Environ. Sci. Technol. 1997, 31, 37
Mass Balance on Surface-Bound, Mineralogic, and Total Lead Concentrations as Related to Industrial Aggregate Bioaccessibility A N D Y D A V I S , * ,† M I C H A E L V . R U B Y , ‡ PHILLIP GOAD,§ STEPHEN EBERLE,| AND STEPHEN CHRYSSOULIS⊥ Geomega, 2995 Baseline Road, Suite 202, Boulder, Colorado 80303, PTI Environmental Services, 4940 Pearl East Circle, Boulder, Colorado 80301, Terranext, 11500 Fairview Road, Little Rock, Arkansas 72212, Poudre Valley Hospital, 1024 South Lemay Avenue, Fort Collins, Colorado 80524, AMTEL, 100 Collip Circle, UWO Research Park, London, Ontario, Canada N6G 4X8
Previous investigations into lead bioavailability have focused on the solubility of the mineralogic fraction. However, the potential for surface-bound (sorbed) lead to contribute to the bioavailable fraction has not yet been determined. This study investigated the Pb distribution between the mineralogic and surface-bound pools in aggregate generated by thermal vitrification of a Pb-bearing organic and metalliferous waste feedstock. Electron microprobe and laser ion mass spectroscopy demonstrated that amorphous slag comprised >95% of the vitreous material, with lead sulfosalts, oxides, and carbonates encapsulated in the refractory slag matrix representing the mineralogic Pb fraction. The maximum potentially bioavailable surfacebound fraction was 27% in the 467 mg/kg primary aggregateimpacted soil and 5% in the 6520 mg/kg slagged aggregate, averaging 14% of the Pb pool for the six samples evaluated. The low concentration of Pb on the surface of aggregate particles, the refractory nature of the Pb-bearing slag, the low percentage of liberated Pb-bearing (non-slag) particles (1:6300 to 1:38 000 of the total grain population), and the large size of the slag particles explain the low bioaccessibility of Pb in modern aggregate (1.5%) as compared to urban Cincinnati soils (51%) and street dusts (77%).
Introduction Recently, interest has focused on the speciation and immobilization of lead (Pb)-bearing phases, driven primarily by a desire to understand constraints on their bioavailability, and consequent human health risks (1-3). For example, the micromineralogy of Pb minerals in mine wastes has been investigated (4, 5), their kinetics of dissolution evaluated (6), and an in vitro test designed as a rapid screening analog for in vivo assessment of Pb bioavailability (7). The in vitro test has been used to estimate bioavailability factors for use in the Integrated Exposure Uptake Biokinetic (IEUBK) model * Author to whom correspondence should be addressed; telephone: 303-938-8115; fax: 303-938-8123. † Geomega. ‡ PTI Environmental Services. § Terranext. | Poudre Valley Hospital. ⊥ AMTEL.
S0013-936X(95)00960-6 CCC: $14.00
1996 American Chemical Society
(8). In addition, phosphate has been proposed as a soil amendment to reduce Pb solubility and, consequently, bioaccessibility (9, 10) in mine wastes, urban dusts and soils, and industrial process wastes. Lead occurring in discrete mineral phases or encapsulated in silicates has been recognized to be less bioaccessible than Pb from urban sources, such as paint or vehicular exhaust (4-6, 9, 12). If all the Pb in the soil is associated with mineral phases, then characterization of soil mineralogy and determination of the relative solubility of the Pb-bearing phases is adequate to determine the relative risk associated with each phase and, in the event of multiple sources (i.e., paint, tailpipe, and industrial emissions), to allocate risk between sources. However, if surface-adsorbed Pb comprises a portion of the bulk Pb pool, its bioavailability may be controlled by desorption of surface Pb in the acidic stomach environment (pH 2-4). Although surface-bound Pb species have been characterized in mine wastes (13), the relative mass contribution of near-surface and surface-bound Pb that is bioavailable has not been described. A mass balance of Pb in soil, as related to the distribution between the mineral and adsorbed fractions, may be represented by
∑Pb (analytical total) )
mineral Pb mass + surface Pb mass (1)
while the bioaccessible fraction, i.e., the proportion of ∑Pb that may be dissolved in the gastrointestinal tract and absorbed into the systemic circulation) is
bioavailable fraction )
∑(Pb
min,sol
+ Pbdesorb)
total mass
(2)
