Organic Photochemistry Simulated Atmospheric Photodecomposition Rates of Methylene Chloride, 1,I,ITrichloroethane, Trichloroethylene, Tetrachloroethylene, and Other Compounds Wendell L. Dilling"?',Corwin J. Bredeweg2, and Nancy B. Tefertiller3 The Dow Chemical Co., Midland, Mich. 48640
T o estimate the decomposition rates of chloro compounds in the atmosphere, we carried out laboratory studies on the photolysis rates of the four title compounds, six other chlorohydrocarbons, and 23 other organic compounds under simulated atmospheric conditions. In the presence of nitric oxide, vinylidene chloride, cis- and trans-dichloroethylene, trichloroethylene, and vinyl chloride decomposed at moderate rates (estimated half-lives of 5-12 h under bright sunlight) when compared with a series of standard hydrocarbons. Chlorobenzene, tetrachloroethylene, and 1,1,2-trichloroethane decomposed slowly (estimated half-lives 20-40 h). Methylene chloride and l,l,ltrichloroethane did not decompose (half-lives >250 and >>1700 h respectively). The photodecomposition rates of several chloro compounds were determined in mixtures with other organic compounds and also in the presence of nitrogen dioxide, ozone, chloral, acetaldehyde, chlorine, and phosgene. The presence of hydrocarbons reduced the decomposition rates of several chloroethylenes. The fate and effect of chloro methanes, ethanes, and ethylenes which may be discharged to the environment (2-7) are of interest from an ecological standpoint. In a previous paper (8) we showed that the major portions of several lowmolecular-weight chlorohydrocarbons evaporated relatively rapidly from dilute aqueous solutions under simulated environmental conditions. We therefore undertook a study of the photodecomposition rates of several of these compounds, methylene chloride (CHzClz), l,l,l-trichloroethane (CH3CC13), trichloroethylene (CHCl=CC12), and tetrachloroethylene (CClp=CC12), which are important because of their current widespread use as solvents, and also of several other related chloro compounds under simulated atmospheric conditions. A previous report indicated that CH2C12, when irradiated under simulated atmospheric conditions, did not disappear a t a significant rate (9). No specific data have been reported on the photodecomposition rates of CH3CC13 under simulated atmospheric conditions. One report (10) indicated a half-life of several months for chlorine-substituted methanes and ethanes, while another report ( 1 1 ) stated that chlorinated saturated hydrocarbons were virtually unreactive. Considerably more work has been done on the behavior of CHCl=CClZ than on the other chlorinated compounds considered in this paper (11-17). In general, CHCl=CC12 decomposed at intermediate rates when compared with other common atmospheric contaminants; CC12=CC12 had a relatively low decomposition rate under simulated atmospheric conditions (11-13). Changes in the decomposition rate of one organic compound in the presence of another have been reported (14, 16, 18, 19). Relatively little work has been reported on the effect of other compounds on the photodecomposition rates
* Environmental Sciences Research. Styrene Molding Polymers Research and Development. Chemicals Processes Research. This is Part XI11 of the series. For Part XII, see Reference 1.
of CH2C12, CH3CC13, CHCl=CC12, and CC12=CClp (9, 14, 16). Experimental
Reactor and Analyses. The reactor (9.58 1.) was a water-jacketed Pyrex cylinder sealed with silicone rubbergasketed quartz windows a t each end. The reactor is shown in Figure 1 which will appear following this article in the microfilm edition of this journal. (See paragraph at end of paper about supplementary material.) The reactor temperature was maintained a t 27 f 1 "C. The ultraviolet light sources were two General Electric 275-W reflector sunlamps (20), each of which had a short wavelength cutoff of 290 nm. The ultraviolet light intensity was -2.6 times that of natural sunlight a t noon on a summer day in Freeport, Tex. The rates of disappearance of the organic compounds in the reactor were determined by flame ionization GC. Materials. Ultrahigh