The reactivity of bentonites: a review. An application to clay barrier stability for nuclear waste storage

Transcription

1 Clay Minerals (1998) 33, The reactivity of bentonites: a review. An application to clay barrier stability for nuclear waste storage A. MEUNIER, B. VELDE* ANO L. GRIFFAULT t Hydrogdotogie, Argiles, Sols, Alterations, CNRS-UMR 6532, Univ. Poitiers, 40 avenue du Recteur Pineau, Poitiers Cedex, France, *Dept. G~ologie, CNRS URA 1316, Ecole Normale Sup~rieure, 24 rue Lhomond, Paris Cedex 05, France, and tandra, DS/HG, Parc de la Croix Blanche, 1-7 rue Jean Monnet, Chatenay-Malabry Cedex, France (Received 25 July 1996; revised 11 April 1997) ABSTRACT: The thermal stability of bentonites is of particular interest for containment barriers in nuclear waste storage facilities. The kinetics of smectite reactions have been investigated under laboratory conditions for some time. The variables of time, chemical composition and temperature have been varied in these experiments. The results of such an assessment are that there are about as many kinetic values deduced from experiments as there are experiments. Experiments using natural bentonite to study the smectite-to-illite conversion have been interpreted as a progressive transformation of montmorillonite to illite. It is highly probable that the initial reaction product is not illite but a high-charge beidellite + saponite + quartz mineral assemblage which gives, then, beidellite-mica interstratified mixed-layer minerals. These experimental reactions are noticeably different from those of diagenesis, being closer to reactions in hydrothermal systems. The transformation of smectite to illite through bentonite alteration reactions has been the subject of much study in the last several decades. The change in smectite content of the illite-smectite (I-S) minerals interpreted from X-ray diffraction (XRD) diagrams has been taken to indicate the change in composition of the clay minerals found in diagenetic sedimentary sequences. Similar changes in mineralogy have been observed in clays found in volcanic ash tufts and hydrothermal alteration zones (Velde, 1985). The change in smectite content of the I-S mineral has been related to the conditions of burial in diagenesis, hence the variables of time and temperature (Perry & Hower, 1970). Following these concepts, several series of experiments have been performed on synthetic clays (Velde, 1969; Eberl & Hower, 1976) and natural clays (Robertson & Lahann, 1981; Howard & Roy, 1985; Whitney & Northrop, 1988; Huang et al., 1993) with the object of establishing the time-temperature relations in the clay transformation. In kinetic studies (Eberl & Hower, 1976; Robertson & Lahann, 1981; Howard & Roy, 1985; Huang et al., 1993) the estimated activation energy has been seen to vary greatly from one series of experiments to another, from 3 to 30 kcal mol -~, even when the starting material was the same in certain instances. Synthesis experiments (Whitney & Velde, 1993) show that the evolution of smectite is not a two-step process, as far as grain morphology is concerned, but appears to be continuous. The analysis which follows attempts to explain the apparent contradictions in experimental results and to define more precisely the reactions and products which have been observed in the experimental studies. This definition can help to use such data to interpret or predict reactions in other systems. It is not clear whether the experiments are relevant to reactions in natural sediments and rocks. However, the kinetics values from experiments can be used with reason The Mineralogical Society

