G. KAHR, F. KRAEHENBUEHL, H. F. STOECKLI AND M. MOLLER-VONMOOS

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1 Clay Minerals (199) 25, STUDY OF THE WATER-BENTONITE SYSTEM BY VAPOUR ADSORPTION, IMMERSION CALORIMETRY AND X-RAY TECHNIQUES: II. HEATS OF IMMERSION, SWELLING PRESSURES AND THERMODYNAMIC PROPERTIES G. KAHR, F. KRAEHENBUEHL*, H. F. STOECKLI* AND M. MOLLER-VONMOOS Institute of Foundation Engineering and Soil Mechanics, Laboratory for Clay Mineralogy, Federal Institute of Technology, CH-892 Ziirich, and *Chemistry Department, University of Neuchftel, CH-2 Neuchdtel, Switzerland (Received 9 November 1989) A B S T R A C T: A number of thermodynamic properties were obtained from the determination of adsorption isotherms and enthatpies of immersion for systems with water and Na- and Ca-bentonites. Entropy differences were calculated by combining the enthalpies of immersion and the changes in free energy derived from the adsorption isotherms. The swelling pressures, also calculated from the water adsorption isotherms, are in satisfactory agreement with the experimental data. In the second part of this study of the water-bentonite system for well characterized natural Na- and Ca-bentonites, the changes in the adsorption entropy, the enthalpy of immersion and the swelling pressure are examined as the amount of water increases (for the first part of the study see Kraehenbuehl et al., 1987). The transport of water in unsaturated clays and in soils depends on various potentials and on transport coefficients (diffusion coefficients, permeabilities, etc.). The potentials are thermodynamic quantities which can be derived from measurements performed at equilibrium, and they are usually defined in terms of partial specific potentials, for example g,w = ( 6Gw/bmw)T,p and hw = (6Hwlbmw)r,e In the present study, gw and hw denote the partial specific free energy and enthalpy of water, whereas G and H represent the total free energy and enthalpy of the water-bentonite system containing mw grams of water. Low & Anderson (1958) have pointed out that gw - gw ~ the difference between the free energies of water in the water-bentonite system and pure water, is related to the swelling pressure Ps through g~ - g~~ = - 9w P~. (1) The quantity ~7 = (5V/6mw).l;e represents the partial specific volume of water in the system. It may differ substantially from the value for pure water near room temperature (Vw ~ = 1-3 m3/kg), in particular for low contents of water in the bentonites (Oliphant & Low, 1982). The difference in free energy is also related to pw, the partial vapour pressure The Mineralogical Society

2 5 G. Kahr et al. of water in the system, and the vapour pressure Po of pure water at the temperature T of the experiment (Oliphant & Low, 1982) gw - gw ~ "- (RT/Mw) In (Pw/Po) (2) In this equation, R is the gas constant (8.31 J/mol K) and Mw represents the molecular weight of water (1.8 x 1 2 kg/mol). From equations (1) and (2) it follows that the changes in free energy of water in the bentonites and the swelling pressure can be determined from the water adsorption isotherm. The swelling pressure as a function of water content is P, = - (RT/Mw 17,,) x In (Pw/Po) (3) This relation has been used by Sposito (1972) for montmorillonites with water contents w > 1% (relative to the weight of the dry montmorillonites). Good agreement was found between the experimental and the calculated values of Ps. For the changes in enthalpy, the following thermodynamic relation applies, (hw - hw ~ = (gw - gw ~ + T (sw - Sw ~ (4) where hw and hw ~ are the partial specific enthalpy of water in the bentonite and the enthalpy of the pure liquid. The partial specific entropy of veater gw is defined by $w = (6s/6mw)r,e (5) The quantities hw, gw and gw are functions of the amount of water present in the bentonite and are expressed either in J/g K or J/kg K. The entropy change (sw - sw ~ in equation (4) can be estimated from the combination of (hw - hw ~ and (gw - gw~ obtained from immersion and adsorption data (see below). For bentonites with low water contents, one may expect a higher degree of order for the water molecules than in the liquid state, and ($w - s~ ~ should be negative. As found experimentally, the enthalpy difference is also negative, since heat is released when bentonites, either dry or with variable amounts of preadsorbed water, are immersed in water (Kijne, 1969). Thermodynamically, when a small amount of water 6mw is added to the water-bentonite system at constant pressure p, the amount of heat released is 6Q = - (6H/Omw - hw ~ (6) If a bentonite with dry mass ms and an initial water content w = mw/ms is immersed in a large amount of water, the integral heat evolved is AQ = - [~(6Hw/6mw - Jw h~ ~ ms dw TABLE 1. Mineralogical data for bentonites MX-8 and Montigel (MiJller- Vonmoos & Kahr, 1983). Results refer to oven-dried (15~ material. MX-8 Montigel Montmorillonite content (%) Calculated bentonite surface (m2/g) Cation exchange capacity (meq/1g) Exchangeable Na (%) 86 3 Ca 1 61 Mg 4 36

