Using microwave along with TiO2 for degradation of 4-chloro-2-nitrophenol in aqueous environment

Article abstract of DOI:10.1134/S1070427216120090

In this project, microwave (MW) irradiation, photolysis, and photo catalyst were used for degradation of 4-chloro-2-nitro phenol (4-C2NP) in aqueous environment. The influence of main operating parameters such as initial pH, initial concentration of 4C2NP

Full text of DOI:10.1134/S1070427216120090

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2016, Vol. 89, No. 12, pp. 1985−1990. © Pleiades Publishing, Ltd., 2016.
VARIOUS TECHNOLOGICAL
PROCESSES
Using Microwave along with TiO2 for Degradation
of 4-Chloro-2-nitrophenol in Aqueous Environment1
Aref Shokri* and Ali Hassani Joshagani
a Young Researchers and Elite Club, Arak Branch, Islamic Azad University, Arak, Iran
b Department of Chemical Engineering, Faculty of Technical Engineering, Arak Branch, Islamic Azad University, Arak, Iran
*e-mail: aref.shokri3@gmail.com
Received September 2, 2016
Abstract—In this project, microwave (MW) irradiation, photolysis, and photo catalyst were used for degradation
of 4-chloro-2-nitro phenol (4-C2NP) in aqueous environment. The influence of main operating parameters such as
initial pH, initial concentration of 4C2NP, power dissipation and the dosage of TiO2 on the degradation efficiency
has been investigated. The optimum conditions was obtained such as initial concentration of 4C2NP at 30 mg L−1,
initial pH at 6, power dissipation at 16 W for UV irradiation, and the amount of TiO2 at 0.2 g L−1. The removal of
4C2NPand chemical oxygen demand (COD) after 100 min of reaction in the combined method (MW/UV/TiO2) was
obtained as 80.5% and 47.3%, respectively. Almost all processes are followed from the pseudo first order kinetics
and the degradation rate of 4C2NP obeyed the following order: UV/TiO2/MW > UV/TiO2 > MW/UV > UV>MW.
DOI: 10.1134/S1070427216120090
INTRODUCTION
water. In recent years, many advanced treatment methods
[4‒8] have been employed for the degradation of phenolic
compounds in wastewater. Moreover, microwave (MW)
irradiation can be operative because it has localized
higher energy densities and constant intense heating [9].
The activation energy was decreased by the nonthermal
effect of absorbed MW energy and the chemical bond
of the pollutants in a solution was weakened [10]. The
energy of MW has been reported to be sufficient to break
the chemical bonds of several organic compounds [11].
Microwave can be used along with hydrogen peroxide
and photo catalyst and the combined system can give
better results for the degradation of organic pollutants
because MW and photo catalyst had a same mechanism
for generating hydroxyl radicals from water.
One of the most important problems of the last century
is environmental pollution according to technological
developments. Industrial use of phenolic compounds
and their products over the last decades has led to the
environmental pollution. These compounds are toxic
and carcinogenic and they can persevere for a long
time in the environment because of their resistance to
microbiological remediation [1].
Advanced oxidation processes (AOPs), which are
an in situ production of highly chemical oxidants such
as hydroxyl radicals, have appeared as a main class of
technologies to accelerate the non-selective oxidation
[2]. 4-Chloro-2-nitrophenol (4C2NP) is a toxic pollutant
that is extended in the environment, including ground
water and soil [3]. The wastewater containing 4C2NP
should be treated before discharge to the environment.
The conventional treatment methods used for the removal
of phenolic compounds are not pleasing according to the
high stability and solubility of phenolic compounds in
In this study, degradation of 4C2NPby MW irradiation
was investigated for the first time. Anovel method based
on the use of MW and UV in the presence of catalyst
TiO2 was used for increasing the degradation efficiency.
The combined method including UV/MW/TiO2, can
increase the amount of radical generation and removal
of 4C2NP. The energy requirements for the microwave
and UV operations can be reduced by combining because
1 The text was submitted by the authors in English.
1985

1986
AREF SHOKRI, ALI HASSANI JOSHAGANI
of the lower required reaction times. The optimization of
operating parameters by one factor at a time and kinetic
analysis were investigated.
samples were analyzed by a UV–Vis spectrophotometer
(Agilent, 5453,American) and the maximum wavelength
was 230 nm.
