Envlron. Sci. Techno/. 1991, 25, 1098-1 103
Dorward, E. J.; Barisas, B. G. Enuiron. Sci. Technol. 1984, 18, 967-972. Bulich, A. A. In Aquatic Toxicology; Markings, L. L., Kimerle, R. A., Eds.; American Society for Testing and Materials: Philadelphia, PA, 1979; p 98. Hand, S. C.; Gnaiger, E. Science 1988, 239, 1425-1427. Eftimiadi, C.; Rialdi, G. Thermochim. Acta 1985, 85, 489-492. Application of Calorimetry in Life Sciences; Lamprecht, I., Schaarschmidt, B., Eds.; Walder de Gruyter & Co.: Berlin, 1977. Eftimiadi, C.; Rialdi, G. Microbiologica 1985,8, 297-301. Schon, A,; Wadso, I. J. Biochem. Biophys. Methods 1986, 135-143. Monti, M.; Brand, L.; Ikommi-kumm, J.;Ohson, H.; Wadso, I. Scand. J . Haematol. 1981, 27, 305. Levin, K. J. Clin. Lab. Inuest. 1973, 32, 67. Krakauer, T.; Krakauer, H. Cell Immunol. 1976,26,242. Miles, R. J.; Beezer, A. E.; Lee, D. H. Microbios 1986,45, 7-19. Monti, M.; Faldt, R.; Anherst, J.; Wadso, I. J . Immunol. Methods 1980, 37, 29-37. Binford, J. S.; Binford, L. F.; Adler, P. Am. J . Clin. Pharmacol. 1973, 59, 86. Mishell, B. B.; Shiigi, S. M.; Henry, C.; Chan, E. L.; North, J.; Gallilt, R.; Somich, M.; Miller, K.; Marbrook, J.; Parks, D.; Good, A. H. In Selected Methods in Cellular Immunology; Mishell, B. B., Shiigi, M., Eds.; W. H. Freeman and Co.: San Francisco, CA, 1980; Chapter 1. Baumann, P.; Baumann, L. In Bergey’s Manual of Systematic Bacteriology; Krieg, N. R., Holt, J. G., Eds.;
Williams and Wilkins: Baltimore, MD, 1984; Vol. 1, pp 539-545. McGuiness, S. M.; Roess, D. A.; Barisas, B. G. Thermochim. Acta 1990, 172, 131-145. Metal Toxicity In Mammals; Venugopal, B., Luckey, T. D.; Plenum Press: New York, 1978; p 68. Ambient Water Quality Criteria for Cadmium; EPA 440/5-80-025; United States Environmental Protection Agency, Office of Water Regulations and Standards: Washington, DC, 1980; p B3. Pickering, Q.H.; Henderson, C. Air Water Pollut. 1966, 10, 453. Hastings, J. W.; Nealson, K. H. Bacterial Bioluminescence. In Annual Review of Microbiology;Starr, M. P., Ingraham, J. L., Balows, A., Eds.; Annual Reviews: Palo Alto,CA, 1977; Vol. 31, p 549. Dorward, E. J. Acute Toxicity Screening of Aquatic Pollutants Using a Bacterial Electrode. Dissertation, Colorado State University, Fort Collins, CO, 1984. Eftimiadi, C.; Rialdi, G. Cell Biophys. 1982, 4, 231-244. Nordmark, M.; Laynez, J.;Schon, A.; Suurkut.uk, J.;Wadso, I. J . Biochem. Biophys. Methods 1984,10, 187-202. Dutka, B. J.; Kwan, K. K. Bull. Environ. Contam. Toxicol. 1981, 27, 753-757. Walgreen, B., Beckman Instruments, Inc., 1990, personal communication.
Received f o r review July 3, 1990. Revised manuscript received January 14,1991. Accepted January 15,1991. This work was supported in part by the Colorado State University Agricultural Experiment Station.
