Environ. Sci. Technol. lQQ2,26, 1354-1360
Seasonal Variation of the Ionic Alkyllead Species Composition of Rainwater Rudy J. A. Van Cleuvenbergent and Fred C. Adams" Department of Chemistry, University of Antwerp (UIA), Universiteitsplein 1, B-2610 Wilrijk, Belgium
A comprehensive long-term survey of the concentration levels of ionic alkyllead species in rainwater at a residential location was carried out, relying on a funnel-in-bottle sampler and a well-assessed analytical procedure based on extraction, derivatization, and gas chromatography-atomic absorption spectrometry (GC-AAS). During the 1.5-year monitoring program (1986-1987), the total dissolved ionic alkyllead content of the samples never exceeded 230 ng L-l Pb and averaged 45 ng L-l Pb. The relative concentrations of the four major species PbMe3+, PbMe22+, PbEt3+,and PbEtz2+corresponded roughly to 4:1:2:2; the mixed methylethyllead species contributed only marginally. From the measurements, the local annual wet deposition of ionic alkyllead could be estimated at -32 pg m-2 year-l Pb. Interestingly, a seasonal variation characterized by lower concentrations in summer, which probably reflect a higher photochemical degradation rate in the atmosphere, was observed; the ethyllead species most clearly underwent this fluctuation. The results are discussed within the framework of previous studies, at this and other locations, of organolead in wet atmospheric deposition. Introduction During the past decade it became gradually recognized that exploring and monitoring the concentration levels of the various ionic organolead species in the environment would be of major importance for a double reason: to elucidate a possible (bio)geochemicalcycle of lead and to point out possible adverse effects, on man and the environment, associated with the widespread use of tetraalkyllead (TAL; PbR4,R = methyl or ethyl) compounds as antiknock agents in gasoline ( I ) . Man-made alkyllead enters the environment in the form of tetraalkyllead and probably also ionic alkyllead (IAL; PbR3+ and PbR22+), emitted into the atmosphere or spilled in the aqueous and terrestrial environment. In both cases the original TAL pollutants have a limited lifetime of a few days at most. Conversion, primarily by reaction with hydroxyl radicals, to ionic trialkyllead (triAL) and dialkyllead (diAL) species extends the atmospheric lifetime and allows transport of atmospheric organolead, mainly in the gaseous phase, to remote areas before complete breakdown to inorganic lead has occurred (2). Aqueous TAL degrades rapidly to trialkyllead, especially under the influence of light, and finally also yields inorganic lead ( 3 , 4 ) . Since the intermediate ionic alkyllead compounds differ from the precursor TAL species in that they are highly water-soluble,wet deposition of these ionic breakdown products could be assumed to be an important sink mechanism for atmospheric alkyllead. At the start of our studies on the significance and fate of ionic organolead compounds in the biosphere, specific data about the occurrence of tri- and dialkyllead compounds in environmental water hardly existed, mainly because most of the analytical methods developed could not detect all the species at environmentally relevant concentration levels. The breakthrough of "hyphenated" speciation techniques enabling their sensitive and specific 'Present address: VITO, Environmental Department, Boeretang 200, B-2400 Mol, Belgium. 1354 Environ. Sci. Technoi., Vol. 26, No. 7, 1992
determination, often based on a chromatographic separation followed by an atomic spectrometric determination, paved the way. One of the first detailed environmental data sets resulted from our first rainwater sampling campaign in the spring of 1984 (5). It covered widely differing sampling sites and provided evidence on a gradual decrease of the ionic alkyllead concentration from central city streets to a nature reserve in a rural area, though less pronounced than in the case of atmospheric tetraalkyllead (6); this agrees with a larger lifetime of the ionic degradation products. Subsequent measurements by other research groups (7-9) provided additional information on the typical abundance and species distribution at a variety of locations. In this article, the results are summarized of a continuous 1.5-year sampling campaign of rainwater at a residential location carried out to get a better insight into the fluctuation of the ionic alkyllead concentrations throughout the year and to elucidate possible seasonal trends. The need for such an extensive program, despite all previous studies, was recognized explicitly after a report by Faulstich and Stournaras (10) claiming potentially toxic trialkyllead concentrations (up to 62 pg L-' Pb) in rainwater from the Black Forest region, FRG. Though their measurements were questioned (11,12),it was nevertheless striking that only 14 out of a series of 51 consecutive samples contained elevated (and thus by bioassay detectable) trialkyllead levels. In the dispute, Faulstich et al. used the lack of long-term consecutive measurements at that time as an argument to support the plausibility of their conclusions (13, 14). Design of the Experiments Sampling. The sampling method employed for collection of rainwater was the same as described earlier by De Jonghe et al. (15) and selected for our previous sampling campaigns, so as to enable a straightforward comparison of data. The collection was thus carried out with 10-L high-density-type polyethylene containers provided with 0.43-m-diameter polyethylene funnels. This ensures a reasonable protection of the collected water from daylight until analysis and in this way effectively retards photochemical degradation of the analytes (16). No preservatives were added at the start of sampling. In this context it should be noted that Radojevic and Harrison adopted a similar, but glass, funnel-in-bottle sampling arrangement and reported that dilute ionic alkyllead solutions show no appreciable adsorption on the container walls (7); our experience from preliminary experiments pointed in the same direction. Our monitoring program covered the period between January 13,1986, and August 3,1987. The container of the funnel-in-bottle sampler was usually changed on a weekly basis (on Mondays at 13 h), but on several occasions (holidays, etc.) remained in the field for longer periods. The entire campaign, in this way, consisted of 73 sampling periods. As the sampling location, the roof of a ca. 15-mhigh building at the UIA campus was selected; it had also been used satisfactorily in the previous campaigns and was preferred mainly for practical reasons (always and easily accessible during working hours and less subject to acci-
