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Smectite alteration and its influence on the barrier properties of smectite clay for a repository
Applied Clay Science 47 (2010) 99–104
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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 e v i e r. c o m / l o c a t e / c l a y
Smectite alteration and its influence on the barrier properties of smectite clay for a repository Jae Owan Lee a,⁎, Il Mo Kang b, Won Jin Cho a a b
Korea Atomic Energy Research Institute, P.O. Box 105, Yusong, Taejon 305-600, Korea Korea National Oil Corporation, 1588-14, Gwanyang-dong, Dongsan-gu, Anyang, Gyeonggi-do 431-711, Korea
a r t i c l e
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Article history: Received 31 December 2007 Received in revised form 10 October 2008 Accepted 13 October 2008 Available online 29 October 2008 Keywords: Smectite Hydrothermal test Smectite alteration Barrier property Repository
a b s t r a c t Hydrothermal tests were conducted to investigate smectite alteration and its influence on the barrier properties of a smectite clay for a repository. Examinations of the X-ray diffraction patterns of the starting material and reacted samples and the silica release rate in the solution revealed that the smectite was transformed into randomly interstratified illite–smectite by a smectite-to-illite conversion when it was hydrothermally treated under a potassium concentration of 0.5 M, maintaining a 1 g/20 ml of solid sampleto-solution ratio. Temperature was observed to be a key factor controlling the conversion reaction. The smectite alteration affected the barrier properties of smectite clay for a repository: when the temperature increased, the percentage of the expandable smectite layers in the randomly interstratified illite–smectite decreased, the layer charge was more negative, and the cation exchange capacity and the sorption capacity for the cesium and nickel ions were reduced. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The major functions of the buffer in a high-level waste (HLW) repository are to inhibit the penetration of groundwater and to retard the release of radionuclides from the radioactive wastes to the surrounding environment. Smectite clay has been considered favorably as such a buffer material because of its high swelling and sorption capacities. However, when the smectite clay is exposed to an elevated temperature due to heat from radioactive decay and the particular geochemical conditions of a repository for a long time, it may be transformed into other minerals (e.g., illite, chlorite, etc.) which leads to a decrease in the swelling and sorption properties of the smectite clay and consequently increases its water penetration and radionuclide transport properties (Pusch and Carlsson, 1985; Bucher and Müller-Vonmoos, 1989). Therefore, an understanding of smectite alteration and its influence on the barrier properties of a smectite clay is essential to evaluate the long-term barrier performance of a buffer for a HLW repository. Smectite alteration, especially smectite-to-illite conversion which may occur under commonly-prevailing pH conditions, has drawn considerable interest in the last few decades due to the implication of its reaction for clay diagenesis and migration of petroleum as well as the long-term barrier performance of a smectitic buffer for a repository. Very many investigators (Eberl, 1978; Roberson and Lahann, 1981; Inoue, 1983; Howard and Roy, 1985; Huang and Otten,
⁎ Corresponding author. Tel.: +82 42 868 2852; fax: +82 42 868 8850. E-mail address: [email protected] (J.O. Lee). 0169-1317/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2008.10.007
1987; Proust et al., 1990; Güven and Huang, 1991; Velde and Vasseur, 1992; Huang et al., 1993; Cuadros and Linares, 1996; Huertas et al., 2001; Bauer et al., 2005; Sato et al., 2005; Bauer et al., 2006) have studied the reaction mechanism and kinetics of smectite-to-illite conversion. On the basis of those studies, smectite-to-illite conversion has been suggested by a general consensus to proceed by two different mechanisms. One is a solid-state one-to-one transformation where T– O–T layers are conserved, and the reaction proceeds with a replacement of the tetrahedral Si4+by Al3+until the layer charge deficiency is sufficiently developed to dehydrate the interlayer cations, and a collapse occurs (Hower et al., 1976): Smectite þ Al3þ þ Kþ –NIllite þ Si4þ The other is a mechanism in which a destruction of the T–O–T layers provides the source of aluminium for the transformation, which thus consumes more smectite than it produces illite (Boles and Franks,1979): Smectite þ Kþ –NIllite þ Si4þ where the ratio of smectite consumed to illite produced is approximately 1.6:1, and no external source of aluminium is required. However, no consensus has been reached on which mechanism prevails because the reaction depends on different test conditions. The kinetics of a smectite-to-illite conversion has been studied to obtain a rate law for the overall conversion process (Pytte and Reynolds, 1989; Velde and Vasseur, 1992; Huang et al., 1993; Wei et al., 1993; Cuadros and Linares, 1996; Huertas et al., 2001). Most of these studies are based on field observation of clay diagenesis, while there
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Table 1 Physicochemical and mineralogical properties of the bentonite and the separated smectite. Sample
Properties
Bentonite
Mineral composition Smectite (78%), feldspar (20.1%), quartz (1.7%) CEC: 61.7 cmol/kg pH: 8.3 Chemical composition (wt.%)(by XRF) SiO2 58.21 Al2O3 18.31 Fe2O3 7.85 TiO2 0.91 CaO 0.15 MgO 3.19 MnO 0.04 Na2O 3.31 K2O 0.21 P2O5 0.10 L.O.I 7.49 Total 99.75 Structural formula (Ca, Na, K)0.93(Al2.57Fe+30.78 Mg0.64)(Si7.71Al0.29)O20(OH)4 CEC: 96.5 cmol/kg
Smectite (b 2 µm)
