Effects of γ radiation on microstructure of montmorillonite

Abstract

Bentonite will be exposed to γ radiation during geological disposal of high-level radioactive waste. Montmorillonite is the main mineral component of bentonite, and the study on how γ radiation affects the microstructure can help us comprehend how bentonite works and how it evolves. Montmorillonite extracted from GaoMiaoZi (GMZ) bentonite was irradiated by a ⁶⁰Co source (average energy is 1.25 MeV) with a dose rate of 2.88 kGy·h⁻¹ and absorbed dose of 1.0, 2.0, and 3.0 MGy. The samples were then tested by X-ray diffraction, synchronous thermal analysis, Mössbauer spectroscopy, infrared spectroscopy, Raman spectroscopy, and a Four-bar mass spectrometer. The results showed that: γ radiation reduced the d₀₀₁-value, and the average grain size decreased from 5.6 to 4.5 nm. The partial Si-O bonds of the tetrahedral structure, Al-O bonds of the octahedral structure, and hydroxyl (OH) groups in the montmorillonite structure were destroyed, and the mass loss of the hydroxyl (OH) groups decreased from 2.64 to 2.30%. The unit cell charge of montmorillonite rose as a result of H radicals produced by the radiolysis of water entering the crystal structure and converting Fe³⁺ to Fe²⁺. The content of Fe³⁺ decreased from 91 to 83%, while the content of Fe²⁺ increased from 9 to 17%. γ radiation damaged the partial microstructure of montmorillonite and induced the changes in the valence state of structural ion.

INTRODUCTION

The disposal of high-level radioactive waste (HLW) is one of the prominent issues in the development of nuclear energy and the utilization of nuclear technology today, and it is also the key and difficult issue in radioactive waste management. At present, geological disposal is acknowledged globally as the most promising engineering disposal plan. The geological disposal is designed based on the principle of multiple barriers (Victoria et al., 2006; Bennett and Gens, 2008), and the buffer materials, connected to containers and excavation disturbed zones or surrounding rocks, play a crucial role in blocking the migration of nuclides from waste form to the biosphere. Bentonite is recognized as the preferred buffer material, due to its long-term stability, good expansibility, extremely low permeability, and excellent adsorption (Liu and Chen, 2001; Liu and Wen, 2003; Wen, 2005).

Radiation, heat, hydrology, mechanics, and chemistry are some of the numerous elements influencing the near field of repository in the geological disposal of HLW (Zhang et al., 2008; Allard and Calas, 2009; Liu et al., 2011). Radiation is one of the inherent characteristics of waste form (Yang, 2012). Buffer materials are inevitably exposed to radiation because they are in close contact with waste container. The types of radiation and cumulatively absorbed doses received by bentonite vary with the scenario. In the normal evolutionary scenario, beta rays dominate the radioactivity for up to approximately 500 years after the closure of repository, followed by alpha rays 1000 years later (Ewing et al., 1995). The maximum absorbed gamma dose during the containment period (1000 years) prior to the container failure is calculated to be 0.7 MGy, with the longer-term cumulatively absorbed dose reaching several MGy (Plötze and Kahr, 2003). The effects of radiation on clay have been the subject of numerous investigations. Plötze and Kahr (2003) investigated bentonites from several U.S. locations and found that the cation exchange capacity of bentonite with an absorbed dose of 1.1 MGy remained basically unchanged. However, after being left for 22 months, the cation exchange capacity of the irradiated sample decreased by 10%. Pushkareva et al. (2002) found that when clay minerals such as montmorillonite, kaolinite, and palygorskite were irradiated by γ rays, their specific surface areas increased, and their solubility changed. The Al³⁺ ions decreased, but the Si⁴⁺ ions increased for montmorillonite and palygorskite. Pente et al. (2010) observed a weakening of CEC and distribution coefficient (Kd value) for ¹³⁷Cs in expansive clays after γ radiation. Holmboe et al. (2011) observed a decrease in Co²⁺ adsorption by MX80 bentonite with γ radiation, without a significant change for Cs⁺. According to the findings of the aforementioned scholars, bentonite’s macro properties, including its cation exchange capacity, specific surface area, solubility and nuclide adsorption capacity, were impacted by γ radiation. However, the effects of γ radiation on the microstructure and the interaction process between γ rays and bentonite had not been further investigated. The microstructure evolution under the disposal environment is of great significance for understanding the macroscopic performance changes of bentonite.

