Degradation of the azo dye Orange G in a fluidized bed reactor using iron oxide as a heterogeneous photo-Fenton catalyst

Article abstract of DOI:10.1039/c5ra04238k

The heterogeneous Fenton process was employed to degrade Orange G (OG) using waste iron oxide as the catalyst in a three-phase fluidized bed reactor (3P-FBR). The morphology and the FTIR spectra of the used BT4 were compared with the fresh catalyst to illustrate the catalyst stability. The catalyst reusability was evaluated by measuring the colour removal in four successive cycles. The effects of major parameters, including pH, H₂O₂ concentration and catalyst addition on the decolourization of OG were investigated. A satisfactory decolourization efficiency (>92%) could be obtained under the conditions tested, and 78.9% of TOC removal was achieved at a 50 mg L^-1 initial OG concentration, 25 mg L^-1 H₂O₂ concentration, pH = 3 and 6 g L^-1 BT4 addition. The decolourization of OG was mainly attributed to the homogeneous photo-Fenton reaction, while the heterogeneous catalytic process played an important role in TOC removal. The main intermediates were identified by the GC/MS technique and the degradation pathway of OG was proposed. This journal is

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Degradation of Azo Dye Orange G in fluidized bed
reactor using iron oxide as a heterogeneous photo-
Fenton catalyst
Cite this: DOI:
a,b,c
c
a,†
c,d,†
Y. Wang , R. Priambodo , H. Zhang and Y.H. Huang
Received,
Accepted
Abstract: The heterogeneous Fenton process was employed to degrade Orange G (OG) using waste iron
oxide as the catalyst in a threeꢀphase fluidized bed reactor (3PꢀFBR). The morphology and the FTIR
spectra of used BT4 were compared with fresh catalyst to illustrate the catalyst stability. The catalyst
reusability was evaluated by measuring colour removal in four successive cycles. The effect of major
parameters, including pH, H O concentration and catalyst addition on the decolorization of OG was
DOI:
www.rsc.org/
2
2
investigated. The satisfactory decolorization efficiency (> 92%) could be obtained under the conditions
ꢀ
1
ꢀ1
tested, and 78.9% of TOC removal was achieved at 50 mg L initial OG concentration, 25 mg L H O
2
2
ꢀ1
concentration, pH = 3 and 6 g L BT4 addition. The decolorization of OG was mainly attributed to the
homogeneous photoꢀFenton reaction, while the heterogeneous catalytic process played an important role
in TOC removal. The main intermediates were identified by GC/MS technique and the degradation
pathway of OG was proposed.
Keywords: waste iron oxide; degradation pathway; heterogeneous Fentonꢀlike catalyst; orange G
9
,12,22ꢀ24
the treatment of dye wastewater containing OG.
catalysts, iron oxides and iron supported catalysts exhibit high
and are considered
In this study, the feasibility
Among these
1
. Introduction
In recent years, the treatment of textile and dyeing wastewater activity to accelerate the decomposition of H O
2 2
4,22ꢀ31
coming from textile industries and other related industries has to be the most promising catalysts.
received the increasing attention because of the high yield, the low of waste iron oxide (labeled as BT4), as a cheap photoꢀFenton
1
ꢀ5
biodegradability and the high toxicity of these wastewaters. As one catalyst, was investigated for the OG degradation. BT4 is a byꢀ
of azo dyes used widely, Orange G (OG) is characterized by one azo product resulting from the nonꢀseeded fluidizedꢀbed Fenton reaction
group (–N=N–) bound to aromatic rings and auxochromes (–OH, – for the treatment of the bioꢀeffluent tannery wastewater from a
6
ꢀ8
32ꢀ34
SO , etc.). Moreover, the azo group is responsible for producing dyeing plant in Taiwan.
