microscale pseudomorphism (length scale ∼10 μm): the morphology of the pentlandite grains is only roughly preserved and internal details are not preserved.This case study illustrates some general principles of replacement reactions proceeding via the coupled dissolution-reprecipitation mechanism: (i) primary mineral dissolution needs to be rate-limiting compared to the secondary mineral precipitation in order to achieve a high degree pseudomorphic replacement; (ii) the effects of solution composition on reaction kinetics can be qualitatively rationalized by considering the rate-limiting step reaction.  相似文献   

14.
The standard enthalpy of formation of thorite and huttonite and the enthalpy of the phase transition between these polymorphs were determined using high-temperature oxide melt solution calorimetry and transposed temperature drop calorimetry. Standard enthalpies of formation of thorite and huttonite are reported for the first time and are −2117.6 ± 4.2 kJ/mol and −2110.9 ± 4.7 kJ/mol, respectively. Based on our measurements, thorite and huttonite are metastable relative to SiO2 (quartz) and ThO2 (thorianite) at standard conditions, but are presumably stabilized at high temperature by the entropy contribution. Based on the measured enthalpy of the thorite-huttonite phase transition of 6.7 ± 2.5 kJ/mol, a dP/dT slope for the transformation was calculated as −1.21 ± 0.45 MPa/K.  相似文献   

15.
The kinetics of calcite dissolution in aqueous KCl-solutions far from equilibrium, between 1 and 62°C in the pH-range 2.7 to 8.4 have been investigated using a rotating disc apparatus. At neutral and alkaline pH in the mixed kinetic regime the empirical apparent activation energy (EAAE) for the surface chemical reaction rate constant is 54 ± 4 kJ mole?1 for Carrara marble and 46 ± 4 kJ mole?1 for Iceland spar. Under similar conditions the EAAE of the transport rate constant increases with decreasing temperature, but has a mean value of 27 ± 2 kJ mole?1. The corresponding diffusion coefficient has a mean EAAE of 37 ± 3 kJ mole?1 and this high EAAE is consistent with transport dependence on product diffusion in this H+-independent regime.In contrast, in acid solutions, where the rate approaches end-member transport control, the EAAE of the diffusion coefficient is 16 kJ mole?1, also decreasing with increasing temperature. This is compatible with H+-diffusion to the surface being rate-controlling.In inhibitor-free natural systems, calcite dissolution kinetics far from equilibrium can be described in terms of three regimes: an H+-dependent regime (pH < 4 at 25°C), a transition regime (4 < pH < 5.5 at 25°C) and an H+-independent regime (pH > 5.5 at 25°C). At lower temperatures these boundaries move to higher pH values. The presence of inhibitors in natural systems may enhance surface controlled kinetics.  相似文献   

16.
本文对辉铜矿、黄铜矿和斑铜矿在ＮａＣｌ溶液中的溶解动力学进行了实验研究，在２５－７０℃，溶解反应活化能分别为辉铜矿３６．４６ｋＪ／ｍｏｌ，黄铜矿４４．０５ｋＪ／ｍｏｌ，斑铜矿５１．９６ｋＪ／ｍｏｌ。最近探讨了动力学实验结果在砂岩铜矿成矿作用中的意义。  相似文献   

17.
We report the application of an in situ method to obtain field dissolution rates of fine mineral particles in soils. Samples with different metal-containing mineral and slag particles (lead oxide, copper concentrate and copper slag) from the mining and smelting industry were buried in the topsoil of an acidic forest soil for up to 18 months. In addition we studied the dissolution of these particles in samples of the same soil, in a sand matrix and in acid solution under constant temperature and moisture conditions in the laboratory. Under field conditions the PbO particles dissolved quite rapidly (2.4 ± 0.7 × 10−10 mol Pb m−2 s−1), whereas the copper concentrate (<1 × 10−11 mol Cu m−2 s−1) and the copper slag particles (4.3 ± 0.8 × 10−11 mol Cu m−2 s−1) proved to be more resistant to weathering. In addition to qualitative information on dissolution features (SEM), the method yielded quantitative data on in situ dissolution rates. The dissolution rates followed the order: sand with acid percolation (pH 3.5; lab) < soil (lab) < soil (field) < acid solution (pH 3.5; lab). Dissolution rates in soil were found to be lower under laboratory than under field conditions. The faster field rates may in part be attributed to the higher biological activity in the field soil compared to the same soil in the laboratory.  相似文献   

18.
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19.
The surface speciation and leaching kinetics of 38- to 75-μm sphalerite (0.45 wt.% Fe) particles reacted in O2 purged perchloric acid (at pH 1.0) at 25, 40, 60, and 85 °C over a leach period of 144 h were investigated. In all cases, an initial rapid leach rate is observed followed by a slower leach rate. These two leach regimes can each be adequately modeled using straight-line interpolation, and thus two activation energies (Ea) have been derived. Ea for the fast and slow Zn dissolution rates were 33 ± 4 kJ mol−1 and 34 ± 4 kJ mol−1 respectively, suggesting the same rate-determining step.  相似文献   

20.
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