Effect of oxidants on photoelectrocatalytic decolourization using α-Fe2O3/TiO2/activated charcoal plate nanocomposite under visible light

Article abstract of DOI:10.1039/c4ra15889j

The present study is to investigate the effect of oxidants H₂O₂, S₂O₈^2-, BrO₃^-, ClO₃^- and IO₄^- with different concentrations on photoelectrocatalytic decolourization of Lanasol yellow 4G (LY4G) as a model contaminant using α-Fe₂O₃/TiO₂/activated charcoal plate (ACP) nanocomposite under visible light. In this system, the decolourization efficiency increased with increasing BrO₃^-, ClO₃^- and IO₄^- doses but reached an optimum amount with H₂O₂ and S₂O₈^2- at 1 mM. Experimental data revealed that the decolourization rate of LY4G in all of the processes obeyed pseudo-first-order kinetics. Total organic carbon (TOC) results indicated that 21% and 100% of organic substrate were mineralized respectively after 80 min and 8 h. The gas chromatography-mass spectrometry (GC-MS) analysis was employed to identify the intermediate products. Also, a plausible degradation pathway was proposed. Finally, the real wastewater treatment was investigated by chemical oxygen demand (COD) measurements. This journal is

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ARTICLE TYPE
Effect of oxidants on photoelectrocatalytic decolourization using αꢀ
Fe2O3/TiO2/Activated charcoal plate nanocomposite under visible light
Baharak AyoubiꢀFeiz, Soheil Aber * and Mohsen Sheydaei
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
ꢀ
The present study is to investigate the effect of oxidants H2O2, S2O82ꢀ, BrO3 , ClO3ꢀ and IO4ꢀ with different
concentrations on photoelectrocatalytic decolourization of Lanasol yellow 4G (LY4G) as a model
contaminant using αꢀFe2O3/TiO2/Activated charcoal plate (ACP) nanocomposite under visible light. In
ꢀ
ꢀ
ꢀ
this system, the decolourization efficiency increased with increasing BrO3 , ClO3 and IO4 doses but
2ꢀ
10 reached an optimum amount with H2O2 and S2O8 at 1 mM. Experimental data revealed that the
decolourization rate of LY4G in all of the processes obeyed pseudoꢀfirstꢀorder kinetics. Total organic
carbon (TOC) results indicated that 21% and 100% of organic substrate was mineralized respectively
after 80 min and 8 h. The gas chromatographyꢀmass spectrometry (GCꢀMS) analysis was employed to
identify the intermediate products. Also, a plausible degradation pathway was proposed. Finally, the real
15 wastewater treatment was investigated by chemical oxygen demand (COD) measurements.
basic problem is higher tendency of photogenerated eꢀ/h+ to
recombine rather than contribution in the formation of reactive
radicals which imposes low efficiency of photocatalytic
degradation.10 The second problem is the wide band gap of TiO2
50 (3.2 eV), so only UV light can be utilized to promote the eꢀ from
the CB to the VB of this semiconductor.11 The third problem is
separation of TiO2 powders from batch slurry photoreactor after
photocatalytic process.12 Numerous efforts have been made to
overcome the problems and promote the efficiency of
55 photocatalytic degradation process.
The aim of this work is to enhance photocatalytic performance
of TiO2 nanoparticles by solving all of the mentioned problems
simultaneously through: 1) Impregnating TiO2 nanoparticles with
Hematite (αꢀFe2O3). This iron oxide has a band gap of 2.2 eV,
60 therefore it is an appropriate sensitizer for TiO2 to improve the
photocatalytic properties under visible light irradiation in addition
to its ability to inhibit the recombination of the photogenerated eꢀ
/h+ 13 2) Immobilization of these nanoparticles on the surface of
activated charcoal plate (ACP) as a conductive support material
65 and applying an anodic bias to drive away photogenerated
electrons from the surface of TiO2 and inhibit the eꢀ/h+
recombination, 3) Using inorganic oxidants with efficient
electron accepting properties to trap the promoted eꢀ from VB of
TiO2, avoid eꢀ/h+ recombination, generate more reactive radicals
70 and other oxidizing species, and consequently enhance the
photocatalytic degradation efficiency.14
1. Introduction
Advanced oxidation processes (AOPs) are a group of available
and promising processes for the removal of almost all organic
pollutants in water and wastewater.1 AOPs are processes in which
20 reactive radicals are produced under different sources of energy
such as electrical or chemical energy. These radicals are
involved in effective removal of persistent hazardous organic
pollutants or changing the pollutants into less toxic
intermediates.4 The rate constants of the oxidative reactions
25 between these radicals and organic compounds are approximately
106–109 M−1 S−1. This high reaction rate has been attributed to
their high oxidative power (1.90 V versus normal hydrogen
electrode (NHE)).5, 6
Heterogeneous photocatalytic degradation process via
30 photoactivation of semiconductors such as TiO2, ZnO and ZnS is
considered as one of the most promising AOPs for destruction of
waterꢀsoluble, nonꢀbiodegradable organic pollutants. UV light
has been normally used as an energy source in this process.
