Molecular interactions of SO2 with carbonate minerals under co-sequestration conditions: A combined experimental and theoretical study

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Geochimica et Cosmochimica Acta 92 (2012) 265–274 www.elsevier.com/locate/gca

Molecular interactions of SO2 with carbonate minerals under co-sequestration conditions: A combined experimental and theoretical study Vassiliki-Alexandra Glezakou a,⇑, B. Peter McGrail b, H. Todd Schaef c a

Chemical Physics and Analysis FCSD, Pacific Northwest National Laboratory, 902 Battelle Blvd., P.O. Box 999, MSIN K1-83, Richland, WA 99352, United States b Energy Processes and Materials EED, Pacific Northwest National Laboratory, 902 Battelle Blvd., P.O. Box 999, MSIN K1-83, Richland, WA 99352, United States c Geochemistry FCSD, Pacific Northwest National Laboratory, 902 Battelle Blvd., P.O. Box 999, MSIN K1-83, Richland, WA 99352, United States Received 8 November 2011; accepted in revised form 20 June 2012; available online 2 July 2012

Abstract We present a combined experimental and theoretical study investigating the reactivity between select and morphologically important surfaces of carbonate minerals with supercritical CO2 and co-existing H2O and SO2. Trace amounts of SO2 cause formation of CaSO3 in the form of hannebachite in the initial stages of SO2 adsorption and transformation. Atomistic simulations based on density functional theory of these initial steps indicate accumulation of water over the magnesium sites, and suggest depletion of Mg over the Ca from the mineral surface. Under co-sequestration conditions with wet scCO2, water is not likely to cause carbonate dissolution of a perfect surface, however, it stabilizes pre-existing low coordination oxygen atoms by creating surface hydroxyl groups on the CO2-defect sites. Formation of bisulfites (surface-SO2OH) occurs with a low barrier of ca 0.5 eV, estimated by the climbing image nudged elastic band method (CI-NEB). Estimates of the effective transformation rates are in the range of 4.0  101 to 4.0  104 s1. The sulfur-containing species bind preferentially on surface calcium atoms creating the first nucleation sites. Molecular dynamics simulations also show dynamic tautomerization of the adsorbed bifulfites (s-SO2OH s-S(H)O3), which is likely to slow down further oxidation to sulfates in less oxidative environments. From the same simulations, we extract local geometries of the resulting CaSO3HOH species, similar to the crystallographic structure of hannebachite. Collectively, the experimental results and ab initio molecular dynamics simulations suggest potential of carbonate reservoirs for in situ chemical scrubbing of CO2 captured from fossil fuel sources, which could be stored permanently for sequestration purposes or extracted and utilized for enhanced oil recovery (EOR). Ó 2012 Published by Elsevier Ltd.

1. INTRODUCTION Controlling emissions from fossil fuel power plants through long term storage in deep geologic formations offers the potential of sequestering significant amounts of carbon dioxide (Bachu and Adams, 2003). This approach,

⇑ Corresponding author.

E-mail addresses: [email protected] (V.-A. Glezakou), [email protected] (B. Peter McGrail). 0016-7037/$ - see front matter Ó 2012 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.gca.2012.06.015

employed with other strategies, such as increased deployment of renewable energy sources and conservation efforts, has the potential of reducing greenhouse gas concentrations in the atmosphere and mitigating global climate change. There are a number of laboratory experiments demonstrating that pure CO2 combined with water and certain reactive minerals at geologic pressures and temperatures results in direct mineral carbonation (Giammar et al., 2005; McGrail et al., 2006; Marini, 2007; Oelkers et al., 2008; Garcia et al., 2010; King et al., 2010; Schaef et al., 2010). However, depending on the CO2 source and gas cleanup technologies

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employed, the CO2 captured and stored can contain traces of other contaminants. Combustion, of coal in particular, generates gas streams containing CO, O2, SOx, NOx, and various Hg and As-containing compounds. The fate of these contaminants is of considerable environmental importance, independent of CO2 emissions. Co-sequestration refers to the capture and geologic sequestration of carbon dioxide together with any contaminants present in the gas stream. Co-sequestration offers the potential of a more economically acceptable carbon management through significant savings in plant and retrofit capital costs, operating costs, and energy savings, as well by eliminating the need for one or more individual pollutant capture systems, for example SO2 scrubbers (Zhang et al., 2011). Co-sequestration may be particularly beneficial when applied to older existing coal plants not currently equipped with SO2 scrubbers, NOx, and Hg control, but which would be considered worthy of carbon capture and sequestration (CCS) retrofits. If CCS could also perform the function of these costly emissions control systems, it would simplify the overall retrofit process, possibly reduce land, area and water requirements, and would, thereby, facilitate CCS deployment on such plants, and achieve significant cost savings. Eliminating the SO2 scrubbers alone during a CCS retrofit would result in a 13% reduction in capital costs and 8% reduction in annual operating expenses based on an amine capture system (Woods et al., 2007). The potential savings as a percentage of total costs become even more significant, if advanced CO2 capture technologies effectuated the promised reductions, relative to amine scrubbers, in the costs of CO2 capture (Freeman and Rhudy, 2007) and additional savings from elimination of capital and operating costs for NOx or Hg control are included. Although the cost saving potential of avoided capture system costs under co-sequestration are relatively straightforward to estimate, the viability of subsurface storage of mixed-gas streams is considerably less certain. Fate and transport of mixed gas streams in the sub-surface are much less understood than for pure CO2 and recently issued regulations (EPA, 2010) for CO2 sequestration projects leave unresolved whether supercritical CO2 mixed with trace gases would constitute a hazardous waste regulated under different underground injection control protocols. Hence, there is a critical need to better understand the fundamental behavior of mixed gas supercritical fluids in the subsurface. Because of the importance of carbonate minerals in various geological and environmental processes, there is a large body of work on their stability (De Leeuw and Parker, 1998; Titiloye et al., 1998; DeLeeuw, 2002; Geysermans and Noguera, 2009; Pina et al., 2010), growth (Raiteri et al., 2010) and dissolution (De Leeuw et al., 1999; Pokrovsky et al., 2009) mostly in aqueous environments and often in the presence of ionic species (Stipp and Hochella, 1991; De Leeuw and Parker, 2000, 2001; Kerisit et al., 2003; Mielczarski et al., 2006; Rahaman et al., 2008; Geysermans and Noguera, 2009; Heberling et al., 2010), or their efficient use as desulfurization media (Bandosz, 2006). Carbonate formations are also of considerable importance in geologic storage of CO2 in their capacity as caprock and injection

