Photodegradation of methyl orange catalyzed by nanoscale zerovalent iron particles supported on natural zeolite

Article abstract of DOI:10.1007/s13738-012-0181-5

A nanoscale catalyst Fe⁰(FeNPs) supported on the natrolite zeolite nanoparticles (NANPs) is successfully synthesized and characterized by FT-IR, X-ray diffraction (XRD) and scanning electron microscopy (SEM) and thermogravimetric-differential thermal analysis (TG-DTA). The photodegradation of methyl orange (MO) is studied in aqueous suspension containing the catalyst under UV irradiation and H₂O₂. The effect of various reaction parameters such as initial dye concentration, irradiation time, pH, H₂O₂ concentration and catalyst dosage on the decolorization of methyl orange is investigated. The degradation study reveals that the reactivity of the catalysts is in order of: photo-NANPs-FeNPs-H ₂O₂ > photo-NANPs-H₂O₂ > photo-NANPs-FeNPs > photo-H₂O₂ > NANPs-FeNPs-H ₂O₂. The results show that methyl orange can be effectively decolorized by NANPs-FeNPs via the pseudo-first-order kinetic model.

Full text of DOI:10.1007/s13738-012-0181-5

J IRAN CHEM SOC (2013) 10:471–479
DOI 10.1007/s13738-012-0181-5
ORIGINAL PAPER
Photodegradation of methyl orange catalyzed by nanoscale
zerovalent iron particles supported on natural zeolite
Hamidreza Naderpour
Mozhgan Khorasani-Motlagh
•
Meissam Noroozifar
•
Received: 17 July 2012 / Accepted: 11 October 2012 / Published online: 7 November 2012
Ó Iranian Chemical Society 2012
0
Abstract A nanoscale catalyst Fe (FeNPs) supported on
the natrolite zeolite nanoparticles (NANPs) is successfully
synthesized and characterized by FT-IR, X-ray diffraction
largest groups of colorants and over 50 % of all the dyes
used in the industry such as textile, printing, tannery, paper
manufacture and photography. Some of these dyes have
documented health and environment hazards [2]. Dyes are
an important class of synthetic organic compounds. Due to
the stability of modern dyes, conventional biological
treatment methods for industrial wastewater are ineffective
resulting often in an intensively colored discharge from the
treatment facilities.
(
XRD) and scanning electron microscopy (SEM) and
thermogravimetric-differential thermal analysis (TG-DTA).
The photodegradation of methyl orange (MO) is studied in
aqueous suspension containing the catalyst under UV
irradiation and H O . The effect of various reaction
2
2
parameters such as initial dye concentration, irradiation
time, pH, H O concentration and catalyst dosage on the
Numerous methods are presently available for decolor-
ization of azo dye effluents from various industries [3].
Some of these methods are: chemical coagulation, air flo-
tation, photolysis, chemical oxidation and reduction, elec-
trochemical treatment and adsorption methods [4]. But
these methods are not suitable due to secondary pollution
in other media, high cost, low efficiency and inapplicability
to a wide variety of dyes. Heterogeneous photocatalysis are
a promising technology for the reduction of global envi-
ronmental pollutants [5]. Recently, heterogeneous catalyst
2
2
decolorization of methyl orange is investigated. The deg-
radation study reveals that the reactivity of the catalysts is
in order of: photo-NANPs–FeNPs–H O [ photo-NANPs–
2
2
H O [ photo-NANPs–FeNPs [ photo-H O [ NANPs–
FeNPs–H O . The results show that methyl orange can be
2
2
2 2
2
2
effectively decolorized by NANPs–FeNPs via the pseudo-
first-order kinetic model.
Keywords Zerovalent iron ꢀ Zeolite ꢀ Azo dye ꢀ
Nanoparticles ꢀ Photodegradation
such as supported titanium (IV) oxide [6–8], Ag/TiO [9],
2
nanoscale zerovalent iron particles [10], Bi TaO I [11],
4
8
Au/TiO [12], C N /Bi WO [13] and nanosized magnetic
6
2
3
4
2
Introduction
[14] composite and radiation have been used for rapid and
easy decolorization of these dyes.