During photocatalytic degradation process, the photocatalyst
35 absorbs energy from irradiated light. This energy leads to the
excitation of electrons (eꢀ) from valence band (VB) of
photocatalyst into the conduction band (CB) and development of
holes (h+) in the VB. The reaction of h+ with H2O and/or OHꢀ
causes to generate reactive radicals.7
Among various photocatalysts, there is meaningful attention
on using TiO2 as an effective and suitable photocatalyst for the
degradation of organic pollutants because of its particular
properties including: low toxicity, chemical stability, insolubility
and low price.8, 9 However, some problems are associated with
2
3
40
With this background, in the present study αꢀFe2O3 and TiO2
nanoparticles were immobilized on the surface of the ACP
(denoted as αꢀFe2O3/TiO2/ACP). The photoelectrocatalytic
75 performance of the αꢀFe2O3/TiO2/ACP nanocomposite in
decolourization of Lanasol yellow 4G (LY4G) solution in the
45 photocatalytic degradation process in the presence of TiO2. The
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ꢀ
ꢀ
ꢀ
presence of H2O2, S2O82ꢀ, BrO3 , IO4 , and ClO3 was evaluated
under visible light irradiation. Mineralization of dye and the
produced intermediates were studied by total organic carbon
(TOC) removal and gas chromatography–mass spectrometry
(GC–MS), respectively. Finally, chemical oxygen demand (COD)
analysis was employed to investigate the mineralization of real
textile wastewater contains LY4G.
2.3. Decolourization experiments
Fig. 1 shows the experimental set up of the photoreactor used for
the treatment of the contaminated solution. It was composed of a
round Pyrex reactor with the capacity of 150 mL, a magnetic
60 stirrer, a pH meter (Eutech pH 510, Malaysia), a potentiostat (CV
320ꢀxh, Hirad, Iran), a visible light lamp (9 W, Nama Noor Co.,
Iran) with intense emission lines at 425, 500, 550 and 600 nm
(Fig. 2) on the top of the reactor, the prepared αꢀFe2O3/TiO2/ACP
as working electrode, Pt plate (3 cm×3 cm) as counter electrode
65 and a saturated calomel electrode (SCE) (+ 0.24 V vs. standard
hydrogen electrode) as reference electrode. The working and
counter electrodes were held horizontally in parallel by a 5ꢀmm
distance. All of experiments were done with a constant αꢀ
Fe2O3/TiO2/ACP nanocomposite electrode (12.60 g). During the
70 experiments, a constant voltage of 700 mV was applied to this
electrode vs. SCE. The distance between the visible light lamp
and the solution surface was 5 cm. The total solution volume was
115 mL and it consisted of 10 mg Lꢀ1 LY4G, 8 g Lꢀ1 Na2SO4 as
electrolyte and different amounts of the oxidant. The initial
75 concentration of the oxidant in the solution ranged from 0.5 to 5
mM. All over the decolourization experiments, pH of the solution
was adjusted to 6±0.3 by H2SO4 and NaOH solutions. 2 mL
sample was withdrawn at predetermined time intervals and
immediately after measuring the concentration of the residual
80 LY4G in the solution by UV–Visible spectroscopy (PerkinꢀElmer
550 SE) at λmax equal to 419 nm, the sample was returned to the
reactor.
5
2. Experimental
2.1. Materials
10 Charcoal as a cheap and easily available material was purchased
from a local market in Tabriz, Iran. TiO2 P25 powder with an
average size of 20ꢀ30 nm (Degussa, Germany), αꢀFe2O3 powder
with an average diameter of 20ꢀ40 nm (US Research
Nanomaterials, Inc., USA), LY4G (CibaꢀGeigy Co.,
15 Switzerland), potassium bromate, (KBrO3, 99%, Fluka),
potassium peroxydisulfate, (K2S2O8, 98%, Fluka), potassium
chlorate, (KClO3, 99%, Fluka), potassium periodate (KIO4,
99.8%, Fluka), and hydrogen peroxide (H2O2, 30% w/w, Merck)
were used in this work. All other chemicals were of analytical
20 reagent grade.
2.2. Preparation and characterization of αꢀFe2O3/TiO2/ACP
nanocomposite
The ACP electrode with the dimensions of 5.7 cm ×3.1 cm ×1.1
cm was produced by physical activation of charcoal under CO2
25 atmosphere at 850 oC using the method described in our previous
work.7 The produced ACP was soaked in 2ꢀpropanol solution
before the immobilization of TiO2 and αꢀFe2O3 on its surface.
Simultaneously, 0.18 g of TiO2 and αꢀFe2O3 powders mixture
were dispersed in 60 mL of 2ꢀpropanol solution containing 0.07 g
30 Mg(NO3)2.6H2O as electrolyte.15 The suspension was sonicated
for 1 h using an ultrasonic bath (Grant, XB6, England). An
electrophoretic cell was designed which consisted of a 150ꢀml
beaker, the prepared ACP electrode as cathode and stainless steel
with the dimensions of 6 cm × 4 cm as anode. The electrodes
35 were placed horizontally with a 5ꢀmm distance of each other.