are not many studies based on systematic laboratory work. From these studies, it is implied that, although the most important parameters for controlling smectite-to-illite conversion are the temperature, potassium concentration, and reaction time, the proposed rate laws have limitations in their application to evaluate the extent of smectite-to-illite conversion under different repository conditions, due to an uncertainty in the parameter values. On the other hand, insufficient data has been reported regarding the influence of smectite alteration on the barrier properties such as the percentage of expandable smectite (MacEwan and Wilson, 1980; Huang et al., 1993), layer charge (Howard and Roy, 1985; Güven and Huang, 1991), cation exchange capacity (CEC) (Oscarson, and Hume, 1993), and sorption capacity (Comans et al., 1991; Ohnuki et al., 1994) of a smectite clay for a buffer of a HLW repository. The present study, in this connection, focuses on investigating to what extent the smectite may be altered when it is hydrothermally treated under a certain potassium concentration and as a result how the smectite alteration may affect the barrier properties of a smectite clay for a Korean HLW repository. 2. Materials and methods
perature of 90, 140, 200 °C, an initial concentration of CsCl of 5×10− 3 M and NiCl2 of 5×10− 4 M, and a reaction time of 3, 7, 15, 28, 50 days. The internal pressure was subject to an autogenous vapor pressure of H2O at a given temperature of each test run. Upon termination of the runs, the vessels were quenched in iced water for their rapid cooling to a temperature of 25 °C to reduce the possibility of silica polymerization. The cooled suspension was centrifuged at 10,000 rpm for 10 min; the solution was filtered on a 0.45 μm membrane; and the filtered solids were freeze-dried. The chemical and mineralogical investigations were conducted by means of inductively coupled plasma mass spectrometry (ICP-MS) for the liquid solutions, and X-ray diffraction (XRD), Fourier transform infrared spectrometry (FT-IR), and Electron Probe Micro Analysis (EPMA) for the solid samples. In preparation for the XRD analyses, each solid sample was resaturated with calcium ions to remove the exchangeable potassium not completely fixed into the illite interlayer during the hydrothermal tests. After extra electrolytes were washed with deionized water, oriented specimens were prepared by sedimenting the treated sample onto a glass slide, followed by ethylene-glycol (EG) treatment. The XRD analyses were performed with a Bragg–Brentano diffractometer (MXP 18A RINT-2500, Mac Science Co., Ltd, Japan): Cu-Kα (40 kV/30 mA), a graphic monochromator, a 1° divergence slit, a 0.15° receiving slit, two soller slits (2.4° and 2.5°), and 1 s /0.05° 2 step-scanning. The FT-IR analyses for investigation of the sorption mechanism of cesium and nickel onto the solid material were conducted using an in-situ IR cell (Graseby, Specac Co.) with the water in the interlayer of the sample removed by vacuuming under the condition of 760 mmHg and 110 °C for 24 h.
2.3. Determination of expandability, layer charge, CEC, and sorption distribution coefficient The expandability (the percentage of the expandable smectite layers in the interstratified I–S, % S) was determined using a saddle/001 peak intensity ratio for the ethylene-glycolated (EG) sample (Inoue et al., 1989). Glycolations were performed under ethylene glycol vapor at 60 °C for 24 h. In this analysis, the NEWMOD program (Reynolds, 1985) was complementarily used for simulating the XRD pattern of interstratified illite–smectite (I–S) and obtaining a calibration curve between the saddle/001 peak intensity ratio and % S. The layer charge was evaluated
Table 2 Experimental design for hydrothermal tests. Run
2.1. Materials The solid material used for the tests was a natural smectite fractioned into a b 2 µm size from a bentonite (Chun et al., 1998) which was taken from Kyeongju, Korea. The original bentonite contains smectite (78%), feldspars (20.1%), quartz (1.7%), and some impurities. The b2 µm fraction of the bentonite was separated by a centrifugation method, and the physicochemical and mineralogical properties of the separated natural smectite are as summarized in Table 1. All the solutions were prepared by adding potassium chloride salt to de-mineralized water, except those for Run#16 and Run#18 in Table 2, which were conducted to investigate the sorption of cesium and nickel onto less altered solid material. 2.2. Hydrothermal experiment Tests were carried out in stainless steel pressure vessels with a Teflon liner by maintaining a 1 g/20 ml of solid sample-to-solution ratio. The test conditions, as summarized in Table 2, were combinations of the following variable values: an initial potassium concentration of 0.5 M, a tem-
Sample
ID ORM 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
Smectite Smectite Smectite Smectite Smectite Smectite Smectite Smectite Smectite Smectite Smectite Smectite Smectite Smectite Smectite Smectite Smectite Smectite Smectite Smectite
Temp. K+conc.
Other ion conc.
(°C)
(M)
(M)
90 90 90 90 90 140 140 140 140 140 200 200 200 200 200 200 200 200 200
5 × x10− 1 5 × 10− 1 5 × 10− 1 5 × 10− 1 5 × 10− 1 5 × 10− 1 5 × 10− 1 5 × 10− 1 5 × 10− 1 5 × 10− 1 5 × 10− 1 5 × 10− 1 5 × 10− 1 5 × 10− 1 5 × 10− 1 DW 5 × 10− 1 DW 5 × 10− 1
None None None None None None None None None None None None None None None 5 × 10− 3 5 × 10− 3 5 × 10− 4 5 × 10− 4
M M M M
Time
Analysis/ (days) measurement
3 7 15 28 50 3 7 15 28 50 3 7 15 28 50 (Cs+) 50 (Cs+) 50 (Ni2+) 50 (Ni2+) 50
Starting material XRD, Si XRD, Si XRD, Si XRD, Si XRD, Si, EP, LC, CEC XRD, Si XRD, Si XRD, Si XRD, Si XRD, Si, EP, LC, CEC XRD, Si XRD, Si XRD, Si XRD, Si XRD, Si, EP, LC, CEC XRD, EP, LC, CEC, Kd XRD, EP, LC, CEC, Kd XRD, EP, LC, CEC, Kd XRD, EP, LC, CEC, Kd
EP: expandability; LC: layer charge; CEC: cation exchange capacity; Kd: sorption distribution coefficient.
