Surface Speciation of Brucite Dissolution in Aqueous Mineral

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Surface Speciation of Brucite Dissolution in Aqueous Mineral Carbonation – Insights from DFT Simulations Dariush Azizi, and Faïçal Larachi J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b09140 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 13, 2019

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The Journal of Physical Chemistry

Surface Speciation of Brucite Dissolution in Aqueous Mineral Carbonation – Insights from DFT Simulations Dariush Azizi, Faïçal Larachi* Department of Chemical Engineering, Université Laval, 1065 Avenue de la Médecine, Québec, Québec G1V 0A6 Canada ABSTRACT: Aqueous mineral carbonation of brucite is an important mineralization route for carbon capture and sequestration. Prerequisite to mineral carbonation are the simultaneous CO2 absorption and brucite dissolution which imply, in the first place, the formation and release in the liquid phase of CO32-, HCO3-, Mg2+, MgOH+, MgHCO3+ ions. To gain insights on the nature of adsorption sites and resulting surface complexes, the affinity of water and of dissolved species for pristine and partially dissolved brucite (001) cleaved surfaces in aqueous mineral carbonation has been investigated using density-functional theory (DFT) simulations. The species’ affinity for uptake by brucite (001) surface is predicted to obey the trend: Mg2+ > MgHCO3+ > MgOH+ > HCO3- > CO32-, whereas the surface acid/base behavior controls affinity following the order: dehydroxylated (001) surface > deprotonated (001) surface > neutral and protonated (001) surfaces. Covalent bonds have been predicted for the following (charge-determining) ion-(001) brucite surface sites: CO32-dehydroxylated site, HCO3-dehydroxylated site, MgOH+dehydroxylated/deprotonated sites, MgHCO3+dehydroxylated/(de)protonated sites, and Mg2+neutral/(de)protonated/dehydroxylated sites. Congruent dissolution of (001) brucite surface leads to a diverse population of coordination-deficient Mg and O centers which are more active to form covalently-bonded surface complexes with aqueous CO32-, HCO3-, Mg2+, MgOH+, MgHCO3+ as compared to the undissolved surface. However, although affinity of the altered surfaces for dissolved ions increases conspicuously, the same affinity trend is predicted for the dissolving surfaces as compared to the pristine (001) brucite surface.

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1. INTRODUCTION Concerns for Earth global warming are increasingly brought to the fore by pointing fingers to the anthropogenic origin of greenhouse gas (GHG) emissions into the atmosphere.1 Technological solutions for mitigation of GHG emissions have thus become important research subjects,2-6 such as, switching to renewable energy,7 ameliorating energy efficiency of internal combustion engines/processes,7 and implementing carbon capture and sequestration.3,4,8 Mineral carbonation is contemplated as one of the most permanent and large-capacity options for withdrawing CO2 from the atmosphere.4 It is based on the premise that the abundant and already-mined mafic/ultramafic mining wastes (e.g., brucite, olivine, serpentine, wollastonite) can be taken advantage of by using low-cost carbon fixation processes.9 The three-phase chemistry of mineral carbonation roughly depends on the acidity-driven dissolution of (ultra)mafic minerals through gaseous CO2 absorption in water followed by mineral dissolution and precipitation of the leached off metal cations in the form of stable carbonate minerals.4,5,8,9-12 Among the Mg-bearing minerals associated with (ultra)mafic mine wastes/tailings, brucite, as the simplest mineral and also the most CO2-reactive for rapid sequestration potential,13,14 straightforwardly lends itself to fundamental studies of the surface phenomena at play during mineral carbonation. Therefore, it is reasonable to conjecture that basic studies on brucite will help understand, without loss of generality, the carbonation behavior of more complex (ultra)mafic minerals, such as serpentines.9,13,14 The multi-step reactions operating during carbonation of brucite consist commonly of CO2 dissociative absorption [R1-3], brucite acidic dissolution [R4], ion-pair formations with leached off metallic cation [R5,R6], and precipitation/crystallization [R7] to yield a range of magnesium hydroxyl-carbonates: Gaseous carbon dioxide dissociative absorption [R1]

CO2 (g) ↔ CO2 (aq)

[R2.1] CO2 (aq) + H2O (l) ↔ H+ (aq) + HCO3- (aq) & [R2.2] [R3]

CO2 (aq) + OH- (aq) ↔ HCO3- (aq)

HCO3- (aq) ↔ H+ (aq) + CO32- (aq)

Acidic leaching of brucite [R4]

Mg(OH)2 (s) + 2 H+ (aq) ↔ Mg2+ (aq) + 2 H2O (l)

Ion-pair formations


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[R5]

Mg2+ (aq) + OH- (aq) ↔ MgOH+ (aq)

[R6]

Mg2+ (aq) + HCO3- (aq) ↔ MgHCO3+ (aq)

Formation of magnesium hydroxyl-carbonates [R7]

Mg2+ (aq) + x CO32- (aq) + (2 + y – x) H2O (l) ↔ MgOxCO2(y+1)H2O (s) + 2(1-x) H+ (aq)

The stoichiometric coefficients, x and y, in [R7] determine the hydroxyl-carbonate being formed, for example, nesquehonite (MgCO33H2O), dypingite ((MgCO3)4Mg(OH)25H2O) or hydromagnesite ((MgCO3)4Mg(OH)24H2O).6,9,13,14 A quantitative description of the interactions with brucite surface of water molecules and the suite of ions (H+, HCO3-, CO32−, Mg2+, MgOH+, MgHCO3+) in aqueous media is essential to the understanding of brucite dissolution in the context of mineral carbonation.15-17 With the advent and maturity of current molecular mechanics and quantum chemistry computational methods, a range of possibilities is opening up to gain insights with atomic-scale resolution on the material structure, the nature of adsorption sites and the accompanying surface complexes forming on mineral surfaces.18-21 The structure and properties of multiple-layer hydroxylated sheet minerals have been the subject of many studies including interfacial and bulk properties of layered minerals, structure and bonding properties, and interaction of various species with the mineral surfaces.18,19,21-24 Molecular dynamics (MD) simulations have also been used to highlight the dependence of brucite dehydroxylation on its cleavage planes.25 Likewise, it was reported that any free space on the brucite (001) surface can turn into a diffusion channel for hydroxide ions and protons in contact with aqueous solution.26 MD study of the interaction between water molecules and brucite (001) surface unveiled the weak hydrogen bond formation between water and hydroxyl groups of brucite surface at the water-brucite interface.26 Quantum chemical simulations based on density-functional theory (DFT) calculations have also been carried out to analyze brucite OH vibrational characteristics, to highlight the role of OH groups during brucite (de)hydroxylation, and to infer stability of brucite edge surface through Mg-OH bond.27-29 Studies on brucite interlayer interactions as well as metal-Mg substitution in brucite structure revealed that Mg cation substitution with Cu cation endows brucite with paramagnetic properties.30 The structure and properties of aqueous interfaces with brucite and other clay minerals is shown to be strongly affected by significant degrees of structural and compositional disorder of the interfaces.31 To unveil the importance of brucite surface in its interaction with aqueous species as well as nucleation of brucite on a hydroxylated surface, DFT studies prove indeed of crucial help.26


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Aqueous mineral carbonation of brucite is an important mineralization route for carbon capture and sequestration. To the best of the authors’ knowledge, atomically-resolved quantum chemical simulations of brucite dissolution in aqueous mineral carbonation are still lacking. This work is therefore offered as a first step placing emphasis on the surface behavior of pristine and dissolving (001) brucite surfaces vis-à-vis water, and CO2 and brucite dissolved species prior to carbonate nucleation/ precipitation to take place. To gain insights on the nature of adsorption sites and resulting surface complexes, the affinity of water and of dissolved (H+, CO32-, HCO3-, Mg2+, MgOH+, MgHCO3+) species for pristine and partially dissolved brucite cleaved surfaces in aqueous mineral carbonation use is made of density-functional theory (DFT) simulations. Both (001) and (110) cleaved surfaces will be considered in terms of surface energy and solvation, before simulating protonated, deprotonated, dehydroxylated and neutral surface slabs in interaction with dissolved (CO32-, HCO3-, Mg2+, MgOH+, MgHCO3+) species in order to emulate brucite mineral carbonation. The dissolution behavior of brucite surface will also be quantitatively analyzed in terms of the consequential coordination losses for Mg and O ion active centers on brucite surface and their increased affinity for adsorbing the dissolved brucite and CO2 ionic species.

