Behavior of By-products during Direct-photodegradation Treatment of Trichloroethylene. Effect of Oxygen Concentration on Production of By-products

Article abstract of DOI:10.1246/cl.2003.1046

During direct-photodegradation treatment of trichloroethylene by a low-pressure mercury lamp in a dry air atmosphere, slightly degradable by-products are produced; this production is the most serious issue associated with such treatment. We observed the acceleration of by-products degradation with decrease in an oxygen concentration in a treatment atmosphere.

Full text of DOI:10.1246/cl.2003.1046

1
046
Chemistry Letters Vol.32, No.11 (2003)
Behavior of By-products during Direct-photodegradation Treatment of Trichloroethylene.
Effect of Oxygen Concentration on Production of By-products
ꢀ
Shin Yamamoto, Takashi Amemiya, Masayuki Murabayashi, and Kiminori Itoh
Graduate School of Environment and Information Sciences, Yokohama National University, Hodogaya-ku, Yokohama 240-8501
(Received July 28, 2003; CL-030685)
During direct-photodegradation treatment of trichloroethyl-
Consequently, a possibility that the light intensity at 185 nm
is reduced wastefully is present and therefore the efficiency of
photodegradation treatment should be raised by the decrease in
an oxygen concentration to less than 21 vol %. The variation
in an oxygen concentration is, however, considered to influence
photodegradation of by-products chemically as well as physical-
ly; in aqueous phase, the efficiencies of dissolved oxygen on
ene by a low-pressure mercury lamp in a dry air atmosphere,
slightly degradable by-products are produced; this production
is the most serious issue associated with such treatment. We ob-
served the acceleration of by-products degradation with decrease
in an oxygen concentration in a treatment atmosphere.
9
VOCCs photodegradation have been reported. We carried out
Soil and groundwater have been polluted with volatile or-
ganic chlorinated compounds (VOCCs) such as trichloroethyl-
ene (TCE) and tetrachloroethylene discharged into the environ-
ment as an industrial waste. These pollutions have been
considered to be of grave environmental concern for several dec-
photodegradation of TCE under conditions with various oxygen
concentrations prepared by mixing oxygen and nitrogen as
1 atm, and investigated into the behavior of the by-products.
The initial concentration of TCE was approximately
ꢂ3
0.06 molꢁm
and it was almost completely degraded for
1
ades. Among many remedies against the pollutions, direct-pho-
todegradation by UV light has been suggested as a effective
< 0:5 min of the treatment at = 0.1 vol % oxygen concentration;
at < 0:1 vol %, however, a small amount of TCE still remained
at 0.5 min after the beginning of the treatment. The infrared ab-
sorption spectrum of TCE observed under an initial condition
and these of by-products at 20min after the beginnings of
TCE photodegradation treatments that are carried out at 100,
20, 1.0, and < 0:1 vol % oxygen concentrations are shown in
Figure 1. Under the conditions of = 0.1 vol % oxygen concentra-
tion, carbon dioxide (CO2), CO, HCl, and phosgene were ob-
served; these amounts of phosgene are < 10% of the maximum
amounts formed under each condition. Then, 100 ꢃ 6% as car-
bon balances were obtained under these conditions. However,
2
,3
method of treating VOCCs since it is well known that chlori-
nated ethylenes such as TCE are degraded very fast in gaseous
phase due to a homogeneous chain reaction with chlorine radi-
4
5
cal. When the photodegradation is used as a method of treating
VOCCs, a principal issue arises, namely, how efficiently slight-
ly-degradable by-products such as carbon monoxide (CO), di-
chloroacetyl chloride (DCAC), and phosgene can be degraded;
if a low-pressure mercury lamp (LML, wavelength: 254 and
185 nm), which emits vacuum UV light, is employed as a light
source for the photodegradation, the treatment of ozone formed
6
from oxygen is also required. In particular, the photodegrada-
7
tion of CO is extremely difficult and is therefore the most seri-
TCE
ous issue regarding the photodegradation of VOCCs. To the best
of our knowledge, however, hardly any studies on the photode-
gradation of these by-products have been reported from an envi-
ronmental perspective.
In this study, we chose to use TCE as an object material of
photodegradation treatment in gaseous phase, and we investigat-
ed the treatment of by-products formed during the TCE photode-
(
a)
CO
2
CO
O3
(b)
gradation. A circulation-type of photodegradation apparatus
(
(
8
made of Pyrex glass and Teflon) was employed and an LML
Sen Lights Corp., 10W, light intensity: 3.75 W at 254 nm and
(
c)
0.75 W at 185 nm) was used as a light source. A FT-IR spectrom-
eter (Perkin Elmer, Spectrum GX) was used to measure the con-
centrations of the reactant and products. The TCE photodegrada-
tion in our experiment was carried out in a dry air atmosphere.
The light intensity at 185 nm is reduced owing to the absorption
by oxygen in the air. While oxygen in a treatment atmosphere is
indispensable for oxidation of TCE as shown in the following Eq
HCl
(
d)
Phosgene
CCl
4
(
e)
1, during the photodegradation treatment carried out in the air at-
mosphere, the oxygen concentration (approximately 21 vol % =
2800
3200
2400
2000
1800
1600
1400
1200
1000
800
-1
Wave Number / cm
ꢂ3
9
.4 molꢁm ) is much higher than that required for the TCE con-
4
ꢂ3
centration at issue (mostly < 10 ppm = 0.45 molꢁm ).
C2HCl3 þ 2O2 ! 2CO2 þ HCl þ Cl2
Figure 1. Infrared absorption spectrum of reactants. (a) is that
of TCE under the initial condition, and (b), (c), (d), and (e) are
these of by-products generated at 100, 20, 1.0, and < 0:1 vol
ð1Þ
%
oxygen concentration, respectively.
Copyright Ó 2003 The Chemical Society of Japan