3
the colour while auxochromes enhance the affinity of the dye
Due to the simplicity of its design and construction, low
towards the fibres and water. The widespread utilization of OG in operating cost, high flexibility for liquid and solid phase residence
different domains has caused a serious environmental problem due times and temperature control, heterogeneous Fenton process
9
,10
8
,10
to its obvious and latent danger for humans and ecosystem.
Therefore, various treatment processes have been employed for the the treatment of wastewater.
combined with fluidized bed reactor (FBR) has been widely used for
To further improve the contact
between different phases, mass transfer capacity and the mixing
35,36
11
OG removal from aqueous solution, such as biological treatment
10
and adsorption . Biological process is the most commonly used properties with low power requirements, a novel threeꢀphase
method for the wastewater treatment, but it has been proved to be fluidized bed reactor (3PꢀFBR) was proposed in our previous
11,12
34, 37
inefficient for dye removal.
Adsorption is also extensively studies.
In this work, OG, as a model azo dye, was treated by the
applied for the dye wastewater treatment, but it only transfers the heterogeneous photoꢀFenton process in 3PꢀFBR. The effect of
contaminants from one medium to another and further disposal is operating conditions on the removal of OG was investigated to
1
3ꢀ15
needed.
oxidation processes is required urgently to remove OG and its byꢀ were identified and the degradation pathway of OG was proposed.
As a result, the development of powerful and effective explore the mechanism of this process. The intermediate products
1
6
products from wastewater.
Over the past decades, advanced oxidation processes (AOPs)
have been proved to be effective for the removal of toxic and/or bioꢀ
refractory organic pollutants.
offers the costꢀeffective source of hydroxyl radicals and is easily
operated and maintained.
homogeneous Fenton process is restricted by the difficulty and
expensive cost for the disposal of the large amount of iron
2
. Experiment
5
ꢀ7,17
2
.1 Materials
Orange G (OG, C H N Na O S ) was purchased from Sigma
Among AOPs, Fenton process
1
8ꢀ20
16 10
2
2
7 2
However, the application of
and had a molecular weight of 452.38. H O2 (50 wt%, Union
2
Chemical Works, Taiwan) was of analytical reagent grade and used
without further purification. Other chemicals used herein, including
sulfuric acid and sodium hydroxide, were of analytical grade and
were used to adjust pH. Deionized and doubly distilled water were
used throughout this investigation.
21,22
sludge.
To overcome these drawbacks, heterogeneous Fenton
process using inexpensive, efficient, stable, and easily prepared and
separated catalysts has been developed, as a potential alternative for
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The iron oxide (BT4), a byꢀproduct of the FBRꢀFenton reaction draft tube), respectively. As shown in Fig. 2, the higher
for the treatment of the bioꢀeffluent from tannery wastewater, was decolorization and TOC removal rates of OG were achieved in 3Pꢀ
withdrawn from FBR after 4 months. It comprises mainly goethite FBR than those in conventional FBR. In addition to the advantages
(
αꢀFeOOH phase) with lower crystallinity, which has been verified of the conventional FBR such as the excellent contact between the
32ꢀ34
in our previous studies.
The physical properties of the BT4 were reactants as well as the high mass and heat transfer rates, the 3Pꢀ
ꢀ
3
ꢀ3
as follows: 1.54 g cm of the bulk density, 2.50 g cm of absolute FBR with an internal draft tube can obtain smooth and steady
2
ꢀ1
34
3,38
density, and 150 m g of the specific surface area.
.2 Experimental procedures
All experiments were carried out in a 3PꢀFBR containing 1.5 L
circulation of the catalyst particles in the bed.
This would
improve the performance of the reactor. Therefore, the 3PꢀFBR was
used in the following experiments.
2
solution at room temperature (25 °C) under the UV irradiation (Fig.
). The reactor consists of coaxial cylinder with inner and outer
diameters of 35 and 70 mm and the heights of these two cylinders
were 300 and 450 mm, respectively. The porous sintered plate was
Fig. 2.
