Efficient removal of atrazine in water with a Fe3O4/MWCNTs nanocomposite as a heterogeneous Fenton-like catalyst

Article abstract of DOI:10.1039/c5ra04249f

Fe₃O₄ and multi-walled carbon nanotube hybrid materials (Fe₃O₄/MWCNTs) were synthesized by a coprecipitation combined hydrothermal method. The nanocomposites were applied for adsorption and degradation of atrazine (ATZ) in the presence of H₂O₂. The obtained catalysts were characterized by TEM, XRD, BET, XPS and Raman spectroscopy. The effects of solution pH, catalysts dosage, H₂O₂ concentration and iron leaching on the degradation of ATZ were investigated. Fe₃O₄/MWCNTs showed a higher utilization efficiency of H₂O₂, higher ability of adsorption for ATZ and higher degradation efficiency of ATZ than Fe₃O₄ nanoparticles in the batch degradation experiment. The degradation efficiency increased with the solution pH decreasing from 8.0 to 3.0. The catalytic results showed that Fe₃O₄/MWCNTs presented good performance for the degradation of ATZ, achieving 81.4% decomposition of ATZ after 120 min at reaction conditions of H₂O₂ concentration 3.0 mmol L^-1, catalysts dosage 0.1 g L^-1, ATZ concentration 10.0 mg L^-1, pH 5.0 and T 30 °C. Three degradation products (desethylatrazine, desisopropylatrazine, and 2-hydroxyatrazine) were detected during a heterogeneous Fenton reaction in solution. The stability, and reusability of Fe₃O₄/MWCNTs for ATZ degradation were also investigated.

Full text of DOI:10.1039/c5ra04249f

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Efficient removal of atrazine in water with a Fe3O4/
MWCNTs nanocomposite as a heterogeneous
Fenton-like catalyst
Cite this: RSC Adv., 2015, 5, 46059
a
a
b
a
*
Lian Yu, Xiaofang Yang, Yushi Ye and Dongsheng Wang
Fe3O4 and multi-walled carbon nanotube hybrid materials (Fe3O4/MWCNTs) were synthesized by a
coprecipitation combined hydrothermal method. The nanocomposites were applied for adsorption and
degradation of atrazine (ATZ) in the presence of H2O2. The obtained catalysts were characterized by
TEM, XRD, BET, XPS and Raman spectroscopy. The effects of solution pH, catalysts dosage, H2O2
concentration and iron leaching on the degradation of ATZ were investigated. Fe3O4/MWCNTs showed a
higher utilization efficiency of H2O2, higher ability of adsorption for ATZ and higher degradation
efficiency of ATZ than Fe3O4 nanoparticles in the batch degradation experiment. The degradation
efficiency increased with the solution pH decreasing from 8.0 to 3.0. The catalytic results showed that
Fe3O4/MWCNTs presented good performance for the degradation of ATZ, achieving 81.4%
decomposition of ATZ after 120 min at reaction conditions of H2O2 concentration 3.0 mmol Lꢀ1
catalysts dosage 0.1 g Lꢀ1, ATZ concentration 10.0 mg Lꢀ1, pH 5.0 and T 30 ꢁC. Three degradation
products (desethylatrazine, desisopropylatrazine, and 2-hydroxyatrazine) were detected during
,
Received 10th March 2015
Accepted 18th May 2015
a
DOI: 10.1039/c5ra04249f
heterogeneous Fenton reaction in solution. The stability, and reusability of Fe3O4/MWCNTs for ATZ
degradation were also investigated.
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banned in the European Union, it is still in use in North
America and China. Thus, it is very important to develop effi-
1. Introduction
cient methods for ATZ removal.