0013-936X/92/0926-1354$03.00/0
0 1992 American Chemical Society
dental disturbance than public sites). Generally the samples were processed within 48 h; if this was not possible, they were stored in a deep freezer and analyzed at the earliest convenience. Prior to extraction, the samples were filtered on a type HA Millipore filter of 0.45-pm pore size. The starting volume of rainwater from which the analytes were extracted depended on the amount of wet deposition collected; whenever possible a 1.5-L sample was used. For the majority of the samples, a portion was acidified and stored in a polyethylene bottle at room temperature pending determination of the total dissolved lead by AAS after wet acid digestion. The sampling apparatus was thoroughly cleaned and rinsed with deionized water after each sampling period, and the containers were periodically soaked in dilute nitric acid. Although the rainwater collector described is designed to catch wet deposition, in addition, an uncontrollable fraction of the dry fallout may be sampled. This way of atmospheric removal is likely to be more important for the diAL species than for the triAL compounds, for their lower volatility can be assumed to promote a more ready condensation on particulate matter. As apparent from earlier experiments (5),most probably, however, the dry fallout is largely retained on the funnel surface, and any organolead present at this location can be expected to be broken down rapidly by the action of direct sunlight. Analytical Methods. The sensitive analytical methodology optimized in our laboratory (17, 18) enables the detection of the triAL and diAL species in water at a concentration as low as a few hundreds of picograms per liter for 5-L samples. A complete speciation including tetraalkylleads could be realized by determining the latter compounds separately with the gas chromatography-atomic absorption spectrometry (GC-AAS) system after preextraction of the sample with hexane. The inorganic lead content, on the other hand, could be estimated independently via a total (dissolved) lead measurement by flame or graphite furnace AAS (GFAAS), depending on the concentration and sample volume available, after wet acid (HN03/HC104)digestion, according to laboratory procedures which have proven their validity (19). A simultaneous determination of inorganic lead as tetrabutyllead during the GC-AAS analysis appeared both inconvenient and a source of interference in the quantification of the ionic alkylleads (18). We did not, however, monitor systematically the tetraalkyllead species in the samples; some of them, including unfiltered and freshly collected rain after showers, were preextracted with hexane, and no TAL compounds were detected in the extract using GC-AAS, with detection limits around 20 ng L-' Pb. Moreover none of the numerous natural water samples investigated so far by several groups was found to contain tetraalkyllead (16, 20-22), with claimed detection limits for hexane-extractable lead as low as 1 ng L-l Pb, except for a report that demonstrated occasional TAL contents in rain of up to ca. 90 ng L-l P b (23). The most probable reason why TAL compounds are not regularly observed in rainwater might be the rapid (within 48 h) and quantitative conversion into the triAL form (7), whereas typical rainwater collectors are left in the field for periods of days to weeks. As we could not detect TAL in the funnel-in-bottle samples or in freshly collected rain (from a roof) during a shower, another reason could be the lower solubility of TAL compounds in water than of triAL and diAL species. Finally, TAL compounds present in water are well-known to be easily subject to, largely unpredictable, adsorption processes onto particulate matter or onto the container walls
(24). Harrison and Radojevic carried out the extraction directly in the sampling bottles without prior filtration. The optimized procedure for speciation of ionic alkyllead compounds in water is described in full detail elsewhere (5, 17, 18). I t relies on a preconcentration of the species by extraction as dithiocarbamate complexes, enrichment in a microvolume of nonane, followed by conversion with n-butyl Grignard reagent to tetraalkyllead derivatives and a subsequent specific measurement by gas chromatography-quartz furnace atomic absorption spectrometry. Whenever higher volumes were analyzed (1.0 or 1.5 L) than the 0.5-L sample for which the procedure was originally ( 5 ) described, the quantity of reagents in the extraction step was accordingly multiplied by a factor 2 or 3. If a 1.5-L sample intake was considered still insufficient, several batches were extracted and the pentane extracts recombined prior to enrichment and derivatization;the latter steps were always carried out following the unmodified procedure. To verify the quantitative recovery of the ionic alkylleads from 0.5- and 1.5-L rainwater samples, duplicates were analyzed of which one had previously been spiked with a mixture of four standard species. The recovery efficiency of the procedure for PbMe3+,PbMe22+,PbEb+, and PbEt:+ averaged 97%, 99%, 95% and 93%, respectively, for a spiking level of ca. 95 ng L-l Pb of each species (average of three replicates). With a lower spiking concentration of ca. 40 ng L-l P b of each species, the yields remained quite satisfactory (95%, 91%, 94%, and 91% for the respective species, average of two replicates). Furthermore, supplementary validation experiments with respect to storage of both samples and extracts, as well as concentration changes due to filtration or evaporation, were included in earlier publications (5, 18).
