Ion exchange properties of hydrous ceria—II

.I. inorg, nacl. Chem. Vol. 43, pp. 1663-1667, 1981 Printed in Great Britain. All rights reserved

0022-1902/81/071663--05502.00/0 Copyright © 1981 Pergamon Press Ltd.

ION EXCHANGE PROPERTIES OF HYDROUS CERIA--II THERMODYNAMICS OF ALKALI CATION EXCHANGE N. Z. MISAK and E. M. MIKHAIL Nuclear Chemistry Department, Atomic Energy Establishment, Cairo, Egypt

(Received 6 May 1978; receivedfor publication 4 June 1980) Abstract--The thermodynamics of Li/Na and Li/Cs exchanges in hydrous ceria, whose cation exchange properties proved to be unaffected by prolonged exposure to air, was studied. The Li/Na exchange is entropy directed while the LilCs exchange is both enthalpy and entropy directed. Application of the Eisenman model to these results and to other results for zirconia and organic resins proved the general validity of the model and the primary importance of oxide acidity in determining selectivity. Quantitative applicability of the model, although possible in some cases is hampered by the significant entropy changes in the solid that are neglected in the model. The basic ideas in the model can be used for obtaining useful conclusions and successful predictions of trends in the values of changes in the thermodynamic functions, accompanying exchange reactions in different exchangers. INTRODUCTION Hydrous oxides are now arousing increasing interest due to the perspectives of their use in the fields of nuclear energy and water desalination[I, 2]. Among these oxides, hydrous ceria is among the least studied[2]. Recently, Bhaduri et a/.[3,4] have reported on the kinetics and equilibria of exchange of the ammine complexes of some transition metal ions on hydrous ceria. In a previous paper[5], we have reported on the general features, kinetics and equilibria of several cations and anions on this exchanger. The selectivity patterns deduced from distribution measurements on hydrous ceria and those obtained by other authors for other hydrous oxides were discussed in the general terms of the electrostatic model of Eisenman[6]. It was envisaged that this model can be applied, at least qualitatively, for the hydrous oxides if due consideration is given to their acidity, porosity, capacity and probably also crystallinity. This paper reports on the thermodynamics of alkali ion exchange on hydrous ceria. The received data are compared with those for hydrous zirconia and organic resins in the light of Eisenman's model whose quantitative application is considered. EXPERIMENTAL Reagents. Boiled distilled water and analytical-reagent chemicals were used in all the preparations. Cesium hydroxide was prepared by adding cesium sulphate in a nitrogen atmosphere to the requisite amount of a barium hydroxide solution. Hydrous ceria. Hydrous ceria used in the present work is that prepared, as already mentioned in the first paper of this series [5], by addition of I : I NH4OH to a saturated solution of ammonium eerie nitrate. The final product obtained after washing with distilled water for more than one month to remove sorbed ions, was dried at 50°C and stored over saturated ammonium chloride in a nitrogen atmosphere. It was amorphous to X-ray, has the empirical formula CeO2.2.7H~O and could be represented by the formula CeOx(OHh-z~'yH20, where x is less than 2 and probably close to it (5). The so-obtained ceria presumably does not contain any significant amounts of NH4+ ions since the exchange values of alkali ions on the H+-form of ceria were practically the same when measured through the H + ions liberated from the solid or the alkali ions sorbed by it[5]. INC Vol. 43, No. 7--P

