Research Journal of Recent Sciences ________________________________________________ ISSN 2277-2502
Vol. 1 (ISC-2011), 191-201 (2012)
Res.J.Recent Sci.

Ultrasound assisted semiconductor mediated catalytic degradation of organic
pollutants in water: Comparative efficacy of ZnO, TiO2 and ZnO-TiO2
Anju S.G., Jyothi K.P., Sindhu Joseph, Suguna Y. and Yesodharan E.P.*
School of Environmental Studies, Cochin University of Science and Technology, Kochi-682022 INDIA

Available online at: www.isca.in
(Received 26th November 2011, revised 6th January 2012, accepted 25th January 2012)

Abstract
Sonocatalytic degradation of organic pollutants in water is investigated using ZnO, TiO 2 and combination of ZnO and TiO2
(ZnO-TiO2) as catalysts, with phenol as the test substrate. The efficacy of the catalysts is in the order ZnO-TiO2 > ZnO > TiO2.
The degradation in presence of ZnO-TiO2 is more than the sum of the degradation achieved in presence of the individual oxides
under identical conditions, thereby demonstrating a synergistic effect. The ratio of the components in the mixed oxide is
optimized. The kinetics of the degradation as well as the influence of various parameters such as catalyst loading, concentration
of the pollutant and pH on the degradation efficiency is evaluated. Maximum degradation is observed in the acidic pH for all
catalysts. H2O2 is formed in the reaction and it undergoes simultaneous decomposition resulting in periodic increase and
decrease in its concentration. This observation of the phenomenon of oscillation in the concentration of H2O2 is the first of its
kind in sonocatalytic systems. A mechanism for the degradation of phenol is proposed based on the observations as well as the
concurrent formation and decomposition of H2O2.
Keywords: Zinc oxide, titanium dioxide, sonocatalysis, phenol, hydrogen peroxide.

Introduction
Advanced Oxidation Processes (AOP) involving Ultraviolet
light, Fenton reagents, Ozone, Ultrasound etc have been
tested individually as well as in combination, in the presence
and absence of catalysts for the treatment of wastewater
containing pesticides, phenols, chlorophenols, dyes and other
pollutants 1-5. The mechanism in all these cases involves the
formation of active .OH radicals which mineralize the
pollutants into carbon dioxide, phosphates, sulphates etc.
Recently, Ultrasonic (US) irradiation mediated by suitable
catalysts (sonocatalysis) has been receiving special attention
as an environment - friendly technique for the treatment of
hazardous organic pollutants in wastewater6. However the
degradation rate is slow compared to other established
methods. Investigations aimed at enhancing the efficiency of
US promoted decontamination of water are in progress in
many laboratories. These include testing a variety of
catalysts with different physico-chemical characteristics,
modification of reactor design and reaction conditions,
combining US with other AOP techniques etc6-9. Coupling
US with Ultraviolet (UV) irradiation enhances the efficiency
of semiconductor mediated degradation of aqueous pollutants
synergistically6,7,10,11.
In liquids US produces cavitation which consists of
nucleation, growth and collapse of bubbles. The collapse of
the bubbles results in localized supercritical condition such
as high temperature, pressure, electrical discharges and
International Science Congress Association

plasma effects10. The temperature of the gaseous contents of
a collapsing cavity can reach approximately 5500 0C and that
of the liquid immediately surrounding the cavity reaches up
to 21000C. The localized pressure is estimated to be around
500 atmospheres resulting in the formation of transient
supercritical water12. The cavities are thus capable of
functioning like high energy micro reactors. The
consequence of these extreme conditions is the cleavage of
dissolved oxygen molecules and water molecules into
radicals such as H., OH. and O. which will react with each
other as well as with H2O and O2 during the rapid cooling
phase giving HO2. and H2O2. In this highly reactive nuclear
environment, organic pollutants can be decomposed and
inorganic pollutants can be oxidised or reduced. This
phenomenon is being explored in the emerging field of
sonocatalysis for the removal of water pollutants.
Most of the studies on the sonocatalytic degradation of water
pollutants are made using TiO2 catalyst, mainly due to its
wide availability, stability, non-toxicity and reactivity.
Another similar semiconductor oxide ZnO has received
relatively less attention possibly due to its corrosive nature
under extreme pH conditions. At the same time ZnO is
reported to be more efficient than TiO2 for the visible light
induced photocatalytic degradation of organic pollutants
because the former can absorb a larger fraction of solar
spectrum compared to the latter 13,14. Earlier studies in our
laboratory showed that ZnO is very efficient as a sono
catalyst and sonophotocatalyst for the degradation of trace
pollutants in water15. Comparative study of the catalytic
191

Research Journal of Recent Sciences ____________________________________________________________ ISSN 2277-2502
Vol. 1 (ISC-2011), 191-201 (2012)
Res. J. Recent Sci.
activity of nano-sized TiO2, ZnO and composite TiO2/ZnO
powders for the sonocatalytic degradation of dyestuffs
showed that the composite powder is more effective 16.
Similarly Tb7O12/TiO2 composite was reported to perform
better as sonocatalyst compared to the individual oxides for
the degradation of amaranth17. In spite of the multitude of
research papers on the sonocatalytic activity of TiO 2 and its
modified forms, not many reports are available on the use of
ZnO or coupled ZnO-TiO2 as sonocatalysts for the removal
of water pollutants. In this paper we report the comparative
assessment of the sonocatalytic activity of ZnO, TiO2 and
ZnO-TiO2 for the removal of trace amounts of phenol in
water and the factors influencing their performance.

was agitated continuously at constant temperature of 27 ±
10C for 2 hrs to achieve equilibrium. This was then
centrifuged at 3000 rpm for 10 min. After centrifugation the
concentration of phenol in the supernatant was determined
colorimetrically. The adsorbate uptake was calculated from
the relation : qe = (C0 – Ce)V / W
where C0 is the initial adsorbate concentration (mg/L), Ce is
the equilibrium adsorbate concentration in solution (mg/L),
V is the volume of the solution in liter, W is the mass of the
adsorbent in gram and qe is the amount adsorbed in mg per
gram of the adsorbent.

