CURRENT RESEARCH Identification of Soil Lead Compounds from Automotive Sources Kenneth W . Olson and Rodney K. Skogerboe* Department of Chemistry, Colorado State University, Fort Collins. Coio. 80521
Gradient density and magnetic separation procedures are described for the preconcentration of lead compounds from automotive sources in soil. These procedures have been used to remove the bulk of the soil matrix from the lead compounds, thereby increasing the applicability and reliability of X-ray powder diffraction as a means of compound identification. The primary lead compounds in eight different lead-contaminated soil samples have been identified through the use of the combined separation and diffraction procedures. Lead sulfate has been determined to be the principal constituent in the soils examined. An examination of possible reactions that could account for the presence of lead sulfate as the major entity is presented in conjunction with supporting circumstantial evidence from the literature. Lead exhausted from automobiles occurs primarily in particulate form (1-6). These reports agree that lead bromochloride (PbBrC1) and ammonium chloride complexes thereof comprise the major portion of the particulate lead compounds in fresh auto exhaust. In the atmospheric transport of these particulates, the lead concentration is reduced through dilution of the exhaust envelope and through fallout. Measurement of the lead content of soil as a function of distance from a thoroughfare reflects the dilution-fallout processes and provides information about the atmospheric mobility of lead. Several reports have indicated that there has been significant contamination of soil by lead originating from vehicular traffic (7-12). Given this contamination, it is obvious that the following questions must be asked. Are the lead species mobile in soil? Can they reach the root zone of plants? Can the lead species be taken up by plants thereby finding a route into the food chain? Laboratory studies generally indicate that the mobility of lead in soils is low (12). Even so, it is patently clear that identification of the compound form(s) involved is essential to the elucidation of the overall problem. Although lead bromochloride is the primary compound emitted from vehicles, Pierrard (13) has reported that photochemical decomposition can occur. He suggests that a lead oxide may be the ultimate product to be expected from the photolysis of the bromochloride in the atmosphere. Dzubay and Stevens (14) have observed a diurnal variation in the bromine to lead concentration ratio in atmospheric particulates that is indicative of the loss of halogens during atmospheric transport. Ter Haar and Bayard (6) have determined that oxides, carbonates, oxycarbonates, sulfates, and oxysulfates become the most prominent constituents of aged exhaust particles-i.e., those collected a t sites more remote from traffic sources. They estimate that particulates collected at a rural site are 30% PbC03, 20% PbO, 27% (PbO)zPbCOs, 5% PbOePbS04,
and 3% PbS04. These independent observations imply that the lead halides are converted to other compounds during atmospheric transport and exposure. In essence, the characterization of atmospheric lead particulates has received considerable attention. Based on these reports, it may be concluded that a reasonably large percentage of the lead from automotive sources may be deposited on the soil surface as the relatively soluble halides; the less soluble oxides and sulfate compounds may comprise a smaller fraction of the total. Whether these compounds remain as such in the soil matrix will obviously depend on the nature and extent of their interactions with atmospheric constituents and with chemical entities contained in both the soil and the ground water. Again, identification of the lead species in contaminated soil must be considered essential in answering the questions implied by these observations. From the identification of lead compounds in soils contaminated by automotive sources, the problem may be generally defined as follows. Soil is a complex matrix comprised of perhaps several hundred different compounds only a small fraction of which are lead compounds. The lead contamination level may range from a few to several hundred ppm. As a result, most analytical identification techniques are not capable of providing a positive identification unless the lead compounds are first separated (preconcentrated) . The separation method(s) used must be such that the identities of the lead compounds are not altered. The present report describes methods which have been utilized for the nondestructive separation/preconcentration of lead compounds from soils. The isolated fractions have been analyzed by X-ray powder diffraction methods, and the identities of the principal lead compounds are reported below.