Atrazine (ATZ) (2-chloro-4-(ethylamino)-6-isopropylamino-s-
triazine) is widely used as a herbicide at 70 000–90 000 tons
annually in the world.1 It is extensively used in agriculture to
control broad leaf and grassy weeds in corn, rice, sorghum and
sugarcane elds. Moreover, ATZ is also widely applied to non-
agricultural elds such as lawns and turf. Because of its wide-
spread use, moderate water solubility and high persistence in
water (the half-life of atrazine is as long as about 100 days), ATZ
has been frequently detected in soil and surface waters in North
America and Europe, where it oen exceeds the 10 mg Lꢀ1 level
of concern for aquatic ecosystems.2
ATZ shows slight toxicity to many sh species, and less
toxicity to aquatic invertebrates.2,3 As a widely used herbicide,
ATZ is also highly toxic to algae and aquatic vascular plants.2 As
far as for human health, the main threat related to ATZ expo-
sure is its endocrine disruption capabilities.2,4 Because of its
endocrine disrupting effect, ATZ is included in the list of prior
substances by the European Union.5,6 Although ATZ has been
Traditional physical and chemical methods (coagulation,
adsorption, reverse osmosis, etc.) can generally be used for ATZ
removal. Nevertheless, ATZ is usually non-destructive aer
being treated with these methods, and the post-treatment of the
adsorbent or the solid wastes is necessary and expensive.
Advanced oxidation processes (AOPs) have been regarded as
effective methods to oxidize these compounds, because they
can produce hydroxyl radicals (cOH), a powerful unselective
oxidants (2.8 V vs. NHE (pH 0)), which can mineralize almost
any organic pollutant.7 Among the AOPs technologies, Fenton
reaction (H2O2 + Fe2+/Fe3+) has been proven to be one of the
most effective methods to degrade organic pollutants in
wastewater. Unfortunately, there are two critical drawbacks in
traditional Fenton system: (1) removal of the ferric ions
remaining in the treated water complicates the whole process
and makes the method uneconomic and even leads to
secondary metal ion pollution easily. (2) A traditional homo-
geneous Fenton system works well only under the highly acidic
conditions (pH 2–3).8,9 In order to overcome the disadvantages
of the homogeneous Fenton reaction, the heterogeneous
Fenton-like systems, in which soluble ferric ions are replaced by
Fe-containing solids (e.g., Fe0, Fe3O4, Fe2O3, FeOOH, and so on),
have been recently developed.10,11 Especially, magnetite (Fe3O4)
has been reported as an efficient catalyst for heterogeneous
aState Key Laboratory of Environmental Aquatic Chemistry, Research Center for
Eco-Environmental Sciences, Chinese Academy of Sciences, P.O. Box 2871, Beijing
100085, China. E-mail: wangdongshengrcees@126.com; Fax: +8601062849144; Tel:
+8601062849144
bChangjiang River Scientic Research Institute, Changjiang Water Resources
Commission, Wuhan 430010, China
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Fenton-like process.12–22 Magnetite exhibits several characteris- further purication. FeCl3$6H2O, FeCl2$4H2O, ammonia
tics that are important for the Fenton reaction: (1) it contains solution, and H2O2 were purchased from Beijing Chemical
Fe2+ that might play an important role as an electron donor to Reagents Company. Atrazine (99.2%) and its degradation
initiate the Fenton reaction; (2) the octahedral site in the products desethyl-desisopropyl-2-hydroxy-atrazine (99.6%),
magnetite structure can easily accommodate both Fe2+ and desisopropyl-2-hydroxy-atrazine (DIHA, 95.4%), desethyl-2-
Fe3+, Fe2+ can be reversibly oxidized and reduced in the same hydroxy-atrazine (DEHA, 98.7%), desethyl-desisopropyl-
structure; and (3) Fe3O4 has peroxidase-like activity which can atrazine (DEIA, 98.3%), desethyl-atrazine (DEA, 99.9%),
active H2O2.12 The excellent catalytic activity, biocompatibility, desisopropyl-atrazine (DIA, 96.1%), 2-hydroxy-atrazine
easy preparation and convenient separation from water by (ATZOH, 94.7%) were purchased from Sigma Aldrich. tert-
external magnetic eld, make Fe3O4 a promising catalyst for Butanol (t-BuOH) was purchased from Sigma Aldrich
wastewater treatment.