Chemistry Letters Vol.32, No.11 (2003)
1047
no amount of CO2 was generated but carbon tetrachloride was
observed at < 0:1 vol % oxygen concentration. Under this con-
dition, further, a carbon balance also reduced to < 50%; this may
be due to low oxygen concentration. The concentrations of CO
and CO2 generated in atmospheres containing various concen-
trations of oxygen (0.1 to 100 vol %) are shown in Figure 2.
The concentration of CO was decreased with decreasing the oxy-
gen concentration from 100 to 3.0 vol %, and a steep increase of
CO production was observed at < 1:0 vol % oxygen concentra-
tion; meanwhile, the concentration of CO2 shows an opposite be-
havior to that of CO toward the oxygen concentration. Conse-
4
0
O3
30
2
1
0
0
quently, the condition of 1.0to 3 v. 0o l
%
oxygen
concentration has been found to be most efficient for oxidation
0
0
20
40
60
80
100
of CO to CO2 during the TCE photodegradation. The minimum
ꢂ3
Concentration of O / %
2
concentration of CO was approximately 0.01 molꢁm observed
at 3.0vol % oxygen concentration; this CO concentration is 1/5
of that generated under the condition of air atmosphere. Since
light from the LML cannot be absorbed by CO, the decrease
of CO observed with decreasing the oxygen concentration would
be incomprehensible; this phenomenon is presumably, however,
because chlorine-atom generated in the treatment atmosphere,
Figure 4. Variation in the production of ozone with changing
oxygen concentration.
these times are for degrading 95% of the maximum amounts
formed under each condition. These results represent that the
highest degradation rate of DCAC and that of phosgene are ob-
tained at ca. 1 vol % and at 3.0to 5.0vol % oxygen concentra-
tions, respectively. The increases in the degradation rates of
these two by-products are considered to be due to not only the
increase in the light intensity at 185 nm but also the influence
on chemical kinetics because increases in the degradation-time
were observed at the region of low oxygen concentration. More-
over, the degradation-time of phosgene increased rapidly at
1
0
which can oxidize CO, increased with decreasing the oxygen
concentration. Figure 3 shows variations in a degradation-time
for 95% of DCAC and in that of phosgene during TCE photode-
gradation treatment with changing the oxygen concentration;
0
0
0
0
0
0
0
.12
.10
.08
.06
.04
.02
.00
<
3:0 vol % oxygen concentration, and the decrease in degrada-
CO2
CO
tion-time of phosgene observed with decreasing the oxygen con-
centration from 100 to 5.0 vol % was more sluggish than that of
DCAC. Such differences between the behavior of DCAC and
that of phosgene are considered to be due to photo-formation
of phosgene; the rate of phosgene photo-formation has been
reported to be higher, the lower an oxygen concentration is. Fi-
nally, the concentrations of ozone formed under the condition of
various oxygen concentrations are shown in Figure 4. A decrease
in the formed concentration of ozone was observed with decreas-
ing oxygen concentration.
11
0
20
40
60
80
100
In consequence of these results, the decrease of oxygen con-
centration to ca. 3 vol % is found to be effective in the by-prod-
ucts degradation during the TCE photodegradation treatment.
Concentration of O / %
2
Figure 2. Variations in the productions of CO and CO2 with
changing oxygen concentration.
References
1
2
J. M. Symons, T. A. Bellar, J. K. Carswell, J. DeMarco, K. L. Kropp, G.
G. Robeck, D. R. Seeger, C. J. Slocum, B. L. Smith, and A. A. Stevens,
J.—Am. Water Works Assoc., 67, 634 (1975).
2
2
1
1
5
0
5
0
M. Bhowmick and M. J. Semmens, J.—Am. Water Works Assoc., 86, 86
(
1994).
Phosgene
DCAC
3
4
W. R. Haag and M. D. Johnson, Environ. Sci. Technol., 30, 414 (1996).
R. G u¨ rtler, U. M o¨ ller, S. Sommer, H. M u¨ ller, and K. Kleinermanns,
Chemosphere, 29, 1671 (1994).
5
6
7
8
E. Sanhueza, I. C. Hisatsune, and J. Heicklen, Chem. Rev., 76, 80 1
1976).
(
H. Matsutani, M. Hashimoto, and M. Hashimoto, Mizu Kankyo
Gakkaishi, 21, 29 (1998).
S.-I. Yamaguti, T. Iwaizumi, Y. Mitsui, and S. Tsuda, Combust. Sci.
Technol., 133, 107 (1998).
S. Yamamoto, K. Itoh, and M. Murabayashi, Electrochemistry, 70, 425
5
0
0
20
40
60
80
100
(
2002).
Concentration of O / %
9
1
H. Shirayama, Y. Tohezo, and S. Taguchi, Water Res., 35, 1941 (2001).
0G. K. Rollefson and C. W. Montgomery, J. Am. Chem. Soc., 55, 4036
(1933).
2
Figure 3. Variations in the degradation-times for 95% of
DCAC and phosgene with changing oxygen concentration.
11 G. K. Rollefson, J. Am. Chem. Soc., 55, 148 (1933).
Published on the web (Advance View) October 20, 2003; DOI 10.1246/cl.2003.1046