Comparative kinetic analysis of silent and ultrasound-assisted catalytic wet peroxide oxidation of phenol

Article abstract of DOI:10.1016/j.ultsonch.2009.10.011

The kinetic study of silent and ultrasound-assisted catalytic wet peroxide oxidation of phenol in water was performed to qualitatively assess the effect of ultrasound on the process kinetics. Various kinetic parameters such as the apparent kinetic rate co

Full text of DOI:10.1016/j.ultsonch.2009.10.011

Ultrasonics Sonochemistry 17 (2010) 541–546
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultsonch
Comparative kinetic analysis of silent and ultrasound-assisted catalytic wet
peroxide oxidation of phenol
b
a
Ekaterina V. Rokhina a, , Eveliina Repo , Jurate Virkutyte
*
a Department of Environmental Science, University of Kuopio, FI-70211 KY, Finland
b Laboratory of Applied Environmental Chemistry, University of Kuopio, FI-50100 MI, Finland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 July 2009
Received in revised form 20 October 2009
Accepted 22 October 2009
Available online 27 October 2009
The kinetic study of silent and ultrasound-assisted catalytic wet peroxide oxidation of phenol in water
was performed to qualitatively assess the effect of ultrasound on the process kinetics. Various kinetic
parameters such as the apparent kinetic rate constants, the surface utilization coefficient and activation
energy of phenol oxidation over RuI3 catalyst were investigated. Comparative analysis revealed that the
use of ultrasound irradiation reduced the energy barrier of the reaction but had no impact on the reac-
tion pathway. The activation energy for the oxidation of phenol over RuI3 catalyst in the presence of
ultrasound was found to be 13 kJ molꢀ1, which was four times smaller in comparison to the silent oxi-
dation process (57 kJ molꢀ1). Finally, ‘figures-of-merit’ was utilized to assess different experimental
strategies such as sonolysis alone, H2O2-enhanced sonolysis and sono-catalytic oxidation of phenol in
order to estimate the electric energy consumption based on the kinetic rate constants of the oxidation
process.
Keywords:
Ultrasound
Phenol
Ruthenium iodide
Activation energy
Figure-of-merit
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
Ultrasound-assisted oxidation of phenol as a model com-
pound is successfully harnessed in the presence of various het-
Over the last decade, the considerable interest has been
expressed in the application of innovative methods to destroy phe-
nol based on the use of ultrasound coupled with traditional ad-
vanced oxidation processes (AOPs) [1]. The ability of sound
waves to catalyze the decomposition of refractory organic com-
pounds in water may have a major advantage over currently prac-
ticed advanced treatment technologies, where intensive chemical
and energy inputs are required for acceptable degrees of destruc-
tion [2]. The application of sonolysis alone is not capable to fully
degrade phenol because it is an extremely stable contaminant that
can only be destroyed by a vicious radical attack. Therefore, to
reach higher removal efficiency, the combination of the catalyst
and ultrasound irradiation can be applied [3] wherein ultrasound
increases the performance of the catalyst in view of the cavitation-
al cleaning if the catalyst surface. Ultrasound irradiation provides
the fragmentation of the catalyst particles with the subsequent in-
crease in the surface area, whereas the presence of solids eases the
cavitation process and hence intensifies the cavitational activity in
the reactor [4].
erogeneous catalysts such as zero-valent iron, TiO2, CuO, Fe-
SBA-15, clays, among others, and therefore attracts an increased
attention as an effective, inexpensive and reliable method to
decompose phenol in aqueous solutions [5–8]. Nikolopoulos
and co-workers studied mixed pillared clays (Al–Fe), FAZA, as
a solid catalyst and reported its beneficial properties in terms
of high oxidation activity and low leachability of the catalyst
[9].
Earlier, we found that ultrasound irradiation greatly enhanced
the efficiency of RuI3/H2O2 system to eliminate phenol due to the
generation of additional radical species and enhanced catalyst per-
formance [10]. However, the impact of ultrasound on the process
in the aforementioned system was studied only from the mecha-
nistic point of view via the detailed study of the catalyst, free rad-
icals formation and the assessment of the reaction scheme
combined with the evaluation of the influence of operational
parameters on the process efficiency.
