Kinetics of phenol oxidation under flash corona electric discharge

Article abstract of DOI:10.1023/A:1015525125122

A study was made of the kinetics of phenol oxidation in a 3.5-1 (liquid) tank at the initial chemical oxygen demand of the solution of 200-6000 mg O l^-1. The total current of flash corona electric discharge from 50 electrodes was 3 mA.

Full text of DOI:10.1023/A:1015525125122

Russian Journal of Applied Chemistry, Vol. 75, No. 1, 2002, pp. 86 89. Translated from Zhurnal Prikladnoi Khimii, Vol. 75, No. 1,
002, pp. 86 89.
Original Russian Text Copyright
2
2002 by Aristova, Piskarev.
ENVIRONMENTAL PROBLEMS
OF CHEMISTRY AND TECHNOLOGY
Kinetics of Phenol Oxidation under Flash Corona
Electric Discharge
N. A. Aristova and I. M. Piskarev
Nizhni Tagil Institute, Ural State Technical University, Nizhni Tagil, Sverdlovsk oblast, Russia
Research Institute of Nuclear Physics, Moscow State University, Moscow, Russia
Received March 6, 2001
Abstract A study was made of the kinetics of phenol oxidation in a 3.5-l (liquid) tank at the initial chemical
1
oxygen demand of the solution of 200 6000 mg O l . The total current of flash corona electric discharge
from 50 electrodes was 3 mA.
The technical and economic requirements to the
quality and cost of wastewater treatment are now so
stringent that neither of the known treatment methods
fully satisfies them. Solving this problem requires the
use of different methods. The place and role of each
method are assessed in the development stage by
mathematical simulation.
on a Teflon sheet arranged under the tank cover. The
electrodes were separated from one another by 5 cm.
The 10-kV negative-polarity voltage was applied to
each electrode via the untying resistance of 10 M .
The total discharge current from all the electrodes
was 3 mA. In the case of measurements in an oxygen
medium, before the voltage was switched on, the tank
was purged with oxygen at a rate of 3 l h 1 for 24 h.
During the experiments, oxygen was supplied at the
same rate. Oxygen was fed through the holes in the
tank bottom (through the liquid), and gases were re-
moved through the holes in the tank cover. During
experiments in air, the ventilation holes were addi-
tionally opened at the tank cover, and oxygen purge
was stopped. During treatment the liquid was mixed
by ion movement and by electric wind , which is
a specific feature of the electric discharge mode
chosen [1]. As working liquids served phenol and KI
solutions of different concentrations in distilled water.
The reagents were of chemically pure grade. The
Phenol-containing wastewaters are widespread;
therefore, it is of interest to develop new processes for
their oxidation. In [1], we considered specific reac-
tions induced by electric discharge, namely, by flash
corona electric discharge generating a small volume
concentration of active particles, extremely high elec-
tric field intensity in the discharge space, and pulse
electric current. Also, we formulated the main features
of the electric discharge utilized for initiating the
reactions [1], described the reaction kinetics [2], and
showed that under certain conditions this reaction
has advantages over ozonation [3]. In this work, we
analyzed the kinetics of phenol oxidation in a tank of
a sufficiently large volume and compared the results
with the characteristics of the known processes.
1
phenol concentration was 84 (COD 200 mg O l ,
1
1
8
6
40 (COD 2000 mg O l ), and 2520 mg l (COD
1
000 mg O l ). The phenol concentration in the ini-
tial and treated solutions was determined by extraction
followed by spectrophotometry.
EXPERIMENTAL
2
The active species yield Y was estimated by us
previously for the treated liquid volumes of 20
The liquid was treated in a 35 60 cm tank with
a polyethylene bottom and 25-cm-high glass walls.
The tank volume was 50 l. Along the bottom perim-
eter and in the central area longitudinally, a contact
electrode made of aluminum wire, 3 mm in diameter,
was mounted. The tank was filled with 3.5 l of liquid.
Fifty discharge electrodes were placed at a distance
of 5 mm from the liquid surface. Each discharge elec-
trode was 12 cm long. The electrodes were mounted
1
00 ml at the discharge current of 0.1 mA [2, 4].
.
These yields were as follows: Y(OH ) = 14.4, Y(H) =
1
8.0, and Y(O) = 2.5 mol mol 1 electrons (or mole-
cules per electron passed in the circuit). In [2] we
showed that in the presence of ozone the H atoms are
.
transformed into OH radicals, and their total yield
.
.
1
Y(OH ) is about 32 mol mol electrons, or 32 OH
1
070-4272/02/7501-0086$27.00 2002 MAIK Nauka/Interperiodica

