New Insights into the Cs Adsorption on Montmorillonite Clay from

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New Insights into the Cs Adsorption on Montmorillonite Clay from Cs Solid-state NMR and Density Functional Theory Calculations 133

Takahiro Ohkubo, Takuya Okamoto, Katsuyuki Kawamura, Régis Guégan, Kenzo Deguchi, Shinobu Ohki, Tadashi Shimizu, Yukio Tachi, and Yasuhiko Iwadate J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b07276 • Publication Date (Web): 01 Nov 2018 Downloaded from http://pubs.acs.org on November 8, 2018

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The Journal of Physical Chemistry

New Insights into the Cs Adsorption on Montmorillonite Clay from

133

Cs Solid-state

NMR and Density Functional Theory Calculations Takahiro Ohkubo,∗,† Takuya Okamoto,† Katsuyuki Kawamura,‡ Régis Guégan,¶,§ Kenzo Deguchi,∥ Shinobu Ohki,∥ Tadashi Shimizu,∥ Yukio Tachi,⊥ and Yasuhiko Iwadate† †Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho Inage-ku, Chiba 263-8522, Japan ‡Graduate School of Environmental and Life Science, Okayama University, 1-1-1 Tsushima-naka Kita-ku, Okayama-shi, Okayama 700-8530, Japan ¶Faculty of Science and Engineering, Global Center for Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan §Institut des Sciences de la Terre d’Orléans, UMR 7327, Univ Orléans, CNRS, BRGM, 1A Rue de la Férollerie, 45071 Orléans, France ∥National Institute of Material Science, Sakura-site, 3-13 Sakura, Tsukuba, Ibaraki 305-0003, Japan ⊥Japan Atomic Energy Agency, Muramatsu 4-33, Tokai, Ibaraki, 319-1194, Japan E-mail: [email protected] Phone: +81 (0)43 2903435. Fax: +81 (0)43 2903435

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Abstract The adsorption sites of Cs on montmorillonite clays were investigated by theoretical

133 Cs

chemical shift calculations,

133 Cs

magic-angle-spinning nuclear magnetic

resonance (MAS NMR) spectroscopy, and X-ray diffraction under controlled relative humidity. The theoretical calculations were carried out for structures with three stacking variations in the clay layers, where hexagonal cavities formed with Si-O bonds in the tetrahedral layers were aligned as monoclinic, parallel, alternated; and various dspacings. After structural optimization, all Cs atoms were positioned around the center of hexagonal cavities in the upper or lower tetrahedral sheets. The calculated

133 Cs

chemical shifts were highly sensitive to the tetrahedral Al (AlT )-Cs distance and dspacing, rather than to the Cs coordination number. Accordingly, three peaks observed in our theoretical spectra were interpreted to be adsorbed Cs around the center of hexagonal cavity with or without AlT and on the surface in the open nano-space. In a series of

133 Cs

MAS NMR spectral changes for partial Cs substituted samples, the Cs

atoms are preferentially adsorbed at sites near AlT for low Cs substituted montmorillonites. The presence of non-hydrated Cs was also confirmed in partially Cs substituted samples, even after being hydrated under high relative humidity. Keywords: montmorillonite;

133 Cs

solid-state nuclear magnetic resonance;

133 Cs

chem-

ical shift; DFT; GIPAW

Introduction The migration of radioactive cesium in compacted bentonites and argillaceous rocks have been widely studied in the context of the geological disposal of radioactive wastes. 1–3 The migration of radioactive cesium is also attracting much attention from the perspectives of environmental preservation and restoration, after the accident at Fukushima Dai-ichi Nuclear Power Plant. 4,5 The mechanism of Cs adsorption and diffusion in these bentonites, argillaceous rocks and soils are a key to understand and predict future radioactive Cs distribution in the environment. It is well known that natural smectites such as montmorillonite 2


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in bentonites, argillaceous rocks and soils are one of key materials that control Cs migration, because of their high surface area and high cation exchange capacities. 6–8 The layered structure of montmorillonite consists of an octahedral layer sheet sandwiched by two tetrahedral layer sheets (TOT). Ionic substitution of AlO6 by MgO6 in the octahedral sheet and SiO4 by AlO4 causes the layers to be negatively charged. In order to compensate for the negative charge, cations are located in the interlayer space. The dominant interlayer cations in natural montmorillonite are Na+ and Ca2+ (Na- and Ca-montmorillonite). Especially, the interlayer Na cation in Na-montmorillonite is easily exchanged with other mono and divalent cations by contact with aqueous solution. Therefore, the exchange and fixation mechanism of Cs+ in the interlayer space is important. Many studies have addressed the adsorption characteristics of Cs on montmorillonite. Using batch experiments under various conditions (e.g., salinity, pH, and Cs concentration), the distribution coefficient Kd or the selectivity coefficient Kc was determined. 1,2,9–14 In order to apply the cation exchange model to a wider range of Cs concentration (from trace-level concentration related to environmental contaminant to higher concentrations for radioactive waste disposal), concentration-dependent sorption has been widely investigated. It is generally accepted that Cs+ adsorbs on montmorillonite through a cation exchange reaction at basal plane sites, which is strongly affected by salinity while not being relatively insensitive to the Cs+ concentration. In addition, non-linear and concentration-dependent Cs sorption is also reported for montmorillonite, indicating the minor contribution of a small amount of frayed edge sites. 6,12 These studies suggested that various Cs sites with different affinity govern Cs adsorption or fixation in the environment. In order to understand the affinity of Cs adsorption sites on montmorillonite, molecular simulation and spectroscopic observations have been dedicated to characterize the local structure of Cs in montmorillonite. X-ray absorption fine structure spectroscopy (XAFS) can obtain the Cs local coordination structure, highlighting the number and distance of the nearest atoms in the first and second coordination shells. 15–18 The XAFS spectra were usu-

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ally analyzed by fitting based on possible Cs coordination models (coordination number and distance) after background and normalization treatments. Variable coordination numbers with Cs-O distances between 3.2 and 4.3 Å were reported for montmorillonites. These Cs sites were recognized for both inner- and outer-sphere adsorption in the interlayer, which correspond to interlayer sites in the hydrated state and directly coordinated to the siloxane group on the tetrahedral layer. While EXAFS results were useful in estimating the inner- and outer-sphere adsorption of Cs, however, the validity of the model used for fitting these species remains unchecked. The molecular dynamics (MD) and Grand Canonical Monte Carlo simulations have been successfully applied to Cs-montmorillonite. 19–32 Complete atomic configuration could be used to simulate micron-meter clay sheets up to the nanosecond timescale. Although Cs diffusivity and adsorption sites in the interlayer space were revealed by these simulations, experimental proofs of the simulation results require a comparison with experimental data, which may be forthcoming. Like EXAFS analysis,

