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Rare-earth disilicate formation under Deep Geological Repository approach conditions
Applied Clay Science 46 (2009) 63–68
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Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y
Rare-earth disilicate formation under Deep Geological Repository approach conditions María D. Alba ⁎, Pablo Chain, M. Mar Orta Instituto Ciencia de los Materiales de Sevilla, CSIC-US, Avda, Americo Vespucio, 49, 41092 Sevilla, Spain
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Article history: Received 13 December 2008 Received in revised form 9 July 2009 Accepted 13 July 2009 Available online 18 July 2009 Keywords: Deep Geological Repository Engineered Barrier System Bentonite Rare earth Hydrothermal Long-lived radiactive waste
a b s t r a c t The Deep Geological Repository (DGR) concept involves the placement of long-lived radioactive waste in rooms excavated deep. The major responsibility of the disposal safety falls on the Engineered Barrier System (EBS). The main constituent of EBS is bentonite that prevents the release of radiactive nuclei by physical and chemical mechanisms. The physical mechanism is expected to fault with the weathering of the bentonite while the chemical mechanisms have been only proved at 300 °C. It is the aim of this paper to explore the feasibility of the chemical mechanism at temperatures closer to the DGP conditions and to shed light on the mechanism of transformation of the argillaceous materials of the EBS in rare-earth disilicate phases. Saponite was submitted to hydrothermal reaction at 175 °C and 150 °C with different solutions of REE3+ cations (REE = Sc, Lu, Y, Sm, Nd and La). The products were analyzed by XRD, NMR and electron microscopy. At conditions close to the DGP, the saponite was able to form rare-earth silicates. The formation of the disilicate phase, as final product, needs a set of stages and oxyorthosilicate as precursor. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The Deep Geological Repository (DGR) concept involves the placement of long-lived radioactive waste, often spent nuclear fuel, in rooms excavated deep within stable, low-permeability bedrock (Savage and Chapman, 1982; Astudillo, 2001). The combination of waste package, engineered seals and bedrock would provide a high level of long-term safety, without relying on on-going future maintenance. One of the challenges facing the supporters of these efforts is to demonstrate that a repository will contain wastes for so long that any release that might take place in the future will pose no significant health or environmental risk. The major responsibility of the safety falls on the Engineered Barrier System (EBS) which is one part of this passive multi-barrier system approach. On one hand, it is expected that most radionuclides will decay in the EBS (e.g. it is conservatively expected that the steel canister will last for over 1000 years). In the other hand, in almost all repository concepts it is assumed that the geosphere surrounding the EBS is saturated with water, and this groundwater will be gradually flowing back into the tunnel and the EBS after the tunnel is closed. During bentonite saturation, it swells and seals the tunnel, restricting almost all flow of water inside the EBS (Bailey, 1980). Additionally, the existence of a simultaneous chemical reaction between the argillaceous component of the EBS and the rare-earth cations, as actinide simulators has been demonstrated (Perdigón,
⁎ Corresponding author. Fax: +34 954460665. E-mail address: [email protected] (M.D. Alba). 0169-1317/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2009.07.012
2002; Alba and Chain, 2005, 2007; Alba et al., 2008a). The formation of REE2Si2O7 at 300 °C, under hydrothermal conditions, is formed from a wide set of REE3+ cations (Alba et al., 2008a). This temperature is an appropriated condition to simulate deep geological disposal conditions and to increase reaction rates (Mather et al., 1982; Savage and Chapman, 1982; Allen and Wood, 1988). However, it is well-known that geochemical processes are developed at 200 °C (Mather et al., 1982; Savage and Chapman, 1982; Allen and Wood, 1988). To evaluate the applicability of the long-immobilization process proposed by Alba et al. (2008a) in real repositories, it is necessary to explore the feasibility of the reaction below 200 °C. It is the aim of this paper to explore the feasibility of the reaction at temperatures closer to the DGP conditions and to shed light on the transformation mechanism of the argillaceous materials of the EBS in the rare-earth disilicate.
2. Experimental 2.1. Materials Saponite was obtained from the Source Clay Minerals Repository University of Missouri (Columbia) and had the following chemical formula: Na0.61K0.02Ca0.09 (Si7.2Al0.8)IV(Mg5.79Fe0.15)VIO20(OH)4 (Alba et al, 2001a). The choice of saponite as starting material has been based on the fact that saponite is the crystalline reaction product of nuclear waste glass altered in brine (Abdelouas et al., 1994, Abdelouas et al., 1997) and it is formed after the reaction of bentonite with the steel corrosion products of the canister of the nuclear repository
