Catalytic oxidation of 4-chlorophenol with magnetic Fe3O4 nanoparticles: mechanisms and particle transformation

Article abstract of DOI:10.1039/c5ra10433e

Magnetite (Fe₃O₄) is usually inert and when combined with metal catalysts or enzymes it forms a composite that exhibits both magnetism and catalytic activity. However, it has been reported that Fe₃O₄ nanoparticles have intrinsic peroxidase-like activity. In this study, super paramagnetic Fe₃O₄ nanoparticles with a diameter of about 30 nm were synthesized using self-designed experimental devices under mild conditions. Moreover, 4-chlorophenol (4-CP), which is a priority pollutant that widely exists in the environment but is recalcitrant towards chemical and biological degradation, was used as a model compound to test the catalytic activity of the synthesized Fe₃O₄ nanoparticles and analyse the mechanisms for 4-CP removal. Besides, surface analysis techniques, such as SEM, XRD and Raman spectroscopy, were used to investigate the transformation of the nanoparticles and further verify the interaction between the nanoparticles and 4-CP. The results revealed that the synthesized Fe₃O₄ nanoparticles show high catalytic activity even after being used several times, and acidic conditions are favourable for the dechlorination of 4-CP. However, 4-CP could also be degraded under neutral and alkali conditions. In the process 4-CP was transformed to formic acid, acetic acid and other byproducts. Adsorption tests indicated that the adsorption process does not play an important role in 4-CP removal, but it occurs between 4-CP and Fe₃O₄. The surface morphology of the Fe₃O₄ nanoparticles changed a lot and the reactive sites on the surface increased, which resulted in the higher activity of the particles after being used. The crystal structure of the nanoparticles did not change, suggesting the role of Fe₃O₄ nanoparticles as catalysts. Moreover, Raman spectra reflected that the adsorption and catalytic oxidation were surface reaction processes. It is proposed that the hydroxyl radical produced during the reaction is the main cause for the degradation of 4-CP. The reaction of H₂O₂ with ferrous to produce hydroxyl radical is the initial step, and is very important for the overall process.

Full text of DOI:10.1039/c5ra10433e

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Catalytic oxidation of 4-chlorophenol with magnetic Fe3O4
nanoparticles: mechanisms and particles transformation
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Rong Cheng∗, Guanꢀqing Li, Can Cheng, Lei Shi, Xiang Zheng, Zhong Ma*
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School of Environment and Natural Resources, Renmin University of China, Beijing 100872,
China
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Abstract: Magnetite (Fe3O4) is usually supposed to be inert and combines with metal
catalysts or enzyme to form a composite both of magnetism and catalytic activity.
However, it’s reported that Fe3O4 nanoparticles was of intrinsic peroxidaseꢀlike
activity. In this study, super paramagnetic Fe3O4 nanoparticles with a diameter of
about 30 nm were synthesized using selfꢀdesigned experimental devices under mild
conditions. And 4ꢀchlorophenol (4ꢀCP), which is a priority pollutant widely exist in
environment but recalcitrant towards chemical and biological degradation, was used
as the model compound to test the catalytic activity of synthesized Fe3O4
nanoparticles and analysis the mechanisms for 4ꢀCP removal. Besides, surface
analysis techniques such as SEM, XRD and Raman spectrum were used to investigate
the transformation of nanoparticles and verify the interaction between nanoparticles
and 4ꢀCP further. The results revealed that the synthesized Fe3O4 nanoparticles
showed high catalytic activity even after being used several times, and acidic
condition was favourable for dechlorination of 4ꢀCP. However, 4ꢀCP could also be
degraded under neutral and alkali conditions. In the process 4ꢀCP was transformed to
formic acid, acetic acid and other byproducts. Adsorption test indicated that the
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∗
To whom correspondence should be addressed. Tel: +86ꢀ010ꢀ62512798; Email: chengrong@ruc.edu.cn;
zhongma@vip.sina.com; fax number: +86ꢀ010ꢀ62512798.