133

Cs solid-state NMR is an atom-selective spectroscopy method

and very sensitive to Cs adsorbed in local environments. 33–35,35–41

133

Cs NMR can provide

well-resolved Cs adsorption sites, however, the correlation between

133

Cs chemical shift and

local structure has been incomplete, because of the poorer spectral resolution for the natural samples and their complexity. It has been reported that

133

Cs NMR experiments of

anhydrous Cs-montmorillonite showed the existence of at least two distinct Cs adsorption sites, 34,35 and the fast exchange between hydrated Cs sites leads to only one peak from averaging the multiple Cs adsorption sites in the hydrated state. The objective of our present work was to systematically investigate the correlation between 133 Cs chemical shift and local structure on montmorillonite. To identify Cs adsorption sites by means of

133

Cs solid-state magic-angle-spinning (MAS) NMR experiments, we have

carried out experimental and theoretical analysis for various conditions and atomic models. The 133 Cs MAS NMR spectra were obtained for synthetic and natural montmorillonite clays with different Cs substitutions and hydrated states. Atomic models were constructed for

4


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theoretical calculations of

133

Cs chemical shift, by taking into account the different stacking

structures where two TOT layers were stacked in parallel, alternated, and perfect monoclinic crystals. The interlayer space was also systematically parameterized. These experimental and theoretical approaches can provide quantitative information of Cs sites on montmorillonite. Based on the results, we propose an adsorption model of Cs on montmorillonite and formulate the general conclusions.

Experimental Section Sample Preparations The starting material for this study was natural Na-montmorillonite (Na-mont) prepared by purification of bentonite from the Tsukinuno deposit (Kunipia-F, Kunimine Industries Co. Ltd.). Its composition and chemical and physical properties are available from the Clay Science Society of Japan. 42 Synthetic Na-montmorillonite (Na-synmont) was also prepared as described in a reference. 43 The compositions of natural and synthetic samples were previously determined by elemental analysis, and their chemical formula are summarized in Table 1. Completely Cs exchanged samples (Cs100-mont and Cs-synmont) were prepared by stirring approximately 2.0 g of the clay samples in polyethylene vessels with 150 ml 1 M CsCl solution for 1 h. Next, the clay and liquid were separated by centrifugation. These procedures were repeated three times to achieve complete Cs exchange. In order to remove excess ions, the clay slurry was washed by stirring with 80% ethanol and separated by centrifugation. This process was repeated until Cl – could not be detected by AgNO3 titration. Partial Cs substituted samples of Na-mont were also prepared using more dilute CsCl solutions (12.2, 7.64, 4.58, and 1.53 mM). In these cases, the samples were only stirred with the solutions for 1 h, and the same washing procedure was applied as that for Cs100-mont. After Cs exchange, powder samples were obtained by drying in a thermostat chamber at 150℃. In order to accurately estimate the Cs exchange fraction, a part of the sample was 5



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completely dissolved in Teflon vessels by 50 ml HNO3 /HF solution (30 ml 65% HNO3 and 20 ml 40% HF) for 12 h at 90℃. Na and Cs contents in the solution were determined by flame atomic absorption spectroscopy (AAS). Cs exchange fractions of the samples were determined from the measured Cs concentration and their chemical formula. The samples prepared using 12.2, 7.64, 4.58, and 1.53 mM CsCl solutions correspond to 79%, 59%, 36%, and 16% Cs exchange fractions (Cs79-mont, Cs59-mont, Cs36-mont, and Cs16-mont), respectively.

X-ray Diffraction X-ray diffraction (XRD) patterns at different relative humidity were recorded using a RintUltima III system (Rigaku Co, Ltd,; Cu Kα radiation) for Na-mont, Cs100-mont, Cs79-mont, Cs59-mont, and Cs36-mont samples. The setup of XRD measurement under humidity control was described in detail elsewhere. 44 Temperature of the sample chamber was controlled at 50℃, which is the same as that used for NMR measurements (described later). First, XRD measurement was carried out for each sample in the dehydrated condition, and then the vapor pressure was increased gradually. After the diffraction pattern stopped changing (i.e., reaching equilibrium), the XRD patterns were recorded in the range from 2 to 10° with 0.05° step.

133

Cs Solid-State NMR

NMR spectra were collected on a JEOL Delta spectrometer using 4-mm double resonance magic angle spinning (MAS) probe and under a magnetic field of 18.8 T (corresponding to 105.0 MHz) at room temperature. The

133

Cs MAS spectrum was recorded with a spinning

frequency of 18 kHz. The actual sample temperature under 18 kHz spinning was approximately 50 ℃, as measured by

207

Pb NMR of solid lead nitrate. 45 Thus, the same sample

temperature was used for the XRD measurements. The excitation pulse was of 2.6 µs, and the chemical shift was referenced to 1.0 M CsCl at 0 ppm.

133

Cs has a nuclear spin 7/2,

which means that second-order quadrupolar interaction under MAS should be considered 6


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to estimate chemical shift in MAS spectra. Fortunately, the estimated directly from the peak position in

133

133

Cs chemical shift can be

Cs MAS NMR spectra, as the second-order

quadrupolar effects are small in this case. Indeed, we observed perfect sine curves in nutation 133

Cs NMR experiment for the dehydrated and hydrated Cs100-mont samples, as shown in

Fig. S1 of Supporting Information. Therefore, the peak positions observed in all spectra were treated as isotropic chemical shifts (δiso ). The fully relaxed recycling delay was 100 ms, and a total of 8192 transients were collected. Dehydrated samples were prepared by drying at 150℃ under vacuum. These samples were carefully packed in the NMR tube in Ar-filled glove box (dew point < −70℃) to avoid adsorption of water from air. Hydrated samples were prepared under controlled humidity (6.4, 11.3, 32.8, 42.3, 57.6, 75.3, and 85.1%RH) using saturated salts. First, the dehydrated sample was put in glove box filled with air at targeted humidity. After reaching equilibrium, the sample was packed in an airtight NMR sample tube to prevent water adsorption and desorption during NMR experiments. In order to ensure the dehydrated and hydrated conditions, the weight of the tube containing sample was measured before and after NMR experiments. We thus confirmed that the mass remained unchanged.

