Enhanced Ion Adsorption on Mineral Nanoparticles - ACS Publications

Article Cite This: Langmuir XXXX, XXX, XXX−XXX

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Enhanced Ion Adsorption on Mineral Nanoparticles Tuan A. Ho,* Jeffery A. Greathouse,* Andrew S. Lee, and Louise J. Criscenti* Geochemistry Department, Sandia National Laboratories, Albuquerque, New Mexico 87185, United States S Supporting Information *

ABSTRACT: Classical molecular dynamics simulation was used to study the adsorption of Na+, Ca2+, Ba2+, and Cl− ions on gibbsite edge (1 0 0), basal (0 0 1), and nanoparticle (NP) surfaces. The gibbsite NP consists of both basal and edge surfaces. Simulation results indicate that Na+ and Cl− ions adsorb on both (1 0 0) and (0 0 1) surfaces as inner-sphere species (i.e., no water molecules between an ion and the surface). Outer-sphere Cl− ions (i.e., one water molecule between an ion and the surface) were also found on these surfaces. On the (1 0 0) edge, Ca2+ ions adsorb as inner-sphere and outer-sphere complexes, whereas on the (0 0 1) surface, outer-sphere Ca2+ ions are the dominant species. Ba2+ ions were found as inner-sphere and outer-sphere complexes on both surfaces. Calculated ion surface coverages indicate that, for all ions, surface coverages are always higher on the basal surface compared to those on the edge surface. More importantly, surface coverages for cations on the gibbsite NP are always higher than those calculated for the (1 0 0) and (0 0 1) surfaces. This enhanced ion adsorption behavior for the NP is due to the significant number of inner-sphere cations found at NP corners. Outer-sphere cations do not contribute to the enhanced surface coverage. In addition, there is no ion adsorption enhancement observed for the Cl− ion. Our work provides a molecular-scale understanding of the relative significance of ion adsorption onto gibbsite basal versus edge surfaces and demonstrates the corner effect on ion adsorption on NPs.

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INTRODUCTION The availability and transport of ions in surface and subsurface environments mainly depends on the adsorption/desorption of ions to/from oxides, hydroxides, and clay minerals.1,2 To fully understand ion adsorption/desorption at mineral interfaces, fundamental molecular-scale understanding of the interface is required, including molecular structure, adsorption sites, and surface protonation/deprotonation states. Experimental data on ion adsorption are largely collected on mineral particles; in contrast, molecular simulations are largely performed to examine the interaction of one mineral surface with an aqueous solution. In this work, we use classical molecular dynamics (MD) simulations to investigate the adsorption of alkali, alkaline earth, and chloride ions on a mineral nanoparticle (NP). We selected gibbsite [γ-Al(OH)3] as our substrate because it is the most commonly formed aluminum oxide in soils.3 In addition, gibbsite is representative of one of the two basal surfaces of a clay mineral, for example, kaolinite. We chose to investigate the adsorption of Na+, Ca2+, Ba2+, and Cl− ions. Sodium chloride is a dominant electrolyte in groundwater; Ca2+ is a major divalent cation in natural waters; and Ca2+ and Ba2+ exhibit different ionic radii and hydration energies. To our knowledge, this is the first time that ion adsorption on a single oxide NP has been studied using classical MD simulation. In the gibbsite structure, octahedral Al atoms are coordinated by three hydroxyl O atoms each above and below the Al sheet. This neutral aluminum hydroxide sheet is similar to the octahedral sheet bonded to silicate sheets in clay minerals such © XXXX American Chemical Society

as kaolinite, illite, and smectite. In the past, it has been assumed that the doubly coordinated −OH groups on the gibbsite basal plane (001) are inactive to deprotonation/protonation and that surface charge and ion adsorption are dominated by singly coordinated aluminol edges.4 However, recent experimental5−7 and simulation8 work provides clear evidence that the gibbsite (0 0 1) face is not inert in aqueous solution. MD simulation of the edge surfaces of layered minerals such as gibbsite and clay minerals is more complex than simulation of basal surfaces. First, the chemistry of the edge surfaces under different pH conditions is not well-understood.9 Second, one of the most widely used force fields to describe mineral-water systems (i.e., ClayFF10) was initially developed to simulate the interaction of water with both unreactive clay basal surfaces and interlayers. Therefore, the majority of MD simulation studies using ClayFF10 for mineral-water systems have been carried out using a simple slit-shaped pore between two clay-mineral basal (001) surfaces, without considering edge surfaces such as the gibbsite (100) and (110) surfaces.9,11−18 Numerous quantum mechanical simulations have tried to address the first issue (pHdependent structure of edge surfaces) by studying the stability9,19,20 and acid−base properties of edge surfaces.21−24 In particular, recent density functional theory simulations of (100) gibbsite surfaces in contact with water indicate that both Received: February 28, 2018 Revised: April 27, 2018