The capacity and equilibria measurements of alkali metal ions were done on two samples of ceria: the first was a rather freshly prepared ceria stored over saturated ammonium chloride in a nitrogen atmosphere and the second was a sample stored for more than two years over saturated ammonium chloride in air. Both samples gave the same results within the reproducibility limit of measurements (-+3%). This showed that the cation exchange properties of hydrous ceria are not affected by prolonged exposure to the atmosphere. Capacity measurements. The capacity of the W-form of ceria for Li+, Na+ and Cs+ ions and of the Li+-form (prepared by repeated equilibration of the W-form with 0.1 M LiOH) for Na+ and Cs÷ ions was determined by repeated equilibration of 0.1 g solid with 10 ml of 0.1 M solutions of COs-free alkali hydroxides at 25, 45 and 65°C. The capacity values were obtained from analysis of both solution and solid phase in the case of Na÷ and Cs+ and of the solid phase in case of Li+. These values were found to be independent of temperature. The values for the H+-form were 0.97, 0.87 and 0.65 meqlg, respectively, for Li+, Na+ and Cs+ ions while those of the Li+-form were 0.90 and 0.65 meq/g, respectively, for Na+ and Cs+ ions. Ion exchange isotherms. The .forward isotherms were done starting from the Li+-form of the exchanger which was shaken overnight (sufficient to attain equilibrium) with COrfree solutions of LiOH + NaOH or LiOH + CsOH of total molarity 0.1 in a mechanical shaker thermostat at 25, 45 and 65-+0.5°C. The reverse isotherms were done at 25°C in the same manner but starting with the Na+- and Cs+-forms of ceria, obtained by saturation of the Li+-formwith these ions. The Vim ratios were generally 100 ml g-t in the case of the forward isotherms and 250 ml g-i for the reverse isotherms. To obtain ionic fractions, in the forward isotherms, higher than about 0.5 and near to unity, larger volumes of solution and smaller amounts of solid were used. Besides, in the case of the reverse Li+/Cs+ isotherm, some points were obtained starting with the Li+-form partially exchanged with Cs+ ions. The equilibrations were made in duplicate and the containers were flushed with nitrogen and sealed during equilibration. In several experiments, the released alkali ion and the sorted one were found equivalent, showing that only ion exchange took place and that the NH4+ content of the prepared ceria is, if present at all, insignificant. Analyses. Li was determined either with a Beckman DU flame spectrophotometer or with an atomic absorption spectrophotometer (type Pye Unicam SP 90 A). Na and Cs were determined either by y-counting of the added tracers Na22 and Cs ~ using a NaI scintillation head connected to a scaler of the type Nuclear

1663

1664

N. Z. MISAKand E. M. MIKHAIL

Chicago, Model 186A, U.S.A. (forward isotherms) or by atomic absorption spectrophotometry(reverse isotherms). In the analysis of the solids, they were separated by filteration and washed with a small amount of alcohol to avoid hydrolysis, The yactivity of the solid was either measured directly or first eluted with 0.1M HCI, while the alkali ion content of the solid was determined after its elution with 0.1 M HCI.

Following Gaines and Thomas [7] and neglecting water activity terms (justifiable due to the non-swelling of ceria), the following equation can be written for the forward exchanges:

RESULTSANDDISCUSSION The ion exchange isotherms for replacement of Li+ at 25, 45 and 65°C in hydrous ceria by Na÷ and Cs + and for the reverse exchanges at 25°C are given in Figs. l and 2 for Li/Na and Li/Cs exchanges, respectively, as the equivalent fraction of the sorbed ion in the solid (qlqo) vs its equivalent fraction in solution (C/Co). qo is the maximum occupancy (capacity) of ceria by the sorbed ion and Co is the total molarity of solution; 4/and C are the concentrations of the sorbed ion in the exchanger (meq/g) and solution (meq/ml), respectively. It is seen from Figs. 1 and 2 that the ion exchange isotherms are reversible.

where K~ is the thermodynamic equilibrium constant, or selectivity constant, and K' is the corrected selectivity coefficient:

IO

log Ka = y j log K'd(q/qo)

K'= K

y24-LiOH y2_ MOH

0 45"C

_qlqo(l -C/Co)

OZl

Q:,

02 l

1 04

I 0.6

I

08

;o

C/Co

Fig. 1. Li/Na exchangeisotherms on hydrousceria at 25, 45 and 65°C. to!