reservoirs (Bacon et al., 2009; Wollenweber et al., 2010; Bachaud et al., 2011). With the growing interest in CO2 sequestration and reactivity in the ensuing supercritical CO2 environments (Lin et al., 2008; Bakri and Zaoui, 2011), there is a need to understand how these transformation reactions occur with carbonate rocks exposed to scCO2-dominated fluids. In this paper, we examine, in detail, the interactions of carbonate rocks with CO2-SO2-H2O mixtures both experimentally and through atomistic simulations. In particular, we will explore reactions occurring between dolomite, calcite, and magnesite with supercritical CO2 containing SO2 and molecular water. 2. EXPERIMENTAL STUDIES 2.1. Experimental methods Large diameter core (15 cm) representing Bonneterre dolomite (southeastern Missouri) was purchased from Core Laboratories (Salt Lake City, UT) and used in this study. Bulk powder X-ray diffraction (XRD) on a sub-sample indicated the material was pure dolomite (PDF# 11-0078). From the original 15 cm diameter core, smaller pieces were prepared (1.9 cm diameter by 1.7 cm in length). Because these cores were still too large for surface analysis, they were further reduced into four equal wedges by a mineral saw. The weights of the sectioned pieces were between 2.45 and 3.20 g. Surface area was determined using nitrogen adsorption/desorption collected with a Quantachrome Autosorb 6B gas sorption system. Prior to these measurements, samples were degassed for 8–16 h under vacuum. The surface area was determined from the isotherm using a 5-point BET method. The surface area obtained from eight different core pieces ranged between 0.13 and 0.26 m2g1. Characterization by scanning electron microscopy (SEM) showed well-developed rhombohedral crystals. Chemistry determined by energy dispersive X-ray spectroscopy (EDXS), showed the dolomite contains Ca (25 wt.%), Mg (11 wt.%), and trace amounts of Fe (2 wt.%) and Mn (<1 wt.%). Large crystals of calcite (CaCO3) and magnesite (MgCO3) were obtained from the Excalibur Mineral Company (New York). X-ray diffraction tracings collected of crushed sub-samples matched PDF files for calcite (050586) and magnesite (36-0383). Sub samples, with surface area 1 cm2, were removed from the larger pieces and used for testing. Water-saturated dolomite cores were placed inside 25 ml stainless steel Parr reaction vessels (Parr Instrument Company) outfitted with valves, pressure gauges, sample ports, and non-reactive gold plated rupture discs were used for all the high-pressure static tests. A schematic drawing of the experimental set up is shown in Fig. 1. Prior to pressurization, residual air in the vessels was evacuated under vacuum. To maintain constant temperature, the vessels were placed inside a Quincy gravity convection oven (Hogentogler and Company, Inc.) preheated to 110 °C after being placed under vacuum. Pressurization to 124 bar with CO2 containing 1% SO2 (Scott’s Specialty Gases) was

V.-A. Glezakou et al. / Geochimica et Cosmochimica Acta 92 (2012) 265–274 Gas Analyzer

267

Tank (CO2-SO2)

Teflon basket H2O/scCO2

Fig. 1. Experimental schematic showing suspension of dolomite samples in a TeflonÒ basket within a ParrÒ pressure vessel connected to both the gas source (CO2 containing 1 wt.% SO2) and the gas analyzer. Fig. 2. SEM microphotograph of dolomite exposed to aqueous CO2 containing 1% wt SO2 for 30 days at 100 °C and 100 bar.

accomplished with a high-pressure syringe pump (Teleydyne ISCO) directly connected to the vessel though 0.07 mm (ID) tubing (PEEK). Collection of gases was done with metering valves (Tescon) located outside the oven. Gas chemistry was tracked with a Stanford Research residual gas analyzer (RGA) positioned directly adjacent to the oven. Masses (amu) corresponding to H2O (18), CO2 (44), and SO2 (64) were monitored for a minimum of 10 min. The relative amounts of SO2 to CO2 were tracked by monitoring the ratio of their corresponding masses. Based on the temperature and pressure conditions of these experiments, water concentration in the scCO2 phase was between 7,500 and 10,000 ppm (Spycher et al., 2003). Sample post-characterization included scanning electron microscopy (SEM) equipped with EDXS, and optical imaging of the reacted samples. Precipitates were removed from the dolomite surface using tweezers and placed inside glass capillaries. Samples were analyzed on a Bruker-AXS Discover D8 diffractometer (Madison, WI) equipped with a ˚ ), HI-STAR GADDS detecrotating Cu anode (k = 1.54 A tor system, and 0.5 beam collimator. Collection of individual XRD tracings required 200 s with power settings of 45 kV and 200 mA. Initially, images were processed with Bruker-AXS GADDSÒ software before importing into JADEÒ XRD software to obtain peak positions (2h) and intensities. Identification of mineral phases was based on mineral powder diffraction files (PDF) published by the JCPDS International Center for Diffraction Data.

liquid water was placed in the bottom of the reactor, while the dolomite was either suspended in a Teflon basket or in direct contact with the liquid water in the bottom of the reactor. The water (1.1 ml) was placed directly on the core in EXP #2 and allowed to saturate into the pore spaces. Finally, in EXP #3, 1.1 ml of water was placed into the reactor bottom with a dolomite core suspended above it in a Teflon basket. EXP #4 was the control test containing only water and no mineral sample. Finally, tests with calcite (EXP #5) and magnesite (EXP #6) were conducted with a small amount of water (listed in Table 1) directly applied to the samples. 2.2.1. Experiments containing excess water Dolomite cores submerged in water were allowed to equilibrate with scCO2 containing 1% SO2 and reacted for 30 days at 100 °C and 100 bar were observed as being coated in delicate white crystals. These precipitates, visible as thin rectangular shaped crystals, were well defined in the SEM, and appear as randomly oriented clusters attached to the dolomite surface, shown in Fig. 2. Chemistries, as determined by SEM–EDXS were dominated by calcium and sulfur. Removal followed by subsequent characterization by lXRD produced a series of reflections matching hannebachite (PDF 39-0725), a hydrated calcium sulfite mineral, [CaSO30.5H2O]. In contrast, dolomite pieces positioned above the water line, suspended within the wet scCO2-SO2 fluid appeared unaltered with the exception of a small white elongated precipitate containing similar amounts of Ca and Mg as well as minor amounts of Fe; EDXS analysis did not detect sulfur. Morphologies were

