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Longevity of Bentonite as Buffer Material in a Nuclear-Waste Repository
Engineering Geology, 28 (1990) 233-247 Elsevier Science Publishers B.V., Amsterdam-- Printed in The Netherlands
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L o n g e v i t y of B e n t o n i t e as Buffer M a t e r i a l in a Nuclear-Waste Repository NECIP GIJVEN Department of C,eosciences, Texas Tech University, Lubbock, TX 79409(U.S.A.) (Accepted for publication February 14, 1989)
ABSTRACT Giiven, N., 1990. Longevity of bentonite as buffer material in a nuclear-waste repository. Eng. Geol., 28: 233-247. Longevity of bentonite as a nuclear waste barrier depends on mineralogical stability of the smectite component of the clay as well as on its abilityto perform as a buffer in the repository for long periods of time. Processes affecting the stabilityof smectites under the geochemical and thermal conditions in a high-level nuclear waste repository can be construed by examining the geological analogues and the resultsof the hydrothermal experiments. The physical and chemical properties of bentonite seem to be related to the nature of smectites in them. Smectites represent a range of expandable layer silicatesof Al, Mg, Fe and other transition metals. Their thermal, hydrothermal, or solution stabilitiesvary from one smectite species to another. This is also true to a certain extent for the physical propertiesof bentonites,i.e.swelling,permeability,plasticityand adsorption. Swelling ability and permeability of naturally altered (metabentonites) and steam-treated bentonites are badly damaged. Temperature distribution in a clay barrier that was simulated in situ under the repository conditions showed a sharp drop from 175°C at the clay/canistercontact to 81°C at a distance 65 m m away. Microstructures of bentonites were noticeably altered after hydrothermal treatment at 200cC.
INTRODUCTION
A backfill material in a nuclear waste repository must perform certain crucial functions in order to isolate and contain radioactive waste. As explained by Pusch (1982) and Pusch and Carlsson (1985), the backfill must provide: (1) an effective (impervious) barrier which can minimize the flow rate of percolating groundwater and retard the migration of radionuclides upon their release from the waste container; (2) a buffer media with appropriate rheology, bulk density and swelling properties to withstand the mechanical stresses in the host rock, and to seal the cracks and fissures; (3) good thermal conductivity to dissipate the heat generated in the waste container. More importantly the buffer material must remain physically and chemically stable to perform the above functions for long periods of time up to a few million years; this is referred to as the longevity of the buffer material. Bentonites have been considered as a prime candidate for backfill material due 0013-7952/90/$03.50
© 1995 Elsevier Science Publishers B.V.
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x ~;l:~~;x
to t h e i r p h y s i c a l a n d r h e o l o g i c a l p r o p e r t i e s s u c h as s w e l l i n g , p l a s t i c i t y , i o n exchange and sorption, sealing capability, and large reactive surface areas. B e n t o n i t e s a r e t h e c l a y s t h a t c o n s i s t p r e d o m i n a n t l y of a s m e c t i t e m i n e r a l . T h e p r o p e r t i e s of b e n t o n i t e s a r e l a r g e l y d e t e r m i n e d by t h e s t r u c t u r a l , c h e m i c a l a n d m o r p h o l o g i c a l f e a t u r e s of s m e c t i t e m i n e r a l s i n t h e m . T h e s e c h a r a c t e r i s t i c s of s m e c t i t e s will t h e r e f o r e be d e s c r i b e d i n t h e n e x t s e c t i o n . VARIATIONS IN CHEMISTRY AND STRUCTURE OF SMECTITES IN BENTONITES
Classification of smectites S m e c t i t e s r e p r e s e n t a s e r i e s o f e x p a n d a b l e ( s w e l l i n g ) 2:1 t y p e l a y e r s i l i c a t e s w i t h a n e x c e s s l a y e r c h a r g e i n t h e r a n g e of 0.2 0.6 e q u i v a l e n t s p e r O~o (OH)2. S m e c t i t e s a r e c l a s s i f i e d i n T a b l e l a c c o r d i n g to t h e c r i t e r i a r e l a t e d to (a) p r e d o m i n a n t o c t a h e d r a i c a t i o n s a n d t h e i r s u b s t i t u t i o n s , (b) di- or t r i - o c t a h e d r a l n a t u r e of o c t a h e d r a l s h e e t s , a n d (c) r e l a t i v e d i s t r i b u t i o n o f e x c e s s c h a r g e s over t e t r a h e d r a l and o c t a h e d r a l sheets. TABLE I Classification of natural and synthetic: smectites (Gfiven. Ratio between tetrahedral (xt) and octahedral (x.) charges
xjx, > 1.0
Dioctahedral smectites Predominant octahedral cation(s)
Trioctahedral smectites ...................... Smectite Predominant Smectite species octahedral species cation(s)
Al(Mg,Fe) 2
Montmorillonite
(Octahedrat charges predominant)
x,/x o> 1.0 (Tetrahedral charges predominant)
1988) I
AI Fe -~-
Beidellite Nontronite
Cr "~"
Volkonskoite
V"~"
Vanadium smectite
Mg( : l ) ' ~ Mg(l,i) 2 AIMgLi
Stevensite Hectoritc Swinefordite
Single or mixed transition metals
Transition metal "defect" trioe. smectites
Mg Fe 2" Zn Co Mn
Saponite Iron saponite Sauconite Cobalt smectite Manganese smectite
Single or mixed transition metals
Transition metal trioct. smectites
IReasonably well-defined species (synthetic or natural) are given as separate entries. :Octahedral substitutions. :_-]Vacancies.
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Smectite crystallites appear in a variety of habits as thin platelets, large and flexible films, laths, ribbons and fibers (Giiven, 1988). These crystallites occur in aggregates with various textures: foliated aggregates of thin films, compact aggregates of platelets, mossy flocks and fine granular (globular) agglomerates. Aluminium dioctahedral smectites (montmorillonite) are by far the most common in nature and they are often found in bentonites. The terms "bentonite" and "montmorillonite" are often used interchangeably (also in this report) although the former is a clay (rock) while the latter is a clay mineral. Montmorillonites as clay mineral display significant chemical and morphological differences (Giiven, 1988). They are subdivided into several varieties according to their type localities or special chemistries: (1) Wyoming-type montmorillonite, (2) Otaytype montmorillonite, (3) Chambers-(Cheto-) type montmorillonite, (4) Tatilla-type montmorillonite, and (5) Fe-rich montmorillonites. Each of these montmorillonites may display different physical properties and thermal stabilities mainly because of the chemical, morphological and textural differences. The hydrothermal stability of smectites ranges from 50°C to 850°C; trioctahedral magnesian smectites are more stable than the dioctahedral aluminian ones. Fe-rich smectites are known to be more easily altered and thermally less stable than the magnesian and aluminian varieties. Synthetic fluorohectorite is probably the most stable smectite; it maintains its structural integrity and water swelling even at 850°C (Barrer and Jones, 1970). FACTORS CONTROLLINGSMECTITE-WATER INTERACTIONS Most of the physical properties of smectites such as swelling, adsorption, plasticity, permeability and shear strength are related to the interactions between electrically charged interlamellar surfaces and the water molecules. Factors controlling smectite hydration were discussed in detail by Suquet et al. (1975, 1977) and Suquet and Pezerat (1987), and are summarized here: (1) layer charge, its density and site (tetrahedral vs. octahedral sheets); (2) nature of interlayer cation; (3) di- or tri~ctahedral character of the layer; (4) mean crystallite size; (5) partial pressure of water; and (6) distribution of tetrahedral (Al, Si) substitutions and that of interlayer cations over the available structural sites. Under the combined affects of the above factors, two kinds of smectitewater complexes can develop between the siloxane surfaces of the layers: (a) interlamellar hydrates with an ordered structure; they can consist of an integral number (0, 1, 2 and maximum 3) of water layers; and thereafter, (b) liquid-like water in amounts that are determined mainly by osmotic effects. Current concepts of the clay-water complexes were recently reviewed by Sposito (1984) and Newman (1987) and they depict the following picture. Smectite's interlayer surfaces exert a influence on the water molecules within a 10-A distance from the layers. Beyond that, water molecules behave just like in liquid water. The interlamellar water does not possess a rigid "ice-like" structure but a dynamic one. Water molecules interact with smectite's basal surfaces and with the interlayer cations within the interlayer space in a dynamic manner. Water's molecular motions (rotational, vibrational and
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• (;()W;N
translational) and its orientation are thereby perturbed. The interactions b e t w e e n t h e i n t e r l a y e r c a t i o n a n d w a t e r m o l e c u l e s is l a r g e l y d e t e r m i n e d by the special ionic properties such as the ionic potential (cation's charge/radius) and its hydration energy. The hydration energies of common interlayer c a t i o n s a n d t h e i r i o n i c p o t e n t i a l s a r e l i s t e d in T a b l e II f o r t w o d i f f e r e n t s m e c t i t e s m o n t m o r i l l o n i t e a n d h e c t o r i t e . T h e h y d r a t i o n e n e r g i e s in T a b l e [| w e r e c a l c u l a t e d b y G a r r e l s a n d T a r d y (1982); t h e y c o r r e l a t e f a i r l y w e l l w i t h i o n i c p o t e n t i a l s e x c e p t for C a - m o n t m o r i l l o n i t e . F u r t h e r m o r e , h e c t o r i t e ' s hyd r a t i o n e n e r g i e s for t h e s a m e c a t i o n s w e r e f o u n d t o b e s i g n i f i c a n t l y h i g h e r t h a n t h o s e in m o n t m o r i l l o n i t e ; i n d i c a t i n g t h a t t h e i n t e r l a y e r c a t i o n w a t e r c o m p l e x e s in h e c t o r i t e w o u l d b e m u c h m o r e s t a b l e t h a n in m o n t m o r i l l o n i t e . DYNAMICS OF CLAY--WATER INTERACTIONS Ionic potential represents a measure of the polarizing effect of a cation on a w a t e r m o l e c u l e in i t s v i c i n i t y , h e n c e o n t h e e x t e n t o f b i n d i n g b e t w e e n t h e m . T h i s b i n d i n g m u s t be c o n s i d e r e d a " d y n a m i c o n e " e s p e c i a l l y w h e n t h e c l a y is saturated with water. This dynamic picture was described early by Samoilov (1957) b y c o m p a r i n g t h e a v e r a g e t i m e (r) a w a t e r m o l e c u l e s p e n d s ( i n t e r a c t s ) w i t h t h e n e x t w a t e r m o l e c u l e in p u r e w a t e r t o t h e a v e r a g e t i m e (ri) a w a t e r m o l e c u l e r e s i d e s in t h e i m m e d i a t e v i c i n i t y o f a n i o n in s o l u t i o n . T h e a v e r a g e e x c h a n g e e n e r g y (E) b e t w e e n t w o w a t e r m o l e c u l e s w i l l b e i n c r e a s e d by a n a d d i t i o n a l A E i b y t h e p r e s e n c e o f ion. T h e n u m e r i c a l v a l u e s o f r a n d A E i w e r e d e t e r m i n e d f r o m t h e s e l f - d i f f u s i o n e x p e r i m e n t s ; r w a s f o u n d to b e 1.7 × | 0 ~ s in p u r e w a t e r . T h e t e m p o r a l r a t i o r d r is d i r e c t l y r e l a t e d to AE~ b y t h e e q u a t i o n : ri/r = e x p ( A E i / R T ) T h e v a l u e s f o r ri/~ a n d A E i a r e l i s t e d in T a b l e II f o r a s e r i e s o f i n t e r l a y e r cations. They correlate fairly well with the ionic potentials and hydration TABLE II Ionic properties of interlayer cations in smectites and additional exchange energies (AE) of water molecule in dilute solutions Interlayer cation
K" Rb" Cs' Na + LiBa 2" Sr-" " Ca 2Mg 2"
Ionic: potential (nm "J)
Water molecule's AE~ (kcal/mole)
temporal ratio ( ~ d ~ )
Interlayer hydration energies (kcal/mole) .............. montmorillonite hectorite
7.5 6.8 6.0 10.3 14.7 14.9 17.9 20.2 30.3
- 0.25 n.d. - 0.33 0.25 0.73 n.d. n.d. 0.45 2.61
0.65 n.d. 0.57 1.46 3.48 n.d. n.d. 2.16 86.30
-- 2.63 --2.65 2.89 -3.97 - 5.59 5.95 - 6.00 - 7.43 6.6{)
....
