st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Pulsed-plasma degradation of phenol in an aqueous solution
Kohki Satoh1, Masaya Murakami1 and Hidenori Itoh1
1
Muroran Institute of Technology, 27-1, Mizumoto, Muroran, 050-8585 JAPAN
Abstract: Byproducts from phenol are investigated by gas chromatography mass spectrometry when pulsed-discharge plasma is generated above a phenol aqueous solution, and then the
degradation process of phenol is deduced. In Ar atmosphere, catechol, hydroquinone and
4-hydroxy-2-cyclohexene-1-one are produced by active species such as OH, O, HO2, H2O2. In
O2 atmosphere, formic acid, maleic acid, succinic acid and 4,6-dihydroxy-2,4-hexadienoic
acid are produced by O3 in addition to catechol and hydroquinone.
Keywords: water purification, pulsed discharge plasma, phenol, degradation process
1. Introduction
Advanced oxidation using OH radicals and other strong
oxidants is attracting attention as a method of purifying
water polluted by tetrachloroethylene and other volatile
organochlorine compounds. We consider a pulsed
-discharge plasma generated above a water surface that
induces complex processes involving OH and other radicals, O3, and UV irradiation.
In this study, we decompose phenol by a pulsed discharge generated above the surface of a phenol aqueous
solution, and analyze the decomposition products, aiming
at clarification of the phenol decomposition processes.
We use Ar or O2 as a background gas (BG gas), and investigate the types and concentrations of phenol decomposition products in the presence of pulsed discharge for
various compositions of the BG gas by using a gas chromatograph mass spectrometer (GCMS). Based on the
results, we examine the phenol decomposition processes
and their relation to the species produced by the discharge.
2. Experimental setup, method and conditions
The experimental setup is shown in Fig. 1. The electrode system consists of a needle electrode and a stainless
steel specimen container. The needle electrode is a stainless steel nail with a diameter of 1.5 mm and a length of
19 mm. The grounded specimen container have an inner
diameter of 119 mm and a depth of 12 mm (maximum
capacity approx. 130 mL). The electrodes are placed inside a discharge chamber with an inner diameter of 140
mm and a height of 100 mm. Model polluted water is
phenol aqueous solution with 3000 ppm, and 70 g of the
model polluted water is then poured into the chamber. The
gap between the water surface and the needle tip is set to
4 mm. The BG gas is composed of O2 with a purity of
99.5% and Ar with a purity of 99.99%. The gas is fed to
the discharge chamber at a flow rate of 1 L/min.
A Blumlein HV pulse generator is used to produce the
pulsed voltage. Using a DC high voltage source (LS40-10,
Max-Electronics), a HF coaxial cable (Fujikura, 50 m,
100 pF/m) is charged at 14.14 kV, and pulses are generated at a repetition frequency of 20 pps by a rotary gap
switch. The pulses are applied to the needle electrode so
as to generate a pulsed discharge above the surface of the
phenol aqueous solution.
Discharge irradiation toward the surface of the phenol
aqueous solution is performed for 120 minutes. Every 15
minutes, 1 mL of the specimen is collected in a vial and
analyzed by GCMS (GCMS-QP2010, Shimadzu Corp.,
column: DB-17 ms) to examine the decomposition products in the aqueous solution. In order to analyze the
off-gas exhausted from the discharge chamber, a Fourier
transform spectrophotometer (FTIR-8900, Shimadzu
Corp.) fitted with a gas cell with an optical path length of
10 m (10-PA, Infrared Analysis) is used.
3. Results and discussion
Photographs of discharges in Ar and O2 atmospheres
are shown in Fig. 2. In the case of Ar, the discharge
branches when it reaches the surface of the phenol aqueous solution and takes a radial shape on the water surface
Fig. 1
Experimental apparatus.
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
as shown in Fig. 2(a). The shape changes over time, and
the discharge length halves after 60 minutes, as shown in
Fig. 2(b). In the case of O2, there are thick discharges
reaching the rim of the specimen container, as shown in
Fig. 2(c). After 15 minutes, there remain only thin discharges that do not reach the rim of the lower electrode,
as shown in Fig. 2(d).
Fig. 3 shows a chromatogram of the sampled phenol
aqueous solution after 120 minutes of irradiation in an Ar
atmosphere. In addition to the peaks for BG gas and phenol that appeared at retention times of 1.2 and 4.2 minutes,
there are also peaks with retention times of (1) 8.3
minutes, (2) 8.4 minutes and (3) 10.2 minutes. These
peaks are assumed to represent phenol decomposition
products.
