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Effect of iron (III) on the swelling pressure of dioctahedral smectites: A molecular dynamics study
Journal Pre-proofs Research paper Effect of Iron (III) on the Swelling Pressure of Dioctahedral Smectites: A Molecular Dynamics Study Aderemi D. Fayoyiwa, Linlin Sun, Janne T. Hirvi, Seppo Kasa, Tapani A. Pakkanen PII: DOI: Reference:
S0009-2614(19)30799-7 https://doi.org/10.1016/j.cplett.2019.136818 CPLETT 136818
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
21 August 2019 27 September 2019 2 October 2019
Please cite this article as: A.D. Fayoyiwa, L. Sun, J.T. Hirvi, S. Kasa, T.A. Pakkanen, Effect of Iron (III) on the Swelling Pressure of Dioctahedral Smectites: A Molecular Dynamics Study, Chemical Physics Letters (2019), doi: https://doi.org/10.1016/j.cplett.2019.136818
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Effect of Iron (III) on the Swelling Pressure of Dioctahedral Smectites: A Molecular Dynamics Study Aderemi D. Fayoyiwaa, Linlin Suna, Janne T. Hirvia, Seppo Kasab and Tapani A. Pakkanena,* aDepartment
of Chemistry, University of Eastern Finland, P.O. Box 111, Joensuu, FI-80101,
Finland bPosiva
Oy, Olkiluoto, Eurajoki, FI-27160, Finland
ABSTRACT Smectites in bentonites contain variable amounts of iron substituting aluminum and silicon in the crystal structure. The swelling pressure of smectite models with varying Fe3+ content, layer charge and charge location were studied using molecular dynamics method. Swelling pressures decrease with increasing iron content at high dry densities, this is more pronounced in montmorillonites than beidellites with same layer charge. This swelling pressure decrease is slightly faster for smectites with lower layer charge. Results obtained from the study is useful in the screening process of iron-containing bentonites intended for use as sealants in High Level Radioactive Waste (HLRW) repositories.
GRAPHICAL ABSTRACT
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Keywords: Montmorillonite, beidellite, HLRW repositories * Corresponding Author, Tel: +358 405028982 E-mail address: [email protected]
1. INTRODUCTION In recent years, steady progress has been made towards construction of underground repositories for deposition of high-level radioactive waste (HLRW). The underground repositories are considered the safest and most suitable disposal option. Spent nuclear waste is placed in copper canisters, which are deposited in the repositories while a compact bentonite buffer is employed to mechanically isolate the canisters from the host rock [1]. Bentonites have been identified in many countries as the best candidates for buffer and back filling agents in several country-specific repository concepts [2–5]. Properties of bentonites such as their high swelling ability, low permeability, high retention capacity, small pore size and high ion exchange capacity [3,5,6] make them suitable for use as buffers in HLRW repositories. Bentonites contain smectite minerals, which are responsible for their swelling properties. In HLRW repositories, bentonite is designed to fill the space between the host rock and the canisters holding the spent nuclear waste. Smectites swell after taking up water, causing effective sealing and forming a barrier between the canister and the environment [1]. Smectites contribute to swelling of other types of soil as well [7]. Smectites are phyllosilicates with 2:1 layer structure, having an octahedral sheet of aluminum oxide sandwiched between two tetrahedral silicon oxide sheets. Substitution of aluminum and silicon ions with ions of lower valency is possible in both the octahedral and tetrahedral sheets. The substitutions cause a total negative layer charge with a typical range from -0.4 to 2
-1.2 in the unit cell [8]. The negative charge is compensated by interlayer cations such as sodium and calcium. Typical smectite minerals are montmorillonites, which are the magnesium-rich end-members of dioctahedral smectites having the layer charge in the octahedral sheet, and beidellites, which are also end-members of dioctahedral smectites but bearing the layer charge in the tetrahedral sheets [9]. Smectites naturally contain iron in the sheet structures, although the content varies [9]. The iron content in montmorillonite is typically approximately 3 mass percent, while it can reach up to 20% in nontronite [10]. Smectites having more than 0.6 Fe atoms per [O20(OH)4] unit are referred to as iron-rich smectites [11]. The iron in smectites can be either in divalent Fe(II) or in trivalent Fe(III) states. A typical iron substitution is trivalent iron in the octahedral sheet. Grim (1939) [12] summarized the factors responsible for the physical properties of clay as the character of exchangeable cation and the composition of clay. However, in a later study, Low and Margheim (1979) [13] explored these factors and concluded that the effect of exchangeable cations on swelling is lower when compared to surface-water interactions. Further studies by Low (1980, 1981) [14,15] considered the effects of other factors including ion concentration in water, specific surface area and exchangeable cations on swelling, but arrived at the same conclusion as the previous study. Low (1980, 1981) [14,15] established that swelling of clay is primarily due to the surface-water interaction, which is affected by the composition of the clay. The presence of iron in smectites leads to effects on their physicochemical properties, such as swelling pressure, hydraulic conductivity and cation exchange capacity. There are experimental studies [16–18] on the effect of iron on swelling ability of smectites. Foster (1953) [17] focused on the effect of octahedral Fe3+ substitution and demonstrated that 3
swelling volume of montmorillonites decreases with increasing Fe3+ content of the studied samples. Interlayer cations in smectites adsorb surrounding water, and the water influx into the interlayer causes swelling [19]. Pressure builds up when the clay swells in a confined environment of the host rock: this is referred to as swelling pressure. Swelling pressures of smectites are affected by several factors such as dry density, layer charge, charge location, interlayer cations, temperature and composition of the surrounding water phase. The swelling pressure is routinely determined experimentally. Molecular dynamics simulations have been recently shown to be useful in the estimation of swelling pressure of montmorillonites and beidellites [20–22]. Previous simulation studies have extensively investigated the swelling properties of smectites and the factors affecting them, but the effect of iron has received less attention. In particular, the iron substitution effect on the swelling pressure of the smectites is virtually unexplored by simulation methods. This study seeks to employ molecular dynamics as a tool to understand how octahedral Fe3+ substitution affects the swelling pressure of montmorillonite and beidellite. The main variables of the swelling pressure will be the dry density, total charge, charge location and iron content of the smectite models.