-- 5.02 n.d. .1.97 - 6.21 - 8.6{) 7.51 - 8.80 - 10.18 10.92
BENTONITE AS B U F F E R M A T E R I A L IN A N U C L E A R W A S T E REPOSITORY
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energies of the interlayer cations. The temporal ratio ~i/z> 1 indicates that the water molecule would spend more time in the immediate vicinity of the ion than in the vicinity of another water molecule. Thus, stronger binding (in dynamic sense) develops between the cation and the water molecules as Ti/z ratio becomes larger than one. For instance M g 2+ with a zi/z ratio of 86.3 would establish an unusually strong binding with water molecules. The situation is rather interesting for K + and Cs + ions; their temporal Ti/zratios are less than 1.0 and their A E i values are negative. This implies that water molecules in a solution would spend less time in the immediate vicinity of K + or Cs + ions than in the vicinity of another water molecule, i.e.water molecules become more mobile around these ions. The above dynamic interactions of water molecules within the interlayer space are expected to have significant bearing on the structure of interlamelfar water and hence on swelling, permeability and rheology, even on the hydrothermal stability of smectites. Swelling of bentonite and the related swelling pressure within a confined volume is mainly generated by dynamically "structured" interlamellar water. Obviously high temperatures can considerably destabilize the structure of the interlamellar water as well as the interactions between water molecules and the lamellar surfaces. PROCESSES A N D FACTORS AFFECTING LONGEVITY OF BENTONITE IN A NUCLEAR WASTE REPOSITORY Longevity of bentonites is synonymous with the stability of the smectites in them. Stability, in this connotation, is not restricted to the chemical and structural survival of the smectite mineral in the repository environment but its continued performance of the buffer functions for a few million years. Temperature in such an environment may range probably from 200°C at the clay/waste container interface to the ambient temperature in the host rock/clay contact. The geochemical parameters are site-specific and vary depending on the chemical and mineralogical make-up of the backfill material, chemistry of pore fluids and the prevailing pH and Eh conditions in them, and the nature of radionuclides that may be released from the waste container. After the initial water saturation of the backfill material, the repository may be considered to a first approximation as a hydrothermal system. Geological literature offers a vast volume of studies on the stability of smectites in sedimentary (diagenetic), hydrothermal and contact metamorphic environments. The conversion of smectite to illite appears as the primary process underlying the longevity of the bentonite for this conversion can destroy smectite's physical properties. The mechanism of this conversion is briefly described next. M E C H A N I S M OF THE SMECTITE-TO-ILLITE CONVERSION Reaction mechanism involved in this transition has been the subject of numerous studies and the conversion requires a significant enhancement of an excess layer charge in smectite.
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The layer charge in smectite s t r uct ur e can be increased by: (a) substitutions of A13+ for Si 4÷ in tetrahedra; (b) substitutions of Mg 2~ or Fe 2÷ for Al 3~ in octahedra; and (c) reduction of Fe 3 ~ to Fe 2 ÷ in octahedra. The mean layer charge in smectites is found to be 0.33 equivalents per O10 (OH) 2 whereas illite layers possess on the average a larger charge of 0.75-0.8. However, successive layers in a smectite crystallite generally show a nonuniform charge distribution ranging from 0.2 to 0.6 equivalents. Two reaction mechanisms have been proposed for the smectite-to-illite conversion. (1) l,ayer-by-layer transformation in solid state according to the reaction: Smectite + K ÷ + A13 ~ = iilite + silica This mechanism was formulated by Hower et al. 0976). It involves tetrahedral substitutions of A1a * for Si 4÷ with little disruption of illite layer and with practically no change of volume. (2) Dissolution-reprecipitation mechanism (neoformation) involves complete dissolution of smectite layers and reprecipitation of iilite layers according to the reaction: 1.5 smectite + K ÷ --, 1.0 illite + silica while other ions such as Mg 2 +, Fe 2", Ca 2 ~, and Na * in the original smectite layers are released to the solution. Hydrothermal experiments with smectites by Eberl and Hower (1976, 1977), studies of Gulf Coast sediments by Boles and Franks (1978) demonstrated the neoformation mechanism in these systems. No matter what reaction mechanism, the smectite-to-illite conversion was t h o u g h t to proceed via the formation of continuous series of illite/smectite mixed-layers with smectite c ont e nt (expandability) progressively reducing from 100 to 0%. This concept was widely approved and accepted until its validity was seriously questioned in recent studies by Inoue and Utada (1983), Nadeau et al. (1984, 1985), Lee et al. (1985), Ahn and Peacor (1986), Nadeau and Bain (1986) and Inoue et al. (1987, 1988). The results of these studies were in apparent conflict with a continuous transformation increasing illite component. Ahn and Peacor (1986) and Lee et al. (1985) obtained lattice images of the apparently interstratified illite/smectites as indicated by X-ray diffraction. These showed the presence of discrete illite domains in a smectite matrix but no evidence for interstratification of illite and smectite layers. Inoue and Utada (1983) and Inoue et al. (1987, 1988) proposed a dual reaction mechanism where the initial stage involves a layer-by-layer solid state transformation and the second stage proceeds as neoformation of illite. Accordingly, the first stage involves K-fixation of smectite layers t hat had developed high enough charges for collapse. The clay loses its expandability gradually from 100 to about 50% and appears as a random interstratification of expandable and non-expandable layers. The non-expandable layers have been identified as illite although highly charged smectite layers could have also collapsed upon K-fixation. In any case, the randomly interstratified illite/smectite displays a flakey morphology just like a smectite with 100% expandability. lnoue et al. (1987) found a drastic change in morphology and in the mode of
BENTONITE AS B U F F E R M A T E R I A L IN A N U C L E A R W A S T E REPOSITORY
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mixing when the expandability of illite/smectite clay was reduced below 50%. Smectites were assumed to dissolve and thin laths of illite precipitate, which develop into wider and thicker laths with continued crystal growth. These thin laths are the product of the neoformation and their coarsening follows the Ostwald ripening process, i.e. simultaneous dissolution of fine particles and growth of the larger ones in a closed system. An oriented aggregate of thin illite crystallites can, however, give an X-ray diffraction pattern like a regular interstratification of I/S layers with a certain expandability; the expandability being caused by the adsorption of organic (or water) molecules between the particle surfaces as suggested by Nadeau et al. (1984, 1985) and Nadeau and Bain (1986). The "apparent" expandability can be a direct function of the thickness of illite crystallites. This relationship is approximated by Eberl and Srodon (1988) by the equation: maximum expandability (in %)= lO0/n with n > 1, where n is the average number of (2:1) layers in illite crystallites. For instance, a glycol saturated sample of pure illite crystallites with an average of 5 layers in thickness may show as much as 20% "apparent" expandability on their X-ray diffraction pattern. HYDROTHERMAL
STABILITY O F BENTONITE: E X P E R I M E N T A L STUDIES
Experimental studies on hydrothermal reactions of aluminous smectites, notably those by Eberl and Hower (1976), Eberl (1978), Eberl et al. (1978), Roberson and Lahann (1981), Howard and Roy (1985) and Whitney and Northrup (1988) have been very useful in understanding hydrothermal stability of bentonites. These studies revealed the reaction kinetics and the importance of factors such as pore fluid chemistry including its pH and redox potential, the interlayer cations and temperature. The role of the interlayer cation was found to be of paramount importance. The smectite-to-illite conversion rate at temperatures up to 400°C was found to be directly related to the interlayer hydration energy of the exchangeable cations (Eberl, 1978). The expandable smectite layers became non-expandable at 300°C within 30 days when smectite was saturated with K +, Rb ÷ or Cs ÷ but no such reaction was observed with Na ÷ or Li ÷. Smectites saturated with alkaline earth cations reacted first at 400°C and generated a regular mixed-layer with 50% smectite content (rectorite-type). Thus, the stability of smectite can be enhanced and its intracrystalline swelling (expandability) can be maintained at high temperatures by the addition of appropriate salts of alkaline earths. Obviously, the presence of K ÷, Rb ÷, and Cs ÷ in pore waters will significantly decrease the stability of smectite in repository environment. Although the potassium content of the ground water is usually negligible, potassium can be leached from K-feldspars and micas by circulating ground water. Eberl and Hower (1976) calculated an activation energy of 19.6_ 3.5 kcal/ mole for the conversion of K-saturated (aluminous) smectite to illite; they expressed the conversion rates by the equation:
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In
N ~;(?v~:x
(a/a - x) = k t
where a is the initial c o n c e n t r a t i o n of smectite, x is the amount of the reaction product illite, t is the time and k is reaction rate for a specific temperature. Accordingly, to convert K-smectite to illite it would take about one million years at 50~:C, about 20,000 years at 100 C, and about 99 days at 393~:C. Similar hydrothermal experiments but buffered at neutral pH conditions were conducted by Roberson and L a ha nn (1981) in mixed cation solutions (K N a , K--Ca, K -Mg), the conversion reaction: smectite + K = illite + silica was inhibited by the presence of these ions; the inhibiting power of Na ", Ca 2 " and Mg 2+ on the equivalent basis was approximately in the ratio 1:10:30. Whitney and N or t hr up (1988) demonstrated that 80 90% of non-expandable "illite" layers which were developed in the temperature range 250 400C reexpanded upon saturation with N a - . These reversibly collapsed layers were therefore not truly illites, the latter is expected to display an irreversible collapse even upon saturation with Na *. Thus, the question may arise whether the "illites" that were identified using K-saturated reaction products in the past were really illites? NATURAL AI,TERATION OF BENTONITES The natural alteration of bentonites may give an idea about the role of time t h at can n o t be simulated in the laboratory experiments. Alteration processes in hydrothermal environments, in cont act zones of magmatic intrusions and in metabentonites r a t h e r closely resemble the reactions of bentonites in waste repositories. Conversion of smectite to illite during burial diagenesis of shales is a prominent phenomenon in sedimentary basins. The burial diagenetic environments are more complex and involve a wide range of components; they display little similarity to the repository environment. Diagenetic reactions of smectite will therefore not be discussed but selected examples of other natural alteration processes will be briefly described. A good example of bentonite alteration by percolating hydrothermal solutions within t e m p e r a t u r e range 80-200"C was reported by Inoue and Utada (1983) and Inoue et al. (1987) from the Shinzan area, Akita Prefecture, Japan. The hydrothermal episode lasted about one million years during the Miocene Period. Within a section of 184 to 453 m with a steep geothermal gradient of 10°C/100 m, smectites were converted to illite/smectite mixed-layers displaying 100-50% expandabilities, and they maintained the flakey morphology of the original smectite. Illite/smectite products with less than 50% expandabilities gave rise to X-ray diffraction patterns similar to those of ordered illite/smectite mixed-layers but in reality, they consisted of thin illite crystallites that were formed by neoformation mechanism as discussed above. The decrease in expandabilities of ordered illite/smectite mixed-layers was assumed to actually reflect the increasing thickness of growing illite crystallites.