DFT Computations Atomic models of Cs-montmorillonite were made according to the reported crystal structure of montmorillonite. 46 We have built two models with different sizes (3×2×1 and 2×1×2 supercells). The 3×2×1 supercell consists of only one set of TOT layers, while the 2×1×2 cell contains two sets of TOT layers. The Cs position in interlayer space was examined for 3×2×1 supercell. On the other hand, three stacking structures of two TOT layers and the interlayer space (d-spacing) dependency were investigated for 2×1×2 supercell. The three kinds of stacking structures will be described later. The atomic composition for 3×2×1 and 2×1×2 supercells are as follows: Cs6 (Al19 Mg5 )(Si47 Al1 )O120 (OH)24 (3×2×1 supercell) 7



Cs5 (Al12 Mg4 )(Si31 Al1 )O80 (OH)16 (2×1×2 supercell) where the octahedral Al sites and 1 tetrahedral Si site were substituted by Mg and Al, respectively. Mg was randomly substituted on octahedral sheets with Al–O–Al avoidance. 47 Before density functional theory (DFT) calculation, classical molecular dynamics (MD) simulation was carried out for the 3×2×1 and 2×1×2 supercells with monoclinic crystal atomic structure using ClayFF 48 force field with NPT (constant number of particles, constant pressure, and constant temperature) ensemble until reaching energy equilibrium at 300 K. Next, the temperature was decreased to 1 K in the classical MD framework. The initial Cs positions for classical MD are equivalent to those in the crystal structure 46 . Plane-wave-based DFT calculations were carried out using Quantum Espresso 5.0.1 code 49 employing the pseudopotential. 50 Theoretical 133 Cs NMR chemical shifts were also calculated by gauge including projected augmented wave (GIPAW) method 51 using QE-GIPAW module. The atomic positions and cell parameters were adjusted for the geometry optimization with variable lattice constants (vc-relax) for 3 × 2 × 1 and 2 × 1 × 2 supercells, where the crystallographic system of 2×1×2 supercell was monoclinic. Hexagonal cavities formed with Si-O bonds in tetrahedral layers were aligned either parallel or alternated for the 2×1×2 cell (referred to as parallel and alternated stacking). The parallel and alternated stacking structures were manually made by calibrating positions in the optimized monoclinic stacking. These structures were optimized with vc-relax under fixed position of Si in TOT layers to keep the stacking structure. Final atomic positions in the three stacking structures are shown in Fig. 7. To vary the interlayer distance, lattice constant c of the monoclinic 2×1×2 supercells was manually changed from the optimized distance (z-opt) to −0.3, +0.3, +1.0, +3.0 and +6.0 Å (z-0.3, z+0.3, z+1.0, z+3.0, and z+6.0, respectively), whereas atomic positions of the TOT layers were unchanged. The geometry optimization of Cs with fixed lattice constants was carried out before calculating the NMR chemical shifts. The final atomic structures of z-0.3, z+1.0, and z+6.0 are shown in Fig. 8 as examples.

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The cutoff energies were 80 and 120 Ry for 3 × 2 × 1 and 2 × 1 × 2 supercells to satisfy the convergence within 10−3 Ry. Geometry optimization was carried out until reaching the convergence threshold of 10−4 Ry/au on the forces. Monkhorst-Pack (MP) grids for sampling the Brillouin zone were 1 × 1 × 1 and 2 × 2 × 1 for 3×2×1 and 2×1×2 supercell, respectively. Since the GIPAW calculation is more sensitive to the energy cutoff and MP grid, the convergence was checked by calculation with higher energy cutoff and more mesh points. The results showed the same chemical shifts (within 3 digits at the least), therefore the calculation of chemical shifts converged well under the above conditions. The reference isotropic shielding for

133

Cs was adjusted by values estimated from GIPAW calculation of

referenced crystal compounds. Here, the chosen reference crystalline systems were CsCl, CsF, and Cs2 SO2 shown in Fig. S2 of Supporting Information. Linear regressions led to the following relationships: gipaw δiso = −0.9226(δiso + 5415.98),

(1)

gipaw where δiso is the chemical shift from GIPAW calculations.

Results XRD Patterns XRD patterns ranging from 2 to 10° obtained under controlled RH for Na-mont, Cs36-mont, Cs59-mont, Cs70-mont, and Cs100-mont are shown in Figure S3 of Supporting Information. All patterns showed the (001) peak corresponding to the d-spacing. Using the position of this peak at maximum intensity, the d-spacing was calculated as a function of relative humidity (Fig. 1). The d-spacing of Na-mont showed well-known step-like expansion with increasing relative humidity according to the 1-, 2-, and 3-hydrated states. 44,52–54 On the other hand, a slight increase of d-spacing in the range of 6 to 18% RH and a plateau at above 20% RH were observed for Cs79-mont and Cs100-mont. After uptaking a small amount of water at

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low RH, the d-spacings of Cs79-mont and Cs100-mont were almost constant (12.0–12.5 Å) against further humidity increase. In these cases, the ions keep the TOT sheets apart, so that water can enter the interlayer space without any lattice expansion. 54 The results of Na and Cs-montmorillonite obtained in the present work agreed quite well with those of previous studies. 52,55 The d-spacing of partially Cs substituted montmorillonite (Cs36-mont and Cs-59) was newly obtained, showing intermediate behavior between Namont and Cs100-mont. It should be noted that the plotted d-spacing data are representative values calculated from the peak top. As shown in Fig. S3 of Support Information, the shape of the (001) peak for Cs36-mont and Cs59-mont imply the coexistence of several d-spacings, which is different from the cases of Na-mont and Cs100-mont. It seems that Cs+ and Na+ in the interlayer compete for water to form hydration shells. Since hydrated Na and Cs have different sizes, their co-existence in the interlayer space will cause the clay sheets to be non-flat (i.e., multiple d-spacings), in contrast to samples containing a single type of cations. Therefore, the d-spacing curves of montmorillonite with different Cs exchange ratios are associated with geometrically heterogeneous interlayer space. However, it would be difficult to discuss the local structure of Cs based on only XRD experiments. Detailed analysis of Cs adsorption structure will be instead examined using a combination of experimental 133 Cs NMR spectra and GIPAW calculations.

