Research Journal of 
Recent 
Sciences 
______
______________________________
______
____
___
 
ISSN 2277
-
2502
 
 
Vol. 
2
(
ISC
-
2012
), 
136
-
149
 
(201
3
)
 
Res.J.
Recent
.Sci.
 
 
 
Internation
al Science Congress Association
 
 
 
 
 
136
 
Periodic Change in the Concentration of Hydrogen peroxide Formed during 
the Semiconductor Mediated Sonocatalytic treatment of Wastewater: 
Investigations on pH Effect and Other Operational Variables
 
 
Jyothi K. P, S
indhu Joseph, Suguna Yesodharan and
 
Yesodha
ran E.P.
 
School of Environmental Studies, Cochin University of Scien
ce and Technology, Kochi
, INDIA
 
 
Available online at: 
www.isca.in
 
Received
 
31
st
 
July
 
2012, revised
 
29
th
 
December 
 
2012, accepted
 
22
nd
 
January
  
201
3
 
 
 
 
Abs
tract
 
Hydrogen peroxide, formed in situ or externally added, is an important Reactive Oxygen Species (ROS) involved in 
Advanced Oxidation Processes (AOP) such as sono, photo and sonophoto catalysis being investigated as environment 
friendly technologies fo
r the treatment of wastewater under ambient conditions.  Among the various ROS such as 
.
OH, HO
2
.
, 
O
2
-
 
.
, H
2
O
2
, O
2
 
etc, H
2
O
2
 
is the most stable and it serves as a reservoir of other ROS. Current investigations on the ZnO and 
TiO
2
 
mediated sonocatalytic degr
adation of phenol pollutant in water reveal that, H
2
O
2
 
formed cannot be quantitatively 
correlated with the degradation of the pollutant.  The concentration of H
2
O
2
 
varies in a wavelike fashion (oscillation) with 
well defined crests and troughs, indicating 
concurrent formation and decomposition. Both processes are sensitive to the 
reaction conditions and depending on the externally forced or in situ situation, either of them can predominate.  The 
degradation of H
2
O
2
 
continues for some more time even after th
e sonication has been put off showing that the catalyst has 
some residual activity. This further confirms that trapped electrons and holes have unusually longer life even after the 
irradiation is off.  Concentration of H
2
O
2
, catalyst loading, dissolved gas
es, concentration of the organic pollutant, pH etc 
influence the oscillation. The degradation of phenol is favored in the acidic range with maximum at pH 5.5. The successive 
maxima and the minima in the oscillation of H
2
O
2
 
concentration also are higher in 
the acidic range. The influence of pH on 
various factors leading to the oscillation in the concentration of H
2
O
2
 
is unequivocally established from a number of 
experiments, for the first time in this paper. An appropriate mechanism to explain the complex ph
enomenon
 
is also proposed.
 
 
Keywords: 
Sonocatalysis, zinc oxide, titanium dioxide, hydrogen peroxide, oscillation, pH effect
.
 
 
Introduction
 
Considerable interest has been shown in the application of 
sonocatalysis, photocatalysis and its combination 
sonop
hotocatalysis using suspended semiconductor oxide 
particles for the environment
–
friendly destruction of organic 
pollutants in water
1
-
6
.  Major advantages of these Advanced 
Oxidation Processes (AOP) include relatively mild reaction 
conditions and their prov
en ability to degrade several toxic 
refractory pollutants. Of these, photocatalysis has been 
investigated extensively with semiconductors such as TiO
2
 
and 
ZnO as catalysts for the removal of a variety of pollutants. 
However, many of these catalysts are act
ive only in the UV 
range, which make them unattractive for solar energy 
harvesting. Several efforts are being made to make visible light 
active photocatalysts. These include dye sensitization, 
semiconductor coupling, impurity doping, use of coordination 
me
tal complexes and metal deposition
7
-
11
.  Composites such as 
TiO
2
/carbon have also been reported
12,13
. Deposition of noble 
metals such as Pt, Pd, Au, Ag etc on TiO
2
 
enhances the catalytic 
oxidation of organic pollutants
14,15
.
 
 
Recently, Ultrasonic (US) irra
diation mediated by suitable 
catalysts (sonocatalysis) has been receiving special attention as 
an environment 
-
 
friendly technique for the treatment of 
hazardous organic pollutants in wastewater
16,17
. 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 o
f 
reactor design and reaction conditions, combining US with other 
AOP techniques etc
18
-
22
. Coupling US with Ultraviolet (UV) 
irradiation enhances the efficiency of semiconductor mediated 
degradation of aqueous pollutants synergistically
5,19
. 
 
 
In liquids U
S 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 plasma effects 
16,22
-
24
. The temperat
ure of the gaseous contents of a collapsing 
cavity can reach approximately 5500
0
C and that of the liquid 
immediately surrounding the cavity reaches up to 2100
0
C. The 
localized pressure is estimated to be around 500 atmospheres 
resulting in the formation of
 
transient supercritical water.  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 
Research Journal of 
Recent 
Sciences 
______
_
_
_______________________________
______________
_
________
 
ISSN 2277
-
2502
 
Vol. 
2
(
ISC
-
2012
), 
136
-
149
 
(201
3
)
 
 
Res.J.Recent.Sci
.
 
 
 
International Science Congress Association
 
          
 
137
 
into radicals such as H
.
, OH
.
 
and O
.
 
which will react with each 
other as well as with H
2
O and O
2
 
during the rapid cooling phase 
giving HO
2
.
 
and H
2
O
2
. In this highly reactive nuclear 
environment, organic pollutants can be decomposed and 
inorganic pollutants can be oxidised or reduced. This 
p
henomenon is being explored in the emerging field of 
sonocatalysis for the removal of water pollutants. 
 
