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1.
The steady state dissolution rate of San Carlos olivine [Mg1.82Fe0.18 SiO4] in dilute aqueous solutions was measured at 90, 120, and 150 °C and pH ranging from 2 to 12.5. Dissolution experiments were performed in a stirred flow-through reactor, under either a nitrogen or carbon dioxide atmosphere at pressures between 15 and 180 bar. Low pH values were achieved either by adding HCl to the solution or by pressurising the reactor with CO2, whereas high pH values were achieved by adding LiOH. Dissolution was stoichiometric for almost all experiments except for a brief start-up period. At all three temperatures, the dissolution rate decreases with increasing pH at acidic to neutral conditions with a slope of close to 0.5; by regressing all data for 2 ? pH ? 8.5 and 90 °C ? T ? 150 °C together, the following correlation for the dissolution rate in CO2-free solutions is obtained:
2.
The stability of yttrium-acetate (Y-Ac) complexes in aqueous solution was determined potentiometrically at temperatures 25-175 °C (at Ps) and pressures 1-1000 bar (at 25 and 75 °C). Measurements were performed using glass H+-selective electrodes in potentiometric cells with a liquid junction. The species YAc2+ and were found to dominate yttrium aqueous speciation in experimental solutions at 25-100 °C (log [Ac−] < −1.5, pH < 5.2), whereas at 125, 150 and 175 °C introduction of into the Y-Ac speciation model was necessary. The overall stability constants βn were determined for the reaction
3.
Forward dissolution rates of Na-Montmorillonite (Wyoming) SWy-2 smectite (Ca0.06Na0.56)[Al3.08Fe(III)0.38Mg0.54] [Si7.93 Al0.07]O20(OH)4 were measured at 25 °C in a mixed-flow reactor equipped with interior dialysis compartment (6-8 kDa membrane) as a function of pH (1-12), dissolved carbonate (0.5-10 mM), phosphate (10−5 to 0.03 M), and nine organic ligands (acetate, oxalate, citrate, EDTA, alginate, glucuronic acid, 3,4-dihydroxybenzoic acid, gluconate, and glucosamine) in the concentration range from 10−5 to 0.03 M. In organic-free solutions, the Si-based rates decrease with increasing pH at 1 ? pH ? 8 with a slope close to −0.2. At 9 ? pH ? 12, the Si-based rates increase with a slope of ∼0.3. In contrast, non-stoichiometric Mg release weakly depends on pH at 1 ? pH ? 12 and decreases with increasing pH. The empirical expression describing Si-release rates [R, mol/cm2/s] obtained in the present study at 25 °C, I = 0.01 M is given by
4.
Steady-state talc dissolution rates, at far-from-equilibrium conditions, were measured as a function of aqueous silica and magnesium activity, pH from 1 to 10.6, and temperature from 25 to 150 °C. All rates were measured in mixed flow reactors and exhibited stoichiometric or close to stoichiometric dissolution. All measured rates at pH > 2 obtained at a fixed ionic strength of 0.02 M can be described to within experimental uncertainty using
5.
Claire Chaïrat Jacques Schott Jean-Eric Lartigue 《Geochimica et cosmochimica acta》2007,71(24):5901-5912
The dissolution rates of natural fluorapatite (FAP), Ca10(PO4)6F2, were measured at 25 °C in mixed-flow reactors as a function of pH from 3.0 to 11.7, and aqueous calcium, phosphorus, and fluoride concentration. After an initial preferential Ca and/or F release, stoichiometric Ca, P, and F release was observed. Measured FAP dissolution rates decrease with increasing pH at 3 ? pH ? 7, FAP dissolution rates are pH independent at 7 ? pH ? 10, and FAP dissolution rates again decrease with increasing pH at pH ? 10. Measured FAP dissolution rates are independent of aqueous Ca, P, and F concentration at pH ≈ 3 and pH ≈ 10.Apatite dissolution appears to be initiated by the relatively rapid removal from the near surface of F and the Ca located in the M1 sites, via proton for Ca exchange reactions. Dissolution rates are controlled by the destruction of this F and Ca depleted surface layer. The destruction of this layer is facilitated by the adsorption/penetration of protons into the surface at acidic conditions, and by surface hydration at neutral and basic conditions. Taking into account these two parallel mechanisms, measured fluorapatite forward dissolution rates can be accurately described using
6.