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adsorption process didn’t play an important role for 4ꢀCP removal, but it did occur
between 4ꢀCP and Fe3O4. The surface morphology of Fe3O4 nanoparticles changed a
lot and the reactive sites on the surface increased which resulted in the higher activity
of particles after being used. The crystal structure of nanoparticles didn’t change,
suggesting the role of Fe3O4 nanoparticles as catalysts. And Raman spectrum reflected
that the adsorption and catalytic oxidation were surface reaction processes. It’s
proposed that hydroxyl radical produced during reaction was the main cause for
degradation of 4ꢀCP. The reaction of H2O2 with ferrous to produce hydroxyl radical
was the initiate step, and very important for the overall process.
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Keywords: Fe3O4 nanoparticles; 4ꢀchlorophenol; catalytic activity; mechanisms; iron
nanoparticles.
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1. Introduction
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As important chemical raw materials, chlorophenols have been widely used in
pesticide, medicine, dyes and synthetic materials (Zhang et al., 2007). The exposure
of chlorophenols poses a serious problem to environment. As a result, chlorophenols
are tagged as the hazardous and top priority pollutants by the USEPA (Environmental
Protection Agency) due to their toxicity, carcinogenicity, and intractability (Ruzgas et
al., 1995; Hwang et al., 2015). In addition, chlorophenols are highly toxic and
recalcitrant towards chemical and biological degradation in the environment. Hence
the treatment of chlorophenols has been one of the hot topics in the field of
environmental science and engineering.
For the past decades, zeroꢀvalent iron has been used for groundwater and
wastewater remediation (Zhang et al., 2011; Choi et al., 2012). And many studies
showed that zeroꢀvalent iron had a capacity to remove refractory organic pollutants
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(Loraine et al., 2001; Dorathi et al., 2012). However, surface passivation seriously
decreases the activity and capacity of zeroꢀvalent iron in terms of removing pollutants
(Liang et al., 2000; Parbs et al., 2007; Cariato et al., 2012). Due to the high specific
surface area and high surface reactivity, nanoscale zeroꢀvalent iron particles were paid
extensive attention (Wang et al., 1997; Cheng et al., 2007). In these years, the study
mainly focuses on the reduction mechanism of iron nanoparticles. And Fe3O4
nanoparticles, which were the main corrosion products of nanosized zeroꢀvalent iron
particles, were received little attention.
Generally, Fe3O4 is supposed to be inert, and used as a magnetic carrier for
catalysts or enzyme. Later, it’s reported that Fe3O4 nanoparticles was of intrinsic
peroxidaseꢀlike activity, and the combination with catalysts was not necessary (Gao et
al., 2007). In our previous work, it’s found that Fe3O4 was the main product of iron
nanoparticles when reacting with pentachlorophenol (Cheng et al., 2010). In addition,
the removal efficiency of 4ꢀchlorophenol (4ꢀCP) was improved when iron
nanoparticles were used for a period of time (Cheng et al., 2007). It indicated that
some product in the system contributed to the removal of 4ꢀCP. In this paper, the role
of nanosized Fe3O4, which is a product of nanosized iron, was studied. 4ꢀCP was used
as the model compound to test the catalytic activity of synthesized Fe3O4
nanoparticles and analysis the mechanisms for 4ꢀCP removal. The results would
provide important complement for the current reduction dechloridation mechanism of
zeroꢀvalent iron nanoparticles, and be helpful for the application of nanosized iron
particles.
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2. Materials and methods
2.1 Chemicals and materials
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4ꢀchlorophenol (4ꢀCP, ≥99%, reagent) was provided by Tianjin Jinke Fine
Chemical Industry Research Institute. Ferric sulfate (Fe2(SO4)3, 99%, reagent) was
purchased from Nankai Fine Chemical Factory. Ferrous sulfate (FeSO4·7H2O,
99.0%ꢀ101.0%) was supplied by Shenyang Reagent Factory. Methanol (99%, reagent),
ethanol (99.7%, reagent), hydrogen peroxide (30%, reagent), sulfuric acid (98%,
reagent) and Sodium hydroxide came from Beijing Chemical Factory. Argon gas (Ar,
99.99%) was supplied by Beijing Aolin Gas Company. All chemicals were of reagent
grade and used without further purification.