Results and Discussion
Material and Methods
ZnO and TiO2 used in the study were supplied by Merck
India Limited. In both cases the particles were approximately
spherical and nonporous with over 99% purity. The surface
areas of TiO2 and ZnO, as determined by the BET method
are 15 and 12 m2/g respectively. ZnO-TiO2 catalysts were
prepared by physically mixing respective components in
required weight ratios and thoroughly mixing for 30 minutes
using mechanical shaker. Phenol AnalaR Grade (99.5%
purity) from Qualigen (India) was used as such without
further purification. Doubly distilled water was used in all
the experiments. All other chemicals were of AnalaR Grade
or equivalent. The average particle size of both ZnO and
TiO2 was 10 µm, unless mentioned otherwise.
The experiments were performed using aqueous solutions of
phenol of the desired concentration. Specified quantity of
the catalyst is suspended in the solution. In the case of US
irradiation experiments, sonication was sufficient to ensure
adequate mixing of the suspension. Additional mechanical
mixing did not make any notable consistent difference in the
US reaction rate. The reactor was a cylindrical Pyrex vessel
of 250 ml capacity. In the case of sonocatalytic experiments,
ultrasonic bath was used as the source of US. The ultrasonic
bath operated at 40 kHz and power of 100 W. Water from the
sonicator was continuously replaced by circulation from a
thermostat maintained at the required temperature. Unless
otherwise mentioned, the reaction temperature was
maintained at 27 ± 10C. The position of the reactor in the
ultrasonic bath was always kept the same. At periodic
intervals samples were drawn, the suspended catalyst
particles were removed by centrifugation and the
concentration of phenol left behind was analyzed by
Spectrophotometry at 500 nm. H2O2 is determined by
standard iodometry. Degradation and mineralization were
identified by the evolution of CO2. Adsorption studies were
performed as follows18:
A fixed amount (0.1 g) of the catalyst was introduced to 100
ml of phenol solution of required concentration in a 250 ml
beaker and the pH was adjusted as required. The suspension

International Science Congress Association

Preliminary investigations on the sonocatalytic degradation
of phenol were made using ZnO and TiO2 catalysts under
identical conditions. The results show that ZnO with 14%
degradation of phenol is more efficient as sonocatalyst than
TiO2 with 7% degradation in 2 hr time under otherwise
identical conditions.
No significant degradation of phenol took place in the
absence of US or the catalyst suggesting that both catalyst
and sound are essential to effect degradation. Small quantity
of phenol degraded under US irradiation even in the absence
of the catalyst. This is understandable since sonolysis of
water is known to produce free radicals H. and OH. (via
reaction 1), which are capable of attacking the organic
compounds in solution.
H2 O

>>>

H. + .OH (1)

>>> refers to sonication

The process is facilitated in a heterogeneous environment
such as the presence of ZnO or TiO2. The presence of the
particles helps to break up the microbubbles created by US
into smaller ones, thus increasing the number of regions of
high temperature and pressure10. This leads to increase in the
number of OH radicals which will interact with the phenol
present in water and oxidise it, resulting in eventual
mineralization.
Coupling of ZnO and TiO2 in the weight ratio 1:1 results in
13.5% phenol degradation which is same as in the case of
ZnO of same mass.
This is more than the sum of
degradation achieved in the presence of individual ZnO and
TiO2 at loadings equivalent to their concentration in the
combination, thereby showing synergistic effect. The %
degradation varies with the composition of ZnO-TiO2 with
maximum degradation of 14% in presence of ZnO/TiO2 at
4:6 as shown in figure 1. Hence further studies with the ZnOTiO2 combination were carried out using this ratio.
The synergy index can be calculated from the rate of
degradation using the following equation: Synergy index =
R(Z-T) / (RZ+ RT)

192

Research Journal of Recent Sciences ____________________________________________________________ ISSN 2277-2502
Vol. 1 (ISC-2011), 191-201 (2012)
Res. J. Recent Sci.
where RZ, RT and R(Z-T) are sono catalytic degradation rates
in presence of ZnO, TiO2 and ZnO-TiO2 respectively. The
maximum synergy index thus calculated is approx. 1.30 for
ZnO concentration of 10% (by mass) in the coupled ZnOTiO2. With increase in the concentration of ZnO in the
couple, the synergy index drops slowly. Beyond 40%
concentration of ZnO, the couple behaves more like pure
ZnO as a sonocatalyst.

The effect of various parameters on the efficiency of the
catalysts is investigated in detail in order to optimize the
conditions of degradation of phenol in presence of each of
them.
Effect of catalyst dosage: The effect of catalyst dosage on
the sonocatalytic degradation of phenol is studied at different
loadings of ZnO, TiO2 and ZnO-TiO2. The results are plotted
in figure 2.

