Article http://pubs.acs.org/journal/aesccq
The Carbonatation of Anhydrite: Kinetics and Reaction Pathways Alexander Altree-Williams,† Allan Pring,‡ Yung Ngothai,† and Joel̈ Brugger*,§ †
School of Chemical Engineering, University of Adelaide, North Terrace, Adelaide, South Australia 5000 Australia School of Chemical and Physical Sciences, Flinders University, Bedford Park, South Australia 5042, Australia § School of Earth, Atmosphere and the Environment, Monash University, Clayton, Victoria 3800 Australia ‡
S Supporting Information *
ABSTRACT: A textural and kinetic investigation of the carbonatation of anhydrite in the presence of carbonate- and bicarbonatebearing solutions under static conditions was conducted. The replacement occurs via a coupled dissolution−precipitation mechanism. Textural and kinetic evidence indicates that the rate-limiting step in the replacement reaction was the dissolution of anhydrite and that the dissolution rate was likely controlled by the diffusion of ionic species in the aqueous phase. Calcium carbonate polymorphism was sensitive to temperature and solution composition. Bicarbonate-bearing solutions up to 80 °C only produced calcite, but aragonite formed alongside calcite in carbonate-bearing solutions, occurring in trace amounts at 25 °C and becoming the dominant polymorph at temperatures ≥60 °C. Furthermore, within the carbonate-bearing solutions (high pH) at elevated temperature, kinetic and textural evidence indicates that competition between calcite and aragonite nucleation and growth plays a greater role in defining the mineralogy and textures of the products than an aragonite to calcite ripening process such as the one previously reported for the carbonatation of gypsum. A lack of crystallographic relationship between the aragonite and calcite that formed at elevated temperatures, along with an apparent stabilization of the calcite/aragonite ratio at the early stages of the replacement, highlight the importance of the kinetics of precipitation (via nucleation and growth) and the role temperature and solution composition can play in stabilizing metastable product phases during mineral replacement reactions. KEYWORDS: Coupled dissolution−precipitation, fluid-mediated replacement, textures, calcium carbonate, calcium sulfate, pseudomorphism, mineral replacement, kinetics
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INTRODUCTION Understanding the (trans-)formation of carbonate and sulfate minerals (reaction mechanism, kinetics, and effect on porosity) under diagenetic conditions is of high interest due to their close association with hydrocarbon reserves. In particular, sulfate-rich evaporites containing gypsum and anhydrite have been linked to the formation of carbonate rocks via interaction with infiltrating carbonate-rich fluids.1,2 Indeed, carbonatation of sulfate-rich evaporite deposits is widespread, playing an important role in the formation of diagenetic carbonates.3−5 Carbonatation of calcium sulfate minerals requires the presence of carbonate species (HCO3−/CO32−) in the fluid phase. Two major natural sources of carbonate have been identified as the main drivers for sulfate mineral alteration. First, infiltrating carbonate-rich meteoric waters that form as a result of the dissolution of atmospheric CO2(g) or upstream carbonate minerals.5,6 Second, carbonate derived from the oxidation of hydrocarbons during thermochemical or bacterial sulfate reduction.4,7 Besides being an important mineral in evaporite deposits, anhydrite is commonly observed in carbonate formations as cap-rocks, nodules, and cements.8 The presence of small amounts of sulfate minerals such as gypsum and anhydrite can © XXXX American Chemical Society
sour hydrocarbons and limit the efficiency of chemical enhanced oil recovery (EOR).9−11 In particular, the effectiveness of alkaline-surfactant polymer (ASP) flooding can be drastically reduced in the presence of sulfate minerals.1,10,12−14 Sodium carbonate, a cost-effective alkali, is primarily injected as a means of controlling carbonate mineral surface charge (zero point of charge occurs near pH 8)15 in order to minimize the adsorption of anionic surfactants during ASP flooding.16−18 An important consequence of the consumption of the injected alkali via the reaction of calcium liberated by the dissolution of calcium sulfate minerals is the loss of control on the surface chemistry. This results in increased absorption of anionic surfactants, lowering the surfactant recovery and decreasing efficiency of the EOR process.16,18,19 Many mineral replacement reactions in nature are fluid-driven and proceed via a coupled dissolution−precipitation (CDP) mechanism.20,21 Re-equilibration of a mineral phase in the presence of Received: Revised: Accepted: Published: A