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K-bentonites (or metabentonites) were originally composed of rather pure smectites that were later partially or almost fully altered to illites depending mainly on the availability of K and on the original chemistry of the parental smectites, and to a lesser extent on the thermal conditions. Most of the Kbentonite beds range in thickness from a few cm to a few meters, and were not subjected to deep burial and high temperatures. Ordovician K-bentonites ( ~ 450 Ma old) were studied by Weaver (1953) from Pennsylvania, Huff and Tiirkmeno$1u (1981) from Cincinnati Arch, Brusewitz (1986, 1988) and earlier by the same investigator (Bystr6m, 1956) from Kinnekulle, Sweden. A relatively younger (50-76 Ma) K-bentonite with a deep burial history was also reported by Altaner et al. (1984). Key factors controlling the extent of alteration of smectite to illite in K-bentonites were found to be the diffusion of potassium (i.e., K-saturation of bentonite), the original smectite chemistry, the presence of the inhibiting cations in the pore solutions, and thickness of bentonite beds. Temperature seems to play a secondary role especially at the low temperature ranges (below 100°C).
Swelling capacity of K-bentonites from Kinnekulle Selected samples from Kinnekulle containing illite/smectite mixed layers with 30-50% expandabilities (as originally determined by Brusewitz) were tested by Mfiller-Vonmoos et al. (1990). The swelling pressure of these altered bentonites of about 450 Ma age was found about 20% of an unaltered reference Ca-bentonite. The swelling pressures of the same K-bentonites increased more than three times after the samples were ultrasonically disintegrated and boiled in 5% Na2CO3 solution. This led Miiller-Vonmoos et al. to conclude that free silica in the K-bentonite was possibly cementing the clay particles in the original (untreated) samples. X-ray diffraction analysis of the Na+-saturated clay after the soda treatment (ultrasonic and boiling) would have clarified the question whether the non-expandable layers in the samples were "true" illites or highly charged K-fixed smectite layers. In any case Miiller-Vonmoos et al.'s study shows to what extent swelling of a bentonite can be reduced when 50-70% of the smectite layers became "non-expandable". ARTIFICIAL A L T E R A T I O N O F B E N T O N I T E S U N D E R S I M U L A T E D R E P O S I T O R Y CONDITIONS
Hydrothermal alteration of a Na-bentonite (Wyoming) was simulated by Wood (1983), Allen et al. (1984) and Allen and Rawson (1986), using site-specific groundwater with pH = 9.7. After 60 days at 200°C under 30 MPa pressure, no changes were observed in smectite except for the exchange of the K + (originally in smectite in small amounts) by Ca 2+ in the solution. The situation was however, different after the runs at 300°C for 217 and 341 days. No structural alteration of the smectite was detected at the end of the run. However, the solution chemistry was found to significantly change during the duration of the experiment. Large amounts of silica, some A1 and K were
242
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(;t~VEN
continuously released to the solution. Solution pH was drastically dropped from 9.7 to 5.7 within an hour and remained steady thereafter. Thus, the solution chemistry indicates some silica (possibly amorphous) was rapidly dissolved from the bentonite while smectite (probably the fine particles) kept undergoing a slow dissolution. Similar hydrothermal experiments by Yau et al. (1987) using Na-bentonite and a synthetic groundw at er at 200 and 300"C up to 537 days in duration showed no changes (structural and morphological} on the smectite particles in the run products except for small amounts of Ca K ion exchange. The solution chemistry was not however analyzed. Wood's (1983) experiments definitely demonstrate the importance of the solution chemistry for understanding the alteration kinetics of smectites.
Hydrothermal alteration of smectite microstructure An attempt was made by Pusch and Gfiven (1990) to assess affects of hydrothermal t r e a t m e n t on bentonite's microstructure. A Na-bentonite from Wyoming with a bulk density of 2.0 g/cm ~ was hydrothermally aged in distilled water for about six months at 150 and 20ffC. Morphology and chemical composition of the smectite particles were determined by an analytical electron microscope equipped with an energy-dispersive X-ray analyzer. No mineralogical or chemical changes were observed in smectite particles but subtle differences in m i c r os t r uct ur e were noticed. The m i crost ruct ure of the Na-bentonite displays a network of undulating and bifurcating thin smectite layers flowing around small and large elongated pores. Locally there are dense patches of smectite platelets which appear like branches of a foliated tree. These dense branches have similar chemistries as the thin films of smectite and they are probably stacks of edge-wise oriented platelets. With increasing temperatures of experiment, these branches appear more compact and pores become larger. This enlargement of the pores may increase the hydraulic conductivity.
Effects of water saturation: dry heating and steam Hydrothermal conditions may dominate in the backfill clay as along as the latter is water saturated. However, if the clay barrier becomes unsaturated, the bentonite may be exposed to water vapor or in extreme case to dry heat. Na-bentonite (Wyoming) was exposed by Allen et al. (1984) to dry heating between 250 and 550:'C for I 365 days at atmospheric pressure and the stability of the bentonite was examined. Na-bentonite showed up to 370"C a dehydration which was fully reversible upon exposure to water vapor. No permanent damage was caused to the s t r u c t u r e of smectite up to 370°C but the bentonite lost irreversibly its swelling ability after exposed to dry heat at 440'C. Bentonites were exposed to water vapor at 260°C and above were found to suffer serious reductions in their swelling capability and increases in their permeability by factors as much as 10 s (Couture, 1985). Oscarson et al.'s experiments (1990) were extremely useful for understanding the magnitude of
BENTONITE AS BUFFER MATERIAL IN A N U C L E A R W A S T E REPOSITORY
243
the damage and the underlying causes. Their steam treatment also significantly reduced the swelling pressure and increased the hydraulic conductivity of the bentonite (especially mixtures of bentonite with sand or other rocks) without any detectable mineralogical changes. This occurred with distilled water or with groundwater and became worse with increasing temperature and time. Oscarson et al. attributed the effects of steam to the possibility of cementation of clay particles by the precipitations of silica, alumina, or iron oxyhydroxides during or after the steaming process. The presence of such cement precipitates was, however, not directly observed. Oscarson et al.'s investigations clearly demonstrated that simple pretreatment of the bentonite by mechanical processing and heating below 120°C greatly enhances the swelling pressure and reduces the permeability of the bentonite, indicating the importance of the exposed surface area of smectite. They found substantial improvement in these properties with the duration (time) of the tests. Furthermore, anisotropy in bentonite microstructure (fabric) was found to give rise to large directional differences in swelling and permeability due to the lamellar morphology of the smectite particles. The steaming process at higher temperatures can affect bentonite's microstructure and disturb the lamellar fabric by generating slightly tilted grain boundaries between smectite aggregates. Moreover, steam may also destabilize the structure of the interlamellar water which primarily contributes to the development of the swelling pressure. Another process that can be initiated by steam treatment is related to a proton dissociation from the hydration shell of the interlayer cations (M ~+) according to the reaction (Frenkel, 1974): Mm *(n20)n = M ~ - 1 O H + H ÷ The dissociated proton is capable of affecting the layer charge by associating with the oxygens in smectite layers. The dissociated protons may also react with Al in smectite and lead to formation of hydroxy-aluminum interlayer complexes. The proton dissociation from the interlamellar c a t i o n - w a t e r complexes is promoted by the interlayer cations with high ionic potentials. Variations in temperature and water saturation in a bentonite barrier
According to a thermal model of a potential waste repository in Columbia River basalt (Allen and Wood, 1988), the maximum temperature in a barrier material would initially be 200°C, which later drops to 150°C and then remains so for 500 years. The pattern of temperature and water saturation in a bentonite barrier was simulated by Atabek et al. (1990). A heating device in a steel casing was placed in the center of a highly compacted clay core with a 20~mm diameter. The temperature was maintained at 175°C at the clay/casing interface for 8 months. Temperature distribution was mapped from the clay/casing interface to the clay/host rock (granite) contact. The temperature showed a sharp drop from 175°C to 81°C within a 65-mm distance from the casing. The water saturation was found to reach a relatively uniform pattern and ranged from 90-95% at the clay/granite contact to 70-80~/o at the clay/casing interface.