1
3
.2. Decolorization of OG by different treatment processes
The experiments were conducted in H O , BT4, UV, H O /BT4,
2
2
2
2
located at a distance of 35 mm from the bottom of the reactor to UV/H O , UV/BT4 and UV/H O /BT4 system, respectively to
2
2
2
2
ensure a uniform distribution of air. The irradiation source was a illustrate the photoꢀcatalytic potential of BT4. As shown in Fig. 3,
5W UV lamp (UVP BLꢀ15 365 nm) fixed inside a cylindrical pyrex the colour hardly decreased when UV alone was applied to OG
1
•
tube (allowing wavelengths λ > 320 nm to pass) and the UV light solution. It is due to the fact that only a small amount of OH is
was turned on for 10 min to stabilize the irradiation before starting formed in the presence of only UV irradiation.3
4,39
The addition of
the experiment. Air bubbles were injected continuously from the H O alone could not result in significant colour removal after 90
bottom of fluidized bed reactor to circulate the BT4 particles min treatment because of the limit oxidizing power of H O (E =
throughout the reactor. Solution pH was adjusted by H SO or NaOH 1.78 V).
2
2
0
2
2
21,34, 39
2
4
In control experiments, it was also confirmed that
and measured with a pH meter (Action, A211). The predetermined little colour removal was achieved when BT4 alone was added to
quantity of BT4 and H O was dripped into the reactor. The reaction solution, indicating that the effect of BT4 adsorption on OG removal
2
2
time was recorded immediately after H O was applied. Samples was not obvious at pH 3. Although H O could be decomposed by
2
2
2
2
•
were taken from the reactor periodically using a pipette, and NaOH UV irradiation to generate OH when the UV wavelength was less
was added immediately to quench the reaction after filtration by a than 300 nm, little amount of OH was produced at UV
40
•
0
.22 ꢀm syringe filter made of polyꢀ(vinylidene fluoride).
wavelength > 300 nm (365 nm in this study). Therefore, the
decolorization efficiency was also insignificant. Similarly, BT4
could not be photocatalyzed by 365 nm UV to produce OH and OG
•
Fig. 1.
decolorization could be neglected in UV/BT4 system. However, BT4
2
.3 Analysis
•
could catalyze H O2 to generate OH, resulting in an 88.6% of
2
decolorization efficiency after 90 min reaction through Eq. (1):
The absorbance of OG was measured at λmax = 480 nm using an
2+
3+
•
−
≡
Fe + H O → ≡Fe + OH + OH
(1)
Agilent 8453 UVꢀVisible spectrophotometer. The intermediate
products were identified by GC/MS with a HPꢀ5MS capillary
column (30 m × 0.25 mm × 0.25 ꢁm) which was employed for
molecule separation (GC HP series 6890). A 200 mL dye solution
was extracted with 25 mL of dichloromethane for 4 times. Then the
organic phases were concentrated to about 2 mL by a rotary
evaporator at 40 °C before GC/MS analyses. A temperature
programmed mode was used. The initial temperature of 40 °C was
2
2
where the symbol ≡ represents the iron species bound to the surface
of the catalyst.
31,41ꢀ42
Moreover, this process could be improved by UV
and the
OG decolorization efficiency rose to 99.1% in the UV/H O /BT4
2
2
system. This will be discussed in the following section.
Fig. 3.
−1
held for 10 min, and ramped at 12 °C min to 100 °C, then to 200
3
.3. The degradation mechanism of OG in UV/H O /BT4 system
2 2
−1
−1
°
°
C with a 5 °C min rate, and finally ramped at 20 °C min to 270
C, followed by an isothermal time of 5 min.
It has been reported that homogeneous catalytic mechanism was
also involved in ironꢀbased heterogeneous Fenton systems due to the
The total iron leached from BT4 was determined by atomic
31,43ꢀ46
leaching of iron from the catalyst.