December 14, 2016 February 6, 2017 February 10, 2017 February 10, 2017 DOI: 10.1021/acsearthspacechem.6b00012 ACS Earth Space Chem. XXXX, XXX, XXX−XXX
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ACS Earth and Space Chemistry a fluid offers a kinetically favorable pathway for equilibration relative to solid-state mechanisms, and CDP reactions are particularly prominent at environmental to hydrothermal conditions (25−500 °C).22,23 The saturation of the interfacial solution with respect to a secondary, more stable phase as a result of the dissolution of the parent phase drives the re-equilibration process. Coupling between dissolution and precipitation occurs over a range of length scales with nanometer-scale replacement and preservation of external dimensions, that is, pseudomorphism, occurring when the rate-limiting step is the dissolution rather than the precipitation step, that is, precipitation is much faster than dissolution.24,25 Continued replacement is facilitated by replacement textures such as reaction-enhanced porosity and reaction-induced fracturing, which maintain continued interaction between the infiltrating fluid and the parent phase. Porosity and fracturing are generally controlled by complex interactions between the volume change of the reaction, nucleation, and growth rates of the product(s) and mineral solubility. Overall, the complexity of CDP arises from the interfacial nature of the replacement. The kinetics involved are highly coupled, facilitating the formation of complex textures even when temperature, pressure, and bulk composition undergo relatively simple evolution.22,26−28 This study has two main goals. First, we quantify the kinetics of the replacement of anhydrite in carbonate-bearing solutions (CO32− and HCO3− dominated) at temperatures relevant for diagenesis (25−80 °C) and aim to elucidate the reaction mechanisms driving the replacement through kinetic and textural observations. This was achieved by conducting a number of isothermal experiments over a range of temperatures from 25 to 80 °C. The second goal is to extend the investigation of the carbonatation of gypsum by Fernández-Dı ́az et al.3 to the anhydrite system and consider the evolution of the products across the range of temperatures considered in the kinetic investigation.
where g(y) represents the empirical rate law that depends on the extent of the reaction, k is the empirical rate constant with dimensions of inverse time, and t is the time of reaction. Full details of the different empirical models considered can be found in Khawam.38 Whereas the models of the form defined by eq 3 allow for an empirical interpretation of replacement kinetics based on the extent of reaction, more physically realistic models need to account for parameters relating to solution chemistry and reactive surface area. In particular, the kinetics of CDP replacements are strongly coupled to the chemistry of the solution at the local solid−fluid interface.20−23 Putnis30 expresses this general form as r = k+f (C)
where r (mol s−1 g−1) is the reaction rate, k+ is the rate constant whose units depend on the kinetic model, and f(C) represents the empirical kinetic model. The kinetic description of CDP replacements is difficult due to their heterogeneous nature. To gain a full understanding of the kinetics, one must gain an understanding of the interaction among the parent phase dissolution, mass transport of species to and from the reaction interface, and precipitation (i.e., nucleation and growth) of the secondary phases. The temperature dependence of the reaction rate constant generally follows an Arrhenius relationship ⎡ EA,app ⎤ k = A exp⎢ − ⎥ ⎣ RT ⎦
KINETICS BACKGROUND The isothermal kinetics of a wide range of mineral transformations have been found to be described by the Avrami equation.29 The Avrami equation is given by (1) −1
where y (dimensionless) is the reaction extent, k (s ) is the Avrami rate constant, and n (dimensionless) is the Avrami exponent, which depends on the reaction mechanism.30−33 By taking the natural logarithm twice, eq 1 can be linearized ⎡ ⎛ 1 ⎞⎤ ln⎢ln⎜ ⎟⎥ = n ln(k) + n ln(t ) ⎢⎣ ⎝ 1 − y ⎠⎥⎦
⎛ A ⎞ EA,app −ln(t ) = ln⎜ ⎟− RT ⎝ g (y ) ⎠
(2)
