Effect of Radiation-Induced Amorphization on Smectite Dissolution

Environ. Sci. Technol. 2010, 44, 2509–2514

Effect of Radiation-Induced Amorphization on Smectite Dissolution C . F O U R D R I N , * ,† T . A L L A R D , † I. MONNET,‡ N. MENGUY,† M. BENEDETTI,§ AND G. CALAS† Institut de Mine´ralogie et de Physique des Milieux Condense´s (IMPMC), CNRS, Universite´ Paris 6, Universite´ Paris 7, IPGP, Institut de Recherche pour le De´veloppement (IRD), Campus Boucicaut, bat. 7, 140, rue de Lourmel, 75015 Paris, France, Centre Interdisciplinaire de Recherche Ions Lasers (CIRIL-GANIL), CEA-CNRS-ENSICAEN, BP 5133, Bd Henri Becquerel, F-14070 Caen cedex 5, France, and Laboratoire de Ge´ochimie des Eaux, IPGP, CNRS, Universite´ Denis Diderot, UMR7154, 4 Place Jussieu, 75005 Paris France

Received October 30, 2009. Revised manuscript received February 15, 2010. Accepted February 26, 2010.

Effects of radiation-induced amorphization of smectite were investigated using artificial irradiation. Beams of 925 MeV Xenon ions with radiation dose reaching 73 MGy were used to simulate the effects generated by alpha recoil nuclei or fission products in the context of high level nuclear waste repository. Amorphization was controlled by X-ray diffraction, transmission electron microscopy, and Fourier transform infrared spectroscopy. An important coalescence of the smectite sheets was observed which lead to a loss of interparticle porosity. The amorphization is revealed by a loss of long-range structure and accompanied by dehydroxylation. The dissolution rate far-fromequilibrium shows that the amount of silica in solution is two times larger in the amorphous sample than in the reference clay, a value which may be enhanced by orders of magnitude when considering the relative surface area of the samples. Irradiation-induced amorphization thus facilitates dissolution of the clay-derived material. This has to be taken into account for the safety assessment of high level nuclear waste repository, particularly in a scenario of leakage of the waste package which would deliver alpha emitters able to amorphize smectite after a limited period of time.

Introduction Bentonite and its dominant component smectite are considered in several concepts of high level nuclear waste repository (HLNWR) in Japan and various European countries. In the near field of a HLNWR, bentonite is a major component of engineered barriers, used for backfilling and sealing. In the far field, deep argillaceous formations may be potential host rock for nuclear waste disposal. Owing to their reactivity, large specific surface area and swelling properties, clays are used to limit the leaching and dissemination of radionuclides in the environment. However, the physicochemical properties of clays may be potentially altered as a * Corresponding author e-mail: [email protected]. † Institut de Mine´ralogie et de Physique des Milieux Condense´s. ‡ Centre Interdisciplinaire de Recherche Ions Lasers. § Laboratoire de Ge´ochimie des Eaux. 10.1021/es903300r

