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Jun 9, 2012 - Dissolution Kinetics and Mechanisms at Dolomite−Water Interfaces: Effects of Electrolyte Specific Ionic Strength. Man Xu,. †. Katie ...
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Dissolution Kinetics and Mechanisms at Dolomite−Water Interfaces: Effects of Electrolyte Specific Ionic Strength Man Xu,† Katie Sullivan,† Garrett VanNess,† Kevin G. Knauss,‡ and Steven R. Higgins*,† †

Department of Chemistry, Wright State University, 3640 Colonel Glenn Highway, Dayton, Ohio 45435, United States Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States

‡

ABSTRACT: Elucidating dissolution kinetics and mechanisms at carbonate mineral−water interfaces is essential to many environmental and geochemical processes, including geologic CO2 sequestration in deep aquifers. In the present work, effects of background electrolytes on dolomite (CaMg(CO3)2) reactivity were investigated by measuring step dissolution rates using in situ hydrothermal atomic force microscopy (HAFM) at 90 °C. Cleaved surfaces of dolomite were exposed to sodium chloride and tetramethylammonium chloride (TMACl) aqueous solutions with ionic strengths (I) ranging from 0 to 0.77 m at pH 4 and pH 9. HAFM results demonstrated that dolomite step retreat rates increased with increasing solution ionic strength and decreasing pH. Comparison of [481̅] and [4̅41] steps revealed that the anisotropy of [481̅] and [4̅41] step speeds became significant as solution ionic strength increased, with NaCl exerting more pronounced effects than TMACl for the same I. To interpret the different trends observed for NaCl and TMACl, a dissolution mechanism involving orientation-dependent ion adsorption and consequent edge free energy changes is proposed.

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INTRODUCTION Dissolution of carbonate minerals is one of the major chemical reactions occurring at the Earth’s surface and subsurface and has significant environmental and geochemical implications. In particular, carbonate dissolution plays a key role in the global carbon cycle and in geologic CO2 sequestration. Chemical weathering of calcite and dolomite, the two major carbonate minerals at the Earth’s surface, accounts for approximately 50% of the chemical denudation of the continent,1 and thus exerts indirect but pronounced influences on global CO2 cycling. Dissolution reactions of carbonate cements in caprocks may facilitate the leakage of CO2 from the sequestration reservoir back to the atmosphere.2 Mg2+ ions liberated from dolomite dissolution reactions are likely incorporated into silicate clay minerals, affecting rock permeability and limiting the sequestration capacity of subsurface reservoirs.3 The chemical reactivity of minerals is known to be largely controlled by processes occurring at the mineral−water interface.4,5 Natural water in contact with minerals often contains various electrolytes. For instance, the ionic strength (I) of pore waters in sedimentary rocks ranges over several orders of magnitude, from dilute meteoric water to water systems with more than 600 g/L of dissolved ions.6,7 The variation in carbonate mineral dissolution rates as a function of solution ionic strength has been studied at room temperature.7−13 The reported experimental results were somewhat contradictory, however. Buhmann and Dreybrodt8 found slightly enhanced dissolution of calcite upon the addition © 2012 American Chemical Society

of up to 5 mM NaCl from both theoretical predictions and experimental results. By using a pH-free-drift method, Gledhill and Morse7 observed ionic strength inhibition on calcite dissolution rates in the I range of 1 to 4 molal (m). Pokrovsky et al.9 measured calcite dissolution kinetics using a batch reactor and results showed minor effect of I on calcite dissolution for NaCl solutions of up to 1 M over pH range of 4−8. Recent atomic force microscopic (AFM) studies by Ruiz-Agudo et al.,11,12 however, revealed that at neutral pH NaCl promoted calcite dissolution when presented in moderate to high concentrations (∼0.01 to ∼6 M). Relative to calcite, the dissolution kinetics and mechanisms of which have been investigated over a wide range of conditions and solution compositions, there have been comparatively few studies on the dissolution processes at dolomite−water interfaces in the presence of dissolved foreign ions. Notable exceptions include a macroscopic study by Pokrovsky et al.9 and a microscopic study by Ruiz-Agudo et al.13 Both were conducted at room temperature and nearly neutral pH. Pokrovsky et al.9 revealed a 2.5-fold increase of bulk dissolution rate for dolomite when I increased from 2 × 10−5 to 0.03 M and little change in dissolution kinetics for 0.03 M ≤ I ≤ 1 M at Special Issue: Carbon Sequestration Received: Revised: Accepted: Published: 110