(>99.5%). All solutions were prepared in ultra-pure water
However, Fe3O4 nanoparticles usually aggregate during the (Milli-Q water, Millipore system). Solutions were subject to a
reaction process, resulting in reduced catalytic activity.15 So, brief period in an ultrasound bath to achieve better
many efforts have been made to improve this situation.23–26 Iron dissolution.
oxides can be immobilized on organic or inorganic supports to
form novel heterogeneous Fenton catalysts. Carbon materials
such as multiwalled carbon nanotubes,25,27 graphene,28,29 and
activated carbon30,31 have attracted great attention because of
their excellent properties, including acid/base resistance and
high thermal stability. Particularly, due to large reactive area,
good dispersion of iron oxides, and high reaction rate, carbon
nanotube-supported iron oxides have attracted much attention
for heterogeneous oxidation of pollutants such as azo dyes and
bisphenol A.27,32 Recently multi-walled carbon nanotube-
supported magnetite (Fe3O4/MWCNT) has been synthesized
and used as the heterogeneous catalyst for Fenton reaction.25,33
Fe3O4/MWCNTs demonstrated high oxidation efficiency of the
contaminants (i.e., 17a-methylestosterone and synthetic dyes)
and can be easily separated from water by external magnetic
eld aer treatment.27,33 Due to the hydrophobic surface,
MWCNTs exhibit strong interactions with organic chemicals.
MWCNTs are an excellent adsorbent for organic contaminants
in water treatment compared with activated carbon and octa-
decyl adsorbent (C18).34 Consequently, MWCNTs are an attrac-
tive and competitive support compared with other materials for
its adsorption property and stability.35 Hu et al.27 prepared the
MWCNTs supported Fe3O4 nanocomposites by in situ growth.
The as-prepared catalyst was used to degrade 17a-methyl-
estosterone with H2O2 by Fenton reaction. In our study, Fe3O4/
MWCNTs catalyst has been prepared by coprecipitation
combined with hydrothermal method. Compared with the
catalysts synthesized by in situ growth, Fe3O4/MWCNTs
prepared by coprecipitation combined with hydrothermal
method showed better stability and crystallinity, the
morphology of Fe3O4 nanoparticles is more uniform and Fe3O4
nanoparticles dispersed well on the surface of MWCNTs. For
2.2 Catalyst synthesis
Purication of MWCNTs. For purication and oxidation the
MWCNTs surface to facilitate a uniform Fe3O4 deposition on
their outer walls, 2.0 g of the pristine MWCNTs were dispersed
in 200 mL 68% HNO3 by exerting ultrasonic dispersion (Kun-
shan, KQ-250DE, 50 kHz, 100 W) and reuxed at 70 ꢁC with
constant stirring for 14 h. Aer cooling to room temperature,
the treated MWCNTs were separated from the black suspension
through a vacuum lter, and then washed with deionized (DI)
water until pH neutral, the obtained MWCNTs were dried under
ꢁ
vacuum at 40 C. The dried MWCNTs were gently milled into
powder for use.
Preparation of Fe3O4/MWCNTs. A mixture containing 0.14 g
FeCl3$6H2O, 0.04 g FeCl2$4H2O, and 0.10 g MWCNTs was
placed in 160 mL deionized water, vigorously stirred at 60 ꢁC in
a three-neck ask under the purge of nitrogen gas. Aer the
addition of 0.2 mL ammonia solution and stirred for 30 min,
the Fe3O4/MWCNTs colloidal solution was formed. The
obtained MWCNTs colloidal solution was then transferred into
a 200 mL Teon-lined autoclave. The autoclave was sealed and
ꢁ
kept at 120 C for 12 h, then cooled naturally to room temper-
ature. The precipitate was separated from the suspension by a
magnet, and then washed with DI water to remove the residual
reagents. Aer repeated washing with DI water and absolute
methanol under ultrasonication for 5 min, the formed Fe3O4/
MWCNTs nanocomposites were dried in a vacuum oven at 60 ꢁC
for 24 h. The cleaned MWCNT/Fe3O4 was used in the subse-
quent characterization and application work.