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(Guillaume et al., 2003, 2004; Wilson et al., 2006a,b) and in hyperalkaline condition resembling cement–bentonite interface (Sánchez et al., 2006). Additionally, Alba et al. (2001a,b) demonstrated that saponite was the most reactive smectite species under hydrothermal conditions and it allows us to obtain results at laboratory time scale. In order to study the behaviour of critical long-lived radionuclides, we chose a set of suitable chemical elements: Sc, Y, La, Nd, Sm, and Lu. Based on similarities in valence state, speciation, complexation, ionic radius, etc., possible chemical analogues for the most important longlived trivalent radionuclides Np, Am and Cm were La3+, Nd3+ and Sm3+ (Chapman, 1986). The study of the hydrothermal reactivity between smectite and these rare-earth cations was complemented with a similar study of Sc3+, Y3+ and Lu3+. 2.2. Hydrothermal reactions The powdered saponite was dispersed in 50 ml of 6.3 · 10− 2 M REE (NO3)3 solution, in a molar ratio REE/Si = 1.25 and was heated at 175 °C for 4 weeks and at 150 °C for 12 weeks in a stainless steel reactor (Perdigón, 2002). The reaction products were collected by filtering, washed with distilled water and dried in air at 60 °C. 2.3. Characterization methods 2.3.1. X-ray powder diffraction (XRD) XRD patterns were obtained with a Bruker D8I instrument (Ni filtered Cu Ka radiation, at 40 kV and 40 mA), at the CITIUS, Universidad de Sevilla. Diffractograms were obtained from 3 to 70° 2θ at a scan step of 0.05° 2θ and with a counting time of 10 s. 2.3.2. Scanning electron microscopy (SEM) The morphology and chemical composition of the samples were analyzed at the Microscopy Service of the Instituto Ciencia de los Materiales de Sevilla (CSIC-US) with a Scanning Electron Microscope (JEOL JSM 5400) equipped with a LINK Pentafet probe and ATW windows for Energy Dispersive X-ray Analysis (EDX). 2.3.3. Nuclear magnetic resonance spectroscopy (MAS NMR) 29 Si, 27Al and 1H single-pulse spectra were recorded on a Bruker DRX400 spectrometer with a magnetic field of 9.36 T and equipped with
a multinuclear probe of the Spectroscopy Service of the Instituto Ciencia de los Materiales de Sevilla (CSIC-US). Powdered samples were packed in 4 mm zirconia rotors and spun at 12 kHz. 1H MAS NMR spectra were obtained at a frequency of 400.13 MHz using a typical π/2 pulse width of 4.5 μs and a pulse space of 5 s. 29Si MAS NMR spectra were obtained at a frequency of 79.49 MHz using a π/6 pulse width of 2.7 μs and a pulse space of 60 s. 27Al MAS NMR spectra were recorded at 104.26 MHz with a π/20 pulse width of 1.1 μs and delay time of 3 s. The chemical shifts were reported in ppm from tetramethylsilane for 29Si and 1H and from 0.1 M solutions of AlCl3 for 27Al. The MAS NMR analysis has not been performed for saponite reacted with Nd3+ or Sm3+ due to the paramagnetic behaviour of both cations. The spectra were simulated using a modified version of the Bruker Winfit program to handle the finite spinning speed in MAS experiments (Massiot et al., 2002). A Gaussian–Lorentzian model was used for all the peaks. The fitted parameters were amplitude, position, linewidth and Gaussian/Lorentzian ratio.
3. Results and discussion Fig. 1 shows the XRD pattern of the saponite before and after hydrothermal reaction with REE3+ solutions at 150 °C for 12 weeks (left) and 175 °C for 4 weeks (right). The original saponite (Fig. 1a) showed a pattern that is made up of the two distinct types of reflections, general and basal reflections. The basal spacing of 12.3 Å of saponite corresponded to the one-layer hydrate and d060 was in agreement with the literature data for trioctahedral smectites (Grim, 1968; Bergaya et al., 2006). The remnant smectite after the hydrothermal reactions, showed basal spacing up to 15.3 Å, in agreement with previous data reported for smectites saturated with multivalent cations (Ravina and Low, 1977), and a slight displacement of the 060 reflection, in the range of trioctahedral smectites (Grim, 1968), to lower 2θ angles. Additional changes in the XRD patterns of saponite reacted with Y3+ and Lu3+ solutions at 150° and 175 °C were not observed (Fig. 1c and d). The XRD of the saponite reacted with Sc3+ at 150 °C (Fig. 1b left) showed a set of small reflections corresponding to X2-Sc2SiO5 (JCPDS chart 40-0035; X in Fig. 1b left) and coexisting with 15 Å-saponite and with a small amount of 11 Å-saponite, i.e. partially dehydrated saponite (marked with asterisk in the Fig. 1b left). The XRD showed a prominent background which indicated the partial disruption of the
Fig. 1. XRD patterns of saponite before (a) and after hydrothermal reaction at 150 °C for 12 weeks (left) and 175 °C for 4 weeks (right) with a solution of RE3+, REE/Si = 1.25: (b) Sc3+, (c) Lu3+, (d) Y3+, (e) La3+, (f) Nd3+, and, (g) Sm3+. X = X2-RE2SiO5, C = CRE2Si2O7, and, * = dehydrated saponite.
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Fig. 2. Rows (dots) and fits (lines) 29Si MAS NMR spectra of saponite (a) and after hydrothermal reaction at 150 °C for 12 weeks (left) and 175 °C for 4 weeks (right) with a solution of RE3+, REE/Si = 1.25: (b) Y3+, (c) Lu3+, (d) La3+, and, (e) Sc3+.
saponite framework. After the treatment at 175 °C, the XRD showed almost exclusively reflections of C-Sc2Si2O7 (JCPDS chart 72-0779; C in Fig. 1b right). The treatment of saponite with La3+, Nd3+ and Sm3+ (Fig. 1e, f and g) did not yield new phases. The only crystalline phase was 15 Åsaponite. The XRD patterns also showed a prominent background due to the partial disruption of the saponite framework, more evident in the 175 °C samples. The 29Si MAS NMR spectrum of the starting material (Fig. 2a) was typical of saponite (Alba et al., 2001b) with three main signals at −95.8, −90.8 and −85.0 ppm corresponding to Q3(mAl), 0 ≤ m ≤ 2, with an intensity ratio of 10:4.5:0.22. From these results, the Si/Al ratio was calculated (Engelhardt et al., 1981) as 8.9, in good agreement with the chemical composition, Si/Al = 9.0. The spectra of saponite reacted with Y3+ and Lu3+ solutions (Fig. 2b and c) showed peak shapes similar to the original saponite but
Fig. 3. Conversion of Si of the saponite into rare-earth silicates as a function of the hydrothermal treatment for (a) La3+, (b) Y3+, (c) Lu3+, and, (d) Sc3+. HT1 = 150 °C, 12 weeks; HT2 = 175 °C, 4 weeks, and, HT3 = 300 °C 48 h. ★ = total conversion into rare-earth silicates, ● = percentage of REE2Si2O7, and, ○ = percentage of REE2SiO5.