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Figure 1. AFM topographic (left) and deflection (right) images of etch pits formed on dolomite (101̅4) surfaces during dissolution in deionized water at pH 4.0 and 90 °C. Image size: 5.18 μm × 5.18 μm.

(resistivity ∼18 MΩ cm). Visual MINTEQ software was employed to calculate the ionic strengths and the ion activities of the reacting fluids. The ionic strengths of NaCl and TMACl aqueous solutions varied from 0 to 0.77 m (I = 0, 0.10, 0.37, and 0.77 m). The pH of these solutions was slightly acidic under ambient conditions. 0.1 mol/L HCl and NaOH were employed to adjust the solution pH to 4.0 and 9.0, respectively. In-situ HAFM dissolution experiments were carried out by passing the prepared reacting fluids of NaCl and TMACl at different ionic strengths over dolomite (CaMg(CO3)2) cleavage surfaces at 90 °C. Optically clear dolomite crystals16 were purchased from Ward’s Natural Science Est., Inc. and were used throughout the study. Analysis using inductively coupled plasma optical emission spectrometry (ICP-OES; Varian 710ES) revealed that dolomite contained less than 1 mol % divalent metal impurities. Dolomite specimens of approximately 4 mm × 4 mm × 1 mm in size were cleaved with a razor blade along the (1014̅ ) cleavage. The surface composition of our cleaved dolomite samples had a Ca/Mg ratio of 0.9−1.1 based on X-ray photoelectron spectroscopic results.17 Detailed information on the self-constructed hydrothermal AFM instrument used in the present study can be found in previous publications.18−20 Briefly, the HAFM equipped with a flowing fluid cell allowed a continuous flow of solution to interact with the mineral specimen and enabled the operation at high pressure and temperature. A “wall-jet” was employed in the fluid cell and thus the flow was due to a fluid jet impinging normally on the mineral surface and spreading radially over the surface.21,22 Fluid flow rate (1.4 μg/s) was controlled by a mass flow controller (Porter Instrument) downstream of the cell. Additional AFM experiments at higher flow rates were carried out (AFM images are not shown in this paper) to ensure that under our experimental conditions dissolution kinetics were surface reaction-controlled at a flow rate of 1.4 μg/s. In the present study, the fluid cell was pressurized to approximately 1 bar above ambient using N2 and heated to 90 °C. AFM images were collected in contact mode using uncoated silicon cantilevers (Nanosensors, typical force constant of 0.2 N/m). The scanning frequency was 2−6 Hz, with 512 sampling points per scan line and scanning areas in the range of 4−64 μm2. Measurements of step retreat rates were made from timesequential AFM images scanned in the same direction. Step speeds of the four edges of etch pits and summed step speeds along different directions were quantified and are reported in

