共查询到20条相似文献，搜索用时 31 毫秒
1.
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
2.
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:
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 magnesite dissolution rates were measured in mixed-flow reactors at 150 and 200 °C and 4.6 < pH < 8.4, as a function of ionic strength (0.001 M ? I ? 1 M), total dissolved carbonate concentration (10−4 M < ΣCO2 < 0.1 M), and distance from equilibrium. Rates were found to increase with increasing ionic strength, but decrease with increasing temperature from 150 to 200 °C, pH, and aqueous CO32− activity. Measured rates were interpreted using the surface complexation model developed by Pokrovsky et al. (1999a) in conjunction with transition state theory (Eyring, 1935). Within this formalism, magnesite dissolution rates are found to be consistent with
5.
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
6.
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:
7.
Dissolution rates of natural illite (Illite du Puy) were measured from Si release rates during closed system experiments at pH ranging from 1.4 to 12.4 and temperatures ranging from 5 to 50°C. Experiments performed at 4<pH<11 exhibited reactive fluid Si/Al concentration ratios that were inconsistent with stoichiometric illite dissolution likely due to secondary phase precipitation. In contrast, after an initial preferential release of aluminum relative to silicon, the reactive fluid Si/Al concentration ratio evolution was consistent with stoichiometric illite dissolution for all experiments conducted at 4>pH>11. Si release rate decreased with time during all experiments; for those experiments performed at 4>pH>11 this observation can be attributed to either 1) changing reactive surface area; 2) the effect of initial fine particle dissolution; or 3) a negative order of the illite dissolution reaction with respect to aqueous Al and/or Si. Measured dissolution rates exhibited a typical variation with pH; rates decrease with increasing pH at acid conditions, minimize at near to neutral pH and increase with increasing pH at basic conditions. An empirical expression describing rates obtained in the present study is given by
8.
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:
9.
James D. Prikryl 《Geochimica et cosmochimica acta》2008,72(18):4508-4520
The dissolution and growth of uranophane [Ca(UO2)2(SiO3OH)2·5H2O] have been examined in Ca- and Si-rich test solutions at low temperatures (20.5 ± 2.0 °C) and near-neutral pH (∼6.0). Uranium-bearing experimental solutions undersaturated and supersaturated with uranophane were prepared in matrices of ∼10−2 M CaCl2 and ∼10−3 M SiO2(aq). The experimental solutions were reacted with synthetic uranophane and analyzed periodically over 10 weeks. Interpretation of the aqueous solution data permitted extraction of a solubility constant for the uranophane dissolution reaction and standard state Gibbs free energy of formation for uranophane ( kJ mol−1). 相似文献
10.
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:
11.
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
12.
The origin of Zn isotope fractionation in sulfides 总被引：2，自引：0，他引：2
Toshiyuki Fujii Frédéric Moynier Francis Albarède 《Geochimica et cosmochimica acta》2011,75(23):7632-7643
Isotope fractionation of Zn between aqueous sulfide, chloride, and carbonate species (Zn2+, Zn(HS)2, , , ZnS(HS)−, ZnCl+, ZnCl2, , and ZnCO3) was investigated using ab initio methods. Only little fractionation is found between the sulfide species, whereas carbonates are up to 1‰ heavier than the parent solution. At pH > 3 and under atmospheric-like CO2 pressures, isotope fractionation of Zn sulfides precipitated from sulfidic solutions is affected by aqueous sulfide species and the δ66Zn of sulfides reflect these in the parent solutions. Under high PCO2 conditions, carbonate species become abundant. In high PCO2 conditions of hydrothermal solutions, Zn precipitated as sulfides is isotopically nearly unfractionated with respect to a low-pH parent fluid. In contrast, negative δ66Zn down to at least −0.6‰ can be expected in sulfides precipitated from solutions with pH > 9. Zinc isotopes in sulfides and rocks therefore represent a potential indicator of mid to high pH in ancient hydrothermal fluids. 相似文献
13.
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
14.
