Effect of a low frequency acoustic field on the photocatlytic
degradation of phenol in synthetic wastewater
W. Van de Moortel, J. Degrève, Chemical and Biochemical Process Technology and Control, Department of
chemical engineering, Katholieke Universiteit Leuven, W. de Croylaan 46, 3001 Heverlee, Belgium
K. Sniegowski, L. Braeken, , Katholieke Hogeschool Limburg, Agoralaan gebouw B bus 3, 3590 Diepenbeek
B. Vanderbeke, J. Luyten, Environmental and Process Technology, Lessius University College - Campus De Nayer,
J.P. De Nayerlaan 5, 2860 Sint-Katelijne-Waver, Belgium
L. Braeken, Laboratory for Applied Physical Chemistry and Environmental Technology, Department of chemical
engineering, Katholieke Universiteit Leuven, W. de Croylaan 46, 3001 Heverlee, Belgium
Abstract
In this paper, the effect of a low frequency acoustic field (20 – 25 kHz) on the degradation of
phenol by photocatalysis was investigated. In a first part the effect of TiO2-concentration was
observed. It can be concluded that there is an optimal concentration. Also the effect of an
increasing power of the acoustic field was investigated. The second part of this research
showed clearly the increasing degradation rate when acoustic field power was increased. A
statistical analysis has been made to identify and quantify the importance of different
operating parameters. The variables considered for this study are: initial phenol concentration,
reaction time, power of the acoustic field and TiO2 concentration. The response variable is the
concentration relative to the initial phenol concentration.
Keywords
Phenol, photo-catalysis, sonolysis, wastewatertreatment
Introduction
Reuse and recycling of wastewaster is a key approach in sustainable water management. The
presence of recalcitrant compounds as there are pesticides and phenolic compounds are
hazardous for nature and men [1].
In the photocatalytic oxidation process, organic pollutants are destroyed in the presence of a
semiconductor, e.g. TiO2, ZnO [2]. This semiconductor is combined with ultraviolet light and
in most cases also an oxidizing agent like oxygen. In many case, TiO2 P – 25, produced by
Degussa is used [3-5]. Ultraviolet light will excite electrons from valence band to conduction
band, in creation of an electron-hole pair. Different reaction can occur at the surface of the
catalyst [6]:
TiO2 + h
 e- + h+
(1)
●
e + O2
 O2
(2)
h+ + R
 intermediates  CO2 + H2O
(3)
+
●
●
h + H2O
 OH + H
(4)
●
OH + R
 intermediates  CO2 + H2O
(5)
Organic pollutants can be degraded in two different ways, but before reaction (3) can occur
adsorption on the catalyst surface is needed. The hydroxyl radical attacks organic compounds
(R) which results in various reaction intermediates depending on the nature of the
compounds. The resulting intermediates further react with the radicals to produce final
degradation products such as CO2 and H2O.
Sonolysis can be divided in low frequency sonolysis (LFS) and high frequency sonolysis
(HFS) [7]. In sonolysis, ultrasonic pressure waves induce cavitation, gas bubbles grow in size
during cycles of compression and expansion. These will implode, and create local hotspots
with high pressure and temperature. Volatile compounds will diffuse into the bubbles and can
be thermally degraded by the implosion.
HFS is capable to create radicals out of water by following reaction:
H2O + energy  H● + ●OH
These radicals can degrade organic pollutants in the water. LFS however can’t create these
radicals and must be combined with other advanced oxidation processes (AOPs) to enhance
degradation.
In this research, LFS (20 – 25 kHz) is combined with photocatalysis to enhance the
degradation rate of phenol.
Materials
Materials
The photocatalytic material TiO2 (crystalline structure: anatase) is obtained from Acros
Organics. The diameter, specific surface area and band gap energy of TiO2 are 100 – 400 nm,
55 m²/g and 3.13 eV (UV absorption at 356 nm or lower), respectively. Phenol (Acros
Organics, 99.5 %) was used without further purification. Synthetic wastewater is prepared
with phenol and ultra pure water. T-butanol (Acros Organics, 99.5 % pure) is used as
hydroxyl radical scavenger. All reagents are of analytical grade and used as purchased.
Chemical Analysis
The samples are analyzed by a DIONEX HPLC with water/methanol (68/32  20/80) as an
eluting solvent at a flow rate of 1 ml/min. The separation is obtained with a C18 column
(Grace 4.6 ID X 250 mm). The wavelength used for the UV detection is 270 nm on an
injected sample of 20 μl. The products are identified by comparison of retention times with
standards.
Experiments
Experiments are run in a 3 l vessel, with 2.0 l of synthetic wastewater. A Low Pressure
Mercury UV-lamp (11 W) was placed slightly off-centre of the vessel, next to an ultrasonic
probe (Bandalin Sonoplus regulator and Bandalin UW 3200 Probe). The ultrasonic probe
power can be varied between 30 and 150 W and produces low frequency (20 kHz) waves.
TiO2 is added to the synthetic wastewater in concentrations of 0.01 g/l, 0.1 g/l, 0.5 g/l and 1
g/l. Samples of 2 ml are taken at 0, 10, 20, 30, 45 and 60 minutes of reaction time. When TiO2
is used, the samples are filtered by cellulose filters (1 μm), prior to analysis.
