Environ. Sci. Technol. 1992, 26, 1242-1248
(17) Hansch, C.; Leo, A. Substituent Constants for Correlation Analysis in Chemistry and Biology; Wiley New York, 1979; pp 172-330. (18) Gossett, J. M. Environ. Sci. Technol. 1987, 21 (2), 202. (19) MacKay, D.; Leinonen, P. J. Environ. Sci. Technol. 1975, 9 (13), 1178. (20) Guttman, I.; Wilks, S. Introductory Engineering Statistics; Wiley: New York, 1965. (21) Karickhoff, S. W.; Brown, D. S.; Scott, T. A. Water Res. 1979, 13, 241. (22) Chiou, C. T.; Peters, L. J.; Freed, V. H. Science 1979,206 (Nov), 831. (23) Crank, J. The Mathematics of Diffusion, 2nd ed.; Clarendon Press: Oxford, U.K.; 1975; pp 49-52. (24) Griffin, R. A.; Shimp, N. F. Environ. Sci. Technol. 1976, 10 (6), 1256. (25) Griffin, R. A; Shimp, N. F.; Steele, J. D.; Ruch, R. R.; White, W. A.; Hughes, G. M. Environ. Sci. Technol. 1976,lO (6), 1262.
(26) Bohn, H. L.; McNeal, B. L.; O'Connor, G. A. Soil Chemistry; Wiley: New York, 1979; pp 141-192. (27) Sposito, G. The Surface Chemistry of Soils; Oxford University Press: New York, 1984; pp 1-35. (28) Griffin, R. A.; Au, A. K.; Frat, R. R. J. Environ. Sci. Health 1977, A12 (8), 431.
Received for review August 5,1991. Revised manuscript received January 28,1992. Accepted February 18,1992. Partial support for this work was provided, in sequence, by Research Grant R811570-01-0 from the U.S. Environmental Protection Agency, by a University of Michigan Rackham Research Fellowship award to H.V.M.,and by Research Grant l-P42-ES04911-01 from the National Institutes of Environmental and Health Sciences. Neither the Environmental Protection Agency nor the National Institutes of Environmental and Health Sciences has reLiewed this article, and the opinions and conclusions set forth herein are not necessarily those held by either agency.
Lead Bioavailability: Dissolution Kinetics under Simulated Gastric Conditions Michael V. Ruby,'vt Andy
J. Houston Kempton,t John W. Drexler,f and Paul D. Bergstroms
PTI Environmental Services, 2995 Baseline Road, Suite 202, Boulder, Colorado 80303,Department of Geological Sciences,
University of Colorado at Boulder, Boulder, Colorado 80309,and Atlantic Richfield Company, 555 17th Street, Denver, Colorado 80202 The lower bioavailability of lead from mining sites compared to urban environments is due partially to the relative solubility of Pb-bearing phases in the respective mineral assemblages and to kinetic limitations relative to the residence time of soil in the gastrointestinal (GI) tract. In this study, P b dissolution kinetics for mine-waste-impacted soil and for pure anglesite were diffusion controlled. Dissolution rates over stomach residence time (2 h) were 0.49 and 0.07 [(mg of Pb/L of solution)/(mg of solid Pb)(h)] for PbS04 and test soil, respectively, and were linearly dependent on HC1 concentration, whereas particle size did not affect dissolution rates except at mean diameters of PbS04 > test soil. Dissolution of only 4% C P b from the test soil after 2 h demonstrates that kinetics of Pb-bearing minerals in mine-waste-impacted soils is an important factor controlling P b solubility in the GI tract.
Introduction The factors controlling anthropogenic lead bioavailability have been investigated by numerous authors (1-8). However, only recently have investigations focused on the geochemical controls of P b bioavailability and the mechanisms controlling P b dissolution from soil particles in the gastrointestinal (GI) tract, such as rinding by precipitation products, dissolution kinetics, and encapsulation by alteration products and inert matrices (e.g., silicates) (9, 10). Dissolution kinetics of an ingested solid Pb-bearing phase chsracteristic of a mining site are affected by both Pb mineralogy and particle size distribution. For soluble P b salts such as Pb(OAc)z,which is often used in toxicological P b studies because it dissolves rapidly (4, 8, 11), the concentration of P b in solution is controlled by the available mass of salt. However, for anglesite (PbS04), which may control P b dissolution from soils impacted by PTI Environmental Services.
* University of Colorado at Boulder. 5 Atlantic
1242
Richfield Company.
Environ. Sci. Technol., Vol. 26, No. 6, 1992
mine waste originating from the reduced zone of the ore body, Pb solubilization is controlled by kinetic constraints, resulting in lower bioavailability relative to Pb(OAe1, (12, 13).
This paper describes in vitro P b dissolution kinetics using both mine-waste-impacted soil and pure, crystalline anglesite to estimate the solubility of Pb-bearing solids during passage through the stomach. The effects of particle size and pH were examined, and kinetic modeLs were used to evaluate the mechanism of P b dissolution. The equilibrium geochemical speciation model MINTEQAB (14) was evaluated to determine if it is capable of predicting realistic aqueous P b concentrations for a nonequilibrium system. Experiments such as these, which examine the mechanistic framework for Pb bioavailability, are necessary to evaluate or construct a predictive model for P b bioavailability from soil. Although the subject of anglesite dissolution kinetics has been studied previously under laboratory conditions Us), there have been no previous investigations under physio. logical conditions, or as a function of particle size and pH. An understanding of the kinetic constraints that limit bioavailability of Pb-bearing mine waste is valuable because human health risk assessments used to define cleanup standards at mine waste sites are particvldY sensitive to this parameter.
Soil Chemistry Oxidation reactions on mineral surfaces result in Brmoring of the primary mineral grain by a secondary reaction product. For example, the weathering of galena (PbS) in the acidic environment generated by pyrite dis. solution in mine-waste rock (16) results in a rind of anglesite around a galena core (17). Ingestion of soil mixed with mine waste that bears anglesite results in a dissolution reaction with HC1 in the stomach to form predominantly PbCl+: PbS04(s) + HCl(aq)
0013-936X/92/0926-1242$03.00/0
F?
PbCl+(aq) + HS04-(aq) (l)
0 1992 American Chemical Soclew
fie rate of this reaction in basal gastric fluid at pH 1.0-2.0 (18) determines the concentration of Pb solubilized during through the stomach. However, the limited residence time of material in the stomach (