Experimental Sample Characterization. The samples selected for the present study were from three discrete geographic areas. One set consisted of Colorado soil samples; two were of local origin while one was from Denver. Each was collected from the top centimeter of soil surface a t sites within 2 meters of a heavily traveled highway or street. Another was a composite surface sample collected in the Missouri Lead Belt along the ore truck route between a mine and the smelter. The third set consisted of four street dust samples vacuumed from curb and/or parking lot areas in Chicago. All samples were 100-500 grams in size and had lead concentrations which were 40 or more times greater than the natural background concentrations for the sites in question. Mechanical characterization of the Chicago and Colorado samples indicated that they should be classed as loamy sands containing nominally 510% silt Volume 9, Number 3, March 1975
227
and similar amounts of clay. The Missouri sample was more than half silt and clay combined. Each of the samples was sieved through a 20-mesh screen to remove extraneous rock, pebbles, and litter. Approximately 5 grams of each sample was taken by quartering and the bulk lead concentration determined by atomic absorption spectrophotometry after dissolution in hot, concentrated nitric acid (9). Separation and Preconcentration of Lead Compounds. A density gradient method in conjunction with a magnetic separation procedure was used to separate the lead compounds from the bulk of the soil matrices. Tengram samples taken from each soil by quartering were centrifuged first in carbon tetrachloride (density = 1.58 g/cc) and then in a series of CCld-diiodomethane mixtures such that the density of each successive mixture was incrementally increased by approximately 0.3-0.4 g/cc. The final centrifugation was in diiodomethane (density = 3.32 g/cc). Consequently, a series of soil fractions was obtained each of which represented a particular density range. The fractions with density greater than 3.32 were subsequently separated into magnetic and nonmagnetic fractions after air drying through the use of a small hand magnet. All fractions obtained from these separation procedures were air dried and portions of each (10-20% by weight) were taken for determination of their respective lead concentrations. These portions were digested in hot concentrated nitric acid, and the lead concentrations were determined by atomic absorption procedures (9). The data were used to calculate the degree of separation and preconcentration as discussed below. Compound Identification. Those soil fractions showing the major lead concentrations and containing the majority of the total lead in the bulk soil were used for compound identification purposes. These were exclusively the fractions with density greater than 3.32. Said fractions were ground in an agate mortar to pass a 200-mesh screen, and samples of this finely powdered material were mounted on fused quartz filaments with Duco cement for X-ray powder diffraction analyses. The powder diffraction patterns were obtained with a General Electric, Model XRD-1, unit using cobalt radiation to increase the dispersion of the d-spacings recorded on film. Spacing measurements were made from the film using a vernier scale system which permitted calculation of the d-spacings with an accuracy-of 0.02-0.04 A depending on the angle. The intensities of the diffraction lines were estimated by visual matching with a standard sodium chloride powder pattern. The compounds represented by each powder pattern were identified by computer search of the Joint Committee on Powder Diffraction Standards (JCPDS) (15) powder diffraction files. In the typical case where two or more compounds were indicated as high reliability choices by the search program, careful manual examinations were carried out as supplemental means of verifying the compound identities. Said verifications were based on both d-spacing and intensity matching. In general, the observed intensities for the primary constituent(s) of any pattern matched the listed intensities without normalization. The intensities observed for minor constituents often had to be normalized, however, to the listed intensities of the principal spacings of the compound(s) indicated in order to carry out intensity matching verifications. Since the accuracy assigned to the listed intensities is generally *IO% ( 1 5 ) , matches that agreed within these limits were considered acceptable. The final verification of the compounds identified were, in every instance, based on a direct comparison of diffraction patterns obtained with pure compounds or mixtures of pure compounds run under the same conditions. 228
Environmental Science & Technology
Table I. Preconcentration of Lead by Density Gradient and Magnetic Separation Methods Fort Collins soil, 2000 ppm Pb
Denver soil 960 ppm P d
Density range, g/cc
Pb, pprn
Percenttage of total Pb
Pb, pprn
Percentage of total Pb
3.32 >3.32, magnetic >3.32, nonmagnetic
4,850 3,560 3,460 365 1,420 66,500 47,100
2.9 1.6 5.7 12.5 1.9 75.4 25.4
1,380 4,060 380 100 710 24,200 14,500
1.4 1.0 8.5 6.8 3.1 79.2 26.8
81,650
50.0
45,200
52.4
Results and Discussion Separation Methods. When the present investigation was initiated, X-ray diffraction was selected as the most definitive means available for compound identification. Early work quickly indicated, however, that the identification success would be minimal even for the relatively high lead concentrations found in the soil samples selected. Thus, efforts were devoted to the development of nondestructive separation-preconcentration procedures that would permit the isolation of a large percentage of the lead compounds present from the bulk of the soil matrices. Although several approaches to separation were investigated, the density gradient and magnetic isolation methods described above were most generally useful and were used for all samples examined. Results obtained for two typical sample separations are summarized in Table I. For both samples, more than 75% of the total lead present was contained in, or associated with, soil particles having mean densities in excess of 3.32 g/cc. Because the majority of soil particles have densities below this, a high degree of preconcentration was obtained simply by centrifugation in diiodomethane. The advantages which may accrue from the use of the density increments indicated in the table all relate to the problem of characterization. For example, the fact that the lowest concentrations in these two instances were in the density fractions generally assigned to clays suggested that the lead was not preferentially associated with such clays. To determine the possible extent of chemical interactions with the solvents used, all solvent mixtures were analyzed for lead following the separations. In every case, the total amount of lead found was less than 1 pg. This strongly implies that compound conversion reactions involving the solvents did not occur. It should be emphasized that attempts to comminute the soils before separation resulted in the lead being retained in the less dense fractions. This was probably due to a “smearing” of the compounds into the lighter silt and clay materials during grinding. The simple further separation of the high density fraction into magnetic and nonmagnetic portions as shown for the examples of Table I resulted in nominally 25% of the total lead being retained with the magnetic soil particles. The lead concentration of the nonmagnetic fraction was accordingly enhanced. This combination of separation methods consequently produced lead concentrations suitable for X-ray diffraction identification purposes through the removal of major amounts of soil constituents not of interest in the present work.
