Degradation of oxalic acid in a photocatalytic ozonation system by means of Pilkington Active glass

Article abstract of DOI:10.1016/j.jphotochem.2010.11.016

This study aims to present the effect of combined ozone and immobilized TiO₂ in the form of a well-known commercial product, Pilkington Active, as a photocatalyst for the degradation of oxalic acid in aqueous solutions which is refractory to conventional ozonation treatments. Inclusive results on the influence of experimental variables such as temperature, pH value and initial concentration of oxalic acid on the rate of oxalic acid degradation and ozone consumption are demonstrated. Total organic carbon analyses were performed to investigate the mineralization of oxalic acid. Due to the enhanced photogeneration of hydroxyl radicals as a consequence of a synergistic interaction between the photo-activated TiO₂ and ozone, the O ₃/Pilkington Active/UVA system showed excellent potential for the oxidation and removal of oxalic acid contaminants in water.

Full text of DOI:10.1016/j.jphotochem.2010.11.016

Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 417–424
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Degradation of oxalic acid in a photocatalytic ozonation system by means of
Pilkington ActiveTM glass
∗
Mohammad Mehrjouei , Siegfried Müller, Detlev Möller
Institute of Soil, Water and Air, Brandenburg University of Technology (BTU Cottbus), Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 June 2010
Received in revised form
This study aims to present the effect of combined ozone and immobilized TiO in the form of a well-known
commercial product, Pilkington Active , as a photocatalyst for the degradation of oxalic acid in aqueous
2
TM
solutions which is refractory to conventional ozonation treatments. Inclusive results on the influence
of experimental variables such as temperature, pH value and initial concentration of oxalic acid on the
rate of oxalic acid degradation and ozone consumption are demonstrated. Total organic carbon analyses
were performed to investigate the mineralization of oxalic acid. Due to the enhanced photogeneration
of hydroxyl radicals as a consequence of a synergistic interaction between the photo-activated TiO2 and
2
3 November 2010
Accepted 25 November 2010
Available online 2 December 2010
Keywords:
Oxalic acid
Photocatalytic ozonation
Pilkington Active
TM
ozone, the O3/Pilkington Active /UVA system showed excellent potential for the oxidation and removal
of oxalic acid contaminants in water.
© 2010 Elsevier B.V. All rights reserved.
1
. Introduction
aqueous solutions to remove different groups of compounds have
been reported [10–12]. This photocatalyst generates electron–hole
0
Ozone is a very powerful oxidizing reagent (E = 2.08 V) which
pairs when irradiated with UV/vis light with energy in the range of
the TiO2 band gap, Ebg = 3–3.2 eV [13]. The active electron (−)–hole
(+) pairs handle the redox reactions with various species adsorbed
onto the semiconductor surface.
reacts with many groups of organic and inorganic compounds
directly and/or indirectly, mainly via the formation of the more
oxidant OH radicals (E = 2.8 V) [1]. Despite these positive points,
0
ozone, besides high production costs, has a relatively low solubil-
ity and stability in water and, furthermore, it reacts slowly with
some organic compounds, such as inactivated aromatics and sat-
urated carboxylic acids, and in many cases it does not completely
oxidize these organic compounds [2]. These disadvantages make
the application of ozone alone in polluted water treatment unfea-
sible from an economic point of view. Therefore, many studies have
focused on the performance enhancement of ozonation systems by
combining them with different categories of ions, metal oxides and
irradiations [3–5].
The immobilization of stable TiO2 films on support materi-
als instead of using such semiconductors in aqueous suspensions
solves two significant problems for researchers: first, the difficul-
ties in filtering and recycling the photocatalyst, and second, the
extinction of UV light due to scattering and absorption of the radi-
ation by the particles themselves. However, better mass transfer
properties and process simplicities are still the most important
advantages in slurry applications [14,15].