Today textile industry utilizes approximately about 10,000
different dyes and pigments and over 0.7 million tones of
dyes are produced annually worldwide [1]. The removal of
the non-biodegradable organic chemicals is a crucial eco-
logical problem. Among these, azo dyes are one of the
Zeolites are microporous aluminosilicates characterized
by high specific surface area and high cation exchange
capacities. Zeolites are of great interest to material science
because of their many valuable properties such as sorption
ability, catalytic, ion-exchange and atom/molecule trapping
capabilities. One of the most important applications of
natural and synthetic zeolites are adsorption of organic
compounds and dyes. For example, Xu and Langford have
H. Naderpour ꢀ M. Noroozifar (&) ꢀ M. Khorasani-Motlagh
Department of Chemistry, University of Sistan and Baluchestan,
P.O. Box 98155-147, Zahedan, Iran
utilized zeolites as supports for TiO , and it was observed
2
that for the degradation of acetophenone or 4-chlorophenol
in aqueous medium, supported titania on a hydrophobic
e-mail: mnoroozifar@chem.usb.ac.ir
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J IRAN CHEM SOC (2013) 10:471–479
-
1
molecular sieve ZSM-5 shows high photoactivity even at
low Ti loading [7, 8].
agitated for 48 h in a 1 mol L NaCl solution at 25 °C,
washed with doubly distilled water (DDW) until there was
Here, we report the synthesis and characterization of
0
new nanoscale catalyst Fe (FeNPs) supported on natural
no traces of free chloride ions in the filtrate (AgNO test)
3
and dried at 105 °C to constant weight. The natrolite–Fe
system was synthesized by following method. The DDW
was purged with highly pure argon for 2 h before use to
remove oxygen. The modified zeolite system was prepared
by mixing 20.0 g of Na-form of zeolite and 100 mL of
zeolite nanoparticles (NANPs). NANPs–FeNPs has decol-
orizing ability for methyl orange as model of azo dyes from
aqueous solution. Also, effects of different parameters,
such as initial dye concentration, irradiation time, pH,
H O concentration and catalyst dosage on the dye photo-
freshly prepared 1 M Fe(NO ) solution. The mixture was
3 3
2
2
oxidation rate were investigated.
agitated for 60 h at 25 °C, washed until there was no traces
3
?
of free Fe ions (KSCN test) and dried at 90 °C to con-
stant weight (the result particles denoted as NA–Fe). The
preparation of iron nanoparticles (FeNPs) was carried out
by mixing 10.0 g NA–Fe in 50 mL DDW with 1.05 g of
NaBH4 in a three-necked round-bottomed flask under
argon atmosphere at 30 °C. After 1 h of stirring with a
magnetic stirrer, a light-brown precipitate was obtained.
The reaction was as follows:
Experimental
Chemicals
All reagents were prepared from analytical reagent grade
chemicals and purchased from Merck Company. All
aqueous solutions were prepared with double-distilled
water (DDW). A suitable volume of acid (HCl) or base
2
FeCl + 6NaBH + 18H O ! 2Fe + 21H "
3
4
2
2
þ 6B(OH) + 6NaCl
ð1Þ
(
NaOH) solutions were added to adjust pH that was mea-
3
sured with pH meter. Natural Iranian natrolite zeolite from
the Hormak area (Zahedan city, Sistan and Baluchestan
province, Iran) was used in this work [15, 16].
The solid turned dark brown as the reaction proceeded, and
finally a black solid was obtained. Over an extended period
of aging, the product particles were separated from the
solution by centrifugation, washed five times with deion-
ized water, and then washed with acetone. The result par-
ticles denoted as NANPs–FeNPs.
