Environ. Scl. Technol. 1985, 19, 462-464
nical Information Service: Springfield,VA, 1979; NTIS PB 292-751, pp 1-67. (2) Wertheimer, A. L. “Modification of Optical Instrument for In-Stack Monitoring of Particle Size”;National Technical Information Service: Springfield, VA, 1981; NTIS PB 81-213 373, pp 1-38. (3) Wertheimer, A. L.; Pfeister, G. J., Jr. “Optical Based,
Microprocessor Controlled, Stack Particulate Monitor”. SPIE-Microcomputers and Microprocessors in Optical Systems, 1980, 230150-155. (4) Wertheimer, A. L.; Wilcock, W. L. Appl. Opt. 1976,15, (6), 1616-1620. ( 5 ) Pilat, M. J.; Raemhild, G. A.; Powell, E. B.; Fioretti, G. M.; Meyer, D. F. “Developmentof a Cascade Impactor System
for Sampling 0.02 to 20 Micron Diameter Particles”.1978, Electric Power Research Institute Report FP-844, Vol. 1, pp 1-91. (6) Moran, M. J. “Simulated Stationary Source Facility Users
Handbook”. prepared for EPA by Northrop Services, Inc. under Contract 68-02-1567, Research Triangle Park, NC, Oct 1975, Report NSI TN-262-1535, pp 1-41. (7) Smith, W. B.; McCain, J. E. In ”Air Pollution Control”; Strauss, W., Ed.; Wiley: New York, 1978; Part 111, pp 124-133.
Received for review May 29,1984. Revised manuscript received November 14, 1984. Accepted December 19, 1984.
NOTES Rate Constants for the Gas-Phase Reaction of Hydroxyl Radicals with Biphenyl and the Monochloroblphenyls at 295 =t 1 K Roger Atkinson’ and Sara M. Aschmann
Statewide Air Pollution Research Center, University of California, Riverside, California 9252 1 Rate constants for the gas-phase reactions of OH radicals with biphenyl and the monochlorobiphenyls have been determined by using a relative rate technique in 1 atm of air at 295 f 1 K. The rate constants obtained, relative to a rate constant for the reaction of OH radicals with cyclohexane of (7.57 f 0.05) X cm3 molecule-’ s-’, were the following (in units of X10-l2 cm3 molecule-’ s-9: biphenyl, 8.5 f 0.8; 2-chlorobiphenyl, 2.9 f 0.4; 3chlorobiphenyl, 5.4 f 0.8; 4-chlorobiphenyl, 3.9 f 0.7. These rate constants lead to estimated atmospheric lifetimes due to reaction with the OH radical of -2.7, -8, -4, and -6 days for biphenyl and 2-, 3-, and 4-chlorobiphenyl, respectively, for a 24-h average OH radical concentration of 5 x IO5 cm-,. H
Introduction Polychlorinated biphenyls (PCB’s) are ubiquitous constituents of the ecosystem, with evidence accumulating that their transport occurs largely through the atmosphere (1-3). However, few data are available concerning their homogeneous gas-phase loss processes under atmospheric conditions, although it has been reported that certain PCB’s are stable to photolysis by ultraviolet light under simulated atmospheric conditions (4). The major gasphase processes responsible for the homogeneous degradation of organics emitted into the atmosphere are now recognized to be photolysis and reaction with hydroxyl (OH) and nitrate (NO,) radicals and with ozone (5,6). For the majority of organics studied to date the reaction with OH radicals is the most important of these loss processes (5, 6). In order to evaluate the homogeneous gas-phase atmospheric lifetimes and fates of the chlorinated biphenyls, in this work we have extended our previous study of the atmospheric reactions of naphthalene and biphenyl (7) to the determination of rate constants for the reaction of OH radicals with the three monochlorobiphenyl isomers. 462
Environ. Sci. Technol., Vol. 19, No. 5, 1985
Biphenyl was also included in this study as a check on the experimental technique.