Montmorillonite which is the main mineral component of bentonite is the main embodiment of its expansion, permeability, and adsorption properties. So, in the present study, particular attention was given to the montmorillonite extracted from GMZ bentonite, which was a reference material used for buffer material in the geological disposal of HLW in China. Radiation was achieved by using a ⁶⁰Co source. It was produced to simulate the γ radiation effect in the period preceding container failure after the closure of the disposal repository. Various techniques such as X-ray diffraction (XRD), thermogravimetry (TGA)-differential scanning calorimetry (DSC), Mössbauer spectrum, infrared (IR) spectrum, Raman spectrum, and a Four-bar mass spectrometer were employed to comprehensively assess the impact of γ radiation on the microstructure of montmorillonite. Finally, the interaction between γ rays and montmorillonite was described.

MATERIALS AND METHODS

Preparation of montmorillonite

Montmorillonite was obtained as follows. Firstly, 10 g of powder GMZ bentonite, whose fundamental properties are detailed in Table 1, was added to 1 L of ultrapure water in a beaker. The beaker was stirred for 3 h at 500 rpm using an electric blender. Low-power ultrasound was used to disperse the bentonite for two hours in order to minimize the damage that high-power ultrasound could do to the bentonite structure. Secondly, the solution was centrifuged at 4000 rpm for 30 min after letting rest for 24 h at room temperature. The centrifuged liquid was taken out and dried at 60 °C. Thirdly, the montmorillonite that contained a small amount of cristobalite and a very small amount of quartz (Fig. 1) was obtained by grounding the solid sample to a particle size of less than 0.074 mm. the mineral compositions and contents are shown in Table 2 which was tested by XRD, and its chemical compositions and contents are shown in Table 3 which was tested by XRF.

Table 1. Basic properties of GMZ bentonite

Sample	Property	Description	Value
Reference	Specific gravity of soil grain (g/cm3)		2.64
	Liquid limit (%)		76
	Plastic limit (%)		32
	Cation exchange capacity (mmol/100 g)		58.14
	Main exchanged cation (mmol/100 g)	E (Na+)	34.39
		E (Ca2+)	11.54
		E (Mg2+)	5.52
		E (K+)	0.53
	Main minerals (%)	Montmorillonite	48.5
		Quartz	26.0
		Albite high	16.0
		Heulandite	4.2

Figure 1. XRD patterns of reference and radiation samples.

Table 2. Mineral compositions and contents of montmorillonite

Composition	Montmorillonite	Cristobalite	Quartz
Content (wt%)	97.4	2.1	0.5

Table 3. Chemical compositions and content of montmorillonite

	SiO2	Al2O3	Fe2O3	CaO	MgO	K2O	Na2O	TiO2	SO3	MnO	L.O.I
Content (wt%)	71.29	16.64	2.98	1.75	2.53	2.61	1.34	0.30	0.18	0.11	0.27

Sample preparation and radiation

To avoid contamination of the samples by other substances in the ⁶⁰Co source facility during radiation, and also to test the radiolytic gas, the prepared sample was placed in hermetically sealed stainless steel receptacles. Subsequently, they were irradiated at ambient temperature using a ⁶⁰Co gamma source (the average energy is 1.25 MeV). The dose rate was 2.88 kGy·h⁻¹, and the absorbed doses ranged from 1.0 to 3.0 MGy in increments of 1.0 MGy.

Analytical methods

Powder XRD data were collected by a DX2700 powder X-ray diffractometer under the following conditions: graphite-monochromatized CuKα radiation (λ = 0.1541 nm), operating voltage of 40 kV, current of 40 mA. The data was collected from 3 to 70° with a step size of 0.02° and a count time of 1 s per step.