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2.2 Preparation of nanoparticles
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Fe3O4 nanoparticles were synthesized by Massart hydrolysis method (Massart,
1981). Briefly, 100 mL mixed solution of 0.07 mol/L Fe2(SO4)3 and 0.07 mol/L
FeSO4 was added dropwise to a fourꢀnecked flask containing 100 mL 1 mol/L NaOH
aqueous solution at 80 ℃. The process was performed in Ar atmosphere. Fe3O4
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nanoparticles were produced via following reaction:
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(1)
Synthesized Fe3O4 nanoparticles were deposited in Ar atmosphere, and then
washed three times with deionized water, and dried in a vacuum dryer.
The morphology of synthesized Fe3O4 nanoparticles were observed with scanning
electron microscope (SEM, Hitachi Sꢀ4500) and transmission electron microscope
(TEM, Hitachi Hꢀ7650B). The crystal structure of the asꢀprepared nanoparticles and
transformation products were characterized by Xꢀray powder diffraction (XRD,
D8ꢀadvance) using a Rigaku D/maxꢀRB Xꢀray diffractometer with Cu Kα radiation
(λ=0.1542 nm). The magnetic properties were determined with vibrating sample
magnetometer (VSM, 730T).
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Besides, microscopic confocal Raman spectrometer (RM2000, Renishaw) was used
to investigate the transformation of nanoparticles and verify the interaction between
nanoparticles and 4ꢀCP.
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2.3 Experimental procedure
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Batch experiments were conducted in 50 mL flask containing 15 mL solution, in
which 4ꢀCP, H2O2 and synthesized Fe3O4 nanoparticles were added with initial
concentrations of 20 mg·Lꢀ1, 1.0‰ and 2 g·Lꢀ1 , respectively. The flasks were sealed
with sealing films and placed on a rotary shaker (TZꢀ2EH, Beijing Wode Company).
The rotate speed and the temperature of reaction were set to be 150 rpm and 30 ℃,
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separately. Samples were withdrawn from various test groups at predetermined time
intervals and then were filtrated with 0.22 ꢁm filter film.
In the adsorption test, Fe3O4 nanoparticles (100 mg) were loaded into a 250ꢀmL
conical flask with cover containing 100 mL of an aqueous solution of 4ꢀCP. The
initial concentration of 4ꢀCP was 20 mg•Lꢀ1.The conical flask was placed on the same
rotary shaker as above. In the reciprocating experiment, the 4ꢀCP solutions were
picked up for detecting and new 4ꢀCP solutions were added into the flask and so on.
And the particles were in the flask all the time. The initial concentration of 4ꢀCP in
each new solution was 20 mg•Lꢀ1 and the volume of the solution was 100 mL, too.
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2.4 Analysis
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4ꢀCP and byproducts were quantified with an Agilent 1100 HPLC (Shanghai
Agilent Ltd) equipped with a C18 column and an Lꢀ4000 UVꢀvis detector. The mobile
phase for 4ꢀCP consisted of 60% methanol and 40% water distilled three times. The
flow rate was 1 mL/min and the detector wavelength was 280 nm for 4 –CP. Chlorine
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ion was quantified with a DXꢀ100 ion chromatogram (IC, DIONEX Company,
Germany). The operational conditions were: eluent at 3.5 mM Na2CO3/1.0 mM
NaHCO3; eluent flow at 1.2 mL/min, sample loop volume at 250 ꢁL, and run time at
6 min. Deionized water was used as blank.
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3. Results and discussion
3.1 Characterization of Fe3O4 nanoparticles
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The morphology of Fe3O4 nanoparticles synthesized was shown in Fig. 1. As seen
in the SEM image, the samples were circular particles and relatively uniform. Most
particles had a diameter of about 30 nm (the statistic data was shown in Fig A1 in
Supplementary Materials). As seen in TEM image, the fine structure of the sample
was fairly uniform.
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Fig. 1. The morphology of synthesized Fe3O4 nanoparticles: (a) SEM image; (b) TEM
image.
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As shown in the XRD pattern (Fig. 2), the peak position and relative intensity of
asꢀprepared particles was consistent with that of A.R. Fe3O4 particles. All of the
diffraction peaks can be assigned to the planes of (220), (311), (400), (422), (511) and
(440) of a Fe3O4 structure, and the three lines were (311), (440) and (511) (JCPDS no.