20

% Degradation of phenol

15

10

[phenol]
Catalyat loading
Time
pH

- 40 mg/L
- 0.1g/L
- 120 min
- 5.5

5

0
--

0

10

20

30

40

ZnO % in

50

70

80

90

100

(ZnO +TiO2)

Figure - 1

Figure-1
Effect
ZnO/TiOratio
ratio
on sonocatalytic
the sonocatalytic
Effectof
of ZnO/TiO2
on the
activity
2

14

ZnO
TiO2
ZnO+TiO2

% Degradation of phenol

12

activity

[phenol]
Time
pH

- 40 mg/L
- 120 min
- 5.5

10
8
6
4
2
0
0.02

0.05

0.1

0.15

0.25

0.4

Catalyst loading (g/L)
Figure - 2

Figure-2
Effect
of catalystdosage
dosage onon
thethe
sonocatalytic
activity ofactivity
ZnO, TiO
ZnO+TiO
Effect
of catalyst
sonocatalytic
of
ZnO,TiO
and
2 and
2
2

International Science Congress Association

ZnO+TiO2

193

Research Journal of Recent Sciences ____________________________________________________________ ISSN 2277-2502
Vol. 1 (ISC-2011), 191-201 (2012)
Res. J. Recent Sci.
In all three cases the degradation increases with increase in
catalyst loading and reaches an optimum range. Beyond this
optimum, the degradation slows down and thereafter remains
more or less steady or even decreases. The enhanced
degradation efficiency with increase in the dosage is
probably due to increased number of catalytic sites, higher
production of OH radicals and more effective interaction
with the substrate. It is known that the addition of particles
of appropriate amount and size into the liquid system results
in increase in the acoustic noise and a rise in temperature in
the irradiated liquid19. Introduction of more catalyst particles
in the solution provide more nucleation sites for cavitation
bubbles at their surface. This will result in decrease in the
cavitation thresholds responsible for the increase in the
number of bubbles when the liquid is irradiated by US. The
increase in the number of cavitation bubbles increases the
pyrolysis of water and the sonocatalytic degradation of
phenol. Any further increase in catalyst concentration beyond
the optimum will only result in the particles coming too close
to each other or aggregating thereby limiting the number of
active sites on the surface. Higher concentration of the
suspended particles may also disturb the transmission of
ultrasound in water medium. Hence no further increase in the
degradation of the pollutant is observed beyond the optimum
dosage. However the number of particles alone or the effect
of ultrasound on them is not the only factor leading to
increased degradation with increase in catalyst dosage, as
seen in the difference in the optimum amount of ZnO , TiO2
or ZnO-TiO2 with comparable particle size and surface area.
Surface and bulk interactions of the reactant molecules play
an important role in the sonocatalytic degradation of organics
in suspended systems.

or its consequences alone. Irradiation of aqueous solution by
ultrasound is known to produce ultraviolet light by
sonoluminescence22. Since ZnO is known to be a better
harvester of light14, the higher sonocatalytic activity can be at
least partly attributed to the photocatalysis occurring during
US irradiation. The presence of suspended particles lead to
better propagation of the ultrasonic wave in the suspended
medium resulting in the production of cavitation bubbles and
emission of light throughout the reactor. This light can
activate ZnO leading to the production of OH radicals which
can either react with phenol and degrade it or recombine to
produce H2O2. Higher adsorption of the pollutant on the
surface of the catalysts is known to retard the absorption of
light resulting in lower degradation. At the same time lower
adsorption can result in decreased reaction rate, prolonged
degradation time and even incomplete degradation. Hence
reasonable degree of adsorption combined with good
absorption of light resulting from sonoluminescence lead to
good sonocatalytic activity of semiconductor oxides. The
higher activity of ZnO-TiO2 indicates that the better
adsorption capability of TiO2 and the light absorption
capability of ZnO can be suitably exploited to achieve
maximum degradation of the pollutant in water by
sonocatalysis.

The increase in the degradation of phenol with increase in
catalyst dosage as observed here is inconsistent with the
report that adsorption of the pollutant molecules on the
surface may protect them from ultrasonic degradation 20. The
adsorption of phenol on the catalysts is determined at the
optimum degradation dosages and the values are 24, 16 and
19 mg/g of the catalyst for TiO2, ZnO and ZnO-TiO2
respectively. TiO2 is a better adsorber compared to the other
two catalysts. However, the sonocatalytic activity is less
compared to ZnO thereby confirming that adsorption is not
the major factor in sonocatalysis. At the same time increase
in the degradation rate with increase in catalyst loading, as
observed in the current study shows that adsorption does not
inhibit the degradation altogether. Adsorption helps the
surface initiated degradation on the one hand and protects at
least partly, the adsorbed species from cavitation effects. At
the same time cavitation is known to alter the
adsorption/desorption/degradation rates 21.

The results indicate that H2O2 formation is more in presence
of UV than US in the case of ZnO and ZnO-TiO2. In the case
of TiO2, the H2O2 formed is more in presence of US
compared to UV, at least in the initial stages. In the case of
US, the concentration of H2O2 is less in the presence of
phenol probably because some of the .OH radicals formed
may be reacting with phenol before they could recombine to
produce H2O2. Also in this case, thermal decomposition of
H2O2 to water and oxygen rather than to reactive radical
species may be occurring23. In the case of TiO2 the difference
in H2O2 concentration in the reaction system, between in the
presence and absence of phenol is not significant. In this
case, the degradation of phenol is also less. The
concentration of H2O2 is increasing and decreasing
periodically showing that it is undergoing simultaneous
formation and decomposition. At the same time, the
degradation of phenol continues without break, though the
rate of degradation slows down with time.

