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



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,

R. Terzano et al. / Applied Clay Science 29 (2005) 99–110100



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.

R. Terzano et al. / Applied Clay Science 29 (2005) 99–110 101



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%




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.




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).




(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.




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


FA6012

10

8

Z e o l i t e s ( %

)

6

4

2


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.




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).




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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Zeolite Synthesis From Pre-treated Coal Fly Ash in Presence of Soil as a Tool for Soil Remediation