2. COMPUTATIONAL DETAILS Periodic Density Functional Theory (DFT)-based calculations were performed using the Dmol3 module32 implemented in Material Studio 2016 software package to simulate interfacial environments akin to aqueous mineral carbonation of brucite in CO2 capture processes. First, the geometric crystal parameters and total energy of the relaxed brucite bulk and relaxed brucite slabs with their (001) and (110) cleaved-plane surfaces were obtained. The optimized structures of relaxed protonated, deprotonated, and dehydroxylated (001) brucite surfaces were also computed. Subsequently, the dissolution behavior of brucite (001) surface was described in terms of coordination energetics of the various adsorbed species stemming from aqueous environments as encountered during mineral carbonation of brucite. The generalized gradient approximation (GGA) with the PW91 functional was used to describe the exchange correlation interactions, and to optimize both cell parameters and atomic positions. It has been already shown that simulation results by PW91 for clay and layered minerals such as brucite are very accurate in terms of bulk properties.33-38 Triple numerical polarization (TNP) basis set was also selected due to its high accuracy and versatile applicability for all atoms.33-38


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The DFT-D3 correction method of Grimme et al.35,39 was applied to account for the interactions between the metal cations and OH- groups within the brucite structure. Self-consistent field (SCF) convergence was imposed to 10-6 and the set of convergence criteria for the energy, maximum force and maximum displacement were, respectively, 10-5 Ha, 510-3 Ha/Å, and 510-3 Å. Finer SCF and convergence criteria did not alter meaningfully the simulation results. No special treatment of core electrons was considered and all the electrons were included in the calculations. Also, a smearing value was kept at 510-3 through calculations. Spin-unrestricted condition was considered in which the calculations were performed by use of various orbitals for different spins. Besides, the initial value for the number of unpaired electrons was taken from the formal spin introduced for each atom with the starting value subsequently optimized throughout calculations. For mineral bulk calculations, the Brillouin zone was sampled using a (3 × 3 × 3) Monkhorst-Pack k-point mesh. Further increases of the k point mesh had a negligible effect on both cell parameters (< 0.001 Å), and on total energies of bulk and slab models (< 1 meV). Depending on the specifics of the simulations being conducted, the mineral slabs were constructed for various sizes of mineral (001) and (110) surface supercells with appropriate vacuum spacing to prevent interaction between image slabs. Dipole corrections cannot be done without adjusting the vacuum size: while smallgap vacuum won’t provide sufficiently accurate results, too large-gap a vacuum will become computationally intensive to handle. To avert artefacts due to large dipoles along the direction of the vacuum gap, the self-consistent dipole correction of Neugebauer and Scheffler40 was enabled in Dmol3. This precaution merely adds up a step function to the electrostatic potential to correct for the nonphysical vacuum potential gradient. Implementation of self-consistent dipole correction also avoids summing charges to infinity for charged slabs upon the application of periodic boundary conditions.41,42 Furthermore, to eliminate spurious/nonphysical interactions between the adsorbate and the periodic images of the bottom layer of the slabs, relatively large vacuum regions are necessary to make sure the repeated slabs are decoupled.43 Precautions were therefore taken to initialize the adsorbate positions below the middle of the box headspace so that in none of the DFT simulations did the adsorbate wander far off the cleaved surface. This has the advantage to prevent adventitious wanderings of the adsorbate close by the upper headspace face of the box. Bookkeeping concerns related to adsorbate crossover from box upper face and the “conservative” adsorbate reentrance from the bottom face of the box, in order to satisfy the upper/lower face periodic boundary condition, are precluded with this prescription. In our study, convergence of the total energy towards some constant value is used for the selection of an appropriate vacuum size where a 40 Å vacuum gap was deemed to be adequate. All the multi-step optimizations were started from converged coarse-quality structures which were further evolved


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through medium-quality re-optimization until the finer quality was reached. Any particular piece of brucite structural information obtained in these simulations can be made available upon request for interested readers. As carbonation of brucite takes place in an aqueous environment, the presence of water has to be described in some way. For the sake of “engineering accuracy”, we adopted an approximate treatment based on an analysis using a relative comparison scale. Relative trend analysis does not require reliably quantifiable absolute values of the interaction energies between the various species and brucite surface inasmuch as the energetic differences within analogous series are statistically significant. In our DFT calculations, i) slab (Eslab in Eq. 1), ii) bare species  headspace (Especies in Eq. 1), and iii) slab-sorbed bare species (Eslab+species in Eq. 1) have been embedded in an effective dielectric continuum. This latter filled the slab headspace by enabling an implicit solvation model (conductor-like screening model, COSMO) whereby bare neutral/charged species form cavities within a dielectric continuum (water permittivity = 78.5). For the purpose of comparison with this simplified approach, a few benchmark cases have also been simulated to assess alternate ways to account for explicit water first-solvation shell in the presence of fully and under-coordinated brucite model surfaces. The species affinities for the mineral surfaces were calculated as follows:19

Ea = Eslab+species – Eslab - Especies

(1)

Where in Eq. 1, Eslab+species is the total slab energy after species adsorption on the mineral surface, Eslab represents the energy of the relaxed bare mineral slab (correspondingly neutral or charged), Especies is the energy of aqueous species after optimization, and Ea is the affinity or apparent adsorption energy of species on the surface. A careful procedure of how these different energy components are calculated is exemplified in Section § 3B. Adsorption of Water on Brucite Surface). Furthermore, surface energy density, Є, of the cleaved slabs was calculated according to the following definition:19 Є = (Eslab - nEbulk)/ 2A

(2)

Where in Eq. 2, Eslab is the converged-slab energy using initialized non-relaxed neutral/charged slabs and implicit solvation in headspace. The unit-cell energy, Ebulk, of bulk brucite crystal is estimated in conventional manner but by selecting neutral and charged brucite bulk assuming periodic boundary conditions. If the number of Mg-unit cells, n, is the same for the neutral/charged brucite slab, the


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corresponding Eslab energy has to be calculated for each case. Finally, A is the slab headspace-facing surface.

3. RESULTS AND DISCUSSION 3A.

Structural Optimization of Mineral’s Crystal & Slab. Brucite crystal is hexagonal and its

structure belongs to the 𝑃3𝑚1 space group.27,28 Brucite is a layered hydroxide mineral consisting of stacked single layers each comprised of a magnesium ion plane tacked in-between two hydroxyl-ion planes through Mg-OH covalent bonds. The Mg ions lie at the centers of puckered hexagons whose corners comprise OH ions alternating from above and below the hydroxyl-ion planes. Brucite unit cell contains one Mg(OH)2 made up with edge-sharing Mg(OH)6 octahedra.44,45 Our DFT-calculated unit cell dimensions for brucite crystal bulk: a = b = 3.149 Å and c = 4.77 Å, and α = β = 90°, γ = 120°, are found to be very closely matching their X-ray diffraction measured counterparts: a = b = 3.15 Å and c = 4.783 Å, and α = β = 90°, γ = 120°.43 Atom coordinates of the optimized brucite crystal bulk are provided in Supporting Information Table S1. Crystal slabs of brucite with cleaved (001) (12.59246.79 Å3, atom coordinates in Supporting Information Table S2) and (110) planes (14.3116.3646.79 Å3, atom coordinates in Supporting Information Table S3) were afterwards optimized as these latter planes are considered as the most common cleavage planes for brucite.27,28 All the slabs were produced through cleaving the DFT-minimized crystal brucite bulk structure which was then re-optimized. As a precaution, the sub-layer atoms were constrained during the slab relaxation to avert unlikely occurrences such as migration of surface hydroxyl ions away from the surface due to full translational motion.47-49 Hence in spite of risks of being trapped in local extrema, constrained relaxation has been shown to affect calculation accuracy of dynamical properties such as clay vibrational spectra,28 which are more demanding than the less stringent geometry and energy properties of interest to our study. Hexagonally-bonded Mg ions layered between two hydroxide layers, with all OH ions parallel to the slab c-axis, are distinguishable from the (001) cleaved surface where each of the six O ions is, in return, coordinated to three Mg ions (Figure 1a). Akin to antiferromagnetic arrangements,25 OH groups from same-side face of brucite layer view along c-axis oppositely-oriented and shifted OH groups from an adjacent (001) layer. This arrangement produces relatively weak interlayer van der Waals interactions (Figure 1a) whereby (001) crystallographic planes become inherently the most facile cleaving planes in brucite. Though less stable and thus less favorable than its (001) counterpart, the (110) crystallographic


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plane is another possible cleavage plane.25,28 Surface stability is confirmed from DFT-calculated surface energy densities of the (110) and (001) cleaved planes, respectively, to 14.1 meV/Å2 and 3.3 meV/Å2. Lack of stability of the former surface can be ascribed to coordination shortage of the (110) surface where each Mg ion has four coordinations with oxygen ions, while, likewise, each oxygen ion is coordinated to two Mg ions (Figure 1b). 3B.

Adsorption of Water on Brucite Surface. Combined explicit-water/COSMO DFT calculations

have been carried to provide estimates of the water affinity for the mineral surfaces using Eq. 1. Hence, solvent interactions with brucite (001) and (110) surfaces are simulated by initially seeding twenty water molecules in the slab headspace. To emulate an aqueous environment in contact with the surface, these individual solvent molecules are assumed to interact with the surface, in simultaneity with the “effective” interaction by the vast pool of an aqueous medium which is accounted for by enabling COSMO as an implicit water solvation model. Estimations among possible (local) minimum energy realizations of Eslab+species, Especies and Eslab were obtained along with their corresponding optimized structures. The slab+20-water-molecule structure was initialized using a brucite slab relaxed with activated-COSMO headspace in which afterwards a set of 20 molecules was initialized in the middle of the box headspace to get an estimate for Eslab+species. The 20-water-molecule structure used a slab-less equally-sized activated-COSMO headspace with exactly the same initial distribution of 20 molecules in the middle of the box headspace to get an estimate for Especies. Finally, estimation of Eslab for the optimized (relaxed) slab structure considered an equally-sized activated-COSMO headspace in absence of water molecules. Provided the same set of initializations is used for estimating Eslab+species, Especies and Eslab, an “internal-standard” bias corrector such as Eq. 1, should in principle attenuate a great deal of statistical bias in estimating the relative trend of water interaction energy with brucite surface. Perwater-molecule interaction energy values can then be estimated by dividing Eq. 1 output with the number of water molecules in the box. In this normalized descriptor, all covalent and non-covalent interaction energies stemming from water-water (H-bonding and van der Waals) and water-surface (covalent bonds, H-bonding and van der Waals) interactions are included. However, in spite of its global character, sensitivity of this descriptor to population size changes proves very informative to assess chemical versus physical interactions of water with brucite surfaces. In the case of the more stable cleaved (001) surface, water molecules interact with brucite OH surface groups mainly through hydrogen bonding (Figure 2a). To adapt to the water molecules dangling at the surface, the brucite surface OH groups tend to tilt by ca. 7° (Figure 2a) with respect to the slab c-axis.