2.2. Experimental results Experimental parameters for high-pressure static tests exposing dolomite cores to scCO2 containing 1% SO2 in the presence of water are shown in Table 1. For EXP #1,

Table 1 Experimental parameters for high-pressure tests exposing minerals (dolomite, calcite, magnesite) to water and scCO2 containing 1% SO2. Exp ID

Pressure (bar)

Duration (days)

Water (g)

Temp (°C)

Mineral type

Mineral (g)

EXP EXP EXP EXP EXP EXP

100 120 120 120 90 90

30 16.7 16.7 16.7 27 27

10 1.1 1.1 1.1 0.02 0.03

100 110 110 110 115 115

Dolomite Dolomite Dolomite None Calcite Magnesite

11.5 15.8 16.1 – 3.4 2.4

#1 #2 #3 #4 #5 #6

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non-distinctive and X-ray diffraction was inconclusive with respect to phase identification. These experimental results demonstrate strong partitioning of SO2 into the aqueous phase at the bottom of the reactor, evident by the absence of sulfur bearing products on suspended dolomite pieces. Dissolution of co-injected SO2 into formation waters is expected due to the high solubility of SO2 (Crandell et al., 2010) to form sulfurous acid (H2SO3), which combines with divalent metal cation (Ca2+) to form insoluble sulfite phases (Apps, 2006). Precipitation of hannebachite, often a by-product of flue gas desulfurization (Solemtishmack et al., 1995; Laperche and Bigham, 2002), is the most likely phase to occur in the aqueous phase under a co-injection scenario (Xu et al., 2007; Crandell et al., 2010). 2.2.2. Experiments containing water-saturated dolomite cores Based on findings from the previous experiment (EXP #1), a series of tests were initiated exposing the dolomite to scCO2–SO2 containing significantly less free water (EXP #2 and #3). Gas chemistry obtained from the RGA provided early indications the sulfur was being consumed in all reactors with the exception of the control (EXP #4). Fig. 3 shows the logarithmic SO2/CO2 ratio as a function of time for each reactor. Initially, the drop in SO2 is faster for the water-saturated dolomite (EXP #2) than for the dry specimen (EXP #3). Over time, the SO2 concentration remains consistent in the control with little variation. Reactors containing dolomite (EXP #2 and #3) experience a steady drop in SO2 concentrations, indicating consumption into a mineral. As shown in Fig. 3, the initial SO2/CO2 ratio decreases from 3.5  103 to below 1.6  104 for EXP #2 and 7.0  103 to below the 4.3  104 for EXP #3. Reference concentrations for SO2 are shown in Fig. 3 and correspond to 12,700 ppm (starting concentration) and 2,200 PPM, which is the detection limit for this technique.

Fig. 3. Ratio of SO2/CO2 as a function of time (hours) for tests containing scCO2 (1% SO2) and excess H2O in the bottom of the reactor (red circles), H2O saturated dolomite (green triangles), and no water (black circles). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Visual observations of post-reacted dolomite from EXP #2 and #3 show subtle changes in sample color from light yellow to grey. The reacted dolomite surface did not, however, appear significantly altered, unlike dolomite surfaces from EXP #1. Precipitates observed by SEM on post-reacted dolomite (EXP #2) indicated well developed circular areas composed of flat rectangular blades, Fig. 4. Similar observations were made on reaction products associated with EXP #3: unlike the un-reacted sections of the dolomite surface, which maintained the original composition (Ca and Mg along with a tiny amount of Fe), the composition chemistry of these crystals was dominated by S and Ca. Tiny amounts of precipitates from the surfaces of these reacted dolomites were removed and subsequently analyzed by lXRD. The dominating phase was the sulfur bearing hannebachite (PDF 39-0725), which was identified in both samples (EXP #2 and #3). Additionally, precipitates analyzed from EXP #3 produced X-ray tracings with reflections matching bassanite [CaSO40.67H2O] (PDF 36-0617) and anhydrite (PDF 03-0496) [CaSO4]. Tiny amounts of precipitates from the surfaces of these reacted dolomites were removed and subsequently analyzed by lXRD. No crystalline Mg and S bearing phases were found. Occurrence of both sulfite and sulfate minerals in reaction products is possible under certain environmental conditions (Valimbe et al., 1995; Halevy et al., 2007; Halevy and Schrag, 2009). If oxidized, sulfite minerals will alter to sulfate, which is one way of producing the mixture of compounds identified by XRD (Halevy and Schrag, 2009). Although we cannot rule out the possibility that sulfate minerals formed after depressurization and exposure of the samples to the atmosphere, spontaneous oxidation of hannebachite at room temperature with little or no water present seems highly unlikely. 2.2.3. Experiments with calcite and magnesite Static high pressure testing conducted with calcite (EXP #6) and magnesite (EXP #7) ran for 27 days at 115 °C and 90 bar CO2 (1% SO2). Following depressurization and removal, the calcite sample was clearly coated with white precipitate, whereas the magnesite specimen appeared less reacted with minimal surface coatings. Associated with

Fig. 4. SEM microphotograph of reacted dolomite removed from Cell 1 after 400 h exposure to CO2 containing 12,700 ppm SO2 at 110 °C and 12.1 MPa.