 
Studies in our laboratory have shown that H
2
O
2
, one of the 
major products of sonocatalytic degradation of organic 
pollutants in water, undergoes simul
taneous formation and 
decomposition during the degradation process
5,23
. This results in 
oscillation in the concentration of H
2
O
2
. The rate of degradation 
of the pollutants and the oscillation phenomenon are influenced 
by various reaction parameters such as
 
catalyst loading, 
substrate concentration, reaction volume, presence of anions, 
reaction intermediates, pH etc.  The effect of pH on 
sonocatalytic degradation of pollutants in water is more 
complex. In this study the effect of pH on sonocatalytically 
form
ed H
2
O
2 
in presence of ZnO is investigated in detail.  
 
 
 
Materials and Methods
 
ZnO and TiO
2
 
used in the study were supplied by Merck India 
Limited. In both cases the purity was over 99%.  The surface 
areas of TiO
2
 
and ZnO, as determined by the BET method 
are 
approximately15 and 12 m
2
/g respectively. The average particle 
size of both ZnO and TiO
2 
was 10 �m.  Phenol AnalaR Grade 
(99% purity) from Qualigen (India) was used as such without 
further purification. All other chemicals were of AnalaR Grade 
or equiv
alent.  The sonocatalytic reactions were performed as 
reported earlier
5
. The concentration of phenol left behind was 
analyzed periodically by Spectrophotometry at 500 nm.  H
2
O
2 
is 
determined by iodometry
23
.  Mineralization was identified by 
the evolution o
f CO
2
.
 
 
Results and Discussion
 
Investigations on the sonocatalytic degradation of phenol using 
ZnO catalysts showed that no significant degradation took place 
in the absence of ultrasound or the catalyst suggesting that both 
catalyst and sound are essentia
l to effect reasonable degradation. 
However, small quantity of phenol degraded under US 
irradiation even in the absence of catalysts (see figure 1).
 
 
 
 
Research Journal of 
Recent 
Sciences 
______
_
_
_______________________________
______________
_
________
 
ISSN 2277
-
2502
 
Vol. 
2
(
ISC
-
2012
), 
136
-
149
 
(201
3
)
 
 
Res.J.Recent.Sci
.
 
 
 
International Science Congress Association
 
          
 
138
 
This is understandable since sonolysis of water is known to 
produce free radicals H
.
 
and OH
.
 
(via rea
ction 1), which are 
capable of attacking the organic compounds in solution
25
. 
 
H
2
O          ���      H
. 
  
+  
.
OH
 
(1)
 
The free radicals thus produced can lead to the formation of 
H
2
O
2
 
and degradation of phenol as follows:
 
H
. 
+ O
2
   
→   HO
2
.
 
 
(2)
 
HO
2
. 
+ HO
2
.
 
→   H
2
O
2
 
 
(3)
 
2 
.
OH→   H
2
O
2
 
 
 
 
(4)
 
Reactive oxygen species (
.
OH, HO
2
. 
etc) + phenol →   
Interediates  →   H
2
O + CO
2
 
(5)
 
However, there was no continuous increase in the concentration 
of H
2
O
2
 
after the initial period possibly d
ue to its parallel 
decomposition into water and oxygen as well as participation in 
the degradation of phenol
5,16,23
.  At the same time the removal of 
phenol continued, though at a very slow rate.   
 
 
The sonocatalytic degradation of phenol in presence of Z
nO and 
TiO
2 
at respective optimized loadings
 
is shown in figure 2a. The 
fate of H
2
O
2
 
formed in presence of these catalysts is shown in 
figure 2b. 
 
Sonochemical processes in aqueous media are facilitated in a 
heterogeneous environment such as the presence o
f suspended 
particles
24,25
. The presence of the  particles help to break up the 
microbubbles created by US into smaller ones, thus increasing 
the number of regions of high temperature and pressure This 
leads to increase in the number of reactive OH radical
s which 
will interact with the organic pollutants present in water and 
oxidise them, resulting in eventual mineralization. The increase 
in the optimum concentration of H
2
O
2
 
in presence of particles 
was reported by Keck etal
26
.  Instances of decrease in the
 
concentration of H
2
O
2
 
in presence of particles have also been 
reported
19
. Our studies presented in this report as well as in 
earlier papers show that both increase and decrease in the 
concentration of H
2
O
2
 
is possible in the same system depending 
on the r
elative concentration of H
2
O
2
 
and the substrate as well 
as other reaction parameters
5,23
.
 
 
The effect of ZnO loading on the sonocatalytic formation of 
H
2
O
2
 
is shown in figure 3a. Catalyst loading for    highest 
maximum in the oscillation curve of H
2
O
2
 
is 5
0 mg/L.  
Optimum loading for the degradation of phenol is 100 mg/L (fig 
3b).
 
 
 
Research Journal of 
Recent 
Sciences 
______
_
_
_______________________________
______________
_
________
 
ISSN 2277
-
2502
 
Vol. 
2
(
ISC
-
2012
), 
136
-
149
 
(201
3
)
 
 
Res.J.Recent.Sci
.
 
 
 
International Science Congress Association
 
          
 
139
 
 
 
Research Journal of 
Recent 
Sciences 
______
_
_
_______________________________
______________
_
________
 
ISSN 2277
-
2502
 
Vol. 
2
(
ISC
-
2012
), 
136
-
149
 
(201
3
)
 
 
Res.J.Recent.Sci
.
 