Instruments
The Fourier-transformed infrared (FT-IR) spectra were
measured in transmission mode using Valor III (JASCO)
equipped with a MCT detector. The energy resolution was set
Experimental procedure
-
1
to 1 cm . X-ray diffraction data were obtained using a
Phillips XPERT diffractometer and was performed with
computer via the Apdw Phillips software’s. Cu Ka emission
was used with a Ni filter. The diffractions were monitored in
the 2h range of 2–60° (h: Bragg angle). Scanning electron
microscope image was taken using a Philips XL30 scanning
electron microscopy. In order to prepare zeolite nanoparticles
from natural natrolite, a planetary ball mill (PM100,
RETSCH Co.) was used. In dry milling stage, 50 g of zeolite
was milled for 10 min at 500 rpm, and for wet milling, the
resultant powder of the dry milling was mixed with 50 mL of
water and milled for 3 h at 450 rpm [17]. Thermogravi-
metric-differential thermal analysis (TG-DTA) was per-
formed using STA 1500? Rheometric Scientific (England).
In order to establish the optimum working configurations,
key parameters that influence the performance of the pro-
posed method such as initial dye concentrations, irradiation
time, pH, H O concentration and the catalyst dosage were
2
2
studied. Pyrex glass reactor (size, 150 9 75 mm; capacity,
2
L; surface area, 176 cm ) was directly exposed to the
1
light source in open air condition. An 80-W high-pressure
mercury lamp fixed at a distance of 10 cm above the
solution’s surface was used as the light source. The entire
photo reactor system was maintained at room temperature
(
25 °C). The pH of the solution was adjusted either by
adding dilute NaOH or dilute H SO . The samples were
2
4
taken at different time intervals and then separated by
centrifugation at 5,000 rpm for 30 min to get the super-
natant and the sediment for analysis. The concentrations of
MO samples were quantified by measuring the absorption
intensity at kmax = 510 nm. If needed, sample solution was
diluted five times before measurement. The decolorization
efficiency of MO was defined as follows:
-1
Theflowrateofairwas120 ml min andthe rampingrateof
-1
sample was 2 °C min . UV–Vis spectrawererecorded on an
analytikjena SPECORD S100 spectrometer with photodiode
array detector.
Preparation of natrolite–Fe nanoparticles
ðC0 ꢁ CtÞ
Na-form of the zeolitic material was prepared prior to
modification. For this purpose the zeolitic material was
Decolorization efficiency ¼
ꢂ 100 %
ð2Þ
C0
1
23

J IRAN CHEM SOC (2013) 10:471–479
473
-
zeolitic water (3,619; 3,440 and 1,640 cm ) and pseudo-
1
where C is the initial concentration of MO, and C is the
0
t
-
lattice vibrations (800–500 cm ).
1
concentration of MO at reaction time t (min).
The experiments were conducted thrice in identical
conditions and difference in the error between consecutive
experiments was less than 3 %.
Camera picture and SEM characterization surface mor-
phology of the zeolite samples are present in Fig. 1a, b,
respectively. Typical SEM micrographs of the ground
natural zeolite nanoparticles (NANPs), in which as a result
of ball milling process in two magnifications are presented
in Fig. 1c, d. Figure 1e shows the SEM images of prepared
NANPs–FeNPs. The SEM images of the NANPs and
NANPs–FeNPs show the uniform shape of the zeolite
granules with particles size smaller than 100 nm.
Results and discussion
FT-IR, SEM, XRD, and TG-DTA characterizations
Natrolite is a tectosilicate mineral species belonging to the
zeolite group which is found in the volcanic rocks (basalt
and phonolite). It is a hydrated sodium and aluminium
silicate with the formula Na Al Si O ꢀ2H O and white
Figure 2a, b show the XRD patterns of the NANPs and
NANPs–FeNPs prepared by the chemical reductive reaction
between sodium borohydride and iron chloride at 30 °C.