purity air (synthetic, 20%, 0 2 , 80% N2) from Matheson showed no impurities by GC. Nitric oxide (NO) and nitrogen dioxide (N02) from Matheson were used as received. Water, CH2C12 (uninhibited), CH3CC13 (uninhibited), and CC12=CC12 were redistilled. CHCl=CC12 (Hi-Tri),which contained 20 ppm of diisopropyl amine, was used without purification. Other materials were either redistilled or shown to be free of impurities by GC. The ozone (03)-air mixtures were prepared as follows. A stream of air was passed into a flask in which was suspended a screen electrode. The electrode was charged by a small Tesla coil. The exit gas which contained a mixture of O3 and air was collected in a 20-1. Saran plastic bag which then was used to fill the previously evacuated photochemical reactor. The 0 3 concentration was determined iodometrically (21). Example of a Typical Photolysis. After the reactor was evacuated to 0.2-0.5 mm for an hour or more, 47.9 pl of NO was added to the reactor through a silicone rubber septum to give a concentration of 5 ppm. While the reactor was still under vacuum, 94 p1 of distilled water was added. Evaporation of the water with a heat gun from outside the reactor gave a calculated relative humidity of 35-40%. Air was then added from a Saran bag to the reactor until atmospheric pressure was reached. Finally, 0.34 111 of CHCl=CCl* was added to the reactor from a syringe to give a concentration of 10 ppm. All concentrations are expressed as a molar or gas volume basis. Gas samples were analyzed periodically by GC. The first samples analyzed, before the lamps were turned on, were used as standards to which subsequent samples were compared during the reaction. All reactions were run at 35% relative humidity unless specified otherwise. Results and Discussion
Table I shows a summary of the results of the photodecomposition rates, based on the time required for 50% disappearance of the compound, of a variety of chloro compounds, some standard hydrocarbons, and other materials. All data are from reactions run under the same conditions. The compounds are listed in order of increasing decomposition rate in the presence of NO. Volume 10, Number 4 , April 1976
351
Table I. Photodecomposition Rates of Chloro Compounds, Standard Hydrocarbons, and Other Materials under Simulated Atmospheric Conditions Time (h) f o r 50% disappearance of compound Compounda
With NOb
With NO,c
CH,CCI, CH,CI, t-Butyl alcohol Epichlorohydrin 1,1,2-Trichloroethane 1-Butene oxide (c-CH EtC H, 0) Ethyl acetate (EtOAc) CCI,=CCI, Methyl ethyl ketone (MeCOEt) Nitromethane Chlorobenzene Cyclohexane (c-C,H,,) Toluene (PhMe) n-Butyl alcohol (n-BuOH) Ethyl benzene Trioxane Vinyl chloride (CH,=CHCI) s-Butyl alcohol
d
e
f
g
34.5h 16.0 15.9i 15.9
15.2k
CHCI=CCI, Methy I isobutyl ketone (MeC Oi-Bu ) Isobutyl alcohol (i-BuOH) Dioxane c - 0 (C, H,) ,Ol 1-Methoxy-2-propanol (MeOCH,CHOHMe) p-Xylenes (p-Me,C,H,) m-Xylenes (m-Me,C,H,) cis- Dich I oroethy lene (cis-CHCI=CHCI) tra ns-Di c h lo roet hy Iene Ethylene (CH,===CH,) Vinylidene chloride (CH,=CCI,) Cyclohexene (c-C,H,,) 2,4,4-Tri met hy I- 1pentene trans-2 -Bu t ene (trans-MeCH=CH N-Methylpyrrole
8.3n
9.2 8.7P 6.9 6.8 6.5
6.5, 7.5k
5.0 4.7 4.3
trans-MeCH=CHMe
CHCI=CCI,
Relative light intensity
Time for 50% disappearance Relative of compound reactivity
11 min 23 40 2.8 h 5.2 10.0 3.5 h 6.3 9.5
1.OOb 0.50 0.25 1.OOb 0.50 0.25 1.OOb 0.50 0.25
1.00 0.48 0.26 1.00 0.52 0.28 1.oo 0.56 0.37
a All reactions carried out with 10 ppm of the compound and 5-ppm NO. b Usual light intensity in this work.
Table Ill. Effect of Concentration on Photodecomposition Rates
2.9 Compound
3.5
trans-MeCH=CHMea
3.4
3.1 CHCI=CCI,
3.1 2.9 3.0
3.0
2.9 2.9 2.1
2.8 2.5
0.87 0.60
0.19k
0.30
0.17k
CCI,=CCI,
Initial concentrations, Time for 50% disPPm aPPearance .. Comof NO compound pound 10 20 40 100 10 20 40 100 10 40 100
5 10 20 50 5 10 20 50 5 20 50
Relative reactivity
26 min 22.5 17 13 3.5 h 3.0 1.5 1.0 11.2b h 7.0 3.8
1.o 1.2 1.5 2.0 1.o 1.2 2.3 3.5 1.o 1.6 3.0
a Reactions with trans-MeCH=CHMe carried out with 0.50 relative light intensity. b Extrapolated from 29% reaction in 6.5 h.