Rate Constants for the Gas-Phase Reactions of Hydroxyl Radicals with Tetramethyllead and Tetraethyllead Ole John Nlelsen*
Section for Chemical Reactivity, R i m National Laboratory, DK-4000 Roskilde, Denmark Denls J. O’Farrell and Jack J. Treacy
Department of Chemlstry, Dublin Institute of Technology, Dublin, Ireland Howard W. Sldebottom
Department of Chemistry, Universlty College Dublin, Ireland Rate constants for the reactions of OH radicals with (CH3)4Pband (C2H5)4Pbhave been determined a t 298 f 2 K and a total pressure of approximately 1 atm. Rate data were obtained by using both the absolute rate technique of pulse radiolysis combined with kinetic spectroscopy and a conventional photolytic relative rate method. Rate constants were also obtained for the analogous reactions with (CH3),C and (C2HJ4C for comparison purposes. The results are discussed in terms of structurereactivity relationships. Values of the rate constants cm3 molecule-’ s-l) are (CHd4C (CZH~)IC (CWJ‘b (CzH,),Pb
pulse radiolysis
relative rate
0.79 f 0.10 5.1 f 0.8 5.9 1.2 68 f 16
0.84 f 0.10 4.8 f 0.3 3.9 f 0.2 61 f 5
(TML) and tetraethyllead (TEL) were first used in the United States. Most of the organic lead compounds are decomposed during the combustion process and emitted as a complex mixture of lead salts (I),although incomplete combustion, spillage, and evaporation results in -2% of the alkyllead content of fuel entering the atmosphere unchanged (2). For several decades the use of tetraalkyllead additives has given rise to environmental concern and has led to legislation to limit their use. However, they are still extensively employed in many countries (3),and the rate and mechanism for their atmospheric degradation is of some importance. Harrison and Laxen (4) carried out an extensive investigation of the decomposition of (CH,),Pb and (C2H&,Pb under atmospheric conditions. They concluded that the main atmospheric removal pathway was initiated by reaction with OH radicals and that removal by photolysis and reaction with O3 and O(3P) atoms was of less importance. Rate constant data for the reaction of OH with tetramethyl- and tetraethyllead were determined by Harrison and Laxen ( 4 ) , employing a relative rate method using a system designed to simulate photochemical smog
Introduction Tetraalkyllead compounds have been added to gasoline to prevent preignition since 1923, when tetramethyllead 1088 Envlron. Scl. Technol., Vol. 25, No. 6, 1991 0013-936X/91/0925-1098$02.50/0 0
1991 American
Chemical Society
conditions, in which the OH radical was the principal reactive species. Nielsen et al. (5) also measured rate coefficients for these reaction systems using the absolute rate technique of pulse radiolysis. Although the reported rate constants for (CH3)4Pbare in reasonable agreement, those for (C2H6),Pbdisagree by a factor of -7. The results from the relative rate study suggest that TEL is -9 times more reactive than TML (4), whereas Nielsen et al. ( 5 ) found a corresponding variance in reactivity of less than a factor of 2. More recently, Hewitt and Harrison (3) redetermined the relative rate of these two reactions and derived a rate constant ratio of -4, which does little to resolve the discrepancy. The aim of this work was to redetermine rate constants at atmospheric pressure and room temperature for the reaction of OH radicals with tetramethyl- and tetraethyllead, using both the absolute technique of pulse radiolysis combined with UV kinetic spectroscopy and a conventional relative rate method. It was hoped that the results would resolve the discrepancy in the previously reported rate constants for these reactions and provide reliable data in order to evaluate the atmospheric lifetimes of these species. The reactions of OH radicals with the analogous carbon compounds (CH3),C and (C2H6)& were also investigated with a view to providing further information on the reactivity trends for these reactions. Experimental Section Relative rate experiments were carried out at 730-750 Torr and 298 f 2 K in an approximately 50-L FEP Teflon cylindrical reaction chamber. Hydroxyl radicals were generated by photolysis of methyl nitrite in air at wavelengths greater than 290 nm: CH,ONO
CH30 + O2 HO2
-
+ hu
+ NO
CH30 + NO
CH20 + HOP OH
+ NO2