Therefore, to ascertain the
absorption spectrophotometry (Sens AA). TOC was measured using
a TOC analyzer (Sievers Innovox Laboratory TOC Analyzer). The
carboxylate ion was analyzed by ion chromatography (IC, Dionex
DXꢀ120). Morphology of BT4 was determined using a Hitachi Sꢀ
contribution of homogeneous photoꢀFenton reaction induced by
leached iron ion, the total iron concentration was monitored during
the reaction. As can be seen in Fig. 4a, the dissolved iron increased
−1
firstly, reached a maximum value of 1.4 mg L , and then decreased
4
00 scanning electron microscope (SEM). The infrared spectra of
3+
gradually. The ≡Fe of BT4 surface could complex with carboxylate
ion formed by the degradation OG. It resulted in the nonꢀreductive
dissolution of BT4 and accompanied by the increasing total iron
BT4 were recorded on KBr pellets by a Fourier transform infrared
spectrometer (FTIR, Nicolet Avatar 330). To avoid moisture, KBr
pellets were prepared by pressing mixtures of dry powered sample
and spectrometryꢀgrade KBr under vacuum. 150 scans were
4
7
concentration in the form of ferric carboxylate complexes. When
3+
−1
carboxylate was further degraded, the free Fe would be released
and then precipitated and/or crystallized onto the carrier (BT4)
surface in 3PꢀFBR, leading to the decrease of dissolved iron
collected for each sample in the range of 400–4000 cm with a
−1
resolution of 2 cm .
35,47
3
. Results and discussion
concentration.
To investigate the role of leached iron on the OG decolorization,
.1 Performance comparison between 3P-FBR and conventional homogeneous photoꢀFenton experiment was carried out with 1.4 mg
3
−1
2+
FBR
L
of soluble Fe . As can be seen from Fig. 4b, 90.4% of
decolorization efficiency was achieved after 90 min reaction. This
The degradation of OG by heterogeneous photoꢀFenton process
was performed in 3PꢀFBR and conventional FBR (without internal
48
phenomenon is consistent with our previous studies. It illustrated
that a satisfactory colour removal could be obtained via Fenton
2
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4
8
2+
•
•
process even at a low iron concentration . However, as low as 9.2% Fe . Then, OG was oxidized by OH to form organic radical R . The
•
•
of mineralization efficiency was removed after 90 min reaction and reaction of R with O yielded peroxoꢀorganic radical ROO , which
2
TOC removal rose to only 20.2% even if the reaction time was was further degraded to carboxylate. The carboxylate ion would
3+
extended to 180 min. This was much lower than 78.9% of TOC complex with ≡Fe to form ferric carboxylate and then produce
2+
removal achieved in heterogeneous photoꢀFenton (UV/H O /BT4) carboxylate radical and ≡Fe under UV irradiation. In presence of
2
2
system. Based on the results, it can be concluded that the O , carboxylate radical could be degraded to smaller molecule
2
decolorization of OG was mainly attributed to the homogeneous compounds.
photoꢀFenton reaction, while the heterogeneous catalytic process
played an important role in TOC removal.
The complete mineralization of OG could be described by Eq.
2) below,
Fig. 6.
(
C H N Na O S + 42H O → 16CO
3.4 Stability and reusability of BT4
16
10
2
2
7
2
2
2
2
+
2HNO + 2NaHSO + 45H O
(2)
3
4
2
−1
The morphology and functional groups of used BT4 were
obtained and compared with those of fresh BT4 through SEM
images and FTIR spectra, respectively. As illustrated in Fig. 7(a), the
rough surface and amorphous structure of used BT4 were observed,
which was similar to fresh BT4. In the meanwhile, there was little
difference of FTIR spectra between fresh and used BT4 as depicted
According to this equation, 157.8 mg L
theoretically needed to completely mineralize 50 mg L of OG,
which is much higher than H O addition used in this study. The
following reasons may explain this phenomenon: (i) The adsorption
of intermediate products on the surface of BT4, (ii) The
of H O is
2 2
−1
2
2
4
8
4
9
contribution of O in photoꢀFenton reactions.
