Goethite as an efficient heterogeneous Fenton catalyst for the degradation of methyl orange

Article abstract of DOI:10.1016/j.cattod.2015.01.012

The heterogeneous Fenton process was employed to degrade methyl orange (MO) using goethite as the catalyst. The catalyst stability was evaluated by measuring color removal in three successive cycles. The goethite samples before and after the reaction were characterized by XRD, SEM and FTIR techniques. The effect of major parameters, including pH, H₂O₂ concentration, catalyst addition and initial dye concentration on the decolorization of MO was investigated. The results indicated that the MO decolorization rate increased with the increase of H₂O₂ concentration and catalyst addition, but decreased with the increase of initial MO concentration. The optimal initial pH for the decolorization was found as 3. The acute toxicity experiments illustrated that the Daphnia magna mortality declined with the progress of the oxidation.

Full text of DOI:10.1016/j.cattod.2015.01.012

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Goethite as an efficient heterogeneous Fenton catalyst for the
degradation of methyl orange
Yan Wanga,b, Yaowen Gaoa, Lu Chena, Hui Zhanga,∗
a Department of Environmental Engineering, Wuhan University, P.O. Box C319, Luoyu Road 129#, Wuhan, 430079, China
b Department of Environmental Science and Engineering, Anhui Science and Technology University, Donghua Road 9#, Fengyang, 233100, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 July 2014
Received in revised form 6 January 2015
Accepted 7 January 2015
Available online xxx
The heterogeneous Fenton process was employed to degrade methyl orange (MO) using goethite as the
catalyst. The catalyst stability was evaluated by measuring color removal in three successive cycles. The
goethite samples before and after the reaction were characterized by XRD, SEM and FTIR techniques. The
effect of major parameters, including pH, H2O2 concentration, catalyst addition and initial dye concen-
tration on the decolorization of MO was investigated. The results indicated that the MO decolorization
rate increased with the increase of H2O2 concentration and catalyst addition, but decreased with the
increase of initial MO concentration. The optimal initial pH for the decolorization was found as 3. The
acute toxicity experiments illustrated that the Daphnia magna mortality declined with the progress of
the oxidation.
Keywords:
Goethite
Decolorization
Acute toxicity
© 2015 Elsevier B.V. All rights reserved.
Heterogeneous Fenton-like catalyst
1. Introduction
Therefore, the effective methods for MO wastewater treatment are
required. Nowadays, more and more attention has been paid to
Recently, there has been an increasing concern about color efflu-
ents coming from different textile industries and other related
industries [1,2], in which approximately different 100,000 types of
synthetic dyes are used commonly and 700,000 tons of synthetic
dyes are produced annually [1,3,4]. Large amounts of wastewater
were generated during the production and utilization process of
these dyes [1,3]. Their presence obstructs light penetration and
oxygen transfer into water bodies [4]. Among these dyes, azo dyes,
the use of advanced oxidation processes (AOPs) in the treatment
of dye wastewater [13,21–23]. AOPs can generate hydroxyl radi-
cals (•OH, E0 = 2.80 V/SHE), which can oxide organic pollutants to
easily biodegradable small molecules or even carbon dioxide, water
and inorganic salts [5,24]. Among the different AOPs, Fenton oxida-
tion process is considered as a promising and potential alternative
due to its high efficiency, simple operation and ability to destruct
a large range of hazardous organics [11,25]. However, the applica-
tion of classic Fenton process has been limited by its disadvantages,
such as tight pH range and large amount of iron sludge produced
[2,5,11,23,25–27].
To overcome these drawbacks, the heterogeneous Fenton
process was proposed for the degradation of MO using vari-
ous catalysts, including Fe2O3–SiO2 [2], H3PW12O40 supported
Fe-bentonite [28], NdFeB magnetic activated carbon [29], and mag-
netic Fe2MO4 activated carbons [30].