Results & Discussion
TiO2 – dosage
The phenol concentration with time follows in good approximation a first-order decay. The
last column of table 1 represents the first order rate constant that fits the experimental values.
Also, the coefficient of determination R² is reported, confirming the first order decay by the
high values. Figure 1 clearly shows the effect of the TiO2 dosage. A higher amount of TiO2
does not correspond to a higher degradation with UV/TiO2 or UV/US/TiO2. A dosage of 0.1
g/l resulted in an optimal degradation of phenol. A higher concentration of solid particles
results in less effective light absorption (due to scattering and shielding). At high TiO2
dosages , the purely photolytic degradation rate is even higher than the photocatalytic rate.
Many photocatalytic reactions follow the Langmuir-Hinshelwood (LH) rate form [8-9], i.e
adsorption of the reagents at the catalyst surface followed by chemical reaction. Additionally,
phenol is also degraded by direct photolysis.
a
b
Figure 1: The degradation of phenol by UV/TiO2 at different TiO2-dosages (a) ; Comparison of the pseudo-first-order rate
constant (b) for different TiO2-dosages.
Table 1: Design matrix and results of the treatment of the synthetic wastewater
assay
c0,phenol
UV
TiO2
US
Time
1
5
11
0
0
60
2
5
11
0,01
0
3
5
11
0,1
4
5
11
0,5
5
5
11
6
5
7
c/c0
k
R²
0,978326
60
0,510825 0,01181
0,42744
0,01463
0
60
0,32987
0,01921
0,995805
0
60
0,622576 0,00777
0,99458
1
0
60
0,689062
0,00665
0,978956
11
0
50
60
0,457749
0,01266
0,977544
5
11
0,01
50
60
0,317432
0,01995
0,998513
8
5
11
0,1
50
60
0,260321
0,02235
0,987096
9
5
11
0,5
50
60
0,337192
0,018
0,968557
10
5
11
1
50
60
0,453557
0,0129
0,996682
11
5
11
0
100
60
0,526266
0,01099
0,987051
12
5
11
0,01
100
60
0,220065
0,02274
0,822039
13
5
11
0,1
100
60
0,113169 0,03633
0,972328
14
5
11
0,5
100
60
0,292562
0,02034
0,984689
15
10
11
0
0
60
0,732537
0,00491
0,999361
16
10
11
0,5
0
60
0,711294
0,00714
0,847677
17
10
11
1
0
60
0,704568
0,00584
0,993068
18
10
11
0
50
60
0,659767
0,00796
0,941927
19
10
11
0,01
50
60
0,454923
0,01225
0,955676
20
10
11
0,1
50
60
0,401891
0,01537
0,964601
21
10
11
0,5
50
60
0,487979 0,01061
0,949162
22
10
11
0,1
100
60
0,555236
0,00341
0,909008
23
10
11
0
100
60
0,520399
0,0102
0,984259
24
5
0
0,1
0
60
0,981196
0,00084
0,555381
25
5
0
0,1
50
60
0,980871
0,00053
0,228015
0,990966
Use of an acoustic field
Figure 2 clearly shows the increase in the degradation efficiency if ultrasonic power is
increased. From Table 1, it can be deduced that sonolysis of phenol under the experimental
conditions used is negligibly small. The effect of ultrasound is therefore attributed to the
synergetic effects of increased mass transfer, cleaning of the catalyst surface and
desaggregation of the particles. The relative importance of each if these synergetic effects Is
further investigated.
Figure 2: The degradation of phenol by UV/TiO2/US: comparison of different acoustic field powers
Pseudo-first order reaction rate constants were obtained for the different conditions (table 2).
Table 2: comparison of the pseudo-first-order rate constants for different acoustic field power levels and
TiO2-dosages
TiO2 [g/l]
0,01
0,1
0,5
US [W]
0
50
100
0
50
100
0
50
100
k [1/s]
0,014628
0,018411
0,036329
0,019214
0,022355
0,038329
0,007772
0,017997
0,020388
Statistical Analysis
A Pareto chart showing both the magnitude and the importance of the main and interaction
effects (variables of interactions) is given in Figure 3. A level of significance of 95% is
selected. The reference points for this Pareto chart are the minimal and maximal values of the
parameters.
1,982
F actor
A
B
C
D
C
A
CD
N ame
TIM E
C0
US
TiO 2
BD
D
Term
BC
BCD
AB
B
ABCD
ABC
ABD
AD
AC
ACD
0
1
2
3
4
5
Figure 3: Pareto chart for the experiments
The acoustic field, and time are the most important parameters. But also the acoustic field
multiplied with TiO2-dosage, initial concentration multiplied by TiO2-dosage and TiO2dosage are important parameters.
Conclusions
In this research, the effect of TiO2 dosage and an acoustic field on the degradation of phenol
by a photo-catalytic degradation, was studied. An optimal catalyst concentration was found
for this reactor.
In the second part of this research the addition of an acoustic field was examined. Tests
clearly showed an increasing degradation rate of phenol by increasing acoustic field.
Acknowledgments
Research funded by a Ph.D. grant of the Agency for Innovation by Science and Technology
(IWT).
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