Configuration, Anion-Specific Effects, Diffusion, and Impact on

Article pubs.acs.org/JPCC

Configuration, Anion-Specific Effects, Diffusion, and Impact on Counterions for Adsorption of Salt Anions at the Interfaces of Clay Minerals Xiong Li, Hang Li,* and Gang Yang* College of Resources and Environment & Chongqing Key Laboratory of Soil Multi-scale Interfacial Process, Southwest University, Chongqing 400715, China S Supporting Information *

ABSTRACT: Interfacial interactions of clay minerals with salt solutions are ubiquitous and play a crucial role in a wide range of fields, where salt cations are the focus while anions are generally regarded as spectators. Here, molecular dynamics simulations show that the various anions are strongly adsorbed on the surfaces of clay minerals, and the resulting anion-specific effects are pronounced. Although constructing only H-bonds, anions form stable inner- and outer-sphere complexes with clay minerals, and F− and OH− can result in even more stable complexes than metal ions. The underlying anion-specific effects abide by the sequence OH− > F− > Cl− > I− and show apparent enhancements with increase of salt concentrations. OH− is particular at relatively high concentrations, forming clusters and capturing metal ions at octahedral AlO6 surfaces and approaching tetrahedral SiO4 surfaces with help of metal ions in addition to the monodispersive inner- and outer-sphere species at octahedral AlO6 surfaces that are similar for all anions. Diffusion coefficients of anions are the same order of magnitude as those of metal ions and are affected by counterions, concentrations, and distances to the surfaces of clay minerals. Diffusion coefficients of both inner- and outer-sphere anions decrease as I− > Cl− > F− > OH−. Adsorption of anions is affected by counterions (metal ions) and vice versa. Impact of anions on the adsorption of counterions also shows ion-specific effects that follow the sequence OH− > F− > Cl− > I−, and OH− can even alter the adsorption structure and distribution of counterions. sphere species; instead, the inner-sphere K+ complexes remain stable with increase of humidity.11−13 Ion-specific effects, whose importance has been regarded to be no less than Gregor Mendel’s work to genetics,24 are closely associated with a plethora of clay-mineral/salt-solution interfacial processes.25 In previous works,26,27 Hofmeister series of anions were reported for the aggregation of protein and colloid particles. To best of our knowledge, an atomistic level description with regard to the adsorption of anions on the surfaces of clay minerals is scarce, in contrast to extensive studies for metal ions. As a matter of fact, anions in salt solutions are concomitant with cations, and a number of anions can also cause serious atmospheric and environmental concerns.28 Here, molecular dynamics (MD) simulations were conducted to address the adsorption behaviors of different anions on kaolinite surfaces, underlying anion-specific effects and impact on the adsorption of counterions: (1) Adsorption of Cl− ions from NaCl and PbCl2 solutions with a wide range of concentrations. Stable anion−mineral complexes were detected, and then adsorption structure, distribution, stability, and

1. INTRODUCTION Clay minerals are generally hydrous layer-type aluminum phyllosilicates and constitute a major portion of soils, sediments, and rocks. Owing to the large surface area, low permeability, and high retention capacity, clay minerals play a crucial role in a wide spectrum of physical, chemical, and geological processes, controlling the transport and bioavailability of ions, nutrients, and contaminants; meanwhile, the interfacial tension, surface properties, and further reactions of clay minerals are significantly affected by adsorbates.1−4 Computational simulations have been performed extensively to tackle the adsorption behavior and mechanism of salt ions on the surfaces of clay minerals.5−23 It is now known that adsorbed metal ions can be classified into inner- and outer-sphere species: Inner-sphere metal ions form direct bonds with the surfaces of clay minerals while outer-sphere ones are separated by one intermediate water molecule. Churakov7 demonstrated that the distribution of Na+ ions on montmorillonite surfaces is dependent on humidity: At relatively high humidity, Na+ ions form exclusively outer-sphere complexes; otherwise, innersphere Na+ complexes will be constructed. Hensen et al.10 further showed that with addition of water molecules, the innersphere Na+ ions can be reversibly transformed to the outer© XXXX American Chemical Society

Received: February 24, 2016 Revised: June 20, 2016

A

DOI: 10.1021/acs.jpcc.6b01886 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

water molecules between the inward tetrahedral SiO4 and octahedral AlO6 surfaces were described by the flexible SPC model.40 In recent years, the combination of CLAYFF force field and SPC model has been sufficiently testified to accurately describe hydrated mineral systems and interfacial interactions of clay minerals and salt solutions.6−18,20−23 The threedimensional periodic boundary conditions (PBC) were applied and long-range electrostatic interactions were treated by the particle mesh Ewald (PME) method. The cutoff radii for van der Waals interactions and Ewald electrostatic summation were chosen to be 12.0 Å, and the leapfrog algorithm was employed with time step of 2.0 fs.41 The temperature (300.0 K) and pressure (1.0 bar) were respectively controlled by the V-rescale thermostat42 and Parrinello−Rahman barostat.43 The 5.0 ns MD simulations were run for each system with the atomic coordinates being saved per 1.0 ps. According to the results of root-mean-square deviations (RMSD), all systems have achieved the equilibrium states since 2.0 ns, and thereby the last 3.0 ns trajectories were used for analyses (Figures S1−S5). In line with previous work,16 the root-mean-square fluctuations (RMSF) during the last 3.0 ns MD simulations were used to quantify the stability of metal ions and classified into four regimes: (i) ≤ 1.2 Å. Strong inner-sphere adsorption. (ii) 1.2−1.7 Å. Relatively weak inner-sphere and strong outersphere adsorption. The upper limit (1.7 Å) was set to include all inner-sphere metal ions. (iii) 1.7−2.7 Å. Relatively weak outer-sphere adsorbed metal ions and those in bulk solutions but with comparable stabilities. All remaining outer-sphere metal ions were guaranteed to fall within this regime. (iv) > 2.7 Å. Remaining metal ions in bulk solutions. The RMSF of anions were classified similarly (vide post). The diffusion coefficients (D) of ions parallel to clay surfaces (x−y plane) can be calculated using the Einstein relation20