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2. MODELS AND METHODS 2.1 Simulation Setup Swelling occurs when exchangeable cations in smectites become hydrated. The hydrated cations cause swelling by forcing the clay layers apart in a series of steps [19]. The swelling process involves flow of water between clay interlayer and surrounding bulk solution. Simulation setup in a periodic boundary condition consists of a clay mineral model in contact with bulk water. The clay mineral model has two layers placed one on top of the other and separated by an interlayer space (Figure 1). Both layers are made up of eight montmorillonite or beidellite unit cells with a 2 x 4 arrangement. The distance between the lower tetrahedral sheets of the bottom and upper layers is defined as the d-spacing. The simulation box has dimensions of 2.079 nm x 10.0 nm x 10.0 nm, containing approximately 6,000 water molecules.
Figure 1. (a) Clay model with two layers placed one on top of the other. (b) Simulation cell where the clay model is in contact with bulk water. Color code: Yellow - Si, Red - O, White H, Tan - Al and Blue - Na. Clay stripes were used for our simulation models. It is necessary to terminate the clay surface because swelling depends on movement and flow of water between interlayer and bulk
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solution. The stacked clay layers have edge termination along (010) surfaces [23]. Clay layers are continuous in x-direction through the periodic boundary conditions and are cut in the ydirection, leaving broken bonds at the edge surfaces that are saturated by H2O, H and OH groups to neutralize the charges. Exchange of interlayer cation and water molecules between solution phase and interlayer occurs when (010) surfaces are saturated by water molecules and hydroxyl groups, leading to the termination of the edge surface [24]. Trial calculations with larger models yielded similar trends, but the computational limitations did not allow exploration of the effect of iron substitution in several smectites.
2.2 Smectite Models Two sodium montmorillonite and two sodium beidellite models were studied with the layer charges of -0.5 and -1.0 per unit cell Details of the models and their compositions, including the iron substitutions, are given in Table 1. M0.5 and M1.0 represent sodium montmorillonites with -0.5 and -1.0 layer charges, respectively, and B0.5 and B1.0 are the corresponding sodium beidellites. The montmorillonites hold all of the charge in the octahedral sheet, while beidellites hold their charges in the tetrahedral sheets. Sodium ions balance out the charges in the clay models; 8 Na+ are needed for M0.5 and B0.5, while 16 Na+ cations balance the charge for M1.0 and B1.0.
