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1.
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
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2.
Sorption and catalytic oxidation of Fe(II) at the surface of calcite   总被引:1,自引:0,他引:1  
The effect of sorption and coprecipitation of Fe(II) with calcite on the kinetics of Fe(II) oxidation was investigated. The interaction of Fe(II) with calcite was studied experimentally in the absence and presence of oxygen. The sorption of Fe(II) on calcite occurred in two distinguishable steps: (a) a rapid adsorption step (seconds-minutes) was followed by (b) a slower incorporation (hours-weeks). The incorporated Fe(II) could not be remobilized by a strong complexing agent (phenanthroline or ferrozine) but the dissolution of the outmost calcite layers with carbonic acid allowed its recovery. Based on results of the latter dissolution experiments, a stoichiometry of 0.4 mol% Fe:Ca and a mixed carbonate layer thickness of 25 nm (after 168 h equilibration) were estimated. Fe(II) sorption on calcite could be successfully described by a surface adsorption and precipitation model (Comans & Middelburg, GCA51 (1987), 2587) and surface complexation modeling (Van Cappellen et al., GCA57 (1993), 3505; Pokrovsky et al., Langmuir16 (2000), 2677). The surface complex model required the consideration of two adsorbed Fe(II) surface species, >CO3Fe+ and >CO3FeCO3H0. For the formation of the latter species, a stability constant is being suggested. The oxidation kinetics of Fe(II) in the presence of calcite depended on the equilibration time of aqueous Fe(II) with the mineral prior to the introduction of oxygen. If pre-equilibrated for >15 h, the oxidation kinetics was comparable to a calcite-free system (t1/2 = 145 ± 15 min). Conversely, if Fe(II) was added to an aerated calcite suspension, the rate of oxidation was higher than in the absence of calcite (t1/2 = 41 ± 1 min and t1/2 = 100 ± 15 min, respectively). This catalysis was due to the greater reactivity of the adsorbed Fe(II) species, >CO3FeCO3H0, for which the species specific rate constant was estimated.  相似文献   

3.
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:
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4.
5.
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
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6.
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:
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7.
The composition of carbonate minerals formed in past and present oceans is assumed to be significantly controlled by temperature and seawater composition. To determine if and how temperature is kinetically responsible for the amount of Mg incorporated in calcite, we quantified the influence of temperature and specific dissolved components on the complex mechanism of calcite precipitation in seawater. A kinetic study was carried out in artificial seawater and NaCl-CaCl2 solutions, each having a total ionic strength of 0.7 M. The constant addition technique was used to maintain [Ca2+] at 10.5 mmol kg−1 while [] was varied to isolate the role of this variable on the precipitation rate of calcite.Our results show that the overall reaction of calcite precipitation in both seawater and NaCl-CaCl2 solutions is dominated by the following reaction:
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8.
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
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9.
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:
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10.
The solubility of natural, near-end-member wollastonite-I (>99.5% CaSiO3) has been determined at temperatures from 400 to 800 °C and pressures between 0.8 and 5 GPa in piston-cylinder apparatus with the weight-loss method. Chemical analysis of quench products and optical monitoring in a hydrothermal diamond anvil cell demonstrates that no additional phases form during dissolution. Wollastonite-I, therefore, dissolves congruently in the pressure-temperature range investigated. The solubility of CaSiO3 varies between 0.175 and 13.485 wt% and increases systematically with both temperature and pressure up to 3.0 GPa. Above 3.0 GPa wollastonite-I reacts rapidly to the high-pressure modification wollastonite-II. No obvious trends are evident in the solubility of wollastonite-II, with values between 1.93 and 10.61 wt%. The systematics of wollastonite-I solubility can be described well by a composite polynomial expression that leads to isothermal linear correlation with the density of water. The molality of dissolved wollastonite-I in pure water is then
log(mwoll)=2.2288-3418.23×T-1+671386.84×T-2+logρH2O×(5.4578+2359.11×T-1).  相似文献   

