H2O interfaces as a function of water content

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Geochimica et Cosmochimica Acta 84 (2012) 137–151 www.elsevier.com/locate/gca

Structure and dynamics of forsterite–scCO2/H2O interfaces as a function of water content Sebastien Kerisit a,⇑, John H. Weare b, Andrew R. Felmy a a

Chemical and Materials Sciences Division, Pacific Northwest National Laboratory, Richland, WA 99352, USA b Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093, USA Received 18 October 2011; accepted in revised form 23 January 2012; available online 2 February 2012

Abstract Molecular dynamics (MD) simulations of forsterite surfaces in contact with supercritical carbon dioxide (scCO2) fluids of varying water content were performed to determine the partition of water between the scCO2 fluid and the mineral surface, the ð2xÞ nature of CO2 and H2O bonding at the interface, and the regions of the interface that may be conducive to Hx CO3 formation. Calculations of the free energy of the associative adsorption of water onto the (0 1 0) forsterite surface from the scCO2 phase indicated that the formation of a water film up to three-monolayer thick can be exothermic even for water contents below the water saturation concentration of the scCO2 fluid. In MD simulations of scCO2/H2O mixtures in contact with the (0 1 0) forsterite surface, H2O was found to readily displace CO2 at the surface and, therefore, CO2 directly contacted the surface only for water coverages below two monolayers. For thicker water films, a two-monolayer hydration layer formed ð2xÞ that CO2 could not penetrate. The MD simulations thus suggest that, in the presence of sufficient water, Hx CO3 formation occurs in the water films and not via direct reaction of CO2 with the forsterite surface. Simulations of the hydroxylated (0 1 0) surface and of the (0 1 1) surface suggested that this conclusion can be extended to forsterite surfaces with different surface structures and/or compositions. The density, diffusion, and degree of hydration of CO2 as well as the extent of CO2/H2O mixing at the interface were all predicted to depend strongly on the thickness of the water-rich film, i.e., on the water content of the scCO2 fluid. Ó 2012 Elsevier Ltd. All rights reserved.

1. INTRODUCTION Carbon dioxide capture and sequestration (CCS), as a supercritical fluid in deep geological formations, is one of the most attractive options for reducing the levels of anthropogenic CO2 emission into the atmosphere (Benson and Cole, 2008). Importantly, evaluating the effectiveness and safety of geologic sequestration depends upon our ability to predict the stability and permeability of geologic reservoirs following injection of supercritical carbon dioxide (scCO2), which, in turn, relies on our understanding of chemical interactions between scCO2 and geologic media.

⇑ Corresponding author.

E-mail address: [email protected] (S. Kerisit). 0016-7037/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2012.01.038

The reaction of CO2 with host silicates to form metal carbonates, i.e., carbonation reactions, is one of the principal families of chemical interactions relevant to CCS, as it leads to stable, long-term sequestration via mineral trapping (Oelkers et al., 2008). Silicates such as olivines and serpentines are highly reactive phases that can undergo carbonation reactions as they are rich in the divalent metal cations necessary to form carbonates such as calcite (CaCO3), magnesite (MgCO3), and siderite (FeCO3). Although an extensive body of work (e.g., (Daval et al., 2009; King et al., 2010a) and references therein) has been published, in the context of CCS, on the carbonation of minerals in contact with a CO2-containing aqueous phase, carbonation reactions involving water-bearing supercritical CO2 fluids (WBSF) have received comparatively limited attention. However, WBSF are equally relevant to CCS. Indeed, migration of the injected, dry scCO2 is likely to

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create dehydration fronts that range in water content from anhydrous scCO2 to CO2-saturated brine (Nordbotten and Celia, 2006). Consequently, as explained by McGrail et al. (2009), WBSF will likely dominate interactions with the caprock in the vicinity of the injection bore well for two principal reasons. Firstly, their lower density relative to water means that WBSF will diffuse upwards and contact the caprock. Secondly, WBSF will easily permeate the caprock pore network because of their high diffusivity and low capillary entry pressure. Therefore, scCO2 containing variable concentrations of water could permeate the pore space of the caprock near the injection bore well, thus creating low water content environments, which effects on the sealing effectiveness of the caprock are presently unknown. The limited number of studies published to date show that WBSF can react with mineral surfaces to form metal carbonates (Regnault et al., 2005; McGrail et al., 2009; Regnault et al., 2009; Kwak et al., 2010,2011; Loring et al., 2011). Interestingly, mineral carbonation reactions with WBSF differ significantly from those in aqueous solutions. Indeed, Felmy et al. (in review) observed the formation of magnesite, following reaction of forsterite nanoparticles with water–saturated scCO2, at temperatures (e.g. 35 °C) thought to be too low to overcome the sluggish precipitation kinetics of magnesite in bulk aqueous solutions. Moreover, these studies all highlighted the dependence of the extent and rate of reaction on the water content of the WBSF. Therefore, a detailed understanding of the structure and dynamics of the WBSF–mineral interface as a function of water content, even for low H2O contents, is critical to determining the role of water and thus elucidating the mechanisms of carbonation reactions in conditions relevant to CCS and, ultimately, the long-term effect of CO2 sequestration. Carbonation reactions involve the initial formation of carbonate from CO2 followed by reaction of carbonate with divalent metal cations. There are two potential pathways for carbonate formation during carbonation reactions: (1) direct oxygen abstraction from the silicate surface, in which CO2 forms the carbonate moiety ðCO2 3 Þ by removing oxygen from Mg2SiO4; and, if water is present, (2) oxygen abstraction from interfacial water (i.e., formation of a carbonic acid/bicarbonate intermediate) by removing oxygen from H2O. Therefore, we hypothesize that, as the water content increases, direct access of CO2 to the surface will be restricted and that the dominant mechanism for carbonate formation could change from (1) to (2). Confirmation of this hypothesis would imply that metal carbonate formation takes place either in the fluid following dissolution of alkaline-earth cations from the forsterite surface or via direct reaction of carbonate/bicarbonate ions with surface alkaline-earth cations. A prerequisite to the interpretation of carbonation reactions is an understanding of the component availability and the structure and dynamics of CO2 and H2O at the WBSF– mineral interface. Indeed, while the state of the fluid phase away from the mineral surface is determined by the composition, temperature, and pressure of the bulk phase, the local composition near the mineral surface is not known without direct measurement or simulation. In particular,