Experimental Section The relative rate technique used was essentially identical with that recently used to determine the OH radical reaction rate constants for naphthalene and biphenyl (7). Hydroxyl radicals were generated by the photolysis of methyl nitrite in air at wavelengths 2300 nm. CH30N0 + hu CH30 + NO CH30 + O2 HCHO + HOz HOz + NO -.+ OH + NO2
-
In order to minimize the formation of 0, and of NO, radicals, NO was included in the reactant mixtures, which had the following concentrations: CH,ONO, (2.4-3.6) X 1014 molecule ~ m - NO, ~ ; -1.2 X 1014 molecule ~ m - ~ ; biphenyl and/or monochlorobiphenyls, (1.2-2.4) X 10l2 molecule cm-,; the reference organic (cyclohexane in this case), -2.4 X lo1, molecule cm-,. Dry purified matrix air (8),at a total pressure of -735 torr, was used as the diluent gas. Providing that biphenyl, the monochlorobiphenyls,and cyclohexane (the reference organic) were removed solely via reaction with the OH radical OH biphenyl products (1) OH cyclohexane products (2) then (7)
-
--
+ +
In
(
[biPhenYlIt, [biphenyl],
-
)
=
In ( y o h e = e l , o ) cyclohexane]
,
(I)
where [biphenyl],, and [cyclohexane],, are the concentrations of biphenyl and cyclohexane, respectively, at time to, [biphenyl], and [cyclohexane], are the corresponding
0013-936X/85/0919-0462$01.50/0
0 1985 American Chemical Society
concentrations a t time t, and kl and k2 are the rate constants for reactions 1 and 2, respectively. Hence, plots of In ([biphenyl],,/ [biphenyl],) vs. In ([cyclohexane],,/ [cyclohexane],) should yield straight lines of slope k l / k 2 and zero intercept. Irradiations were carried out in an ~ 6 4 0 0 - Lall-Teflon chamber equipped with blacklight irradiation. In this study, 20% of the maximum light intensity was used, corresponding to a photolytic half-life of CH30N0 of 15 min. Biphenyl and the monochlorobiphenyls were introduced into the chamber by flowing ultrahigh purity N2 at known flow rates through Pyrex tubes filled with the solid biphenyls or, for 3-chlorobiphenyl, through a U-tube containing the 3-chlorobiphenyl coated on glass beads. The vapor pressures of biphenyl and 4-chlorobiphenyl at 295 K are reported to be 6 X lo-, and 9 X torr, respectively (9), and, on the basis of our preirradiation gas chromatographic analyses for the monochlorobiphenyls and the total amounts of N2 flowed through the Pyrex tubes containing these isomers, those of 2- and 3-chlorotorr (to within f a factor of -2). biphenyls are -6 X The other reactants were introduced into the chamber as described previously (7). Biphenyl and the monochlorobiphenyls were monitored prior to and during the irradiations by gas chromatography with flame ionization detection (GC-FID) as described previously (3, except that 10-L gas samples were withdrawn from the chamber and the column temperature was programmed from 383 to 443 K at 4 K min-l. Cyclohexane was analyzed by GC-FID using a 20 ft X 0.125 in. stainless steel column of 5% DC703/C20M on 100/120 mesh AW, DMCS Chromosorb G, operated at 333 K. Nitric oxide was monitored by a chemiluminescence instrument. Biphenyl and cyclohexane were obtained from Aldrich Chemical Co., with stated purity levels of 299%. 2Chlorobiphenyl was obtained from Pfaltz & Bauer, Inc., and 3- and 4-chlorobiphenyls were from Alfa Products. Methyl nitrite was prepared as described previously (10).
c
-
-
-
/6 3-CHLOROBIPHENYL
1.2'
-
Results and Discussion A series of CH30NO-NO-biphenyl and/or chlorobiphenyl-cyclohexane-air irradiations were carried out at 295 f 1 K. In addition, analogous irradiations in the absence of CH,ONO were carried out to determine whether photolysis or wall adsorption/desorption processes were occurring to any significant extent for biphenyl and the monochlorobiphenyls. All irradiations were of 60-min duration, and samples for analyses were taken every 15, 20, or 30 min, depending on the particular irradiation. The OH radical concentrations during these irradiations could be estimated from the cyclohexane decay rates and were in the range -(l-10) x lo7 cm-, for the irradiations involving CH30N0. For the irradiations carried out in the absence of CH,ONO, the OH radical concentrations were a factor of 10-100 lower (i.e., 1X lo6 cm-7, and these irradiations showed that no gas-phase loss or appearance of biphenyl or the monochlorobiphenyls (to within the analytical error limits for these organics of -3-5%) occurred during a 60-min irradiation. Additionally, no dark loss (to within f3-5%) of these biphenyls was observed over an -60-min duration. The data obtained from irradiations of CH,ONO-NObiphenyl and/or chlorobiphenyl-cyclohexane-air mixtures are plotted in accordance with eq I in Figure 1. Compared to the amounts of biphenyl, chlorobiphenyl, and cyclohexane consumed during these irradiations, any effects of photolysis and/or wall adsorption or desorption of biphenyl and the chlorobiphenyls were negligible. Since NO was observed to be present at the termination of these
9
BIPHENYL n
0.8'
C
/ w
77 /
4-CHLOROBIPHENYL
E
__
2-CHLOROBIPHENYL
0
0.4
0
In
0.8
I .6
I .2
( [CYCLOHEXANE] t,,/[CYCLOHEXANE]
t)
Flgure 1. Plot of eq I for biphenyl and 2-, 3-, and 4-chloroblphenyl.