concentration dependence were elaborated. (2) Investigation of other halide (F−, I−) and hydroxide (OH−) ions to unravel anion-specific effects, where the particularities of OH− ions such as occurrence at tetrahedral SiO4 surfaces were discussed. (3) Impact of anion-specific effects on the adsorption of counterions. (4) Calculations of diffusion coefficients for anions and counterions with a wide range of concentrations, which are useful to comprehend their mobility at the surfaces of clay minerals and high-level radioactive waste managements.

2. COMPUTATIONAL DETAILS 2.1. Models. Kaolinite is composed of alternative tetrahedral SiO4 and octahedral AlO6 layers that are connected by bridging O atoms. It has a chemical formulas of Al2Si2O5(OH)4 with the unit-cell parameters being a = 5.19 Å, b = 8.96 Å, c = 7.36 Å, α = 90.77°, β = 104.17°, and γ = 90.40°.29 In accord with previous works,15,16 orthogonal transformation was conducted for the crystal structure of kaolinite that causes its (001) and (001̅) planes to correspond to the octahedral AlO6 and tetrahedral SiO4 surfaces, respectively. Model of kaolinite presently used was constructed by combining 324 unit cells (9 × 9 × 4 respectively along x, y, and z directions) into a supercell. Isomorphous substitutions are ubiquitous in clay minerals such as montmorillonite30,31 and kaolinite,32,33 and as suggested, nine Al3+/Si4+ replacements were introduced to the tetrahedral SiO4 surfaces of kaolinite and the resulting negative charges were electrically neutralized by protons.34−36 Then the inward tetrahedral SiO4 and octahedral AlO6 surfaces were set apart by a vacuum layer (40.0 Å thick) that was filled with 5097 water molecules to maintain a density of 1.0 g/cm3. Note that the inward tetrahedral SiO4 and octahedral AlO6 surfaces that interact interfacially with salt solutions are both external surfaces. Finally, NaCl solutions of 0.16, 0.32, 0.48, 0.64, 0.80, and 0.96 mol/L were prepared by replacing certain numbers of water molecules with salt ions, where the ion pairs (Na+−Cl−) are counted to be 14, 28, 42, 56, 70, and 84, respectively. Figure 1

⟨Δx(τ )2 + Δy(τ )2 ⟩ = 4Dτ

where ⟨Δx(τ)2 + Δy(τ)2⟩ refers to the mean-square displacement over a time interval (τ).

3. RESULTS AND DISCUSSION 3.1. Anion−Mineral Complexes. 3.1.1. Observence of Stable Complexes. Equilibrium configurations of NaCl and PbCl2 solutions (0.16−0.96 mol/L) interacting interfacially with kaolinite are shown in Figure 2 as well as Figures S6 and S7. In all cases, metal ions (Na+, Pb2+) and anions (Cl−) are inclined to approach respectively the negatively charged tetrahedral SiO4 and positively charged octahedral AlO6 surfaces, which are in agreement with previous results.15,16 At 0.16 mol/L, 11.0 and 3.2 Na+ ions are respectively inner- and outer-sphere adsorbed while Pb2+ ions form only 12.2 outersphere complexes (Tables 1 and 2). Both inner- and outersphere complexes show a gradual increase with concentrations until saturated, except the inner-sphere Pb2+ species that retain null over the whole concentration range. The time-evolution trajectories in Figure S8 indicate that a portion of inner- and outer-sphere Na+ and Pb2+ ions are stabilized at tetrahedral SiO4 surfaces and can be regarded as long-lived adsorption species.5,7,13,15,16,21 Although only Hbonds are constructed with octahedral AlO6 surfaces, a considerable proportion of Cl− ions remain focused at the adsorption sites during MD simulations, and their stabilities are comparable to those of metal ions. The stabilities of anions are quantitatively characterized by classification of RMSFs, similar

Figure 1. Initial configuration of 0.16 mol/L NaCl solutions in contact with kaolinite surfaces (upper: tetrahedral SiO4 surfaces; bottom: octahedral AlO6 surfaces). Na+ and Cl− ions are presented as blue and green balls, respectively. Dashed red lines are used to divide the salt solutions schematically into three regions.

