Calcite-Carbonic Acid

We will compute for pure water in equilibrium with calcite in a closed system with constant total carbonate, Cj. Dissolved Ca + is thus controlled by calcite saturation, as defined by (Ca ) = K,J COl ) where K. = 10 . With an HCl titrant Q = Cl , and the charge-balance equation is 

The first bracketed term is the contribution of water and dissolved carbonate species. The second term in brackets is the calcite contribution. Equation (5.134) is plotted in Fig. 5.11 which shows that, in equilibrium with calcite below about pH 8.5 for Cj = 10 , system buffer capacity is 100 times or more greater than it is without calcite. Because calcite can both dissolve and precipitate under ambient conditions, its buffer capacity provides resistance to a pH change in either direction. 

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SYNS AGRICULTURAL LIMESTONE AGSTONE ARAGONITE ATOMIT BELL MINE PULVERIZED limestone CALCITE CARBONIC ACID, CALCIUM SALT (1 1) CHALK DOLOMITE FRANKLIN D LIMESTONE (FCC) LITHOGRAPHIC STONE MARBLE NATURAL CALCIUM CARBONATE PORTLAND STONE SOHNHOFEN STONE VATERITE... 

The hydrogen ion flux that is provided by carbonic acid dissociation also can attack calcite (CaCO ) ... 

Carbon dioxide has a dominant effect on the dissolution of carbonate minerals, such as calcite and dolomite (Table 2.1). If a carbonate mineral dissolves in water that is equilibrated with a constant source of CO, then the concentration of dissolved carbonic acid remains constant and high. However, when calcite dissolution is accompanied by consumption of carbonic acid and a continuous source of CO is not maintained, the reaction proceeds further to achieve equilibrium. 

Ca(HC03)2 (aq.). Randall and White3 reviewed the data of Backstrom,3 Cameron and Brezeate,1 Cameron and Robinson,1 2 Cavazzi,1 Ehlert and Hempel,1 Engel,1 Haehnel,1 2 Johnston,3 Kendall,2 McCoy and Smith,1 Schloesing,1 Wells,1 and Frear, Johnston, and Kline,1 on the solubility of calcium carbonate in aqueous carbonic acid, and concluded that Q25=8.6 for the reaction, CaC03 (c, calcite)+C02 (g)+H20 (liq.) = Ca(HC03)2 (aq.). 

Limestone is mostly made up of the mineral calcite, or calcium carbonate (CaC03). As the calcium carbonate rock dissolves in the slightly acidic water, spaces and even caves develop underground. If carbonic acid dissolves all the way through the rock and into a cave below the Earth s surface, the resulting solution contains calcium hydrogen carbonate (calcium bicarbonate). 

He S, Morse JW (1993) The carbonic acid system and calcite solubility in aqueous Na-K-Ca-Mg-Cl-S04 solutions from 0 to 90°C. Geochim Cosmochim Acta 57 3533-3555... 

Temperature and pressure variations in natural systems exert major influences on carbonate mineral solubility and the distribution of carbonic acid chemical species. For example, the solubility of calcite decreases with increasing temperature, as does the solubility of CO2 gas in water. These two effects on solubilities can lead to precipitation of calcite as a cement in a marine sediment-pore water system that undergoes moderate burial. 

Table 1.12. Variation of carbonic acid components and calcite saturation in seawater under different conditions.

Jacobsen and Langmuir (1974) determined a value for pKSp (25°C) for calcite of 8.42 0.01, whereas Berner s (1976) value was 8.45 0.01. Berner also determined the pKSp for aragonite at 25°C to be 8.28 0.03. An aspect of particular interest of both studies was that to obtain internal consistency for the carbonic acid system or constant values for the solubility products over the range of conditions studied, it was necessary to neglect ion pair formation. The potentially important ion pairs that could have formed in the experimental solutions are CaHCC>3+ and CaC03°. The former is by far the most important species, and a vast body of previous literature supported its existence (see Plummer and Busenberg, 1982, for summary). 