X-ray diffraction pattern of the zeolite reveals a natrolite
pattern using Apdw software. Compared with XRD spectrum
of NANPs, the XRD spectrum of NANPs–FeNPsshowed one
new broad diffraction peak at 44.6°, which are due to the
(110) plane of iron particles [19]. Also, Fig. 2b showed that
the peaks heights (intensity) of NANPs in NANPs–FeNPS
decreased. It is speculated the FeNPs attached to NANPs.
TG-DTA data of the NANPs is presence in Fig. 2c. Gradual
weight decrease is observed between 100 and 300 °C due to
desorption of surface water and then an abrupt weight
decrease between 300 and 400 °C is associated with exo-
thermic peak due to the desorption of lattice water.
2
2
3
10
2
color (or colorless) [18]. Larger crystals most commonly
have the form of a square prism terminated by a low pyr-
amid, the prism angle being nearly a right angle. The
crystals are tetragonal in appearance, though actually
orthorhombic. There are perfect cleavages parallel to the
faces of the prism. The mineral also often occurs in com-
pact fibrous aggregates, the fibers having a divergent or
radial arrangement. Natrolite is readily distinguished from
other fibrous zeolites by its optical characteristics.
The FT-IR spectrum of NANPs contains bands due to
-
Si–O–Si and Si–O–Al vibrations (1,200–950 cm ), the
1
Fig. 1 a Camera picture of natural parent natrolite zeolite, b SEM image of the NA, c SEM image of the NANPs, d part figure, c with different
magnification and e SEM image of the NANPs–FeNPs
123

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J IRAN CHEM SOC (2013) 10:471–479
Fig. 2 X-ray diffraction
pattern of a NANPs, b NANPs–
FeNPs and c TG–DTA data
measured of the NANPs–FeNPs
Decolorization of MO in different conditions
and in presence of NANPs–FeNPs and H O , no appre-
2
2
ciable photobleaching (about 19.7 %) was observed during
the 60 min. Under UV illumination in presence of H O , a
Figure 3 illustrates the typical UV–Vis spectra of MO during
decolorization process using UV radiation in the presence of
H O and NANPs–FeNPS. The strong absorbance band at
2
2
certain decrease of MO takes place, e.g., 37.9 % of the
initial substrate disappeared after 75 min of continuous
stirring due to the adsorption phenomena. Over this time,
no appreciable change [abatement of 38.0 % after 240 min
(6 h)] was detected. The difference in the two degradation
rates might be related to the dissimilarity in the experi-
mental conditions, mainly the UV radiation intensity. It has
been reported that the presence of H O in solutions
2
2
5
10 nm is due to conjugated structure formed by the azo
bond under the strong influence of the electron-donating
dimethylamino group [20]. For MO, the strong absorbance
band at 510 nm was ascribed to the n–p* transition of the azo
bond. After 5 min of the reaction, the band at 510 nm became
weaker. The spectra revealed the cleavage of the azo bond
and the formation of the products after 25 min; the photo-
catalytic reaction process was summarized in Scheme 1.
As the photocatalytic mechanism suggests, NANPs–
FeNPs, H O and a light source are necessary for the
2
2
exposed to UV radiation, the generation of hydroxyl radi-
cals OHꢀ according to the Eq. (3) [21]:
UV
H2O2 ꢁ! 2 OH
ꢀ
2
2
ð3Þ
photo-oxidation reaction to occur. A control experiment
was conducted on five different conditions: (a) with
NANPs–FeNPs and H O , (b) under UV illumination in
Therefore, it is believed that in this case, ꢀOH radicals
were generated by the photolysis of H O molecules and,
2 2
2
2
presence of H O , (c) under UV illumination in presence of
2
consequently, these radicals oxidized the dye molecules
leading to intermediate compounds and the complete
decolorization of the solution.