where Pbmin,sol represents Pb in the mineral fraction (i.e., PbCO3, PbS, etc.) that may be soluble on ingestion, and Pbdesorb is the fraction of surface-bound Pb released in the GI tract by desorption from non-Pb-bearing mineral surfaces, e.g., clay, quartz, feldspars, etc. The Pb bioavailability factor used in human health risk assessments to represent the proportion of ingested Pb absorbed into the blood stream is actually the soluble Pb mass less that portion not absorbed due to precipitation and sorption reactions in the small intestine, incomplete absorption across the intestinal epithelium, and other pharmacokinetic factors. Therefore, because only a portion of the Pb that dissolves in the stomach will be absorbed into the body, the bioavailable fraction will by necessity be less than the bioaccessible Pb fraction, i.e., that which is soluble in the GI tract. The focus of this research was to independently measure all variables in eq 1, to assess the relative contributions of surface-bound and mineralogic Pb forms to the Pb pool, and to investigate the relationship between surface-bound Pb and total bioaccessible Pb in industrial materials. In addition, the Pb bioaccessibility of urban soils and street dust was compared with aggregate materials of different ages, sources, and environmental exposure histories. Aggregate Source Materials. Incineration of hazardous waste to fix metals in a refractory aggregate, while simultaneously decomposing organic compounds, is a recent technology designed to chemically convert hazardous substances to inert construction materials (e.g., fill, asphalt, or cement filler). In the process investigated in this study, metals were sequestered in sintered material (primary aggregate) by processing feedstock through a 84 m long, 3.6 m diameter rotary kiln for approximately 2.5 h at 1800 °F and 1 rpm (Figure
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FIGURE 1. Schematic of slagging process. 1). In the event that the material leaches >0.5 mg/L Pb after processing, the entire pile would be slagged at a higher temperature. Slagged aggregate is produced by processing the primary aggregate, along with disentrained oxidizer solids and baghouse dusts (Figure 1), through the kiln oxidizer at 25003000 °F, producing a siliceous melt that is water quenched to generate an angular, conchoidal slag with a nominal particle diameter of 1-500 mm. In 1988, the slagging system was upgraded by incorporation of external slag boxes with hightemperature O2 lances, resulting in a more homogeneous, refractory product. The primary aggregate evaluated during this study was produced from organic-contaminated soils containing minimal Pb, while the slagged aggregate was produced from sludges and numerous industrial Pb-containing process wastes. Fresh (2-months-old) primary (PA-1) and slagged aggregates (SA-1) were collected together with three samples of older slagged aggregate from piles that had been exposed to surface weathering for approximately 5 years and a sample of primary aggregate (C-2), which had been buried for 6 years beneath a 1 m thick layer of shell sand (Table 1). The weathered slagged aggregate samples included a composite sample from the 30-50-cm depth interval of a large pile of slagged aggregate (LP-1) and samples from the 0-5-cm and 0-8-cm depth intervals of two unpaved roadways near the pile (LL-1 and LL-2).
Methods Each sample of aggregate was oven-dried at 80 °C for 24 h and sieved to 100-µm size fraction, while 68% of the non-slag Pb particles occur in the 0-10-µm fraction. The difference between slag and other Pb phase particle sizes suggests that the oxide and carbonate fractions may represent external Pb sources deposited regionally by both wet and dry mechanisms, after atmospheric transport from other industrial sources in the area. Furthermore, the soluble oxide and carbonates are predominantly 100 µm) to prevent inhalation or accidental ingestion (due to mouthing behavior) in children, precluding a reasonable exposure pathway. The non-slag Pb-bearing particles are predominantly encapsulated (nonbioaccessible) within other refractory phases, with only 24% of the non-slag Pb-bearing particles and 10% of the oxide/ carbonate fraction liberated. These data demonstrate that the majority of the mineralbound Pb in the slag is distributed homogeneously throughout the slag grains, is refractory and insoluble, and from a geochemical perspective, is unlikely to be bioaccessible over the residence time in the GI tract (2 h in the stomach where Pb may be solubilized and 4 h in the small intestine, the segment of the GI tract where Pb is absorbed; 23, 24). The encapsulation of Pb in a siliceous matrix has been demonstrated to reduce bioavailability and leachability (25). Therefore, based on the characteristics of PA and SA, we hypothesized that Pb bioaccessibility/bioavailability would be lower than the 30% that is usually assumed in applying the IEUBK model (8). In Vitro Testing. The slag and sinter-enriched soil demonstrated the same Pb dissolution trends observed in