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Table 1. Information on studied clay models Fe content
Structural formula of clay
(%)
Layer charge
Charge in tetrahedral
per unit cell
sheet (%)
Model M0.5
0
Si8(Al3.5Mg0.5)O20(OH)4
-0.5
0
12.5
Si8(Al3.0Fe0.5Mg0.5)O20(OH)4
-0.5
0
25.0
Si8(Al2.5Fe1.0Mg0.5)O20(OH)4
-0.5
0
50.0
Si8(Al1.5Fe2.0Mg0.5)O20(OH)4
-0.5
0
75.0
Si8(Al0.5Fe3.0Mg0.5)O20(OH)4
-0.5
0
87.5
Si8(Fe3.5Mg0.5)O20(OH)4
-0.5
0
Model B0.5
0
(Si7.5Al0.5)(Al4)O20(OH)4
-0.5
100
12.5
(Si7.5Al0.5)(Al3.5Fe0.5)O20(OH)4
-0.5
100
25.0
(Si7.5Al0.5)(Al3.0Fe1.0)O20(OH)4
-0.5
100
50.0
(Si7.5Al0.5)(Al2.0Fe2.0)O20(OH)4
-0.5
100
75.0
(Si7.5Al0.5)(Al1.0Fe3.0)O20(OH)4
-0.5
100
7
87.5
(Si7.5Al0.5)(Al0.5Fe3.5)O20(OH)4
-0.5
100
Model M1.0
0
Si8(Al3.0Mg1.0)O20(OH)4
-1.00
0
12.5
Si8(Al2.5Fe0.5Mg1.0)O20(OH)4
-1.00
0
25.0
Si8(Al2.0Fe1.0Mg1.0)O20(OH)4
-1.00
0
50.0
Si8(Al1.0Fe2.0Mg1.0)O20(OH)4
-1.00
0
75.0
Si8(Fe3.0Mg1.0)O20(OH)4
-1.00
0
Model B1.0
0
(Si7Al1.0)(Al4)O20(OH)4
-1.00
100
12.5
(Si7Al1.0)(Al3.5Fe0.5)O20(OH)4
-1.00
100
25.0
(Si7Al1.0)(Al3.0Fe1.0)O20(OH)4
-1.00
100
50.0
(Si7Al1.0)(Al2.0Fe2.0)O20(OH)4
-1.00
100
75.0
(Si7Al1.0)(Al1.0Fe3.0)O20(OH)4
-1.00
100
87.5
(Si7Al1.0)(Al0.5Fe3.5)O20(OH)4
-1.00
100
All four models were studied with and without iron content. For each model, Fe3+ replaced Al3+ in the octahedral sheet gradually until the substitution limit was reached. For each of the
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four smectite models, there are models with 0%, 12.5%, 25%, 50%, 75% and 87.5% iron content, except for M1.0, which has a 75% iron substitution limit. No charge deficiency occurred due to substitution in the octahedral sheet. The stripe method of substitution was used to replace Al3+ with Fe3+. Substitutions were made such that the edge sites were avoided as much as possible, and the substitutions followed a pattern such that they have a mirror image relationship [21]. The octahedral sheets (8 unit cells) of model M0.5 with varying iron content along the stripe are depicted in Figure 2.
Figure 2. Octahedral sheets showing iron distribution in M0.5 with (a) 0%, (b) 12.5%, (c) 25%, (d) 50%, (e) 75%, and (f) 87.5% iron content. Color code: Purple - Fe, Tan - Al and Teal - Mg.
2.3 Computational Details Molecular dynamics simulations were carried out with Gromacs simulation package version 5.1.4 [25]. All interactions were described with the CLAYFF force field [26], which represents water molecules with the flexible Simple Point Charge (SPC) model [27]. The Verlet cut-off scheme was employed with a cut-off distance of 0.9 nm for short-range Coulomb and
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Lennard-Jones interactions. Long-range Coulomb interactions were calculated with particlemesh Ewald (PME) electrostatics [28]. In preparation for MD simulation, it is important to ensure stablility of the model by performing energy minimization. Initial energy minimization allowed for full relaxation of the clay mineral model in bulk water, while the water phase equilibrated around the fixed clay model during the following simulation of 10 ns with a time step of 0.5 fs in the NPT ensemble at 300 K and 1 bar. Temperature and pressure were controlled by the velocity rescaling thermostat [29] and Berendsen barostat [30] respectively. Actual swelling pressure simulation was performed as a direct continuation without pressure control and utilizing the same 0.5 fs time step. The length of the production run was 50 ns, but only the last 40 ns were used for the analysis of swelling pressure with a data collection frequency of 5 ps. During the simulation of the clay mineral models, the lower layer remained fixed in all directions. The upper layer was fixed in the lateral directions, and allowed a restricted movement in the vertical direction. The spring model introduced by Sun et al. (2015) [20] focuses on the swelling effect of the clay interlayer. Mechanical springs, which possess force constants and obey Hooke’s law, were used as opposing force for the swelling. During the swelling, displacement of the upper clay mineral layer from its initial position causes compression of the spring from its equilibrium position, and an increase in spring force restricts the movement of the upper layer. The force of the spring and the force of swelling are taken to be equivalent at equilibrium, which allows calculation of the swelling pressure based on average spring deformation in the vertical direction. The simulation model takes into account the pressure
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due to the processes in the interlayer space which is the primary component in the experimentally determined swelling pressure. A series of simulations with spring constants of 0.5 - 10 kJ mol-1 nm-2 and initial d-spacings of 1.4 nm - 3.0 nm were used to obtain the swelling pressure curve for each clay mineral model. The choice of the force constant – initial d-spacing combinations was based on standardized elongation range optimizing the sensitivity of the force probe and minimizing the scatter in the swelling pressure curves. The dry density used in analysis was calculated from the mass and volume of the smectite models as given in equation (I) below. Dry density =
𝑚𝑎𝑠𝑠 𝑜𝑓 𝑠𝑚𝑒𝑐𝑡𝑖𝑡𝑒 𝑙𝑎𝑦𝑒𝑟 + 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑖𝑛𝑡𝑒𝑟𝑙𝑎𝑦𝑒𝑟 𝑐𝑎𝑡𝑖𝑜𝑛𝑠 𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑠𝑚𝑒𝑐𝑡𝑖𝑡𝑒 𝑙𝑎𝑦𝑒𝑟 + 𝑖𝑛𝑡𝑒𝑟𝑙𝑎𝑦𝑒𝑟 𝑠𝑝𝑎𝑐𝑒
(I)
3. RESULTS AND DISCUSSION 3.1 Swelling pressure as a function of d-spacing and dry density The swelling pressure of smectites with varied iron content was studied with d-spacing and dry density as variables. The exchangeable cation used was Na+ for all models. The effect of Fe3+ on swelling pressures of M0.5 smectite with varying iron content was compared (Figure 3). The smectite M0.5 was chosen because it has the highest swelling pressure of all four models studied. An increase in d-spacing leads to a decrease in the swelling pressure, while swelling pressure increases as dry density increases. This is in line with the experimental studies by Gens and Alonso, 1992; Komine and Ogata, 1994 [31,32]. As presented in Figure 3, the effect of iron on the swelling pressure as a function of d-spacing was found to be small. A more pronounced effect was observed as a function of dry density due to the substitution of aluminum atoms by heavier iron atoms in the octahedral sheets. Data points shown in Figure 11
3 are presented using exponential fittings, the dry density curve (Figure 3b) has 6 fittings representing 0%, 12.5%, 25%, 50%, 75% and 87.5% Fe-contents with 0.96, 0.88, 0.92, 0.82, 0.85 and 0.83 R2 values respectively.
