High-dispersive FeS2 on graphene oxide for effective degradation of 4-chlorophenol

Article abstract of DOI:10.1039/c4ra11354c

A high-dispersive FeS₂ micro-cube crystal on graphene oxide (FeS₂@GO) was fabricated by a one-pot hydrothermal method. The catalytic degradation of 4-chlorophenol (4-CP) and its mechanism in a FeS₂@GO-based Fenton system was investigated. Under acidic to slight alkaline conditions, FeS₂@GO demonstrated an excellent capacity to remove 4-CP. More than 97% of 4-CP was eliminated within 60 min in pH 7.0 reaction solutions initially containing 0.2 g L^-1 FeS₂@GO, 128.6 mg L^-1 4-CP and 100 mM H₂O₂ at 25 ± 1°C, and the removal of 4-CP was further enhanced with increasing FeS₂@GO loadings. In the meantime, the FeS₂@GO also achieved lower iron leaching and a more complete TOC removal compared with pure synthetic FeS₂ without graphene oxide. Furthermore, acetic acid and oxalic acid were identified as the primary products. The remarkable capacity of the FeS₂@GO-based Fenton system in removing 4-CP displays its potential application in the treatment of organic compound-contaminated water.

Full text of DOI:10.1039/c4ra11354c

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High-dispersive FeS2 on graphene oxide for
effective degradation of 4-chlorophenol
Cite this: RSC Adv., 2015, 5, 2449
a
a
a
b
b
a
*
Wanpeng Liu, Lili Xu, Xingfa Li, Chensi Shen, Sadia Rashid, Yuezhong Wen,
Weiping Liua and Xiaohua Wuc
A high-dispersive FeS2 micro-cube crystal on graphene oxide (FeS2@GO) was fabricated by a one-pot
hydrothermal method. The catalytic degradation of 4-chlorophenol (4-CP) and its mechanism in a
FeS2@GO-based Fenton system was investigated. Under acidic to slight alkaline conditions, FeS2@GO
demonstrated an excellent capacity to remove 4-CP. More than 97% of 4-CP was eliminated within 60
min in pH 7.0 reaction solutions initially containing 0.2 g Lꢀ1 FeS2@GO, 128.6 mg Lꢀ1 4-CP and 100 mM
H2O2 at 25 ꢁ 1 ꢂC, and the removal of 4-CP was further enhanced with increasing FeS2@GO loadings. In
the meantime, the FeS2@GO also achieved lower iron leaching and a more complete TOC removal
compared with pure synthetic FeS2 without graphene oxide. Furthermore, acetic acid and oxalic acid
were identified as the primary products. The remarkable capacity of the FeS2@GO-based Fenton
system in removing 4-CP displays its potential application in the treatment of organic compound-
contaminated water.
Received 2nd October 2014
Accepted 28th November 2014
DOI: 10.1039/c4ra11354c
www.rsc.org/advances
The hydrothermal method was an alternative way for
obtaining more pure and small particles. Up to now, FeS2
Introduction
nanocrystallines with different morphologies have been
synthesized via various solventhermal methods. One-
dimensional nanowires of FeS2 were synthesized in large
quantities by solvothermal process at relatively low temperature
with different with morphologies by Kar and Chaudhuri,12 and
single phase FeS2 nanocrystals with cubic shape were synthe-
sized but in a relative complicated solventhermal process.13
Aerward well-dened FeS2 micro-cubes and micro-octahedras
with high-yield and good uniformity were synthesized by a more
simple polymer-assisted hydrothermal method.14,15 However,
these FeS2 particles are prone to aggregate and form large
particles during the hydrothermal synthesis process, thus
losing their dispersibility and specic area which eventually
diminish their activity. Therefore, it is necessary to prepare
high-dispersive FeS2 on a suitable support to preserve or even
improve their unique properties.16–18
In the past decade, graphene and its derivatives are widely
investigated as promising materials for the immobilization of
nanoparticles. However, the lack of surface functionalities in
graphene to directly immobilize the nanoparticles onto its
surfaces has led to favorable utilization of graphene oxide (GO)
as an alternative support for the assembly of graphene based
nanocomposites.19 GO is fabricated by exfoliating of graphite
oxide and is abundant of oxygenated functional groups, such as
hydroxyl and epoxides on the plane with carbonyl and carboxyl
groups at the edges. These oxygenated functional groups can
serve as nucleation sites for metal ions to form GO/
Pyrite (FeS2) is one of the most abundant metal sulde minerals
on Earth, possessing a degradation capacity for contaminants.1
For example, degradation kinetics and the mechanism of
aqueous trichloroethylene (TCE) in aerobic pyrite suspensions