Consequently, the scope of the current work is to delineate the
effect of low frequency ultrasound on the kinetics of chemical oxi-
dation and, in particular, to evaluate the difference in kinetic
parameters for the silent and ultrasound-assisted process. The ef-
fect of ultrasound on the reaction rates is described by the kinetic
analysis of the reduced complexity, restricted to the analysis of
phenol removal mechanism, initial rates of phenol oxidation and
* Corresponding author. Address: Department of Environmental Science, Univer-
sity of Kuopio, Yliopistonranta 1 E, D Snellmania, 4krs., 70211 Kuopio, Finland. Tel.:
+358 44 017 6196; fax: +358 15 355 6363.
E-mail address: katja.rokhina@uku.fi (E.V. Rokhina).
1350-4177/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ultsonch.2009.10.011

542
E.V. Rokhina et al. / Ultrasonics Sonochemistry 17 (2010) 541–546
its related parameters including the variation in the reaction en-
ergy barrier.
data points was within 5% based on the triplicate runs.
The reported uncertainty was based on the standard deviation.
2. Materials and methods
3. Results and discussion
2.1. Materials
3.1. Assessment of the reaction mechanism
Phenol, RuI3, H2O2 (30%), and other chemicals, from Sigma–Al-
drich and Merck and Co., Inc., were of laboratory reagent grade
and used without further purification. Doubly distilled water and
Three different reaction zones and therefore three reaction
pathways are generally accepted for the removal of organic com-
pounds in the presence of ultrasound irradiation in aqueous solu-
tions. The first is a gas cavity with the extreme conditions
(temperatures and pressures above 5000 K and 1000 atm, respec-
tively), the second zone is interfacial zone between the gas phase
and the bulk solution (temperatures around 2000 K and pressure
gradient exists) and the third zone is bulk solution [12]. Accord-
ingly, the main decomposition pathway of a volatile pollutant oc-
curs via pyrolysis within the cavitation bubble, whereas a non-
volatile pollutant is mostly subjected to the radical attack in the
bulk solution. Moreover, the combined process is reported to take
place in the interfacial zone [2]. In the current study, the direct
sonolysis of phenol (100 ppm) did not result in any significant phe-
nol removal. Moreover, the production of hydrogen peroxide dur-
ing the sonolysis was also negligible (0.04 mM). The addition of
hydrogen peroxide to the reaction mixture showed 25% increase
in the removal rates of phenol in the presence of the ultrasound,
whereas the combined application of the catalyst, hydrogen perox-
ide and ultrasound gave a 4-fold increase in the process efficiency
in comparison to the sonolysis alone. It is reported that phenol can-
not be destroyed by a thermal degradation alone due to its semi-
volatile nature [10]. Moreover, the combined pyrolysis and oxida-
tion by the free radical species was reported mainly for the 300–
500 kHz frequencies [13]. Thus, based on the reaction conditions
and a nature of the contaminant, phenol degradation was mainly
attributed to the radical attack.
The vigorous analysis of the reaction intermediates is necessary
to fully assess the oxidation mechanism of a target contaminant.
Evolution and identification of aromatic intermediates of phenol
has been widely reported and explained for various oxidation pro-
cesses [14–16]. Both, silent and ultrasound-assisted catalytic wet
peroxide oxidation of phenol followed the traditional reaction
pathway via the formation of catechol and hydroquinone, which
were detected after the electrophilic addition of hydroxyl radical
to the aromatic ring [2]. Consequently, as briefly mentioned above,
phenol degradation can be attributed to the reaction with ÅOH rad-
icals rather than to the combined effect of oxidation and pyrolysis.
The aforementioned primary oxidation products were subse-
quently oxidized to carboxylic acids, from which acetic, formic
and oxalic acids were identified as main products by the GC–MS
procedure (Fig. 1). Similar intermediates and terminal products
were detected for both, silent and ultrasound-assisted processes.