 
 
International Science Congress Association
 
          
 
140
 
 
 
 
 
Research Journal of 
Recent 
Sciences 
______
_
_
_______________________________
______________
_
________
 
ISSN 2277
-
2502
 
Vol. 
2
(
ISC
-
2012
), 
136
-
149
 
(201
3
)
 
 
Res.J.Recent.Sci
.
 
 
 
International Science Congress Association
 
          
 
141
 
Since the formation and decomposition o
f H
2
O
2
 
is occurring in 
parallel all the time, optimization of catalyst loading with 
respect to H
2
O
2
 
formation  may not be reliable. Hence the 
optimum loading of ZnO for phenol degradation is taken as the 
basis for further studies on the oscillation in the 
concentration of 
H
2
O
2
. 
 
 
At the optimized loading for phenol degradation, the effect of 
particle size on the H
2
O
2
 
formation is examined.  The 
sonocatalytic rate of degradation of phenol increases with 
decrease in the particle size upto an optimum value
5
.  
The effect 
of particle size of ZnO on the H
2
O
2
 
formed in the system is 
shown in fig
ure
-
4. 
 
 
With increase in particle size in the range of 6
-
60 micron, the 
initial rate of H
2
O
2
 
formation decreases slightly from 1.4x 10
-
3
  
to 1.1 x 10
-
3
 
mg/L/sec possibly du
e to lower surface area and 
decrease in the rate of generation of OH radicals.  However, the 
nature of the oscillation curve remains the same in all cases 
even though the maxima of H
2
O
2
 
concentration in the 
oscillation curve are higher at lower particle si
zes. The 
successive maxima and minima of the oscillation curve remain 
more or less identical irrespective of the particle size of the 
catalyst. This confirms our earlier observation that the single 
most important factor that determines the rate of formatio
n or 
decomposition of H
2
O
2
 
is its concentration and either of them 
will predominate once a critical lower or upper concentration is 
reached respectively
5,21
. 
 
 
Simple quartz or alumina particles have very little effect on the 
sonochemical removal of phenol
 
thereby indicating that it is not 
just the particle effect that promotes sonocatalytic degradation 
of phenol in the case of semiconductor oxides such as ZnO, 
which can act catalytically. However the concentration of 
sonochemically formed H
2
O
2
 
increases in
itially in the presence 
of quartz, Al
2
O
3
 
and ZnO in the order ZnO � quartz � alumina � 
no particles.  
 
 
The higher yield of H
2
O
2
 
by sonolysis in presence of particles is 
explained by Keck etal
26
 
on the assumption that the bubble size 
and collapse time are 
not influenced by the nature and 
concentrations of the particles used. But the shape of the 
bubbles may have changed from spherical to asymmetric.  The 
larger surface of these bubbles enable more radicals to escape 
into the bulk forming more H
2
O
2
 
or react 
with more o
rganic 
molecules in the bulk.
 
 
Effect of Phenol on H
2
O
2
:
 
Formation of H
2
O
2
 
is reported in the 
case of sonocatalytic and photocatalytic degradation of phenol 
in presence of ZnO, TiO
2
 
and ZnO
-
TiO
2
 
catalysts
23
. In the case 
of US irradiation, H
2
O
2 
i
s produced even in the absence of 
phenol indicating the formation of free radicals OH and HO
2
 
in 
liquid water by US. The concentration of H
2
O
2
 
is less in the 
presence of phenol probably because some of the 
.
OH radicals 
formed may be reacting with phenol be
fore they could 
recombine to produce H
2
O
2
. This is confirmed by experiments 
with added H
2
O
2
 
which show that H
2
O
2
 
enhances the 
degradation of phenol significantly in the beginning [Table 1].
 
 
However this high rate of enhancement is not sustained later on, 
probably because thermal decomposition of H
2
O
2
 
to water and 
oxygen rather than to reactive radical species may be occurring 
in presence of US
19
. The decomposition and consequent 
decrease in the concentration of H
2
O
2
 
is more evident in the 
initial stages in
 
the case of TiO
2
27
. The maximum concentration 
of H
2
O
2
 
reached is different for different catalysts under 
otherwise identical conditions, indicating that it is dependent on 
the nature of the catalyst.  It also implies that the role of the 
semiconductor oxi
de is not limited to just particle effect.  In the 
beginning, added H
2
O
2 
decomposes faster   producing maximum 
OH radicals which can degrade phenol. However, the 
decomposition of H
2
O
2
 
to water and oxygen also occurs in 
parallel which restricts the continue
d availability of the 
oxidizing species for phenol degradation. Further, even in those 
experiments without externally added H
2
O
2
, the H
2
O
2
 
formed 
insitu will be accelerating the reaction rate. Hence the effect of 
initially added H
2
O
2
 
is not that prominent 
in the later
 
stages of 
the reaction.  
 
 
The effect of concentration of phenol on the rate of degradation 
has been investigated earlier
23
. The degradation increases with 
increase in the concentration of phenol upto 40 mg/L beyond 
which it levels off or decr
eases slightly, due to saturation of the 
catalyst surface.  In the case of oscillation of H
2
O
2
 
also the 
effect of concentration is fairly similar.  At the optimum 
concentration for phenol degradation (30
-
40 mg/L) (figure 5), 
the maxima   in H
2
O
2
 
concentrat
ion in the oscillation curve also 
are the highest indicating a direct correlation between the rate of 
phenol d
egradation and the oscillation.
 
 
Effect of pH
: 
The pH of the reaction medium is known to have 
strong influence on   US or UV
-
induced degradation o
f 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.  Previous studies
5 
have sho
wn that the 
degradation is more efficient in the acidic region than in the 
alkaline region and 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 TiO
2
 
also similar trend follows with th
e maximum 
at pH 6. The effect of pH on the fate of H
2
O
2
, especially the 
oscillation in its concentration resulting from simultaneous 
formation and decomposition has not been investigated so far.  
Fig 6 shows the concentration of H
2
O
2
 
in the system at diffe
rent 
times during the sonocatalytic degradation of phenol on ZnO at 
different pH.
 
