Bioaccessibility of lead sequestered to corundum and ferrihydrite in a simulated gastrointestinal system

Lead (Pb) sorption onto oxide surfaces in soils may strongly influence the risk posed from incidental ingestion of Pb-contaminated soil. Lead was sorbed to model oxide minerals of corundum (alpha-Al2O3) and ferrihydrite (Fe(5)HO(8)(.)4H(2)O). The Pb-sorbed minerals were placed in a simulated gastrointestinal tract (in vitro) to simulate ingestion of Pb-contaminated soil. The changes in Pb speciation were determined using extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge spectroscopy (XANES). Both corundum (sorption maximum of 2.13 g kg(-1)) and ferrihydrite (sorption maximum of 38.6 g kg(-1)) have been shown to sorb Pb, with ferrihydrite having a very high affinity for Pb. The gastric bioaccessible Pb for corundum was > 85% for corundum when the concentration of Pb was > 200 mg kg(-1). Bioaccessible Pb was not detectable at <= 200 mg kg(-1). Bioaccessible Pb ranged from 53 to 88% for ferrihydrite. The bioaccessible Pb was below detection limits for the intestinal phase in the ferrihydrite system. Solid phase speciation identified both inner- (mononuclear bidentate) and outer-sphere species for Pb sorbed to corundum, while only an inner-sphere (mononuclear bidentate) complex was found for ferrihydrite. Although corundum and ferrihydrite can bind Pb, they fail to significantly reduce gastric bioaccessible Pb but do reduce intestinal bioaccessible Pb. Treatment of Pb-contaminated soil with corundum or ferrihydrite may reduce Pb solubility under field soil conditions of pH > 4. However, much of the sorbed Pb will become bioaccessible under gastric conditions (pH 1.5-2.5) if this soil is ingested. Caution should be used before using these materials to remediate a soil where soil ingestion is an important exposure pathway.