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Zeolite Synthesis From Pre-treated Coal Fly Ash in Presence of Soil as a Tool for Soil Remediation
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Transcript of Zeolite Synthesis From Pre-treated Coal Fly Ash in Presence of Soil as a Tool for Soil Remediation
7/18/2019 Zeolite Synthesis From Pre-treated Coal Fly Ash in Presence of Soil as a Tool for Soil Remediation
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Zeolite synthesis from pre-treated coal fly ash in
presence of soil as a tool for soil remediation
R. Terzanoa,*, M. Spagnuoloa ,1, L. Medici b,2, F. Tateoc,3, P. Ruggieroa ,1
a Dipartimento di Biologia e Chimica Agroforestale ed Ambientale, Universita degli Studi di Bari, Via Amendola, 165/A, I-70126 Bari, Italy bConsiglio Nazionale delle Ricerche (CNR), Istituto di Metodologie per l’Analisi Ambientale (IMAA),
Contrada S. Loja, I-85050 Tito Scalo (Potenza), ItalycConsiglio Nazionale delle Ricerche (CNR), Istituto di Geoscienze e Georisorse (IGG)-sezione di Padova, c/o Dipartimento di Geologia,
Paleontologia e Geofisica, UniversitaT degli Studi di Padova, Via Giotto, 1, I-35137 Padova, Italy
Received 7 May 2004; received in revised form 13 December 2004; accepted 16 December 2004
Available online 25 January 2005
Abstract
The study reports the synthesis of zeolites from pre-treated coal fly ash in presence of a natural agricultural soil. The
synthetic process of zeolites formation in soil was studied for a period of 6 months at 30 and 60 8C. The synthesis of zeolite P
(zeolite belonging to the Gismondine series) and zeolite X (zeolite belonging to the Faujasite series) was observed for the first
time directly in soil in the presence of organic matter (2.4% C) and several mineral phases. Zeolites were characterized andquantified by means of XRD and SEM-EDX analysis. Moreover, correlations between Si/Al molar ratio in solution, curing
temperature and the type of synthesized zeolite were found. A Si/Al ratio lower than 1 and higher curing temperatures favored
the synthesis of zeolite P, while a Si/Al higher than 1 and lower curing temperatures drove the synthesis preferentially towards
zeolite X. The results obtained in the present research could be useful for a better comprehension and long-term assessment of
the physical and chemical processes which are at the basis of many solidification/stabilization (S/S) soil remediation
technologies for the stabilization of heavy metals or for the catalytic degradation of organic pollutants.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Soil; Fly ash; Zeolites; Soil remediation
1. Introduction
Many soil remediation technologies based on
solidification/stabilization (S/S) principles adopt com-
plex mixtures of inorganic compounds (cement, lime,
sodium silicates, clays, phosphates, metal oxides, coal
fly ash, blast furnace slag, kiln dust, etc.) for the
remediation of soils polluted both by organic and
0169-1317/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.clay.2004.12.006
* Corresponding author. Tel.: +39 80 5442847; fax: +39 80
5442850.
E-mail addresses: [email protected] (R. Terzano)8
[email protected] (M. Spagnuolo)8 [email protected]
(L. Medici)8 [email protected] (F. Tateo)8 [email protected]
(P. Ruggiero).1 Fax: +39 80 5442850.2 Fax: +39 971 427222.3 Fax: +39 49 8272070.
Applied Clay Science 29 (2005) 99–110
www.elsevier.com/locate/clay
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inorganic pollutants (Conner and Hoeffner, 1998).
Alkalizing agents are often added to these mixtures in
order to increase the stabilizing properties of treated
soils in particular toward heavy metals.S/S technologies do not remove heavy metals from
the polluted soil but have the purpose to physically as
well as chemically bfix Q them in a solid matrix in
order to reduce their mobility so as to minimize the
threat to the environment and also to ensure com-
pliance with existing regulatory standards.
Among the constituents of the mixtures employed
in many S/S techniques, coal fly ash is widely adopted
because of its inexpensiveness and its very good
pozzolanic properties (Dermatas and Meng, 2003).
Coal fly ash is a byproduct of coal combustion inthermoelectric power plants and is constituted mainly
by crystalline phases like quartz and mullite as well as
amorphous glass phases aside other minor constitu-
ents such as hematite and magnetite (Mohapatra and
Rao, 2001).