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x (;{JVEN
The clay used in the barrier was a Ca-bentonite from Paris Basin (France) and contained a Ca-smectite/kaolinite mixed-layer as the predominant component. The variations in water saturation and t em perat ure did not cause any detectable mineralogical changes (they were within the errors of observations) except at the clay/casing interface. Sections of the clay a few mm thick at this interface showed excess iron which may have migrated from the steel casing and precipitated on the clay as oxides. The bentonite had an unusually high iron co n ten t and depending on the redox conditions the oxidation state of Fe within the smectite s t r uc t ur e may change; this can alter smectite's layer charge. CONCLUSIONS l,ongevity of a bentonite barrier is directly related to the stability of smectites under the site-specific conditions. Hydrothermal stabilities of smectires considerably vary according to their chemical and structural characteristics. Trioctahedral smectites with Mg 2+, Li ~ octahedral compositions are known to have very high hydrothermal stabilities (up to 850°C) whereas ironrich smectites represent the least stable phases. Montmorillonites are the most common smectites in bentonites and they can be stabilized up to 400' C. The stability of the aluminous smectites, hence the longevity of bentonite, is primarily dependent on the i nt e r l ayer cations and to a lesser extent on the octahedral substitutions. Octahedral substitutions in aluminous smectites are commonly limited to about 25% of the total octahedral composition (Giiven, 1988) with Mg 2÷ and Fe 3÷ being the main proxying ions. Mg 2+ substitution in octahedra enhance the stability of the montmorillonite while Fe "~÷ substitutions decrease it. The role of interlayer cation seems to be related to their ionic properties affecting their interactions with the interlamellar water molecules. K*, Cs(and possibly Rb +) are characterized by low ionic potential, low hydration energies, and temporal rdr ratios below 1.0. These i nt erl ayer cations weakly interact with water molecules, facilitate the dehydration of the interlamellar water, and greatly promote the illitization of aluminous smectite. In the presence of K ~-bearing pore waters the stability of smectite can be as low as 50' C. The effects of K + on the physical properties and rheological behavior of bentonite must be tested especially at high temperatures in order to evaluate the damages to bentonite's performance. [ n t e r l a y e r cations like Na +, L i ' , Ca 2+, and Mg" ~, on the o t h e r hand. e n h a n c e the stability of aluminous smectite above 300°C. This has been d e m o n s t r a t e d in the l a b o r a t o r y for s hor t terms (few months up to a year). However, the long-term stabilities of Na *, Li ÷, Ca +, and Mg2+-smectites need to be calcu l a t e d as it was done for K+-smectite by Eberl and Hower (1976). Na ~ is well-known for its role in greatly improving various physical properties of bentonites. Mg 2 ~ has the highest ionic potential and temporal rdr value and can form the strongest s m e c t i t e i n t e r l a m e l l a r w at er complexes. The h y d r o t h e r m a l stability of be n t oni t e s a t u r a t e d with Mg 2÷ is
BENTONITE AS B U F F E R M A T E R I A L IN A N U C L E A R W A S T E REPOSITORY
245
expected to be much higher than other interlayer cations. Hydrothermal experiments (Gfiven and Huang, unpublished) demonstrated that Mg+-mont morillonite remains intact at 300°C for 90 days with no loss of swelling. The properties and rheological properties of Mg 2 +-saturated bentonites have not yet been systematically evaluated but they are expected to be superior to Ca 2 +-saturated bentonites. Thus, the contributions of the various interlayer cations to the stability and properties of aluminous smectites should be seriously considered in designing the barrier materials for radioactive waste. These interlayer cations can be easily added during the processing or during the preparation of bentonites. Bentonites that will be placed in the hot zone of the repository can be saturated by an interlayer cation like Mg 2+ to enhance its thermal stability. Similarly, the host rock contact of the barrier can be lined with Na-saturated bentonite in order to establish high swelling and low permeability. Moreover, special hydroxy metal complexes can be intercalated into smectite interlayers to enhance the adsorption capacity of the bentonite in order to retard radionuclide migration. Bentonite's situation in a nuclear repository seems to be somewhat analogous to that of a batallion attacked by enemy from both sides with different weapons. Bentonite has to retard the migration of " h o t " radionuclides on one side and to hinder the flow of "cold" groundwater on the other side. Bentonites equipped with proper interlayer ammunition can control the situation. Thus, a multilayered barrier consisting of slices of bentonites saturated with different interlayer cations may deserve some consideration in the future. For an extremely hot repository (above 300°C), trioctahedral smectites like saponites and hectorites may be used, especially in the hot zone of the repository. Again the physical properties of these smectites need to be evaluated under the expected conditions of a nuclear waste repository. Finally, a note of disclaimer may be attached to this report. The above predictions about the stability of bentonite are reasonable as long as pH and Eh conditions in the pore waters remain within moderate regimes. Acidic (pH < 5) and alkaline (pH/> 10) conditions or any unusual chemical component may severely and rapidly destroy bentonites if the conditions are not controlled by buffers. ACKNOWLEDGEMENTS I a m thankful to Professor Roland Pusch for the invitation to present this lecture at Lund workshop on "Artificial Clay Barriers for High Level Radioactive Waste Repositories". I would like also to express m y sincere thanks to Mr. Anders BergstrSm and to Swedish Nuclear Fuel and Management Co. for the perfectly organized excursion to Stripa Research Mine. The on-going experimentation in the Mine left a profound impression on m e and made m e realize the nature and magnitude of the nuclear waste problem. I a m convinced that Stripa Mine will be serving the scientists in their struggle with nuclear waste like a lighthouse serves the sailors.
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REFERENCES
Ahn, J.H. and Peacor, D.R., 1986. Transmission and analytical electron microscopy of the smectite-to-illite transition. Clays Clay Miner., 34:165 179. Allen, C.C., Lane, D.L., Palmer, R.A. and Johnston, R.G. 1984. Experimental studies of packing material stability. In: G.L. McVay (Editor), Materials Research Society Symposia Proceedings, Vol. 26. North-Holland, New York, N.Y., pp.105 112. Allen, C.C. and Rawson, S.A., 1986. Effects of irradiation and dry heating on bentonite: a transmission electron microscopy and X-ray diffraction study. In: A.D. Roming and W.F. Chambers (Editors), Microbeam analysis 1986. San Francisco Press, San Francisco, Calif., pp.135-137. Allen, C.C. and Wood, M.C., 1988. Bentonite in nuclear waste disposal: A review of research in support of the Basalt Waste Isolation Project. Appl. Clay Sci., 3:11 30. Altaner, S.P., Hower, J., Whitney, G. and Aranson, J.l,., 1984. Model for K-bentonite |brmation: Evidence from zoned K-bentonites in the disturbed belt, Montana. Geology, 12:412 415. Atabek, R., Lajudie, A., Lechelle, J. and Pusch, R., 19,90. Pilot field experiment with canisterembedding clay under simulated repository conditions. Eng. Geol.. 28:291 302. B a r r e l R.M. and Jones, D.L., 1970. Synthesis and properties of flurohectorites. J. Chem. Soc. A.: 1531- 1537. Boles, J.R. and Franks, S.G., 1978. Clay diagenesis in Wilcox Sand sandstones of southwest Texas: implications of smectite diagenesis on sandstone cementation. J. Sediment. Petrol., 49: 55 70. Brusewitz, A.M., 1986. Chemical and physical properties of Paleozoic potassium bentonites from Kinnekulle, Sweden. Clays Clay Miner., 34:442 454. Brusewitz, A.M., 1988. Asymmetric zonation of a thick Ordovician K-bentonite bed at Kinnekulle. Sweden. Clays Clay Miner., 36:349 353. Bystrom, A.M., 1956. Mineralogy of the Ordovician bentonite beds at Kinnekulle, Sweden. Sver. Geol. Unders. Ser. C, No. 540, 62 pp. Couture, R.A., 1985. Rapid increases in permeability and porosity of bentonite sand mixture due to alteration by water vapor. In: C.M. Jantzen, J.A. Stone and R.C. Ewing, (Editors), Materials Research Society Symposia Proceedings, Vol. 44. Materials Research Society, Pittsburgh. pp.515 522. Eberl, D., 1978. The reaction of montmorillonite to mixed-layer clay: the effect of interlayer alkali and alkaline earth cations. Geochim. Cosmochim. Acta, 42:1 7. Eberl, D. and Hower, J., 1976. Kinetics of illite formation. Geol. Soc. Am. Bull., 87:1326 1330. Eberl, D. and Hower, J., 1977. The hydrothermal transformation of sodium and potassium smectite into mixed-layer clay. Clays Clay Miner., 25: 215-217. Eberl, D.D., Whitney, G. and Khoury, H., 1978. Hydrothermal reactivity of smectite. Am. Mineral.. 63:401 409. Eberl, D. and Srodon, J., 1988. Ostwald ripening and interparticle-diffraction effects for illite crystals. Am. Mineral., 73:1335 1345. Frenkel, M., 1974. Surface acidity of montmorillonites. Clays Clay Miner., 22:435 441. Garrels, R.M. and Tardy, Y., 1982. Born Haber cycles for interlayer cations of micas. In: H. van Olphen and F. Veniale (Editors), International Clay Conference 1981. Elsevier, New York, N.Y., pp.423- 440. Giiven, N., 1988. Smectites. In: S.W. Bailey (Editor), Hydrous Phyllosilicates. Reviews in Mineralogy, 19, Mineralogical Society of America, pp.497 559. Howard, J.d. and Roy, D.M., 1985. Development of layer charge and kinetics of experimental smectite alteration. Clays Clay Miner., 33:81 -89. Hower, J., Eslinger, E., Hower, M.E. and Berry, E.A., 1976. Mechanism of burial metamorphism of argillaceous sediment: 1. Mineralogical and chemical evidence. Geol. Soc. Am. Bull., 87: 725 737. tluff, W.D. and Tiirkmenoi~lu, A.G., 1981. Chemical characteristic and origin of Ordovian Kbentonites along the Cincinnati Arch. Clays Clay Miner., 29: 113-123. lnoue, A. and Utada, M., 1983. Further investigations of a conversion series of dioctahedral mica