Oxalic acid is one of the toxic pollutants existing in alumina
processing liquors [16] and textile industrial wastewaters [17]; fur-
thermore, oxalate is a detectable intermediate in the mineralization
of many pesticides and other organic compounds [18–20] and, at
the same time, it is oxidized directly to CO2 without the forma-
tion of any stable intermediate products [21,22]. This compound
is recalcitrant to the direct attack of ozone alone, specially at an
acidic pH [23]. The four abovementioned properties were the rea-
sons why oxalic acid was chosen as the model compound in our
work.
Due to the low toxicity and costs, commercial availability, chem-
ical inertness and high photoconductivity of TiO , its use in the
2
anatase-rutile form as a photocatalyst in the oxidation and removal
of innumerable pollutant chemical compounds in water and air
has received an increasing amount of attention in recent years
[
6–9]. Furthermore, several scientific works in the field of TiO het-
2
erogeneous photocatalytic activities in combination with ozone in
Despite many investigations reporting on the degradation of
oxalic acid in aqueous solutions by various treatment systems,
there is a lack of studies investigating the degradation behaviour
of this compound when a well-known commercial immobilized
∗ Corresponding author at: Environment Science and Process Engineering, Bran-
denburg University of Technology, Volmerstrasse 13, D-12489 Berlin, Germany.
Tel.: +49 30 63925652; fax: +49 30 63925654.
TiO2 combined with O /UVA is used. Introduction of this com-
3
E-mail addresses: m mehr1356@yahoo.com, meh@btu-lc.fta-berlin.de
(
M. Mehrjouei).
mercialized product and assessment of its performance combined
1
010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.11.016

4
18
M. Mehrjouei et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 417–424
Table 1
Setup and application details.
with ozone and UVA light in degradation of special compounds
under different conditions is a new concept might open more doors
toward application of such products in the field of water treatment.
Therefore, the present paper aims to investigate the synergistic
Photocatalyst surface area
Thickness of solution over the photocatalyst
Ozonation chamber volume
Reactor volume
150 cm2
3 mm
500 ml
effects among ozone, immobilized TiO2 in the form of a commer-
160 cm3
1 l/min
400 ml
5 ml
10 l/h
TM
cialized product, Pilkington Active
glass and near UV light, in
Solution recycling rate
the degradation of oxalic acid by a heterogeneous photocatalytic
ozonation treatment.
Solution volume for each treatment
Sampling volume
Oxygen input flow rate
Input ozone concentration
145 ± 2 mg/l-O2
2
. Experimental details
2
.1. Reagents, materials and analyses
tor in the dark for 10 min by bubbling oxygen into the ozonation
chamber in order to achieve steady state conditions. The UV light
and the ozone generator were switched on 30 min before connect-
ing to the system for the same reason. More installation details are
available in Table 1.
The oxalic acid solutions were prepared from Merck (Germany)
analytical grade substances. All treatments were performed using
pH 2.1 ± 0.1, except for the attempts to show the influence of differ-
ent pH values on the degradation rate, when NaOH (Lachema, Czech
Republic) was used to increment the pH value (pH 3–9 ± 0.1) of the
solutions. The pH was measured using a pH-196 Microprocessor
pH-meter (WTW, Germany).
The initial oxalic acid concentration for all attempts was 10 mM;
while to investigate the concentration effect on degradation rate,
oxalic acid solutions of 1–50 mM were treated. Samplings and anal-
yses have been carried out every 20 min in 5 periods of time, so that
each treatment has lasted 100 min.
A UV lamp (Narva Lichtquellen GmbH & Co. KG, LT 30 W/009)
providing a range of wavelengths between 300 nm and 420 nm, and
a pronounced maximum wavelength of about 360 nm was used as
irradiation source. The light intensity of the UVA light source was
2
ca. 1 mW/cm .
The ozone generator (Fischer OZ 502/10, Germany) produced
ozone from pure, dry oxygen (Air Liquide, >99.5 vol.%, H O
2
<
200 ppmv) on demand. The oxygen/ozone mixture input rate was
adjusted to 10 l/h (1.45 ± 0.02 g O /h or 145 ± 2 mg O /l-O ).
