Sonolysis of an oxalic acid solution under xenon lamp irradiation

Article abstract of DOI:10.1016/j.ultsonch.2010.02.008

The photosonolysis of oxalic acid was carried out in an Ar atmosphere. The detectable products of sonolysis were CO₂, CO, H₂, and H₂O₂. The yield of CO₂ was higher than that for the sum of sonolysis and photolysis reactions. Namely, a synergistic effect was observed during simultaneous irradiations of 200 kHz ultrasound and Xe lamp. The degradation of oxalic acid was promoted by active species such as H₂O₂ produced from water by sonolysis. An oxalic acid-H₂O₂ complex is likely to be present in the solution, but could not be detected. The effects of not only the photo-irradiation but also the thermal or incident energy during Xe lamp illumination were also considered.

Full text of DOI:10.1016/j.ultsonch.2010.02.008

Ultrasonics Sonochemistry 17 (2010) 770–772
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultsonch
Short Communication
Sonolysis of an oxalic acid solution under xenon lamp irradiation
Hisashi Tanaka, Hisashi Harada *
Graduate School of Meisei University, College of Chemistry, 2-1-1, Hodokubo, Hino-shi, Tokyo 191-8506, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The photosonolysis of oxalic acid was carried out in an Ar atmosphere. The detectable products of son-
Received 10 September 2009
Received in revised form 9 February 2010
Accepted 10 February 2010
Available online 17 February 2010
olysis were CO
tolysis reactions. Namely, a synergistic effect was observed during simultaneous irradiations of 200 kHz
ultrasound and Xe lamp. The degradation of oxalic acid was promoted by active species such as H pro-
duced from water by sonolysis. An oxalic acid–H complex is likely to be present in the solution, but
2 2 2 2 2
, CO, H , and H O . The yield of CO was higher than that for the sum of sonolysis and pho-
2 2
O
2 2
O
could not be detected. The effects of not only the photo-irradiation but also the thermal or incident
energy during Xe lamp illumination were also considered.
Keywords:
Hydrogen peroxide
Oxalic acid
Ó 2010 Elsevier B.V. All rights reserved.
Sonolysis
Photosonolysis
Cavitation
1
. Introduction
It is known that sonolysis coupled with other chemical reaction
was passed through an aqueous 0.8 mol/L solution of oxalic acid
supplied by Wako Pure Chemicals (special grade) and used as re-
ceived. The reactor was then kept at 25 °C in a temperature-con-
trolled bath.
techniques often improve chemical reactivity. There have been
many reports about such systems in recent years. We also have re-
ported sonolysis in combination with photocatalysis [1] or cataly-
sis [2] from the standpoint of water splitting. Hydrogen peroxide,
The solution was sonicated using an ultrasonic generator (Kaijo
TA-4021-4611, 200 kHz, 200 W). For photolysis and photosonoly-
sis, a 500 W Xe lamp (Ushio, UXL 500 D-O, operation 20 A) were
used. Photolysis was usually performed with stirring by a mechan-
ical mixer. Photosonolysis was irradiated from one side with a Xe
lamp and from the bottom surface with an ultrasonic generator.
The amounts of gaseous products in the reactor were deter-
mined by using a gas chromatograph (Shimadzu GC8AIT equipped
with a Molecular Sieve 5A or a Porapak Q column). Hydrogen
peroxide in the solution was analyzed by a colorimetric method
(JASCO V-530, 407 nm) using a titanium sulfate solution (Nacalai
Tesque) [7].
2 2
O , and H were obtained as stable products when sonolysis of
water was performed. The hydrogen peroxide produced would be
expected to act as an oxidizing reagent in an aqueous solution. In
most cases, however, such reactions do not proceed without a
stimulus. Furthermore, Duhkkanci and Guhnduhz reported that
H
2
O
2
had a negative effect on the degradation of an oxalic acid
solution during sonolysis [6]. On the other hand, Selli insisted that
the H played a key role for degradation of a dye in an aqueous
solution treated by sonophotocatalysis [3]. We also have reported
2 2
O
2 2
the importance of the role of H O in the case of sonophotocataly-
sis of oxalic acid [4,5].
Oxalic acid is a reducing reagent and it is a typical water-solu-
ble, non-volatile, dicarboxylic acid. In this paper, we point out the
3
. Results and discussion
3.1. Sonolysis of an oxalic acid solution
importance of presence of H
2 2
O in a solution of oxalic acid under
photo-irradiation.
Fig. 1 shows the products obtained from the sonication of an
oxalic acid solution in an Ar atmosphere. Trials at similar experi-
mental conditions were conducted six times and the values aver-
aged. The maximum and minimum values obtained are indicated
by error bars. No other products other than those shown here could
be detected. Almost all of H and H O are produced from the son-
2 2 2
olysis of water (Eq. (1)). This reaction continues indefinitely during
sonication.
2
. Experimental
As a reactor, a Pyrex glass, short-neck, Kjeldahl flask (actual vol-
3
ume approximately 300 cm ) was used. Before irradiation, pure Ar
*
Corresponding author. Tel.: +81 42 591 7462; fax: +81 42 591 7419.
E-mail address: harada@chem.meisei-u.ac.jp (H. Harada).
1
350-4177/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ultsonch.2010.02.008