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the calcite, precipitates contained a variety of compounds including bassanite (PDF 36-0617), hannebachite (PDF 39-0725), gypsum (PDF 21-0816) [CaSO42H2O], and anhydrite (PDF 03-0496). In contrast, coatings on the magnesite were limited to hexahydrite, a hydrated magnesium sulfate (PDF 08-0479) [MgSO46H2O]. 3. ATOMISTIC SIMULATIONS 3.1. Computational details To better understand the reaction steps that strip the SO2 from the supercritical CO2 phase, periodic DFT atomistic simulations were performed with periodic boundary conditions (3D PBC). Calculations were carried out with the CP2K suit of codes (CP2K, 2011), using the GGA exchange correlation functional of Perdue, Burke and Erzenhoff (Perdew et al., 1996), including Grimme’s dispersion corrections (Grimme, 2006). The core electrons were represented by norm-preserving potentials (Goedecker et al., 1996), while the valence electrons are treated by well-conditioned molecularly-optimized Gaussian-type functions of double-zeta quality (VandeVondele and Hutter, 2007). An auxiliary plane wave basis with 500 Ry cut-off used for the density grid (VandeVondele et al., 2005). The unit cells for calcite and dolomite were optimized and fitted to the Birch–Murnaghan equation of state (Murnaghan, 1944; Birch, 1947). The optimized parameters thus obtained, and the corresponding experimental ones from the literature are summarized in Table 2. Using these optimized cell parameters, we constructed the {10 1 4} surfaces ˚ of calcite and dolomite and {10 1 0} of dolomite with 12 A vacuum used for the periodic boundary calculations. Earlier studies by Titiloye et al. (Titiloye et al., 1998) have shown that for calcite, the most stable surface is the {1014}, and for dolomite the most stable surface is the {1010}. However, we included both surfaces of dolomite for comparison. MD simulations with the scCO2/H2O/ SO2 mixture were performed only with the dolomite surface {1014}, because under higher pressure, dolomite adopts a more calcite-like structure (Bakri and Zaoui, 2011). The vaccuum slab was filled with 32 CO2/SO2/H2O molecules corresponding to supercritical CO2 densities of 0.7 g/ cm3 at T = 350 K, see NIST web book “Thermophysical Properties of Fluid Systems”, http://webbook.nist.gov/ chemistry/fluid/. The top two and bottom two layers of the slab were allowed to relax during the MD simulations to account for the interactions with the condensed phase. Geometry optimization of the slabs with only H2O and SO2 molecules were performed to determine the binding

Table 2 Optimized cell parameters for bulk calcite and dolomite. Experimental values from reference 43 and Refs. therein. ˚) ˚) ˚ 3) a (A c (A V0 (A B0 (GPa) Calcite Exp. Dolomite Exp.

5.07 4.99 4.87 4.82

17.32 17.061 16.18 16.01

381.3 367.9 332.3 321.2

70.77 72.0 128.2 94.0

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energies of these species to different binding sites. Minimum energy pathways were determined with the climbing image nudged elastic band method (CI-NEB) (Henkelman et al., 2000). Following the AIMD simulations, we extracted radial distribution functions (RDF’s) describing the variation of atomic density as a function of distance from a given atom. This type of analysis was performed on well-equilibrated molecular trajectories of at least 5 ps and with a time step of 1.0 fs. Graphics were done using VMD (Humphrey et al., 1996) graphical interface. 3.2. Mechanistic insights on SO2 stripping by carbonate minerals under low-water content The experiments discussed previously clearly demonstrate the ability of carbonate rocks to strip SO2 from water–wet scCO2. Our molecular dynamics simulations tested whether such transformation can occur at lower water contents. At temperatures below the decomposition temperature of calcareous rocks, a simple reaction can be written to produce sulfites/sulfates from calcite (Malaga-Starzec et al., 2004; Al-Hosney and Grassian, 2005; Baltrusaitis et al., 2007; Olaru et al., 2010): CaCO3 ðsÞ þ SO2 þ 0:5H2 OðgÞ ! CaSO3  0:5H2 OðsÞ þ CO2 ðgÞ

ð1Þ

Once sulfites are formed, further oxidation steps will produce sulfates: CaSO3  0:5H2 OðsÞ þ 1=2O2 ðgÞ þ 1:5H2 O ! CaSO4  2H2 O

ð2Þ

In order to understand the initial steps that lead to Ca sulfite formation in the scCO2 environments and quantify the type and magnitude of interactions between the different sites on the mineral surfaces and H2O and SO2, we determined their respective binding energies on the perfect calcite and dolomite {10 14} and dolomite {10 10} surfaces. The results are summarized in Table 3. Short MD trajectories, in parallel with geometry optimizations, showed that water binds on the metal sites through its oxygen and forms hydrogen bonds with the oxygens of the carbonate moieties. On calcite, the water tends to orient itself with its plane parallel to the surface, while on the dolomite, the Mgbound waters tend to be more upright with only one OH bond pointing towards neighboring OCO2-moieties. The binding energy of water on Ca sites of dolomite is reduced by 30% compared to that on calcite. Although this trend

Table 3 Binding energies (in eV) of H2O and SO2 on perfect calcite {1014} and dolomite surfaces {1014} (Dol. I) and {1010} (Dol.II).

Calcite Dol. I (@Ca) Dol. I (@Mg) Dol. II (@Ca) Dol. II (@Mg)

H2O

SO2

1.1 0.70 1.0 1.4 1.6

0.88 1.0 0.62 1.3 1.2

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holds true for the dolomite {1010} surface, the binding energies of water are almost 50% higher than the corresponding binding energies on the less reactive dolomite {1014} surface. Dissociation of water on the perfect {1014} surfaces is highly unlikely, especially on calcite. The {1010} dolomite surface however undergoes extreme reconstruction and water spontaneously dissociates on it, producing (surf)-CO3H and (surf)-OH. The binding energy for the dissociated species becomes as low as 2.0 eV. SO2 mainly binds on the metal sites through the oxygens. Its binding energy is somewhat less than that of water, ranging between 0.6 and 1.3 eV. On dolomite, SO2 shows a clear preference for the calcium sites. The binding energies for water agree well with those reported in the literature, see for example work by Lardge (Lardge, 2009), which range between 0.63 to 0.91 eV depending on the binding site and water coverage. To our knowledge, the binding energies for SO2 on calcite or dolomite are the first ones reported here.