 
Research Journal of 
Recent 
Sciences 
______
_
_
_______________________________
______________
_
________
 
ISSN 2277
-
2502
 
Vol. 
2
(
ISC
-
2012
), 
136
-
149
 
(201
3
)
 
 
Res.J.Recent.Sci
.
 
 
 
International Science Congress Association
 
          
 
142
 
Table
-
1
 
Effect of added H
2
O
2
 
on the sonocatalytic degradation of phenol in presence of ZnO and TiO
2
 
Reaction
 
Condition
 
% Degradation of phenol
 
without added H
2
O
2
 
% Degradat
ion of phenol
 
with added H
2
O
2
 
% enhancement by added 
H
2
O
2
 
30 min
 
60 min
 
90 min
 
30 min
 
60 min
 
90 min
 
30 min
 
60 min
 
90 min
 
US (ZnO)
 
1.1
 
6.0
 
9.5
 
3.0
 
7.1
 
11.2
 
172.7
 
18.3
 
17.9
 
US (TiO
2
)
 
0.8
 
3.7
 
5.2
 
1.8
 
4.7
 
5.8
 
125.0
 
27.0
 
11.5
 
[Catalyst]: 0.1g/L
 
 
pH: 5.5
 
 
R
eaction Volume: 50 ml
 
[Phenol]: 40 mg/L
 
 
 
 
Research Journal of 
Recent 
Sciences 
______
_
_
_______________________________
______________
_
________
 
ISSN 2277
-
2502
 
Vol. 
2
(
ISC
-
2012
), 
136
-
149
 
(201
3
)
 
 
Res.J.Recent.Sci
.
 
 
 
International Science Congress Association
 
          
 
143
 
The oscillation as well as the maxima in the concentration of 
H
2
O
2
 
is more pronounced in the acidic region. This is consistent 
with the observations on the sono
catalytic degradation of phenol 
on ZnO in which maximum degradation is observed at in the 
range of pH 5.5 to 6. The relatively lower maxima and minima 
at pH 3 are probably because of the corrosion of ZnO which 
reduces the effective surface sites for the fo
rmation of OH 
radicals and subsequent interactions.  In order to decipher the 
effect of phenol on the oscillation, the sonocatalytic 
decomposition of H
2
O
2
 
on ZnO was investigated (in the absence 
of phenol) under identical conditions (Fig
ure
 
7) 
 
 
In this ca
se also the concurrent formation and decomposition of 
H
2
O
2
 
is seen at all pH.  Comparison of the results in the 
presence as well as the absence of phenol shows that the 
oscillation is more significant in presence of phenol. This 
illustrates the importance 
of interaction between the free 
radicals generated and phenol.  This is further verified by 
adding phenol in between to the sonocatalytic system containing 
only H
2
O
2
 
(see Fig
ure
 
8)
 
 
At pH5.5 where the adsorption and degradation of phenol is 
maximum, the H
2
O
2
 
formation is accelerated by phenol 
addition. At pH 3 and 11 where the degradation of phenol is 
relatively less, addition of phenol in between does not influence 
the oscillation as much as at pH 5.5. This reiterates the influence 
of phenol on the phenome
non of oscillation.  Addition of H
2
O
2
 
into the phenol
-
ZnO sonocatalytic system in between the 
reaction does not alter the course of oscillation significantly at 
pH 3,
 
5.5 or 11. This indicates that there is a critical range of 
H
2
O
2
 
concentration in which t
he oscillation is quite facile.  
Beyond this, any addition of H
2
O
2
 
will make the system over
-
saturated and it is difficult to distinguish the relatively smaller 
increase or decrease in the concentration of insitu formed H
2
O
2
.  
Addition of extra amounts of 
ZnO in between to an oscillating 
system containing phenol
 
enhances the maxima in the 
oscillation curve. This suggests that the formation of OH 
radicals is accelerated, which in turn can result in enhanced 
degradation of phenol and production of more H
2
O
2
. 
Thereafter 
the oscillation continues with a hi
gher maxima and minima 
(fig
ure
 
9).
 
 
 
 
Research Journal of 
Recent 
Sciences 
______
_
_
_______________________________
______________
_
________
 
ISSN 2277
-
2502
 
Vol. 
2
(
ISC
-
2012
), 
136
-
149
 
(201
3
)
 
 
Res.J.Recent.Sci
.
 
 
 
International Science Congress Association
 
          
 
144
 
 
 
 
Research Journal of 
Recent 
Sciences 
______
_
_
_______________________________
______________
_
________
 
ISSN 2277
-
2502
 
Vol. 
2
(
ISC
-
2012
), 
136
-
149
 
(201
3
)
 
 
Res.J.Recent.Sci
.
 
 
 
International Science Congress Association
 
          
 
145
 
 
Figure 10
 
 
Research Journal of 
Recent 
Sciences 
______
_
_
_______________________________
______________
_
________
 
ISSN 2277
-
2502
 
Vol. 
2
(
ISC
-
2012
), 
136
-
149
 
(201
3
)
 
 
Res.J.Recent.Sci
.
 
 
 
International Science Congress Association
 
          
 
146
 
It has been reported that
 
trapped electrons and holes on the 
surface of TiO
2
 
have an unusually long life time extending to 
hours after the irradiation source is put off
 
28
. Such long lived 
electrons and holes can react with H
2
O
2
 
resulting in minor 
change in its concentration even 
after the US is off. However, 
the results are not exactly reproducible and the memory effect 
does not seem to be significant enough to merit any detailed 
investigation at this stage. Deaeration by bubbling the system 
with nitrogen does not affect the oscil
lation significantly (see 
figure 11). 
 
