The physiologically based extraction test (PBET) is an in vitro test system for predicting the bioavailability of metals from a solid matrix and incorporates gastrointestinal tract parameters representative of a human (including stomach and small intestinal pH and chemistry, soil-to-solution ratio, stomach mixing, and stomach emptying rates). For lead (Pb), the results of the PBET are linearly correlated with results from a Sprague-Dawley rat model (r 2 ) 0.93 between in vitro and in vivo results, n ) 7). For arsenic (As), the results of the PBET are overpredicting bioavailability study results in rabbit and primate models (2-11% difference between in vitro and in vivo results, depending on the animal model). The PBET was not designed to supplant bioavailability studies using animal models, but rather to estimate Pb and As bioavailability when animal study results are not available. Dissolution of Pb in the acidic stomach environment was strongly pH dependent; the extent of dissolution decreased by 65% when stomach pH was increased from 1.3 to 2.5. Arsenic solubility decreased by only 16% over the same pH range. Lead was removed from solution to a greater extent than As by neutralization during the small intestinal simulation, consistent with adsorption and precipitation reactions occurring for Pbsbut not Assat neutral pH values. In addition to providing mechanistic explanations for controls on Pb and As bioavailability, the PBET allows estimates of site-specific Pb and As bioavailability from soil for the purpose of exposure assessment.

A screening-level in vitro test was developed to evaluate the relative solubility of ingested lead (Pb) from different mine wastes in the gastrointestinal (GI) tract. The in vitro method, modeled after assay methods for available iron from food, used a laboratory digestion procedure designed to reproduce GI tract chemistry and function. The in vitro method was independently calibrated against a rabbit feeding study, demonstrating that only 1-6% of the total Pb in four mine-waste samples with disparate Pb mineralogy was bioaccessible. In vitro method development tests indicated that H+ concentration and Clc omplexation control dissolution of Pb-bearing minerals in the stomach and that both GI tract enzymes and organic acids are necessary to maintain Pb in the soluble form on entering the small intestine. The experimental results indicate that ingestion of Pb-bearing mine wastes results in limited Pb solubility and that the in vitro test provides a screening-level estimate of the maximum available Pb from mine wastes.

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Electron microprobe analysis of soil and waste rock mineralogy helps explain the low blood Pb levels observed in young children living in Butte, MT. The sulfide/sulfate assemblage consists primarily of galena, anglesite, and lead jarosite,while the oxide/phosphate assemblage was principally manganese lead oxide, lead phosphate solid-solution series, and lead oxide. Modeled paragenetic sequences initiated using galena and lead oxide demonstrated that both primary phases weather to less soluble anglesite and pyromorphite end members, respectively. If these soils were ingested, Pb solubility would be constrained by alteration and encapsulation of the Pb-bearing minerals, which limits the available Pb-bearing surface area. This premise is supported by the results of the Butte-Silver Bow environmental health Pb study, which indicate that no statistical relation exists between blood-Pb levels and the presence of individual Pb-bearing phases.