reaction of excited argon atoms with H 2 0 was extremely rapid compared with the reaction times for OH radical decay. On the basis of previous experiments in this laboratory, the initial OH radical concentration was estimated to be in the region of 1013molecules cmS. The irradiation cell was a 1-L stainless-steel cell mounted directly onto the accelerator. A set of White mirrors provided variable optical path lengths. An optical path length of 120 cm was used in all experiments. Each substrate compound was premixed with Ar in a 50-1, FEP Teflon bag, since if the substrate compound was admitted directly to the cell it is possible that loss due to adsorption on the walls could occur. Periodic sampling over 1h and GLC analysis of the reaction mixtures showed that the samples prepared in this way were stable in the Teflon chamber. The stainless-steel reaction cell was repeatedly flushed with the reaction mixture between each experiment. Hydroxyl radical decays recorded from samples left in the cell for up to 30 min were reproducible, indicating that wall loss of the substrate in the cell was unimportant. Water and the argon/substrate mixture were admitted directly one at a time and the partial pressures read by using an MKS Baratron 170 absolute membrane manometer with a resolution of Torr. The analyzing light source was a pulsed 150-W high-pressure xenon arc lamp. A Hilger and Watts grating spectrograph, a Hamamatsu photomultiplier, and a Biomation-8100waveform digitizer were used to detect and record the light intensity at the desired wavelength. Transfer and storage of raw data on a mainframe computer was controlled by a P D P l l minicomputer. Following irradiation, the formation and decay of OH radicals is governed by reactions 4-6: Ar* + H 2 0 OH + H Ar (4)
-
-
(1)
OH
(2)
H
(3)
Irradiation was provided by 20 fluorescent lamps; 10 black lamps (Philips TL 20/08), and 10 sunlamps (Philips TL 20/09), giving a photolytic half-life for CH30N0 of approximately 30 min. Light intensity was varied by switching off various sets of lamps. An electric fan positioned in front of the reaction chamber helped maintain a uniform reaction temperature during photolysis. In order to prevent prephotolysis of the reactants, the chamber was covered prior to commencement of irradiation. Pressure measurements were made with an MKS Baratron 220A capacitance manometer. Measured amounts of the reagents were flushed from Pyrex bulbs into the reaction chamber by a stream of zero-grade N2 (BOC) and the bag was filled with zero-grade air (BOC). Reaction mixtures were allowed to mix for -30 min prior to photolysis. All quantitative analyses were carried out using a Perkin-Elmer F11 gas chromatograph incorporating a flame ionization detector. Gas-tight syringes or a Valco gas sampling valve was used to remove reaction mixtures from the Teflon bag. Analysis of the samples was carried out using a 2 m X 3 mm Teflon analytical column packed with 20% DC 200 on Chromosorb-WHP (80-100 mesh). The pulse radiolysis-W kinetic spectroscopy technique has been described in detail previously (6, 7). Hydroxyl radicals were produced from the pulsed radiolysis of mixtures of 12 Torr H 2 0 and argon at a total pressure of 1 atm. Excited Ar atoms were formed in a single 30-ns pulse of 2-MeV electrons from a Febetron 705B field emission accelerator. Formation of OH radicals from the
+
+ OH + M + H 2 0 2 + M
(5)
+ OH + M - H 2 0 + M
(6)
In the presence of a reactive substrate, S, OH radicals will also be removed in reaction 7: OH + S products (7)
-
Removal of OH radicals can be represented by the differential equation -d[OH]/dt = 2k6[OH12[Ml + k6[OH1[HI [MI + k7[S] [OH] (1) For the experimental conditions employed in this work, the pseudo-first-order conditions k7[S] >> 2k5[OH][MI + k6[H][MI was established and simple first-order kinetics were obeyed: In ([OH],/[OH],) = k7[S]t = k’t (11) The decay of OH radicals at 298 f 2 K was studied by monitoring the transient absorbance at 309 nm, using a ~ . version of modified version of Beer’s law A = ( ~ l c ) This Beer’s law is required when the spectral bandpass is wide compared to the line width of the spectral features. The value of n was determined from the function log A = n log (clc) by varying the optical path length and carrying out a linear regression best fit analysis. For a spectral bandpass of 0.08 nm, n was found to be 0.70 f 0.04. The observed transient absorbance is a direct measure of the OH concentration at any time during the decay. The experimental error in the determination of n, used to calculate the absorbance, introduces an error in the pseudo-first-order rate constants of approximately 3%. The argon diluent (AGA) had a state6 purity of >99.9% and was used without further purification. Nitric oxide Environ. Sci. Technol., Vol. 25, No. 6, 1991
1099
Table I. Reactant Concentrations' and Slope, k,/ks, for the Reaction of OH Radicals with Tetraalkyl Compounds of Carbon and Lead at Room Temperature and Atmospheric Pressure
substrate (CH.314 (C$&)IC (CHJJ'b
PPm 5-10 5-15 5-10 5-10 5-10 5-10
(C&)$b a
n-C4H10 c-C6H12 n-C6H14
c-C6H12
n-C9Hm C3H6
PPm
CHSONO, PPm
NO, ppm
kilka
5-15 5-15 3-7 3-7 5-10 5-10
10-30 10-20 10-20 10-20 10-20 10-20
0-10 0-20 0-10 0-10 0-10 0-10
0.33 f 0.04 0.64 f 0.04 0.71 f 0.05 0.51 f 0.05 5.38 f 1.4 2.52 f 0.04
molecules cm-3 at 298 K and 740 Torr total pressure.