In addition, the samples were tested by high perfor-
mance liquid chromatography (HPLC), Knauer Germany.
It was equipped with a spectrophotometer (Platm blue
Germany) and a reverse phase column, filled with 3 μm
Separon C18. The column was 4.6 mm in diameter and
150 mm in length. The isocratic method with a solvent
mixture of 75% methanol and 25% deionized water was
used with a flow rate of 1 mL min‒1. The samples were
measured by both spectrophotometric and HPLC methods
and both of them gave similar results, approximately.
The chemical oxygen demand (COD) was determined
by the standard closed reflux and colorimetric method
[12] and the absorbance of samples was measured by
spectrophotometer at 600 nm.
EXPERIMENTAL
Materials. 4C2NP is purchased from Fluka Co.,
Germany, sulfuric acid (98%) and sodium hydroxide
of analytical reagent grade were obtained from Merck.
TiO2 in anatase form is purchased from Aldrich. All the
chemicals were used as received from the supplier without
further purification. Distillate water has been used to
prepare aqueous solutions of the pollutant at required
concentrations.
Experimental methodology. The experimental setup
of the reactor is comprised of a cylindrical glass reactor
(800 mL) equipped with the required features for the
progress of the different processes. For all microwave
tests, the power dissipation was kept constant at 150W.
The removal percent of 4C2NPand COD as a function
of reaction time are introduced by the following equations
(Eqs. 1, 2):
In each run, about 500 mL of an aqueous solution
of 4C2NP with preset pH value was transferred to the
reactor. The low-pressure mercury vapor lamps were used
as the radiation source which was emitted a wavelength
about 254 nm. The temperature of the solution in the
reactor was kept fixed at 25°C by a thermal bath. The
UV/TiO2 system without microwave was performed in a
stainless steel photochemical reactor with the continuous
aeration system. Before the photo catalysis tests, the
requisite TiO2 was added into the reactor and kept in the
dark for 30 min for equilibrium. At first, the withdrawn
photo catalysis samples were centrifuged to remove the
TiO2 prior to tests.
(1)
(2)
where [4C2NP]0 and [COD]0 are the initial concentrations
of 4C2NP and COD at the start of the reaction, and
[4C2NP] and [COD] are the concentration of the pollutant
and the extent of COD at time t, respectively.
RESULTS AND DISCUSSION
The impact of initial concentration of the 4C2NP has
been explored over the range of 30–90 mg L‒1 and the
effect of UV power on photo-degradation of 4C2NP has
been examined over the range of 8–32 W. The pH of the
solution was adjusted using H2SO4 (0.1 M) and NaOH
(0.1 M) solution. In the microwave related investigates,
a glass reactor with the solution were placed at the
center of the rotating plate directly below the magnetron
source. After irradiation, the temperature of the solution
increased, but it was cooled to 25°C using cold water bath
and 3 mL samples were withdrawn each time. A process
strengthening additive such as TiO2 was employed at
optimized conditions of both microwave and UV to
increase the degradation efficiency.
Effect of UV power. In Photolysis process, the UV
light was interacted with the molecules of pollutants
and water, causing their dissociation to the following
fragments (Eq. 3) [13].
4C2NP + hv → intermediates + hv
→ CO2 + H2O + Cl−
(3)
The result from the influence of UV power (8–32 W)
on the amount of degradation at an initial concentration of
4C2NP as 30 mg L‒1, natural pH of 6 and a temperature
of 25°C is presented in Fig. 1. It is obvious that with
an increase in UV power from 8 to 16 W the amount of
degradation rises from 30.2% to 47.5% and then slightly
decreased at UV power of 24 and 32 W. The maximum
All tests were executed for the 100 min of reaction and
samples were reserved at periodic intervals of 20 min. The
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 89 No. 12 2016

USING MICROWAVE ALONG WITH TiO2 FOR DEGRADATION
1987
5 0
40
5 0
40
30
20
10
0
30 mg L‒1
50 mg L‒1
70 mg L‒1
90 mg L‒1
8 W
30
20
10
0
16 W
24 W
32 W
0
20
40
60
80
100
120
0
20
40
60
80
100
120
Time, min
Time, min
Fig. 1. Effect of the power of UV on the removal of 4C2NP in
photolysis process (initial concentration of 4C2NPat 30 mg L‒1,
temperature at 25°C, and pH at 6).
