Calcite equilibrium

The simulated C02 fugacity matches the initial reservoir C02 content and indicates that the pH is buffered by C02-calcite equilibrium. Further modelling was carried out using the Geochemists Workbench React and Tact modules with the thermodynamic database modified to reflect the elevated P conditions and kinetic rate parameters consistent with the Waarre C mineralogy. The Waarre C shows low reactivity and short-term predictive modelling of the system under elevated C02 content changes little with time (Fig. 1). 

Figure 8.18 Relationship between pH of aqueous solution and fco2 of gaseous phase in equilibrium with water and calcite. Equilibrium pH is in correspondance with zero on ordinate axis.

It should be emphasized that an equilibrium can be established between a metastable phase(s) and its environment. Aragonite, for example, can be precipitated from seawater at 25°C, but it is unstable at Earth-surface T and P, and can persist metastably because of kinetic reasons. This statement is illustrated by the following calculation. We can use the free energies of formation of Table 6.1, and calculate the Gibbs free energy of reaction for the mass action equation representing aragonite-calcite equilibrium ... 

Fig. 38 shows that settling of the calcite equilibrium is very rapid at low C02 partial pressures (in the example 0.03 Vol-%), but distinctly slower at increased C02 partial pressures (in the example 1 Vol-%). 

Considering calcite equilibrium a pHc of 7.076 results, that is 0.376 pH units above the measured pH value of 6.7. The permitted deviation of 0.2 is exceeded. Since pH-pHc is negative, the water is calcite aggressive, i.e., it can still dissolve calcite and present a danger for pipe corrosion. Undersaturation can also be determined without calculation of the pHc, because within initial solution calculations in the PHREEQC output, calcite already shows a saturation index of -0.63 (= 23% saturation). 

After aeration by adjustment of an equilibrium with the atmospheric C02 partial pressure (0.03 Vol%, C02(g) -3.52) the pH value increases to 8.783, the pHc to 7.57. Thus ApH is +1.213, i.e., the water is supersaturated with regard to calcite and calcite precipitation might occur in the pipe systems. The SI calcite (under batch reaction calculations ) is +1.35. Thus, aeration deteriorates the initial conditions regarding calcite equilibrium. The drinking water standards are exceeded by far. 

To display the secondary Y-axis for the calcite saturation index besides the primary Y-axis with the concentrations for Ca and C, Chart options / Show secondary y-axis must be chosen by click on the right mouse button in the graph. The result of the modeling can be seen in Fig. 72. The figure depicts a convergence to the calcite equilibrium. However, the saturation index shows that... 

Fig. 72 Corrosion in a fracture showing convergence to the calcite equilibrium with...

Figure 17. Log of the dissolution rate vs. total carbonate ion concentration for synthetic aragonite, pteropods, calcitic Pacific Ocean sediment, and foraminifera in the 125-500 iim size fraction. (A) indicates ihe aragonite equilibrium total carbonate ion concentration at 25°C, 1 atm (26). (C) indicates the calcite equilibrium total carbonate ion concentration at 25°C, 1 atm (25).

Theoretical calcite equilibrium pH at bulk fluid PCO2 (open system). 

Theoretical calcite equilibrium pH in a sea water system closed to CO2. 

The surface equilibrium pH values implied by our rate equation and Morse s rates are all within 0.01 pH of the theoretical pH for calcite equilibrium in sea water closed to CO2 (Table III). In a closed system, calcite equilibrium determines both surface pH and PCO2, and rate depends, in part, on the flux of CO2 to the surface. Sjoberg (23) noted a stirring dependence of rate at pH 8 and very low CO2 partial pressures, where calcite dissolution has previously been attributed to surface reaction alone. 

Most natural water systems in contact with calcite (oceans, rivers, lakes, carbonate rock aquifiers) are, however, near equilibrium, and PCO2 dependence cannot be ignored. According to our model, the rate of backward reaction is a significant function of surface pH, and surface pH is determined by calcite equilibrium at the surface PCO2. At the relatively high pH, low PCO2 conditions of most natural waters, the surface pH is least well defined and may depend, in part, on the flux of CO2 between the surface and bulk fluid. 

The problem has 2 parts. In the first part the sample T-93 is speciated on the Macinnes, and unsealed conventions, respectively, using the measured pH. In the second part an attempt is made to resolve uncertainties in the carbonate system by assuming the brine is in equilibrium with calcite, on the Macinnes and unsealed conventions, respectively. Use of calcite equilibrium is preferable to alternate means of defining pH, such as through charge balance, owing to uncertainties in the analytical data. 

Two geochemical models were used to quantify the exchangeable C reservoir (1) a theoretical model based on calcite equilibrium control (calcite equilibrium model), and (2) an empirical model based on measured losses of CO2 from a surrogate unsaturated zone atmosphere to unsaturated water-sediment mixtures (CO2 retention model). 

According to the calcite equilibrium model, Pick s second law for C02 diffusion in the unsaturated zone can be generalized as ... 

III. P C02 values that were calculated using the calcite equilibrium model (column A) were within 1 standard deviation (S.D.) of onsite mean P C02 data (column B) only at the piezometers located in the borehole nearest the trench. Simulated P C02 values for piezometers at the second and third borehole were substantially larger than measured values. Decreasing the source term for C02 at the trench produced a fit to the onsite data at either the second or third borehole, but simulated P C02 values at the two remaining boreholes were unlike the onsite data. 

Laboratory measurements of the losses of CO2 and C02 from a surrogate unsaturated zone atmosphere to unsaturated sediments indicate the presence of an adsorbed C phase that can retard C02 transport in the unsaturated zone. Measured losses of CO2 from the atmosphere were 8 to 17 times greater than those predicted by calcite equilibrium calculations. Modeled predictions of C02 transport in a cross section near buried low-level radioactive waste support the presence of the adsorbed C phase distribution of P C02 was more accurately simulated using a model of C02 retention based on measured CO2 -loss isotherms than with a model based on calcite equilibrium control. Failure to account for the adsorbed C phase can lead to substantial errors when using models to estimate C transport and exchange in the unsaturated zone. 

Svensson, U. and Dreybrodt, W., 1992. Dissolution kinetics of natural calcite minerals in C02-water systems approaching calcite equilibrium. Chemical Geology, 100 129-145. 

Fig. 15.3 Geochemical model calculation using the program PHREEQC. To an oxic seawater with calcite in equilibrium (cf. Table 15.4) an organic substance is gradually added automatically leading to a redox reaction. The system should continue to be open to calcite equilibrium, but sealed from the gaseous phase...

Fig. 15.4 Geochemical model calculation using the program PHREEQC. In an anoxic system (state at the end of the model calculation from Fig. 15.3), the gradual addition of organic matter to the redox reaction is continued, whereby the system is kept open for calcite equilibrium and sealed from the gaseous phase. Initially, the dissolved sulfate will be consumed, in the course of which low amounts (logarithmic scale) of methane will emerge. Only after the sulfate concentration has become sufficiently low, will the generation of methane display its distinct increase.

As expected, the sulfate concentration in solution initially decreases at a swift rate and, concomitantly, low amounts of methane already emerge. Only after the concentration of sulfate falls under a certain level, will methane be released in substantial amounts. The permanently maintained calcite equilibrium will result, due to precipitation, in the depression of the concentration of dissolved calcium ions (compare Schulz et al. 1994). 

Utilisation following rapid decarbonisation Excessive CO2 and/or calcium-hydrogen-carbonate can be eliminated by adding calcium hydroxide. After calcite precipitation, the pH can be adjusted as appropriate by adding small amounts of CO2 (cf. calcite equilibrium)... 

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