1 ppm = 2.40 X
1.2,
reference
I
I
/
1
I
!
I I
I
Table 11. Summary of Reported Rate Constants for the Reactions of OH Radicals with Tetraalkyl Compounds of Carbon and Lead at Room Temperature and Atmospheric Pressure
substrate (CHJ4C (CzHdJ
06
(CH3),Pb 0 4
(C&)dPb
02
00 00
0.2
04
06
08
1.3
In([RHlo/[RHl,)
Figure 1. Plots of In ([S]o/[S],) versus In ([RH],/[RH],) from the relative rate studies. Data for (CH,),C with n-C4H,o as RH, (C,H,),C with c-C,H,, as RH, (CH,),Pb with n-C8H,, as RH, and (C2H,),Pb with C3H, as RH.
(99.6%), n-butane (99%), 2,2-dimethylpropane (>99%), and propene (99%) from Matheson Gas Products were further purified by repeated freeze-pump-thaw cycles. 3,&Diethylpentane, n-hexane, cyclohexane, and n-nonane were from Aldrich, with stated purities better than 99%. They were further purified by vacuum distillation until gas chromatographicanalysis showed no impurities. Water samples were always at least triple distilled. The samples of tetramethyl- and tetraethyllead were a gift from the Associated Octel Co. and showed no impurities on IR or GLC analysis. Methyl nitrite was prepared by dropwise addition of 50% H 3 0 4to methanol saturated with sodium nitrite.
Results In the relative rate studies hydroxyl radicals generated in reactions 1-3 react with the substrate, S, and the reference hydrocarbon, RH: OH + S products (7) OH
-
+ RH
products
(8)
Provided that reaction with OH is the only significant loss process for both substrate and reference compound, then In ([SIO/[SIJ= k 7 / k 8 ln([RHl~/[RHlh (111) Mixtures of the substrates and reference compounds with methyl nitrite were stable in the dark for several hours in the Teflon reaction chamber. Under the photolysis con1100 Envlron. Scl. Technol., Vol. 25, No. 6, 1991
10i2ka'
techniqueh
ref
0.79 f 0.10 0.84 f 0.10 0.85 5.1 f 0.8 4.8 f 0.3 5.1 9.0 f 1.6d 6.3 f 1.3 5.9 f 1.2 3.9 f 0.2e 80 f 12' 11.6 f 1.7 19 f 28 68 f 16 61 f 5e
PR-KS RR evaluated PR-KS RR estimate RR PR-KS PR-KS RR RR PR-KS RR PR-KS RR
this work this work 8, 9
this work this work 8, 9 4 5
this work this work 4 5 3
this work this work
In units of cm3 molecule-' s-l. *Errors are two standard deviations and represent precision only. CAll relative rate data have been recalculated from the original rate constant ratios by using the recent OH rate constant evaluation by Atkinson (9). dMeasured relative to toluene. uAverage of rate constants derived from both reference compounds. 'Measured relative to m-xylene. #Measured relative to (CH3!4Pb, k(OH + (CH3)4Pb)= 4.9 x cm3 molecule-' s-l from this work. "RR, relative rate; PR-KS, Dulse radiolvsis-kinetic sDectroscoov.