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Fig. 4. 27Al MAS NMR spectra of saponite (a) and after hydrothermal reaction at 150 °C for 12 weeks (left) and 175 °C for 4 weeks (right) with a solution of RE3+, REE/Si=1.25: (b) Y3+, (c) Lu3+, (d) La3+, and, (e) Sc3+. SSB = spinning side bands; * = aluminium in octahedral coordination.
with a slight asymmetric signal at higher frequency in the range of Q0 (Libau, 1985). The fit of the spectra indicated that, besides the two signals of Q3(0Al) and Q3(1Al) of saponite, a signal at −82.3 ppm for Y3+ and at − 84.6 ppm for Lu3+ was observed. These signals correspond to X1-REE2SiO5 (Becerro et al., 2004). In the case of Y3+, the X1-phase involved 5.7% Si after treatment at 150 °C and 10.9% Si at 175 °C in the case of Lu3+, the X1-phase involved 8.3% Si at 150 °C and 16.0% Si at 175 °C. No 29Si environment of REE2Si2O7 was observed (Becerro et al., 2004; Ohashi et al., 2007). The reaction of saponite with the La3+ solution led to more complex 29 Si MAS NMR spectra (Fig. 2d) resulting from the overlapping of several signals between −100 and −70 ppm, which cover the Q3(mAl), Q1 and Q0 Si environments (Libau, 1985). The deconvolution of the spectra showed four signals at −79.8 ppm, due to two X1-La2SiO5 (Becerro
Fig. 5. 1H MAS NMR spectra of saponite (a) and after hydrothermal reaction at 150 °C for 12 weeks (left) and 175 °C for 4 weeks (right) with a solution of RE3+, REE/Si = 1.25: (b) Y3+, (c) Lu3+, (d) La3+, and, (e) Sc3+.
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et al., 2004), at −83.5 ppm due to G-La2Si2O7 (Mägi et al., 1984) and at ca. − 95.5 and − 90.5 ppm corresponding to the remnant saponite (76.8% after the reaction at 150 °C and 68.8% at 150 °C). The amount of Si nuclei in the two new phases increased with temperature, the X1-phase increased from 2.4% up to 3.8% and the G-La2Si2O7 from 20.5% up to 27.6%. The 29Si signals of the remnant saponite after the hydrothermal reaction at 175 °C (Fig. 2d right) were broader than the original one, which agrees with the amorphous character indicated by XRD (Fig. 1e right). In the case of Sc3+, the reaction at 150 °C led to a 29Si MAS NMR spectrum (Fig 2e left) similar to those showed by Y3+ and Lu3+ (Fig. 2b and c). The fit of the spectrum indicated that, besides the two signals of saponite, Q3(0Al) and Q3(1Al), a new signal at −79.7 ppm, 10.3% of Si, was observed corresponding to X2-Sc2SiO5 (Alba et al., 2008b). The reaction at 175 °C promoted the total disruption of the saponite structure and the 29Si MAS NMR spectrum consisted of a set of three signals at −94.9 ppm, C-Sc2Si2O7 (Ohashi et al., 2007), −92.6 ppm, α-Na2Si2O5 (Mackenzie and Smith, 2002), and −85.7 ppm, X2-Sc2SiO5 (Becerro et al., 2004) in a proportion of 30.8%, 36.6% and 32.6% respectively.
The evaluation of the Si transformation into rare-earth silicates with the different hydrothermal treatments is shown in Fig. 3; the results of the hydrothermal treatment at 300 °C (HT3) were included (Alba et al., 2008a). At the softest treatment (HT1), the main rare-earth silicate was the oxyorthosilicate, except for La3+, and, this phase diminished as temperature increased. At the highest temperature used in this research, the disilicate phase was dominant (HT2). The transformation kinetics of oxyorthosilicate into the disilicate phase and the total conversion of saponite into rare-earth silicates depended on the rare-earth cation. In general, the total conversion of saponite into rare-earth silicates showed the trend: Sc N LuN LaN Y (stars in Fig. 3). Thus, reactivity was enhanced as the cation size decreased with exception of La3+. That two different mechanisms operate for small (Sc, Lu and Y) and large REE cations (La) (Alba et al., 2008a). The formation of an inner-sphere complex was favoured for small ionic radii (Sc, Y, Lu), because the electrostatic attraction between REE3+ and OH at the saponite edges increases. For the large cations (La), the main adsorption mechanism is the formation of an outer-sphere complex. The affinity of the exchanger for REE3+ increases when the radius of the hydrated ion decreases from
Fig. 6. (a,b) Micrographs of the saponite hydrothermally reacted at 150 °C for 12 weeks with a solution of Nd3+, Nd/Si = 1.25; (c) EDX of the lamellar particles shown in (a); (d) EDX of the block particles shown in (a); and (e) EDX of the particles shown in (b). (f,g) Micrographs of the saponite hydrothermally reacted at 175 °C for 4 weeks with a solution of Nd3+, Nd/Si = 1.25; (h) EDX of the original saponite; (i) EDX of the particles shown in (f); and; (j) EDX of the particles shown in (g).