Van Jaarsveld et al. (1997) showed that this waste
material, owing to its large amount of amorphous
aluminosilicates, could readily dissolve in alkali
media and promote the so-called bgeopolymerisation Q
reactions. Moreover, Chang and Shih (1998) demon-
strated that the amount of these highly reactive
amorphous aluminosilicates could be largely
increased if coal fly ash is pre-treated with NaOH at
high temperatures, before its utilization.
Geopolymers can be viewed as the amorphous
equivalent of certain synthetic zeolites and would
have more or less the same chemical composition
although the absence of the distinctive long-range
zeolite structure makes them amorphous to X-rays
(Van Jaarsveld et al., 1997).
During the geopolymerisation process, once the
aluminosilicate powder is mixed with alkaline
solution, a paste forms which quickly transformsinto hard geopolymers. In such a situation, there is
not sufficient time and space for the gel or paste to
grow into a well-crystallized structure such as in the
case of zeolite formation (Xu and Van Deventer,
2000).
Geopolymers can be obtained under alkaline
conditions also from many other different aluminosili-
ceous minerals (e.g. feldspars, kaolinite, illite, etc.; Xu
and Van Deventer, 2000) as well as from any available
source of Si and Al (Xu and Van Deventer, 2002).
It has been shown that geopolymerisation reactions
can lead to the immobilization of toxic metals like Cu
and Pb inside a solid phase. Heavy metals immobi-
lization could proceed through a combination of physical encapsulation and chemical bonding into
the amorphous phase of the geopolymeric matrix (Van
Jaarsveld et al., 1999).
However, amorphous aluminosilicates could, over
time, undergo transformation into crystalline com-
pounds in the same way as it happens for other
amorphous phases such as amorphous alumina
transforming into gibbsite (Martinez and McBride,
2000) or iron hydroxide transforming into goethite
(Bigham et al., 1996) or hematite (Sorensen et al.,
2000). Very often these crystallization processes arelong-term transformation reactions so that they
cannot be observed if the system under investigation
is studied for a limited period of time. Moreover,
thermal treatment can be used to simulate sponta-
neous long-term transformation of the solid phases
(Martinez et al., 2001). The transformation from the
amorphous to the crystalline state may be advanta-
geous because it could increase the stability of the
solid phase (Martinez and McBride, 2000; Sorensen
et al., 2000). Nonetheless it could happen that the
crystalline solids are likely to have a lower capacity
to bind heavy metals (Sorensen et al., 2000). In
contrast, when the fate of heavy metals during the
process of crystallization is followed, a decrease in
the solubility of the metals with aging can be
achieved due to changes in structural location and
chemical form of heavy metals (Martinez and
McBride, 2000).
It is probable that the crystallization of more
loosely packed phases could bring to an improvement
in the toxic metal stabilization process, entrapping
part of them inside stable minerals.
In this sense, the crystallization of minerals withhighly porous structures like zeolites and its environ-
mental consequences deserve attention.
Newton et al. (1999) observed that, following to
the application of a S/S mixture of inorganic
constituents containing coal fly ash (Georeme-
diationk) to a soil polluted by hydrocarbons, a small
amount (not detectable by XRD but only by SEM
analysis) of zeolite crystals (mordenite) was formed.
The occurred synthesis of zeolites from an alumi-
nosiliceous source like coal fly ash (or any other Si,
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Al source), added to a polluted soil in a mixture
together with other constituents for remediation
purposes, could be involved in the stabilization
processes of toxic metals or in the catalytic degrada-tion reactions of organic xenobiotics.
Zeolites are widely employed in environmental
a pplications for the decontamination of polluted soils
(Shanableh and Kharabsheh, 1996; Lin et al., 1998;
Edwards et al., 1999; Cama et al., 2002; Oste et al.,
2002; Coppola et al., 2003) and waters (Pansini et al.,
1991; Ruiz et al., 1997; Moreno et al., 2001; Alvarez-
Ayuso et al., 2003). In particular, due to their high
cation exchange capacity, they are largely used for the
removal of toxic metals (Shih and Chang, 1996;
Curkovic et al., 1997; Chang and Shih, 2000; deGennaro et al., 2003). Moreover, opportunely synthe-
sized zeolites can be used also for the photo-catalytic
degradation of organic pollutants (Calza et al., 2001;
Xamena et al., 2003).
