The effect of ionic strength (1), pCO(2), and temperature on the dissolution rate of calcite was investigated in magnesium-free, phosphate-free, low calcium (m(Ca)2+approximate to 0.01 m) simple KCl and NaCl solutions over the undersaturation range of 0.4 <=Omega(calcite)<= 0.8. First-order kinetics were found sufficient to describe the rate data where the rate constant (k) is dependent on the solution composition. Rates decreased with increasing I and were faster in KCl than NaCl solutions at the same I indicating that Na+ interacts more strongly with the calcite surface than K+ or that water is less available in NaCl solutions. Rates increased with increasing pCO(2) and temperature, and their influences diminished at high L Arrhenius plots yielded a relatively high activation energy (E-a approximate to 20 +/- 2 kJ mol(-1)) which indicated that dissolution was dominated by surface-controlled processes. The multiple regression model (MR) of Gledhill and Morse (2006a) was found to adequately describe the results at high I in NaCl solutions, but caution must be used when extrapolating to low I or pCO(2) values. These results are consistent with the hypothesis that the mole fraction of "free" solvent X-"free"H2O) plays a significant role in the dissolution kinetics of calcite with a minimum value of similar to 45-55% required for dissolution to proceed in undersaturated solutions at 25-55 degrees C and pCO(2)=0.1-1 atm. This hypothesis has been incorporated into a modified version of the MR model of Gledhill and Morse (2006a) where X-"free"H2O has replaced I and the Ca2+ and Mg2+ terms have been dropped: k(pred) = beta(0) + beta T-1 + beta(2)pCO(2) + beta X-3("free"H2O).
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