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Their now more pronounced shadow on the slab surface further reduces clearance for the water molecules to intrude deep into the non-smooth indentations of the cleaved surface to meet the electron-accepting Mg cations. Thus, excepting the weaker H-bonding events reminiscent of physical adsorption of water26 on brucite (001) surface, stronger covalent-bond interactions between H2O molecules and brucite surface are in all likelihood averted. On the contrary, the individual H2O molecules nearby the (110) surface interact both weakly via similar H-bonding for the water upper layers, and strongly via covalent bonding of first-layer water molecules with coordination-deficient surface magnesium cations (Figure 2b). DFT simulation predicts that four (out of 20) H2O molecules chemically bond to the surface to form four new penta-coordinated Mg cation(s) (Figure 2b). Similarly, two other H2O molecules also chemically bond to the surface but yield one new hexa-coordinated Mg cation (Figure 2b). The normalized interaction energy, expressed on water molar basis, amounts to, respectively, -46.7 kJ/mol and -83.9 kJ/mol for brucite (001) and (110) surfaces. These numbers help comparing the partner-indifferent non-covalent interactions of all the water molecules in the case of brucite (001) surface (Figure 2a) to the mixture of covalent/noncovalent among all water molecules in the case of brucite (100) surface (Figure 2b). If the former value is consistent with weak solvent-surface interactions in agreement with literature findings,26 the latter is more in line with chemisorbed water. These findings accredit the naïve observation that the narrow (Figure 1a) versus wide (Figure 1b) indentations of the surface in terms of magnesium accessibility to solvent molecules agrees with the more stable (001) brucite cleaved surface (Figure 1a) would be less hydrophilic than the less stable (110) brucite cleaved surface (Figure 1b). Thus, on account of stability of brucite surfaces, slabs with (001) cleavage plane are selected from now on to undertake the rest of this investigation. 3C.

(Non-)Specific Adsorption/Charge-Determining Species at Aqueous-Brucite (001) Interface.

Distinguishing the nature of interactions between brucite (001) surface and the various species resulting from aqueous mineral carbonation of brucite is essential to establish the charge structure of the electrical double layer at the brucite-water interface.50 From the standpoint of surface complexation, significance of the Stern compact layer in mineral carbonation should be paid attention to in priority. This latter is customarily divided into inner and outer Stern sub-layers hosting, respectively, inner- and outer-sphere surface coordinated complexes between the surface and the aqueous species. In the context of our DFT simulations of brucite aqueous carbonation, the respective average thicknesses of these sublayers are estimated to 2.6 Å and 3.7 Å. If specific adsorptions via covalent and hydrogen


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bonding on brucite surface are evocative of the inner Stern sublayer, species populating the outer sublayer, more than often, showcase non-specific interactions.50-55 As an illustration, non-solvated magnesium cation chemisorbed in the inner Stern sub-layer appears to be surface-charge determining for (001) brucite surface owing to Mg covalent bond with surface hydroxyl O (Figure S1[Supporting Information], case (i)). However, despite it is also an inner-sphere coordinated surface complex, adsorbed non-solvated carbonate anion involves weaker H-bond with the surface of brucite and thus does probably not qualify for being a charge-determining ion for the (001) brucite surface (Figure SI_1, case (ii)).49-54 The solvated carbonate and brucite surface OH groups bridged via adsorbed water molecules (Figure SI_1, case (iii)) exemplify otherwise the diverse range of possibilities to form outersphere coordination complexes. Here, the non-specific interactions of carbonate with the surface of brucite cannot directly modify the (001) brucite surface charge. Thus having specified a terminology regarding the types of adsorptions of the various surface complexes and, whether or not, they directly affect brucite surface charge, the next sections will scrutinize in-depth the speciation of the (001) brucite surface under mineral carbonation. 3D.

Protonation/Deprotonation/Dehydroxylation of (001) Brucite Surface. Depending on pH of the

brucite aqueous system, charged MgOH2+ and MgO- surface species are, respectively, induced by surface protonation and deprotonation of the neutral MgOH0 surface sites prompted by brucite surface hydroxyl groups gaining or losing protons.15,17 More abundant below the point-of-zero charge pH, the fraction of protonated Mg sites (MgOH2+) declines with increasing pH at the expense of the deprotonated Mg sites (MgO-) which become overwhelming at pH > 12.15,17 Removal of hydroxyl anion from brucite (001) surface can also lead, apart from proton adsorption as in a protonation step, to a positively-charged surface where the loss of OH triple coordination results in three five-coordination Mg cations in lieu of the usual hexagonally-bonded Mg ions. Optimized structures of neutral (Figure 3a), single-site deprotonated (Figure 3b: O highlighted in pink color), single-site protonated (Figure 3c: OH highlighted in yellow color) brucite (001), and multiple-Mg-site dehydroxylated (Figure 3d: three Mg highlighted in black color) surface slabs have been computed to reflect the brucite net surface charges around point-of-zero charge conditions (pH ≈ 11).15 These slabs would be tantamount to brucite aqueous systems with net surface charges of zero at pH 11 (Figure 3a), -1.2 mol/m2 at pH 11.5 (Figure 3b) and +1.2 mol/m2 at pH 10 (Figures 3c,d). COSMO approximation was used to account for the implicit solvating effect of water.


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Subsequent to site deprotonation, the surface OH groups enclosing surface MgO- tilt inwardly by ca. 8° (Figure 3b) with respect to the OH orientation for the neutral-site slab (Figure 3a). Furthermore, clearance of the MgO-centered area after proton expulsion results in a more exposed brucite-water interface. Atomic charge analysis based on Hirshfeld population charge50,51 for the OH groups in the periphery of MgO- site indicates changes from 0.045e (neutral slab) to 0.037e (deprotonated slab) for hydrogen, and likewise from -0.21e to -0.3e for OH oxygen. On the other hand, the MgO- oxygen charge is estimated to drop from -0.21e to -0.49e. Significant charge alteration for MgO- oxygen is to be paralleled with the increasing surface energy density from a neutral MgOH0bearing brucite slab (3.3 meV/Å2) to a slab with single-site deprotonated species MgO- (6.91 meV/Å2). Such dissimilarity vouches for MgO- augmented reactivity to form new surface complex species as will be discussed later. On the other hand, the DFT-predicted H+OHsurface interaction precludes formation of a covalent bond between solution proton and brucite surface (Figure 3c). The protonated surface groups, MgOH2+, are likely to involve relatively weakly bonded protons to the surface via hydrogen bond or van der Waals interactions. This is also supported by the unchanged atomic charges, with respect to neutral site, according to the Hirshfeld population charge analysis of the MgOH2+bearing slab, as well as from the DFT-computed slab-normalized surface energies of neutral brucite MgOH0 species (3.3 meV/Å2) versus single-site protonated species MgOH2+ (3.63 meV/Å2). According to the terminology introduced in Section 3C. above, MgOH2+ species can hardly be labelled as charge-determining of the (001) brucite surface in spite they are the resultant of specific adsorptions in the inner Stern sub-layer. Regarding the multiple-Mg-site dehydroxylation scheme (Figure 3d), the charge of surface magnesium cations increases from +0.39e (6-coordination) to +0.47e (5-coordination) and suggests that incompletely coordinated Mg sites may become active centers for covalent bonding with dissolved species. In this slab configuration, surface energy density dehydroxylated species (8.36 meV/Å2) is predicted to even surpass that of a slab with single-site deprotonated species MgO- protonated species MgOH2+ (6.91 meV/Å2). 3E.