133

Cs MAS NMR

The

133

Cs MAS NMR spectra of dehydrated Cs100-mont, Cs79-mont, Cs59-mont, Cs36-

mont, and Cs16-mont samples are shown in Fig. 2. The line shape of

133

Cs NMR spectra

clearly depends on the Cs exchange fraction. The spectrum for Cs16-mont has only a broad peak in the range of 100 to −10 ppm (peak-A), while this peak is broader for Cs36-mont. On the other hand,

133

Cs MAS NMR spectra for Cs59-mont, Cs70-mont, and Cs-100 indicated

an additional shoulder and a narrow peak in two ranges: −30 to −70 ppm (peak-B) and the range −100 to −140 ppm (peak-C). There were previous reports of two distinct peaks in 10


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133

Cs MAS NMR spectra of montmorillonite dried at 450℃ under 11.7 T. 33,34 Weiss et al.

tentatively assigned the peaks at lower and higher fields to the more shielded Cs sites with coordination numbers of 12 or less (possibly 9). 33 Although such peak assignments related to coordination number was reasonable, the local Cs environments on the clay sheets were not clear. To the best of our knowledge, our result is the first observation of three distinct peaks in 133 Cs MAS spectra of dehydrated clays. This is due to the benefit of NMR measurements in a relatively higher field (18.8 T). Detailed assignments of these peaks and explanation for the spectral change with different Cs exchange fractions are discussed together with theoretical calculations of 133

133

Cs NMR chemical shifts.

Cs MAS NMR spectra of Cs59-mont and Cs100-mont prepared with different RH are

shown in Figs. 3 and 4. Gray regions for peak-A, -B and -C in Fig. 2 are reproduced here as a guide for comparison. The single peak at −30 ppm observed in the sample prepared at the highest RH (85.1%) is attributed to the averaging of motion at the possible Cs sites, indicating that these Cs sites under dehydrated conditions have the same chemical environments within the NMR timescale. Two possibilities of the mechanism for the averaging of motion are considered. One is the chemical exchange among the possible Cs sites, and the other one is equivalent hydration strcutre on each Cs sites without the chemical exchange. These possibilities are examined in the following discussion section. A specific difference in spectral shape was observed in Cs59-mont and Cs100-mont compared to the samples with lower RH. The motion averaging is not apparent for Cs59-mont with 6.4% RH, since two distinct peaks were observed there instead. The peak corresponding to peak-A for Cs59mont decreases with increasing RH. The main peaks for Cs59-mont and Cs100-mont under 85.1% RH seem to have identical line shapes, meaning that these Cs are present in the same hydrated state. It should be noted that the peak corresponding to peak-A for Cs59-mont persisted slightly even at 85.1% RH. The result suggested that both non-hydrated and hydrated Cs were present in the interlayer. This may be due to the preferential hydration of Na+ , which prevents the hydration of all Cs+ despite the higher RH.

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Fig. 5 shows the

133

Cs MAS NMR spectra of Cs-synmont with different humidity. The

gray regions are the same as Figs. 2-4 for comparison. The trend of spectral change with increasing humidity is similar to natural montmorillonites: two peak observed in the dehydrated condition changed to one peak with increasing RH due to the Cs hydration. The intensity of peak-C for dehydrated Cs-synmont was relatively higher than that of peak-A, which is different from natural montmorillonite. Natural and synthetic montmorillonites differ in particle size and chemical composition, as shown in Table 1. The amount of Si substitution with Al in the tetrahedral sheet is smaller in synthetic montmorillonite. In addition, the particle size of synthetic montmorillonite is relatively smaller according to the external surface area (excluding that of interlayer space) estimated from N2 BET experiments, being 7.08 and 87.1 m2 /g for natural and synthetic montmorillonites, respectively. 43,56 It is expected that these chemical and geometrical differences of clay sheets affect Cs distribution at the adsorption sites in the dehydrated condition. Detailed discussion is given after peak assignment based on the calculated

133

Cs chemical shifts.

The effect of paramagnetic ions on 133 Cs MAS NMR spectra is also considered by comparing natural and synthetic montmorillonites. Generally, strong interactions with the electron spin of paramagnetic ions such as Fe3+ cause line broadening. Since no significant narrowing of the peaks was observed in the

133

Cs MAS NMR spectrum for Cs-synmont, a major

contribution to the peak width is the structural distribution of Cs adsorption sites.

GIPAW DFT Calculation of

133

Cs

The final structure of 3×2×1 supercell after the geometry optimization with variable lattice constants is displayed in Fig. 6. Three illustrations show the ac plane (left) with unit-cell boundary (gray lines), only the tetrahedral interlayers with Cs atoms on upper (center) and lower (right) are shown. Labels on the Cs atom correspond to labels in Table 3. The lattice constants and energy are summarized in Table 2. The obtained lattice constant c (11.30 Å) is smaller than experimental one (11.71 Å) for Cs100-mont with 6.1% RH, and it agrees with 12


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the reported experimental data (in the range of 10.7–11.5Å). 55 As shown in the center and right illustrations of Fig. 6, all Cs atoms are located around the center of hexagonal cavities formed by SiO4 and AlO4 on the upper or lower tetrahedral sheets. The distances between Cs and basal oxygen planes, calculated from the average basal oxygen positions on the upper and lower tetrahedral layers, were listed in Table 3. Cs1, Cs2, and Cs3 are close to the upper tetrahedral sheets, and Cs4, Cs5, and Cs6 are positioned near the center of the interlayer space. The center positions of hexagonal rings are categorized into two types (site-A and site-B). For site-A, the hexagonal ring includes tetrahedral Al (AlT ) substitution, while that for site-B does not include AlT . All site-A are filled by Cs1, Cs2, and Cs3, whose positions are closer to the tetrahedral layers compared with those of Cs4, Cs5, and Cs6. It is expected that the AlT can attract positively charged Cs by Coulombic interaction. All site-A are preferentially filled with Cs, and the distance between the tetrahedral layer and Cs is shorter than that for site-B. For the 3 × 2 × 1 supercell, the calculated δiso of