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Sc3+ to La3+ because smaller hydrated cations approach the negative charge more easily and were more strongly fixed (Takahashi et al., 2004). A relationship was also observed between the conversion of the saponite into the disilicate phase and the formation of oxyorthosilicate (solid and open circles in Fig. 3). The amount of oxyorthosilicate decreased as the disilicate amount increased as a function of the temperature and the total conversion. Thus, these results point to oxyorthosilicate as a precursor of the disilicate phase when saponite reacts with the REE3+ solutions. The 27Al MAS NMR spectrum of the untreated saponite (Fig. 4a) showed a unique signal centred at 65 ppm due to tetrahedral aluminium, Q3(3Si) (Engelhardt and Michel, 1987), which remained almost unchanged during the reaction at 150 °C (Fig. 4 left). A very small signal at ca. 0 ppm due to octahedral aluminium after the reaction with La3+ and Sc3+ appeared. With Y3+ and Lu3+, the reaction at 175 °C (Fig. 4b and c right) only led to a very small signal at ca. 0 ppm. Apart from the 0 ppm signal, a new small signal at ca. 35 ppm, due to pentacoordinated aluminium, was observed after the reaction with La3+ at 175 °C (Fig. 4d right). The major change was observed with Sc3+ at 175 °C (Fig. 4e left) where the signal of tetrahedral aluminium disappears. The spectrum was characterized by a low S/N ratio with a very broad signal in the range of pentacoordinated aluminium, in agreement with the absence of saponite indicated by XRD and 29Si MAS NMR. The 1H MAS NMR spectrum of the untreated saponite (Fig. 5a) showed two signals at 3.70 ppm and 0.63 ppm due to the interlayer water and the framework hydroxyl groups (Alba et al., 2003). The reaction of saponite with Y3+, Lu3+ and La3+ solutions at both temperatures led to 1H MAS NMR spectra (Fig. 5b, c and d) where the hydroxyl proton signal diminished drastically and the water proton signal shifted towards higher frequencies, in agreement with the exchange of Na+ by REE3+ (Alba et al., 2003). In the case of Sc3+, a
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similar spectrum to those described for the others cations was observed at the lowest temperature (Fig. 5e, left) but an almost complete disappearance of saponite at 175 °C was indicated by the 1H MAS NMR spectrum (Fig. 5e, right) where the lower frequency signal was not observed. Regarding the saponite reacted with Nd3+ and Sm3+, the MAS NMR study was not possible due to the paramagnetic character of these cations. In these samples the information obtained through XRD was poor due to the presence of a considerable amount of amorphous phases. Thus, the samples were carefully studied by electron scanning microscopy (Figs. 6 and 7). After the reaction of saponite with Nd3+ at 150° and 175 °C, most of the particles showed a lamellar morphology (Fig. 6a and f) and EDX spectra (Fig. 6c and i) characterized by the Kα1 lines of Si, Al and Mg and Mα, Lα and Lβ lines of Nd due to Nd-saponite. The decrease of Mg and the absence of Na, in comparison with the original saponite (Fig. 6h), were due to the leaching of Mg2+ from the framework and the exchanged of Na+ by Nd3+ in the interlayer space. The XRD patterns showed also degradation of the layers and increased basal spacing (Fig. 1f). Besides those lamellar particles, some compact particles with brilliant appearance under the backscattering electron beam were observed (Fig. 6a and g) with a chemical composition compatible with Nd2Si2O7 or Nd2SiO5 (Fig. 6d and j). Finally, saponite reacted with Nd3+ at 150 °C showed block particles (Fig. 6b) with an EDX spectrum (Fig. 6e) only showing Mα, Lα and Lβ lines of Nd, may be of Nd2O3. The hydrothermal reaction of saponite with Sm3+ at 150° and 175 °C led to a majority of the particles with lamellar structure (Fig. 7a and d) and an EDX spectra (Fig. 7b and e) compatible with saponite exchanged with Sm3+. The lamellar particles showed some small brilliant particles at their surfaces (marked with a circle in Fig. 7a and d) with a chemical composition compatible with Sm2Si2O7 or Sm2SiO5 (Fig. 7c and f). These particles could not be observed as isolated particles, as in the reaction with Nd3+.
Fig. 7. (a) Micrograph of the saponite hydrothermally reacted at 150 °C for 12 weeks with a solution of Sm3+, Sm/Si = 1.25; (b) EDX of the original saponite (red dashed line) and lamellar particles shown in (a); and; (c) EDX of the brilliant particles shown in (a). (d) Micrograph of the saponite hydrothermally reacted at 175 °C for 4 weeks with a solution of Sm3+, Sm/Si = 1.25; (e) EDX of the original saponite (red dash line) and lamellar particles shown in (d); and; (f) EDX of the brilliant particles shown in (d). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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4. Conclusions The results indicated formation of rare-earth silicate at temperatures as low as 150 °C, close to the expected temperature in the DGR (Astudillo, 2001). The temperature influenced the degrees of conversion much more than time did. Saponite reacted with Sc3+ at 175 °C for 4 weeks showed a total conversion, similar to previously described reactions at 300 °C for 48 h (63.4% vs. 70.8%) (Alba et al., 2008a). Therefore, it can be foreseen that the chemical reaction of saponite with actinide simulators will be limited kinetically but not limited thermodynamically at the conditions expected in the DGR. The oxyorthosilicate phase was formed as precursor of disilicate formation, in good agreement with previous work by Maier et al. (2006) who observed that even starting with the stoichiometric relation Si:REE 1:1 large amounts of oxyorthosilicate were obtained. They postulated that the rare-earth disilicate did not form directly, but in two stages with oxyorthosilicate as intermediate phase: RE2 O3 þ SiO2 →RE2 SiO5 RE2 SiO5 þ SiO2 →RE2 Si2 O7 Although the starting materials employed by us differ from those shown in this mechanism, they changed in a similar manner. This implies the existence of common points in both mechanisms of reaction. At this point, it was not possible to postulate a complete reaction mechanism but the obtained results reveal some stages of this mechanism: (i) Ion exchange of interlayer cations, Na+ by REE3+. (ii) Leaching of cations from the saponite framework and generation of amorphous phases. (iii) Formation of the oxyorthosilicate phases in the early stage which transform into disilicates. Acknowledgments We gratefully acknowledge financial support from DGICYT Projects no. CTQ2007-63297, Junta de Andalucía Project no. P06-FQM-02179, PETRI project PTR95-0996.OP and from EC for the project funded within the 6th Framework Programme as an HRM activity under contract number MRTN-CT-2006-035957. References Abdelouas, A., Crovisier, J.L., Lutze, W., Fritz, B., Mosser, A., Müller, R., 1994. Formation of hydrotalcite-like compounds during R7T7 nuclear waste glass and basaltic glass alteration. Clays and Clay Minerals 42, 526–533. Abdelouas, A., Crovisier, J.L., Lutze, W., Grambow, B., Dran, J.C., Müller, R., 1997. Surface layers on a borosilicate nuclear waste glass corroded in MgCl2 solution. Journal of Nuclear Materials 240, 100–111. Alba, M.D., Becerro, A.I., Castro, M.A., Perdigón, A.C., 2001a. Hydrothermal reactivity of Lu-saturated smectites: part I. a long-range order study. American Mineralogist 86, 115–123. Alba, M.D., Becerro, A.I., Castro, M.A., Perdigón, A.C., 2001b. Hydrothermal reactivity of Lu-saturated smectites: part II. a short-range order study. American Mineralogist 86, 124–131.
