Transformation and bioaccessibility of lead during physiologically based extraction test: effects of phosphate amendment and extract fluid components

Abstract

In this study, a physiologically based extraction test (PBET) was used to determine the effects of the fluid components and soluble phosphate amendment on the transformation and bioaccessibility of Pb(NO₃)₂ and PbCO₃(s) during the extraction. In the gastric phase, phosphate amendment reduced up to 95% bioaccessible Pb in both Pb(NO₃)₂ and PbCO₃(s) within 10 min, with the decrease being continued with time. The decrease in Pb bioaccessibility was attributed to the formation of insoluble Pb phosphate precipitates (i.e., Pb₅(PO₄)₃Cl(s) and PbHPO₄(s)), which were verified and quantified by X-ray diffraction. The continuous decrease of Pb bioaccessibility resulted from the lasting transformation of Pb phosphate precipitates over the extraction time. In the intestinal phase, the decrease of Pb bioaccessibility with P amendment was over 60%, but it increased slightly during the extraction process. This was probably due to dissolution of formed Pb carbonates or phosphates through competitive complexation of the fluid-inherent pepsin or organic acid with Pb under neutral condition. Complexation of free Pb²⁺ with organic components in the fluid, including pepsin, organic acid, pancreatin or bile inhibited the formation of insoluble Pb phosphate and resulted in an increase of Pb bioaccessibility in both gastric and intestinal phases. Results from this study indicated that both phosphate amendment and fluid components impacted the transformation and bioaccessibility of Pb, and that the one-time measurement using PBET assay may not accurately estimate Pb bioaccessibility in the P-rich soil or the contaminated soil amended with P.