lXQ rT
Physicochemical Characterization of Lead in Urban Dusts. A Microanalytical Approach to Lead Tracing R. W. Linton Department of Chemistry, University of North Carolina, Chapel Hill, N.C. 27514
D. F. S. Natusch” Department of Chemistry, Colorado State University, Fort Collins, Colo. 80523
R. L. Solomon Institute for Environmental Studies, University of Illinois, Urbana, 111. 61801
C. A. Evans, Jr. School of Chemical Sciences and Materials Research Laboratory, University of Illinois, Urbana, 111. 6 1801
Scanning electron microscopy associated with energy dispersive X-ray analysis is used to identify the sources of leadcontaining particles in urban dust. The identities of individual particles (automobile exhaust, paint chips) are established on the basis of characteristic particle morphology and chemical composition. Lead source tracing is greatly facilitated by physically separating and preconcentrating lead-containing particles, but quantitation of the contributions of specific sources requires the combined use of individual particle analysis and determination of lead in physically fractionated dust samples. It is established that automobile exhaust particles are the main contributors of lead to roadway dusts and that they also contribute substantially to dusts collected in the vicinity of buildings having lead-painted trim and situated a t some distance from a roadway. The utility of this microscopic approach to lead tracing is assessed. The majority of lead poisoning cases currently encountered in the United States involve young children who are more frequently exposed to sources of available lead than adults, and are more susceptible to lead intoxication (1).The literature suggests that children who exhibit elevated blood lead concentrations most commonly accumulate lead by ingestion of lead-containing paint chips, street dust, soil, and various other nonfood wbstances ( 2 ) .However, the evidence implicating a given lead source is usually circumstantial, and the primary sources and transport process responsible for the acquisition of lead by children have not been clearly established. Settled indoor and outdoor dusts present in urban areas have been shown to contain considerable amounts of lead and have been suggested as important environmental sources to which children have access ( I , 3 ) . For example, repeated ingestion of as little as 0.4 g of street dust containing 2000 pg/g of lead can eventually result in clinical lead poisoning in young children ( I ) . Furthermore, there is considerable evidence that a relationship exists between elevated blood lead levels in children and high lead levels in local outdoor dusts and soils (4-7).
A number of studies have attempted to establish the origins of lead found in urban dusts (3-13);however, to date none has 0013-936X/80/0914-0159$01.00/0
provided definitive information. The procedures employed can be classified into these groups: Determination of the topographic distribution of lead in dusts and soils so as to establish concentration profiles which relate sources and exposure points (3,5, 6,8, 9). Determination of relationships between lead and one or more “tracer” elements characteristic of a specific source of lead (10-13). Determination of lead isotope ratios so as to identify the contribution of one or more sources having characteristic lead isotope ratios. Topographic distribution studies are useful for the purpose of source identification in selected areas where only one source of lead is present or where potential sources are well defined (5-9). However, such studies are unable to assess the relative contributions of lead-containing dust particles from different sources and frequently provide equivocal information ( 3 ) . Pairwise correlations between bulk concentrations of lead and a tracer element in dusts, can, in principle, provide both qualitative and quantitative definition of lead sources. However, such definition requires that the tracer be unique to a given source, and that it maintains an invariant relationship with lead. Unfortunately, these criteria are rarely fulfilled. For example, the concentration ratios Br/Pb and Fe/Pb have been used in attempts to distinguish automotive and industrial sources of lead, respectively (10-13). However, while bromine is fairly unique to automobile exhaust particles in soils and dusts (though not in biological systems), there is considerable evidence that suggests that bromine is lost from such particles following emission (14-19). Similarly, the use of iron as a tracer for industrial lead is discredited on the grounds that automobile exhaust particles often have an extremely high iron content (14, 20, 21), and iron is a major constituent of many soils. It should, however, be noted that the use of pattern recognition techniques to match multielemental patterns in environmental and source samples can provide remarkable insight into both the identities and contributions of different lead sources ( 2 1 ) . The utility of lead isotope ratio determination for source tracing requires that contributing sources have a sufficiently unique isotope ratio to enable their distinction. This requires that the original lead associated with each source be of dif-
@ 1980 American Chemical Society
Volume 14, Number 2, February 1980 159
ferent geochemical origin. In cases where these criteria are fulfilled quite spectacular source definition can be achieved (22);however, the necessary uniqueness of lead isotope ratios is rarely encountered in those sources (e.g., paint, automobile emissions), which are normally expected to contribute lead to urban dusts. It is apparent from this brief summary that successful identification and quantitation of an environmental source of lead (or of any other pollutant for that matter) require that one or more distinguishing characteristics be associated with each source. These characteristics must, of course, be observable and measurable in a highly heterogeneous composite sample such as street dust. In this paper we describe an approach to environmental lead tracing, which, in essence, combines two methods for assigning distinguishing characteristics. The first involves fractionation of each dust sample into a number of subsamples each of which has distinct particle size, density, and ferromagnetic characteristics. This results in significant physical separation of lead-containing particles derived from different environmental sources. The second method involves scanning electron microscopic examination and energy dispersive X-ray analysis (SEM/EDS) of individual particles in each subsample. This enables the morphological and chemical characteristics of individual lead-containing particles to be determined and related to those observed in source particulates. The extent to which this procedure can preconcentrate lead-containing particles and its ability to identify and quantify the sources of such particles is described in the following sections.