133

Cs and distance between Cs and

substituted AlT on tetrahedral sheets are summarized in Table 3. These two parameters are strongly correlated. A 20–40 ppm lower shift was observed for site-A (Cs1, Cs2, and Cs3) compared to site-B (Cs4, Cs5, and Cs6). The electron cloud of

133

Cs is drawn towards the

AlT , leading to less shielding on Cs on site-A and therefore a lower shift compared to that on site-B. The distribution of δiso for site-A and site-B was approximately 10 ppm, which came from slight structural discrepancy among each type of adsorption sites. Therefore, we cannot explain the experimentally observed wide distribution of chemical shift for site-A and site-B by the AlT position alone. Next, we discuss the effects of stacking structures in order to understand the origin of the

133

Cs chemical shift, by using supercell structures with two

TOT layers (the 2×1×2 supercell). Fig. 7 illustrates the optimized structure for 2×1×2 supercell with three stacking structures: monoclinic, parallel, and alternated stacking. Slice selections with red and blue dotted lines of the ab plane are also displayed for each illustration. Upper interlayers (surrounded

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by red dotted line) show the targeted stacking structure, i.e. hexagonal cavities on upper and lower tetrahedral layers are stacked to be monoclinic, parallel, and alternated. Lower interlayers (surrounded by blue dotted line) are slightly off from the perfect parallel and alternated stacking, due to asymmetry of the TOT layers for the ab plane. The optimized lattice constants and energy for the three stacking structures are shown in Table 4. Here, two d-spacings (dupper and dlower ) were individually calculated from the average basal oxygen positions on the upper and lower tetrahedral layers. The lattice constant c of supercell for three stacking structures can be ordered as parallel < monoclinic < alternated. It is clear that the structure with two hexagonal cavities sandwiching the Cs changes the lattice constant c. As described in the results of the 3×2×1 supercell, all Cs are located around the center of hexagonal cavities regardless of the stacking structure. The alternated stacking had the maximum c because of the steric hindrance of a basal oxygen against an opposed sheet. On the other hand, Cs sandwiched by upper and lower hexagonal cavities (parallel stacking) shows the minimum d-spacing due to their weaker interaction with basal O. The obtained total energies are −8211.566, −8210.394, and −8210.326 Ry for the monoclinic, parallel, and alternated stacking structures, respectively. Although monoclinic stacking appears to be the most stable energetically, the difference in total energy among the three stacking structures is small (< 0.01 Ry/atoms). Therefore, it is not surprising that these stacking structures can coexist in the real clay. Such heterogeneity in sheet stacking structure brings about the broadening of (001) XRD peak and the 133

133

Cs MAS NMR spectral peaks.

Cs chemical shift calculated for the three optimized stacking structures are summarized

in Table 5. The trend regarding the distance to AlT is the same as results obtained from calculations of 3×2×1 supercell, that is, Cs sites around the center of hexagonal rings that include AlT are shifted to the lower field. The coordination numbers of Cs for neighbor basal oxygens on the upper and lower tetrahedral sheets are 12 and 9 for parallel and alternated stacking structures, respectively. Among the three stacking models, we have not seen any general trend of

133

Cs chemical shift against the coordination number. Therefore, the

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133

Cs

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chemical shift is sensitive to the Cs-AlT distance rather than to their coordination number. This does not support the tentative assignment by Weiss et al. 33 DFT and GIPAW calculations were also carried out for the structures with expanded and shrunken interlayer distance, which were built from the optimized monoclinic structure as described in the experimental section. Only the z-0.3, z+1.0, and z+6.0 structures are displayed in Fig. 8. For z+3.0 and z+6.0 structures, all Cs lie to either surface of upper or lower tetrahedral sheets. The structural parameters and energy are summarized in Table 6. Here, the two d-spacing values (dupper and dlower ) were individually calculated from the averaged basal oxygen positions of upper and lower tetrahedral layers, respectively. The energy change upon varying the interlayer distance is small (< 0.002 Ry/atoms), but the 133

Cs chemical shift strongly depends on the d-spacing as summarized in Table 7. The

higher field shift (up to approximately ∼ −160 ppm) is confirmed for the z+3.0 and z+6.0 structures. On the other hand, the shrunken structure z-0.3 showed lower field shift for all Cs sites.

Discussion First, the peak assignments of the

133

Cs MAS NMR spectra for dehydrated montmorillonite

(Fig. 2) are considered in basis of the GIPAW calculation results. The

133

Cs chemical

shift of 3×2×1 (one TOT layer) showed clear dependence on the distance between Cs and substituted AlT on the tetrahedral layers. Three Cs sites (Cs1, Cs2, and Cs3) in Fig. 6 are located around the center of the hexagonal cavities (site-A), indicating that the lower field shift of

133

Cs chemical shift is caused by the electron-withdrawing effect of positively

charged AlT . Peak-A was observed at the lowest field in Fig. 2, therefore it was assigned to adsorbed Cs on site-A. On the other hand, Cs4 and Cs5 in Fig. 6 are located around the center of hexagonal cavities without AlT (site-B), and they correspond to spectral peak-B on the higher field side. These assignments are consistent with Cs sites for the 2×1×2 supercell

15



Page 16 of 40

with three different stacking structures. The considerable broadening of peak-A (in a range from approximately 100 to −10 ppm) cannot be explained from the theoretical results for 3×2×1. The second-order quadrupole and paramagnetic broadening are negligibly small in these cases, as confirmed from the observed perfect nutation curve and comparison with Cssynmont without paramagnetic ions. Rather, the peak width is determined by a distribution of chemical shifts owing to the structural heterogeneity. Such heterogeneity can also explain the broadened (001) peak in the XRD patterns (Fig. S3 of Supporting Information). A possible origin of the structural heterogeneity is deviation from perfect monoclinic stacking. The change of 133 Cs chemical shift resulting from a variety of stacking structures is approximately 60 ppm (Table 5), and they also cause variation in the d-spacing. Theoretical 133

Cs chemical shifts for 2×1×2 with expanded and shrunken d-spacings help to understand

the distribution of experimental 133 Cs chemical shift. The 133 Cs chemical shifts as a function of lattice constant c showed increased shielding with increasing d-spacing. A good linear correlation of

