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Microporous and Mesoporous Materials 61 (2003) 159–165

Evaluation of an intermediate-silica sedimentary chabaziteas exchanger for potentially radioactive cations

Bruno de Gennaro a,*, Abner Colella b, Paolo Aprea a, Carmine Colella a,*

a Dipartimento d’Ingegneria dei Materiali e della Produzione, Universit�aa Federico II, Piazzale V. Tecchio 80, I-80125 Napoli, Italyb Dipartimento di Scienze della Terra, Universit�aa Federico II, Via Mezzocannone 8, I-80134 Napoli, Italy

Received 22 July 2002; received in revised form 2 December 2002; accepted 3 December 2002

Abstract

Intermediate-silica sedimentary chabazite contained in two chabazite-rich tuffaceous rocks has been evaluated as

potential cation exchanger for radioactive cation removal from nuclear waste streams. Exchange isotherms have been

obtained for the cationic couples Na/Ba, Na/Co, Na/Cs and Na/Sr and the relevant thermodynamic parameters cal-

culated with the help of a computer program. Sedimentary chabazite turns out very selective for Csþ, fairly selective for

Ba2þ and Sr2þ, especially at low equivalent fraction of these cations in solution, and unselective for Co2þ. An expla-

nation of the observed selectivities is attempted on the basis of the structural features of chabazite.

� 2003 Elsevier Inc. All rights reserved.

Keywords: Cation exchange; Chabazite; Radioactive cations; Zeolitic tuff

1. Introduction

Chabazite is not frequent as a predominant

phase in sedimentary deposits, but important oc-

currences of this zeolite have been reported in

several locations in the United States and in a few

other countries in the world [1]. On the contrary,chabazite is somewhat common in joint occurrence

with phillipsite in several deposits in Italy, and, in

minor extent, in Germany and on Canary Islands

(Spain) [2].

Given its moderate to good selectivity for var-

ious toxic and noxious cations [3,4], chabazite

represents an interesting natural resource for use

as cation exchanger in the purification of liquid

* Corresponding authors.

1387-1811/03/$ - see front matter � 2003 Elsevier Inc. All rights rese

doi:10.1016/S1387-1811(03)00363-9

wastes. Applications have been devised, in partic-

ular, in the removal of some heavy metal cations

or ammonium from industrial or municipal

wastewaters, respectively, [5] and in the treatment

of nuclear waste streams, contaminated by Cs and

Sr [6]. A sedimentary chabazite from Bowie, Ari-

zona, [7] has been used in admixture with thesynthetic zeolite A (LTA) to clean up after the

Three Mile Island nuclear accident in 1979 [8].

Several papers have been published on the

fundamentals of ion exchange in chabazite [9–18].

Most research involving natural chabazite has

been carried out on samples coming from two

sources: a hydrothermal occurrence from Nova

Scotia, Canada, [19], having a medium Si/Al ratio(�2.5) and the already mentioned sedimentary

occurrence from Bowie, Arizona, [7], presenting a

high silica content (Si/Al� 4). Investigation about

rved.

160 B. de Gennaro et al. / Microporous and Mesoporous Materials 61 (2003) 159–165

the cation exchange properties of the Italian

sedimentary chabazites, contained in rocks of

trachytic nature and therefore regarded as inter-

mediate-silica chabazites (Si/Al ranging roughly

from 2.2 to 2.6), has been limited to date to am-

monium, the main alkaline and alkaline-earth ca-tions, and some heavy metal cations [12,14,15,

17,18]. No investigation, on the contrary, has been

performed to date on the ion exchange equilibria

involving Italian chabazite and the cations that are

usual contaminants of the nuclear power plants

wastewaters.

In view of a possible use of the Italian mixed

chabazite–phillipsite sedimentary materials in thetreatment of nuclear waste streams, this paper

aims, as already made for phillipsite [20], to extend

the study of the cation exchange selectivity of the

‘‘trachytic’’ chabazite to the most common radio

nuclides, such as Ba2þ, Co2þ, Csþ and Sr2þ [21].

Additional objectives of this study are (1) to elu-

cidate the thermodynamic parameters of the ion

exchange reactions and to compare them to pastwork and (2) to relate ion exchange properties of

chabazite to its structural features.