2 188 A. Meunier et al. able confidence to predict the life span of the manmade clay barriers such as those in the nuclear waste systems. REACTIONS UNDER NATURAL CONDITIONS Bentonites initially comprise pure smectite (alteration of volcanic ash at or near the surface), and then, the smectite gradually transforms into illite. The reaction steps, although almost identical when observed by XRD giving mixed-layer I-S minerals, are in fact different. The I-S mixed-layer series has been studied in contact metamorphosed Cretaceous bentonites in the proximity of basaltic dykes (Pytte & Reynolds, 1989). The concentration of illite was shown to increase towards the dykes, demonstrating the compositional response of I-S to temperature. However, the pattern of smectite content as a function of distance from the heat source is sufficiently different in each case so that a complicated reaction model was necessary to explain the different cases investigated. This natural situation obviously incorporates several variables which lead to complex results. Smectite has been seen to persist in very old bentonite beds (Ordovician) even though the shales have I-S minerals with high illite contents (Velde, 1985). There is a difference in the evolution of smectites according to the statigraphic thickness and the type of the surrounding rocks when the beds are thick (>1 m). Pusch & Madsen (1995) have proposed models for this process of illitization based upon diffusion. Sucha et al. (1993) have described the evolution of smectite from thin bentonite beds and compared these I-S clay minerals to those from shales in the same diagenetically affected series of rocks. Here there is a regular change in I-S composition with depth (time and temperature) in bentonites but there is a difference in reaction progress compared to that of I-S in shales (Fig. 1). It is apparent that the smectite in bentonites and the smectite formed in shales do not react at the same rate under the same thermal conditions. This can be due to the differences in the chemical processes (diffusion necessary to provide the K for the conversion of smectite to illite for instance) or there could be a fundamental mineralogical difference in the conversion process. Whatever the reason, bentonites and shales do not give the same kinetic response to thermal change Reaction progress (% illite) FIG. 1. Smectite-to-illite reaction in shales and bentonites under diagenetic conditions in the Tertiary East Slovak basin, from ~ucha et al. (1993). The reaction progress is expressed as % illite in I-S. REACTION EXPERIMENTAL UNDER CONDITIONS Most studies of smectite stability in laboratory experiments use smectites of bentonitic origin and in fact a type called Wyoming bentonite is used almost exclusively. It is a montmorillonite, the layer charge originating through substitution in the octahedral sheet. The values of the activation energies derived from four studies are given in Table 1. They range from 4 to 30 kcal mo1-1. The other values for different reactions producing illite are for the greatest part in the kcal mo1-1 range. The differences in the reaction constants obtained indicate that there is something which has not been assessed in the studies up to this time. The study of Whitney & Northrop (1988) is the most exhaustive concerning the range of experimental conditions and the analysis of the run products ( ~ days duration). It gives us the greatest insight into the reactions produced under experimental conditions. In the figures which summarize the run results (Whitney & Northrop, 1988; Fig. 3, p. 81) it is evident that there are two reactions, a more rapid rate for the short periods of time and a much slower rate of change at longer times. A plot of the results on In(a/a-x) vs. time coordinates, where a is the initial 100% smectite concentration and x the amount of non-expanding layers in the K-saturated state clearly shows the differences in reaction rates (Fig. 2). This plot follows the usage of Howard & Roy (1985). 100

3 The reactivity q[ bentonites: a review 189 TABLE I. Estimation of the activation energies (Ea; kcal mo1-1) for the smectite (or kaolinite) to illite reaction. Experiments Natural smectite E a Synthetic beidellite Ea Kaolinite E a Howard & Roy (1985) 4 Robertson & Lahann (1981) 30 Whitney & Northrop (1988) 18 Huang et al. (1993) 28 Eberl & Hower (1976) 20 Chermak (1989) 30 Small (1993) 28 Bentonite smectite Reactions in nature Ea Shale smectite (diagenesis) E~ Shale smectite Ea Pusch & Madsen (1995) 25 Bethke & Altaner (1986) 20 Elliott et al. (1991) 30 Velde & Vasseur (1992) 27 & 7 Pytte & Reynolds (1989) 25 Esposito & Whitney (1995) 27 & 7 Velde & Lanson (1993) 28 The most striking observation which can be made on these data is that, from the tables of run results and XRD diagrams presented in the studies, the reaction products are not the same for the two reactions. In the first, rapid reaction the run product is a random mixed-layered mineral composed of smectite and 10,~ non-expanding layers in the K-saturated state. These layers expand upon Na saturation. The mixed-layer mineral is associated with quartz and small amounts of mica. Mica and quartz were considered by the authors to have been components of the starting material. However, the XRD patterns of run products from the 300~ and 400~ series (Whitney & Northrop, 1988, Fig. 2, p.. 79) show a strong increase in the 4.26 and 3.34 A peak intensities suggesting that quartz is produced during the hydrothermal reaction. Experiments by Eberl & Hower (1976), Eberl (1978), Eberl et al. (1978), Inoue (1983) and Howard & Roy (1985) show exactly the same sequences of mineral phases under hydrothermal treatment. It is difficult to determine if the amount of the mica phase present in the short run products increases; nevertheless, it is obvious that the mica phase disappeared from the longer run products in which chlorite was formed. Mixed-layer clays and quartz can thus be considered to be part of the first reaction product assemblage. The second reaction, found after -40 days at 250~ and above, produces an R = 1 (ordered) mixed-layer mineral composed of 10 A nonexpandable layers, both in K- and Na-saturated states, and smectite layers. It is associated with quartz and chlorite. Whitney & Northrop (1988) indicate that smectites of varying properties are interstratified with illite in mixed-layer phases in the random I-S run products. A variable number of layers collapses in the K-saturated state and re-expand after Na saturation. This is commonly assumed to characterize high-charge smectite layers (Schultz, 1969; Howard, 1981). This signifies that illite contents of the mixed-layered phase determined in the K-saturated state are largely overestimated. This c o,s- Mixed-layer (R=I) + chlorite + quartz Ko 16o I~0 2~0 2;0 Days FIG. 2. Plot of hydrothermal reaction progress In(a/a-x) of smectite to non-expanding layers in the K-saturated state (high-charge smectite + illite) according to the data of Whitney & Northrop (1988). Two rates are apparent, those concerning the R = 0 and the R = 1 structural I-S mineral type respectively. The run product assemblage is different for the two apparent conversion rates. Temperature conditions were 250~