Experimental Sample Collection. Two types of settled outdoor dusts were collected in the small, nonindustrial community of Champaign-Urbana, Ill. (population 100 000). The curb sample was a composite of dust collected from the curb at four similar sites characterized by moderately high traffic density (12 000 to 20 000 cars per day). The building line sample was collected from the ground next to a three-story brick building directly facing and 50 ft removed from a major street (20 000 cars per day). The painted window trim of this building contained approximately 14% lead by weight. All samples were collected from selected test areas of 0.5 m X 0.5 m using techniques described previously ( 3 ) . Sample Fractionation. Both the curb and building line samples were fractionated sequentially according to the scheme illustrated in Figure 1.The following procedures were employed. Samples were fractionated into four physical size fractions (600 pm) sieve was discarded. Sieved fractions were next separated into nonmagnetic and magnetic fractions by repeated passage through a vertical glass tube held between the poles of a 3-kG electromagnet. (It will be noted that the designation of particles as being magnetic or nonmagnetic is thus entirely a function of the separation method employed.) Finally, each subsample was further separated into three particle density fractions (3.3 g/cm3) using chloroform and diiodomethane and a mixture thereof as flotation liquids. The resulting sample set contained 24 subsamples, although several of these were subsequently recombined to obtain sufficient material for bulk analysis. Multielemental Analysis. Portions of each separated dust fraction were subjected to elemental analysis using atomic absorption spectrometry (for Pb) and instrumental neutron activation analysis (for Al, Ag, Ba, Br, Ca, Ce, Cs, Cr, Co, Dy, 160
Environmental Science & Technology
I
BULK SAMPLE
I
OF URBAN DUST
/ PHYSICAL SIZE SEPARATIONS
DENSITY SEPARATIONS
/I\
I
/I\
QUANTITATIVE ANALYSIS FOR LEAD BY AA I
I
V
V
I 'I'
SUBSAMPLES (INCLUDING THOSE WITH HIGH LEAD CONCENTRATIONS) CHOSEN FOR FURTHER STUDY
Quoiitative Elemental Analysis and Morphology of Individual Particles
Quantitative Multielemental Bulk Analysis
Identification of Major Compounds (Including Laad Species)
Figure 1. Flow chart for urban dust fractionation
Eu, Ga, Hf, Hg, Fe, K, La, Lu, Mn, Na, Ni, Rb, Sb, Sm, Sc, Se, Sr, Tb, Th, U, Yb, Zn, and Zr). Specific analytical procedures have been reported elsewhere ( 3 , 2 1 ) . Scanning Electron Microscopy/Energy Dispersive X-ray Spectrometry (SEM/EDS).A JEOL JSM-U3 scanning electron microscope (SEM) equipped with an Ortec Si(Li) detector and Model 6200 multichemical analyzer was employed for morphological examination and X-ray analysis of individual dust particles. Particles were mounted on aluminum stubs with double-sided adhesive tape and were subsequently coated with a thin layer of carbon to render them electronically conducting for SEM observation. X-ray analyses were performed with a 25-keV electron beam using two different modes of operation. First, the electron beam was rastered over a field of particles, and the X-ray emission characteristic of a single element was monitored. This raster mode of operation is useful for locating leadcontaining particles and for establishing the interparticle distribution of selected elements. Once lead-containing particles have been located, the electron beam is focused on a single particle and held stationary while an X-ray spectrum is recorded in the spot mode. In the present study spot mode spectra were acquired for 200-s live counting time, which enabled detection of elements present a t concentrations greater than about 1%by weight within the analytical volume. X-ray Powder Diffraction (XRPD). X-ray powder diffraction patterns were recorded for those subsamples containing substantial amounts of lead. Particles were ground to sizes of a few micrometers in cross section, mounted on a glass slide, and irradiated with a monochromatic beam of copper ( K a )X-rays using a Phillips Norelco X-ray powder diffractometer. The intensities of diffracted X-rays were measured between 10 and 60' 20 angles using a proportional counter.