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Fig. 1. XRD patterns of the initial material and samples reacted with the 0.5 M KCl solution at 90, 140 and 200 °C.
from a structural formula calculated based on 22 anion equivalents and four tetrahedral cations (Newman and Brown, 1987). EPMA was implemented for acquiring the chemical composition data to be required for the structural formulae calculation (Ylagan et al., 2000). CEC was measured according to Sumner and Miller's (2002) procedure using a NaCl/NH4Cl solution. The distribution coefficients (Kd) of cesium and nickel ions for their sorption were determined from the following equation (Lee et al., 1997): Kd =
Mass of an ion sorbed on a unit mass of a solid ðw0 − w1 Þ =w = ½L=g ð1Þ Mass of an ion per unit volume of solution w1 =V
ratio method (Inoue et al., 1989). It follows from this that the starting smectite transforms into randomly interstratified I–S under the given hydrothermal conditions, and eventually into illite with increasing time. An examination of the Si released into a solution after hydrothermal reaction has been employed as a sensitive method to detect a small change in the smectite-to-illite conversion. The values of the Si concentration released into the solutions after the hydrothermal reactions are shown in Fig. 2. The profiles of the Si concentration for 90 °C and 140 °C showed a rapid increase within a short time period and then a nearly constant value, while that for 200 °C, indicating a
where w0 is the mass of an ion in the initial solution, w1 the mass of an ion in the solution after sorption equilibrium, w the mass of the solid, and V the volume of the solution. 3. Results and discussion 3.1. Smectite alteration The smectite alteration caused by the hydrothermal reaction under the potassium concentration of 0.5 M was identified by examining the data sets of XRD and dissolved silica concentration. Fig. 1 demonstrates the XRD patterns of EG samples, each of which is subjected to a different reaction time and temperature. With increasing reaction time and temperature, the first-and third-order reflections of EG samples progressively weakened and broadened, the second-order reflections moved toward the lower 2θ angle region, and the fifthorder reflection intensities increased relative to the first-order's. These features were obvious above all in the samples hydrothermally reacted at 200 °C. Based on Mering's principle (Moore and Reynolds, 1997), the peak variations shown in Fig. 1 correspond to those for the randomly interstratified illite and smectite layers. Moreover, because the reacted samples were resaturated with calcium ions to remove exchangeable potassium ions prior to the XRD measurement, the interstratificational features support the occurrence of potassium ions which had been changed into a non-exchangeable form by being fixed within the newly formed illite-like layers during the hydrothermal reactions. The percentage of the illite layer in the randomly interstratified I–S was reduced to a minimum of 56.8% under the experimental condition of this study, when it was determined using the saddle/001 peak intensity
Fig. 2. Silica released into the solution from the smectite samples reacted with the 0.5 M KCl solution at 90, 140 and 200 °C.
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Table 3 Changes in the barrier properties of reacted smectite after hydrothermal tests. Run ID
Expandability (%S)
Tet.
Layer charge Oct.
Net
CEC (cmol/kg)
Sorption distribution coefficient, Kd (L/g)
ORM 5 10 15 16 17 18 19
94.2 88.5 86.2 61.3 92.3 56.8 93.1 61.6
− 0.3 − 0.36 − 0.41 − 0.52 − 0.40 − 0.53 − 0.43 − 0.50
− 0.68 − 0.71 − 0.72 − 0.72 − 0.73 −0.72 −0.64 −0.73
− 0.98 − 1.07 − 1.13 − 1.24 − 1.13 − 1.25 − 1.07 − 1.23
96.5 87 77 57 95 52 81 61
0.62 0.03 130.4 0.348
(at pH(⁎) = 7.5) (at pH(⁎) = 4.1) (at pH(⁎) = 5.5) (at pH(⁎) = 4.0)
(⁎) represents an equilibrium pH value after sorption test under hydrothermal conditions.
much higher silica concentration than those for 90 °C and 140 °C, showed two distinguishable stages: a rapid increase in the silica concentration during the first 15 days and thereafter a more slowly increasing silica concentration. It is thought for the reaction at 200 °C that the first stage corresponds to a dissolution of the non-crystalline siliceous phase and the second one to silica release from the tetrahedral sheet in the structural scheme of smectite due to the process of smectiteto-illite conversion (Cuadros and Linares, 1996).
3.2. Influence of smectite alteration on the barrier properties of smectite clay for a repository A change in the expandability (%S), layer charge, CEC, and sorption distribution coefficient (Kd) as material properties which may affect the barrier performance of smectite clay was investigated by comparing the values of these properties of the starting material and reacted samples. After the hydrothermal reaction, the percentage of expandable smectite layers in the randomly interstratified I–S (%S) decreased with an increasing temperature, as shown in Table 3 and Fig. 3. With a temperature increase, there was a negligible decrease up to 140 °C, but a much lower value of % S was found at 200 °C. Their interlayer orderings were all zero regardless of temperature under the given conditions. This indicates that the smectite alteration occurred within the same structural scheme of interlayer stacking of the smectite and illite units and thus the reacted sample existed as a randomly interstrafied I–S.
Fig. 3. Change of expandability with respect to the temperature of hydrothermal reactions with the 0.5 M KCl solution.
Fig. 4 shows a change in the layer charges of the reacted samples as a function of the temperature. As shown in the figure, the net charge values for the reacted samples were more negative than that for the initial material, and they became more negative with an increasing temperature. The increase in the net charge, as seen in Table 3, was dominated by a tetrahedral charge, which probably resulted from an Al for a Si substitution in the tetrahedral sheet, as seen from the previous investigations (refer to the introduction of this paper). On the other hand, there was no observable change in the octahedral charge. Fig. 5 shows that the CEC of the reacted samples is lower than that of the starting material and decreases with an increasing temperature, implying that the smectite alteration affected the CEC of the reacted smectite. However, this was against the expectation that the CEC would increase given the more negative layer charge of the reacted samples when compared with the starting material. It is likely that such a CEC decrease with an increasing temperature was attributed to some cation exchange sites being blocked by a collapse of the interlayers due to the smectite alteration (Oscarson, and Hume, 1993) as well as a decrease in the reacted solution pH (Allard et al., 1983). In our hydrothermal tests, the solution pH increased from 5.6 (an initial value) to 7.3 and 6.8 for 90 °C and 140 °C, respectively, while it decreased from 5.6 to 4.0 for 200 °C. A change in the sorption capacity of the smectite by hydrothermal reaction is as shown in Table 3 and Fig. 6((a) cesium, (b) nickel). The sorption distribution coefficients (Kd) of the cesium and nickel ions for the reacted samples with a lower % S (Run#17 for cesium, Run#19 for nickel) are considerably lower in comparison with those of the samples with a higher % S (Run#16 for cesium, Run#18 for nickel),
Fig. 4. Change of layer charge with respect to temperature of hydrothermal reactions with the 0.5 M KCl solution.