strongly attractive H2O–mineral interactions could lead to the formation of water films even at very low levels of water saturation of the scCO2 fluid. Indeed, water thin films have been invoked in several instances to explain the high reactivity of WBSF (Lin et al., 2008; Loring et al., 2011; Shao et al., 2011; Felmy et al., in review). Importantly, the differences in reactivity between WBSF and aqueous solutions strongly suggest that these thin water films can provide unique conditions for mineral reactivity (Felmy et al., in review). To begin the development of such an understanding, we performed molecular dynamics (MD) simulations of scCO2/H2O mixtures in contact with forsterite surfaces to determine the composition, structure, and dynamics of the interface as a function of water content of WBSF. MD simulation is a powerful tool for investigating the atomic-level structure of solid–fluid interfaces (Kerisit, 2011). Forsterite was selected as it is a potential source mineral for carbonation reaction (Oelkers et al., 2008) and is being used as a model system in studies conducted at this laboratory (Kwak et al., 2010,2011; Loring et al., 2011). The (0 1 0) surface was used as a model surface as it is one of the main cleavage planes of forsterite and it has been shown computationally to be one of the most stable forsterite surfaces (Watson et al., 1997; de Leeuw et al., 2000). To investigate to what extent our findings are dependent on the surface structure and composition, a hydroxylated (0 1 0) model surface as well as the (0 1 1) surface were also considered. The (0 1 1) surface was also calculated by de Leeuw et al. (2000) to have one of the lowest surface energies of the forsterite surfaces in hydrous conditions. The specific objectives of this contribution are to determine the nature of CO2 and H2O bonding at the surface and how it changes with H2O content, the distribution of H2O between the scCO2 fluid and the mineral surface, and where carbonic acid/bicarbonate formation reactions are most likely to take place. In addition, some estimates of the thickness of water films on forsterite as a function of water content in WBSF were reported by Loring et al. (2011); however, the reliability of these estimates is unknown. Therefore, there is a need for accurate predictions of the extent of water surface coverage as a function of water content in WBSF. Finally, MD simulations can provide representations of surface and interfacial molecular structures for cluster calculations evaluating reaction barrier heights with high-level ab initio methods (Nguyen et al., 2008; Morrow et al., 2010). The long-term goal of this research is to determine the role of interfacial H2O and CO2 on the mechanisms of reaction of orthosilicate minerals in contact with scCO2 containing variable amount of H2O. 2. COMPUTATIONAL METHODS Potential model and parameters. In the model used in this work, atoms are represented as point-charge particles that interact via long-range Coulombic forces and short-range interactions. The latter are described by parameterized functions and represent the repulsion between electroncharge clouds, van der Waals attractive forces, and, where applicable, covalent effects. The potential model used in this work combines the SPC/E water model (Berendsen et al.,

Forsterite-scCO2/H2O interfaces

1987), the EPM2 carbon dioxide model (Harris and Yung, 1995), and a modified version of the CLAYFF model (Cygan et al., 2004) for simulating forsterite. In the SPC/E model, the O–H bond length and the HOH bond angle are kept fixed. In the EPM2 model, the C–O bond length is kept fixed but the OCO angle is not constrained. In the CLAYFF model, all the atoms of the forsterite lattice are free to move. The CLAYFF model was modified by changing the magnesium charge from the original partial charge of +1.36 e to the formal charge, +2.0 e. This change was done for two reasons. First, it allows us to use the potential parameters of Kerisit and Parker (2004) for simulating magnesium–water interactions. These parameters were shown to give a satisfactory description of the structure and dynamics of water around magnesium (Kerisit and Parker, 2004; Kerisit and Liu, 2010). Second, it will allow for future simulations of the adsorption and diffusion of alkaline-earth cations near orthosilicate surfaces. Indeed, using partial charges for modeling the alkaline-earth cations present in orthosilicate minerals creates an electrostatic imbalance with respect to the formal charge used for alkaline-earth cations adsorbing from aqueous solutions. The Si charge was kept constant and the oxygen charge was modified to make the Mg2SiO4 unit charge neutral. In addition, the R parameter of Si (Cygan et al., 2004), which is the distance parameter in the Lennard–Jones formulation (see Eqs. (1)–(3) below), was increased by 16% (from 3.7064 ˚ ) to fit the experimental Si-O bond lengths reto 4.2994 A ported by Hazen (1976) (the lowest temperature reported in that study, 196 °C, was used for comparison). The bond lengths, calculated from an energy minimization of the forserite bulk structure with the computer code METADISE (Watson et al., 1996), are (percentage differences with respect to the experimental values are shown in parenthe˚ (0.8%), 1.653 A ˚ (+0.2%), and ses): Si-O = 1.603 A ˚  2 (+0.1%). The R parameter of Mg (Cygan 1.635 A et al., 2004) was increased by 8% (from 5.9090 to ˚ ) to reproduce the forsterite experimental lattice 6.3817 A parameters (Hazen, 1976). The calculated lattice parame˚ (0.2%), b = 10.17 A ˚ ters are (Pbnm): a = 4.736 A ˚ (+0.3%). (0.1%), c = 5.992 A A thermodynamic integration calculation of a single Mg2+ ion in a simulation cell containing 511 H2O molecules was performed following the approach described in a previous publication (Kerisit and Rosso, 2009) and, in particular, included a series of corrections described by Kastenholz and Hu¨nenberger (2006a,b). The free energy of hydration thus calculated (1848 kJ mol1) was within 1% of the experimental value reported by Marcus (1991) (1830 kJ mol1), indicating that the energetics of magnesium hydration are well reproduced by the potential model used in this work. To evaluate the effects of hydroxylation on the results of the MD simulations, interfacial water molecules were dissociated to form [SiO3(OH)]3 surface species and OH groups coordinated to surface magnesium ions. The potential parameters and ionic charges for the hydrogen atoms of both hydroxyl species and for the oxygen of the hydroxyl group of the [SiO3(OH)]3 surface species were taken from

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the CLAYFF model. The potential parameters for the hydroxyl oxygen–hydroxyl oxygen interactions of the OH groups coordinated to surface magnesium ions were derived using the brucite (Mg(OH)2) bulk structure while keeping the CLAYFF parameters to describe the magnesium–magnesium and magnesium–hydroxyl oxygen interactions constant. The oxygen partial charge was set to make the Mg(OH)2 unit charge neutral. Starting from the CLAYFF parameters for the hydroxyl oxygen–hydroxyl oxygen interactions, the A parameter (see Eq. (1)) was increased to fit the lattice parameters of brucite. The experimental (Desgranges et al., 1996) and calculated lattice parameters of bru˚ , c = 4.779 A ˚ and a = b = 3.163 A ˚ cite are: a = b = 3.148 A ˚ (2.0%), respectively. (+0.5%), c = 4.685 A Following the approach used in CLAYFF and in other force fields, the arithmetic mean rule and the geometric mean rule were used for the distance parameter R and energy parameter D, respectively, to calculate the Lennard– Jones interactions between all atoms from the atomic parameters (Cygan et al., 2004): ELJ ij ¼

Aij Bij  6 r12 rij ij

 12 pffiffiffiffiffiffiffiffiffiffi 1 ðRi þ Rj Þ Di Dj  2  6 pffiffiffiffiffiffiffiffiffiffi 1 ðRi þ Rj Þ Bij ¼ 2 Di Dj  2 Aij ¼

ð1Þ ð2Þ ð3Þ

There were three exceptions: (1) the magnesium–water oxygen, for which previously derived Buckingham potentials were used as described above; (2) the A parameter of the OX–OW Lennard–Jones potential, where OX = O, OH or OHS, was scaled to the oxygen charge, q, (i.e., q AOX OW ¼ AOW OW qOOX ) to avoid short contacts of water W with the surface following the approach used in a previous study (Kerisit et al., 2008) with the SPC/E and CLAYFF potential model parameters (the atom labels are defined in the Electronic Annex); and (3) similarly, the A parameters of the O–OHS and OHS–OH Lennard–Jones potentials were set to those of the O–OW and OHS–OHS potentials, respectively, to avoid short contacts of the surface hydroxyl groups with the surface. All the ionic charges and potential parameters used in this work are reported in the Electronic Annex. Molecular dynamics simulations. All the calculations were carried out with the computer program DL_POLY (Smith and Forester, 1996). The geometry of the water molecules and the carbon–oxygen bonds were held fixed using the SHAKE algorithm (Ryckaert et al., 1977). The temperature and pressure were kept constant via the Nose´-Hoover thermostat (Hoover, 1985) and the Hoover barostat (Melchionna et al., 1993), respectively, with relaxation time parameters of 0.5 ps. The electrostatic forces were calculated by means of the Ewald summation method (Ewald, ˚ cutoff was used for the short-range interac1921). A 9 A tions and the real part of the Ewald sum. The Ewald sum parameters were chosen to achieve a relative error on the electrostatic energy of at most 107. The Verlet leapfrog integration algorithm was used to integrate the equations