pH 7. Ruiz-Agudo et al.13 investigated dolomite dissolution in the presence of different background electrolytes using AFM and found that the etch pit spreading velocities at dolomite cleavage surfaces varied with the solution I and the nature of the background electrolyte. Based on the ion-specific and concentration dependent ionic strength effects, the authors proposed different mechanisms in dilute and concentrated electrolyte solutions: at low I (I = 0.001), dolomite dissolution was dominated by the stabilization of crystal building cation hydration by background electrolytes, whereas at high I (I = 1), the impact of background anions on water structure dynamics controlled dolomite dissolution kinetics. In regard to NaCl, the etch pit spreading rates obtained on dolomite surfaces generally increased with I. Yet, due to the limited data points, the relationship between dolomite step kinetics and solution ionic strength remains unclear, particularly under moderate to high salinity conditions where electrolyte hydration near the surface of the mineral may perturb the interfacial water structure. The objective of the current study was to address the role of interfacial water structure through different perturbations of electrolyte cation hydration spheres and the subsequent effects on step edge dissolution kinetics. In the present work, in situ dissolution experiments were carried out using hydrothermal AFM (HAFM) on dolomite cleavage surfaces in contact with NaCl or tetramethylammonium chloride (TMACl) solution at 90 °C. The elevated temperature employed in this work was similar to the temperature of CO2 sequestration reservoirs, enabling the application of present experimental results to the CO2 sequestration modeling.14,15 Moreover, due to the relatively slow kinetics of dolomite dissolution at room temperature, the elevated temperature increased step retreat rates such that the rate differences became more significant. Solutions at pH 4 and pH 9 were employed to represent the initial stage of CO2-rich fluid injection and the postinjection period in geologic CO2 sequestration.15 To elucidate the roles of background Na+ and Cl− in dolomite dissolution kinetics, parallel HAFM experiments were completed by exposing dolomite surfaces to TMACl aqueous solutions. Orientation dependent step kinetics and etch pit morphology changes are discussed.

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EXPERIMENTAL SECTION Reacting fluids were prepared by dissolving NaCl (high-purity, 99.99+%) or TMACl (reagent grade) into deionized water 111

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this paper. All step speed data were averaged from more than three replicate measurements.

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RESULTS At the dolomite (101̅4)−water interface, dissolution occurs by the formation, spreading, and coalescence of etch pits and by the retreat of the original cleavage steps, as illustrated in Figure 1, which shows a representative AFM image of the etch pits formed in deionized water at pH 4.0 and 90 °C. The etch pits display a rhombic shape, bounded by [4̅41] and [481̅] edges, as typically observed in rhombohedral carbonate minerals. However, unlike calcite (R3̅c) for which the positive (or obtuse) step pair along [4̅41] and [481̅] directions, [4̅41]+ and [481̅]+, are bisected by a c-glide plane, a lack of such plane in dolomite (R3̅) leads to structurally distinct positive steps in [4̅41]+ and [481̅]+ directions.23 This is also true for the two negative (or acute) steps, [4̅41]− and [481̅]−. Therefore, there are four rather than two distinct step speeds on dolomite surfaces that define the kinetic shape of the etch pits. Due to the lack of fixed surface landmarks and lateral drift in AFM experiments, it was impossible in some experiments to determine the retreat rates for the four individual steps. However, the summed speed of two parallel steps was calculated by measuring the distance increase between these steps in time sequential images. The summed step speeds for [4̅41]+ and [4̅41]−, [481̅]+ and [481̅]−, and all four steps, referred to as V[−441], V[48−1], and Vsum, are summarized in Table 1 and Figures 2 and 3. Table 1. Step Retreat Rates (V, nm/s) of Dolomite as a Function of Solution Ionic Strength (I, molal) and pH for NaCl and TMACl IS 0 0.10 0.37 0.77 0.10 0.37 0.77 0 0.10 0.37 0.77 0.10 0.37 0.77

salt NaCl NaCl NaCl TMACl TMACl TMACl NaCl NaCl NaCl TMACl TMACl TMACl

pH 4.0 4.0 4.0 4.0 4.0 4.0 4.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0

V[48−1] 1.8 3.2 5.8 5.2 2.1 3.2 3.4 0.85 1.7 1.8 2.0 1.1 1.6 1.8

± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.2 0.2 1.3 0.3 0.1 0.5 0.4 0.32 0.1 0.1 0.3 0.2 0.3 0.2

V[‑441] 1.7 3.2 3.3 3.1 1.9 3.4 3.1 0.72 1.0 1.2 1.3 0.82 1.2 1.8

± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.5 0.2 0.7 0.2 0.1 0.7 0.5 0.19 0.1 0.1 0.2 0.10 0.2 0.2

Figure 2. Summed step retreat rates on dolomite cleavage (101̅4) surfaces as a function of solution ionic strength (molal) for NaCl and TMACl at pH 4.0 (a) and pH 9.0 (b) at 90 °C.