During the electrophoretic deposition, constant deposition voltage
of 40 V was applied by a DC power supplier (Micro, Iran) for 7
min. Finally, the prepared nanocomposite was dried at room
temperature for 24 h.
Fig. 1 The experimental set up for photoelectrocatalytic process.
40
Scanning electron microscopy (SEM) image and energy
dispersive Xꢀray (EDX) analysis of αꢀFe2O3/TiO2/ACP surface
were performed using a MIRA3 FEGꢀSEM (Tescan, Czech)
microscope. The compositions of the nanocomposite were
detected by Xꢀray fluorescence (XRF) using a Philips model
45 PW1480 (the Netherlands) instrument. The BrunauerꢀEmmettꢀ
Teller (BET) surface area of the ACP and αꢀFe2O3/TiO2/ACP
nanocomposite was determined through N2 adsorption at 77 K in
the relative pressure range from 0.05 to 0.9 using a BelsorpꢀMini
(Japan) surface analyser. UVꢀVisible diffuse reflectance spectra
50 (DRS) of TiO2/ACP and αꢀFe2O3/TiO2/ACP nanocomposite
samples were measured by using Sinco (S4100, Korea) UVꢀ
Visible spectrophotometer. Furthermore, photoluminescence
emission spectra of TiO2/ACP and αꢀFe2O3/TiO2/ACP
nanocomposites were recorded with an excitation wavelength of
55 285 nm on a spectrofluorometer (Jasco, FPꢀ6200, Japan).
85
Fig. 2 Emission spectrum of the visible light lamp.
Decolourization efficiency (%) = [(C0 ꢀ Ct)/C0] × 100 was used
to determine the percent of decolourization of LY4G, where C0
(mg Lꢀ1) is the initial concentration of LY4G and Ct (mg Lꢀ1) is its
90 concentration after certain irradiation time.
2
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is mainly due to the immobilization of TiO2 and αꢀFe2O3 nano particles
within the porous structure of the
ACP.17
It is noted that after each photoelectrocatalytic degradation run,
the used nanocomposite was regenerated by applying reverse
voltage of ꢀ240 mV in 0.01 M NaOH solution with the volume of
115 mL (electrodesorption). This process lasted for 30 min.
5
2.4. Characterization techniques on the solution
GCꢀMS was used in order to identify produced intermediates
during photoelectrocatalytic process. N,Oꢀbisꢀ(trimethylsilyl)
acetamide was used after extraction of intermediates to obtain
silylated compounds. These compounds could be detected more
10 convenient by GCꢀMS method. The GCꢀMS equipped with an
Agilent 6890 gas chromatograph with a 30 m–0.25 mm HPꢀ5MS
capillary column and an Agilent 5973 mass spectrometer (Agilent
Technologies, Palo Alto, Canada). The value of TOC in the
solution was analysed with a TOC analyser (TOC, VCHS,
15 Shimadzu, Japan). In order to check the leaching of iron during
treatment process, the amount of total iron was analysed after
photoelectrocatalytic experiments in the solution by colorimetric
method using 1,10ꢀphenantroline.16
2.5. Treatment of real textile wastewater
20 To compare the decolourization efficiency in the real textile
wastewater
with
the
synthetic
dye
solution,
the
photoelectrocatalytic experiment for the removal of LY4G from
the real wastewater was done according to section 2.3 without
adding any electrolyte due to the wastewater conductivity. The
25 wastewater sample containing LY4G was obtained from Farsh &
Patu textile factory in Tabriz, Iran. Concentration of LY4G,
COD, pH, and conductivity of filtered wastewater were 29 mg Lꢀ
1, 330 mg Lꢀ1, 6.5 and 2.21 mS cmꢀ1, respectively.
3. Results and discussion
30 3.1. Morphology and characterization of αꢀFe2O3/TiO2/ACP
55
Fig. 3 SEM images of (a) ACP and (b) αꢀFe2O3/TiO2/ACP.
The morphology of the ACP and αꢀFe2O3/TiO2/ACP
nanocomposite is illustrated in Fig. 3. The SEM micrograph of αꢀ
Fe2O3/TiO2/ACP nanocomposite (Fig. 3b) showed the deposition
of TiO2 and αꢀFe2O3 particles on the surface of ACP compared
35 with that of nonꢀcoated ACP (Fig. 3a). The EDX microanalyses
of the αꢀFe2O3/TiO2/ACP nanocomposite are listed in Table 1. It
can be clearly seen that this nanocomposite was rich in carbon,
oxygen, titanium and iron which approved the immobilization of
TiO2 and αꢀFe2O3 nanoparticles on the surface of the ACP.
40 Furthermore, existing a low amount of Mg in EDX microanalyses
may be attributed to Mg(NO3)2 salt which was used as the
electrolyte in the electrophoretic deposition method.