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of motion with a time step of 0.001 ps. The temperature and pressure conditions employed in the experiments of Loring et al. (2011), i.e., 50 °C and 180 atm, were used as reference conditions in this work. Two system sizes, referred to hereafter as small and large, were used to model scCO2/H2O mixtures in contact with the forsterite surfaces. The large system size was required to encompass water films thicker than three monolayers. As the large system is computationally expensive, the small system size was used for water films below three monolayers. For the (0 1 0) surface, after orientation of the crystal, the surface vectors were scaled as to generate ˚ 2 and slabs with surface areas of 18.945  17.977 A 2 ˚ for the small and large systems, respec28.418  29.961 A tively. For the (1 1 0) surface, the slab surface areas were ˚ 2 and 28.418  35.413 A ˚ 2, for the small 18.945  23.609 A and large systems, respectively. The slab thicknesses were ˚ and 13/25 A ˚ (small/large), for the approximately 20/30 A (0 1 0) and (1 1 0) surfaces, respectively. In the small system, ˚ gap was introduced between the two faces of the a 20-A ˚ gap was used for the large mineral slab, whereas a 100-A system. The compositions of the fluid phase in the MD simulations of scCO2/H2O mixtures at forsterite surfaces are summarized in Table SV of the Electronic Annex. 3. RESULTS AND DISCUSSION The remainder of this work is divided into three sections. In the first section, the H2O-forsterite interactions are validated against experimental data on the energetics of water adsorption at forsterite surfaces. In the second section, the thermodynamic driving force for H2O adsorption at the forsterite (0 1 0) surface is determined as a function of water content in scCO2 to give insights into the extent of surface hydration as a function of water saturation of the scCO2 fluid. Finally, in the last section, structural and dynamical properties of forsterite–scCO2/H2O interfaces are investigated as a function of water content and the effects of surface structure and surface hydroxylation are also evaluated. We note that the ability of the model to reproduce accurately the H2O–CO2 interactions was assessed by comparing the results of bulk MD calculations with experimental data and ab initio molecular dynamics (AIMD) simulations and that these results are described in detail in the Electronic Annex. H2O adsorption at the forsterite (0 1 0) surface from the gas phase. In this section, the energetics of water associative adsorption from the gas phase are calculated and compared to the calorimetric data of Chen and Navrotsky (2010) as well as to previous computational work to validate the model ability to reproduce H2O–forsterite interactions. The calculations made use of the small system size and the gap between the two surfaces of the forsterite slab was filled with varying amounts of water. Calculations were performed for 0, 1/4, 1/2, 1, 2, 3, 4, and 5 water monolayers as well as for the case where the gap is filled with sufficient water to obtain a water density of 1.0 g cm3 in the middle of the water slab. We note that density functional theory plane-wave calculations (de Leeuw et al., 2001) showed that an isolated dissociated water molecule spontaneously

recombined at this surface, which suggests that associative adsorption is energetically favored over dissociative adsorption at this surface at low water coverage as was also predicted from classical atomistic models (de Leeuw et al., 2000; Stimpfl et al., 2006; King et al., 2010b). Therefore, we only consider associative water adsorption for the purposes of this section. All the calculations were carried out in the NVT ensemble for 1 ns, after a 100 ps equilibration period, at 25 °C to enable comparison with the experimental data of Chen and Navrotsky (2010). The adsorption energy was obtained as follows:   Eads: ¼ hEix  hEi0 =n ð4Þ where hEix is the ensemble average energy of the system with x monolayers and n is the total number of water molecules in the system. The initial configurations were generated by deleting the appropriate number of water molecules from the equilibrated bulk H2O simulation. In Fig. 1, the calculated energies of adsorption are compared to the integral heat of water adsorption measured by Chen and Navrotsky (2010). Good agreement is obtained throughout but especially at coverages higher than 20 H2O/nm2. As noted by Chen and Navrotsky (2010), this value corresponds to the coverage at which additional water molecules are physisorbed on this surface. Chen and Navrotsky (2010) estimated that this coverage translates to two monolayers of water based on a cross-sectional area of 0.1 nm2 for water; however, our simulations reveal that, for the (0 1 0) surface, this coverage actually corresponds to three monolayers of water. The difference is due to the fact that, as noted in their paper, Chen and Navrotsky (2010) used a rough approximation for the water cross-sectional area. We note that Chen and Navrotsky (2010) do not include a description of how the water cross-sectional area was approximated.

Fig. 1. Comparison of the calculated water adsorption energy on the forsterite (0 1 0) surface at 25 °C as a function of surface coverage with the experimental data of Chen and Navrotsky (2010). Error bars on the calculated water adsorption energies (calculated as ±one standard deviation of the mean when the simulations were divided into five equivalent blocks) are smaller than data points. Also shown are adsorption energies obtained in previous computational studies (de Leeuw, 2001; Stimpfl et al., 2006).

Forsterite-scCO2/H2O interfaces

We also note that the data of Chen and Navrotsky (2010) corresponds to an average over all the surfaces exposed by the nano-powders, whereas our results are for the (0 1 0) surface only. Adsorption energies obtained for 1/2 a monolayer with a different potential model (Stimpfl et al., 2006) and from a density functional theory calculation (de Leeuw, 2001) are also shown in Fig. 1. These results indicate that the model used in this work satisfactorily reproduces the energetics of H2O–forsterite interactions. H2O adsorption at the forsterite (0 1 0) surface in scCO2. The aim of this section is to determine the thermodynamic driving force for water adsorption as a function of the level of water saturation in the scCO2 phase. To achieve this goal, a series of potential of mean force (PMF) simulations was carried out to calculate the free energy of adsorption of water on the forsterite (0 1 0) surface for different extents of surface hydration. The reaction of interest is the adsorption of H2O from the scCO2 phase onto the forsterite surface: H2 OðscÞ ! H2 Oðads:Þ

ð5Þ

with free energy of reaction, DAR. Because the size of the MD simulation cell does not allow for simulating low concentrations of H2O in scCO2, the following two reactions were used instead in order to evaluate DAR: H2 OðscÞ ! H2 OðscMDÞ ð6Þ H2 OðscMDÞ ! H2 Oðads:Þ