Fig. 5a–e show the radial distribution functions between the top layer of the metal atoms (Ca or Mg) to the water oxygens, for the calcite {10 14} (Fig. 5a), dolomite {10 14} (Fig. 5b, c) and dolomite {10 10} (Fig. 5d, e) surfaces. The inset snapshots show a typical water distribution in the equilibrated systems. The distribution of waters on the calcite surface (Fig. 5a) is uniform, showing the first layer of waters with their plane parallel to the surface, while any excess waters start forming a second layer with the O–H bond point towards the surface. The picture is consistent with previously reported results in literature, (De Leeuw and Parker, 1997; Kerisit et al., 2003; Rahaman et al., 2008; Lardge, 2009; Heberling et al., 2010) and it is reflected in the Ca-Ow radial distribution function (RDF) of the top calcium atoms with the water oxygens, Fig. 5a, showing a ˚ and a smaller one at about 4.5 A ˚ . Besharp peak at 2.435 A yond that, it seems devoid of further structure. Fig. 5b, c shows the RDF’s for the top Ca and Mg atoms of the dolomite {10 14} surface with the water oxygen atoms. At first

Fig. 5. Radial distribution functions of M-Ow, M = Ca, Mg and snapshots from equilibrated MD simulations of carbonate minerals surfaces and water, showing the propensity of water to preferentially solvate the Mg sites: (a) gCa-Ow(R) on calcite {1014} (b) gCaOw(R) and (c) gMgOw(R) on dolomite {1014} (d) gCaOw(R) and (e) gMgOw(R) on dolomite {1010} dolomite.

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glance, these are similar, however at closer inspection, it is evident that the Mg-Ow RDF exhibits a maximum at a ˚ , than the Ca-Ow RDF, which shorter distance, 2.175 A ˚ peaks at 2.405 A. Compared to the pure calcite surface, the Ca-Ow distance in dolomite is also shorter. This observation seems to be in agreement with recent studies (Stashans and Chamba, 2011) that conclude the presence of Mg in the carbonate minerals, even as impurity, tends to make chemical bonding more ionic resulting in shorter bonds. Another interesting feature of the RDF’s is the better-defined outer-shell structure, beyond the 2nd shell, around the Mg sites in dolomite. Snapshots from the equilibrated trajectories also show an accumulation of water molecules over the MgCO3 areas of the dolomite and a more layered water structure. The latter, can also be seen at longer distances of the radial distribution functions, R ˚ , Fig. 5b, c: the Mg-Ow RDF shows well-defined > 3.8 A ˚ . This preference of water for peaks at 4.4, 6.0 and 7.5 A the Mg sites is even more pronounced in the corresponding radial distribution functions of the dolomite {1010} surface, Fig. 5d, e. In spite of the drastic reconstruction of the top layers of the {1010} surface, with the top CO3 moieties twisting by as much as 60 deg from their original, perpendicular orientation, the differences in the RDF’s of the top metal with water are equally distinct. The corresponding radial distribution functions show a significantly ˚ for Ca-Ow, while the Mg-Ow radial smaller peak at 2.4 A distribution function peaks at shorter distances and displays well-defined second and third-shell structure. Hydrogen bonds between water hydrogens and surface oxygens are within the normal H-bonding range, between 1.6– ˚ for both minerals, however in the case of dolomite 1.9 A ˚ ) are also observed. some slightly shorter H-bonds (1.52 A On perfect {1010} dolomite surfaces, 2SO2 + H2O form a strongly bound precursor, with a binding energy that exceeds 2 eV. Despite the lack of direct M–S bonding interactions, our simulations also showed that SO2 will not readily occupy metal vacancies, instead CO2/CO3 vacancies have to pre-exist before an SO3 moiety takes up their place, according to the general scheme: ðsurfÞ  CaCO3  MgCO3  CaCO3 þ H2 O þ 2SO2 ! ðsurfÞ  CaCO3  MgCO3  CaCO3  ð2SO2  H2 OÞ ðstable pre-complexÞ ! ðsurfÞ  CaSO3  MgCO3 ðH2 OÞ  CaSO3    2CO2 ð3Þ On the one hand, given surface defects, sulphonation of the carbonate mineral is a thermodynamically favorable transformation. On the other hand, one can question how releasing more CO2 into a scCO2 environment could be a feasible transformation. In our previous study (Glezakou et al., 2010) we have shown that the presence of small quantities of water in scCO2 does not lead into formation of H2CO3/HCO3, which could possibly facilitate CO3 dissolution and/or carbonate precipitation. Given the propensity of water to cluster around the Mg sites, it is possible that it may cause some carbonate dissolution. We tested this hypothesis on a pristine dolomite {1014} surface according to the general scheme:

271

Fig. 6. Schematic of a CO2-defect formation on pristine Dolomite {1014}surface under scCO2 phase, in the presence of H2O only (green curve), or H2O/SO2 (red curve). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

ðsurfÞ  MgCO3 þ H2 O ! ðsurfÞ  ½MgOH2 þ CO2

ð4Þ

This is depicted schematically in Fig. 6 (green curve) by showing the energy requirements to create a CO2 defect in a pristine surface, assisted by water. The barrier is on the order of 3.0 eV and incorporation of the CO2 into the scCO2 matrix results in a stabilization of about 0.5 eV, even though this implies adding more CO2 into the scCO2 phase. The same reaction was repeated in the presence of SO2, replacing a surface CO2 with SO2, to form surface SO3, Fig. 6 (red curve). The presence of SO2 downshifts the entire reactive profile, with the transition state being lower by 0.6 eV, however it is still significant (2.4 eV) since it is still associated mainly with the departure of CO2 form the surface. Once the CO2 defect is formed, SO3 formation is fast and results in an almost isoenergetic product, (surf)SO3 H OH. The weathering of carbonate minerals that releases CO2 is a well-established phenomenon and it is the result of various thermal (Goldin and Kulikova, 1984) or physical (Agrinier et al., 2001) transformations that create surface CO2 surface defects with remaining low coordination oxygen atoms. The importance of low-coordination surface oxygens has been noted in earlier studies of SO2 adsorption at surface and step sites of MgO surfaces by Pacchioni et al. (Pacchioni et al., 1994a; Pacchioni et al., 1994b). We confirmed the enhanced surface activity by reactivity studies on CO2-defect bearing surfaces interacting with scCO2/ H2O/SO2 phase. In this case, water quickly reacted with the low-coordination oxygens at the CO2-defect sites of the carbonate surface, to form adsorbed OH’s. Any co-adsorbed SO2 at nearby Ca sites, reacted with the OH’s to form CaSO3 H OwH. Fig. 7 shows the calculated reactive pathway that connects the hydrated CO2-defect with coadsorbed SO2 to form the first nucleus of hannebachite, in a low-barrier transformation. Note that the barrier to form SO3 is only 0.5 eV. The average distances within

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Fig. 7. Reactive profile of the initial stages of sulfation reactions on CO2-defect bearing dolomite {1014}surface.