Reflectance spectra of TiO2/ACP and αꢀFe2O3/TiO2/ACP
nanocomposite samples are shown in Fig. 4. It can be seen that
the absorption for αꢀFe2O3/TiO2/ACP was higher than that of
60 TiO2/ACP in the range of 400–700 nm. This makes it possible to
use the αꢀFe2O3/TiO2/ACP as a photoactive catalyst under visible
light irradiation in the photoelectrocatalytic process.
Table 1 EDX microanalyses of the αꢀFe2O3/TiO2/ACP nanocomposite.
Elements
Weight (%)
Atomic (%)
C
5.11
13.37
O
38.71
59.25
Ti
36.84
18.83
Fe
17.45
7.65
Mg
1.89
0.90
Total (%)
100
100
45
Results obtained from XRF analysis indicated that chemical
compositions of the prepared nanocomposite include 6.8% TiO2,
4.1% Fe2O3, 1% MgO, 86.1% C and 2% other compounds.
Specific surface area of the ACP and the αꢀFe2O3/TiO2/ACP
Fig. 4 DRS spectra of TiO2/ACP and αꢀFe2O3/TiO2/ACP.
50 nanocomposite samples were 462 m2 gꢀ1 and 291.4 m2 gꢀ1,
respectively. The reduced surface area of the αꢀFe2O3/TiO2/ACP
65
As shown in Fig. 5, the photoluminescence intensity of αꢀ
Fe2O3/TiO2/ACP nanocomposite was lower than that of
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TiO2/ACP nanocomposite. This indicates that separation of
photogenerated charge in αꢀFe2O3/TiO2/ACP sample was higher
than TiO2/ACP.18
heterogeneous Fe3+ on the surface of αꢀFe2O3 to produce reactive
radicals (Eqs. 3 and 4).21
H2O2 + eC−B → OH− +•OH
(1)
(2)
(3)
(4)
30
H2O2 + O2•− → OH− +•OH + O2
Feα3+
+ H2O2 → Feα2+
+ HO2• + H +
3
ꢀFe
O
ꢀFe
O
2
3
2
Feα2+
+ H2O2 → Feα3+
+ HO• + OH−
ꢀFe
O
ꢀFe O
2
2
3
3
As can be seen in Fig. 6, further increase in the initial H2O2
concentration led to decrease in decolourization efficiency. It is
35 well known that in the presence of excess H2O2, the amount of
available •OH decreases due to the scavenger effect of H2O2 on
•OH which leads to the production of other radicals with low
oxidation potential such as HO2• (Eq. 5).22, 23 In addition, H2O2 is
a hole scavenger (Eq. 6) and at high concentrations suppresses
40 the contribution of photogenerated holes in reaction with H2O
and/or OHꢀ leading to
concentration.24
a decrease in OH free radical
H2O2 +•OH → H2O + HO2•
H2O2 + 2hV+B → O2 + 2H +
(5)
(6)
5
Fig. 5 Photoluminescence spectra of TiO2/ACP and αꢀFe2O3/TiO2/ACP
nanocomposites excited by 285 nm irradiation.
3.2. Decolourization process
In this process the effect of inorganic oxidants such as H2O2,
45
Similar result has been reported by Govindan et al.25 for
photocatalytic degradation of Pentachlorophenol by visible light
sensitive NꢀFꢀcodoped TiO2 photocatalyst. They investigated the
effect of H2O2 concentration from 0.02 to 0.14 mM and obtained
a high degradation at 0.1 mM.
2
ꢀ
S2O8 , BrO3 , ClO3ꢀ and IO4ꢀ with different dosages (0.5 to 5 mM)
10 was investigated on the photoelectrocatalytic decolourization of
LY4G by αꢀFe2O3/TiO2/ ACP nanocomposite under visible light.
3.2.1. Effect of H2O2
50
2ꢀ
The effect of H2O2 as a strong oxidant was studied on the
15 decolourization of LY4G under visible light irradiation on the αꢀ
Fe2O3/TiO2/ACP nanocomposite (Fig. 6). As can be seen in Fig.
6, increase in the concentration of H2O2 up to 1 mM led to an
increase in decolourization efficiency.
3.2.2. Effect of S2O8
Results obtained from experiments conducted to determine the
2ꢀ
effect of S2O8 concentration on decolourization efficiency of
LY4G are represented in Fig. 7. It can be seen from this figure
55 that the decolourization efficiency generally increased with
2ꢀ
2ꢀ
increasing the initial S2O8 concentration up to 1 mM. S2O8
inhibits the eꢀ/h+ recombination by accepting the conduction band
electron (Eq. 7). Moreover, SO4 is a selective reactive radical
•−
which is produced according to equation 8:
60
S2O82− + eC−B → SO4•− + SO42−
(7)
•−
The SO4 can react with organic molecules by three different
mechanisms: electron transfer, hydrogen abstraction and addition
on double bond.26 This radical also convert H2O to •OH according
to the equation 8:
65
SO4•− + H2O → SO42− +•OH + H +
(8)
2ꢀ
20
Fig. 6 Effect of H2O2 on the decolourization efficiency of LY4G
As can be seen in Fig. 7, further increase in S2O8
([dye]0= 10 mg Lꢀ1, Voltage = 700 mV, pH = 6, and [Na2SO4] = 8 g Lꢀ1).