H. Tanaka, H. Harada / Ultrasonics Sonochemistry 17 (2010) 770–772
771
the sum of the rates of sonolysis and photolysis, thereby confirm-
ing the existence of a synergistic effect of combining the two types
of excitation. The product distribution under photo-irradiation was
also significantly different.
As also shown in Fig. 2, under photosonolysis, CO
2
became an
increasingly large fraction of the products. Photo-irradiation would
promote the production of CO
Eq. (5).
2
via the reaction as shown below as
ðCOOHÞ þ H
2
O
2
þ h
m
! 2CO
2
þ 2H
2
O
ð5Þ
2
hh : Photo-irradiationi
m
The smaller increase in CO production might be explained by
the sonochemical reduction of CO [10,11].
To prove the effect of photo-irradiation, photolysis of an oxalic
acid solution after 2 h of sonication was carried out. Under these
circumstances, hydrogen peroxide would have accumulated during
2
Fig. 1. Time dependences of sonochemical reaction products from an oxalic acid
solution under sonication (average of six runs). Maximum and minimum values are
shown as bars. Oxalic acid: 800 mM; ultrasound: 200 kHz, 200 W; atmosphere: Ar;
bath temperature: 25 °C.
the sonication. Thus, an oxalic acid–H
before photolysis. Fig. 3 shows the promotion effect of photolysis
on the reaction. Namely H was consumed and CO was pro-
duced by photo-irradiation. The amount of H present after son-
olysis, that is, before photolysis commenced, was 50.4 mol. All of
the H was consumed during photolysis. The amount of CO pro-
duced (96 mol) corresponded to the H loss in accordance with
Eq. (5). The amounts of carbon monoxide and H , on the other
hand, showed no increase during photolysis. It was clear that
2 2
O solution was prepared
2
H
2
O ! H
2
þ H
2
O
2
ð1Þ
2
O
2
2
2 2
O
Oxalic acid and oxalate ions do not enter cavitation bubbles be-
l
cause of their non-volatile nature. Thus, two kinds of reactions
need to be considered with regard to cavitation; those are pyrolysis
close to walls of the cavitation bubbles and oxidation by active spe-
cies produced from cavitation. Because similar amounts of CO
CO were evolved, the former process, as shown in Eq. (2), is prob-
ably the main reaction. Since the average amount of CO was
2
O
2
2
l
2 2
O
2
2
and
photo-irradiation was responsible for the increased rate of CO
production during photosonolysis.
2
2
slightly more than that of CO, oxidation by oxygen produced from
cavitation as shown in Eq. (3), might also have occurred.
Hirano et al. reported that pre-treatment by sonication was
effective for the mineralization of trichloroacetic acid because of
the initial formation of some intermediates sonochemically [12].
To confirm the effect of photo-irradiation on the oxidation of oxalic
ðCOOHÞ ! CO þ CO
2
2
þ H
þ 2H
þ 2H
2
O
ð2Þ
ð3Þ
ð4Þ
2
ðCOOHÞ þ O
2
! 2CO
! 2CO
2
2
O
2
acid in the presence of H
solution was also performed. Fig. 4 shows the evolution of CO
rors the consumption of H . Thus, the presence of oxalic acid and
promoted CO evolution during photo-irradiation.
It is possible that H in the solution changed into active spe-
2
O
2
, the photolysis of an oxalic acid–H
2 2
O
ðCOOHÞ þ H
2
O
2
2
2
O
2
2
mir-
The oxidation of oxalic acid by H
centration of H
In our case, however, only a trace amount of CO
little consumption of H were observed although both oxalic
acid and H were present in the solution.
2
O
2
takes place when the con-
is far in excess of that required by Eq. (4) [8,9].
increased and a
2 2
O
2
O
2
2
H O
2
2
2
2 2
O
2 2
O
cies such as hydroxyl radicals by photo-irradiation. Then, these
species might attack oxalic acid. Baxendale et al. reported on the
2 2
O
2 2
photolysis of H O at high intensities [13]. It is certain that the hy-
3
.2. Photosonolysis of oxalic acid–H
O
2 2
system
droxyl radical is generated by UV irradiation at 254 nm [14]. How-
ever, deep-UV light could not reach the solution in our
experimental setup because of the use of a Pyrex flask and a Xe
To obtain a higher reaction rate, we attempted to make the best
use of the remaining H in the solution by adding photolysis to
O
2
2 2
lamp. Thus, H O could not photo-excite directly in our system be-
2
the system. Fig. 2 shows all detectable products from an oxalic acid
solution during sonication with or without photo-irradiation. In
this combined ‘‘photosonolysis” system, higher yields of carbon
products were obtained compared with sonolysis on its own. Over-
all, the rate of reaction for the combined system was larger than
cause of the lower photon energy employed. The presence of both
oxalic acid and hydrogen peroxide is the key in the photosonolysis.
Fig. 2. Effect of photo-irradiation on the yield of products from an oxalic acid
solution under sonication. Oxalic acid: 800 mM; ultrasound: 200 kHz, 200 W; light
source: 500 W Xe lamp; atmosphere: Ar; bath temperature: 25 °C.
Fig. 3. Effect of illumination on the yield of products from an oxalic acid solution
after 2-h sonication. Oxalic acid: 800 mM; ultrasound: 200 kHz, 200 W; light
source: 500 W Xe lamp; atmosphere: Ar; bath temperature: 25 °C.