Natural carbonate minerals do not form from pure solutions where the only components are water, calcium, and the carbonic acid system species. Because of the general phenomenon known as coprecipitation, at least trace amounts of all components present in the solution from which a carbonate mineral forms can be incorporated into the solid. Natural carbonates contain such coprecipitates in concentrations ranging from trace (e.g., heavy metals), to minor (e.g., Sr), to major (e.g., Mg). When the concentration of the coprecipitate reaches major (>1%) concentrations, it can significantly alter the chemical properties of the carbonate mineral, such as its solubility. The most important example of this mineral property in marine sediments is the magnesian calcites, which commonly contain in excess of 12 mole % Mg. The fact that natural carbonate minerals contain coprecipitates whose concentrations reflect the composition of the solution and conditions, such as temperature, under which their formation took place, means that there is potentially a large amount of information which can be obtained from the study of carbonate mineral composition. This type of information allied with stable isotope ratio data, which are influenced by many of the same environmental factors, has become a major area of study in carbonate geochemistry. 

It should be kept in mind that, in spite of these major variations in the CO2-carbonic acid system, virtually all surface seawater is supersaturated with respect to calcite and aragonite. However, variations in the composition of surface waters can have a major influence on the depth at which deep seawater becomes undersaturated with respect to these minerals. The CO2 content of the water is the primary factor controlling its initial saturation state. The productivity and temperature of surface seawater also play major roles, in determining the types and amounts of biogenic carbonates that are produced. Later it will be shown that there is a definite relation between the saturation state of deep seawater, the rain rate of biogenic material and the accumulation of calcium carbonate in deep sea sediments. 

Here we will use a simplified example to illustrate some basic aspects of the mass transport process for carbonates that avoids most of the more complex relationships. In this example, the calcium and carbonate ion concentrations are set equal, and values of the activity coefficients, temperature, and pressure are held constant. The carbonate ion concentration is considered to be independent of the carbonic acid system. The resulting simple (and approximate) relation between the change in saturation state of a solution and volume of calcite that can be dissolved or precipitated (Vc) is given by equation 7.4, where v is the molar volume of calcite. 

The carbonic acid-promoted dissolution of calcite (Eq. 3.59b) can be described in an open system by the sequence of heterogeneous reactions ... 

Figure 2.4 Reaction mechanism contributions to the rate of calcite dissolution as a function of pH and PCO, at 25°C. Although H carbonic acid, and water reaction with calcite occur simultaneously throughout (far from equilibrium, as well as at equilibrium), the forward reaction is dominated by reaction with single species in the fields shown. More than one species contributes significantly to the forward rate in the stippled area. Along the lines labeled 1,2, and 3, the forward rate attributable to one species balances that of the other two. After Plummer et al. (1979). Reprinted with permission from Chemical modeling in aqueous systems, Am. Chem. Soc. Symp. 93. 1979, American Chemical Society.

Figure 5.11 A log plot of the buffer capacity due to carbonic acid species for = 10 M (see Fig. 5.10) at saturation with respect to calcite for = I0 M and for equilibrium between the clays illite and kaolinite. The lower curve is...

The Ca—Mg—HCO3- and Ca—Mg—SO4-type groundwater from the glacial drift aquifers reflects the dissolution of calcite and dolomite by carbonic acid formed in the soil zone, and the production and leaching of secondary gypsum through oxidation of sulfide in the presence of calcite or dolomite under conditions of partial saturation. In cases where the content of carbonates is low, silicate mineral weathering potentially occurs. 

Using the reaction from part (a), estimate the quantity of Ca " (kg/ha) that could be leached out of a calcareous soil in one year by reaction with the carbonic acid in pristine rainfall. Assume a moist temperate climate with 90 cm of rainfall per year. Calculate the equilibrium concentration of Ca " that would be present in the solution of a calcareous soil with a pH of 7.8 and a CO2 gas pressure of 10 milliatmospheres. How much annual Ca loss by leaching could this dissolution reaction cause under the same conditions described in part (b) (Assume rapid kinetics of calcite dissolution.)... 

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