Fig. 2. The effect of initial concentration of 4C2NP on the
degradation rate in potolysis (16W UV power, temperature at
25°C, and pH of 6).
Effect of initial pH of the solution. It is essential
to investigate the effect of pH on the degradation rate,
because the initial pH has a main influence on the
degradation of organic pollutants. The results for the
effect of pH on the amount of degradation at an initial
concentration of 30 mg L‒1 and at a temperature of 25°C
over the range of pH at 3–9 are shown in Fig. 3. It has
been seen that the amount of degradation rises from
pH 3 to pH 7 and then drops in pH 9. The maximum
degradation of 47.5 and 48.4% has been obtained at pH 6
and 7, respectively. The amount of degradation at pH 3,
5, 9 was 24.3, 35 and 27.2%, respectively. The extent
of degradation obtained under the alkaline media is less
than acidic conditions. Perhaps the oxidation potential
and reactivity of the free radicals is poorer under basic
conditions, so the degradation efficiency of 4C2NP is
lower at pH 9. The structure of 4C2NP molecules helps
the photon attack satisfactorily in mild acidic conditions
[16] and greater degradation is achieved. The strong acid
used in acidic condition which causes interference in the
photon interaction with the pollutant resulting in lower
degradation at pH of 3. These achievements are in good
agreement with the previous study [17]. The difference
in degradation between pH 6 and 7 was very low, so the
natural pH of the solution at 6 was selected as an optimum
pH from an economic point of view, and it was used as
an optimum in the rest of the experiments.
degradation efficiency of 47.5% was achieved at 16 W.
As it can be seen from Fig. 1, the degradation rate of
4C2NP at UV power of 24 and 32W was low. It can
be ascribed to the existence of extreme UV photons at
higher UV power that leads to the formation of more
intermediate compounds which disturb the photons
to attack the initial pollutant [14]. In the presence of
more UV light water splits more to generate hydroxyl
radicals and larger amount of hydroxyl radicals cause
a recombination reaction to form H2O2 which hinders
the degradation efficiency. It is essential to note that the
aims for an amount of degradation as a function of power
dissipation in the case of photolysis was based on the
type of compound.
Effect of initial concentration of 4C2NP. The
influence of initial concentration of 4C2NP on the
degradation efficiency has been explored over the range
of 30–90 mg L‒1 at optimized UV power (16W), pH at
6 and temperatures of 25°C. It can be seen from Fig. 2
that the amount of degradation was dropped from 47.5%
at 30 mg L‒1 to 21.5 % at 90 mg L‒1 of the pollutant.
Fundamentally during Photolysis, the degradation of
pollutant occurs by direct attack of the UV photons
and oxidation with hydroxyl radicals. Subsequently, at
higher concentrations of the original compound, the
availability of the hydroxyl radicals as well as UV photons
is limited so, the amount of degradation falls. Shokri et
al. [15] also stated higher degradation rates at a lower
initial concentration of 2-Nitrophenol. Accordingly, it
is proven that Photolytic oxidation is improved at low
concentrations of pollutants and therefore 30 mg L‒1 of
initial concentration has been selected as an optimum
concentration and employed in the rest tests.
Influence of titanium dioxide dosage in UV
photolysis process. Various concentrations of TiO2 over
the range of 0.1–0.4 g L‒1 were employed at an initial
pH of 6 and initial concentration of 4C2NP at 30 mg L‒1.
The results in the degradation efficiency of 4C2NP in
different TiO2 concentration at UV power dissipation of
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1988
AREF SHOKRI, ALI HASSANI JOSHAGANI
60
5 0
40
30
20
10
0
pH = 3
pH = 5
pH = 6
pH = 7
pH = 9
80
60
40
20
0
0.1 g L‒1
0.2 g L‒1
0.3 g L‒1
0.4 g L‒1
0
20
40
60
80
100
120
0
20
40
60
80
100
120
Time, min
Time, min
Fig. 4. Degradation of 4C2NP at different TiO2 loading in
photolysis process (the power of UV at 16 W, temperature at
25°C, initial concentration of pollutant at 30 mg L–1 and pH
of 6).