11.
The solubility of FeSm, synthetic nanoparticulate mackinawite, in aqueous solution was measured at 23 °C from pH 3-10 using an in situ precipitation and dissolution procedure and the solution species was investigated voltammetrically. The solubility is described by a pH-dependent reaction and a pH-independent reaction. The pH-dependent dissolution reaction can be described by
FeSm+2H+→Fe2++H2S  相似文献   

12.
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   

13.
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   

14.
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
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15.
Ca isotope fractionation during inorganic calcite formation was experimentally studied by spontaneous precipitation at various precipitation rates (1.8 < log R < 4.4 μmol/m2/h) and temperatures (5, 25, and 40 °C) with traces of Sr using the CO2 diffusion technique.Results show that in analogy to Sr/Ca [see Tang J., Köhler S. J. and Dietzel M. (2008) Sr2+/Ca2+ and 44Ca/40Ca fractionation during inorganic calcite formation: I. Sr incorporation. Geochim. Cosmochim. Acta] the 44Ca/40Ca fractionation during calcite formation can be followed by the Surface Entrapment Model (SEMO). According to the SEMO calculations at isotopic equilibrium no fractionation occurs (i.e., the fractionation coefficient αcalcite-aq = (44Ca/40Ca)s/(44Ca/40Ca)aq = 1 and Δ44/40Cacalcite-aq = 0‰), whereas at disequilibrium 44Ca is fractionated in a primary surface layer (i.e., the surface entrapment factor of 44Ca, F44Ca < 1). As a crystal grows at disequilibrium, the surface-depleted 44Ca is entrapped into the newly formed crystal lattice. 44Ca depletion in calcite can be counteracted by ion diffusion within the surface region. Our experimental results show elevated 44Ca fractionation in calcite grown at high precipitation rates due to limited time for Ca isotope re-equilibration by ion diffusion. Elevated temperature results in an increase of 44Ca ion diffusion and less 44Ca fractionation in the surface region. Thus, it is predicted from the SEMO that an increase in temperature results in less 44Ca fractionation and the impact of precipitation rate on 44Ca fractionation is reduced.A highly significant positive linear relationship between absolute 44Ca/40Ca fractionation and the apparent Sr distribution coefficient during calcite formation according to the equation
Δ44/40Cacalcite-aq=(1.90±0.26)·logDSr2.83±0.28  相似文献   

16.
Stable oxygen isotopic fractionation during inorganic calcite precipitation was experimentally studied by spontaneous precipitation at various pH (8.3 < pH < 10.5), precipitation rates (1.8 < log R < 4.4 μmol m− 2 h− 1) and temperatures (5, 25, and 40 °C) using the CO2 diffusion technique.The results show that the apparent stable oxygen isotopic fractionation factor between calcite and water (αcalcite–water) is affected by temperature, the pH of the solution, and the precipitation rate of calcite. Isotopic equilibrium is not maintained during spontaneous precipitation from the solution. Under isotopic non-equilibrium conditions, at a constant temperature and precipitation rate, apparent 1000lnαcalcite–water decreases with increasing pH of the solution. If the temperature and pH are held constant, apparent 1000lnαcalcite–water values decrease with elevated precipitation rates of calcite. At pH = 8.3, oxygen isotopic fractionation between inorganically precipitated calcite and water as a function of the precipitation rate (R) can be described by the expressions
at 5, 25, and 40 °C, respectively.The impact of precipitation rate on 1000lnαcalcite–water value in our experiments clearly indicates a kinetic effect on oxygen isotopic fractionation during calcite precipitation from aqueous solution, even if calcite precipitated slowly from aqueous solution at the given temperature range. Our results support Coplen's work [Coplen T. B. (2007) Calibration of the calcite–water oxygen isotope geothermometer at Devils Hole, Nevada, a natural laboratory. Geochim. Cosmochim. Acta 71, 3948–3957], which indicates that the equilibrium oxygen isotopic fractionation factor might be greater than the commonly accepted value.  相似文献   