As a first step, hydroxylation of phenol to hydroquinone and cate-
chol occurred, then both dihydroxylbenzenes followed the same
oxidation route regardless the presence or the absence of ultra-
sound irradiation. However, the reaction induction period greatly
decreased in the presence of ultrasound and the first intermediates
were already detected after few minutes the reaction was initiated,
whereas intermediate products were detected only after 30 min of
the silent treatment.
0.45
lm hydrophilic Millipore filters were used throughout the
work.
2.2. Catalytic oxidation of phenol and analytic procedures
Irradiation of 100 mL aqueous phenol solutions (100 ppm) was
performed in a batch reactor equipped with a water-circulating
unit and a temperature controller. Ultrasonic transducer UP 400
H (Hielscher, Germany) with a titanium horn-type sonicator, capa-
ble of operating either continuously or in a pulse mode at a fixed
frequency of 24 kHz and a maximum electric power output of
400 W was used to pass ultrasonic waves through the solution.
The effective ultrasonic power was estimated by the calorimetric
method [10]. The calculated total power input, power density
and power intensity to the system were 20 W, 9.5 ꢁ 10ꢀ2 W cmꢀ3
,
and 14.3 W cmꢀ2, respectively. The fixed power has been used
throughout the experiments. Immersion circulator unit (Mo
1112A, VWR, UK) was used to maintain various experimental tem-
peratures throughout the study.
Previously optimized doses of reagents [10] were used, unless
otherwise specified: 1 g Lꢀ1 of the catalyst and 600 ppm of H2O2
(30%) at about neutral pH, atmospheric pressure and a tempera-
ture range from 298 to 343 K. Samples were taken at regular
intervals for the subsequent analysis and filtered through the syr-
inge filter to separate the catalyst particles from the solution. The
pH of the filtrate was then determined by a pH meter (3401
WTW, Germany). H2O2 was measured spectrophotometrically
using a dual-wavelength method by UV–vis spectrophotometer
[11].
The removal of phenol was monitored by UV–vis spectroscopy
at k = 268.4 nm (Perkin–Elmer UV–vis Spectrometer Lambda 45,
US) [10]. GC–MS analysis was performed to identify the reaction
products with an Agilent system comprising a model 6890 gas
chromatograph, a model 5975 mass selective detector, and an
Agilent ChemStation data system. Compounds were separated
on a 30 m ꢁ 0.25 mm i.d. fused-silica capillary column coated
with a 0.25
(without further dilution) in a split mode with a split ratio of
1:60. The sample was diluted 30-fold and 1 L of the dilute solu-
lm film of HP-5 ms. Samples (1 lL) were injected
l
tion was analyzed by GC–MS with the same split ratio. After
injection the oven temperature was maintained at 60 °C for
5 min then programmed at 5 °C minꢀ1 to 250 °C. The injector
and detector temperatures were 250 and 265 °C, respectively. He-
lium was used as carrier gas at a linear velocity of 30 cm sꢀ1. For
mass spectrometry the ionization energy was 70 eV and five
scans sꢀ1 were acquired in the mass range 40–540 amu. The com-
ponents were identified by the comparison of their mass spectra
with those in a computer library (2000 NIST database). The re-
sults obtained were confirmed by the comparison with data pub-
lished in the literature.
3.2. Kinetic study
The specific surface area was determined using BET method on
a Quantachrome Autosorb 1 analyzer (Quantachorome instru-
ments, UK) with liquid nitrogen at ꢀ196 °C.
Prior to any kinetic studies, the effect of heat and mass transfer
limitations during the oxidation process has to be established. Ear-
lier, we observed the strong dependence between the conversion
rate and the rate of agitation [10]. Heat transfer limitation was de-
Statistical treatment of data was performed by STATISTICA
software version 7.0 (USA). The reproducibility of the individual

E.V. Rokhina et al. / Ultrasonics Sonochemistry 17 (2010) 541–546
543
Fig. 1. Scheme of ultrasound-assisted catalytic wet peroxide oxidation of phenol.
apparent rate constant (minꢀ1). The slope of a linear plot of ln
(Co/C) versus time gives the apparent removal rate constant. The
reaction rate constants (kapp) and correlation coefficients (R) are gi-
ven in Table 1. It is evident from Table 1, that the reaction rate con-
stant for ultrasound-assisted process is much higher than for the
silent process as has been explained in our earlier comprehensive
study on the physical and chemical effects of cavitation [10].