Adsorption of Mineral Carbonation Species on (001) Brucite Surface. To gain insights into the

uptake by the mineral surface of dissolved species, adsorption on neutral, protonated, deprotonated and dehydroxylated (001) brucite surfaces of CO32- (Figure 4), HCO3- (Figure 5), Mg2+ (Figure 6), MgOH+ ion pair (Figure 7) and MgHCO3+ ion pair (Figure 8), as encountered during the aqueous dissolution/carbonation process of brucite, is then investigated via DFT calculations to infer the resulting surface complex configurations and corresponding apparent adsorption energies (Table 1), in addition to


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distinguishing specific from non-specific adsorptions, inner- and outer-sphere coordinations and chargedetermining species among these complexes. Note that DFT calculations predict the (Mg2+,OH-) and (Mg2+,HCO3-) ion pairs to be covalently bonded. Table 1 also compares results from the implicit-solvation COSMO approach (cf. § 2. Computational Details above) with more elaborate simulation scenarii i) of explicitly-solvated Mg2+ cations interacting with neutral, (de)protonated and dehydroxylated brucite surface (Mg & O full coordination and Mg under-coordination), and ii) of explicitly-solvated CO32-, HCO3-, Mg2+, MgOH+ and MgHCO3+ interacting with dehydroxylated brucite surface (Mg under-coordination). In these treatments, the “explicitly solvated” ions and the “bare” brucite surfaces were treated as inclusions in the dielectric continuum. Comparison of the results ascertains the trend for the interaction energies as predicted for both bare ions and surfaces embedded in a dielectric continuum (Table 1). An even more accurate solvation picture could have been built had explicit-solvation of the brucite surface been simulated. This would have resulted in that the slightly lower energies predicted by the simplified COSMO approach (Table 1) would have receded even further. Notwithstanding, similar gradation of the interaction energies would have been arrived at. Therefore, the simpler implicit-solvation COSMO approach was adopted to discuss the relative trend of interaction energies in the context of brucite aqueous carbonation. A concern may be raised that the interaction energies upon involving adsorbing charged species will be dependent on the system’s charge. To alleviate such effects, relatively large vacuum space (40 Å) was applied in all the simulations.56 In addition a few simulations were run using the counter-ions method56 to check for charge independence. For this purpose, counter ions were inserted in the head space to neutralize the charge of the dissolved species to be adsorbed on brucite (001) surface. Mirroring the adsorbing species, CO32-, HCO3-, Mg2+, MgOH+, and MgHCO3+, the following counter-ions were, respectively, considered, Mg2+, H+, O2-, OH-, and OH-. Counter ions were initialized at quite a large distance of 20 Å (half of vacuum space size) above the brucite slab to ensure zero-charge systems while preventing interactions of the counter-ions with the mineral surface. Energy calculations used Eq. 1 in which the terms Eslab+species and Especies embrace both the interacting and the (remote) counter ions. The apparent adsorption energies, with and without the counter-ion artifice, are compared in Table 1. It can be concluded that choosing large vacuum space would be sufficient to rank confidently the affinities of adsorbing charged species as the calculated energies with and without counter ions keep very close. Carbonate ion interacts with neutral (001) brucite surface through hydrogen bonding and van der Waals forces by involving the CO32- oxygens and the hydrogens from the surface OH groups (Figure 4a). This


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non-covalent interaction is also confirmed from the Hirshfeld population charge analysis which reveals unchanged atomic charges either for brucite surface or carbonate. In spite of its location between the 0plane and iHp (Figure SI_1) and while being specifically adsorbed on the (001) brucite surface, carbonate cannot be a charge-determining species. Lack of active centers on the (de)protonated (001) brucite surfaces averts covalent bond with carbonate ions (Figures 4b,c) which only interact with the surface via hydrogen bonding and van der Waals forces. However, our DFT simulations predict formation of bidentate inner-sphere coordination carbonate surface complexes by bringing penta-coordinated magnesium cations to the forefront following brucite dehydroxylation (Figure 3d). Full coordination for two out of three penta-coordinated Mg cations is retrieved after covalently bonding to two carbonate oxygens with an average bond length of 2.8 Å whereby electrons from carbonate HOMOs are shared with surface-Mg LUMOs to form new coordinations. The third carbonate O is left to only allow hydrogen bonding with H atoms from two adjacent OH groups on the brucite surface which also undergo changes in orientation. Unlike above neutral and (de)protonated surfaces (Figures 4a-c), the sorption of carbonate on (001) dehydroxylated brucite surface is surface charge determining. This is also confirmed from the Hirshfeld population charge analysis which points to Mg charges decreasing from 0.45e to 0.37e with change of Mg coordination from five to six. The apparent adsorption energies (Table 1) of carbonate on the four different surfaces follow in a consistent manner the predicted surface complexes. Similarly to carbonate, DFT simulations and ensuing Hirshfeld population charge analysis predict that bicarbonate ion also interacts with neutral (Figure 5a), deprotonated (Figure 5b) and protonated (Figure 5c) (001) brucite surface through hydrogen bonding and van der Waals forces with surface OH groups. Likewise, lack of accessibility to metal cations as active centers precludes covalent bond between these brucite surfaces and bicarbonate. Upon dehydroxylation, bicarbonate covalently bond to coordinationdeficient magnesium cations as a bidentate surface complex (Figure 3d) akin to the carbonate ion just described above. The average length of these two covalent bonds is shorter by 0.5 Å and the final Mg charge is lower by 0.3e with respect to the carbonate ion bonds. Both such features point to the specifically adsorbed bicarbonate interacting with dehydroxylated brucite surface more strongly than carbonate thus also taking on the role of surface charge determining anion. The apparent adsorption energies (Table 1) of bicarbonate also follow the trends highlighted in the case of carbonate. DFT simulations predict that regardless of the examined surface modality (Figures 6a-d), the uptake of Mg2+ from solution occurs through covalent bonding with surface oxygen and is in all cases accompanied with site deprotonations. Protons thus expelled from the surface OH groups undergo in return


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adsorption on nearby surface hydroxyl groups through hydrogen bonding and van der Waals forces. The average Mg-O bond length is 1.8 Å demonstrating the stronger interactions of Mg cations with brucite surfaces as compared with the previously discussed anionic species. This confirms that chemisorbed Mg2+ cations are indeed charge-determining ions for aqueous brucite pulps.57 Except the bidentate inner-sphere surface complex formed on a deprotonated brucite surface (Figure 6d), solution Mg2+ tend to privilege monodentate complexes (Figures 6a,c,d) though all sorbed structures exhibit very high apparent adsorption energies (Table 1). The diversity of surface complex structures is astonishing in terms of availability of active centers with ability of electron exchanges for MgOH+ ion pair adsorption. If neutral (Figure 7a) and protonated (Figure 7c) (001) brucite surfaces prompt H-bonding/van der Waals forces with MgOH+ ion pair, deprotonated (Figure 7b) and dehydroxylated (Figure 7d) (001) brucite surfaces promote, on the contrary, covalent bond formation with MgOH+. Covalent bonding involves either MgOH+ magnesium cation with surface oxygen for the deprotonated surface (Figure 7b), or MgOH+ oxygen and one out of three penta-coordinated surface magnesium cations for the dehydroxylated surface (Figure 7d). The average bond length for these two cases is 2.3 Å reflecting charge changes for O and Mg, respectively, from -0.3e and 0.45e to -0.2e and 0.36e. Here too, chemisorbed MgOH+ ion pairs are likely to be surface charge-determining ions, consistent with the estimated apparent adsorption energies (Table 1). The sorbed structures of MgHCO3+ ion pair on the various brucite surfaces differ very much from those just described for MgOH+ ion pair. Different structures are possible for the adsorbed MgHCO3+ ion pair on neutral (Figure 8a), deprotonated (Figure 8b), protonated (Figure 8c) and dehydroxylated (Figure 8d) (001) brucite surfaces. This ion pair yields inner-sphere coordinated surface complexes forming covalent bonds between MgHCO3+ magnesium and surface OH oxygen (Figures 8a-c) in agreement with computed affinities (Table 1). However, contrary to the reactivity vis-à-vis solution anions, the dehydroxylated brucite surface is not predicted to involve more than H-bonding/van der Waals interactions with MgHCO3+ (Figure 8d). Except the deprotonated surface (Figure 8b), deprotonation of surface hydroxyl groups (Figures 8a,c) is a compulsory step before magnesium cation in the ion pair covalently bond with surface oxygen. This results in surface O atom charge increase from -0.29e to 0.19e after surface bonding according to Hirshfeld population charge analysis. The orphan protons expelled from OH to enable formation of this surface complex are left to interact via H-bonding/van der Waals forces either with other adjacent surface OH groups or water molecules. The average bond length


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for the three cases is 2.1 Å indicating chemisorbed MgHCO3+ as an inner-sphere coordinated surface complex also taking on the role of surface charge determining cation. Sorption on brucite of the species enumerated above, contrary to loose electrostatic interactions, ranges minimally from H-bond surface interactions to covalent bonds and is summarized in Table S4 (Supporting Information). Both vouch for specific adsorptions within the inner Stern sub-layer.50-55 Even if H-bonding does not alter the atomic charge distribution before and after formation of the surface species, the computed bond distances (Table S4, Supporting Information) are coherent with complexes in the inner Stern sub-layer albeit these do not fulfill the function of surface charge-determining ions (CDI).52 In the instances where ion chemisorption occurs, the resulting specific adsorption is accompanied by changes in atomic charges disclosing the covalent nature of the surface bonds and thus the charge determining role of the adsorbed ion.55 The simulated surfaces are shown to exhibit the following affinity towards species adsorption: dehydroxylated surface (001) > deprotonated surface (001) > surface (001) and protonated surface (001). Likewise, the species sorption affinity for a given surface is found to obey the following gradation: Mg2+ > MgHCO3+ > MgOH+ > HCO3- > CO32-. 3F.