0.8

l~ • 0 X •

25°C 450C 65"C Reverse isot"herrn- 2 5 ° C

(96 O.,:

02

f

~jQ 04

0.6

C/Co

y2 +_LiOH d(q/qo). log y2_+MOH

The value of the last integral was evaluated from the known values of y± of the alkali hydroxides at this ionic strength [8] by the use of Glueckauf equation [9] to obtain these values in the mixed solutions. In the case of Li+/Cs + exchange, the value of the integral was equal to - 0.022, which increases the value of AF ° of the Li+/Cs + reaction (Table l) by 0.03 kcal/mole. In the case of the Li+/Na + exchange, the value of the integral is very close to zero, and therefore this integral has been neglected in all the calculations. The variation of log K with loading (q/qo) is given in Figs. 3 and 4 for the Li/Na and Li/Cs exchanges, respectively. These figures show the great complexity of dependence of log K on loading. Such complexity is encountered in the case of zeolites[10, 11] and indicates, according to Barrer and Klinowski [ l l ], the heterogeneous nature of ion siting in the exchanger. This conclusion, which follows also from the isotherms in Figs. l and 2, is in conformity with the base titration curves of ceria[5]. The values of the thermodynamic equilibrium constants are calculated by integration under the curves in Figs. 3 and 4, applying eqn (4). The values of the changes in the thermodynamic quantities (at 25°C) are calculated from these constants as follows:

RT In K~

(5)

don K.) AH °= - R d(l/T)

(6)

AS ° = (AH o - AF°)/T

(7)

AF ° = -

0.2

fo

(4)

06

0

(3)

y 4- LiOH is the mean activity coefficient of LiOH and y - M O H is the activity coefficient of NaOH or CsOH, all at the constant ionic strength 0.1M. The thermodynamic equilibrium constant is then equal to: log K~= fo' log Kd(q/qo)+

X 65*C I Reverse i s o t h e r m - 2 5 * C

(2)

K is the selectivity coefficient:

• 25"C 08

(1)

0 8

I0

Fig. 2. Li/Cs exchange isotherms on hydrous ceria at 25, 45 and 65°C.

AH ° is obtained from Fig. 5 (expressing the variation of log K,, with I/T) for the Li/Cs exchange, using equation 6; for the Li/Na exchange, AH ° is equal to zero, as is clear from Fig. 3 (no change of K with temperature).

1665

Ion exchange propertiesof hydrous ceria--lI

q/qo

q/qo 02

-02

04

0.6

0.8

I

I

I0

x

0,2.

04

06

08

[

1

I

f

LO

-0~'

'x~ -04

- 04

-OE

-0.6

x

-0.8

-QE

-I.0

-I0

•

-

25oc -1.2

O - 45%

-12

/

-1.4

X

-

65°C -14

"

-16

•

- 25oC

0

-45"C

-18

-I.8

Fig. 3. Variationof log K with Na÷-Ioadingfor Li/Na exchange on hydrousceria at 25, 45 and 65°C. Eisenman[6] separated the free energy of exchange into two components: one belonging to the solution phase and involving the difference in the hydration energies of the two exchanging ions and the other belonging to the solid phase and involving the difference in their electrostatic interaction energies with the exchanger matrix. Thus, for the exchange: R-A+B+~R-B+A

+

-24

Fig. 4. Variation of log K with Cs+-Ioadingfor Li/Cs exchange on hydrousceria at 25, 45 and 65°C.

AF ° = (AFA+h - AFB+h) + (AFs+" - AFA+') AF h + AF'.

-22

- 2.~

(R represents the exchanger)

=

X - 65 °C - 20

-I.64

(8)

./"

-t.60

We have done this separation for all the thermodynamic functions. AF h, AH h, AS h are obtained from the data of Rosseinsky[12] on ion hydration while AF', AH" and AS" are obtained by subtraction of these quantities from AF °, AH ° and AS° obtained experimentally. The values o3~_AF°, AH °, AS°, AF h, AH h, AS h, AF', AH" and AS" for exchanges in ceria are given in Table I. For a more meaningful discussion, these values are given also for similar exchanges in two different samples of zirconia, a carboxylate and a sulphonate resin of two different crosslinkings (%DVB). In all cases, Li+ is the ion initially present in the exchanger. Table 1 shows that in ceria, Li + is preferred to both Na + and Cs +, and specially to Cs+. Besides, the Li/Na exchange is an entropy directed reaction (AH ° = 0) while

- 1.56

-I.48

/

-144

-I.40 0.0029

i

I

L

00031

]

0.0033

I

O.OO35

I/T

Fig. 5. Variation of log Ko with liT for Li/Cs exchange on hydrous cerla.