3
3
2
◦
All treatments were performed at 25 C, except for those in
After each attempt, the reactor was washed with 2 l of deionized
water and prepared for the next treatment.
the temperature effects study. The solution temperature was fixed
◦
at the required temperatures, 10–70 ± 0.2 C, using a thermostatic
bath (LAUDA model B, Germany).
3. Results and discussion
The Pilkington ActiveTM glass sheets [24–26] were used as an
immobilized photocatalyst after cutting without any further prepa-
ration processes.
The deviation in the results (oxalic acid concentration and ozone
consumption level) after reproducing our study was lower than
%. Table 2 shows the abbreviations used in the illustration of the
results.
Before starting the functional assessment of the parameters (ini-
Oxalic acid concentration measurements were performed by
ionic chromatography using the Dionex DX 500 with conductiv-
ity detection connected to an Ion Pac AG4A (guard column) and
an AS14 anion exchange column with a 4 mm format (Dionex). The
samples were pumped through a sample loop at a volume of 100 ␮l.
5
tial concentration of pollutant, solution pH value and temperature),
a general experimental study was performed to investigate the
oxalic acid removal rate under our reactor conditions for six differ-
ent oxidation systems (Fig. 2). Fig. 3 shows the ozone consumption
levels in three of the six treatment systems utilizing ozone. The UVA
irradiation (photolysis) in the presence of oxygen had no influence
on the degradation of oxalic acid, while photocatalytic treatment
showed a negligible effect: less than a 5% decrease in oxalic acid
concentration after 100 min. Krysa et al. [8] proposed the mecha-
nism of photocatalytic degradation of oxalic acid as below:
The flow rate of the mobile phase, NaHCO3 (1.7 mM)/Na CO3
2
(
1.8 mM), was fixed at 1.2 ml/min. The evaluation of the mineral-
ization level of oxalic acid was performed by means of a TOC-5000
Shimadzu (total organic carbon) analyzer (Japan).
The ozone analyzer, Anseros Ozomat GM model RT1 (Germany),
measured ozone existing in the gas phase in the ozonation chamber
during the process. The ozone consumption level was calculated by
comparing the input ozone concentration after reaching the steady
state conditions with the output values during treatment. The
ozone concentration in the liquid phase was measured by Indigo
method [27]. After about 10 min, the equilibrium was achieved
between liquid phase and gaseous phase. A maximum ozone con-
centration of 14 ± 1 mg/l was measured in the liquid phase.
−
+
•
−
•
TiO (e + h ) + O + H O → O2 + OH
(1)
(2)
(3)
(4)
(5)
(6)
2
2
2
HOOC-COO + OH → HOOC-COO + OH−
And/or HOOC-COO− + h+ → HOOC-COO•
HOOC-COO → H+ + CO2 + CO2
−
•
•
•
•
−
2.2. Installation
•
−
•
CO2 + O → CO + O
O2 + H+ → HO2•
2
2
2
The setup details (a) and structure of the planar reactor (b)
•
−
used in this work are shown schematically in Fig. 1. A Pilking-
ton Active glass sheet, 30 cm × 5 cm × 0.5 cm, was embedded and
TM
fixed in a polymethylmethacrylate box so that UV light could reach
the semiconductor surface easily. After passing through the ozona-
tion chamber the oxalic acid solution was injected through the
inlet point at the bottom of reactor to make a thin layer over the
photocatalyst surface and it left the reactor through the top out-
let point. The solution was transferred to the ozonation chamber
to be recycled again. The recycling of oxalic acid solution was per-
formed using a Micropump 75211-15 gear pump. Before sampling
was started, the oxalic acid solution was recycled through the reac-
Table 2
Abbreviations used in this paper.
C
C0
Oxalic acid concentration
Initial oxalic acid concentration
Total organic carbon
Initial total organic carbon
Pilkington Active glass
Borosilicate glass
TOC
TOC0
PAG
BSG
UVA
Near UV-light

M. Mehrjouei et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 417–424
419
Fig. 1. (a) Setup details and (b) planar reactor’s structure.