Structure of Dissolved Brucite Surface. DFT simulations of the pristine (001) brucite surface

would be incomplete without touching upon the surface behavior of brucite which undergoes dissolution in carbonating aqueous environments. A twelve-step congruent dissolution path of Mg(OH)2 moieties has been simulated (Figure 9). For this purpose, a bigger (001) brucite slab (22.04246.79 Å3, atom coordinates in Supporting Information Table S5) was obtained from cleaving the DFT-minimized crystal brucite bulk structure and then was optimized (Figure 9) in order to induce representative distributions of surface coordinations for Mg and O ions as prerequisite condition to dissolving (001) brucite surface (Table S6, Supporting Information). Note that the congruent dissolution path retained for carving the (001) brucite surface is not unique and many other paths could as well have been chosen. One important aspect not considered in our dissolution treatment concerns the simultaneous hydration steps which will compete for low-coordination Mg and O crystal surface ions. The issue of Mg- & Odeficient coordinations of brucite surface has been addressed in above § 3B. Adsorption of Water on Brucite Surface. Stable hydrated structures, predicted from explicit-solvation calculations, can still bear an incomplete coordination character, e.g., 4-coordination (110) cleaved-surface Mg converting to 5coordination of Mg (see Figure 2). Therefore, the congruent dissolution structures, even though formed in an effective polar dielectric medium, can still be viewed as precursors for many different possible hydration-stabilized structures with varying coordinations.


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Adjacent to the hole left after dissolution of the first Mg(OH)2 moiety from (001) surface (step 1: Figures 9, S2, Supporting Information), four penta-coordinated Mg cations and four di-coordinated OH groups are formed (Table SI_6). One of the four di-coordinated OH groups (shown as empty diamond in step 1: Figure SI_2) points outwardly (Table SI_6) towards the slab headspace suggesting differentiated surface protonation/deprotonation steps depending on OH group orientations. Due to dissolution, incomplete coordination for these Mg and O surface ions increases the surface reactivity as a result of atomic charge alterations: from -0.29e to -0.40e for O and from 0.39e to 0.46e for Mg according to Hirshfeld population charge analysis. Dissolution of a second Mg(OH)2 moiety (step 2: Figures 9, SI_2) increases up to six the number of coordination-deficient surface ions for each of Mg and O, among which one new tetra-coordinated Mg and one new single-coordinated O are formed (Table SI_6). The drop in coordination, from 6 to 4 (resp., 3 to 1) for Mg (resp., O), results in an increase of the atomic charge from 0.39e to 0.49e (resp., -0.29e to -0.42e). Removal of a third surface Mg(OH)2 increases the number of low-coordination surface Mg and O ions (step 3: Figures 9, SI_2) without affecting either their coordination rank (Table SI_6) or the atomic charge variations as compared to step 2. Dissolutions of steps 2 and 3 lead to more positively (negatively) charged 4-coordination Mg (1-coordination O) ions. These lower-coordination surface species are expected to prompt even stronger interactions through accepting or donating their electrons with solution anions and cations. Further dissolutions (steps 4 to 10: Figures 9, SI_2) give rise to new 3-coordinated Mg cations. Upon incremental removal of 4 to 10 Mg(OH)2 surface moieties, the number of coordination-deficient surface species increases remarkably for 3-, 4- and 5-coordinated Mg (1- and 2-coordinated O) (Table SI_6). The charge corresponding to the newly formed 3-coordinated Mg cations has jumped to +0.52e anticipating an increased electron accepting ability of Mg by further dissolutions and lesser coordinations. Step 11 dissolution (Figures 9, SI_2) unveils formation of doubly-coordinated surface Mg cation (Table SI_6) with a corresponding charge of +0.57e. Removal of a twelfth Mg(OH)2 leads to the complete spectrum of coordination ranks (Table SI_6) including the most reactive single-coordinated Mg adatom species (step 12: Figures 9, SI_2) with charge of +0.65e. It is worth noting that the Hirshfeld population charge analysis predicted approximately the same changes in atomic charge of Mg and O ions for the same coordination rank in the successive dissolution steps. By sharing less electron density with their immediate Mg neighbors in the brucite structures in steps 1 to 12, the singly- and doubly-coordinated OH oxygen ions become potentially available to set new covalent bonds and transfer electrons from their Highest Occupied Molecular Orbital (HOMO) to the Lowest Unoccupied Molecular Orbital (LUMO) of aqueous species. Similarly, with the drop in


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coordination from 6 to 1, the increasingly positively-charged lower-coordinated Mg cations become more propitious to accept electrons from aqueous species HOMOs. Therefore, the dissolving brucite surface, owing to these Mg and O coordination-deficient surface species, is likely to prompt more reactive centers for surface interactions with the dissolved species. 3G.

Adsorption of Mineral Carbonation Species on Dissolved Brucite Surface. One might anticipate

that the CO32-, HCO3-, Mg2+, and MgOH+ and MgHCO3+ uptakes by the dissolved brucite surface will be sensitive to the topography of the reacting surface during the aqueous dissolution/carbonation process. That these aqueous ions’ uptake by pristine (001) brucite surface would not fully render the complex reality of the surface interactions as a result of the diversity of coordinations of surface Mg and O ions (Table S6, Supporting Information) to occur during brucite dissolution is obvious. Apparent adsorption energies have thus been computed according to Eq.1 for the brucite carbonation induced-species, CO32-, HCO3-, Mg2+, and MgOH+ and MgHCO3+ on various partially dissolved neutral (001) brucite surfaces after congruent dissolution steps 1, 2, 5, 11 and 12 (Table 2). Similar trends are expected in the case of protonated/deprotonated surface sites as discussed in Section 3D. above (results not shown here). The apparent adsorption energies of the same ions on the pristine neutral (001) brucite surface are also summarized for comparison along with the prevailing Mg and O coordinations for dissolution steps selected to reflect the entire range of coordinations for surface magnesium and oxygen ions. Through dissolution and thus reduction of Mg and O coordinations on brucite surface, affinity of the surface whether for dissolved anions or cations increases dramatically. It is worth reminding that survival of lower-coordination Mg decreases with decreasing Mg coordination. Therefore, the interaction energetics summarized in Table 2 are to be taken cum grano salis while sliding from left to right, as more thorough DFT calculations would be required for taking into account the simultaneously competing hydration steps. For all the adsorbing ions, jumps of the apparent adsorption energies are conspicuous upon dissolution from the pristine brucite surface to the step 1 dissolved brucite surface: -77.6, -82.7, -48, -110 and -22.8 kJ/mol, respectively, for CO32-, HCO3-, Mg2+, and MgOH+ and MgHCO3+. The largest contrasts are observed in the case of MgOH+ and MgHCO3+ ion pairs where the maximum and minimum differences have, respectively, been scored. There is also a general tendency for the increase of adsorption energies with reduction of Mg and O coordinations as shown for the subsequent dissolution steps (Table 2). Nonetheless, such effects are of secondary importance as compared with those stemming from first dissolution. Furthermore, occasional disruptions of the monotony of adsorption energies with decreased


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Mg coordination are presumably ascribed to smearing effects due to the differing statistics of the Mg and O coordinations (Table S6, Supporting Information) for the interrogated dissolution steps. Interestingly, gradation of strengths of the apparent adsorption energies for CO32-, HCO3-, Mg2+, and MgOH+ and MgHCO3+ for each given dissolution step follows the same trend as the one inferred for the DFT-simulated apparent adsorption energies in the case of the pristine (001) brucite surface: Mg2+ > MgHCO3+ > MgOH+ > HCO3- > CO32-. Adsorption configurations of representative anionic and cationic species, namely, CO32- and Mg2+, on the different dissolving brucite surfaces are illustrated in Figures 10, 11, respectively. The dissolved anionic species are expected to chemically bond to the surface Mg cations whereas, conversely, the cationic species are predicted to interact with surface O anions. Hence, CO32- anions only interact with Mg cations on the surface via monodentate covalent bond with 5- and 4-coordinated surface Mg cations (Figures 10a,b) or bidentate covalent bonds with 3- and single-coordinated surface Mg cations (Figures 10c,d). After chemisorption of CO32- on the dissolving brucite surfaces, Mg charges drop from 0.46e to 0.42e, from 0.49e to 0.40e, from 0.52e to 0.44e and from 0.65e to 0.55e, respectively, through dissolution steps one, two, five and twelve. Bidentate bond configuration with surface O ions occurs with dissolved Mg2+ regardless of whether hydroxyl group is bi- (Figures 11a,b) or mono-coordinated (Figures 11c,d) to the surface. After chemisorption of Mg2+ on the dissolving brucite surfaces, O charges increase from -0.40e to -0.30e in steps 1 and 2 (Figures 11a,b) and from -0.42e to -0.30e in steps 5 and 12 (Figures 11c,d). Due to their covalent character while all occurring in inner Stern layer, all these interactions correspond to CO32- and Mg2+ specific adsorptions whereby both dissolved species act as surface charge-determining ions (Table S7, Supporting Information). Our DFT study of brucite mineral dissolution predicts various new surface species and active sites to potentially participate in complex carbonation mechanisms. It suggests that following surface dissolution of brucite, the released lattice magnesium cation and surface hydroxyl groups are likely to interact in aqueous solution with the primary (bi)carbonates ((H)CO32-(-)) resulting from the dissolved carbon dioxide via reactions [R2.1], [R2.2] and [R3] (Introduction Section). Then species (H)CO32-(-) will yield a range of solvated (metal) carbonate species (e.g., CO32-, HCO3-, MgHCO3+, MgCO3,) which can in turn adsorb on the partially dissolved brucite surface. The dissolving brucite surface itself becomes more favorable to creating surface sites acting as new centers for promoted/inhibited interactions between the mineral surfaces and carbonate species. Hence, the present computational predictions would require experimental verification in future works to pinpoint which among these identified surface