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DOI: 10.1021/acs.langmuir.8b00680 Langmuir XXXX, XXX, XXX−XXX

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Figure 1. Simulation snapshot of an aqueous solution (water molecules are removed for clarity) confined in a gibbsite slit pore (A). The gibbsite surfaces used in the slit-pore simulations are either (1 0 0) or (0 0 1) faces. Structure of the (1 0 0) gibbsite edge (B). Simulation snapshot of the gibbsite NP surrounded by an aqueous solution (water molecules are removed for clarity) (C). Top and side views of the NP (D). Pink, cyan, and purple spheres are gibbsite Al, O, and H atoms, respectively. Green and blue spheres are cations and Cl− ions, respectively. Gibbsite NP Immersed in Aqueous Solution. In the second approach, a hexagonal gibbsite NP was placed in the center of a simulation box of dimensions 90 × 90 × 60 Å3 (Figure 1C). The method to build the hexagonal gibbsite NP was detailed in our previous work.28 Briefly, the NP was cleaved from a gibbsite slab with a thickness of three layers along the (1 0 0) and (1 1 0) directions (Figure 1D). The (1 0 0) and (1 1 0) faces were selected because they are the most commonly observed experimentally for gibbsite NPs that usually exhibit a pseudo-hexagonal shape.29 Because the (1 0 0) and (1 1 0) faces are equivalent,30 we hereafter only refer to the (1 0 0) surface. Additional OH groups were added to undercoordinated Al atoms on the edges, resulting in 5-fold coordination of edge Al atoms (Figure 1B). On the basal (0 0 1) surface, there are only bridging OH groups (i.e., doubly coordinated OH group, OH group bonded to two Al atoms), but on the (1 0 0) edge surface, there are both singly coordinated (i.e., OH group bonded to one Al atom) and doubly coordinated OH groups. The structure of the basal and edge surfaces in the NP is consistent with experimental data.6 The hexagonal NP has 2016 atoms including 288 Al, 864 O, and 864 H atoms; 21.0 Å edges; and a thickness of 13.4 Å (3 gibbsite layers, Figure 1D). The lateral dimension of our NP is much smaller than those of the NPs synthesized in the laboratory or found in nature.6,31−33 However, this work focuses on comparing the properties of water and ion adsorption on different gibbsite surfaces, and in consideration of computational resources and simulation time, our particle size selection is a reasonable choice. Simulation Details. The total potential energy expression for interatomic interactions in gibbsite was described using ClayFF,10 in which interatomic interaction energies are described by nonbonded Lennard-Jones and electrostatic energy terms. Partial charges of +1.575e, −0.95e, and +0.425e are assigned to the Al, O, and H atoms, respectively, which results in a charge-neutral slit pore and NP. Recent microscopy experiments indicate that the surface charge of the gibbsite basal surfaces is variable.5,34,35 Negatively charged surfaces can be simulated in classical MD by removing hydrogen atoms of hydroxyl groups and redistributing the resulting negative charge.36−38 For this fundamental study of ion adsorption on gibbsite, because there is no parameterization within ClayFF available for charged gibbsite surfaces, we chose to study only neutral surfaces. Similarly, water dissociation near the surface39 is not allowed during the course of simulation. The only bonded energy term used in ClayFF for the mineral phase is the OH bond stretch. ClayFF was first developed to simulate clay mineral basal surfaces with periodic boundary conditions (i.e., no edges). Recent efforts further developed ClayFF by defining harmonic Mg− O−H and Al−O−H angle bending terms to simulate specific edges of brucite and gibbsite, respectively.25,26 The Al−O−H angle bending