4 190 A. Meunier et al. situation is frequently the case for the random I-S structures in which high-charge layers are erroneously assimilated to illite ones. The amount of the high-charge smectite layers (collapsed upon K saturation) varies with the temperature of the experiment (Fig. 3) and the duration of the run. The random I-S run products are three-component mixed-layer minerals composed of illite, low-charge and high-charge smectite. The number of high-charge smectite layers varies with time and temperature while the number of illite layers does not exceed 11% (random I-S). This reaction proceeds in the presence of quartz and mica. INTERPRETATION: THE REACTION PRODUCTS The Whitney and Northrop hypothesis: smectite = montmorillonite Whitney & Northrop (1988) stated that there are clearly two mineral reactions in their experiments. The first one was considered to produce a random I-S in which many of the 10.A layers were not illite because interlayer K ions were not fixed (exchangeable ions). They consider that I-S crystals are produced by a 'transformation process' (opposed to a dissolution-precipitation process; Nadeau et al., 1985; Pusch & Madsen, 1995 among others). It is important to note that Whitney & Northrop (1988) showed that there is a difference in the exchange of oxygen (through oxygen isotope measurements) in the two different series of run products. This suggests that there are two different reaction mechanisms according to the reaction products and reaction rates. In spite of the fact that they showed the presence of high-charge smectite layers, they deduced a simplistic first reaction involving random I-S mixed-layer minerals in which all smectite had the same layer charge (see Figs. 9 and 12). This is confusing because the progress of the reaction is not directly proportional to the %1 in I-S. The first reaction products are not simple random I-S but more complex mixed-layered minerals which are composed of ~10% illite z 50 ~ % ~ -- % o Oo% i! i,, IIIIIli * % iliite FIG. 3. Plot of high-charge montmoril]onite layers (non-expanding in the K-saturated state but re-expanding in the Na-saturated state) to illite (non-expanding layers in the Na-saturated state) according to the data of Whitney & Northrop (1988). Temperature conditions: 250~ (diamonds); 300~ (squares); 350~ (full stars); 400~ (empty stars); 450~ (full circles); 1 & 2: reactions la and 2a (see text).