has been researched with O2 as the common oxidant.2–4 Liang
et al.5 investigated oxidative degradation of methyl tert-butyl
ether (MTBE) by activated persulfate, using pyrite as the source
of ferrous ion activators. Recently, pyrite has been considered as
a potential and promising heterogeneous iron source for
Fenton-like systems to treat various environmental organic
pollutants in wastewater and groundwater.6–10 However, for the
limitations of pool purity of natural pyrite, its Fenton-like
catalytic capacity can't be further improved. It is known that
the catalytic activity of catalysts depends on the size distribu-
tions and morphologies of the particles.11 Hence, reducing the
diameter of the material to the nanometer or micrometer scale
may result in enhanced its efficacy. But, the difficulty in
obtaining ne particles because of its high Mons' hardness
scale is the other limitation for further improvement in
efficiency.
aInstitute of Environmental Science, College of Environmental and Resource Sciences,
Zhejiang University, Hangzhou 310058, China. E-mail: wenyuezhong@zju.edu.cn;
Fax: +86-571-8898-2421; Tel: +86-571-8898-2421
bCollege of Environmental Science and Engineering, Donghua University, Shanghai
201620, China
cCollege of Chemistry and Life Science, Zhejiang Normal University, Jinhua 310024,
China
nanoparticles composites. As
a result, GO used as an
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attractive material in this eld owing to its unique two- then cooled to room temperature naturally. The resulting black
dimensional lamellar structure, large surface area, and full solid was collected by centrifugation, washed alternately with
surface accessibility.20 The graphene or GO is not only able to ultrapure water and ethanol several times to remove the excess
prevent the aggregation of immobilized particles but also surfactant and nally dried in vacuum at 50 ꢂC for 10 h
improve the overall catalytic activity owing to the synergistic before further use. Iron disulde–graphene oxide composite
effects between both components.21,22 For example, several (FeS2@GO) was synthesized according to the above procedures
recent studies have been reported using GO for the support of with appropriate amount of GO (0.03, 0.06, 0.12 g) dispersed in
Fe3O4 NPs in catalysis for the oxidation of cysteine23 and 3,3,5,5- EG by ultrasonication in advance.
tetramethylbenzidine,24 and the reduction of nitrobenzene.25
Furthermore, the reported enhancement in catalytic activity was
attributed to the synergistic effects between GO sheets and
3. Reaction setup
Fe3O4 nanoparticles.
All degradation experiments of 4-CP were carried out in 50 mL
The objective of the present work was to explore the degra- glass asks with a total solution volume of 40 mL under magnetic
dation of 4-CP with FeS2@GO by investigating removal efficacy stirring (400 r minꢀ1) and 25 ꢁ 1 ꢂC. A 500 W xenon lamp
and inuencing factors such as FeS2@GO loading, solution pH, (Trustech Inc., Beijing, China) was used as the light source for the
and H2O2 concentration. The proposed mechanism was given experiments conducted in the presence of visible light. The light
and the principal reaction intermediates were also identied.
radiates to the solutions through a glass slide to obtain visible
light without UV wave band. The initial pH (pHi) of the solutions
was adjusted to the designated value with 1 M H2SO4 and 1 M
NaOH standard solution. A 40 mL aliquot of 1.0 M 4-CP stock
solution was added to make a nominal initial concentration of 1.0
mM (128.6 mg Lꢀ1) and appropriate volume of H2O2 solution was
injected to make a demanded concentration. Reactions were
initiated by adding a predetermined amount of pyrite, FeS2 or
FeS2@GO into the pre-equilibrated and constantly stirred solu-
tions. Aliquots of 1.0 mL sample were periodically withdrawn and
ltrated through 0.45 mm syringe lter. The supernatant was
transferred to 2 mL vials containing 10 mL of tert-butanol (as a
radical scavenger) and subjected to HPLC analysis to determine
the remaining 4-CP concentration and the formation of carbonyl
acids. For the runs of cyclic reaction, the post-reaction FeS2@GO
was collected by ltration and dried in 50 ꢂC. For TOC measure-
ment, aliquots of 5 mL sample were periodically withdrawn and
ltrated through 0.45 mm syringe lter, but no radical scavenger
was added to avoid background TOC interference. All samples
were stored at 4 ꢂC and analyzed within 24 h.