Fig. 4 shows the mass spectra of peaks observed in the
chromatogram (Fig. 3). Comparison with reference spectra shows that the peaks respectively represent catechol,
4-hydroxy-2-cyclohexene-1-one and hydroquinone.
Fig. 5 illustrates the process of phenol decomposition
in Ar atmosphere as can be inferred from the decomposition products. The results of off-gas analysis do not indicate the presence of O3 in the case of the Ar atmosphere,
and one can conclude that phenol is decomposed by active species other than O3. Joshi et al. [1] report that OH
radicals, O radicals, HO2 radicals, H2O2 and other active
species derived from water molecules, as well as
(a) Ar (0 min)
(b) Ar (60 min)
(c) O2 (0 min)
(d) O2 (15 min)
Fig.2
Photographs of a pulsed discharge.
6
intensity [a.u.]
8x10
6
Ar
After discharge (120 min)
phenol
4
2
(1)
①
BG gas
0
0
2
Fig. 3
4
6
retention time [min]
③
(3)
(2)
②
8
10
12
Chromatogram of phenol aqueous solution after the plasma exposure in Ar atmosphere.
(2)
(1)
(3)
Fig. 4
Mass spectra at the peaks of (1) ~ (3).
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
high-energy electrons produced by discharge in water,
contribute to phenol decomposition. Hoeben et al. [2]
believe that OH radicals produced by a pulsed discharge
cause the generation of dihydroxycyclohexadienyl radicals (structures in which a hydroxyl group is added to one
of the C atoms making up the benzene ring of phenol);
after that, catechol and hydroquinone are generated via
some intermediate products. In this study, the electrical
discharge is generated above a water surface, and hence
one can assume that OH radicals, O radicals, HO2 radicals,
H2O2 and other active species derived from water molecules as well as high-energy electrons contribute to phenol decomposition. The 4-hydroxy-2-cyclohexene-1-one
detected in this study is a structure very similar to the
intermediate products assumed by Hoeben et al. [2].
Therefore, phenol is likely to be converted to 4-hydroxy
-2-cyclohexene-1-one, and then to catechol and hydroquinone. In addition, when O2 was included in the BG gas,
the generation of catechol and hydroquinone is confirmed
but 4-hydroxy-2-cyclohexene-1-one is not detected. Thus
we can assume a process of generation of catechol and
hydroquinone that does not involve 4-hydroxy-2
-cyclohexene-1-one as an intermediate stage.
Fig. 6 shows a chromatogram of the sampled phenol
aqueous solution after 120 minutes of irradiation in an O 2
atmosphere. Here the substances identified previously in
the Ar atmosphere are shown in the diagram. In addition
OH
OH
OH
OH
●
.
catechol
HO
phenol
OH
H
4-hydroxy-2cyclohexene-1-one
HO
hydroquinone
Fig. 5
Decomposition process of phenol when Ar
is used as a background gas.
to the peaks related to catechol and hydroquinone, there
are peaks with retention times of•(1) 1.3 minutes, (2) 3.6
minutes, (3) 7.3 minutes, and (4) 7.6 minutes, which presumable represent products of phenol decomposition in an
O2 atmosphere.
Fig. 7 shows mass spectra of peaks observed in the
chromatogram (Fig. 6). Comparison with the reference
spectra shows that the peaks respectively represented
formic acid, maleic anhydride, succinic anhydride, and
5-(2-hydroxyethylidene)-2(5H)-furanone. In the GCMS
employed in this study, liquid samples are vaporized in a
chamber heated up to 150 °C, then separated in a column
for mass spectrometry. Therefore, we may conclude that
6
intensity [a.u.]
8x10
6
O2
After discharge (120 min)
4
2
BG gas
0
0
(1)
①
phenol
(3)
③
(2)
②
2
Fig. 6
4
6
retention time [min]
catechol
(4)
④
hydroquinone
8
10
12
Chromatogram of phenol aqueous solution after the plasma exposure in O2 atmosphere.
Fig. 7
(1)
(2)
(3)
(4)
Mass spectra at the peaks of (1) ~ (4).
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
substances with two hydroxyl groups located close to
each other undergo dehydration due to intramolecular
reaction, that is, the two hydroxyl groups form an ether
bond, and one water molecule is separated. Hence maleic
acid, succinic acid, and 4,6-dihydroxy-2,4-hexadienoic
acid are likely to exist in the aqueous solution.
