Photocatalytic ozonation for degradation of 2-sec-butyl-4,6-dinitrophenol (DNBP) using titanium dioxide: Effect of operational parameters and wastewater treatment

Article abstract of DOI:10.1007/s11164-012-0996-9

The results of degradation efficiency of 2-sec-butyl-4,6-dinitrophenol (DNBP) in a batch system by various advanced oxidation processes revealed the order of TiO₂/UV/O₃ > TiO₂/O₃ > UV/O₃ > O₃ > UV/TiO₂. All processes followed pseudo-first order kinetics. The influence of operational parameters such as initial pH, initial concentration of DNBP, ozone and catalyst dosage on the TiO₂/UV/O₃ process, which was the most significant investigated method. The ozone dosage was found to have the noticeable impact on the process; however, initial pH and TiO₂ dosage were less effective. The mineralization of 40 mg/L of DNBP and petrochemical wastewater under the obtained optimal conditions was monitored by total organic carbon and chemical oxygen demand, respectively. The results demonstrated that the TiO ₂/UV/O₃ process was a very effective method for degradation and mineralization of DNBP in aqueous solutions and industrial wastewater. The degradation intermediates were identified by GC-MS.

Full text of DOI:10.1007/s11164-012-0996-9

Res Chem Intermed
DOI 10.1007/s11164-012-0996-9
Photocatalytic ozonation for degradation of 2-sec-butyl-
4,6-dinitrophenol (DNBP) using titanium dioxide: effect
of operational parameters and wastewater treatment
T. Mousanejad
A. R. Khataee
•
M. Khosravi
K. Zare
•
S. M. Tabatabaii
•
•
Received: 6 November 2012 / Accepted: 19 December 2012
Springer Science+Business Media Dordrecht 2013
ꢀ
Abstract The results of degradation efficiency of 2-sec-butyl-4,6-dinitrophenol
DNBP) in a batch system by various advanced oxidation processes revealed the
order of TiO /UV/O [ TiO /O [ UV/O [ O [ UV/TiO . All processes fol-
(
2
3
2
3
3
3
2
lowed pseudo-first order kinetics. The influence of operational parameters such as
initial pH, initial concentration of DNBP, ozone and catalyst dosage on the TiO2/
UV/O process, which was the most significant investigated method. The ozone
3
dosage was found to have the noticeable impact on the process; however, initial pH
and TiO dosage were less effective. The mineralization of 40 mg/L of DNBP and
2
petrochemical wastewater under the obtained optimal conditions was monitored by
total organic carbon and chemical oxygen demand, respectively. The results dem-
onstrated that the TiO /UV/O process was a very effective method for degradation
2
3
and mineralization of DNBP in aqueous solutions and industrial wastewater.
The degradation intermediates were identified by GC–MS.
Keywords 2-Sec-butyl-4,6-dinitrophenol ꢀ Advanced oxidation processes ꢀ
Photodegradation ꢀ Photocatalytic ozonation ꢀ TiO2
T. Mousanejad (&) ꢀ M. Khosravi ꢀ S. M. Tabatabaii
Department of Chemistry, North Tehran Branch, Islamic Azad University, Tehran, Iran
e-mail: mosanegad589@yahoo.com
A. R. Khataee
Department of Applied Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran
K. Zare
Department of Chemistry, Science and Research Branch, Islamic Azad University, Tehran, Iran
123