Various zeolites can be synthesized from different
source materials and under different hydrothermal
conditions. Coal fly ash is one of the most used
starting materials due to its inexpensiveness and to the
opportunit y of partly solving the problem of its
disposal (Shigemoto et al., 1993; Berkgaut and
Singer, 1996; Hollman et al., 1999; Poole et al.,
2000; Murayama et al., 2002). Different clay minerals
have been used as a starting material for zeolites
synthesis: kaolinite (Murat et al., 1992; Chandrase-
khar and Pramada, 1999; Lee et al., 2002), montmor-
illonite (Lee et al., 2002), bentonite (Ruiz et al., 1997;
de la Villa et al., 2001; Ramirez et al., 2002),
halloysite (Gualtieri, 2001), interstratified illite–smec-
tite (Baccouche et al., 1998), etc. Some natural
zeolites have been also used to synthesize zeolites
possessing properties better than those of the starting
material (Kang et al., 1998).
The main zeolitic phases usually obtained byhydrothermally treating these materials at temperatures
ranging from 80 to 150 8C are sodalite, hydroxisoda-
lite, zeolite X, P, A and Y. The type of zeolite obtained
depends on many factors like starting material
characteristics, temperature, alkali concentration, reac-
tion time, pressure, and Si/Al molar ratio in the starting
solution (Barth-Wirsching and Holler, 1989). Shih and
Chang (1996) reported the synthesis of zeolite X
(zeolite belonging to the Faujasite series) from coal fly
ash also at low temperature (38 8C) in 5 days.
The purpose of this research is to find the
experimental conditions under which zeolites can be
directly synthesized in soil after the addition of coal
fly ash, even at low temperature (30 8C). Differentlyfrom other works published on the synthesis of
zeolites from coal fly ash or from many different soil
minerals, this study deals with the synthesis of
zeolites from fly ash achieved in the presence of a
natural agricultural soil characterized by a mineralog-
ical complexity and a rather high amount of organic
matter.
2. Materials and methods
2.1. Coal fly ash
Coal fly ash was obtained from the ENEL thermoelectric
power plant of Cerano (Brindisi, Italy). Fly ash chemical
composition (Table 1) was determined by the combined use
of X-ray fluorescence (XRF) analysis (Philips spectrometer
PW2400; powders fused with lithium tetraborate with 1:10
w/w ratio and quantitative determination obtained against
about 30 international geologic standards) and total acidic
dissolution of the samples followed by Inductive Coupled
Plasma-Optical Emission Spectroscopy (ICP-OES) analysis
(Thermo Jarrel Ash, Tracescan). Fly ash mean particle size
(Table 1) was determined by laser granulometry (MalvernMastersizer/E). Before its application to the soil, coal fly ash
was fused by mixing with NaOH powder (fly ash/NaOH: 1/
Table 1
Coal fly ash and soil characteristics (on dry weight basis)
Coal fly ash Soil
SiO2a 48.1% SiO2
a 49.7%
TiO2a 1.2% TiO2
a 0.9%
Al2O3
a
24.6% Al2O3
a
19.7%Fe2O3
a 5.4% Fe2O3a 7.0%
MnO b 0.2%
MgO b 2.8% MgO b 1.2%
CaO b 8.1% CaO b 1.6%
Na2O b 2.8% Na2O b 0.6%
K 2O b 0.6% K 2O b 2.7%
SO3a 1.0%
L.O.I. 4.6% L.O.I. 15.8%
Other 0.8% Other 0.6%
Mean particle size: 21 Am Total organic carbon: 2.4%
a Determined by XRF. b Determined by ICP-OES.
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1.2 w/w) and treating the result ing mixture in air at 550 8C
for 1 h (Chang and Shih, 1998).
2.2. Soil
Samples were collected from an agricultural soil in Turi
(Bari, Italy) and sieved at 2 mm. Soil chemical composition
(Table 1), was determined by XRF and ICP-OES analysis as
previously reported for coal fly ash. Soil organic carbon
(Table 1) was determined following the Walkley–Black
procedure ( Nelson and Sommers, 1982).