2
NANPs–FeNPs, (d) under UV illumination in presence of
NANPs and H O and (e) under UV illumination in pres-
2
2
ence of NANPs–FeNPs and H O . In Fig. 4, the normal-
2
When the dye with NANPs–FeNPs in aqueous disper-
sion was exposed to UV light, only 40 % of the dye was
decomposed. Under UV illumination in presence of H O
2
ized degradation of MO is plotted as a function of reaction
time. As can be seen in Fig. 4, without UV illumination
2
2
1
23

J IRAN CHEM SOC (2013) 10:471–479
475
Methyl Orange (MO)
N=N
(
CH ) N
SO Na
3
3
2
2 .
OH
(
CH ) N
OH
+
HO
SO Na
3
3
2
+
NH4+
+
NO3
-
Scheme 1 The proposed photocatalytic reaction process
Fig. 3 UV–Vis spectra of MO during the decolorization process:
-
5
-1
initial MO concentration = 5.5 9 10 mol L , NANPs–FeNPs
-
1
-2
-1
dosage = 0.1 g L ; [H
temperature = 25 °C
2 2
O ]0 = 4.5 9 10 mol L , initial pH = 2,
Therefore, sulfanilic acid and p-dimethylaminoaniline
investigated as the first possible intermediates. Moreover, the
0
intermediate products of Fe such as Fe , Fe , Fe(OH)y
2?
3?
2-y
and NANPs, a certain decrease of MO takes place, e.g.,
5 % of the initial substrate disappeared after 75 min of
3
x
-x
and Fe(OH) were thermodynamically unstable and active
4
[25]. The formed passive iron oxides layers (Fe O , Fe O ,
3 4 2 3
continuous stirring due to the adsorption phenomena and
over this time, no appreciable change was detected.
Finally, when the dye with NANPs–FeNPs and H O in
Fe(OH) , and FeOOH) [25–27] could also adsorb dye mol-
3
ecules via the sulfonic group and reduce the colority of the
dye wastewater through the formation of a bridged bidentate
complex [28].
2
2
aqueous dispersion was exposed to UV light, the concen-
tration of MeO decrease markedly. About 92 % of the dye
was bleached within 60 min of time experiment, respec-
tively. This indicates that the system is working in a pure
photocatalytic regime. The difference in the two later
cases, degradation rates might be related to the dissimi-
larity in the experimental conditions, mainly the FeNPs.
Based on the previous literatures [22–24], the suggested
degradation pathways of MO through the cleavage of the
azo bond by the reductive processes. It is well known that
Effect of the initial dye concentration
In order to investigate the effect of initial dye concentration
on the dye degradation rate, various MO solutions of
-
5
different concentrations ranging from 1.0 9 10
to
.0 9 10 mol L were prepared and treated by NANPs–
FeNPs, UV and H O while keeping [H O ] constant at
-4
-1
1
2
2
2 2 0
-
2
-1
0
4.5 9 10 mol L . Figure 5a shows the dye concentra-
tion ratio C variation with time for the three studied initial
the reduction of azo dyes using Fe occurs on the surface of
metal iron:
Ci
SO Na
3
NH2
SO Na
3
N
+
+ 2Fe2+
+
2Fe + 4H
+
N
NH2
N(CH3)2
N(CH3)2
123

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J IRAN CHEM SOC (2013) 10:471–479
1
00
dosages was shown in Fig. 5b. As the reaction occurred at
0
the Fe –H O interface, the more the NZVI loading, more is
2
the iron surface area and the adsorptive and active sites. It
can be seen from Fig. 5b that at the same reaction time,
higher decolorization efficiency was obtained under higher
NANPs–FeNPs loading. For example, within 60-min
reaction, the decolorization efficiency was found to be
8
6
4
2
0
0
0
0
0
47.3, 80.6 and 91.1 %, respectively, for the NANPs–FeNPs
dosages of 0.05, 0.1 and 0.2 g. However, it is not necessary
to use a much higher iron dosage because the difference in
the decolorization efficiency was rather small as the iron
dosage increased to a certain degree.