2. Experimental

2.1. Materials

Enriched chabazite samples were prepared by

the separation methods commonly used in miner-

alogy [22], starting from two parent rocks, one

obtained through a drilling core from a fine tuff

formation (FT) beneath the Neapolitan yellow tuff

[23] in Parco Margherita (Naples, South Italy) and

the other coming from a quarry in Palombara-

Lubriano (Viterbo), belonging to the formation ofthe Orvieto-Bagnoregio ignimbrite (OBI) (Vulsini

volcanic district, Central Italy), locally called red

tuff with black scoriae [23].

The two chabazite samples, after abundant

washing with distilled water and drying overnight

at about 80 �C, were exchanged in Naþ form (see

below) and stored at room temperature over sat-

urated Ca(NO3)2 solution (relative humidity near50%), before using them for ion exchange runs. In

particular, the FT sample was used for the equi-

librium studies involving the Na/Ba and Na/Co

cation pairs, whereas the OBI samples served for

the Na/Cs and Na/Sr cation pairs.

2.2. X-ray and chemical characterisation

The FT and OBI chabazite samples were ex-

amined by qualitative and quantitative X-ray dif-

fractometry (XRD, Philips PW 1730 apparatus,

equipped with a Philips 3710 count unit). The

mineral composition was determined using the

reference intensity ratio (RIR) procedure [24],

which is an improved version of the well known

XRD technique based on the use of the internalstandard [25].

The chemical composition of the two chabazites

in the selected samples was obtained by electron

microprobe analysis (Link AN 10000 apparatus

connected to a Jeol JSM 5310 scanning electron

microscope). Water content was estimated by

thermogravimetry (Netzsch STA 409 thermo

analyser).The cation exchange capacity (CEC) of the two

chabazite samples was determined using the cross-

exchange method [26]. Accordingly, two 1-g zeolite

samples, placed on gooch filters, were percolated

at about 60 �C up to exhaustion by 1 M NaCl or

KCl solutions, prepared by using Carlo Erba re-

agent-grade chemicals (purity 99.5%). The ob-

tained mono-cationic forms (Naþ or Kþ) werethen re-exchanged under the same conditions with

potassium and sodium, respectively. Naþ and Kþ

concentrations in the effluents of the second ex-

change cycle, evaluated by atomic absorption

spectrophotometry (AAS, Perkin Elmer AA 2100

apparatus), were used to calculate the mean CEC

values.

2.3. Ion exchange runs

The FT and OBI chabazite samples were used

for the equilibrium studies. Sodium forms of the

two chabazites, obtained through an exhaustive

procedure, analogous to that used in the estima-

tion of the CEC, were allowed to react at 25� 0.1�C in sealed teflon test tubes with solutions, con-taining varying amounts of Naþ and one of the

cations Ba2þ, Co2þ, Csþ or Sr2þ at 0.1 total nor-

Table 1

Mineral composition of the chabazite-rich materials

Phase Enriched chabazite samples

FT OBI

Chabazite 73.8 91.2

Phillipsite 1.3 –

Analcime 1.7 –

Feldspar 6.8 1.6

Biotite 0.6 –

Glass 15.8 7.2

B. de Gennaro et al. / Microporous and Mesoporous Materials 61 (2003) 159–165 161

mality, prepared starting from the relevant re-

agent-grade Carlo Erba RPE chlorides. Revers-

ibility ion exchange tests were performed following

the recommendations of Fletcher and Townsend

[27]. The solid-to-liquid ratio was varied between

1/100 and 1/500. The reaction time was fixed at 3days, which was beforehand proved to be sufficient

to attain equilibrium. Csþ and Naþ concentrations

in the liquid phase were measured by AAS,

whereas the Sr2þ, Ba2þ or Co2þ concentration was

evaluated by titration with EDTA [28]. The con-

centration of the exchangeable cations in solids

was calculated by mass balance.

2.4. Computation of the thermodynamic parameters

The measured equilibrium data were plotted

under the form of ion exchange isotherms, re-

porting the equivalent fraction of the ingoing ca-

tions in the solid phase as a function of the

equivalent fraction of the same cations in solution.

The thermodynamic exchange parameters, i.e., theequilibrium constant Ka and the standard free en-ergy per g-equiv of exchange DG0, were calculatedfrom the same data with the help of a computer

program following a standard procedure [15,26].

