Photoelectro-Fenton/photocatalytic process for decolorization of an organic compound by Ag:Cd-1,4-BDOAH2 nano-photocatalyst: Response surface modeling and central composite design optimization

Article abstract of DOI:10.1016/j.molliq.2020.113689

In the present research, the synthesis of the cadmium-benzene-1,4-dioxyacetic acid (Cd-1,4-BDOAH₂) was carried out by the assistance of ultrasound waves, and then it was loaded with the low amount of Ag nano-particles. Subsequently, the Ag:Cd-1,4-BDOAH₂ nano-photocatalyst was applied for photoelectro-Fenton/photocatalytic (PEF-PH) degradation of brilliant green (BG) in the existence of visible light in aqueous media. The synthesized composite was characterized by FTIR, SEM, EDX, TEM, XRD, DRS, and PL. The best operational conditions including time, initial BG concentration, pH, amount of photocatalyst, current and amount of FeCl₃ on degradation efficiency (main and interactive) were as follows: 4 mg L^?1 of BG at pH of 5, 0.4 g L^?1 of Ag:Cd-1,4-BDOAH₂, 250 mA of current, and 0.3 mmol L^?1 of FeCl₃ obtained by central composite design. The PEF-PH degradation kinetic data of BG were successfully fitted to Langmuir–Hinshelwood (L–H) model with the values of 1.16 mg L^?1 min^?1 and 0.018 L mg^?1 for the rate constant (k_r) and (K_A), respectively.

Full text of DOI:10.1016/j.molliq.2020.113689

Journal Pre-proof
Photoelectro-Fenton/photocatalytic process for decolorization of
an organic compound by Ag:Cd-1,4-BDOAH2 nano-
photocatalyst: Response surface modeling and central composite
design optimization
Tahere Taghipour, Gholamreza Karimipour, Mehrorang Ghaedi,
Arash Asfaram, Hamedreza Javadian, Mohammad Mehdi
Sabzehmeidani, Hajir Karimi
PII:
S0167-7322(20)31778-5
DOI:
https://doi.org/10.1016/j.molliq.2020.113689
MOLLIQ 113689
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
24 March 2020
23 June 2020
25 June 2020
Please cite this article as: T. Taghipour, G. Karimipour, M. Ghaedi, et al., Photoelectro-
Fenton/photocatalytic process for decolorization of an organic compound by
Ag:Cd-1,4-BDOAH2 nano-photocatalyst: Response surface modeling and central
composite design optimization, Journal of Molecular Liquids (2020), https://doi.org/
1
0.1016/j.molliq.2020.113689
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Photoelectro-Fenton/photocatalytic process for decolorization of an organic
compound by Ag:Cd-1,4-BDOAH nano-photocatalyst: Response surface
2
modeling and central composite design optimization
a
a
a
Tahere Taghipour , Gholamreza Karimipour * , Mehrorang Ghaedi *, Arash
b
c
d
Asfaram , Hamedreza Javadian , Mohammad Mehdi Sabzehmeidani , and Hajir
d
Karimi
a
Chemistry Department, Yasouj University, Yasouj 75918-74831, Iran.
b
Medicinal Plants Research Center, Yasuj University of Medical Sciences, Yasuj, Iran.
c
Universitat Politècnica de Catalunya, Department of Chemical Engineering, ETSEIB,
Diagonal 647, 08028 Barcelona, Spain.
d
Chemical Engineering Department, Yasouj University, Yasouj, Iran.
*
Corresponding authors: E-mail address:
gh_kar@mail.yu.ac.ir (G. Karimipour);
m_ghaedi@mail.yu.ac.ir (M. Ghaedi)
1