Goethite is one of the most thermodynamically stable iron oxide
and formed close-packed array of O2− and OH− anions with Fe3+
[31]. As an effective catalyst, it plays a crucial important role
in the heterogeneous Fenton processes due to its ubiquity, high
catalytic activity, wide range of operating pH, controllable leach-
ing of iron into the solution, large surface area and high surface
hydroxyl content [23,31,32]. Both of natural goethite and synthetic
goethite have been employed for the treatment of various contam-
inants, such as C.I. Acid Orange 7, Orange G, PCB28, 2-chlorophenol,
benzenesulfonic acid, and so on [13,22,23,31–37]. However, there
characterized by nitrogen-to-nitrogen double-bonds (
N N ),
contribute to more than 50% of the world production of dyes
[5]. Methyl orange (MO) is a representative azo dye which is
widely used for colorization in textile, paper and chemical indus-
tries [6–9]. Due to their toxicity and carcinogenicity, MO and
its degradation products threaten to human health and aquatic
life safety [9,10]. Therefore, the treatment of MO effluents has
attracted the increasing attention [2,6–8]. Unfortunately, MO can-
not be effectively destructed by traditional biological methods
on account of its bio-recalcitrant and toxic properties [2,11–13].
Although MO could be removed from wastewater by physical
processes such as flocculation/coagulation [14,15] and adsorp-
tion [16–19], it was just transferred from one phase to another
phase, which required further treatment and disposal [2,3,5,20].
∗
Corresponding author. Tel.: +86 27 68775837; fax: +86 27 68778893.
E-mail address: eeng@whu.edu.cn (H. Zhang).
http://dx.doi.org/10.1016/j.cattod.2015.01.012
0920-5861/© 2015 Elsevier B.V. All rights reserved.
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was little report on the degradation of MO by heterogeneous Fen-
ton process using goethite as catalyst [13]. It was reported that
the daughter compounds generated during AOPs might be more
toxic compared with their parent compound [35,38,39]. Herein, it
is necessary to identify the intermediate products and monitor the
change of the toxicity during the degradation of MO by heteroge-
neous Fenton process. In this study, the goethite was prepared as
heterogeneous Fenton catalyst. The effects of various conditions
on the decolorization efficiency were investigated. The fresh and
used catalysts were characterized by X-ray diffraction (XRD), scan-
ning electron microscope (SEM) and Fourier transform infrared
spectrometer (FTIR). The changes of TOC and acute toxicity were
monitored.
(a)
1.0
0.8
0.6
0.4
0.2
0.0
H O
2
2
α−FeOOH
α−FeOOH/H O
2
2
0
20
40
60
80
t/min
2. Experiment
(b)
4
1.0
0.8
0.6
0.4
0.2
0.0
[Fe]
2.1. Materials
[H2O2]
3
2
1
0
Methyl orange was purchased from Shanghai Shiyi Chemicals
Reagent Co. Ltd. (China) and used as received. Hydrogen per-
oxide (analytical grade, 30%, w/w) was obtained from Shanghai
Sinopharm Chemical Reagent Co., Ltd. H2SO4 and NaOH were
obtained from Chengdu Institute of the Joint Chemical Reagent
(China) and used as received. All solutions were prepared with
deionized water.
0
10
20
30
40
50
60
70
2.2. Preparation and characterization of the catalyst
t/min
Goethite was prepared based on our previous study [23]. The
morphology of the catalyst was carried out using a scanning elec-
tron microscope (SEM, Zeiss EVO LS-185, UK) operated at 20.0 kV.
X-ray diffraction (XRD) patterns were recorded on a D/Max-2550
PC diffractometer in ꢀ–2ꢀ configuration to identify the crystal
phase and structure. The wide angle data were collected from
20◦ to 90◦ on 2ꢀ scale, when the operated condition was set
at 36 kV/24 mA, using Cu K␣1 radiation with a wavelength of
Fig. 1. (a) Degradation of MO under different conditions, (b) H2O2 decomposition
and the iron leaching with reaction time ([MO] = 75 mg L−1, [H2O2] = 3.88 mmol L−1
pH = 3, [goethite] = 0.3 g L−1).