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26ꢀ1136). The diffraction peaks become broadened as the particle size decreases,
from pattern a for A.R. Fe3O4 particles to pattern b for synthesized sample. No other
peaks were detected in the XRD pattern, indicating the high purity of the sample.
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Fig. 2. XRD pattern of A.R. Fe3O4 particles and synthesized nanoparticles.
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The magnetic property of asꢀprepared particles was also tested. As seen in the
magnetization curve (Fig. A2 in Supplementary Materials), the saturation
magnetization was 50 emu/g, and the remanence and coercivity was zero. It indicated
that the synthesized particles are superparamagnetic. It meant that the synthesized
particles could be reused by an external magnetic field. Actually, the particles were
separated from the solution with a magnet in our experiments.
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3.2 Adsorption performance of Fe3O4 nanoparticles
The adsorption test showed that 4ꢀCP removed through adsorption process was
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rather limited (Fig. 3). And no more than 10% of 4ꢀCP was removed from solution by
synthesized nanoscale Fe3O4 particles. It is generally acknowledged that the specific
surface area will increase with the decrease of the particle size. And the adsorption of
heavy metals by nanoscale Fe3O4 particles had received great attention (Shen et al.
2009). This test showed a different result, which could due to the different surface
properties of the particles. As known, 4ꢀCP is an electron acceptor. There is an extra
electron in Fe3O4, but Fe3O4 is not a good electron donor. As a result, the direct
electron transfer is not so easily between 4ꢀCP and Fe3O4 (Gopalakrishnan et al. 2011).
In the other hand, Fe3O4 is magnetic and 4ꢀCP is dipole like, there would be some
magnetic attraction between Fe3O4 and 4ꢀCP. However, the removal efficiency of
4ꢀCP did not figure it out.
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Fig. 3. Adsorption of 4ꢀCP with synthesized Fe3O4 nanoparticles.
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To understand the surface property of asꢀprepared Fe3O4 nanoparticles and the
interaction between 4ꢀCP and Fe3O4, the Raman spectrum of Fe3O4 nanoparticles
before and after adsorption was determined. As shown in Fig. 4, there was a strong
peak at 665 cmꢀ1 in both of the samples, which was the characteristic peak of Fe3O4
(Xue et al., 2009). And in the spectrum of the sample after reaction, there were other
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two weak peaks at 380 cmꢀ1 and 295 cmꢀ1, respectively. It might be produced by
molecular vibration of 4ꢀCP adsorbed on the surface of Fe3O4 particles. The signal
from Raman spectrum indicated that there was some interaction between 4ꢀCP and
Fe3O4 particles, although the action was really weak. It’s speculated that the physical
adsorption was occurred between 4ꢀCP and Fe3O4 particles, and the low amount of
adsorption led to the weak peak (Lin et al. 2015).
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Fig. 4. Raman spectrum of Fe3O4 nanoparticles before and after adsorption.
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3.3 Removal of 4ꢀCP
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It is well known that pH value has a significant effect on the formation of hydroxyl
radicals (·OH) and the removal of pollutants in Fenton/Fentonꢀlike systems. The
removal of 4ꢀCP with synthesized Fe3O4 nanoparticles at different pH values were
tested in this study. As shown in Fig. 5, the compound was completely removed
within 4 h in acid condition. On the contrary, the removal rate at pH=7.0 or 8.0 was
about 40% even though the reaction extended to 30 h. Therefore, the degradation rate
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of 4ꢀCP at low pH value was absolutely higher than that at high pH value. It was
similar to the Fenton systems (Zhou et al., 2008; Li et al., 2015). Different from the
conventional Fenton system, the organic pollutant could still be degraded to a certain
extent in neutral and alkaline conditions. At low pH value, the results can be
attributed to: (1) the high oxidation capacity of hydroxyl radicals (·OH), which was
responsible for the removal and oxidation of chlorophenol; (2) more hydroxyl radicals
(·OH) were generated via Fenton reaction (Kim et al., 2010); (3) less adsorption of
iron hydroxides on the surface of particles (Masomboon et al., 2009); (4) relatively
stability of H2O2 (Zhang et al., 2009).
