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Monarca, S.; Meier, J. R.; Bull, R. J. Water Res. 1983, 17, 1015-1026.
Meier, J. R.; Ringhand, H. P.; Coleman, W. E.; Munch, J. W.; Streicher, R. P.; Kaylor, W. H.; Schenck, K. M. Mutat. Res. 1985, 157, 111-122. Ulitzur, S.; Weiser, I.; Yannai, S. Mutat. Res. 1980, 74, 113-124.
Weiser, I.; Ulitzur, S.; Yannai, S. Mutat. Res. 1981, 91, 443-450.
Mantura, R. F. L.; Riley, J. Anal. Chim. Acta 1975, 76, 97-100.
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Barton, D. H. R.; Schnitzer, M. Nature (London)1963,198, 217-218.
American Public Health Association Standard Methods for the Examination of Water and Wastewater, 15th ed.; American Public Health Association: New York, 1980.
(47) Masschelein,W. J. In Chlorine Dioxide; Ann Arbor Science: Ann Arbor, MI, 1979; p 129. (48) Wheeler, G. L.; Lott, P. F.; Yau, F. W. Microchem. J. 1978, 23, 160-164. (49) Maron, D. M.; Ames, B. N. Mutat. Res. 1983,113,173-215. (50) Marnett, L. J.; Hurd, H. K.; Hollstein, M. C.; Levin, D. E.; Esterbauer, E.; Ames, B. N. Mutat. Res. 1985,148,25-34. (51) Visser, S. A. Freshwater Biol. 1984, 14, 79-87. (52) Rav-Acha,Ch.; Blits, R., Environmental Health Laboratory, Hebrew University, Jerusalem,unpublished results, 1985. (53) Reichert, J. K. Arch. Hyg. Bakteriol. 1968, 152, 265-276. (54) Trevors, J. T.; Bursuraba, J. Bull. Environ. Contam. Toxicol. 1980, 25, 672-675.
Received for review February 4,1986. Revised manuscript received September 9,1986. Accepted October 21,1986. M.B.-A. was partially supported by a grant from the Israel Interior Ministry.
Atmospheric Concentrations and Chemistry of Alkyllead Compounds and Environmental Alkylation of Lead C. Nicholas Hewltt
Department of Environmental Science, University of Lancaster, Lancaster LA 1 4YQ, U.K. Roy M. Harrison"
Department of Chemistry, University of Essex, Coichester C04 3SQ, U.K. The atmospheric chemistry of alkyllead compounds was investigated by determining the ratio of alkyllead to total lead in a variety of different air masses. Maritime air was found to contain significantly more alkyllead relative to total lead than continental or urban air. The presence of gas-phase trialkyllead in the rural atmosphere was confirmed, with trialkyllead becoming progressively more important relative to other lead species with increasing distance from urban source areas. On the basis of estimates of the atmospheric lifethnes of the various lead species, it is concluded that these enhanced alkyllead ratios are explicable in terms of the atmospheric chemistry of the various species, with pollutant tetraalkyllead decomposing in the atmosphere to the relatively stable trialkyllead derivatives. It is therefore probably not necessary to invoke the hypothesis of the natural alkylation of lead to explain these enhanced ratios. Notwithstanding this, evidence is also presented from experiments using intertidal sediments both with and without the addition of a labeled lead tracer that indicates that lead(I1) nitrate can be inefficiently alkylated by a sediment system. Introduction Evidence for the environmental formation of alkyllead compounds from inorganic lead is, a t the present time, available from several sources, but despite extensive research is mainly circumstantial in nature. Experiments with environmental media (1, 2) and experiments with chemical systems (3) and environmental monitoring (4) all provide evidence that has been used to support the hypothesis that the alkylation of lead takes place in the environment. We present here the results of a series of experiments and atmospheric monitoring that indicate that the atmospheric chemistry of alkyllead is considerably more complex than was previously believed, with species other than tetraalkyllead (TAL) being present in the gas phase in both urban and rural air. Smog-chamber studies in260
Environ. Sci. Technoi., Vol. 21, No. 3, 1987
dicate that these other alkyllead compounds, the ionic trialkyl and dialkyl species (TriAL and DiAL), are formed from TAL by reaction with hydroxyl (HO). They are more stable in the atmosphere than TAL, which casts doubts on the validity of the use of alkyllead-in-air data as an indicator of environmental alkylation. Notwithstanding this, we have also obtained experimental evidence using intertidal sediments both with and without the addition of a labeled lead tracer that indicates that lead(I1) nitrate can be inefficiently alkylated by a sediment system. Prior to this study little was known of the atmospheric chemistry of alkyllead apart from the concentrations of TAL in the urban and rural atmosphere (5) and the rates of the main TAL decomposition reactions (6). No information was available concerning the nature or atmospheric concentrations of the products of these reactions, and it was assumed that these intermediate compounds (formed in the inevitable decay of TAL to inorganic Pb) were not significant in the atmosphere compared with TAL. Consequently, it was anticipated that measuring the ratios of alkyllead to total P b in different air masses, both in continental air close to urban areas and in maritime air remote from anthropogenic lead sources, would be a suitable method for determining whether or not the natural alkylation of lead takes place in the environment (4).This method assumes that both the gas-phase organic lead and the inorganic lead aerosol have similar atmospheric lifetimes, of the order of a few days. However, we show here that this is not the case. The concentration of particulate lead found in the at mospheric aerosol in rural and maritime regions is now well established,with several recent papers containing extensive data sets of such measurements (7-10).Unfortunately, however, the same cannot be said of the organic lead species for which only a very few data are available for rural air (11-15), and none, to date, have been reported for true maritime air. We present here data on organic lead concentrations obtained at several rural sites in N.W.