A synchronous thermal analyzer (NETZSCH STA449F3) was used to obtain DSC and TGA curves. Each measurement utilized a precise amount of 20.0 ± 0.5 mg, which was carefully deposited in an alumina crucible atop the thermobalance with microgram accuracy. The experiment was conducted under a dynamic nitrogen atmosphere with a flow rate of 50 mL·min⁻¹ and a heating rate of 20 °C·min⁻¹. The temperature range spanned from 30 to 1000 °C.

Mössbauer measurements were carried out with a standard constant acceleration spectrometer and a ⁵⁷Co source. Mosswin software was used to conduct spectral fitting, with Lorentzian line shapes.

Fourier transform infrared spectra (FT-IR) were obtained using a Nicolet IS50 FT-IR spectrometer at room temperature, covering the range from 400 to 4000 cm⁻¹ with a resolution of 4 cm⁻¹. The test samples were obtained by pressing a mixture of 1 mg of montmorillonite diluted in 200 mg of dried KBr. The spectrums were fitted by LabSpec6, with the Gaussian Lorentz function and a linear baseline correction applied.

The Raman spectra were collected by using the HORIBA LabRAM HR Evolution spectrometer, which was equipped with a CCD detector, and the samples were not processed. The spectra range was 70-1200 cm⁻¹, with a laser wavelength of 633 nm for excitation at ambient temperature. Spectra were acquired with a 400 s exposure time and three accumulations. All spectra were fitted by LabSpec6 software, with the Gaussian Lorentz function and a linear baseline correction applied.

RESULTS AND DISCUSSION

XRD analyses

The primary mineral composition of the purified GMZ bentonite was montmorillonite, and it included quartz and cristobalite that existed in physical and mechanical contact with montmorillonite (Zhang and Liao, 2010) (Fig. 1). For all samples, the positions and intensities of reflections had no significant change. So, γ radiation had little influence on mineral compositions. The d₀₀₁-value of montmorillonite was between 14 and 15 Å, and there was two water layer hydrate in the interlayer space (Cases et al., 2002; Wilson et al., 2004; Oueslati et al., 2012), but γ radiation changed the d₀₀₁-value and average grain size of montmorillonite which was calculated by the Scheler formula, and they decreased with the increase of absorbed dose (Table 4).

Table 4. The interlayer space and average grain size of montmorillonite

Sample ID	d001-Value/nm	Average grain size/nm
Reference	1.458	5.6
1.0 MGy	1.455	5.3
2.0 MGy	1.433	4.7
3.0 MGy	1.417	4.5

TG-DSC analysis

The TG-DSC curves for the reference and irradiated samples are displayed in Figure 2. There were the following reactions in the range of 30-1000 °C.

Figure 2. The synchronous thermal analysis carves of reference and radiation samples [(a) DSC carve with a heating rate of 20 °C·min⁻¹ and temperature range from 30 to 1000 °C; (b) TG carve with a heating rate of 20 °C·min⁻¹ and temperature range from 30 to 1000 °C].

In the range of 50-200 °C, there were two endothermic peaks in the DSC curves (Fig. 2a), with peak temperatures of around 110 and 169 °C. The TG curves (Fig. 2b) showed two phases of mass loss, which were similar to the findings of Zhang et al. (2015) and Fernandez et al. (2017). The process corresponding to the peak temperature of 110 °C loses mass about 10.5-11.5 wt%, and the adsorption of water on the surface of montmorillonite was removed. Another process that eliminated the interlayer water of montmorillonite at 169 °C resulted in a mass loss of about 0.5-0.8 wt%, and the mass loss decreased as the absorbed dose increased. The findings indicated that the γ radiation resulted in a decrease of interlayer water of montmorillonite, and decreased with an increase in absorbed dose.

In the range of 550-800 °C, there was an endothermic peak in the DSC curve (Fig. 2a), corresponding to a step of mass loss in the TG curve (Fig. 2b), and it was associated with the loss of constitutional water (Górniak et al., 2016; Fernandez et al., 2017). The peak temperatures of endothermic peaks for the reference and radiation samples were between 677 and 683 °C, the endothermic enthalpies were 74.54-93.31 J/g, and the mass losses were 2.30-2.64 wt%. The peak temperature moved towards a higher temperature as the absorbed dose increased, and the endothermic enthalpy and mass loss decreased with an increase in absorbed dose. The reductions amounted to 20.12 and 12.88% when the sample was exposed to up to 3.0 MGy. These results suggested that the constitutional water of montmorillonite was effectively damaged by γ radiation.