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Fig. 5. Removal of 4ꢀCP at different pH values
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Based on previous studies, the removal mechanism was involved in dechlorination
in the system (Xu et al., 2009; Jia et al., 2012; Xu et al., 2013). So the chlorine ion in
the system was determined. And a similar phenomenon was attained from the
perspective of chlorine ion release. A significant higher dechlorination rate was
obtained in acid condition compared to neutral and alkaline conditions (Fig. 6). And
the dechlorination rate showed a trend of growth in acid condition. Especially, when
4ꢀCP was completely removed after reacting for 4 h, the dechlorination rate was just
48.8%, and later dechlorination continued. When the time extended to 30 h, the
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dechlorination rate was 83.9%.
Comparing the dechlorination rate of 4ꢀCP (Fig. 6) with the removal rate (Fig. 5) at
different pH values, apparently, the dechlorination rate was lower than the removal
rate in each system, which suggested that there were other processes involved in the
removal of 4ꢀCP in addition to dechlorination. On the one hand, it could be physical
processes such as adsorption, volatilization, etc; on the other hand, chlorine
compounds may be produced. As determined in section 3.2, the physical adsorption
contributed little to the removal of 4ꢀCP. Besides, the volatilization of 4ꢀCP is so
weak that the volatilization process could be neglected. Therefore, the suspect was
likely to be chlorine compounds, which will be elaborated in section 3.6.
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Fig. 6. Dechlorination of 4ꢀCP at different pH values.
3.4 Removal of 4ꢀCP by reused Fe3O4 nanoparticles
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In order to study whether the residual particles had the catalysis ability to remove
4ꢀCP, a series of tests were carried out at different pH values. As shown in Fig 7(a),
4ꢀCP can be completely removed in 0.5 h with the reused Fe3O4 particles in repeated
experiments. It indicated that synthesized Fe3O4 nanoparticles still had an excellent
catalysis activity even after being used for a few times. When the initial pH value was
8, the removal rate was obviously lower (Fig. 7(b)). However, the results showed
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similar trend when Fe3O4 particles were reused. The removal of 4ꢀCP was enhanced
when Fe3O4 nanoparticles were reused in the systems. It was speculated that the
reaction sites were changed on the surface of Fe3O4 nanoparticles, which improved
the oxidation of 4ꢀCP. Besides, long time reaction made a significant contribution to
the morphological change of Fe3O4 nanoparticles (details were demonstrated in
section 3.5).
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Fig .7. Removal of 4ꢀCP with reused Fe3O4 nanoparticles: (a) initial pH=5.0; (b)
initial pH=8.0.
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3.5 The transformation of Fe3O4 nanoparticles
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The morphology of Fe3O4 nanoparticles presented little change after reaction
without hydrogen peroxide and were still evenly dispersed small particles (Fig A3 in
Supplementary Materials). However, things were totally different when hydrogen
peroxide was added into the system. Although Fe3O4 remained as scattered small
particles in a short period, the nanoparticles would develop into chain, flower
structure as reaction proceed, namely formed rough surface. As shown in Fig. 8,
particles were eroded, aggregated, finally larger flakes were generated. The existence
of erosion points were the sources of surface active sites of Fe3O4 particles. And in the
other studies, some surface defects of nanoscale iron were also formed, which could
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be used as active sites in the process of dechlorination (Gotpagar et al., 1999; Zhou et
al., 2008).
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Fig. 8. SEM images of Fe3O4 nanoparticles after reaction with H2O2 present. (a)~(c)
showed the particles after reacting for different periods. (d) was the amplification of
image (c).
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The XRD pattern revealed that the composition of Fe3O4 nanoparticles had little
change before and after reaction (Fig 9). And the result further confirmed that Fe3O4
nanoparticles acted as a catalyst in the system. There was no impurity peak detected,
indicating there was no new solid matter produced, and no phrase change in the
reaction process occurred to nanoparticles.
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Fig. 9. XRD pattern of Fe3O4 after reacting with 4ꢀCP (with H2O2 present).