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Wilson, J., Cressey, G., Cressey, B., Cuadros, J., Ragnardsdottir, K.V., Savage, D., Shibata, M., 2006a. The effect of iron on montmorillonite stability. (II). Experimental investigation. Geochimica et Cosmochimica Acta 70, 323–336. Wilson, J., Savage, D., Cuadros, J., Shibata, M., Ragnardsdottir, K.V., 2006b. The effect of iron on montmorillonite stability. (I). Background and thermodynamic considerations. Geochimica et Cosmochimica Acta 70, 306–322.
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Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y
Rare-earth disilicate formation under Deep Geological Repository approach conditions María D. Alba ⁎, Pablo Chain, M. Mar Orta Instituto Ciencia de los Materiales de Sevilla, CSIC-US, Avda, Americo Vespucio, 49, 41092 Sevilla, Spain
a r t i c l e
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Article history: Received 13 December 2008 Received in revised form 9 July 2009 Accepted 13 July 2009 Available online 18 July 2009 Keywords: Deep Geological Repository Engineered Barrier System Bentonite Rare earth Hydrothermal Long-lived radiactive waste
a b s t r a c t The Deep Geological Repository (DGR) concept involves the placement of long-lived radioactive waste in rooms excavated deep. The major responsibility of the disposal safety falls on the Engineered Barrier System (EBS). The main constituent of EBS is bentonite that prevents the release of radiactive nuclei by physical and chemical mechanisms. The physical mechanism is expected to fault with the weathering of the bentonite while the chemical mechanisms have been only proved at 300 °C. It is the aim of this paper to explore the feasibility of the chemical mechanism at temperatures closer to the DGP conditions and to shed light on the mechanism of transformation of the argillaceous materials of the EBS in rare-earth disilicate phases. Saponite was submitted to hydrothermal reaction at 175 °C and 150 °C with different solutions of REE3+ cations (REE = Sc, Lu, Y, Sm, Nd and La). The products were analyzed by XRD, NMR and electron microscopy. At conditions close to the DGP, the saponite was able to form rare-earth silicates. The formation of the disilicate phase, as final product, needs a set of stages and oxyorthosilicate as precursor. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The Deep Geological Repository (DGR) concept involves the placement of long-lived radioactive waste, often spent nuclear fuel, in rooms excavated deep within stable, low-permeability bedrock (Savage and Chapman, 1982; Astudillo, 2001). The combination of waste package, engineered seals and bedrock would provide a high level of long-term safety, without relying on on-going future maintenance. One of the challenges facing the supporters of these efforts is to demonstrate that a repository will contain wastes for so long that any release that might take place in the future will pose no significant health or environmental risk. The major responsibility of the safety falls on the Engineered Barrier System (EBS) which is one part of this passive multi-barrier system approach. On one hand, it is expected that most radionuclides will decay in the EBS (e.g. it is conservatively expected that the steel canister will last for over 1000 years). In the other hand, in almost all repository concepts it is assumed that the geosphere surrounding the EBS is saturated with water, and this groundwater will be gradually flowing back into the tunnel and the EBS after the tunnel is closed. During bentonite saturation, it swells and seals the tunnel, restricting almost all flow of water inside the EBS (Bailey, 1980). Additionally, the existence of a simultaneous chemical reaction between the argillaceous component of the EBS and the rare-earth cations, as actinide simulators has been demonstrated (Perdigón,
⁎ Corresponding author. Fax: +34 954460665. E-mail address: [email protected] (M.D. Alba). 0169-1317/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2009.07.012
2002; Alba and Chain, 2005, 2007; Alba et al., 2008a). The formation of REE2Si2O7 at 300 °C, under hydrothermal conditions, is formed from a wide set of REE3+ cations (Alba et al., 2008a). This temperature is an appropriated condition to simulate deep geological disposal conditions and to increase reaction rates (Mather et al., 1982; Savage and Chapman, 1982; Allen and Wood, 1988). However, it is well-known that geochemical processes are developed at 200 °C (Mather et al., 1982; Savage and Chapman, 1982; Allen and Wood, 1988). To evaluate the applicability of the long-immobilization process proposed by Alba et al. (2008a) in real repositories, it is necessary to explore the feasibility of the reaction below 200 °C. It is the aim of this paper to explore the feasibility of the reaction at temperatures closer to the DGP conditions and to shed light on the transformation mechanism of the argillaceous materials of the EBS in the rare-earth disilicate.
2. Experimental 2.1. Materials Saponite was obtained from the Source Clay Minerals Repository University of Missouri (Columbia) and had the following chemical formula: Na0.61K0.02Ca0.09 (Si7.2Al0.8)IV(Mg5.79Fe0.15)VIO20(OH)4 (Alba et al, 2001a). The choice of saponite as starting material has been based on the fact that saponite is the crystalline reaction product of nuclear waste glass altered in brine (Abdelouas et al., 1994, Abdelouas et al., 1997) and it is formed after the reaction of bentonite with the steel corrosion products of the canister of the nuclear repository