Figure 3. Comparison of swelling pressure of smectite M0.5 with varying iron content as a function of (a) d-spacing and (b) dry density using exponential fitting.
3.2 Effect of iron content on swelling pressure of montmorillonite and beidellite The effect of octahedral Fe3+ on swelling pressure of clay models studied was analyzed by comparing their swelling pressures with varying iron contents at certain dry densities. Three
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different dry densities (1,500 kg m-3, 1,200 kg m-3, and 900 kg m-3) representing high, intermediate, and low dry density values were used in the analysis of swelling pressure (Figure 4).
Figure 4. Swelling pressure as a function of iron content at (a) 1,500 kg m-3, (b) 1,200 kg m-3 and (c) 900 kg m-3 dry density As a general trend, the swelling pressure decreases with increasing iron content (Figure 4). The effect of iron substitution is more pronounced with higher dry densities, and hence Figure 4(a) allows a more detailed analysis of structural effects. 13
The effect of Fe3+ substitution in the octahedral sheet is in general more pronounced in the montmorillonites than in the beidellites if the total charge is the same in the models. Swelling pressures of models M0.5 and M1.0 decrease more rapidly with increasing Fe3+ content at high dry density (1,500 kg m-3) than the respective swelling pressures of models B0.5 and B1.0 (Figure 4a). This can be rationalized by the structural differences between the montmorillonite and beidellite. Since the charge is located in the octahedral sheet in the montmorillonite models, the iron substitution taking place in the same sheet causes a direct effect on swelling pressure. In the beidellite models, the charge is in the tetrahedral sheet; since the substitution is in the octahedral sheet, the effect is less pronounced. This is similar to the trends observed for the Belle Fourche and Tatatila samples reported by Foster (1953) [17]. In the montmorillonite samples, the swelling volume was markedly affected by increasing iron content in the octahedral sheet, while there was little effect on the swelling volume of beidellite type smectites. The ionic radius of Fe3+ is 11% larger than the ionic radius of Al3+, expanding the clay mineral structure and decreasing the surface charge density with higher iron contents. The lower charge has a slight effect on the swelling pressure. Overall, regardless of the clay mineral composition, the swelling pressure differences decrease with very high iron contents. However, the most common iron substitution range in natural clay samples is up to 20% of the octahedral aluminum sites.
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4. CONCLUSION Molecular dynamics simulations have been employed to demonstrate the effect of octahedral iron substitution on the swelling pressure of sodium smectites. The swelling pressure of smectites depends primarily on the dry density. For all smectites studied, swelling pressure decreases with increase in iron content. The trend is more evident at high dry densities. Iron substitution more substantially reduced the swelling pressure in montmorillonite than in beidellite. This is because the iron substitution occurred in the octahedral sheet, where the layer charge is located in the montmorillonite. The total charge of the smectite was found to exert only a minor effect on the swelling pressure when iron substitutes aluminum in octahedral sheets. The study presents atomic level information on the effect of Fe3+ on swelling pressure of two smectite types. The results obtained can be used for screening clays intended for use as buffers and backfilling agents in geo-disposal structures for spent nuclear waste deposition. ACKNOWLEDGEMENTS Financial supports given by Posiva Oy, the Finnish Research Programme on Nuclear Waste Management (KYT) 2015-2018, and the Finnish Cultural Foundation (2018) are acknowledged. Provision of computer capacity from the Finnish Grid and Cloud Infrastructure (persistent identifier urn:nbn:fi:research-infras-2016072533) is also acknowledged.