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Abstract
In the present research, the synthesis of the cadmium-benzene-1,4-dioxyacetic acid (Cd-1,4-
BDOAH ) was carried out by the assistance of ultrasound waves, and then it was loaded with
2
the low amount of Ag nano-particles. Subsequently, the Ag:Cd-1,4-BDOAH nano-
2
photocatalyst was applied for photoelectro-Fenton/photocatalytic (PEF-PH) degradation of
brilliant green (BG) in the existence of visible light in aqueous media. The synthesized
composite was characterized by FTIR, SEM, EDX, TEM, XRD, DRS, and PL. The best
operational conditions including time, initial BG concentration, pH, amount of photocatalyst,
current and amount of FeCl on degradation efficiency (main and interactive) were as
3
-1
-1
follows: 4 mg L of BG at pH of 5, 0.4 g L of Ag:Cd-1,4-BDOAH , 250 mA of current, and
2
-1
0
.3 mmol L of FeCl obtained by central composite design. The PEF-PH degradation kinetic
3
data of BG were successfully fitted to Langmuir–Hinshelwood (L–H) model with the values
−
1
−1
−1
of 1.16 mg L min and 0.018 Lmg for the rate constant (k ) and (K ), respectively.
r
A
Keywords: Photoelectro-Fenton/photocatalytic, Decolorization, Ag:Cd-1,4-BDOAH2,
Response surface methodology, Light-emitting diode irradiation.
1
. Introduction
From past decades, organic pollutants such as dyes have caused a series of global
environmental problems, while various industries have an ever-greater role in contaminating
wastewaters [1-4]. In particular, the release of textile wastewater into the environment causes
serious threat to human health; consequently, synthetic dyes removal from colored effluent is
essential [5, 6]. Among all technologies for colored effluents treatment, advanced oxidation
processes (AOPs) are reported as effective technologies to remove toxic species and improve
water quality of industrial effluents [7-10]. The most popular Fenton’s chemistry method is
2

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2
+
based on treatment by advanced oxidation processes, and accordingly, Fenton reagents (Fe
and H O ) have more ability to degrade organic pollutants based on the formation of
2
2
•
hydroxyl radicals ( OH) [11]. The main properties of carbon based cathodes such as low
toxicity, stability, and good conductivity caused them to be applied in the E-Fenton process
[
12-14] for the degradation of organic materials from aqueous solutions [15-18].
Photoelectro-Fenton (PEF) is based on simultaneous irradiation of solution with visible or
ultra-violet (UV) light under PEF conditions, while accordingly light lead to acceleration in
3
+
•
2+
the photolysis of the complexes of Fe and enhancement in the OH and Fe ions as product
[
19, 20]. The combination of PEF and heterogeneous photocatalysts is the integrated
approach for efficient AOPs treatment.
In the past few years, the most popular photocatalysts are based on inorganic
semiconductors such as TiO and ZnO, which show unique advantages. Nevertheless, their
2
high and wide band gaps cause low efficiency, especially in the presence of visible light.
These kinds of photocatalysts are also expensive and consume high energy sources,
accordingly their usage are limited [21]. Designing and synthesizing such photocatalysts have
increased attentions in term of green heterogeneous catalysts. The combination of metal ions
and the organic ligands leads to different ligand-to-metal charge-transfer (LMCT) transitions.
This phenomenon plays a fundamental role in generating and manipulating band gaps that
influences on photocatalytic degradation efficiency [22, 23]. Due to these reasons, metal
organic photocatalysts have various functions, properties, and applications in electronics,
optics, magnetism, sensors, catalysis, etc. [24, 25]. For instance, Cd-based metal organic
materials are very promising candidates for photocatalytic application in terms of their
relatively narrow band gaps, exposure of high-density active sites, large pore size, facile
loading of highly dispersed heteroatoms, and high stability [26, 27].
3