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not correspond with ~30% reduction in CEC. It is thought that there may be the possibility of an impact of the experimental conditions such as pH reduction or carbonate variations etc., as well as CEC on the sorption distribution coefficient, but an investigation of the possibility was not included in this study. Fig. 7, which is a plot of the in-situ FT-IR spectra for the reacted samples after the sorption tests, indicates that the cesium ions were adsorbed onto the randomly interstratified I–S by a different mechanism from that of the nickel ions. The reacted sample which has sorbed cesium ions has a distinctive band at ~685 cm− 1 as compared with that of nickel ions. This is probably because the cesium ions may be adsorbed by a fixation to specific sorption sites of the “ditrigonal cavity” yielded in the tetrahedral layers due to a smectite-to-illite conversion (Comans et al., 1991; Ohnuki et al., 1994), in addition to their sorption onto the interlayer and/or edge sites by ion-exchange reaction. 4. Conclusions
Fig. 5. Change of CEC with respect to temperature of hydrothermal reactions with the 0.5 M KCl solution.
indicating that the sorption capacity of the smectite may be affected by the extent of a smectite alteration. These results are likely attributed to the reduction of the CEC due to the smectite-to-illite conversion. However, the change of Kd is more than 90%, which does
This study identified that when the smectite was hydrothermally treated under the potassium concentration of 0.5 M it was transformed into randomly interstratified I–S by a smectite-to-illite conversion. The temperature was a key factor controlling the conversion reaction. Also, it was observed that such a smectite conversion might affect the barrier properties of smectite clay: the percentage of the expandable smectite layers in the randomly interstratified I–S decreased, the layer charge was more negative, and the CEC and the sorption capacity for the cesium and nickel ions were reduced, when the temperature was increased. Although care
Fig. 6. Change of Kd with respect to the degree of smectite alteration due to hydrothermal reactions.
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Fig. 7. In-situ FT-IR spectra for the sorption of Cs
+
and Ni
should be taken, the results obtained in this study may be used to extrapolate to the expected buffer temperature, because there is no evidence that the reaction mechanisms are fundamentally different at higher temperatures. References Allard, B., Karsson, M., Tullborg, E.L., Larson, S.A., 1983. Ion exchange capacities and surface areas of some major components and common fracture filling materials of igneous rocks. KBS Technical Report 83-64, SKB, Sweden. Bauer, A., Lanson, B., Ferrage, E., Emerlich, K., Velde, B., Fanghänel, T., 2005. The fate of smectite before becoming illite in bentonite experiments. International meeting: Clays in natural and engineering barriers for nuclear waste confinement, March 14–18, 2005, Tours, France. Bauer, A., Lanson, B., Ferrage, E., Emmerich, K., Taubald, H., Schild, D., Velde, B., 2006. The fate of smectite in KOH solution. American Mineralogist 91, 1313–1322. Boles, J.R., Franks, S.G., 1979. Clay diagenesis in Wilcox sandstones of southwest Texas: implications of smectite diagenesis on sandstone cementation. Journal of Sedimentary Petrology 49, 55. Bucher, F., Müller-Vonmoos, M., 1989. Bentonite as a containment barrier for the disposal of highly radioactive wastes. Applied Clay Science 4, 157–178. Chun, K.S., Cho, W.J., Lee, J.O., Kim, S.S., Kang, M.J., 1998. High-level waste disposal technology development: Engineered barrier development. KAERI/RR-1897/98, Korea Atomic Energy Research Institute. Comans, R.N.J., Haller, M., Preter, P., 1991. Sorption of cesium on illite: non-equilibrium behavior and reversibility. Geochimica et Cosmochimica Acta 55, 433–440. Cuadros, J., Linares, J., 1996. Experimental kinetics study of the smectite-to-illite transformation. Geochimica et Cosmochimica Acta 60, 439. Eberl, D.D., 1978. The reaction of montmorillonite to mixed-layer clay: the effect of interlayer alkali and alkaline-earth cations. Geochimica et Ccosmochimica Acta 42,1–7. Güven, N., Huang, W.L., 1991. Effect of octahedral Mg2+and Fe3+substitutions on hydrothermal illitization reactions. Clays and Clay Minerals 39, 397. Howard, J.J., Roy, D.M., 1985. Development of layer charge and kinetics of experimental smectite alteration. Clays and Clay Minerals 33, 81. Hower, J., Eslinger, E.V., Hower, M.E., Perry Jr., E.A., 1976. Mechanism of burial metamorphism of argillaceous sediment, I. mineralogical and chemical evidence. Geological Society of America Bulletin 85, 827. Huang, W.L., Otten, G.A., 1987. Smectite illitization: effect of smectite composition. In program and abstracts, 26th Annual Meeting. The Clay Minerals Society, Sacramento, California, p. 94. Huang, W.L., Longo, J.M., Pevear, D.R., 1993. An experimentally derived kinetic models for smectite-to-illite conversion and its use as a geothermometer. Clays and Clay Minerals 41, 162–177. Huertas, F.J., Caballero, E., Jimenes de Cisneros, C., Huertas, F., Linares, J., 2001. Kinetics of montmorillonite dissolution in granitic solution. Applied Geochemistry 16, 397–407.
