Dissolution kinetics and topographic relaxation on celestite (0 0 1) surfaces: The effect of solution saturation state studied using Atomic Force Microscopy
Introduction
Mineral dissolution processes have significant geochemical, environmental and biological impacts; and thus exert essential controls over geochemical cycles. Of all minerals, reactions at the interfaces of naturally occurring sulfate minerals and water control important aspects of the sulfur geochemical cycle. Geochemical transformations of sulfur in sediments impact processes like early sedimentary diagenesis, conditions for mineral deposition and the global cycling of sulfur (Goldhaber, 2004). Celestite (SrSO4), a naturally occurring sulfate mineral, is usually found in marine sedimentary deposits and sometimes in groundwater aquifers (Hanor, 2000, Chapelle, 2004). Celestite is highly reactive at room temperature compared to the isostructural mineral barite (e.g., Dove and Czank, 1995) and dissolution and precipitation of celestite likely plays an important role in determining the Sr composition in natural waters.
Understanding the kinetics of dissolution of minerals at different experimental conditions permits a description of mineral–water interactions in natural systems, for example, in ground and surface water, or in soils and sediments. The general form of the rate equation used in most mineral dissolution studies is (Lasaga, 1981):where rate is typically in units of mol cm−2 s−1 and Ω in Eq. (1) represents the solution saturation state:In the above expressions, k is the rate coefficient with units depending on the reaction order, n, a constant which may relate to the reaction mechanism if appropriate constraints are placed on the system of interest (Teng et al., 2000). [Sr2+] and are the activities of Sr2+ and ions, respectively, and Ksp represents the solubility product of the celestite. In Eq. (1), the kinetic behavior of a mineral reaction with n = 1 describes either adsorption-controlled or transport-controlled growth and dissolution (Nielsen, 1983). With n = 2, the rate equation may be indicative of spiral growth and dissolution at screw dislocations (Brantley, 2004). From a fundamental mechanistic perspective, simple integer power laws such as Eq. (1) provide a test for experimental consistency with certain proposed mechanisms (Nancollas et al., 1973), however, n has often been used as an adjustable parameter for empirical fitting of data (Reddy and Nancollas, 1973, Reddy, 1977, House, 1981, Blum and Lasaga, 1987, Shiraki and Brantley, 1995).
To model trends in dissolution rate as a function of saturation state in which Eq. (1) models do not apply, Lasaga and Lüttge (2001) proposed a different rate equation on the basis of dissolution of steps originating from dissolution etch pits. The model features non-linear variation in the reaction rate with chemical potential and is expressed as:Three parameters, A, B and Ωcrit describe the relationship between dissolution rate and Ω where Ωcrit is the critical saturation state below which etch pit formation at linear defects is spontaneous, A is the dissolution rate far from equilibrium, and B is related to the surface diffusion distance, xs, expressed in molecular or lattice units (Lasaga and Lüttge, 2001).
Dissolution generally occurs on mineral surfaces by the nucleation of dissolution etch pits and by the retreat of surface steps. Previous studies show that formation of etch pits promoting mineral dissolution occurs on the exposed mineral surfaces at discrete active sites (Helgeson et al., 1984, Brantley et al., 1986). The “active sites” include cleavage fractures, dislocations and grain boundaries (Helgeson et al., 1984). Studies of celestite dissolution have characterized both the mineral’s microscopic and macroscopic dissolution characteristics. The rate of celestite (0 0 1) dissolution was measured by AFM (6.3 × 10−12 mol cm−2 s−1) in deionized water at ∼30 °C (Dove and Platt, 1996). Related studies on sulfate powder specimens by Dove and Czank (1995) described the dissolution kinetics of celestite, barite (BaSO4) and anglesite (PbSO4) over a wide range of pH and temperatures. Because AFM-based rates are typically sampled from surface areas of order 100 μm2 whereas powder-based rates result from areas often exceeding 100 cm2, and mineral surfaces may possess a wide range of heterogeneities such as microfacets, grain boundaries, dislocations and microfractures (Schott et al., 1989), comparison of rates in the two methods is not immediately valid.
Mineral surface behavior and reaction kinetics are influenced by solution saturation state, affecting both nucleation of etch pits and dissolution at step edges. A recent study on calcite demonstrated the effect of saturation state on the generation of etch pits (Teng, 2004), however, prior studies on celestite dissolution have not explored this effect. In the calcite study, in nearly saturated solutions (Ω > 0.54), no etch pit formation was observed and dissolution proceeded at the existing steps. Further from equilibrium (Ω ≅ 0.50), etch pits were generated at presumed linear defect sites. A dramatic rise in etch pit density was noted at Ω ≅ 0.007 where random two-dimensional etch pit nucleation was observed. The findings presented above in light of the theoretical dissolution kinetics models indicate a need to examine effects on dissolution kinetics of other minerals to eventually formulate more generally applicable rate equations describing dissolution in natural systems (Teng, 2004).
Here, the investigations are aimed at addressing the basic hypothesis that dissolution models based on step edge dissolution do not adequately predict behavior under conditions where defect-driven etch pit nucleation is significant. To test the applicability of the models discussed above in the dissolution of celestite, a series of experiments were conducted using fluid cell AFM as a means to simultaneously measure dissolution rates and observe the surface microtopography at varying degrees of solution undersaturation.
Section snippets
Experimental
The celestite (SrSO4, space group: Pnma; a = 8.360 Å, b = 5.352 Å, c = 6.858 Å and Z = 4 (e.g., Seo and Shindo, 1994)) mineral specimens used in the experiments were colorless and clear to slightly bluish green in color. Fresh (0 0 1) surfaces of celestite were obtained by cleaving the crystals with the aid of a knife-edge along the natural cleavage planes. The dimensions of the mineral specimens prepared by this method were approximately 0.3 cm × 0.5 cm × 0.1 cm.
The instruments used to conduct the experiments
Results
The dissolution rate of celestite (0 0 1) as a function of undersaturation (Ω = 0.01–0.63) was determined using AFM by observing the displacement of step positions as functions of time in combination with the step density. By varying the flow rates during experiments, the effect of mass transport on dissolution rates was determined. Fig. 1 shows a graphical representation of the dissolution rates (Ω = 0.01 and Ω = 0.63) at different flow rates, indicating that flow rate did not significantly affect
Discussion
AFM observations on etch pit formation (Fig. 2) and dissolution processes (Fig. 3) showed that the surface reaction occurred by the retreat of steps parallel to 〈0 1 0〉 and 〈1 2 0〉 directions. The step speeds along both these crystallographic directions increased with increase in solution undersaturation (Fig. 4). The observation that 〈0 1 0〉 step velocities were lower than 〈1 2 0〉 step velocities, at fixed Ω, can be explained by the atomic structure along the two step directions.
The 〈1 2 0〉 steps are
Summary
The AFM investigations presented above demonstrate that linear rate equations for dissolution do not apply over broad ranges of undersaturation for the celestite–water interface. With regard to the dissolution of 〈120〉 and 〈010〉 step edges, the undersaturation at which step speeds trended toward zero was far below the bulk equilibrium condition, suggesting a likely influence from impurities at the interface. Therefore, prediction of near-equilibrium step speeds presents significant challenges
Acknowledgments
The authors gratefully acknowledge the financial support of this work by the American Chemical Society Petroleum Research Fund, the Chemical Sciences, Geosciences and Biosciences Division, Basic Energy Sciences, Office of Science, Department of Energy, and the National Science Foundation, Geoscience Division. Three anonymous reviewers are acknowledged for providing very helpful criticisms and suggestions for improving the original manuscript.
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