Calcite dissolution kinetics in saline waters

The effect of ionic strength (I), pCO2, and temperature on the dissolution rate of calcite was investigated in magnesium-free, phosphate-free, low calcium (mCa2+ ≈ 0.01 m) simple KCl and NaCl solutions over the undersaturation range of 0.4 ≤ Ωcalcite ≤ 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 pCO2 and temperature, and their influences diminished at high I. Arrhenius plots yielded a relatively high activation energy (Ea ≈ 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 pCO2 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 ∼ 45–55% required for dissolution to proceed in undersaturated solutions at 25–55 °C and pCO2 = 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: d = β 0 + β 1 T + β 2 p C O 2 + β 3 X ”free”H 2 O .