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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 e v i e r. c o m / l o c a t e / c l a y
Smectite alteration and its influence on the barrier properties of smectite clay for a repository Jae Owan Lee a,⁎, Il Mo Kang b, Won Jin Cho a a b
Korea Atomic Energy Research Institute, P.O. Box 105, Yusong, Taejon 305-600, Korea Korea National Oil Corporation, 1588-14, Gwanyang-dong, Dongsan-gu, Anyang, Gyeonggi-do 431-711, Korea
a r t i c l e
i n f o
Article history: Received 31 December 2007 Received in revised form 10 October 2008 Accepted 13 October 2008 Available online 29 October 2008 Keywords: Smectite Hydrothermal test Smectite alteration Barrier property Repository
a b s t r a c t Hydrothermal tests were conducted to investigate smectite alteration and its influence on the barrier properties of a smectite clay for a repository. Examinations of the X-ray diffraction patterns of the starting material and reacted samples and the silica release rate in the solution revealed that the smectite was transformed into randomly interstratified illite–smectite by a smectite-to-illite conversion when it was hydrothermally treated under a potassium concentration of 0.5 M, maintaining a 1 g/20 ml of solid sampleto-solution ratio. Temperature was observed to be a key factor controlling the conversion reaction. The smectite alteration affected the barrier properties of smectite clay for a repository: when the temperature increased, the percentage of the expandable smectite layers in the randomly interstratified illite–smectite decreased, the layer charge was more negative, and the cation exchange capacity and the sorption capacity for the cesium and nickel ions were reduced. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The major functions of the buffer in a high-level waste (HLW) repository are to inhibit the penetration of groundwater and to retard the release of radionuclides from the radioactive wastes to the surrounding environment. Smectite clay has been considered favorably as such a buffer material because of its high swelling and sorption capacities. However, when the smectite clay is exposed to an elevated temperature due to heat from radioactive decay and the particular geochemical conditions of a repository for a long time, it may be transformed into other minerals (e.g., illite, chlorite, etc.) which leads to a decrease in the swelling and sorption properties of the smectite clay and consequently increases its water penetration and radionuclide transport properties (Pusch and Carlsson, 1985; Bucher and Müller-Vonmoos, 1989). Therefore, an understanding of smectite alteration and its influence on the barrier properties of a smectite clay is essential to evaluate the long-term barrier performance of a buffer for a HLW repository. Smectite alteration, especially smectite-to-illite conversion which may occur under commonly-prevailing pH conditions, has drawn considerable interest in the last few decades due to the implication of its reaction for clay diagenesis and migration of petroleum as well as the long-term barrier performance of a smectitic buffer for a repository. Very many investigators (Eberl, 1978; Roberson and Lahann, 1981; Inoue, 1983; Howard and Roy, 1985; Huang and Otten,
⁎ Corresponding author. Tel.: +82 42 868 2852; fax: +82 42 868 8850. E-mail address: [email protected] (J.O. Lee). 0169-1317/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2008.10.007
1987; Proust et al., 1990; Güven and Huang, 1991; Velde and Vasseur, 1992; Huang et al., 1993; Cuadros and Linares, 1996; Huertas et al., 2001; Bauer et al., 2005; Sato et al., 2005; Bauer et al., 2006) have studied the reaction mechanism and kinetics of smectite-to-illite conversion. On the basis of those studies, smectite-to-illite conversion has been suggested by a general consensus to proceed by two different mechanisms. One is a solid-state one-to-one transformation where T– O–T layers are conserved, and the reaction proceeds with a replacement of the tetrahedral Si4+by Al3+until the layer charge deficiency is sufficiently developed to dehydrate the interlayer cations, and a collapse occurs (Hower et al., 1976): Smectite þ Al3þ þ Kþ –NIllite þ Si4þ The other is a mechanism in which a destruction of the T–O–T layers provides the source of aluminium for the transformation, which thus consumes more smectite than it produces illite (Boles and Franks,1979): Smectite þ Kþ –NIllite þ Si4þ where the ratio of smectite consumed to illite produced is approximately 1.6:1, and no external source of aluminium is required. However, no consensus has been reached on which mechanism prevails because the reaction depends on different test conditions. The kinetics of a smectite-to-illite conversion has been studied to obtain a rate law for the overall conversion process (Pytte and Reynolds, 1989; Velde and Vasseur, 1992; Huang et al., 1993; Wei et al., 1993; Cuadros and Linares, 1996; Huertas et al., 2001). Most of these studies are based on field observation of clay diagenesis, while there
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Table 1 Physicochemical and mineralogical properties of the bentonite and the separated smectite. Sample
Properties
Bentonite
Mineral composition Smectite (78%), feldspar (20.1%), quartz (1.7%) CEC: 61.7 cmol/kg pH: 8.3 Chemical composition (wt.%)(by XRF) SiO2 58.21 Al2O3 18.31 Fe2O3 7.85 TiO2 0.91 CaO 0.15 MgO 3.19 MnO 0.04 Na2O 3.31 K2O 0.21 P2O5 0.10 L.O.I 7.49 Total 99.75 Structural formula (Ca, Na, K)0.93(Al2.57Fe+30.78 Mg0.64)(Si7.71Al0.29)O20(OH)4 CEC: 96.5 cmol/kg
Smectite (b 2 µm)
