An evaluation of rate equations for calcite precipitation kinetics at [formula omitted] less than 0.01 atm and pH greater than 8

Abstract

We used a reproducible seeded growth technique with a pH-stat to study the kinetics of calcite precipitation at 25°C. We performed different experiments at initial Ca 2+ and HCO 3 − concentrations ranging from 0.7–2 and 4–7 mmol L −1, pH values ranging from 8.25 to 8.70, pCO 2 values ranging from 0.0006 to 0.01 atm, and ionic strengths ranging from 0.015 to 0.10 mol L −1. With this experimental data set, we used initial rate measurements and integral methods to test several precipitation rate equations. Rate equations that possess a disequilibrium functional dependence, such as the BURTON et al. (1951) dislocation model, forms of the Davies and Jones (1955) model, and the model used by Reddy and Nancollas (1973), did not adequately describe the kinetics of calcite precipitation at pH greater than 8 and pCO 2 less than 0.01 atm. Rate equations that describe independent dissolution and precipitation mechanisms with elementary reactions, such as the equation presented by Plummer et al. (1978), and nancollas and Reddy (1971) were more successful. However, Plummer's model did not adequately describe the rate of all experiments due to the presence of an OH − surface term in the precipitation rate equation. The elementary reaction of the Nancollas and Reddy model is written in terms of bulk Ca 2+ and CO 3 − concentrations, and appears to be the most successful model which describes calcite precipitation at pH > 8 and pCO 2 < 0.01 atm. The Nancollas and Reddy model, altered to account for varying ionic strengths, adequately described the rate of all experiments and yielded a precipitation rate constant of 118.2 ± 13.9 dm 6 mol −1 m −2 s −1, with an apparent Arrhenius activation energy of 48.1 kJ mol −1.