illustrates the initial configuration of 0.16 mol/L NaCl solutions in contact with kaolinite surfaces. Preparations of other salt solutions (NaF, NaI, NaOH, PbF2, and PbCl2) in contact with kaolinite surfaces followed those of NaCl solutions. 2.2. Methods. MD simulations were conducted using the Gromacs-4.6.5 software package.37 The CLAYFF force field that consists mainly of nonbonded potentials (electrostatics and van der Waals) was used for kaolinite and salt ions,38,39 while B


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formation of stable anion−mineral complexes (Table 3). In particular, the RMSFs of metal ions are all beyond 0.6 Å (Table 4), whereas a non-negligible portion of F− and OH− ions as elaborated subsequently have RMSF ≤ 0.6 Å, especially in relatively dense solutions. This explicitly states that the corresponding anion−mineral complexes are even more stable than those of inner-sphere metal ions. 3.1.2. Adsorption Behaviors. Two distinct adsorption structures are observed for Cl− ions at octahedral AlO6 surfaces and tentatively defined as (a) inner-sphere mode where direct H-bonds are constructed between anions and clay minerals (Figure 3a) and (b) outer-sphere mode that is separated from the surfaces of clay minerals by one intermediate water molecule (Figure 3b). Note that the definition of inner-sphere anions differs from that of metal ions. As discussed earlier, Cl− ions of these two adsorption modes have apparently higher stabilities than those remaining in bulk solutions and respectively form inner- or outer-sphere complexes. Innersphere Cl− ions are situated at the Al vacancies and associated closely with three surface −OH groups with an average distance of 2.6 Å; see the radial distribution functions (RDF) in Figure 4 as well as Figures S9 and S10. The formation of H-bonds between Cl− ions and surface hydroxyls has been evidenced experimentally.44,45 Accordingly, the inner-sphere Cl− ions are strongly anchored at octahedral AlO6 surfaces, while the outersphere Cl− ions are relatively far from surface −OH groups (4.2 Å) and show an increased coordination number with water molecules (3.7 vs 2.8; see the RDF plots in Figure 4 as well as Figures S9 and S10). Note that only the first-shell water molecules (generally ≤3.0 Å) are counted here and elsewhere. With increase of salt concentrations, the numbers of innerand outer-sphere Cl− complexes show an increase until

Figure 2. Equilibrium configurations of 0.16 and 0.96 mol/L NaCl/ PbCl2 solutions interacting interfacially with the basal surfaces of kaolinite. Salt concentrations (mol/L) are indicated in the parentheses of legends. Na+, Pb2+, and Cl− ions are presented as blue, dark yellow, and green balls, respectively.

to the case of metal ions: (i) ≤ 0.6 Å. Especially strong innersphere adsorption. (ii) 0.6−1.2 Å. Strong inner-sphere adsorption. (iii) 1.2−1.7 Å. Relatively weak inner-sphere and strong outer-sphere adsorption. (iv) 1.7−2.7 Å. Relatively weak outer-sphere anions and those in bulk solutions but with comparable stabilities. (v) > 2.7 Å. Remaining anions in bulk solutions. A number of Cl− ions fall within regimes ii (RMSF: 0.6−1.2 Å) and iii (RMSF: 1.2−1.7 Å), suggesting the

Table 1. Average Numbers (N) and Percentages (X) of Na+ Ions for the Adsorption of NaA Solutions (A− = F−, Cl−, I−, OH−) on Tetrahedral SiO4 Surfaces of Kaolinite as Well as of the Corresponding A− Ions on Octahedral AlO6 Surfacesa,b inner-sphere adsorption NaCl

NaF NaI NaOH

outer-sphere adsorption

c (mol/L)

N(Na+)

X(Na+) (%)

N(A−)

X(A−) (%)

N(Na+)

X(Na+) (%)

N(A−)

X(A−) (%)

0.16 0.32 0.48 0.64 0.80 0.96 0.16 0.96 0.16 0.96 0.16

11.0 15.8 17.1 18.2 18.4 18.5 13.1 22.8 12.2 17.3 11.2 (0.0) 20.7 (0.0) 19.6 (4.9) 18.3 (5.1) 16.6 (5.8) 15.2 (6.7)

78.6 56.4 40.7 32.5 26.3 22.0 93.6 27.1 87.1 20.6 80.0 (0.0) 73.9 (0.0) 46.7 (11.7) 32.7 (9.1) 23.7 (8.3) 18.1 (8.0)

11.2 16.5 19.4 20.2 20.8 22.4 12.1 27.4 10.0 19.2 13.6 (0.0) 24.2 (0.0) 31.1 (1.2) 37.4 (2.2) 35.1 (3.2) 34.8 (4.1)

80.0 58.9 46.2 36.1 29.7 26.7 86.4 32.6 71.4 22.9 97.1 (0.0) 86.4 (0.0) 74.0 (2.9) 66.8 (3.9) 50.1 (4.6) 41.4 (4.9)

3.2 11.4 14.2 14.8 16.5 17.2 1.2 16.7 2.1 13.8 3.1 (0.0) 4.6 (0.0) 9.2 (1.8) 15.7 (5.2) 18.3 (7.2) 20.5 (11.6)

22.9 40.7 33.8 26.4 23.6 20.5 8.6 19.9 15.0 16.4 22.1 (0.0) 16.4 (0.0) 21.9 (4.3) 28.0 (9.3) 26.1 (10.3) 24.4 (13.8)