2.3 Characterization methods
the rst time, the Fe3O4/MWCNTs catalyst had been used in A transmission electron microscope (TEM, JEOL-2010) were
Fenton reaction to adsorb and degrade pesticide atrazine in employed to characterize the morphology of the catalysts and
water.
the distribution of magnetic nanoparticles in MWCNTs. The
crystalline phase of the synthesized samples were determined
by X-ray diffraction (XRD, Phillips PW 1050-3710 Diffractom-
2. Experimental
2.1 Materials
˚
eter) with Cu Ka radiation (l ¼ 1.5406 A). The sample magne-
tization curves were determined using a vibrating sample
The MWCNTs (diameter, 30–50 nm; length, ꢂ20 mm) used magnetometer (VSM, Quantum Design MPMS-5S). The surface
in this work were purchased from Chengdu Institute of properties of the catalysts were measured using nitrogen
Organic Chemicals, Chinese Academy of Science. All the adsorption–desorption experiments at 77 K. The surface area
other chemicals were analytic grade and used without was calculated using the standard Brunauer–Emmett–Teller
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(BET) equation. All of the calculations were automatically per-
formed using an accelerated surface area and porosimeter
system (ASAP 2010, Micromeritics). X-ray photo-electron spec-
troscopy (XPS, Al K-Alpha, Thermo Fisher Scientic) with
monochromatic Al Ka X-ray radiation at 1486.71 eV was used to
identify metal oxidation states of the nanocomposites. Raman
spectra were recorded with a JY-HR800 (Jobin Yvon) spectrom-
eter equipped with a confocal microscope.
3. Results and discussion
3.1 Characterization of catalyst
It can be seen from Fig. 1a that there was a distortion in the
linear structure of MWCNTs, indicating that, in some situa-
tions, the damage extended beyond the outermost graphene
sheet and into the underlying sidewalls. Fig. 1b showed that the
distribution of diameter of the synthesized Fe3O4 nanoparticles
ranging from 10 nm to 30 nm. As can be seen from Fig. 1c, Fe3O4
nanoparticles grew regularly on the surface of MWCNTs with
diameters ranging from 10 to 30 nm. Although the nano-
composites had been washed with water and methanol for
several times, and underwent ultrasonication before TEM
measurement, most of the Fe3O4 nanoparticles were still found
on MWCNTs surface. This reected the strong interaction
between MWCNTs and Fe3O4 nanoparticles. As can be seen in
Fig. 1d, the Fe3O4 nanoparticles in Fe3O4/MWCNTs nano-
composites showed some but not serious agglomeration aer
being reused for three times for oxidation reaction.
Fig. 2 shows the XRD patterns of MWCNTs, Fe3O4 nano-
particles and Fe3O4/MWCNTs nanocomposites. A diffraction
peak at 2q ¼ 25.9ꢁ, which is assigned to MWCNTs,25 can be seen
for treated MWCNTs and Fe3O4/MWCNTs nanocomposites. As
shown in Fig. 2b and c, the characteristic peaks for Fe3O4 (2q ¼
18.3ꢁ, 30.3ꢁ, 35.7ꢁ, 43.5ꢁ, 53.4ꢁ, 57.4ꢁ, 62.8ꢁ marked by their
indices d(111), d(220), d(311), d(400), d(422), d(511) and
d(440)),36 can be observed for the synthesized Fe3O4 nano-
particles and Fe3O4/MWCNTs nanocomposites. As can be seen
from Fig. 2c, the peak at 2q ¼ 25.9ꢁ for Fe3O4/MWCNTs is
smaller than that of MWCNTs, this is because that the
MWCNTs content in Fe3O4/MWCNTs is 50 wt%.
2.4 Degradation of ATZ by heterogeneous Fenton
experiments
All experiments were conducted in the dark in a 500 mL conical
ask (with 200 mL aqueous solution) placed in a thermostated
water bath (TZ-2EH) with an agitation of 150 rpm. The reactions
were initiated by adding a desired dosage of H2O2 to a pH-
adjusted solution (by H2SO4 or NaOH) containing Fe3O4/
MWCNTs and atrazine. The suspension was sampled at pre-
determined time intervals, meanwhile, 2.0 mL t-BuOH was
added into 2.0 mL sample to quench the reaction. The aqueous
phase was sampled for the analysis of pH, the concentrations of
ATZ, H2O2, Fe(II), and total soluble Fe. The solid catalyst sepa-
rated from aqueous phase was rinsed by 5 mL methanol for
three times. The rinsed methanol was mixed for analysis. The
residual ATZ amount is the summation of that in aqueous
phase and solid phase. As in ATZ adsorption experiment, the
concentration of ATZ is just the ATZ remained in aqueous
phase. The reusability of the catalyst was evaluated by washing
the catalyst with methanol and DI water, drying the used cata-
lyst under vacuum, and using it for the next reaction under
similar experimental conditions. Experiments were carried out
at least in duplicate, and all results were expressed as a mean
value. In addition, control experiments and the effects of pH,
initial H2O2 concentration, Fe3O4/MWCNTs loading and dis-
solved iron on ATZ degradation were carried out according to
the same steps as above.