KINETICS OF PHENOL OXIDATION
radicals per electron passed in the circuit. The voltage
on the electrode relative to the liquid surface is 10 kV,
which means that the energy spent for transfer of this
87
COD, mg O l 1
C, mg l 1
electron is equal to 10 keV. Then the average energy
.
cost of formation of one OH radical will be 312 eV,
1
or 0.32 (100 eV) . Additionally, the yield of ozone
under the actual experimental conditions was deter-
mined. To this end, the tank was filled with 3.5 l of
0
.1 M KI solution, and the iodine oxidation yield was
measured. The oxidation of I and the accumulation in
solution of 1/2I were calculated in terms of the proc-
2
ess model considered previously [2]. In the calculation
we used the above-cited yields of active species, the
liquid volume V = 3.5 l, the gas phase volume V =
l
g
3
2
4
6.5 l, the liquid surface area S = 2.1 10 cm , and
l
the discharge current I = 3 mA. The ozone yield was
treated as a free parameter. The calculated and ex-
perimental data proved to be consistent at the ozone
yield of 135 mol mol 1 electrons. This value is
somewhat lower than that obtained in [2], namely,
t, h
1
Fig. 1. Variation of the COD of the 840 mg l phenol
1
solution (COD = 2000 mg O l ) with time t of treat-
ment in the tank at the discharge current of 3 mA. Experi-
ment no.: (1) 1, (2) 2, and (3) 3; (4) variation of the phenol
0
80 mol mol 1 electrons for one electrode. A decrease
1
1
concentration C, mg l , in experiment no. 1.
in the ozone yield on changing from one to a system
of 50 electrodes is probably due to inaccurate arrange-
ment of the electrodes and to the scatter in the dis-
tance between the electrodes and the surface. Namely,
for a significant number of electrodes the distance
from the liquid surface is not optimal.
We studied the decrease in the chemical oxygen
demand (COD) for the phenol solution contained in
the tank under the following conditions: experiment
no. 1, under oxygen; during the discharge treatment,
alkali (NaOH) was added to maintain pH within 6 7;
experiment no. 2, under oxygen in the absence of
alkali; and experiment no. 3, in air without alkali. For
1
all the solutions the initial COD was 2000 mg O l .
t, h
The results are shown in Fig. 1. The concentration of
phenol itself was monitored in experiment no. 1.
The amount of the alkali utilized for maintaining pH
in experiment no. 1 was 14 mol of NaOH per mole
of electrons.
Fig. 2. Variation with treatment time t of the pH of the
phenol solutions with the initial COD of (1 3) 2000 and
4) 6000 mg O l . Medium: (1, 3) oxygen and (2, 4) air.
1
(
In the case of oxidation under oxygen without adding
alkali (Fig. 1, experiment no. 2), within the first 10
2 h the oxidation rate is identical to that in the case
of adding alkali (experiment no. 1), but then the oxi-
dation rate tends to decrease with decreasing pH. Fig-
Figure 1 shows that within about 60 h the COD
decreases by on order of magnitude, whereas the con-
tent of phenol itself decreases 1000-fold within about
1
5
h. Rapid consumption of phenol proper in the reac-
tion under study is confirmed by our previous experi- ure 2 shows how pH varies with the treatment time
ments [4]. Among intermediate reaction products, the
most easily identifiable is benzoquinone. Its presence
is confirmed by the fact that the initially colorless
solution turns reddish-yellow within several hours of
treatment, and this color is retained virtually until the
experiment is complete. Within 45 h of treatment the
solution turns colorless again.
in experiment nos. 1 3. It is seen that within 15 20 h
in experiment nos. 2 and 3 pH decreases from 5 to
3 4. The patterns of variation of pH with the time of
treatment under oxygen and in air (experiment nos. 2
and 3) are virtually identical. This suggests a little
significance of nitric acid formation under discharge
in air. Accumulation of nitric acid under the same
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 75 No. 1 2002