5 The reactivity of bentonites: a review 191 interstratified with variable amounts of high-charge and low-charge smectite layers. Based on X-ray patterns, the first reaction can be written as follows: low-charge montmorillonite + K + (mica) ---, high-charge montmorillonite + illite + quartz (la) In spite of the fact that the chemical compositions of the phyllosilicates produced in each run are not known with precision, it is possible to determine the trends of the reaction using a graphical projection in the M+-Si-R 2+ system (see Meunier & Velde, 1989 for details of the coordinates of this plot). The layer charge of the high-charge montmorillonite (which collapses on K saturation) varies between 0.45 and 0.66 per Olo(OH)2 (Proust et al., 1990). Assuming a highcharge smectite layer of charge per O10(OH)2, a low-charge one of and an illite (10,& nonexpandable layers of 0.75+), it is possible to plot the run products as determined by XRD identification in chemical coordinates. As an example, the coordinates of the run product A132 in Whitney & Northrop (1988, Table 2) are the following: 9% high-charge montmorillonite (Exp. Na - Exp. K); 83% low-charge montmorillonite (Exp. Na - Highcharge mont.); 8% illite (100 - Exp. Na). All of the points for montmorillonite reaction products are given in Fig. 4. It can be seen that their composition evolves between high- and lowcharge montmorillonite due to the constant nature of illite (fully collapsed) layers. As the experiments are in a closed system, it is possible to estimate the evolution of the composition of the phases which were identified in the run products. The starting material is a mixture of lowcharge montmorillonite with quartz and mica available as Wyoming bentonite. This assemblage is represented in Fig. 5a. Quartz is produced in the reaction, indicated by an increase in the quartz peaks on the XRD patterns, as in the experiments of Eberl & Hower (1976), Eberl (1978), Eberl et al. (1978), Inoue (1983) and Howard & Roy (1985). According to Whitney & Northrop's hypothesis, random I-S is produced by a transformation mechanism. This means that the high-charge smectite layers must be of montmorillonitic composition because the RZ+(Mg2+,Fe 2+) components remain in the structure. The increase in the charge on the unit layer results in the loss of silica. In the second reaction the random illite/highcharge montmorillonite mixed-layered mineral is replaced by an ordered I-S. The compositions of'the I-S minerals, assumed to be illite and high-charge montmorillonite, are plotted in Fig. 4. The mineral assemblages change: mica disappears when chlorite occurs (Fig. 5b). The phase association of the reaction shows that the micaceous phase present in the starting material disappears when the illite percent in the I-S mixed-layered phase increases beyond 11%. The mineral which replaces mica in the triphase assemblages is chlorite. Thus, the I-S phase is enriched in A1. These I-S minerals formed in longer runs and at the highest temperature are more illitic as the number of high-charge smectite layers decreases (Fig. 2, part 2). Therefore in the second reaction, the charge increase is due to the production of illite layers via increased tetrahedral A1. high-charge montmorillonite layers illite + chlorite + quartz An alternative: smectite = saponite + beidellite (2a) Whitney & Northrop (1988) have not established beyond doubt that the smectite component in the random and the ordered I-S mixed-layers preserve the montmorillonitic structure of the original Wyoming clay. The stoichiometry of mineral reactions in a closed system allows an alternative. Another reaction should be envisaged for the first step of the illitization process: the low-charge montmorillonite could have been transformed into a high-charge beidellite + saponite + quartz mineral assemblage as indicated in Fig. 6a. In such a case, the Fe and Mg chemical components are concentrated into a separate phase as it was shown in experimental synthesis of clays in the Mg-A1-Si system (Grauby et al., 1993). Then, the smectite component in the ordered smectite-illite mixedlayers is beidellitic; I-S chemical composition must be located on the beidellite-muscovite join rather than on the montmorillonite-illite one. In such a case, the first reaction would be written as follows: low-charge montmorillonite high-charge beidellite + saponite + quartz (lb) Such a reaction was studied by Yamada et al. (1991) and Yamada & Nakasawa (1993) who showed that the saponite-beidellite assemblage crystallized from smectites of the montmorillonitebeidellite series under hydrothermal experimental conditions. As a consequence, the second reaction