 
Sono and photocatalytic reactions are known to require the 
presence of efficient electron acceptors so that the 
recombination of the electrons and holes at the surface can be 
prevented. Fast recombination between ele
ctrons and holes 
inhibits the interfacial charge transfer and the reactions that 
follow. Dioxygen molecules are efficient electron acceptors and 
hence sono or photocatalytic reactions do not occur in the 
absence of oxygen
29
.  The observation that the oscil
lation in the 
concentration of H
2
O
2
 
is not affected by deaeration shows that 
in the absence of O
2
, H
2
O
2
 
will serve as an electron acceptor as 
follows:
 
H
2
O
2
 
(ad)
 
+ e
-
 
  
→ OH
(ad)
 
+ HO
-
(ad)     
 
 
 
         
(6)
 
 
The minimum in the oscillation curve is lower in
 
the case of 
deaerated system which indicates that the H
2
O
2 
decomposition is 
proceeding even in the absence of oxygen. Hence the 
recombination of electrons and holes is not taking place. That 
means H
2
O
2
 
behaves uniquely by playing the role of electron 
and 
hole scavenger in the same system as in reactions 6 and 
7.H
2
O
2 (ad)
 
+ .OH/h+
 
  
→ 
 
HO
2
(
ad)  
+
 
H
2
O/H
+ 
  
 
 
 
         
(7)
 
 
Such behavior by H
2
O
2
 
has been proven in the case of 
photocatalysis by using Cavity Ring Down Spectroscopy 
(CRDS)
30
. 
 
 
The electron or hole transfer to H
2
O
2
 
generates HO
2
.
 
or 
.
OH as 
above which may further  react o
n the surface or get desorbed  
into the bulk. Such desorption of OH radicals from the TiO
2
 
surface has been confirmed by single molecule imaging using 
Fluorescence Microscopy
31
 
as well as Laser Induced 
Fluorescence Spectroscopy
32
. The desorption of HO
2
. 
(f
ormed 
by the decomposition of H
2
O
2
) from the surface was confirmed 
by CRDS
33
. Once the concentration of H
2
O
2
 
has fallen below a 
critical point, O
2
 
can compete effectively as an electron acceptor 
and the cycle of oscillation continues.
 
 
The behavior of phen
ol is different at different pH.  This can 
influence the fate of H
2
O
2 
also at respective pH. In weakly 
acidic solution, most of the phenol molecules remain un
-
dissociated. Hence maximum number of phenol molecules can 
be adsorbed onto the surface resulting 
in increased degradation 
and correspondingly more H
2
O
2
. Hence the amount of H
2
O
2
 
at 
the maximum of the curve is more. In the alkaline medium, the 
surface of TiO
2
 
is negatively charged and the phenolate 
intermediate may be repelled away from the surface
34
. 
Further, 
sonication effects also favor higher degradation in the acidic pH. 
When the pH exceeds 10 (pK
a
 
value of phenol at 25
0
C), ionic 
species of phenol will be predominant. When the pH is less than 
the pK
a
, molecular species will dominate. Under sonicati
on, the 
phenolate ions are concentrated in the gas
-
water interface of the 
bubbles where the hydrophobicity is strong and cannot vaporize 
into the cavitation bubbles
35
. They can react only outside of the 
bubble film with the OH radicals cleaved from water. 
However 
in the molecular state, phenol enters the gas
-
water interface of 
bubbles and even vaporizes into cavitation bubbles. They can 
react both inside by thermal cleavage and outside with OH 
radicals. This results in higher degradation of phenol as well a
s 
enhanced formation and decomposition of H
2
O
2
. Hence the 
concentration of H
2
O
2
 
is more at the maxima and minima under 
acidic conditions. 
 
 
The pH of the reaction medium has significant effect on the 
surface properties of semiconductor oxide particles, inc
luding 
the surface charge, size of the aggregation and the band edge 
position
23
. 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 leadi
ng to 
the degradation for reasons explained earlier. 
 
 
The complex behavior of H
2
O
2
 
is further aggravated by the 
different mechanisms of sonodegradation of phenol at different 
pH. Sonochemical degradation of phenol proceeds through 
catechol (CC), hydroquin
one (HQ) and p
-
benzoquinone (BQ) 
intermediates at pH 3, CC and HQ at pH 5.7and no detectable 
intermediate at pH 12. Further the sonication of HQ produces 
BQ while the sonication of BQ produces HQ and in both cases 
hydroxyl
-
p
-
benzoquinone is formed in trace
s as another 
intermediate
36
. In this complex system consisting of too many 
intermediates, the exact influence of pH on the oscillatory 
behavior cannot be deciphered precisely. 
 
 
Our results show that H
2
O
2
 
decomposition is negligible in the 
dark or by photo
lysis or sonolysis in the absence of a catalyst. 
This is in contrast with the findings of Augugliaro etal
37
 
as well 
as Jenny and Pichat
38
.  Jenny and Pichat reported that the 
heterogeneous H
2
O
2 
decomposition is only twice faster than 
homogeneous decomposit
ion while Augugliaro etal did not find 
any significant difference in the decomposition with or without 
suspended catalyst. However Ilisz etal
39
 
observed that efficient 
degradation of H
2
O
2
 
takes place only in presence of catalysts in 
photochemical systems. 
Their observation that the initial rate of 
decomposition of H
2
O
2
 
decreases with decreasing concentration  
is reconfirmed by the current findings that in the oscillation 
curve of H
2
O
2
 
the decomposition of H
2
O
2
 
begins only when a 
critical maximum concentrati
on is reached and the formation 
process takes over when a critical lower concentration is 
reached. 
 