•
2
HO2 → O + H O
(7)
(8)
to that of ozone alone, our system showed an effective degradation
of oxalic acid during the photo-ozonation treatment, indicating that
UVA irradiation played a more significant role in our setup condi-
tions. The effect of UVA could be attributed to the higher amount
of hydroxyl radicals produced during direct reaction of ozone with
hydrogen peroxide molecules (reaction (13)) [28].
2
2
2
H O + HO → •OH + H O + O2
•
2
2
2
2
Likewise, the ozonation and catalytic ozonation treatments
reduced the initial oxalic acid concentration by 9–12% after the
same time period, indicating that no remarkable heterogeneous
catalytic ozonation occurred on the semiconductor surface. Fur-
thermore, since Hoigné and Bader [23] reported that at acidic pH
values the direct reaction rate constant between the ozone and
oxalic acid molecules is very low, it is reasonable to conclude that
the small ozone effect observed here was because of hydroxyl rad-
icals generated by ozone decomposition [2] (reactions (9)–(11))
and the oxidation of oxalic acid molecules by these more powerful
oxidants (reaction (2)).
O + H O → OH + HO2• + O2
•
(13)
(14)
3
2
2
O + H O + hv → H O + O
3
2
2
2
2
Hydrogen peroxide molecules are generated as an intermediate in
ozone decomposition chain reactions (reactions (9)–(12)) and by
irradiation of ozone molecules (reaction (14)) [28]. Despite the neg-
ligible adsorption of radiations with ꢀ > 300 by hydrogen peroxide,
very slight generation of hydroxyl radicals is probably occurring
by adsorption of the short wavelengths existing in the spectrum of
UV-light source used in the present study.
The higher level of ozone consumption in photo-ozonation com-
pared to ozonation alone (Fig. 3) is in agreement with the two above
mentioned possibilities; however, the ozone photolysis seemed to
be more effective due to the generation of two hydroxyl radicals
by the decomposition of one ozone molecule, while three ozone
molecules are required to produce two hydroxyl radicals via ozone
attack on water molecules.
•
O + H O → 2HO + O2
(9)
(10)
(11)
(12)
3
2
•
•
•
O2 + H+
−
O + HO → O + HO
ꢀ
3
2
2
•
•
O + HO ꢀ 2O + HO
3
2
2
•
2
HO2 → O + H O
2
2
2
In spite of the fact that Addamo et al. [22] reported no notice-
able decrease in the concentration of oxalic acid by the presence of
ozone and UV light together in the reactor medium when compared

4
20
M. Mehrjouei et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 417–424
1
1
0
0
0
0
0
.2
.0
.8
.6
.4
.2
.0
0
20
40
60
80
100
Process time, min
BSG/O3/UVA
PAG/O3
BSG/O2/UVA
PAG/O3/UVA
BSG/O3
PAG/UVA
Fig. 2. Degradation of oxalic acid vs. time in
C0 = 10 mM, T = 25 C and pH = 2.1.
6 different oxidation systems,
◦
◦
C
Fig. 5. Degradation of oxalic acid vs. time at different initial concentrations, T = 25
and pH = 2.1 (inset graph: degradation rate vs. concentration).
4
3
3
2
2
1
1
0
5
0
5
0
5
0
5
0
Fig. 4 shows that the mineralization of oxalic acid by O /UVA
represents a similar trend to that found in the degradation of this
compound.
Nevertheless, the highest degradation rate of oxalic acid and
the highest level of ozone consumption occurred during photo-
catalytic ozonation, would indicate an enhancement of hydroxyl
radical generation on the photocatalyst surface in addition to that
3
brought by catalytic ozone decomposition in the TiO /ozone/UVA
2
oxidation system. Considering that ozone reacts directly with the
nucleophilic positions of organic compounds, and since oxalic acid
molecules do not have such positions, a direct reaction between
ozone and oxalic acid could be considered as negligible [29].