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(Guillaume et al., 2003, 2004; Wilson et al., 2006a,b) and in hyperalkaline condition resembling cement–bentonite interface (Sánchez et al., 2006). Additionally, Alba et al. (2001a,b) demonstrated that saponite was the most reactive smectite species under hydrothermal conditions and it allows us to obtain results at laboratory time scale. In order to study the behaviour of critical long-lived radionuclides, we chose a set of suitable chemical elements: Sc, Y, La, Nd, Sm, and Lu. Based on similarities in valence state, speciation, complexation, ionic radius, etc., possible chemical analogues for the most important longlived trivalent radionuclides Np, Am and Cm were La3+, Nd3+ and Sm3+ (Chapman, 1986). The study of the hydrothermal reactivity between smectite and these rare-earth cations was complemented with a similar study of Sc3+, Y3+ and Lu3+. 2.2. Hydrothermal reactions The powdered saponite was dispersed in 50 ml of 6.3 · 10− 2 M REE (NO3)3 solution, in a molar ratio REE/Si = 1.25 and was heated at 175 °C for 4 weeks and at 150 °C for 12 weeks in a stainless steel reactor (Perdigón, 2002). The reaction products were collected by filtering, washed with distilled water and dried in air at 60 °C. 2.3. Characterization methods 2.3.1. X-ray powder diffraction (XRD) XRD patterns were obtained with a Bruker D8I instrument (Ni filtered Cu Ka radiation, at 40 kV and 40 mA), at the CITIUS, Universidad de Sevilla. Diffractograms were obtained from 3 to 70° 2θ at a scan step of 0.05° 2θ and with a counting time of 10 s. 2.3.2. Scanning electron microscopy (SEM) The morphology and chemical composition of the samples were analyzed at the Microscopy Service of the Instituto Ciencia de los Materiales de Sevilla (CSIC-US) with a Scanning Electron Microscope (JEOL JSM 5400) equipped with a LINK Pentafet probe and ATW windows for Energy Dispersive X-ray Analysis (EDX). 2.3.3. Nuclear magnetic resonance spectroscopy (MAS NMR) 29 Si, 27Al and 1H single-pulse spectra were recorded on a Bruker DRX400 spectrometer with a magnetic field of 9.36 T and equipped with
a multinuclear probe of the Spectroscopy Service of the Instituto Ciencia de los Materiales de Sevilla (CSIC-US). Powdered samples were packed in 4 mm zirconia rotors and spun at 12 kHz. 1H MAS NMR spectra were obtained at a frequency of 400.13 MHz using a typical π/2 pulse width of 4.5 μs and a pulse space of 5 s. 29Si MAS NMR spectra were obtained at a frequency of 79.49 MHz using a π/6 pulse width of 2.7 μs and a pulse space of 60 s. 27Al MAS NMR spectra were recorded at 104.26 MHz with a π/20 pulse width of 1.1 μs and delay time of 3 s. The chemical shifts were reported in ppm from tetramethylsilane for 29Si and 1H and from 0.1 M solutions of AlCl3 for 27Al. The MAS NMR analysis has not been performed for saponite reacted with Nd3+ or Sm3+ due to the paramagnetic behaviour of both cations. The spectra were simulated using a modified version of the Bruker Winfit program to handle the finite spinning speed in MAS experiments (Massiot et al., 2002). A Gaussian–Lorentzian model was used for all the peaks. The fitted parameters were amplitude, position, linewidth and Gaussian/Lorentzian ratio.
3. Results and discussion Fig. 1 shows the XRD pattern of the saponite before and after hydrothermal reaction with REE3+ solutions at 150 °C for 12 weeks (left) and 175 °C for 4 weeks (right). The original saponite (Fig. 1a) showed a pattern that is made up of the two distinct types of reflections, general and basal reflections. The basal spacing of 12.3 Å of saponite corresponded to the one-layer hydrate and d060 was in agreement with the literature data for trioctahedral smectites (Grim, 1968; Bergaya et al., 2006). The remnant smectite after the hydrothermal reactions, showed basal spacing up to 15.3 Å, in agreement with previous data reported for smectites saturated with multivalent cations (Ravina and Low, 1977), and a slight displacement of the 060 reflection, in the range of trioctahedral smectites (Grim, 1968), to lower 2θ angles. Additional changes in the XRD patterns of saponite reacted with Y3+ and Lu3+ solutions at 150° and 175 °C were not observed (Fig. 1c and d). The XRD of the saponite reacted with Sc3+ at 150 °C (Fig. 1b left) showed a set of small reflections corresponding to X2-Sc2SiO5 (JCPDS chart 40-0035; X in Fig. 1b left) and coexisting with 15 Å-saponite and with a small amount of 11 Å-saponite, i.e. partially dehydrated saponite (marked with asterisk in the Fig. 1b left). The XRD showed a prominent background which indicated the partial disruption of the
Fig. 1. XRD patterns of saponite before (a) and after hydrothermal reaction at 150 °C for 12 weeks (left) and 175 °C for 4 weeks (right) with a solution of RE3+, REE/Si = 1.25: (b) Sc3+, (c) Lu3+, (d) Y3+, (e) La3+, (f) Nd3+, and, (g) Sm3+. X = X2-RE2SiO5, C = CRE2Si2O7, and, * = dehydrated saponite.
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Fig. 2. Rows (dots) and fits (lines) 29Si MAS NMR spectra of saponite (a) and after hydrothermal reaction at 150 °C for 12 weeks (left) and 175 °C for 4 weeks (right) with a solution of RE3+, REE/Si = 1.25: (b) Y3+, (c) Lu3+, (d) La3+, and, (e) Sc3+.
saponite framework. After the treatment at 175 °C, the XRD showed almost exclusively reflections of C-Sc2Si2O7 (JCPDS chart 72-0779; C in Fig. 1b right). The treatment of saponite with La3+, Nd3+ and Sm3+ (Fig. 1e, f and g) did not yield new phases. The only crystalline phase was 15 Åsaponite. The XRD patterns also showed a prominent background due to the partial disruption of the saponite framework, more evident in the 175 °C samples. The 29Si MAS NMR spectrum of the starting material (Fig. 2a) was typical of saponite (Alba et al., 2001b) with three main signals at −95.8, −90.8 and −85.0 ppm corresponding to Q3(mAl), 0 ≤ m ≤ 2, with an intensity ratio of 10:4.5:0.22. From these results, the Si/Al ratio was calculated (Engelhardt et al., 1981) as 8.9, in good agreement with the chemical composition, Si/Al = 9.0. The spectra of saponite reacted with Y3+ and Lu3+ solutions (Fig. 2b and c) showed peak shapes similar to the original saponite but
Fig. 3. Conversion of Si of the saponite into rare-earth silicates as a function of the hydrothermal treatment for (a) La3+, (b) Y3+, (c) Lu3+, and, (d) Sc3+. HT1 = 150 °C, 12 weeks; HT2 = 175 °C, 4 weeks, and, HT3 = 300 °C 48 h. ★ = total conversion into rare-earth silicates, ● = percentage of REE2Si2O7, and, ○ = percentage of REE2SiO5.