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The heterogeneous photocatalyst particle needs the necessary amount of light for
activation under low operating costs and environmentally sustainable conditions, accordingly
the light-emitting diode (LED) lamps are extremely better than UV lamps [28, 29]. As
promising technology, photocatalysis has been widely studied for decomposition of very
toxic contaminants; therefore, efficient visible-light photocatalysts have been synthesized via
strategies such as doping and coupling with other semiconductors [30]. It is well known that
Ag nano-particles have high conductivity properties, which quickly and effectively transfer
photo-generated electron to inhibit recombination of electron hole pairs and improve the
photocatalytic activity [31, 32]. The combination between inorganic and organic building
blocks leads to the synthesis of metal organic materials with widely different compositions,
morphologies, and sizes. An effective path to design and synthesis such materials is based on
the application of metal-based ions/clusters with multidentate ligands (pre-designed or in situ
generated) that contain N- or O-donor atoms. Usually, binding benzene multicarboxylate
ligands with metal centers forms appropriate materials [33].
In the present work, the Ag:Cd-1,4-BDOAH as a new nano-photpcatalyst was
2
synthesized under green conditions, and its usage with blue LED irradiation for BG
degradation was studied in details. The central composite design (CCD) was utilized for
optimizing and estimating the main variables that are efficient and interaction effects of the
main variables (time, BG dye concentration, photocatalyst dosage, pH, current, and FeCl3
dosage) in a batch mode. Additionally, the PEF-PH kinetic data of BG degradation in LED
light/Ag:Cd-1,4-BDOAH instrument were tested with L–H kinetic model.
2
2
. Experimental
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2
.1. Materials and instruments
In this study, all chemicals and reagents such as hydroquinone, hydrochloric acid (37%), Iron
III) chloride, sodium sulfate anhydrous (99.0%), sodium hydroxide (97.0%), and BG (Table
) were obtained from Merck. Cadmium nitrate dehydrate (Cd (NO ) .2H O) was from
(
1
3
2
2
Sigma-Aldrich. Dae-Jung chemicals and metals Co., LTD provided chloroacetic acid. The
analytical grade of the reagents was used without further purification.
UV–Vis spectrophotometer model V-530, Jasco, Japan, was utilized for determining UV
spectrum of BG. pH/Redox/Temperature meter model AL20 pH (AQUALYTIC, Germany)
was applied for measuring pH. For recording XRD patterns, Bruker AXS (D8, Advance)
instrument was applied by the employment of CuK (1.54 Å) as the source of X-ray. The
recordance of the FTIR spectra of the 1,4-BDOAH , Cd-1,4-BDOAH , and Ag:Cd-1,4-
2
2
BDOAH was carried out using a spectrometer (Perkin Elmer RX-IFTIR) over the range of
2
-
1
4
1
000-500 cm . The determination of the average sizes of the of Cd-1,4-BDOAH and Ag:Cd-
2
,4-BDOAH and their morphologies were investigated by FESEM (ZEISS, model Sigma,
2
Germany). Philips CM-10 TEM working at 100 kV was applied to further investigate the
morphology and microstructure of the synthesized materials. The PL spectra of the samples
were obtained by a Perkin Elmer.
<
Table.1>
2
2
.2. Photocatalyst synthesis
.2.1. Synthesis of the 1,4-BDOAH by ultrasound-assisted method
2
A mixture was prepared by 7.1 g of chloroacetic acid and 2.75 g of hydroquinone, and then
NaOH (48 g in 97.5 mL water) was added dropwise to the prepared mixture and heated in an
oil bath at the temperature of 80 °C. After 1.5 h sonication of the obtained mixture at this
°
temperature, its temperature was decreased to 25 C, and it was kept in an ice bath for 30 min.
5

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To this mixture, hydrochloric acid was added dropwise until reaching neutral pH, and
subsequently, the temperature of the flask was naturally increased to room temperature. The
final white precipitates were filtered, washed with deionized water several times and finally
1
13
1
recrystallized. Subsequently, it was identified by HNMR and C NMR as follow: H NMR
1
3
(
D O, 250 MHz): δ = 4.34 (s, 4H), 6.81 (s, 4H). C NMR (D O, 62.9 MHz): δ = 67.24,
2
2
1
15.42, 152.12, 147.7, 177.02.
2
.2.2. Cd-1,4-BDOAH synthesis by ultrasound-assisted method
2
Under sonication condition, 0.4 mmol of 1,4-BDOAH was dispersed in 50 mL of distillated
2
water, and then it was added to 50 mL of cadmium nitrate dehydrate solution (24 mmol). The
mixture was sonicated at 25 °C for 30 min that led to the formation of white precipitates. The
final product was filtered and washed with deionized water.
2
.2.3. Ag:Cd-1,4-BDOAH synthesis by ultrasound assisted method
2
The cadmium salt solution was added to 1,4-phenylenedioxy diacetic acid solution while
sonicated at 25 °C for 10 min. Afterwards, the solution was thoroughly mixed with Ag(NO3)
(0.025 g). The sonication of the mixture was carried out for 10 min, which causes the reaction
and mixing to be done completely. Subsequently, the suspension was transferred to an
autoclave at 100 ºC for 24 h. After filtration of the obtained material, it was washed with
deionized water several times and finally dried in an oven at 60 ºC.
2
.3. Setup of experiment
General schematic of the experimental setup shown in Fig. 1 was used for BG degradation in
aqueous solution. As can be seen in the schematic, it includes power supply, graphite
cylindrical cathode (with 20 cm length and 9 mm diameter), platinum wire anode (76×1 mm
6