Eric H. Oelkers Jacques Schott Jean-Marie Gauthier Teresa Herrero-Roncal 《Geochimica et cosmochimica acta》2008,72(20):4948-4961
Steady-state muscovite dissolution rates have been measured at temperatures from 60 to 201 °C and 1 ? pH ? 10.3 as a function of reactive solution K, Si, and Al concentration. The pegmatitic muscovite used in these experiments has a composition consistent with (Na0.09, K0.86)Fe0.05Al2.92Si3.05O10(OH1.95, F0.06). All experiments were performed in titanium mixed-flow reactors. All experiments were performed at far-from-equilibrium conditions with respect to muscovite. All reactive solutions were undersaturated with respect to secondary product phases other than for some experiments which were supersaturated with respect to bohemite and diaspore; steady-state dissolution was stoichiometric for all experiments that were undersaturated with respect to these phases.The variation of rates with reactive solution composition depends on the solution pH. At pH ? 7 rates were found to decrease significantly with increasing reactive fluid Al activity but be independent of aqueous SiO2 activity. pH < 7 rates measured in the present study from 60 to 175 °C are consistent with
7.
8.
Dissolution kinetics as a function of the Gibbs free energy of reaction: An experimental study based on albite feldspar 总被引：1，自引：0，他引：1
Here we report on an experimental investigation of the relation between the dissolution rate of albite feldspar and the Gibbs free energy of reaction, ΔGr. The experiments were carried out in a continuously stirred flow-through reactor at 150 °C and pH(150 °C) 9.2. The dissolution rates R are based on steady-state Si and Al concentrations and sample mass loss. The overall relation between ΔGr and R was determined over a free energy range of −150 < ΔGr < −15.6 kJ mol−1. The data define a continuous and highly non-linear, sigmoidal relation between R and ΔGr that is characterized by three distinct free energy regions. The region furthest from equilibrium, delimited by −150 < ΔGr < −70 kJ mol−1, represents an extensive dissolution rate plateau with an average rate . In this free energy range the rates of dissolution are constant and independent of ΔGr, as well as [Si] and [Al]. The free energy range delimited by −70 ? ΔGr ? −25 kJ mol−1, referred to as the ‘transition equilibrium’ region, is characterized by a sharp decrease in dissolution rates with increasing ΔGr, indicating a very strong inverse dependence of the rates on free energy. Dissolution nearest equilibrium, defined by ΔGr > −25 kJ mol−1, represents the ‘near equilibrium’ region where the rates decrease as chemical equilibrium is approached, but with a much weaker dependence on ΔGr. The lowest rate measured in this study, R = 6.2 × 10−11 mol m−2 s−1 at ΔGr = −16.3 kJ mol−1, is more than two orders of magnitude slower than the plateau rate. The data have been fitted to a rate equation (adapted from Burch et al. [Burch, T. E., Nagy, K. L., Lasaga, A. C., 1993. Free energy dependence of albite dissolution kinetics at 80 °C and pH 8.8. Chem. Geol.105, 137-162]) that represents the sum of two parallel reactions
R=k1[1-exp(-ngm1)]+k2[1-exp(-g)]m2, 相似文献
9.
Dissolution and precipitation rates of brucite (Mg(OH)2) were measured at 25°C in a mixed-flow reactor as a function of pH (2.5 to 12), ionic strength (10−4 to 3 M), saturation index (−12 < log Ω < 0.4) and aqueous magnesium concentrations (10−6 to 5·10−4 M). Brucite surface charge and isoelectric point (pHIEP) were determined by surface titrations in a limited residence time reactor and electrophoretic measurements, respectively. The pH of zero charge and pHIEP were close to 11. A two-pK, one site surface speciation model which assumes a constant capacitance of the electric double layer (5 F/m2) and lack of dependence on ionic strength predicts the dominance of >MgOH2+ species at pH < 8 and their progressive replacement by >MgOH° and >MgO− as pH increases to 10-12. Rates are proportional to the square of >MgOH2+ surface concentration at pH from 2.5 to 12. In accord with surface speciation predictions, dissolution rates do not depend on ionic strength at pH 6.5 to 11. Brucite dissolution and precipitation rates at close to equilibrium conditions obeyed TST-derived rate laws. At constant saturation indices, brucite precipitation rates were proportional to the square of >MgOH2+ concentration. The following rate equation, consistent with transition state theory, describes brucite dissolution and precipitation kinetics over a wide range of solution composition and chemical affinity:
10.
Barry R. Bickmore Justin C. Wheeler Kathryn L. Nagy 《Geochimica et cosmochimica acta》2008,72(18):4521-4536
In light of recent work on the reactivity of specific sites on large (hydr)oxo-molecules and the evolution of surface topography during dissolution, we examined the ability to extract molecular-scale reaction pathways from macroscopic dissolution and surface charge measurements of powdered minerals using an approach that involved regression of multiple datasets and statistical graphical analysis of model fits. The test case (far-from-equilibrium quartz dissolution from 25 to 300 °C, pH 1-12, in solutions with [Na+] ? 0.5 M) avoids the objections to this goal raised in these recent studies. The strategy was used to assess several mechanistic rate laws, and was more powerful in distinguishing between models than the statistical approaches employed previously. The best-fit model included three mechanisms—two involving hydrolysis of Si centers by H2O next to neutral (>Si-OH0) and deprotonated (>Si-O−) silanol groups, and one involving hydrolysis of Si centers by OH−. The model rate law is
11.