3 Physical properties of water-bentonite systems 51 i, 9 "I 8-7- O Montigel Ahi(w) = 9.14 exp - (8-66 w ~2) MX - 8 Ahi(w) = exp - (8.53 w w 2) 6o z 5 4o ~: 3o i PREADSORBED WATER IN g / gbent(~nite FIG. 1. Heat of immersion at 293 K as a function of pre-adsorbed water for bentonites MX-8 and Montigel. provided that the enthalpy of the solid does not change. It follows that AQ/m, = - (~(h-~ - hw ~ dw Jw (7) EXPERIMENTAL The investigations were carried out using MX-8, a Na-bentonite from Wyoming, USA, and Montigel, a Ca-bentonite from Byern, BRD. Some data for MX-8 and Montigel are TABLE 2. Parameters for the best fit of the experimental enthalpies of immersion Ahi(w) to equation (8). A B C MX Montigel

4 52 G. Kahr et al. 2 IOOC 2, partial specific free enthalpy (Ow - gw ~ = In (P/Po) + swelling pressure calculated P~ = - (Ow - gw~ / vw ~ (vw ~ = 1 3ma/kg) ~+ swelling pressure measured "~k regression ~ + Ps = exp ( w) + *%. ++ ~ + z ioo e_ q * Adsorption + Desorption ~ + CO Oo 2 o + lo ~ + I I I I I I I I * ] I I I I J I I I I I I I I I I I I I O~ I S i WATER CONTENT IN PERCENT Fl6. 2. Swelling pressures calculated from adsorption and desorption isotherms, and measured swelling pressures as a function of water content of MX-8. given in Table 1. For further information, see Kraehenbuehl et al. (1987) and Mtiller-Vonmoos & Kahr (1983). Adsorption measurements were carried out in a gravimetric apparatus of the McBaine type (Gregg & Sing, 1982) using.7--8 g sample. Prior to adsorption, the samples were outgassed for 24 h at 373 K and pressures of mbar. During adsorption the solids were thermostatically controlled in a water-bath at 293 K. The equilibrium pressure was recorded with two transducer gauges of the Barocell type covering the ranges of 1 =-1, and 1-1 mbar, respectively. Enthalpy of immersion in water was determined in a calorimeter described by Stoeckli & Kraehenbuehl (1981) using samples of --.3 g immersed in 5 ml water at 293 K. Prior to immersion, the samples were outgassed, as described above, and pre-determined amounts of water were adsorbed at 293 K from the vapour phase. The required equilibrium pressure was known from the corresponding adsorption isotherms. The swelling pressure has been determined by Bucher & Spiegel (1984) using compacted cylindrical samples 25 mm high and 56 mm in diameter. The

5 Physical properties of water-bentonite systems 53 2 I z i 2 i,~ partial specific free enthalpy (gw - gw ~ = 134"34 In (p/po) k,~ ~ + swelling pressure calculated ~& + ms = - (Ow -gw ~ / vw ~ "~, + (vw ~ = lo-3m3/kg) "~- swelling pressure measured regression ~ + Ps = exp ( w) w-'k + ~ + + % + * Adsorption ~'~ + + Desorption ~ W W ~ + J 2 o WW + WWW+ O K \ I I I I I I I I I I I I I I I t t I t I I I I I I I I ~ t 5 1O WATER CONTENT IN PERCENT Fi6.3. Swelling pressures calculated from adsorption and desorption isotherms and experimental data as a function of water content of Montigel. o swelling pressure apparatus was described by Bucher & Spiegel (1984) and Bucher & M~ller-Vonmoos (1989). RESULTS AND DISCUSSION The heats of immersion of the bentonites Mx-8 and Montigel as a function of the preadsorbed water measured at 293 K are presented in Fig. 1. The quantity AQ/m~ in equation (7) represents the heat of immersion, or, more accurately, the enthalpy immersion z~h i of the bentonite in water. Usually, Ah i is given in J/g of dry bentonite and is a function of w. The enthalpy change (-hw - hw ~ can be obtained from the enthalpies of immersion by using a suitable analytical expression for the experimental data of Ahi(w). The relation gives a good fit, and it follows that zlhi(w ) = A exp {-Bw - Cw 2} (8)