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smectites in the Shinzan hydrothermal alteration area, northeast Japan. Clays Clay Miner., 31: 401-412. Inoue, A., Kohyama, N., Kitagawa, R. and Watanabe, T., 1987. Chemical and morphological evidence for the conversion of smectite to illite.Clays Clay Miner., 35: 111-120. Inoue, A., Velde, B., Mennier, A. and Touchard, G., 1988. Mechanism of illiteformation during smectite-t@illite conversion in a hydrothermal system. Am. Mineral., 73: 1325-1334. Lee, J.H., Ahn, J.H. and Peacor, D.R., 1985. Textures in layered silicates:progressive changes through diagenesis and low-temperature metamorphism. J. Sediment. Petrol., 55: 532-540. Miiller-Vonmoos, M., Kahr, G., Bucher, F. and Madsen, F.T., 1990. Investigation of the Kinnekulle K-bentonite aimed at assessing the long-term stability of bentonites under repository conditions. Eng. Geol., 28: 269-280. Nadeau, P.H., Wilson, M.J., McHardy, W.J. and Tait, J.W., 1984. Interparticle diffraction: A new concept for interstratified clays. Clay Miner., 19:757 769. Nadeau, P.H., Wilson, M.J., McHardy, W.J. and Tait, J.W., 1985. The conversion of smectite to illite during diagenesis: Evidence from some illitic clays from bentonites and sandstones. Miner. Mag., 49: 397-400. Nadeau, P.H. and Bain, D.C., 1986. Composition of some smectites and diagenetic illitic clays and implications for their origin. Clays Clay Miner., 34: 455-464. Newman, A.C.D., 1987. The interaction of water with clay mineral surfaces. In: A.C.D. Newman (Editor), Chemistry of Clays and Clay Minerals. Monogr. 6, Mineral. Soc., Wiley, New York, N.Y. pp.237-274. Oscarson, D.W., Dixon, D.A. and Gray, M.N., 1990. Swelling capacity and permeability of an unprocessed and a processed bentonitic clay. Eng. Geol., 26: 261-289. Pusch, R., 1982. Mineral-water interactions and their influence on the physical behavior of highly compacted Na-bentonite. Can. Geotech. J., 19: 381-387. Pusch, R. and Carlsson, T., 1985. The physical state of Na-smectite used as barrier component. Eng. Geol., 21: 257-265. Pusch, R. and G/iven, N., 1990. Electron microscopic examination of hydrothermally treated bentonite clay. Eng. Geol., 28: ~?-~?. Roberson, H.E. and Lahann, R.W., 1981. Smectite to illiteconversion rates: effects of solution chemistry. Clays Clay Miner., 29: 129-135. Samoilov, O. Ya., 1957. A new approach to the study of hydration of ions in aqueous solutions. Discuss. Faraday Soc., 24:141 146. Sposito, G., 1984. The Surface Chemistry of Soils, Oxford Univ. Pred., N e w York, Clarendon, Oxford, 234 pp. Suquet, H., De La Calle, C. and Pezerat, H., 1975. Swelling and structural organization of saponitc. Clays Clay Miner., 23: 1-9. Suquet, H., liyama, J.T., Kodama, H. and Pezerat, H., 1977. Synthesis and swelling properties of saponites with increasing layer charge. Clays Clay Miner., 25: 231-242. Suquet, H. and Pezerat, H., 1987. Parameters influencing layer stacking types in saponite and vermiculite: a review. Clays Clay Miner., 35: 353-362. Weaver, C.W., 1953. Mineralogy and petrology of some Ordovician K-bentonites and related limestones. Geol. Soc. Am. Bull., 64: 921-964. Whitney, G., 1983. Hydrothermal reactivity of saponite. Clays Clay Miner., 31: I-8. Whitney, G. and Northrop, H.R., 1988. Experimental investigation of the smectite to illite reaction: Dual reaction mechanisms and oxygen-isotope systematics. Am. Mineral., 73: 77-90. Wood, M.I., 1983. Experimental investigation of sodium bentonite stabilityin Hanford Basalt. In: D.G. Brookins (Editor), Materials Research Society Symposia Proceedings, Vol. 15. NorthHolland, Amsterdam, pp.727-743. Yau, Y.-C., Peacor, D.R., Essene, E.J., Lee, J.H., Kuo, L.-C. and Cosca, M.A., 1987. Hydrothermal treatment of smectite, illite, and basalt to 460°C: comparison of natural with hydrothermally formed clay minerals. Clays Clay Miner., 35: 241-250.
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L o n g e v i t y of B e n t o n i t e as Buffer M a t e r i a l in a Nuclear-Waste Repository NECIP GIJVEN Department of C,eosciences, Texas Tech University, Lubbock, TX 79409(U.S.A.) (Accepted for publication February 14, 1989)
ABSTRACT Giiven, N., 1990. Longevity of bentonite as buffer material in a nuclear-waste repository. Eng. Geol., 28: 233-247. Longevity of bentonite as a nuclear waste barrier depends on mineralogical stability of the smectite component of the clay as well as on its abilityto perform as a buffer in the repository for long periods of time. Processes affecting the stabilityof smectites under the geochemical and thermal conditions in a high-level nuclear waste repository can be construed by examining the geological analogues and the resultsof the hydrothermal experiments. The physical and chemical properties of bentonite seem to be related to the nature of smectites in them. Smectites represent a range of expandable layer silicatesof Al, Mg, Fe and other transition metals. Their thermal, hydrothermal, or solution stabilitiesvary from one smectite species to another. This is also true to a certain extent for the physical propertiesof bentonites,i.e.swelling,permeability,plasticityand adsorption. Swelling ability and permeability of naturally altered (metabentonites) and steam-treated bentonites are badly damaged. Temperature distribution in a clay barrier that was simulated in situ under the repository conditions showed a sharp drop from 175°C at the clay/canistercontact to 81°C at a distance 65 m m away. Microstructures of bentonites were noticeably altered after hydrothermal treatment at 200cC.
INTRODUCTION
A backfill material in a nuclear waste repository must perform certain crucial functions in order to isolate and contain radioactive waste. As explained by Pusch (1982) and Pusch and Carlsson (1985), the backfill must provide: (1) an effective (impervious) barrier which can minimize the flow rate of percolating groundwater and retard the migration of radionuclides upon their release from the waste container; (2) a buffer media with appropriate rheology, bulk density and swelling properties to withstand the mechanical stresses in the host rock, and to seal the cracks and fissures; (3) good thermal conductivity to dissipate the heat generated in the waste container. More importantly the buffer material must remain physically and chemically stable to perform the above functions for long periods of time up to a few million years; this is referred to as the longevity of the buffer material. Bentonites have been considered as a prime candidate for backfill material due 0013-7952/90/$03.50
© 1995 Elsevier Science Publishers B.V.
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to t h e i r p h y s i c a l a n d r h e o l o g i c a l p r o p e r t i e s s u c h as s w e l l i n g , p l a s t i c i t y , i o n exchange and sorption, sealing capability, and large reactive surface areas. B e n t o n i t e s a r e t h e c l a y s t h a t c o n s i s t p r e d o m i n a n t l y of a s m e c t i t e m i n e r a l . T h e p r o p e r t i e s of b e n t o n i t e s a r e l a r g e l y d e t e r m i n e d by t h e s t r u c t u r a l , c h e m i c a l a n d m o r p h o l o g i c a l f e a t u r e s of s m e c t i t e m i n e r a l s i n t h e m . T h e s e c h a r a c t e r i s t i c s of s m e c t i t e s will t h e r e f o r e be d e s c r i b e d i n t h e n e x t s e c t i o n . VARIATIONS IN CHEMISTRY AND STRUCTURE OF SMECTITES IN BENTONITES
Classification of smectites S m e c t i t e s r e p r e s e n t a s e r i e s o f e x p a n d a b l e ( s w e l l i n g ) 2:1 t y p e l a y e r s i l i c a t e s w i t h a n e x c e s s l a y e r c h a r g e i n t h e r a n g e of 0.2 0.6 e q u i v a l e n t s p e r O~o (OH)2. S m e c t i t e s a r e c l a s s i f i e d i n T a b l e l a c c o r d i n g to t h e c r i t e r i a r e l a t e d to (a) p r e d o m i n a n t o c t a h e d r a i c a t i o n s a n d t h e i r s u b s t i t u t i o n s , (b) di- or t r i - o c t a h e d r a l n a t u r e of o c t a h e d r a l s h e e t s , a n d (c) r e l a t i v e d i s t r i b u t i o n o f e x c e s s c h a r g e s over t e t r a h e d r a l and o c t a h e d r a l sheets. TABLE I Classification of natural and synthetic: smectites (Gfiven. Ratio between tetrahedral (xt) and octahedral (x.) charges
xjx, > 1.0
Dioctahedral smectites Predominant octahedral cation(s)
Trioctahedral smectites ...................... Smectite Predominant Smectite species octahedral species cation(s)
Al(Mg,Fe) 2
Montmorillonite
(Octahedrat charges predominant)
x,/x o> 1.0 (Tetrahedral charges predominant)
1988) I
AI Fe -~-
Beidellite Nontronite
Cr "~"
Volkonskoite
V"~"
Vanadium smectite
Mg( : l ) ' ~ Mg(l,i) 2 AIMgLi
Stevensite Hectoritc Swinefordite
Single or mixed transition metals
Transition metal "defect" trioe. smectites
Mg Fe 2" Zn Co Mn
Saponite Iron saponite Sauconite Cobalt smectite Manganese smectite
Single or mixed transition metals
Transition metal trioct. smectites
IReasonably well-defined species (synthetic or natural) are given as separate entries. :Octahedral substitutions. :_-]Vacancies.