Table I. Rate Constant Ratios k l / k zand Rate Constants k l for the Reaction of OH Radicals with Biphenyl and the Monochlorobiphenyls at 295 f 1 K and 735 torr Total Pressure of Air, together with Available Reported Data biphenyl biphenyl
kl/kza
1012kl, cm3 molecule-' s-' this workb literature
1.12 f 0.10
8.5 f 0.8
2-chlorobiphenyl 0.38 f 0.05 3-chlorobiphenyl 0.71 f 0.11 4-chlorobiphenyl 0.52 f 0.09
2.9 f 0.4 5.4 f 0.8 3.9 f 0.7
5.8 f 0.8 (12) 8.06 f 0.77c (7) 5.9 f 1.6d (13) 11.3 f 3.1d (13) 12.8 f 3.6d (13)
Error limits are two least-squares standard deviations of the slopes of the plots shown in Figure 1. bPlaced on an absolute basis using a rate constant for the reaction of OH radicals with cyclohexane of kz = (7.57 f 0.05) X cm3 molecule-' s-' ( I O ) , which in turn is based upon a rate constant for the reaction of OH radicals with n-butane of 2.58 X om3 molecule-' s-l (10). The error limits are two least-squares standard deviations. C A t294 f 2 K. Relative to a rate constant for the reaction of OH radicals with n-nonane of (1.07 f 0.04) X lo-'' cm3 molecule-' (7) which in turn is based on a rate constant for the reaction of OH radicals cm3 molecule-'s-' ( I O ) . d A t 302 f 2 with n-butane of 2.58 X K. Relative to a rate constant for the reaction of OH radicals with benzene of (1.28 f 0.22) X cm3 molecule-' s-' (13).
irradiations at concentrations of 1 X 10l2 to 8 X 1013 molecule cm-,, negligible formation of 0,or of NO, radicals occurred. Furthermore, by analogy with biphenyl (7) and chlorobenzene ( l l ) , any reactions of 0,with the monochlorobiphenyls are expected to be negligibly slow, and thus, the disappearance of biphenyl and the monochlorobiphenyls during these irradiations was essentially solely due to reaction with the OH radical. The rate constant ratios kl/k2 determined from leastsquares analyses of the slopes of the plots shown in Figure 1 are given in Table I. The least-squares intercepts of these plots were within two standard deviations of zero. These rate constant ratios k 1 / k 2can be placed on an absolute basis by using a rate constant for the reaction of OH radicals with cyclohexane of kz = (7.57 f 0.05) X cm3 molecule-l s-l at room temperature (IO). This rate constant k 2 is in turn based upon a rate constant for the reaction of OH radicals with n-butane of 2.58 X 10-l2cm3molecule-l s-l (10). The rate constants kl so derived are also giveq in Table I, together with the available reported values. It can be seen that the present rate constant kl for biphenyl is, within the experimental error limits, in excellent Environ. Sci. Technol., Vol. 19, No. 5, 1985
463
agreement with our previous value (7), but is -50% higher than that reported by Zetzsch (12) usitlg a flash photolysis technique, with the reasons for this latter discrepancy not being known at the present time. For the monochlorobiphenyl isomers, the sole reported data concerning their OH radical rate constants arise from the relative rate study of Dilling et al. (13), carried out in a 2-L Pyrex reaction vessel. Their rate constants kl are higher by factors of -2-3 than those determined here, possibly due to wall adsorptian/desorption problems with the small reaction vessel used (13). Our rate constant data lead to estimated atmospheric lifetimes due to reaction with the OH radical of -2.7 days for biphenyl and -8, -4, and -6 days for 2-, 3-, and 4-chlorabiphenyl, respectively, assuming an average atmospheric OH radical concentration of 5 X lo5 cm-3 (14). Since it is expected that the OH radical reactions will be the major homogeneous reaction loss process for these biphenyls, these lifetimes will then correspond to their overall atmospheric lifetimes with respect to chemical reaction. These rate constants klfor the monochlorobiphenyls are lower, by factors of 1.5-3, than that for biphenyl, with 3-chlorobiphenyl being the most reactive of the