0
50
100
150
200
250
300
Time (min)
Effect of initial pH
Fig. 4 Time-course of MO under different conditions: (1) NANPs–
FeNPs and H (filled square), (2) H , UV (filled circle), (3)
NANPs–FeNPs, UV (filled square), (4) NANPs, H and UV (filled
diamond), and (5) NANPs–FeNPs, H and UV (filled triangle).
2
O
2
2
O
2
Solution pH has been considered as one of the important
0
factors in Fe -contaminant systems [29]. The decoloriza-
O
2 2
2 2
O
-5
-1
tion of MO was carried out at different initial pH of the
solution ranging from 1.0 to 12. The effect of pH on degree
of decolorization is presented in Fig. 6a. It was found that
with increasing pH, the degree of decolorization of MO
takes place, e.g., \45 % above pH 5. This may be due to
the fact that H O decomposes in basic medium to
Initial MO concentration = 1.0 9 10 mol L , zeolite dosage =
-
2
-1
0
.1 g, [H
2 2
O ]0 = 4.5 9 10 mol L , pH = 2, agitation speed =
1
00 rpm and temperature = 25 °C)
dye concentrations. As the initial dye concentration
increased, the time required to achieve a specific conversion
increases as well. As shown in Fig. 5a, at a given experi-
mental condition, decolorization time increased as the initial
MO concentration increased. In general, the reaction pro-
ceeded rapidly within the first 50 min, and then slowed down.
The possible reason is that at the early stage, dye molecules
2
2
molecular oxygen and water and hence, loses its oxidizing
ability. In addition, at higher pH, the catalyst surface
becomes negatively charged and adsorption rate of MO
-
(having SO3 group) is very slow on the catalyst surface
for successive decolorization. For the catalytic system the
optimum pH was found to be 1–3 for maximum decolor-
ization (C90 %) of MO within a period of 40 min. The
need for lower pH may be due to the formation of more
oxidant, i.e., the hydroxyl radical. At lower pH, MO prefers
the quinoid structure which undergoes degradation by ꢀOH
and ꢀOOH radicals easily than the azo structure [30]. Again
also at lower pH the catalyst surface is positively charged
which facilitates the adsorption of the negatively charged
dye and hence enhancing the decolorization [31].
0
could easily transport to Fe surface because of the strong
adsorption and reduction ability of NANPs–FeNPs.
Effect of the catalyst dosage
To evaluate the effect of the NANPs–FeNPs dosage on the
decolorization efficiency, MO concentration was fixed
-
at 5.5 9 10 mol L . In the reaction process, the vari-
5
-1
ation of MO concentration with different NANPs–FeNPs
1
1
00
80
60
40
20
0
0
0
0
0
.8
.6
.4
.2
0
a
300
b
0
50
100
150
200
250
0
50
100
150
200
250
Time (min)
Time (min)
-
Fig. 5 a Effect of the initial MO concentrations 1.0 9 10 (filled
5
NANPs–FeNPs dosage 0.05 (filled triangle), 0.1 (filled square) and
0.2 g (filled diamond) on decolorization efficiency of MO. [MO] =
1.5 9 10 M, pH = 2, temperature = 25 °C and agitation speed =
100 rpm
-4
triangle), 5.5 9 10 - 5 (filled square) and 1.0 9 10
(filled
on decolorization efficiency of MO: NANPs–
FeNPs dosage, 0.1 g, pH = 2, temperature = 25 °C, [MO], [H ]0 =
.5 9 10 mol L , and agitation speed = 100 rpm. b Effect of
-
1
-5
diamond) mol L
O
2 2
-2
-1
4
1
23

J IRAN CHEM SOC (2013) 10:471–479
477
a
100
b 100
80
60
40
20
0
8
6
4
2
0
0
0
0
0
10
20
30
40
50
1
60
70
0
2
4
6
8
10
12
-
2 2
[H O ] mmol L
pH
Fig. 6 a Effect of pH on decolorization efficiency of MO:
=
2 2
100 rpm. b Effect of H O concentration on degree of decolourization
-
MO] = 1.5 9 10 M, NANPs–FeNPs dosage = 0.2 g, [H
5
-5
[
2 2
O ]
0
of MO: [MO] = 1.5 9 10 M, NANPs–FeNPs dosage = 0.2 g,
temperature = 25 °C and agitation speed = 100 rpm
-
2
-1
4
.5 9 10 mol L , temperature = 25 °C and agitation speed =
Effect of H O concentration
2
hydroxyl radical to generate perhydroxy radical which has
lower oxidation potential than the former [32].