concentration from 1 mM to 5 mM led to a decrease in
decolourization efficiency. This can be attributed to more
H2O2 is contributed in decolourization process through three
basic ways. The first is the trapping of eꢀ in conduction band of
TiO2 (Eq. 1) in order to decrease eꢀ/h+ recombination.19 The
increase in concentration of SO4•−. Excessive SO4 can act as
•−
70 HO• and SO4 scavenger, reducing the degradation efficiency
•−
28
(Eqs. 9 and 10).27,
Similar result has been reported for
sonochemical degradation of Rhodamine B, Methylene Blue,
25 second is the reaction of H2O2 with the superoxide radical anion
forming •OH (Eq. 2).20 The other way is the reaction with
4
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ꢀ
3.2.4. Effect of BrO3
Acid Orange II and Acid Scarlet Red 3R dyes in aqueous solution
using sulphate radicals activated by immobilized cobalt ions. 29
ꢀ
30 Fig. 9 shows the effect of BrO3 concentration on decolourization
efficiency of LY4G. It indicates that decolourization efficiency
remarkably increased with increasing BrO3 concentration from
ꢀ
SO4•− +•OH → SO42− + 0.5O2 + H +
SO4•− + SO4•− → S2O82−
(9)
ꢀ
0.5 to 5 mM. BrO3 is an efficient electron acceptor. So, it can
prevent the eꢀ/h+ recombination at the semiconductor surface for
35 efficiently production of •OH (Eq. 12).32 Moreover, production of
(10)
•
BrO2 as an oxidant promotes decolourization efficiency (Eq.
13).14 Yu et al. investigated the effect of BrO3ꢀ on the degradation
rate of Methylene Blue by UV/TiO2. They found that with
increasing BrO3ꢀ concentration from 1 to 24 mM at pH=7, the rate
40 constant increased from 0.101 to 0.479 (minꢀ1).30
BrO3− + 6H + + 6eC−B → Br− + 3H2O
BrO3− + 2H + + eC−B → BrO2• + H2O
(12)
(13)
5
Fig. 7 Effect of S2O82ꢀ concentration on the decolourization efficiency of
LY4G ([dye]0= 10 mg Lꢀ1, Voltage = 700 mV, pH = 6, and [Na2SO4] = 8
g Lꢀ1).
ꢀ
3.2.3. Effect of IO4
10 The obtained results for decolourization efficiency of LY4G as a
ꢀ
function of the IO4 concentration are shown in Fig. 8. Increase in
ꢀ
IO4 concentration led to enhance the decolourization efficiency.
This phenomenon can be explained by the fact that with increase
in IO4 concentration, probability of recombination of eꢀ/h+
ꢀ
15 decreases due to the capturing the photogenerated electrons of the
excited TiO2 (Eq. 11).30 So, the available number of h+ enhances
which causes to produce more hydroxyl radicals. The effect of
ꢀ
Fig. 9 Effect of BrO3 on the decolourization efficiency of LY4G ([dye]0=
45
10 mg Lꢀ1, Voltage = 700 mV, pH = 6, and [Na2SO4] = 8 g Lꢀ1).
ꢀ
IO4 concentration in the range of 1.0–10.0 mM on degradation of
Basic Red 46 and Basic Yellow 28 dyes was investigated in
ꢀ
3.2.5. Effect of ClO3
20 UV/TiO2/IO4 system by Gözmen et al.31. They also found that the
−
ꢀ
The effect of ClO3 concentration on decolourization efficiency of
50 LY4G is illustrated in Fig. 10.
degradation efficiency of both dyes was slightly enhanced by
ꢀ
increasing IO4 concentration.
IO4− + 8eC−B + 8H + → 4H2O + I −
(11)
ꢀ
Fig. 10 Effect of ClO3 on the decolourization efficiency of LY4G
([dye]0= 10 mg Lꢀ1, Voltage = 700 mV, pH = 6, and [Na2SO4] = 8 g Lꢀ1).
As it has been indicated in Fig. 10, decolourization efficiency
55 increased gently with increasing the initial ClO3 concentration
ꢀ
ꢀ
25 Fig. 8 Effect of IO4 on the decolourization efficiency of LY4G ([dye]0=
10 mg Lꢀ1, Voltage = 700 mV, pH = 6, and [Na2SO4] = 8 g Lꢀ1).
from 0.5 to 5 mM. Like to the other investigated inorganic
ꢀ
oxidants, ClO3 causes to separation of eꢀ/h+ by accepting the
conduction band electron through Eq. 14.33 More increase in the
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ꢀ
ꢀ
ꢀ
initial ClO3 concentration led to the enhancement of reactive
radicals production through the reaction of h+ with H2O and/or
OHꢀ in solution. Similar result has been reported by Seyedꢀ
IO4 > H2O2> S2O82ꢀ > ClO3 .