88
ARISTOVA, PISKAREV
1
Table 1. Experimental rates of oxygen uptake (COD decrease) for phenol solutions (COD 2000 mg O l , total discharge
0
current 3 mA, 50 electrodes)
Oxygen uptake rate
Experiment no.
oxidation beginning
oxidation end
mol O : mol electrons
mol O : mol electrons
(100 eV) 1
(100 eV) 1
1
2
3
210 20
190 20
80 10
2.1 0.2
1.9 0.2
0.8 0.1
20 5
12 3
13 3
0.2 0.05
0.12 0.03
0.13 0.03
Table 2. Comparison of the oxidation yields of phenol and organic substances, obtained in this work for discharge under
1
oxygen in the aqueous solution with COD 2000 mg O l , with those in [7, 12, 13] at the phenol concentration of 1 mM
and COD0 30 mg O l
0
1
Yield
This work
Pulse corona [12]
Ozonation [13]*
Catalytic ozonation*
Organic substances:
1
(
100 eV)
0.14
5
1
1
g kW
h
Phenol:
1
1
39
1
1
(100 eV)
1.5 (0.8)**
50 (27)
0.2
7.1
0.5
18
1
g kW
h
Ozone, (100 eV) 1
1.35 1.8
0.32
1.8
6
6
.
1
OH radicals (100 eV)
0.06 0.29
Up to 6
*
Under assumption that 1 kW h goes for production of 100 g of ozone
1
.
*
* Given in parentheses are the data for the solution with COD 200 mg O l
0
type of discharge over the distilled water surface is
about 5 mol mol of electrons [5], and the rate con-
stants of oxidation of atomic nitrogen and phenol are
of the same order of magnitude [6]. However, the
ent stages of phenol oxidation. At the beginning of
oxidation under discharge in oxygen COD decreases
at a rate of 190 210 mol O mol electrons, irrespec-
1
1
tive of adding the alkali.
concentration of the oxidizable impurities in water
at COD > 2000 mg O l 1 is much greater than that
In the case of discharge in air (experiment no. 3)
the oxidation rate is lower owing to a decrease in the
ozone yield with decreasing oxygen concentration. At
the end of all the experiments the oxidation rate de-
creases by an order of magnitude. The reason is two-
of atomic nitrogen forming under electric discharge.
Therefore, at a high concentration of the impurities
the active species are consumed, above all, for oxida-
tion of these impurities, and nitric acid virtually does
not form under discharge in air. Nitric acid can be
accumulated in noticeable amounts in later stages,
when slowly oxidizable products remain in solution
and their concentration is low.
fold: accumulation in the solution of products oxi-
.
dized at a lower rate by both ozone and OH radicals
and decrease in the concentration of the oxidizable
substances. At the end of the experiment, the yields
for discharges under oxygen and in air in the absence
Experiment no. 4 for the phenol solution with the
initial COD 6000 demonstrates how the pH is influ-
enced by the phenol oxidation products (Fig. 2). It is
seen that in this case the decrease in pH of solution is
more pronounced than in the case of COD 2000. This
is due to accumulation in solution of a large amount
of organic acids. Table 1 presents the experimental
rates of oxygen uptake (COD decrease) in the differ-
of alkali (experiment nos. 2 and 3) are approximately
the same, namely, 12 13 mol O mol 1 electrons. This
suggests that in this stage the major role belongs to
.
OH radicals whose yield is determined by the partial
pressure of the water vapor [4].
Let us compare the oxidation rates for organic im-
purities obtained in different works (Table 2). The
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 75 No. 1 2002