Sorption of phosphate onto calcite; results from batch experiments and surface complexation modeling 总被引：3，自引：0，他引：3
The adsorption of phosphate onto calcite was studied in a series of batch experiments. To avoid the precipitation of phosphate-containing minerals the experiments were conducted using a short reaction time (3 h) and low concentrations of phosphate (?50 μM). Sorption of phosphate on calcite was studied in 11 different calcite-equilibrated solutions that varied in pH, PCO2, ionic strength and activity of Ca2+, and . Our results show strong sorption of phosphate onto calcite. The kinetics of phosphate sorption onto calcite are fast; adsorption is complete within 2-3 h while desorption is complete in less than 0.5 h. The reversibility of the sorption process indicates that phosphate is not incorporated into the calcite crystal lattice under our experimental conditions. Precipitation of phosphate-containing phases does not seem to take place in systems with ?50 μM total phosphate, in spite of a high degree of super-saturation with respect to hydroxyapatite (SIHAP ? 7.83). The amount of phosphate adsorbed varied with the solution composition, in particular, adsorption increases as the activity decreases (at constant pH) and as pH increases (at constant activity). The primary effect of ionic strength on phosphate sorption onto calcite is its influence on the activity of the different aqueous phosphate species. The experimental results were modeled satisfactorily using the constant capacitance model with >CaPO4Ca0 and either >CaHPO4Ca+ or > as the adsorbed surface species. Generally the model captures the variation in phosphate adsorption onto calcite as a function of solution composition, though it was necessary to include two types of sorption sites (strong and weak) in the model to reproduce the convex shape of the sorption isotherms. 相似文献
15.
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
16.
Effects of magnesium ions on near-equilibrium calcite dissolution: Step kinetics and morphology 总被引：1，自引：0，他引：1
Dissolution kinetics at the aqueous solution-calcite interface at 50 °C were investigated using in situ atomic force microscopy (AFM) to reveal the influence of magnesium concentration and solution saturation state on calcite dissolution kinetics and surface morphology. Under near-equilibrium conditions, dissolved Mg2+ displayed negligible inhibitory effects on calcite dissolution even at concentrations of . Upon the introduction of , the solution saturation state with respect to calcite, , acted as a “switch” for magnesium inhibition whereby no significant changes in step kinetics were observed at Ωcalcite<0.2, whereas a sudden inhibition from Mg2+ was activated at Ωcalcite?0.2. The presence of the Ω-switch in dissolution kinetics indicates the presence of critical undersaturation in accordance with thermodynamic principles. The etch pits formed in solutions with exhibited a unique distorted rhombic profile, different from those formed in Mg-free solutions and in de-ionized water. Such unique etch pit morphology may be associated with the anisotropy in net detachment rates of counter-propagating kink sites upon the addition of Mg2+. 相似文献
17.
Mechanisms of iron oxide transformations in hydrothermal systems 总被引：2，自引：0，他引：2
Tsubasa Otake David J. Wesolowski Lawrence F. Allard 《Geochimica et cosmochimica acta》2010,74(21):6141-6156
Coexistence of magnetite and hematite in hydrothermal systems has often been used to constrain the redox potential of fluids, assuming that the redox equilibrium is attained among all minerals and aqueous species. However, as temperature decreases, disequilibrium mineral assemblages may occur due to the slow kinetics of reaction involving the minerals and fluids. In this study, we conducted a series of experiments in which hematite or magnetite was reacted with an acidic solution under H2-rich hydrothermal conditions (T = 100-250 °C, ) to investigate the kinetics of redox and non-redox transformations between hematite and magnetite, and the mechanisms of iron oxide transformation under hydrothermal conditions. The formation of euhedral crystals of hematite in 150 and 200 °C experiments, in which magnetite was used as the starting material, indicates that non-redox transformation of magnetite to hematite occurred within 24 h. The chemical composition of the experimental solutions was controlled by the non-redox transformation between magnetite and hematite throughout the experiments. While solution compositions were controlled by the non-redox transformation in the first 3 days in a 250 °C experiment, reductive dissolution of magnetite became important after 5 days and affected the solution chemistry. At 100 °C, the presence of maghemite was indicated in the first 7 days. Based on these results, equilibrium constants of non-redox transformation between magnetite and hematite and those of non-redox transformation between magnetite and maghemite were calculated. Our results suggest that the redox transformation of hematite to magnetite occurs in the following steps: (1) reductive dissolution of hematite to and (2) non-redox transformation of hematite and to magnetite. 相似文献
18.
