Calcite Dissolution and Precipitation

According to Plummer et al. (1979) the rate of solution or precipitation of calcite is given by 

At concentrations well below saturation with calcite, the term dominates the rate equation up to pHs between about 4.5 and 5.5. At higher pHs when Pco. 0.1 bar, the 2 term dominates rates. At higher pHs and lower CO2 pressures, the kj term dominates above pH 5.5. In freshwater IH2O] = 1, and because [H2CO3] = /fco, co, he rate law may be written 

The low activation energy for it, suggests diffusion control is rate limiting at low pH s. Sjoberg and Rickard (1984) have suggested that for acid pHs (where k, is dominant), the rate of cal-cite dissolution is given by 

Based on the rate equations of Plummer et al. (1979), we can compute that for groundwater pH s above 6 and 7 co, values less than 0.1 bar at 25°C and below, the solution rate of calcite far from equilibrium reduces toR = ky In other words, for these conditions (which are typical of many shallow groundwaters) the reaction is zero-order, as long as the surface area of the calcite is constant. This assumes no catalysis or inhibition of the rate by adsorbed substances. (Sc, Cu, and PO4 are strong inhibitors according to Sjoberg and Rickard 1984.) As equilibrium is approached, the rate equation becomes 

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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. 

FIG. 3.3. Influence of pH on the rates of calcite dissolution and precipitation (left, adapted from Chou et al.7 with the permission of Elsevier Science Publishers), and an initial-rate plot for calcite precipitation indicating the applicability of Eq. 3.9 (right, data from W. P. Inskeep and P. R. Bloom,8 adapted with permission. Copyright by Pergamon Press Ltd., Headington Hill Hall, Oxford 0X3 OBW, England.)... 

Critical Review of the Kinetics of Calcite Dissolution and Precipitation... 

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. 

This paper reviews the subject of the kinetics of calcite dissolution and precipitation by comparing predictions made by our mechanistic model with published laboratory results. 

A complex composite rate law may describe the rate of an overall reaction as the sum of rates of its constituent elementary reactions. Such a rate law may be written for calcite dissolution and precipitation, for which the overall rate equals... 

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. 

Results from this multi-component, multi-species transport model show that the transport of some components are in inter-locked step fashion. For example, the transport of sulfate is closely tied to calcite dissolution and precipitation. 

The numerical model CoTReM was applied to investigate the depth dependent effects of respiration and redox processes related to CaCO dissolntion (Pfeifer et al. 2002 cf. Fig. 15.16 in chapter 15). Interestingly, if calculated until a steady-state situation is reached, the model-derived calcite dissolution and precipitation rates produce an almost perfect fit to the measured CaC03 profile in the sediment (Fig. 9.8), which suggests that 90 % of the CaC03 flux to the sea floor is redissolved in the sediment. 

Fig. 9.8 Measured and simulated pore water profiles at station GeoB 4906 and corresponding rates of primary and secondary redox processes, (b) Corresponding calcite dissolution and precipitation rates and the resulting steady-state distribution of sedimentary CaCO for input fluxes of 40, 42, and 44 g m yr, respectively, compared to measured concentrations (bars) (after Pfeifer et al. 2002).

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