6 192 A. Meunier et al. b M %~c^ illite ~ celadonite 50 % exp. X R D ~ MU %1t_ run product A 132 /~~l~nt. h=gh charge ~O "1J:r" 'so% Low /..~ont. Low charge MO *C o 300 *C * 350 *C r 400 *C *C Q ~ R 2+ 45i FIG. 4. Plot of run results for the data of Whitney & Northrop (1988) according to the phase identifications (XRD) and the starting composition of the reacting clay (SWY). (a) Coordinates are M + = (K, Na and 2Ca) content, 4Si and R 2+ (Mg + Fe z+) as developed in Meunier & Velde (1989); MU: muscovite; BE: beidellite; MO" montmorillonite; IL: illite. Numbers indicate the layer charge per OI0(OH)z. (b) Plot of the A132 run (see text). which produced chlorite and ordered I-S mixedlayers must be significantly different from that envisaged by Whitney & Northrop (1988). The illite component resulting from beidellite destabilization is richer in A1 than the illite resulting from montmorillonite as seen in diagenetic series (Meunier & Velde, 1989). Besides, the random to ordered transition processes in I-S are not identical for montmorillonite and beidellite because of the location of the layer charge. high-charge beidellite + saponite + K+ ---) ordered I-S + chlorite + quartz (2b) The Wyoming bentonite is classically used as a standard smectite in a great number of experimental studies especially in recent experiments devoted to illitization process kinetics (Huang et al., 1993). It is, of course, of great interest to determine the actual reaction because kinetic laws are calculated for a strict transformation of smectite to illite ignoring that other processes are working. Interpretation: the illite formation The kinetics of illite formation from reaction la can be detailed in the experiments of Whitney &

7 The reactivity of bentonites: a review 193 a M* M + MU... 4, _ / ///" 3 PHASES: Q/S"i~ "S" I-S + chlorite + quartz Si R 4Si R 2 Whitney & Northrop (1988) FIG. 5. Interpretation of experiments from Whitney & Northrop (1988): the representation of the phase assemblages in the M+4Si-R 2 coordinates for the two portions of the smectite to illite reaction according to the assumption that the smectite component in I-S is of montmorillonitic type. A three-phase assemblage is imposed by the stoichiometry. (a) Formation of high-charge montmorillonite layers + quartz (+ mica?). (b) Formation of ordered illite -montmorillonite mixed-layered minerals + quartz + chlorite. Northrop (1988). Indeed, it is possible to calculate the activation energy of the reaction which produces the 10 A non-expandable layers (i.e. illite layers + K-saturated high-charge smectite layers) from the runs corresponding to parts 1 and 2 in Fig. 3. The values of In(a/a-y) where a is 100% smectite and y the percent of non-expandable layers in the Na-saturated state, are linearly correlated with time. The lines do not intercept at the origin (Fig. 7a) but at In(a/a-y) = 0.9 which correspond to a value of y = 8%. This means that the starting material which is apparently homogeneous (100% expanding layers at 25~ is heterogeneous at elevated temperature in the experimental conditions: 8% layers collapse to 10 A before reaction 1 proceeds. The Arrhenius plot (Fig. 7b, Table 2) gives an activation energy of 18.1-I-2.2 kcal mol -I. This value is close to that of TABLE 2. Experimentally determined rate constants and maximum errors for the formation of 10 A non-expanding layers in the Na-saturated state (i.e. illite layers). Data are from Whitney & Northrop (1988). Each rate value is associated with a function constant (values of ln(a/a+y) at time = 0 indicated by y in % illite layers) given with maximum errors and a correlation coefficient (r2). Temperature (~ Number of runs Rate constants % illite layers r t t t _ t I