1666

N.Z. MISAKand E. M. MIKHAIL Table 1. Thermodynamicdata, separated into hydrationand electrostatic interactionterms, for alkali ion exchange in ceria, zirconia and organic resins

Exchanger

Exchange reaction

_ ~ -Li/Na

Electrostatic interaction terms

AF° kcal/ mole

AH° kcal/ mole

Hydration terms

AS° e.u.

AF h

AH h

AS h

1.36

0.00

-4.5

-23.9

-26.1

-7.5

2.17 0.57 0.49

1.45 -0.17 -2.06

-2.4 -2.4 -8.6

-54.3 -23.9 -23.9

-60.1 -26.1 -26.1

-19.6 56.5 61.5 17.2 -7.5 24.5 25.9 5.1 -7.5 24.4 24.0 -I.1

-0.004

-5.49

-18.4

-54.3

-60.1

-19.6 54.3 54.6

0.13

-0.40

-1.8

-54.3

-60.1

-19.6 54.4 59.7 17.8

0.62

-I.22

-6.2

-54.3

"60.1 -19.6 54.9 58.9 13.4

-0.06

-0.33

-0.9

-23.9

-26.1

-7.5

23.8 25.8

6.6

-0.46

- 1.7

-4.0

-23.9

-26.1

-7.5

23.4 24.4

3.5

-0.29

-0.69

-I.3

-54.3

-60.1

-19.6 54.0 59.4 18.3

-0.83

-2.89

-6.7

-54.3

-60.1

-19.6

AF"

AH e

25.3 26.1

AS"

3.0

Ceria ~. ~" Li/Cs ZirconiaI - -- --Li/Na';" ZirconiaII ~ L i / N a ' + ~Li/Cs~ Carboxylate resin (1%) Li/Cs§ Carboxylate resin (15%) Li/Cs§ Sulphonate resin (0.5%) Li/Na¶ Sulphonate resin (16%) Li/Na¶ Sulphonate resin (0.5%) Li/Cs¶ Sulphonate resin (16%) Li/Cs¶

53.5

1.2

57.2 12.9

tBy calculationfrom the results of Britz and Nancollas[13]. *From data by Ghoneimy[14]. §From the data of Lindenbaum and Boyd[15]. ¶From the data of Boyd[16]. the Li/Cs exchange is directed by both the enthalpy and entropy changes. This is different from zirconia[13, 14] where the entropy change, leading also to a preference of Li + over Na ÷, is high enough to overcome the unfavourable enthalpy change and where Cs ~ is slightly preferred to Li ÷ due to the enthalpic term. Similar to zirconia[13, 14] substantial dehydration of the alkali ions seems to occur in ceria, since as in zirconia, the capacity of ceria increases with the decrease of the crystallographic radius of the alkali ion and the water content of its different forms is similar (20.8, 17.9 and 19% for the Li ÷-, Na ÷- and K÷-forms, respectively). According to Eisenman's model (eqn (8)), the alkali ion with the lesser hydration energy (larger relatively weakly hydrated ion) will be preferred if the hydration free energy term predominates while the smaller ion will be preferred in the case of predominance of the electrostatic interaction free energy term. The first situation is expected to be met in the case of exchangers with low field strength and low exchange site density while the second situation will correspond to the revers_& conditions. Eisenman assumes that AF" = AH', i.e. A S " = O, and that there are no water molecules between the cations and the anionic exchanger sites. The field strength of an acid exchanger is higher the weaker its acidity[17] while the site density may be related, when other things are equal, to the ion exchange capacity[18]. Before trying to draw out conclusions from the consideration of the Eisenman model and its main ideas in the given exchange systems, it should be first said that the capacities of the Li+-forms of zirconia (I and II) and ceria for Na ÷ and Cs ÷ ions are rather close (1.1 and 0.68 meq/g for zirconia[13, 14] and 0.9 and 0.65 meq/g for ceria, respectively). If the differences in the porosities of the two oxides can be ignored, their comparison would