The current study shows that ZnO is more efficient than TiO2
for the degradation of phenol. Appropriate combination of
ZnO -TiO2 is having the same activity as ZnO. This also
shows that the effect of particles is not limited to cavitation

The decomposition and consequent decrease in the
concentration of H2O2 is more evident even in the initial
stages in the case of TiO2. It is also pertinent to note that the
maxima and minima attained in the case of respective

International Science Congress Association

H2O2 formation and decomposition: Formation of H2O2 is
observed in the case of sonocatalytic and photocatalytic
degradation of phenol in presence of ZnO, TiO2 and ZnOTiO2. Hydrogen peroxide is produced even in the absence of
phenol indicating the formation of free radicals OH and HO 2
in liquid water by US/UV. The results are shown in figures 3
and 4.

194

Research Journal of Recent Sciences ____________________________________________________________ ISSN 2277-2502
Vol. 1 (ISC-2011), 191-201 (2012)
Res. J. Recent Sci.
catalysts remain more or less in the same range, irrespective
of the number of crests and valleys. This suggests that there
is some kind of an equilibrium concentration for H2O2 in
each system, at which the rate of decomposition and
formation balances each other. The maximum concentration
of H2O2 reached is different for different catalysts indicating
that it is dependent on the catalyst. Similar oscillatory

behavior in the concentration of H2O2 during photocatalysis
24
and sonophotocatalysis15 has been reported earlier. It is
known25 that H2O2 decomposes and produces OH radicals
during, sono, photo and sonophotocatalysis. These radicals
can accelerate the degradation of phenol. This is tested by
adding H2O2 in the beginning of the experiments. The results
are shown in table 1.

ZnO(US)
ZnO(UV)
TiO2(US)
TiO2(UV)
ZnO+TiO2(US)
ZnO+TiO2(UV)

14

12

[phenol]
Time
pH
Catalyst dosage

H2O2 (mg/L)

10

- 40 mg/L
- 120 min
- 5.5
- ZnO (0.1 g/L)
TiO2 (0.25 g/L)
ZnO+TiO2(0.1 g/L)

8

6

4

2

0
30

60

90

120

150

180

Time (min)

Figure-3
Figure - 3
Oscillation in the concentration of H2O2 in the presence of phenol under sono and photo catalytic condition on various
Oscillation in the concentration
catalysts of H2O2 in the presence of phenol
under sono and photo catalytic condition on various catalysts

ZnO(US)
ZnO(UV)
TiO2(US)
TiO2(UV)
ZnO+TiO2(US)
ZnO+TiO2(UV)

14
12

[phenol]
Time
pH
Catalyst dosage

- 40 mg/L
- 120 min
- 5.5
- ZnO (0.1 g/L)
TiO2 (0.25 g/L)
ZnO+TiO2(0.1 g/L)

H2O2 (mg/L)

10
8
6
4
2
0
0

30

60

90
120
Time (min)

150

180

Figure-4
- 4 sono and photo catalytic condition on various
Oscillation in the concentration of H2O2 in the absence of Figure
phenol under
catalysts
Oscillation in the concentration of H2O2 in the absence of phenol
under sono and photo catalytic conditon on various catalysts

International Science Congress Association

195

Research Journal of Recent Sciences ____________________________________________________________ ISSN 2277-2502
Vol. 1 (ISC-2011), 191-201 (2012)
Res. J. Recent Sci.

Reaction
Condition
US (ZnO)

Table 1
Effect of added H2O2 on the degradation of phenol under US, UV and (US + UV)
[Catalyst]: 0.1g/L
pH: 5.5
Reaction Volume: 50 ml [Phenol]: 40 mg/L
% Degradation of phenol
% Degradation of phenol
% enhancement by added
without added H2O2
with added H2O2
H2O2
30 min
60 min
90 min
30 min
60 min
90 min
30 min 60 min 90 min
1.1
6.0
9.5
3.0
7.1
11.2
172.7
18.3
17.9

UV (ZnO)

7.0

17.5

33.2

14.3

31.2

44.7

104.3

78.3

34.6

US (TiO2)

0.8

3.7

5.2

1.8

4.7

5.8

125.0

27.0

11.5

UV (TiO2)

5.8

14.6

26.3

10.5

24.8

34.6

81.0

70.0

31.6

US (ZnO-TiO2)

1.1

6.2

10.1

2.7

7.4

11.4

145.5

19.4

12.9

UV (ZnO-TiO2)

9.2

18.3

36.6

16.1

29.5

45.8

75.0

61.2

25.1

H2O2 enhances the degradation of phenol significantly in the
beginning. However this high rate of enhancement is not
sustained later on. This can be explained as follows:
In the beginning, added H2O2 decomposes faster in presence
of UV and US producing maximum OH radicals which can
degrade phenol. However, the decomposition of H 2O2 to
water and oxygen also occurs in parallel which restricts the
continued availability of the oxidizing species for phenol
degradation. Further, even in those experiments without
externally added H2O2, the H2O2 formed insitu will be
accelerating the reaction rate. Hence the effect of initially
added H2O2 is not that prominent in the later stages of the
reaction. The decrease in the enhancement of degradation
with time is relatively less in the case of UV. Here the
decomposition of H2O2 is occurring slowly thereby making
the OH radicals available for degradation reaction for
extended period. The thermal decomposition of H2O2 into
inactive H2O and O2 also is lower in the case of UV
irradiation compared to US.
H2O2 accelerates the degradation in all cases following a
fairly uniform pattern. The enhancement effect is comparable
in the case of ZnO and ZnO-TiO2. This shows that in the
case of the coupled catalyst, the mechanism of degradation of
phenol as well as the formation and decomposition of H 2O2
is more or less dictated by ZnO since it has higher sono and
photocatalytic activity compared to TiO2.
Concentration Effect: The effect of concentration of phenol
on the rate of degradation is investigated. The results are
plotted in figure 5.
In the case of ZnO, TiO2 as well as ZnO-TiO2 the rate
increases linearly with increase in concentration at lower
concentration range of 10-40 mg/L.
At higher
concentrations, the rate slows down as the concentration
increases. Thus the degradation follows first order kinetics at
lower concentration which changes to lower order at higher
concentration. Decrease in the rate of degradation and hence