 2010 American Chemical Society

Published on Web 03/12/2010

result of irradiation (1, 2). As a matter of fact, small variations of cation exchange capacity (CEC), specific surface area and solubility were observed on several clay minerals after ionizing irradiation (3, 4), and the conditions for experimental amorphization with heavy ions were reported to be consistent with a HLNWR in a scenario of leakage of the waste package (5). Thus, in the engineered barriers, severe irradiation and ultimately amorphization may result from the alpha recoil nuclei of long-lived actinides and the ionizing radiation of fission products. Effects of amorphization on properties of clay are expected to be significant, particularly on sorption capacity, swelling, and dissolution kinetics (2, 6). For instance it was shown that thermally produced amorphization of zeolites induced a higher retention of Cs (7). Furthermore, the ion exchange capacity of thermally amorphized smectite was found to be greatly decreased (8). However, despite its environmental relevance, the radiation damage and its consequences in clay minerals including smectites are not fully understood. The dose of amorphization of smectite with electron or heavy ions has been determined but the macroscopic dissolution kinetic was not studied (5, 9). Realistic models used for the long-term prediction of radionuclides transport have to take into account radiation damage in containment materials. Nevertheless, the radiation induced crystalline to amorphous transitions of several minerals were intensively studied the last decades (10–12) and such investigations are necessary to determine the radiation-resistance of materials for the isolation and disposal of radioactive waste (1, 4, 13, 14). These studies were most often performed at the microscopic scale, because of an experimental limitation due to the small range in matter of heavy ions with moderate energy. This limitation strongly hinders the possibility of investigating macroscopic properties such as, for example, dissolution kinetics. This can be overcome by using irradiation conditions providing larger ranges in matter more appropriate for amorphization. Such amorphization can be performed using high energy beams, typically tens or hundreds MeV, of heavy ions such as those provided by the GANIL (15). Besides, highly alkaline environments are likely to occur in concrete-containing radioactive waste repositories (16). An alkaline plume is expected to take place at the interface between the smectite-rich material (bentonite) and the cement constructions. This plume might alter the physicochemical properties of smectite and particularly enhances dissolution. Indeed, alkaline conditions are known to increase the dissolution rate of smectite (17). The mechanisms and kinetics of dissolution of clay minerals were widely studied under various conditions of pH (18–20). In previous works (21, 22), the dissolution and morphology of reference sample of smectite were studied by macroscopic measurements as well as atomic force microscopy: it was observed that dissolution principally occurred on edge faces of smectites. However, the initial condition of the smectite could also affect the dissolution rates. Thus, radiation-induced amorphization of smectite is expected to induce a strong change in smectite dissolution rate as a result of the loss of periodic structure. In the present study, a particular attention was given to the solubility of amorphized smectite extracted from the MX80 bentonite, a reference material for the engineered barrier (5). Amorphization was achieved using 925 MeV xenon ions at the GANIL facility (France). The irradiation-induced amorphization was produced to simulate the ionizing effect of alpha recoils, albeit with a sufficient energy to amorphize a large quantity of material. Various techniques were used to characterize the structure of the sample after irradiation VOL. 44, NO. 7, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Energy loss by ionization and Xe implantation with the depth penetration in the sample for 925 MeV 124Xe42+ particles in smectite calculated with the TRIM code (26). experiments. Furthermore, a macroscopic study of dissolution kinetics was performed at ambient temperature and in alkaline conditions to simulate the environmental conditions of the waste package.

Material and Methods Sample Preparation. The investigated sample was extracted from the Wyoming bentonite MX-80, a widely used montmorillonite-rich clay reference for studies dealing with the backfill material (23). A size fraction below 0.5 µm was prepared by centrifugation allowing the separation of the clay from the associated minerals such as quartz and feldspars (5). A “gel-like” fraction from the centrifugation residue was further selected within this size fraction. It corresponded to a hydrated material composed of the finest crystalline particles. Ancillary iron oxides were removed with citratedithionite bicarbonate (DCB) treatment (24). Removal of carbonates was performed with acetic buffer (pH 5) (25). Both DCB and acetic treatments were performed at 80 °C. The resulting sample was sodium-exchanged by stirring the clay in 1 mol · L-1 of chloride solution. The excess salts were removed by repeated washing with deionized water. Purity of the sample was checked by X-ray diffraction and infrared spectroscopy. Only a minor impurity of cristobalite remained in the “gel-like” fraction. The chemical composition of the resulting sample was (5):

FIGURE 2. SEM image of the samples (b) reference, (a,c) irradiated film. The thickness of the film is 40 ( 2 µm. of 40 µm, implantation of Xe atoms in particles is negligible (Figure 1). Accordingly, 200 mg of reference sample were deposited on each plate. The film thickness was checked with scanning electron microscopy (Figure 2). The energy losses dominantly correspond to ionization processes (more than 99%) and the error on the deposited dose was estimated at (10%. The dose was calculated by the following formula: D)

eφS m

dx ∫ ( dE dx ) Rp

0

where e is the conversion factor for MeV to J, φ the fluence, S is the surface of irradiated sample, m the mass of irradiated sample, (dE/dx) is the deposited energy per unit of length, and Rp is the projected path. For the following experiments, the dose corresponding to a total amorphization of the sample was 73 MGy (1013 ions/cm2). Amorphization of the sample was controlled by transmission electron spectroscopy and X-ray diffraction (Figure 3e).