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species are the dominant form(s) under actual carbonating conditions. One direction to take could be through identification of the fate of 18O isotopically-labeled brucite as recently monitored by Pesce et al.58 during their carbonation studies of portlandite using Raman spectroscopy, time-of-flight secondary ion mass spectrometry and electron microscopy. Their study demonstrated that during the early stages of Ca(OH)2 carbonation, surface hydroxyl groups provide O for the formation of carbonate ions in the solution through surface dissolution. Carbonate crystal growth is preceded by the dissolution of hydroxide moieties from the metal hydroxide, e.g., portlandite or brucite, by chemically reacting with dissolved carbon dioxide to yield (bi)carbonates, see reaction [R2.2] in Introduction Section. Then the metal cations leach off from the metal hydroxide surface to react with (bi)carbonates to form metastable calcium carbonate.58 In a subsequent step, evolution of the metastable carbonate phase into stable calcite was observed to occur through the release of water which is also source of oxygen interaction with CO2 (see reaction [R2.1] in Introduction Section) to form new carbonate ions during the progress of carbonation.58

4. CONCLUSION Brucite slabs cleaved along (001) and (110) crystallographic planes have been simulated using densityfunctional theory to provide an understanding about the surface interactions of brucite in aqueous carbonation. Solvent-surface interactions reveal weak hydrogen bonding between water molecules and surface OH-group hydrogens on (001) brucite surface. Furthermore, dehydroxylated, protonated and deprotonated brucite slabs have higher surface energy as compared with neutral brucite surface and thus suggest differently reacting brucite surfaces depending on the acid/base conditions to prevail for a carbonation reaction. Also, the dissolution species present in solution under carbonation conditions, i.e., CO32-, HCO3-, MgOH+, MgHCO3+ and Mg2+ interact differently with pristine (undissolved) cleaved brucite surfaces depending on the presence of neutral, protonated, deprotonated or dehydroxylated surface sites. These species are involved in covalent bonds with active centers on the surface, but hydrogen bonding and van der Waals forces as weaker interaction forces are also possible as exemplified for (001) brucite slabs. Hence, the following affinity trends have been predicted from DFT simulations: i) Mg2+ > MgHCO3+ > MgOH+ > HCO3- > CO32- for the adsorbing species, and ii) dehydroxylated (001) surface > deprotonated (001) surface > neutral and protonated (001) surfaces. Only Mg2+ specifically adsorbs through covalent bond formation on the different types of (001) brucite surfaces and can be considered as a surface charge determining ion (CDI). On the other hand, covalent bonds have also been predicted for the following ion-(001) brucite surface combinations: CO32-dehydroxylated site, HCO3-


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dehydroxylated site, MgOH+dehydroxylated or deprotonated sites and MgHCO3+dehydroxylated or deprotonated or protonated sites. These latter species can be considered in those instances as surface charge determining ions as well similarly to Mg2+. Dissolution of (001) brucite surface also results in less stable surface species due to changes in Mg and O coordination numbers thus impacting directly surface reactivity. Surface interactions during dissolution of (001) brucite surface of these dissolved species are predicted to increase due to proliferation of active centers on the surface. However, the same trend for surface affinity of these dissolved species is predicted for the dissolving (001) brucite surface. The active centers for enhanced surface interactions have been identified as the magnesium and oxygen surface ions with deficient coordination to form covalently-bonded surface complexes with the dissolved species. All these species are predicted to specifically adsorb on dissolved brucite (001) surface via covalent bond formation and therefore can be viewed as surface charge determining ions.

 ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: ??.????/acs.jpca.??????? as a pdf file: Figure S1. Schematic structure of Stern compact layer forming on top of (001) brucite surface: x-plane = crystal truncation through surface hydroxyl O atoms; 0-plane = cutting through surface hydroxyl H atoms; 1-plane = inner Helmholtz plane (iHp) delineating the inner Stern sub-layer for inner-sphere surface-coordinated complexes; 2-plane = outer Helmholtz plane (oHp) delineating the outer Stern sublayer for outer-sphere surface-coordinated complexes; (i) covalent bond in inner Stern sub-layer between leached/desolvated Mg2+ and surface O; (ii) H-bond in inner Stern sub-layer between CO32- and surface OH; (iii) H-bond in outer Stern sub-layer between CO32- aqua-complex, adsorbed water molecules and surface OH (Mg: green, O: red, H: white, C: gray). Figure S2. Top view of the (001) brucite surface following dissolution steps 1 to 12 each corresponding to one-at-a-time removal of 12 successive Mg(OH)2 moieties: Mg = empty circle; upwardly pointing OH = empty diamond, downwardly-pointing OH =black diamond, target area for dissolution = red-edge rhombus, Mg-O covalent bonds shown in blue, hexagons locating Mg cations shown in gold color. Table S1. Atom coordinates of brucite crystal bulk


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Table S2. Atom coordinates of brucite slab (001) Table S3. Atom coordinates of brucite slab (110) Table S4. Summary of species adsorptions on (001) brucite surface Table S5. Atom coordinates of larger brucite slab (001) Table S6. Distribution of Mg & O coordination numbers, CN, for successive congruent dissolution steps of (001) brucite surface Table S7. Summary of aqueous species adsorptions, CO32- and Mg2+, on partially dissolved neutral (001) brucite surface after congruent dissolution steps 1, 2, 5, 11 and 12 (see Table SI_6 and Figure SI_2 for other details)

 AUTHOR INFORMATION Corresponding Author Email: [email protected] ; Phone +1-418-656-3566 ORCID F. Larachi: 0000-0002-0127-4738

Present Address

Department of Chemical Engineering, Université Laval, 1065 Avenue de la Médecine, Québec, Québec G1V 0A6 Canada Notes The authors declare no competing financial interest.

 ACKNOWLEDGEMENTS The authors gratefully acknowledge the Natural Sciences and Engineering Research Council of Canada and the Canada Research Chairs on Sustainable Energy Processes and Materials for their financial support. We are also indebted to Compute Canada for the HPC platform without which the present DFT calculations would not have been possible.

 REFERENCES


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1. Rogelj, J.; Den Elzen, M.; Höhne, N.; Fransen, T.; Fekete, H.; Winkler, H.; Schaeffer, R.; Sha, F.; Riahi, K.; Meinshausen, M., Paris Agreement climate proposals need a boost to keep warming well below 2 C. Nature 2016, 534, 631-639. 2. Larachi, F.; Aksenova, D.; Yousefi, B.; Maldague, X.; Beaudoin, G., Thermochemical Monitoring of Brucite Carbonation Using Passive Infrared Thermography. Chem. Eng. Process. 2018, 130, 43-52. 3. Hamilton, J. L.; Wilson, S. A.; Morgan, B.; Turvey, C. C.; Paterson, D. J.; Jowitt, S. M.; McCutcheon, J.; Southam, G., Fate of transition metals during passive carbonation of ultramafic mine tailings via air capture with potential for metal resource recovery. Int. J. Greenh. Gas. Con. 2018, 71, 155-167. 4. Chen, Z. Y.; O'Connor, W. K.; Gerdemann, S., Chemistry of aqueous mineral carbonation for carbon sequestration and explanation of experimental results. Environ. Prog. 2006, 25, 161-166. 5. Harrison, A. L.; Power, I. M.; Dipple, G. M., Accelerated carbonation of brucite in mine tailings for carbon sequestration. Environ. Sci. Technol. 2012, 47, 126-134. 6. Zhao, L.; Sang, L.; Chen, J.; Ji, J.; Teng, H. H., Aqueous carbonation of natural brucite: relevance to CO2 sequestration. Environ. Sci. Technol. 2009, 44, 406-411. 7. Nejat, P.; Jomehzadeh, F.; Taheri, M. M.; Gohari, M.; Majid, M. Z. A, A global review of energy consumption, CO2 emissions and policy in the residential sector (with an overview of the top ten CO2 emitting countries). Renew. Sust. Energ. Rev. 2015, 43, 843-862 8. Hövelmann, J.; Putnis, C.; Ruiz-Agudo, E.; Austrheim, H., Direct nanoscale observations of CO2 sequestration during brucite [Mg (OH)2] dissolution. Environ. Sci. Technol. 2012, 46, 5253-5260. 9. Zarandi, A.E.; Larachi, F.; Beaudoin, G.; Plante, B. and Sciortino, M., Ambient mineral carbonation of different lithologies of mafic to ultramafic mining wastes/tailings–A comparative study. Int. J. Greenh. Gas. Con. 2017, 63, 392-400. 10. Chaka, A. M.; Felmy, A. R.; Qafoku, O., Ab initio thermodynamics of magnesium carbonates and hydrates in water-saturated supercritical CO2 and CO2-rich regions. Chem. Geol. 2016, 434, 1-11. 11. Ruiz-Agudo, E. n.; Kudłacz, K.; Putnis, C. V.; Putnis, A.; Rodriguez-Navarro, C., Dissolution and carbonation of portlandite [Ca(OH)2] single crystals. Environ. Sci. Technol. 2013, 47, 11342-11349; 12. Pesce, G. L.; Fletcher, I. W.; Grant, J.; Molinari, M.; Parker, S. C.; Ball, R. J., Carbonation of Hydrous Materials at the Molecular Level: A Time of Flight-Secondary Ion Mass Spectrometry, Raman and Density Functional Theory Study. Crys. Growth Des. 2017, 17, 1036-1044. 13. Assima, G. P.; Larachi, F.; Molson, J.; Beaudoin, G., Accurate and direct quantification of native brucite in serpentine ores—new methodology and implications for CO2 sequestration by mining residues. Thermochim. acta 2013, 566, 281-291.


