Human Bioavailability Of Lead And Arsenic In Contaminated Soils

Ingestion of contaminated soil is a major nondietary exposure pathway of arsenic (As) and lead (Pb). To more accurately quantify As or Pb eaposure via soil ingestion, determination of the bioavailability is required. Number of animal models have been used to measure As/Pb relative bioavailability (RBA) in soil. However, given the long time, cost and ethical issues, there is great demand for simple, rapid, and inexpensive in vitro assay for estimating RBA. At present, a variety of in vitro assays from different countries have been applied for the determination of bioaccessibility. However, before these assays used to predict RBA, a correlation between in vitro bioaccessibility and in vivo bioavailability is a mandatory prerequisite. Except in vitro assays, different animals and biomarkers have been used to measure As RBA, however, the influences of different animal models, dosing schemes, and biological end points on As-RBA measurement are lacking. For this study, the IVIVC between As/Pb bioaccessibility via different in vitro assays and RBA measured by mouse blood model was established of contaminated soil from China. In addition, we compared As-RBA in contaminated soils determined by use of different animal models, different feeding schemes, and different biomarkers. The main results were as follows:1. Twelve Pb-contaminated soils representing different contamination sources from China were analyzed for Pb bioaccessibility using four in vitro methods (UBM, SBRC, IVG, and PBET), Pb-RBA using a mouse blood model, and Pb fractionation using sequential extraction. Lead bioaccessibility in the gastric phase (GP) and Pb-RBA was generally lower in mining soils (0.46-29% and 7.0-26%) than smelting (19-92% and 31-84%) and farming soils (13-99% and 51-61%), with more Pb in the residual fraction in mining soils. Lead bioaccessibility varied with assays, with SBRC (3.0-99%) producing significantly higher bioaccessible Pb than other assays (0.46-84%) in the gastric phase. However, Pb bioaccessibility in the intestinal phase (IP) of all assays sharply decreased to 0.01-20% possibly due to Pb sorption to solid phase at higher pH.2. Lead bioaccessibility by UBM-GP assay was best correlated with Pb-RBA (R2= 0.67), followed by IVG-GP (R2=0.55). Among different Pb fractions, strong correlation was found between Pb bioaccessibility/Pb-RBA and the sum of exchangeable and carbonate fractions. Our study suggested that UBM-GP assay has potential to determine Pb bioaccessibility in contaminated soils in China.3. As bioaccessibility and As-RBA in 12 As-contaminated soils (22.2-4172 mg kg-1As) were measured usingfive assays (SBRC, IVG, DIN, PBET, and UBM) and a mouse blood model. Arsenic RBA in the soils ranged from 6.38±2.80% to 73.1±17.7% with soils containing higher extractable Fe showing lower values. Arsenic bioaccessibility varied within and between assays. Arsenic bioaccessibility was used as input values into established IVIVC to predict As-RBA in soils. There were signi ficant differences between predicted and measured As-RBA for the 12 soils, illustrating the inability of established IVIVC to predict As-RBA in those contaminated soils.4. A new IVIVC was established by correlating measured As-RBA and As bioaccessibility for the 12 soils. The strength of the predictive models varied from R2= 0.50 for PBET to R2=0.83 for IVG, with IVG assay providing the best prediction of As-RBA. When IVIVC were compared to those of Juhasz et al. (2014a), slopes of the relationships were signi ficantly higher possibly due to different As-RBA measurements. Our research showed that IVG has potential to measure As bioavailability in contaminated soils from China though UBM and SBRC assays were also suitable. More research is needed to verify their suitability to predict As-RBA in soils for refining health risk assessment.5. As-RBA in 12 As-contaminated soils with known As-RBA via swine blood AUC model were measured by mouse blood AUC, SSUE, and liver and kidney analyses. Comparison As-RBA based on different animals (i.e., swine and mouse) and biomarkers [area under blood As concentration curve (AUC) after a single gavaged dose vs steady-state As urinary excretion (SSUE) and As accumulation in liver or kidney after multiple doses via diet]. In this study, As-RBA ranges for the four mouse assays were 2.8-61%,3.6-64%,3.9-74%, and 3.4-61%. Compared to swine blood AUC assay (7.0-81%), though well correlated (R2=0.83), the mouse blood AUC assay yielded lower values (2.8-61%). Similarly, strong correlations of As-RBA were observed between mouse blood AUC and mouse SSUE (R2=0.86) and between urine, liver, and kidney (R2=0.75-0.89), suggesting As-RBA was congruent among different animals and end points.6. Different animals and biomarkers had little impact on the outcome of in vivo assays to validate in vitro assays. On the basis of its simplicity, mouse liver or kidney assay following repeated doses of soil-amended diet is recommended for future As-RBA studies.