,
was evaluated by phenanthroline spectrophotometry during cat-
alytic tests [40]. The active toxicity was determined with Daphnia
magna (D. magna) immobilization essays [41]. D. magna was cul-
tured in laboratory for more than three generations. The acute
toxicity experiments were performed using 25 24-h-old D. magna
in 50-mL-capacity test beakers which were divided into five groups.
Four groups were used for the test while one group for the blank.
They were set in the incubator which was set at 20 ◦C in a 16 h light-
8 h dark cycle. No foods were given during the acute toxic test. The
number of immobilized D. magna was counted after 24 h, and the
experiments were repeated five times.
˚
1.5406 A.
The infrared spectra of synthesized goethite were recorded
on KBr pellets by a Fourier transform infrared spectrometer
(FTIR, Nicolet Avatar 330). To avoid moisture uptake, KBr pel-
lets were prepared by pressing mixtures of dry powered sample
and spectrometry-grade KBr under vacuum. One hundred and fifty
scans were collected for each sample in the range of 400–4000 cm−1
with a resolution of 2 cm−1
.
2.3. Procedure
3. Results and discussion
In all experiments, a stock solution of MO was prepared fresh
before each run and the initial concentration (C0) was kept at
75 mg L−1. The solution pH was measured with a Mettler Toledo
FE20 pH-meter.
The experiments were performed in a glass beaker contain-
ing 200 mL solution at room temperature (20 ◦C). A given amount
of hydrogen peroxide and goethite were added into the reactor.
A magnetic stirrer provided complete mixing of the solution in
the reactor. At pre-selected time intervals, samples were removed
by a syringe and filtered through 0.22 ␮m membranes before the
absorbance was measured.
3.1. Decolorization of MO under different systems
To investigate the removal of MO under different conditions, the
experiments were carried out with H2O2 alone, goethite alone and
goethite/H2O2, respectively.
As presented in Fig. 1(a), the color of MO was hardly removed
by either H2O2 or goethite alone. It is due to the limited oxida-
tion power of H2O2 and the lower adsorption capacity of goethite.
When goethite was added with H2O2 simultaneously, the decol-
orization efficiency reached 98.9% within 70 min reaction, and this
goethite catalyst showed higher catalytic activity compared with
other catalysts [13,29]. For example, Li and Zhang [13] reported
that the decolorization efficiency could reach nearly 98% within
80 min reaction when MO was oxidized by heterogeneous Fenton in
combination with UV using the amorphous FeOOH catalyst. How-
ever, 2.5 g L−1 catalyst and 15.8 mmol L−1 H2O2 were required by
the oxidation process. Yang et al. [29] also reported that the MO
removal efficiency could reach 97.1% by Fenton-like process with
2.4. Analysis methods
The absorbance of MO was measured at ꢁmax = 464 nm using a
Shimadzu UV-1700 spectrophotometer. Total organic carbon (TOC)
was analyzed using a TOC analyzer (Shimadzu TOC-L) to evalu-
ate the mineralization of MO. The total iron leached from goethite
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3
1.0
1st run
2nd run
3rd run
0.8
0.6
0.4
0.2
0.0
0
50
100
t/min
150
200
Fig. 3. Reusability of goethite for the decolorization of MO ([MO] = 75 mg L−1
,
[H2O2] = 3.88 mmol L−1, pH = 3, [goethite] = 0.3 g L−1).
The peaks at the angle 2ꢀ of 21.223◦, 33.241◦, 34.700◦, 36.649◦,
39.984◦, 41.186◦, 50.613◦, 53.237◦, and 59.023◦, are specific of
goethite [42,43]. The FTIR spectra of fresh catalyst were shown in
Fig. 2(c), and various functional groups represented in the IR spec-
tra were the same as that of goethite. The band between 3000 and
3500 cm−1 is generated by the stretching of free/bonded hydroxyl
groups [44]. The peaks at about 890 and 793 cm−1 are attributed
to O H bending bands in goethite (Fe OH) and are respectively on
behalf of the vibration in and out of the plane [44]. The band absorp-
tion at 600 cm−1 is assigned to Fe O stretching vibrations [44,45].