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TABLE 3. Example Calculation of Pb Bioaccessibility for Slag Samples and Cincinnati Street Dust and Soil Samples Based on in Vitro Test Data lead mass solubilized (mg)
Cincinnati
time (h)
SA-1
LP-1
LL-1
LL-2
PA-1
C-2
street dust
soil
0.5 1.0 1.5 2.0 5 (small intestine) Pb mass in flask (mg) (based on 4 g of material used) % bioaccessible Pb % bioaccessible Pb for all in vitro tests on sampleb n predicted % bioavailable Pb based on comparison to feeding studies Pb (mg/kg) bioavailable Pb concn (mg/kg)
0.40 0.54 0.68 0.79 0.07 26.1 3a 3 ( 0.0 4 1.5
2.0 3.2 3.8 4.2 NAd 15.4 27 27 ( 0.0 2 13.5
4.2 5.8 6.8 6.6 0.005 18.5 37 51 ( 12 3 25.5
0.50 1.3 2.0 2.2 0.003 8.3 26 30 ( 3.5 2 15
0.03 0.02 0.02 0.02 0.002 1.7 2 3 ( 0.7 6 1.5
0.06 0.06 0.06 0.06 0.008 1.9 3 3 ( 0.0 2 1.5
5.28 7.2 6.8 8.0 0.08 10.5 77 77c 38.5
5.8 5.5 5.6 5.3 0.04 11.1 51 51 ( 0.5 2 25.5
6490 97
3850 520
4620 1180
2080 310
422 6.3
467 7.0
2630 1000
2780 710
a Quadruplicate in vitro analyses for SA-1 resulted in a bioaccessibility of 3 ( 0.7. b Average ( standard deviation for all in vitro tests (1-6 per sample) conducted on each sample. c No standard deviation reported indicates that only one in vitro test was conducted. d NA, Not analyzed due to ruptured dialysis bag.
TABLE 4. Comparison of HCl Lead Extractability from Aggregate and Other Materials sample
% of lead extracted
road dusts Cincinnati street dust street dusts street dusts Cincinnati soil av lowlands slag aggregate (pre-1989 manufacture) slag aggregate (av) primary aggregate (av) av lowlands slag aggregate (pre-1989 manufacture) 1993 slag aggregate 1993 primary aggregate primary aggregate (pre-1989) slag aggregate primary & slag aggregate (av) slag aggregate (4-h extract) primary aggregatec primary aggregate
90 77 77 60 51 38 11.8a 5.6a 3.3 3.0 3.0 2.7 1.8 1.1 0.93 0.63 0.009
study parameters ha
pH 1, 1 pH 1.3, 2 hb pH 1.2, 1 h NA pH 1.3, 2 hb pH 1.3, 2 hb pH 1.7, 6 ha pH 1.7, 6 ha pH 1.5, 20 minb pH 1.3, 2 hb pH 1.3, 2 hb pH 1.3, 2 hb pH 1.0, 24 ha pH 1.3, 8 ha pH 1.0, 4 ha pH 1.0, 4 ha pH 1.0, 4 ha
investigator(s) Day et al. (31) this study Harrison (32) Duggan & Williams (33) this study this study Kendall (34) Kendall (34) Angle (35) this study this study this study Cheng et al. (36) Drexler (37) Means (38) Forrest (39) Stockton (40)
a The studies were carried out with variable pH (pH not controlled during experiment). b These studies were carried out with pH maintained at initial level during the course of the experiment. c Assumes 1000 mg/kg total lead in aggregate.
FIGURE 4. Particle size histogram for slag and non-slag Pb-bearing particles. previous in vivo and in vitro analyses (4, 6, 7). The greatest Pb mass was solubilized in the acid (pH 1.3) stomach analog. Bioaccessibility was computed based on the highest stomach analog Pb concentration by dividing solubilized Pb by the total Pb mass in the flask, based on the reasoning outlined in Ruby et al. (7). This interpretation is conservative in that the temporal dissolution curve demonstrates a substantial (10-fold) decrease between the solubilized Pb in the stomach (0.79 mg) and small intestinal (0.074 mg) compartments (Figure 5).
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FIGURE 5. Temporal dissolution of Pb from duplicate in vitro analyses of slagged aggregate (SA-1). Bioaccessible Pb ranged from 3 ( 0.7% in PA-1 to 51 ( 12% in LL-1. The corresponding bioavailability factor, derived by applying a 2-fold decrease to the bioaccessibility data, based on comparison of retained versus ingested soluble Pb (% uptake) by children (26, 27) results in a Pb bioavailability estimate between 1.5 and 25.5% (Table 3). Lead bioavailability from slag generated following installation of the slag box (i.e., SA-1; 1.5%) samples was lower than pre-1988 slags LP-1, LL-
Rick Nelson, respectively. A.D. and M.V.R. appreciate the support of K. Davis and P. Diehl, respectively.