2
−1
in Fig. 7(b). The band between 3000 and 3500 cm is generated by
55
the stretching of free/bonded hydroxyl groups. The peaks at about
Fig. 4.
ꢀ1
−1
7
81 cm and 789 cm are attributed to O–H bending bands in
To quantify the adsorption of intermediate products on the
surface of BT4, the solution pH was adjusted to 11 at the end of the
reaction and the TOC removal efficiency dropped to about 60%,
indicating approximate 18.9% of TOC was adsorbed on the surface
of BT4.
goethite (Fe–OH) and are respectively on behalf of the vibration in
5
5
−1
and out of the plane. The band absorption at 506 cm is assigned
55,56
to Fe–O stretching vibrations.
Fig. 7.
The role of oxygen in the degradation of OG was investigated
by bubbling nitrogen instead of air in the UV/H O /BT4 system. As
2
2
In order to further confirm the stability of the BT4 catalyst, the
reuse experiments were performed. Under the same conditions, the
experiments of the catalytic degradation of OG were repeated for
four cycles and each experiment was lasted for 90 min. The reused
catalyst was filtered and rinsed with deionized water after each
repeated experiment, then dried around 100 °C. As shown in Fig.
7(c), 99.1%, 89.4%, 91.1% and 91.1% of the OG removal were
obtained in four successive cycles, respectively. The decolorization
efficiency of OG is about 10% higher in the first cycle than that in
the last three successive cycles. It was due to less leaching iron ion
in the last three successive cycles. However, the decolorization
efficiencies of OG were nearly the same in the last three successive
cycles, indicating BT4 was an excellent longꢀterm stable catalyst for
the UV/H O /BT4 system.
shown in Fig. 5, both decolorization and TOC removal rates of OG
decreased when air was replaced by nitrogen. The attack of organic
species (RH) by OH generates the organic radical (R ) via Eq. (3). In
•
•
the presence of O (air), the organic radical would be converted to a
2
•
peroxoꢀorganic radical ROO as indicated in Eq. (4), which in turn
3
7,46, 50ꢀ52
could participate in the series of reactions through Eq. (5):
•
•
RH + OH → R + H O
(3)
(4)
(5)
2
•
•
R + O → ROO
2
•
•
ROO + H O→ intermediates + HO
2
2
Therefore, after 180 min reaction, 78.9% of TOC removal
efficiency was achieved by UV/H O /BT4 when oxygen was present,
2
2
while only 38.4% TOC was removed in the presence of nitrogen and
more organics (23.0% of TOC) would be desorbed from catalyst
particles when the solution pH was adjusted to 11 at the end of the
reaction.
2
2
3
.5 Effects of important parameters on the decolorization of OG
Experiments under various reaction conditions (pH, H O and
Fig. 5.
2
2
catalyst addition) were conducted and the results were shown in Fig.
. It can be seen from Fig. 8(a), the decolorization rate increased
The carboxylic acids, such as adipic acid (G), citric acid (H),
8
and oxalic acid (I), were detected by IC during the degradation of
3+
appreciably with initial pH decreasing from 6 to 3, but further
decrease of initial pH to 2 led to an unpronounced increase of
decolorization rate. Despite of this, the decolorization efficiency of
OG was almost the same after 90 min reaction. It indicated that the
proposed heterogeneous photoꢀFenton process is efficient for colour
removal under the pH range investigated.
OG. These carboxylic acids would complex with ≡Fe to form
(
nꢀ3)−
≡
FeR(COO)n
. The ferric carboxylate complexes could be
2
+
•−
photolyzed to generate ≡Fe and carboxylate radical (R(COO)n ) in
acid solution via Eq. (6).