Me) 0.16
a All compounds at 10 ppm in air initially, unless specified otherwise. b All initial concentrations, 5 ppm. C All initial concentrations, 16.8 ppm, except as noted. d Reaction, ,
I
Em
'0
m
3
0
U C
4-
m
U
K 3
0
E 8
c 0
.5
U 0
R
Ol
C W
P
09
V W
S
a
:
0
U
0
W
Q Y
b
v)
9
0
2
Q Q
.-Ua
2
3
:
s
0
m
x
i
13 9
H
bc
h
s
Volume 10, Number 4, April 1976
353
Table V I . Effect of Organic Compound Ratio on Photodecomposition Rates of CHCI=CCI, Ratio [CHCI=CCI,]
Initial concentration, p p m CHCI=CCI,
c-C,H,,
10 10 10 10 10 10 2 0 10 10
21
Lc-'6
0 0.0 1 0.1 0.2 2 10 10 10 2 10
Initial N O concent rat ion, PPm
and c-C6H,, T i m e , h, for 50% disappearance CHCI=CCI,
5 5 5 5 5 5 5 5 10 10
c x
1000 100 50 5 1 0.2
0 5 1
12
-
3.5 3.3 2.7 3.5 5.6 > 16 >20
-0.9 1.3 2.2 5.5 6.7 6.9 2.8 4.9
6.6
8.7
Table V I I . Photodecomposition Rates of Organic Materials in t h e Presence of NO, NO,, and 0, T i m e h for 50% disappearance Compound0
C-CH Et CH, 0 c-C6H1
2
CHCI=CCI, CH,=CH,
Relative reactivity
N0b.c
NO,b,d
03e
NO
NO2
0 3
15.9 6.9 3.5 2.9 0.87 0.30
15.2 7.5 2.9f 2.5f 0.19 0.17
0.83
0.23 0.51 1.o 1.2 4.0 11.7
0.19 0.38 1.o 1.2 15.1 16.9
0.5 1 0.42 1.o
1.o
0.42
-
c-C,H,o trans-MeCH=CHMe 0 All compounds at 10 p p m initially. b Data from Table I . C All initial concentrations, 5 p p m . p p m , except a s noted. e Initial concentration, 15 p p m . f Initial NO, concentration, 16.8 p p m .
A comparison of the relative photodecomposition rates of the standard hydrocarbons ( c - C ~ H IPhMe, ~, CH2=CH2, and trans-MeCH=CHMe) in Table I with those obtained by other workers, who used different types and sizes of irradiation chambers, shows a satisfactory correlation (Table X in the microfilm edition of this journal). CH3CC13 and CH2C12 did not decompose at a significant rate with either NO or NO2 present. CC12=CC12 decomposed more slowly than c-C&12 while the decomposition rates of the remaining chloroethylenes, including CHCl=CC12, were comparable to that of CH2=CH2. In general, the relative decomposition rates of the materials in Table I roughly parallel the known or expected reactivities of these materials with electrophilic radicals. In several reactions, the light intensity was varied to determine its effect on the reaction rate since our reactions were carried out with a light intensity higher than that of sunlight. Table I1 shows a good linear correlation between decomposition rate and light intensity with the exception of CHCl=CC12 at the lowest light intensity. Estimated half-lives under bright sunlight conditions can be calculated by multiplying the half-lives given in Table I by 2.6. For three of the olefins studied, a change in the absolute concentrations of compound and NO from 1 0 5 to 100:50 (pprn) resulted in a 2 to 3.5-fold increase in decomposition rate (Table 111). The maximum decomposition rate of CHCl=CC12 occurred at a -4:l ratio of CHCl=CC12 to NO at 35% relative humidity (Table IV). There was about a 50% decrease in the decomposition rate of CHCl=CC12 on increasing the relative humidity from 0 to 70% (Table IV). Photodecomposition rates were determined for a number of the compounds listed in Table I, each in combination with other organic materials that had a wide range of decomposition rates. The data are shown in Table V. 354
Environmental Science 8 Technology
d All
-
initial concentrations, 5
CHCl=CC12 was studied more extensively than the other compounds. In all combinations, except those which involved CHCl=CC12 with CC12=CC12 and with CH3CC13, the photodecomposition rate of CHCl=CC12 decreased compared with the rate of CHCl=CC12 in the absence of other organic compounds. For example, in the absence of other organic compounds, CHCl=CC12 decomposed faster than c-CgH12 while in the mixture of CHCl=CClz and c C6H12, c-CsH12 decomposed faster than CHCl=CC12. More extensive data on the CHCl=CC12-c-CgH12 system are presented in Table VI. In general, as the CHCl=CC12:cC6H12 ratio decreased, the decomposition rate of CHCl=CC12 decreased. In contrast, as the c-CgH12: CHCl=CC12 ratio decreased, the decomposition rate of cincreased. These results suggest that there is a reactive intermediate involved when CHCl=CC12 is present that is not involved when c-CgH12 is photolyzed in the absence of CHCI=CC12. These results are qualitatively consistent with the generation of chlorine