ditions employed the substrates were photochemically stable. Addition of NO to the reaction mixtures minimized formation of 03.The rate constant ratios were shown to be independent of the concentration of NO added, indicating that reactions of both substrate and reference compounds with O3are negligible under the conditions employed. Harrison and Laxen (4) have provided evidence for a surface reaction between NOz and tetraalkyllead compounds and hence it is possible that NO2 formed in reaction 3 could lead to enhanced removal of (CH3),Pb and (CzH6)4Pb.Mixtures of 5 ppm NO2 with 10 ppm of the tetraalkyllead compounds in 1atm air were shown to be stable in the dark for several hours in the Teflon bag, indicating that problems associated with NO2 surface catalyzed loss of (CH,),Pb and (C2H6),Pbwere unimportant over the time scale of the kinetic experiments. Initial reactant concentrations and reference compounds used for each substrate are given in Table I. Representative concentration-time data for the various compounds investigated are shown plotted in the form of eq I11 in Figure 1. The derived rate constant ratios, k,/k,, were found to be independent of reaction time, relative reactant concentrations, and light intensity, in agreement with the proposed mechanism. Rate constants for the reactions of OH radicals with 2,2-dimethylpropane, 3,3-diethylpentane, tetramethyllead, and tetraethyllead given in Table I1 were calculated by using the following values [at 298 K in units of cm3 molecule-' s-' (8, 9)]:k(OH + n-C4Hlo)= 2.54 X
1
0
LA,..." 0
1
I
I
,
,
'
time
'
/
40 0
psec
1
0 ' -
,
0
time
/
I
psec
400
I
h
01 0
time
/
400
psec
Flgur. 2. Typical OH decay kinetics from the pulse radiolysis experiments at 298 K. Upper left 12 Torr H20 with Ar to 1 atm. Lower left: with 0.012 Torr of (C2H6),Pb added. The right-hand figures are the corresponding logarithm of absorbance versus tlme plots.
16
I
i
* /
l4I 1 12
x
k
8
6
i
4
2 2
0
[SI x
1
I
l
4
6
8
1 0 - l ~
/
molecules cm
1i 10
k(OH n-C&,,) = 5.61 X k(OH + c-CeH12) = 7.49 X 10-l2; k(OH n-CgHzo)= 10.2 X 10-l2; k(OH + C,H,) = 26.3 X 10-l2. The errors quoted are the standard deviations of the least-squares fit of the straight lines and do not include an estimate of the error in the reference rate constants. The estimated errors in the reference rate constants probably add a further 25% to the uncertainty of the determined rate constants. All experiments using the pulse radiolysis technique were performed under pseudo-first-order conditions. In the absence of added substrate the OH decays were nonexponential; however, for the hydrocarbon and tetraalkyllead concentrations used in this study all decays were clearly exponential over at least 3 half-lives. Typical OH absorbance decay curves in the absence and presence of tetraethyllead are shown in Figure 2, together with the corresponding logarithm of absorbance versus time plots. Plots of k' versus the hydrocarbon and tetraalkyllead concentrations are shown in Figures 3 and 4. The rate constants, k7,determined from the slopes of these plots were insensitive to variations in the pulse energy by a
+
0
-3
Figure 9. Plot of k'versus substrate concentration from the pulse radiolysis experiments with (CH,),C and (CH,),Pb.
+
0 3
[SI x
6 10-l~
9
/
m o l e c u l e s cm
12
15
-3
Flgure 4. Plot of k'versus substrate concentration from the pulse radiolysis experiments with (C2H6),C and (C,H,),Pb.
