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X-Ray Absorption Fine Structure at the Cesium L Absorption Edge for Cesium Sorbed in Clay Minerals Mitsunori Honda, Iwao Shimoyama, Yoshihiro Okamoto, Yuji Baba, Shinichi Suzuki, and Tsuyoshi Yaita J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b12378 • Publication Date (Web): 22 Feb 2016 Downloaded from http://pubs.acs.org on March 6, 2016

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The Journal of Physical Chemistry

X-Ray Absorption Fine Structure at the Cesium L3 Absorption Edge for Cesium Sorbed in Clay Minerals

Mitsunori Honda*,†, Iwao Shimoyama†, Yoshihiro Okamoto†, Yuji Baba†, Shinichi Suzuki†§, and Tsuyoshi Yaita†§ †

Quantum Beam Science Center (Qubs), Japan Atomic Energy Agency (JAEA), 2-4,

Shirane, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan. §

Fukushima Environmental Safety Center, Japan Atomic Energy Agency (JAEA), 6-6

Sakae-machi, Fukushima-shi, Fukushima 960-8031, Japan *Corresponding Author: Email: [email protected] Telephone number:+81-29-284-3928 Fax number:+81-29-284-3747

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Abstract We present the use of near-edge X-ray absorption fine structure (NEXAFS) to investigate local electronic structures of cesium ions sorbed in two types of clay minerals (vermiculite and kaolinite) with a different capacity to fix Cs. NEXAFS is element specific because X-ray absorption edges of different elements have different energies. However, the energy of the Cs L3 absorption edge is close to that of the K-edge of titanium generally contained in clay minerals. Therefore, Cs L3-edge NEXAFS measurements of Cs in clay minerals have not yet succeeded. In this study, we confirmed the peak intensity between vermiculite and kaolinite in the Cs L3-edge NEXAFS spectra by separately monitoring T Kα and Cs Lα fluorescence X-rays. To clarify the identification of NEXAFS spectra, theoretical calculations were performed using the discrete variational Xα molecular orbital method (DV-Xα), and peak identification was achieved. The difference in peak intensity was explained by the difference in the electron density of unoccupied molecular orbitals. We studied the influence of water molecules and found a change in the electron densities of unoccupied molecular orbitals caused by the coordination of water molecules.

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Introduction Contamination of soil with radioactive materials released in the Fukushima Daiichi Nuclear Power Plant accident caused by the Great East Japan Earthquake on March 11, 2011 is still a significant problem. Contamination of radioactive materials also occurred from the Chernobyl Nuclear Power Plant accident in 1989. Research results from after these accidents revealed that radioactive cesium released into the atmosphere diffused into the soil over time. Reports showed that radioactive cesium in soil is fixed in particular clay minerals1,2,3,4. Many researchers have suggested, mainly based on laboratory experiments, that micaceous minerals are important for sorption and retention of Ce in the ground or soil. Clay mineral is a silicate compound having a layered structure that can absorb water molecules or various ions in its interlayers. Radioactive cesium is selectively associated with the frayed-edge sites located at the layer edges of clay mineral. Among these ions, those with large hydration energies such as Ca2+, Mg2+, and Sr2+ can be easily removed through ion exchange; however, those with small hydration energies such as NH4+, K+, Rb+, and Cs+ are strongly fixed in the interlayers,1,5. The ability to fix ions is highly dependent on the type of clay minerals6. Although both vermiculite and montmorillonite are swelling clay minerals with a 2:1 layer structure, radioactive cesium is more strongly trapped in vermiculite than in montmorillonite7. This difference is thought to be due to the difference in the charge of the layers in the clay minerals. Vermiculite, having a large layer charge, is considered to have a high ability to fix Cs. According to a recent soil survey in the central region of Fukushima Prefecture, Japan, most radioactive cesium present in the soil is within 5 cm of the surface, mostly sorbed in vermiculite7.

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To reduce the volume of radioactive waste, various methods have been developed for the decontamination of radioactive cesium from soil. However, it is difficult to desorb radioactive cesium in soil when it is strongly bound in vermiculite, the main soil component in Fukushima Prefecture. Therefore, it is important to elucidate the adsorption structure of cesium in clay minerals. Clay mineral is an environmental sample; therefore, it consists of multiple types of elements. To date, various spectroscopic methods have been applied to the analysis of the adsorption structure of cesium in clay minerals8,9,10,11,12. However, the precise structure has not yet been elucidated because the structure of such a multi-element material is complex. X-ray absorption fine structure (XAFS) is a powerful tool for understanding the structure of a material. XAFS has the advantage to determine the local structure around the specific element, even in a multi-element material. XAFS can be expanded to two classifications: extended X-ray absorption fine structure (EXAFS) and near-edge X-ray absorption fine structure (NEXAFS). The information obtained using these two methods differ. In EXAFS analysis, information about the coordination environment is obtained from the vibrational structure of the high-energy regions resulting from scattering of excited electrons from the surrounding atoms and some investigations was reported5,13. Although EXAFS analysis provides coordination number and bond length, this information do not explain why Cs is strongly fixed in a specific clay mineral, e.g. vermiculite which would be involved in chemical states of Cs. In addition, a small irregular feature caused by a multi-electron excitation influences the EXAFS region since multi-electron features related to a specific primary transition are observed at higher photon energy than the NEXAFS region.