˚, the resulting CaSO3H. . .OwH moieties are: R(Ca–S) 3 A ˚ , R(OsOw)  2.6 A ˚ . These are very R(CaOw)  4.7 A similar to the corresponding structural parameters for han˚ nebachite crystallographic data, at ca 3.3, 4.3 and 3.1 A respectively. 4. IN SITU CHEMICAL STRIPPING BY CARBONATE MINERALS Both the experimental and atomistic simulations of the interactions of CO2-SO2-H2O mixtures with carbonate minerals suggest potential for exploiting these reactions in the subsurface as a means for integrated management of greenhouse gas emissions. Criteria pollutants could be stripped and permanently sequestered via contact with appropriate reactive carbonate reservoir rocks or carbonate bearing

Fig. 8. Conceptual drawing of reverse flood reactive reservoir management. Contours illustrate stripping of SO2 from the supercritical CO2.

sandstones (Kummerow and Spangenberg, 2011). The resulting nearly pure CO2 stream could either be stored permanently as in conventional CCS or extracted for use in EOR. One potential configuration of the concept, as illustrated in Fig. 8, involves injection of the mixed gas stream into a suitable reactive bed or beds (carbonate breccias, porous dolomite, hydraulically fractured carbonates) that would be actively managed over time to remove SO2 and other criteria pollutants from the CO2 stream. A collector well or wells would convey the clean CO2 via hydrostatic pressure into a depleted oil reservoir, or the clean CO2 could simply be brought back to the surface for pipeline transport to a nearby oil field. Given a favorable geological context, the clean CO2 could be trapped in an anticlinal structure for later retrieval when needed. Essentially, this concept is an advanced form of a reverse dump flood, which is sometimes used for waterfloods in the oil industry. 5. CONCLUSIONS We have presented a combined experimental and theoretical study discussing the reactions occurring between Mg-rich carbonate minerals and scCO2 streams under cosequestration conditions with concomitant residual SO2. The experiments with excess water or wet dolomite cores show that hannebachite is formed at least in the first stages of exposure. A combination of DFT-based atomistic and molecular dynamics simulations was used to determine the necessary atomistic rearrangements driving reactivity of carbonates with SO2 and H2O solvated in scCO2. Our calculations determined that both H2O and SO2 bind quite strongly on calcite 1.0 eV, however the binding energies can increase by as much as 50% on the more reactive {10 10} surface of dolomite, and slightly less on the more calcite-like {10 14} surface. Water preferentially binds on Mg and SO2 on Ca sites. On pure calcite, water dissociation almost certainly requires defects, while it is spontaneous on dolomite {10 10}. In general, on dolomite, the presence of Mg facilitates the formation of surface hydroxyl groups and subsequent reaction with SO2 to form (surf)SO3 + CO2. The barrier for this transformation on defectfree surface is mainly associated by the release of CO2 with a substantial barrier of 2.5 eV, however, the forming sulfites correspond to an equally stable system. In CO2-defect bearing surfaces, the same transformation proceeds with a very low barrier of only 0.5 eV. A qualitative estimate of the effective rate for SO3 formation was calculated, using the simple Arrhenius expression with a pre-factor  1010-1013, to be in the range of 4.0  101 to 4.0  104 s1, which points at a facile and effective SO2 capture by carbonate mineral surfaces. Combustion gases from burning coal, oil, biomass, or natural gas have SO2 concentrations that range from 6000 to 100 ppmw in each ton CO2 produced. Facile SO2 stripping on carbonates suggests that power plants being retrofitted for CO2 capture and geologic sequestration that are proximal to a suitable carbonate reservoir could avoid large capital and operating expenses for surface-based gas cleanup operations. In situ chemical scrubbing could provide pipeline grade CO2 for either permanent storage

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or extraction for use in EOR with very little additional marginal cost to the overall integrated CCS system. ACKNOWLEDGMENTS The authors are grateful to A. T. Owen for her assistance in the preparation of the samples. The authors have benefited by useful discussions with Drs. R. Rousseau, S. Raugei and C. F. Windisch of PNNL. This work was supported by the U.S. Department of Energy (DOE), Office of Fossil Energy and Office of Science. The Pacific Northwest National Laboratory (PNNL) is operated by Battelle for the DOE under Contract DE-AC05-76RL01830. A portion of the research was performed using EMSL, a national science user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research located at PNNL.

REFERENCES Agrinier P., Deutsch A., Scha˚rer U. and Martinez I. (2001) Fast back-reactions of shock-released CO2 from carbonates: An experimental approach. Geochim. Cosmochim. Acta 65, 2615– 2632. Al-Hosney H. A. and Grassian V. H. (2005) Water, sulfur dioxide and nitric acid adsorption on calcium carbonate: A transmission and ATR-FTIR study. Phys. Chem. Chem. Phys. 7, 1266–1276. Apps J. A. (2006). A review of hazardous chemical species associated with CO2 capture from coal-fired power plants and their potential fate in CO2 geologic storage. LBNL-59731, Lawrence Berkeley National Laboratory. Bachaud P., Berne P., Renard F., Sardin M. and Leclerc J. R. (2011) Use of tracers to characterize the effects of a CO2-saturated brine on the petrophysical properties of a low permeability carbonate caprock. Chem. Eng. Res. Des. 89, 1817–1826. Bachu S. and Adams J. J. (2003) Sequestration of CO2 in geological media in response to climate change: Capacity of deep saline aquifers to sequester CO2 in solution. Energy Convers. Manage. 44, 3151–3175. Bacon D. H., Sass B. M., Bhargava M., Sminchak J. and Gupta N. (2009) Reactive transport modeling of CO2 and SO2 injection into deep saline formations and their effect on the hydraulic properties of host rocks. Energy Procedia 1, 3283–3290. Bakri Z. and Zaoui A. (2011) Structural and mechanical properties of dolomite rock under high pressure conditions: A firstprinciples study. Phys. Status Solidi. B 1, 894–1900. Baltrusaitis J., Usher C. R. and Grassian V. H. (2007) Reactions of sulfur dioxide on calcium carbonate single crystal and particle surfaces at the adsorbed water carbonate interface. Phys. Chem. Chem. Phys. 9, 3011–3024. Bandosz T. J. (2006). Carboneceous Materials as Desulfurization Media. In Combined and Hybrid Adsorbents: Fundamentals and Applications, (Eds. M. Kartel, J.-M. Loureiro), Springer, New York 1996. Birch F. (1947) Finite elastic strain of cubic crystals. Phys. Rev. 71, 809–824. CP2K. (2011) CP2K is a freely available (GPL) program, written in Fortran 95, to perform atomistic and molecular simulations of solid state, liquid, molecular and biological systems. It provides a general framework for different methods such as e.g. density functional theory (DFT) using a mixed Gaussian and plane waves approach (GPW), and classical pair and many-body potentials. http://cp2k.berlios.de/. Crandell L. E., Ellis B. R. and Peters C. A. (2010) Dissolution potential of SO2 co-injected with CO2 in geologic sequestration. Environ. Sci. Technol. 44, 349–355.