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Fig. 4. 27Al MAS NMR spectra of saponite (a) and after hydrothermal reaction at 150 °C for 12 weeks (left) and 175 °C for 4 weeks (right) with a solution of RE3+, REE/Si=1.25: (b) Y3+, (c) Lu3+, (d) La3+, and, (e) Sc3+. SSB = spinning side bands; * = aluminium in octahedral coordination.
with a slight asymmetric signal at higher frequency in the range of Q0 (Libau, 1985). The fit of the spectra indicated that, besides the two signals of Q3(0Al) and Q3(1Al) of saponite, a signal at −82.3 ppm for Y3+ and at − 84.6 ppm for Lu3+ was observed. These signals correspond to X1-REE2SiO5 (Becerro et al., 2004). In the case of Y3+, the X1-phase involved 5.7% Si after treatment at 150 °C and 10.9% Si at 175 °C in the case of Lu3+, the X1-phase involved 8.3% Si at 150 °C and 16.0% Si at 175 °C. No 29Si environment of REE2Si2O7 was observed (Becerro et al., 2004; Ohashi et al., 2007). The reaction of saponite with the La3+ solution led to more complex 29 Si MAS NMR spectra (Fig. 2d) resulting from the overlapping of several signals between −100 and −70 ppm, which cover the Q3(mAl), Q1 and Q0 Si environments (Libau, 1985). The deconvolution of the spectra showed four signals at −79.8 ppm, due to two X1-La2SiO5 (Becerro
Fig. 5. 1H MAS NMR spectra of saponite (a) and after hydrothermal reaction at 150 °C for 12 weeks (left) and 175 °C for 4 weeks (right) with a solution of RE3+, REE/Si = 1.25: (b) Y3+, (c) Lu3+, (d) La3+, and, (e) Sc3+.
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et al., 2004), at −83.5 ppm due to G-La2Si2O7 (Mägi et al., 1984) and at ca. − 95.5 and − 90.5 ppm corresponding to the remnant saponite (76.8% after the reaction at 150 °C and 68.8% at 150 °C). The amount of Si nuclei in the two new phases increased with temperature, the X1-phase increased from 2.4% up to 3.8% and the G-La2Si2O7 from 20.5% up to 27.6%. The 29Si signals of the remnant saponite after the hydrothermal reaction at 175 °C (Fig. 2d right) were broader than the original one, which agrees with the amorphous character indicated by XRD (Fig. 1e right). In the case of Sc3+, the reaction at 150 °C led to a 29Si MAS NMR spectrum (Fig 2e left) similar to those showed by Y3+ and Lu3+ (Fig. 2b and c). The fit of the spectrum indicated that, besides the two signals of saponite, Q3(0Al) and Q3(1Al), a new signal at −79.7 ppm, 10.3% of Si, was observed corresponding to X2-Sc2SiO5 (Alba et al., 2008b). The reaction at 175 °C promoted the total disruption of the saponite structure and the 29Si MAS NMR spectrum consisted of a set of three signals at −94.9 ppm, C-Sc2Si2O7 (Ohashi et al., 2007), −92.6 ppm, α-Na2Si2O5 (Mackenzie and Smith, 2002), and −85.7 ppm, X2-Sc2SiO5 (Becerro et al., 2004) in a proportion of 30.8%, 36.6% and 32.6% respectively.
The evaluation of the Si transformation into rare-earth silicates with the different hydrothermal treatments is shown in Fig. 3; the results of the hydrothermal treatment at 300 °C (HT3) were included (Alba et al., 2008a). At the softest treatment (HT1), the main rare-earth silicate was the oxyorthosilicate, except for La3+, and, this phase diminished as temperature increased. At the highest temperature used in this research, the disilicate phase was dominant (HT2). The transformation kinetics of oxyorthosilicate into the disilicate phase and the total conversion of saponite into rare-earth silicates depended on the rare-earth cation. In general, the total conversion of saponite into rare-earth silicates showed the trend: Sc N LuN LaN Y (stars in Fig. 3). Thus, reactivity was enhanced as the cation size decreased with exception of La3+. That two different mechanisms operate for small (Sc, Lu and Y) and large REE cations (La) (Alba et al., 2008a). The formation of an inner-sphere complex was favoured for small ionic radii (Sc, Y, Lu), because the electrostatic attraction between REE3+ and OH at the saponite edges increases. For the large cations (La), the main adsorption mechanism is the formation of an outer-sphere complex. The affinity of the exchanger for REE3+ increases when the radius of the hydrated ion decreases from
Fig. 6. (a,b) Micrographs of the saponite hydrothermally reacted at 150 °C for 12 weeks with a solution of Nd3+, Nd/Si = 1.25; (c) EDX of the lamellar particles shown in (a); (d) EDX of the block particles shown in (a); and (e) EDX of the particles shown in (b). (f,g) Micrographs of the saponite hydrothermally reacted at 175 °C for 4 weeks with a solution of Nd3+, Nd/Si = 1.25; (h) EDX of the original saponite; (i) EDX of the particles shown in (f); and; (j) EDX of the particles shown in (g).