Also, there is a correlation between the reaction half-life and the
reaction rate constant in the first and pseudo-first-order reactions
that can be expressed as following:
tected presumably due to any external resistance to the heat trans-
fer from the bulk liquid phase to the external surface of the cata-
lyst. An increase in the reaction rate of phenol oxidation with an
increase in the catalyst weight was also reported. Thus, the resis-
tance to the mass transfer must have been negligible between
the liquid bulk of the reactant and the outer surface of the catalyst
particles [17]. Moreover, the application of ultrasound decreased
both, the heat and mass transfer limitations due to the physical ef-
fects induced by cavitation [10]. It is also well known that the lim-
itations of the internal and external diffusion must be excluded
before measuring the kinetic data. The conversion of phenol is
hardly known to alter by the particle size of the catalyst when it
is less than 0.30–0.45 mm, thus, the effect of an internal diffusion
on the catalytic reaction could be ignored [18].
In 2 0:693
t1=2
¼
¼
ð2Þ
kapp
kapp
where kapp is the apparent reaction rate constant (minꢀ1).
Fig. 2 demonstrates a profile of phenol oxidation during silent
and ultrasound-assisted process conducted at the optimized pro-
cess conditions [10]. Both oxidation processes fit the pseudo-
first-order kinetics and thus, the integral equation may be applied:
As it is seen from Table 1, half-life for ultrasound-assisted sys-
tem is nearly 50% less than for the silent process, which is consis-
tent with the empirical data, obtained earlier [10].
ꢀ
ꢁ
3.3. Efficient surface area of the catalyst
Co
C
In
¼ kapp ꢁ t
ð1Þ
As heterogeneous catalysis is a surface phenomenon, it is well
documented that the overall kinetics of the oxidation process is
highly dependent on the surface area that participates in the reac-
tion [7]. In practice, only some part, so-called active sites of the cat-
alyst surface area is available for the reaction. However, it is quite
difficult to estimate the amount of active sites that are present in
the catalytic system due to the catalyst deactivation processes,
which lead to low catalytic efficiency. When ultrasound irradiation
is applied, the significant variation in the surface area occurs with
the possible increase in the number of active ‘clean’ sites and con-
sequently, the increase in the reaction rates may be observed.
The novel catalytic parameter (Ksurf), reported by Sakkas and
co-workers describes the participation of the surface area in the
process [7]. It expresses the removal efficiency per unit area of
the catalyst and estimates the area of the catalyst that can be effi-
ciently utilized for the process. Ksurf is derived by the partition of
the apparent kinetic rate, estimated for the studied process with
the value of the surface area of the catalyst:
where Co and C are the initial and final concentrations of phenol
(ppm) at time 0 and t (min), respectively and kapp is the observed
110
100
silent
90
US
80
70
60
50
40
30
20
10
0
Table 1
0
1
2
3
4
5
6
7
8
9
Relating kinetic parameters of the catalytic wet peroxide oxidation of phenol.
Time (h)
Process
k (minꢀ1
)
t1/2 (min)
R
Ksurf (x10ꢀ5 minꢀ1 cmꢀ2
)
Fig. 2. Oxidation profile of phenol (100 ppm) during silent and ultrasound-assisted
catalytic oxidation process (1 g Lꢀ1 of RuI3, 600 ppm of H2O2 (30%), pH = 6.98,
t = 298 1 K and atmospheric pressure in 0.1 L).
Dark
US-assisted
0.0021
0.0038
330
182
0.996
0.936
0.9296
1.936

544
E.V. Rokhina et al. / Ultrasonics Sonochemistry 17 (2010) 541–546
kapp
S
1.1
a
Ksurf
¼
ð3Þ
293K
313K
323K
333K
343K
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
where S is the surface area of the catalyst (m2 gꢀ1).