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Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Highlights Effect of iron on the swelling pressure of sodium smectites has been modeled. The variables are dry density, total charge, charge location and iron content. Increase in iron content results in decrease in the swelling pressure of smectites. Effect is more pronounced in montmorillonite than beidellite at high dry densities.
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S0009-2614(19)30799-7 https://doi.org/10.1016/j.cplett.2019.136818 CPLETT 136818
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
21 August 2019 27 September 2019 2 October 2019
Please cite this article as: A.D. Fayoyiwa, L. Sun, J.T. Hirvi, S. Kasa, T.A. Pakkanen, Effect of Iron (III) on the Swelling Pressure of Dioctahedral Smectites: A Molecular Dynamics Study, Chemical Physics Letters (2019), doi: https://doi.org/10.1016/j.cplett.2019.136818
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Effect of Iron (III) on the Swelling Pressure of Dioctahedral Smectites: A Molecular Dynamics Study Aderemi D. Fayoyiwaa, Linlin Suna, Janne T. Hirvia, Seppo Kasab and Tapani A. Pakkanena,* aDepartment
of Chemistry, University of Eastern Finland, P.O. Box 111, Joensuu, FI-80101,
Finland bPosiva
Oy, Olkiluoto, Eurajoki, FI-27160, Finland
ABSTRACT Smectites in bentonites contain variable amounts of iron substituting aluminum and silicon in the crystal structure. The swelling pressure of smectite models with varying Fe3+ content, layer charge and charge location were studied using molecular dynamics method. Swelling pressures decrease with increasing iron content at high dry densities, this is more pronounced in montmorillonites than beidellites with same layer charge. This swelling pressure decrease is slightly faster for smectites with lower layer charge. Results obtained from the study is useful in the screening process of iron-containing bentonites intended for use as sealants in High Level Radioactive Waste (HLRW) repositories.
GRAPHICAL ABSTRACT
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Keywords: Montmorillonite, beidellite, HLRW repositories * Corresponding Author, Tel: +358 405028982 E-mail address: [email protected]
1. INTRODUCTION In recent years, steady progress has been made towards construction of underground repositories for deposition of high-level radioactive waste (HLRW). The underground repositories are considered the safest and most suitable disposal option. Spent nuclear waste is placed in copper canisters, which are deposited in the repositories while a compact bentonite buffer is employed to mechanically isolate the canisters from the host rock [1]. Bentonites have been identified in many countries as the best candidates for buffer and back filling agents in several country-specific repository concepts [2–5]. Properties of bentonites such as their high swelling ability, low permeability, high retention capacity, small pore size and high ion exchange capacity [3,5,6] make them suitable for use as buffers in HLRW repositories. Bentonites contain smectite minerals, which are responsible for their swelling properties. In HLRW repositories, bentonite is designed to fill the space between the host rock and the canisters holding the spent nuclear waste. Smectites swell after taking up water, causing effective sealing and forming a barrier between the canister and the environment [1]. Smectites contribute to swelling of other types of soil as well [7]. Smectites are phyllosilicates with 2:1 layer structure, having an octahedral sheet of aluminum oxide sandwiched between two tetrahedral silicon oxide sheets. Substitution of aluminum and silicon ions with ions of lower valency is possible in both the octahedral and tetrahedral sheets. The substitutions cause a total negative layer charge with a typical range from -0.4 to 2
-1.2 in the unit cell [8]. The negative charge is compensated by interlayer cations such as sodium and calcium. Typical smectite minerals are montmorillonites, which are the magnesium-rich end-members of dioctahedral smectites having the layer charge in the octahedral sheet, and beidellites, which are also end-members of dioctahedral smectites but bearing the layer charge in the tetrahedral sheets [9]. Smectites naturally contain iron in the sheet structures, although the content varies [9]. The iron content in montmorillonite is typically approximately 3 mass percent, while it can reach up to 20% in nontronite [10]. Smectites having more than 0.6 Fe atoms per [O20(OH)4] unit are referred to as iron-rich smectites [11]. The iron in smectites can be either in divalent Fe(II) or in trivalent Fe(III) states. A typical iron substitution is trivalent iron in the octahedral sheet. Grim (1939) [12] summarized the factors responsible for the physical properties of clay as the character of exchangeable cation and the composition of clay. However, in a later study, Low and Margheim (1979) [13] explored these factors and concluded that the effect of exchangeable cations on swelling is lower when compared to surface-water interactions. Further studies by Low (1980, 1981) [14,15] considered the effects of other factors including ion concentration in water, specific surface area and exchangeable cations on swelling, but arrived at the same conclusion as the previous study. Low (1980, 1981) [14,15] established that swelling of clay is primarily due to the surface-water interaction, which is affected by the composition of the clay. The presence of iron in smectites leads to effects on their physicochemical properties, such as swelling pressure, hydraulic conductivity and cation exchange capacity. There are experimental studies [16–18] on the effect of iron on swelling ability of smectites. Foster (1953) [17] focused on the effect of octahedral Fe3+ substitution and demonstrated that 3
swelling volume of montmorillonites decreases with increasing Fe3+ content of the studied samples. Interlayer cations in smectites adsorb surrounding water, and the water influx into the interlayer causes swelling [19]. Pressure builds up when the clay swells in a confined environment of the host rock: this is referred to as swelling pressure. Swelling pressures of smectites are affected by several factors such as dry density, layer charge, charge location, interlayer cations, temperature and composition of the surrounding water phase. The swelling pressure is routinely determined experimentally. Molecular dynamics simulations have been recently shown to be useful in the estimation of swelling pressure of montmorillonites and beidellites [20–22]. Previous simulation studies have extensively investigated the swelling properties of smectites and the factors affecting them, but the effect of iron has received less attention. In particular, the iron substitution effect on the swelling pressure of the smectites is virtually unexplored by simulation methods. This study seeks to employ molecular dynamics as a tool to understand how octahedral Fe3+ substitution affects the swelling pressure of montmorillonite and beidellite. The main variables of the swelling pressure will be the dry density, total charge, charge location and iron content of the smectite models.