are not many studies based on systematic laboratory work. From these studies, it is implied that, although the most important parameters for controlling smectite-to-illite conversion are the temperature, potassium concentration, and reaction time, the proposed rate laws have limitations in their application to evaluate the extent of smectite-to-illite conversion under different repository conditions, due to an uncertainty in the parameter values. On the other hand, insufficient data has been reported regarding the influence of smectite alteration on the barrier properties such as the percentage of expandable smectite (MacEwan and Wilson, 1980; Huang et al., 1993), layer charge (Howard and Roy, 1985; Güven and Huang, 1991), cation exchange capacity (CEC) (Oscarson, and Hume, 1993), and sorption capacity (Comans et al., 1991; Ohnuki et al., 1994) of a smectite clay for a buffer of a HLW repository. The present study, in this connection, focuses on investigating to what extent the smectite may be altered when it is hydrothermally treated under a certain potassium concentration and as a result how the smectite alteration may affect the barrier properties of a smectite clay for a Korean HLW repository. 2. Materials and methods
perature of 90, 140, 200 °C, an initial concentration of CsCl of 5×10− 3 M and NiCl2 of 5×10− 4 M, and a reaction time of 3, 7, 15, 28, 50 days. The internal pressure was subject to an autogenous vapor pressure of H2O at a given temperature of each test run. Upon termination of the runs, the vessels were quenched in iced water for their rapid cooling to a temperature of 25 °C to reduce the possibility of silica polymerization. The cooled suspension was centrifuged at 10,000 rpm for 10 min; the solution was filtered on a 0.45 μm membrane; and the filtered solids were freeze-dried. The chemical and mineralogical investigations were conducted by means of inductively coupled plasma mass spectrometry (ICP-MS) for the liquid solutions, and X-ray diffraction (XRD), Fourier transform infrared spectrometry (FT-IR), and Electron Probe Micro Analysis (EPMA) for the solid samples. In preparation for the XRD analyses, each solid sample was resaturated with calcium ions to remove the exchangeable potassium not completely fixed into the illite interlayer during the hydrothermal tests. After extra electrolytes were washed with deionized water, oriented specimens were prepared by sedimenting the treated sample onto a glass slide, followed by ethylene-glycol (EG) treatment. The XRD analyses were performed with a Bragg–Brentano diffractometer (MXP 18A RINT-2500, Mac Science Co., Ltd, Japan): Cu-Kα (40 kV/30 mA), a graphic monochromator, a 1° divergence slit, a 0.15° receiving slit, two soller slits (2.4° and 2.5°), and 1 s /0.05° 2 step-scanning. The FT-IR analyses for investigation of the sorption mechanism of cesium and nickel onto the solid material were conducted using an in-situ IR cell (Graseby, Specac Co.) with the water in the interlayer of the sample removed by vacuuming under the condition of 760 mmHg and 110 °C for 24 h.
2.3. Determination of expandability, layer charge, CEC, and sorption distribution coefficient The expandability (the percentage of the expandable smectite layers in the interstratified I–S, % S) was determined using a saddle/001 peak intensity ratio for the ethylene-glycolated (EG) sample (Inoue et al., 1989). Glycolations were performed under ethylene glycol vapor at 60 °C for 24 h. In this analysis, the NEWMOD program (Reynolds, 1985) was complementarily used for simulating the XRD pattern of interstratified illite–smectite (I–S) and obtaining a calibration curve between the saddle/001 peak intensity ratio and % S. The layer charge was evaluated
Table 2 Experimental design for hydrothermal tests. Run
2.1. Materials The solid material used for the tests was a natural smectite fractioned into a b 2 µm size from a bentonite (Chun et al., 1998) which was taken from Kyeongju, Korea. The original bentonite contains smectite (78%), feldspars (20.1%), quartz (1.7%), and some impurities. The b2 µm fraction of the bentonite was separated by a centrifugation method, and the physicochemical and mineralogical properties of the separated natural smectite are as summarized in Table 1. All the solutions were prepared by adding potassium chloride salt to de-mineralized water, except those for Run#16 and Run#18 in Table 2, which were conducted to investigate the sorption of cesium and nickel onto less altered solid material. 2.2. Hydrothermal experiment Tests were carried out in stainless steel pressure vessels with a Teflon liner by maintaining a 1 g/20 ml of solid sample-to-solution ratio. The test conditions, as summarized in Table 2, were combinations of the following variable values: an initial potassium concentration of 0.5 M, a tem-
Sample
ID ORM 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
Smectite Smectite Smectite Smectite Smectite Smectite Smectite Smectite Smectite Smectite Smectite Smectite Smectite Smectite Smectite Smectite Smectite Smectite Smectite Smectite
Temp. K+conc.
Other ion conc.
(°C)
(M)
(M)
90 90 90 90 90 140 140 140 140 140 200 200 200 200 200 200 200 200 200
5 × x10− 1 5 × 10− 1 5 × 10− 1 5 × 10− 1 5 × 10− 1 5 × 10− 1 5 × 10− 1 5 × 10− 1 5 × 10− 1 5 × 10− 1 5 × 10− 1 5 × 10− 1 5 × 10− 1 5 × 10− 1 5 × 10− 1 DW 5 × 10− 1 DW 5 × 10− 1
None None None None None None None None None None None None None None None 5 × 10− 3 5 × 10− 3 5 × 10− 4 5 × 10− 4
M M M M
Time
Analysis/ (days) measurement
3 7 15 28 50 3 7 15 28 50 3 7 15 28 50 (Cs+) 50 (Cs+) 50 (Ni2+) 50 (Ni2+) 50
Starting material XRD, Si XRD, Si XRD, Si XRD, Si XRD, Si, EP, LC, CEC XRD, Si XRD, Si XRD, Si XRD, Si XRD, Si, EP, LC, CEC XRD, Si XRD, Si XRD, Si XRD, Si XRD, Si, EP, LC, CEC XRD, EP, LC, CEC, Kd XRD, EP, LC, CEC, Kd XRD, EP, LC, CEC, Kd XRD, EP, LC, CEC, Kd
EP: expandability; LC: layer charge; CEC: cation exchange capacity; Kd: sorption distribution coefficient.