Soil semiquantitative miner alogical composition of the
mineralogical phases (Table 2) was determined by X-ray
powder diffraction (XRD); data collection was recorded on
a Rigaku MiniFlex diffractometer operating at 30 kV and
15 mA, with a nickel filtered CuK a radiation, variable slit,
2h range 28 –638; different oriented mounts were prepared
and analyzed: air-dried, glycolated at 60 8C, and heated at
375 8C; the amount of the miner alogical phases was
estimated following Barahona (1974). The content of each
phyllosilicate was evaluated measuring the peak areas.
WinFit software (Krumm, 1997) was used for peak
decomposition.
2.3. Soil treatment
The fused fly ash (4.4 g) was grinded and mixed with 20
g of soil (fly ash/soil equal to 1/10 w/w) in closed
polypropylene vessels and 42 mL of deionized water wereadded. Mixtures were stirred for 1 h and then incubated in
an electrical oven at 30 (sample FA30) or 60 8C (sample
FA60) at atmospheric pressure, in two sets of separate
experiments.
In order to compare the effect of the pre-treated coal fly
ash on the synthesis of zeolites in soil with that due only to
the pH increase of the suspensions, 2.4 g of NaOH powder
was mixed with 20 g of soil in closed polypropylene vessels
and 42 mL of deionized water was added. In this way
samples at the same pH (about 13.0) in the absence and in
the presence of pre-treated coal fly ash were obtained.
Mixtures were stirred for 1 h and then incubated in an
electrical oven at 30 (sample NaOH30) or at 60 8C (sample
NaOH60) at atmospheric pressure.
2.4. Samples analysis
Samples of the FA30, FA60, NaOH30 and NaOH60
mixtures were collected at regular intervals (1 h, 24 h, 1
week, 1 month, 3 months and 6 months) and centrifuged at
20,600 g for 10 min.
Supernatants were separated and analysed by ICP-OES
to determine silicon and aluminium concentrations in soil
solutions. Each experiment was conducted in triplicate.
Pellets were washed three times with deionized water
followed by centrifugation and dried at 80 8C for 12 h. Themineralogical phases in the dried solids were identified by
XRD and Scanning Electron Microscopy-Energy Dispersive
X-ray analysis (SEM-EDX; LEO Stereoscan 440).
XRD patterns were collected using a Rigaku D-max
Rapid micro-diffractometer operating at 40 kV and 30 mA
with CuK a radiation and flat graphite monochromator.
Zeolites quantification was obtained by Rietveld refinement,
using EXPGUI software (Toby, 2001), after X-ray powder
diffraction collection with corundum NIST 676 as internal
standard.
Subtractions of XRD spectra were obtained from the
original raw files and then converted for a 2h8 presentation.
3. Results
As shown in Fig. 1 after only 1 week of incubation
at 60 8C and atmospheric pressure, XRD analysis of
the soil treated with pre-treated coal fly ash (FA60)
revealed the occurred synthesis of zeolite P (zeolite
belonging to the Gismondine series) and zeolite X
(zeolite belonging to the Faujasite series). The
amount of the two synthesized zeolites increased
with the incubation time as can be clearly seen inFigs. 1 and 8.
The occurred ex novo synthesis of these two
zeolites becomes more evident by subtracting the
XRD spectrum of the soil treated for 1 h at 60 8C from
that obtained after 1 month of curing at 60 8C. In this
spectrum only the characteristic XRD peaks of zeolite
P and zeolite X are clearly visible (Fig. 2).
In addition, SEM micrographs of the treated soil
confirmed, after 1 month of curing at 60 8C, the
occurred synthesis of zeolite P, whose typical spher-
Table 2
Soil mineralogical composition (percent dry weight)
Illite/Smectite 37%
Illite 29%
Kaolinite 9%
Chlorite/Smectite 8%
Quartz 9%
K-Feldspar 5%
Plagioclase 2%
Hematite 1%
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ical granular clusters are evident in Fig. 3a, and zeolite
X, whose characteristic octahedral forms are visible in
Fig. 3 b.
These zeolites, aside Na, Si and Al, contained also
other major elements such as Ca, K and Fe as can be
seen from the EDX spectra in Fig. 3a and b.
Zeolite synthesis was also obtained when the soil
was treated only with NaOH and cured at 60 8C
(NaOH60). However, in this case, XRD analysis
revealed only the synthesis of zeolite P whereas the
synthesis of zeolite X was not observed (Fig. 4). The
amount of the synthesized zeolite P increased with the
incubation time (Figs. 4 and 8).
