Simple preparation and catalytic properties of ZnO for ozonation degradation of phenol in water

Article abstract of DOI:10.1016/j.cclet.2010.10.010

ZnO particles about 200 nm were prepared through a facile hydrothermal method. Compared with single ozonation, the degradation efficiency of phenol increased about 23.7% and the degradation efficiency of intermediates improved about four times in the presence of ZnO at 298 K. In addition, the catalyst had good stability in the ozonation process. The influence of temperature was investigated and it was found that the better catalysis efficiency could be obtained at lower temperature.

Full text of DOI:10.1016/j.cclet.2010.10.010

Available online at www.sciencedirect.com
Chinese Chemical Letters 22 (2011) 209–212
www.elsevier.com/locate/cclet
Simple preparation and catalytic properties of ZnO
for ozonation degradation of phenol in water
a,
Yu Ming Dong , Guang Li Wang , Ping Ping Jiang , Ai Min Zhang
a
a
b,*
,
*
c
Lin Yue , Xiao Ming Zhang
c
a
School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, China
b
School of Chemistry and Chemical Engineering, Key Laboratory of Mesoscopic Chemistry, Nanjing University, Nanjing 210093, China
c
School of Food Science and Technology, Jiangnan University, Wuxi 214122, China
Received 28 June 2010
Abstract
ZnO particles about 200 nm were prepared through a facile hydrothermal method. Compared with single ozonation, the
degradation efficiency of phenol increased about 23.7% and the degradation efficiency of intermediates improved about four times
in the presence of ZnO at 298 K. In addition, the catalyst had good stability in the ozonation process. The influence of temperature
was investigated and it was found that the better catalysis efficiency could be obtained at lower temperature.
#
2010 Yu Ming Dong. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Organic pollutants; Water treatment; Ozone; Catalysis
Nowadays, pollution from highly toxic organic compounds in the aquatic environment is a serious concern to
consider because of its dangers to humans and ecological health. Phenol and phenol derivates have received increased
attention as pollutants in the past years due to their high toxicity. Several catalytic oxidation technologies have been
developed for the treatment of wastewaters containing phenol and phenol derivates, such as photocatalysis, catalytic
wet air/peroxide oxidation and catalytic ozonation [1–4]. Among these methods, catalytic ozonation is a promising
process for organic wastewater treatment [2]. First, ozone is an environmentally friendly and practical oxidant since it
generates from O and decomposes to O , which lacks of secondary pollution. Secondly, the ozonation process can be
2
2
carried out under normal temperature and pressure. For the treatment of organic pollutants, the search for an active,
stable and durable catalyst is an important task.
Nanomaterials, which can exhibit unusual chemical and physical properties, are now gaining attention as
heterogeneous ozonation catalysts. The high surface area and good dispersivity of nano-catalyst are favorable to
catalysis [5,6]. At the same time, most nano-catalysts are difficult to separate from water. This becomes the main
limitation for reuse. It is very significant to develop novel catalysts with good separability as well as remarkable
catalysis for water treatment.
*
Corresponding authors.
E-mail addresses: dongym@jiangnan.edu.cn (Y.M. Dong), zhangam@nju.edu.cn (A.M. Zhang).
1001-8417/$ – see front matter # 2010 Yu Ming Dong. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2010.10.010

210
Y.M. Dong et al. / Chinese Chemical Letters 22 (2011) 209–212
Zinc oxide (ZnO) has wide applications in electronics, catalysis, optoelectronic and sensors. Jung and Choi
reported the catalytic decomposition of ozone and para-chlorobenzoic acid ( p-CBA) in the presence of nanosized
ZnO, and Huang et al. found a nanometer-ZnO catalyst to enhance the ozonation of 2,4,6-trichlorophenol in water
[7,8]. However, both ZnO nano-catalysts were supplied from commercial source. The synthesis of ZnO as ozonation
catalyst was rarely reported. The aim of this work is to develop a new route to obtain desirable ZnO ozonation catalyst.
1
. Experimental
All chemicals used (ZnSO , carbamide, NaOH, alcohol and phenol) were of analytical grade, without further
4
purification. All aqueous solutions were prepared with distilled water.
In our experiment, 100 mL of 0.1 mol/L ZnSO aqueous solution, 20 mL of 0.5 mol/L carbamide aqueous solution
4
and 20 mL of alcohol were mixed. Then 5 mol/L NaOH aqueous solution was added dropwise to the above solution
until the pH of the solution was 11.0. In this process, white suspension formed. Subsequently, the white suspension was
placed in sealed Teflon autoclave, heated at 433 K for 12 h and cooled naturally. The resulting white precipitates were
washed by water several times and dried at 383 K. The characterization conditions were same as literature [5]. The
catalytic degradation of phenol was performed in a semicontinuous flow mode, and the flow rate of ozone was
0
.55 mg/min. In a typical catalytic degradation procedure, certain amount of ZnO and 200 mL of simulated
wastewater (the initial concentration of phenol was 100 mg/L) were mixed in a flask under thermostatic control. The
concentrations of phenol and its oxidized intermediates were determined by a Waters 515 high performance liquid
chromatography (HPLC, Waters, USA).
2
. Results and discussion
As can be shown in Fig. 1, all the XRD patterns can be readily indexed to pure ZnO (JCPDS 79-0208). No
characteristic peaks of impurities were observed. The results indicate the high purity of the products prepared by this
approach. TEM images of the as-prepared ZnO are shown in Fig. 2. The products were made up of particles with
average size of 200 nm. According to the results of the nitrogen adsorption and desorption isotherms, the BET surface
2
area of ZnO was 36.48 m /g and pore structure was not observed.
Fig. 3 shows the degradation of phenol at different ozonation time in single ozonation and catalytic ozonation. It can
be seen that the ZnO demonstrated remarkable catalysis efficiency on the ozonation degradation of phenol in water.
For example, the residual concentration of phenol was 27.7 mg/L after single ozonation for 60 min, while it was
merely 10.5 mg/L in catalytic ozonation. Compared with the single ozonation, the degradation efficiency of phenol
increased about 23.7% in presence of catalyst.
Before being degraded to small organic acids, phenol is easily oxidized to more toxic intermediates such as
catechol, hydroquinone and p-benzoquinone [9–13]. The time dependent concentrations of these intermediates in the
single ozonation and the catalytic ozonation were investigated. It can be found that the concentrations of them in
catalytic ozonation were lower than those in single ozonation. When the ozonation time reached 60 min, the
concentrations of catechol, hydroquinone and p-benzoquinone in the catalytic process were 1.1, 2.4 and 0.3 mg/L,
[()TD$FIG]
1
01
1
00
0
02
1
10
1
03
1
02
1
12
2
01 202
2
0
30
40
50
60
70
80
Two theta (degree)
Fig. 1. XRD patterns of as-prepared ZnO.