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2. MODELS AND METHODS 2.1 Simulation Setup Swelling occurs when exchangeable cations in smectites become hydrated. The hydrated cations cause swelling by forcing the clay layers apart in a series of steps [19]. The swelling process involves flow of water between clay interlayer and surrounding bulk solution. Simulation setup in a periodic boundary condition consists of a clay mineral model in contact with bulk water. The clay mineral model has two layers placed one on top of the other and separated by an interlayer space (Figure 1). Both layers are made up of eight montmorillonite or beidellite unit cells with a 2 x 4 arrangement. The distance between the lower tetrahedral sheets of the bottom and upper layers is defined as the d-spacing. The simulation box has dimensions of 2.079 nm x 10.0 nm x 10.0 nm, containing approximately 6,000 water molecules.
Figure 1. (a) Clay model with two layers placed one on top of the other. (b) Simulation cell where the clay model is in contact with bulk water. Color code: Yellow - Si, Red - O, White H, Tan - Al and Blue - Na. Clay stripes were used for our simulation models. It is necessary to terminate the clay surface because swelling depends on movement and flow of water between interlayer and bulk
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solution. The stacked clay layers have edge termination along (010) surfaces [23]. Clay layers are continuous in x-direction through the periodic boundary conditions and are cut in the ydirection, leaving broken bonds at the edge surfaces that are saturated by H2O, H and OH groups to neutralize the charges. Exchange of interlayer cation and water molecules between solution phase and interlayer occurs when (010) surfaces are saturated by water molecules and hydroxyl groups, leading to the termination of the edge surface [24]. Trial calculations with larger models yielded similar trends, but the computational limitations did not allow exploration of the effect of iron substitution in several smectites.
2.2 Smectite Models Two sodium montmorillonite and two sodium beidellite models were studied with the layer charges of -0.5 and -1.0 per unit cell Details of the models and their compositions, including the iron substitutions, are given in Table 1. M0.5 and M1.0 represent sodium montmorillonites with -0.5 and -1.0 layer charges, respectively, and B0.5 and B1.0 are the corresponding sodium beidellites. The montmorillonites hold all of the charge in the octahedral sheet, while beidellites hold their charges in the tetrahedral sheets. Sodium ions balance out the charges in the clay models; 8 Na+ are needed for M0.5 and B0.5, while 16 Na+ cations balance the charge for M1.0 and B1.0.
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Table 1. Information on studied clay models Fe content
Structural formula of clay
(%)
Layer charge
Charge in tetrahedral
per unit cell
sheet (%)
Model M0.5
0
Si8(Al3.5Mg0.5)O20(OH)4
-0.5
0
12.5
Si8(Al3.0Fe0.5Mg0.5)O20(OH)4
-0.5
0
25.0
Si8(Al2.5Fe1.0Mg0.5)O20(OH)4
-0.5
0
50.0
Si8(Al1.5Fe2.0Mg0.5)O20(OH)4
-0.5
0
75.0
Si8(Al0.5Fe3.0Mg0.5)O20(OH)4
-0.5
0
87.5
Si8(Fe3.5Mg0.5)O20(OH)4
-0.5
0
Model B0.5
0
(Si7.5Al0.5)(Al4)O20(OH)4
-0.5
100
12.5
(Si7.5Al0.5)(Al3.5Fe0.5)O20(OH)4
-0.5
100
25.0
(Si7.5Al0.5)(Al3.0Fe1.0)O20(OH)4
-0.5
100
50.0
(Si7.5Al0.5)(Al2.0Fe2.0)O20(OH)4
-0.5
100
75.0
(Si7.5Al0.5)(Al1.0Fe3.0)O20(OH)4
-0.5
100
7
87.5
(Si7.5Al0.5)(Al0.5Fe3.5)O20(OH)4
-0.5
100
Model M1.0
0
Si8(Al3.0Mg1.0)O20(OH)4
-1.00
0
12.5
Si8(Al2.5Fe0.5Mg1.0)O20(OH)4
-1.00
0
25.0
Si8(Al2.0Fe1.0Mg1.0)O20(OH)4
-1.00
0
50.0
Si8(Al1.0Fe2.0Mg1.0)O20(OH)4
-1.00
0
75.0
Si8(Fe3.0Mg1.0)O20(OH)4
-1.00
0
Model B1.0
0
(Si7Al1.0)(Al4)O20(OH)4
-1.00
100
12.5
(Si7Al1.0)(Al3.5Fe0.5)O20(OH)4
-1.00
100
25.0
(Si7Al1.0)(Al3.0Fe1.0)O20(OH)4
-1.00
100
50.0
(Si7Al1.0)(Al2.0Fe2.0)O20(OH)4
-1.00
100
75.0
(Si7Al1.0)(Al1.0Fe3.0)O20(OH)4
-1.00
100
87.5
(Si7Al1.0)(Al0.5Fe3.5)O20(OH)4
-1.00
100
All four models were studied with and without iron content. For each model, Fe3+ replaced Al3+ in the octahedral sheet gradually until the substitution limit was reached. For each of the
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four smectite models, there are models with 0%, 12.5%, 25%, 50%, 75% and 87.5% iron content, except for M1.0, which has a 75% iron substitution limit. No charge deficiency occurred due to substitution in the octahedral sheet. The stripe method of substitution was used to replace Al3+ with Fe3+. Substitutions were made such that the edge sites were avoided as much as possible, and the substitutions followed a pattern such that they have a mirror image relationship [21]. The octahedral sheets (8 unit cells) of model M0.5 with varying iron content along the stripe are depicted in Figure 2.