Fig. 8 reports the total amount of zeolites formed by treating the soil with pre-treated coal fly ash or
with NaOH. After 6 months of incubation at 60 8C,
the amount of zeolites obtained in the presence of pre-
treated coal fly ash accounted to about 12% of the soil
total dry weight and was 2 times the amount obtained
by treating the soil with NaOH alone. It seems that the
presence of coal fly ash stimulated the synthesis of
zeolite X since the amount of zeolite P obtained in the
presence and absence of coal fly ash is almost
equivalent. It is worth of notice that the amount of
pre-treated coal fly ash added to soil at the beginningof the experiments was 10% of the soil total dry
weight. Therefore, almost all the fly ash added should
have been converted into zeolite.
XRD analysis revealed appreciable zeolite forma-
tion also after 3 months of incubation of the soil cured
with pre-treated coal fly ash at 30 8C (FA30). Under
these conditions only zeolite X was detected (Fig. 5).
The XRD pattern of t he ex novo synthesized zeolite X
is clearly visible in Fig. 6 where the XRD spectrum
obtained by subtracting the XRD spectrum of the soil
treated for 1 h with pre-treated fly ash at 30 8C from
that recorded after 3 months of incubation is reported.
Under these experimental conditions zeolite syn-
thesis was also observed after 3 months if the soil was
treated with NaOH alone, in order to reach pH 13, and
10 20 30 40
Cu 2Θo
X
X
X
X
X
X
X
X
X
X
P
P
P
P
P
Fig. 2. XRD patterns of zeolite X (X) and zeolite P (P) obtained by
subtracting the XRD spectrum of the soil treated for 1 h at 60 8C
with pre-treated coal fly ash from the one recorded after 1 month of
curing at 60 8C (FA60, fly ash/soil: 1/10 w/w).
10 20 30 40
1 hour
Cu 2Θo
Qz
Qz
XX X
XX
X
X
X XX
P P
P
P
P
1 week
1 month
3 months
6 months
Ph
Fig. 1. XRD patterns of an agricultural soil mixed with pre-treated
coal fly ash and incubated at 60 8C (FA60, fly ash/soil: 1/10 w/w).
X: zeolite X; P: zeolite P; Qz: quartz; Ph: phyllosilicates.
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cured at 30 8C (NaOH30; Fig. 7). However, in
contrast with the result obtained treating the soil with
NaOH and incubated at 60 8C, where only zeolite P
was obtained, under these conditions (30 8C) thesynthesis of both zeolite X and zeolite P was
observed. The total amount of zeolites formed by
treating the soil with pre-treated fly ash is, after 6
months of incubation at 30 8C, more than 5% of the
soil total dry weight and, in the same way, as it was
observed for the samples treated at 60 8C, about 2
times the amount obtained by treating the soil with
NaOH alone (Fig. 8).
Si/Al molar ratio in solution is an extremely
important factor in zeolite synthesis (Barth-Wirsching
and Holler, 1989) together with other parameters such
as starting material composition, temperature, alkali
concentrat ion, reaction time and pressure.
Fig. 9 shows the differences in the Si/Al molar ratio in solution, as a function of the incubation time,
between the soil treated with pre-treated coal fly ash
and the soil treated simply with NaOH, both at 30 8C
10 20 30 40
1 hour
Cu 2Θo
Ph
Qz
Qz
P
PP
PP
1 week
1 month
3 months
6 months
Fig. 4. XRD patterns of an agricultural soil mixed with NaOH (pH
13) and incubated at 60 8C (NaOH60). P: zeolite P; Qz: quartz; Ph:
phyllosilicates.
Fig. 3. SEM micrographs and EDX spectra of zeolite P (a) and
zeolite X (b) synthesized in soil after 1 month of curing at 60 8C
with pre-treated coal fly ash (FA60, fly ash/soil: 1/10 w/w).
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(Fig. 9a) and 60 8C (Fig. 9 b). If the period before the
beginning of zeolite crystallization is considered (Fig.
9a and b, expanded regions), Si/Al ratio in the
samples treated with NaOH alone was always lower
than that observed in the samples cured in the
presence of pre-treated coal fly ash.
4. Discussion and conclusions
The differences in the observed synthesis of
zeolites in the four samples (FA30, FA60, NaOH30
and NaOH60) could be explained if the Si/Al molar
ratio in the soil solution, which is dependent on
starting material and pH of the solution, and the
temperature at which the experiments were carried
out, are considered (Fig. 10).