3.4 10.1 14.8 15.2 16.3 17.7 1.8 25.4 4.2 11.2 0.0 (0.0) 3.8 (0.0) 6.2 (1.1) 8.7 (1.3) 9.6 (3.8) 10.3 (5.7)

24.3 36.1 35.2 27.1 23.3 21.1 12.9 30.2 30.0 13.3 0.0 (0.0) 13.6 (0.0) 14.8 (2.6) 15.5 (2.3) 13.7 (5.4) 12.3 (6.8)

0.32 0.48 0.64 0.80 0.96

For Na+ ions, data in parentheses for adsorption on octahedral AlO6 surfaces. bFor A− ions, data in parentheses for adsorption on tetrahedral SiO4 surfaces. a

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Table 2. Average Numbers (N) and Percentages (X) of Pb2+ Ions for the Adsorption of PbA2 Solutions (A− = F−, Cl−) on Tetrahedral SiO4 Surfaces of Kaolinite as Well as of the Corresponding A− Ions on Octahedral AlO6 Surfaces inner-sphere adsorption PbCl2

PbF2

outer-sphere adsorption

c (mol/L)

N(Pb )

X(Pb ) (%)

N(A )

X(A ) (%)

N(Pb )

X(Pb2+) (%)

N(A−)

X(A−) (%)

0.16 0.32 0.48 0.64 0.80 0.96 0.16 0.96

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

12.8 20.4 24.1 24.2 25.8 26.6 13.1 36.6

45.7 36.4 28.7 21.6 18.4 15.8 46.8 21.8

12.2 18.5 22.5 22.2 23.4 24.7 12.4 27.9

87.1 66.1 53.6 39.6 33.4 29.4 88.6 33.2

13.1 20.4 22.8 20.4 21.5 22.6 13.2 32.8

46.8 36.4 27.1 18.2 15.4 13.5 47.1 19.5

2+

2+

−

−

2+

Table 3. Numbers of Anions for NaA and PbA2 Solutions (A− = F−, Cl−, I−, OH−) in Equilibrium with Octahedral AlO6 Surfaces of Kaolinite That Fall within the Specific RMSF Ranges c (mol/L)

RMSF (Å)

NaF

NaCl

NaI

NaOHa

PbF2

PbCl2

0.16

≤0.6 0.6−1.2 1.2−1.7 1.7−2.7 >2.7 ≤0.6 0.6−1.2 1.2−1.7 1.7−2.7 >2.7

2 7 4 1 0 6 11 24 20 23

0 6 6 2 0 0 12 20 22 30

0 4 7 3 0 0 9 18 23 34

4 9 1 0 0 12 (2) 13 (2) 18 (6) 16 15

4 10 8 5 1 12 15 31 27 83

0 9 10 5 4 0 19 24 28 97

0.96

a

Data in parentheses for adsorption on tetrahedral SiO4 surfaces.

Table 4. Numbers of Na+ and Pb2+ Ions for NaA and PbA2 Solutions (A− = F−, Cl−, I−, OH−) in Equilibrium with Tetrahedral SiO4 Surfaces of Kaolinite That Fall within the Specific RMSF Ranges c (mol/L)

RMSF (Å)

NaF

NaCl

NaI

NaOHa

PbF2

PbCl2

0.16

0.6−1.2 1.2−1.7 1.7−2.7 >2.7 0.6−1.2 1.2−1.7 1.7−2.7 >2.7

4 10 0 0 13 22 21 28

4 9 1 0 9 20 21 34

4 9 1 0 7 18 22 37

5 9 0 0 10 (6) 24 (12) 17 15

0 5 9 1 0 20 36 28

0 4 8 2 0 13 38 33

0.96

a

Figure 3. Local adsorption structures of (a) inner-sphere Cl−, (b) outer-sphere Cl−, (c) inner-sphere F−, and (d) inner-sphere I− on octahedral AlO6 surfaces of kaolinite. H-bonds are marked as dashed lines. Water molecules in the first-coordination shell of anions are presented in ball and stick while the rest in stick.

more than in 0.16 mol/L solutions, and the growing rates are more obvious for regime iii (RMSF: 1.2−1.7 Å). 3.2. Anion-Specific Effects. 3.2.1. Halide Ions. Equilibrium configurations of 0.16 and 0.96 mol/L NaA/PbA2 solutions (A− = F−, I−) in contact with kaolinite surfaces are shown in Figure 5. A considerable portion of F− and I− ions occur at octahedral AlO6 surfaces, in line with the results of NaCl/PbCl2 solutions. Although for 0.16 mol/L NaA solutions, the total adsorption numbers of different halide ions are very close to each other, anion-specific effects can still be observed in that the allocation to inner- and outer-sphere complexes is somewhat different: 12.1 and 1.8 for F−, 11.2 and 3.4 for Cl−, and 10.0 and 4.2 for I−, respectively (Table 1 and Figure 6a). The distances between inner-sphere halide ions and octahedral AlO6 surfaces are dependent on ionic radii and equal to 2.0 Å for F−, 2.3 Å for Cl−, and 2.8 Å for I−. The numbers of adsorbed halide ions increase with salt concentrations, and more apparent anion specificities are observed at higher concentrations, e.g., at 0.96 mol/L NaA solutions, inner-/ outer-sphere complexes are counted at 27.4/25.4 for F−, 22.4/ 17.7 for Cl−, and 19.2/11.2 for I− (Figure 6b and Table 1); that is, inner- and outer-sphere anions both show clear specificities

