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DIVISION S-2-SOIL CHEMISTRY

Solubility and Dissolution Kinetics of Dolomite in Ca–Mg–HCO₃/CO₃ Solutions at 25°C and 0.1 MPa Carbon Dioxide

^a Dep. of Chemistry, Washington College, 300 Washington Ave., Chestertown, MD 21620-1197 USA
^b Dep. of Soil Science, Univ. of Wisconsin-Madison, 1525 Observatory Dr., Madison, WI 53706-1299 USA

leslie.sherman{at}washcoll.edu

Dolomite solubility in water has been measured by a number of different methods during the past several decades, yielding inconsistent and unreliable results that vary more than three orders of magnitude. The most commonly used best value for dolomite solubility in water is based on HCl solution calorimetry at 300.15 K, which is not confirmed by earlier determinations based on heat capacity of dolomite nor by more recent acid solution and metal oxide melt calorimetric measurements. In this study, the solubility of a high purity dolomite was measured directly by monitoring dissolution in Ca-Mg-HCO₃/CO₃ solutions designed to bracket the presumed solubility product of dolomite, pK_{s dolomite} [= -log (Ca²⁺)(Mg²⁺)(CO^2-₃)², where the values in parentheses are activities at equilibrium], between 16.0 and 19.0, at 0.101 MPa (1 atm) CO₂ and 25°C. The use of gas-permeable, water-impermeable membranes over the dissolution vessels allowed for maintenance of an open system for CO₂, with minimal water loss during the course of the 672-d experimental period. The dolomite dissolved congruently in Ca-Mg-HCO₃/CO₃ solutions with initial ion activity products [pIAP_dolomite = -log (Ca²⁺)(Mg²⁺)(CO^2-₃)², where the values in parentheses are measured activities] greater than 17.5. Both calcite and magnesian calcites can be ruled out as controlling solubility in these measurements. Based on statistical inference by comparison of alkalinity in unseeded (control) and dolomite-seeded solutions, the pK_{s dolomite} is between 17.4 and 17.0, expressed as 17.2 ± 0.2. A previously proposed kinetic model of successive reactions for dolomite dissolution near equilibrium—a fairly rapid dissolution of the CaCO₃ component in equilibrium and a rate-limiting protonation reaction dependent on the activity of the MgCO₃ component—appears to fit the experimental data.

Abbreviations: BET, Brunauer–Emmett–Teller • EDX, energy dispersive x-ray • IAP, ion activity product • XRD, x-ray diffraction

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