10 20 30 40
1 hour
Cu 2Θo
Ph
Qz
Qz
XX XP P
P
PP
X X
X
X
X
X
3 months
6 months
Fig. 7. XRD patterns of an agricultural soil mixed with NaOH (pH
13) and incubated at 30 8C (NaOH30). X: zeolite X; P: zeolite P;
Qz: quartz; Ph: phyllosilicates.
10 20 30 40
Cu 2Θo
X
XX
X
X
XX
X
XX
Fig. 6. XRD patterns of zeolite X (X) obtained by subtracting the
XRD spectrum of the soil treated for 1 h at 30 8C with pre-treated
coal fly ash from the one recorded after 3 months of curing at 30 8C
(FA30, fly ash/soil: 1/10 w/w).
10 20 30 40
1 hour
Cu 2Θo
PhQz
Qz
XX
X
X
X
XXXXX
3 months
6 months
Fig. 5. XRD patterns of an agricultural soil mixed with pre-treated
coal fly ash and incubated at 30 8C (FA30, fly ash/soil: 1/10 w/w).
X: zeolite X; Qz: quartz; Ph: phyllosilicates.
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High temperature and low Si concentration in
solution favor the formation of monomer and small
oligomer silicate species such as S4R, which zeolite P
is based on, while low temperature and high Si
concentration favor the formation of larger silicate
species such as D6R and h-cages, which zeolite X is
based on (Kinrade and Swaddle, 1988; Chang and
Shih, 1998). Zeolite P is thermodinamically morestable than zeolite X (Petrovic et al., 1993; Chang and
Shih, 1998). Therefore, higher curing temperatures
allow the more stable state of zeolite P to be reached
while lower curing temperatures may provide a
kinetic path that stabilizes the zeolite X phase.
According to these findings, it can be seen that
when the starting Si/Al molar ratio in solution was
lower than 1 and the curing temperature was 60 8C
(Fig. 9 b, sample NaOH60), only zeolite P synthesis
was observed (Fig. 8, NaOH60). In these conditions
zeolite P synthesis is favored both by temperature and
the low Si content.
On the other side, if the Si/Al molar ratio in
solution was higher than 1 and the curing temperature
was 30 8C (Fig. 9a, sample FA30), only zeolite X
synthesis was observed (Fig. 8, FA30). In these
conditions zeolite X synthesis was favored both by
the low temperature and the high Si content.As long as concerns sample NaOH30, both zeolite
X and zeolite P were formed and the amount of
synthesized zeolite X was bigger than that of zeolite P.
In these conditions zeolite X formation was promoted
by the low temperature (30 8C). However, the low Si
content in solution (Si/Al molar ratio lower than 1,
Fig. 9a) allowed zeolite P also to be synthesized even
if in a lower amount (NaOH30, Fig. 8).
At last, the results obtained for sample FA60 show
that zeolite X formation was favored by the high Si
FA3012
10
8
Z e o l i t e s ( %
)
6
4
2
01 hour 1 week 1 month 3 months6 months
NaOH3012
10
8
Z e o l i t e s ( % )
6
4
2
01 hour 1 week 1 month 3 months 6 months
NaOH6012
10
8
Z e o l i t e s ( % )
6
4
2
01 hour 1 week 1 month 3 months 6 months
FA6012
10
8
Z e o l i t e s ( %
)
6
4
2
01 hour 1 week 1 month 3 months 6 months
Zeolite XZeolite P
Fig. 8. Amount of zeolites, as a percentage of the soil total dry weight, formed after curing the soil with pre-treated coal fly ash at 30 8C (FA30,
fly ash/soil: 1/10 w/w) and 60 8
C (FA60, fly ash/soil: 1/10 w/w) and with NaOH in the absence of coal fly ash at 30 8
C (NaOH30) and 60 8
C(NaOH60), at different incubation times.
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content in solution (Si/Al molar ratio higher than 1,Fig. 9 b), but, since the temperature of incubation was
60 8C, also zeolite P was synthesized and, after 3
months it became the main zeolitic phase formed
(FA60, Fig. 8).
These results suggest that there is a sort of
competition in the zeolite P and zeolite X synthesis
that seems to be regulated, in the experimental
conditions adopted in this work, by both temperature
and Si/Al molar ratio in solution, before the beginning
of zeolite crystallization.