0013-936X/87/0921-0260$01.50/0
0 1987 American Chemical Society
England and N.W. Scotland at varying distances from motor-trafficked roads. By relating concentrations to wind direction sectors and backward air mass trajectory analysis, we believe that we are able to distinguish between “rural” and “maritime” air, that is, between air which has recently passed over land and that which has arrived at the sampling site after a long fetch over the sea. Rigorously clean sample collection and analytical techniques were used to minimize the possibilities of sample contamination. Nevertheless, our data do not, of course, represent the concentrations of organic lead to be found in the cleanest oceanic air such as might be expected in the mid South Pacific or other truly remote areas. The data obtained at these sites are interpreted in light of experiments we have carried out on the photochemical decomposition and reaction rates of the alkyllead species. We also present the results obtained from respiration experiments using intertidal sediments, both with and without the addition of a labeled lead tracer. Experimental Section
Gas-Phase Alkyllead and Particulate Inorganic h a d . Air was sampled at two sites near Lancaster in N.W. England, at the Hazlerigg field station of the University of Lancaster in an elevated, rural, very well ventilated position with open aspects in all directions surrounded by pastures (grid reference SD 492573) and a t Cockerham (grid reference SD 451530). This latter site is 1 km west of the nearest road on a lightly trafficked agricultural lane, surrounded by meadows, and is -200 m from the River Lune estuary shore. Two sampling sites were used on the island of Harris in the Outer Hebrides, N.W. Scotland. One was at Husinish (grid reference NA 981118) on the extreme west coast of the island and was as remote as possible from vehicles. The sampling equipment was positioned -400 m upwind (west) of the nearest track and -15 km west of the nearest road carrying more than a few cars per day. The equipment was elevated 20 m above sea level and was positioned such that air arriving from the western half of the compass would do so without receiving any fresh anthropogenic inputs of lead after a long sea passage. Two locations were also used at Luskintyre (grid reference NG 087976) about 500 m from a minor road. Samples were collected and analyzed with equipment and methods already described (12, 16). Particulate P b data were obtained by filtration, acid extraction, and atomic absorption spectroscopy (AAS). Total gas-phase alkyllead (TAL plus any gas-phase ionic species present) was obtained by collection in iodine monochloride solution, a selective extraction procedure, and AAS while TAL was specifically determined by adsorption on a porous polymer followed by gas chromatography (GC)-AAS. Some samples were also collected in IC1 solution following filtration of the air through 0.25 g of iron(I1) sulfate (which removes -50% of any gas-phase ionic TriAL salt present). The differences in concentrations obtained by simultaneous sampling with and without this filter could therefore be used to infer the relative proportions of TAL and ionic TriAL salt in the atmosphere. Entirely separate particulate samples were simultaneously obtained during the 1983 Harris sampling period by another worker using Fluoropore filters in Gelman holders (17). Particulate sodium analyses were carried out on both seta of filters by the same method (flame photometry with a Corning 400 instrument), and good correlation between the air concentrations was found. This provides confidence in the airflow rate measurements and hence in the derived air sample volumes. Blank filter papers, IC1 solutions, and adsorption tubes were transported, handled, stored, and
