J. Phys. Chem. 1983, 87, 1213-1219
we can predict ita rate constant to be near 65 X cm3 molecule-'s-'. A larger value for 2-pentene (mixture of cis, trans), 90.1 X cm3 molecule-' s-' a t 298 K, was reported in 1971 by Morris et al.le To check the discrepancy, we measured the relative rate constant against l,&butadiene and obtained 1.00 f 0.03, and the absolute cm3 molecule-' s-l. The rate constant was 68.5 X value showed good agreement with the expected value. The additivity rule was not observed for cyclohexene and 1,4-cyclohexadiene. Rate constants for allenes did not follow the rule, either. In conclusion, our results verified the hypothesis of Hendry and Kenley and showed that the reactivity of diolefins, except for allenes and cyclic diolefins, can be expressed by summing up the contributions from each carbon-carbon double bond. And our estimation method for the reactivity of each double bond from the rate constant for the corresponding monoolefin is proved to be much more accurate and more widely applicable than the (16)E.D. Morris, Jr., and H. Niki, J. Phys. Chem., 75, 3640 (1971).
1213
method of Hendry and K e n l e ~since , ~ they proposed fixed rate constants for unit double bonds and ignored conjugation effects. Acknowledgment. I thank Mitsubishi Gas Corp. for supplying 90% H202. I also thank Dr. H. Akimoto and Professors 0. Kajimoto and S. Sato for helpful discussions. I am also grateful to Dr. I. Mizoguchi for his interest and encouragement. Registry No. OH, 3352-57-6;1,3-butadiene,106-99-0;trans1,3-hexadiene,20237-34-7;trans-1,4-hexadiene,7319-00-8; 1,5hexadiene, 592-42-7; cis,cis-2,4-hexadiene, 6108-61-8; trans,5194-50-3; trans-2,4-hexadiene,5194-51-4;trans,cis-2,4-hexadiene, cis-1,3-pentadiene,1574-41-0;2-methyl-1,3-butadiene,78-79-5; 2,3-dimethyl-1,3-butadiene, 513-81-5; 1,4-pentadiene,591-93-5; 2-methyl-1,4-pentadiene, 763-30-4; 3-methyl-1,3-pentadiene, 4549-74-0; 4-methyl-l,3-pentadiene,926-56-7; trans-2-butene, 624-64-6; 2-methyl-l,5-hexadiene,4049-81-4; 2,5-dimethyl-1,5hexadiene, 627-58-7; 1,4-cyclohexadiene, 628-41-1; 2,5-dimethyl-2,4-hexadiene,764-13-6;propadiene, 463-49-0;1,2-butadiene, 590-19-2;l,e-pentadiene,591-95-7;3-methyl-l,2-butadiene, 598-25-4; trans-2-pentene, 646-04-8; propylene, 115-07-1; 2methyl-2-butene,513-35-9;cyclohexene, 110-83-8.
Cesium and Rubidium Ion Equillbria in Illite Clay E. Brouwer,t B. Baeyens, A. Maes, and A. Cremers' Centrum voor Opperviaktescheikunde en Coib-&le Scheikunde, Kathoiieke UniversiteitLeuven, de Croyiaan 42, 8-3030 Leuven (Heveriee), Belgium (Received: M r c h 1, 1982; I n Final Form: November 4, 1982)
The ion-exchange selectivity of cesium and rubidium ions is studied in illite clay, saturated with either calcium, strontium, barium, sodium or potassium ions. The cesium- and rubidium-exchange levels studied vary between 50% and vanishingly low values. It is shown that the equilibria can be described in terms of three kinds of sites-corresponding to 0.5%,3%, and 96.5% of the exchange capacity-each of which shows a characteristic selectivity coefficient. The site representing 0.5% of the capacity exhibits an exceedingly high selectivity for cesium ions (33 and 40 kJ/equiv with respect to sodium and calcium ions) and discriminates strongly between cesium and rubidium (6 kJ/equiv). The equilibria on this site are thermodynamically reversible, as shown from three kinds of evidence: (1)the Cs-Rb trace selectivity difference, obtained from the equilibria in Ca, Sr, and Ba clay, agrees with the value obtained in the Na or K clay; (2) the same result is obtained from direct Cs-Rb trace mixture selectivity studies in Sr and Na clay; (3) the K-Na selectivity difference for this site as calculated from trace Cs equilibria agrees with the result obtained from Rb equilibria (11kJ/equiv). Finally, it is shown that the driving force for the high selectivity on this site is exclusively enthalpic.