factor of 3 and to the number of pulses up to at least 5, indicating the lack of complications due to secondary kinetics in the system. Reaction of OH radicals with impurities in either the substrate or the argon buffer gas cannot be discounted by the above analytical procedures. The rate constants derived from the linear least-squares fits of the straight lines are given in Table 11. The error limits are two standard deviations and refer only to precision of the It' versus [SI data and do not include any contribution from systematic errors, estimated to be less than 20% in the present work. Discussion Rate constants for the reaction of hydroxyl radicals with tetramethyl and tetraethyl compounds of carbon and lead obtained at 1 atm total pressure and 298 K in this work are compared with available data from the literature in Table 11. In general, the results obtained in this investigation from the pulse radiolysis technique and the relative rate method are in reasonable agreement, although the Envlron. Sci. Technol., Vol. 25, No. 6, 1991
1101
pulse radiolysis data for (C2H5),Pbexhibit considerable scatter. Excellent agreement between the rate constants for the OH + (CH3),C reaction and previously published results was obtained. The first kinetic data for reaction of OH with (CZH5),Care reported in this work and agree well with the rate constant calculated by using Atkinson’s (8,9) recommended group rate constants for OH radical attack on organic compounds. The rate constant for the OH (CH3)4Pbreaction obtained in this work from the pulse radiolysis technique is somewhat higher than that determined from the relative rate measurements. The differences in the reported rate constants are outside the precision of the two studies but within the stated accuracies. The reason for this discrepancy is not clear; however, it is possible that the slightly higher value from the pulse radiolysis experiments may be due to low levels of reactive impurities in the (CH3I4Pbsample. The older relative rate data from Harrison and Laxen ( 4 ) were obtained in a smog chamber in which photolysis of a complex mixture of hydrocarbons and oxides of nitrogen was used to generate OH radicals in the presence of tetraalkyllead and reference compounds. The rate constant obtained from this work appears to be a little high. I t is possible that in such a complex reaction system other reactive species besides OH radicals may be generated and lead to the removal of the lead and the reference compounds. The high value of the rate constant obtained may be a consequence of preferential reaction of some reactive species with the lead compounds. It is also possible that the large variety of products generated in the system could give rise to interference problems during measurements of the loss of the tetraalkyllead and reference compounds. Initial experiments using the pulse radiolysis method of Nielsen et al. (5) performed during the course of this Ftudy reproduced their original value for k(OH + (C2H5),Pb) of approximately 12 X cm3molecule-’ s-l. However, the possibility exists that direct filling of the evacuated reaction vessel followed by argon pressurizing could cause some wall loss due to the “sticky” nature of (C2H6),Pb. As a consequence, the lowered reactant concentration will give a low rate constant measurement. In this work, careful premixing of (CZH6)$b with argon was carried out to minimize this effect. The results given in Table I1 clearly show the low rate constant reported by Nielsen et al. (5) to be in error. As indicated previously, rate data obtained by Harrison and Laxen (4) using smog chamber conditions tend to give rise to slightly higher rate constant values than obtained in this work. Further evidence for problems arising in these complex systems comes from a more recent smog chamber study by Hewitt and Harrison (3). In this work, the decays of (C2H6),Pbwere measured in the same system. The results indicated that the relative rates of the two reactions with OH radicals were 4:l compared to 9:l when their decays were measured separately relative to m-xylene and toluene, respectively, under similar experimental conditions. It is evident from the rate constant data that there is a pronounced increase in reactivity for the tetraalkyl compounds in going from carbon to lead. Reaction with the carbon compounds undoubtedly involves a hydrogen atom abstraction process (8,9):
+
The enhanced reactivity toward OH radical attack of (C2Hd4Ccompared to (CH3),C can be rationalized in terms 1102