To quantify the consumption of H2O2 and leaching of Fe ions
in the reaction systems, an additional reaction solution without
4-CP was also prepared. For H2O2 consumption quantication,
100 mL samples were periodically withdrawn and ltrated
through 0.45 mm syringe lter. Aliquots of 50 mL supernatant
was transferred to 5 mL volumetric ask and diluted to 500 mL.
The dilution was used for H2O2 measurement using a method
according to Kasaka et al.27 The determination of H2O2 is based
on a spectrophotometric method via the stoichiometric reaction
of H2O2 with copper(II) ion and DMP. Briey, 0.5 mL each of
Experimental
1. Chemicals
The 4-chlorophenol (4-CP) standard (purity
> 99%) was
purchased from Aladdin Chemistry (Shanghai, China). Gra-
phene oxide (purity > 99%, single layer ratio > 99%, diameter
1–5 mm, thickness 0.8–1.2 nm) was purchased from XFNano Inc.
(Nanjing, China). Pyrite (purity 95%) was purchased from Strem
Chemicals (Newburyport, MA, USA). Triton™ X-100 (TX-100),
sulfur
(purity
99.5–100.5%)
and
2,9-dimethyl-1,10-
phenanthroline (DMP) were purchased from Sigma-Aldrich
(St. Louis, MO, USA). Other chemicals and solvents used in
this study were of analytical grade or high performance liquid
chromatography (HPLC) grade. The concentration of the
purchased hydrogen peroxide solution (30 wt%) was calibrated
by titration with potassium permanganate26 and the purchased
pyrite was grounded, sieved through 150 mm mesh and washed
with 0.1 M HCl prior to use. Other chemicals were used as
received. Ultrapure water (18.2 MU cm resistivity) was prepared
using a Millipore® purication system and used throughout the
experiments. Stock solution of 1.0 M 4-CP was prepared in
methanol. The 1% (w/v) DMP solution was prepared in ethanol
in a brown bottle. A 0.01 M copper(II) sulfate (CuSO4) solution
was prepared by dissolving CuSO4$5H2O in ultrapure water. All
ꢂ
solutions were stored at 4 C prior to use.
2. Preparation of FeS2 and FeS2@GO
Iron disulde microcube crystal (FeS2) was synthesized using DMP, CuSO4 and phosphate buffer was added to a 5 mL volu-
FeSO4 and sulfur in alkaline solution based on Wang's metric ask containing 0.5 mL diluted sample supernatant and
method.14 Briey, 16 mL of non-ionic surfactant TX-100 was the ask was lled up to 5 mL with water. Aer mixing, the
added in 44 mL of ethylene glycol (EG) at room temperature solution was transferred to 1 cm cells and the absorbance was
followed by addition of 0.39 g of ferrous sulfate heptahydrate measured at 454 nm on a Shimadzu UV-2401 PC UV/Vis spec-
(FeSO4$7H2O), forming a homogenous solution under vigorous trophotometer (Tokyo, Japan). While, for Fe ions leaching
stirring. Then 0.4 g of sulfur was added to the solution under quantication, aliquots 2 mL sample were withdrawn and l-
magnetic stirring for 1 h, aer complete dispersion of sulfur trated through 0.45 mm syringe lter. The supernatant was
powder, 10 mL of 1.5 M NaOH was added and stirred for 30 min. transferred to 5 mL vials containing 10 mL HNO3 (65 wt%) and
Then the nal mixture was sealed in a 100 mL Teon-lined subjected to inductively coupled plasma mass spectrometry
stainless steel autoclave, and maintained at 180 ꢂC for 12 h, (ICP-MS) analysis to determine the Fe ions concentration.