A Sharp-Hancock plot, which plots ln(ln[1/(1 − y)]) against ln(t), will yield a straight line when the kinetics conform to the Avrami equation with a constant slope indicating a constant reaction mechanism throughout a transformation.30,34 The Avrami exponent n and rate constant k can be obtained from the slope and intercept, respectively.35,36 Interpretation of the reaction exponent, n, obtained from the Sharp-Hancock plots can provide useful information for characterizing nucleation processes if a detailed understanding of the reaction mechanism exists.37 In addition to the Avrami equation, several other empirical models are considered. Integrated rate equations are represented by the relation30
g (y) = kt
(5)
where A is the pre-exponential factor with the same units as k, EA,app (J mol−1) is the apparent activation energy, R (8.314 J mol−1 K−1) is the gas constant, and T (K) is the absolute temperature. As noted by Putnis,30 mineral replacements involve a number of elementary steps, each with their own activation energy, and EA,app likely represents either the activation energy of the rate-limiting process or is some combination of the activation energies of a number of parallel processes. The activation energy and pre-exponential factor can be found from a plot of ln(k) against 1/T. As discussed by several authors, the major limitation of this method is that the calculated activation energy depends on the model selected and represents the average value throughout the reaction.30,39,40 An alternative, model-independent method determines the activation energy using an isoconversional technique in which the activation energy is determined for several different values of the reaction extent. Inserting eq 5 into eq 3, taking the natural logarithm of both sides, and rearranging yields
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y = 1 − exp[−(kt )n ]
(4)
(6)
For a given reaction extent, the plot of −ln(t) against 1/T will yield the activation energy independent of any empirical kinetic model, although the method by itself is unable to determine the value of the pre-exponential value.30 By combining the isoconversional and model-based activation energy methods it is possible to select the appropriate model using the activation energy calculated from the isoconversional technique. Full details can be found in Khawam and Flanagan39 and Pedrosa et al.40
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METHODOLOGY Samples. Natural anhydrite (South Australian Museum sample number G18740) from B-Block, Olympic Dam, Roxby
(3) B
DOI: 10.1021/acsearthspacechem.6b00012 ACS Earth Space Chem. XXXX, XXX, XXX−XXX
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where yt, y0, and y∞ are the fractions of calcium carbonate at times t, initially (t = 0), and at the end of the reaction (taken as y∞ = 1 in this case as the reaction is irreversible under the given conditions). Error bars on determined reaction extents represent the uncertainty of the quantitative phase analysis. Scanning Electron Microscopy (SEM). The textural features of the replaced grains were investigated using a Philips XL30 Field Emission Scanning Electron Microscope (FESEM) at Adelaide Microscopy, University of Adelaide. The FESEM operating voltage was 20 kV with a working distance of 10 mm. Textural investigation of partially replaced grains were carried out by embedding the sample in epoxy resin, sectioning, and polishing down to 1 μm diamond paste. The sample was then carbon-coated. Surface features were investigated using secondary electrons with the sample mounted onto aluminum stubs (12 mm diameter) using double-sided carbon tape. Electron Backscatter Diffraction (EBSD). EBSD was utilized to differentiate the crystalline calcium carbonate polymorphs that formed during experiments at elevated temperature and investigate textural relationships between phases. Analysis was performed using an Oxford HKL EBSD system fitted to a Phillips XL30 FESEM at Adelaide Microscopy, the University of Adelaide. Samples were polished with 3 μm diamond paste for 6 min, followed by 0.04 μm colloidal silica for 60−120 min using a Struers TegraPol automatic polisher with tegraForce-1 attachment. A very thin layer of carbon was evaporatively applied to the sample surface to prevent charging during the scanning process. Samples were placed at a tilt angle of 70° prior to sealing the chamber with a working distance of 20 mm and accelerating voltage of 20 kV used to capture patterns. EBSD patterns were recorded using a Nordlys camera and the HKL channel 5 software package was used for data collection. Crystal structures were taken from ICSD (see above). MTEX,42 a MATLAB toolbox for quantitative texture analysis, was utilized to evaluate the collected EBSD data.