Introduction The study of the selectivity of ions such as potassium and cesium in clay minerals has been a matter of interest for a long time. It is well-known that, depending on clay mineral ~tructure,l-~ ions with low hydration energy are very selectively retained by clays, a process which is sometimes loosely referred to as an irreversible one or as one of fixation. The interest in the behavior of K+ relates, of course, to soil-K+ availability4 whereas the treatment of radioactive effluents promoted the interest in C S + . ~ More V ~ recently, the problem of risk evaluation in geological disposal of high-level radioactive provided a new incentive for such studies. Illite clay, which often forms a substantial part of the exchange complex of shales which are considered as potential repositories for such waste, is well-known for its On leave from the Department of Soils and Fertilizers, Landbouwhogeschool Wageningen, The Netherlands. 0022-3654/83/2087-1213$01.50/0
selective sorption of cesium ions. Its sorption properties are considered to result from the involvement of two or more exchange sites2i5J0of decreasing selectivity. In surveys of the literature on the subject, however, it appears that some problems have not been resolved satisfactorily, a t least not from a purely quantitative point of view. Perhaps the most obvious and important one is the question of whether the highly selective sorption of B. L. Sawhney, Clays Clay Miner., 18,47 (1970). B. L. Sawhney, Clays Clay Miner., 20, 93 (1972). D. D.Eberl, Clays Clay Miner., 28, 161 (1980). M. E. Sumner and G. H. Bolt, Soil Sci. SOC.Am. h o c . , 26, 541 (1962). (5) D. G. Jacobs and T. Tamura, Trans. Int. Congr. Soil Sci., 7th, 1960,2, 206 (1960). (6)T.Tamura and D. G. Jacobs, Health Phys., 2, 391 (1960). (7)S.Komarneni and D. M. Rov. Clays Clay Miner.. 28, 142 (1980). (8)S. Komarneni, J. Inorg. Nu& Chem., 4i,397 (1979). (9)The National Research Council, "Geological Criteria for Repositories for High-Level Radioactive Wastes", National Academy of Sciences, Washineton. DC. 1978. (IO) 6. H:Bolt, M. E. Sumner, and A. Kamphorst, Soil Sci. Soc. Am. Proc., 27,294 (1963).
0 1983 American Chemical Society
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The Journal
of Physical Chemistry, Vol. 87, No. 7, 7983
cesium in illite is a thermodynamically reversible process and, if so, what is the magnitude of the thermodynamic state functions for the exchange of ions on the highly selective sites. In studying the behavior of cesium in a shale near Mol, Belgium, which is taken as a potential burial site, we observed the well-known high selectivity at low cesium levels which we ascribed to the presence of illite. This paper attempts a quantitative study of the "fixation" process by studying the ion-exchange selectivity of Cs+ and Rb+ down to extremely low exchange levels in a reference clay mineral, Morris illite (Wards Natural Science Establishment) saturated with either calcium, strontium, barium, sodium, or potassium ions.
Experimental and Theoretical Aspects Materials. The clay is finely ground in a mortar, dispersed in distilled water, and mixed with nitrate solutions of either Ca2+,Sr2+,Ba2+,Na+, or K+. After repeated saturation and centrifugation, the homoionic clays are finally diluted to about 20 g/L and equilibrated dialytically with solutions of selected concentrations used in the ion equilibria measurements. The cation-exchange capacities are determined by isotopic dilution (Wa, @Sr,'=Ba, 22Na) methods1' or NH40Ac displacement (K). The values, reproducible within 2-3%, are 18.2 (Ca), 20.1 (Sr),24.2 (Ba), 20.0 (K), and 15.3 (Na) mequiv/(100 g). As expected,12 the data are dependent on the nature of the cation, since only part of the clay charge is due to isomorphic substitution. Ion Equilibria. The Cs+ and Rb+ ion equilibria are measured at high and low loading in Ca2+,Sr2+,and Ba2+ illite by single batch equilibria. At high loading, both ions are monitored in duplicate batches, containing either the Ca (Sr, Ba) or 137Cs(86Rb)label. Equilibria are carried out by mixing known volumes of clay suspension with mixed solutions of constant total normality (0.001 N) and varying M+/M2+ ratios. Systems are centrifuged (at constant temperature) after 24-h mixing at 25 "C and the supernatants assayed radiometrically. The procedure used at low Cs+ (or Rb+) loadings is identical except that now only Cs+ (or Rb') is monitored and the measurements are extended to values of 0.04 and 0.8 N total normality. For each pair of ions (and each value of the total normality studied) six batch equilibria are carried out, correspondingto Cs+ (or Rb+) additions in the range of 20-2 x lo4 mequiv/(100 g of clay). All data refer to a 25 "C temperature. The procedure used for the Cs+ and Rb+ equilibria in Na+ and K+ illite is entirely similar except that the measurements are limited to 0.002 and 0.02 N total normality values. In two cases, additional measurements are made at three (Cs-Sr) or four (Cs-Na) temperature values in order to assess effects of loading on enthalpy effects. Finally, the cesium-to-rubidium selectivity is measured in the low loading region in Sr2+illite (at 0.04 N Sr2+)and Na+ illite (at 0.1 N Na+) by adding mixtures of cesium and rubidium in amounts varying between 1 mequiv/ (100 g) to vanishingly low values. Both Cs+ and Rb+ are monitored in separately labeled batch equilibria. Theoretical Aspects. Ion-exchange adsorption data may be analyzed in either one of two ways: l3 firstly on the basis (11) P. Peigneur, A. Maes, and A. Cremers, Clays Clay Miner., 23,71 (1975). (12) P. W. Arnold, in "The Chemistry of Soil Constituents", D. J. Greenland and M. H. B. Hayes, Eds., Wiley, New York, 1978, Chapter 5. (13) K. Harmsen in "Soil Chemistry, B. Physico-chemical Models", G. H. Bolt, Ed., Elsevier, Amsterdam, 1979, Chapter 4.