 
The overall mechanism depicting the formation of ROS and 
degradation of phenol is schematically presented in scheme 1. 
Possible steps involved in the concu
rrent formation and 
decomposition of H
2
O
2
 
are given in reactions 8 to 19.
Research Journal of 
Recent 
Sciences 
______
_
_
_______________________________
______________
_
________
 
ISSN 2277
-
2502
 
Vol. 
2
(
ISC
-
2012
), 
136
-
149
 
(201
3
)
 
 
Res.J.Recent.Sci
.
 
 
 
International Science Congress Association
 
          
 
147
 
 
 
 
Scheme
-
1
 
Sono catalytic activation of semiconductor oxides 
and the formation of ROS
 
 
The formation of H
2
O
2 
from various ROS takes place as 
follows:
 
OH
 
+ 
.
OH → H
2
O
2  
              
 
 
 
         
(8)
 
O
2
 
+ 2e
cb
-
 
+ 2H
+
 
→ H
2
O
2 
              
 
 
 
         
(9
)
 
2H
2
O + 2h
vb
+
 
→ H
2
O
2
 
+ 2H
+ 
        
 
 
 
         
(10
)
 
O
2
-
.
 
+ H
+
 
→ HO
2
.                                          
 
 
 
         
(11)
 
HO
2
.
 
+ HO
2
.  
→ H
2
O
2 
               
      
 
 
 
         
(12
)
 
O
2
-
.
 
+ HO
2
.
 
→ O
2
 
+ HO
2
-
. 
              
 
 
 
         
(13)
 
HO
2
-
.
  
+ H
+
 
→ H
2
O
2 
                       
 
 
 
         
(14)
 
The concurrent decomposition of H
2
O
2
 
takes place as follows:
 
H
2
O
2
 
+ hν (λ<380 n) → 2 
.
OH   
 
 
 
         
(15
)
 
2 h
vb
+ + 
H
2
O
2
 
→   O
2
 
+ 2 H
+ 
       
 
 
 
         
(16)
 
2 e
cb
-
 
+ 2 H
+
 
+ H
2
O
2
 
→ 2 H
2
O      
 
 
 
         
(17
)
 
H
2
O
2
 
+ 
.
OH→ H
2
O +HO
2
. 
            
 
 
 
         
(18)
 
H
2
O
2
 
+ HO
2
.
 
→ H
2
O + 
.
OH + O
2 
 
 
 
 
         
(19)
 
 
Being a complex free radical system, other interactions leading
 
to the formation and decomposition of H
2
O
2
 
are also possible.
 
 
Conclusion
 
Hydrogen peroxide formed during the sonocatalytic degradation 
of   phenol in water in presence of semiconductor oxides such as 
ZnO and TiO
2
 
undergoes concurrent formation and 
decomp
osition resulting in oscillation in its concentration. 
Various reaction parameters such as catalyst loading, substrate 
concentration, particle size, pH, externally added H
2
O
2 
and 
presence of air/O
2
 
influence the phenomenon in general and the 
maxima and min
ima in the oscillation curve in particular. H
2
O
2
 
plays a unique role in the process as acceptor of both electrons 
and holes. The oscillation phenomenon continues for some more 
time even after the sono source is switched off, indicating the 
presence of memo
ry effect in the semiconductor.  A reaction 
mechanism based on the observations is proposed and 
discussed. 
 
 
Acknowledgement
 
The authors wish to acknowledge the financial support to JKP 
and SJ from the University Grants Commission, India by way of 
Junior R
esearch Fellowships.
 
 
References
 
1.
 
Ying
-
Shih M
.
, Chi
-
Fanga S
.
 
and  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.
 
Entewrzar
i M.H
.
, Masoud M
.
 
and Ali S.Y
.
, A combination 
of ultrasound and biocatalyst: Removal of 2
-
chlorophenol 
from aqueous solution, 
Ultrason. Sonochem.
,
 
13, 
37
-
 
41
(2006)
 
 
O
2
 
VB  
 
(h
+
)
 
  
CB
  
(
e
-
)
 
Band Gap     
       
(E
b
)
 
 
Re
-
combination 
 
of 
 
h
+
 
and e
-
 
 
Ultrasound
 
(US)
 
 
 
O
2
-
 
Reduction
 
Oxidation
 
 
P
+
 
 
 
 
P
 
P
+
 
H
2
O
 
H
+
 
+ OH
.
 
Degradation products
 
 
P
 
P
 
VB: Valence Band
 
 
CB: Conduction Band
 
P: Pollutant
 
Research Journal of 
Recent 
Sciences 
______
_
_
_______________________________
______________
_
________
 
ISSN 2277
-
2502
 
Vol. 
2
(
ISC
-
2012
), 
136
-
149
 
(201
3
)
 
 
Res.J.Recent.Sci
.
 
 
 
International Science Congress Association
 
          
 
148
 
3.
 
Devipriya S
.
, Yesodharan S, Photocatalytic degradation of 
pesticide pollutants in water, 
So
lar Energy Mater and 
Solar Cells,  
86
, 309
-
348 
(2005)
 
 
4.
 
Chong M.N
.
, Jin B
.
, Chow C.W.K
.
 
and Saint C
., 
New 
developments in photocatalytic water t
reatment 
technology: A review,
 
Water
 
Res.
,
 
44,
 
2997
-
3027 
(2010)
 
 
5.
 