273

De Leeuw N. H. and Parker S. C. (1997) Atomistic simulation of the effect of molecular adsorption of water on the surface structure and energies of calcite surfaces. J. Chem. Soc., Faraday Trans. 93, 467–475. De Leeuw N. H. and Parker S. C. (1998) Surface structure and morphology of calcium carbonate polymorphs calcite, aragonite, and vaterite: An atomistic approach. J. Phys. Chem. B 102, 2914–2922. De Leeuw N. H., Parker S. C. and Harding J. H. (1999) Molecular dynamics simulation of crystal dissolution from calcite steps. Phys. Rev. B 60, 13792–13799. De Leeuw N. H. and Parker S. C. (2000) Atomistic simulation of mineral surfaces. Mol. Simul. 24, 71–86. De Leeuw N. H. and Parker S. C. (2001) Surface-water interactions in the dolomite problem. Phys. Chem. Chem. Phys. 3, 3217– 3221. DeLeeuw N. H. (2002) Surface structures, stabilities, and growth of magneisan calcites: A computational investigation from the perspective of dolomite formation. Am. Mineral. 87, 679–689. EPA (2010). Federal requirements under the Underground Injection Control (UIC) program for carbon dioxide (CO2) Geologic Sequestration (GS) Wells. Federal register, http:// water.epa.gov/type/groundwater/uic/class6/gsregulations.cfm. Freeman B. and Rhudy R. (2007) Assessment of Post-Combustion Carbon Capture Technology Developments. Electric Power Research Institute, Palo Alto, California. Garcia B., Beaumont V., Perfetti E., Rouchon V., Blanchet D., Oger P., Dromart G., Huc A. Y. and Haeseler F. (2010) Experiments and geochemical modelling of CO2 sequestration by olivine: Potential, quantification. Appl. Geochem. 25, 1383– 1396. Geysermans P. and Noguera C. (2009) Advances in atomistic simulations of mineral surfaces. J. Mater. Chem. 19, 7807–7821. Giammar D. E., Bruant R. G. and Peters C. A. (2005) Forsterite dissolution and magnesite precipitation at conditions relevant for deep saline aquifer storage and sequestration of carbon dioxide. Chem. Geol. 217, 257–276. Glezakou V.-A., Rousseau R. J., Dang L. X. and McGrail B. P. (2010) Structure, dynamics and vibrational spectrum os scCO2/ H2O mixtures from ab initio molecular dynamics as a function of water cluster formation. Physical Chemistry Chemical Physics 12, 8759–8771. Goedecker S., Teter M. and Hutter J. (1996) Separable dual-space Gaussian Pseudopotentials. Phys. Rev. B 54, 1703–1710. Goldin D. M. and Kulikova G. V. (1984) On the dissociation mechanism of carbonates and their isomorphous mixture. J. Therm. Anal. 29, 139–145. Grimme S. (2006) Semi-empirical GGA-type density funstional constructed with a long-range dispersion correction. J. Comput. Chem. 27, 1787–1799. Halevy I., Zuber M. T. and Schrag D. P. (2007) A sulfur dioxide cliamte feedback on early mars. Science 318, 1903–1907. Halevy I. and Schrag D. P. (2009) Sulfur dioxide inhibits calcium carbonate precipitation: Implications for early Mars and Earth. Geophys. Res. Lett. 36, L23201. Heberling F., Trainor T. P., Lutzenkirchen J., Eng P., Denecke M. A. and Bosbach D. (2010) Structure and reactivity of the calcite-water interface. J. Colloid Interface Sci. 354, 843–857. Henkelman G., Uberuaga B. and Jo´nsson H. (2000) A climbing image NEB method for finding minimum energy paths and saddle points. Journal Chemical Physics 113, 9901–9904. Humphrey W., Dalke A. and Schulten K. (1996) VMD - Visual Molecular Dynamics. J. Mol. Graph. 14, 33–38. Kerisit S., Parker S. C. and Harding J. H. (2003) Atomistic simulation of the dissociative adsorption of water on calcite surfaces. J. Phys. Chem. B 107, 7676–7682.