Data in parentheses for adsorption on octahedral AlO6 surfaces.

saturatedrapidly at dilute solutions and then mildly, while their proportions decline gradually, which are consistent with the results of metal ions (Tables 1 and 2). Adsorption of Cl− ions from NaCl solutions seems to reach the saturation point since ca. 0.64 mol/L. Cl− ions in PbCl2 solutions are 2 mol equiv and hence correspond to a lower saturation point (ca. 0.48 mol/L). For 0.16 mol/L NaCl/PbCl2 solutions, a majority of Cl− ions are adsorbed at octahedral AlO6 surfaces: 6/9 and 6/10 Cl− ions are respectively ascribed to strong inner-sphere species (RMSF: 0.6−1.2 Å) and weak inner-sphere/strong outer-sphere species (RMSF: 1.2−1.7 Å) (see Table 3). This clearly indicates the formation of stable anion−mineral complexes even when counterions (e.g., Pb2+) correspond to only the outer-sphere mode. Strong inner-sphere Cl − complexes in 0.96 mol/L NaCl/PbCl2 solutions are twice D


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Figure 5. Equilibration configurations of 0.16 and 0.96 mol/L NaA/ PbA2 solutions (A− = F−, I−, OH−) interacting interfacially with the basal surfaces of kaolinite. Salt concentrations (mol/L) are indicated in the legends. Na+, Pb2+, F−, I−, O2− (in OH−), and H+ (in OH−) ions are presented as blue, dark yellow, cyan, gray, red, and white balls, respectively.

Figure 4. Radial distribution functions (RDF, g(r)) and coordination numbers (CN, plotted as dashed lines) for inner-sphere and outersphere anions of 0.96 mol/L NaA solutions (A− = F−, Cl−, I−, and OH−) adsorbed on octahedral AlO6 surfaces of kaolinite. H atoms from kaolinite and O atoms from water are referred to as HK and OW, respectively. With regard to OH− ions, the comprising O atoms are used for calculating RDFs.

distances of 2.2, 2.6, and 2.8 Å when interacting with surface −OH groups while of 2.6, 3.2, and 3.4 Å when interacting with water molecules, respectively. Accordingly, F− has the most compact and stable adsorption structures, and this is substantialized by time-evolution trajectories and RMSF. The numbers of halide ions centralized at adsorption sites decrease as F− > Cl− > I− (Figures S8, S11, and S13). The stability differences become more apparent at higher concentrations: 41/58 F−, 32/43 Cl−, and 27 I− for 0.96 mol/L NaA/PbA2 solutions have RMSF ≤ 1.7 Å, in contrast to 13/22 F−, 12/19 Cl−, and 11 I− for 0.16 mol/L solutions that are similar for different halide ions (Table 3). Moreover, among these halide ions, only the adsorption of F− ions can result in especially stable anion−mineral complexes (RMSF ≤ 0.6 Å). 3.2.2. Hydroxide Ion. Inner- and outer-sphere OH− ions are defined similarly to those of halide ions, and the comprising H or O atoms that are closer to kaolinite surfaces are used as benchmarks; e.g., for OH− ions with H atoms being closer to surfaces, inner-sphere adsorption forms if direct H-bonds are constructed between H atoms and kaolinite surfaces while outer-sphere adsorption forms if H atoms are separated by only one intermediate water molecule. The adsorption behaviors of

and abide by the same Hofmeister series (F− > Cl− > I−). The results of PbA2 solutions provide further evidence to anionspecific effects (F− > Cl− for both inner- and outer-sphere species; see Figure S12 and Tables 2 and 3. Anion-specific effects are also featured in terms of adsorption structures and stabilities. As indicated in Figure 3, inner-sphere F− and Cl− ions construct octahedral environments by forming six H-bonds, while trigonal-bipyramid environments are formed for inner-sphere I− ions. The RDF plots (Figure 4 as well as Figures S9 and S10) show that the coordination numbers with surface −OH groups decrease as 3.8 for F− > 3.0 for Cl− > 2.0 for I−, while those with water molecules have an opposite trend (2.6 for F− < 2.8 for Cl− < 3.0 for I−). In addition, H-bonding strengths that are largely determined by atomic electronegativity follow the sequence F− > Cl− > I−, with their E