Anju S.G
.
, Suguna Yesodharan and Yesodharan E.P.
 
Zinc 
oxide mediated sonophotocatalytic degradation of phenol 
in water, Chem. Eng. J. 
189
-
190
, 84
-
93 
(2012)
 
 
6.
 
Joseph C.G, Puma G.L, Bono A and Krishniah D, 
Sonophotocatalysis in advanced oxidation process:  A 
short review, 
Ultrason., Sonochem. 
16
, 583
-
589 
(
2009)
 
 
7.
 
 
Moon J,  Yun C.Y, Chung K.W, Kang M.S and  Yi J,  
Photocatalytic activation of TiO
2
 
under visible light using 
Acid red, 
Catal. Today
, 
87
, 77
-
86 
(2003)
 
 
8.
 
Pei D, and Luan J, Development of visible light
-
responsive 
sensitized photocatalyst,  
Int. J. of
 
Photoenergy
,   
2012
, 
article id 262831, 13 pages 
(2012)
 
 
9.
 
Wu C.G, Chao C.C
.
 
and Kuo F.T
.
, Enhancement of the 
photocatalytic performance of TiO
2
 
catalysts via transition 
metal modification, 
Catal. Today, 
 
97
,
 
103
-
112 
(2004)
 
 
10.
 
Pellegrin Y
.
, Le Pleux L
.
, Blart
 
E
.
, Renaud A
.
, Chavilion 
B
.
, Szuwarski N, Boujitita M, Cario L, Jobic S,      
Jacquemin D and Odobel F, Ruthenium polypyridine 
complexes as sensitisers in NiO based p
-
type dye
-
sensitized solar cells: Effects of the anchoring groups, 
J 
Photochem. Photobiol
. A
-
Chem.
, 
219
, 235
-
242 (
2011)
 
 
11.
 
Youngblood J, Lee S.H.A, Maeda K and Mallouk T.E
.
, 
Visible light water splitting using dye sensitized oxide 
semiconductors, 
Acc. Chem.  Res
.
,
 
42
, 1966
-
1973 
(2009)
 
 
12.
 
Lee D
.
K
.
, Kim S
.
C
.
, Cho I
.
C
.
, Kim S
.
J
.
 
and Kim S
.
W
.
, 
Photoca
talytic oxidation of microcystine LR in a fluidized 
bed reactor having TiO
2
 
coated activated carbon, 
Purification  Technology,
 
34,
 
59
-
66 
(2004)
 
 
 
13.
 
Li Y
.
, Sun S
.
, Ma M
.
, Ouyang Y
.
 
and  Yan W
.
,  Kinetic 
study and model of the photocatalytic degradation of 
Rho
damine B  by a TiO
2
-
coated activated carbon catalyst: 
Effects of initial RhB content, light intensity and TiO
2
 
content in the catalyst, 
Chemical  Eng.  J.
,
 
142
, 147
-
155 
(2008)
 
 
14.
 
Sakthivel S, Shankar M.V, Palanichamy M, Arabindoo A, 
Bahnemann D.M and Muruges
an B.V, Enhancement of 
photocatalytic activity by metal deposition: 
characterization and photonic efficiency of Pt, Au, and Pd 
deposited on TiO
2
 
catalyst, 
Wat. Res.
,
 
38
, 3001
-
3008 
(2004)
 
 
15.
 
Li F.B
.
 
and Li X.Z
.
, Enhancement of photodegradation 
efficiency usin
g Pt/TiO
2
 
catalyst, 
Chemosphere
,
 
48
  
1103
-
1111 
(2002)
 
 
16.
 
Kotronatou A
.
, Mills G
.
, Hoffmann M
.
R
.
, Ultrasonic 
irradiation of p
-
n
itrophenol in aqueous solution,
 
Phys. 
Chem.
,
 
95
, 3630
-
3638 
(1991)
 
 
17.
 
Hartmann J
.
, Bartels P
.
, Mau U
.
, Witter M
.
, Tumpling 
W.V
.
, Hofman
n J
.
, and Nietzschmann E
.
, Degradation of 
the drug diclofenac in water by sonolysis in presence of 
catalysts, 
Chemosphere
,
 
70
, 453
-
461
 
(2008)
 
 
18.
 
Gogate P.R
.
, Treatment of wastewater streams containing 
phenolic compounds using hybrid techniques based on 
cavit
ation: a review of the current status and the way 
forward, 
Ultrason. Sonochem.
, 
15
, 1
-
15 
(2008)
 
 
19.
 
Torres
-
Palma R.A
.
, Nieto J.I
.
, Combet E
.
, Petrier C
.
 
and  
Pulgarin C
.
, An innovative ultrasaound, Fe
2+
 
and TiO
2
 
photo assisted process for bisphenol a minerali
zation, 
Water Res. 
44
, 2245
-
2252 
(2010)
 
 
20.
 
Davydov L
.
, Reddy E.P
.
, France P
.
 
and  Smirniotis P
.
, 
Sonophotocatalytic destruction of organic contaminants in 
aqueous systems on TiO
2
 
powders, 
Appl. Catal.B: 
Environmental
 
32
, 95
-
105 
(2001)
 
 
21.
 
Chen Y.C
.
 
and Smirniot
is P
.
,  Enhancement of 
photocatalytic degradation of phenol and chlorophenols by 
ultrasound,  
Ind. Eng. Chem. Res.
,
 
41
,5958
-
 
5965 
(2002)
 
 
22.
 
Kritikos D.E
.
, Xekoukoulotakis N.P
.
, Psillakis E
.
, 
Mantzavinos D
.
, Photocatalytic degradation of reactive 
black 5 in a
queous solutionds: Effect of operating 
conditions and coupling with ultrasound irradiation, 
Water 
Res. 
41
, 2236
-
2246 
(2007)
 
 
23.
 