5
0,53
In airꢀsaturated acid solution, the
carboxylate radical reacts with molecular oxygen to form the
•−
hydroperoxyl radical (O ) through Eq. (7), which disproportionates
2
5
0,54
to produce H O .
The generated H O , as the additional oxidant,
2
2
2 2
•
Fig. 8.
is decomposed to generate OH, resulting in the improvement of the
TOC removal. Therefore, the UV/H O /BT4 system could achieve
The effect of BT4 addition on the OG decolourization is
presented in Fig. 8(b). It was found that the decolourization rate
2
2
higher decolorization efficiency than dark system, as illustrated in
Fig. 3.
−1
increased with the increase of BT4 addition from 4 to 6 g L . The
increase of BT4 addition corresponds to the higher total specific area,
(
nꢀ3)− h v
2+
•−
≡
FeR(COO)
 → ≡Fe + R(COO)n + intermediates (6)
•
n
resulting in the faster decomposition of H O to generate OH for the
•−
•−
2
2
R(COO)n + O → intermediates +CO + O
(7)
2
2
2
decolorization of OG. However, the overdosed catalyst brought
about the turbidity of the suspension and consequently the decrease
Based on the results and previous literatures, a possible
mechanism of colour and TOC removal in the UV/BT4/H O system
−1
2
2
of UV light penetration when BT4 addition increased over 6 g L .
•
was proposed and illustrated in Fig. 6. The OH was generated by
activation of H O with ≡Fe in the surface of BT4 or dissolved
Therefore, the removal rate of OG increased insignificantly with
2
+
−1
2
2
BT4 addition increasing from 6 to 8 g L .
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Fig. 8(c) illustrated that the decolorization rate increased with appears as a promising process for the dye wastewater treatment.
−1
an increase in H O concentration of up to 25 mg L . Afterwards,
2
2
the removal rate of OG was not improved obviously. It could be
Acknowledgements
•
noticed that H O is a source of OH in the UV/H O /BT4 system
2
2
2
2
•
and the increase of H O concentration would lead to more OH
However, when H O was excess, the generated OH Hubei Province, China (Grant 2012FFA089) and the National
would be consumed via the following reaction,
This work was supported by Natural Science Foundation of
2
2
21,57
•
produced.
2
2
21,57
Science Council of the Republic of China (No.NSC102ꢀ2622ꢀEꢀ006ꢀ
•
•
H O + OH → HO + H O
(8) 004ꢀCC2).
2
2
2
2
After 90 min reaction, the decolorization efficiencies were
almost the same with H O concentration ranging from 25 to 100 mg
L , indicating that the generated OH were sufficient enough to
achieve the almost complete colour removal.
2
2
Notes references
−1
•
a
Department of Environmental Engineering, Wuhan University, P.O.
Box C319, Luoyu Road 129#, Wuhan 430079, China
3
.6 The degradation pathway of OG
b
Department of Environmental science and Engineering, Anhui science
The changes of the absorption bands reflect the evolution of the and technology University, Donghua Road 9#, Fengyang 233100, China
chromophores of OG and useful information could be provided by
the analysis of the UVꢀvis spectra. The UVꢀvis spectral changes as
the reaction time are shown in Fig. 9(a). As can be seen from Fig.
c
Department of Chemical Engineering, National Cheng Kung
University, Tainan 701, Taiwan
Sustainable Environmental Research Center, National ChengꢀKung
University, Tainan 701, Taiwan
Corresponding author. Tel.: +86 27 68775837; fax: +86 27 68778893;
was attributed to the absorption of the π → π transition related to Eꢀmail address: eeng@whu.edu.cn (H. Zhang).
d
9
(a), The UVꢀvis spectrum of the untreated initial solution of OG
recorded at pH 3 shows three main absorption peaks at 480, 330, and
50 nm, as well as a shoulder peak at 400 nm. The peak at 480 nm
†
2
*
the –N=N– group, while additional bands at 250 and 330 nm were
Corresponding author. Tel:+ 886ꢀ2757575x62636; fax: + 886ꢀ6ꢀ
*
assigned to the π → π transition of benzene and naphthalene rings
2
344496; Eꢀmail address: yhhuang@mail.ncku.edu.tw (Y.H. Huang).