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Sc3+ to La3+ because smaller hydrated cations approach the negative charge more easily and were more strongly fixed (Takahashi et al., 2004). A relationship was also observed between the conversion of the saponite into the disilicate phase and the formation of oxyorthosilicate (solid and open circles in Fig. 3). The amount of oxyorthosilicate decreased as the disilicate amount increased as a function of the temperature and the total conversion. Thus, these results point to oxyorthosilicate as a precursor of the disilicate phase when saponite reacts with the REE3+ solutions. The 27Al MAS NMR spectrum of the untreated saponite (Fig. 4a) showed a unique signal centred at 65 ppm due to tetrahedral aluminium, Q3(3Si) (Engelhardt and Michel, 1987), which remained almost unchanged during the reaction at 150 °C (Fig. 4 left). A very small signal at ca. 0 ppm due to octahedral aluminium after the reaction with La3+ and Sc3+ appeared. With Y3+ and Lu3+, the reaction at 175 °C (Fig. 4b and c right) only led to a very small signal at ca. 0 ppm. Apart from the 0 ppm signal, a new small signal at ca. 35 ppm, due to pentacoordinated aluminium, was observed after the reaction with La3+ at 175 °C (Fig. 4d right). The major change was observed with Sc3+ at 175 °C (Fig. 4e left) where the signal of tetrahedral aluminium disappears. The spectrum was characterized by a low S/N ratio with a very broad signal in the range of pentacoordinated aluminium, in agreement with the absence of saponite indicated by XRD and 29Si MAS NMR. The 1H MAS NMR spectrum of the untreated saponite (Fig. 5a) showed two signals at 3.70 ppm and 0.63 ppm due to the interlayer water and the framework hydroxyl groups (Alba et al., 2003). The reaction of saponite with Y3+, Lu3+ and La3+ solutions at both temperatures led to 1H MAS NMR spectra (Fig. 5b, c and d) where the hydroxyl proton signal diminished drastically and the water proton signal shifted towards higher frequencies, in agreement with the exchange of Na+ by REE3+ (Alba et al., 2003). In the case of Sc3+, a
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similar spectrum to those described for the others cations was observed at the lowest temperature (Fig. 5e, left) but an almost complete disappearance of saponite at 175 °C was indicated by the 1H MAS NMR spectrum (Fig. 5e, right) where the lower frequency signal was not observed. Regarding the saponite reacted with Nd3+ and Sm3+, the MAS NMR study was not possible due to the paramagnetic character of these cations. In these samples the information obtained through XRD was poor due to the presence of a considerable amount of amorphous phases. Thus, the samples were carefully studied by electron scanning microscopy (Figs. 6 and 7). After the reaction of saponite with Nd3+ at 150° and 175 °C, most of the particles showed a lamellar morphology (Fig. 6a and f) and EDX spectra (Fig. 6c and i) characterized by the Kα1 lines of Si, Al and Mg and Mα, Lα and Lβ lines of Nd due to Nd-saponite. The decrease of Mg and the absence of Na, in comparison with the original saponite (Fig. 6h), were due to the leaching of Mg2+ from the framework and the exchanged of Na+ by Nd3+ in the interlayer space. The XRD patterns showed also degradation of the layers and increased basal spacing (Fig. 1f). Besides those lamellar particles, some compact particles with brilliant appearance under the backscattering electron beam were observed (Fig. 6a and g) with a chemical composition compatible with Nd2Si2O7 or Nd2SiO5 (Fig. 6d and j). Finally, saponite reacted with Nd3+ at 150 °C showed block particles (Fig. 6b) with an EDX spectrum (Fig. 6e) only showing Mα, Lα and Lβ lines of Nd, may be of Nd2O3. The hydrothermal reaction of saponite with Sm3+ at 150° and 175 °C led to a majority of the particles with lamellar structure (Fig. 7a and d) and an EDX spectra (Fig. 7b and e) compatible with saponite exchanged with Sm3+. The lamellar particles showed some small brilliant particles at their surfaces (marked with a circle in Fig. 7a and d) with a chemical composition compatible with Sm2Si2O7 or Sm2SiO5 (Fig. 7c and f). These particles could not be observed as isolated particles, as in the reaction with Nd3+.
Fig. 7. (a) Micrograph of the saponite hydrothermally reacted at 150 °C for 12 weeks with a solution of Sm3+, Sm/Si = 1.25; (b) EDX of the original saponite (red dashed line) and lamellar particles shown in (a); and; (c) EDX of the brilliant particles shown in (a). (d) Micrograph of the saponite hydrothermally reacted at 175 °C for 4 weeks with a solution of Sm3+, Sm/Si = 1.25; (e) EDX of the original saponite (red dash line) and lamellar particles shown in (d); and; (f) EDX of the brilliant particles shown in (d). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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4. Conclusions The results indicated formation of rare-earth silicate at temperatures as low as 150 °C, close to the expected temperature in the DGR (Astudillo, 2001). The temperature influenced the degrees of conversion much more than time did. Saponite reacted with Sc3+ at 175 °C for 4 weeks showed a total conversion, similar to previously described reactions at 300 °C for 48 h (63.4% vs. 70.8%) (Alba et al., 2008a). Therefore, it can be foreseen that the chemical reaction of saponite with actinide simulators will be limited kinetically but not limited thermodynamically at the conditions expected in the DGR. The oxyorthosilicate phase was formed as precursor of disilicate formation, in good agreement with previous work by Maier et al. (2006) who observed that even starting with the stoichiometric relation Si:REE 1:1 large amounts of oxyorthosilicate were obtained. They postulated that the rare-earth disilicate did not form directly, but in two stages with oxyorthosilicate as intermediate phase: RE2 O3 þ SiO2 →RE2 SiO5 RE2 SiO5 þ SiO2 →RE2 Si2 O7 Although the starting materials employed by us differ from those shown in this mechanism, they changed in a similar manner. This implies the existence of common points in both mechanisms of reaction. At this point, it was not possible to postulate a complete reaction mechanism but the obtained results reveal some stages of this mechanism: (i) Ion exchange of interlayer cations, Na+ by REE3+. (ii) Leaching of cations from the saponite framework and generation of amorphous phases. (iii) Formation of the oxyorthosilicate phases in the early stage which transform into disilicates. Acknowledgments We gratefully acknowledge financial support from DGICYT Projects no. CTQ2007-63297, Junta de Andalucía Project no. P06-FQM-02179, PETRI project PTR95-0996.OP and from EC for the project funded within the 6th Framework Programme as an HRM activity under contract number MRTN-CT-2006-035957. References Abdelouas, A., Crovisier, J.L., Lutze, W., Fritz, B., Mosser, A., Müller, R., 1994. Formation of hydrotalcite-like compounds during R7T7 nuclear waste glass and basaltic glass alteration. Clays and Clay Minerals 42, 526–533. Abdelouas, A., Crovisier, J.L., Lutze, W., Grambow, B., Dran, J.C., Müller, R., 1997. Surface layers on a borosilicate nuclear waste glass corroded in MgCl2 solution. Journal of Nuclear Materials 240, 100–111. Alba, M.D., Becerro, A.I., Castro, M.A., Perdigón, A.C., 2001a. Hydrothermal reactivity of Lu-saturated smectites: part I. a long-range order study. American Mineralogist 86, 115–123. Alba, M.D., Becerro, A.I., Castro, M.A., Perdigón, A.C., 2001b. Hydrothermal reactivity of Lu-saturated smectites: part II. a short-range order study. American Mineralogist 86, 124–131.
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