274

V.-A. Glezakou et al. / Geochimica et Cosmochimica Acta 92 (2012) 265–274

King H. E., Plumper O. and Putnis A. (2010) Effect of Secondary Phase Formation on the Carbonation of Olivine. Environ. Sci. Technol. 44, 6503–6509. Kummerow J. and Spangenberg E. (2011) Experimental evaluation of the impact of the interactions of CO2-SO2, brine, and reservoir rock on petrophysical properties: A case study from the Ketzin test site, Germany. Geochem. Geophys. Geosyst. 12, Q5010. Laperche V. and Bigham J. M. (2002) Quantitative, chemical, and mineralogical characterization of flue gas desulfurization byproducts. J. Environ. Qual. 31, 979–988. Lardge J. S. (2009) Investigation of the interaction of water with the calcite {104} surface using ab initio simulation. Doctoral Thesis, University. College London. Lin H., Fujii T., Takisawa R., Takahashi T. and Hashida T. (2008) Experimental evaluation of interactions in supercritical CO2/ water/rock minerals system under geologic CO2 sequestration conditions. J. Mater. Sci. 43, 2307–2315. Malaga-Starzec K., Panas I. and Lindqvist O. (2004) Model study of initial adsorption of SO2 on calcite and dolomite. Appl. Surf. Sci. 222, 82–88. Marini L. (2007) Geological Sequestration of Carbon Dioxide. Thermodynamics, Kinetics and Reaction Path Modeling. In Developments in Geochemistry, 11, Elsevier, Amsterdam, The Netherlands. McGrail B. P., Schaef H. T., Ho A. M., Chien Y. J., Dooley J. J., and Davidson C. L. (2006) Potential for carbon dioxide sequestration in flood basalts. J. Geophys. Res.-Solid Earth 111, ARTN B12201. Mielczarski J. A., Schott J. and Pokrovsky O. S. (2006) Surface Speciation of Dolomite and Calcite in Aqueous Solutions. In Encyclopedia of Surface and Colloid Science, Taylor & Francis. Murnaghan F. D. (1944) The Compressibility of Media Under Extreme Pressures. Proceedings of the National Academy of Sciences USA 30, 244–247. Oelkers E. H., Gislason S. R. and Matter J. (2008) Mineral carbonation of CO2. Elements 4, 333–337. Olaru M., Aflori M., Simionescu B., Doroftei F. and Stratulat L. (2010) Effect of SO2 Dry Deposition on Porous Dolomitic Limestones. Materials 3, 216–231. Pacchioni G., Clotet A. and Ricart J. M. (1994a) A theoretical study of the adsorption and reaction of SO2 at surface and step site of the MgO(100) surface. Surf. Sci. 315, 337–350. Pacchioni G., Ricart J. M. and Illas F. (1994b) Ab initio cluster modle calculations on the chemisorption of CO2 and SO2 probe molecules on MgO and CaO(100) surfaces. A theoretical measure of oxide basicity. J. Am. Chem. Soc. 116, 10152–10158. Perdew J. P., Burke K. and Erzenhof M. (1996) Generalized Gradient Approximation Made Simple. Phys. Rev. B 77, 3865– 3868. Pina C. M., Pimentel C. and Garcia-Merino M. (2010) High resolution imaging of the dolomite (104) cleavage surface by atomic force microscopy. Surf. Sci. 604, 1877–1881. Pokrovsky O. S., Golubev S. V., Schott J. and Castillo A. (2009) Calcite, dolomite and magnesite dissolution kinetics in aqueous solutions at acid to circumneutral pH, 25 to 150 C and 1 to 55 atm pCO2: New constraints on CO2 sequestration in sedimentary basins. Chem. Geol. 265, 20–32. Rahaman A., Grassian V. H. and Margulis C. J. (2008) Dynamics of water adsorption onto a calcite surface as a function of relative humidity. J. Phys. Chem. C 112, 2109–2115.

Raiteri P., Gale J. D., Quigley D. and Rodger P. M. (2010) Derivation of an accurate force-field for simulating the growth of calcium carbonate from aqueous solution: A new model for the calcite-water interface. J. Phys. Chem. C 114, 5997– 6010. Schaef H. T., McGrail B. P. and Owen A. T. (2010) Carbonate mineralization of volcanic province basalts. Int. J. Greenhouse Gas Control 4, 249–261. Solemtishmack J. K., McCarthy G. J., Docktor B., Eylands K. E., Thompson J. S., and Hassett D. J. (1995) High-calcium coal combustion by-products - engineering properties, ettringite formation, and potential application in solidification and stabilization of selenium and boron. Cem. Conc. Res. 25, 658670. Spycher N., Pruess K. and Ennis-King J. (2003) CO2-H2O mixtures in the geological sequestration of CO2. I. Assessment and calculation of mutual solubilities from 12 to 100°C and up to 600 bar. Geochim. Cosmochim. Acta 67, 3015–3031. Stashans A. and Chamba G. (2011) A new insight on the role of Mg in calcite. Int. J. Quantum Chem. 111, 2436–2443. Stipp S. L. S. and Hochella M. F. (1991) Structure and bonding environments at the calcite surface as observed with X-ray spectroscopy (XPS) and low energy electron diffraction (LEED). Geochim. Cosmochim. Acta 55, 1723–1736. Titiloye J. O., de Leeuw N. H. and Parker S. C. (1998) Atomistic simulation f the differences between calcite and dolomite surfaces. Geochim. Cosmochim. Acta 62, 2637–2641. Valimbe P. S., Malhotra V. M. and Banerjee D. D. (1995) Structural and thermal behavior of coal combustion and gasification by-products: SEM, FTIR, DSC, and DTA measurements. Journal Name: Preprints of Papers, American Chemical Society, Division of Fuel Chemistry; Journal Volume: 40; Journal Issue: 4; Conference: 210. national meeting of the American Chemical Society (ACS), Chicago, IL (United States), 20-25 Aug 1995; Other Information: PBD: 1995, Medium: X; Size: pp. 776-782. VandeVondele J., Krack M., Fawzi M., Parrinello M., Chassaing T. and Hutter J. (2005) QUICKSTEP: Fast and accurate density functional calculations using a mixed Gaussian and plane waves approach. Comput. Phys. Commun. 167, 103– 128. VandeVondele J. and Hutter J. (2007) Gaussian basis sets for accurate calculations on molecular systems in gas and condensed phases. J. Chem. Phys.. Wollenweber J., Alles S., Busch A., Krooss B. M., Stanjek H. and Littke R. (2010) Experimental investigation of the CO2 sealing efficiency of caprocks. Int. J. Greenh. Gas Con. 4, 231–241. Woods M. C., Capicotto P. J., Haslbeck J. L., Kuehn N. J., Matuszewski M., Pinkerton L. L., Rutkowski M. D., Schoff R. L. and Vaysman V. (2007) Cost and performance baseline for fossil energy plants. Volume 1: bituminous coal and natural gas to electricity. National Energy Technology Laboratory, Pittsburgh, Pennsylvania. Xu T., Apps J. A., Pruess K. and Yamamoto H. (2007) Numerical modeling of injection and mineral trapping of CO2 with H2S and SO2 in a sandstone formation. Chem. Geol. 242, 319–346. Zhang W., Xu T. and Li Y. (2011) Modeling of fate and transport of co-injection of H2S with CO2 in deep saline formations. J. Geophys. Res. 116, 7652–7665. Associate editor: David J. Vaughan