Results and Discussion 1. Synopsis of the Measured Concentrations and Discussion. A full compilation of the individual results of the 57 samples analyzed is presented in Table I. During 12 of the intermediate periods not mentioned in Table I, no precipitation could be collected because of dry weather conditions within the entire period; the samples of the 4 remaining periods (39, 48, 49, and 52) could not be analyzed for diverse reasons. Table I1 contains some summarizing statistics on the individual alkyllead species, the total tri- and dialkyllead and ionic alkyllead, the total dissolved lead, and the ratio of the two latter variables. These statistics include the sample size, the (arithmetic) mean and median, the standard deviation, and finally the range. From the point of view of alkyllead abundance and distribution, the following statements briefly highlight the main outcome of this long-term rainwater sampling campaign: (a) During the 1.5-year monitoring program, the total dissolved ionic alkyllead content of the samples never exceeded 230 ng L-l P b and averaged 45 ng L-' Pb. (b) Trialkyllead compounds contributed about twothirds to the total ionic alkyllead burden, and dialkyllead compounds about one-third. (c) Generally speaking, trimethyllead was the principal ionic alkyllead species in rainwater. (d) The relative concentrations of the four major species, PbMe3+, PbMe22+,PbEt3+, and PbEt22+,corresponded roughly to 4:1:2:2; the mixed methylethyllead species contributed only marginally. (e) From the measurements, the annual wet deposition of ionic alkyllead at the suburban site can be estimated at -32 pg m-2 year-l Pb. Environ. Sci. Technol., Vol. 26, No. 7, 1992
1355
Table I. Concentrations of the Individual Ionic Alkyllead Species and of Total Dissolved Lead in t h e Rainwater Samples Collected in 1986-1987
no.
period
1 2 3 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 25 26 27 28 29 30 31 32 33 34 38 40 41 42 43 44 45 46 47 50 51 53 55 56 57 59 60 62 63 64 65 66 67 68 69 71 72 73
1301-200186 2001-270186 2701-030286 0303-100386 1003-170386 1703-240386 2403-310386 3103-070486 0704-140486 1404-210486 2104-280486 2804-050586 0505-120586 1205-200586 2005-260586 2605-020686 0206-090686 0906-160686 1606-240686 3006-070786 0707-140786 1407-280786 2807-040886 0408-180886 1808-250886 2508-010986 0109-080986 0809-150986 1509-220986 1310-211086 2710-031186 0311-121186 1211-171186 1711-251 186 2511-011286 0112-081286 0812-151286 1512-050187 0302-090287 0902-170287 2302-020387 0903-160387 1603-230387 2303-300387 0604-130487 1304-210487 2704-040587 0405-180587 1805-250587 2505-010687 0106-090687 0906-150687 1506-230687 2306-300687 1307-200787 2007-270787 2707-030887
precip, L rn+ 41.32 39.60 3.10 20.31 2.07 40.63 59.56 5.51 4.82 39.60 12.74 13.08 11.71 5.92 10.33 16.94 66.31 6.61 40.49 16.87 11.22 3.99 2.82 5.85 25.48 38.70 13.08 17.49 24.24 26.10 32.92 11.36 3.37 60.39 4.48 14.94 13.77
ndb 8.19 19.63 43.45 1.10 36.29 44.76 15.08 3.17 7.02 39.53 18.59 8.95 63.77 11.22 39.53 9.02
nd 63.28 13.70
PbMe3+ 9.2 4.0 21.3 27.1 63.3 11.5 8.0 9.9 30.1 20.3 14.1 10.0 35.5 32.2 10.5 21.3 2.3 7.5 12.3 33.8 29.3 26.4 28.4 12.3 15.9 18.0 3.1 12.6 3.3 6.9 10.9 28.5 29.0 9.5 27.1 17.4 17.4 10.6 39.8 27.3 10.7 90.6 14.7 14.2 33.3 18.6 16.8 10.6 3.7 44.1 12.6 17.2 14.3 21.9 3.7 4.7 24.7
PbMezEt+
concn," ng L-' Pb PbMeEtzt PbMe?+ PbEt,+
1.8 1.0 2.6 0.7 3.7 0.8 0.6 1.1 2.1
0.6 0.5 1.9 0.6