International Science Congress Association

in the order of the reaction at higher concentration of the
reactant has been reported in the case of photocatalysis26,27,
sonocatalysis28 as well as sonophotocatalysis15. In the
present study the kinetics observed in the case of all three
catalysts is similar indicating that the mechanism of
degradation may be the same. However, the change of
reaction order takes place at slightly lower concentration
ranges in the case of TiO2 showing the role of surface
characteristics and adsorption on the rate of reaction.
Sonocatalytic reactions occur at the surface, in the bulk as
well as at the interface of the cavitation bubble. At the
surface of the collapsed bubble, the concentration of the OH
radicals is relatively high. At low concentration, when the
amount of phenol at the surface or in the bulk is low, a
considerable part of the OH radicals will recombine yielding
H2O2. Only about 10% of the OH radicals generated in the
bubble can diffuse into the bulk solution29. These factors
result in lower degradation of phenol. With increase in
concentration, the probability of interaction of OH radicals
with phenol increases on the surface as well as in the bulk
resulting in increased rate of degradation. The degradation
rate slows down and reaches almost a constant level when
the concentration of phenol on the catalyst surface as well as
at the bubble surface reach a saturation limit during the
persistence of the bubble. This is in agreement with earlier
findings30.
The general mechanism of sonocatalytic degradation in
aqueous medium involves the formation of OH radicals and
their attack on the organic substrate. This can also explain
the decrease in the recombination of OH radicals resulting in
lower concentration of H2O2 at higher concentration of
phenol31. At higher concentration of the substrate, the
surface is fully covered as a result of which it cannot
effectively absorb the light produced by ultrasound, resulting
in decreases photocatalytic effect and eventual stabilization.
Also at higher concentrations, the phenol molecules can act
as mutual screens thereby preventing effective interaction of
all molecules with the ultrasound 32.

196

Research Journal of Recent Sciences ____________________________________________________________ ISSN 2277-2502
Vol. 1 (ISC-2011), 191-201 (2012)
Res. J. Recent Sci.
Effect of pH: The pH of the reaction medium is known to
have strong influence on US or UV-induced degradation of
organic pollutants. In photolysis, the possibility of bond
breakage and the site might be different at different pH due
to difference in the distribution of molecular charges. In
sonocatalytic reaction, pH can alter the distribution of the
pollutants in the bulk region, on the surface and at the site of

the cavity collapse. The surface charge of semiconductors
and the interfacial electron transfer and the photoredox
processes occurring in their presence are also affected by pH.
Hence the effect of pH on sonocatalytic degradation of
phenol was investigated in the range 3-11. The pH of the
suspension was adjusted initially and it was not controlled
during the irradiation. The results are presented in figure 6.

75
ZnO
TiO2
ZnO+TiO2

Rate (mM/hr) x 10

3

60

45
pH
Time
Catalyst dosage

30

- 5.5
- 120 min
- ZnO (0.1 g/L)
TiO2 (0.25 g/L)
ZnO+TiO2(0.1 g/L)

15

0
10

20

30

40

50

60

Concentration of phenol (mg/L)

Figure-5
Figure - 5
Effect of concentration of phenol on the intial rate of sonocatalytic degradation on various catalysts
Effect of concentration of phenol on the intial rate of sonocatalytic degradation on various catalysts

15
ZnO
TiO2
ZnO+TiO2

% Degradation of phenol

12
[phenol]
Time
Catalyst dosage

9

- 40 mg/L
- 120 min
- ZnO (0.1 g/L)
TiO2 (0.25 g/L)
ZnO+TiO2(0.1 g/L)

6

3

0
0

2

4

6

8

10

12

14

pH

Figure-6
Figure - 6
Effect of pH on the sonocatalytic degradation
of phenol on various catalysts
Effect of pH on the sonocatalytic degradation of phenol on various catalysts

International Science Congress Association

197

Research Journal of Recent Sciences ____________________________________________________________ ISSN 2277-2502
Vol. 1 (ISC-2011), 191-201 (2012)
Res. J. Recent Sci.
The degradation is more efficient in the acidic region than in
the alkaline region in the case of the three catalysts tested
here. In the case of ZnO, maximum degradation is observed
in the acidic pH range of 4-6, which peaks at pH 5.5. In the
case of TiO2 also similar trend follows with the maximum at
pH 6. For ZnO-TiO2, the pH effect is quite similar to that of
ZnO as expected. The optimum pH in all these cases is 5.56. Higher degradation efficiency in the acidic range has been
reported by other authors also 33-35 with different types of
phenol using TiO2 as the catalyst. The steep fall in
degradation rate below pH 4 in the case of ZnO and ZnOTiO2 can be attributed to the corrosion of ZnO under acidic
conditions.

