Dissolution kinetics of natural calcite minerals in CO 2-water systems approaching calcite equilibrium

Abstract

We have measured dissolution rates of various natural calcite samples, e.g marbles, limestones and marine pelagic sediments, in CO 2H 2O solutions of fixed P CO 2 and temperature during their approach to equilibrium with respect to calcite. The runs have been carried out as batch experiments using the free-drift technique ( P CO 2 : 5·10 −3atm; T: 20°C), where size fractioned particles of 100 μm were kept in suspension by turbulently stirring the solution. In each experiment three natural specimens and for reference one of synthetic NBS Calcite (SRM 915) have been investigated. The dissolution rates observed for all natural samples can be represented by an empirical rate law given as R=α 1(1 − C C s ) n1 at Ca 2+ concentrations C< xC s ( n 1 ≈ 1.5−2.3, α 1 ≈ 1.6·10 −7−2.2·10 −7 mmol cm −2 s −1, x ≈ 0.6−0.8). Above this concentration the empirical reaction order switches to a higher value of n 2 ≈ 3–4. In contrast, NBS Calcite exhibits a linear rate law of R=α(1- C C s ) with α ≈ 1.6·10 −7 mmol cm −2s −1, which closely follows the dissolution rates predicted by the mechanistic dissolution model (PWP model) of Plummer et al. (1978). Experiments on NBS Calcite in a solution containing 10 μM KH 2PO 4 exhibited results very similar to natural specimens, displaying inhibited dissolution with respect to synthetic calcite, when approaching equilibrium. Only minor influence of phosphate was observed on the dissolution rates of natural samples. From this observation we suggest a model in which Ca 2+ ions are adsorbed to lattice sites (kinks) active to dissolution, blocking further dissolution there. By an attractive interaction between these ions and impurities (heavy metals or phosphate) on the surface of natural materials their adsorption enthalpy increases with increasing coverage Θ of physisorbed Ca 2+ ions on the surface. From this we derive a rate law: R EMP (C)=(1−θ)R PWP ; C=[ θ (1−θ) ]ƒ exp [ −(U o+U o′θ) K B T ] where R PWP is the rate predicted by the PWP model; and (1 − Θ) gives the amount of surface sites still active to dissolution, which is represented by the Fowler-Frumkin isotherm. Our results show that the binding enthalpy U o is a property of the pure calcite surface, whereas U o′ is dependent on the origin of the materials. Using this rate equation we have examined a variety of calcite dissolution data published in the literature and, in all cases, have found them to obey closely the suggested rate law. Especially the earlier published data on inhibition of calcite dissolution by heavy-metal ions, e.g. Cu 2+ and Sc 3+, fit well into our model. It reconciles with many apparently conflicting results and explains the dissolution rates from the chemical reactions proposed by L.N. Plummer and coworkers and adsorption processes of the solute, both acting simultaneously.