7
72
H. Tanaka, H. Harada / Ultrasonics Sonochemistry 17 (2010) 770–772
It is certain that thermal energy is concentrated along a narrow
path. It is possible that some local heating might occur, even in
the bulk solution, and heating promotes the reaction.
4
. Conclusions
Sonolysis of oxalic acid solution was carried out under photo-
irradiation in an Ar atmosphere. The rate of reaction of this com-
bined photosonolysis system was larger than the sum of rates of
sonolysis and photolysis. Thus, this increased rate is a synergetic
effect of combining these two excitation sources, rather than the
‘
‘effect” of photo-irradiation on sonolysis. The degradation of oxalic
acid was promoted by active species from H , which was pro-
duced by sonolysis of water. Although the literature suggests the
2 2
O
Fig. 4. Effect of addition of H
2 2 2
O on evolution of CO . Oxalic acid: 800 mM; light
source: 500 W Xe lamp; atmosphere: Ar; bath temperature: 25 °C.
presence of oxalic acid–H
detected.
2 2
O complexes, they could not be
The Xenon lamp used emits not only light but also heat. Ther-
mal or incident energy effects on the decomposition of H
also examined.
2 2
O were
2 2
Our results confirmed that the production of H O during soni-
cation and photolysis in solution are both key factors in the
improvement of the reactivity of this combined system.
Acknowledgement
The authors would like to thank Mr. Ikuo Aburai for his great
contribution to this work.
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Fig. 5. Thermal decomposition of oxalic acid–H
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