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Alternatively, information about the orbital hybridization can be obtained from NEXAFS analysis because NEXAFS spectra reflect the electronic states of unoccupied orbitals. Therefore, when we want to determine the electronic structure around cesium atoms in clay minerals, NEXAFS is an advantageous method in comparison with EXAFS. NEXAFS measurements at the Cs L3 absorption edge (approximately 5.01 keV) are more suitable for the elucidation of the electronic structure of Cs sorbed in clay minerals than those at the Cs K-edge (～35.98 keV) because of the higher energy resolution in the L-edge region. Cs L-edge NEXAFS has several advantages over K-edge NEXAFS. Firstly, since the electron configuration of cesium atom is (Xe)6s1, the outermost electron in cesium ion is considered to be in Cs 5d orbital. In order to clarify the electronic structure of Cs 5d orbitals, Cs L-edge (2p) NEXAFS is superior to Cs K-edge (1s) NEXAFS considering the dipole selection rule. Secondly, the energy resolution of X-rays in Cs L-edge region is fairly better than that in Cs K-edge region. However, Cs L3-edge NEXAFS for clay minerals is difficult because the energy of the K absorption edge of Ti (4.97 keV), which is always present as a trace element, is close to the Cs L3 absorption edge. In this research, we employed Cs L3 edge NEXAFS to determine the local electronic structures of Cs sorbed in two types of clay minerals: vermiculite and kaolinite. These two types of clay minerals were chosen because they have different structures and Cs-fixing ability. Vermiculite is a 2:1 phyllosilicate structure, with 2 tetrahedral silica sheets for every one octahedral alumina sheet and a high Cs-fixing ability. Kaolinite has a 1:1 type structure with a low Cs-fixing ability1. Furthermore, vermiculite have both internal and external site on the other hand, kaolinite has external surface only. We succeeded in measuring Cs L3-edge NEXAFS spectra of these clay minerals using 5



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the fluoresce-XAFS system14 that was recently developed. As a demonstration of this method, we present NEXAFS spectral changes that are dependent on the clay minerals and water environment. Experimental The vermiculite samples were produced in Ono-machi, Fukushima Prefecture, Japan. The sample had a particle size of 2 µm or less and was a mixed crystal of weathered biotite that adsorbs Cs well. The detailed structure of this sample was reported by Motokawa et al.15. Kaolinite was purchased from Geo-Science Materials Nichika Co., Ltd. (Kyoto, Japan). Reagent-grade CsCl was purchased from Wako Pure Chemical Industries, Co., Ltd. (Osaka, Japan) and used without further purification. The water used in this study was deionized with a Milli-Q purification system (Merck Millipore, Billerica, MA). The Cs-sorbed samples were prepared as follows: First, clay minerals were dispersed into a 1.0 × 10−1 mol L−1 CsCl solution for 24 h while slowly stirring. Then, the solution was centrifuged to separate solid and liquid, and the supernatant was discarded. Next, the 1.0 × 10−1 mol L−1 CsCl solution was added again, and the above procedure was repeated five times. After that, the remaining clay mineral was washed with distilled water and negligible CsCl was confirmed by adding a silver nitrate solution and verifying that the suspension was free of white turbidity. The sample was then dried by a freeze-drying machine. All experiments were conducted at BL-27A of the soft X-ray synchrotron radiation beam line of Photon Factory, KEK. In recent years, we have developed a fluorescence XAFS measurement system on this beam line14. The sample was sandwiched in two polypropylene foils with a thickness of 6 µm. Using a thin polypropylene film of 6 µm, 6