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FeS2 is irregular, and it is unhomologous in size. Besides, the
FeS2 crystal aggregates as clusters, making an obscure micro-
cube structure and a low surface area (3.48 m2 gꢀ1). On the other
hand, GO facilitates the dispersion of FeS2 during the synthesis
process. The FeS2 synthesized with GO sheets spiking was found
to be single crystal with distinct microcubic structures. It is also
homologous in size (800–1000 nm), which has a much larger
surface area (23.26 m2 gꢀ1) compared with FeS2 synthesized
without GO. The microstructures of FeS2 and FeS2@GO were
further examined by transmission electron microscopy (TEM)
images (Fig. 1c and d) and drawn a same result. Furthermore,
the composition and phase purity of natural pyrite and
synthetic FeS2 were examined by XRD (Fig. 2). It can be seen that
all reection peaks in the red line can be readily indexed as a
pure cubic phase of FeS2, which is consistent with value given in
Joint Committee on Powder Diffraction Standards (JCPDS)
diffraction data les (no. 71-2219). However, a lower purity of
pyrite can be easily observed in the black line with relatively
poor clearness in reection peaks.
4. Chemical analysis
The 4-chlorophenol and by-products concentrations in reaction
samples were determined using a Waters® e2695 reverse-phase
HPLC coupled with a Waters® 2998 photodiode array detector
(Milford, MA, USA). The detection wavelength was both 210 nm.
A Waters® XBridge™ Phenyl column (250 ꢃ 4.6 mm, 5 mm) was
employed for the separation of 4-chlorophenol. The isocratic
mobile phase consisted of 70% methanol and 30% water with a
ow rate of 0.8 mL minꢀ1. The injection volume was 20 mL.
Under these conditions, the typical retention time for 4-CP was
5.5 min. For the products such as carbonyl acids, a Waters®
Atlantis™ T3 column (250 ꢃ 4.6 mm, 5 mm) was employed for
the separation. The isocratic mobile phase was 10 mM NaH2PO4
aqueous solution (adjusting pH to 3.0 with H3PO4) with a ow
rate of 0.5 mL minꢀ1. The injection volume was 50 mL. Under
these conditions, the typical retention time for acetic acid,
oxalic acid and chloroacetic were 6.1, 12.7 and 14.1 min,
respectively.
5. Characterization
2. Removal of 4-chlorophenol by FeS2@GO
Surface morphology studies of FeS2 and FeS2@GO were ach-
ieved with a eld emission scanning electron microscope (FEI,
SIRON) at a voltage of 25.0 kV to test. The sample surfaces were
gold-coated before analysis. The microstructures were further
examined by transmission electron microscopy (TEM,
JEM1200EX, JEOL). X-ray diffraction (XRD) measurements were
conducted using a D8 Advance (Bruker, Germany) X-ray
A typical degradation prole of 4-CP by FeS2 and FeS2@GO is
given in Fig. 3a. For verifying and comparing the capacity of iron
disulde in organics removal, commercially available natural
pyrite crystals (the major ingredient is iron disulde) were also
employed for the removal of 4-CP. According to the results,
FeS2@GO possesses a fairly high capacity in removing 4-CP in
H2O2 solutions compared to FeS2 particles and pyrite, both the
synthesis and none-synthesis iron disulde. For example, 86%
of 4-CP was removed in 30 min in a pH 5.0 reaction system
originally containing 0.8 g Lꢀ1 FeS2@GO, 50 mM H2O2 and 1.0
mM 4-CP, while only 58% and 19% 4-CP was removed by FeS2
and pyrite under the same conditions, respectively. Up to 99%
of 4-CP disappeared as the reaction was prolonged to 60 min as
well as 97% for FeS2. However, the inevitable aggregation of
FeS2 during the reaction tends to inhibit the removal rate and
can't be ignored. As a result, natural pyrite have the lowest
efficiency for 4-CP removal as compared to FeS2 and FeS2@GO.
And FeS2@GO possesses the most remarkable dispersibility as
˚
diffractometer with Cu Ka radiation (l ¼ 1.5418 A).