17.
Spectrophotometric measurements of the pH in natural waters such as seawater have been shown to yield precise results. In this paper, the sulfonephthalein indicator m-cresol purple (mCP, H2I) has been used to determine the pH of NaCl brines. The indicator has been calibrated in NaCl solutions from 5 to 45 °C and ionic strengths from 0.03 to 5.5 m. The calibrations were made using TRIS buffers (0.03 m, TRIS/TRIS-HCl) with known dissociation constants pKTRIS in NaCl solutions [Foti C., Rigano C. and Sammartano S. (1999) Analysis of thermodynamic data for complex formation: protonation of THAM and fluoride ion at different temperatures and ionic strength. Ann. Chim. 89, 1-12]. The values of pH were determined from
pH=pKmCP+log{(R-e1)/(e2-Re3)}  相似文献   

18.
19.
Understanding the relationships between speleothem stable isotopes (δ13C δ18O) and in situ cave forcing mechanisms is important to interpreting ancient stalagmite paleoclimate records. Cave studies have demonstrated that the δ18O of inorganically precipitated (low temperature) speleothem calcite is systematically heavier than the δ18O of laboratory-grown calcite for a given temperature. To understand this apparent offset, rainwater, cave drip water, groundwater, and modern naturally precipitated calcite (farmed in situ) were grown at multiple locations inside Hollow Ridge Cave in Marianna, Florida. High resolution micrometeorological, air chemistry time series and ventilation regimes were also monitored continuously at two locations inside the cave, supplemented with periodic bi-monthly air gas grab sample transects throughout the cave.Cave air chemistry and isotope monitoring reveal density-driven airflow pathways through Hollow Ridge Cave at velocities of up to 1.2 m s−1 in winter and 0.4 m s−1 in summer. Hollow Ridge Cave displays a strong ventilation gradient in the front of the cave near the entrances, resulting in cave air that is a mixture of soil gas and atmospheric CO2. A clear relationship is found between calcite δ13C and cave air ventilation rates estimated by proxies pCO2 and 222Rn. Calcite δ13C decreased linearly with distance from the front entrance to the interior of the cave during all seasons, with a maximum entrance-to-interior gradient of Δδ13CCaCO3 = −7‰. A whole-cave “Hendy test” at multiple contemporaneous farming sites reveals that ventilation induces a +1.9 ± 0.96‰ δ13C offset between calcite precipitated in a ventilation flow path and calcite precipitated on the edge or out of flow paths. This interpretation of the “Hendy test” has implications for interpreting δ13C records in ancient speleothems. Calcite δ13CCaCO3 may be a proxy not only for atmospheric CO2 or overlying vegetation shifts but also for changes in cave ventilation due to dissolution fissures and ceiling collapse creating and plugging ventilation windows.Farmed calcite δ18O was found to exhibit a +0.82 ± 0.24‰ offset from values predicted by both theoretical calculations and laboratory-grown inorganic calcite. Unlike δ13CCaCO3, oxygen isotopes showed no ventilation effects, i.e. Δδ18OCaCO3 appears to be a function of growth temperature only although we cannot rule out a small effect of (unmeasured) gradients in relative humidity (evaporation) accompanying ventilation. Our results support the findings of other cave investigators that water-calcite fractionation factors observed in speleothem calcite are higher that those measured in laboratory experiments. Cave and laboratory calcite precipitates may differ mainly in the complex effects of kinetic isotope fractionation. Combining our data with other recent speleothem studies, we find a new empirical relationship for cave-specific water-calcite oxygen isotope fractionation across a range of temperatures and cave environments:
1000lnα=16.1(103T-1)-24.6  相似文献   

20.
The relationship between stable isotope composition (δ13C and δ18O) in seawater and in larval shell aragonite of the sea scallop, Placopecten magellanicus, was investigated in a controlled experiment to determine whether isotopes in larval shell aragonite can be used as a reliable proxy for environmental conditions. The linear relationship between δ13CDIC and δ13Caragonite (r2 = 0.97, p < 0.0001, RMSE = 0.18) was:
δ13CDIC=1.15(±0.05)∗δ13Caragonite-0.85(±0.04)  相似文献   

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