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it is possible to prevent the attenuation of the X-rays in the measurement range. NEXAFS was measured by fluorescence yield. A silicon drift detector (SDD) (Amptek Co., Ltd, United States of America. FAST SDD) was used as a detector for the fluorescent X-rays. The resolution of the detector is 125 eV FWHM at 5.9 keV. Furthermore, the sample and the detector were set at special location. At this location, the intensity of diffuse scattering X-rays is weak. In addition, elastic scattering from the sample is controlled in case of this experimental geometry. A multi-channel analyzer (MCA) was used for multi-element analysis. For NEXAFS measurements, X-rays were scanned at 0.5 eV steps from 4980 eV to 5150 eV. At this time, SDD was used in the multi-channel scalar (MCS) mode. By plotting the fluorescent X-ray intensity from the sample for each incident X-ray, a NEXAFS spectrum was obtained. In this case, we set a region of interest (ROI) at a respective fluorescence peak to detect only the characteristic X-rays from the element of interest. The energy of the X-rays was calibrated by a Ti K-edge pre-edge peak at 4970 eV of a single crystal of TiO2 (anatase)16. Result and Discussion Figure 1 shows the X-ray fluorescence analysis results for Cs-sorbed vermiculite measured in the MCA mode. Figure 1(a) shows the fluorescence X-ray spectrum of the Cs-sorbed vermiculite excited by X-rays from 4990 eV to 5020 eV. A large number of chemical elements have been easily identified in the energy range between 1.8 keV and 5 keV. Figure 1(b) shows the same spectra as Figure 1(a) in an enlarged energy scale around the fluorescence of Cs, Ti, and an elastic scattering peak. It is noted that Cs Lα and Ti Kα can be clearly separated.

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From the above results, Cs Lα and Ti Kα peaks were found to be completely separable by setting ROI from 4230 to 4400 eV. This confirmed that we can obtain a pure Cs L3-edge NEXAFS spectrum, without the contribution of titanium, by setting ROI at the Cs Lα peak. Next, to confirm the energy of the Cs L3-edge, the fluorescence spectra around Cs Lα and Ti Kα regions were measured at an incident energy from 4990 eV to 5020 eV. The results are shown in Figure 1(c). Consequently, it was found that Cs Lα fluorescence X-rays are emitted by the excitation at an energy higher than 5012 eV; therefore, the energy of the Cs L3 absorption edge for cesium in vermiculite was determined to be 5012 eV. The NEXAFS spectra at the Cs L3-edge for (a) Cs-sorbed vermiculite (black line) and (b) Cs-sorbed kaolinite (red line) are shown in Figure 2. The difference spectrum of the above two spectra is shown in the lower figure by the blue line. The black and red lines in Figure 2 were normalized at a high energy region around 5120 eV, where the specific chemical state of Cs should not affect the signal intensity. In the Cs-sorbed vermiculite sample, a single main peak (Figure 2, peak A) was observed at 5023.1 eV. The same main peak was observed around the peak A in the Cs-sorbed kaolinite sample. The difference spetrum shows that peak B for vermiculite (Figure 2) is stronger than that for kaolinite.

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(a) 5020 eV

Intensity [arb. units]

Elastic peak

K-Kα Ca-Kα Ar-Kα Si-Kα Al-Kα

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4500

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(c) 5020 eV 5019 eV 5018 eV


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5017 eV 5016 eV 5015 eV 5014 eV 5013 eV 5012 eV 5011 eV 5010 eV 5000 eV 4990 eV

4100 4200 4300 4400 4500 4600 4700 4800

Energy /eV

Figure 1. X-ray fluorescence spectra for CsCs-saturated vermiculite. (a) Spectra in wide energy region, (b) spectra in enlarged enlarged energy scale around the characteristic X-ray of CsCs- Lα, TiTi- Kα, and elastic peak, and (c) same as (b) but expanded for the analysis of threshold energy of Cs L3 absorption edge. edge.

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2.0

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Normalized Intensity


0.0

Difference spectrum -0.2 5000

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Photon energy / eV

Figure 2. Comparison of Cs L3-edge NEXAFS spectra for Cs in vermiculite (black line), Cs in kaolinite (red line), line), and difference spectra (blue line).

Next, to interpret the peaks in the NEXAFS spectra shown in Figure 2, theoretical calculations were performed using a discrete variational Xα molecular orbital method (DV-Xα). The DV-Xα method is a molecular orbital calculation based on density functional theory and can be applied to the calculation of electronic structures at a surface and interface. The advantage of the DV-Xα method is the possibility to obtain realistic molecular potentials and wave functions with relatively small basis set. This method is also applicable to molecules or clusters both for ground and excited states, which have no translational symmetry. This method can be also adapted to core hole

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spectroscopy like NEXAFS and it is effective for simulating an electronic transition process17. Figure 3 shows the results of a calculation using the DV-Xα method for a Cs adsorption model in vermiculite. The calculated model of Cs sorbed in vermiculite is shown in Figure 3(a). Figure 3(b) shows the partial density of states (PDOS) of Cs. DV-Xα is a method that can elucidate the atomic orbital components in a chemical bond. As shown in Figure 3(b), PDOS below 0 eV are mainly composed of an occupied Cs 5p orbital. Above 0 eV, PDOS are mainly composed of unoccupied Cs 5d, 6s, and 4f orbitals. Figure 3(c) shows the photoabsorption cross section (PACS). Considering the order of unoccupied orbitals shown in Figure 3(b), the main peak in Figure 3(c) is attributed to the transition from the Cs 2p3/2 to the Cs 5d orbitals.