However, the calculation of Ksurf for ultrasound-assisted process
is more complicated due to the fact that the surface area continu-
ously changes during the process as a result of the fragmentation of
the catalyst particles and thus, producing a substantial increase in
the surface area. The surface area of the catalyst increased from
22.5 to 110.4 m2 gꢀ1 during the ultrasound-assisted process. Since
kapp and S are both functions of time we have to sequentially differ-
entiate and integrate Eq. (3) to obtain Ksurf for the ultrasound-as-
sisted process.
kapp
S
Ks ꢁ S ¼ kapp
Ks ¼
ð3aÞ
ð3bÞ
0
2
4
6
8
dS
Time (h)
Ks ꢁ
¼ kapp
ð3cÞ
ð3dÞ
dt
Ks ꢁ dS ¼ kapp ꢁ dt
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
b
293K
313K
323K
333K
343K
The domain of integration was determined as following: for t from 0
to t and for S from S1 (initial surface area of the catalyst) to S2
(resulting surface area of the catalyst). After the integration of Eq.
(3d), the calculated values were summarized in Table 1. According
to Table 1, the percentage of utilized catalyst surface area is much
higher for the ultrasound-assisted in comparison to the silent oxi-
dation process. Thus, it can be presumed that an increase in the ac-
tive sites takes place and the overall increase in the rate constant
results from the effectively utilized surface area of the catalyst.
3.4. Reaction temperature and activation energy
Four important parameters are affected by the temperature
during the sonolysis: the cavitation energy, the threshold limit of
cavitation, the quantity of dissolved gases and vapor pressure
[19]. Jiang et al. argued that at low frequency (20 kHz), due to
the large number of formed cavitation bubbles, it was expected
that an increase in temperature would lead to an increase in the
possibility of coalescence among the bubbles, resulting in some
of the bubbles losing their activity. Therefore, the contaminant re-
moval rate would decrease with an increase in the solution tem-
perature [19]. However, the presence of the catalyst solid
particles can drastically alter the situation. The presence of solid
particles provides additional nuclei for the cavitation phenomena
and hence the number of cavitation events occurring in the reactor
is enhanced, resulting in a subsequent enhancement in the cavita-
tional activity and the net chemical effects [4]. It is known that in
case of low frequency ultrasound irradiation, the majority of hy-
droxyl radicals are located very close to the sonicator horn. On
the contrary, catalysts particles available for the generation of
ꢂOH radicals via decomposition of hydrogen peroxide over the ac-
tive Ru sites are widely distributed in bulk solution [20]. Moreover,
it is generally accepted that oxidation of phenol is faster at high
temperatures due to higher activation of hydrogen peroxide during
catalytic wet peroxidation processes [21]. However, the optimal
temperature range is below 350 K in order to prevent the destruc-
tion of H2O2 to O2 and H2O [22]. Fig. 3a (ultrasound) and b (silent)
show the profiles of phenol degradation observed at various tem-
peratures. A stepwise improvement in the degradation rate is ob-
served with an increase in temperature from 293 to 343 K, which
is in accordance with the consecutive increase in kapp from
0.0021 to 0.0055 minꢀ1 for silent and 0.0038–0.0099 minꢀ1 for
ultrasound-assisted catalytic oxidation process. The increase in
the removal rate with an increase in temperature has also been re-
ported by Entezari et al. during the ultrasound-assisted catalytic
wet peroxide oxidation of 2-chlorophenol [23]. Catalytic reactions
0
2
4
6
8
Time (h)
Fig. 3. Oxidation profile of phenol at various temperatures during (a) silent and (b)
ultrasound-assisted catalytic oxidation process (1 g Lꢀ1 of RuI3, 600 ppm of H2O2
(30%), pH = 6.98 and atmospheric pressure in 0.1 L).
in the bulk are facilitated by increasing temperature due to the
higher mass transfer of different species at higher temperatures
leading to an enhancement of the reaction rates between radicals
and a target pollutant.