The influence of three significant practical variables (initial con-
centration of pollutant, solution pH value and temperature) on the
degradation rate and ozone consumption level over the treatment
time period was studied. It is worth noting that in such heteroge-
neous systems, where various simultaneous events happen, such
as electron–hole pair generation on the irradiated semiconductor
surface, their transfer and reaction with adsorbed molecules (ozone
and/or oxalic acid) or further recombination with each other, dif-
fusion of oxalic acid and ozone molecules into solution and their
adsorption on the photocatalyst surface, ozone decomposition by
irradiation, the direct attack of ozone on oxalic acid in the bulk of
the solution or on the photocatalyst surface, formation of hydroxyl
radicals as stronger oxidants for attacking the pollutants indirectly;
to present a comprehensive and precise description of the involved
mechanisms is a complicated task that is out of the scope of the
present work.
0
20
40
60
80
100
Process time, min
BSG/O3/UVA
BSG/O3
PAG/O3/UVA
Fig. 3. Ozone consumption level vs. time in
C0 = 10 mM, T = 25 C and pH = 2.1.
3
different oxidation systems,
◦
1
0
0
,0
,5
,0
3
.1. Effect of the initial concentration of oxalic acid
The decomposition rate of oxalic acid was strongly affected by
its initial concentration in our heterogeneous oxidation system. As
expected, Fig. 5 shows that complete oxalic acid degradation was
achievable in a shorter time for lower initial concentrations; never-
theless, a surprising result was observed where the biggest average
degradation rate over the first 20 min was found at the highest ini-
tial concentration (Table 3), indicating that the oxidation capacity
of our system was still open for the treatment of higher concen-
trations; however, the typical oxalic acid concentration in Bayer
0
20
40
60
80
100
Process time, min
−
5
−2
◦
liquor varies between 10 M and 10 M [16,31]. Moreover, after
reaching ozone saturation conditions (supposed to be after 20 min
Fig. 4. TOC/TOC0 level vs. time, T = 25 C and pH = 2.1.

M. Mehrjouei et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 417–424
421
Table 3
Initial degradation rate at different conditions.
◦
Initial concentration (mM)
pH
Temperature ( C)
Average degradation rate over the first 20 min (mM/min)
1
5
2.1
2.1
2.1
2.1
3
5
7
9
2.1
2.1
2.1
2.1
25
25
25
25
25
25
25
25
10
40
55
70
0.047
0.088
0.081
0.167
0.225
0.075
0.066
0.048
0.046
0.138
0.213
0.124
1
5
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
0
0
from the treatment start point) the degradation rate for concentra-
tions higher than 3 mM represented a linear trend vs. concentration
this compound by hydroxyl radicals are highly dependent on the
interactions between the existing forms of these molecules in solu-
tion and the surface groups on the photocatalyst. In the present
(
Fig. 5, inset graph). In addition, Fig. 4 shows the initial concentra-
−
tion effect on total mineralization of oxalic acid. It is evident that in
our setup conditions, solutions of 5 mM oxalic acid were mineral-
izable after 1 h treatment, as rapid as the degradation in the same
period of time.
Fig. 6 illustrates an evident decrease in the ozone consumption
after 40 min and 60 min for concentrations of 1 mM and 5 mM,
respectively, where no more oxalic acid was detectable accord-
ing to Fig. 5 for each one of these two conditions. At the same
time, a remarkable increase in the ozone consumption over the
time period was observed in the case of 50 mM which matches the
results mentioned above.
study conditions the oxalic acid was mainly comprised of C O H ,
2
4
2−
87–98% between pH 2.1 and pH 3, although C O4 was the main
2
form of oxalic acid in the solution at pH >5 (84%). Fig. 7 depicts the
influence of solution pH on the degradation of oxalic acid. As far as
the adsorption levels of oxalate and hydrogen oxalate at the photo-
catalyst surface are concerned, oxalic acid degradation trends were
definitely different for the two pH ranges mentioned above, pH >4
and pH <4. Total removal of oxalic acid was achievable at the end of
the process time at pH 2.1, and the degradation was almost 2 and
2.5 times higher at pH 3 compared to pH 5–7 and pH 9, respec-
tively. However, treatment at pH 3 showed unexpected results
by revealing a sharp decrease in the concentration during the
first 20 min and continued with a slight slope. Furthermore, ozone
consumption levels during the treatment at pH 3 determined an
anomalous developing at the same time, Fig. 8. This behaviour could
be attributed to the coupling of catalytic ozone decomposition and
the generation of hydroxyl radicals beside effective hydroxyl rad-
ical generation from the non-catalytic ozone decomposition at pH
3, which was limited at more acidic pH values [2]. The next slight
slope at these conditions (pH 3) agreed with our assumption that
non-catalytic hydroxyl radical generation is stopped by when more
acidic conditions are reached during the oxidation process. Her-
rmann et al. [30] also reported a similar maximum performance
in the oxidation of oxalic acid at pH 2.34 during a photocatalytic
process in an aqueous suspension.