Literature Cited
FIGURE 6. Lead bioaccessibility as a function of surface-bound Pb reported by LIMS technique. 1, and LL-2 (13.5%, 25.2%, and 15%, respectively). The bioavailable Pb concentration, determined from the product of the bioavailability factor and the bulk Pb concentration, ranged from 6.3 mg/kg in PA-1 to 1200 mg/kg in LL-1, with no obvious relation between surface-bound Pb and bioaccessibility (Figure 6). The cause of higher Pb bioavailability for historical (pre-1988) slagged aggregate may be due to a nugget of soluble lead sulfosalt. Also, there is no obvious reason for the 15% bioavailability for LL-2 in which surface Pb accounts for only 4% of the ∑Pb and where mineralogically-bound Pb occurred predominantly (95.6%) in the slag pool. The presence of galena (4.4%, equivalent to 36 mg/kg) in this sample fails to account for the bioavailable Pb (1200 mg/kg) either on a mass basis or on a solubility basis (4), although LP-1, LL-1, and LL-2 Pb budgets were all deficient (69-82% of total Pb), suggesting that the balance (possibly a result of an unknown soluble mineralogic nugget) may have contributed to the bioavailability. This was the case for LL-1, where a mass balance of 60% was reported initially, which on reanalysis was revised upward to 78% following the discovery of a large grain of galena in the sample. Despite the absence of a quantitative relationship, these data are consistent with other leach tests, demonstrating the inert nature of the aggregate in comparison with street dusts and Pb-bearing soils (Table 4). Soil and street dust samples obtained from Cincinnati generated 51% and 77% bioaccessible Pb, respectively, substantially higher than for the recently generated slag SA-1 (1.5%). These data are consistent with the premise that Pb species typically associated with an urban soil matrix are more soluble (28, 29). For comparison, 89% of an equivalent mass (3850 mg Pb/kg, as in LP-1) of Pb(OAc) was recovered during the in vitro test. The Pb bioavailability for slag is also consistent with that of Butte mine waste (4%; 7) where a “trigger level” of 1200 mg/kg bulk Pb has been selected to require evaluation of child blood Pb levels, but not automatic yard cleanup (30). This approach is more technically defensible than yard soil replacement because, as demonstrated in the Tri-Cities study (Baltimore, Boston, and Cincinnati), (1) soil removal results in only minimal statistical reduction in blood Pb concentrations and (2) clean replacement topfill is rapidly recontaminated by urban Pb, often returning to original soil Pb concentrations within a few months.
Acknowledgments The authors would like to thank GTX Inc. for funding this research, and they appreciate the contributions of Dr. Bob Bornschein in making the Cincinnati samples available, the thorough reviews of this manuscript by Dr. Rosalind Schoof and two anonymous reviewers, and the secretarial, graphical, and editorial support of BarBara Kyvik, Wendy Hawkins, and
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(31) Day, J. P.; Ferguson, J. E.; Chen, T. M. Bull. Environ. Contam. Toxicol. 1979, 23, 497. (32) Harrison, R. M. Sci. Total Environ. 1979, 1, 89. (33) Duggan, M. J.; Williams, S. Sci. Total Environ. 1977, 7, 91. (34) Kendall, D. 1991. Extraction of metals with dilute hydrochloric acid from Marine Shale primary aggregate and slagged aggregate. Memorandum to Tom Clark, Department of Justice, U.S. Environmental Protection Agency, Office of Enforcement, Denver, CO, 1991. (35) Angle, C. R. Lead releases by weathered MSP products to children's hands, ambient air, and simulated gastric juice; Lead Research Laboratory, University of Nebraska Medical Center: Omaha, NE, 1994. (36) Cheng, Y.; Preslan, J. E.; Anderson, M. B.; George, W. J. J. Hazard. Mater. 1991, 27, 137. (37) Drexler, J. W. Metal speciation and bioaccessibility of metals from: Marine Shale Processors waste materials; Department of Geological Sciences, University of Colorado: Boulder, CO, 1994.
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(38) Means, J. Slagged aggregate leachate studies; Department of Environmental Chemistry and Toxicology, Louisiana State University: Baton Rouge, LA, 1991. (39) Forrest, R. Houston laboratory results for Dexter Leonard residence. Memorandum to Russell Rhoades, Environmental Services Division, 1990. (40) Stockton, D. C. Laboratory results for Dexter Leonard residence. Memorandum to D. Ayers, U.S. Environmental Protection Agency, Region VI, Houston, TX, 1990.
Received for review December 29, 1995. Revised manuscript received June 26, 1996. Accepted August 7, 1996.X ES950960Z X
Abstract published in Advance ACS Abstracts, October 15, 1996.