Nicolas C.M. Marty Jordi Cama Daisuke Chino Angelina Razafitianamaharavo Jocelyne Brendlé Josep M. Soler Christophe Tournassat 《Geochimica et cosmochimica acta》2011,75(20):5849-5864
The effect of pH and Gibbs energy on the dissolution rate of a synthetic Na-montmorillonite was investigated by means of flow-through experiments at 25 and 80 °C at pH of 7 and 9. The dissolution reaction took place stoichiometrically at 80 °C, whereas at 25 °C preferential release of Mg over Si and Al was observed. The TEM-EDX analyses (transmission electronic microscopy with quantitative chemical analysis) of the dissolved synthetic phase at 25 °C showed the presence of newly formed Si-rich phases, which accounts for the Si deficit. At low temperature, depletion of Si concentration was attributed to incongruent clay dissolution with the formation of detached Si tetrahedral sheets (i.e., alteration product) whereas the Al behaviour remains uncertain (e.g., possible incorporation into Al-rich phases). Hence, steady-state rates were based on the release of Mg. Ex situ AFM measurements were used to investigate the variations in reactive surface area. Accordingly, steady-state rates were normalized to the initial edge surface area (11.2 m2 g−1) and used to propose the dissolution rate law for the dissolution reactions as a function of ΔGr at 25 °C and pH∼9:
12.
Emilie Pourtier Jean-Luc Devidal François Gibert 《Geochimica et cosmochimica acta》2010,74(6):1872-66
The solubility of synthetic NdPO4 monazite end-member was experimentally determined from 300 up to 800 °C, at 2000 bars in pure water, and in aqueous chloride or phosphate solutions. Both the classical weight-loss method and a new method based on isotope dilution coupled with thermal ionization mass spectrometer were used. In the range of temperature studied monazite showed a prograde solubility from 10−5.4 m at 300 °C up to 10−2.57 m at 800 °C. Experiments in H2O-H3PO4-NaCl-HCl solutions suggested Nd(OH)30 was the major species that was formed at high temperature and pressure. The equilibrium constants (log K) for the reaction:
13.
The effects of organic acids on the dissolution of silicate minerals: A case study of oxalate catalysis of kaolinite dissolution 总被引：1，自引：0，他引：1
Jordi Cama 《Geochimica et cosmochimica acta》2006,70(9):2191-2209
Most studies agree that the dissolution rate of aluminosilicates in the presence of oxalic and other simple carboxylic acids is faster than the rate with non-organic acid under the same pH. However, the mechanisms by which organic ligands enhance the dissolution of minerals are in debate. The main goal of this paper was to study the mechanism that controls the dissolution rate of kaolinite in the presence of oxalate under far from equilibrium conditions (−29 < ΔGr < −18 kcal mol−1). Two types of experiments were performed: non-stirred flow-through dissolution experiments and batch type adsorption isotherms. All the experiments were conducted at pH 2.5-3.5 in a thermostatic water-bath held at a constant temperature of 25.0, 50.0 or 70.0 ± 0.1 °C. Kaolinite 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 kaolinite dissolution rate. At constant temperature, there is a general trend of increase in the overall dissolution rate as a function of the total concentration of oxalate in solution. The overall kaolinite dissolution rates in the presence of oxalate was up to 30 times faster than the dissolution rate of kaolinite at the same temperature and pH without oxalate as was observed in our previous study. Therefore, these rate differences are related to differences in oxalate and aluminum concentrations. Within the experimental variability, the oxalate adsorption at 25, 50, and 70 °C showed the same dependence on the sum of the activities of oxalate and bioxalate in solution. The change of oxalate concentration on the kaolinite surface (Cs,ox) as a function of the sum of the activities of the oxalate and bioxalate in solution may be described by the general adsorption isotherm:
14.
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):
15.