19.
Pyromorphite Pb5(PO4)3Cl and mimetite Pb5(AsO4)3Cl are isostructural minerals with apatite. Due to their high environmental stability, they have gained considerable attention as metals sequestration agents in water treatment and contaminated soil remediation. Pyromorphite and mimetite can form a continuous solid solution series in near-Earth surface environments. Precipitation of the end members and intermediate members of the series is likely to occur in the areas where the cost-effective in situ immobilization reclamation method, based on phosphate amendments, is applied. In contrast to the widely studied thermodynamic parameters of pyromorphite and mimetite, knowledge of the thermodynamics of their solid solutions is sparse. To supplement the data, a number of compounds from the pyromorphite-mimetite series were synthesized at room temperature using a method to simulate the conditions in the near-Earth surface environments. Afterwards, batch dissolution and dissolution-recrystallization experiments of seven synthesized precipitates were conducted at 25 °C, pH = 2 and in a 0.05 M KNO3 background electrolyte. The experiments were carried out for a period of 6 (dissolution) and 14 (dissolution-recrystallization) months. A plateau in the [Pb] evolution patterns was used to determine equilibrium. All seven dissolutions were congruent, and the ionic activity products (IAP) of the minerals from the pyromorphite-mimetite solid solution series were calculated based on the dissolution reaction: . The IAPs for pyromorphite and mimetite exhibit a significant difference in values over three orders of magnitude between approximately 10−79 for pyromorphite and approximately 10−76 for mimetite. The series appeared to be ideal, and Lippmann and Roozboom diagrams were used for better understanding of its thermodynamics. The results indicated a strong tendency of pyromorphite to partition into the solid phase in the series, which explains some of the naturally observed phenomena. The improvement of the lattice stability of the mimetite due to isostructural phosphate substitutions in anionic sites was observed. The thermodynamic data reported in this study supplement existing databases used in geochemical modeling. 相似文献
20.
Metal loading effect on rare earth element binding to humic acid: Experimental and modelling evidence 总被引：1，自引：0，他引：1
Rémi Marsac Mélanie Davranche Gérard Gruau Aline Dia 《Geochimica et cosmochimica acta》2010,74(6):1749-5637
The effect of metal loading on the binding of rare earth elements (REE) to humic acid (HA) was studied by combining ultrafiltration and Inductively Coupled Plasma Mass Spectrometry techniques. REE-HA complexation experiments were performed at pH 3 for REE/C molar ratios ranging from ca 4 × 10−4 to 2.7 × 10−2. Results show that the relative amount of REE bound to HA strongly increases with decreasing REE/C. A middle-REE (MREE) downward concavity is shown by patterns at high metal loading, whereas patterns at low metal loading display a regular increase from La to Lu. Humic Ion Model VI modelling are close to the experimental data variations, provided that (i) the ΔLK2 parameter (i.e. the Model VI parameter taken into account the presence of strong but low density binding sites) is allowed to increase regularly from La to Lu (from 1.1 to 2.1) and (ii) the published log KMA values (i.e. the REE-HA binding constants specific to Model VI) are slightly modified, in particular with respect to heavy REE. Modelling approach provided evidence that patterns with varying REE/C likely arises because REE binding to HA occurs through two types of binding sites in different density: (i) a few strong sites that preferentially complex the heavy REE and thus control the atterns at low REE/C; (ii) a larger amount of weaker binding sites that preferentially complex the middle-REE and thus control the pattern at high REE/C. Hence, metal loading exerts a major effect on HA-mediated REE binding, which could explain the diversity of published conditional constants for REE binding with HA. A literature survey suggests that the few strong sites activated at low REE/C could be multidentate carboxylic sites, or perhaps N-, or P-functional groups. Finally, an examination of the literature field data proposed that the described loading effect could account for much of the variation in REE patterns observed in natural organic-rich waters (DOC > 5 mg L−1 and 4 ? pH ? 7). 相似文献