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Fig. 1. XRD patterns of the initial material and samples reacted with the 0.5 M KCl solution at 90, 140 and 200 °C.
from a structural formula calculated based on 22 anion equivalents and four tetrahedral cations (Newman and Brown, 1987). EPMA was implemented for acquiring the chemical composition data to be required for the structural formulae calculation (Ylagan et al., 2000). CEC was measured according to Sumner and Miller's (2002) procedure using a NaCl/NH4Cl solution. The distribution coefficients (Kd) of cesium and nickel ions for their sorption were determined from the following equation (Lee et al., 1997): Kd =
Mass of an ion sorbed on a unit mass of a solid ðw0 − w1 Þ =w = ½L=g ð1Þ Mass of an ion per unit volume of solution w1 =V
ratio method (Inoue et al., 1989). It follows from this that the starting smectite transforms into randomly interstratified I–S under the given hydrothermal conditions, and eventually into illite with increasing time. An examination of the Si released into a solution after hydrothermal reaction has been employed as a sensitive method to detect a small change in the smectite-to-illite conversion. The values of the Si concentration released into the solutions after the hydrothermal reactions are shown in Fig. 2. The profiles of the Si concentration for 90 °C and 140 °C showed a rapid increase within a short time period and then a nearly constant value, while that for 200 °C, indicating a
where w0 is the mass of an ion in the initial solution, w1 the mass of an ion in the solution after sorption equilibrium, w the mass of the solid, and V the volume of the solution. 3. Results and discussion 3.1. Smectite alteration The smectite alteration caused by the hydrothermal reaction under the potassium concentration of 0.5 M was identified by examining the data sets of XRD and dissolved silica concentration. Fig. 1 demonstrates the XRD patterns of EG samples, each of which is subjected to a different reaction time and temperature. With increasing reaction time and temperature, the first-and third-order reflections of EG samples progressively weakened and broadened, the second-order reflections moved toward the lower 2θ angle region, and the fifthorder reflection intensities increased relative to the first-order's. These features were obvious above all in the samples hydrothermally reacted at 200 °C. Based on Mering's principle (Moore and Reynolds, 1997), the peak variations shown in Fig. 1 correspond to those for the randomly interstratified illite and smectite layers. Moreover, because the reacted samples were resaturated with calcium ions to remove exchangeable potassium ions prior to the XRD measurement, the interstratificational features support the occurrence of potassium ions which had been changed into a non-exchangeable form by being fixed within the newly formed illite-like layers during the hydrothermal reactions. The percentage of the illite layer in the randomly interstratified I–S was reduced to a minimum of 56.8% under the experimental condition of this study, when it was determined using the saddle/001 peak intensity
Fig. 2. Silica released into the solution from the smectite samples reacted with the 0.5 M KCl solution at 90, 140 and 200 °C.
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Table 3 Changes in the barrier properties of reacted smectite after hydrothermal tests. Run ID
Expandability (%S)
Tet.
Layer charge Oct.
Net
CEC (cmol/kg)
Sorption distribution coefficient, Kd (L/g)
ORM 5 10 15 16 17 18 19
94.2 88.5 86.2 61.3 92.3 56.8 93.1 61.6
− 0.3 − 0.36 − 0.41 − 0.52 − 0.40 − 0.53 − 0.43 − 0.50
− 0.68 − 0.71 − 0.72 − 0.72 − 0.73 −0.72 −0.64 −0.73
− 0.98 − 1.07 − 1.13 − 1.24 − 1.13 − 1.25 − 1.07 − 1.23
96.5 87 77 57 95 52 81 61
0.62 0.03 130.4 0.348
(at pH(⁎) = 7.5) (at pH(⁎) = 4.1) (at pH(⁎) = 5.5) (at pH(⁎) = 4.0)
(⁎) represents an equilibrium pH value after sorption test under hydrothermal conditions.
much higher silica concentration than those for 90 °C and 140 °C, showed two distinguishable stages: a rapid increase in the silica concentration during the first 15 days and thereafter a more slowly increasing silica concentration. It is thought for the reaction at 200 °C that the first stage corresponds to a dissolution of the non-crystalline siliceous phase and the second one to silica release from the tetrahedral sheet in the structural scheme of smectite due to the process of smectiteto-illite conversion (Cuadros and Linares, 1996).
3.2. Influence of smectite alteration on the barrier properties of smectite clay for a repository A change in the expandability (%S), layer charge, CEC, and sorption distribution coefficient (Kd) as material properties which may affect the barrier performance of smectite clay was investigated by comparing the values of these properties of the starting material and reacted samples. After the hydrothermal reaction, the percentage of expandable smectite layers in the randomly interstratified I–S (%S) decreased with an increasing temperature, as shown in Table 3 and Fig. 3. With a temperature increase, there was a negligible decrease up to 140 °C, but a much lower value of % S was found at 200 °C. Their interlayer orderings were all zero regardless of temperature under the given conditions. This indicates that the smectite alteration occurred within the same structural scheme of interlayer stacking of the smectite and illite units and thus the reacted sample existed as a randomly interstrafied I–S.
Fig. 3. Change of expandability with respect to the temperature of hydrothermal reactions with the 0.5 M KCl solution.
Fig. 4 shows a change in the layer charges of the reacted samples as a function of the temperature. As shown in the figure, the net charge values for the reacted samples were more negative than that for the initial material, and they became more negative with an increasing temperature. The increase in the net charge, as seen in Table 3, was dominated by a tetrahedral charge, which probably resulted from an Al for a Si substitution in the tetrahedral sheet, as seen from the previous investigations (refer to the introduction of this paper). On the other hand, there was no observable change in the octahedral charge. Fig. 5 shows that the CEC of the reacted samples is lower than that of the starting material and decreases with an increasing temperature, implying that the smectite alteration affected the CEC of the reacted smectite. However, this was against the expectation that the CEC would increase given the more negative layer charge of the reacted samples when compared with the starting material. It is likely that such a CEC decrease with an increasing temperature was attributed to some cation exchange sites being blocked by a collapse of the interlayers due to the smectite alteration (Oscarson, and Hume, 1993) as well as a decrease in the reacted solution pH (Allard et al., 1983). In our hydrothermal tests, the solution pH increased from 5.6 (an initial value) to 7.3 and 6.8 for 90 °C and 140 °C, respectively, while it decreased from 5.6 to 4.0 for 200 °C. A change in the sorption capacity of the smectite by hydrothermal reaction is as shown in Table 3 and Fig. 6((a) cesium, (b) nickel). The sorption distribution coefficients (Kd) of the cesium and nickel ions for the reacted samples with a lower % S (Run#17 for cesium, Run#19 for nickel) are considerably lower in comparison with those of the samples with a higher % S (Run#16 for cesium, Run#18 for nickel),
Fig. 4. Change of layer charge with respect to temperature of hydrothermal reactions with the 0.5 M KCl solution.
