Chemical affinity and pH effects on chlorite dissolution kinetics under geological CO2 sequestration related conditions

- The kinetic dissolution of Flagstaff Hill chlorite was investigated.
- The dissolution rate is determined as a function of pH and chemical affinity.
- The critical Gibbs free energy is larger than expected from a linear TST rate law.
- The rate law is applicable in subsurface CO2 sequestration.

The kinetic dissolution of Flagstaff Hill chlorite (CCa-2 from Clay Minerals Society), (Mg4.55Al1.23Fe0.12)(Al1.04Si2.96)O10(OH)8, was investigated using a well-mixed flow-through reactor at 100 °C, pH values ranging from 3.0 to 7.5, and variable saturation states. The objective was to measure the dependence of chlorite dissolution rate on pH as well as on degree of undersaturation under conditions applicable to geologic carbon storage. A batch experiment was conducted to determine the equilibrium constant of the dissolution reaction at 100 °C for the chlorite mineral phase used in this study. A series of experiments was run at varying CO2 partial pressures (0-60 bars) and different flow rates (0.01-0.25 ml/min). We use our experimental results in conjunction with previously published data to separate the effects of pH and solution saturation state. At constant pH, the chlorite dissolution rate decreases slowly as a function of the Gibbs free energy of reaction (∆Gr), which is a quantitative measure of the degree of undersaturation. The undersaturation required (− ΔGr > 100 kJ/mol) to reach a limiting far-from-equilibrium rate (the “dissolution plateau”) is roughly an order of magnitude larger than expected from transition state theory, but similar to that determined for smectite (− ΔGr > 130 kJ/mol) (Cama et al., 2000). Hence, chlorite dissolution behaves as if the chlorite were close to equilibrium even in solutions that are strongly undersaturated. The chlorite dissolution rate (R) as a function of pH and chemical affinity (ΔGr) at 100 °C can be fit reasonably well with the following expression: R=10−7.64aH1.05+10−11.561−exp−0.01677ΔGrRT1.3The rate law obtained in this study provides a basis for modeling chlorite mineral dissolution over a range of pH and saturation conditions, which is essential in applications such as subsurface CO2 sequestration.