Downs, South Australia, was used throughout the study. Mineral phases were confirmed by X-ray diffraction. Samples were ground and sieved into appropriate size fractions. Fractions utilized were +1.4−2.0, +0.6−1.0, and +0.3−0.56 mm. Note that some initial microfractures observed when viewing samples in cross-section formed as a result of sample preparation. Solutions. Solutions were prepared using ultrapure Milli-Q water (resistivity 18.2 MΩ cm−1; Direct-Q3 system, Millipore Corp) and analytical grade reagents (>99.0% sodium carbonate, Sigma-Aldrich; >99.0% sodium bicarbonate, Chem-Supply Pty. Ltd.). The pH was measured using a temperature-corrected pH meter (EUTECH Scientific, model cyber-scan 510) with a Ag/AgCl pH electrode. The ionic strength was not buffered during the experiments with differing solutions. Table 1 summarizes the solutions utilized during carbonatation experiments. Table 1. Outline of the Reaction Conditions for the Kinetics Studies solution
component
C (mol/kg H2O)
temperature (°C)
pH
X
Na2CO3
0.5
Y Z
Na2CO3 NaHCO3
0.1 0.5
25 40 60 80 25 25 80
11.53 11.20 10.70 10.03 10.80 8.17 8.03
Water-Bath Experiments. Approximately 10−15 mg of sieved anhydrite was added to a 5 mL test tube into which a temperature-equilibrated carbonate solution of volume sufficient to achieve the desired solid to fluid ratio was added. During the experiments, two water-baths were used. The first made use of a Grant heat circulator (model no. GD120-S38) with temperature stability of ±0.05 °C. The second water-bath (SEM (SA) Pty. Ltd. model no. WB7D) used a PID controller to provide a stability of ±0.2 °C. Samples were left within the water-baths for the specified reaction periods. The samples were not stirred or agitated during the reaction period. Postreaction, reacted solids were removed from solution and quickly dried. Multiple samples were run for each time period in order to quantify uncertainty of the kinetic parameters during curve fitting. Reaction periods ranged from 5 min to 28 days. X-ray Diffraction. Room-temperature powder X-ray diffraction (XRD) patterns of samples were obtained using a Huber Guiner Image Plate G670 with CoKα1 radiation (λ = 1.78892 Å). Approximately 5−10 mg of sample was ground under acetone in an agate mortar and spread thinly across a polyethylene terephthalate film. Diffraction data were collected for 10 min. The extent of reaction was calculated using the mole fractions of calcium carbonate minerals and anhydrite obtained by quantitative phase analysis using the program TOPAS,41 making use of data in the 2θ range of 4−100°. A pseudo-Voigt function was used to fit peak shapes, whereas a sixth-order Chebychev polynomial modeled the background. The zero-shift and scale factors were refined with crystal structure data taken from the Inorganic Crystal Structure Database (ICSD; anhydrite no. 183918; calcite no. 16710; aragonite no. 157993). Reaction extent (y) was calculated using the mineral mole fractions according to eq 7 y − y0 y= t y∞ − y0 (7)
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RESULTS Textures and Nature of the Products. The reaction between anhydrite and the carbonate-bearing fluid is clearly seen in the optical micrographs presented in Figure 1. The unreacted anhydrite (a) is clear and colorless, whereas the post reaction sample (b) is cloudy and white. The reaction progress can be quantified using powder XRD patterns (Figure 2), which clearly indicate the partial
Figure 1. Optical microscope images of (a) unreacted anhydrite and (b) partially reacted anhydrite covered in porous calcium carbonate polymorphs. Preservation of external dimensions indicates a pseudomorphic replacement. Reaction conditions: 60 min, 0.5 mol/kg Na2CO3, 80 °C. C
DOI: 10.1021/acsearthspacechem.6b00012 ACS Earth Space Chem. XXXX, XXX, XXX−XXX
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replacement front. Comparing Figure 4 panels b and e,f, it is evident that the size of the calcium carbonate crystallites growing on the outside of the grains is larger for the lower-temperature replacements. Similarly, the low-temperature replacement results in the formation of much coarser porosity compared to the higher temperature replacement (compare Figure 4 panel a and c). Both calcite and aragonite formed as a result of anhydrite carbonatation in our experiments, but the nature of the products was found to be sensitive to solution composition and temperature. Experiments conducted using solutions of sodium bicarbonate showed no sign of aragonite based on SEM micrographs (Figure 5c) as well as powder XRD results. XRD analysis suggests that aragonite is the predominant form of calcium carbonate for experiments conducted at 60 and 80 °C in sodium carbonate solutions (Figure 2). The typical aragonite morphology was also seen in SEM micrographs of the products obtained at 60−80 °C (Figure 4e,f). However, SEM micrographs also reveal the