T. Mousanejad et al.
Introduction
Discharge of persistent organic pollutants (POPs) such as phenolic compounds
poses serious threats to the environment and human health. Nitro-substituted
phenols are classified as one of the most toxic and carcinogenic compounds. 2-Sec-
butyl-4,6-dinitrophenol (DNBP) is a typical example of this class, which is
manufactured in large quantities and used as an inhibitor for vinyl aromatics
polymerization in the petrochemical industry and as a herbicide in agriculture [1, 2].
During DNBP synthesis and application processes, it can be introduced into the
surface water. Due to the undesirable side effects of DNBP, governments restrict its
maximum concentration level (MCL) in water. The MCL of DNBP in drinking
water is 7 ng/mL according to the environmental protection agency (EPA) of the
USA [3]. Thus, the environmental fate of DNBP and its removal in water sources
are of great public concern. DNBP is resistant to biological treatment and other
processes like adsorption [4], which merely transfers the contaminant from aqueous
solution to secondary waste. So, the application of more effective methods like
advanced oxidation processes (AOPs) are more desirable, because they cannot only
oxidize and degrade the pollutants non-selectively but also mineralize them [5]. The
0
main oxidizing agent in these processes is hydroxyl radical (E = 2.8 eV), which
can be produced under ultraviolet (UV) light-catalyzed reactions [6–8]. Titanium
dioxide (TiO ) has been extensively utilized as a heterogeneous catalyst in AOPs.
2
When TiO absorbs UV light, photo-induced TiO is generated and electrons are
2
2
excited from the valence band of TiO to the exciting band, which results in
2
electron–hole pairs. Oxygen molecules act as electron acceptors to form superoxide
•
-
ions ( O ), and hydroxyl ions or water molecules function as electron donors to
2
produce hydroxyl radicals [9] as follows:
þ
ꢁ
TiO þ hv ! TiO þ h þ e
ð1Þ
ð2Þ
ð3Þ
ð4Þ
ð5Þ
2
2
ꢁ
ꢂ
ꢁ
2
O þ e ! O
2
ꢂ
ꢁ
ꢂ
ꢁ
3
O þ O ! O þ O
OH þ h ! OH
H2O þ h ! OH þ H
2
3
2
ꢁ
þ
ꢂ
þ
ꢂ
þ
Although the UV/TiO process can degrade contaminants in aqueous solutions,
the degradation rate is not very rapid [10]. The high reactivity and disinfection of
2
0
ozone (O ), which is a stronger oxidant than oxygen (E = 2.07 V) makes it
3
suitable for application in various water treatment processes [11]. Hence, ozonation
has been used for treatment of wastewaters containing phenolic compounds [12].
However, it has been proven that in the conventional ozonation method complete
degradation of organic compounds is not achieved. The degradation efficiency of
humic acid and oxalic acid was made more effective by TiO -catalyzed ozonation
2
(
TiO /O ) in comparison with ozone alone [13]. This can be attributed to both the
2 3
enhancement of ozone dissolution and its decomposition in the presence of TiO2
particles [14]. The combination of ozone with photocatalytic oxidation elicits the
advantages of all the above-mentioned processes. The production of extra hydroxyl
1
23