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14. Zarandi, A. E.; Larachi, F.; Beaudoin, G.; Plante, B.; Sciortino, M., Multivariate study of the dynamics of CO2 reaction with brucite-rich ultramafic mine tailings. Int. J. Greenh. Gas. Con. 2016, 52, 110-119. 15. Pokrovsky, O. S.; Schott, J., Experimental study of brucite dissolution and precipitation in aqueous solutions: surface speciation and chemical affinity control. Geochim. Cosmochim. Acta 2004, 68, 31-45. 16. Bharadwaj, H. K.; Lee, J.-Y.; Li, X.; Liu, Z.; Keener, T. C., Dissolution kinetics of magnesium hydroxide for CO2 separation from coal-fired power plants. J. Hazard. Mater. 2013, 250, 292-297. 17. Pokrovsky, O. S.; Schott, J.; Castillo, A., Kinetics of brucite dissolution at 25 C in the presence of organic and inorganic ligands and divalent metals. Geochim. Cosmochim. Acta 2005, 69, 905-918. 18. Cygan, R. T.; Myshakin, E. M., Advances in Molecular Simulation Studies of Clay Minerals. In Greenhouse Gases and Clay Minerals, Springer, Cham, 2018, US. 19. Rai, B., Molecular modeling for the design of novel performance chemicals and materials. CRC Press, 2012, New York, US. 20. Meunier, M., Industrial applications of molecular simulations. CRC Press, 2016, New York, US. 21. Zhao, H.; Yang, Y.; Shu, X.; Wang, Y.; Ran, Q., Adsorption of organic molecules on mineral surfaces studied by first-principle calculations: A review. Adv. Colloid. Interface. Sci. 2018. 256, 230-241. 22. Cygan, R. T.; Greathouse, J. A.; Heinz, H.; Kalinichev, A. G., Molecular models and simulations of layered materials. J. Mater. Chem. 2009, 19, 2470-2481. 23. Greenwell, H. C.; Jones, W.; Coveney, P. V.; Stackhouse, S., On the application of computer simulation techniques to anionic and cationic clays: A materials chemistry perspective. J. Mater. Chem. 2006, 16, 708-723. 24. Liu, G.; Yang, X.; Zhong, H., Molecular design of flotation collectors: A recent progress. Adv. Colloid. Interface. Sci. 2017, 246, 181-195. 25. Churakov, S. V.; Iannuzzi, M.; Parrinello, M., Ab initio study of dehydroxylation− carbonation reaction on brucite surface. J. Phys. Chem. B 2004, 108, 11567-11574. 26. Sakuma, H.; Tsuchiya, T.; Kawamura, K.; Otsuki, K., Large self-diffusion of water on brucite surface by ab initio potential energy surface and molecular dynamics simulations. Surf. Sci. 2003, 536, L396-L402. 27. Masini, P.; Bernasconi, M., Ab initio simulations of hydroxylation and dehydroxylation reactions at surfaces: amorphous silica and brucite. J. Phys. Condens. Matter. 2002, 14, 4133–4144. 28. Zeitler, T. R.; Greathouse, J. A.; Gale, J. D.; Cygan, R. T., Vibrational analysis of brucite surfaces and the development of an improved force field for molecular simulation of interfaces. J. Phys. Chem. C 2014, 118, 7946-7953.


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29. Hermansson, K.; Probst, M. M.; Gajewski, G.; Mitev, P. D., Anharmonic OH vibrations in Mg(OH)2 (brucite): Two-dimensional calculations and crystal-induced blueshift. J. Chem. Phys. 2009, 131, 244517. 30. Costa, D. G.; Rocha, A. B.; Souza, W. F.; Chiaro, S. S. X.; Leitao, A. A., Structural and Energetic Analysis of Mg x M1− x (OH) 2 (M= Zn, Cu or Ca) Brucite-Like Compounds by DFT Calculations. J. Phys. Chem. C 2008, 112, 10681-10687. 31. Pouvreau, M.; Greathouse, J. A.; Cygan, R. T.; Kalinichev, A. G., Structure of hydrated gibbsite and brucite edge surfaces: DFT results and further development of the ClayFF classical force field with metal–O–H angle bending terms. J. Phys. Chem. C 2017, 121, 14757-14771. 32. Delley, B., From molecules to solids with the DMol 3 approach. J. Chem. Phys. 2000, 113, 7756-7764. 33. Cygan, R. T.; Liang, J.-J.; Kalinichev, A. G., Molecular Models of Hydroxide, Oxyhydroxide, and Clay Phases and the Development of a General Force Field. J. Phys. Chem. B 2004, 108, 1255-1266. 34. Pascale, F.; Tosoni, S.; Zicovich-Wilson, C.; Ugliengo, P.; Orlando, R.; Dovesi, R., Vibrational spectrum of brucite, Mg(OH)2: a periodic ab initio quantum mechanical calculation including OH anharmonicity. Chem. Phys. Lett. 2004, 396, 308-315. 35. Grimme, S., Semiempirical GGA-type density functional constructed with a long-range dispersion correction. J. Comput. Chem. 2006, 27, 1787-1799. 36. Santra, B.; Michaelides, A.; Scheffler, M., On the accuracy of density-functional theory exchangecorrelation functionals for H bonds in small water clusters: Benchmarks approaching the complete basis set limit. J. Chem. Phys. 2007, 127, 1841041 - 1841049. 37. Tunega, D.; Bučko, T.; Zaoui, A., Assessment of ten DFT methods in predicting structures of sheet silicates: Importance of dispersion corrections. J. Chem. Phys. 2012, 137, 1141051- 1141059. 38. Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865-3868. 39. Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H., A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 2010, 132, 1541041-15410419. 40. Neugebauer, J.; Scheffler, M., Adsorbate-substrate and adsorbate-adsorbate interactions of Na and K adlayers on Al(111). Phys. Rev. B. 1992, 46, 16067-16080. 41. Hine, N. D. M.; Dziedzic, J.; Haynes, P. D.; Skylaris, C.-K., Electrostatic interactions in finite systems treated with periodic boundary conditions: Application to linear-scaling density functional theory. J. Chem. Phys. 2011, 135, 204103-204120.


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42. Yu, L.; Ranjan, V.; Lu, W.; Bernholc, J.; Nardelli, M. B. Equivalence of dipole correction and Coulomb cutoff techniques in supercell calculations. Phys. Rev. B. 2008, 77, 245102-24108. 43. Roscioni, O. M.; Dyke, J. M.; Evans, J., Structural characterization of supported RhI 397 (CO)2/γ-Al2O3 catalysts by periodic DFT calculations. J. Phys. Chem. C. 2013, 117, 19464-19470. 44. Zigan, F.; Rothbauer, R., Neutronenbeugungsmessungen am brucit. Neues Jahrbuch für Mineralogie Monatshefte 1967, 1967, 137-143. 45. Jug, K.; Heidberg, B.; Bredow, T., Cyclic cluster study on the formation of brucite from periclase and water. J. Phys. Chem. C 2007, 111, 13103-13108. 46. Catti, M.; Ferraris, G.; Hull, S.; Pavese, A., Static compression and H disorder in brucite, Mg (OH) 2, to 11 GPa: a powder neutron diffraction study. Phys. Chem. Miner. 1995, 22, 200-206. 47. Greathouse, J. A.; O'Brien, R. J.; Bemis, G.; Pabalan, R. T., Molecular dynamics study of aqueous uranyl interactions with quartz (010). J. Phys. Chem. B 2002, 106, 1646-1655. 48. Greathouse, J. A.; Hart, D. B.; Ochs, M. E., Alcohol and thiol adsorption on (oxy) hydroxide and carbon surfaces: Molecular dynamics simulation and desorption experiments. J. Phys. Chem. C 2012, 116, 26756-26764. 49. Fu, Y.-T.; Zartman, G. D.; Yoonessi, M.; Drummy, L. F.; Heinz, H., Bending of layered silicates on the nanometer scale: mechanism, stored energy, and curvature limits. J. Phys. Chem. C 2011, 115, 2229222300. 50. Wolthers, M.; Charlet, L.; Van Cappellen, P., The surface chemistry of divalent metal carbonate minerals; a critical assessment of surface charge and potential data using the charge distribution multisite ion complexation model. Am. J. Sci. 2008, 308, 905-941. 51. Dobias, B.; Stechemesser, H.; Coagulation and flocculation: theory and application. CRC Press, 2005. New York, US. 52. Lyklema, J., Fundamentals of interface and colloid science: Solid-liquid interface. Academic Press, Elsevier, 1995, London, UK. 53. Bockris, J. O. M.; Conway, B. E.; Yeager, E.; White, R. E., Comprehensive treatise of electrochemistry: the double layer. Springer, 1980, New York, US. 54. Kiselev, A., Non-specific and specific interactions of molecules of different electronic structures with solid surfaces. Discuss. Faraday. Soc. 1965, 40, 205-218. 55. Yong, R. N., Geoenvironmental engineering: Contaminated soils, pollutant fate, and mitigation. CRC press, 2000, New York, US.


