Human Bioavailability Of Typical Heavy Metals In Contaminated Soils

Soil ingestion by hand-to-mouth activity is an important pathway for children’s exposure to heavy metals.Risk assessment based on total concentration may overestimate their risks.In order to refine exposure risk,bioavailability of heavy metals should be considered.In vivo assays were considered as direct approach to determine heavy metal relative bioavailability(RBA).Based on linear relationship with RBA.in vitro assays,which were more economic and easier to handle,were developed.Swine and mouse were two commonly used animal models.Comparing with swine,mouse model was more economic and easier to handle.But relative standard deviation(RSD)of RBA determined by the usually single gavaged exposure in mouse model can not meet the recommended standard(RSD<20%).Therefore,it is necessary to optimize the mouse assay.In present study,diversity of different assays were discussed,mouse assays were optimized by changing exposure way and selecting different endpoints.What’s more,the suitability of in vitro assays to predict RBA of heavy metals in original and P treated soils was assessed.The main results were as follows:1.Arsenic bioaccessibility in 11 contaminated soils were determined by SBRC,PBET,IVG,and UBM.Following the pH increasing in gastric phase,As bioaccessibility in the four assays generally decreased.Compared with gastric phase,As bioaccessibility in intestinal phase were generally lower.Based on an improved sequential extraction procedure,As in 11 contaminated soil was mainly distributed in amorphous Fe/Al(AF3)and crystallized Fe/Al bounded fractions(CF4),while distributed less in non-specifically sorbed(NS 1),specifically sorbed(SS2),and residual(RS5)fraction.Correlation analysis between As bioaccessibility and fraction data showed that extractable As in PBET mainly from NS 1+SS2,while NS I+SS2+AF3 and NS1+SS2 contributed extractable As in gastric and intestinal phase of UBM.However,bad correlations were found between fraction data and IVG/SBRC.2.After optimizing exposure method and endpoint selecting,Pb RBA in 12 soils were assessed using Pb accumulation in mouse liver,kidneys,or femur following a 10-d steady state soil dose via diet.Pb-RBA based on the combined end-point(liver +kidneys + femur)increased data consistency in different endpoints,while decresed RSD from 11.9-15.8%in single endpints to 10.8%in combined endpoints.Comparing with mouse blood AUC,Pb RBA in ccombined endpints were comparable,but RSD was much lower.Therefore,considering the economic and operability of experiment process and robustness of results,the steady state dosing approach with Pb accumulation in combined tissues is recommended to determine Pb RBA in mouse model.3.The optimized exposure expetiment was used to determine Cd RBA,and suitability of in vitro assays to predict Cd RBA was assedded.Cadmium bioaccessibility in soils varied with assays.Based on the optimized in vivo assay,Cd RBA was measured based on Cd concentrations in the mouse liver and/or kidneys.Compared with single endpint,Cd RBA in liver+kidneys provided a better repetablity.In vitro-in vitro correlation showed that the PBET have potential to predict Cd-RBA in contaminated soils.4.This study improved the in vivo experiment further.Based on the mouse kidneys bioassay following repeated daily single gavage dose of soil to fasted mice for 10 d,effect of P addition on changing of Pb and As RBA was investigated.By comparing with in vitro assays,the ability of in vitro assays to predict Pb and As RBA changing after soils being treated with P was assessed.After being treated with P,Pb RBA in amended soils decreased.In contrast,As RBA in soil B decreased while it increased in soil D,with limited changes in 3 other soils.Based on in vitro assays,Pb bioaccessibility generally decreased in amending soils.while As bioaccessibility generally increased.When comparing effects of PA amendment on Pb RBA and bioaccessibility,gastric phase of PBET and intestinal phased of SBRC had the potential to predict effect of P on Pb RBA.For As,its bioaccessibility increased in PA-amended soils,inconsistent with in vivo data,implicating unsuitability of current assays.