Vsum 3.5 6.4 9.2 8.3 3.9 6.6 6.5 1.6 2.7 3.0 3.3 1.9 2.8 3.6

± ± ± ± ± ± ± ± ± ± ± ± ± ±

rates for [4̅41] steps. Figure 4 further demonstrates the anisotropy of step speeds in the two crystallographic directions by plotting the step speed ratios (V[48−1]/V [−441]) as a function of solution I and pH. Comparison of [481̅] and [4̅41] step speeds reveals that NaCl has stronger “anisotropic” effects than TMACl. At pH 4.0 (Figures 3a,b and 4a), V[48−1] and V [−441] appear to become significantly different as NaCl solution ionic strength increases. Steps along [481̅] dissolve at nearly twice the speed of the [4̅41] steps for I = 0.37 and 0.77 m. However, when TMACl instead of NaCl is used, [481]̅ and [44̅ 1] steps dissolve at comparable rates in the I range of 0 to 0.77 m. The differences in step speed isotropy for NaCl and TMACl mainly come from the preferential ion detachment along dolomite [481]̅ direction in the presence of NaCl (Figure 3a). For dolomite dissolution in contact with pH 9.0 aqueous solutions, such differences between NaCl and TMACl are less pronounced (Figures 3c,d and 4b). Etch pit morphology and step kinetics are closely correlated with each other. Figure 5 shows representative AFM images of etch pits formed in aqueous solutions of NaCl and TMACl. In contrast to the typical rhombic shaped pits observed on calcite surfaces, the etch pits on dolomite surfaces display a rhombic shape or a rhomboid shape, depending on the solution chemistry. For instance, etch pits formed in 0.77 m NaCl at pH 4.0 (Figure 5a), where V[48−1] is 1.7 times V[−441] (Figure 4a), lose their characteristic rhombic shape and become elongated, acquiring a rhomboid appearance. At the same pH and ionic strength, dolomite etch pits in contact with the TMACl solution exhibit a nearly rhombic shape (Figure 5b),

0.5 0.3 1.4 0.4 0.1 0.8 0.6 0.4 0.2 0.2 0.4 0.2 0.3 0.3

As shown in Figure 2 and Table 1, in general, dolomite step speeds are larger in acidic (pH 4.0) solutions than in alkaline (pH 9.0) solutions. Upon the addition of background electrolytes, step speeds increase for both NaCl and TMACl solutions. At pH 4.0 (Figure 2a), Vsum in the presence of NaCl increases ∼2.5 times as I varies from 0 to 0.37 m with no increase when I is further elevated to 0.77 m. A similar trend is found for TMACl, with TMACl causing smaller increases in Vsum than NaCl. At pH 9.0, NaCl and TMACl produce similar step speeds at a given concentration, as illustrated in Figure 2b. Interestingly, monolayer step dissolution is not increased by the same factor for all crystallographic directions. Figure 3a and c present retreat rates for steps along [481̅] direction at pH 4.0 and pH 9.0, respectively, and Figure 3b and d refer to retreat 112

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Figure 3. Retreat rates for steps along [481̅] and [4̅41] directions on dolomite surfaces as a function of solution ionic strength (molal) for NaCl and TMACl at 90 °C: (a) retreat rates for [481̅] steps at pH 4.0; (b) retreat rates for [4̅41] steps at pH 4.0; (c) retreat rates for [481̅] steps at pH 9.0; and (d) retreat rates for [4̅41] steps at pH 9.0.

1 NaCl,13 which is in qualitative agreement with the observations from the present study. In the range of 0.1 ≤ I ≤ 1, the authors reported a less than 3% enhancement in etch pit spreading speeds at pH 7 and room temperature.13 Based on our AFM results at 90 °C, the catalytic effects of I on step speeds depend on crystallographic directions. As I increased from 0.1 to 0.77 m NaCl, step speeds increased ∼60% for [481]̅ steps and ∼0% for [4̅41] steps at pH 4.0, and ∼ 15% for [481̅] steps and ∼30% for [4̅41] steps at pH 9.0 (Table 1 and Figure 3). Whether pit spreading speeds were determined from the motion of [481]̅ steps, [44̅ 1] steps, or both would result in quantitatively different catalytic effects of ionic strength. This factor as well as the differences in experimental conditions, such as pH and temperature, may contribute to the observed differences in step speed enhancement between Ruiz-Agudo et al.13 and the present work.