[()TD$FIG]
Y.M. Dong et al. / Chinese Chemical Letters 22 (2011) 209–212
211
Fig. 2. TEM images of as-prepared ZnO.
[()TD$FIG]
100
a
8
6
4
2
0
0
0
0
0
b
0
10
20
30
40
50
60
Time (min)
Fig. 3. Time evolution of concentrations of phenol under single ozonation (a) and catalytic ozonation of ZnO (b). Catalyst dose: 0.5 g, reaction
temperature: 298 K.
while those in the single ozonation were 7.5, 4.5 and 5.6 mg/L. The total residual concentration of the three
intermediates in the catalytic ozonation was only 21.6% of that in the single ozonation. From the above results, we can
see that ZnO exhibited high catalytic activity for the ozonation degradation of organic pollutants. According to the
decomposition of ozone on surface of ZnO [7,8], the catalysis mechanism may be concluded that ZnO enhanced the
transformation of ozone into hydroxyl radicals.
The reusability of the catalyst is important for its practical application. In order to evaluate the activity stability of
the catalyst, the reuse experiment was carried out. From Fig. 4, it can be seen that the catalytic activity kept constant
and no obvious deactivation was observed when the ZnO was used for six times. That is, the ZnO showed considerable
stability in the catalytic process.
As is known, operation temperature influences the ozonation degradation of organic pollutants. Besides 298 K
Fig. 3), we also carried out the catalytic ozonation experiments at 283 and 313 K (Fig. 5). Compared with the results at
98 K, the degradation rate of phenol decreased at lower temperature (283 K), while the degradation rate at 313 K was
(
2
[()TD$FIG]
1
00
a
8
6
4
2
0
0
0
0
0
b
1
2
3
4
5
6
Reused times
Fig. 4. Degradation of phenol in reuse experiments: (a) single ozonation and (b) catalytic ozonation of ZnO. Catalyst dose: 1.0 g, reaction
temperature: 298 K, reaction time: 60 min.

[()TD$FIG]
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Y.M. Dong et al. / Chinese Chemical Letters 22 (2011) 209–212
(
A) 100
(B)
1
00
80
60
40
20
a
b
a
b
8
6
4
2
0
0
0
0
0
0
0
10
20
30
40
50
60
0
10
20
30
40
50
60
Time (min)
Time (min)
Fig. 5. Time evolution of concentrations of phenol at 283 K (A) and 313 K (B) under single ozonation (a) and catalytic ozonation of ZnO (b).
Catalyst dose: 0.50 g.
similar with that of 298 K. The rise of temperature can influence the catalytic ozonation process in three ways: (1) the
concentration of ozone in solution is reduced; (2) the transformation from ozone molecules to hydroxyl radicals is
accelerated, and the degradation of phenol by hydroxyl radicals is faster than that by ozone molecules [14]; (3)
diffusion rate of the reacting substances is enhanced. The efficiency increase in degradation of phenol from 283 to
298 K may be explained that (2) and (3) is predominant. When the temperature increased from 298 to 313 K, (1) was
almost equivalent to (2) and (3), then the degradation rate of phenol did not change.
After the single ozonation of 60 min at 283, 298 and 313 K, the removal of phenol were 55.5, 72.3 and 77.3 mg/L,
respectively. While after the catalytic ozonation of 60 min at 283, 298 and 313 K, the removal of phenol were 76.0,
8
2
9.5 and 87.5 mg/L. That is, the degradation efficiencies of phenol were increased by about 37%, 24% and 13% at 283,
98 and 313 K due to the presence of ZnO. It can be found that the ZnO exhibited certain catalysis activity in the
temperatures ranges of 283–313 K. At lower temperature, ZnO exhibited higher catalysis efficiency.
3
. Conclusions
In conclusion, using inexpensive raw materials, ZnO particles with good separability were synthesized through a
facile hydrothermal method. As-prepared ZnO revealed higher catalysis on ozonation degradation of phenol and its
intermediates. In the reuse experiments, the catalytic activity kept constant and no obvious deactivation was observed
after six ozonation processes. This result proved the good stability of the catalyst. Through the control experiments
carried out at 283, 298 and 313 K, it was found that the better catalysis efficiency could be obtained at lower temperature.
Acknowledgments
The authors gratefully acknowledge the support from the National Natural Science Foundation of China (Nos.
0903048, 20771046 and 21005031), the Fundamental Research Funds for the Central Universities (No.
2
JUSRP30903), the Startup Foundation for Introduced Talents (Nos. 20081789 and 20091207), the Science Foundation
for Youth (Nos. 2008LQN002 and 2009LQN11).
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