To assess the effect of ultrasound on the process kinetics, asso-
ciated energetic aspects such as activation energy play a crucial
role. Traditionally, activation energy may be obtained by plotting
data from temperature and kapp using Arrhenius equation, however
such plot may deviate from a straight line:
ꢀ
ꢁ
Ea
RT
k ¼ A ꢁ exp ꢀ
ð4Þ
Hence, the apparent activation energy may only be valid for a lim-
ited temperature range. The activation energies of phenol removal
over RuI3 in the presence and absence of ultrasound irradiation
are calculated for 298–343 K temperature range. The reaction rate
plotted against the 1/T is shown in Fig. 4. For this temperature
range, the quality of data is statistically significant to extract appar-
ent activation energies. To calculate activation energy (Ea), two
assumptions are considered for the experimental approach: (i) phe-
nol oxidation proceeded mainly in the bulk solution and (ii) cavita-
tion effects did not vary regardless of temperatures applied.
The activation energy values are 13 and 57 kJ molꢀ1 for ultra-
sound-assisted and silent catalytic oxidation systems, respectively
(a linear correlation between ln kapp and 1/T may be seen in Fig. 4).

E.V. Rokhina et al. / Ultrasonics Sonochemistry 17 (2010) 541–546
545
Thus, calculated EE/O values for different processes such as son-
-2.3
-2.4
-2.5
-2.6
-2.7
-2.8
-2.9
-3.0
-3.1
-3.2
-3.3
-3.4
-3.5
-3.6
-3.7
olysis alone, H2O2 sonolysis and sono-catalytic oxidation of phenol
silent
US
are found to be, 31265.5, 2297.8, and 501.8 kW h mꢀ3 orderꢀ1
,
respectively. It is obvious that the energetically beneficial process
is the process with the highest removal efficiency and the highest
kinetic constant in accordance with Eq. (5).
4. Conclusions
The comparative kinetic analysis of both, silent and ultrasound-
assisted catalytic wet peroxide oxidation of phenol was performed.
Ultrasound acted as the oxidation process accelerator but was un-
able to direct the current reaction to an alternative pathway. It
substantially decreased the induction period and consequently de-
creased the half-life of the oxidation reaction. Moreover, it also sig-
nificantly lowered the energy barrier required for the oxidation of
phenol in comparison to the silent catalytic process. The activation
energy for the ultrasound-assisted process was 13 kJ molꢀ1 in com-
parison to 57 kJ molꢀ1 obtained for the silent catalytic oxidation
process. All the kinetic parameters efficiently described the studied
process. ‘Figures-of-merit’, calculated for sonolysis, H2O2 sonolysis
and sono-catalytic oxidation of phenol were reaction rate depen-
dent and thus could be used to rationally develop and improve
the treatment method.
0.0029
0.0030
0.0031
0.0032
1/T
0.0033
0.0034
0.0035
Fig. 4. Arrhenius plot demonstrating temperature dependence on apparent rate
constants (kapp).
It is consistent with the average activation energies of
30–60 kJ molꢀ1 for Fenton-like processes reported in literature
[24,25]. Lower activation energy for ultrasound-assisted process
may be due to that the rate of the sonochemical reaction is much
larger in comparison to the silent processes. Both, the activation
energy and the reaction rate depend on the conversion and the
direction of the temperature change [26]. However, the use of
ultrasound changes the kinetics but not the thermodynamics of
the process. This is also valid for ultrasound-assisted processes
where ultrasound irradiation decreases the energy barrier required
for the oxidation of phenol but does not change the nature of the
reaction. It is clearly seen from Fig. 3b that heat has a cooperative
effect with ultrasound when irradiation is applied at high
temperatures.
Acknowledgements
Ekokem Oy Foundation and Academy of Finland (Decision No.
212649) are greatly acknowledged for their financial support of
the research.
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V ꢁ kapp
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ð5Þ
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where P is the power (kW), V is the reactor volume (m3) and kapp is
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