Briefly, given a generic exchange reaction:

zBAzAþðsÞ þ zAB

zBþðzÞ ¢ zBA

zAþðzÞ þ zAB

zBþðsÞ ; ð1Þ

where A and B are two cations, zA and zB are va-lences of A and B, subscripts (s) and (z) denote

solution and zeolite phase, calculations were per-

formed as follows:

(a) the ratios between the ion activity coefficients

in mixed solutions were evaluated from themean ionic activity coefficients, calculated by

the method proposed by Ciavatta [29] and cor-

rected by the Glueckauf�s equation [30];(b) selectivity coefficients, Kc, were calculated as:

Kc ¼AzB

z azAB

BzAz azBA

; ð2Þ

where Az and Bz are the equivalent fractions of

A and B in the solid phase and aA and aB arethe activities of the same cations in solution;

(c) equilibrium constants, Ka, and standard freeenergies of exchange per equivalent of ex-

changer, DG0, were calculated, at last, accord-ing to the Gaines and Thomas approach [31]:

logKa ¼ 0:4343ðzB � zAÞ þZ 1

0

logKc dAz; ð3Þ

DG0 ¼ � RTzAzB

lnKa: ð4Þ

3. Results

3.1. Material characterisation

The X-ray diffraction patterns of the two chabaz-

ite-rich samples revealed the presence of chabazite

as the only zeolitic component. The only other

crystalline phase found in both samples was feld-

spar. Actually, exiguous amounts of phillipsite and

analcime were demonstrated to be present in the FTsample by the X-ray quantitative analysis (Table 1).

Table 2 reports the chemical analysis of the two

chabazites and the calculated chemical formulae.

The FT and OBI samples appear to be very similar

in composition, so that the produced ion exchange

data may be considered unaffected by the frame-

work composition [32] and therefore comparable

with each other.The experimental CEC values turned out to be

2.70 and 3.28 mequiv/g for the FT and OBI sam-

ples, respectively. From the data in Tables 1 and 2

the CEC can also be calculated, obtaining the val-

ues of 2.66 mequiv/g for FT-chabazite and 3.34

mequiv/g for OBI-chabazite, respectively. The

substantial agreement of the observed and calcu-

lated data is a proof of the reliability of the exper-imental results and therefore of the completeness of

chabazite�s exchange for Naþ and Kþ.

Table 2

Chemical analysis of the FT and OBI chabazites

FT chabazite (%) OBI chabazite (%)

SiO2 51.40 51.48

Al2O3 17.20 17.90

Fe2O3 0.14 0.37

MgO 0.15 0.57

CaO 4.74 6.24

Na2O 0.85 0.72

K2O 7.07 4.30

H2O 18.45 18.42

Si/Al 2.54 2.44

E%a )4.50 )2.76

Formulae

FT chabazite:

(Na0:28K1:50Ca0:85Mg0:04)[Al3:38Fe0:02Si8:57O24]10.27H2OOBI chabazite:

(Na0:23K0:91Ca1:10Mg0:14)[Al3:47Fe0:05Si8:46O24]10.09H2OaE% is a measure of the unbalance between the content of

the trivalent framework cations (essentially Al) and that of the

extra-framework cations.

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

EB

a(z

)

EBa(s)

Fig. 1. Isotherm at 25 �C for the exchange of Ba2þ into Na-chabazite (FT sample, Table 1) at 0.1 total normality. EBaðsÞ: Baequivalent fraction in solution; EBaðzÞ: Ba equivalent fraction inthe zeolite. Open circles: forward points; filled circles: reverse

points.

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

EC

o(z

)

ECo(s)

Fig. 2. Isotherm at 25 �C for the exchange of Co2þ into Na-chabazite (FT sample, Table 1) at 0.1 total normality. ECoðsÞ: Coequivalent fraction in solution; ECoðzÞ: Co equivalent fraction inthe zeolite. Open circles: forward points; filled circles: reverse

points.

162 B. de Gennaro et al. / Microporous and Mesoporous Materials 61 (2003) 159–165

It is to be observed that the CEC values are

practically unaffected by the presence of impurities

in the two chabazite samples (see Table 1), because

of their limited amount (phillipsite and analcime)

and/or low exchange rate (analcime and glass) [33].

3.2. Ion exchange equilibria

Figs. 1–4 report the profiles of the isotherms

obtained for the four examined exchange equilib-

ria. A close inspection and comparison of the

various curves enable to make a number of ob-

servations that are reported in the following.