Fig. 3 shows the Raman spectrogram of treated MWCNTs
and Fe3O4/MWCNTs nanocomposites. As can be seen from
2.5 Analytic methods
Concentration of ATZ and its degradation intermediates was
tested by a high-performance liquid chromatography HPLC
equipped with a UV diode array detector (DAD). Separation was
achieved using a Lichrocart C18-RP Purospher Star (250 mm ꢃ
4.6 mm, 5 mm) column and a water/methanol mobile phase. The
mobile phase ow was 1 mL minꢀ1 and the composition was
gradually changed from 50 : 50 to 30 : 70 v/v in 21 min. ATZ
concentration was calibrate and measured at 222 nm and ATZ
intermediates at 215 nm. Total organic carbon (TOC) was
analyzed using a Multi TOC/TN Analyzer (2100, Analytik Jena AG
Corporation). The solution pH was measured by a Thermo
Orion model 8103BN pH-meter. Ferrous ion and total dissolved
iron concentrations were measured according to the 1,10-phe-
nanthroline method,11 using a UV-VIS spectrophotometer
((UVmini-1240, Shimadzu)) at 510 nm. H2O2 concentration was
measured at l ¼ 400 nm with a UV-VIS spectrophotometer
(UVmini-1240, Shimadzu) aer adding titanyl sulfate solution (a
yellow complex is formed).
Fig. 1 TEM images of the treated MWCNTs (a), Fe3O4 (b), Fe3O4/
MWCNTs (c) and reused Fe3O4/MWCNTs (d).
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Fig.
5
Nitrogen adsorption/desorption isotherms for Fe3O4 and
Fig. 2 X-ray diffraction patterns of MWCNTs (a), Fe3O4 (b) and Fe3O4/
MWCNTs (c).
Fe3O4/MWCNTs nanocomposites.
respectively. There are two peaks at binding energies of 710.9
and 725.1 eV for Fe 2p XPS spectrum (Fig. 4b), and these can be
ascribed to Fe 2p1/2 and Fe 2p3/2 respectively. The results are in
accordance with literature for magnetite37 and agreed with the
XRD results.
As can be seen in Fig. 5, N2 adsorption/desorption isotherms
for Fe3O4 nanoparticles and Fe3O4/MWCNTs nanocomposites
displayed type II isotherms. The isotherm of Fe3O4 nano-
particles is below that of Fe3O4/MWCNTs nanocomposites,
indicating lower surface area (67.35 m2 gꢀ1 for Fe3O4 nano-
particles, and 112.81 m2 gꢀ1 for Fe3O4/MWCNTs nano-
composites) and pore volume of the former. This can be
attributed to the porosity of the treated MWCNTs that were used
as support.
Fig. 3 Raman spectra of the treated MWCNTs and Fe3O4/MWCNTs
nanocomposites.
Fig. 3, the crystallinity of MWCNTs in treated MWCNTs and
Fe3O4/MWCNTs is different. Some extra peaks in Fe3O4/
3.2 Atrazine degradation experiments by heterogeneous
Fenton reaction
MWCNTs at lower wavenumbers (271.0, 481.3 and 662.5 cmꢀ1
)
can be attributed to the Fe–O bonds and the Fe–C bonds, con-
rming that Fe3O4 nanoparticles were anchored on the surface
of MWCNTs.27 As is known, G (1571.1 cmꢀ1) band can reect the
purity and regular structure of MWCNTs, while D band (1340.3
cmꢀ1) can reect the defects at the surface of MWCNTs.27 As can
be seen in Fig. 3, the IG/ID ratio of MWCNTs changed from 1.34
to 1.44 aer Fe3O4 loading. The increase of the ratio suggests
that the atomic ordering of the MWCNTs was enhanced and the
structure defects were reduced. The peak centered at 2682.4
cmꢀ1 can be assigned to D* band of MWCNTs.