6-fold and 5-fold coordination states of edge Al atoms are probable, with the former being much more stable. For 6-fold aluminum coordination, the pKa value of the Al−(OH2)2 group was calculated to be 9.520 (note that charged surfaces were not studied in this work). For the second issue, Al−O−H and Mg− O−H angle bending terms compatible with ClayFF were recently developed specifically for simulating mineral edge surfaces with Al−O−H and Mg−O−H groups.25,26 These recent force field development efforts pave the way for simulations of aqueous fluids interacting with different gibbsite surfaces and with gibbsite NPs. In this paper, we use ClayFF to first investigate ion adsorption on the gibbsite basal (001) and edge surfaces (100) and then investigate ion adsorption on a hexagonal gibbsite NP complete with basal and edge surfaces, as well as finite boundaries between the surfaces, and corners where the surfaces intersect. This work illustrates the impact of ion adsorption on the corners that terminate each NP face.

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METHODS

Two simulation approaches were designed to study the adsorption of aqueous ions on gibbsite surfaces. In the first approach, the aqueous solution was confined in a (0 0 1) or (1 0 0) gibbsite slit pore. In the second approach, a gibbsite NP was immersed in aqueous solution. Aqueous Solution Confined in Gibbsite Slit Pores. Two gibbsite slit-pore models (Figure 1A) were built. The first slit pore was constructed from the gibbsite basal surface [i.e., the (0 0 1) face]. An orthogonalized Al(OH)3 unit cell with dimensions a = 8.783 Å, b = 5.136 Å, and c = 9.847 Å was repeated 4 × 6 × 3 times to form a slab with dimensions 34.7 × 30.4 × 29.2 Å3 using Materials Studio.27 This slab was placed in a simulation box with dimensions 34.7 × 30.4 × 107 Å3 with the z direction perpendicular to the gibbsite basal (0 0 1) surface. The second gibbsite slit-pore model was built from the (1 0 0) surface. The orthogonalized Al(OH)3 unit cell was repeated 3 × 7 × 4 times to form a slab with dimensions 26.3 × 35.2 × 38.6 Å3. This gibbsite slab was placed in a periodic box with dimensions 35.2 × 38.6 × 105 Å3 with the z direction perpendicular to the (1 0 0) surface. Note that additional −OH groups were added to edge Al atoms on the (1 0 0) surface, resulting in 5-fold edge Al atoms (Figure 1B), consistent with previous simulations of this surface.25 For both gibbsite surfaces, periodic boundary conditions were applied in all directions, creating slit pores with an approximate width of 80 Å (Figure 1A). B


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Langmuir Table 1. Number of NaCl and Water Molecules Simulated for Each Systema

a

system

∼0.5 M

∼1 M

∼2 M

(1 0 0) slit pore (0 0 1) slit pore NP

30 NaCl/3432 H2O 25 NaCl/2860 H2O 137 NaCl/15161 H2O



For BaCl2 and CaCl2 solutions, the number of Cl− ions for each concentration was doubled.

Figure 2. Normalized density profiles of water oxygen Ow (A), Na+ (B), Ca2+ (C), Ba2+ (D), and Cl− (E) as a function of shortest distance to gibbsite O, H, and Al atoms calculated for the (1 0 0), (0 0 1), and NP surfaces. Results are shown for simulations of 1 M solutions. Note that the scale of the y-axis is different for each panel. Dang and Smith force field41 was implemented to simulate Na+ and Cl− ions. Ca2+ and Ba2+ ions were modeled using the parameters developed by Koneshan et al.42 and Aqvist,43 respectively. Note that all ion and water force fields used in this work are described in ClayFF.10 After energy minimization, all systems were equilibrated in the NVT ensemble (constant number of atoms, volume, and temperature) for 200 ps and then in the NPT ensemble (constant number of atoms, pressure, and temperature) for 40 ns. All simulations were performed at 298 K and 1 atm using the Nose−Hoover thermostat and barostat.44,45 For the slit-pore systems, pressure was controlled in the z direction, and for the NP simulations, pressure was anisotropically controlled in three directions. Short-range interactions were calculated using a cut-off distance of 10 Å. Long-range electrostatic interactions were calculated using the particle−particle−particle−mesh solver.46 To keep the gibbsite slab in slit-pore and NP systems stationary to simplify the postprocessing, six atoms in the gibbsite slab and nine atoms in the NP were fixed by excluding them from the integration of the equations of motion. The time step was 1 fs. Trajectories from the last 20 ns of the NPT simulations were recorded every 1000 steps and used to compute the results reported herein. All simulations were conducted using the LAMMPS simulation package.47