Brouwer et ai.
of statistical thermodynamic theory of mixtures, considering the adsorbent as an array of equivalent exchange sites; secondly, on the basis of multisite models, in which case the energy of mixing within each site group is assumed to be zero. In the first approach, selectivity changes are rationalized in terms of ion-ion interactions, whereas, in the second one, such changes are assigned to differences in ion-site interactions. The question whether the adsorbent is in fact heterogeneous cannot, in general, be resolved on the basis of ion-exchange evidence alone, particularly when measurements are limited to only one temperature and selectivity changes are small. In such cases, the question should preferably be settled by more direct means such as X-ray diffraction. Site blocking may also be an additional tool in such cases.14J5 However, when temperature data are available and when selectivity changes are very large and occur over minute changes in composition, ion exchange may provide conclusive evidence for charge heterogeneity. A rather convincing argument in this respect is the excellent agreement between the prediction of ion siting, based on ion-exchange and X-ray evidence in Y zeolite~.'~J~ The theory for multisite ion exchange is well covered in the literat~re'~J'-'~ and only bare essentials, needed for our data analysis, will be given. The simplest possible case is an ion exchanger carrying two groups of sites, one of which (H) is highly selective for M+, the other one (L) being poorly selective. When one chooses the equivalent fraction scale for the adsorbent,20the overall selectivity coefficient K , takes the form (heterovalent case) K, = (HM; + LML)2aMzt/[(HMh++ LMf,+)aM+'I (1) The activities in the liquid phase are defined on the molar scale and can be calculated from the solution composition by using Debye-Huckel theory21 provided the ionic strength is sufficiently low. H and L represent the equivalent fractions of the high- and low-selectivity groups (i.e., H L = 1)and M+ and M2+are the corresponding equivalent fractions of the ions in the L and H groups. The characteristic selectivity coefficients are similarly defined as KCH= (M~)'UM*t/{(M~')aM.2) (2) K: = (Mt)2aMzt/{(M~+)aM+2)
+
Expressing aM2+/aMt2 of eq 1 in terms of KcH,one can readily show that (remembering that mMt + 2mMzt = constant) the limiting value of K , when mMt 0 is given by lim K , H2KCH Therefore, the characteristic selectivity coefficient for the H sites is obtained by dividing this limiting K , value by the square of the equivalent fraction of H sites. The number of H sites can be obtained from the fol1 at some very lowing: If again KCH>> K:, then M$ low aMtvalue at which ML remains essentially zero (on account of the low KCLvalue and the high mM2tvalue in
-
-
-
(14) A. Maes and A. Cremers, J. Chem. Soc., Faraday Trans. I , 74,136 (1978). (15) A. Maes, J. Verlinden, and A. Cremers, J . Chem. Soc., Faraday Trans. 1 , 74, 440 (1978). (16) M. Costenoble and A. Maes, J. Chem. SOC.,Faraday Trans. I , 74, 131 (1978). (17) R. M. Barrer and J. Klinowski, J. Chem. Soc., Faraday Trans. I , 68, 73 (1972). (18) R. M. Barrer and J. Klinowski, J . Chem. SOC.,Faraday Trans. 1, 75, 247 (1979). (19) H. E. Jensen, Agrochimica, 19, 257 (1975). (20) G. L. Gaines and H. C. Thomas, J. Chem. Phys., 21,714 (1953). (21) R. A. Robinson and R. H. Stokes, "Electrolyte Solutions", Butterworths, London, 1965.