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bond acceptors and donors, and H-bonding networks can thus be constructed as in the case of water molecules. (c) OH− clusters at octahedral AlO6 surfaces that have captured Na+ ions (Figures 7d and 7e). Without OH− clusters, Na+ ions are unlikely to occur at octahedral AlO6 surfaces. (d) OH− ions at tetrahedral SiO4 surfaces that are driven by H-bonding networks and interactions with Na+ ions (Figures 7f−7h). Na+ ions are involved in all structures of this type and the indispensable role is evident. Owing to the stabilization by H-bonding networks, OH− ions at octahedral AlO6 surfaces possess superior stabilities as compared to halide ions; see time-evolution trajectories in Figures S8, S11, and S13 and RMSFs in Table 3. For 0.16 and 0.96 mol/L NaOH solutions, OH− ions with RMSF ≤ 0.6 Å that correspond to especially strong inner-sphere complexes are respectively counted to be 4 and 12, twice as F− ions in 0.16 and 0.96 mol/L NaF solutions. Neither Cl− nor I− ions are detected within this RMSF regime. OH− ions at tetrahedral SiO4 surfaces may have comparable stabilities as those at octahedral AlO6 surfaces: At 0.96 mol/L, the numbers with RMSF ≤ 0.6, 0.6−1.2, and 1.2−1.7 Å are 2, 2, and 6, respectively (Table 3). 3.2.3. Diffusion Coefficients. To best of our knowledge, almost all previous diffusion studies on the surfaces of clay minerals have been given regarding to metal ions rather than anions.2,46−52 The diffusion coefficients of metal ions (DM) along tetrahedral SiO4 surfaces of kaolinite are calculated for a wide range of salt concentrations (see Figure 8). At a given salt concentration, the DM values at tetrahedral SiO4 surfaces are apparently lower than those in bulk solutions, in line with the “surface disruption” phenomena reported before.2,46 The DM values also show good agreement with previous works; e.g., the bulk DNa+ values of 0.16−0.96 mol/L NaCl solutions are (1.19 ± 0.1)−(1.25 ± 0.1) × 10−9 m2 s−1 that are comparable to the experimental data (1.33× 10−9 m2 s−1) and calculated results of orthoclase (1.1 × 10−9 m2 s−1) and smectite (1.15 × 10−9 m2 s−1).2,47−49,53 As indicated in Figure 8, diffusion coefficients are concentration-dependent, and the DCl− data (units in 10−9 m2 s−1) of 0.16, 0.32, 0.48, 0.64, 0.89, and 0.96 mol/L NaCl solutions are respectively 0.11 ± 0.03, 0.13 ± 0.04, 0.32 ± 0.03, 0.40 ± 0.03, 0.45 ± 0.06, and 0.55 ± 0.07 when inner-sphere adsorbed and 0.29 ± 0.05, 0.31 ± 0.07, 0.49 ± 0.08, 0.58 ± 0.08, 0.62 ± 0.08, and 0.74 ± 0.08 when outer-sphere adsorbed, which are the same order of magnitude as those of metal ions (DNa+) reported previously2,46,48 and presently. Accordingly, adsorbed Cl− ions have comparable mobility and stability with adsorbed metal ions, in line with the results of time-evolution trajectories and RMSF. Both inner- and outer-sphere DCl− values show a steady increase with salt concentrations implying the increase of mobility; instead, the bulk DCl− data are obviously less sensitive to the change of salt concentrations ((1.21 ± 0.13)−(1.35 ± 0.17) × 10−9 m2 s−1 for 0.16−0.96 mol/L). Consistent trends for DA data vs salt concentrations are found for PbA2 and other NaA solutions (see Figures 8 and 9). For each NaA/PbA2 solution (Figure 9), the DA data abide by the sequence of bulk > outer-sphere > inner-sphere.2,8,46,54 Both inner- or outer-sphere DA values decrease as I− > Cl− > F− > OH−, and this sequence is not affected by choice of counterions. As compared to halide ions, OH− ions are additionally stabilized by H-bonding networks and thereby correspond to lower DA values (Figures 8 and 9 and Figure S15).

Figure 6. Atomic density profiles for (a) 0.16 and (b) 0.96 mol/L NaA solutions (A− = F−, Cl−, I−, OH−) in contact with the basal surfaces of kaolinite. Plane that passes through the hydroxyl-H atoms of octahedral AlO6 surfaces is referred to as z = 0. The coordinates of O atoms are used for calculating OH− densities. Color scheme: F− (cyan), Cl− (green), I− (gray), O2− in OH− (red), and Na+ (blue).

OH− ions in 0.16 mol/L NaOH solutions (Figure 5) are in general agreement with those of halide ions, except that all OH− ions are inner-sphere adsorbed with only one sharp peak in atomic density profiles (Figure 6a). At 0.96 mol/L, 34.8 and 10.3 OH− ions are respectively presented as inner- and outersphere complexes, and at the same time, we are surprised to find that an appreciable number of OH− ions occur at tetrahedral SiO4 surfaces (see Figure 6b and Table 2). Further studies of a wide range of NaOH concentrations indicate the strong concentration dependence for OH− adsorption on kaolinite surfaces (see Figure 5, Figure S14, and Table 1). Both inner- and outer-sphere OH− ions at octahedral AlO6 surfaces show a gradual increase with NaOH concentrations until saturated. Adsorption of OH− ions at tetrahedral SiO4 surfaces begins at ca. 0.48 mol/L and can also be grouped into inner- and outer-sphere species with their numbers increasing with concentrations: 1.2 and 1.1 at 0.48 mol/L, 2.2 and 1.3 at 0.64 mol/L, 3.2 and 3.8 at 0.80 mol/L, and 4.1 and 5.7 at 0.96 mol/L, respectively. Owing to the extra adsorption at tetrahedral SiO4 surfaces, OH− ions remaining in bulk solutions reduce substantially when compared to halide ions (Figure 6). The adsorption structures of OH− ions are diverse: (a) Monodispersive inner- and outer-sphere modes at octahedral AlO6 surfaces (Figures 7a and 7b) that resemble those of halide ions. Anion specificities can be detected therein: Inner-sphere OH− ions at the Al vacancies are stabilized by three H-bonds with surface −OH groups, and shorter H-bond distances than F− ions (1.8 vs 2.2 Å) suggest stronger interactions with octahedral AlO6 surfaces (Figure 4 and Figure S9). (b) OH− clusters at octahedral AlO6 surfaces (Figure 7c). Distinct from halide ions, OH− ions are both HF