ð7Þ

with free energies of reaction, DAC and DAPMF, respectively, and where the subscript “scMD” refers to the adsorbing water molecule in the center of the scCO2 phase in the MD simulation cell (i.e., the furthest point away from the forsterite surface). Assuming that the MD simulation cell is large enough so that DH = 0 for Eq. (6) (see the Electronic Annex for an evaluation of this assumption), DAC reduces to the entropy change due to the difference between the water concentration in the MD simulation cell, C2, and that in the macroscopic scCO2 phase, C1, written as the product of the equilibrium concentration, C0, and the saturation level, S. Therefore, ð8Þ DAR ¼ DAPMF þ DAC ¼ DAPMF þ RT ln ðC 2 =ðC 0  SÞÞ   ads: where N ads: C2 is taken as N ads: H2 O = N H2 O þ N CO2 H2 O is the number of water molecules adsorbing from the scCO2 phase (i.e., 1) and N CO2 is the number of CO2 molecules in the MD simulation cell, C0 is the water solubility in the scCO2 phase in molecules per molecule of fluid calculated from a MD simulation of a H2O/scCO2 interface carried out at the same temperature and density conditions, and DAPMF is obtained from the PMF calculations. Constrained PMF calculations were performed for a H2O molecule adsorbing on the (0 1 0) forsterite surface with 0, 1, and 2 water monolayers already adsorbed. In each case, the PMF was constructed from 71 1-ns NVT MD simulations with the small system size. All calculations were performed at 50 °C and qscCO2 ¼ 0:707g cm3 , which corresponds to a pressure of 180 atm. The resulting free energy profiles are shown in Fig. 2. Water adsorption on a bare surface is highly exothermic (DAPMF = 95.3 kJ mol1) and shows only one minimum

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Fig. 2. Potential of mean force of a H2O molecule adsorbing from the scCO2 phase onto the forsterite (0 1 0) surface for different extents of surface hydration. T = 50 °C and qscCO2 ¼ 0:707g cm3 .

˚ above the surface. When adsorbing on a surface alat 0.9 A ready covered with one or two water monolayers, water ad˚ above the surface, respectively. sorbs at 1.8 and 4.1 A DAPMF for these two sites are 46.7 and 19.7 kJ mol1, respectively. For each of these three values of DAPMF, the free energy of adsorption on surfaces with different extents of hydration as a function of water saturation was calculated using Eq. (8), as shown in Fig. 3. Fig. 3 indicates that adsorption on a bare surface or on a surface with a single monolayer is always exothermic whereas adsorption on a surface with two monolayers will depend upon the water content of the scCO2 phase. In addition, the water free energy profiles for a forsterite surface in contact with 3 monolayers and with a 2-nm film were calculated using the water density obtained from 10-ns NVT MD simulations performed at 50 °C and qscCO2 ¼ 0:707g cm3 . The large system size was used because the small system size was not large enough to encompass the H2O/scCO2 interface. The following formula was employed to determine DA(z) (Marrink and Berendsen, 1994): DAðzÞ ¼ RT ln ðhqðzÞi=hq0 iÞ

ð9Þ

Fig. 3. Free energy of adsorption of a H2O molecule from the scCO2 phase onto the (0 1 0) forsterite surface as a function of water saturation in the scCO2 phase for different extents of surface hydration. T = 50 °C and qscCO2 ¼ 0:707g cm3 .

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where hqz i and hq0 i are the ensemble averages of the water density at height z and in the bulk of the scCO2 phase, respectively. As shown in Fig. 2 for the 3-monolayer simulation, the minimum free energy of a H2O molecule in the third monolayer relative to that in the scCO2 phase is 24.9 kJ mol1. This value is more exothermic than the free energy difference between H2O in the H2O-rich phase and H2O in the CO2-rich phase (23.6 kJ mol1) obtained from the H2O/scCO2 interface simulation mentioned above. Consequently, the MD simulations indicate that the forsterite surface can provide an additional driving force for water condensation up to three monolayers and that, therefore, a 3-monolayer water film may form even in an undersaturated scCO2 phase. This is consistent with the nuclear magnetic resonance results of Kwak et al. (2011), who observed the formation of hydrated carbonate phases following the carbonation reaction of forsterite with a scCO2 fluid containing water below the saturation concentration. It is also consistent with the water thicknesses of a few a˚ngstroms predicted by Loring et al. (2011) below water saturation from infrared spectroscopy. The 2-nm film simulation indicates that DA(z) converges to the value obtained in the H2O/scCO2 interface simulation beyond the third monolayer, which suggests that the presence of the surface does not affect the energetics of water condensation beyond the third monolayer. Therefore, water saturation

levels of at least 100% are expected to be required to form water films thicker than 3 monolayers. Forsterite–scCO2/H2O interfaces as a function of water content. The aim of this section is to determine the structural and dynamical properties of H2O and CO2 at the interface with forsterite surfaces and elucidate the nature of H2O and CO2 bonding at the surface as a function of water content. As a reference, NVT MD simulations were performed for a bulk aqueous phase and a bulk scCO2 phase in contact with the forsterite (0 1 0) surface. Additionally, a series of MD simulations was carried out with a scCO2 phase in contact with the (0 1 0) forsterite surface with 1/4, 1/2, 1, 2, and 3 monolayers of water adsorbed on the surface as well as with a 2-nm water film. Moreover, the simulations with 1, 2, and 3 monolayers of water and that with a 2-nm water film were repeated for the (0 1 1) surface and for the hydroxylated (0 1 0) surface to investigate the effects of surface structure and surface hydroxylation on the nature of H2O and CO2 bonding. The small system size was used for all simulations except for the 3-monolayer and 2-nm simulations, for which the large system size was employed. The MD simulations were run for 10 ns after an equilibration period of 100 ps. Again, the temperature was kept fixed at 50 °C and the scCO2 density was set to 0.707 g cm3, which corresponds to a pressure of 180 atm. Snapshots of the simulations are shown in Fig. 4. In the fol-

Fig. 4. Snapshots from the MD simulations of (a) bulk H2O in contact with the (0 1 0) surface, (b) bulk H2O in contact with the hydroxylated (0 1 0) surface, (c) bulk scCO2 in contact with the (0 1 0) surface, (d) 3-monolayer film in contact with the (0 1 0) surface, and (e) 3-monolayer film in contact with the (0 1 1) surface. Silicon atoms are shown in yellow, magnesium atoms in green, oxygen atoms in red, carbon atoms in gray, and hydrogen atoms in white. The forsterite surfaces are represented by a stick and ball model whereas H2O and CO2 molecules are represented by a stick model.