Results and discussion
1. Characterization of FeS2 and FeS2@GO
The morphology and microstructures of synthetic FeS2 and
FeS2@GO were investigated by a scanning electron microscope
(SEM). Fig. 1a and b showed the SEM images of FeS2 and
FeS2@GO, respectively. It illustrates that the shape of synthetic
Fig. 1 SEM image of FeS2 (a) and FeS2@GO (b), and TEM images of
FeS2 (c) and FeS2@GO (d).
Fig. 2 X-ray diffraction patterns of natural pyrite and synthetic FeS2.
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Fig. 3 Time profiles of 4-chlorophenol (4-CP) removal by pyrite, FeS2
FeS2@GO (a), and time profiles of 4-CP removal by FeS2@GO
with different GO content (b) under the conditions of pH 5.0, 25 ꢁ 1 ꢂC,
[4-CP] ¼ 128.6 mg Lꢀ1 (1.0 mM), [H2O2] ¼ 50 mM and [catalyst] ¼ 0.8
g Lꢀ1. Data points are given as means ꢁ standard deviations (n ¼ 3).
Fig. 4 Iron leaching of FeS2 and FeS2@GO during the reaction with
4-hlorophenol (4-CP) (a) and time profiles of 4-CP in classic Fenton
system (b) under the conditions of pH 5.0, 25 ꢁ 1 ꢂC, [4-CP] ¼ 128.6
mg Lꢀ1 (1.0 mM), [H2O2] ¼ 50 mM, [catalyst] ¼ 0.8 g Lꢀ1 and [Fe2+] ¼
5.4 mg Lꢀ1. Data for classic Fenton treatment are from single
measurement, while the other data points are given as means ꢁ
standard deviations (n ¼ 3).
well as 4-CP removal efficiency in the reaction process. To study
the effect of GO content on the efficiency of FeS2@GO for
removal of 4-CP and x an appropriate GO amount during the
synthesis process, FeS2@GO with different GO content was
employed for 4-CP removal. As shown in Fig. 3b, efficacy of 4-CP
removal by FeS2@GO in H2O2 solutions was slightly improved
as the GO content of FeS2@GO increased. Take both economy
and efficiency into consideration, 0.03 g was chosen as the
optimal amount for spiking in FeS2@GO synthesis, and was
employed throughout all experiments below.
98.9% of that in FeS2. It is reported that the charges on GO
surface are highly negative when dispersed in water attribute to
the ionization of the carboxylic acid and the phenolic hydroxyl
groups.28 Hence, the most plausible reason was that Fe3+/Fe2+
was adsorbed on the GO surface through static interaction or
complex with oxygenated functional groups.29 But the leaching
iron makes fairly nite contribution to 4-CP removal. As shown
in Fig. 4b, only 42% of 4-CP was eliminated within 60 min in a
Fenton system with 5.4 mg Lꢀ1 Fe2+ and 50 mM H2O2.
3. Leaching of iron
4. Effect of FeS2@GO loading, pH and H2O2 concentration
The level on the iron leaching is another essential factor to The removal rate of 4-CP increased with the initial FeS2@GO
evaluate the Fenton-like catalytic performance. The aqueous content (Fig. 5a). For example, 4-CP removal at 60 min increased
iron content in the FeS2–H2O2 or FeS2@GO–H2O2 suspension from 76% for a solution with 0.2 g Lꢀ1 FeS2@GO to 100% with
was detected by inductively coupled plasma mass spectrometry 0.8 g Lꢀ1 FeS2@GO. Two factors are responsible for the
(ICP-MS) during a 2 h reaction. According to the results, the enhanced removal of 4-CP with increasing loading. It is repor-
leaching rate of FeS2 was a little faster than that of FeS2@GO ted by Bae et al.7 the hydroxyl radicals (cOH), which are the
(Fig. 4a). For example, aer 120 min reaction, the concentration dominantly oxidant for the decomposition of organic
of aqueous iron was 5.4 mg Lꢀ1 for FeS2, while it was 4.5 mg Lꢀ1 compounds in pyrite Fenton reaction, were generated through
for FeS2@GO, equaling to only 0.81% and 0.68% of the initial reduction of H2O2 by Fe2+ dissolved from FeS2 under aerobic
iron content. Meanwhile, the total iron content in FeS2@GO was condition (eqn (1)).