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2
dimensions), LED (light strip flexible SMD 5050 RGB, 14.4 W/m ), sparger (diffuser),
power source, and oxygen cylinder. The anode and cathode electrodes were plunged in the
aqueous solution. The constant electric current between a platinum anode and graphite
cathode is produced using a DC power supply for the electrochemical process. Around the
outer cylindrical pipe, a flexible RGB LED light strip is wrapped. The LED lamp is located in
a glass tube and put in the central of Pyrex (Simax, Czech) cylindrical photo reactor with the
volume of 3 L. A box covering with aluminum foil for the avoidance of light dispersion is
used to seal all parts of the system. In the present operation, the flexible RGB LED strip is
used to provide a blue light, and the spectral distribution of the light with the wavelength of
irradiation in the range of 465–470 nm. The working volume of the photo reactor is 1 L
solution consisting photocatalyst. To supply oxygen demand and solution mixing, an oxygen
diffuser is used.
<
Fig.1 >
2
.4. Experimental procedure
The PEF-PH process for BG degradation was conducted in a batch reactor at the temperature
of 25 ºC. Before each experiment run, the reactor glass and the tube glass of LED lamp were
filled with deionized water. According to CCD, specific amounts of FeCl and BG were
3
added to the aqueous solution containing 0.05 M Na SO as the supporting electrolyte and
2
4
well mixed for carrying out each experiment run. The solution pH in the range of 3.0-11 was
adjusted by adding HCl and/or NaOH, and it was transferred to the photo reactor equipped
with the lamp of LED with glass tube at its middle while the distribution of oxygen in
aqueous media was at a certain flow rate. After putting the solution of the reactor in a dark
condition for 20 min for the complete establishment of adsorption and desorption equilibrium
of BG dye over the surface area of the photocatalyst, the DC power supply, oxygen
distribution and LED irradiation were switched. At a specific time in each experiment run,
7

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the sample was brought out, and then centrifugation was done at 2000 rpm for 2 min for the
separation of the photocatalyst particles. Analyzing the sample was performed using a UV-
Vis spectrophotometer for determining the PEF-PH degradation of BG. The calculation of
degradation efficiency (R%) was done by Eq. 1 as following [34, 35]:
C -C


×100
0
t
R%BG
=
(1)
C0
-1
-1
Where C (mg L ) is the concentration of target at initial time and C (mg L ) is the
0
t
concentration of target at time t. The typical changes in the absorption spectra of UV–Vis
during the degradation by PEF-PH is shown in Fig. 2.
<
Fig.2 >
2
.5. Response surface methodology
For the evaluation of experimental model and recognition of the main factors that are
effective on the process and their interactions with the least number of experimental runs, and
also the optimization of multiple variables by a suitable model, response surface
methodology (RSM) is extensively used [36, 37]. RSM approach usually includes three
stages: design and experiments, response surface modeling by regression, and optimization. It
is used to evaluate the coefficients in a second order (quadratic) mathematical model [38, 39].
In the present work, CCD was applied for modeling the understudy PEF-PH degradation
process (Table 2). The effective factors including time (X ), BG concentration (X ), pH (X ),
1
2
3
photocatalyst dosage (X ), current (X ) and amount of FeCl (X ) were designed by
4
5
3
6
STATISTICA (version. 10.0) (Table 2). Based on the method of small CCD experiments, 33
individual experimental runs were proposed, and a quadratic polynomial equation was
8

