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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.
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