The dissolution of calcite in aqueous solution at pH < 4: kinetics and mechanism

Abstract

The requirements of any experimental investigation of a reaction, such as dissolution or precipitation, at the solid-liquid interface are delineated, and it is shown that no previously adopted experimental method successfully meets all of these criteria. A new strategy for studying such reactions is therefore proposed, and described specifically for the dissolution kinetics of calcium carbonate in acidic aqueous solution. This technique involves locating a calcite crystal in part of one wall of a rectangular duct, through which reactant flows under laminar conditions, and positioning either an amperometric or potentiometric detector electrode immediately adjacent to and downstream of the crystal. The rate of the reaction at the crystal surface is then followed by measuring the concentration of the reactant or products reaching the electrode as a function of solution flow rate. In this way both transport (to the crystal and detection system) and the surface topography of the crystal are defined and controllable, and, most importantly, the concentrations interrogated by the detector electrode are those pertaining at the crystal surface and not the bulk values, which are those probed with other (previous) methods. A mechanistic model for the dissolution reaction at pH &lt; 4, H + + CaCO 3 → C a 2 + + HCO 3 − , is developed, and it is recognized that half-, first- and second-order reactions in surface H + constitute possible candidate heterogeneous processes. The model also takes the following homogeneous (solution) reactions into account: HCO 3 − + H + ⇌ H 2 C O 3 ⟶ k H 2 C O 3 C O 2 + H 2 O , K H 2 C O 3 = a H + a HCO 3 − / a H 2 C O 3 The backward implicit finite difference method is used to provide theory for the proposed flow cell experiment. Specifically this relates the rate of an n th order process at the crystal surface to the transport-limited current at an amperometric detector electrode, as a function of solution flow rate and cell-crystal-electrode geometry. The computational method selected allows both the full parabolic velocity profile and any value of n to be considered. The validity of the numerical approach is checked by invoking the Lévêque approximation (linearizing the parabolic velocity profile) and solving the problem analytically for n = 0 and 1. The numerical method is extended so that both parallel heterogeneous and homogeneous reactions are covered, specifically so that theory describing the mechanistic model can be derived as related to the flow cell experiment. The design and construction of the necessary experimental apparatus is described, along with experiments devised to prove the validity of the experimental approach. The results of flow cell dissolution experiments carried out with 0.25–1.0 mM HCl solutions are presented, and it is shown for the first time that the reaction between H + and calcite is not transport-controlled. Assuming k H 2 C O 3 = 20 s − 1 and K H 2 C O 3 = 3.06 × 1 0 − 4 M , a best fit to the data is obtained for a first-order heterogeneous process with a rate constant of (0.043 ± 0.015) cm s -1 .

The fate of calcium carbonate nanoparticles administered by oral route: absorption and their interaction with biological matrices

“…The higher dissolution of CaCO 3 under gastric condition is in good agreement with previous reports on pH dependent dissolution of CaCO 3 . [33][34][35] On the other hand, the dissolution of Ca 2+ was not significantly higher for N-Cal in vitro, whereas N-Cal was dissolved markedly more by ex vivo and in vivo fluids, in-line with expectation based on its considerably larger specific surface area (specific surface area =15.8 m 2 /g versus 0.83 m 2 /g). It is likely that in vitro simulated fluids do not completely reflect in vivo biological conditions, and thus caution is required to interpret in vitro results.…”

Evidence and Potential Implications of Exponential Tails to Concentration Versus Time Plots for the Batch Dissolution of Calcite

“…Morse 1974Morse , 1978Berner and Morse 1974;Sjöberg 1978;Dreybrodt 1981;Buhmann and Dreybrodt 1985a, b;Compton et al 1986;Compton and Daly 1987;Compton and Pritchard 1990;Compton and Unwin 1990;Brown et al 1993;Raines and Dewers 1997;Morse and Arvidson 2002;Arvidson et al 2003;Cubillas et al 2005;Morse et al 2007; Arvidson and Lüttge 2010;Xu et al 2012), with application both to the maintenance of the quality of drinking and agricultural water, and to the study of landforms, e.g. karst scenery (Wicander and Monroe 2009;Berner and Berner 2012).…”

“…Also, recent accelerated interest in massive injections of carbon dioxide into geologic formations as part of long-term carbon sequestration must depend upon reliable information on the dissolution kinetics of both bulk minerals and their cements (Noiriel et al 2009). The dissolution kinetics of calcium carbonate have been studied in widely different ways including, for example: simple and mixed-flow batch reactors (Cubillas et al 2005); other reactors (Kaufmann and Dreybrodt 2007;Noiriel et al 2009), free-drift experiments (Sjöberg 1976;Plummer et al 1978;MacInnis and Brantley 1992;Arakaki and Mucci 1995;Eisenlohr et al 1999), reactors with or without bubbled CO 2 (Plummer and Wigley 1976;Plummer et al 1979;Svensson and Dreybrodt 1992;Dreybrodt et al 1996), the chemostat (Morse 1974(Morse , 1978Dreybrodt 1981;Buhmann and Dreybrodt 1985a, b;Compton et al 1986;Compton and Daly 1987;Compton and Pritchard 1990;Compton and Unwin 1990;Brown et al 1993; Morse and Arvidson 2002;Arvidson et al 2003;Cubillas et al 2005;Morse et al 2007); the pH-stat (Morse 1974(Morse , 1978Rickard and Sjöberg 1983); atomic force microscopy (AFM) (Liang et al 1996;Shiraki et al 2000;Lea et al 2001), and the rotating disc apparatus (Sjöberg 1976;MacInnis and Brantley 1992;Rickard and Sjöberg 1983;Sjöberg and Rickard 1983…”

Direct Determination of Dissolution Rates at Crystal Surfaces Using 3D X-ray Microtomography

“…Study of the cleaved {101̅4} surface of calcite, for instance, has permitted investigation of the role of solution chemistry in the formation of etch pits and the kinematics of step retreat during dissolution under a large variety of experimental conditions. ,,,,− However, face-specific measurements are often restricted to small surface areas (typically less than 500 × 500 μm 2 ) at the single crystal face, may have limited vertical range (e.g., about 5 μm for AFM), and ignore the contribution of the crystal edges and corners to the whole process, so that the possible variation in reaction mechanisms is restricted to nanometer- to micrometer-scale observations, and upscaling of the observations and rates determined from 2D techniques on single faces to mineral, porous rock, or reservoir scale remains challenging.…”