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not correspond with ~30% reduction in CEC. It is thought that there may be the possibility of an impact of the experimental conditions such as pH reduction or carbonate variations etc., as well as CEC on the sorption distribution coefficient, but an investigation of the possibility was not included in this study. Fig. 7, which is a plot of the in-situ FT-IR spectra for the reacted samples after the sorption tests, indicates that the cesium ions were adsorbed onto the randomly interstratified I–S by a different mechanism from that of the nickel ions. The reacted sample which has sorbed cesium ions has a distinctive band at ~685 cm− 1 as compared with that of nickel ions. This is probably because the cesium ions may be adsorbed by a fixation to specific sorption sites of the “ditrigonal cavity” yielded in the tetrahedral layers due to a smectite-to-illite conversion (Comans et al., 1991; Ohnuki et al., 1994), in addition to their sorption onto the interlayer and/or edge sites by ion-exchange reaction. 4. Conclusions
Fig. 5. Change of CEC with respect to temperature of hydrothermal reactions with the 0.5 M KCl solution.
indicating that the sorption capacity of the smectite may be affected by the extent of a smectite alteration. These results are likely attributed to the reduction of the CEC due to the smectite-to-illite conversion. However, the change of Kd is more than 90%, which does
This study identified that when the smectite was hydrothermally treated under the potassium concentration of 0.5 M it was transformed into randomly interstratified I–S by a smectite-to-illite conversion. The temperature was a key factor controlling the conversion reaction. Also, it was observed that such a smectite conversion might affect the barrier properties of smectite clay: the percentage of the expandable smectite layers in the randomly interstratified I–S decreased, the layer charge was more negative, and the CEC and the sorption capacity for the cesium and nickel ions were reduced, when the temperature was increased. Although care
Fig. 6. Change of Kd with respect to the degree of smectite alteration due to hydrothermal reactions.
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Fig. 7. In-situ FT-IR spectra for the sorption of Cs
+
and Ni
should be taken, the results obtained in this study may be used to extrapolate to the expected buffer temperature, because there is no evidence that the reaction mechanisms are fundamentally different at higher temperatures. References Allard, B., Karsson, M., Tullborg, E.L., Larson, S.A., 1983. Ion exchange capacities and surface areas of some major components and common fracture filling materials of igneous rocks. KBS Technical Report 83-64, SKB, Sweden. Bauer, A., Lanson, B., Ferrage, E., Emerlich, K., Velde, B., Fanghänel, T., 2005. The fate of smectite before becoming illite in bentonite experiments. International meeting: Clays in natural and engineering barriers for nuclear waste confinement, March 14–18, 2005, Tours, France. Bauer, A., Lanson, B., Ferrage, E., Emmerich, K., Taubald, H., Schild, D., Velde, B., 2006. The fate of smectite in KOH solution. American Mineralogist 91, 1313–1322. Boles, J.R., Franks, S.G., 1979. Clay diagenesis in Wilcox sandstones of southwest Texas: implications of smectite diagenesis on sandstone cementation. Journal of Sedimentary Petrology 49, 55. Bucher, F., Müller-Vonmoos, M., 1989. Bentonite as a containment barrier for the disposal of highly radioactive wastes. Applied Clay Science 4, 157–178. Chun, K.S., Cho, W.J., Lee, J.O., Kim, S.S., Kang, M.J., 1998. High-level waste disposal technology development: Engineered barrier development. KAERI/RR-1897/98, Korea Atomic Energy Research Institute. Comans, R.N.J., Haller, M., Preter, P., 1991. Sorption of cesium on illite: non-equilibrium behavior and reversibility. Geochimica et Cosmochimica Acta 55, 433–440. Cuadros, J., Linares, J., 1996. Experimental kinetics study of the smectite-to-illite transformation. Geochimica et Cosmochimica Acta 60, 439. Eberl, D.D., 1978. The reaction of montmorillonite to mixed-layer clay: the effect of interlayer alkali and alkaline-earth cations. Geochimica et Ccosmochimica Acta 42,1–7. Güven, N., Huang, W.L., 1991. Effect of octahedral Mg2+and Fe3+substitutions on hydrothermal illitization reactions. Clays and Clay Minerals 39, 397. Howard, J.J., Roy, D.M., 1985. Development of layer charge and kinetics of experimental smectite alteration. Clays and Clay Minerals 33, 81. Hower, J., Eslinger, E.V., Hower, M.E., Perry Jr., E.A., 1976. Mechanism of burial metamorphism of argillaceous sediment, I. mineralogical and chemical evidence. Geological Society of America Bulletin 85, 827. Huang, W.L., Otten, G.A., 1987. Smectite illitization: effect of smectite composition. In program and abstracts, 26th Annual Meeting. The Clay Minerals Society, Sacramento, California, p. 94. Huang, W.L., Longo, J.M., Pevear, D.R., 1993. An experimentally derived kinetic models for smectite-to-illite conversion and its use as a geothermometer. Clays and Clay Minerals 41, 162–177. Huertas, F.J., Caballero, E., Jimenes de Cisneros, C., Huertas, F., Linares, J., 2001. Kinetics of montmorillonite dissolution in granitic solution. Applied Geochemistry 16, 397–407.
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