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To investigate the changes of surface properties of particles, Raman spectrum of
Fe3O4 nanoparticles before and after reaction was determined. Similarly to Fig 4,
strong peaks (Fe3O4) were detected in all samples at 665 cmꢀ1 (Fig. 10). Besides, weak
peaks at 358 cmꢀ1 and 488 cmꢀ1 were detected in the sample after being used once,
which might be brought by 4ꢀCP and its products. However, the peaks were not
detected in the sample after being used twice. It revealed that there were certain
interactions, but very weak physical adsorption happened between the surface of
nanoparticles and chlorophenol. And there were no peaks detected yet in the sample
after reacting for 30 h without H2O2 except the 665 cmꢀ1 one, which was different
with the previous result (Fig. 4). It meant that the peaks disappeared brought by 4ꢀCP
adsorbed on the surface of Fe3O4 particles after a long time reaction. The result
confirmed that it’s a physical adsorption between 4ꢀCP and Fe3O4 particles, and
desorption could easily occur. That’s a dynamic process, so the weak peaks could just
be detected sometimes.
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Fig. 10. Raman spectrum of Fe3O4 nanoparticles before and after reaction: (a) after
being used once; (b) after being used twice; (c) without H2O2; (d) before reaction.
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3.6 The mechanism of 4ꢀCP removal
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Based on the above analysis, the catalytic oxidation of 4ꢀCP by Fe3O4 nanoparticles
was a surface reaction process. The Fentonꢀlike reaction between Fe3O4 nanoparticles
and hydrogen peroxide occurred as following:
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(2)
(3)
(4)
(5)
(6)
(7)
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(8)
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(9)
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The reaction (1) played a significant role in the 4ꢀCP removal due to the generation
of hydroxyl radical. And it also explained that acid condition was conducive to chain
reactions and greatly improved the oxidation rate of 4ꢀCP.
In the section 3.3, chlorine ion was detected. It well explained (in Eq. 8) that the
chlorine atom of 4ꢀCP was attacked by hydroxyl radical, which led to the chlorine
atom located in the paraꢀposition of 4ꢀCP was substituted (Li et al., 1999; Zhou et al.,
2008; Huang et al., 2015). As a result, the chlorine ions were released into solution.
Besides, the dechlorination rate was absolutely lower than the removal rate, which
suggested that chlorine compounds were generated through different reaction
pathways (Zhou et al., 2008; Huang et al., 2015). And the byproducts in the solution
might affect the dechlorination of 4ꢀCP by Fe3O4 nanoparticles (Xu et al., 2013). Fig.
11 demonstrated the mechanism of catalytic oxidation of 4ꢀCP by Fe3O4
nanoparticles.
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Fig. 11. The mechanism sketch of catalytic oxidation of 4ꢀCP by Fe3O4 nanoparticles.
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Moreover, there was an obvious decline of pH value in each solution system over
time in above studies (Fig. A4 in Supplementary Materials), which indicated amount
of acidic matter was generated during reaction, in accordance with published study
(Zhou et al., 2008). In fact, formic acid and acetic acid were detected in the system,
which indicated that the carbonꢀcarbon double bond of benzene ring was broken down
and then made the formation of acid matter be possible during the reaction (Zazo et al.,
2005; Zhou et al., 2008).
In addition, intermediates were also detected from HPLC spectrum (Fig. A5 in
Supplementary Materials).
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4. Conclusions
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(1) The removal rate and dechlorination rate of 4ꢀCP in acid condition was absolutely
higher than that in neutral and alkline conditions. However, 4ꢀCP could also be
degraded under neutral and alkali conditions.
(2) Synthesized Fe3O4 nanoparticles showed high catalytic activity even after being
used several times. Especially, the activity was improved after being used.
(3) The surface morphology of Fe3O4 nanoparticles changed a lot and the reactive
sites on the surface increased which resulted in the higher activity of particles
after being used.
(4) Adsorption process didn’t play an important role for 4ꢀCP removal, but it did
occur between 4ꢀCP and Fe3O4. Hydroxyl radical produced through Fenton
reaction made an outstanding contribution to the degradation of 4ꢀCP.
317
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Acknowledgement
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This work was supported by the Fundamental Research Funds for the Central
Universities, and the Research Funds of Renmin University of China (Grant No.
11XNK016), which are greatly acknowledged.
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