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applied for the approximation of the mathematical relationship between the independent
parameters as follows [40]:
6
6
6
6
2
i
R%     X 
 X X   X 
(2)
0

i
i
 ij
i
j

ii
i1
i1 j1
i1
Where R% is the response predicted by the model (degradation efficiency of BG), and the Xi
and X are known independent variables for each experimental run. β parameter is the
j
0
constant of the model, and β , βii, and βij are respectively the linear coefficient, quadratic
i
coefficient, and the coefficient of cross-product [41]. The fitted model quality can be
evaluated by analysis of variance (ANOVA) that is reliable. The determination of model
significance is based on the values of P-value and F-value obtained by ANOVA, while the
best optimum levels for each factors to attain maximum degradation efficiency are
determined by desirability function (DF) with value between 0.0 and 1.0 (low and high
desirability, respectively) [42].
<
Table.2>
3
3
. Result and discussion
.1. Photocatalyst characterization
The FTIR spectrum in Fig. 3 for 1,4-BDOAH , Cd-1,4-BDOAH , and Ag:Cd-1,4-BDOAH
2
2
2
-1
-1
recorded from 4000 to 500 cm indicates a large broad band in the range of 2000-3400 cm ,
-1
attributing to the acidic O-H group stretching vibrations. The band at 1604 cm assigns to the
-1
groups of carboxylate (C=O), and the observed band at 1220 cm relates to the C-O
-1
stretching vibrations. In addition, the bands shown at 1058 cm is related to the symmetric
and asymmetric stretching vibrations of O-C=O and the stretching vibration of C-O, as well.
-1
The bending of aromatic C-H is observed at 684 and 813 cm [43]. A large shift in the FTIR
spectra of the Cd-1,4-BDOAH and Ag:Cd-1,4-BDOAH is assigned to the O-H, C=O and
2
2
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O-C=O groups transmittance as compared with the free ligand, showing the interaction
2
+
-1
possibility of Cd ion with the OH group of free ligand. The peak observed at 1600 cm for
the benzene ring skeleton stretching of 1,4-BDOAH can be seen at the same wavenumber
2
for the Cd-1,4-BDOAH and Ag:Cd-1,4-BDOAH , proving the successful synthesis of the
2
2
materials.
<
Fig.3>
The analysis of the surface morphology for the Cd-1,4-BDOAH and Ag:Cd-1,4-BDOAH
2
2
was examined using SEM in Figs. 4 (a-c) and (d-f), respectively. As it is observed, the Cd-
,4-BDOAH is a nano-powder with the typical needle shape of the particles.
1
2
<
Fig.4>
The EDX spectrum (Fig. 5a) clearly shows the existence of O, C, and Cd in the sample, and
the changes observed in the chemical composition of the sample (Fig. 5b) containing Ag
clearly shows the incorporation of Ag in Ag:Cd-1,4-BDOAH . The A% of the elements in the
2
obtained samples is in good agreement with the compositions, indicating the favorable
synthesis of understudy materials.
<
Fig.5>
The Cd-1,4-BDOAH and Ag:Cd-1,4-BDOAH in terms of shape and size are respectively
2
2
shown in Fig. 6 (a) and (b). The approximate size of the Cd-1,4-BDOAH particles is about
2
1
00 nm and Ag nano-particles in the Ag:Cd-1,4-BDOAH are in the range of 30-80 nm (Fig.
2
6
b). The TEM results are in agreement with those of SEM.
<
Fig.6>
The XRD patterns of the samples were applied to determine their crystalline structures. Fig. 7
a) reveals the absence of CdO and CdO . The XRD peaks prove that the Cd-1,4-BDOAH is
(
2
2
successfully synthesized [44, 45]. Comparing the XRD patterns of the Cd-1,4-BDOAH and
2
1
0