According to the result of the XRD patterns, SEM image and FTIR
spectra of the fresh catalyst, the powder synthesized is goethite.
As can be seen in Fig. 2(a), the SEM image of used goethite
was obviously different from that of fresh one. For fresh catalyst,
lots of the sample particles with rough surface and the common
acicular structure were observed. The surface of used catalyst
became smooth and the catalyst particles were aggregated in an
irregular shape. Despite of this, the surface of agglomerate catalyst
still showed the acicular pattern. However, almost no change of
the structure was discovered between fresh and used goethite in
Fig. 2(b). Moreover, the FTIR spectra of used catalyst were nearly
the same as that of fresh catalyst in Fig. 2(c).
In order to further confirm the stability of the goethite catalyst,
the reuse experiments were performed. Under the same conditions
(75 mg L−1 MO, 0.3 g L−1 goethite, 3.88 mmol L−1 H2O2, and pH 3),
the experiments of the catalytic degradation of MO was repeated
for three cycles and each experiment was lasted for 70 min. The
reused catalyst was filtered and rinsed with deionized water after
each repeated experiment, then dried around 100 ◦C. As shown in
Fig. 3, 99.3, 99.4 and 97.1% of the MO removal were obtained in
three successive cycles, respectively. The decolorization efficien-
cies of MO are nearly the same in three successive cycles, indicating
synthesized goethite is an excellent long-term stable catalyst for
the goethite/H2O2 system.
Fig. 2. Characterization of fresh and used goethite: (a) SEM image, (b) XRD pattern,
(c) FTIR spectra.
NdFeB magnetic activated carbon catalyst, but the catalyst dosage
was as high as 10 g L−1
.
It could be known from Fig. 1(b) that H2O2 concentration
decreased with the increasing reaction time, clearly illustrating that
H2O2 could be activated by goethite to generate •OH which further
oxidized MO. After 40 min reaction, the bleaching of MO was com-
pleted basically, but H2O2 still remained and tended to decompose
gradually. It could be understood that the complete bleaching did
not denote the whole mineralization of MO to CO2, H2O, and so on,
and H2O2 was still required for further degradation of intermediate
products. The iron leaching increased gradually during the oxida-
tion process, but the maximum concentration of the iron leaching
was less than 0.2 mg L−1 and the homogeneous Fenton reaction
could be ignored for the MO degradation.
3.3. Effect of major parameters on the MO removal
Experiments under various reaction conditions (pH, H2O2, cata-
lyst addition and initial MO concentration) were conducted and the
results were shown in Fig. 4. As can be seen in Fig. 4(a), the decol-
orization efficiency increased when initial pH was decreased from
7 to 3. The surface functional groups of goethite are influenced by
the solution pH according to the following equation [31,46,47]:
3.2. Characterization of fresh and used catalyst
The morphology of fresh goethite was observed through SEM
images. As illustrated in Fig. 2(a), the sample particles with rough
surface and the common acicular structure were observed for fresh
catalyst. The XRD analysis was applied to define the structure of
fresh goethite and the obtained patterns were shown on Fig. 2(b).
≡ FeOH2 →≡ FeOH + H+ pKa = 6.4
(1)
+
The ratio of positively charged FeOH2+ groups increased as pH
decreased. The MO is generally negatively charged since the sul-
fonated group ( SO3−) in its structure is hydrolyzed [2]. On account
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(a)
1.0
(c)
1.0
7.77 mmol L-1
3.88 mmol L-1
1.54 mmol L-1
0.78 mmol L-1
0.39 mmol L-1
0.8
0.6
0.4
0.2
0.8
0.6
0.4
0.2
0.0
pH 2
pH 3
pH 4
pH 5
pH 7
0.0
0
10
20
30
40
t/min
50
60
70
0
10
20
30
40
t/min
50
60
70
(d)
(b)
1.0
37.5 mg L-1
75.0 mg L-1
150 mg L-1
1.0
0.8
0.6
0.4
0.2
0.0
0.2 g L-1
0.3 g L-1
0.4 g L-1
0.8
0.6
0.4
0.2
0.0
0
0
10
20
30
40
50
60
70
10
20
30
40
50
60
70
t/min
t/min
Fig. 4. Effect of major parameters on the MO removal: (a) pH, (b) goethite addition, (c) H2O2 concentration and (d) initial MO concentration ([MO] = 75 mg L−1, pH = 3,
[H2O2] = 3.88 mmol L−1, [goethite] = 0.3 g L−1).