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rate at 30 min decreased from 86% for the reaction solution
with pHi 5.0% to 59% for that with pHi 9.0. When pHi was
increased to 11.0, 4-CP removal rate was inhibited to a relatively
low value of 21%, which is attribute to the decreasing formation
of cOH caused by low Fe2+ concentration at high pH.1 Similarly,
pH was a major factor that affected the removal of other
organics (e.g. trichloroethylene and 2,4,6-trinitrotoluene) in
pyrite Fenton systems.7,8 However, it is well known that in a
classic Fenton system aqueous Fe(II) leading to decomposition
of H2O2 to hydroxyl radicals was only efficiently occurred at the
pH below 4.5.30 In the present work, the FeS2@GO-mediated
removal of 4-CP stayed in a high rate in a slight acidic to alka-
line conditions (pH 5.0–9.0), even in a strong alkaline solution
(pH 11.0), 49% of 4-CP was removed in 60 min yet. As reported
by Bonnissel-Gissinger,1 pyrite suspension pH declined and
reached an acidic equilibrium point during the oxidation by
releasing iron and hydrogen, providing an appropriate pH for
the subsequent Fenton reaction. Additionally, the presence of
H2O2 extremely exacerbated pH decreasing, since pyrite disso-
lution kinetics by H2O2 is much faster than that by molecular
oxygen in suspensions,31,32 which leaded to a relatively high
removal efficiency for 4-CP under weak acidic to weak alkaline
conditions (pH 5.0 to pH 9.0). The similar result was observed
by Che et al.7 and Bae et al.9 in the removal of trichloroethylene
and diclofenac, respectively, by pyrite-based Fenton's reaction.
Furthermore, the numerous carboxyl groups (–COOH) and
hydroxyl groups (–OH) on GO surface also provided an acidic
interface for the occurrence of Fenton's reaction.33
It is well know that in a Fenton system H2O2 simultaneously
plays as the roles of both of OHc producer (eqn (2)) and OHc
scavenger (eqn (3)).34,35 Therefore, the concentration of H2O2 is a
crucial parameter for 4-CP removal in FeS2@GO Fenton system.
H2O2 + Fe2+ / Fe3+ + OHꢀ + HOc
H2O2 + OHc / OH2c + H2O
(2)
(3)
As Fig. 5c showed that the removal of 4-CP was increased
with the increasing concentration of H2O2 in the FeS2@GO
Fenton system. The 4-CP removal rates declined as the
concentration of H2O2 varies from 12–50 mM. For example, 4-
CP removal rate at 30 min decreased from 17% at the H2O2
concentration of 12 mM to 56% and 86% at that of 25 mM and
50 mM, respectively. With the reaction proceeding to 60 min, 4-
CP removal rate was respectively increased to 39%, 93% and
100% for H2O2 concentrations of 12 mM, 25 mM and 50 mM. As
Fig.
5 Effect of FeS2@GO loading (a), initial pH (b) and H2O2
concentration (c) on 4-chlorophenol (4-CP) removal at 25 ꢁ 1 ꢂC in pH
5.0 reaction solutions initially containing 128.6 mg Lꢀ1 (1.0 mM) 4-CP
and 50 mM H2O2 and 0.8 g Lꢀ1 FeS2@GO. Data for FeS2@GO-only
treatment are from single measurements, whereas the other data McKibben and Barnes31 reported, the general mechanism of
Fe2+ dissolution from the pyrite surface in an aerobic H2O2
solution includes two different pathways (eqn (1), (4) and (5))
and the second pathway with two consecutive reactions is
considered as the dominant way for Fe2+ generation.
points are given as means ꢁ standard deviations (n ¼ 3).
2FeS2 + 7O2 + 2H2O / 2Fe2+ + 4SO4 + 4H+
(1)
2ꢀ
2ꢀ
2FeS2 + 15H2O2 / 2Fe3+ + 4SO4 +2H+ + 14H2O
(4)
(5)
FeS2 + 14Fe3+ + 8H2O / 15Fe2+ + 2SO4 + 16H+
2ꢀ
The removal of 4-CP in the FeS2@GO-based Fenton system
was obviously pH-dependent. Overall, 4-CP removal rates
decreased with increasing pHi (Fig. 5b), suggesting that acidic
Therefore, the increasing of H2O2 concentration accelerates
conditions facilitated 4-CP removal. For example, 4-CP removal the 4-CP removal in the FeS2@GO Fenton system. However, the
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self scavenging of HOc caused by H2O2 concentration increasing
seemed to be more signicant during the reaction. As a result,
further addition of H2O2 to 200 mM did not signicantly affect
4-CP removal rates (data was not shown) and H2O2 concentra-
tion was xed at an optimal level in the following experiments.