III II (Si7.96Al0.04)(Al3.10Mg0.56Fe0.18 Fe0.16 )O20(OH)4Na0.76

Batch Experiments Irradiation Experiments Sample for irradiation were prepared by sedimentation on aluminum plates. Irradiation experiments were performed with a 124Xe42+ beam at high fluence using the SME line at GANIL (Caen, France). The beam energy was 925 MeV and the flux delivered at the line was stabilized around 4 × 108 ions.cm-2.s-1 (fixed mode) in order to reach a final fluence of 1013 ions/cm2. This flux provides enough irradiated smectite and prevents heating during irradiation. The amorphization fluence was previously evaluated with a 74 MeV Kr18+ beam. In order to avoid inhomogeneity of the irradiation, the thickness of the film was previously calculated to ensure a quasi-constant energy deposition within the films and enough irradiated material for the dissolution experiment (Figure 1). Default values of the SRIM code (10, 26) were taken for displacement energy (20 eV): for a sample thickness 2510

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Batch experiments were carried out in Teflon bottles at ambient temperature. The initial pH was set to 12. The mass of irradiated and reference samples used for the dissolution, were 8.5 mg and 10 mg, respectively. The irradiated sample was not grinded to form a powder but let under a film form. This irradiated film was deposited in a Teflon holder in the bottle. On the other hand, reference sample was under the powder form, in order to maintain the initial specific surface area which is 34 m2 · g-1 (27). The two samples were immersed in 85 mL of 0.01 M NaOH solution. Low magnetic agitation was done with a coated Teflon stirring bar. The pH was measured at the beginning (11.6) of the experiment and 100 h after and did not vary more than 0.1 pH units. The analyzed samples were extracted from 20 mL of the solution by centrifugation (20 min at 16 000 rpm) in order to separate the clayey fraction in suspension. Subsampling was per-

measurements were obtained by pressing a mixture of about 1 mg of clay diluted in 300 mg of dried KBr.

Results and Discussion

FIGURE 3. Transmission electron microscopy on smectite: (a) reference MX-80 sample and electronic diffraction (b1)(b2); (b1) corresponds to the patterns obtained by Herbert et al. (30), whereas (b2) corresponds to the present study; (c) 1013 ions.cm-2 irradiated sample with 925 MeV 124Xe42+ ions and electronic diffraction (d). The scale bar on (a) and (c) corresponds to 500 nm. (e) X-ray diffraction of reference and irradiated MX-80 sample. formed with great care, by taking 2 mL aliquots of the suspension. Concentrations of silica as a function of the time were measured after a 10-fold dilution colorimetrically with an UV-visible spectrophotometer using the molybdate blue method (28, 19) with an uncertainty of 3%.

Characterization of the Reference and Irradiated clay X-ray Diffraction. Powder X-ray patterns of both irradiated and reference samples deposited on Si holders were recorded using a Philips PW3050/60 diffractometer with Co KR tube operating at 40 kV and 40 mA. Transmission Electron Microscopy. Transmission electron microscope (TEM) observations were performed on a JEOL 2100F microscope operating at 200 kV and equipped with a field emission gun, a high-resolution UHR pole piece, and a Gatan US4000 CCD camera. The clay suspension was prepared after ultrasonic treatment on carbon coated Cu grids by air drying. The particles were characterized at first by selected area electron diffraction (SAED) and then by means of morphology. Scanning Electron Microscopy. Images were obtained using a SEM/FEG ZEISS Ultra55 microscope on broken edges of the deposited films. FTIR Spectrocopy. Fourier transform infrared spectra were performed in transmission mode with a Nicolet Magna 560 spectrometer at room temperature, between 400 and 4000 cm-1, with a resolution of 1 cm-1. Pellets for transmission