Forsterite-scCO2/H2O interfaces

lowing discussion, the average bond distance between two atom types was determined from the position of the first maximum in the corresponding radial distribution function. Three distinct hydration layers form in the simulation of bulk water at the (0 1 0) surface, as shown in Fig. 4a and Fig. 5. The first hydration layer corresponds to a surface coverage of 7.0 molecules/nm2 (i.e., two water molecules per surface magnesium ions) and is composed of one type of water molecule with an average bond distance of ˚ to a surface magnesium ion and also forming, on 2.19 A average, 1.1 hydrogen bonds with an average bond distance ˚ with a surface oxygen atom. The fact that the of 1.82 A average number of hydrogen bonds is greater than 1.0 indicates that a fraction of the water molecules in the first hydration layer can have both of their hydrogen atoms involved in hydrogen bonding with surface oxygens at any one time. There is no exchange in the first layer indicating that the water residence time in this layer is greater than the time scale of the MD simulation. The second hydration layer integrates to a surface coverage of 6.8 molecules/nm2 and is composed of two types of water molecules. The first molecule binds to a surface magnesium ion with a bond dis˚ but does not form any hydrogen bond with tance of 2.13 A the surface. Again, this molecule does not exchange during the entire simulation. The second molecule is not coordinated to a surface magnesium ion and donates hydrogen bonds to surface oxygens and first-layer and bulk H2O molecules. This molecule is calculated to have a residence time of 110 ps. The third hydration layer is wider, shows a shoulder, and has a surface coverage value of 8.8 molecules/nm2. The (0 1 0) forsterite surface was hydroxylated by protonating one of the two upward-pointing oxygens of the surface SiO4 4 groups and adsorbing a hydroxyl group on a nearest-neighbor surface magnesium ion (Fig. 4b). The first hydration layer, which consists of water molecules and hydroxyl groups in a 1:1 ratio, has the same value of surface coverage as the first hydration layer of the non-

Fig. 5. O(H2O), O(H2O + OH), C(CO2), and O(CO2) density profiles as a function of distance away from the surface from separate 10-ns NVT MD simulations of the non-hydroxylated (0 1 0) surface in contact with bulk H2O, the hydroxylated surface in contact with bulk H2O, and the non-hydroxylated surface in contact with scCO2.

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hydroxylated surface, whereas the second hydration layer only has a reduced value of 3.5 molecules/nm2 (Fig. 5). Unlike for the non-hydroxylated surface, water molecules in the first hydration layer have, in addition to one hydrogen atom pointing down towards a surface oxygen, one hydrogen atom pointing up towards the bulk of the solution be-

Fig. 6. O(H2O), C(CO2), and O(CO2) density profiles as a function of distance away from the (010) forsterite surface for (a) 1/4, (b) 1/ 2, (c) 1 and (d) 2 monolayers of water adsorbed on the surface.

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cause of the repulsion due to the presence of the hydrogen atoms of the [SiO3(OH)]3 surface groups. This has the effect of pushing the water molecule referred to as the second molecule of the second hydration layer in the non-hydroxylated case further from the surface and into the third hydration layer, which, as shown in Fig. 5, is now found closer to the mineral surface. The second hydration layer of the non-hydroxylated (0 1 0) surface is less dense as a result. Two distinct solvation layers are predicted to form in the simulation where the (0 1 0) forsterite surface is in contact with dry scCO2 (Fig. 4c and Fig. 5). The first layer corresponds to a surface coverage of 3.7 molecules/nm2 or

approximately half that of the first hydration layer. The CO2 molecules lie flat on the surface with their carbon atom ˚ above the surface oxygen of positioned, on average, 2.83 A the Si–O bond that lies in the plane parallel to the surface, and their oxygen atoms pointing towards surface magne˚ . The residence time of CO2 sium at a distance of 2.53 A in this layer is 600 ps. The second layer integrates to a surface coverage of 2.5 molecules/nm2. In this layer, most of the CO2 molecules are pointing down the surface at varying angles and are coordinated to surface magnesium atoms through one of their oxygen atoms with an average bond ˚ . Integration of the first peak of the distance of 2.48 A O(CO2)–Mg RDF reveals that only 70% of the CO2

Fig. 7. O(H2O), O(OH), C(CO2), and O(CO2) density profiles as a function of distance away from the (0 1 0) (top), hydroxylated (0 1 0) (middle), and (0 1 1) (bottom) forsterite surfaces for 1 (left) and 2 (right) monolayers of water adsorbed on the surfaces.

Forsterite-scCO2/H2O interfaces

molecules in the second layer have one of their oxygen atoms interacting with the surface, suggesting that the remaining 30% do not adopt a specific orientation. The residence time of CO2 in this layer is 19 ps. In the simulations of the (0 1 0) forsterite surface in contact with scCO2 and increasing amounts of water (Fig. 6), the strong binding of water to the surface means that water easily displaces CO2 at the surface and that the water molecules did not escape to the scCO2 phase except in the 2monolayer MD simulation. In the 1/4- and 1/2-monolayer simulations (Fig. 6a and b), most of the water molecules occupy the first layer with only a very small percentage (0.2 and 0.3%, respectively) of the water molecules in the second layer. Due to the strong water–forsterite interactions, the water molecules are not able to form clusters on the surface at these water contents. In the 1-monolayer simulation (Fig. 6c), there is an appreciable amount of water in the second hydration layer with the first and second layer occupancies being 89% and 12% that in the bulk water simulation, respectively. These occupancies increase to 100% and 98% for the 2-monolayer simulation (Fig. 6d) with the remaining water molecules in the third layer and the bulk of the scCO2 phase. CO2 is able to adsorb on the surface, in the 1/4- and 1/2-monolayer simulations (Fig. 6a and b), in the configuration observed in the bulk calculation with occupancies of 80% and 36% of that in the bulk calculation, respectively. Compared to the bulk calculation, a new peak appears, at a height of approxi˚ , due to the CO2 molecules interacting with admately 2.6 A sorbed water molecules (Fig. 6a and b). In the 1-monolayer simulation (Fig. 6c), the C(CO2) density profile differs significantly from the bulk case. There is a small density of CO2 molecules in the region where the first solvation peak was in the bulk simulation, but there is no noticeable peak. Additionally, the first C(CO2) peak, although in the same region as the second solvation peak in the bulk calculation, is much wider and shifted to longer distances. In the 2monolayer simulation (Fig. 6d), H2O completely displaced

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CO2 from the surface. In all cases, the position and shape of the first two O(H2O) peaks are essentially identical to the bulk case, thus suggesting that the structure of water in the hydration layer is the same as in bulk water. Again, this can be explained by the strong water–forsterite interactions. Fig. 7 illustrates the influence of surface hydroxylation and surface structure on the density profiles. When the surface is hydroxylated and 1 monolayer is adsorbed on the surface, CO2 molecules are able to approach the forsterite surface closer than when the surface is not hydroxylated (Fig. 7a and b, Fig. 8a and b). SiO4 4 groups on the surface are protonated and water molecules in the first hydration layer donate fewer hydrogen bonds to these groups as a result. Therefore, adsorbed water molecules are found more frequently directly above surface magnesium ions instead of on either side of the surface magnesium rows (Fig. 8a and b). This configuration allows CO2 molecules to approach the rows of [SiO3(OH)]3 closer (Fig. 8a and b). However, when 2 monolayers are adsorbed on the surface, CO2 molecules are displaced by water, which is more strongly attached to the surface via hydrogen bonds with surface oxygens (Fig. 7d and e). When the (0 1 1) surface is in contact with 1 monolayer of water, 77% of the water molecules are found in the first hydration layer and are coordinated to a surface magnesium ion whereas the remaining water molecules are positioned in the second hydration layer and interact via hydrogen bonds with surface oxygens and other water molecules. As a result, the surface SiO44 groups are mostly exposed and can interact ˚ in the with CO2 molecules as evidenced by a peak at 2.93 A C(CO2)–O(SiO44) RDF (data not shown). However, when the surface is put in contact with 2 monolayers of water, H2O displaces the CO2 molecules previously positioned above the rows of SiO44 groups as it can interact more strongly with these groups via hydrogen bonds. Therefore, CO2 does not contact the surface when two monolayers are adsorbed in all three cases considered in this work. To con-

Fig. 8. Snapshots from the MD simulations of the (a) (010) and (b) hydroxylated (010) surfaces in contact with scCO2 and hydrated with one monolayer.