Table II. Summary of Soil Lead Preconcentration Results Lead concentration, r g f g (percent of total lead present) Sample identity
Fort Collins-1 Fort Collins-2 De nver-1 Chicago-10 Chicago-20 Chicago-30 Chicago-40 Missouri-1
Before sepn
Dense fraction"
Magnetic fraction"
Nonmagnetic fractionfb
2000 1900 960 7000 2100 4800 2400 1540
66,500 (75.4) 58,600 (60.3) 24,200 (79.2) 120,800 (65.6) 27,800 (33.4) 75,900 (74.6) 46,700 (70.0) 44,300 (15.8)
47,100 (25.4) 47,700 (19.9) 14,500 (26.8) 106,400 (47.1) 11,300 (12.8) 42,400 (30.8) 22,700 (39.9) 21,000 (1.4)
81,600 (50.0) 66,200 (40.4) 45,200 (52.4) 184,700 (18.5) 127,100 (20.6) 168,700 (43.8) 83,500 (30.1) 47,700 (14.4)
Density greater t h a n 3.32 g/cc.
A summary of the separation results obtained is given in Table 11. For each sample, a major portion of the total lead was in the most dense fraction. The separation differences observed may be related to variations in the types of lead compounds and soil constituents present. The identification studies have been based on the X-ray examination of the dense magnetic and nonmagnetic portions individually to determine if discrete differences exist with regard to the compound identities in these fractions. Compound Identification. One or more samples from each soil fraction were run on the X-ray powder diffraction unit, and the compounds present were identified by the procedures described in the experimental section above. In addition to lead compounds, other compounds consistently present were of iron. These included m-Fez03, Fe304, and (less frequently) Fe3Alz(Si04)~.For each diffraction pattern obtained, there were typically 1-5 d-spacings observed that could not be assigned to the major or minor compounds present. Such d-spacings were all of low intensity (5-1070 on a scale of 100) and were probably due to compounds present a t minor or trace levels. The lead compounds in each soil fraction examined are listed in Table 111. The concentration estimates given are based on visual comparisons obtained from powder patterns of mixtures of the pure compounds with the other principal constituent (usually a-Fe203 or Fe304). While such estimates are only semiquantitative, it is tacitly apparent that lead sulfate accounts for the major portion of lead present in those soils subjected to insult from automotive exhaust. In nearly every instance, more than 50% of the lead present occurs as the sulfate. Microscopic examination of the separated fractions indicated the presence of several visually distinct types of agglomerates and crystalline materials. Samples of each of these were handpicked from the respective fractions and qualitatively analyzed by emission spectrography. These analyses indicated that two readily identified crystalline types contained high concentrations of lead. The first was a dark red crystalline material usually agglomerated with magnetite. The second was a typically clear crystalline material that often occurred with a white (powder) coating. Samples of these two materials selected from the two local soils were analyzed by powder diffraction. Lead sulfate was the principal constituent in each case but the white material contained lead oxysulfate while the red material contained lead oxide. While such materials were visible in all soils examined, except that from Missouri, the laborious sorting procedure was not repeated for the other soils. It can only be inferred, therefore, that these compounds were present in the other soils. Having established by these studies that lead sulfate is the principal compound found in the soils examined, it is appropriate to consider its possible sources and/or the
Table Ill. Lead Compounds Identified
Sample identity
Fort Collins-1 a n d Fort Collins-2
Denver-] Chicago-10 Chicago-20, 30. and 40 Mi ssou ri-1
S o i l fraction
Compounds found
Concentration estimates"
Magnetic
PbS04
Major
Nonmagnetic
PbS04 PbO.PbSO4 PbOz PbOb PbS04 PbS04 PbS04 Pb" PbS04 PbSOa
Major Minor Trace Trace
PbS04
Major
Nonec
.. Major Minor
Magnetic Nonmagnetic Magnetic Nonmagnetic Magnetic Nonmagnetic Magnetic Nonmagnetic
PbS PbSOa
Major
Major Major Major Minor Major