Fabrice Fraysse Oleg S. Pokrovsky Jacques Schott 《Geochimica et cosmochimica acta》2006,70(8):1939-1951
Although phytoliths, constituted mainly by micrometric opal, exhibit an important control on silicon cycle in superficial continental environments, their thermodynamic properties and reactivity in aqueous solution are still poorly known. In this work, we determined the solubility and dissolution rates of bamboo phytoliths collected in the Réunion Island and characterized their surface properties via electrophoretic measurements and potentiometric titrations in a wide range of pH. The solubility product of “soil” phytoliths ( at 25 °C) is equal to that of vitreous silica and is 17 times higher than that of quartz. Similarly, the enthalpy of phytoliths dissolution reaction is close to that of amorphous silica but is significantly lower than the enthalpy of quartz dissolution. Electrophoretic measurements yield isoelectric point pHIEP = 1.2 ± 0.1 and 2.5 ± 0.2 for “soil” (native) and “heated” (450 °C heating to remove organic matter) phytoliths, respectively. Surface acid-base titrations allowed generation of a 2-pK surface complexation model. Phytoliths dissolution rates, measured in mixed-flow reactors at far from equilibrium conditions at 2 ? pH ? 12, were found to be intermediate between those of quartz and vitreous silica. The dissolution rate dependence on pH was modeled within the concept of surface coordination theory using the equation:
16.
17.
The speciation of cobalt (II) in Cl− and H2S-bearing solutions was investigated spectrophotometrically at temperatures of 200, 250, and 300 °C and a pressure of 100 bars, and by measuring the solubility of cobaltpentlandite at temperatures of 120-300 °C and variable pressures of H2S. From the results of these experiments, it is evident that CoHS+ and predominate in the solutions except at 150 °C, for which the dominant chloride complex is CoCl3−. The logarithms of the stability constant for CoHS+ show moderate variation with temperature, decreasing from 6.24 at 120 °C to 5.84 at 200 °C, and increasing to 6.52 at 300 °C. Formation constants for chloride species increase smoothly with temperature and at 300°C their logarithms reach 8.33 for , 6.44 for CoCl3−, 4.94 to 5.36 for , and 2.42 for CoCl+. Calculations based on the composition of a model hydrothermal fluid (Ksp-Mu-Qz, KCl = 0.25 m, NaCl = 0.75 m, ΣS = 0.3 m) suggest that at temperatures ?200 °C, cobalt occurs dominantly as CoHS+, whereas at higher temperatures the dominant species is . 相似文献
18.
Dionisis Katsikopoulos Ángeles Fernández-González Manuel Prieto 《Geochimica et cosmochimica acta》2009,73(20):6147-455
The enthalpy of mixing of the calcite-rhodochrosite (Ca,Mn)CO3 solid solution was determined at 25 °C from calorimetric measurements of the enthalpy of precipitation of solids with different compositions. A detailed study of the broadening of powder X-ray diffraction peaks shows that most of the precipitates are compositionally homogeneous. All the experimental enthalpy of mixing (ΔHm) values are positive and fit reasonably well (R2 = 0.86) to a Guggenheim function of three terms:
19.
Dissolution rates of limestone covered by a water film open to a CO2-containing atmosphere are controlled by the chemical composition of the CaCO3-H2O-CO2 solution at the water-mineral interface. This composition is determined by the Ca2+-concentration at this boundary, conversion of CO2 into H+ and in the solution, and by diffusional mass transport of the dissolved species from and towards the water-limestone interface. A system of coupled diffusion-reaction equations for Ca2+, , and CO2 is derived. The Ca2+ flux rates at the surface of the mineral are defined by the PWP-empirical rate law. These flux rates by the rules of stoichiometry must be equal to the flux rates of CO2 across the air-water interface. In the solution, CO2 is converted into H+ and . At low water-film thickness this reaction becomes rate limiting. The time dependent diffusion-reaction equations are solved for free drift dissolution by a finite-difference scheme, to obtain the dissolution rate of calcite as a function of the average calcium concentration in the water film. Dissolution rates are obtained for high undersaturation. The results reveal two regimes of linear dissolution kinetics, which can be described by a rate law F = αi(miceq − c), where c is the calcium concentration in the water film, ceq the equilibrium concentration with respect to calcite. For index i = 0, a fast rate law, which here is reported for the first time, is found with α0 = 3 × 10−6 m s−1 and m0 = 0.3. For c > m0ceq, a slow rate law is valid with α1 = 3 × 10−7 m s−1 and m1 = 1, which confirms earlier work. The numbers given above are valid for film thickness of several tenths of a millimetre and at 20 °C. These rates are proven experimentally, using a flat inclined limestone plate covered by a laminar flowing water film injected at an input point with known flow rate Q and calcium concentration. From the concentration measured after flow distance x the dissolution rates are determined. These experiments have been performed at a carbon-dioxide pressure of 0.00035 atm and also of 0.01 atm. The results are in good agreement to the theoretical predictions. 相似文献
20.
Partitioning of strontium during spontaneous calcite formation was experimentally studied using an advanced CO2-diffusion technique. Results at different precipitation rates and T = 5, 25, and 40 °C show that at constant temperature Sr incorporation into calcite is controlled by the precipitation rate (R in μmol/m2/h) according to the individual expressions