Amorphization. Scanning electron images show the deposited smectite film before and after irradiation experiments (Figure 2). One can observe the stack of platy particles in the reference sample, which induces a significant microporosity (Figure 2b). By contrast, the irradiated film exhibits a loss of platy-particle structure and a corresponding loss of porosity (Figure 2c). From these observations, it seems likely that the specific surface area of the sample is affected by the irradiation. In solution, the reference montmorillonite under film form disperses into water, as expected, whereas the irradiated sample keeps its thin film morphology, showing that swelling is also altered by the irradiation. The structure of the montmorillonite was investigated using X-ray diffraction (Figure 3e). The reference sample exhibits (001) and (004) diffraction peaks typical of smectite, with particularly a major diffraction peak corresponding to the 12 Å interlayer space characteristic of the (001) reflection of an air-dried Na-exchanged smectite (29). The major effect of irradiation is the disappearance of the diffraction peaks to the benefit of a broad diffuse peak located at 12 Å. This pattern is characteristic of an X-ray amorphous structure indicating a loss of long-range atomic periodicity. Transmission electron microscopy was performed to investigate the structure of materials at the scale of particles (Figure 3). Figure 3 presents the reference particle before (a) and after (c) irradiation. On Figure 3a, large amount of small tactoids are collapsed to form a cloud. The reference montmorillonite exhibits electron diffraction peaks which are similar to those previously published for MX-80 montmorillonite (30) (Figure 3b). On the other hand, the irradiated sample gently grinded before microscopic observation presents blocky and compact particles. Corresponding electron diffraction gives a poorly defined diffuse halo which is characteristic of an amorphous structure at local scale, in accordance with XRD (Figure 3d). The effect of irradiation was also investigated at the molecular scale using Fourier-transform infrared spectroscopy (Figure 4). The infrared spectra of montmorillonite present two distinct zones: the first one between 3300 and 3650 cm-1 corresponds to the OH stretching vibrations and the second zone between 400 and 1700 cm-1 corresponds to various stretching and bending modes involving mainly Si and Al (31). In the first zone, the peak at 3631 cm-1 corresponds to the stretching vibration of structural OH bond, while the large band observed at lower wavenumbers around 3400 cm-1 is assigned to physisorbed water. The reference sample spectrum shows in the second zone an intense band at 1048 cm-1 attributed to the SisO stretching vibrations, and bands at 524 cm-1 and 466 cm-1 assigned to AlsOsSi (octahedral Al) and SisOsSi vibrations respectively (32). Three peaks in the hydroxyl-bending region at 914 cm-1 for Al2OH, 881 cm-1 for AlFeOH, and 842 cm-1 for AlMgOH reflect that octahedral Al3+ is partially replaced by Fe3+ and Mg2+ (33). The peak at 799 cm-1 indicates the presence of cristobalite impurity in the sample. Infrared spectrum of irradiated MX-80 is highly affected by swift Xe ions. In the second zone (Figure 4), the bands corresponding to the Al2OH, AlFeOH and AlMgOH bending vibrations disappear completely after irradiation. Furthermore, one observes an important broadening over the whole spectrum, as it was previously observed on kaolinite as a result of ionizing irradiation (34). The above-mentioned modifications of spectra can be ascribed to either (i) a dehydroxylation, or (ii) a distribution of the phonon vibrations frequency raised by irradiation due to variations of the atomic positions, that is, a loss of organization even at short-range. VOL. 44, NO. 7, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. FTIR of reference MX montmorillonite and Xe-irradiated samples. Both interpretations are consistent with the results. As a matter of fact, dehydroxylation is assigned to the loss of OH vibration bands, either in the bending (around 900 cm-1) or the stretching (around 3600 cm-1) regions. Moreover, the band at 524 cm-1 corresponding to AlsOsSi is absent on the spectrum of the irradiated sample, suggesting that the connections between the octahedral and tetrahedral sheets no longer exist. At 566 cm-1, the peak corresponding to SisOsSi vibration is still present but a lower wavenumbers and also broadened. This provides information on the connection between tetrahedral units which remains after irradiation. In addition, the position for the SisO stretching bond is found to be at 1043 cm-1 on the irradiated sample, which is similar to the reference sample, indicating that the main polymerization degree of silicates remains probably unaffected. However, it cannot be excluded that the broadening in this region arises from a significant variation in the distribution of the polymerization degree. Smectite Dissolution. In the kinetics approach, the dissolution rates are classically expressed by reference of the surface area and it was recently shown that the dissolution takes place at the edges surface of clay particles (22). However, as irradiation gives rise to a coalescence of smectite sheets, the specific surface area cannot be taken equal to the one of the initial reference sample. Nevertheless, without any possible measurement of the specific surface area of the irradiated sample, we will compare bulk chemical Si concentrations for the reference and the irradiated samples. As previously stated (17), only Si release rates are considered in priority because Si is less likely to be incorporated into secondary phases in contrast to Al which can exhibit non stoichiometric behavior. One observes in Figure 5 that the dissolved fraction is much more important for the irradiated sample, a difference which starts after 40 h at ambient temperature. The concentration of Si for the irradiated sample continuously increases within the range 0-200 h, whereas the reference sample shows saturation from about 100 h. Yokoyama et al. (20) also performed dissolution of smectite sample in flow-trough (pH 13.3 at 30 °C) reactor and concluded that the silica concentration reached a steady state after 136 h. The amount of dissolved Si is also consistent with the previous investigations in 2512