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Figure 3. (a) Model structure of Cs absorbed in vermiculite. (b) PDOS at cesium sites of CsCs-sorbed vermiculite. (c) Theoretical NEXAFS spectra of Cs absorbed model in vermiculite.

From experimental and theoretical studies of the Cs L3-edge NEXAFS of Cs-sorbed vermiculite, we assume that peak A can be attributed to resonance excitation from the Cs 2p3/2 to the Cs 5d orbitals. Peak B can be attributed to resonance excitations to orbitals other than the 5d orbital. However, the origin of peak B is not clear because the energy of the calculated value is different from the experimentally observed energy. The difference spectrum in Figure 2 shows that peak B of vermiculite is enhanced in comparison with that of kaolinite. The identification of peak B is not clear. Therefore, in this report, we suggest that the intensity of the peak B represents the occupancy of electrons in unoccupied orbitals other than the 5d orbitals on the basis of the DV-Xα

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calculation. The results show that the electron density of the 5d states for vermiculite does not change much in comparison with that of kaolinite.


2.5

A

B

2.0 1.5 1.0 0.5

vermiculite vermiculite in water

0.0 0.6 0.4

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0.2 0.0 -0.2

Difference spectrum

-0.4

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Figure 4. Comparison of Cs L3-edge NEXAFS spectra for CsCs-sorbed orbed vermiculite (black line) and CsCs-sorbed vermiculite in water (red line) line) and difference spectra (blue line).

To investigate the influence of water molecules on the electronic structure of cesium, we also performed NEXAFS measurements for Cs-sorbed vermiculite immersed in water. Figure 4(a) shows the NEXAFS spectra for Cs-sorbed vermiculite with and 13



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without water. The difference in the above two spectra is shown in the lower figure by the blue line. The difference spetrum shows that peak A of vermiculite in water is stronger than that of vermiculite without water. The intensity of peak A represents the occupancy of electrons in the unoccupied Cs 5d orbital, on the basis of the DV-Xα calculation. This result shows that the occupancy of 5d states for vermiculite changes considerably in comparison with that without water. From these results, we succeeded in measuring the pure Cs L3-edge NEXAFS spectra by completely separating Ti Kα and Cs Lα fluorescence X-rays using a fluorescence method. We revealed, for the first time, the difference in the Cs adsorption state in clay minerals and how the presence of water influences the electronic state of Cs sorbed in vermiculite on the origin of the strong interaction between vermiculite and Cs ions on the basis of NEXAFS and calculation results in conjunction with the electron state. Conclusion NEXAFS was used to investigate the local electronic structures of cesium ions sorbed in two types of clay minerals (vermiculite and kaolinite) that have a different capacity to fix cesium. We succeeded in measuring pure Cs L3-edge NEXAFS spectra for cesium sorbed in clay minerals by completely separating Ti Kα and Cs Lα fluorescence X-rays through a fluorescence method. Accordingly, we confirmed the difference in peak intensities of vermiculite and kaolinite in the NEXAFS spectra. To clarify the identification in the NEXAFS spectra, theoretical calculations were performed using DV-Xα, and peak identification was achieved. In addition, the difference in peak intensity was explained by the difference in the electron densities of unoccupied molecular orbitals. Our results reveal, for the first time, that the influence of water molecules changes the electron densities of unoccupied molecular orbital. 14


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Specifically the Cs 5d states for vermiculite in water considerably changes in comparison with that without water. On the basis of the results for molecular orbital calculation, it was elucidated that the change is caused by the hybridization of O 2p states in water molecules with the Cs 5d states. ACKNOWLEDGMENT This work was performed under the JAEA project, “Cs sorption-desorption mechanism on clay minerals”, based on the special account for Fukushima environment recovery from the Ministry of Education, Culture, Sports, Science and Technology of Japan. The authors thank the staff of KEK-PF for their kind support in the experiments using synchrotron radiation. This work was also supported with the approval of KEK-PF (Proposal 2014G118 and No. 2014G088) and we are grateful to the (Grant in Aid for Scientific Research young person B 25790059).