Photocatalytic ozonation for degradation of DNBP
radicals enhances not only by reduction of dissolved O3 molecules with
photogenerated electrons at conduction band of TiO but also by absorption of
2
UV light by ozone itself which is called the UV/O process and is applied to degrade
3
organic pollutants [15] (Eqs. 6–10).
ꢁ
ꢂ
ꢁ
3
O3 þ e ! O
ð6Þ
ð7Þ
ð8Þ
ð9Þ
ð10Þ
þ
ꢂ
ꢁ
ꢂ
H þ O ! O3H
3
ꢂ
ꢂ
O3H ! OH þ O2
ꢂ
O3 þ hv ! O þ O2
O þ H2O ! 2 OH
ꢂ
ꢂ
The utilization of ozone combined with the photocatalytic oxidation process for
degradation of organic pollutants like monochloroacetic acid, pyridine, neonicot-
inoid insecticides, toluene, sulfamethoxazole and 4-chloronitrobenzene has been
reported [16–21]. However, to the best of our knowledge, there is no report for the
degradation of DNBP by this process. The aim of this study was to compare various
AOPs processes including UV/TiO , O , UV/O , TiO /O , and TiO /UV/O to find
2
3
3
2
3
2
3
the synergetic effect of ozonation and photocatalysis on the degradation of DNBP as
a model contaminant from phenolic compounds. Thus, to investigate the effect of
operational parameters such as initial DNBP concentration, initial pH, ozone and
catalyst dosage in TiO /UV/O , process experiments were carried out. Eventually,
2
3
the mineralization of DNBP in aqueous solution and in the wastewater of the Tabriz
Petrochemical Complex was investigated.
Experimental
Materials
2
-Sec-butyl-4,6-dinitrophenol (DNBP) was used as an alkyl dinitro phenol
compound and obtained from Retell Fine Chemical (Tianjin, China). An aqueous
solution of DNBP was prepared in deionized water. Titanium dioxide (Degussa P25,
Germany) which has 80 % anatase and 20 % rutile, with average particle size of
2
21 nm and surface area of 50 ± 15 m /g, was applied as catalyst [22]. All other
chemicals were purchased from Merck, Germany.
Experimental set-up and procedure
Experiments were carried out in a batch cylindrical quartz reactor with inner
diameter of 50 mm and volume of 500 mL. In each experiment, 250 mL of the
reaction mixture containing desired amounts of DNBP and TiO were poured into
2
the reactor. The pH of the solution was adjusted by adding perchloric acid or sodium
hydroxide. The solution was agitated by a magnetic stirrer. The mixture of ozone–
oxygen which was supplied from the ozone generator (Donali, Iran) was fed into the
reactor by a diffuser from its bottom. The concentration of ozone was adjusted by
123

T. Mousanejad et al.
(a)
(b)
Fig. 1 a Degradation curves of DNBP by different oxidation process, b removal of TOC. Experimental
conditions: pH = 9.5, 50 mg/L TiO , C = 40 mg/L, ozone gas concentration 5.2 mg/L
2
0
Fig. 2 Kinetics of DNBP degradation by various AOPs. Experimental conditions: pH = 9.5,
= 40 mg/L, ozone gas concentration 5.2 mg/L
C
0
1
23

Photocatalytic ozonation for degradation of DNBP
varying the voltage and measured by the KI method [23]. A UV lamp (10 W,
2
54 nm, GPH212T5L/4, Germany) was placed next to the quartz reactor to provide
2
UV irradiation. The light intensity on the outer side of the reactor was 2.3 mW/cm ,
which was measured by UV-Lux-IR meter (Leybold, Germany).
Analytical methods
High performance liquid chromatography (HPLC) (Shimadzu, SCL-6A, Japan) was
applied to the determination of the concentration of DNBP (C) in the solution.
A ZOBAX column (5 lm, C18) and mobile phase of methanol/water (7:3 v/v) with
flow rate of 1.0 mL/min were employed in the HPLC analysis. To investigate the
mineralization of DNBP, total organic carbon (TOC) by catalytic oxidation
(
Shimadzu TOC-5000, Japan) and chemical oxygen demand (COD) by the open
reflux method [6], were used for the DNBP aqueous solution and petrochemical
wastewater containing DNBP. The generated intermediates during DNBP degra-
dation by TiO /UV/O were identified by GC–MS analysis (Agilent Technologies,
2
3
Palo Alto, CA, USA).
Results and discussion
Comparison of various AOPs for degradation of DNBP and kinetics study
The individual effect of UV irradiation and TiO addition is negligible for the
2
degradation of DNBP in aqueous solution. The degradation efficiency (DE%) at a
certain time is defined as the percentage ratio of the degraded amount (C –C) to its
0
initial concentration (C ). The decreasing order of DE% for DNBP (40 mg/L) by
0
different AOPs processes after 15 min of treatment was TiO /UV/O (99.1 %), UV/
3
2
O (94.5 %), TiO /O (92.8 %), O (91.9 %), and UV/TiO (14.2 %) (Fig. 1a).
3
2
3
3
2
Moreover, mineralization of DNBP during the above-mentioned AOPs was
investigated by monitoring of the TOC. Although the order of the mineralization
was the same as the degradation, significant differences for TOC removal were
observed after 45 min of the processes (Fig. 1b). The TiO /UV/O process
2
3
demonstrated 71.9 % TOC elimination, which was 57, 45.9, 26.9, and 19.9 %
higher than that of the UV/TiO , O , TiO /O , and UV/O processes, respectively.
The results indicate that simultaneous application of heterogeneous photocatalysis
2
3
2
3
3
Table 1 Pseudo-first-order
degradation rate constant of
DNBP in different processes
Treatment
process
Rate constant,
-
Correlation
coefficient, R
1
2
k(min
)
TiO
2
/UV
0.0105
0.1537
0.1835
0.1991
0.3057
0.9864
0.9916
0.9848
0.9861
0.9852
O
3
TiO
2
/O
3
UV/O
3
TiO /UV/O
2
3
123