133

Cs chemical shift in ppm to the d-spacing in Å was obtained for 5 of the

structures (z-0.3, z-opt, z+0.3, z+0.5, and z+1.0) (Fig. 9) as

δiso = −190.0 × (d-spacing) + 2093

(2)

On the other hand, the more expanded structures (z+3.0 and z+6.0) showed poor correlation in the linear fitting line. It is worth noting that the

133

Cs chemical shift range of linear

correlation is in good agreement with the experimentally observed range of peak-A in Fig. 2. The reason of the deviation from linear correlation with more expanded d-spacing (z+3.0 and z+6.0) is the absence of contribution from double basal oxygen. After optimization of Cs positions, Cs were located on the surface on either lower or higher tetrahedral sheets. As a result, electron withdrawing effect from basal oxygen on the single sheet is lower, and the increased shielding on Cs leads to higher field shift. As for the z+3.0 structure, the

133

Cs

chemical shift ranges from −100 to −150 ppm, which corresponds to the range of peak-C

16


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in Figs. 2-5. Therefore, the peak with the highest field shift is assigned to the Cs sites interacting with only one single sheet (compared to being sandwiched between two sheets). Previous studies using positronium lifetime spectroscopy also showed Cs adsorption on the surface of open nanospaces with the size of 0.3 and 0.9 nm, which were formed by inserting one and two clay nanosheets into the interlayer space. 57–59 The chemical state of Cs in the open nanospace is surface adsorption on one sheet or on edges, i.e. the Cs local structure is analogous to the non-sandwiched situations of z+3.0 and z+6.0. From these results, we assigned peak-C to the adsorbed Cs on the open nanopore surface (site-C). The populations of peak-A, -B, and -C for all dehydrated samples were estimated from peak deconvolution by three Gaussian functions. Deconvoluted spectra and these populations are shown in Fig. S4 of Supporting Information and Table 8. With a low amount of Cs substitution (Cs16-mont and Cs36-mont),

133

Cs MAS NMR spectra could be accounted for

by peak-A alone, which corresponds to site-A. It can be imagined that the preferential adsorption site at low Cs substitutions is the center of the hexagonal cavity including AlT . If all site-A are occupied by Cs, then site-B and site-C can be new adsorption sites for excess Cs. It seems that there is no preference in adsorption between site-B and site-C. A schematic mechanism for Cs adsorption in samples from low to perfect Cs substitution is displayed in Fig. 10. The population of site-A for Cs-synmont is smaller than that of Cs100-mont, which is reasonable for their chemical formula (Table 1). The reason is that AlT substitutions on the tetrahedral sheets are lower for Cs-synmont than for Cs100-mont. As a result, the population of Cs adsorbed on site-B can be the highest among all Cs adsorption sites. Motion averaging of the three peaks involving hydration was observed for all samples, indicating that Cs on site-A, -B, and -C are hydrated. Structural change of TOT stacking involving Cs hydration may affect

133

Cs chemical shift. However, NMR chemical shifts are

generally sensitive to the first coordination shell or Cs hydration structure. Therefore, we discuss two possible mechanisms, that is, chemical exchange among different Cs sites and

17



Page 18 of 40

equivalent hydration structure on each Cs site. Chemical exchange for peak-A, -B, and -C may be a possible reason for spectral change for hydrated samples. The NMR time scale (dwell time, ∆) is approximately 1.0 µs, and Cs self-diffusion coefficient under the hydrated state (D) was D ∼ 10−10 m2 /s, which was also reported from molecular dynamics simu√ lations. 19 Mean-diffusion distance (L) during dwell time can be obtained from L = D∆, which leads to L ∼ 10 nm. This distance is likely longer than the distance between two Cs adsorption sites in the interlayer. To the best of our knowledge, there is unfortunately no experimental data of the exchange rate between these Cs adsorption sites. In order to confirm the chemical exchange, we carried out

133

Cs MAS NMR experiments under lower tempera-

ture (173-306 K) for Cs100-mont prepared at room temperature and humidity (∼25℃ and ∼40%RH) (Fig. S5 of Supporting Information). Clear peak splitting was not observed in the 133 Cs MAS NMR spectra, indicating that chemical exchange for peak-A, -B, and -C may not exist. Therefore, it is claimed that no chemical exchange of hydrated Cs occurs, and Cs sites in dehydrated condition are changed to similar chemical environments within the NMR timescale by hydration. In other words, the 133 Cs NMR chemical shift is insensitive to hydrated Cs on site-A, -B, and C. However, a small peak-A was observed in the spectrum of Cs59-mont with RH-85.1% (Fig. 3). This result indicates that some Cs atoms on site-A are non-hydrated. As a result, these Cs cannot be removed even in a water-rich environment.

Conclusions 133

Cs MAS NMR spectra of partially substituted Cs-montmorillonite in dehydrated condition

show the existence of at least three different Cs adsorption sites. From a comparison of line shapes between natural and synthetic montmorillonite, the peak widths observed in MAS NMR spectra were caused by the distribution of

133

133

Cs

Cs chemical shift.

To assign these peaks, we carried out theoretical calculations of

133

Cs chemical shifts

using DFT-GIPAW methods. All Cs adsorption sites after optimization are around the

18


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center of hexagonal cavities formed by SiO4 and AlO4 , and Cs atoms are located at these cavities on either the upper or lower tetrahedral sheets. A clear trend of

133

Cs chemical

shifts with the Cs-AlT distance was observed. For Cs atoms with the distance of Cs-AlT within 4 Å located around the center of hexagonal cavities including Al substitution, their 133

Cs chemical shifts are at lower field compared to those not including Al substitution.

The theoretical calculation of

133

Cs chemical shift for three stacking variations (monoclinic,

parallel, and alternated) demonstrated a

133

Cs chemical shift range of approximately 60

ppm, which is a factor for the peak broadening in the experimental spectra. The theoretical 133

Cs chemical shift calculations were also performed for different d-spacings, and the results

showed a good linear correlation when the d-spacing difference was in the range of −0.3 to 1.0 Å from the optimized value.