Fig. 3. Effect of initial pH on the degradation rate of 4C2NP
for photolysis process (the power of UV at 16W, temperature
at 25°C, and initial concentration of 30 mg L–1).
Degradation and mineralization of 4C2NPby MW,
MW/UV and MW/UV/TiO2 processes. An aqueous
solution containing 30 mg L‒1 of 4C2NP, at 25°C and
optimum pH of 6, has been treated by microwave (MW)
using power dissipation of 150 W. As it can be seen from
Fig. 5, in the only microwave system, the degradation
efficiency of 4C2NP was 21.5% at 100 min of reaction.
Water is a polar compound and it has the capacity to
absorb microwave energy. The dipole movement of the
atoms or molecules in dielectric compounds is produced
by friction and it causes dispersion of energy as heat.
A large penetration depth directs poor absorbance
of the radiation, whereas a short penetration depth
identifies the majority of surface heating. The suitable
penetration depth of water makes the bond of organic
pollutants in the solution to be broken with the aid of
non-thermal influence of microwave radiation and it
16 W and a temperature at 25°C are presented in Fig. 4.
As it can be seen from Eqs. 4–7, the catalyst TiO2 gets
excited in the presence of UV, and the electron-hole pairs
(e‒CB, h+vb) are generated on the surface of the catalyst [18].
The molecular oxygen in the solution acts as an electron
‒
·
acceptor to form Superoxide radical anions (O2 ) and
prevents the recombination of electrons and holes. In
addition, the valence band holes (h+vb) and Superoxide
radical anions interact with the water molecule giving
reactive hydroxyl radicals. These hydroxyl radicals
oxidize the organic pollutant, non-selectively.
TiO2 + hv < 387 nm → e‒CB + h+vb,
(4)
h+vb + H2O(ads) → OH(ads)
,
(5)
(6)
‒
h+vb + OH(ads) → OH(ads)
,
OH(ads) + 4C2NP(ads) → intermediates +OH(ads)
→ CO2 + H2O.
(7)
With an increase in the dosage of TiO2 from 0.1 to
0.2, the amount of degradation of 4C2NP was increased
from 63.5 to 77% and then decreased to 60.5% at 0.4 g L–1
of TiO2. However, direct photolysis (without TiO2) gave
47.5% degradation of 4C2NP. This phenomenon can be
originated from this fact that the production of hydroxyl
radicals was high at suitable amount of catalyst. But, the
extra amount of TiO2 delays the light penetration into the
solution because of an increase in turbidity of the solution
and leads to lower degradation efficiency [19–20]. The
difference between 0.2 and 0.3 g L–1 of catalyst was low;
therefore 0.2 g L–1 of TiO2 was selected as an optimum
dosage.
80
60
40
20
0
MW
UV
MW/UV
UV/TiO2
UV/TiO2/MW
0
20
40
60
80
100
120
Time, min
Fig. 5. Degradation of 4C2NPin different processes (MW power
at 150 W, temperature at 25°C, initial concentration of pollutant
–1
–1
at 30 mg L , 0.2 g L of TiO2 and pH of 6).
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 89 No. 12 2016

USING MICROWAVE ALONG WITH TiO2 FOR DEGRADATION
2
1989
y = 0.0164x + 0.0812
R2 = 0.9893
MW
UV
1.6
1.2
0.8
0.4
0
y = 0.0146x + 0.0714
R2 = 0.9902
y = 0.0076x + 0.0554
R2 = 0.982
y = 0.0062x + 0.047
R2 = 0.982
y = 0.0024x + 0.0181
R2 = 0.982
MW/UV
UV/TiO2
UV/TiO2/MW
0
20
40
60
80
100
120
Time, min
Fig. 6. Kinetic study in degradation of 4C2NP versus time for different process at optimum condition.
causes polarization of parts of macromolecules aligning
with the poles of the electromagnetic field [21–22]. In
MW/TiO2 technique, TiO2 was absorbed heat energy
through water molecules and it was excited to produce
electron-hole pairs resulting in the consequent formation
of hydroxyl radicals and improved the degradation
efficiency of 4C2NP. The degradation of Parathion by
MW in the presence and absence of TiO2 was inspected
by Zhang et al. [23] and it was stated higher degradation
in MW/TiO2 system.