Anju S.
G
.
, Jyothi K
.
P
.
, Sindhu Joseph, Suguna 
Yesodharan and Yesodharan E
.
P
.
, Ultrasound assisted 
semiconductor mediated catalytic 
degradation of organic 
pollutants in water: Comarative efficacy of ZnO, TiO
2 
and 
ZnO
-
TiO
2
, 
Res. J. Recent  Sci.
 
1
, 191
-
201 
(2012)
 
 
24.
 
Nepiras E.A
.
, Acoustic cavitation: An introduction, 
Ultrasonics, 
22
, 25
-
40 
(1984)
 
 
25.
 
Suslick K.S
.
, Crum L.A
.
, in Crocker M.J (E
d.), 
Encyclopedia of Acoustics, 
1,
 
Wiley Inter5science, New 
York, 
271
-
282
 
(1997)
 
 
26.
 
Keck A
.
, Gilbert E
.
 
and Koster R
.
, Influence of particles 
on sonochemical reactions in aqueous solutions, 
Ultrasonics
 
40
, 661
-
665 
(2002)
 
 
27.
 
Pulgarin C
.
, Kiwi J
.
, Overview on ph
otocatalytic and 
electro catalytic pretreatment of industrial non
-
biodegradable pollutants and pesticides, 
Chimia, 
50
, 50
-
55 
(1996)  
 
 
28.
 
Szczepankiewicz S
.
, Moss J
.
A
.
 
and Hoffmann M.R
.
, Slow 
surface charge trapping on irradiated TiO
2
, 
J Phys Chem. 
B
.,
 
106
, 2
922
-
2927 
(2002)
 
 
Research Journal of 
Recent 
Sciences 
______
_
_
_______________________________
______________
_
________
 
ISSN 2277
-
2502
 
Vol. 
2
(
ISC
-
2012
), 
136
-
149
 
(201
3
)
 
 
Res.J.Recent.Sci
.
 
 
 
International Science Congress Association
 
          
 
149
 
29.
 
Gerischer H and Heller A, The role of oxygen in 
photooxidation of organic molecules on semiconductor 
particles, 
J Phys Chem
,
  
95(13),
 
5261
-
5267 
(1991)
 
 
30.
 
Yi J
.
, Bahrini C
.
, Schoemaecker C
.
, Fittschen C
.
, Choi W, 
Photocatalytic decomposition 
of H
2
O
2
 
on different TiO
2
 
surfaces along with the concurrent generation of HO
2 
radicals monitored using cavity ring down spectroscopy, 
J 
Phys Chem.
 
C
,
 
116
, 10090
-
10097 
(2012)
 
 
31.
 
Tachikawa T
.
 
and Majima T
.
, Single molecule 
fluorescence imaging of TiO
2
 
photoca
talytic reactions, 
Langmuir
, 
25
, 7791
-
7802 
(2009)
 
 
32.
 
Murakami Y
.
, Ohta L
.
, Hirakawa T
.
 
and Nosaka Y
.
, Direct 
detection of OH radicals in the gas phase diffused from the 
Pt/TiO
2
 
and WO
3
/TiO
2
 
photocatalysts, 
Chem.  Phys. Lett
., 
493
, 292
-
295 
(2010)
 
 
33.
 
Bahrini C
.
,
 
Parker A
.
, schoemaecker C
.
 
and Fittschen C
.
, 
Direct detection of HO
2
 
radicals in the vicinity of TiO
2
 
photocatalytic surfaces using CW
-
CRDS, 
Appl. Catal.
 
B, 
Environ.
,
 
99
, 413
-
419
 
(2010)
 
 
34.
 
Pardeshi S.K
.
 
and Patil A.B
.
, A simple route for 
photocatalytic degradation of phenol in aqueous zinc 
ioxide suspension using solar energy, 
Solar Energy
,
 
82
, 
700
-
705 
(2008)
 
 
35.
 
Wu C
.
, Liu X
.
, Wei D
.
, Fan J
.
 
and Wang L
.
, 
Photosonochemical degradation of phenol in water, 
Wat. 
Res
.
, 
35 
392
7
-
3933 
( 2001)
 
 
36.
 
Serpone N
.
, Terzian R
.
 
and Colarusso P
.
, Sonochemical 
oxidation of phenol and three of its intermediate products 
in aqueous media: Catechol, hydroquinone and 
benzoquinone. Kinetic and mechanistic aspects
,
 
Res Chem 
Intermed. 
18
,
 
183
-
202 
(199
2)
 
 
37.
 
Augugliaro V
.
, Davi E
.
, Palmisano L
.
, Schiavello M
.
 
and 
Sclafani A
.
, Influence of hydrogen peroxide on the kinetics 
of phenol photodegradation in aqueous titanium dioxide 
dispersions, 
Appl. Catal
.
,
 
65
, 101
-
109 
(1990)
 
 
38.
 
Jenny B
.
 
and Pichat P
.
, Determinat
ion of the actual 
photocatalytic rate of hydrogen peroxide decomposition 
over suspended titania. Fitting to the Langmuir
-
Hinshelwood form, 
Langmuir
, 
 
7,
 
947
-
949 
(1991)
 
 
39.
 
Ilisz I
.
, Foglein K
.
 
and Dombi A
.
, The photochemical 
behavior of H
2
O
2
 
in near UV
-
irradi
ated aqueous TiO
2
 
suspensions, 
J Mol Catal. A:Chem
,
 
135
,  55
-
61
 
(1998)
 
 
<end><end>