of the electrostatic attraction, the acidic solution condition favors
the adsorption of MO onto the goethite surface [6]. Moreover, the
MO prefers the quinoid structure which undergoes oxidation by
•OH more easily than the azo structure at lower initial pH values
[2].
in Fig. 4(b). This is because the amount of generated •OH was only
dependent on H2O2 dosage, and 3.88 mmol L−1 H2O2 was sufficient
to generate •OH when only decolorization was concerned.
In order to achieve the maximum color removal, the effect of
the H2O2 concentration on the decolorization of MO was investi-
gated when the pH was 3, the MO concentration was 75 mg L−1
and goethite addition was 0.3 g L−1. Hydrogen peroxide is a source
of hydroxyl radicals in the goethite/H2O2 system, and more •OH
would be generated to oxidize MO at higher H2O2 concentration.
As shown in Fig. 4(c), the MO removal efficiency increased with an
increase in H2O2 concentration from 0.39 to 3.88 mmol L−1. How-
ever, the further increase in H2O2 concentration could not result
in an increase of MO removal efficiency. This is due to the fact that
the overdosed H2O2 would scavenge •OH at H2O2 concentration
higher than 3.88 mmol L−1 [25,50],
Fig. 4(a) illustrated that the decolorization was suppressed as
the initial pH was further decreased to 2. When pH value was below
3, H2O2 could capture a proton and turn into more stable oxonium
+
ion H3O2 [48,49],
+
H2O2 + H+ → H3O2
(2)
On the other hand, hydrogen ion would act as the scavenger of
hydroxyl radicals at a very low pH [23,46],
•OH + H+ + e− → H2O
(3)
where electrons may be gained from ferrous ions. Therefore, the
decolorization rate decreased as the pH dropped from 3 to 2.
Fig. 4(b) depicted the decomposition of MO at goethite addi-
tion of 0.2 − 0.4 g L−1 when the pH was 3, the MO concentration
was 75 mg L−1 and the H2O2 concentration was 3.88 mmol L−1. As
illustrated in Fig. 4(b), the decolorization efficiency of MO signif-
icantly increased from 40.8 to 96.4% after 20 min reaction when
goethite addition increased from 0.2 to 0.4 g L−1. The decoloriza-
tion of azo dyes is mainly through heterogeneous Fenton reaction,
which depends on active sites in the specific area of the catalyst
[20]. The increase of goethite addition corresponds to the increase
of the total area, which would result in the faster H2O2 decompo-
sition to generate •OH through the increase of active sites. Thus,
the removal rate of organic pollutants is improved with increasing
goethite addition [32]. Although the increase of goethite addition
resulted in the increase of decolorization rate, the decolorization
efficiency after 70 min reaction was nearly the same at the fixed
H2O2 concentration, which was higher than 96.0% as illustrated
H2O2 + •OH → HO2• + H2O
(4)
The generated hydroperoxy radicals were less active than
hydroxyl radicals and could not improve decolorization efficiency.
The initial dye concentration is usually considered as one of the
most important parameters for the dye removal. To elucidate the
effect of the initial dye concentration on the decolorization of MO,
the initial MO concentrations were set at 37.5, 75.0, 150 mg L−1
,
respectively, when the pH was 3, the H2O2 concentration was
3.88 mmol L−1 and goethite addition was 0.3 g L−1. As shown in
Fig. 4(d), the MO removal efficiency decreased significantly with
increasing initial dye concentration. After 20 min reaction, the
decolorization efficiency reached 94.8 and 67.0% when the ini-
tial MO concentrations were 37.5 and 150 mg L−1, respectively.