presence of small amounts of aragonite based on the morphology of the crystallites3,43 in sodium carbonate solutions at 25 °C (Figure 5a,b) but not in quantities sufficient for it to be detected using powder XRD (detection limit ∼1 mol %). From Figure 6 it is evident that aragonite is the predominant carbonate polymorph at all times during replacement in sodium carbonate solutions at elevated temperature. Additionally, a higher percentage of aragonite formed at 80 °C than at 60 °C. EBSD data (Figure 7a,b) confirm the coexistence of aragonite and calcite in the samples replaced at elevated temperature. The misorientation distribution plots (Figure S-1) show no consistent relative orientation relationship between the calcite and aragonite grains (no epitaxial relationship as described for the replacement of aragonite by calcite by Perdikouri et al.,44 that is, [001]aragonite//[0001]calcite and [100]aragonite//[101̅0]calcite). The EBSD data also highlight the coarser nature of the calcite grains compared to the aragonite grains (Figure 7c,d), and the fact that calcite grains tend to form “islands” within the finegrained aragonite matrix. SEM imaging (Figure 8) reveals fine porosity within the calcite cores, as well as a recrystallized, less porous rim. Kinetics of the Replacement Reaction. A series of waterbath experiments were conducted in order to investigate the kinetics and reaction mechanism of the replacement of anhydrite by calcium carbonate polymorphs. Table 2 summarizes the reaction variables investigated and the calculated Avrami parameters. Sharp-Hancock plots were constructed by fitting the linearized version of the Avrami equation (eq 2) to the experimental reaction extent values in order to determine the Avrami parameters. In addition to the Avrami model, several other empirical models outlined in Khawam and Flanagan39 and Pedrosa et al.40 were also investigated. The models that fit the data best are listed in Table S-1. When performing linear regression, values corresponding to extents less than 5% or greater than 90% were excluded due to uncertainty in the quantitative phase analysis when a phase was only present in limited quantities.35 Effect of Temperature. The isothermal water bath experiments were conducted over the temperature range 25−80 °C with particle size (+1.4−2.0 mm), carbonate concentration (0.5 mol/kg), and solid−fluid ratio (4 g/L) held constant. The pH of the solution varied with temperature, decreasing from 11.53 at 25 °C to 10.03 at 80 °C. Figure 9 presents the SharpHancock plot of the isothermal experiments. The averaged activation energy calculated using the modelindependent isoconversional method based on two different reaction extents was found to be 46.2 ± 7.6 kJ/mol (see Figure 10a
Figure 2. Reaction progress as followed by X-ray diffraction patterns for (a) 25 °C and (b) 80 °C. All reactions were conducted with a 0.5 mol/kg Na2CO3 solution, grain size of 1.4−2.0 mm, and a solid to fluid ratio of 4 g/L. The mineral phases identified are anhydrite (An), calcite (Cc), and aragonite (Ar). The times selected represent similar reaction extents for the two temperatures.
replacement of the anhydrite by the calcium carbonate polymorphs aragonite and calcite. Figure 1 also illustrates the preservation of the external shape of the anhydrite by the product phases. The product itself is highly porous as indicated by its opacity in these optical micrographs. Figures 3 and 4 provide textural evidence characteristic of a fluid-mediated dissolution−precipitation replacement of
Figure 3. Backscatter electron (BSE) images showing partially replaced anhydrite (25 °C for 24 h in 0.5 mol/kg Na2CO3). (a) BSE image of partially reacted anhydrite grain. (b) BSE image of replacement products forming within pre-existing fractures within the anhydrite grain.
anhydrite by calcium carbonate polymorphs.20,21 It is evident that replacement proceeds from the exterior of the grain toward its core (Figure 3a), as well as along microfractures already present (Figure 3b). In both low temperature (25 °C, Figure 4a) and high temperature samples (80 °C, Figure 4c), a small gap develops at the D
DOI: 10.1021/acsearthspacechem.6b00012 ACS Earth Space Chem. XXXX, XXX, XXX−XXX
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ACS Earth and Space Chemistry
Figure 4. Backscatter electron (BSE) and secondary electron (SE) images of calcium carbonate partially replacing anhydrite. The phases present are labeled: anhydrite (An), calcite (Cc), and aragonite (Ar). (a,b) Replacement at 25 °C for 24 h in 0.5 mol/kg Na2CO3. (a) BSE image of the reaction interface highlighting the large porosity and slight gap that forms at the replacement interface. (b) SE image of the surface showing the calcite rhombohedra that replace the anhydrite. (c,d) Replacement at 80 °C for 1 h (c) and 28 days (d) in 0.5 mol/kg Na2CO3. (c) BSE image highlighting the extensive, but fine (