Fig. 8 illustrates the process of phenol decomposition
in an O2 atmosphere as can be inferred from the decomposition products. Hoeben et al. [2] believe that O3 and
phenol enter into a reaction of 1,3-dipolar addition so that
the benzene ring is cleaved, and unsaturated aliphatics are
produced. These are then further decomposed into formic
acid, oxalic acid, glyoxylic acid, and glyoxal. In our study,
O3 is detected in the off-gas at about 100 ppm when O2 is
used as the BG gas; thus we may infer the existence of
decomposition processes initiated from benzene ring
cleavage caused by O3. The 4,6-dihydroxy-2,4hexadienoic acid observed in this study is similar in
structure to unsaturated aliphatics, and therefore this acid
is likely to be a product of a 1,3-dipolar addition reaction
between phenol and O3. In addition O3 reacts selectively
with C=C double bonds; and therefore, 4,6-dihydroxy-2,
4-hexadienoic acid may be decomposed into maleic acid
or succinic acid with 4 carbon atoms, or oxalic acid with 2
carbon atoms. Considering that formic acid is detected in
this study while oxalic acid is not, and that formic acid is
reported to be generated from oxalic acid in accordance
with Eq. (1) [3], we may infer that oxalic acid is produced
by the same processes as formic acid.
HOOCCOOH → HCOOH + CO2
(1)
The generation of catechol and hydroquinone in an O2
atmosphere has been confirmed. Lukes et al. [4] report
that dihydroxycyclohexadienyl radicals are generated
from phenol due to OH radicals, after which catechol and
hydroquinone are generated due to O2 in the water. In our
study, too, catechol and hydroquinone may be generated
in that manner when O2 is used as BG gas.
at varied compositions of the BG gas, inferred the decomposition processes from the results, and examined the
effects of the BG gas composition on these processes. The
results of the study are summarized below.
(1) Phenol is decomposed in aqueous solution due to a
pulsed discharge above the water surface. Catechol,
4-hydroxy-2-cyclohexene-1-one and hydroquinone are
produced in an Ar atmosphere. In addition, in an O2 atmosphere, catechol, hydroquinone, CO2, formic acid, oxalic acid, maleic acid, succinic acid, and 4,6-dihydroxy-2,
4-hexadienoic acid are produced.
(2) When a pulsed discharge is applied above the surface of an phenol aqueous solution in an Ar atmosphere,
phenol is likely to be converted into 4-hydroxy-2
-cyclohexene-1-one, and then into catechol and hydroquinone. In addition one can assume a process of generation of catechol and hydroquinone that does not involve
4-hydroxy-2-cyclohexene-1-one as an intermediate stage.
(3) When a pulsed discharge is applied above the surface of a phenol aqueous solution in an O2 atmosphere,
the benzene rings of the phenol are cleaved by 1,3-dipolar
addition involving O3, thus producing 4,6-dihydroxy-2,4
-hexadienoic acid. Subsequently, oxalic acid, maleic acid
and succinic acid are likely to be produced by reaction
between C=C double bonds and O3. In addition we may
infer the decomposition of oxalic acid into formic acid
and CO2, and the generation of catechol and hydroquinone from phenol due to OH radicals and O2 molecules
present in the water.
5. References
[1] Joshi AA, Locke BR, Arce P, Finney WC, J.
Hazardous Materials, Vol. 41, pp. 3–30 (1995).
[2] Hoeben WFLM, van Veldhuizen EM, Rutgers WR,
Cramers CAMG, Kroesen GMW, Plasma Sources Sci.
Technol., Vol. 9, pp. 361–369 (2000).
[3] Lapidus G, Barton D, Yankwich PE, J. Phys. Chem.,
Vol. 67, No. 7, pp. 1863–1865 (1964).
[4] Lukes P, Clupek M, Sunka P, Peterka F, Sano T,
Negishi N, Matsuzawa S, Takeuchi K, Res. Chem.
Intermed., Vol. 31, pp. 285–294 (2005).
4. Conclusions
In this study we identified the decomposition products
of phenol after a pulsed discharge above a water surface
O
OH
HO
O3
OH
HO
hydroquinone
Fig. 8
C
O
H
oxalic acid
(predicted)
O3
O
maleic acid
OH
formic acid
O
HO
HO
OH
catechol
O
C
HO
4,6-dihydroxy
-2,4-hexadienoic acid
OH
CO2
C
O3
phenol
OH
O
or
HO
OH
O
succinic acid
Decomposition process of phenol in O2 atmosphere.