term was used in all simulated systems including the (1 0 0) and (0 0 1) slit pores and NP. Although the (1 0 0) surface in the slit-pore system was successfully simulated using the new Al−O−H angle bending term, a hexagonal gibbsite particle with the (1 0 0), (1 1 0), and (0 0 1) surfaces was not numerically stable. To overcome this issue, in our previous work,28 an Al−O−Al angle bending term was explicitly defined using a harmonic potential (k(θ − θ0)2) for the Al− O−Al angles (θ) at the edge surface (only for NP simulation). The equilibrium value for the Al−O−Al angle (θ0) is 100° and k = 800 kcal·mol−1·rad−2. Note that the use of a large k value will affect the calculated vibrational properties, but it is unlikely to affect the properties of water and ions as Al−O−H angles and O−H bonds are fully flexible in this work. Water molecules and ions were randomly placed into the simulation box containing a gibbsite slit pore or gibbsite NP. The number of water molecules was selected to achieve a density of ∼1 g·mL−1. The number of ions added to each system depends on the simulated concentration. We simulated NaCl, CaCl2, and BaCl2 aqueous solutions at 0.5, 1.0, and 2.0 M concentrations, respectively. In Table 1, we report, as an example, the number of water and NaCl molecules used for each simulated system. Water molecules were simulated using the flexible simple point-charge water model.40 The C


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Figure 3. Simulation snapshots (looking down from the z direction) (A,C) illustrate the adsorption sites for water on the (0 0 1) and (1 0 0) surfaces, respectively. Planar density distributions (B,D) of the first water layer (i.e., water molecules with oxygen atom within 3 Å from the surface) on the (0 0 1) and (1 0 0) surfaces, respectively. Gibbsite atoms are colored as in Figure 1B. Water O and H atoms are colored red and white, respectively. In panel C, five-coordinated Al atoms are highlighted by pink spheres.

Figure 4. Simulation snapshots demonstrate the adsorption of aqueous NaCl (A1,A2), CaCl2 (B1,B2), and BaCl2 (C1,C2) solutions on the (0 0 1) and (1 0 0) gibbsite surfaces. Water oxygen and hydrogen atoms are represented as red and white frameworks, respectively. Other atoms are colored as in Figure 1.

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profiles on flat surfaces such as our (1 0 0) and (0 0 1) slit-pore systems.3,8,48,49 However, because the NP has multiple surfaces aligning along multiple directions, we used the shortest distance of aqueous atoms to gibbsite atoms to provide consistent comparisons. For example, for the Ow−O density profile, the distances of an Ow atom to all surface O atoms were calculated, and the shortest distance was recorded. The density profile was

RESULTS AND DISCUSSION

In Figure 2, we report the normalized density profiles of water oxygen Ow and Na+, Ca2+, Ba2+, and Cl− ions as a function of shortest distance to gibbsite O, H, and Al atoms calculated near the (1 0 0), (0 0 1), and NP surfaces to compare the structural properties of aqueous solutions on different gibbsite surfaces. Such comparisons are usually made via traditional 1-D density D