Fig. 6 shows the ATZ degradation (a) and H2O2 decomposition (b)
with time under different experimental conditions. As shown in
Fig. 6a, the adsorption processes proceed very quickly, and the
equilibrium concentrations are reached in about 60 min for
Fe3O4 and Fe3O4/MWCNTs. The percentages of adsorbed ATZ
were 18.4% and 49.6% respectively for Fe3O4 and Fe3O4/
MWCNTs. As can be seen in Fig. 6a, little ATZ was degraded in
the presence of H2O2 only, this can be ascribed to the low
oxidation potential of H2O2 compared with hydroxyl and perhy-
droxyl radicals. In the presence of H2O2 and Fe3O4/MWCNTs, a
conversion efficiency of 81.4% for ATZ can be achieved aer 120
min reaction. There are two stages in the heterogeneous Fenton
As shown in Fig. 4a, the peaks at binding energy of 285.1,
530.1 and 711.1 eV can be attributed to C 1s, O 1s, and Fe 2p,
Fig. 4 XPS spectra of the Fe3O4/MWCNTs nanocomposites (a) and high-resolution scan of Fe 2p region (b).
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Fig. 6 Degradation of atrazine (a) and decomposition of H2O2 (b) along with time under different conditions ([ATZ]0 ¼ 10 mg Lꢀ1; [H2O2]0 ¼ 3.0
mmol Lꢀ1; [Fe3O4] ¼ 0.1 g Lꢀ1; [Fe3O4/MWCNTs] ¼ 0.1 g Lꢀ1; pH 5.0; T ¼ 30 ꢁC).
reaction: induction period and rapid degradation stage, the the catalyst concentration increased from 0.1 g Lꢀ1 to 1.0 g Lꢀ1
.
degradation rates were accelerated aer reaction for about 30 The increased efficiency was mainly due to the increased active
minutes. As can be seen from Fig. 6a, Fe3O4/MWCNTs were much sites when more catalyst was added to the solution, and more
more efficient than Fe3O4 nanoparticles. Some researchers found active sites is favorable for generating more free radical species
that activated carbon and graphite could generate free radicals which can promote the degradation reaction. A catalyst dosage
such as superoxide ion and activate hydrogen peroxide, carbon of 1.0 g Lꢀ1 led to ATZ being almost completely degraded within
materials have been used in heterogeneous Fenton reactions.38,39 120 min.
But Hu et al.27 reported that the contribution of direct catalysis of
MWCNTs in Fe3O4/MWCNTs to the improved degradation
performance was very limited.
3.5 Effect of H2O2 concentration
The inuence of H2O2 concentration on the degradation of ATZ
was illustrated in Fig. 9. The degradation efficiency increased
from 60.5% to 92.7% when H2O2 concentration increased from
1.0 mmol Lꢀ1 to 10.0 mmol Lꢀ1. At lower concentrations of
H2O2, an adequate number of cOH radicals can not generate,
and this slowed the oxidation rate and further reduced the
As shown in Fig. 6b, H2O2 was not converted without the
presence of catalysts. In the presence of Fe3O4 and Fe3O4/
MWCNTs, 61.8% and 47.9% H2O2 were decomposed in 120 min,
respectively. Taking the degradation efficiencies of ATZ into
consideration, Fe3O4/MWCNTs showed higher utilization effi-
ciency of H2O2 than Fe3O4 nanoparticles. It can be deduced that
the adsorbed ATZ molecules which were closed to Fe-ions
immobilized on WMCNTs, were easily attacked by the produced
cOH. The synergistic effect resulting from the adsorption perfor-
mance of MWCNTs caused the different degradation efficiency.