After the nucleation of zeolite X or P (or both),the amount of Al in solution strongly decreased,
since almost all Al was depleted during the synthesis
of zeolites, so that Si/Al molar ratio became
extremely high (Fig. 9a and b). It is well known
(Ginter et al., 1992; Chang and Shih, 1998) that Al
is the controlling species in the synthesis of some
zeolites like, for example, faujasites. In fact, the
formation of Al rich nuclei can explain why Al
species were consumed more rapidly than Si in the
solution.
3
10000
1000
100
S i / A l
10
1
0.1 1 hour 1 week 1 month 3 months 6 months24 hours
2
1
0
1 h 24 h 1 w 1 m 3 m
FA 30
NaOH 30
a
310000
1000
100
S i / A l
10
1
0.11 hour 1 week 1 month 3 months 6 months24 hours
2
1
0
1 h 24 h 1 w
FA 60
NaOH 60
b
Fig. 9. Si/Al molar ratio in solution, as a function of the incubation time, for soil cured at 30 8C (a) with pre-treated coal fly ash (FA30, fly ash/
soil: 1/10 w/w) or NaOH alone (NaOH30) and for soil cured at 60 8C (b) with pre-treated coal fly ash (FA60, fly ash/soil: 1/10 w/w) or NaOH
alone (NaOH60).
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As can be clearly seen from sample NaOH30 and
NaOH60, zeolite synthesis was achieved, both at 30
and 60 8C, also in absence of coal fly ash. Under
these conditions, Si and Al species for zeolite
synthesis were supplied by the partial dissolution
of soil minerals in the alkaline medium. The same
process could be partly involved also in zeolite
synthesis in samples FA30 and FA60 and this could
explain why, after treating the soil with pre-treated
coal fly ash at 60 8C for 6 months, an amount of
zeolites higher than the amount of the initially added
fly ash was obtained.
However, the addition of pre-treated coal fly ash to
soil not only allowed a greater amount of zeolites to
be synthesized if compared to the simple addition of
NaOH but also directed the synthesis toward an
higher amount of zeolite X both at 30 and in particular
at 60 8C (Fig. 8).The addition of pre-treated coal fly ash to soil,
could result in a further source of sodium silicate and
amorphous aluminosilicates, which can be easily
dissolved in the aqueous solution. So, the added
amounts of Si and Al species could change the Si/Al
molar ratio in solution giving rise to a higher yield of
synthesized zeolites and, more significantly, a prefer-
ential path towards zeolite X synthesis.
The induction of this preferential path towards
zeolite X synthesis could be extremely useful
because zeolite X, if compared to zeolite P, possesses
more suitable properties for environmental applica-
tions. It has a higher cation exchange capacity owing
to its lower Si/Al ratio and a larger specific surfacearea. Moreover, zeolite X, having larger pore sizes
associated with the D6R and h-cage unit in its
structure, could be used even to entrap inorganic
pollutants of big dimension like Cs+ (Chang and
Shih, 1998).
In conclusion, the alkaline treatment of an agricul-
tural soil in the presence or absence of pre-treated coal
fly ash led to the synthesis of different types of
zeolites in function of different parameters such as the
temperature and the Si/Al ratio in solution. It should
be pointed out that the synthesis of zeolite was not hindered at all by the presence of organic matter (the
soil carbon content was equal to 2.4%) or by other
mineral phases (smectite, kaolinite, chlorite, illite,
quartz, k-feldspar, plagioclase, hematite). The
amounts of zeolites synthesized in soil after mixing
it with pre-treated fly ash (10% loading) were about
5% and 12% at 30 8C and 60 8C, respectively.
Therefore, the study evidenced the actual possibility
for zeolites to be synthesized directly in situ, even at
low temperature (30 8C). Zeolite crystallization could
contribute to the stabilization processes involved in
solidification/stabilization (S/S) remediation technol-
ogies. It is conceivable that the synthesis of zeolites in
soils polluted by heavy metals could reduce the
availability of the pollutants through their coprecipi-
tation, together with the amorphous aluminosilicates
of pre-treated coal fly ash, and a subsequent immobi-
lization, within the characteristic spatial structure of
zeolites. Finally, this work could be useful as a basic
knowledge for planning new technologies for the on
site remediation of polluted soils.
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