At pH less than ~7, when the TiO2 surface is positively
charged, phenol which is in neutral form can get closer to the
surface or weakly adsorbed. At pH > 7, when the surface is
negatively charged, phenol in neutral or ionized form will
keep away from the surface. In the case of ZnO, weak
adsorption or al least phenol coming closer to the surface is
possible upto pH 9. Hence the surface promoted
sonocatalytic degradation is more in the case of ZnO than
TiO2. As expected, the pH effect on the ZnO-TiO2 is more or
less similar to that on ZnO. The results clearly indicate that
there is no well defined correlation between PZC of the
semiconductor oxide catalyst and the sonocatalytic
degradation rate.

The pH of the reaction medium has significant effect on the
surface properties of semiconductor oxide particles,
including the surface charge, size of the aggregation and the
band edge position36. Hence pH can affect the adsorption –
desorption characteristics of the surface of the catalyst.
However, in the case of sonocatalysis, adsorption is not the
only factor leading to the degradation for reasons explained
earlier.

Possible mechanism: Sonocatalytic degradation is generally
explained based on sonoluminescence and hot spot theory.
Ultrasonic irradiation results in the formation of light of a
comparatively wide wavelength range of 200 -500 nm. Those
lights with wavelength below 375 nm can excite the
semiconductor catalyst and generate highly active OH
radicals on the surface. Thus the basic mechanism is partly
that of photocatalysis. At the same time the more complex
phenomenon of formation of hotspots upon implosion of
some bubbles on the catalyst surface also leads to the
formation of electron-hole pairs and excess OH radicals14.
Since the formation of electron-hole pairs is the first step in
both photocatalysis and sonocatalysis, the efficiency of the
process depends on the ability to prevent their
recombination. This is achieved to some extent by combining
the semiconductor oxides ZnO and TiO2 which is one of the
reasons for the observed synergy here. Under ultrasonic
irradiation, a series of thermal and photochemical reactions
take place on the surface of composite TiO2/ZnO particles16.
Because of the difference in adsorption capacity, the TiO2
part is inclined to the hole oxidation and ZnO tends towards
radical oxidation. The electron transport in the TiO2/ZnO
prevents the electron – hole recombination and increases the
sonocatalytic activity. Because the TiO2 and ZnO possesses
similar energy band gap (3.2 eV) the electrons can transfer
easily from TiO2 to ZnO through the crystal interface
between the two which results in complete separation of
electrons and holes. Such electron transport through the
crystal interface of composite oxides has been reported
earlier also17.

The acid-base property of metal oxides can influence their
photocatalytic activity significantly. The Point of Zero
Charge (PZC) of ZnO and TiO2 are 9.3 and 6.8
approximately14. This means that the catalyst surface is
positively charged when the pH is lower than respective PZC
value and negatively charged when the pH is higher.
Solution pH influences the ionization state of ZnO surface
according to the reaction:
---Zn – OH + H+ ↔ ZnOH2+
-- Zn – OH + OH-- ↔ Zn-O- + H2O

(2)
(3)

In the alkaline pH range, where phenol is expected to be in
the ionized form, the adsorption on ZnO will be weaker.
Hence the surface mediated degradation will be less.
However under acidic conditions, phenol which remains
mainly in the neutral form can get adsorbed or come closer to
the catalyst surface, resulting in its degradation via active
surface species or bulk hydroxyl radicals produced in the
aqueous media. Further, the presence of more protons can
facilitate the formation of reactive OH radicals from the
available OH ions.
Significant enhancement in the
degradation can also be attributed to the effect of US in
reducing the distance between the substrate molecule and the
surface of the catalyst particles. This is not feasible in the
alkaline range where repulsion between like charges of the
substrate and the catalyst particles is much greater 16.
Similarly in the case of TiO2, solution pH influences the
ionization state of TiO2 surface according to the reaction11
Ti – OH + H+ ↔
Ti – OH + OH-

Ti-OH2+
↔
Ti-O-

(4)
+ H2O (5)

International Science Congress Association

The overall mechanism of H2O2 formation and the
decomposition of phenol under sonocatalytic conditions can
be explained as follows:
Acoustic cavitation produces highly reactive primary radicals
such as OH and H as in reaction (6). Recombination and a
number of other reactions occur within the bubble as in
reactions (7) to (11) following this primary radical generation
>>>
H2 O
→
H. + .OH
(6)
.
.
OH + H → H2O
(7)

198

Research Journal of Recent Sciences ____________________________________________________________ ISSN 2277-2502
Vol. 1 (ISC-2011), 191-201 (2012)
Res. J. Recent Sci.
.

OH + .OH → H2O2
(8)
H. + H2O
→ .OH + H2 (9)
H. + O 2
→
HO2.
(10)
.
.
HO2
+ HO2 →
H2O2
(11)
OH radical is a nonselective oxidant with a high redox
potential (2.8 eV) which is able to oxidise most organic
pollutants 37.
Similarly the photocatalytic reaction initiated by ultrasound
can be represented as follows38:
SC (Semiconductor) + hυ → h+ + e(12)
h+ + e- → Heat (Recombination)
(13)
h+ + OH→ OH.
(14)
h+ + H2O
→ OH. + H+
(15)
Scavenging of conduction band electrons
e- + O 2
→ .O2(16)
Formation of multiple peroxide species
H+ + .O2→ HO2.
(17)
.
+
HO2 + e + H → H2O2
(18)
Various reactive species produced as above react with phenol
as in reaction (19) below
Phenol + Reactive species (.O2-, HO2., OH.)
→
Intermediates → H2O + CO2 (19)
Once sufficient concentration of H2O2 is reached, its
decomposition also sets in as follows:
H2O2 + .OH
→
HO2. + H2O
(20)
.
.
HO2 + OH
→
H2O + O2
(21)
H2O2 can also lead to reduction in charge recombination by
taking up the electron
SC (e-) + H2O2 →
SC + HO. + OH- (22)
H2O2 can also produce OH radicals directly or reaction with
superoxide anion
hν
H2O2
→
2 .OH
(23)
.
H2O2 + O2
→
2 .OH + O2
(24)