Table 2 The correlation coefficients and the reaction rate constants for
visible light/αꢀFe2O3/TiO2/ACP system with and without oxidants.
ꢀ
Dorraji et al. in UV/ZnO/ClO3 system for the removal of
Parameter Concentration (mM) k × 100 (min ꢀ1
)
R2
0.995
5
Diazinon.33
With the
absence of
oxidant
H2O2
ꢀ
1.72
ClO3− + 6H + + 6eC−B → Cl− + 3H2O
(14)
0.5
1
2.41
3.57
2.62
2.19
1.86
2.94
2.15
1.81
2.10
2.33
2.84
4.31
1.91
2.10
2.47
7.54
1.88
2.02
2.20
2.45
0.997
0.994
0.996
0.994
0.990
0.999
0.995
0.994
0.997
0.993
0.996
0.992
0.981
0.984
0.979
0.932
0.997
0.998
0.997
0.997
3.3. Kinetic study and comparing the oxidants
2.5
5
0.5
1
Pseudoꢀfirstꢀorder kinetic equation is most widely used to
describe heterogeneous photocatalysis reactions.34 Therefore, the
2ꢀ
S2O8
10 experimental data of decolourization of LY4G in visible light/αꢀ
Fe2O3/TiO2/ACP system at different time intervals were
examined to fit pseudoꢀfirstꢀorder kinetic model. The rate
constant values, k (minꢀ1), as a function of oxidants concentration
were calculated from the slopes of the straightꢀline portion of the
15 pseudoꢀfirstꢀorder plots of ln (C0/Ct) against t where, C0 (mg Lꢀ1)
is the initial concentration of dye and Ct (mg Lꢀ1) is the
concentration at time t. The obtained results are listed in Table 2.
The correlation coefficients (R2) resulted from the pseudoꢀfirstꢀ
order model are high (>0.93). Therefore, the pseudoꢀfirstꢀorder
20 model can describe the kinetics of the photoelectrocatalytic
decolourization of LY4G under visible light using αꢀ
Fe2O3/TiO2/ACP nanocomposite.
2.5
5
0.5
1
ꢀ
IO4
2.5
5
0.5
1
ꢀ
BrO3
2.5
5
0.5
1
ꢀ
ClO3
2.5
5
60
Based on the rate constant values in Table 2, it can be seen that
the decolourization rate of LY4G in the presence of the oxidants
25 is more than that in their absence. It means that presence of
oxidants in all investigated concentrations had a positive effect on
the decolourization process and enhanced the decolourization
efficiency. In addition, the rate constant values increased with
ꢀ
ꢀ
ꢀ
increasing the concentration of IO4 , BrO3 and ClO3 . However,
30 H2O2 and S2O82ꢀ have the optimum amount of 1 mM in this study.
These results are in agreement with the experimental data shown
in Figs. 4ꢀ8.
NezamzadehꢀEjhieh and Khorsandi20 indicated that the
photocatalytic degradation of 4ꢀnitrophenol using ZnO/nanoꢀ
35 clinoptilolite under UV irradiation was well described by the
pseudoꢀfirstꢀorder kinetic model and the rate constants of the
ꢀ
reaction were 0.0012, 0.0016 and 0.12 minꢀ1 when BrO3
Fig. 11 Comparison of decolourization efficiency in the presence of
oxidants using αꢀFe2O3/TiO2/ACP nanocomposite under visible light in
photoelectrocatalytic process ([dye]0= 10 mg Lꢀ1, Voltage = 700 mV, pH
= 6, [Na2SO4] = 8 g Lꢀ1 and Time=80 min).
concentrations were 0.5, 1 and 5 mM, respectively. Furthermore,
the pseudoꢀfirstꢀorder rate constants for photocatalytic
40 degradation of Pyridine with 10 mM BrO3 under UV irradiation
using TiO2 and Ag/TiO2 were 3.59×10−3 and 5.53×10−3,
65
respectively as reported by Tian et al. 35
.
3.4. Comparison of photoelectrocatalytic activity of αꢀ
ꢀ
Fe2O3/TiO2/ACP/BrO3 , αꢀFe2O3/TiO2/ACP and TiO2/ACP
The values of decolourization efficiency in visible light/αꢀ
Fe2O3/TiO2/ACP photoelectrocatalytic process in the presence of
45 oxidants were compared in Fig.11. The best concentration
according to the obtained results was chosen for all of the
oxidants. The results show that among oxidants, the most
effective one for decolourization of LY4G by αꢀFe2O3/TiO2/ACP
Fig. 12 shows decolourization efficiency of LY4G with initial
concentration of 10 mg Lꢀ1 at pH of
6 in different
70 photoelectrocatalytic degradation processes. Comparison of the
results indicates that the photoelectrocatalytic performance of αꢀ
Fe2O3/TiO2/ACP sample was higher than that of the TiO2/ACP.