5. The reuse of FeS2@GO
The reuse and stability were two major concerns for a catalyst to
be used in practical applications.36–38 In the durability test, the
same FeS2@GO samples were repeatedly employed for 4-CP
removal for consecutive ve times. It showed that there were no
obvious attenuation in the extent of 4-CP removal was observed
(Fig. 6). However, a slight decline inevitably occurred for the
loss of catalyst during the reaction and recovery procedures. For
Fig.
7
Formation of intermediates oxalic acid, acetic acid and
example, the removal rate of 4-CP was over 99% for the initial chloroacetic acid.
reaction, but it slightly dropped to about 95% for the h
cycling. It is worth noticing that the recovery of FeS2@GO is
FeS2@GO Fenton system. It is shown in Fig. 8 that aer 60 min
reaction 91%, 70% and 12% of TOC was removed for FeS2@GO,
FeS2 and pyrite, respectively.
difficult for its high dispersibility in the reaction solutions.
Therefore, further research would still need to be conducted to
generate enhanced separation properties for FeS2@GO.
7. Proposed mechanism
6. Identied products
Previous study has proved that FeS2 has a higher optical absorp-
tion coefficient than silicon but has a comparable band gap
energy (Eg ¼ 0.95 eV).40 Additionally, single layered GO have a high
optical transmittance, especially for visible light.41 In our experi-
ments, the removal process of 4-CP was exposed to indoor natural
light, so we conducted a control experiment in the dark. As a
result, no obvious diversity on 4-CP removal was observed under
natural light and dark condition, indicating that FeS2@GO–H2O2
system also works well in dark. Furthermore, for the photo-
oxidation efficiency of photocatalysts was found to generally
depend on light intensity,42 the intense light source xenon lamp
with a UV-light lter was employed in the experiment for 4-CP
removal. Similarly, the concentration of 4-CP declined the same as
in dark (Fig. 9). Therefore, it is considered that the FeS2@GO–
H2O2 system without the photo-catalytic process would avoid the
drawbacks found in light sensitive system.
It is well known that organic compounds were expected to
oxidize to short-chain carboxylic acids (e.g., formic acid, acetic
acid, oxalic acid and maleic acid) and then nally minerals, CO2
and H2O.39 In this study, three carboxylic acids including oxalic
acid, acetic acid and chloroacetic acid were identied as the
primary intermediates through HPLC-DAD analysis and the
time prole of the intermediates were given in Fig. 7. It is shown
that oxalic acid and acetic acid accumulated in the early stage,
and then attenuated aer reaching a peak value. For example,
oxalic acid accumulated to a peak concentration of 0.35 mM at 5
min and then declined, no oxalic acid was detected aer 30 min.
Similarly, acetic acid accumulated to a peak concentration of
0.89 mM at 30 min and was not detected aer 60 min. However,
the same course was not observed on chloroacetic acid. It kept
on a relative low level compared with oxalic acid and acetic acid.
To investigate the mineralization of 4-CP, concentrations of
TOC were measured during the oxidative degradation of 4-CP by
Fig. 6 Reuse of FeS2@GO at 25 ꢁ 1 ꢂC in pH 5.0 reaction solutions Fig. 8 Removal of TOC at 25 ꢁ 1 ꢂC in pH 5.0 reaction solutions
initially containing 128.6 mg Lꢀ1 (1.0 mM) 4-CP, 50 mM H2O2 and initially containing 128.6 mg Lꢀ1 (1.0 mM) 4-CP, 50 mM H2O2 and
0.8 g Lꢀ1 FeS2@GO. (a–e) show 4-CP residue in five degradation 0.8 g Lꢀ1 FeS2@GO (pyrite or FeS2). Data points are from single
reactions. Data points are from single measurement.
measurement.