Environ. Scl. Technol., Vol. 25, No. 6, 1991
of the presence of secondary hydrogen atoms in the tetraethyl compound. It would seem reasonable to assume that the C-H bond energies in (CH3),Pb and (C2H6),Pb are similar to those in the analogous carbon compounds. The similarity of the C-H stretching frequencies for (CH3)4Cand (CHJ4Pb provides support for this conclusion (10). I t therefore seems unlikely that an increase in the rate of H atom abstraction could be responsible for the increase in reactivity by a factor of -6 in going from (CH3),C to (CH3),Pb and an increase by a factor of -13 for the analogous tetraethyl compounds. It is suggested that a new reaction channel may become important for the lead-containing species and provide an explanation for the marked increase in reactivity of these compounds. Kikuchi et al. (11) demonstrated direct attack on the lead-carbon bond in the reaction of C1 atom8 with (CHd4Pb and (C2H,),Pb, where CHBCland C2H&1, respectively, were formed in significant amounts. No mechanistic details for the reaction were discussed, but the weakness of the C-Pb bond (12) and the availability of d orbitals suggest that a reaction pathway involving attack at the site of the P b atom could be important. Similarly, Niki et al. (13)have shown from an FTIR study that products arising from the OH-initiated oxidation of (CH3)2Hgwere consistent with the displacement reaction OH + (CH3)zHg CH3HgOH + CH3. A number of reaction channels are possible for the reaction of OH with tetraalkyllead compounds, for example, with (CH3),Pb:
-
Reaction 11 represents the simple H atom abstraction process; however, as discussed previously, this process is unlikely to be responsible for the increased reactivity of the tetraalkyllead compounds relative to their carbon analogues. We therefore propose that the dominant reaction channel for the reaction of OH radicals with tetraalkyllead compounds involves addition at the P b atom, reaction 13. Preliminary results from this laboratory have shown that CH30H is not a product of the reaction and hence direct displacement of CH3 by OH in reaction 12 cannot be important. Reaction 13 involves formation of an addition complex followed by either loss of a methyl radical (13a) or rearrangement and elimination of HzO (13b) to give the same reaction products as the direct H atom abstraction process. Information on the relative importance of these various reaction channels must await further product studies. However, an addition process involving attack of the electrophilic OH radical at the P b atom is consistent with the increase in reactivity shown by (C2H6),Pbrelative to (CH3)4Pb. I t would be expected that the increased electron-donating effect of the ethyl groups would favor such an electrophilic addition process. For moderately polluted atmospheric conditions the atmospheric lifetime of tetraalkyllead compounds is likely to be determined by reaction with OH radicals ( 4 ) . Assuming a tropospheric concentration of 1 X lo6 molecules for OH radicals under such conditions, the rate coefficients determined in this work give lifetimes of -50 h for (CH,),Pb and 4 h for (C2H6),Pb. The stability of TML in ambient air is hence high enough to cause transportation over relatively large distances.
Environ. Sci. Technol. 1991, 25. 1103-1 111
(6) Hansen, K. B.; Wilbrandt, R.; Pagsberg, P. Rev. Sci. Instrum. 1979,50,1532. (7) Nielsen, 0.J.; Sidebottom,H. W.; Nelson, L.; Treacy, J. J.; O'Farrell, D. J. Znt. J . Chem. Kinet. 1989,21, 1101. (8) Atkinson, R. Chem. Rev. 1986,86,69. (9) Atkinson, R.J.Phys. Chem. Ref.Data 1989,Monograph 1. Steele, W. V. J. Chem. Thermodyn. 1983,15,595. Kikuchi, M.; Lee, F. S. C.; Rowland, F. S. J . Phys. Chem. 1981,85, 84. (12) McMillen, D. F.; Golden, D. M. Annu. Rev. Phys. Chem. 1982,33, 49. (13) Niki, H.;Maker, P. D.; Savage, C. M.; Breitenbach, L. P. J. Phys. Chem. 1983,87,4978.
Acknowledgments Thanks are due to Jette Munk and the late Preben Genske from the Chemistry Department a t R i s ~for technical assistance.
Literature Cited (1) . , Biaains. P. D.E.; Harrison, R. M. Environ. Sci. Technol.
19?5,13,558. . (2) Hewitt, C. N.; Harrison, R. M. In Organometallics in the Environment; Craig, P. J., Ed.; Longmans: London, 1986; Chapter 4. (3) Hewitt, C. N.; Harrison, R. M. Environ. Sci. Technol. 1986, 20, 797. (4) Harrison, R.M.;Laxen, D. P. H. Environ. Sci. Technol. 1978,12, 1384. ( 5 ) Nielsen, 0.J.; Nielsen, T.; Pagsberg, P. Risa Report R-463; Rim National Laboratory, Denmark, 1982.