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the Cl− and the gibbsite surface, is also seen from the broad Cl−H peak between 3 and 6 Å. The orientation of −OH surface groups changes with the adsorption of Na+ and Cl− ions. Upon Na+ ion adsorption, the −OH group aligns parallel to the surface (H atoms directed away from Na+), whereas for Cl− ion adsorption, the −OH group is more perpendicular to the surface with H atoms directed toward the Cl− ion (Figure 5A). On the (1 0 0) surface, Na+ ions were usually found directly over six-coordinated Al atoms and coordinated by four surface O atoms and two water molecules (Figure 5B). The adsorption of Na+ ions on the (1 0 0) surface does not affect the tendency of water molecules to adsorb at five-coordinated Al sites (Figures 5B and S1 in the Supporting Information). For Ca2+ ion adsorption, two pronounced peaks were observed at 2.4 and 4.5 Å for the Ca2+−O density profile near the (1 0 0) surface (Figure 2C1), indicating both innersphere and outer-sphere Ca2+ complexes (see also the snapshot in Figure 4B2). On the (0 0 1) surface, we observed a small peak at 2.4 Å and a pronounced peak at 4.5 Å in the Ca2+−O density profile (Figure 2C2). This result suggests that adsorbed Ca2+ ions on the (0 0 1) surface are predominantly outersphere species, in agreement with the proposed surface structure for adsorbed Ca2+ ions from atomic force microscopy (AFM) studies on the gibbsite basal surface5 and the gibbsitelike surface of kaolinite.51 A limited number of adsorbed Ca2+ ions form inner-sphere surface complexes (see Figure 4B1). Even at 2 M CaCl2 concentration, Ca2+ ions do not adsorb in a regular pattern on the gibbsite (0 0 1) surface as seen in the AFM studies,5 probably because water dissociation to form charge-balancing hydroxide ions is not possible in our classical simulations. For NP simulations with both (0 0 1) and (1 0 0) surfaces simultaneously present in the solution, adsorbed Ca2+ ions were found as inner-sphere and outer-sphere complexes, as expected (Figure 2C3). For Ba2+ ions on the (1 0 0), (0 0 1), and NP surfaces, we observed two pronounced peaks at 2.8 and 4.8 Å (Figure 2D1− D3), indicating the formation of both inner-sphere and outersphere Ba2+ surface complexes on all modeled surfaces (see also the snapshots in Figure 4C1,C2). Ba2+ ions form inner-sphere complexes more easily than Ca2+ on the (0 0 1) surface, which is in good agreement with previous molecular modeling and experimental results suggesting that with increasing ionic radii and decreasing free energy of hydration, water molecules of hydration surrounding the cation are removed more readily, and hence it is easier to form inner-sphere complexes.3 The important effect of enhanced cation adsorption on NP surfaces is shown in Figure 6. Surface coverages for Cl− indicate greater adsorption on the (0 0 1) surface compared to the (1 0 0) surface (Figure 6A). The Cl− surface coverage on the NP is between the coverage values for the slit-pore (0 0 1) and (1 0 0) surfaces, as expected, because the NP contains both surfaces. However, for Na+ (Figure 6B), Ca2+ (Figure 6C), and Ba2+ (Figure 6D) ions, we observed an enhancement of cation adsorption on the NP at all considered concentrations (i.e., the surface coverages for all cations are higher for the NP, compared to those found for the (0 0 1) and (1 0 0) surfaces in the slit-pore systems). As discussed previously, adsorbed Ca2+ and Ba2+ ions form both inner-sphere and outer-sphere complexes on the (1 0 0), (0 0 1), and NP surfaces. Contributions from each type of surface complex (inner-sphere vs outer-sphere) to the surface coverage of these cations are shown in Figure 7. Inner-sphere Ca2+ (Ba2+) complexes are those found within 3.0 (3.5) Å from