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Smectite crystallites appear in a variety of habits as thin platelets, large and flexible films, laths, ribbons and fibers (Giiven, 1988). These crystallites occur in aggregates with various textures: foliated aggregates of thin films, compact aggregates of platelets, mossy flocks and fine granular (globular) agglomerates. Aluminium dioctahedral smectites (montmorillonite) are by far the most common in nature and they are often found in bentonites. The terms "bentonite" and "montmorillonite" are often used interchangeably (also in this report) although the former is a clay (rock) while the latter is a clay mineral. Montmorillonites as clay mineral display significant chemical and morphological differences (Giiven, 1988). They are subdivided into several varieties according to their type localities or special chemistries: (1) Wyoming-type montmorillonite, (2) Otaytype montmorillonite, (3) Chambers-(Cheto-) type montmorillonite, (4) Tatilla-type montmorillonite, and (5) Fe-rich montmorillonites. Each of these montmorillonites may display different physical properties and thermal stabilities mainly because of the chemical, morphological and textural differences. The hydrothermal stability of smectites ranges from 50°C to 850°C; trioctahedral magnesian smectites are more stable than the dioctahedral aluminian ones. Fe-rich smectites are known to be more easily altered and thermally less stable than the magnesian and aluminian varieties. Synthetic fluorohectorite is probably the most stable smectite; it maintains its structural integrity and water swelling even at 850°C (Barrer and Jones, 1970). FACTORS CONTROLLINGSMECTITE-WATER INTERACTIONS Most of the physical properties of smectites such as swelling, adsorption, plasticity, permeability and shear strength are related to the interactions between electrically charged interlamellar surfaces and the water molecules. Factors controlling smectite hydration were discussed in detail by Suquet et al. (1975, 1977) and Suquet and Pezerat (1987), and are summarized here: (1) layer charge, its density and site (tetrahedral vs. octahedral sheets); (2) nature of interlayer cation; (3) di- or tri~ctahedral character of the layer; (4) mean crystallite size; (5) partial pressure of water; and (6) distribution of tetrahedral (Al, Si) substitutions and that of interlayer cations over the available structural sites. Under the combined affects of the above factors, two kinds of smectitewater complexes can develop between the siloxane surfaces of the layers: (a) interlamellar hydrates with an ordered structure; they can consist of an integral number (0, 1, 2 and maximum 3) of water layers; and thereafter, (b) liquid-like water in amounts that are determined mainly by osmotic effects. Current concepts of the clay-water complexes were recently reviewed by Sposito (1984) and Newman (1987) and they depict the following picture. Smectite's interlayer surfaces exert a influence on the water molecules within a 10-A distance from the layers. Beyond that, water molecules behave just like in liquid water. The interlamellar water does not possess a rigid "ice-like" structure but a dynamic one. Water molecules interact with smectite's basal surfaces and with the interlayer cations within the interlayer space in a dynamic manner. Water's molecular motions (rotational, vibrational and
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translational) and its orientation are thereby perturbed. The interactions b e t w e e n t h e i n t e r l a y e r c a t i o n a n d w a t e r m o l e c u l e s is l a r g e l y d e t e r m i n e d by the special ionic properties such as the ionic potential (cation's charge/radius) and its hydration energy. The hydration energies of common interlayer c a t i o n s a n d t h e i r i o n i c p o t e n t i a l s a r e l i s t e d in T a b l e II f o r t w o d i f f e r e n t s m e c t i t e s m o n t m o r i l l o n i t e a n d h e c t o r i t e . T h e h y d r a t i o n e n e r g i e s in T a b l e [| w e r e c a l c u l a t e d b y G a r r e l s a n d T a r d y (1982); t h e y c o r r e l a t e f a i r l y w e l l w i t h i o n i c p o t e n t i a l s e x c e p t for C a - m o n t m o r i l l o n i t e . F u r t h e r m o r e , h e c t o r i t e ' s hyd r a t i o n e n e r g i e s for t h e s a m e c a t i o n s w e r e f o u n d t o b e s i g n i f i c a n t l y h i g h e r t h a n t h o s e in m o n t m o r i l l o n i t e ; i n d i c a t i n g t h a t t h e i n t e r l a y e r c a t i o n w a t e r c o m p l e x e s in h e c t o r i t e w o u l d b e m u c h m o r e s t a b l e t h a n in m o n t m o r i l l o n i t e . DYNAMICS OF CLAY--WATER INTERACTIONS Ionic potential represents a measure of the polarizing effect of a cation on a w a t e r m o l e c u l e in i t s v i c i n i t y , h e n c e o n t h e e x t e n t o f b i n d i n g b e t w e e n t h e m . T h i s b i n d i n g m u s t be c o n s i d e r e d a " d y n a m i c o n e " e s p e c i a l l y w h e n t h e c l a y is saturated with water. This dynamic picture was described early by Samoilov (1957) b y c o m p a r i n g t h e a v e r a g e t i m e (r) a w a t e r m o l e c u l e s p e n d s ( i n t e r a c t s ) w i t h t h e n e x t w a t e r m o l e c u l e in p u r e w a t e r t o t h e a v e r a g e t i m e (ri) a w a t e r m o l e c u l e r e s i d e s in t h e i m m e d i a t e v i c i n i t y o f a n i o n in s o l u t i o n . T h e a v e r a g e e x c h a n g e e n e r g y (E) b e t w e e n t w o w a t e r m o l e c u l e s w i l l b e i n c r e a s e d by a n a d d i t i o n a l A E i b y t h e p r e s e n c e o f ion. T h e n u m e r i c a l v a l u e s o f r a n d A E i w e r e d e t e r m i n e d f r o m t h e s e l f - d i f f u s i o n e x p e r i m e n t s ; r w a s f o u n d to b e 1.7 × | 0 ~ s in p u r e w a t e r . T h e t e m p o r a l r a t i o r d r is d i r e c t l y r e l a t e d to AE~ b y t h e e q u a t i o n : ri/r = e x p ( A E i / R T ) T h e v a l u e s f o r ri/~ a n d A E i a r e l i s t e d in T a b l e II f o r a s e r i e s o f i n t e r l a y e r cations. They correlate fairly well with the ionic potentials and hydration TABLE II Ionic properties of interlayer cations in smectites and additional exchange energies (AE) of water molecule in dilute solutions Interlayer cation
K" Rb" Cs' Na + LiBa 2" Sr-" " Ca 2Mg 2"
Ionic: potential (nm "J)
Water molecule's AE~ (kcal/mole)
temporal ratio ( ~ d ~ )
Interlayer hydration energies (kcal/mole) .............. montmorillonite hectorite
7.5 6.8 6.0 10.3 14.7 14.9 17.9 20.2 30.3
- 0.25 n.d. - 0.33 0.25 0.73 n.d. n.d. 0.45 2.61
0.65 n.d. 0.57 1.46 3.48 n.d. n.d. 2.16 86.30
-- 2.63 --2.65 2.89 -3.97 - 5.59 5.95 - 6.00 - 7.43 6.6{)
....
-- 5.02 n.d. .1.97 - 6.21 - 8.6{) 7.51 - 8.80 - 10.18 10.92
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energies of the interlayer cations. The temporal ratio ~i/z> 1 indicates that the water molecule would spend more time in the immediate vicinity of the ion than in the vicinity of another water molecule. Thus, stronger binding (in dynamic sense) develops between the cation and the water molecules as Ti/z ratio becomes larger than one. For instance M g 2+ with a zi/z ratio of 86.3 would establish an unusually strong binding with water molecules. The situation is rather interesting for K + and Cs + ions; their temporal Ti/zratios are less than 1.0 and their A E i values are negative. This implies that water molecules in a solution would spend less time in the immediate vicinity of K + or Cs + ions than in the vicinity of another water molecule, i.e.water molecules become more mobile around these ions. The above dynamic interactions of water molecules within the interlayer space are expected to have significant bearing on the structure of interlamelfar water and hence on swelling, permeability and rheology, even on the hydrothermal stability of smectites. Swelling of bentonite and the related swelling pressure within a confined volume is mainly generated by dynamically "structured" interlamellar water. Obviously high temperatures can considerably destabilize the structure of the interlamellar water as well as the interactions between water molecules and the lamellar surfaces. PROCESSES A N D FACTORS AFFECTING LONGEVITY OF BENTONITE IN A NUCLEAR WASTE REPOSITORY Longevity of bentonites is synonymous with the stability of the smectites in them. Stability, in this connotation, is not restricted to the chemical and structural survival of the smectite mineral in the repository environment but its continued performance of the buffer functions for a few million years. Temperature in such an environment may range probably from 200°C at the clay/waste container interface to the ambient temperature in the host rock/clay contact. The geochemical parameters are site-specific and vary depending on the chemical and mineralogical make-up of the backfill material, chemistry of pore fluids and the prevailing pH and Eh conditions in them, and the nature of radionuclides that may be released from the waste container. After the initial water saturation of the backfill material, the repository may be considered to a first approximation as a hydrothermal system. Geological literature offers a vast volume of studies on the stability of smectites in sedimentary (diagenetic), hydrothermal and contact metamorphic environments. The conversion of smectite to illite appears as the primary process underlying the longevity of the bentonite for this conversion can destroy smectite's physical properties. The mechanism of this conversion is briefly described next. M E C H A N I S M OF THE SMECTITE-TO-ILLITE CONVERSION Reaction mechanism involved in this transition has been the subject of numerous studies and the conversion requires a significant enhancement of an excess layer charge in smectite.