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Ag:Cd-1,4-BDOAH indicates no extra peaks corresponding to secondary phases, while the
2
absence of silver and silver oxide peaks in the synthesized photocatalysts is probably due to
the low amount of the loaded Ag [46]. According to the XRD, the average particle size is 80
nm using Debye-Scherrer equation.
<
Fig.7>
In general, photocatalytic efficiency can be seriously influenced by rapid recombination rate
of the photo-generated charge carriers. The direct measurement of the electron-hole
recombination rate is done by photoluminescence (PL) intensity and accordingly increasing
in intensity of spectrum denotes faster recombination. The samples were investigated by PL
spectra and the results in Fig. 8 for the Cd-1,4-BDOAH and Ag:Cd-1,4-BDOAH (using an
2
2
excitation wavelength of 360 nm) denote a lower intensity of fluorescence emission for the
Ag:Cd-1,4-BDOAH , implying a lower rate of electron-hole recombination and higher
2
photocatalytic activity in comparison with the Cd-1,4-BDOAH2.
<
Fig.8>
3
.2. Comparison of photocatalytic activity of the Cd-1,4-BDOAH and Ag:Cd-1,4
2
BDOAH2
The degradation efficiency of BG using the Cd-1,4-BDOAH and Ag:Cd-1,4 BDOAH (Fig.
2
2
9
) reveals higher degradation efficiency using the Ag:Cd-1,4 BDOAH in comparison with
2
the Cd-1,4-BDOAH , which prove that the incorporation of Ag leads to enhance in the PEF-
2
PH activity.
<
Fig.9>
3
.3. Analysis of central composite design
1
1

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ANOVA analysis for CCD is applied to find the best operational conditions and construct
mathematical equation, which are applicable to correlate signal to effective terms. The error
functions were applied to fit the model to empirical data for calculating the significance of
each term based on the comparison of the calculated F-value and P-value [47, 48]. The
results represented in Table 3 for BG degradation by the Ag:Cd-1,4-BDOAH reveals the F-
2
value of 81.57 obtained by the model. The lack-of-fit (LOF) of the proposed model with the
amount of 0.0502 for BG photo degradation by the Ag:Cd-1,4-BDOAH is not significant,
2
and "Lack of Fit F-value" is 6.15. The reduced model equation in terms of the coded factors
was given as follows:
R%BG = + 65.4 +16.8 X - 4.0 X -5.1X +5.6X +11.3X +8.7X +1.9X X +
1
2
3
4
5
6
1
2
6
3
.8X X + 7.3X X +5.0X X + 0.9X X + 8.3X X + 2.5X X + 7.9X X -
(3)
1
3
1
4
1
5
1
6
2
3
2
4
2
6
2
2
2
2
2
.3X X +5.7 X X +1.0X X -0.6 X + 6.6X +1.5X +1.4X + 4.1X
3
4
4
5
5
6
1
3
4
5
6
<
Table. 3>
3
.4. Response surface plots
D RSM plots were applied to indicate the combined effects and main interactions between
3
the factors on PEF-PH of BG by the Ag:Cd-1,4-BDOAH (Fig. 10a-c). The fitted response
2
surface plots (R%) versus the main variables (Fig. 10a) shows the presence of interaction
between reaction time and BG concentration on the PEF-PH degradation efficiency.
Increasing BG degradation efficiency of BG with raising time is due to higher contact time
between the Ag:Cd-1,4-BDOAH and BG. Increasing initial dye concentration leads to
2
reduce in degradation efficiency. Lower BG concentration is proportional with higher ratio of
reaction sites to BG concentration, which causes an increase in degradation efficiency of the
understudy system. The effects of pH and photocatalyst dosage on PEF-PH degradation
efficiency and their interactions are shown in Fig. 10(b). The reactor probably enhances the
1
2

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+
production of H ion species at lower range of pH, which mainly causes an increase in
degradation efficiency. In addition, PEF-PH degradation efficiency increases by the
-
production of •OH ion species due to increase in pH value. An enhance in degradation
efficiency of BG is observed when photocatalyst dosage raised due to the higher specific
surface of photocatalyst at higher dosage. The results show that current and the amount of
FeCl have a positive effect on PEF-PH degradation efficiency (Fig. 10b). The higher value
3
of current and FeCl amount cause an increase in PEF rate in the aqueous solution.
3
<
Fig. 10>
3
.5. Optimization condition study
The present study used a desirability function (DF) approach based on CCD model for the
optimization of the parameters in the PEF-PH process [38, 39]. The predicated values and DF
for the PEF-PH degradation of BG dye by the Ag:Cd-1,4-BDOAH is displayed in Fig. 11.
2
The maximum degradation efficiency represents the desired conditions while desirability is
close to 1. The desirability profile represents the maximum PEF-PH degradation of 100%
(
DF = 1) for BG using the Ag:Cd-1,4-BDOAH . The optimum conditions for this percentage
2
-1
are the process time (X ) of 50 (min), BG concentration (X ) of 4 (mg L ), pH (X ) of 5,
1
2
3
-1
Ag:Cd-1,4-BDOAH (X ) dosage of 0.4 (g L ), current (X ) of 250 (mA), and FeCl amount
2
4
5
3
-1
(
X ) of 0.3 (mmol L ).
6
<
Fig.11>
3
.6. Kinetic effect study
To design a PEF-PH reactor, the degradation rate assigning to the studies of kinetic is
considered as an important stage. The degradation kinetic in aqueous solutions can be
expressed by first-order kinetic model by the bellowing equation:
1
3