Conclusion
The sonocatalytic activity of ZnO, TiO2 and ZnO-TiO2 for
the degradation of phenol pollutant in water is investigated.
The efficacy of the catalysts for the degradation is in the
order ZnO-TiO2 > ZnO > TiO2. At lower concentrations of
ZnO, the percentage degradation in the presence of coupled
ZnO-TiO2 is more than the sum of the degradation achieved
in the presence of individual oxides under identical
conditions, implying a synergistic effect. The catalyst
loading, irradiation time, initial pH and concentration of the
substrate have profound effect on the rate of degradation.
H2O2 formed during the degradation of phenol undergoes
simultaneous decomposition as well. After initial
accumulation upto certain concentration, decomposition of
H2O2 also sets in resulting in oscillation in its concentration.
Possible mechanism for the sonocatalytic degradation of
phenol, formation and decomposition of H2O2 and the
enhanced activity of coupled ZnO-TiO2 is discussed.

International Science Congress Association

Acknowledgement
ASG acknowledges the financial support from the Cochin
University of S &T, Kochi, India. JKP and SJ acknowledge
the financial support from the CSIR, New Delhi, India.
References
1.

Ying-Shih M, Chi-Fanga S, Jih-Gaw L, Degradation of
carbofuran in aqueous solution by ultrasound and Fenton
processes: Effect of system parameters and kinetic
study, J Hazardous Mater., 178, 320-325 (2010)

2.

Zouaghi R, David B. Suptil J, Djebbar K, Boutiti A,
Guittonneau S, Sonochemical and sonocatalytic
degradation
of
monolinuron
in
water,
Ultrason.Sonochem. 18, 1107-1112 (2011)

3.

Torres R.A, Abdelmalek F, Combet E, Petrier
C,Pulgarin C, A comparative study of ultrasonic
cavitation and Fenton’s reagent for bisphenol A:
Degradation in deionised and natural waters, J Hazard.
Mater 146, 546-555 (2007)

4.

Devipriya S, Yesodharan S, Photocatalytic degradation
of pesticide pollutants in water, Solar Energy Mater and
Solar Cells, 86, 309-348 (2005)

5.

Malato S, Blanco J, Alarcon D.C, Maldonado M.I, ,
Fernandez-Ibanez P, Gernjak W, Photocatalytic
decontamination and disinfection of water with solar
collectors, Catalysis Today 122, 137-149 (2007)

6.

Joseph C.G, Puma G.L, Bono A, Krishniah D,
Sonophotocatalysis in advanced oxidation process: A
short review, Ultrason., Sonochem. 16, 583-589 (2009)

7.

Gogate P.R, Treatment of wastewater streams containing
phenolic compounds using hybrid techniques based on
cavitation: a review of the current status and the way
forward, Ultrason. Sonochem.15, 1-15 (2008)

8.

Torres-Palma R.A, Nieto J.I, Combet E, Petrier C,
Pulgarin C, An innovative ultrasaound, Fe2+ and TiO2
photo assisted process for bisphenol a mineralization,
Water Res. 44, 2245-2252 (2010)

9.

Davydov L, Reddy E.P, France P, Smirniotis P,
Sonophotocatalytic destruction of organic contaminants
in aqueous systems on TiO2 powders, Appl. Catal.B:
Environmental 32, 95-105 (2001)

10. Chen Y.C, Smirniotis P, Enhancement of photocatalytic
degradation of phenol and chlorophenols by ultrasound,
Ind. Eng. Chem. Res. 41,5958- 5965 (2002)

199

Research Journal of Recent Sciences ____________________________________________________________ ISSN 2277-2502
Vol. 1 (ISC-2011), 191-201 (2012)
Res. J. Recent Sci.
11. Kritikos D.E, Xekoukoulotakis N.P, Psillakis E,
Mantzavinos D, Photocatalytic degradation of reactive
black 5 in aqueous solutionds: Effect of operating
conditions and coupling with ultrasound irradiation,
Water Res. 41, 2236-2246 (2007)
12. Nepiras E.A, Acoustic cavitation: An introduction,
Ultrasonics, 22, 25-40 (1984)
13. Poulios I, Avranas A, Rekliti E, Zouboulis A,
Photocatalytic oxidation of Auramine O in the presence
of semiconducting oxides, J Chem Technol. Biotechnol.
75, 205-212 (2000)
14. Sakthivel S, Neppolian B, Shankar M V, Arabindoo B,
Palanichamy M, Murugesan V, Solar photocatalytic
degradation of azodye: Comparison of photocatalytic
efficiency of ZnO and TiO2, Solar Energy Mater. and
Solar Cells. 77, 65-82 (2003)
15. Anju S.G, Yesodharan S, Yesodharan E.P,
Semiconductor mediated sonophotocatalytic degradation
of organic pollutants in water, Proc. 23 rd Kerala Science
Congress, Trivandrum 156-157 (2011)
16. Wang J, Jiang Z, Zhang L, Kang P, Xie Y, Lv Y, Xu R,
Zhang X, Sonocatalytic degradation of some dyestuffs
and comparison of catalytic activities of nanosized
TiO2, nano-sized ZnO and composite TiO2/ZnO
powders under ultrasonic irradiation, Ultrasonics
Sonochem. 16, 225-231 (2009)
17. Song L, Chen C, Zhang S, Sonocatalytic performance of
Tb7O12/TiO2 composite under ultrasonic irradiation,
Ultrason. Sonochem. 18, 713-717 (2011)
18. Jain S, Yamgar R, Jayram R.V, Photolytic and
photocatalytic degradation of atrazine in the presence of
activated carbon, Chem. Eng. Journal, 148, 342-347
(2009)
19. Tuziuti T, Yasui K, Sivakumar M, Iida Y, Correlation
between acoustic cavitation nopise and yiel
enhancement of sonochemical reaction by particle
addition, J Phys Chem. A 109. 4869-4872 (2005)
20. Pandit A.B, Gogate P.R, Majumdar S, Ultrasonic
degradation of 2.4.6 trichlorophenol in presence of TiO 2
catalyst, Ultrason.Sonochem. 8, 227-231(2001)
21. Hamdaoui O, Naffrechoux E, Adsorption kinetics of 4chlorophenol on granulated activated carbon in the
presence
of
high
frequency
ultrasound,
Ultrason.Sonochem 16, 15-22 (2009)

