Transformation And Bioaccessibility Of Contaminated Soil Lead In Simulated Human Gastrointestinal Tract

Incidental hand-to-mouth ingestion of heavy metal-contaminated soil is one of a primary pathways of heavy metal exposure,especially for children.The bioavailability of heavy metals in the human body depends on the bioaccessibility of heavy metals in the soil,and besides total amount,the speciation of heavy metals also plays an important role in controlling the bioaccessibility.In recent years,a variety of in vitro digestion methods have been established to study the bioaccessibility of soil heavy metals corresponding to in vivo animal exposure experiments.However,differences between methods and heavy metals transformation during methods conduction have been rarely reported.In this dissertation,the bioaccessibility of lead?Pb?in contaminated soils was studied using three kinds of in vitro digestion methods.The XRD spectroscopy and Visual MINTEQ,a chemical equilibrium model,were used to explore speciation and transformation of Pb from real soil environment to the simulated gastric and intestinal fluid.The effects of simulated methods,Pb compounds,soil properties,and combined heavy metal pollution were investigated to evaluate the transformation and bioavailability of Pb.The main findings are as follows:?1?The bioaccessibility and transformation of Pb in contaminated soils with and without phosphorus-containing materials were studied using various in vitro simulation methods?SBRC,PBET and IVG?.The effects of difference in simulation methods on Pb transformation and bioavailability discussed.The results showed that the order of bioaccessibility of Pb in the three simulation methods was SBRC>PBET>IVG in simulated gastric fluid,and PBET>SBRC>IVG in simulated intestinal fluid.The pH value and organic components of the simulant were considered as the major causes of differences in Pb bioaccessibility.In the acidic gastric fluid,Pb was dissolved into free ion and organic complex,and converted into carbonate precipitation state in the neutral intestinal fluid.With the addition of phosphorus-containing material?TSP?,Pb bioaccessibility could be effectively reduced:from 71.33%⁹6.06%to 37.91%⁸4.14%in gastric stage,and from 0.63%⁸.68%to 0.10².12%in intestinal stage,which could be explained by the formation of insoluble Pb₅?PO₄?₃Cl from part of PbCO₃.In addition,the IVG method might underestimate the health risk of contaminated soils or overestimate the performance of remediation materials,while the PBET method was more suitable for strict evaluation of the health risk of Pb.?2?The transformation and bioaccessibility of Pb during the PBET assay process for(Pb?NO₃?₂ and PbCO₃ upon phosphorus-containing materials amendment was studied,and the impact of P amendment and fluid components on Pb bioaccessibility during extraction procedure was discussed.The results showed that soluble Pb?NO₃?₂had higher bioaccessibility than less soluble PbCO₃,and the bioaccessibility of both Pb compounds was significantly reduced with the addition of P material.In the acidic gastric phase,the bioaccessibility of Pb rapidly decreased by more than 95%in 10minutes with the continuous formation of Pb₅?PO₄?₃Cl or PbHPO₄ precipitates.In the neutral intestinal phase,the bioaccessibility of Pb was reduced by more than 60%.However,the Pb bioaccessibility in the intestinal phase slightly increased during the extraction,probably due to the complexation of the fluid-inherent organic matter with Pb.As for fluid components,the presence of pepsin,organic acid,pancreatin or bile salt in gastric and intestinal phases increased Pb bioaccessibility,probably due to their competitive complexation with Pb²⁺,and inhibiting formation of insoluble Pb-P mineral.?3?The transformation and bioaccessibility of Pb in four simulated sudden polluted soils during the extraction procedure by the PBET method was studied.The effects of soil properties,such as soil pH,mineral composition,organic matter,and inorganic anions,on speciation and transformation of Pb were investigated from real soil environment to the simulated gastric and intestinal fluid.The results showed that the speciation of Pb in contaminant-spiked soil were different by various soil properties,and therefore the bioaccessibility of Pb was varied.Pb bioaccessibility of the four test soils in the gastric fluid was 70.9%⁸4.7%and in the order by YT>AS>CS>FQ,while in the intestinal fluid was 9.25%³4.8%and in the order by AS<FQ<YT<CS.In the strongly acid gastric phase,Pb in various forms,even like insoluble PbCO₃,could be dissolved and/or desorbed rapidly into free Pb ion and complex,resulting in an increase of Pb bioaccessibility.And in the neutral intestinal phase,the dissolved Pb could be converted quickly into the adsorbed or precipitated state.In consideration of strict criteria,the health risk of red earth was the highest,and in consideration of being absorbed by the human body,paddy soil had a highest health risk.?4?The morphological conversion and bioaccessibility of Pb and As during the extraction process of PBET were studied in a historically Pb-As polluted soil with and without P amendment,and the impact of combined pollution on the transformation and bioaccessibility of Pb was investigated.The results showed that the main forms of Pb/As in the historically co-contaminated soil were PbCO₃,PbSO₄ and Pb₅?AsO₄?₃Cl.The bioaccessibility of Pb and As in the gastric fluid was below 12%,and further decreased to less than 1%in the intestinal fluid.In the gastro-intestinal phase,the dissolution of Pb and AsO₄^3-in the co-contaminated soil could be enhanced for the precipitation of Pb₅?AsO₄?₃Cl and therefore,the bioaccessibility of Pb and As decreased with the increase of extraction time.With the addition of phosphorus-containing material,the bioaccessibility of Pb could be effectively reduced in the gastric phase,but the bioaccessibility of As in the intestinal phase increased from 1.01%to 2.61%.The addition of P amendment could provide plenty of PO₄^3-for the substitution of AsO₄^3-,resulting in the formation of Pb₅?PO₄?₃Cl mineral.Therefore,phosphate-induced immobilization of Pb-As co-contaminated soil could reduce the health risk of Pb but increase the risk of As.In summary,the effects of various in vitro simulation methods,Pb compounds,soil properties,and combined heavy metal pollution were investigated in this study,to evaluate the mechanisms of speciation transformation and bioavailability of Pb,which could provide a more reliable and scientific basis for the establishment of heavy metal-contaminated soil remediation standards and health risk assessment.