then constructed by counting the number of Ow atoms as a function of shortest distance to surface O atoms. We only considered the Ow atoms within 10 Å from the surface (above which water can be considered bulk water). The density profile was then normalized by the total number of water molecules within 10 Å of the surface. The comparison of the Ow profile on different surfaces (Figure 2A1−A3) highlights the difference in water properties on the (1 0 0) and (0 0 1) gibbsite surfaces. In Figure 2A1,A3, a pronounced peak was observed on the Ow−Al profile (green line) at 2 Å. This peak demonstrates the interaction of water molecules with 5-fold Al atoms on the (1 0 0) surface. This Ow−Al peak was not observed on the (0 0 1) surface (Figure 2A2). In the (0 0 1) direction, each gibbsite Al atom is coordinated by three hydroxyl (−OH) groups each above and below the Al sheets. Water molecules interact directly with −OH groups above each Al atom (see Figure 3A). Some −OH groups point into the solution and donate a hydrogen bond to a water molecule (water marked number 2 in Figure 3A). Some OH bonds align parallel to the surface and form a hydrogen bond with water hydrogen (water marked number 1 in Figure 3A). The planar density distribution of interfacial water (Figure 3B) indicates the preferential position of water oxygen atoms on the (0 0 1) surface. On the (1 0 0) surface, exposed 5-fold Al atoms (see Figure 1B) are always coordinated by water molecules (Figure 3C). Depending on the orientation of the −OH groups that bond to the 5-fold Al atom, water molecules can be on either side of the Al plane (see water marked number 1 and 2 in Figure 3C). Because of the strong interaction between water molecules and 5-fold Al atoms, water molecules accumulate at very precise locations along the surface (Figure 3D). We do not observe a significant difference regarding the relative position of Na+ (Figure 2B1−B3) and Cl− (Figure 2E1−E3) ions with respect to gibbsite atoms on the basal (0 0 1) and edge (1 0 0) surfaces. The Na+ ions adsorb on the (0 0 1) and (1 0 0) gibbsite surfaces as inner-sphere complexes, with no water molecules between the Na+ ion and the gibbsite surface (see the snapshots in Figure 4A1,A2), in agreement with previous work.8 On the (0 0 1) surface, at distances in agreement with previous simulations of 2 M NaCl solutions on the gibbsite (001) surface,8 Na+ ions were usually found above Al atoms and coordinated by three surface O atoms and three water molecules (see the snapshots in Figure 5A). Such a Na+ ion adsorption behavior was also observed on calcium silicate hydrate gel.50 Inner-sphere Cl− ions were also found above Al atoms and usually coordinated by three surface H atoms and three water molecules (Figure 5A); however, evidence of outersphere coordination, where one water molecule sits between

Figure 5. Simulation snapshots showing adsorption sites for Na+ (green) and Cl− (blue) ions on the (0 0 1) (A) and (1 0 0) (B) surfaces. In B, five-coordinated Al atoms are highlighted by pink spheres. E


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Figure 6. Surface coverage of adsorbed ions Cl− (A), Na+ (B), Ca2+ (C), and Ba2+ (D) as a function of surface type and ion concentration. In panel A, Cl− ion surface coverage is shown for NaCl, CaCl2, and BaCl2 solutions, all at 1 M. All Cl− ions within 3 Å from surface H atoms (based on the Cl−−H profile in Figure 2) were considered in this calculation. Surface coverages were calculated for Na+, Ca2+, and Ba2+ ions within 2.8, 5.5, and 5.8 Å from surface oxygen atoms (based on the cation-O profiles in Figure 2), respectively.

Figure 7. Surface coverage of inner-sphere (red) and outer-sphere (blue) ions calculated for Ca2+ in 1 M CaCl2 solution (A) and Ba2+ in 2 M BaCl2 solution (B) for (1 0 0), (0 0 1), and NP surfaces.

a surface O atom, and outer-sphere Ca2+ (Ba2+) complexes are those found between 3.0 (3.5) and 5.5 (5.8) Å from a surface O atom. The results indicate that ion adsorption enhancement observed for the NP is mainly due to enhancement of innersphere complexes (red bars in Figure 7). There is no enhancement observed on the NP surface for outer-sphere species (blue bars in Figure 7). Enhanced Na+ ion adsorption at NP surfaces is explained in a similar manner because Na+ ions adsorb almost exclusively as inner-sphere surface complexes (Figure 2). We also expect that changing the ratio of the edge to basal surface areas will affect the degree of ion adsorption enhancement. Our particle aspect ratio (i.e., diameter/thickness) is ∼3 compared to ∼20 for synthetic gibbsite NPs.6,32 A larger particle aspect ratio would reduce the ion adsorption enhancement. However, our corner model could also be representative of other defective sites present in hexagonal or irregularly shaped NPs. On the basis of our results, we predict

that ion adsorption should also be enhanced at these defect sites. Simulation snapshots in Figure 8 help to explain the enhanced cation adsorption observed for the NP. Visualization of NP snapshots in 1 M NaCl (Figure 8A), 2 M CaCl2 (Figure 8B), and 0.5 M BaCl2 (Figure 8C) solutions reveals that a considerable number of cations (i.e., those highlighted in red circles) are coordinated to −OH groups at NP corners (a corner is where the (1 0 0) and (1 1 0) edges meet; each gibbsite layer has corner −OH groups). The high-density region (yellow to red) in Figure 8D confirms the presence of Na+ ions near the corners and edges of the NP over long simulation times, which results in the adsorption enhancement for cations reported in Figure 6. One could also calculate time correlation functions50,52,53 to provide additional evidence of the presence of the Na+ ions near NP corners, but such an analysis on unconventional NP geometries is beyond the scope of the present work. The simulation snapshot shown in Figure F