consistent with the close to one V[48−1]/V[−441] ratio. At pH 9.0, kinetic anisotropy in steps along [481̅] and [4̅41] correspond to elongated etch pits in NaCl and TMACl systems (Figure 5c for example). Xu et al.24 reported in their Monte Carlo simulations of calcite step orientation that changes in pit morphology could be significant with even minor changes in the relative rates of counter-propagating kink sites so long as the kink density was sufficiently large. In this work, no changes in the pit morphology in terms of the straight step directions were observed across the variety of experimental solution chemistries, which suggests that there could be minimal difference in the detachment rates for counter-propagating kinks. To our knowledge, only a few studies have explored dolomite dissolution kinetics at the nanometer scale.13,16,23,25 Among these studies, Hu et al.16 investigated dolomite step dissolution kinetics in the presence of near-equilibrium aqueous solutions at 25 °C, pH 9, and I = 0.01 M, Urosevic et al.25 probed the effects of solution pH on dolomite dissolution under far-fromequilibrium conditions and room temperature, and Ruiz-Agudo et al.13 examined the effects of background electrolytes at room temperature and pH ∼ 7. None of them, however, has reported the orientation dependent anisotropy in step speeds. Higgins et al.23 found that [481̅] steps retreated at about twice the speed of the [4̅41] steps for near-equilibrium dolomite dissolution at pH 9 and observed etch pit shape deviation from the typical rhombic profile. Their observations generally agree with the results from the present study, although direct comparison is impossible due to different experimental conditions. In Ruiz-Agudo et al.’s AFM study of dolomite dissolution kinetics as a function of ionic strength, enhancement in etch pit spreading speed has been observed when I varied from 0.001 to

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DISCUSSION Traditional Homogeneous Considerations. To understand the enhanced dolomite step retreat rates in contact with NaCl and TMACl aqueous solutions, we first consider how the background electrolytes affect the thermodynamic driving force for the dissolution reaction. The electrostatic shielding from background electrolytes could lead to changes in ion activity coefficients and mineral solubility.8,11,13,26 In accordance with the BCF theory27 and Zhang and Nancollas,28,29 the step retreat rate, V, at the mineral−water interface is a function of the driving force of the reaction V = k(S − 1)/ 2 − S

(1)

where k is the step kinetic coefficient, and S, the saturation ratio, is defined by 113

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equilibrium with CO2 was not reached,20,30 the activity of H2CO3* in solutions would be lower than 10−5. Our ICP experiments revealed trace amounts of Ca 2+ and Mg 2+ impurities ( TMACl > water at pH 4.0 (Figure 2 and Table 1). Figure 3a,b illustrate dissolution rates for steps parallel to [481̅] and [4̅41], while Figure 4a shows the V[48−1] to V [−441] ratios at pH 4.0. Clearly, the differences in the summed step speeds in the presence of NaCl and TMACl (Figure 2a) are mainly attributed to different retreat rates for steps along [481̅]. These observations suggest that background cations, particularly Na+, preferentially interact at certain crystallographically oriented step edges and kink sites, in our case, the [481̅] step edges and kinks along the steps. In the case of TMA+, the fact that V[48−1] to V [−441] ratio remains at 1 (Figure 4a) in our concentration range suggests that the preferential adsorption of TMA+ in the vicinity of [481̅] steps may not occur. Another possibility is that although TMA+ ions tend to accumulate

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AUTHOR INFORMATION

Corresponding Author

*Phone: 1-937-775-2479; fax 1-937-775-2717; e-mail steven. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by the United States Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences and Biosciences Division. We thank Dr. David Dzombak, Dr. Daniel Giammar, and four anonymous reviewers for their valuable comments.

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dx.doi.org/10.1021/es301284h | Environ. Sci. Technol. 2013, 47, 110−118