involve the difference in their acidities. Hydrous ceria is less acidic than zirconia[19] and is therefore expected to have a higher field strength of its O- anionic sites. The two samples of zirconia have the same capacity[13, 14] and therefore, differences in their behaviour may be attributed to differences in their porosities. Zirconia I is presumably less porous than zirconia II since the water content of the different cationic forms is about 24-27% for the first sample[13] and 31-33% for the second one[14]. More porosity means less site density since it may indicate a higher average spatial separation of the exchange sites. The comparison of the weakly acidic exchangers with the strongly acidic sulphonic resin should give a primary consideration to the field strength of the anionic sites which is much lower in that resin[17]. Finally, it should be mentioned that in the hydrous oxides where, as already mentioned, substantial dehydration of the alkali ions takes place, the calculated thermodynamic hydration functions (and consequently the electrostatic interaction ones) can be more or less representative of the actual situation. In the carboxylic resin, substantial dehydration is claimed for the Li + ion but not for Cs+[15]. For the sulphonic resin, Reichenberg[17] declares that probably only one quarter-one half of the hydration energy of ions is involved and so, the calculated A P , AH h and ASh may be significantly exaggerated. Thus, the calculated values of AH" or A P may be considerably in error in the case of organic resins. However, the final conclusions made here with reference to these values remain valid since these conclusions are based only o_n_nthe trends shown by calculation and revealing that AH ~ is positive and higher for the carboxylic resin than for the sulphonic one, for the given exchanges. These trends are either unaffected or even reinforced if one assumes that

1667

Ion exchange properties of hydrous ceria--lI m

Cs + is retained in the fully hydrated state by the carboxylic resin and that only one quarter of the hydration energies of the alkali ions is to be considered for the sulphonic resin. This is so since in the carboxylic resin, AH h (or AF') will be more negative rel_~ative to the values given in Table 1, and AH" (or AF t) will be consequently more positive (eqn (8)). In the case of the sulphonic resin, AH ~ will be less negative but ~--H-ewill still be a positive, though a less, quantity due to the comparatively very low values of AH °. A more positive AH" in the case of the carboxylic resin and a less positive one for the sulphonic resin clearly reinforces the trend, shown by calculation, that AH" is higher for the carboxylic resin than for the sulphonic one. Taking the above-mentioned observations into consideration and applying the ideas of Eisenman to the exchanges in Table 1, the following expectations may be mentioned. First, the hydrous oxides and the carboxylic resin should show at least a less preference for the larger alkali ion than the sulphonic resin. A preference for the smaller ion by the former exchangers is highly likely. Second, such behaviour would be more sharply expressed for the less acidic ceria compared to zirconia (if porosity is of secondary importance relative to acidity) and for the less porous zirconia I compared to zirconia II. Third, the contribution of AH ~ is expected to be higher for the carboxylic resin than for the sulphonic one, for ceria than for zirconia, and for zirconia I than for zirconia II. Since AH" is positive for the given exchanges (Table 1), AH ° should be less negative or more positive for the carboxylic resin compared to the sulphonic one, for ceria compared to zirconia, and for zirconia I compared to zirconia II. Table 1 shows that all the mentioned expectations are fulfilled for the given exchanges. Similar differences between the carboxylate and the sulphonate resins, deduced from the Eisenman model, were indicated by Reichenberg[17] who considered only the AF ° (or selectivity) of alkali ion exchanges in these resins. The fact that the AF° values in ceria are much more positive than in both samples of zirconia emphasizes the primary role of oxide acidity in determining selectivity. From Table 1, it may be deduced that the Eisenman model seems to apply almost quantitatively for_!the Li/Na and Li/Cs exchanges in zirconia II, where AS" is very small and equal to -1.1 and 1.2 e.u., respectively. However, its apparent validity in all the other cases is only qualitative since AS" is large and contributes considerably to AS° and, consequently, to AF ° of the reaction. Such large entropy changes in the solid have been also found in many examples by Sherry[18] in his application of the Eisenman model to zeolites. Sherry declares that entropy changes in the solid phase can be attributed to changes of the entropy of ions and of intracrystalline water in this phase and besides, to water transfer between the solution and the zeolite, which would affect the entropy of the whole system.