Major indicates t h e principal portion of lead present in t h e soil fraction indicated a n d therefore t h e principal portion of t h e soil sample; m i n o r refers t o approximately 1-10% of t h e P b in t h e respective fractions: trace quantities are less t h a n approximately 1% of t h e t o t a l in each fraction. b A s s i e n m e n t based on t h e Dresence of o n l v t h e m o s t i n t e n s e d -
means by which it is formed. As indicated above, Hirschler et al. ( I ) , Habibi et al. ( 3 ) ,and Habibi ( 4 ) have indicated the presence of lead sulfate in exhaust system deposits and in emitted particulates. Habibi ( 5 ) indicates that lead sulfate may account for 5 6 % of these deposits and particles. Similarly, Sampson and Springer ( 1 6 ) , Ganley and Springer ( I 7 ) , and Lamb and Niebylski (18) have established that the oxysulfate is present as a minor constituent in combustion chamber deposits and emitted particles. These findings indicate that a portion of the lead sulfate in the soils was formed in the combustion chamber and exhaust system and deposited directly on the soil surface. Because the studies cited above have all agreed that lead bromochloride compounds are the principal particulate constituents in fresh auto exhaust, the in-vehicle formation and subsequent deposition mechanism cannot account for all of the lead sulfate present in soils. Rather, indirect conversions occurring during the atmospheric transport and atmospheric exposure periods or conversions occurring in the soil, groundwater system must be considered as important additional routes. The atmospheric photolysis of lead halides has been mentioned as a possible means of conversion (13). While Volume 9 , Number 3, March 1975
229
the data of Ter Haar and Bayard (6) support the suggested formation of oxides via this route, the present results indicate that oxides present in the soils are minor constituents relative to the sulfate concentrations. This implies that oxide formation by photolysis is of minimal importance. Lee et al. (19) have measured the concentration and size distributions of particulate emission in auto exhaust. Their results show that the percentage of water-soluble particulate lead increases when the diluted exhaust is irradiated by 3000-6000 8, light. The percentage of watersoluble lead in the diluted exhaust is also increased by irradiation in the presence of 0.5 ppm of SOz. Irradiation also dramatically increases the sulfate concentration of particulates with a pronounced shift to smaller particle sizes. This suggests the formation of small sulfate particles from a reaction sequence involving sulfur dioxide and denies the prominence of oxide formation by photolysis because such conversions would tend to decrease solubilit y in water. Similarly, they showed that irradiation caused a shift of nitrate bearing particles to smaller sizes with and without added SOz, but the amount of NO2 and nitrate formed was decreased by addition of SOz. This may be due to competitive inhibition from sulfate formation. Irradiation increased the particulate chloride concentration and decreased the mean particle size particularly in the presence of S O z . It may be inferred from these results that irradiation produces increased concentrations of sulfate and nitrate very likely involving the oxidation of SOz, NO, and NOz followed by reaction with water to form acid droplets or condensates on particulate surfaces. These can obviously react with particulate and organic lead to produce lead sulfate and nitrate. Because lead sulfate is somewhat less soluble than the halide compounds, the formation of the nitrate is the most logical explanation for the increase in water solubility they observed. These results and those previously discussed clearly show that lead halides may be readily converted in an urban atmosphere to other compounds including lead sulfate. Moreover, because lead compounds dispersed from traffic generally reside a t the soil horizon for long periods, further conversion may occur during the soil residency period via exposure to the atmosphere. If lead nitrate is responsible for the increased water solubility noted by Lee et al. (19), it should be emphasized that lead sulfate may also be formed in the soil