Figure 2. Octahedral sheets showing iron distribution in M0.5 with (a) 0%, (b) 12.5%, (c) 25%, (d) 50%, (e) 75%, and (f) 87.5% iron content. Color code: Purple - Fe, Tan - Al and Teal - Mg.
2.3 Computational Details Molecular dynamics simulations were carried out with Gromacs simulation package version 5.1.4 [25]. All interactions were described with the CLAYFF force field [26], which represents water molecules with the flexible Simple Point Charge (SPC) model [27]. The Verlet cut-off scheme was employed with a cut-off distance of 0.9 nm for short-range Coulomb and
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Lennard-Jones interactions. Long-range Coulomb interactions were calculated with particlemesh Ewald (PME) electrostatics [28]. In preparation for MD simulation, it is important to ensure stablility of the model by performing energy minimization. Initial energy minimization allowed for full relaxation of the clay mineral model in bulk water, while the water phase equilibrated around the fixed clay model during the following simulation of 10 ns with a time step of 0.5 fs in the NPT ensemble at 300 K and 1 bar. Temperature and pressure were controlled by the velocity rescaling thermostat [29] and Berendsen barostat [30] respectively. Actual swelling pressure simulation was performed as a direct continuation without pressure control and utilizing the same 0.5 fs time step. The length of the production run was 50 ns, but only the last 40 ns were used for the analysis of swelling pressure with a data collection frequency of 5 ps. During the simulation of the clay mineral models, the lower layer remained fixed in all directions. The upper layer was fixed in the lateral directions, and allowed a restricted movement in the vertical direction. The spring model introduced by Sun et al. (2015) [20] focuses on the swelling effect of the clay interlayer. Mechanical springs, which possess force constants and obey Hooke’s law, were used as opposing force for the swelling. During the swelling, displacement of the upper clay mineral layer from its initial position causes compression of the spring from its equilibrium position, and an increase in spring force restricts the movement of the upper layer. The force of the spring and the force of swelling are taken to be equivalent at equilibrium, which allows calculation of the swelling pressure based on average spring deformation in the vertical direction. The simulation model takes into account the pressure
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due to the processes in the interlayer space which is the primary component in the experimentally determined swelling pressure. A series of simulations with spring constants of 0.5 - 10 kJ mol-1 nm-2 and initial d-spacings of 1.4 nm - 3.0 nm were used to obtain the swelling pressure curve for each clay mineral model. The choice of the force constant – initial d-spacing combinations was based on standardized elongation range optimizing the sensitivity of the force probe and minimizing the scatter in the swelling pressure curves. The dry density used in analysis was calculated from the mass and volume of the smectite models as given in equation (I) below. Dry density =
𝑚𝑎𝑠𝑠 𝑜𝑓 𝑠𝑚𝑒𝑐𝑡𝑖𝑡𝑒 𝑙𝑎𝑦𝑒𝑟 + 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑖𝑛𝑡𝑒𝑟𝑙𝑎𝑦𝑒𝑟 𝑐𝑎𝑡𝑖𝑜𝑛𝑠 𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑠𝑚𝑒𝑐𝑡𝑖𝑡𝑒 𝑙𝑎𝑦𝑒𝑟 + 𝑖𝑛𝑡𝑒𝑟𝑙𝑎𝑦𝑒𝑟 𝑠𝑝𝑎𝑐𝑒
(I)
3. RESULTS AND DISCUSSION 3.1 Swelling pressure as a function of d-spacing and dry density The swelling pressure of smectites with varied iron content was studied with d-spacing and dry density as variables. The exchangeable cation used was Na+ for all models. The effect of Fe3+ on swelling pressures of M0.5 smectite with varying iron content was compared (Figure 3). The smectite M0.5 was chosen because it has the highest swelling pressure of all four models studied. An increase in d-spacing leads to a decrease in the swelling pressure, while swelling pressure increases as dry density increases. This is in line with the experimental studies by Gens and Alonso, 1992; Komine and Ogata, 1994 [31,32]. As presented in Figure 3, the effect of iron on the swelling pressure as a function of d-spacing was found to be small. A more pronounced effect was observed as a function of dry density due to the substitution of aluminum atoms by heavier iron atoms in the octahedral sheets. Data points shown in Figure 11
3 are presented using exponential fittings, the dry density curve (Figure 3b) has 6 fittings representing 0%, 12.5%, 25%, 50%, 75% and 87.5% Fe-contents with 0.96, 0.88, 0.92, 0.82, 0.85 and 0.83 R2 values respectively.