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The layer charge in smectite s t r uct ur e can be increased by: (a) substitutions of A13+ for Si 4÷ in tetrahedra; (b) substitutions of Mg 2~ or Fe 2÷ for Al 3~ in octahedra; and (c) reduction of Fe 3 ~ to Fe 2 ÷ in octahedra. The mean layer charge in smectites is found to be 0.33 equivalents per O10 (OH) 2 whereas illite layers possess on the average a larger charge of 0.75-0.8. However, successive layers in a smectite crystallite generally show a nonuniform charge distribution ranging from 0.2 to 0.6 equivalents. Two reaction mechanisms have been proposed for the smectite-to-illite conversion. (1) l,ayer-by-layer transformation in solid state according to the reaction: Smectite + K ÷ + A13 ~ = iilite + silica This mechanism was formulated by Hower et al. 0976). It involves tetrahedral substitutions of A1a * for Si 4÷ with little disruption of illite layer and with practically no change of volume. (2) Dissolution-reprecipitation mechanism (neoformation) involves complete dissolution of smectite layers and reprecipitation of iilite layers according to the reaction: 1.5 smectite + K ÷ --, 1.0 illite + silica while other ions such as Mg 2 +, Fe 2", Ca 2 ~, and Na * in the original smectite layers are released to the solution. Hydrothermal experiments with smectites by Eberl and Hower (1976, 1977), studies of Gulf Coast sediments by Boles and Franks (1978) demonstrated the neoformation mechanism in these systems. No matter what reaction mechanism, the smectite-to-illite conversion was t h o u g h t to proceed via the formation of continuous series of illite/smectite mixed-layers with smectite c ont e nt (expandability) progressively reducing from 100 to 0%. This concept was widely approved and accepted until its validity was seriously questioned in recent studies by Inoue and Utada (1983), Nadeau et al. (1984, 1985), Lee et al. (1985), Ahn and Peacor (1986), Nadeau and Bain (1986) and Inoue et al. (1987, 1988). The results of these studies were in apparent conflict with a continuous transformation increasing illite component. Ahn and Peacor (1986) and Lee et al. (1985) obtained lattice images of the apparently interstratified illite/smectites as indicated by X-ray diffraction. These showed the presence of discrete illite domains in a smectite matrix but no evidence for interstratification of illite and smectite layers. Inoue and Utada (1983) and Inoue et al. (1987, 1988) proposed a dual reaction mechanism where the initial stage involves a layer-by-layer solid state transformation and the second stage proceeds as neoformation of illite. Accordingly, the first stage involves K-fixation of smectite layers t hat had developed high enough charges for collapse. The clay loses its expandability gradually from 100 to about 50% and appears as a random interstratification of expandable and non-expandable layers. The non-expandable layers have been identified as illite although highly charged smectite layers could have also collapsed upon K-fixation. In any case, the randomly interstratified illite/smectite displays a flakey morphology just like a smectite with 100% expandability. lnoue et al. (1987) found a drastic change in morphology and in the mode of
BENTONITE AS B U F F E R M A T E R I A L IN A N U C L E A R W A S T E REPOSITORY
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mixing when the expandability of illite/smectite clay was reduced below 50%. Smectites were assumed to dissolve and thin laths of illite precipitate, which develop into wider and thicker laths with continued crystal growth. These thin laths are the product of the neoformation and their coarsening follows the Ostwald ripening process, i.e. simultaneous dissolution of fine particles and growth of the larger ones in a closed system. An oriented aggregate of thin illite crystallites can, however, give an X-ray diffraction pattern like a regular interstratification of I/S layers with a certain expandability; the expandability being caused by the adsorption of organic (or water) molecules between the particle surfaces as suggested by Nadeau et al. (1984, 1985) and Nadeau and Bain (1986). The "apparent" expandability can be a direct function of the thickness of illite crystallites. This relationship is approximated by Eberl and Srodon (1988) by the equation: maximum expandability (in %)= lO0/n with n > 1, where n is the average number of (2:1) layers in illite crystallites. For instance, a glycol saturated sample of pure illite crystallites with an average of 5 layers in thickness may show as much as 20% "apparent" expandability on their X-ray diffraction pattern. HYDROTHERMAL
STABILITY O F BENTONITE: E X P E R I M E N T A L STUDIES
Experimental studies on hydrothermal reactions of aluminous smectites, notably those by Eberl and Hower (1976), Eberl (1978), Eberl et al. (1978), Roberson and Lahann (1981), Howard and Roy (1985) and Whitney and Northrup (1988) have been very useful in understanding hydrothermal stability of bentonites. These studies revealed the reaction kinetics and the importance of factors such as pore fluid chemistry including its pH and redox potential, the interlayer cations and temperature. The role of the interlayer cation was found to be of paramount importance. The smectite-to-illite conversion rate at temperatures up to 400°C was found to be directly related to the interlayer hydration energy of the exchangeable cations (Eberl, 1978). The expandable smectite layers became non-expandable at 300°C within 30 days when smectite was saturated with K +, Rb ÷ or Cs ÷ but no such reaction was observed with Na ÷ or Li ÷. Smectites saturated with alkaline earth cations reacted first at 400°C and generated a regular mixed-layer with 50% smectite content (rectorite-type). Thus, the stability of smectite can be enhanced and its intracrystalline swelling (expandability) can be maintained at high temperatures by the addition of appropriate salts of alkaline earths. Obviously, the presence of K ÷, Rb ÷, and Cs ÷ in pore waters will significantly decrease the stability of smectite in repository environment. Although the potassium content of the ground water is usually negligible, potassium can be leached from K-feldspars and micas by circulating ground water. Eberl and Hower (1976) calculated an activation energy of 19.6_ 3.5 kcal/ mole for the conversion of K-saturated (aluminous) smectite to illite; they expressed the conversion rates by the equation:
240
In
N ~;(?v~:x
(a/a - x) = k t
where a is the initial c o n c e n t r a t i o n of smectite, x is the amount of the reaction product illite, t is the time and k is reaction rate for a specific temperature. Accordingly, to convert K-smectite to illite it would take about one million years at 50~:C, about 20,000 years at 100 C, and about 99 days at 393~:C. Similar hydrothermal experiments but buffered at neutral pH conditions were conducted by Roberson and L a ha nn (1981) in mixed cation solutions (K N a , K--Ca, K -Mg), the conversion reaction: smectite + K = illite + silica was inhibited by the presence of these ions; the inhibiting power of Na ", Ca 2 " and Mg 2+ on the equivalent basis was approximately in the ratio 1:10:30. Whitney and N or t hr up (1988) demonstrated that 80 90% of non-expandable "illite" layers which were developed in the temperature range 250 400C reexpanded upon saturation with N a - . These reversibly collapsed layers were therefore not truly illites, the latter is expected to display an irreversible collapse even upon saturation with Na *. Thus, the question may arise whether the "illites" that were identified using K-saturated reaction products in the past were really illites? NATURAL AI,TERATION OF BENTONITES The natural alteration of bentonites may give an idea about the role of time t h at can n o t be simulated in the laboratory experiments. Alteration processes in hydrothermal environments, in cont act zones of magmatic intrusions and in metabentonites r a t h e r closely resemble the reactions of bentonites in waste repositories. Conversion of smectite to illite during burial diagenesis of shales is a prominent phenomenon in sedimentary basins. The burial diagenetic environments are more complex and involve a wide range of components; they display little similarity to the repository environment. Diagenetic reactions of smectite will therefore not be discussed but selected examples of other natural alteration processes will be briefly described. A good example of bentonite alteration by percolating hydrothermal solutions within t e m p e r a t u r e range 80-200"C was reported by Inoue and Utada (1983) and Inoue et al. (1987) from the Shinzan area, Akita Prefecture, Japan. The hydrothermal episode lasted about one million years during the Miocene Period. Within a section of 184 to 453 m with a steep geothermal gradient of 10°C/100 m, smectites were converted to illite/smectite mixed-layers displaying 100-50% expandabilities, and they maintained the flakey morphology of the original smectite. Illite/smectite products with less than 50% expandabilities gave rise to X-ray diffraction patterns similar to those of ordered illite/smectite mixed-layers but in reality, they consisted of thin illite crystallites that were formed by neoformation mechanism as discussed above. The decrease in expandabilities of ordered illite/smectite mixed-layers was assumed to actually reflect the increasing thickness of growing illite crystallites.
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K-bentonites (or metabentonites) were originally composed of rather pure smectites that were later partially or almost fully altered to illites depending mainly on the availability of K and on the original chemistry of the parental smectites, and to a lesser extent on the thermal conditions. Most of the Kbentonite beds range in thickness from a few cm to a few meters, and were not subjected to deep burial and high temperatures. Ordovician K-bentonites ( ~ 450 Ma old) were studied by Weaver (1953) from Pennsylvania, Huff and Tiirkmeno$1u (1981) from Cincinnati Arch, Brusewitz (1986, 1988) and earlier by the same investigator (Bystr6m, 1956) from Kinnekulle, Sweden. A relatively younger (50-76 Ma) K-bentonite with a deep burial history was also reported by Altaner et al. (1984). Key factors controlling the extent of alteration of smectite to illite in K-bentonites were found to be the diffusion of potassium (i.e., K-saturation of bentonite), the original smectite chemistry, the presence of the inhibiting cations in the pore solutions, and thickness of bentonite beds. Temperature seems to play a secondary role especially at the low temperature ranges (below 100°C).
Swelling capacity of K-bentonites from Kinnekulle Selected samples from Kinnekulle containing illite/smectite mixed layers with 30-50% expandabilities (as originally determined by Brusewitz) were tested by Mfiller-Vonmoos et al. (1990). The swelling pressure of these altered bentonites of about 450 Ma age was found about 20% of an unaltered reference Ca-bentonite. The swelling pressures of the same K-bentonites increased more than three times after the samples were ultrasonically disintegrated and boiled in 5% Na2CO3 solution. This led Miiller-Vonmoos et al. to conclude that free silica in the K-bentonite was possibly cementing the clay particles in the original (untreated) samples. X-ray diffraction analysis of the Na+-saturated clay after the soda treatment (ultrasonic and boiling) would have clarified the question whether the non-expandable layers in the samples were "true" illites or highly charged K-fixed smectite layers. In any case Miiller-Vonmoos et al.'s study shows to what extent swelling of a bentonite can be reduced when 50-70% of the smectite layers became "non-expandable". ARTIFICIAL A L T E R A T I O N O F B E N T O N I T E S U N D E R S I M U L A T E D R E P O S I T O R Y CONDITIONS
Hydrothermal alteration of a Na-bentonite (Wyoming) was simulated by Wood (1983), Allen et al. (1984) and Allen and Rawson (1986), using site-specific groundwater with pH = 9.7. After 60 days at 200°C under 30 MPa pressure, no changes were observed in smectite except for the exchange of the K + (originally in smectite in small amounts) by Ca 2+ in the solution. The situation was however, different after the runs at 300°C for 217 and 341 days. No structural alteration of the smectite was detected at the end of the run. However, the solution chemistry was found to significantly change during the duration of the experiment. Large amounts of silica, some A1 and K were
242
N
(;t~VEN
continuously released to the solution. Solution pH was drastically dropped from 9.7 to 5.7 within an hour and remained steady thereafter. Thus, the solution chemistry indicates some silica (possibly amorphous) was rapidly dissolved from the bentonite while smectite (probably the fine particles) kept undergoing a slow dissolution. Similar hydrothermal experiments by Yau et al. (1987) using Na-bentonite and a synthetic groundw at er at 200 and 300"C up to 537 days in duration showed no changes (structural and morphological} on the smectite particles in the run products except for small amounts of Ca K ion exchange. The solution chemistry was not however analyzed. Wood's (1983) experiments definitely demonstrate the importance of the solution chemistry for understanding the alteration kinetics of smectites.