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


C 
0
ln
 kt
(4)

Ct

All parameters corresponding to this equation have been described elsewhere [9, 42]. Fig.
2(a) shows the fitness of the experimental data by pseudo-first order model with a high
order of linearity. The L–H model was employed to analyze the degradation kinetic of BG
49-51]. By using the definition of surface coverage factor (θ), the reaction between the
1
[
interface of solid-liquid with the photocatalyst and FeCl for BG degradation was described
3
by L-H kinetic model:
dC
dt
KC0
R  
 k   k
(5)
(6)
r
r
1 KC  K C
0
s
s
All parameters corresponding to this equation have been described elsewhere [9, 42]. The
final form of the kinetic rate is as follows:
1
1
1


R
k K C
0
k r
r
A
Where K is defined by K =K/ (1+K C ). The result in Fig. 12(b) shows a straight line
A
A
s
s
formed by the plot of 1/R versus 1/C according to L–H model with the coefficient of
0
determination value of 0.998. The plot gives the intercept of 1/k and slope of 1/k K . The
r
r
A
−
1
−1
rate constant values of PEF-PH degradation were calculated to be k =1.164 mg L min and
r
−
1
K =0.0181 Lmg .
A
<
Fig.12>
3
.7. Effect of procedure on the treatment efficiency
To evaluate the performance of individual and combinational procedures, different treatment
processes including catalysis (adsorption), photolysis (light irradiation without catalyst),
photocatalysis (light irradiation + catalyst), Fenton (electro Fenton + catalyst), and
1
4

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photoelectro-Fenton (electro Fenton + light irradiation + catalyst) were investigated (Fig.
1
3a). The experimental results indicate the treatment percentages of 6.74, 19.22, 32.22,
5
8.54, and 76.45% using adsorption, photolysis, photocatalyst, Fenton, and photoelectro-
Fenton, respectively. The contribution of Fenton and photocatalytic processes leads to an
increase in BG degradation efficiency due to the generation of the hydroxyl radicals. Fig. 13b
indicates the reusability of the photoelectron Fenton for the degradation of BG in three cycles
under visible-light (LED) irradiation, and the experimental results reveal that the photo-
catalyst can be reused three times, indicating the good stability of understudy photoelectron
Fenton.
<
Fig.13 >
3
.8. Proposed mechanism of BG degradation by the PEF-PH process
The mechanism of PEF-PH degradation using the synthesized photocatalyst under the
irradiation of blue LED is illustrated in Fig. 14. Since the blue LED provides energy higher
than the energy band gap of the synthesized photocatalyst, it can be excited by light energy,
+
leading to the formation of electron-hole pairs. The valence holes (h VB ) are strong oxidants
and are able to oxidize various contaminants and form hydroxyl radicals, while the
_
conduction band electrons (e CB ) are good reductants and can reduce the dissolved oxygen
.
-
to O [52, 53].
2
-
+
Ag:Cd-1,4-BDOAH
2
+ h  Ag:Cd-1,4-BDOAH
2
(e + h )
(7)
+
.
H O
+
h OH
(8)
(9)
2
-
.-
2
O + e  O
2
3
+
On the other hand, iron is added as a Fe salt and H O is produced by reduction of oxygen
2
2
at the cathode.
3
+
-
2+
Fe + e  Fe
(10)
1
5