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FIGURE 5. Dissolution kinetics (Si concentration in µmol/g/L) of reference (O) and irradiated (b) montmorillonites. flow-through experiments (17). The concentrations of dissolved Si are lower than the solubility of amorphous silica suggesting that the steady-state reached is an intermediary step. After 192 h, the concentration of dissolved Si is almost twice the value measured for the irradiated sample assuming that the specific surface area remains unchanged after irradiation. Taking into account the edge specific surface area would probably increase significantly the discrepancy up to several orders of magnitude. This result, showing an effect of irradiation upon dissolution, is consistent with previous observations: an increase of the solubility of 3 × 107 Gy gamma irradiated montmorillonite (3) related to a modification of the specific surface area. It is also in agreement with results of implantations in mica with low energy Pb ions (35) which show that irradiated surface dissolves faster than unimplanted ones. Faster dissolution rates were also evidenced in metamict zircon (36, 37) or in other silicates minerals through experimental irradiations (35, 38). This result may be a consequence of the increased strain in irradiated materials (34), although the amorphization processes are still unknown. With swift heavy ions, fission tracks will be produced in the sample and the energy of the incident

particle is mainly lost by ionization (more than 99%). Gu et al. performed electron irradiation of smectite (8, 11) and assumed the amorphization is reached via the radiolysis of the physisorbed water. This decomposition of water under radiation is known to produce free radicals which can in turn break the bond of the structure (11). The issue of the difference between the amorphous compounds produced by different techniques is still poorly documented. Investigations on the amorphous structure must therefore be performed to differentiate the samples produced by acid activation amorphization (39), thermal treatment, and irradiations with low and high energy ions. The challenge is to determine the appropriate conditions to simulate the waste package environment. It was previously observed that the amorphous silica produced by neutron, electron, photon or swift heavy ion irradiation (40) has the same structure and that the amorphization process is triggered by a critical point defect concentration. The present data demonstrate that the effect of radiationinduced amorphization on dissolution is significant. The consequences for the HLNWR might be important and should be taken into account in its safety assessment. However, the dose rate used for this study is very high compared to what is encountered in HLNWR. The effect of dose rate on the structure is not known. Thus, it is also necessary to investigate the dissolution kinetics for a sample amorphized with a lower dose rate. Therefore, in situ irradiations in cells containing radioelements adsorbed on smectite have to be performed. Indeed, the present results are relevant only in case of leakage of alpha-emitters from the waste package, because the mean range of alpha recoil nuclei is small in matter. Corresponding radiations would thus alter the clay structure only for an intimate association of radioelements and clays, that is, through sorption or precipitation on clay surfaces. Dissolution in alkaline water of amorphous zones would produce preferential paths for solutions and dissemination of radionuclides in the environment. In previous studies, it was shown experimentally that radiation-induced amorphization would potentially occur in about 1000 years in case of contact of smectite with alpha emitters (5, 9). Consequently, in this worst-case scenario, the local alteration of the waste package would be very fast at the scale of the expected durability of the HLWNR (expected around 1 M years, ref 13). The limiting process would thus be the first release of alpha emitters from the waste package.

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