There was the fourth endothermic peak in the range of 800-950 °C (Fig. 2a), with a peak temperature between 873 and 876 °C, no corresponding mass loss step in the TG curve (Fig. 2b), and structure disintegration occurred in this process (Caglar et al., 2009). The endothermic enthalpy ranged from 11.44 to 15.30 J/g, and it decreased with the increase of absorbed dose, while the peak temperature increased with the increase of absorbed dose. Compared with the reference sample, the endothermic enthalpy decreased by 25.20% for the sample which was irradiated up to 3.0 MGy. This indicated that the enthalpy required for phase transformation of montmorillonite decreased with the increase of the absorbed dose. This also suggested that γ rays destroyed some of the crystal structure of montmorillonite, thereby causing a decrease in the enthalpy required for phase transformation.

Mössbauer results

Figure 3 shows the ⁵⁷Fe Mössbauer spectra of the montmorillonite sample before (a) and after (b) γ radiation with a total absorbed dose of 3.0 MGy. The calculated values of half of width (Γ/2), isomer shift (IS), quadrupole splitting (QS), and the percentage area (%) for trivalent iron in sites M₁ and M₂, and divalent iron were listed in Table 5. In the reference sample, trivalent iron accounted for 91% (with 78% of the total iron content located in M₁), and divalent iron accounted for 9% of the total iron content. However, in the radiation sample with the total absorbed dose of 3.0 MGy, the content of trivalent iron decreased (accounted for 83% of the total iron), and divalent iron increased (accounted for 17% of the total iron content). It indicated that, after radiation, the trivalent iron in the montmorillonite structure transformed into divalent iron.

Figure 3. Mössbauer spectra from the montmorillonite sample before (a) and after (b) γ radiation with a total absorbed dose of 3.0 MGy.

Table 5. Mössbauer parameters of montmorillonite before and after radiation

Sample	Fe3+					Fe2+
	Site	Γ/2a	ISb	QSc	Area (%)	LW	IS	QS	Area (%)
Reference	M1	0.30	0.36	0.60	79	0.21	1.24	3.43	9
	M2	0.28	0.40	1.51	12
3.0 MGy	M1	0.29	0.34	0.59	76	0.20	1.31	3.49	17
	M2	0.26	0.37	1.47	7

^aHalf line width, Γ/2, (in mm⁻¹)

^bIsomer shift, relative to metallic Fe, (in mm⁻¹)

^cQuadrupole splitting, (in mm⁻¹)

The study by Gournis et al. (2000), Caër (2011), and Jonsson (2012) indicated that γ radiation caused the radiolysis of water in bentonite and produced H radicals, OH radicals, hydrated electrons, hydrogen. Meanwhile, hydrogen (Table 6) was detected by a Four-bar mass spectrometer (Pfeiffer Hiquad 700) by using the Multiple Concentration Determination (MCD) mode in the laboratory for the irradiated GMZ bentonite. According to the studies by Gournis et al. (2000), Holmboe and Jonsson (2012), and Chikkamath et al. (2020), due to the entry of reducing substances from the radiolysis products into the crystal structure of montmorillonite, reducing Fe³⁺ to Fe²⁺, the structural Fe³⁺ content decreased and structural Fe²⁺ content increased. Hydrated electrons and H radicals that are produced by radiolysis of water are strongly reducing substances. Hydrated electron is an electron surrounded by a group of water molecules with a certain orientation in the action of an electric field, so its radius is greater than that of one water molecule (1.4 Å) at least (Pan, 1983). The radius of H radicals should be equivalent to that of an H atom which is around 0.50 Å. The spatial radius available for coordination ions is about 0.61 Å in octahedral structure of montmorillonite (Wang, 1980). Therefore, H radicals can enter the octahedral structure of montmorillonite and reduce Fe³⁺ to Fe²⁺, but they can also transform into H ions and diffuse out of the octahedral structure.