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firm this conclusion, the potential of mean force of a CO2 molecule adsorbing on the (0 1 0) forsterite surface with two water monolayers was calculated using the same approach described above for water, as described in the Electronic Annex. The PMF curve (Fig. A4 of the Electronic Annex) shows no free energy minimum in the hydration layer thus confirming that CO2 cannot penetrate the twomonolayer hydration layer. This finding validates the hypothesis, stated in the Introduction, that oxygen abstraction from interfacial water is the dominant mechanism for carbonate formation in the presence of sufficient water in the WBSF.

The large system size was used to investigate scCO2/H2O mixtures in contact with the (0 1 0) forsterite surface for higher water coverages, namely, a 3-monolayer film and 2-nm film (Fig. 9). A snapshot of the 3-monolayer simulation is shown in Fig. 4d. In the simulation of a 3-monolayer film, the shape and positions of the first three O(H2O) peaks are similar to that in the bulk water simulation, as shown in Fig. 9a. The occupancies of the first, second and third hydration layers are 100%, 97% and 79% that of the same layers in the bulk water simulation. The remaining water molecules are found either at the interface with scCO2 or dissolved in the scCO2 phase. The C(CO2) density profile

Fig. 9. O(H2O), O(OH), C(CO2), and O(CO2) density profiles as a function of distance away from the (0 1 0) (top), hydroxylated (0 1 0) (middle), (0 1 1) (bottom) forsterite surfaces for a 3-monolayer (left) and 2-nm (right) water film. The vertical solid lines and the boxed numbers designate the regions of different CO2 bonding environments as explained in the text.

Forsterite-scCO2/H2O interfaces

˚ from the surface, shows a first peak centered at 5.5 A approximately where the third hydration layer ends (Fig. 9a). A second peak is observed on the CO2 side of ˚ from the surface. The presence of this the interface at 7.9 A second peak is consistent with the results of da Rocha et al. (2001) and our own simulation of the scCO2/H2O interface (data not shown), which reveal an increase in CO2 density relative to the bulk on the CO2-rich side of the interface. The two peaks just described, the interfacial region up to the point where both the CO2 and H2O densities reach a constant value, and the bulk of the scCO2 phase constitutes the four regions of the interface at this water content (Fig. 9a). When the surface is hydroxylated, there is no effect on how close CO2 molecules can approach the surface at this water content (Fig. 9b). However, the C(CO2) and O(CO2) density peaks in the third hydration layer disappear. As discussed in the above, the three-dimensional arrangement of water at the interface with a hydroxylated surface is different from that obtained when only associative adsorption is considered. Fig. 9b therefore suggests that, although the closest distance of approach is not affected by surface hydroxylation, the density of CO2 molecules in the vicinity of the interface is significantly influenced by surface hydroxylation through its effects on the water arrangement at the interface. Fig. 9c shows the density profiles obtained with the (0 1 1) surface. Again, CO2 cannot contact the forsterite surface directly and the details of its density profile in the vicinity of the mineral surface differ from those obtained for the (0 1 0) surface. In the simulation of a 2-nm water film in contact with the (0 1 0) forsterite surface, the O(H2O) density profile shows the three distinct hydration layers, as expected (Fig. 9d). The C(CO2) density profile displays two distinct peaks with positions similar to those calculated in the 3-monolayer film ˚ . A total of six different simulation, i.e., at 5.0 and 8.0 A interfacial regions are predicted for a 2-nm water film as shown in Fig. 9d. Similar conclusions to those drawn for the 3-monolayer film are obtained when examining the density profiles of the hydroxylated (0 1 0) surface and those of the (0 1 1) surface. Indeed, the C(CO2) and O(CO2) density profiles are essentially identical to those of the non-hydroxylated (0 1 0) surface with the exception of regions 1 and 2 as these regions are most sensitive to the arrangement of water molecules at the interface and thus to the structure and composition of the mineral surface. To identify the region of the interface with the greatest extent of mixing between H2O and CO2 and thus provide information on where carbonic acid/bicarbonate is most likely to form, we show in Fig. 10 the product of the O(H2O) and C(CO2) densities. For the non-hydroxylated (0 1 0) surface, the region of greatest mixing is more intense for the 3-monolayer film simulation but narrower than that in the 2-nm film simulation. A similar picture is obtained for the (0 1 1) surface whereas, for the hydroxylated (0 1 0) surface, the absence of a C(CO2) density peak inside the third monolayer reduces the mixing peak maximum in the 3-monolayer simulation. In all three cases, however, the region of greatest mixing is found further away from the mineral surface the thicker the water film.

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Fig. 10. Extent of CO2/H2O mixing as a function of distance away from (a) the (0 1 0) surface, (b) the hydroxylated (0 1 0) surface, and (c) the (0 1 1) surface for a 3-monolayer and 2-nm water film.

C(CO2)–O(H2O) RDF were calculated in each of the different interfacial regions identified above in the 3-monolayer and 2-nm simulations. Integration of the RDF up to either the shoulder of the first peak or to the first minimum yields the CO2 coordination with H2O for each region. The results are shown in Table 1 as a percentage of the bulk coordination number (i.e., 100% corresponds to fully hydrated CO2). This analysis was performed to evaluate the extent of hydration of CO2 in the different regions of the interface. For a 3-monolayer water film, CO2 is hydrated, at most, by approximately half the number of H2O molecules for the bulk case, whereas, for a 2-nm film, there is an extensive region where CO2 solvation is equal or greater to the bulk case. Assuming that a greater extent of hydration favors carbonic acid/bicarbonate formation (Nguyen et al., 2008), this analysis suggest that the reaction of CO2 with H2O to form carbonic acid/bicarbonate would be more extensive for the 2-nm water film. However, the over-

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Table 1 CO2 coordination number with H2O (in percentage of the coordination number of a single CO2 molecule in bulk H2O, i.e., 2.22 and 18.71 for the “Shoulder” and “First Min.” cases) for different regions of the forsterite (0 1 0)-scCO2/H2O interface, as obtained from integration of the C(CO2)–O(H2O) RDF to the ˚ ) and the first minimum (5.25 A ˚ ). shoulder of the first peak (3.40 A Region

1 2 3 4 5 6

3-monolayer film

2-nm film

Shoulder

First min.

Shoulder

First min.

47 17 1 0.03

56 22 2 0.02

107 98 97 40 5 0.02

106 99 97 45 7 0.02

all extent of reaction will also be dependent on other parameters, such as the extent of mixing, and the interplay between the different parameters cannot be probed with the current model.

