Calcite dissolution rates

Mg2+ influences calcite dissolution rates the same way, but not to the same extent as Ca2+. The inhibition effects of Mg2+ can be described in terms of a Langmuir adsorption isotherm. Sjoberg (1978) found he could model results for the combined influences of Ca2+ and Mg2+ in terms of site competition consistent with ion exchange equilibrium. The inhibition effects of Mg2+ in calcite powder runs increase with increasing Mg2+ concentration and as equilibrium is approached. 

Normalized to unstrained calcite. Dissolution rate calculation as a function of radius r of the hollow core, based on Eq. 11, for p = 10locm-2 (ST = total perfect surface area Sd = total surface area of hollow cores). 

In the last decade, however, in-situ techniques have been developed to overcome these problems. Profiling lander systems were deployed to record the pore water microprofiles of oxygen, pH and pCOj, and Ca whereas benthic chambers were deployed to measure solute fluxes across the sediment-water interface directly. Very often, reactive-transport models are used to explain the interrelation between measured microprofiles, to predict overall calcite dissolution rates by defining the dissolution rate constants, and to distinguish between dissolution driven by organic matter oxidation and by the undersaturation of the bottom water. 

According to this model, CO, H3CO3 and H O relatively easily penetrate to the interface, not forming concentration gradients, and calcite dissolution rate depends mostly on CO partial pressure andbegins drastically growing at pH <4-5.5 due to increase in H+ activity. Plummer at al. proposed an equation of this correlation... 

On the whole, under normal conditions max calcite dissolution rate changes between n lO mole m s in neutral water and n lO mole-m" s S in acidic. 

Table 2.21 Calcite dissolution rate constants and activation energy.

Ultimately calcite dissolution rate and solubility depend on pH values, which in the natural water are closely associated with the CO content. For this reason calcite usually dissolves until it reaches certain equilibrium with the content or partial pressure of CO. ... 

The dissolution rate for calcite and aragonite have been described in terms of the following rate law (Plummer et al., 1978 Busenberg and Plummer, 1986 Chou and Wollast, 1989. ... 

As with the calcareous tests, BSi dissolution rates depend on (1) the susceptibility of a particular shell type to dissolution and (2) the degree to which a water mass is undersaturated with respect to opaline silica. Susceptibility to dissolution is related to chemical and physical factors. For example, various trace metals lower the solubility of BSi. (See Table 11.6 for the trace metal composition of siliceous shells.) From the physical perspective, denser shells sink fester. They also tend to have thicker walls and lower surface-area-to-volume ratios, all of which contribute to slower dissolution rates. As with calcivun carbonate, the degree of saturation of seawater with respect to BSi decreases with depth. The greater the thermodynamic driving force for dissolution, the fester the dissolution rate. As shown in Table 16.1, vertical and horizontal segregation of DSi does not significantly coimter the effect of pressure in increasing the saturation concentration DSi. Thus, unlike calcite, there is no deep water that is more thermodynamically favorable for BSi preservation they are all corrosive to BSi. 

Figure 8.33 Reaction mechanism contribution to the total rate of calcite dissolution reaction as a function of pH and Pco2 25 °C. From L. N. Plummer, T. M. L. Wigley, and D. L. Pankhurst (1978), American Journal of Science, 278, 179-216. Reprinted with permission of American Journal of Science.

Figure 4-6 Interface reaction rate as a function of temperature, pressure, and composition. The vertical dashed line indicates the equilibrium condition (growth rate is zero), (a) Diopside growth and melting in its own melt as a function of temperature with the following parameters Te= 1664K at 0.1 MPa, A5m-c = 82.76J mol K , E/R —30000 K, 4 = 12.8 ms K, and AV c l. l x 10 m /mol. The dots are experimental data on diopside melting (Kuo and Kirkpatrick, 1985). (b) Diopside growth and melting in its own melt as a function of pressure at 1810 K (Tg = 1810 K at 1 GPa from the equilibrium temperature at 0.1 MPa and the Clapeyron slope for diopside). (c) Calcite growth and dissolution rate in water at 25 °C as a function of Ca " and CO concentrations.

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). 

Figure 2.10. Dissolution rates as a function of pH for aragonite, calcite, witherite, dolomite, and magnesite. (After Chou et al., 1989.)...

Phosphate has been found to be an extremely strong inhibitor of carbonate reaction kinetics, even at micromolar concentrations. This constituent has been of considerable interest in seawater because of its variability in concentration. It has been observed that phosphate changes the critical undersaturation necessary for the onset of rapid calcite dissolution (e.g., Berner and Morse, 1974), and alters the empirical reaction order by approximately a factor of 6 in going from 0 to 10 mM orthophosphate solutions. Less influence was found on the log of the rate constant. Walter and Burton (1986) observed a smaller influence of phosphate on calcite... 

The influence of heavy metals on calcium carbonate reaction rates has not been extensively studied. Experiments have shown that many metals exhibit inhibitory effects on calcite dissolution. Ions tested by Terjesen et al. (1961), in decreasing order of effectiveness, were Pb2+, La3+, Y3+, Sc3+, Cd2+, Cu2+, Au3+, Zn2+, Ge4+, and Mn2+, and those found to be about equal were Ni2+, Ba2+, Mg2+, and Co2+. The general trend follows the solubility of the metal carbonate minerals, with the exception of Zn2+ and the "about equal" group whose solubilities are all greater than calcite. 

Figure 7.6. Relative dissolution rates of biogenic carbonates in seawater undersaturated with respect to calcite. (After Walter, 1985.)...

The rate of reaction is dependent on the nucleation and growth rates of calcite, not the dissolution rate of aragonite. Curiously, it has also been observed that absolute rates are strongly dependent on the aragonitic material used. This observation appears to contradict the generally held conclusion that rates are strictly dependent on calcite nucleation and precipitation rates, not the dissolution rate of aragonite. 

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