This can be attributed to the interfacial charge transfer between αꢀ
Fe2O3 and TiO2 semiconductors. It is assumed that photon of
75 visible irradiation excites electron from valence band of αꢀFe2O3
to the conduction band, leaving holes in the valence band.
Simultaneously, under this irradiation, electron in valence band of
TiO2 tends to transfer to the nearest band with the lowest energy
(valence band of αꢀFe2O3). This transformation between
80 semiconductors can suppress the high rate of eꢀ/h+ recombination
ꢀ
nanocomposite under visible irradiation was BrO3 with almost
50 100% decolourization yield in 80 min. Another effective oxidant
ꢀ
was IO4 which had a decolourization efficiency of 98% at the
2ꢀ
ꢀ
same time. Application of H2O2, S2O8 and ClO3 led to 96.5%,
93%, and 89% decolourization of LY4G after 80 min,
respectively. Based on the results, decolourization efficiencies of
55 LY4G for all of the investigated oxidants used in visible light/αꢀ
ꢀ
Fe2O3/TiO2/ACP system were found to be in the order of BrO3 >
6
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DOI: 10.1039/C4RA15889J
and consequently increase the yield of photoelectrocatalytic
process.36 A possible mechanism has been proposed in Fig. 13.
ꢀ
Furthermore, as can be seen in Fig.12, presence of BrO3 anion
along with αꢀFe2O3/TiO2/ACP nanocomposite led to more
ꢀ
5
increase in decolourization efficiency. The BrO3 anion as an
effective electron accepter hinders the photogenerated eꢀ/h+
recombination which causes more increase in production of
reactive radicals.
35 Fig. 14 UVꢀVisible absorbance spectra of (a) untreated dye solution, (b)
solution treated by photoelectrocatalytic degradation process, (c) the
solution contains pollutants electrodesorbed from the surface of the used
nanocomposite.
3.5. Leaching test and reusability of αꢀFe2O3/TiO2/ACP
40 According to Pourbaix diagram 37, αꢀFe2O3 is the most stable
form of iron oxides and iron leaching can only occur at low pH
and low potential values. Since all of the experiments in this work
were carried out in approximately neutral pH (6) and potential of
700 mV, so iron leaching is impossible theoretically. However, at
45 the end of the all experiments, the total amount of iron in solution
was measured to assess the durability of αꢀFe2O3/TiO2/ACP
nanocomposite. No iron was detected in the solution after the
experiments thus confirmed the absence of leaching.
10 Fig. 12 Comparison of photoelectrocatalytic decolourization efficiency of
ꢀ
LY4G in the presence of αꢀFe2O3/TiO2/ACP/BrO3 , αꢀFe2O3/TiO2/ACP
and TiO2/ACP.
The performance of αꢀFe2O3/TiO2/ACP nanocomposite was
50 investigated in three subsequent decolourization cycles under
identical conditions ([dye]0= 10 mg Lꢀ1, Voltage = 700 mV, pH =
ꢀ
6, [BrO3 ] = 5 mM and [Na2SO4] = 8 g Lꢀ1). In order to regenerate
the nanocomposite after each experiment, it was separated from
treated solution and immersed in 0.01 M NaOH solution with the
55 volume of 115 mL and the electrodesorption was conducted at
voltage of −240 mV for 30 min. Then the regenerated
nanocomposite electrode was washed with distilled water and
then used for next run. The values of decolourization efficiency
in three successive cycles are shown in Fig. 15. The obtained
60 results show that the nanocomposite can be effectively reused as
a catalyst several times without significant activity loss.
Fig. 13 Proposed mechanism of electronꢀhole separation in αꢀ
Fe2O3/TiO2/ACP during photoelectrocatalysis under visible light
irradiation (electronꢀhole recombination process is not shown here).
15
To confirm the contribution of developed reactive radicals in
degradation of dye molecules, UVꢀVisible absorbance spectra of
untreated dye solution, solution treated by photoelectrocatalytic
20 degradation process and the solution contains pollutants
electrodesorbed from the surface of the used nanocomposite were
recorded as can be seen in Fig. 14. The UVꢀVisible spectrum of
the treated solution by photoelectrocatalytic degradation process
shows that the absorption peak around 419 nm corresponding to
25 LY4G decreased through treatment process (Spectrum b) due to
the removal of dye from the solution. Furthermore, the spectrum
of solution contains pollutants electrodesorbed from the surface
of the used nanocomposite (Spectrum c) shows that the
concentration of dye in this solution was too low compared to its
30 concentration in untreated dye solution (Spectrum a). This
indicates that during the treatment process, most part of adsorbed
dye molecules were degraded through photoelectrocatalytic
degradation process.
Fig. 15 The reusability of αꢀFe2O3/TiO2/ACP nanocomposite within three
consecutive decolourization cycles. Experimental conditions: [dye]0= 10
ꢀ
65 mg Lꢀ1, Voltage = 700 mV, pH = 6, [BrO3 ] = 5 mM and [Na2SO4] = 8 g
Lꢀ1.