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Figure 8. Cation adsorption at NP corners. Simulation snapshots showing the adsorption of Na+ (A), Ca2+ (B), and Ba2+ (C) ions highlighted in red circles near corners of the NP. Planar density distribution of Na+ ions near the NP in a 1 M NaCl solution (D), calculated from the final 10 ns of the simulation. Only Na+ ions in the region defined by the z coordinates of the NP were considered in this calculation. Simulation snapshot showing the adsorption of a Na+ ion at a NP corner along with first-shell water molecules (E).

8E illustrates that a Na+ ion adsorbed at the NP corner is coordinated by three surface O atoms and three water molecules. Note that no Cl− ion adsorption was observed at NP corners, which explains the lack of enhanced Cl− ion surface coverage (Figure 6A).

reported data are collected on mineral particles. In addition, these results have implications for colloid- or NP-assisted migration of metal cations in the subsurface. Results from laboratory batch adsorption experiments combined with results from X-ray absorption fine-structure (EXAFS) spectroscopy are typically used to develop databases of adsorption complexes for reactive transport modeling. Molecular simulations are often used to support or interpret spectroscopic data; however, these simulations are incomplete without the consideration of particle edges, corners, and defects. Simulations of NPs should improve our interpretation of batch adsorption experiments, provide more detailed information on adsorption complexes, and ultimately improve the use of reactive transport models to predict the fate and transport of aqueous species under different geochemical conditions.

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CONCLUSIONS We applied MD simulation to investigate the adsorption properties of Na+, Ca2+, Ba2+, and Cl− ions on the gibbsite (1 0 0), (0 0 1), and NP surfaces. Two simulated systems were built: (1) aqueous solution confined in gibbsite slit pores to study adsorption on either (1 0 0) or (0 0 1) surfaces and (2) a hexagonal gibbsite NP immersed in aqueous solution to investigate adsorption on two surfaces (1 0 0) and (0 0 1) simultaneously present in aqueous solution. The simulation results indicate that Na+ and Cl− ions adsorb on both (1 0 0) and (0 0 1) surfaces as inner-sphere species. Outer-sphere Cl− ions were also found on these surfaces. On the (1 0 0) edge, Ca2+ ions adsorb as inner-sphere and outer-sphere complexes, whereas on the (0 0 1) surface, outer-sphere Ca2+ ions are the dominant species. Ba2+ ions formed inner-sphere and outersphere complexes on both surfaces. Calculated ion surface coverages indicate that for all ions, surface coverages are always higher on the basal surface compared to the edge surface, demonstrating the high activity of the gibbsite basal surface for ion adsorption. More importantly, surface coverages for cations on the gibbsite NP are always higher than those calculated on the (1 0 0) and (0 0 1) surfaces. This enhanced ion adsorption behavior is due to the significant number of inner-sphere cations found at NP corners. Outer-sphere cations do not contribute to the enhanced surface coverages. The results presented in this paper suggest that molecular simulation studies performed to investigate cation adsorption to one mineral surface at a time cannot be directly compared either to laboratory batch adsorption experiments or most reported spectroscopic results on ion adsorption because these

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b00680. Planar density distribution of interfacial water with and without Na+ ions (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T.A.H.). *E-mail: [email protected] (J.A.G.). *E-mail: [email protected] (L.J.C.). ORCID

Tuan A. Ho: 0000-0002-8129-1027 Jeffery A. Greathouse: 0000-0002-4247-3362 Louise J. Criscenti: 0000-0002-5212-7201 Notes

The authors declare no competing financial interest. G


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ACKNOWLEDGMENTS Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. This material is based upon work supported by the Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. The views expressed in the article do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

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