2
The effect of H O initial concentration on the dye degra-
2
2
H2O2 þ ꢀOH ! H2O + ꢀOOH
ð4Þ
dation rate was also investigated and the result is shown in
Fig. 6b. The decomposition of MO increases 46–94.5 %
Kinetic study of decolorization
(
Fig. 6b) with increasing initial concentration of H O (5.0–
2 2
-1
2
0.0 mmol L ). This is due to the fact that at higher H O2
2
Pseudo-first-order and pseudo-second-order models were
used to test dynamical experimental data. It is found that
the concentration of the dye rapidly decreases in the first
30 min and subsequently slows down. The rate of degra-
dation of the dye is represented by Eq. (5):
concentration enough hydroxyl radicals are produced leading
to almost complete decolorization. The optimum concentra-
-1
tion of H O of about 20–30 mmol L was obtained for
2
2
efficient decolorization of MO. However, increase of H O
2
2
-1
(
30–60 mmol L ) further results in decrease in degradation
dC
n
ꢁ dt ¼ kC
ð5Þ
process because surplus H O molecules act as scavenger of
2
2
Table 1 Comparison of the proposed method with the different reported methods for degradation of MO
Method
Catalyst
Decolorization Degradation Order
(
Problem
References
%)
time (min)
kinetic
model
Photocatalytic
degradation
Nanocrystal catalyst Ag/ZnO
–
60
–
–
–
[33]
[34]
[35]
Photocatalytic
discoloration
Innovative parylene–TiO
flexible thin film
2
[95
3,600
180–300
Long time
Long time
Long time
6
0
Radiation
Co-c-source irradiation
91–97 %
Pseudo
first
Photocatalytic
Au/TiO
2
[95
75
300
–
–
[36]
[37]
Microbial fuel cell
Rutile-cathode
3,600
Very long time and
low degradation
efficiency
Ozonation in
combination with
ultrasonic irradiation
–
–
–
Pseudo
first
Complex system
[38]
Catalytic
Fenton-like mesoporous
C90
*50
92
20
–
–
–
–
–
[39]
Fe
Palladium/hydroxyapatite/
Fe nanocatalyst
2 3 2
O –SiO composite
Catalytic
200
60
[40]
3 4
O
Photocatalytic
Nanoscale zerovalent iron
particles supported on natural
zeolite
Pseudo
first
This work
123

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J IRAN CHEM SOC (2013) 10:471–479
where C is concentration, k is rate constant, n is order of
reaction and t is time. For first order reaction the semi-log
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At
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of residual color at 507 nm (ln ) versus reaction time (t)
was plotted. Similarly, the 1 versus t was plotted for sec-
At
ond order reaction. A comparison of two kinetic models
indicates that the first order kinetics fits well with the data
3
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2
2
(
R = 0.99) than the second order kinetics (R = 0.84).
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-
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Recycling efficiency of catalyst
(
The efficiency of the nano catalyst was tested by recycling
the catalyst for decolorizing the MO in the optimum con-
ditions as reported. After each run, fresh MO solution and
oxidant is replaced in the reactor using the same zeolitic
catalyst. In the presence of H O and UV irradiation, for
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the first two runs of the experiment, the efficiency of MO
decolorization was higher than 80 %. For the third and
fourth runs, the efficiency of the catalyst decreased to 74
and 62 % and from fifth to sixth run the efficiency were
only 47 and 30 % of initial decolorization. Beyond this
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3
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2
2
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