6
,12,24
in the OG molecule, respectively.
Moreover, the absorption
peak at 480 nm decreased rapidly and essentially disappeared with
the reaction proceeded after 90 min reaction, indicating that the azo
bonds (–N=N–) and conjugated π systems were completely
Reference
*
1
2
3
4
N. Panda, H. Sahoo and S. Mohapatra, J. Hazard. Mater., 2011, 185,
59–365.
destroyed. In addition, the other two absorption peaks around 250
and 330 nm declined slowly, illustrating that the aromatic rings were
still present.
3
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http://dx.doi.org/10.1016/j.apcatb.2014.11.005.
GC/MS analysis was employed to further identify the
intermediate products during the OG degradation process. The byꢀ
products, such as aniline (A), 2ꢀaminophenol (B), 4ꢀnitrophenol (C),
phenol (D), benzoquinone (E), and 3ꢀnitropropanoic acid (F), were
detected. Based on the IC and GC/MS results as well as previous
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Environ., 2014, 146, 192– 200.
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Sivaiah, F. Baron, J. Barrault, S. Petit and S. Valange, J. Hazard.
Mater., 2015, 287, 32–41.
8
,58ꢀ62
studies,
the plausible degradation pathway of OG by
UV/H O /BT4 is proposed in Fig. 9(b).
2
2
5
6
7
8
9
1
1
1
A. Ajmal, I. Majeed, R.N. Malik, H. Idriss and M.A. Nadeem, RSC
Adv., 2014, 4, 37003–37026.
Fig. 9.
As indicated in Fig. 9(b), the first step of oxidation is the cleavage of
N=N−, resulting in the production of aniline (A) and 7ꢀhydroxyꢀ8ꢀ
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•
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ꢀnitrophenol (C) and phenol (D). These products were then
1
068–1076.
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and environmental benign for the heterogeneous activation of H O2
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was still over 89% after four cycles. 78.9% of TOC removal was
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ꢀ1
obtained at 50 mg L initial OG concentration, 25 mg L H O
2
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4
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UVꢀvis, IC and GC/MS analysis, OG quickly reacted with free
radical, resulting in the cleavage of azoꢀbond, followed by a series of
degradation reactions of OG. The coupled UV/H O /BT4 system
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RSC Advances, 2015, 00, 1ꢀ3 | 5

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Glass vessel
Pyrex tube
5 W UVA lamp
1
Compressed air
Fig. 1. The experimental set up
100
pH=11
80
60
40
20
0
Colour (3P-FBR)
Colour (FBR)
TOC (3P-FBR)
TOC (FBR)
0
30
60
90
120 150 180 210 240
t (min)
-
1
-1
Fig. 2. Comparison of OG degradation in 3P-FBR and conventional FBR ([OG] = 50 mg L , [H
2
O
2
] = 25 mg L ,
-1
pH = 3, [BT4] = 6 g L )

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1.0
0.8
0.6
0.4
0.2
0.0
H O
2
2
BT4
UV
UV/H O
2
2
UV/BT4
H O /BT4
2
2
UV/H O /BT4
2
2
0