Table 6. Hydrogen content of the reference and radiation samples for GMZ bentonite

Sample ID	Reference	3.0 MGy
H2 content (vol%)	0.04	12.25

FT-IR analysis

Figure 4a presented the IR spectrum of the reference and γ radiation sample. Notably, two bonds at 3619 and 3449 cm⁻¹ corresponded to the stretching vibration of hydroxyl (OH) groups. The bond located at 1636 cm⁻¹ corresponded to the bending vibration of interlayer water, and the bonds at 1035 and 1111 cm⁻¹ were related to the Si-O stretching vibration and perpendicular vibration of montmorillonite (Madejova and Komadel, 2001; Bishop and Murad, 2004). The bonds observed at 914 and 846 cm⁻¹ corresponded to the Al-OH and AlMg-OH shearing vibration of montmorillonite (Madejova and Komadel, 2001; Górniak et al., 2016). The bonds at 797 and 783 cm⁻¹ were related to the Si-O symmetric stretching vibration of quartz (Caglar et al., 2009). Furthermore, the bond located at 626 cm⁻¹ corresponded to the out of plane vibration of Al-O and stretching vibration of Si-O, the bond at 519 and 466 cm⁻¹ related to the bending vibration of Al-O-Si and Si-O-Si (Madejova and Komadel, 2001). In the IR spectrum, the absorption peaks of montmorillonite and weak characteristic absorption peaks of quartz were observed; it was consistent with the XRD results that the mineral compositions of the purified GMZ bentonite were montmorillonite, quartz, and cristobalite (Fig. 1).

Figure 4. The results of the infrared spectrum [(a) tested spectrum of reference and radiation samples; (b) the fitting spectrum of reference sample; (c) the fitting spectrum of Si-O stretching vibration at 1035 cm⁻¹ for reference and radiation sample; (d) the fitting spectrum of Al-OH shearing vibration at 914 cm⁻¹ for reference and radiation sample].

There were two changes in the radiation samples in the IR spectrum. Firstly, the stretching vibration of Si-O was broadening. This observation was consistent with findings obtained under the ionizing radiation of kaolinite, as reported by Fourdrin et al. (2009). Meanwhile, the intensities of stretching and bending vibration of Si-O were weakened. Secondly, the intensities of shearing and bending vibration of Al-O were also weakened.

LabSpec6 was used to fit the IR spectrum in the 750-2000 cm⁻¹ region in order to comprehend the impact of γ radiation on the secondary structure of montmorillonite (Fig. 4b). The fitting spectra of the shearing vibration of Al-O at 914 cm⁻¹ and the stretching vibration of Si-O at 1035 cm⁻¹ were displayed in Figures 4c and 4d, respectively. The areas of the Si-O stretching vibration and the Al-O shearing vibration both decreased with the increase of absorbed dose. So, γ radiation damaged part structures of the Si-O tetrahedral and Al-O octahedral for montmorillonite, which was consistent with the γ radiation-induced change of crystal chemistry of montmorillonite obtained from TG-DSC analysis (Fig. 2).

Raman spectroscopy analysis

Figure 5a presents Raman spectrum of montmorillonite. The bond observed at 917 and 797 cm⁻¹ corresponded to the deformation and swing vibration of AlOH. The bond at 710 cm⁻¹ corresponded to the stretching vibration of SiO₄. Furthermore, the bond at 435 cm⁻¹ was attributed to the SiO₃ stretching vibration. The bond at 202 cm⁻¹ represented the stretching vibration of AlO₆. The bond at approximately 145 cm⁻¹ corresponded to the ring breathing mode of the Si₂O₅, and the strong bond at 99 cm⁻¹ was ascribed to the vibrations of the interlayer cations (Bishop and Murad, 2004; Gate et al., 2017).

Figure 5. The Raman spectrum of montmorillonite [(a) tested spectrum of reference and radiation samples; (b) fitting spectrum of SiO₄ stretching vibration at 710 cm⁻¹ for reference and radiation sample; (c) fitting spectrum of AlO₆ stretching vibration at 202 cm⁻¹ for reference and radiation sample; (d) fitting spectrum of Si₂O₅ ring breathing mode at 145 cm⁻¹ for reference and radiation sample].