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3.6. Identification of intermediates of LY4G decolourization
by photoelectrocatalytic process using BrO3 /visible light/αꢀ
Fe2O3/TiO2/ACP system and proposed degradation
mechanism
analysis are listed in Table 3. It should be pointed out that quick
oxidation of the process prevented the detection of some
intermediates with large molecular structure.
ꢀ
The possible reaction pathway of LY4G decolourization can
5
In order to distinguish intermediates produced through 15 be concluded by Fig. 16. Degradation could take place by
decolourization of LY4G solution, 100 mL of 20 mg Lꢀ1 LY4G
cleavage of C–S, C–N, C–C or N=N bonds. Short chained
compounds such as organic acids were produced after opening
the aromatic rings through successive attacks by OH. Finally,
ꢀ
solution by adding 5 mM BrO3 , applied potential of 700 mV and
•
pH of 6 was treated using αꢀFe2O3/TiO2/ACP system under
visible light for 4 min.
these intermediates could be mineralized to CO2 and H2O.
10
Molecular structure and main fragments identified by GCꢀMS
20
ꢀ
Table 3 Identified intermediates during photoelectrocatalytic decolourization of LYG4 using BrO3 /visible light/αꢀFe2O3/TiO2/ACP system.
Structure
Compound name
Acetic acid
Retention time (min)
4.58
Main fragments a
117, 75, 45
Acetamide
4.59
5.06
116, 75
Ethanimidic acid
203, 147, 114, 73, 45
Hydroxy acrylic acid
Hydroxypyruvic acid
Pyruvic acid oxime
5.78
5.78
5.78
217, 147, 73, 45
217, 147, 73, 45
217, 147, 73, 45
2,5ꢀDihydroxybenzoic acid
2,4ꢀDihydroxybenzoic acid
20.43
26.63
355, 73
355, 73
a Corresponding values for the trimethylsilyl derivative.
analysis show that the TOC of LY4G was decreased from 1.793
mg Lꢀ1 to 1.411 mg Lꢀ1 after 80 min and reached to 1.335 µg Lꢀ1
after 8 h. These results indicate that approximately 21% of the
35 carbon in LY4G was mineralized within 80 min, while this
solution was completely decolourized within this reaction time.
Comparison of the initial TOC value with TOC8h indicated the
complete mineralization of LY4G during photoelectrocatalytic
process under visible irradiation for 8 h.
3.7. Mineralization analysis
25 One of the basic advantages of the AOPs is that these processes
eventually destruct the organic compounds to CO2 and H2O. In
this work, the mineralization efficiency of 10 mg Lꢀ1 LY4G at
pH=6, applied potential of 700 mV and electrolyte concentration
of 8 g Lꢀ1 by adding 5 mM BrO3 during photoelectrocatalytic
ꢀ
30 process using αꢀFe2O3/TiO2/ACP under visible light was
determined by TOC measurement analysis. Results of TOC
8
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Fig. 16 Probable decolourization mechanism of LY4G.
ꢀ
BrO3 at contact time of 80 min removed approximately 59% of
LY4G in the sample with the volume of 115 mL. Results showed
that the decolourization efficiency from real wastewater was
10 lower than that of the synthetic sample at the same conditions.
This can be explained by the presence of various organic and
3.8. Colour and COD reduction of real wastewater
Studies on real textile wastewater sample containing LY4G
showed that αꢀFe2O3/TiO2/ACP nanocomposite under visible
light at pH=6, applied potential of 700 mV by adding 5 mM
5
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60
65
70
12.
13.
Furthermore, the efficiency of αꢀFe2O3/TiO2/ACP nanocomposite
at mentioned conditions under visible light was evaluated by the
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5
ꢀ
the photoelectrocatalytic process using BrO3 /visible light/αꢀ
14.
15.
Fe2O3/TiO2/ACP system can effectively degrade textile
wastewater
16.
17.
18.
Conclusions
10 The photoelectrocatalytic decolourization of LY4G using the αꢀ
Fe2O3/TiO2/ACP nanocomposite under visible light was found to
be an efficient technique. The obtained results indicated that the
decolourization efficiency was obviously affected by different
75 19.
20.
ꢀ
ꢀ
ꢀ
concentrations of H2O2, S2O82ꢀ, BrO3 , ClO3 and IO4 and these
15 oxidants improved the performance of αꢀFe2O3/TiO2/ACP under
visible irradiation. The TOC results proved that the designed
photoelectrocatalytic system had appropriate ability for
degradation and mineralization of the model contaminant. Some
of the degradation intermediate compounds were identified by
20 GCꢀMS technique. Eventually, COD measurement confirmed the
proper treatment of real wastewater by photoelectrocatalytic
21.
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Acknowledgment
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This paper is published as part of a research project supported by
25 the University of Tabriz Research Affairs Office. The authors
would like to express their gratitude to the University of Tabriz
for financial supports and Dr. I. Ahadzadeh from Research
Laboratory of Electrochemical Instrumentation and Energy
Systems, Faculty of Chemistry, University of Tabriz for
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This journal is © The Royal Society of Chemistry [year]