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56. Makkos, E.; Kerridge, A.; Austin, J.; Kaltsoyannis, N. Ionic adsorption on the brucite (0001) surface: a periodic electrostatic embedded cluster method study. J. Chem. Phys. 2016, 145, 2047081 – 20470813. 57. Schott, H., Electrokinetic studies of magnesium hydroxide. J. Pharm. Sci. 1981, 70, 486-489. 58. Pesce, G. L.; Fletcher, I. W.; Grant, J.; Molinari, M.; Parker, S. C.; Ball, R. J. Carbonation of Hydrous Materials at the Molecular Level: A Time of Flight-Secondary Ion Mass Spectrometry, Raman and Density Functional Theory Study. Cryst. Growth Des. 2017, 17, 1036−1044.


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Table Captions Table 1. Apparent adsorption energies (affinities) (kJ/mol) according to Eq.1 of brucite carbonation induced-species on the four (001) brucite surface modalities Table 2. Apparent adsorption energies (affinities) (kJ/mol) according to Eq.1 of brucite carbonation induced-species on partially dissolved neutral (001) brucite surface after congruent dissolution steps 1, 2, 5, 11 and 12 (see Table SI_6 and Figure 10 for other details)


27


Page 28 of 42

Table 1. Apparent adsorption energies (affinities) (kJ/mol) according to Eq.1 of brucite carbonation induced-species on the four (001) brucite surface modalities Species

CO32-

Brucite surface

MgOH0

MgO-

MgOH2+

Mg+1/3

-40.5

-123.4

Solvated

-

-

-

-118.8

-45.3

-

-

-

Non-solvated

-44.6

-47.3

-45.1

-129.5

Solvated

-

-

-

-124.3

-43.1

-

-

-

Non-solvated

-173.1

-202.1

-161.2

-165.7

Solvated

-160.3

-192.1

-155.1

-158.1

-174.9

-

-

-

Non-solvated

-46.4

-132.4

-42.1

-142.1

Solvated

-

-

-

-138.2

-45.8

-

-

-

Non-solvated

-140.3

-155.5

-135.2

-54.3

Solvated

-

-

-

-54.1

-142.4

-

-

-

(H+)

With counter ion (O2-)

With counter ion (OH-)

MgHCO3+

dehydroxylated (001)

-42.7

With counter ion

MgOH+

protonated (001)

-45.1

(Mg2+)

Mg2+

deprotonated (001)

Non-solvated

With counter ion

HCO3-

neutral (001)

With counter ion (OH-)


28

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Table 2. Apparent adsorption energies (affinities) (kJ/mol) according to Eq.1 of brucite carbonation induced-species on partially dissolved neutral (001) brucite surface after congruent dissolution steps 1, 2, 5, 11 and 12 (see Table S6 and Figure 10 for other details) Coordination number of Mg & O atoms on brucite (001) for different dissolution steps CN(Mg) = 6

CN(Mg) = 5,6

CN(Mg) = 4-6

CN(Mg) = 3-6

CN(O) = 3

CN(O) = 2, 3

CN(O) = 1-3

CN(O) = 1-3

CO32-

(001) surface -45.1

Step 1 -122.7

Step 2 -126.1

Step 5 -133.4

HCO3-

-44.6

-127.3

-130.4

-136.3

-145.1

-152.8

Mg2+

-173.1

-221.1

-228.8

-232.2

-244.7

-250.2

MgOH+

-46.4

-156.4

-157.8

-160.7

-163.3

-172.5

MgHCO3+

-140.3

-163.1

-165.9

-169.1

-171.3

-177.6

Species


CN(Mg) = 2-6 CN(O) = 13 Step 11 -143.1

CN(Mg) = 1, 3-6 CN(O) = 1-3 Step 12 -149.9

29

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 30 of 42

Figure 1

(a)

(b)

Figure 1. Optimized structure of (a) brucite slab (001), and (b) brucite slab (110) (Mg: green, O: red, H: white).


30

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Figure 2

(a)

(b)

Figure 2. Adsorption on (001) brucite (44 supercell) surface of twenty water molecules (Mg: green, O: red, H: white).


31


Page 32 of 42

Figure 3

(a)

(c)

OH-protonated site

(b)

OH-deprotonated site

(d)

Mg- dehydroxylated sites

Figure 3. Optimized structure of (a) neutral, (b) single-site deprotonated, (c) single-site protonated, (d) multiple-Mg-site dehydroxylated brucite (001) slab (Mg: green, O: red, H: white, deprotonated O: pink, protonated OH group: yellow, dehydroxylated Mg: black).


32

Page 33 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


Figure 4

(b)

(a)

(c)

OH-protonated site

(d)



Figure 4. Interaction of CO32- with a) neutral, b) single-site deprotonated, c) single-site protonated, and d) multiple-Mg-site dehydroxylated brucite (001) surface slab (C: gray, Mg: green, O: red, H: white, deprotonated O: pink, protonated OH group: yellow, dehydroxylated Mg: black).


33


Page 34 of 42

Figure 5

(a)

(c)


(b)

OH-protonated site

(d)


Figure 5. Interaction of HCO3- with a) neutral, b) single-site deprotonated, c) single-site protonated, and d) multiple-Mg-site dehydroxylated brucite (001) surface slab (C: gray, Mg: green, O: red, H: white, deprotonated O: pink, protonated OH group: yellow, dehydroxylated Mg: black).


34

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Figure 6

(a)

(c)

OH-protonated site

(b)


(d)


Figure 6. Interaction of Mg2+ with a) neutral, b) single-site deprotonated, c) single-site protonated, and d) multiple-Mg-site dehydroxylated brucite (001) surface slab (C: gray, Mg: green, O: red, H: white, deprotonated O: pink, protonated OH group: yellow, dehydroxylated Mg: black).


35


Page 36 of 42

Figure 7

(a)

(c)

OH-protonated site

(b)


(d)


Figure 7. Interaction of MgOH+ ion pair with a) neutral, b) single-site deprotonated, c) single-site protonated, and d) multiple-Mg-site dehydroxylated brucite (001) surface slab (C: gray, Mg: green, O: red, H: white, deprotonated O: pink, protonated OH group: yellow, dehydroxylated Mg: black).


36

Page 37 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


Figure 8

(a)

(c)

OH-protonated sites

(b)


(d)


Figure 8. Interaction of MgHCO3+ ion pair with a) neutral (expelled proton on the right of Mg), b) single-site deprotonated, c) single-site protonated (two expelled protons: one below carbonate moiety of the ion pair, on above two surface OH groups highlighted in yellow), and d) multiple-Mg-site dehydroxylated brucite (001) surface slab (C: gray, Mg: green, O: red, H: white, deprotonated O: pink, protonated OH group: yellow, dehydroxylated Mg: black).


37


Page 38 of 42

Figure 9

Step 1

Step 4

CN(O↑)=2 CN(Mg)=5

CN(O↓)=1

CN(Mg)=3

Step 2

Step 3 CN(Mg)=4

Step 5 CN(O↑)=1

CN(O↓)=1

CN(Mg)=3

Step 6

CN(Mg)=4

CN(O↓)=1

CN(Mg)=3

Figure 9. Optimized structures of dissolved (001) brucite surface following steps 1 to 12 of Figure S2 (Supporting Information): downwardly (upwardly) pointing OH = ↓ (↑) (Mg: green, O: red, H: white, CN : coordination number).


38

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Figure 9 cont’d

Step 8

Step 7

CN(Mg)=3

CN(Mg)=3

Step 10 CN(Mg)=3

Step 9

Step 11

CN(Mg)=2

CN(Mg)=3

Step 12

CN(Mg)=1

Figure 9. Optimized structures of dissolved (001) brucite surface following steps 1 to 12 of Figure S2 (Supporting Information): downwardly (upwardly) pointing OH = ↓ (↑) (Mg: green, O: red, H: white, CN : coordination number).


39


Page 40 of 42

Figure 10

(b)

(a) CN(Mg)=5

(c)

CN(Mg)=3

CN(Mg)=4

(d)

CN(Mg)=1

Figure 10. Interaction of CO32- with neutral partially dissolving brucite surface a) step 1, b) step 2, c) step 5, and d) step 12 (C: gray, Mg: green, O: red, H: white).


40

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Figure 11

(a)

(c)

CN(O↓)=2

CN(O↓)=1

(b)

(d)

CN(O↓)=2

CN(O↓)=1

Figure 11. Interaction of Mg2+ with neutral partially dissolving brucite surface a) step 1, b) step 2, c) step 5, and d) step 12 : downwardly pointing OH = ↓ (Mg: green, O: red, H: white, Mg from solution: blue).


41


x0

Outer Stern layer

Brucite (001)

Inner Stern layer

GRAPHICAL ABSTRACT

1

2

Carbonating Brucite – Aqueous Interface

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 42 of 42

Stern layer


42

Surface Speciation of Brucite Dissolution in Aqueous Mineral

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