atoms as the reactive intermediates. In solution, c-CgHI2is about five times more reactive toward chlorine atoms than is CHCl=CClz (22). In all of the reactions which involved both c-CsHl2 and a chloroethylene except that with CC12=CC12, the photodecomposition rate of c-CgH12 increased while the rate of the chloroethylenes decreased. Experiments with mixtures of CHCl=CC12 and c-CHEtCH2O gave results which were similar to those observed in the CHC1=CC12-c-C6H12 system (Tables XI and XI1 in the microfilm edition of this journal). Experiments were carried out in an attempt to identify the reactive species present in these reactions. These studies involved the use of NO, NO2, 0 3 , and other potentially reactive species with CHCl=CC12 and other organic materials in individual and competitive reactions. The data in Table I indicate that in general, with materi-
Table VI1 I . Photodecomposition Rates of Organic Materials in Competition Reactions Which Involve N O , NO,, and 0, T i m e , h , for 50% disappearance of compound in combination Compound paira C-C6H12
CHCI=
CI, c-CHEtC H, 0 CHCI=CC12
Table I X . Photodecomposition Rates of CHCI=CCI, t h e Presence of Possible Photoxidation Products and Other Coreactants
Reactivity ratio
NO,
NOb
NO,b
O,b
NO
4.9
5.0 10.5
0.73 0.87
1.8 2.1
6.4 7.0
8’o 10.5 0.13 0.15
0‘5 0.5
1.1
8.7
1.3
0,
1.0
trans-MeCH=CHMe 0.37 - 1.0 1.1 0.37 c-c 6 H 1 0 a Initial concentration, 10 p p m each. b Initial concentration, 10 ppm.
als which decompose slowly, both NO and NO2 gave similar results, while with materials which decompose rapidly, such as trans-MeCH=CHMe and C - C ~ H INO2 ~ , produced an increased decomposition rate over that observed when NO was used. Reactions with O3 indicate that it causes a much faster decomposition of the organic material than either NO or NO2 (Table VII). In competition reactions, the ratios of the photodecomposition rates of the compound pairs were similar even though the absolute rates varied greatly as the “initiator” was changed from NO to NO2 to O3 (Table VIII). O3 appeared to have a slightly greater leveling effect than did NO or Nos. Comparison of the data in Tables VI1 and VIII further suggests that CHCl=CC12 caused the formation of some reactive species other than NO2 or O3 in the photochemical reactions. Whether NO, Nos, or 0 3 was used as the “initiator’’, CHCl=CC12 decomposed faster than c-CHEtCHzO and c-CgH12, whereas in competition reactions, CHCl=CC12 decomposed slower. The unknown reactive species appeared to be highly reactive toward CHCl=CC12 in individual experiments, but was even more reactive toward C-CcHl:! and c-CHEtCH20 when either of the latter two compounds was present. This led us to consider, as possible intermediates or their precursors, possible photochemical oxidation products of CHCl=CC12 such as phosgene (COCIZ), chlorine (C12), chloral (CC13CHO) (23), dichloroacetyl chloride (CHClZCOCl), and so forth ( 2 4 ) . These products could lead to free radicals such as chlorine atoms, trichloromethyl radicals (.CC13), and acyl radicals, and so forth, under the reaction conditions. Table IX shows the photodecomposition rates of CHCl=CC12 with a number of these possible products and also with acetaldehyde (MeCHO) and bromotrichloromethane (CBrC13).CC13CHO significantly increased the decomposition rate of CHCl=CC12 in comparison with the effect of NO or NO2. CC13CHO would be expected to lead to two types of radicals, trichloroacetyl and .CC13. MeCHO was used to determine if the acetyl radical ( 2 5 ) would lead to similar results. MeCHO decreased the decomposition rate of CHCl=CC12 whereas CBrC13, in combination with NO, did increase the decomposition rate of CHCl=CCl:!. From these experiments, it is not clear why CC13CHO was so active, Chlorine atoms also were potential intermediates in the photochemical oxidation of CHCl=CC12. Indeed, CHCl=CC12 reacted extremely rapidly in the photochemical reaction with Clz, even faster than with 03.C12 in combination with NO in-
Co-reactants (concentration i n p p m p
in
T i m e , h , f o r 50% disappearance of CHCI=CCI,
NO (5) 3.5 ~o~’(i6.8) 2.9 3.2 CCI,CHO 12) CCI~CHO(5j 0.93 CCI,CHO (IO) 0.45 MeCHO (10) 15.7b MeCHO (10) + NO (5) 10.7C CBrCI, (10) 5.0 CBrCI, (IO) + NO (IO) 1.3 CI2 (1) 3. I d CI, (1) + NO (5) 0.43 CI, (2) 0.18 CI, (5) < 0.05e CI, (10)