T. Mousanejad et al.
with ozone, which has high oxidative ability, is an efficient coupled process for the
degradation and mineralization of DNBP. These findings can be explained by (1)
more dissolution and decomposition of O in water in the presence of TiO [13], (2)
3
2
O acts as an electron acceptor from the conduction band of TiO [14], preventing
3
2
recombination of electron–hole pairs, and (3) O absorbs UV light (O /UV) [15].
3
3
All the mentioned reasons cause the formation of extra hydroxyl radicals resulting
in the enhancement of DE% and mineralization of DNBP.
All applied AOPs in this study follow pseudo-first-order kinetics (Fig. 2):
ln ðC =CÞ ¼ k
ꢃ t
The apparent pseudo-first-order reaction rate constant (kapp) was determined from
ð11Þ
0
app
the slope of ln (C /C) versus process time (t) (Table 1). The high correlation
0
coefficients obtained confirmed the assumed kinetics. The synergistic effect of O3
(a)
(b)
Fig. 3 Effect of initial DNBP concentration on the a degradation of DNBP, b removal of TOC by the
TiO /UV/O process. Experimental conditions: 50 mg/L TiO , pH = 9.5, ozone dosage 5.2 mg/L
2
3
2
1
23

Photocatalytic ozonation for degradation of DNBP
(a)
(b)
Fig. 4 Effect of ozone dosage on the a degradation of DNBP, b removal of TOC by the TiO
process. Experimental conditions: pH = 9.5, 50 mg/L TiO , C = 40 mg/L
2 3
/UV/O
2
0
and UV/TiO is considerable and can be measured in the apparent pseudo-first-order
2
rate constant in the following equation as 46 %:
Synergy ¼ ðk
ꢁ (k þ k
ÞÞ=k
ð12Þ
TiO2=UV=O3
O3
UV=TiO2
TiO2=UV=O3
As a result, other experiments for DNBP degradation were carried out by the
coupled photocatalytic ozonation process.
Influence of operational parameters on photocatalytic ozonation
and intermediates identification
The effects of experimental parameters including ozone and TiO dosage, initial
2
DNBP concentration, and pH on the degradation and mineralization of DNBP were
investigated. Degradation and mineralization of DNBP declines by increasing the
123

T. Mousanejad et al.
(a)
(b)
Fig. 5 Effect of catalyst dosage on the a degradation of DNBP, b removal of TOC by the TiO
process. Experimental conditions: pH = 9.5, C = 40 mg/L, ozone dosage 5.2 mg/L
2 3
/UV/O
0
DNBP concentration (Fig. 3), because the identical amount of oxidizing species,
which are generated under the same conditions, have to react with more DNBP
molecules and its degradation intermediates [24]. By increasing the O dosage, the
3
degradation and mineralization of DNBP increase due to the formation of more
oxidizing species like hydroxyl radicals (Fig. 4) [11]. However, as can be seen from
Fig. 4, a further increase in O dosage does not have a remarkable effect on DE%
3
because of unreacted O being released in the system [25]. The effect of TiO2
3
dosage on the DE% and TOC is depicted in Fig. 5. It can be concluded that neither
DE% and TOC vary noticeably by increasing the TiO dosage. This result is
2
consistent with the degradation of 4-chloronitrobenzene by the TiO /UV/O process
2
3
[
21]. The DE% and TOC removal of DNBP was studied in pH of 3.5, 6.5, and 9.5
Fig. 6). The degradation of DNBP shows approximately the same results in various
(
1
23