The Journal of Physical Chemistry, Vol. 87,
Cesium and Rubldlum Ion Equilibria In Illite Clay
I
I
1
- Sr
-8
-I
-8
-1
-5
0.4
0.5
CS(Rb)
Flgure 1. in K , for the Cs+ and Rb+ exchange (25 OC) in Ca2+, Sr2+, and Ba2+Illite vs. clay composition (in equlvalent fractions Z,,,ZRb) at 0.001 N total normality.
solution). Therefore, increasing aM+in the solution by 1 order of magnitude will result in a 2 order of magnitude drop in K , since the overall surface composition is not affected until aM+is sufficiently high to lead to a response in the low-selectivity group. Consequently, a very sharp decrease is expected at some critical loading in M+, the number of H sites. In the case of homovalent exchange, the limiting value of K , KCHHand the change in K, at the critical loading should be less sharp. Finally, the limiting value for K , at the other end of the composition scale is lim K , K>/L for both homovalent and heterovalent cases.
-
-
Results and Discussion Cesium and Rubidium Selectivity at High Exchange Levels in Calcium,Strontium, and Barium Illite. Figure 1 shows the effect of clay composition on the (logarithm of) selectivity coefficient of Cs+ and Rb+ in Ca2+,Sr2+,and Ba2+illite. In all systems, the distribution of both ions is monitored and the total normality is kept constant throughout N). The exchange reaction appears to be stoichiometric, as evidenced by the constancy of the clay ion occupancies, which were identical with the values of
-2
-1
the starting materials (values are in mequiv/(100 g)): 18.1 f 0.5 for Ca Cs (or Rb); 19.9 f 0.4 for Sr Cs (or Rb); 24.0 f 0.3 for Ba Cs (or Rb). The cation-exchange capacities were 18.2 (Ca), 20.1 (Sr), and 24.2 (Ba) mequiv/ (100 g). It appears that, in all cases, there is a gradual decrease in K , with increasing Cs or Rb loading, the In K , values leveling off at about 3.5 (Rb) and 5 (Cs) (overlookingthe minor differences which exist between Ca, Sr, and Ba). These differences correspond to AG values of about -5.5 (Ca, Sr, Ba Rb) and -7.5 (Ca, Sr, Ba Cs) kJ/equiv or a value of -2 kJ/equiv for the Rb-Cs exchange at high loading. These results are identical with what is commonly found in montmorillonite22and support the current views that we are here dealing with the “planar” sites. A rather remarkable feature of the data shown in Figure 1 is that, irrespective of the initial form of the ion exchanger (Ca, Sr, Ba), the numerical values of the selectivity coefficients for Cs and Rb converge toward a value of about 5.5-6.0 (In K,) at a loading of 0.05, i.e., 1 mequiv/(100 g). This would indicate the involvement of a high-selectivity site which fails to discriminate between Cs and Rb (the same applies evidently to the ions Ca, Sr, and Ba). Cesium and Rubidium Selectivity at Low Loading Levels in Calcium,Strontium, and Barium Illite. Figure 2 shows the set of ion-exchange isotherms (in double-log form) for the Ca-Rb exchange at three different values of the total normality (0.001,0.04, and 0.8 N). The range of Rb-exchange levels varies from ZRb= lo-’ to Le., between 2 and 2 X mequiv/(100 g). It is apparent from this figure that, at increasing values of the total electrolyte concentration in the solution, increasingly lower values are required for the equivalent fraction of Rb in solution, SRb, to reach a given exchange level. This is an illustration of the well-known electrovalence effect, favoring the sorption of the ion of lower valency at increasing electrolyte levels (keeping the fractional composition of the liquid phase constant). This seems to indicate that we are dealing with a reversible ion-exchange reaction over the entire composition range. Six sets of similar isotherms were obtained for the exchange of Rb+ and Cs+ in the Ca2+,Sr2+ and Ba2+illite under exactly similar conditions. Since, for all six cases, identical initial conditions were chosen, it is possible to condense all the data in a single figure, showing the ranges in liquid- and solid-phase compositions for each particular combination. The result is shown in Figure 3. This figure
-
3t
-3
SRL
Figure 2. Ion-exchange Isotherms (25 “C) of Rb+ in Ca2+ lllle at low Rb+-exchange levels and three values of the total ionic concentration: 0.001 (O), 0.04 (A),0.8 (W) N. Z and S refer to equivalent fractions in the clay and the equilibrium solution.
+
0.3
,9,
,