6 54 G. Kahr et al N 6 ~ 5oQ Lo enthalpy changes from heat of immersion (hw - hw ~ = ( w), exp ( 8.53 w w 2) free energy from adsorption isotherm - (gw - gw ~ = In (P/Po) + free energy from swelling pressure measured - (~ gw ~ = exp ( w) x adsorption entropy changes (g~ - sw ~ {(h~ hw ~ - (gw - gw~ # entropy changes from enthalpy differences and free energy from swelling pressure LJ 3 % 2o ~. ++ o lo + o + i; o o J J f r i i i i i i i i i ~1 i~1 i ~1 i i~1 i i i i i i i o ~-i. I I I I I I [ I I l I I l I I I l I I I~1 I I ~ I I I I [ /5.'i.'15. ~.25.3 WATER CONTENT IN g / gbentonite FIG. 4. Enthalpy, free enthalpy and entropy changes as a function of water content of MX-8. (hw - hw ~ = d(ahi(w))/dw = (B + 2Cw) Ahi(w) (9) The right-hand side of equation (9) is purely empirical, but it provides a good estimate of the enthalpy change of water during adsorption by the bentonites. Table 2 gives the values of parameters A, B and C for the systems (water + MX-8) and (water + Montigel) at 293 K. A similar procedure was adopted by Oliphant & Low (1982) for the calculation of (hw - hw ~ from the immersion data represented by Mooney et al. (1952). There is good agreement between their results and the present data. As shown in Figs. 2 and 3, ttlere is a fair agreement between the swelling pressures measured by Bucher & Spiegel (1984) for MX-8 and Montigel, and the values calculated in the present study by equation (3) and the water adsorption data determined earlier (Kraehenbuehl et al. 1987). By using equations (2) and (7), our adsorption and immersion data lead to the variations of (gw - go) and (hw - h~ ~ for the systems (water + MX-8) and (water + Montigel) at 293 K shown in Figs. 4 and 5. Both functions are negative and approach zero as the relative water content w

7 Physical properties of water-bentonite systems 55 '~176 8OO enthalpy changes from heat of immersion (hw hw ~ = ( w) 9 exp (-8.66 w w 2) 7 ~ ~" 5 ~ +~,.= 4 -# 3 2OO i i I i i free energy from adsorption isotherm - (gw gw ~ = In (p/po) + free energy from swelling pressure measured - (gw - gw ~ = exp (@ w) adsorption entropy changes (g~ - sw ~ = {(h~ hw ~ - (~ - gw~ # entropy changes from enthalpy differences and free energy from swelling pressure oo ~ o < * +~ + ~ + * +~ ~ v # ### ^ XXx%& ' ++::::?**~ O ~* ++*~-~ ~,~,,~,~,,~,, ~,~, i i I I i i i t i t i i i, p >- LLJ.5.'i /15 ] WATER CONTENT IN g / gbentonii[ FIG. 5. Enthalpy, free enthalpy and entropy changes as a function of water content of Montigel. increases. The entropy differences calculated from (gw - gw ~ and (hw - hw ~ using equation (4) are plotted in Figs. 4 and 5. It appears that the determination of the entropy difference through equation (4) and using Ahi is easier than calculation from variation of (gw - gw~ with T; b(gw - gw~ = - (Sw - s~ ~ This approach requires at least two isotherms. For both montmorillonites the entropy loss displays a maximum near w = -5~)-7 g water per g of bentonite. This corresponds to the break in the Dubinin plots (Kraehenbuehl et al., 1987) and is related to the completion of the first adsorption layer. The largest difference (- 1-2 J/K and g of water), observed with Montigel, is close to (Sice ~ - Sw~ the entropy of freezing for pure water at 273 K. This suggests a high degree of order in the first monolayer. However, the results obtained should not be overestimated, as the differences in free energies (g~ - gw ~ are based on the adsorption branch of the water isotherms which show a slight hysteresis, in particular for