Results and Discussion Distribution of Particulate Lead. The overall fractionation and analysis scheme employed in this study is illustrated in Figure 1.The resulting distribution of lead concentration and mass determined for the curb and building line samples are presented in Tables I and 11, respectively. I t is apparent
Table 1. Preconcentration of Lead in Urban Dust by Size, Magnetic, and Density Separations for the Curb Sample density, g/cm3 size, prn
51.5b
1.5-3.3
>3.3b
Lead Concentrations, ppm by W t a 250-600 45-250 3.3 g/cm3) in the curb sample (86'%)than in the building line sample (59%).This was expected since the densities of automobile exhaust particulates are generally somewhat greater than those of lead paint chips (-3 g/cm3) though both are substantially greater than natural crustal dusts ( 2 3 ) .The presence of lead a t concentrations substantially higher than natural crustal abundance ( 2 4 ) in fractions of density 3.3 glcm"), nonmagnetic. laree size (90-600 um) fraction were identified as paint is approximately
I
c (1 FIgure 3. SEMIEDS elemental maps for building line: nonmagnetic. >3.3 g/cm3, 90-600 pm (subsample E in Table 111) 1
Figure 4. SEM/EDS f?lementalmaps for paint chips obtained from the window trim of the t)uilding
(4) Fairey, F. S., Gray, J. W., J . Sci. Med. Assoc., 66,79 (1970). (5) Ter Haar, G., Aronow, R., Enuiron. Health Perspect., 7, 83
blad, V., Creason, J., Lagerwerff, J. V., Ferrard, E. F., paper presented at the 100th Annual Meeting of the American Public Health Association, Atlantic City, N.J., 1972. (9) Bodgen, J. D., Louia, D. B., Bull. Enuiron. Contam. Toxicol., 14, 289 (1975). (10) Wedberg, G. H., Chan, K., Cohen, B. L., Frohlinger, J. O., Enuiron. Sci. Technol., 8, 1090 (1974). (11) Bowman, H. R., Conway,J. G., Asaro, F., Enuiron. Sci. Technol., 6,558 (1972). (12) Rhodes, J. R., Pradzynski, A. H., Hunter, C. B., Payne, J. S., Lindgren, J. L., Enuiron. Sci. Technol., 6,922 (1972). (13) Paciga, J. J., Roberts, T. M., Jervis, R. E., Enuiron. Sci. Technol., 9,1141 (1975). (14) Olson, K. W., Skogerboe, R. K., Enuiron. Sci. Technol., 9,227 (1975). 115) Ter Haar. G.. Bavard, M., Nature (London).232,553 (1971). (16) Boyer, K. W.’, La-itinen, H. A , , Enuiron. Sci. Techno/., 8, 1093 (1974). (17) Moyers, J. L., Zoller, W. H., Duce, R. A,, Hoffman, G. L., Enuiron. Sci. Techn’ol.,6,68 (1972). (18) Pierrard, J. M., Enuiron. Sci. Technol., 3,48 (1969). (19) Robbins, J. A., Snitz, F. L., Enuiron. Sci. Technol., 6, 164 (1972). (20) Lamb, R. E., Ph.D. Thesis, University of Illinois, Urbana, Ill., 1975. (21) , . Hoake. . P. K.. Lamb. R. E.. Natusch. D. F. S., Enuiron. Sci. Technol.,’followingpaper in this issue. (22) Rabinowitz, M. B., Wetherhill, G. W., Enuiron. Sci. Technol., 6, 705 (1972). (23) McCrone, W. C., Delly, J. G., “The Particle Atlas”, Ann Arbor Science, Ann Arbor, Mich:, 1973. (24) Mason, B., “Principles of Geochemistry”, Wiley, New York, 1966. (25) Linton, R. W., Ph.D. Thesis, University of Illinois, Urbana, Ill., 1977. (26) Joint Committee on Powder Diffraction Standards, “Powder Diffraction File-Inorganic Compounds”, Swarthmore, Pa., 1976. (27) Keyser, T. R., Natusch, D. F. S., Evans, C. A., Jr., Linton R. W., Environ. Sci. Technol., 12,768 (1978).