Photocatalytic ozonation for degradation of DNBP
(a)
(b)
Fig. 6 Effect of initial solution pH on the a degradation of DNBP, b removal of TOC by the TiO
O
2
/UV/
3
process. Experimental conditions: 50 mg/L TiO , C = 40 mg/L, ozone dosage 5.2 mg/L
0
2
pHs, but the pH of 9.5 shows the best performance for the mineralization of DNBP.
In acidic medium (pH = 3.5), the predominant form of ozone molecule is in the
solution, which can degrade organic pollutants by direct electrophilic attack [26]. In
the basic medium (pH = 9.5), indirect attack of hydroxyl radicals, which were
generated by O3 decomposition [27], can degrade DNBP more efficiently.
•
Furthermore, the reaction of OH with DNBP might be more efficient at high pH
rather than at low pH. In order to identify intermediates in the photocatalytic
ozonation, 250 mL of DNBP (40 mg/L) was treated for 15 min. Then, the
intermediates were extracted and detected by the GC–MS method [28, 29]. Five
intermediates, which were recognized by comparison with commercial standards
when the match factor was above 90 %, are presented in Table 2. However, several
intermediates could not be identified because of their rapid oxidization to other
derivatives or low match factors of chromatographic peaks.
123

T. Mousanejad et al.
Table 2 Identified intermediates during photoassisted electrochemical process
Name
Chemical structure
Dinitro catechol
O-
O
N+
O
O
N+
O
O-
Dinitro benzoquinone
O
O
N+
O-
O-
N+
O
O
Nitrophenol
O
N+
-O
HO
Maleic acid
Oxalic acid
HO
O
O
OH
O
HO
OH
O
Study of photocatalytic ozonation treatment of a real wastewater
The application of the TiO /UV/O process for mineralization of a real wastewater
2
3
containing DNBP is essential from the practical point of view. Wastewater with a
yellow color, which has approximately 40 mg/L of DNBP, was provided by the
Petrochemical Complex (Tabriz, Iran). The initial COD of the wastewater was
1
,053 mg/L with pH of 8.5. After 60 min of treatment, 67.2 % of the COD was
eliminated, which proved the destruction of organic molecules in the wastewater
Fig. 7).
(
1
23

Photocatalytic ozonation for degradation of DNBP
2 3
Fig. 7 COD removal for real wastewater (Petrochemical Complex, with DNBP) by the TiO /UV/O
process. Experimental conditions: pH = 9.5, 50 mg/L TiO , ozone gas concentration 5.2 mg/L
2
Conclusion
The photocatalytic ozonation process for degradation of DNBP, as a model pollutant
from phenolic compounds, was compared with UV/TiO , O , UV/O , and TiO /O
3
2
3
3
2
processes. All processes followed the pseudo-first-order kinetics. Moreover, the
operational parameters including ozone and TiO dosage, initial DNBP concentra-
2
tion, and pH on the degradation and mineralization of DNBP were investigated in
the TiO /UV/O process, and some of the degradation intermediates were identified
2
3
by GC–MS. The mineralization of DNBP in aqueous solution and petrochemical
wastewater was investigated by TOC, and the COD showed the proper ability of the
TiO /UV/O process for the destruction of phenolic compounds.
2
3
Acknowledgments The authors sincerely appreciate the Water and Wastewater Company of East
Azerbaijan, Tabriz, Iran. We also thank Dr. B. Vahid, Mr. Fatholahnejad and Mrs. Movahed for their
valuable comments and help.
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