8 56 G. Kahr et al. MX-8. This point has been discussed in detail by Sposito & Prost (1982). In any case the higher degree of order in the first monolayer is in agreement with conclusions of other authors (Oliphant & Low, 1982; Kijne, 1969; Fripiat et al., 1982). SUMMARY The changes in free energy, enthalpy and entropy of water in bentonites as function of the degree of adsorption were calculated from measurements at the same temperature T of the water adsorption isotherm and the enthalpies of immersion for different degrees of preadsorption. A maximum of entropy loss at the completion of the first layer of water indicates a high degree of order, also found in other systems (Gregg & Sing, 1982). The hydration of the interlamellar space proceeds in steps, corresponding approximately to the intercalation of one, two, and three to four sheets of water (Kraehenbuel et al., 1987). The relative changes in energy of the water uptake vary with the cation forms of the bentonites. The exchangeable Ca 2+ in Montigel has a higher hydration energy than Na + in MX-8, in agreement with the classical results of Mooney et al. (1952). The use of immersion calorimetry reduces the effort in measuring at least two isotherms at different temperatures, and the uncertainties linked with this alternative approach based on the variation of the Gibbs free energy difference with temperature. In the present examples, satisfactory agreement is also found between the experimental data for the swelling pressure as a function of the amount of water adsorbed and the values calculated from the adsorption isotherms at 293 K. REFERENCES BUCHER F. & MOLLER-VONMOOS M. (1989) Bentonite as a containment barrier for the disposal of highly radioactive wastes. Applied Clay Sci. 4, BUCHER F. & SPIECEL U. (1984) Quelldruck von hochverdichteten Bentoniten. NAGRA Techn. Bericht FRIPIAT J., CASES J., FRANCOIS M. & LETELLIER M. (1982) Thermodynamic and microdynamic behavior of water in clay suspensions and gels. J. Coll. Interf. Sci. 89, GREGG S.J. & SING K.S.W. (1982) Adsorption, Surface Area and Porosity. 2rid ed. Academic Press, London. KRAEHENBUEHE F., STOECKLI H.F., BRUNNER F., KAHR G. & MOELER-VONMOOS M. (1987) Study of the waterbentonite system by vapour adsorption, immersion calorimetry and X-ray techniques: I. Micropore volumes and internal surface area, following Dubinin's theory. Clay Miner. 22, 1-9. KIJNE J.W. (1969) On the interaction of water molecules and montmorillonite surfaces. Soil Sci. Soc. Am. Proc. 33, Low P.F. & ANDERSON D.M. (1958) Osmotic pressure equation for determining thermodynamic properties of water. Soil Sci. 86, 251. MOONEY R.W., KEENAN A.G. & WOOD L.A. (1952) Adsorption of water vapor by montmoriuonite. J. Am. Chem. Soc. 74, MOEEER-VONMOOS M. KAHR G. (1983) Mineralogische Untersuchungen von Wyoming-Bentonit MX-8 und Montigel. NAGRA Techn. Bericht OLIPHANT J.L. & LOW P.F. (1982) The relative partial specific enthalpy of water in montmorillonite-water systems and its relation to the swelling of these systems. J. Coll. Interf. Sci. 89, STOECKEI H.F. & KRAEHENBUEHL F. (1981) The enthalpies of immersion of active carbons in relation to the Dubinin theory for the volume filling of micropores. Carbon 19, SPOSITO G. (1972) Thermodynamics of swelling clay-water systems. Soil Sci. ll4, SPosrro G. & PROST R. (1982) Structure of water adsorbed on smectites. Chem. Rev. 82,

A NOVEL DIFFERENTIAL THERMAL ANALYSIS METHOD FOR QUANTIFYING THE SORPTION CAPACITY OF SMECTITE CLAY

Clays and Clay Minerals, Vol. 38, No. 3, 322-326, 1990. A NOVEL DIFFERENTIAL THERMAL ANALYSIS METHOD FOR QUANTIFYING THE SORPTION CAPACITY OF SMECTITE CLAY PAUL O'BRIEN AND COLIN J. WILLIAMSON Department