In the case of the hydrous oxides, AS~ is generally positive. Since the water contents of the different cationic forms of either ceria or zirconia are almost the same, the increase of solid phase entropy can be probably attributed to more degrees of freedom of a less strongly bound larger alkali ion (Na + or Cs+) insid._ðe solid. Table ! shows that for the same exchange AS" is smaller for zirconia II than for zirconia I and for zirconia II than for ceria, which may be in conformity with less differences in the strengths of electrostatic interaction of ions with decrease of site density and field strength of the exchanger. However, contrary to this, AS" is higher for zirconia I than for ceria. This probably shows that even small changes in the hydration of ions inside th____e hydrous oxides may have a profound effect on ~S e which can assume large values on this account. Acknowledgements--The authors wish to thank deeply Prof. G.

R. Choppin,Chemistry Department, Florida State University,for encouragement and useful discussions during his brief stay in Cairo. Thanks are also due to Dr. A. Youssef, Faculty of Science, Mansoura Universityfor atomic absorption analyses.

REFERENCES

1. C. B. Amphlett, Inorganic Ion Exchangers. Elsevier, Amsterdam (1%4). 2. V. Veselyand V. Pekarek, Talanta 19, 219 (1972). 3. A. K. Bhaduri, K. P. Kar and K. B. Pandeya, Curr. Sci. 45, % (1976). 4. K. P. Kar, K. B. Pandeya and A. K. Bhaduri, J, lnorg. Nucl. Chem. 38, 1211(1976). 5. N. Z. Misak and E. M. Mikhail, J. Appl. Chem. Biotech. 28, 499 (1978). 6. G. Eisenman, Biophys. J. 2,259 (1%2). 7. G. L. Gaines and H. C. Thomas, J. Chem. Phys. 21,714 (1953). 8. H. S. Harned and B. B. Owen, The Physical Chemistry of Electrolyte Solutions, p. 498. Reinhold, New York (1958). 9. E. Glueckauf, Nature 163,414 0949). 10. Z. Dizdar, J. lnorg. Nucl. Chem, 34, 1069(1972). II. R. M. Barrer and J. Klinowski, Trans. Farad. Soc. I, 73 (1972). 12. D. R. Rosseinsky, Chem. Rev. 65,467 (1965). 13. D. Britz and G. H. Nancollas, 3, lnorg. Nucl. Chem. 31, 3861 (1%9). 14. H. F. Ghoneimy, M.Sc. Thesis, Ion Exchange Behaviour of Hydrous Zirconia in Mixed Solvents, Ain-ShamsUniversity, Cairo (1978). 15. S. Lindenbaum and G. E. Boy& J. Phys. Chem. 69, 2374 (1%5). 16. G. E. Boyd, lon Exchange Process Ind. Conf., Soc. Chem. Indian p. 261, London(1970). 17. D. Reichenberg,In Ion Exchange (Edited by J. A. Marinsky), Vol. 1, Chap. 7. Dekker, New York (1%6). 18. H. S. Sherry, In Ion Exchange (Edited by J. A. Marinsky), Vol. 2, Chap. 3. Dekker, New York (1%9). 19. N. V. Sidgwick, The Chemical Elements and Their Compounds, p. 451. Oxford UniversityPress, London(1952).