3.3 Effect of pH
The experiments were carried out under four different pH
values of 2.5, 3.0, 5.0 and 8.0. It can be seen from Fig. 7 that
solution pH has a crucial inuence on the removal of ATZ by
Fenton-like reaction, lower pH was benecial for the degrada-
tion of ATZ. At pH near neutrality (pH ¼ 5.0), Fe3O4/MWCNTs
are still active but the ATZ removal efficiency decreased to
81.4%. At pH ¼ 8.0, the reaction rate was very slow and 39.7%
ATZ can be removed in 120 min. This phenomenon is consis-
tent with other iron oxide based heterogeneous Fenton
systems.40 As can be seen, when solution pH # 3.0, the reaction
follows a pseudo-rst order law, this is because homogeneous
Fenton reaction may take place in the acidic conditions (Fe3O4
nanoparticles dissolved). The existence of high catalytic activity
near neutral pH, allows these catalysts to be applied at neutral
pH, which is impossible for homogeneous Fenton catalysts.
Fig. 7 The effect of pH on ATZ degradation ([ATZ]0 ¼ 10 mg Lꢀ1
;
[H2O2]0 ¼ 3.0 mmol Lꢀ1; [Fe3O4/MWCNTs] ¼ 0.1 g Lꢀ1; T ¼ 30 ꢁC).
3.4 Effect of the catalyst dosage
Fig. 8 shows the inuence of the catalyst dosage on the
heterogeneous Fenton degradation of ATZ by Fe3O4/MWCNTs.
Fig. 8 The effect of catalyst dosage on ATZ degradation ([ATZ]0 ¼ 10
The degradation efficiency increased from 81.4% to 97.3% as mg Lꢀ1; [H2O2]0 ¼ 3.0 mmol Lꢀ1; pH 5.0; T ¼ 30 ꢁC).
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removal efficiency. However, when the concentration of H2O2 reaction mainly proceed in the surface of Fe3O4/MWCNTs.
increased to 10.0 mmol Lꢀ1, a signicant improvement did not MWCNTs are excellent adsorbent for ATZ, ATZ molecules
appeared. There are two main disadvantages for using high adsorbed by MWCNTs were closed to Fe-ions immobilized on
concentrations of H2O2. First, as H2O2 was excess to the WMCNTs, they were easily attacked by the produced cOH, so
pollutant, the excess H2O2 would not have enough substrate to Fe3O4/MWCNTs show excellent catalytic activity. The interme-
act upon, most of H2O2 would therefore be wasted. Second, diates derived from ATZ decomposition were analyzed by HPLC.
higher concentrations of H2O2 can result in the scavenging of As can be seen from Fig. 11, the main intermediates were DEA,
cOH radicals (eqn (1) and (2)).23 Therefore, the concentration of DIA, ATZ-OH (also some unidentied oxidation products). The
H2O2 should maintain at its optimal level.
concentration of the intermediates increased slowly at rst 30
min, reached peak values at about 60 min, and then decreased
in the next 60 min. Barreiro et al.41 studied the mechanisms of
the ATZ oxidation in solution by Fenton reaction. They found
that it was cOH that initiated the oxidation of ATZ, the oxidation
could be initiated through dealkylation (alkylic sidechain
cleavage), alkylic-oxidation (alkylamino side-chain oxidation),
and/or dechlorination (hydroxylation at the chlorine site), and
hence the related intermediates were observed in the present
study.41
H2O2 + cOH / H2O + cOOH
(1)
(2)
cOOH + cOH / H2O + O2
3.6 Iron leaching
As can be seen in Fig. 10, the Fe ions concentration during ATZ
degradation were investigated. As can be seen, Fe3O4 nano-
particles in Fe3O4/MWCNTs dissolved gradually in the solution
during the reaction, the concentration of the total dissolved
iron increased to 0.48 mg Lꢀ1 aer 120 min. This demonstrated
that homogeneous Fenton reaction occurred during ATZ
degradation in the bulk solution. As ferrous ions can be oxi-
dated to ferric ions by the remaining oxidants (such as cOH and
H2O2) in the solution, the concentration of ferrous reached a
peak value at 60 min, and then decreased to 0.081 mg Lꢀ1 aer
120 min of reaction.