133

Cs chemical shift with much expanded d-spacing cannot

be determined by this correlation, since these Cs are close to only one tetrahedral sheet. This situation is no longer different from the sandwiched structure by double tetrahedral sheets, leading to the highest field shift. The possible site for single-side adsorption is Cs being in the open nanospace. Based on the discussions, we assigned the experimentally observed three peaks as follows:

• adsorbed Cs around center of hexagonal cavity with AlT (100 to −10 ppm) • adsorbed Cs around center of hexagonal cavity without AlT (−30 to −70 ppm) • adsorbed Cs on the open nanopore surface (−100 to −140 ppm) At low Cs exchange fractions, the preferred adsorption site is the center of the hexagonal cavities including Al substitution. For higher Cs exchange fractions, the other two locations (i.e., center of the hexagonal cavities that do not include Al and the surface on the open nanospace) can be new adsorption sites. The samples prepared under controlled relative humidity showed a narrow peak resulting from Cs hydration. This means that the Cs hydration structure was identical for the resolved 19



Page 20 of 40

three sites due to motion averaging. A peak corresponding to Cs around the center of the hexagonal cavities including Al substitution persisted, even when the sample was prepared under 85.1 % RH. These Cs atoms are fixed on the surface even in a water-rich environment.

Supporting Information Nutation curves of 133 Cs NMR experiments for the dehydrated and hydrated Cs-montmorillonite (Figure S1), GIPAW NMR parameters for the reference crystalline compounds (Figure S2 and Table S1), XRD patterns of Cs-montmorillonite samples under controlled relative humidity (Figure S3), Deconvolution of samples (Figure S4), and

133

133

Cs NMR spectra for all dehydrated Cs-montmorillonite

Cs NMR spectra of hydrated Cs-montmorillonite under lower

temperature (173-306 K) (Figure S5) (PDF)

Acknowledgments This study was partly conducted as “The project for validating assessment methodology in geological disposal system” funded by the Ministry of Economy, Trade and Industry of Japan. A part of theoretical calculations were performed in the computer facilities at the Research Institute for Information Technology, Kyushu University.

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Table 1: Chemical formula of natural and synthetic montmorillonites used as the starting materials in this study. samples

Natural Synthetic

chemical formula 2+ (Na0.64 K0.01 H0.04 Mg0.01 Ca0.07 )(Al3.35 Mg0.5 Fe3+ 0.09 Fe0.06 )(Si7.61 Al0.39 )O20 (OH)4 (Na0.68 Mg0.01 K0.01 H0.04 )(Al3.35 Mg0.65 )(Si7.9 Al0.1 )O20 (OH)4

Table 2: Optimized lattice parameters and energy of 3×2×1 supercell. monoclinic

a/Å b/Å c/Å α β γ 15.7991 18.1524 11.30 90.00 99.86 90.00

Energy/Ry −11702.850

Table 3: 133 Cs chemical shift in ppm and Cs-AlT distance in Å of Cs sites assigned in Fig. 6. The distances between Cs and lower (Ol ) and upper (Ou ) basal oxgen planes are also listed in Å. δiso Cs-AlT Cs-Ol Cs-Ou

Cs1 9.54 4.00 2.40 2.18

Cs2 7.24 4.10 2.41 2.17

Cs3 Cs4 Cs5 Cs6 −0.702 −17.14 −11.04 −17.53 4.04 8.34 6.90 8.74 2.38 2.26 2.30 2.31 2.21 2.32 2.28 2.27

28


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Table 4: Optimized lattice parameters (a, b, c, α, β, γ) and energy of 2×1×2 supercell with different stacking structures (monoclinic, parallel, and alternated) as shown in Fig. 7. dlower and dupper indicate d-spacing of lower and upper layers, respectively. monoclinic parallel alternated

a/Å b/Å c/Å α β γ dlower 10.538 9.094 22.429 90.00 99.89 90.00 11.03 10.414 9.368 22.273 90.00 90.00 90.00 11.17 10.411 9.366 23.594 90.00 90.00 90.00 11.80

dupper 11.01 11.11 11.79

Energy/Ry −8211.566 −8210.394 −8210.326

Table 5: 133 Cs chemical shift in ppm of Cs sites in 2×1×2 supercell with different stacking structures (monoclinic, parallel, and alternated) as shown in Fig. 7. The Cs-AlT distance in Å is also listed. monoclinic parallel alternated

δiso Cs-AlT δiso Cs-AlT δiso Cs-AlT

Cs1 3.68 3.99 −31.0 6.36 −4.90 6.45

Cs2 4.11 3.91 13.0 3.92 14.5 3.81

Cs3 6.28 3.99 8.61 3.90 4.67 4.15

Cs4 Cs5 −40.9 −25.2 9.39 8.34 −39.9 −38.5 9.78 8.48 −73.1 −46.9 10.5 8.34

Table 6: Optimized lattice parameters and energy of 2 × 1 × 2 supercell with different interlayer spacings. All optimizations were performed under Si fixation. The label (zX) means the initial c values, where X corresponds to difference in Å from optimized c. Optimized structure (z-opt) without Si fixation is also listed as a reference. d-Spacing of upper and lower layers were individually calculated from basal oxygen positions (dupper and dlower ). z-0.3 z-opt z+0.3 z+0.5 z+1.0 z+3.0 z+6.0

a/Å 10.538 10.538 10.538 10.538 10.538 10.538 10.538

b/Å 9.094 9.094 9.094 9.094 9.094 9.094 9.094

c/Å 22.129 22.429 22.729 22.273 23.429 25.429 28.429

α 90.00 90.00 90.00 90.00 90.00 90.00 90.00

β 99.89 99.89 99.89 99.89 99.89 99.89 99.89

29

γ 90.00 90.00 90.00 90.00 90.00 90.00 90.00


dlower 10.95 11.03 11.26 11.36 11.61 12.61 14.11

dupper 10.85 11.01 11.14 11.23 11.47 12.44 13.90

Energy/Ry −8211.520 −8211.566 −8211.523 −8211.521 −8211.507 −8211.417 −8211.297


Page 30 of 40

Table 7: 133 Cs chemical shift in ppm of five Cs in 2×1×2 supercell with different interlayer spacings as shown in Fig. 8. The label (zX) means initial c value, where X corresponds to difference in Å from optimized c. The values for the optimized structure (z-opt) without Si fixation is also listed as a reference. z-0.3 z-opt z+0.3 z+0.5 z+1.0 z+3.0 z+6.0