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Fig. 9 Effect of visible light on 4-chlorophenol (4-CP) at 25 ꢁ 1 ꢂC in
pH 5.0 reaction solutions initially containing 128.6 mg Lꢀ1 (1.0 mM) 4-
CP, 50 mM H2O2, and 0.8 g Lꢀ1 FeS2@GO. Data for visible light
treatment are from single measurement, while the other data points
are given as means ꢁ standard deviations (n ¼ 3).
Fig. 11 Removal of 4-chlorophenol (4-CP) at 25 ꢁ 1 ꢂC in pH 5.0
reaction solutions in the presence of 2.5% wt. methanol (cOH radical
scavenger) and in the absent of O2 initially containing 128.6 mg Lꢀ1 4-
CP, 50 mM H2O2 and 0.8 g Lꢀ1 FeS2@GO (or FeS2). Data for anaerobic
treatment are from single measurement, while the other data points
are given as means ꢁ standard deviations (n ¼ 3).
According to the previous studies,4,7,8,10 the Fenton reaction
stimulated by the dissolved Fe2+ from pyrite surface is the
dominant mechanism for 4-CP removal. Overall, H2O2
consumption rates in different reaction systems followed the
order of pyrite > FeS2@GO > FeS2 (Fig. 10), because GO highly
facilitated dispersion of FeS2 and provided a huge interface for
the transformation of H2O2, enhancing the 4-CP removal.
The role of H2O2 in the removal of 4-CP was precursor of
HOc. Because H2O2 itself is not an efficient oxidizer of 4-CP, HOc
generated from H2O2 are the actual oxidant in the FeS2 and
FeS2@GO Fenton systems. It is showed in Fig. 11 that addition
of methanol as a HOc scavenger substantially retarded the
removal of 4-CP both in FeS2 and FeS2@GO Fenton systems.
Besides, it is worth to notice that in an anaerobic reaction
system 4-CP removal was also inhibited to a certain degree
(Fig. 11). Thus, it is believed oxygen played an indispensable
role in the FeS2@GO Fenton system for 4-CP removal. First, it is
proved that the transformation of halogenated compounds in
pyrite suspension was consistent with the nding that pyrite
oxidation by O2 follows a Fenton-like mechanism, in which the
reduction of O2 on the pyrite surface can induce HOc forma-
tion.43–48 On the other hand, the dissolved O2 in reaction solu-
tions oxide the FeS2@GO surface, releasing Fe2+ for the further
Fenton reaction (eqn (3)). Therefore, dissolved O2 is an essential
factor in the FeS2@GO Fenton system. In addition, it is
considered that the surface disulde groups have been the
proposed electron donor in reductive dehalogenation.49
Conclusions
This study demonstrated that high-dispersive FeS2 on GO
possesses a relatively high oxidative capacity for removing
aqueous 4-CP in H2O2 solution, offering a good alternative for
the treatment of aqueous organic contaminants. The contin-
uous dissolution of Fe2+ from FeS2@GO surface is considered as
the key factor to achieve the oxidation of 4-CP by continuously
producing the cOH. And oxygen played an indispensable role in
the FeS2@GO Fenton system for 4-CP removal by facilitating the
dissolution of Fe2+. Besides, no distinguishable diversity on
removal of 4-CP was observed in presence and absence of visible
light. The results obtained from this study can provide basic
knowledge to understand the oxidative degradation mechanism
of 4-CP by high-dispersive FeS2 in H2O2 solution and to design a
high-dispersive and efficient heterogeneous Fenton catalyst for
treatment of organic-contaminated water.
Fig. 10 Hydrogen peroxide consumption by pyrite, FeS2 and
FeS2@GO at 25 ꢁ 1 ꢂC in pH 5.0 reaction solutions initially containing
50 mM H2O2 and 0.8 g Lꢀ1 FeS2@GO (pyrite or FeS2). Data points are
given as means ꢁ standard deviations (n ¼ 3).
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Acknowledgements
The authors acknowledge nancial support from “The Opening
Foundation of the Environmental Engineering Key Discipline”,
the Fundamental Research Funds for the Central Universities
and the Zhejiang Provincial Natural Science Foundation of
China (no. LY12B05001).
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2456 | RSC Adv., 2015, 5, 2449–2456
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