The distance dependence of the diffusion coefficient of both molecules was also calculated to investigate the dynamical properties of the interface (Fig. 11). In both simulations, the water molecules in the first two hydration layers do not diffuse on the timescale of the simulation. Beyond the first two hydration layers, DH2 O increases as the distance from the surface increases. In the 3-monolayer simulation, the water film is not thick enough to observe a convergence of DH2 O to its bulk value and, instead, it increases continuously across the H2O–scCO2 interface. In the scCO2 phase, it is similar to that of CO2, albeit with much larger fluctuations due to the limited number of H2O molecules that visit this region during the simulation. In the 2-nm film simulation, DH2 O starts to converge to its bulk value but rises again because of the presence of the H2O–scCO2 interface. Away from the interface, DCO2 is similar but systematically lower than DH2 O , which is consistent with the results obtained for a lone CO2 molecule in H2O. Although DH2 O and DCO2 are similar in the H2O- and CO2-rich phases, DCO2 is much greater than DH2 O in the interfacial region. This can be explained by the extent of

Fig. 11. O(H2O) and C(CO2) diffusion profiles as a function of distance away from the (0 1 0) forsterite surface for (a) a 3-monolayer water film and (b) a 2-nm water film. Also shown is the O(H2O) density profile.

Forsterite-scCO2/H2O interfaces

hydration of the two molecules as a function of their position across the interface. As shown in Fig. 11b for the 2-nm film simulation, DH2 O begins to depart significantly from DCO2 at the lower boundary of region 4. Its slope then increases in the lower section of region 5 and DH2 O becomes similar to DCO2 again approximately in the middle of region 5. The region with the lower slope was divided into two equal layers and the region with the higher slope was used as a third layer for analysis. Integration of the O(H2O)– O(H2O) RDF up to the first minimum yielded H2O coordination numbers of 95, 78, and 50% that of the H2O coordination number in bulk water in the three layers, respectively. According to Table 1, the CO2 coordination number with H2O in the same region of the interface ranges from 40–45% to 5–7%. Therefore, H2O is more hydrated than CO2 at the interface relative to their respective bulk environments. The H2O–H2O interactions being more strongly attractive than the H2O–CO2 interactions, H2O molecules prefer to solvate other H2O molecules than CO2 molecules. In addition, the full coordination numbers of CO2 and H2O by H2O are 18.71 and 4.78, respectively. Consequently, the volume of the CO2 first hydration shell is much larger than that of H2O and CO2 must penetrate deeper into the water film to be able to acquire a full hydration shell. As a result, DCO2 is calculated to be much greater than DH2 O in the liquid–supercritical fluid interfacial region. Although the analysis of the diffusivity of H2O and CO2 in the different regions of the interface does not relate directly ð2xÞ to the likelihood for Hx CO3 formation, it does provide information on both the conditions required for bulk conditions to be reached and the regions of the interface that show bulk conditions. Therefore, this analysis can provide guidance for the applicability of models developed for preð2xÞ dicting Hx CO3 formation in bulk conditions. 4. CONCLUSIONS The environment experienced by CO2 at the interface depends strongly upon the water content. At very low water contents, CO2 can access the surface directly and interact with surface ions leading to surface residence times that can reach several hundreds of picoseconds. Additionally, the strong interactions between water and the forsterite surface prevent the formation of water clusters at the solid– WBSF interface. As the water content increases, the forsterite surface is readily hydrated by two water layers that CO2 ð2xÞ cannot penetrate, which indicates that Hx CO3 formation most likely occurs away from the solid–WBSF interface by abstraction of oxygen from water. Surface hydroxylation alters the structure of water at the interface allowing CO2 to penetrate closer to the forsterite surface at very low water contents. Such an effect could inð2xÞ duce Hx CO3 formation at the forsterite surface. However, even small amounts of additional water (e.g., 2 monolayers) would displace CO2 away from the forsterite surface. Free energy calculations of the associative adsorption of water from the scCO2 phase onto the surface indicated that forsterite is sufficiently hydrophilic to form a water film up to three monolayers even for water contents below the

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water saturation concentration of the scCO2 fluid. This result is consistent with the experimental observation of Loring et al. (2011). In such cases, H2O molecules beyond the first two hydration layers can diffuse and CO2 can be dissolved in the water film increasing the likelihood of ð2xÞ Hx CO3 formation in this region. The propensity of forsterite for forming water films supports the experimental observations that addition of water to the WBSF increases ð2xÞ reactivity as water films can provide a locus for Hx CO3 formation, metal cation dissolution, and metal carbonate formation. For water films thicker than three monolayers, four CO2 environments are predicted: (1) close to the forsterite–H2O interface, CO2 density is structured and sensitive to the surface structure and composition, its diffusion is reduced, and its H2O coordination number is similar to that in the bulk; (2) in the bulk of the water film, CO2 environment is comparable to that in bulk water; (3) at the H2O/scCO2 interface, CO2 diffusion increases, the extent of hydration diminishes, and the mixing between CO2 and H2O is enhanced; and (4) in the scCO2 phase, CO2 diffusion is very fast and interaction with H2O is limited. As a result of these different structural and dynamical environments of CO2, ð2xÞ the rate and extent of Hx CO3 formation are likely to vary in these different regions of the forsterite–scCO2/ H2O interface. ð2xÞ Given that this study indicates that Hx CO3 formation is likely not to occur as a result of direct interaction of CO2 with the forsterite surface, future work will focus on elucidating whether metal carbonate formation takes place in the fluid following dissolution of alkaline-earth cations from the forsterite surface or whether carbonate/bicarbonate ions are able to penetrate the forsterite hydration layer to react directly with the surface. ACKNOWLEDGMENTS The authors acknowledge Prof. James R. Rustad for insightful discussions. This research was supported by the U.S. Department of Energy (DOE) Office of Basic Energy Sciences-Geosciences program through a Single Investigator Small Group Research grant. The computer simulations were performed in part using the Molecular Science Computing (MSC) facilities in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research (OBER) and located at Pacific Northwest National Laboratory (PNNL). PNNL is operated for the DOE by Battelle Memorial Institute under Contract DEAC05-76RL01830.

APPENDIX A. SUPPLEMENTARY DATA Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.gca. 2012.01.038. REFERENCES Benson S. M. and Cole D. R. (2008) CO2 sequestration in deep sedimentary formations. Elements 4, 325–331.