3.8 Catalytic stability of Fe3O4/MWCNTs
Successive experiments were carried out to evaluate the stability
of the catalyst. From Fig. 12, it can be seen that the activity
3.7 Oxidation products
At the solution pH 5.0, the leaching of iron ions can be ignored,
heterogeneous Fenton reaction plays the dominant role, the
Fig. 11 Variation of the concentration of degradation intermediates
detected by HPLC equipped with a UV DAD ([ATZ]0 ¼ 10 mg Lꢀ1
;
[H2O2]0 ¼ 3.0 mmol Lꢀ1; [Fe3O4/MWCNTs] ¼ 0.1 g Lꢀ1; pH 5.0; T ¼
30 ꢁC).
Fig. 9 The effect of H2O2 dosage on ATZ degradation ([ATZ]0 ¼ 10 mg
L
ꢀ1; [Fe3O4/MWCNTs] ¼ 0.1 g Lꢀ1; pH 5.0; T ¼ 30 ꢁC).
Fig. 10 Investigation of iron dissolution during ATZ degradation Fig. 12 The catalytic activity of reused Fe3O4/MWCNTs on ATZ
([ATZ]0 ¼ 10 mg Lꢀ1; [H2O2]0 ¼ 3.0 mmol Lꢀ1; [Fe3O4/MWCNTs] ¼ 0.1 degradation ([ATZ]0 ¼ 10 mg Lꢀ1; [H2O2]0 ¼ 3.0 mmol Lꢀ1; [Fe3O4/
g Lꢀ1; pH 3.0; T ¼ 30 ꢁC).
MWCNTs] ¼ 0.1 g Lꢀ1; pH 5.0; T ¼ 30 ꢁC).
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decreased gradually during three consecutive runs. The loss of
activity can be attributed to the dissolution of Fe3O4 nano-
particles from the surface of the catalyst as described in Section
3.6. In addition, the agglomeration of Fe3O4 nanoparticles in
8 G. K. Zhang, Y. Y. Gao, Y. L. Zhang and Y. D. Guo, Environ.
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of Fig. 1d) can also lead to the decrease of the catalytic activity of
Fe3O4/MWCNTs.
242, 1.
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´ ˆ
12 L. W. Hou, Q. H. Zhang, F. Jerome, D. Duprez, H. Zhang and
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4. Conclusions
Fe3O4/MWCNTs nanocomposites were successfully synthesized
by coprecipitation and hydrothermal method. Fe3O4/MWCNTs
showed a strong ability for the adsorption of ATZ in aqueous
solution. Fe3O4/MWCNTs can be used as an efficient heteroge-
neous Fenton-like catalyst to degrade ATZ in aqueous solution.
The degradation efficiency strongly depends on the solution pH
with a sharp increase in oxidation rate from pH 5.0 to 3.0 which
is the pH range where Fe3O4 dissolution is strongly increased,
and the soluble Fe(III) and Fe(II) species in solution initiate the
homogeneous Fenton reaction. ATZ removal efficiencies are
found not to increase much with the increasing concentrations
of Fe3O4/MWCNTs. Fe3O4/MWCNTs showed higher utilization
efficiency of H2O2 than Fe3O4 nanoparticles. The enhanced
catalytic activity of Fe3O4/MWCNTs in heterogeneous Fenton
system could be attributed to the well dispersion of Fe3O4
nanoparticles on MWCNTs, positive effect of MWCNTs via
adsorption of pollutant molecules.
Acknowledgements
23 L. C. Zhou, Y. M. Shao, J. R. Liu, Z. F. Ye, H. Zhang, J. J. Ma,
Y. Jia, W. J. Gao and Y. F. Li, ACS Appl. Mater. Interfaces, 2014,
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25 X. B. Hu, Y. H. Deng, Z. Q. Gao, B. Z. Liu and C. Sun, Appl.
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This work has been supported by National Nature Science
Foundation of China (Grant no. 51338010, 21107125 and
51221892), National Basic Research Program of China (973
Program, Grant no. 2011CB933704), and the National Natural
Science Funds for Distinguished Yong Scholar (Grant no.
51025830).
26 L. R. Kong, X. F. Lu, X. J. Bian, W. J. Zhang and C. Wang, ACS
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