20
40
60
80
100
t (min)
-
1
-1
2 2
Fig. 3. Decolorization of OG by different processes ([OG] = 50 mg L , [H O ] = 25 mg L , pH = 3, [BT4] = 6 g
-
1
L )
(a)
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
20
40
t (min)
60
80
100
Fig. 4. (a) The changes of total dissolved iron concentration with reaction time, (b) the decolorization and the
mineralization of OG in homogenous photo-Fenton process and heterogeneous photo-Fenton process ([OG] = 50
-
1
-1
-1
2+
-1
2 2
mg L , [H O ] = 25 mg L , pH = 3, [BT4] = 6 g L , [Fe ] = 1.4 mg L )

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(
b)
100
Colour (BT4)
2
+
Colour (Fe )
TOC (BT4)
2+
80
60
40
20
0
TOC (Fe )
pH=11
0
30
60
90
120 150 180 210 240
t (min)
Fig. 4. Continue
100
Colour (N2)
Colour (Air)
TOC (N2)
8
6
4
2
0
TOC (Air)
pH=11
0
0
0
0
0
30
60
90
120 150 180 210 240
t (min)
-
1
2 2
Fig. 5. Effect of dissolved gas on the decolorization and mineralization of OG ([OG] = 50 mg L , [H O ] = 25 mg
-1
-1
L , pH = 3, [BT4] = 6 g L )

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RH(OG)
Fe3+
Fe2+
carboxylate
carboxylate radical
O2
•OH
H O
•OH
H O
2
2
●
R
intermediates
Fe2+
H O
ROO•
2
2
O2
O2•−
H O
intermediates+CO
2
HO2•
2
2
carboxylate
Fe3+
BT4
3+
Fe −carboxylate
dissolution
Fe2+
2 2
Fig. 6. Possible mechanism under UV/H O /BT4 system
(
a)
used
fresh
Fig. 7. (a) SEM image of fresh and used BT4, (b) FTIR spectra of fresh and used BT4 (c) the reusability of BT4
-
1
-1
-1
(
2 2
[OG] = 50 mg L , [H O ] = 25 mg L , pH = 3, [BT4] = 6 g L )

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(
b)
used
fresh
500 1000 1500 2000 2500 3000 3500 4000 4500 5000
-
1
Wavenumber (cm )
(
c)
1.0
0.8
0.6
0.4
0.2
0.0
First
Second
Third
Fourth
0
30 60 90 0 30 60 90 0 30 60 90 0 30 60 90
t (min)
Fig. 7.Continue

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1.0
0.8
0.6
0.4
0.2
0.0
(
a)
pH 2
pH 3
pH 4
pH 6
0
20
40
60
80
100
t (min)
-1
1.0
4 g L BT4
(b)
-
1
5
6
8
g L BT4
-1
0.8
0.6
0.4
0.2
0.0
g L BT4
-1
g L BT4
0
20
40
60
80
100
t (min)
2 2
Fig. 8. Effect of major parameters on the OG removal: (a) pH, (b) BT4 addition, and (c) H O concentration ([OG]
-
1
-1
-1
=
2 2
50 mg L , [H O ] = 25 mg L , pH = 3, [BT4] = 6 g L )

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-
1
1.0
0.8
0.6
0.4
0.2
0.0
10 mg L H O2
2
-1
(
c)
25 mg L H O2
2
-1
5
0 mg L H O2
2
-1
1
00 mg L H O2
2
0
20
40
60
80
100
t (min)
Fig. 8.Continue
(
a) 3.5
3
.0
.5
.0
.5
.0
.5
.0
0 min
1
3
5
min
min
min
2
2
1
1
2
3
4
6
9
0 min
5 min
0 min
0 min
0 min
0 min
0 min
1
1
0
0
200
300
400
500
600
700
800
Wavelength (nm)
Fig. 9. (a) UV–vis spectral changes with reaction time and (b) proposed degradation pathway of OG ([OG] = 50
-
1
-1
-1
2 2
mg L , [H O ] = 25 mg L , pH = 3, [BT4] = 6 g L )

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OH
(
b)
N
N
3
SO Na
–
N=N–cleavage
3
SO Na
+
A
NO
2
OH
OH
NH
2
D
B
OHC
E
OH
O
O
2
N
OH
O
O
O
OH
O
HO
O
O
OH
O
HO
OH
HO
G
OH
H
I
F
SO 2-,CO , H O, et al.
4
2
2
Fig. 9.Continue