Received for review September 19, 1990. Revised manuscript received January28,lBl. Accepted January 29,1991. Financial support was provided by the Commission of the European
Interhemisphere Exchange of Hexachlorocyclohexanes, Hexachlorobenzene, Polychlorobiphenyls, and 1,I,I-Trichloro-2,2-bis(p -chlorophenyl)ethane in the Lower Troposphere Karlhelnz Ballschmlter and Rolf Wlttllngert
Department of Analytical and Environmental Chemistry, University of Ulm, Albert-Einstein-Allee 11, D-7900 Ulm, Germany
rn
Two nearly independent hemispheric compartments exist in the atmosphere for hexachlorobenzene [HCB; south/north (S/N) ratio of 0.061and a-hexachlorocyclohexane (a-HCH; S/N = 0.09), as base-line measurements in both hemispheres indicate. The interhemispheric differences in the profile of the a and y isomers of hexachlorocyclohexane can be explained best by the different HCH products applied worldwide. Both isomers occur in technical HCH, the so-called benzene hexachloride (BHC), which is preferably used in the Northern Hemisphere, and the purified HCH "lindane", consisting of >99% y H C H , which is used worldwide. The distribution of the PCB congeners between gas phase and aerosol particles in dependence of the degree of chlorination is examined in urban air at a temperature of -8 "C, which is close to the global medium temperature of -11 "C at 4000 m. In clean air the interhemispheric contrast, the south/north ratio, is -0.7 or even close to unity for the PCB. 4,4'-DDT is detected at 8 times higher levels in the southern as compared to the Northern Hemisphere. 4,4'-DDE, the transformation product of 4,4'-DDT, follows the same pattern (S/N = 3). The quotient of Henry constant and octanol-water distribution coefficient H/K,,correlates to the S / N ratio of semivolatiles in the troposphere. These results require the conclusion that the interhemispheric exchange of other compounds with similar physicochemical data as given by HCB and a-HCH must be very slow in the atmosphere if it occurs at all at a significant level. This can have far-reaching consequences in the understanding of both the interhemispheric and finally the global distribution of xenobiotics, as the oceanic system has to be discussed as the major long-range transport medium. 1. Introduction
During the last two decades organochlorines were recognized as pollutants to be found in even the most remote t Present
address: BASF-DUU/O,
many. 0013-938X/91/0925-1103$02.50/0
D-7900 Ludwigshafen, Ger-
regions of the globe (1-3)- Dispersion of xenobiotics can occur (1)via trade, (2)via the mass flow in the atmosphere and (3) in the oceans, and (4) by the mobility of biota. The global environmental fate-dispersion, accumulation, and transformation-of chemically stable semivolatile organochalogens, which react little or not all with OH, 0 , or H 2 0 [e.g., 1,2,3,4,5,6-hexachlorocyclohexanes(HCH), hexachlorobenzene (HCB), polychlorobiphenyls (PCB), (DDE)], and l,l-dichloro-2,2-bis(p-chlorophenyl)ethene depends mainly on their mobility in the different environmental compartments. In this paper we discuss the levels and the mobility in the lower troposphere of the a and y isomers of 1,2,3,4,5,6-hexachlorocyclohexane (a-HCH, 7-HCH), hexachlorobenzene, seven principal polychlorobiphenyl congeners, and the DDT group of compounds, l,l,l-trichloro-2,2-bis(p-chlorophenyl)ethane (4,4'-DDT) and 1,ldichloro-2,2-bis(p-chlorophenyl) ethene (4,4'-DDE), under hemispheric and interhemispheric dimensions. In the atmosphere as well as in the oceans the mobility of sparsely water soluble, semivolatile compounds will be mainly regulated by the ratio of particle-bonded to nonparticle-bonded molecules. This ratio is regulated in the atmosphere by the surface of particles offered per volume unit and the mean temperature of the specific environment ( 4 ) . The adsorbed portion will follow the transport routes of the aerosols, which after aggregation are mainly regulated, besides dry deposition, by the process of wet deposition as condensation nuclei together with impact scavanging by the rain droplets. Most authors favor dry sedimentation as the carry-down process for semivolatile compounds in the atmosphere (5-8). In areas and at times of no precipitation, dry sedimentation is surely a correct approach. The discussion of the mobility in the troposphere is based in this paper on results obtained from depositions on snow and air samples of the Northern (9) and Southern Hemisphere (IO), including previous measurements of other authors.
@ 1991 American Chemical Society
Environ. Sci. Technol., Vol. 25, No. 6, 1991 1103