(1974). (6) Lepow, M. L., Bruckman, L., Gillette, M., Markowitz,S.,Robino, R., Kapish, J., Enuiron. Res., 10,415 (1975). (7) Strehlow, C. D., Barltrop, D., Pediatri. Congr. Int., 14th, II(3), 173 (1975). (8) Pinkerton, C., Hammer, K. E., Hinners, T.,Kent, J. L., Hassel-
Received for reuieu: J u l y 13, 1977. Resubmitted October 14, 1978. Accepted January 15,1979. R. Linton u:as the recipient ofa National Science Foundation energy-related Graduate Traineeship. This research UQS supported by National Science Foundation Grants ERT 74-24276 and DMR 76-01058.
of surface-associated material, which is often highly characteristic of particles derived from a specific source. Thus, automobile exhaust particles have been shown to contain the elements Br, C1, Cr, Mn, Ni, P, Pb, and T1 on their outer surface (25,27). In terms of the actual results obtained, the most important conclusions are that lead derived from automobile exhaust particulates can contribute significantly t o the total lead present in soils and dusts a t a considerable distance from a roadway. Otherwise, it would appear that roadway dusts themselves are likely to contain lead that is almost exclusively derived from automobile exhausts. Acknowledgment The technical assistance of the following individuals is gratefully acknowledged: Professor P. K. Hopke for assistance with INAA procedures; Mr. J. Hartford for sample collection and separations; Professor J. F. Young for access to XRPD instrumentation; Professor H. A. Laitinen for supplying samples of the auto exhaust particles; the technical staff of the University of Illinois Institute for Environmental Studies for assistance with AA and INAA determinations; and the University of Illinois Center for Electron Microscopy for access to SEM/EDS instrumentation. Literature Cited (1) National Academy of Sciences, “Lead: Airborne Lead in Per-
spective”, Washington, D.C., 1972.
(2) Kinnison, R. R., Enuiron. Sci. Technol., 10, 644 (1976). (3) Solomon, R. L., Hartford, J. W., Enuiron. Sei. Technol., 10,773
(1976).
Multielemental Characterization of Urban Roadway Dust Philip K. Hopke Institute for Environmental Studies, University of Illinois, Urbana, Ill. 61801
Robert E. Lamb’ and David F. S. Natusch2* School of Chemical Sciences and Institute for Environmental Studies, University of Illinois, Urbana, 111. 61801
It has now been well established that aerosols and deposited dusts found in urban areas are substantially enriched in many potentially toxic trace elements by comparison with those found in nonurban areas ( I , 2). Consequently, people who reside in urban locations are exposed to much larger amounts of potentially hazardous elements than their rural counterparts. Several studies have been conducted (3-6) to determine the composition of particles derived from those sources which may contribute trace elements to urban aerosols and a number of attempts have been made to relate the observed elemental concentrations in collected aerosol samples to these sources (7-14). However, there still remains considerable uncertainty with respect to the origins of elements such as As, Cd, Cr, Pb, Mn, Ni, and Zn, which are known to be enriched in several ~
Present address, Department of Chemistry, Ohio Northern University, Ada, Ohio 45810. Present address, Department of Chemistry, Colorado State University. Fort Collins, Colo. 80523. I
164
Environmental Science & Technology
types of deposited dusts encountered in the urban environment (15-17). Since such dusts may be inhaled, following reentrainment into the air (16, I 8 ) , ingested by children (19), or removed to the aquatic environment by runoff following precipitation ( 2 0 ) ,it is of some importance to determine the sources from which these dusts are derived. Of all of the types of dusts found in the urban environment, one of those most highly enriched in toxic trace elements is roadway dust. Lead concentrations have been reported ( I 7) to reach the percent level in such dusts and several other elements, including Cr, Mn, Ni, and Zn, are present a t disturbingly high levels (16). It is the purpose of this present work, therefore, to investigate one approach to the identification of sources that contribute potentially hazardous elements to roadway dusts. Our approach to elemental source tracing is based on the assumptions that particles derived from a given source exhibit distinguishable size, density, and ferromagnetic characteristics, and that they also exhibit distinguishable interrelation0013-936X/80/0914-0164$01 .OO/O
@ 1980 American Chemical Society