KINETICS OF PHENOL OXIDATION
yield of oxygen uptake (decrease in COD at COD
89
CONCLUSION
0
1
2
000 mg O l ) obtained in this work, i.e., approxi-
1
The yield of phenol oxidation under flash corona
electric discharge at the maximal electric field in-
tensity in the discharge space and small density of
generated active species is of the same order of mag-
nitude as in the case of catalytic ozonation and ex-
ceeds that achieved by conventional ozonation.
mately 2 (100 eV) , corresponds to the uptake of
11.9 g of oxygen per 1 kW h. The COD for phenol is
.38 g O g 1 phenol. Therefore, when recalculated for
2
phenol, the yield of oxidation of organic substances
1
1
1
will be equal to 5 g kW h , or 0.14 (100 eV) . In
many works, the CODs of phenol solutions during
treatment are not given; presented is the content of
phenol proper and of selected compounds, namely, its
oxidation products. As phenol proper is oxidized in
the reaction under study at an approximately 10-fold
higher rate than its degradation products, the yield of
oxidation of phenol proper at COD 2000 mg O l
will be equal to 50 g kW h , or 1.5 (100 eV) . At
lower phenol concentrations, COD 200 mg O l , the
initial yield of oxidation determined in an additional
REFERENCES
1
2
3
. Piskarev, I.M., Zh. Fiz. Khim., 1998, vol. 72, no. 11,
pp. 1976 1983.
. Piskarev, I.M., Zh. Fiz. Khim., 2000, vol. 74, no. 3,
pp. 546 551.
1
0
1
1
1
. Piskarev, I.M., Teor. Osn. Khim. Tekhnol., 2000,
1
0
vol. 34, no. 3, pp. 333 336.
4. Piskarev, I.M., Kinet. Katal., 1999, vol. 40, no. 4,
1
experiment was about 27 g phenol (kW h) , or 0.8
pp. 505 511.
1
(100 eV) . It should be emphasized that the initial
5
6
. Piskarev, I.M., Khim. Vys. Energ., 2000, vol. 34, no. 6,
pp. 475 476.
yield of oxidation is connected with consumption
of phenol proper.
. Pikaev, A.K. and Kabakchi, S.A., Reaktsionnaya spo-
sobnost’ pervichnykh produktov radioliza vody: Spra-
vochnik (Reactivity of the Primary Products of Water
Radiolysis: Reference Book), Moscow: Energoatomiz-
dat, 1982.
The phenol oxidation products were identified in
[7 11]. For oxidation of phenol to hydroquinone,
benzoquinone, and muconic acid the COD recalcu-
lated for one molecule decreases by 7.5, 17, and 28%,
respectively. Further oxidation of these compounds
is a slower process, and the total oxidation rate de-
creases. The data presented in other works for the
7. Al-Hayek, N., Legube, B., and Dore, M., Environ.
Technol. Lett., 1989, vol. 10, pp. 415 426.
8. Fajerwerg, K., Foussard, J.N., Perrard, A., and Debel-
lefontaine, H., Water Sci. Technol., 1997, vol. 35,
no. 4, pp. 103 110.
1
phenol concentration of 1 mM (COD0 230 mg O l )
are listed in Table 2. It is seen that among the works
cited, the highest yield of phenol oxidation was
achieved by catalytic ozonation [7]: 5 mol of ozone
per mol of phenol. It should be noted, however, that
the use of catalysts (e.g., Fe in that work) poses addi-
tional problems connected with contamination of the
water with the catalyst itself. The yield achieved in
electrode-free reaction is of the same order of magni-
tude as in catalytic ozonation, though the yield of
ozone in the process under study per unit energy
consumed is approximately 3 times lower than in
conventional ozonizers [2].
9. Singer, P.C. and Gurol, M.D., Water Res., 1983,
vol. 17, no. 9, pp. 1163 1171.
10. Gurol, M.D. and Singer, P.C., Water Res., 1983,
vol. 17, no. 9, pp. 1173 1183.
11. Hoigne, J. and Bader, H., Water Res., 1983, vol. 17,
no. 1, pp. 185 194.
12. Hoeben, W.F.L.M., Van Veldhuizen, E.M., Rut-
gers, W.R., and Kroesen, G.M.W., J. Phys. D.: Appl.
Phys., 1999, vol. 32, pp. L133 L137.
13. Gurol, M.D. and Vatistas, R., Water Res., 1987,
vol. 21, no. 8, pp. 895 900.
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