3.2. Influence of solution pH
Despite being a carboxylic acid, oxalic acid is categorized among
some relatively powerful acids.
C O H ꢀ C O H + H+, pKa1 = 1.27
−
(15)
(16)
2
4
2
2
4
−
2−
+ H+, pKa2 = 4.28
C O H ꢀ C O
2
4
2
4
The pH value appoints the predominant existing chemical
species present in an aqueous solution of a relatively strong dicar-
boxylic acid. In our case, the oxalic acid could be present as a
neutral molecule, as hydrogen oxalate anion or as oxalate anion.
On the other hand, the adsorption rate of oxalic acid molecules
on the photocatalyst surface and the consequent degradation of
1
1
0
0
0
0
0
.2
.0
.8
.6
.4
.2
.0
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
20
40
60
80
100
0
20
40
60
80
100
Process time, min
Process time, min
50 mM
10 mM
5 mM
1 mM
pH= 2.1
pH= 3
pH= 5
pH= 7
pH= 9
Fig. 6. Ozone consumption level vs. time at different initial concentrations of oxalic
acid, T = 25 C and pH = 2.1.
Fig. 7. Degradation of oxalic acid vs. time at different pH values, C0 = 10 mM and
T = 25 C.
◦
◦

4
22
M. Mehrjouei et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 417–424
4
3
3
2
2
1
1
0
5
0
5
0
5
0
5
0
1.2
1
0
.0
.8
0.6
0
0
.4
.2
0.0
0
20
40
60
80
100
0
20
40
60
80
100
Process time, min
Process time, min
ph= 2.1
pH= 3
pH= 5
pH= 7
pH= 9
10 C
25 C
40 C
55 C
70 C
Fig. 8. Ozone consumption level vs. time at different pH values, C0 = 10 mM and
T = 25 C.
Fig. 10. Degradation of oxalic acid vs. time at different temperatures, C0 = 10 mM
and pH = 2.1.
◦
Fig. 9 illustrates the influence of pH on the mineralization of
oxalic acid. According to Figs. 7 and 8, the decreased oxalic acid
degradation rate and contemporary lower ozone consumption level
undlich adsorption isotherm, x/m = kP1/n where x, m, P are the
quantity adsorbed, the mass of the adsorbent and the pressure
(or concentration), respectively, and k and n are empirical con-
2−
at pH ≥ 5 is explained by the weaker adsorption of C O
, which
2
4
stants for each adsorbent–adsorbate pair at a given temperature,
as the temperature increases at a constant pressure (or concentra-
tion) the adsorption rate is negatively induced and the quantity
adsorbed raises more slowly, leading, as a result, to a decrease in
the degradation of adsorbed particles on the photocatalyst surface.
Furthermore, an increase in temperature decreases ozone solubility
in water, so that negligible ozone solubility in water under con-
is the main form of oxalic acid on the photocatalyst surface at this
pH range, accompanied by the decreased catalytic ozone decom-
position. However, it is proposed that the generation of hydroxyl
radicals from non-catalytic ozone decomposition is likely to be the
main pathway for promoting the oxidation process under these
conditions.