MW process with 0.2 g L–1 of TiO2 was 80.5%. The
degradation of 4C2NP was dominant in UV photolysis
(47.5%) rather than microwave (21.5%) because 4C2NP
is more photosensitive according to the ortho and para
substitution groups [24]. After 100 min of reaction, the
removal of COD in UV/TiO2/MW, UV/TiO2, MW/UV,
UV and MW processes was 47.3%, 41.5%, 27%, 17.6%
and trace, respectively. In all procedures the removal of
COD was lower than the removal of 4C2NP, because
the intermediate compounds were produced during
remediation that had COD themselves.
The progressive experiments of photolysis followed
by microwave (UV + MW) were investigated and results
have been presented in Fig. 5 and the table. The amount
of degradation and kinetic rate constant (54.5%, 7.6 ×
10–3 min–1) in MW+UV process were higher than the
single operationofUV(47.5%, 6.2×10–310–3 min–1)and
MW (21.5%, 2.4 × 10–3 min–1) method. The interaction
of microwave energy with the aqueous solution of
4C2NP leads to the weakening of the bond that is easily
broken in the presence of UV photons to form hydroxyl
radicals. The degradation extent of 4C2NP in UV/TiO2/
Kinetic study. From the experimental results, it was
clear that the numbers of hydroxyl radicals were higher
than 4C2NP so hydroxyl radicals can be supposed to be
approximately fixed, thus kinetic constants of reaction
was gained by fitting the experimental data using
Pseudo first order kinetic equations. This approach is in
agreement with previous studies [25, 26].
As it can be seen from Fig. 6, for evaluating these rate
constants, the terms ln [4C2NP]0/[4C2NP] vs. reaction
time were plotted, and after linear regression analysis,
the first order rate constants and half-life of reaction
were determined as shown in the table:
Rate constant and half-life of different processes for the
removal of 4C2NP at optimum conditions
(8)
= kappt,
k × 10‒3
min‒1
Process
MW
t1/2, min
R2
t
288.7
111.8
91.2
2.4
0.9773
0.982
where [4C2NP]0 and [4C2NP]t are the concentration of
4C2NP at times 0 and t and kapp is a pseudo-first-order
rate constant and t is the time in minutes.Approximately,
all processes tailed the pseudo first order kinetics and
the degradation rate of 4C2NP followed the order:
UV/TiO2/MW > UV/TiO2 > MW/UV > UV>MW.
UV
6.2
MW/UV
UV/TiO2
UV/TiO2/MW
7.6
0.9822
0.9902
0.9893
47.5
14.6
16.4
42.2
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 89 No. 12 2016

1990
AREF SHOKRI, ALI HASSANI JOSHAGANI
6. Bashiri, H. and Rafiee M., J. Saud. Chem. Soc., 2016,
CONCLUSIONS
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7. Daneshvar, N., Behnajady, M.A., and ZorriyehAsghar, Y.,
Various methodologies established on MW, UV, and
the progressive combination of MW and UV along with
TiO2 were used for degradation of 4C2NP. It was clear
that the sequential combination of MW and UV with the
addition of TiO2 is more operative as compared to the
single processes. In this study, the optimum conditions
were gained at pH of 6, initial concentration of 4C2NP
at 30 mg L–1, UV power of 16 W and 0.2 g L–1 of TiO2.
The UV technique has higher degradation than MW
because the molecules of 4C2NP are photosensitive. In
the combined process of MW + UV + TiO2, maximum
degradation of 80.5 % with COD removal of 47.3% was
achieved in 100 min of reaction. Therefore, it is proven
that the consecutive combination of MW and UV with the
presence of TiO2 is a well-organized treatment technique
for degradation of 4C2NP.Almost all processes followed
the pseudo first order kinetics and the degradation rate of
4C2NP followed the order: UV/TiO2/MW > UV/TiO2 >
MW/UV > UV>MW. After 100 min of treatment, the
removal of COD in UV/TiO2/MW, UV/TiO2, MW/UV,
UV and MW processes was 47.3%, 41.5%, 27%, 17.6%
and trace, respectively.
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