Cs1 Cs2 Cs3 Cs4 Cs5 4.40 9.83 37.2 52.2 43.6 3.68 4.11 6.28 −40.9 −25.2 −58.4 −54.3 −25.8 −12.7 −17.3 −76.9 −73.5 −44.3 −32.1 −35.1 −117 −117 −82.9 −75.3 −72.4 −156 −156 −113 −128 −102 −138 −149 −44.4 −50.2 −34.3

Table 8: Population (%) of each Cs component obtained from deconvolution of 133 Cs MAS NMR spectra for dehydrated Cs16-mont, Cs36-mont, Cs59-mont, Cs79-mont, and Cs-mont. Peak-A, -B, and -C correspond to peaks in Fig. 2. Deconvoluted spectra are shown in Fig. S4 of Supporting Information. Cs16-mont Cs36-mont Cs59-mont Cs79-mont Cs-mont Cs-synmont

peak-A 100 100 96.2 94.0 71.7 35.5

30

peak-B

peak-C

1.1 3.7 22.4 56.2

2.6 2.4 5.9 8.4


Page 31 of 40

16

Na-mont Cs36-mont Cs59-mont Cs79-mont Cs100-mont

15

14

d-spacing/Å

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


13

12

11

10

9

0

20

40

60

80

100

Relative humidity/% Figure 1: d-Spacing calculated from (001) peak of XRD patterns for Na montmorillonite (Na-mont) and 36, 59, 79, and 100% Cs exchanged montmorillonite (Cs36-mont, Cs59-mont, Cs70-mont, and Cs100-mont) as a function of relative humidity.

31



Page 32 of 40

peak-A

peak-B peak-C Cs100-mont

Cs79-mont

Cs59-mont

Cs36-mont

Cs16-mont

Figure 2: 133 Cs MAS NMR spectra of dehydrated montmorillonite with 36, 59, 79 and 100% Cs exchange. The symbol * denotes the spinning side bands.

32


Page 33 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


peak-B

peak-A RH-85.1%

RH-75.3%

RH-57.6%

RH-42.3%

RH-32.8%

RH-11.3%

RH-6.4%

peak-C dehydrated

Figure 3: 133 Cs MAS NMR spectra of 59% Cs exchanged montmorillonite (Cs59-mont) with different humidity. The symbol * denotes the spinning side bands.

33



Page 34 of 40

peak-B

peak-A RH-85.1%

RH-75.3%

RH-57.6%

RH-42.3%

RH-32.8%

RH-6.4%

peak-C dehydrated

Figure 4: 133 Cs MAS NMR spectra of 100% Cs exchanged montmorillonite (Cs100-mont) with different humidity. The symbol * denotes the spinning side bands.

34


Page 35 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


peak-B

peak-A RH-85.1%

RH-75.3%

RH-57.6%

RH-42.3%

RH-32.8%

RH-11.3%

peak-C RH-6.4%

dehydrated

Figure 5: 133 Cs MAS NMR spectra of synthetic montmorillonite (Cs-synmont) with different humidity.

35



Page 36 of 40

Al(IV) (backside) Cs5 (backside) Cs3

Cs1 Cs1 Cs2 Cs3

Cs4

Cs6

Cs2

Cs1

Cs4

Cs3

Cs2

Cs5

Cs4

Cs5

c Cs6

b

a

(A)

a

(B)

Cs6

b a

(C)

Figure 6: Illustration of optimized structure of a Cs-montmorillonite (3 × 2 × 1 supercell). The rectangle in gray lines shows the unit cell. Only Cs and the upper and lower tetrahedral layers are shown in (B) and (C). Mg, Si, Al, O and H are represented by ocher, blue, silver, red, cyan and white balls, respectively. Cs are labeled to match Table 3.

36


Page 37 of 40

Cs5 Cs4 Cs5

dupper

Cs4 b a

(B)

Cs1 Cs2 Cs3

Cs3

dlower

Cs1

Cs2 b c

(A)

a

(C)

a

monoclinic stacking

Cs4

Cs5

Cs5

b

Cs4 a

Cs5 (B)

Cs1 Cs2 Cs3

Cs4

dupper

dupper

Cs4

b

Cs2

(A)

Cs3 b

a

a

Cs2 Cs1

b

c

(B)

Cs3

dlower

Cs1 Cs2 Cs1

Cs5 a

Cs3

dlower

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(C)

c

(A) a

parallel stacking

a

(C)

alternated stacking

Figure 7: Illustration of optimized structure of a Cs-montmorillonite (2×1×2 supercell) with three stacking structures. Only Cs and tetrahedral layers are shown in (B) and (C) for easy visualization. The color scheme is the same as Fig. 6. Labeled numbers on Cs correspond to items in Table 5.

37


Cs4

Cs5

Cs5

Cs4

Cs5

Cs1 Cs2 Cs3

dlower

Cs1 Cs2 Cs3

c a

z-0.3

z+1.0

dlower

dupper

dupper

Cs4

dlower

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 40

dupper


Cs1 Cs2 Cs3

z+6.0

Figure 8: Illustration of optimized monoclinic structure of a Cs-montmorillonite 2 × 1 × 2 supercell with different interlayer spaces (zX). The structure and cell parameters were optimized under fixed Si positions. X means the difference in Å from optimized c value without Si fixation. The ball colors are the same as Fig. 6. Labeled numbers on Cs correspond to items in Table 7.

38


Page 39 of 40

.3,

z-0 z-o pt, z+6.0

. z+0 +1. 5, z

+0.

3, z z+3.0

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 9: 133 Cs chemical shift as a function of d-spacing obtained from different values of the lattice constant c. The label (zX) indicates the c values, where X corresponds to difference in Å from the optimized c value. Sold line shows linear regression of data (red circles).

site-A site-B

site-A

site-B

site-C

Figure 10: Schematic illustration of Cs adsorption with increasing Cs substitution. Cyan bars correspond to Al positions in tetrahedral sheets. Open nanospace was formed by onenanosheet insertion into interlayer spaces (left). Red balls on the sheet represent Na, while other atoms are coded in the same colors as in Figs. 6 and 7 (right). Cs at site-A, -B, and -C are outlined in black, red, and yellow, respectively. 39



A Graphic for Table of Contents site-A

, opt

z.3, z-0 site-A

z+0

site-B

site-C

.3, .5,

z+0 . z+1 0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 40

z+3.0

40


z+6.0

site-B

New Insights into the Cs Adsorption on Montmorillonite Clay from

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