Hydrothermal alteration of smectite microstructure An attempt was made by Pusch and Gfiven (1990) to assess affects of hydrothermal t r e a t m e n t on bentonite's microstructure. A Na-bentonite from Wyoming with a bulk density of 2.0 g/cm ~ was hydrothermally aged in distilled water for about six months at 150 and 20ffC. Morphology and chemical composition of the smectite particles were determined by an analytical electron microscope equipped with an energy-dispersive X-ray analyzer. No mineralogical or chemical changes were observed in smectite particles but subtle differences in m i c r os t r uct ur e were noticed. The m i crost ruct ure of the Na-bentonite displays a network of undulating and bifurcating thin smectite layers flowing around small and large elongated pores. Locally there are dense patches of smectite platelets which appear like branches of a foliated tree. These dense branches have similar chemistries as the thin films of smectite and they are probably stacks of edge-wise oriented platelets. With increasing temperatures of experiment, these branches appear more compact and pores become larger. This enlargement of the pores may increase the hydraulic conductivity.
Effects of water saturation: dry heating and steam Hydrothermal conditions may dominate in the backfill clay as along as the latter is water saturated. However, if the clay barrier becomes unsaturated, the bentonite may be exposed to water vapor or in extreme case to dry heat. Na-bentonite (Wyoming) was exposed by Allen et al. (1984) to dry heating between 250 and 550:'C for I 365 days at atmospheric pressure and the stability of the bentonite was examined. Na-bentonite showed up to 370"C a dehydration which was fully reversible upon exposure to water vapor. No permanent damage was caused to the s t r u c t u r e of smectite up to 370°C but the bentonite lost irreversibly its swelling ability after exposed to dry heat at 440'C. Bentonites were exposed to water vapor at 260°C and above were found to suffer serious reductions in their swelling capability and increases in their permeability by factors as much as 10 s (Couture, 1985). Oscarson et al.'s experiments (1990) were extremely useful for understanding the magnitude of
BENTONITE AS BUFFER MATERIAL IN A N U C L E A R W A S T E REPOSITORY
243
the damage and the underlying causes. Their steam treatment also significantly reduced the swelling pressure and increased the hydraulic conductivity of the bentonite (especially mixtures of bentonite with sand or other rocks) without any detectable mineralogical changes. This occurred with distilled water or with groundwater and became worse with increasing temperature and time. Oscarson et al. attributed the effects of steam to the possibility of cementation of clay particles by the precipitations of silica, alumina, or iron oxyhydroxides during or after the steaming process. The presence of such cement precipitates was, however, not directly observed. Oscarson et al.'s investigations clearly demonstrated that simple pretreatment of the bentonite by mechanical processing and heating below 120°C greatly enhances the swelling pressure and reduces the permeability of the bentonite, indicating the importance of the exposed surface area of smectite. They found substantial improvement in these properties with the duration (time) of the tests. Furthermore, anisotropy in bentonite microstructure (fabric) was found to give rise to large directional differences in swelling and permeability due to the lamellar morphology of the smectite particles. The steaming process at higher temperatures can affect bentonite's microstructure and disturb the lamellar fabric by generating slightly tilted grain boundaries between smectite aggregates. Moreover, steam may also destabilize the structure of the interlamellar water which primarily contributes to the development of the swelling pressure. Another process that can be initiated by steam treatment is related to a proton dissociation from the hydration shell of the interlayer cations (M ~+) according to the reaction (Frenkel, 1974): Mm *(n20)n = M ~ - 1 O H + H ÷ The dissociated proton is capable of affecting the layer charge by associating with the oxygens in smectite layers. The dissociated protons may also react with Al in smectite and lead to formation of hydroxy-aluminum interlayer complexes. The proton dissociation from the interlamellar c a t i o n - w a t e r complexes is promoted by the interlayer cations with high ionic potentials. Variations in temperature and water saturation in a bentonite barrier
According to a thermal model of a potential waste repository in Columbia River basalt (Allen and Wood, 1988), the maximum temperature in a barrier material would initially be 200°C, which later drops to 150°C and then remains so for 500 years. The pattern of temperature and water saturation in a bentonite barrier was simulated by Atabek et al. (1990). A heating device in a steel casing was placed in the center of a highly compacted clay core with a 20~mm diameter. The temperature was maintained at 175°C at the clay/casing interface for 8 months. Temperature distribution was mapped from the clay/casing interface to the clay/host rock (granite) contact. The temperature showed a sharp drop from 175°C to 81°C within a 65-mm distance from the casing. The water saturation was found to reach a relatively uniform pattern and ranged from 90-95% at the clay/granite contact to 70-80~/o at the clay/casing interface.
244
x (;{JVEN
The clay used in the barrier was a Ca-bentonite from Paris Basin (France) and contained a Ca-smectite/kaolinite mixed-layer as the predominant component. The variations in water saturation and t em perat ure did not cause any detectable mineralogical changes (they were within the errors of observations) except at the clay/casing interface. Sections of the clay a few mm thick at this interface showed excess iron which may have migrated from the steel casing and precipitated on the clay as oxides. The bentonite had an unusually high iron co n ten t and depending on the redox conditions the oxidation state of Fe within the smectite s t r uc t ur e may change; this can alter smectite's layer charge. CONCLUSIONS l,ongevity of a bentonite barrier is directly related to the stability of smectites under the site-specific conditions. Hydrothermal stabilities of smectires considerably vary according to their chemical and structural characteristics. Trioctahedral smectites with Mg 2+, Li ~ octahedral compositions are known to have very high hydrothermal stabilities (up to 850°C) whereas ironrich smectites represent the least stable phases. Montmorillonites are the most common smectites in bentonites and they can be stabilized up to 400' C. The stability of the aluminous smectites, hence the longevity of bentonite, is primarily dependent on the i nt e r l ayer cations and to a lesser extent on the octahedral substitutions. Octahedral substitutions in aluminous smectites are commonly limited to about 25% of the total octahedral composition (Giiven, 1988) with Mg 2÷ and Fe 3÷ being the main proxying ions. Mg 2+ substitution in octahedra enhance the stability of the montmorillonite while Fe "~÷ substitutions decrease it. The role of interlayer cation seems to be related to their ionic properties affecting their interactions with the interlamellar water molecules. K*, Cs(and possibly Rb +) are characterized by low ionic potential, low hydration energies, and temporal rdr ratios below 1.0. These i nt erl ayer cations weakly interact with water molecules, facilitate the dehydration of the interlamellar water, and greatly promote the illitization of aluminous smectite. In the presence of K ~-bearing pore waters the stability of smectite can be as low as 50' C. The effects of K + on the physical properties and rheological behavior of bentonite must be tested especially at high temperatures in order to evaluate the damages to bentonite's performance. [ n t e r l a y e r cations like Na +, L i ' , Ca 2+, and Mg" ~, on the o t h e r hand. e n h a n c e the stability of aluminous smectite above 300°C. This has been d e m o n s t r a t e d in the l a b o r a t o r y for s hor t terms (few months up to a year). However, the long-term stabilities of Na *, Li ÷, Ca +, and Mg2+-smectites need to be calcu l a t e d as it was done for K+-smectite by Eberl and Hower (1976). Na ~ is well-known for its role in greatly improving various physical properties of bentonites. Mg 2 ~ has the highest ionic potential and temporal rdr value and can form the strongest s m e c t i t e i n t e r l a m e l l a r w at er complexes. The h y d r o t h e r m a l stability of be n t oni t e s a t u r a t e d with Mg 2÷ is
BENTONITE AS B U F F E R M A T E R I A L IN A N U C L E A R W A S T E REPOSITORY
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expected to be much higher than other interlayer cations. Hydrothermal experiments (Gfiven and Huang, unpublished) demonstrated that Mg+-mont morillonite remains intact at 300°C for 90 days with no loss of swelling. The properties and rheological properties of Mg 2 +-saturated bentonites have not yet been systematically evaluated but they are expected to be superior to Ca 2 +-saturated bentonites. Thus, the contributions of the various interlayer cations to the stability and properties of aluminous smectites should be seriously considered in designing the barrier materials for radioactive waste. These interlayer cations can be easily added during the processing or during the preparation of bentonites. Bentonites that will be placed in the hot zone of the repository can be saturated by an interlayer cation like Mg 2+ to enhance its thermal stability. Similarly, the host rock contact of the barrier can be lined with Na-saturated bentonite in order to establish high swelling and low permeability. Moreover, special hydroxy metal complexes can be intercalated into smectite interlayers to enhance the adsorption capacity of the bentonite in order to retard radionuclide migration. Bentonite's situation in a nuclear repository seems to be somewhat analogous to that of a batallion attacked by enemy from both sides with different weapons. Bentonite has to retard the migration of " h o t " radionuclides on one side and to hinder the flow of "cold" groundwater on the other side. Bentonites equipped with proper interlayer ammunition can control the situation. Thus, a multilayered barrier consisting of slices of bentonites saturated with different interlayer cations may deserve some consideration in the future. For an extremely hot repository (above 300°C), trioctahedral smectites like saponites and hectorites may be used, especially in the hot zone of the repository. Again the physical properties of these smectites need to be evaluated under the expected conditions of a nuclear waste repository. Finally, a note of disclaimer may be attached to this report. The above predictions about the stability of bentonite are reasonable as long as pH and Eh conditions in the pore waters remain within moderate regimes. Acidic (pH < 5) and alkaline (pH/> 10) conditions or any unusual chemical component may severely and rapidly destroy bentonites if the conditions are not controlled by buffers. ACKNOWLEDGEMENTS I a m thankful to Professor Roland Pusch for the invitation to present this lecture at Lund workshop on "Artificial Clay Barriers for High Level Radioactive Waste Repositories". I would like also to express m y sincere thanks to Mr. Anders BergstrSm and to Swedish Nuclear Fuel and Management Co. for the perfectly organized excursion to Stripa Research Mine. The on-going experimentation in the Mine left a profound impression on m e and made m e realize the nature and magnitude of the nuclear waste problem. I a m convinced that Stripa Mine will be serving the scientists in their struggle with nuclear waste like a lighthouse serves the sailors.
246
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