◦
ventional conditions is reported above 43 C. This effect causes a
3.3. Temperature effect
shortage in the ozone molecules available to handle the photo-
catalytic ozonation process, and this is possibly the main factor
hindering the efficiency of the process at higher temperatures. On
the other hand, any temperature increase should result in higher
rates for all chemical reactions involved in such heterogeneous
treatments. The unexpected increasing trend of the oxalic acid
According to Fig. 10, an increase in the temperature from
◦
◦
1
0 C to 55 C increased the degradation rate of oxalic acid, while
◦
even higher temperatures of up to 70 C had a negative effect
and reduced the degradation rate to that observed at 40 C,
◦
approximately. Temperature variations influence heterogeneous
oxidation systems in several ways which are usually consid-
ered as being opposite to each other and act either to develop
or to inhibit the treatment. In other words, the observed influ-
ence of temperature is a consequence of many co-current and/or
counter-current functions. On one hand, according to the Fre-
◦
◦
degradation rate between 40 C and 55 C could be explained by
the high ozone content in the input oxygen–ozone mixture gas (ca.
%10 .wt) and the accelerated rate of ozone decomposition produc-
ing hydroxyl radicals under these conditions.
It is worth noting that since the oxalic acid photocatalytic
oxidation with Ea = 45.8 kJ/mol and the consequent transfer of pho-
togenerated holes and electrons to the adsorbed particles on the
photocatalyst surface are categorized as low activation energy pro-
cesses [31], this factor does not seem to be vital in promoting oxalic
acid photocatalytic ozonation by shifting to higher temperatures.
1
0
0
0
0
0
,0
,8
,6
,4
,2
,0
pH=2.1
pH=3
pH=5
pH=7
◦
In Fig. 11 it is evident that increased temperatures of up to 55 C
enhanced the rate of oxalic acid decomposition and also increased
the ozone consumption in our heterogeneous system. As temper-
◦
atures were higher than 40 C, the oxalic acid content was almost
completely degraded after approximately 60 min (Fig. 10), lead-
ing to a significant reduction of ozone consumption during the last
4
0 min of oxidation.
Just like for the oxalic acid initial concentration and for the solu-
tion pH, Fig. 12 shows that the effect of solution temperature on the
mineralization rate of oxalic acid was similar to the effect observed
for the degradation rate of this compound, which emphasizes the
fact that oxalic acid degradation leads to direct mineralization with-
out the production of stable intermediates.
0
20
40
60
80
100
Considering the Arrhenius plot given in Fig. 13, the activation
energy related to the degradation of oxalic acid in our heteroge-
neous photocatalytic ozonation system was about 26.5 kJ/mol.
Process time, min
◦
Fig. 9. TOC/TOC0 level vs. time at different pH values, C0 = 10 mM and T = 25 C.

M. Mehrjouei et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 417–424
423
4
4
3
3
2
2
1
1
5
0
5
0
5
0
5
0
5
0
molecule pollution in water. The obtained results show that under
present experimental conditions, the lower the initial oxalic acid
concentrations, the shorter the degradation time periods in such
heterogeneous photocatalytic ozonation treatments. However, the
highest investigated initial concentration gave the best average
degradation rate over the first 20 min. Furthermore, increased tem-
◦
peratures of up to 55 C have raised the degradation rate and the
◦
ozone consumption level, whereas temperatures higher than 55 C
caused a negative influence. Acidic pH conditions increased the
treatment performance since pH 2.1 caused a total disappearance
of oxalic acid after 100 min and the most noticeable average degra-
dation rate over the first 20 min was found at pH 3. The TOC results
in our study agree with the fact that oxidation of oxalic acid to
CO2 and H O is directly attainable without any formation of stable
2
intermediate products.
0
20
40
60
80
100
Acknowledgments
Process time, min
10 C
25 C
40 C
55 C
70 C
This work was supported financially by the chair of Atmospheric
Chemistry and Air Pollution Control, Brandenburg University of
Technology (BTU Cottbus) and DBU (Deutsche Bundesstiftung
Umwelt).
Fig. 11. Ozone consumption level vs. time at different temperatures, C0 = 10 mM
and pH = 2.1.
pH=2.1
1,0
0,8
0,6
0,4
0,2
0,0
T=40C
T=55C
T=70C
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