2Naþ ¢ Ba2þ exchange. The isotherm exhibits

a very distinct plateau with an inversion of selec-tivity at about 82% of the equivalent fraction of

the ingoing cation, in substantial agreement with

the results obtained by Barrer et al. [11] with a

hydrothermal sample, having a similar Si/Al ratio.

To be observed that the shape of the Na!Ba

isotherm resembles very closely that of the Na/Pb

couple with the same type of material [17].

2Naþ ¢ Co2þ exchange. The isotherm evi-dences a clear unselectivity for Co2þ, in fact it lies

beneath the diagonal for the entire compositional

range in the liquid phase. It is notable the presence

of a plateau at a ECoðzÞ value equal to about 0.44.

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

EC

s(z)

ECs(s)

Fig. 3. Isotherm at 25 �C for the exchange of Csþ into Na-chabazite (OBI sample, Table 1) at 0.1 total normality. ECsðsÞ:Cs equivalent fraction in solution; ECsðzÞ: Cs equivalent fractionin the zeolite. Open circles: forward points; filled circles: reverse

points.

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

ES

r(z)

ESr(s)

Fig. 4. Isotherm at 25 �C for the exchange of Sr2þ into Na-chabazite (OBI sample, Table 1) at 0.1 total normality. ESrðsÞ: Srequivalent fraction in solution; ESrðzÞ: Sr equivalent fraction inthe zeolite. Open circles: forward points; filled circles: reverse

points.

B. de Gennaro et al. / Microporous and Mesoporous Materials 61 (2003) 159–165 163

No literature data are available for this cation

couple in chabazite.

Naþ ¢ Csþ exchanges. The isotherm is convex,

demonstrating a good selectivity for Csþ, in very

good agreement with the results obtained pre-

viously by several authors, irrespective of theframework composition of the investigated samples

[9,11,13,16]. To be observed that in one case [11] a

partial exchange, i.e., an incomplete occupancy of

the exchange sites of chabazite, was reported.

2Naþ ¢ Sr2þ exchange. The isotherm is S-

shaped for the presence of a selectivity reversal at

about 45% of the ingoing cation substitution in the

framework. Dissimilar isotherms� shapes are re-ported in the literature for the same system: in one

case (hydrothermal sample from Nova Scotia) a

very distinct plateau with an inversion of selectiv-

ity at about 80% of the equivalent fraction of the

ingoing cation is reported [11], in another case

(sedimentary sample from Bowie) the curve is re-

ported to be perfectly convex [16].

Table 3 summarizes the equilibrium constants,Ka, and the standard free energies of exchange,DG0, calculated according to the procedure de-scribed in the experimental [15,25]. The resulting

selectivity series, based on the Ka, values, is:Cs�Na>Ba>Sr>Co.

4. Discussion

An interpretation of the data collected in the

light of the Eisenman�s theory of cation exchangeselectivity [34,35] may be attempted. The prefer-

ence shown by Na-exchanged phillipsite for Csþ is

a confirmation that zeolites characterized by me-

dium or weak anionic fields (medium-high Si/Al

ratios) prefer in uni-univalent exchanges cationswith low charge density. Also the moderate to

Table 3

Thermodynamic quantities of investigated equilibria in

chabazite at 25 �C

Cation pairs Ka DG0 (kJ/equiv)

2Naþ !Ba2þ 0.62 0.59

2Naþ !Co2þ 0.02 5.01

Naþ !Csþ 86.56 )10.982Naþ !Sr2þ 0.14 2.38

164 B. de Gennaro et al. / Microporous and Mesoporous Materials 61 (2003) 159–165

poor selectivity exhibited by chabazite for the

divalent cations Ba2þ, Sr2þ and Co2þ is well

explained according to the same theory, which

predicts that zeolites having medium-high Si/Al

ratios prefer in uni-divalent exchanges monovalent

cations or divalent cations with low energy ofhydration. Accordingly, the selectivity sequence

found (see Section 3) is in the order of the de-

creasing enthalpies of hydration, which are )1305,)1443 and )1996 kJ/mol for the three divalentcations reported above, respectively.

An interpretation of the data is also possible on

the basis of the known extra-framework cation

sites in chabazite [36,37]: site C1 is located alongthe ternary axis [1 1 1] in the centre of the double 6-

ring; C2 is located along the ternary axis [1 1 1],

outside the double 6-ring, in contact with three

oxygens; C3 is located along the ternary axis

[1 1 1], nearly at the centre of the large cage; C4 is

nearly the centre of the 8-tetrahedra ring which is

the port between the large cages.