The overall shape of the Raman spectrum stayed consistent after radiation, indicating that no significant structural change occurred. However, three types of alterations were observed. Firstly, at 710 cm⁻¹, there was a shift towards the lower wave number with an increase in absorbed dose (Fig. 5b). A movement of 4.72 cm⁻¹ was observed when the sample was radiated up to 3.0 MGy. Furthermore, it was evident that the peak area decreased with the increase of absorbed dose, indicating that the SiO₄ tetrahedral unit was destructed by γ radiation, Gournis et al. (2001) had also concluded the same result. Secondly, the bond at 200 cm⁻¹ was the same position for reference and radiation samples, but its intensity decreased with the increase of absorbed dose (Fig. 5c). Thus, γ radiation also caused damage to the AlO₆ octahedral unit. This finding aligned with that of Gournis et al. (2001). The last one was the bond at 145 cm⁻¹ (Fig. 4d), the position shifted towards a lower wave number. The wave number was reduced by 2.55 cm⁻¹ for the sample that absorbed dose was up to 3.0 MGy. It was demonstrated by Gate et al. (2017) and Frost and Rintoul (1996) that the unit cell charge had a significant impact on the position. Specifically, the position moved towards the lower wave number at a high charge. Thus, the movement of the bond at 145 cm⁻¹ indicated an increase in the unit cell charge after γ radiation. The discrepancy between the bond at 202 and 710 cm⁻¹ can be explained by the following: γ rays break some of the Al-O and Si-O bonds, but Al-O bonds are located in the octahedron, and while the Si-O bonds are located in the tetrahedron of montmorillonite. The destruction of some Al-O bonds had a smaller impact on the bond force of AlO₆, while the destruction of some Si-O bonds had on significant impact on the bond force of SiO₄. So, it resulted in a red shift of the stretching vibration of SiO₄, and AlO₆ maintained its original position of vibration bond. The change at the wave number of 145 cm⁻¹ would be explained that: according to results of Mössbauer spectra, after γ radiation, the transformation of Fe³⁺ to Fe²⁺ in the montmorillonite structure changed the charge of the cell, and it also changed the electron cloud of chemical bond, so it resulted in the shift of the Raman bond at 145 cm⁻¹. During the transformation from Fe³⁺ to Fe²⁺, the cell got electrons, while the cell itself is negatively charged, so the cell of montmorillonite was even more negatively charged. However, the change of electron cloud did not change the amount of Si₂O₅, therefore, there is no significant correlation between bond intensity and absorbed dose.

THE INTERACTIVE PROCESS BETWEEN γ RAYS AND MONTMORILLONITE

The primary mechanism by which γ rays interacted with montmorillonite was that: γ radiation partially broke the bonds of Si-O and Al-O in the structure, as well as caused the radiolysis of the hydroxyl (OH) groups and adsorbed interlayer water. The generated H radicals entered the octahedral structure of montmorillonite, reducing Fe³⁺ to Fe²⁺ and diffusing out of the structure. The schematic diagram of the interaction mechanism between γ rays and montmorillonite is shown in Figure 6.

Figure 6. The interactive process between γ rays and montmorillonite.

CONCLUSIONS

The investigation into the γ radiation effect on montmorillonite yields the following conclusions:

1. (1)    The d₀₀₁-value and average grain size of montmorillonite decreased with the increase of absorbed dose, and the average grain size decreased from 5.6 nm for the reference sample to 4.5 nm for the radiation sample with the absorbed dose up to 3.0 MGy.
2. (2)    Some of hydroxyl (OH) groups, Si-O bonds, and Al-O bonds were broken by γ rays.
3. (3)    The H radicals that were generated by the radiolysis of adsorption water and hydroxyl (OH) groups reduced Fe³⁺ to Fe²⁺ in the montmorillonite structure.

ACKNOWLEDGMENTS

This study was supported by the founding: CAEA Innovation Center for Geological Disposal of High-Level Radioactive Waste (CXJJ21102208).

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