Systems calcite dissolution

Before proceeding, a few general comments will be helpful. In most cases at least one of the four carbonate system analytical quantities (total alkalinity, total CO2, carbon dioxide partial pressure, and pH) will be known. The equilibrium equations relating these quantities, along with those for water and calcite, will be used frequently in calculations. Also, in cases where calcite dissolution and... 

A number of factors have been investigated that influence calcite dissolution in relatively simple systems. These include the important variables of temperature, pH, and the partial pressure of CO2. The equations describing the net dissolution rate of carbonate minerals in simple experimental solutions differ somewhat, depending on experimental conditions and the interpretations placed on the experimental results by various investigators (cf., for example, Plummer et al., 1978 Chou et al., 1989). 

Buhmann D. and Dreybrodt W. (1985a) The kinetics of calcite dissolution and precipitation in geologically relevant situations of Karst areas. 1. Open System. Chem. Geol. 48, 189-211. 

Plummer L.N., Parkhurst D.L. and Wigley T.M.L. (1979) Critical review of the kinetics of calcite dissolution and precipitation. In Chemical Modelling--Speciation, Sorption, Solubility and Kinetics in Aqueous Systems (ed. E. Jenne), pp. 537-573. American Chemical Society, Washington, D.C. 

Boudreau, B.P., and Canfield, D.E. (1993) A comparison of closed- and open-system models for porewater pH and calcite dissolution. Geochim. Cosmochim. Acta 57, 317-334. 

At a P(C02) of 2 vol% the calcite dissolution is lower in the open system (hence the pH value higher) than in the closed system. At 20 vol% P(C02) it is the other way around, in the open system the calcite dissolution is higher, the pH value is lower (Table 43). 

Table 43 Calcite dissolution in an open and a closed system at P(C02) = 2 vol%, respectively P(C02) = 20 vol%...

C02(g) under batch reaction calculations - saturation indices" - P(C02) = 3.02 vol%), since only a limited amount of gas (lliter) is assumed for the reaction. Because in the open system the partial pressure P(C02) = 2 vol% is lower than in the closed system ( 3.02 vol%), the calcite dissolution is less. 

The rate of calcite dissolution is known to depend on the hydrodynamic conditions of the environment and on the rate of heterogeneous reaction at the mineral surface. Numerous laboratory studies demonstrate transport and surface-controlled aspects of calcite reactions in aqueous solutions, but until recently, no study has been comprehensive enough to enable comparison of kinetic results among differing hydro-chemical systems. 

We have studied the dissolution kinetics of calcite in stirred CO2 water systems at CO2 partial pressures between 0.0003 and 0.97 atm and between 5° and 60°C, using pH-stat and free drift methods (J ) Our results suggest a mechanistic model for reactions at the calcite-aqueous solution interface that has broad implications to the controls on calcite dissolution and precipitation under diverse chemical and hydrodynamic conditions. 

The second type of thermodynamic problem is concerned with comparing parallel reactions in pure and impure systems. In the case of calcite dissolution we ask, how will rate change at constant pH and PCO2 owing to the presence or absence of an impurity Equation 14 shows that thermodynamic effects of impurities on the rate of calcite dissolution are accounted for by calculation of the bulk fluid saturation (f ) and the equilibrium activity of H in the adsorption layer (aH+(s)). The following example demonstrates one possibility. 

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. 

Buhmann, D. and Dreybrodt, W., 1987, Calcite dissolution kinetics in the system H20-C02-CaC03 with the participation of foreign ions, Chem. Geol 64 89-102. 

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.

Plummer, L. N., T. M. L. Wigley, and D. L. Parkhurst (1979), Critical Review of the Kinetics of Calcite Dissolution and Precipitation, in E. A. Jenne, Ed., Chemical Modeling in Aqueous Systems—Speciation. Sorption. Solubility, and Kinetics, (ACS Symposium Series No. 93), American Chemical Society, Washington, DC, pp. 

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