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Figure 7. Local adsorption structures of (a) inner-sphere and (b) outer-sphere OH−, (c) several proximal OH− ions with construction of H-bonds on octahedral AlO6 surfaces as well as OH− and Na+ ions coexistent on (d, e) octahedral AlO6 and (f−h) tetrahedral SiO4 surfaces. H-bonds are marked as dashed lines. Water molecules in the first-coordination shell of anions are presented in ball and stick while the rest in stick. O atoms from water, OH−, and kaolinite are referred to as OW, OH, and OK, respectively.

salt concentrations implying the increase of mobility, while those of bulk metal ions remain almost invariable, which are consistent with the results of anions (DA). Outer-sphere Pb2+ rather than Na+ ions correspond to lower DM values and higher stability due to stronger interactions with water molecules.15,16 3.3.2. Adsorption Behaviors. Figures 2 and 5 as well as Figures S6 and S7 and Table 4 indicate that the distribution and stability of metal ions are substantially affected by the choice of halide ions (A− = F−, Cl−, I−); nonetheless, their adsorption modes are not altered: inner-sphere (mainly) and outer-sphere for Na+ ions and exclusively outer-sphere for Pb2+ ions. For 0.16 mol/L NaA/PbA2 solutions, the various halide ions cause slight differences in the number of inner-/outer-sphere metal ions, while clear differences are detected in dense solutions; e.g., at 0.96 mol/L, the inner-/outer-sphere Na + ions are respectively counted to be 22.8/16.7, 18.5/17.2, and 17.3/ 13.8 in NaF, NaCl, and NaI solutions (Figure 6 and Tables 1 and 2), which are consistent with anion-specific effects (F− > Cl− > I−). The stability differences of metal ions in the various NaA/PbA2 solutions (A− = F−, Cl−, I−) are characterized by time-evolution trajectories (Figures S8, S11, S16, and S17) and RMSF (Table 4). At 0.96 mol/L, Na+ ions of regimes i (RMSF: 0.6−1.2 Å) and ii (RMSF: 1.2−1.7 Å) are counted to be 13 and 22 in NaF solutions, 9 and 20 in NaCl solutions, and 7 and 18

On the other hand, diffusion coefficients of anions depend significantly on counterions, and PbA2 instead of NaA solutions respond more sluggishly to the change of salt concentrations; e.g., when increasing from 0.16 to 0.96 mol/L, the outer-sphere DCl− values ascend from 0.46 ± 0.05 to 0.65 ± 0.06 for PbCl2 solutions instead of from 0.29 ± 0.05 to 0.74 ± 0.08 for NaCl solutions (units in 10−9 m2 s−1). 3.3. Impact on Counterions. Salt anions adsorbed on the surfaces of clay minerals can form stable anion−mineral complexes and exhibit strong anion-specific effects. In this section, we will show the impact of anion-specific effects on the adsorption and diffusion of counterions (metal ions). 3.3.1. Diffusion Coefficients. As discussed above, diffusion coefficients of anions are significantly affected by the choice of counterions. In dilute solutions, diffusion coefficients of metal ions (DM) seem insensitive to the change of anions; e.g., the inner-sphere DNa+ values of 0.16 mol/L NaA solutions (units in 10−9 m2 s−1) are 0.11 ± 0.04 (I−) ≈ 0.12 ± 0.02 (Cl−) ≈ 0.14 ± 0.03 (F−) ≈ 0.11 ± 0.04 (OH−); however, the DM values in dense solutions are more affected by anions; e.g., for 0.96 mol/ L NaA solutions, the inner-sphere DNa+ values (units in 10−9 m2 s−1) decline as 0.74 ± 0.06 (I−) > 0.67 ± 0.04 (Cl−) > 0.42 ± 0.06 (F−) > 0.28 ± 0.03 (OH−) (see Figures 8 and 9). The inner- and outer-sphere DM values show a gradual increase with G


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inner- and outer-sphere Na+ ions increase with NaOH concentrations; however, the proportions of outer-sphere Na+ ions in NaOH rather than other NaA solutions (A− = F−, Cl−, I−) increase more rapidly, and at 0.96 mol/L, the numbers of inner- and outer-sphere Na+ ions are respectively 15.2 and 20.5 at tetrahedral SiO4 surfaces and 6.7 and 11.6 at octahedral AlO6 surfaces (Table 1), indicating that the outer-sphere mode predominates at both surfaces. These are distinct from other NaA solutions, where Na+ ions will not occur at octahedral AlO6 surfaces and the proportions of outer-sphere Na+ ions ascend more slowly with increase of salt concentrations. In addition to the familiar inner- and outer-sphere species at tetrahedral SiO4 surfaces,5−23 Na+ ions in dense NaOH solutions can generate new adsorption species; see Figures 7f−h (at tetrahedral SiO4 surfaces) and Figures 7d,e (at octahedral AlO6 surfaces) where the Na+ ions are all closely coordinated to OH− ions. Hence, these new adsorption species are caused by the specificity of OH− as discussed earlier. RMSF in Table 4 indicate that adsorbed Na+ ions from NaOH solutions show higher stability than from other NaA solutions, especially in dense solutions; e.g., at 0.96 mol/L, Na+ ions of regime ii (RMSF: 1.2−1.7 Å) are counted to be 36 (OH−) > 22 (F−) > 20 (Cl−) > 18 (I−), which can partially be due to the adsorption at both surfaces (Figure 7b). Accordingly, among these anions, OH− exerts the largest influences on the adsorption structure, distribution, and stability of counterions.