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(4)

Ohnuki, T.; Kozai, N. Adsorption Behavior of Radioactive Cesium by Non-Mica Minerals. J. Nucl. Sci. Technol. 2013, 50, 369–375.

(5)

Bostick, B. C.; Vairavamurthy, M. A.; Karthikeyan, K. G.; Chorover, J. Cesium Adsorption on Clay Minerals: An EXAFS Spectroscopic Investigation. Environ. Sci. Technol. 2002, 36, 2670–2676.

(6)

Schulz, R. K.; Overstreet, R.; Barshar, I. On the Soil Chemistry of Cesium 137. Soil Sci. 1960, 89, 16–27.

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Tanaka, K.; Iwatani, H.; Takahashi, Y.; Sakaguchi, A.; Yoshimura, K.; Onda, Y. Investigation of Spatial Distribution of Radiocesium in a Paddy Field as a Potential Sink. PLoS One 2013, 8, e80794–e80797.

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Shimoyama, I.; Hirao, N.; Baba, Y.; Izumi, T.; Okamoto, Y.; Yaita, T.; Suzuki, S. Low-Pressure Sublimation Method for Cesium Decontamination of Clay Minerals. Clay Sci. 2014, 18, 71–77.

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Tsuji, T.; Matsumura, D.; Kobayashi, T.; Suzuki, S.; Yoshii, K.; Nishihata, Y.; Yaita, T. Local Structure around Cesium in Montmorillonite, Vermiculite and Zeolite under Wet Condition. Clay Sci. 2014, 97, 93–97.

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Matsumura, D.; Kobayashi, T.; Miyazaki, Y.; Okajima, Y.; Nishihata, Y.; Yaita, T. Real-Time-Resolved X-Ray Absorption Fine Structure Spectroscopy for Cesium Adsorption on Some Clay Minerals. Clay Sci. 2014, 105, 99–105.

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McKinley, J. P.; Zachara, J. M.; Heald, S. M.; Dohnalkova, A.; Newville, M. G.; Sutton, S. R. Microscale Distribution of Cesium Sorbed to Biotite and Muscovite. Environ. Sci. Technol. 2004, 38, 1017–1023.

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Fukushi, K.; Sakai, H.; Itono, T.; Tamura, A.; Arai, S. Desorption of Intrinsic Cesium from Smectite: Inhibitive Effects of Clay Particle Organization on Cesium Desorption. Environ. Sci. Technol. 2014, 48, 10743–10749.

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Fan, Q. H.; Tanaka, M.; Tanaka, K.; Sakaguchi, A.; Takahashi, Y. An EXAFS Study on the Effects of Natural Organic Matter and the Expandability of Clay Minerals on Cesium Adsorption and Mobility. Geochim. Cosmochim. Acta 2014, 135, 49–65.

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Motokawa, R.; Endo, H.; Yokoyama, S.; Ogawa, H.; Kobayashi, T.; Suzuki, S.; Yaita, T. Mesoscopic Structures of Vermiculite and Weathered Biotite Clays in Suspension with and without Cesium Ions. Langmuir 2014, 30, 15127–15134.

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Farges, F.; Brown, G. E.; Rehr, J. J. Coordination Chemistry of Ti(IV) in Silicate Glasses and Melts: I. XAFS Study of Titanium Coordination in Oxide Model Compounds. Geochim. Cosmochim. Acta 1996, 60, 3023–3038.

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Adachi, H.; Mukoyama, T.; Kawai, J. Gartree-Fock-Slater Method for Materials Science, the DV-Xα Method for Design and Characterization of Materials. Heidelberg. Springer. Berlin, 2005, 3-20 16


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(a) 5020 eV


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5017 eV 5016 eV 5015 eV 5014 eV 5013 eV 5012 eV 5011 eV 5010 eV 5000 eV 4990 eV

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Figure 1 M. Honda et al.

A

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The Journal of Physical Chemistry 5

(a)

Cs PDOS / (1/eV)

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Cs 5p Cs 6s Cs 4f Cs 5d

4

0

(c) PACS / Mb

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

Page 20 of 21

1.0

0.5

0.0 5040

5045

5050

5055

Energy /eV



Page 21 of 21


2.5

A

B

2.0 1.5 1.0 0.5

vermiculite vermiculite in water

0.0 0.6 0.4

Intensity

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


0.2 0.0 -0.2

Difference spectrum

-0.4

5000

5050

5100

Photon energy /eV



X-ray Absorption Fine Structure at the Cesium L3 ... - ACS Publications

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