matrix via precipitation reactions. Sulfate concentrations characteristic of many soils are typically in the lO-3-lO-5M range. For such soil-water systems in equilibrium with atmospheric C O z , calculations by Lindsay (20) indicate that lead sulfate would be the least soluble lead species for soil pH levels below 7-7.5; above this p H range lead carbonate becomes the least soluble species. Thus, although lead may initially reach the soil system in another form, e.g., PbBrC1, any dissolution process has a high probability of resulting in precipitation of insoluble lead sulfate. In essence, the presence of PbS04 as the primary compound in soil may be rationalized on the basis of conversions occurring in either the atmosphere or the soil. The presence of lead oxides is not unexpected while the presence of
230
Environmental Science &. Technology
PbO.PbS04 may be explained by formation in hot exhaust or by atmospheric reactions. The metallic lead found in the one Chicago sample (a parking lot) may be due to exhaust system flakeoff during cold starts. I t is to be noted that generally 70% or more of the total lead in the soils examined was contained in the dense fractions and that the majority of this lead was present as the sulfate. These results indicate that most of the lead particulates emitted by autos undergo conversion to the sulfate. Attempts to identify lead compounds in less dense soil fractions were not successful. It is inferred from this that the lead present in these fractions may exist in ionic form, adsorbed or held by ion-exchange sites, which would preclude X-ray identification. Alternatively, it may exist in numerous compound forms a t concentration levels too low for characterization by the methods used herein. Certainly the identification of lead sulfate as a primary soil form must be considered in formulating experiments dealing with lead mobility in soils, uptake of lead by plants, effects of lead on soil microbes, and related questions.
Acknowledgment S. R. Koirtyohann, University of Missouri, and P. R. Harrison, Chicago Board of Health, provided samples from their respective areas. L i t e r a t u r e Cited (1) Hirschler, D. A,, Gilbert, L. F., Lamb, F. W., Niebylski, L. M., Ind. Eng. Chen., 49, 1131 (1957). ( 2 ) Hirschler. D. A , . Gilbert. L. F.. “Nature of Lead in Automotive Exhaust Gas,” Archives of Environmental Health, Symposium on Lead, February 1964. (3) Habibi, K., Jacobs, E . S., Kunz. W. G . Jr., Pastell, D . L., “Characterization and Control of Gaseous and Particulate Exhaust Emissions from Vehicles,” presented at the 5th Technical Meetine of the Air Pollution Control Association. San Francisco, Octiber 8,9, 1970. (4) Habibi. K.. Enuiron. Sci. Technol.. 4, 239 (1970). (5) Habibi, K., ibid., 7, 223 (1973). (6) Ter Haar, G. L., Bayard, M . A,, Nature, 232,553 (1971). (7) Schuck, E . A,, Locke, J . K., Enuiron. Sci. Technol., 4 , 325 (1970). ( 8 ) Davies, R. H., Matto, H., Chilko, D. M., ibid., p 318. (9) Seeley, J. L., Dick, D., Arvik, J. H., Zimdahl, R. L., Skogerboe, R. K., Appl. Spectres., 26, 456 (1972). (10) Puhling, A., Tyler, G., Bataniska Natiseu, 121,21(1968). (11) Ibid.,122,16 (1969). (12) Zimdahl, R. L., unpublished data, Department of Botany and Plant Pathology, Colorado State University, March 1974. 13) Pierrard, J . M., Enuiron. Sci. Technol., 3,48 (1969). 14) Dzubay, T. G., Stevens, R. K., “Applications of X-ray Fluorescence to Particulate Measurements,” presented at 2nd Joint Conference on Sensing of Environmental Pollutants, Washington, D.C., December 1973. 15) Joint Committee on Powder Diffraction Standards, Powder Diffraction File and Search Program, 1845 Walnut St., Philadelphia, Pa., 1970. 16) Sampson, R. E., Springer, G. S., Enuiron. Sci. Technol., 7, 55 (1973). 17) Ganley, J. T., Springer, G. S., ibid., 8, 340 (1974). 18) Lamb, F. W., Niebylski, L. M., Anal. Chem., 10, 1388 (1951). (19) Lee, R. E. J r . , Patterson, R. K., Crider, W. L., Wagman, J . , A t n o s . Enuiron., 5,225 (1971). (20) Lindsay, W. L., unpublished data, Department of Agronomy, Colorado State University, April 1973.
Receiced for reuieu: May 6, 1974. Accepted October 2, 1974. Work supported by N S F Grant No. GI 34813X.