Comparing these structural data with theshapes and the features of the relevant isotherms,

it can be deduced or postulated that:

• Csþ can likely be accommodated in every cationsite without any difficulty, except in the site C1,

that should be empty, in agreement with the re-

sults of previous crystal chemistry studies in the

case of exchange with large cations, such as Ba2þ

and Cd2þ [38];

• Ba2þ can also occupy all types of sites, except C1[38], but the presence of a well defined plateau in

the isotherm indicates a difficulty to enter some

sites, possibly the inner sites;

• Sr2þ can also occupy all types of sites, in agree-ment with the reported crystal structure of a Sr-

exchanged chabazite [37]. The presence of asmooth inversion of selectivity, however, indi-

cates also in this case that the occupancy of

some sites can occur only forcedly;

• lastly, Co2þ can possibly be located in all typesof cationic sites with difficulty that is progres-

sively greater for the tighter sites (clear evidence

of a plateau).

More in general, the shapes of the present iso-

therms and those found with other cationic sys-

tems in previous research on the same material

[14,17,18] suggest that two major structural re-

strictions may prevent chabazite from the ready

attainment of complete exchange. These con-

straints, evidenced by incomplete exchange, in-

version of selectivity and/or presence of a plateau,occur very frequently at about 80% of the equiv-

alent fraction of the ingoing cation and less fre-

quently at about 45%. The first occurrence, typical

of the Kþ, NHþ4 , Ba

2þ, Pb2þ, Zn2þ and possibly

Cd2þ isotherms is most likely an evidence of the

difficulty or impossibility to have access to the C1

site and therefore of the necessity to find a more

tight position in a reduced number of sites. Thesecond occurrence, presented by the isotherms of

Sr2þ and Co2þ, is more difficult to interpret without

a structural analysis. It is to be noticed, however,

that the sites displaying the higher occupancies in

Sr-exchanged chabazites are C2 and C3 [37].

5. Conclusion

The Ka values computed from the investigated

isotherms (Table 3) and those available in litera-

ture [14,15,17,18] allow the following selectivity

series to be worked out: Cs > NH4 > K > Pb >Na > Ba > Cd > Sr > Cu > Zn > Co.The cations preceding Na are preferred to var-

ious extent by chabazite with respect to sodium,but the relevant isotherms� shapes point out thatchabazite displays a moderate selectivity also for

Ba2þ, Cd2þ and Sr2þ, especially at low equivalent

fractions of these cations in solution.

Considering the selectivity series exhibited by

the intermediate-silica sedimentary phillipsite [20],

it may be concluded that Italian tuffs, containing

either phillipsite or chabazite, present good possi-bilities of use concerning the removal of cations

such as Csþ, NHþ4 , Ba

2þ or Pb2þ, provided the

interfering cation matrix is lacking or poor in Kþ,

for which both zeolites exhibit a good selectivity.

References

[1] G. Gottardi, E. Galli, Natural Zeolites, Springer-Verlag,

Berlin, Heidelberg, 1985, p. 187.

[2] M. de� Gennaro, M. Adabbo, A. Langella, in: D.W. Ming,F.A. Mumpton (Eds.), Natural Zeolites �93, International

B. de Gennaro et al. / Microporous and Mesoporous Materials 61 (2003) 159–165 165

Committee on Natural Zeolites, Brockport, NY, 1995, p.

51.

[3] C. Colella, Miner. Deposita 31 (1996) 554.

[4] R.T. Pabalan, F.P. Bertetti, in: D.L. Bish, D.W. Ming

(Eds.), Natural Zeolites: Mineralogy, Occurrence, Proper-

ties, Applications, Reviews in Mineralogy & Geochemistry,

vol. 45, Mineralogical Society of America, Washington,

DC, 2001, p. 453.

[5] M. Pansini, Miner. Deposita 31 (1996) 563.

[6] S.M. Robinson, T.E. Kent, W.D. Arnold, in: D.W. Ming,

F.A. Mumpton (Eds.), Natural Zeolites �93, InternationalCommittee on Natural Zeolites, Brockport, NY, 1995, p.

579.

[7] R.A. Sheppard, A.J. Gude Jr., G.M. Edson, in: L.B. Sand,

F.A. Mumpton (Eds.), Natural Zeolites, Occurrence,

Properties, Use, Pergamon Press, Elmsford, NY, p. 319.