8 194 A. Meunier et al. a M" ^ nuscovite 4Si ~... ~ R 2+ chlorite b M ~ 4Si R 2" chlorite FIG. 6. An alternative to the interpretation of the experiments of Whitney & Northrop (1988). (a) The starting montmorillonite produces first a high-charge beidellite + saponite + quartz assemblage. (b) Increased temperature and time conditions lead to the formation of an ordered (R=I) beidellite-mica mixedlayer + chlorite + quartz assemblage. the illite formation in the synthetic mineral system K-A1-Si calculated by Eberl & Hower (1976): kcal mol -~. DISCUSSION Given the scatter in estimations of activation energy determinations and differences in models for reaction series, it is desirable to compare the experimental data with those observed for similar reactions in natural settings. Essentially two sets of data can be used for natural settings: those of high thermal regimes, geothermal areas such as the Saton Sea, and those of the burial diagenesis. The data summarized by Velde & Lanson (1993) for the high-temperature regime in the Salton Sea area can be used to estimate the reaction progress to be expected when heating occurs over a time span of about years. On another scale, one can use the data of Sucha et al. (1993) for thin bentonite beds in the East Slovak Basin where the time scale is on the order of 10 Ma. In this last case, the effect of burial does complicate the problem of comparison slightly because each layer of sediments has experienced a range of temperatures during its burial history. However, the high thermal gradient in the basin (above 50~ km -1) ensures relatively rapid reaction at depths below 1000 m (Velde & Lanson, 1993). Figure 8 gives the reaction rate vs. time relation. The smectite components formed during experimental alteration of pure montmorillonitic clays from bentonite deposits are presumably a mixture of beidellite and saponite. The silica in excess produces quartz in closed systems. The crystallization of a trioctahedral Mg-rich clay phase seems to be typical of short duration-high temperature conditions. It was observed in the Stripa experiments in which a kaolinite-smectite mixed-layer clay material has been heated for four years in a granitic environment (Bouchet et ai., 1992). The beidellite + saponite assemblage was described in natural systems where hot hydrothermal fluids generate clay deposits in fractures. Beaufort et al. (1995) showed that the beidellite + saponite assemblage is strictly observed in high-temperature ( ~ vein deposits while the clay fraction of the surrounding rocks is composed of illitesmectite or chlorite-smectite mixed-layer minerals. The experimental reactions using bentonite clays as starting material do not produce an I-S sequence similar to that observed in the montmorillonite-toillite conversion series which is typical of the diagenetic environments. It is known that I-S minerals found in bentonite deposits, diagenetic shales and sandstones, metasomatized sediments and hydrothermal veins have different overall composition trends (Velde & Brusewitz, 1986; Meunier & Velde, 1989). These compositional differences will undoubtedly result in differences in reaction kinetics. In the experiments of Whitney & Northrop (1988), a different product (illitemontmorillonite or beidellite-mica) can change the rate of reaction to a great degree. It is therefore

9 The reactivity of bentonites." a review a 4oo.c -2- b 0.3-,r 0._1 ~ ~ 350-C a 300"C = 9 250"C i u ) z i i i = u i i ) -4 I I I I I I Days 1 / T 103 FIG. 7. Determination of the activation energy of the illite layer formation according to the data of Whitney & Northrop (1988). (a) First-order kinetic plots these data. (b) Arrhenius plot of the rate constants found in Table 2. The slope of the curve is which correspond to an activation energy of 18.1 q- 2.2 kcal/mole. necessary to compare experimental results with minerals having the same type of interstratified mineral. Only in this way can we derive correct time, temperature values of the transformation of smectite to illite or mica which are so common in diagenesis and other geological processes. In manmade systems where the starting material is bentonite smectite, such as in barriers around nuclear waste, the experimental data on Wyoming bentonite can be used to predict the behaviour and life span of the clays when subjected to increased temperature. Since the desired life span of such systems is of the order of 1 myr, the rapid reactions are more pertinent than those of diagenesis. ACKNOWLEDGMENTS The financial support for this study was provided by the Agence pour la gestion des D6chets Radioactifs (ANDRA, France). 6 ESB REFERENCES E i EXP SLS I I I I I I dc / dt (% reaction per day, 10-") F1o. 8. Reaction rate (10n years) vs. time (% reaction per day 10 -n) relation. This relation shows that the more time the reaction has taken to occur, the slower it is. ESB: Tertiary East Slovak basin. SLS: Salton Sea active geothermal area. EXP: Experimental smectiteto-illite conversion results. Beaufort D., Papapanagiotou P., Patrier P., Fujimoto K. & Kasai K. (1995) High temperature smectites in active geothermal field. Proc. 8th Int. Sym. Water- Rock Interact., Bethke C.M. & Altaner S.P. (1986) Layer-by-layer mechanisms of smectite illitization and application to a new rate law. Clays Clay Miner. 34, Bouchet A., Lajudie A., Rassineux F., Meunier A. & Atabek R. (1992) Mineralogy and kinetics of alteration of a mixed-layer kaolinite/smectite in nuclear waste disposal simulation experiment (Stripa site, Sweden). Pp in: Clays and Hydrosilicate Gels in Nuclear Fields (A. Meunier, editor). Chermak J.A. (1989) The kinetics and thermodynamics of clay mineral reactions. PhD thesis, Virginia Polytechnic Institute, Blacksburg, Va, USA.

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