monochlorobiphenyls. This is consistent with the fact that C1 atom substituents are electron withdrawing (15),thus leading to a decrease in reactitity toward such electrophilic reactants as the OH radical. Thus, for example, chlorobenzene has been shown to react less rapidly with OH radicals than does benzene, by a factor of -1.5 (11,12). Thus, the successive chlorination of biphenyl is expected to lead to progressively lower OH radical reaction rate constants, and hence longer atmospheric lifetimes, with the precise rate constants depending on the number and positions of the C1 atom substituents. Zetzsch (12)has shown that for benzene, biphenyl, and a series of substituted monocyclic aromatics the room temperature rate constants for the addition of OH radicals to the aromatic ring correlate well with the sum of the electrophilic substituent constants r ~ + ,with (12) log k (cm3 molecule-1 s-l) = -11.6 - 1.39zu’ [These electrophilic substituent constants u+ are derived and tabulated by Brown and Okamoto (15).] However, while the OH radical rate constant obtained by Zetzsch (12) for biphenyl fits this correlation teasonably well (to a factor of 1.8), for biphenyl and the chlorobiphenyls OH radical addition can obviously occur on either of the two aromatic rings. For the monochlorobiphenyls,OH radical addition is expected to occur mainly on the aromatic ring
404
Envlron. Scl. Technol., Vol. 19, No. 5, 1985
containing the least number of chlorine atom substituents. Thus, the extension of this extremely useful correlation derived by Zetzsch (12) will probably need some modification for the estimation of rate constants for substituted biphenyls, and further work, both experimental and theoretical, is necessary concerning the gas-phase rate constants for the reaction of the OH radical with the polychlorobiphenyls. Registry No. OH, 3352-57-6; biphenyl, 92-52-4; 2-chlorobiphenyl, 2051-60-7; 3-chlorobiphenyl, 2051-61-8; 4-chlorobiphenyl, 2051-62-9.
Literature Cited Eisenreich, S, J.; Looney, B. B.; Thornton, J. D. Enuiron. Sei. Technol. 1981,15, 30-38. Atlas, E.; Giam, C. S. Science (Washington, D E . ) 1981,211, 163-165. Buckley, E. H. Science (Washington, D.C.) 1982, 216, 520-522. Crosby, D. G.; Moilanen, K. W. Chemosphere 1977, 6, 167-172. Atkinson, R.; Darnall, K. R.; Lloyd, A. C.; Winer, A. M.; Pitts, J. N., Jr. Adu, Photochem. 1979, 11, 375-488. Atkinson, R.; Carter, W. P. L. Chem. Rev. 1984,84,437-470. Atkinson, R.; Aschmann, S. M.; Pitts, J. N., Jr. Enuiron. ScI. Technol. 1984, 18, 110-113. Doyle, G. J.; Bekowies, P. J.; Winer, A. M.; Pitts, J. N., Jr. Environ. Sei. Technol. 1977, 1 1 , 45-51. Burkhard, L. P.; Armstrong, D. E.; Andren, A. W. J . Chem. Eng. Data 1984, 29, 248-250. Atkinson, R.; Aschmann, S. M.; Winer, A. M.; Pitts, J. N., Jr. Znt. J . Chem. Kinet. 1982, 14, 507-516. Winer, A. M.; Pitts, J. N., Atkinson, R.; Aschmann, S. Ma; Jr. Arch. Enuirofi. Contam. Toxicol., in press. Zetzsch, C. 15th International Conference Photochemistry, Stanford, CA, June 27-July 1, 1982. Dilling, W. L.; Miracle, G. E.; Boggs, G. U. “Abstracts of Papers”, 186th National Meeting of the American Chemical Society, Washihgton, Dc, Aug 1983; American Chemical Society: Washington, DC, 1983; ENVR 126. Crutzen, P. J. In “Atmospheric Chemistry”; Goldberg, E. D., Ed.; Springer-Verlag: Berlin, 1982; pp 313-328. Brown, H. C.; Okamoto, Y. J . Am. Chem. SOC.1958,80, 4979-4987.
Received for review September 24, 1984. Accepted December 27, 1984. This work was financially supported by the US. Environmental Protection Agency under Cooperative Agreement CR809247-03 (Project Monitor, Bruce W . Gay, J r . ) . Although the research described in this article has been funded by the Environmental Protection Agency, it has not been subjected to agency review and therefore does not necessarily reflect the view of the Agency, and no official endorsement should be inferred.