[8] E.D. Collins, D.O. Campbell, L.J. King, J.B. Knauer,

AIChE Symp. Ser. 78 (213) (1982) 9.

[9] L.L. Ames Jr., Am. Mineral. 49 (1964) 127.

[10] L.L. Ames Jr., Am. Mineral. 49 (1964) 1099.

[11] R.M. Barrer, J.A. Davies, L.V.C. Rees, J. Inorg. Nucl.

Chem. 31 (1969) 219.

[12] M. Pansini, C. Colella, M. de� Gennaro, Desalination 83(1991) 145.

[13] J.J. Perona, AIChE J. 39 (1993) 1716.

[14] C. Colella, M. de� Gennaro, A. Langella, M. Pansini, in:D.W. Ming, F.A. Mumpton (Eds.), Natural Zeolites �93,International Committee on Natural Zeolites, Brockport,

NY, 1995, p. 377.

[15] D. Caputo, R. Dattilo, M. Pansini, in: R. Aiello (Ed.),

Proceedings of III Convegno Naz. Scienza e Tecnologia

delle Zeoliti, Associazione Italiana Zeoliti, Napoli, 1995, p.

143.

[16] A. Dyer, M. Zubair, Micropor. Mesopor. Mater. 22 (1998)

135.

[17] E. Torracca, P. Galli, M. Pansini, C. Colella, Micropor.

Mesopor. Mater. 20 (1998) 119.

[18] C. Colella, M. de� Gennaro, A. Langella, M. Pansini, Sep.Sci. Technol. 33 (1998) 467.

[19] F. Aumento, Can. Min. 8 (1964) 132.

[20] C. Colella, E. Torracca, A. Colella, B. de Gennaro, D.

Caputo, M. de� Gennaro, in: A. Galarneau, F. Di Renzo,F. Fajula, J. Vedrine (Eds.), Zeolites and Mesoporous

Materials at the Dawn of the 21st Century, Studies in

Surface Science and Catalysis, vol. 135, Elsevier, Amster-

dam, 2001, paper 01-O-05 (CD ROM).

[21] A. Colella, B. de Gennaro, in: A. Gamba, C. Colella, S.

Coluccia (Eds.), Oxide-based Systems at the Crossroads of

Chemistry, Studies in Surface Science and Catalysis, vol.

140, Elsevier, Amsterdam, 2001, p. 153.

[22] M. de� Gennaro, E. Franco, L�Ind. Miner. (Rome) 30(1979) 329.

[23] M. de� Gennaro, A. Langella, Miner. Deposita 31 (1996)452.

[24] S.J. Chipera, D.L. Bish, Powder Diffr. 10 (1995) 47.

[25] F.H. Chung, J. Appl. Cryst. 7 (1974) 519.

[26] M. Pansini, C. Colella, D. Caputo, M. de� Gennaro, A.Langella, Micropor. Mater. 5 (1996) 357.

[27] P. Fletcher, R.P. Townsend, J. Chem. Soc., Faraday Trans.

77 (1981) 497.

[28] G. Schwarzenbach, H. Flaschka, Complexometric Titra-

tion, Methuen, London, 1969, pp. 181–182, 244–245.

[29] L. Ciavatta, Ann. Chim. (Rome) 70 (1980) 551.

[30] E. Glueckauf, Nature 163 (1949) 414.

[31] L.G. Gaines, H.C. Thomas, J. Chem. Phys. 21 (1953) 714.

[32] M. Adabbo, D. Caputo, B. de Gennaro, M. Pansini, C.

Colella, Micropor. Mesopor. Mater. 28 (1999) 315.

[33] C. Colella, M. de� Gennaro, E. Franco, R. Aiello, Rend.Soc. Ital. Min. Petr. (Milan) 38 (1982–83) 1423.

[34] G. Eisenman, Biophys. J. 2 (1962) 259.

[35] H.S. Sherry, in: J.A. Marinsky (Ed.), Ion Exchange: A

Series of Advances, vol. 2, Marcel Dekker, New York,

1962, p. 89.

[36] M. Calligaris, G. Nardin, L. Randaccio, P. Comin

Chiaromonti, Acta Cryst. B 38 (1982) 602.

[37] A. Alberti, E. Galli, G. Vezzalini, E. Passaglia, P.F.

Zanazzi, Zeolites 2 (1983) 303.

[38] M. Calligaris, G. Nardin, Zeolites 2 (1982) 200.