International Science Congress Association

22. Beckett M.A, Hua I, Impact of ultrasonic frequency on
aqueous
sonoluminescence
and
sonochemistry,
J.Phys.Chem A 105, 3796-3802 (2001)
23. Chand R, Ince N.H, Gogate P.R, Bremner D.H, Phenol
degradation using 20,300 and 520 kHz ultrasonic
reactors with hydrogen peroxide, ozone and zerovalent
metals, Separation and Purification Technology 67, 103109 (2009)
24. Kuriacose J.C, Ramakrishnan V, Yesodharan E.P,
Photoinduced catalytic reactions of alcohols on ZnO
suspensions in cyclohexane: Oscillation in the
concentration of H2O2 formed, Indian J. Chem. 19A,
254-256 (1978)
25. Hoffmann A.J,
Carraway E.R, Hoffmann M.R,
Photocatalytic production of H2O2 and Organic
peroxides` on quantum sized semiconductor colloids,
Environ. Sci. Technol. 28, 776-785 (1994)
26. Rabindranathan S, Devipriya S,Yesodharan S,
Photocatalytic degradation of phosphamidon on
semiconductor oxides, J Hazard. Mater. 102, 217-229
(2003)
27. Daneshvar N, Aber S, Dorraji M.S.S, Khataee A.R,
Rasoulifard, Preparation and investigation of
photocatalytic properties of ZnO nanocrystals: Effect of
operational parameters and kinetic study, World Acad of
Sci. Eng. And Technol. 29, 267-272 (2007)
28. Zheng W, Maurin M, Tarr M A, Enhancement of
sonochemical degradation of phenol using hydrogen
atom scavengers, Ultrason.Sonochem 12, 313-317
(2005)
29. Goel M, Hongqiang H, Majumdar A.S, Ray M.B,
Sonochemical decomposition of volatile and nonvolatile
organic compound: A comparative study, Water Res. 38,
4247-4261 (2004)
30. Marouani S, Hamdaoui O, Saoudi F, Chiha M,
Sonochemical degradation of Rhodamine B in aqueous
phase: effect of additives, Chem. Eng. J. 158, 550-557
(2010)
31. Madhavan J, Grieser F, Ashokkumar M, Combined
advanced oxidation processes for the synergistic
degradation of ibuprofen in aqueous environments, J
Hazardous Mater 178, 202-208 (2010)
32. Wang J, Jiang Y, Zhang Z, Zhao G, Zhang G, Ma T, Sun
W, Investigations on the sonocatalytic degradation of
Congo red catalysed by nanometer rutile powder and

200

Research Journal of Recent Sciences ____________________________________________________________ ISSN 2277-2502
Vol. 1 (ISC-2011), 191-201 (2012)
Res. J. Recent Sci.
various influencing factors, Desalination 216, 196-208
(2007)
33. Kaur S, Singh V, Visible light induced sonocatalytic
degradation of Reactive Red dye 198 using dye
sensitized TiO2, Ultrason. Sonochem. 14, 531-537
(2007)
34. Vijaya laxmi P.N, Saritha Rambabu P.N, Himabindu V,
Anjaneyulu Y, Sonochemical degradation of 2 chloro5methyl phenol assisted by TiO2 and H2O2, J Hazard
Mater. 174, 151-155 (2010)
35. Ou X.H, Lo S.L, Wu C. H, Exploring the interparticular
electron transfer process in the photocatalytic oxidation
of 4-chlorophenol, J Hazardous Mater. 137, 1362-1370
(2006)

International Science Congress Association

36. Zhow S, Ray A.K, Kinetic studies for photocatalytic
degradation of Eosin B on a thin film of Titanium
dioxide; Ind. Eng. Chem. Res. 42, 6020-6033(2003)
37. Neppolian B, Ciceri L, Bianchi C L, Grieser F, Ashok
kumar M, Sonophotocatalytic degradation of 4chlorophenol using Bi2O3/TiZrO4 as a visible light
responsive photocatalyst, Ultrason. Sonochem 18, 135139 (2011)
38. Davydov L, Smirniotis P.G, Quantification of the
primary processes
in
aqueous
heterogeneous
photocatalysis using single stage oxidation reactions, J
Catal. 191, 105-112 (2000)

201