Figure 8. Diffusion coefficients (D) of Na+/Pb2+ and Cl− ions of NaCl/PbCl2 solutions in contact respectively with tetrahedral SiO4 surfaces and octahedral AlO6 surfaces of kaolinite.

4. CONCLUSIONS The interfacial interactions of clay minerals with salt solutions play a crucial role in a wide range of physical, chemical, and geological processes. For the first time, the adsorption behavior and mechanism of different anions from salt solutions on the surfaces of clay minerals have been demonstrated at an atomistic level; meanwhile, anion-specific effects that underlie such interactions and further chemical reactivity are deciphered. The adsorption behaviors of different anions, with respect to adsorption structure, distribution, stability, and concentration dependence, are discussed. Anions construct only H-bonds with clay minerals while form stable inner- and outer-sphere complexes, even if counterions (e.g., Pb2+) are only outersphere adsorbed. Stabilities of anion−mineral complexes are comparable to those of metal ions, and an appreciable number of F− and OH− ions show even higher stabilities (RMSF ≤ 0.6 Å) corresponding to especially stable anion−mineral complexes. The resulting anion-specific effects abide by the sequence OH− > F− > Cl− > I− and are enhanced with increase of salt concentrations. OH− ions at low concentrations behave similarly as halide ions while at higher concentrations (≥ca. 0.48 mol/L) can form clusters and capture metal ions at octahedral AlO6 surfaces and approach tetrahedral SiO4 surfaces with help of metal ions, in addition to the monodispersive inner- and outer-sphere species at octahedral AlO6 surfaces that are similar for all anions. Inner- and outersphere complexes show a gradual increase with salt concentrations until saturated, and the adsorption trends of OH− ions are consistent at both surfaces. The distribution and stability of metal ions are affected by the choice of halide ions, especially at higher salt concentrations, while their adsorption modes are not altered. OH− ions can also affect the adsorption structures of metal ions: Na+ ions from dense NaOH solutions can occur at octahedral AlO6 surfaces and be coordinated to OH− ions at tetrahedral SiO4 surfaces; in addition, increase of NaOH rather than other NaA

Figure 9. Diffusion coefficients of anions (DA) and metal ions (DM) for NaA/PbA2 solutions (A− = F−, Cl−, I−, OH−) in contact with the basal surfaces of kaolinite. Unless otherwise specified, anions are on octahedral AlO6 (Al) surfaces while metal ions on tetrahedral SiO4 (Si) surfaces.

in NaI solutions, respectively. Accordingly, anion specificities affect significantly the stabilities of counterions, which are corroborated by the results of PbA2 solutions. These are in line with experimental observations that the adsorption quantities and affinities of Cu2+ ions on clay soils are dependent on the choice of anions.55 As aforementioned, Na+ ions in dilute NaOH solutions are adsorbed only at tetrahedral SiO4 surfaces, while since ca. 0.48 mol/L, a portion of Na+ ions emerge at octahedral AlO6 surfaces (Figure 5 and Figure S14). Before saturation, both H


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concentrations enlarges the proportion of outer- vs innersphere Na+ ions more obviously. In consequence, impact of anions on the adsorption behaviors and stability of metal ions decreases in the order OH− > F− > Cl− > I−. For both metal ions and anions, the inner- and outer-sphere diffusion coefficients increase gradually with salt concentrations, while the bulk values remain almost invariant. Diffusion coefficients of metal ions from more dense solutions are more affected by the choice of anions. Adsorbed anions have comparable diffusion coefficients with metal ions, in line with the results of time-evolution trajectories and RMSF. Diffusion coefficients of anions depend significantly on the choice of counterions, and those from PbA2 instead of NaA solutions respond more sluggishly to the change of salt concentrations. Both inner- and outer-sphere diffusion coefficients decrease as I− > Cl− > F− > OH−, and this sequence is not altered by the change of counterions.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b01886. RMSD plots for MD simulations, equilibrium configurations for salt solutions interacting interfacially with clay mineral surfaces, radial distribution functions (g(r)) and coordination numbers (CN) for anions, trajectory maps of anions and metal ions, diffusion coefficients of OH− and Na+ at the basal surfaces of kaolinite (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]; Ph 086-023-68251504; Fax 086023-68250444 (H.L.). *E-mail [email protected]; Ph 086-023-68251504; Fax 086-023-68250444 (G.Y.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (41371249) and the Fourth Excellent Talents Program of Higher Education in Chongqing (2014-03).

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REFERENCES

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Configuration, Anion-Specific Effects, Diffusion, and Impact on

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