Figure 3. Comparison of swelling pressure of smectite M0.5 with varying iron content as a function of (a) d-spacing and (b) dry density using exponential fitting.
3.2 Effect of iron content on swelling pressure of montmorillonite and beidellite The effect of octahedral Fe3+ on swelling pressure of clay models studied was analyzed by comparing their swelling pressures with varying iron contents at certain dry densities. Three
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different dry densities (1,500 kg m-3, 1,200 kg m-3, and 900 kg m-3) representing high, intermediate, and low dry density values were used in the analysis of swelling pressure (Figure 4).
Figure 4. Swelling pressure as a function of iron content at (a) 1,500 kg m-3, (b) 1,200 kg m-3 and (c) 900 kg m-3 dry density As a general trend, the swelling pressure decreases with increasing iron content (Figure 4). The effect of iron substitution is more pronounced with higher dry densities, and hence Figure 4(a) allows a more detailed analysis of structural effects. 13
The effect of Fe3+ substitution in the octahedral sheet is in general more pronounced in the montmorillonites than in the beidellites if the total charge is the same in the models. Swelling pressures of models M0.5 and M1.0 decrease more rapidly with increasing Fe3+ content at high dry density (1,500 kg m-3) than the respective swelling pressures of models B0.5 and B1.0 (Figure 4a). This can be rationalized by the structural differences between the montmorillonite and beidellite. Since the charge is located in the octahedral sheet in the montmorillonite models, the iron substitution taking place in the same sheet causes a direct effect on swelling pressure. In the beidellite models, the charge is in the tetrahedral sheet; since the substitution is in the octahedral sheet, the effect is less pronounced. This is similar to the trends observed for the Belle Fourche and Tatatila samples reported by Foster (1953) [17]. In the montmorillonite samples, the swelling volume was markedly affected by increasing iron content in the octahedral sheet, while there was little effect on the swelling volume of beidellite type smectites. The ionic radius of Fe3+ is 11% larger than the ionic radius of Al3+, expanding the clay mineral structure and decreasing the surface charge density with higher iron contents. The lower charge has a slight effect on the swelling pressure. Overall, regardless of the clay mineral composition, the swelling pressure differences decrease with very high iron contents. However, the most common iron substitution range in natural clay samples is up to 20% of the octahedral aluminum sites.
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4. CONCLUSION Molecular dynamics simulations have been employed to demonstrate the effect of octahedral iron substitution on the swelling pressure of sodium smectites. The swelling pressure of smectites depends primarily on the dry density. For all smectites studied, swelling pressure decreases with increase in iron content. The trend is more evident at high dry densities. Iron substitution more substantially reduced the swelling pressure in montmorillonite than in beidellite. This is because the iron substitution occurred in the octahedral sheet, where the layer charge is located in the montmorillonite. The total charge of the smectite was found to exert only a minor effect on the swelling pressure when iron substitutes aluminum in octahedral sheets. The study presents atomic level information on the effect of Fe3+ on swelling pressure of two smectite types. The results obtained can be used for screening clays intended for use as buffers and backfilling agents in geo-disposal structures for spent nuclear waste deposition. ACKNOWLEDGEMENTS Financial supports given by Posiva Oy, the Finnish Research Programme on Nuclear Waste Management (KYT) 2015-2018, and the Finnish Cultural Foundation (2018) are acknowledged. Provision of computer capacity from the Finnish Grid and Cloud Infrastructure (persistent identifier urn:nbn:fi:research-infras-2016072533) is also acknowledged.
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Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Highlights Effect of iron on the swelling pressure of sodium smectites has been modeled. The variables are dry density, total charge, charge location and iron content. Increase in iron content results in decrease in the swelling pressure of smectites. Effect is more pronounced in montmorillonite than beidellite at high dry densities.
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