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+
-
O + 2H + 2e  H O
(11)
2
2
2
In the presence of light, the photo-Fenton process can be more efficient by the photo
3
‏+
2+
reduction of Fe to Fe ‏and the generation of additional hydroxyl radicals.
2
+
3+
-
.
Fe + H O  Fe + OH + OH
(12)
(13)
2
2
3

2

.
Fe  H O  Fe  H OOH
2
2
-•
•
In both processes, a large number of O and OH is produced that can act as strong oxidants
2
and play an efficient role in BG degradation.
<
Fig.14 >
3
.9. Comparison of the PEF-PH with other systems
BG degradation efficiency by the PEF-PH was compared with formerly reported photocatalyst, and
the results is represented in Table 4. The results indicate that the PEF for Ag:Cd-1,4-BDOAH is
2
suitable and shows reasonable degradation performance for BG.
<
Table. 4>
4
. Conclusion
In this work, the synthesis of a photocatalyst (Cd-1,4-BDOAH ) with a low band gap was
2
performed without using any toxic and expensive solvents. By loading this photocatalyst with
Ag, a better photocatalyst, namely Ag:Cd-1,4-BDOAH , was synthesized with a lower band
2
gap that showed higher efficiency in PEF-PH degradation of BG under blue LED irradiation.
The combination of photoelectro-Fenton and photocatalytic processes showed an efficient
approach for degradation of BG dye. The optimum values to reach the maximum degradation
-1
efficiency using the Ag:Cd-1,4-BDOAH were obtained at 50 min of time, 4 mg L of BG
2
-1
concentration, 5 of pH, 0.4 g L of the Ag:Cd-1,4-BDOAH , 250 mA of current, and 0.3
2
1
6

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-1
mmol L of FeCl . At these conditions, the maximum degradation efficiency of BG by the
3
PEF-PH was obtained (100%). The treatment percentages of 6.74, 19.22, 32.22, 58.54, and
7
6.45% obtained by adsorption, photolysis, photocatalyst, Fenton, and photoelectro-Fenton,
respectively. All hypotheses about the applicability of the PEF-PH were achieved, and it
showed to be high efficient. As a result, it could be considered as a promising approach for
the treatment of dye wastewater.
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Conflict of Interest Statement
The authors declare that they have no known competing financial interests or
personal relationships that could have appeared to influence the work reported in
this paper.

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Credit Author Statement

Dr. Tahere Taghipour
Methodology; Writing - original draft, Validation, Investigation, Software; Formal analysis

Dr. Gholamreza Karimipour
Supervision, Conceptualization, Methodology; Writing - review & editing, Validation,
Project administration; Resources; Funding acquisition.

Prof. Mehrorang Ghaedi
Supervision, Conceptualization, Methodology; Writing - review & editing, Validation,
Project administration; Resources; Funding acquisition.

Dr. Arash Asfaram
Investigation, Software; Formal analysis; Writing - Original Draft; Writing - review &
editing, Validation.

Dr. Hamedreza Javadian
Methodology; Writing - review & editing; Validation, Investigation

Dr. Mohammad Mehdi Sabzehmeidani
Investigation, Software; Formal analysis; Validation; Writing - original draft, Writing -
review & editing

Dr. Hajir Karimi
Methodology, Investigation, Software; Formal analysis.
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Fig. 1. Schematic diagram of the reactor setup for electrolytic–photocatalytic process: (1)
power supply, (2) the reaction flask, (3) Pt anode, (4) graphite cathode, (5) LED lamp, (6)
sparger (diffuser), (7) power source, (8) flow meter, and (9) oxygen cylinder.
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Fig. 2. UV–Vis absorption spectra of photoelectro-Fenton/photocatalytic degradation of BG.
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Fig. 3. FT-IR spectra of 1,4-BDOAH , Cd-1,4-BDOAH , and Ag:Cd-1,4-BDOAH .
2
2
2
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Fig. 4. SEM images of (a-c) Cd-1,4-BDOAH and (d-f) Ag:Cd-1,4-BDOAH .
2
2
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Fig. 5. EDX spectra of the prepared (a) Cd-1,4-BDOAH and (b) Ag:Cd-1,4-BDOAH .
2
2
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Fig. 6. TEM images of (a) Cd-1,4-BDOAH and (b) Ag:Cd-1,4-BDOAH .
2
2
2
9