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Berendsen H. J. C., Grigera J. R. and Straatsma T. P. (1987) The missing term in effective pair potentials. J. Phys. Chem. 91, 6269–6271. Chen S. and Navrotsky A. (2010) Calorimetric study of the surface energy of forsterite. Am. Mineral. 95, 112–117. Cygan R. T., Liang J.-J. and Kalinichev A. G. (2004) Molecular models of hydroxide, oxyhydroxide, and clay phases and the development of a general force field. J. Phys. Chem. B 108, 1255–1266. da Rocha S. R. P., Johnston K. P., Westacott R. E. and Rossky P. J. (2001) Molecular structure of the water–supercritical CO2 interface. J. Phys. Chem. B 105, 12092–12104. Daval D., Martinez I., Corvisier J., Findling N., Goffe´ B. and Guyot F. (2009) Carbonation of Ca-bearing silicates, the case of wollastonite: experimental investigations and kinetic modeling. Chem. Geol. 265, 63–78. de Leeuw N. H. (2001) Density functional theory calculations of hydrogen-containing defects in forsterite, periclase, and aquartz. J. Phys. Chem. B 105, 9747–9754. de Leeuw N. H., Parker S. C., Catlow C. R. A. and Price G. D. (2000) Modelling the effect of water on the surface structure and stability of forsterite. Phys. Chem. Mineral. 27, 332–341. Desgranges L., Calvarin G. and Chevrier G. (1996) Interlayer interactions in M(OH)2: a neutron diffraction study of Mg(OH)2. Acta Crystallogr. B 52, 82–86. Ewald P. P. (1921) Die Berechnung optischer und elektrostatischer Gitterpotentiale. Annalen der Physik 64, 253–287. Felmy A. R., Qafoku O., Arey B. W., Hu J. Z., Hu M., Schaef H. T., Ilton E. S., Hess N. J., Pearce C. I., Feng J. and Rosso K. M. (in review) Reaction of water–saturated supercritical CO2 with forsterite: evidence for magnesite formation at low temperatures. Harris J. G. and Yung K. H. (1995) Carbon dioxide’s liquid–vapor coexistence curve and critical properties as predicted by a simple molecular model. J. Phys. Chem. 99, 12021–12024. Hazen R. M. (1976) Effects of temperature and pressure on the crystal structure of forsterite. Am. Mineral. 61, 1280–1293. Hoover W. G. (1985) Canonical dynamics - equilibrium phasespace distributions. Phys. Rev. A 31, 1695–1697. Kastenholz M. A. and Hu¨nenberger P. H. (2006a) Computation of methodology-independent ionic solvation free energies from molecular simulations. I. The electrostatic potential in molecular liquids. J. Chem. Phys. 124, 124106. Kastenholz M. A. and Hu¨nenberger P. H. (2006b) Computation of methodology-independent ionic solvation free energies from molecular simulations. II. The hydration free energy of the sodium cation. J. Chem. Phys. 124, 224501. Kerisit S. (2011) Water structure at hematite–water interfaces. Geochim. Cosmochim. Acta 75, 2043–2061. Kerisit S. and Liu C. X. (2010) Molecular simulation of the diffusion of uranyl carbonate species in aqueous solution. Geochim. Cosmochim. Acta 74, 4937–4952. Kerisit S. and Parker S. C. (2004) Free energy of adsorption of water and metal ions on the 10.4 calcite surface. J. Am. Chem. Soc. 126, 10152–10161. Kerisit S. and Rosso K. M. (2009) Transition path sampling of water exchange rates and mechanisms around aqueous ions. J. Chem. Phys. 131, 114512. Kerisit S., Liu C. and Ilton E. S. (2008) Molecular dynamics simulations of the orthoclase (0 0 1)- and (0 1 0)-water interfaces. Geochim. Cosmochim. Acta 72, 1481–1497. King H. E., Plu¨mper O. and Putnis A. (2010a) Effect of secondary phase formation on the carbonation of olivine. Environ. Sci. Technol. 44, 6503–6509. King H. E., Stimpfl M., Deymier P., Drake M. J., Catlow C. R. A., Putnis A. and de Leeuw N. H. (2010b) Computer simulations of

water interactions with low-coordinated forsterite surface sites: implications of the origin of water in the inner solar system. Earth Planet. Sci. Lett. 300, 11–18. Kwak J. H., Hu J. Z., Hoyt D. W., Sears J. A., Wang C., Rosso K. M. and Felmy A. R. (2010) Metal carbonation of forsterite in supercritical CO2 and H2O using solid state 29Si, 13C NMR spectroscopy. J. Phys. Chem. C 114, 4126–4134. Kwak J. H., Hu J. Z., Turcu R. V. F., Rosso K. M., Ilton E. S., Wang C., Sears J. A., Engelhard M. H., Felmy A. R. and Hoyt D. W. (2011) The role of H2O in the carbonation of forsterite in supercritical CO2. Int. J. Greenhouse Gas Control 5, 1081–1092. 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. Mat. Sci. 43, 2307–2315. Loring J. S., Thompson C. J., Wang Z., Joly A. G., Sklarew D. S., Schaef H. T., Ilton E. S., Rosso K. M. and Felmy A. R. (2011) Investigation of forsterite carbonation in wet supercritical CO2 by in situ infrared spectroscopy: the role of water. Environ. Sci. Technol. 45, 6204–6210. Marcus Y. (1991) Thermodynamics of solvation on ions. Part 5. Gibbs free energy of hydration at 298.15 K. J. Chem. Soc. Faraday Trans. 87, 2995–2999. Marrink S. J. and Berendsen J. C. (1994) Simulation of water transport through a lipid membrane. J. Phys. Chem. 98, 4155– 4168. McGrail B. P., Schaef H. T., Glezakou V.-A., Dang L. X. and Owen A. T. (2009) Water reactivity in the liquid and supercritical CO2 phase: has half the story been neglected? Energy Procedia 1, 3415–3419. Melchionna S., Ciccotti G. and Holian B. L. (1993) Hoover NPT dynamics for systems varying in shape and size. Mol. Phys. 78, 533–544. Morrow C. P., Kubicki J. D., Mueller K. T. and Cole D. R. (2010) Description of Mg2+ release from forsterite using ab initio methods. J. Phys. Chem. C 114, 5417–5428. Nguyen M. T., Matus M. H., Jackson V. E., Ngan V. T., Rustad J. R. and Dixon D. A. (2008) Mechanism of the hydration of carbon dioxide: direct participation of H2O versus microsolvation. J. Phys. Chem. A 112, 10386–10398. Nordbotten J. M. and Celia M. A. (2006) Similarity solutions for fluid injection into confined aquifers. J. Fluid Mech. 561, 307– 327. Oelkers E. H., Gislason S. R. and Matter J. M. (2008) Mineral carbonation of CO2. Elements 4, 333–337. Regnault O., Lagneau V., Catalette H. and Schneider H. (2005) E´tude expe´rimentale de la re´activite´ du CO2 supercritique vis-a`vis de phases mine´rales pures. Implications pour la se´questration ge´ologique de CO2. C.R. Geosci. 337, 1331–1339. Regnault O., Lagneau V. and Schneider H. (2009) Experimental measurement of portlandite carbonation kinetics with supercritical CO2. Chem. Geol. 265, 113–121. Ryckaert J. P., Ciccotti G. and Berendsen H. J. C. (1977) Numerical-integration of cartesian equations of motion of a system with constraints - molecular-dynamics of n-alkanes. J. Comput. Phys. 23, 327–341. Shao H., Ray J. R. and Jun Y.-S. (2011) Effects of salinity and the extent of water on supercritical CO2-induced phlogopite dissolution and secondary mineral formation. Environ. Sci. Technol. 45, 1737–1743. Smith W. and Forester T. R. (1996) DL_POLY is a package of molecular simulation routines, copyright The Council for the Central Laboratory of the Research Councils. Daresbury Laboratory at Daresbury, Nr. Warrington. Stimpfl M., Walker A. M., Drake M. J., de Leeuw N. H. and Deymier P. (2006) An a˚ngstro¨m-size window on the origin of

Forsterite-scCO2/H2O interfaces water in the inner solar system: atomistic simulation of adsorption of water on olivine. J. Cryst. Growth 294, 83–95. Watson G. W., Kelsey E. T., de Leeuw N. H., Harris D. J. and Parker S. C. (1996) Atomistic simulation of dislocations, surfaces and interfaces in MgO. J. Chem. Soc. - Faraday Trans. 92, 433–438.

151

Watson G. W., Oliver P. M. and Parker S. C. (1997) Computer simulation of the structure and stability of forsterite surfaces. Phys. Chem. Mineral. 25, 70–78. Associate editor: William H. Casey