analyzed in the same way as the samples. No significant contamination of any of the blanks was observed, with minimal, acceptable, blank values being found. These were deducted from the sample concentrations in the normal manner. Laboratory Experiments Using Sediments. Intertidal sediments were taken with plastic core tubes from the River Lune estuary, N.W. England (grid reference SD 456561). Composites of -9 kg were immediately placed in Pyrex containers and subsequently kept in the dark at room temperature. Charcoal-filtered air was passed through the head space above the sediment, filtered through a 0.45-pm Millipore membrane, and then bubbled through 0.1 M IC1 solution or through an adsorption tube packed with a porous polymer. Alkyllead evolved by the sediments was thus collected and either analyzed by dithizone extraction-AAS (16) (giving a measure of the total alkyllead present) or by thermal desorption-GC-AAS (12) (which allows determination of the five TAL species individually). Four sediment vessels and a fifth “blank” collector were used in parallel, utilizing the same filtered air supply. Experiments were also carried out in which labeled lead(I1) nitrate solution containing 210Pbwas added to sediments in the apparatus described above. A Millipore 0.45-pm filter in a stainless steel holder with a Teflon gasket was placed between each container and its collection bubbler in order to prevent the transfer of particulate or liquid 210Pb(N03)2to the IC1 solution. This was an additional safeguard as the selective extraction procedure would, in any case, prevent interference from nonalkylated labeled lead. An activity of 9.678 MBq of 210Pb(N03)2 in 100 cm3 of deionized water was added to each 9-kg mass of sediment and well mixed. Gas-phase organic lead was collected and extracted as above and a 20-pL aliquot of the final acid solution used for lead analysis by flameless AAS. The zloPb activity of the bulk of the acid solution was then determined by y spectroscopy [using an Intertechnique IN90 with NaI(T1) detector] and the amount of zloPbpresent determined by comparison with standards prepared in the same medium. A blank experiment was carried out by counting the y activity of the organic lead evolved from a set of sediments to which no labeled lead had been added. These sediments were quite productive, three out of four releasing alkyllead, but no 210Pbactivity was detected from any. Results and Discussion
R u r a l Air Sampling (Easterly Air Masses). Simultaneous sampling was carried out at Hazlerigg and Cockerham in April 1984 during a 7-day period of wellestablished easterly (continental) airflow over the U.K. The results obtained are summarized in Table IA and shown in Figure 1. Mean particulate P b concentrations of 115 and 101 ng of P b m-3 were found at Hazlerigg and Cockerham (with ranges of 57-213 and 52-207 ng of P b m-3, respectively), in close accord with values found previously at these sites (18). Both sites had very similar mean P b concentrations, indicating that local sources of particulate P b were unimportant. This was also the case for the mean total organic lead and TAL concentrations, these being 2.4 and 3.3 ng of Pb m-3 for total organic lead (with ranges of 1.1-5.3 ng of P b m-3 at Hazlerigg and 1.1-6.2 ng of Pb m-3 at Cockerham) and 0.7 and 0.8 ng of P b mb3for TAL (with ranges of 0.4-1.5 ng of P b m-3 at Hazlerigg and 0.3-1.8 ng of P b m-3 at Cockerham). The mean ratios of organic Pb/total P b found at the two sites were also in close agreement, being 2.7 and 3.5% for total organic Pb/total P b and 0.6 and 0.9% for TAL/total Pb Environ. Sci. Technol., Vol. 21, No. 3, 1987
281
Table I. Atmospheric Lead Concentrations Obtained at Various Sites in
N.W.England and N.W.Scotland
Part A April 1984 (7 days easterly air mass) Hazlerigg Cockerham mean range mean range P b aerosol, ng m-3 total alkyllead, ng m-3 TAL, ng m-3 alkyllead/total Pb, % TAL/total Pb, %
51-213 1.1-5.3 0.4-1.5 0.6-7.2 0.1-1.1
115 2.4 0.7 2.7 0.6
101 3.3 0.8 3.5 0.9
52-208 1.1-6.2 0.3-1.8 0.5-6.7 0.1-1.6
June 1984 (18 days westerly air mass) Hazlerigg Cockerham mean range mean range 56 2.9 0.7 5.3 1.5
6.3-89 0.4-13.1 0.2-2.0 0.5-19.5 0.4-4.1
24 2.8 0.8 13.1 4.3
5.7-76 0.9-11.6 0.2-1.8 3.3-37.9 1.2-10.0
Part B Harris 1983 (n = 19) mean range
Harris 1984 (n = 24) mean range
16.4 3.2
1.3-45 1.0-9.5
22
4-48
16.5 7.3 0.1 32
P b aerosol, ng m-3 total alkyllead, ng TAL, ng m-s alkyllead/total Pb, % TAL/total Pb, %
3.9-36 0.7-12.4