As the non-selective oxidants, •OH could oxidize not only target
pollutants, but also the intermediates generated during the reac-
tion process. When hydrogen peroxide concentration and amount
of goethite are fixed in the goethite/H2O2 system, the amount
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5
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
100
100
80
60
40
20
0
Immobilization
TOC
Color
0min
80
2min
5min
10min
20min
30min
40min
50min
60min
70min
60
40
20
0
10
30
60
0
t/min
200
300
400
500
600
700
800
Wavelength/nm
Fig. 6. The mineralization and the changes of active toxicity with reaction time
([MO] = 75 mg L−1, [H2O2] = 3.88 mmol L−1, pH = 3, [goethite] = 0.3 g L−1).
Fig. 5. UV–Vis spectral changes with reaction time ([MO] = 75 mg L−1
,
[H2O2] = 3.88 mmol L−1, pH = 3, [goethite] = 0.3 g L−1).
process proceeded, the acute toxicity gradually decreased com-
pared with the initial MO solution, indicating the toxic structures of
MO and the immediate products were degraded into the less toxic
products.
of formed •OH in this process is also constant. However, the
probability of reaction between MO molecules and the hydroxyl
radicals would be decreased with the increase of initial MO con-
centration [20]. In addition, more intermediate products would
compete with MO for •OH. These resulted in the decline of the
MO decolorization rate. Despite of the competitive consumption
of hydroxyl radicals, the color was nearly removed completely
at different initial dye concentration, indicating hydroxyl radicals
generated from the goethite/H2O2 oxidation system were sufficient
enough to bleach MO completely after 70 min reaction.
4. Conclusions
The results obtained from this work indicate that goethite is
an efficient catalyst for the degradation of MO by heterogeneous
Fenton. The decolorization was mainly attributed by the hetero-
geneous Fenton reaction and the homogeneous Fenton reaction
could be ignored. The degradation rate of MO is improved with the
increase of goethite addition and H2O2 concentration but decreased
with increasing of initial MO concentration. Recycle experiments
showed goethite was stable and can be reused. As clearly proved by
UV–Vis changes with the reaction time, MO quickly reacted with
free radical to form by-products. Toxicity to D. magna decreased
gradually with reaction time. Therefore, the goethite/H2O2 system
is a promising process for the treatment of dye wastewater.
3.4. Analysis of UV–visible spectra changes
To investigate the degradation of MO in the heterogeneous Fen-
ton process, representative UV–Vis spectra changes of MO solution
as a function of reaction time were shown in Fig. 5. It can be seen
that the absorption spectrum of MO in aqueous solution is mainly
characterized by two bands. One main band in the visible region
located at 464 nm resulted from an extended chromophore com-
prising of both aromatic rings, which connected through the azo
bond. The other band in the ultraviolet region located at 272 nm
was attributed to the ␲→␲* transition related to aromatic rings
in the MO molecule [2,13,51]. As the reaction proceeded, the
two characteristic absorption peaks at 272 and 464 nm decreased
gradually and essentially disappeared after 70 min. The contin-
uously decrease of the band at 464 nm with the reaction time
indicated the azo-band chromophore and conjugated ␲* systems
were destroyed. Meanwhile, the decline of zone around 272 nm
illustrated that the aromatic rings were also decomposed.
Acknowledgment
This study was supported by National Natural Science Founda-
tion of China (Grant No. 20977069).
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3.5. The changes of TOC and acute toxicity with reaction time
As is well known, the complete decolorization of azo dye
does not mean that it is completely oxidized to small inorganic
molecules and the toxicity disappears. Thus, the mineralization and
the acute toxic tests were determined before and after degradation
of MO by goethite/H2O2. As shown in Fig. 6, less than 13% TOC was
removed after 10 min reaction compared with 45% MO decoloriza-
tion efficiency. When the decolorization efficiency of MO nearly
reached 100% after 30 min reaction, the TOC removal rate was still
less than 40%. When the reaction time was extended to 60 min, 55%
of TOC removal efficiency could be achieved.
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orange, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.01.012