Bioavailability And Their Toxic Effects Of Heavy Metal In Soils From An Electronic Disassembly Plant

Large quantities of electronic waste(e-waste)have been widely recycled due to the considerable quantities of valuable and reusable metals and plastics they contain.However,recyclable electronic components are separated using a wide range of primitive e-waste disposal methods,in which the heavy metals released can be deposited directly into the soil by irrigation water and air resulting in soil contamination.Studies have shown that e-wastes release toxic metals including Cd,Cu,Ni,Pb and Zn into nearby soils during dismantling process.Therefore,environmental and human health problems caused by heavy metals in e-waste are attracting more and more attention.In addition,accidental oral ingestion of soil is one of the main pathways for human exposure to heavy metals.However,most of existing health risk assessment models are dependent on the total concentrations of heavy metals,which may overestimate the risk.Recently,some scientists demonstrated that bioavailable metals are more accurate to predict its health risk since it reflects actual fraction of human absorption.Given that,soil samples from an electronic disassembly plant in Wenling,Zhejiang Province were sampled to characterize the physical and chemical properties of the soil and clarify the characteristics of heavy metal pollution.Caco-2 cell model was established in vitro to determine the bioavailability of soil heavy metals,and its absorption and transport mechanism was explored from the perspective of molecular biology.Moreover,the toxic effects of soil extracts on human intestinal epithelial cells were also evaluated.It is of great significance to accurately assess its health risks and provide a scientific basis for the health risk assessment of heavy metals in soil in China.The main results of this study are as follows:(1)To characterize the physical and chemical properties of the soil and clarify its heavy metal pollution characteristics.The results showed that the physical and chemical properties of soil in this site were similar,with pH ranging from 4.4 to 4.9.Total organic carbon is 1.2%-1.5%.Silt is mainly particle size of soil.Total concentrations of Cd,Ni,Pb,and Zn in some soils exceeded the risk screening values based on soil environmental quality standard for contaminated soils(GB15618-2018),while Cd and Cu in all samples exceeded the risk screening values.Among metals,Cd and Ni were of the most concern as total Cd and Ni exceeded the risk screening values in soils,being 12.8–29.2 and 0.71–2.6 times of the screening value,respectively.The extractable Cd and Ni were 20%–67% and 3.6%–29% for total concentration,respectively.In addition,the results of the carcinogenic risk coefficient calculated based on the total and extractable Cd and Ni in the soil showed that both Cd and Ni had carcinogenic risks for children and adults.(2)Bioavailability and absorption and transport mechanism of soil total metals based on Caco-2 cell model were investigated.The bioavailability of Cd was relatively high(6.30%–25.08%),followed by Ni(6.60%)and Cu(3.22%).In addition,the expressions of metal transporters DMT1,ZIP8,ATP7 A and CTR1 were altered after exposure to soil extracts in Caco-2 cells,and the up-regulation of DMT1 expression might mediate the increase of Ni content in cells.The increased expression level of ATP7 A maintained intracellular Cu content in response to soil extracts.The down-regulation of ZIP8 expression and up-regulation of CTR1 expression caused by soil extracts may be one of the mechanisms of soil Cd with high bioavailability.In addition,down-regulation of CLDN3 expression and decreased fluorescence intensity may be one of the targets for soil extracts to destroy Caco-2 cell tight junctions.Therefore,the damage of soil extracts to the tight connection of intestinal epithelium cells may also be one of the reasons that Caco-2cells absorb heavy metals in soil.(3)The mechanism of toxic effects of soil extracts on Caco-2 cell viability,oxidative damage,inflammatory response and apoptosis were investigated.Cell morphology showed that with the increase of soil extracts concentration,cell membrane structure distortion and suspension increased.Cell viability results showed that three soil samples showed Soil-2 was high toxicity followed by Soil-1and Soil-3.The cytotoxicity of Caco-2 induced by soil extracts was related to oxidative damage,inflammatory response and apoptosis.Oxidative damage caused by down-regulation of the expression of antioxidant enzymes SOD1 and CAT genes resulted in increased intracellular reactive oxygen species.Inflammatory responses,activation of Caspase family genes and increased GADD45 α expression were also observed during Caco-2 cell apoptosis.And we found that the toxic index and extractable Cd and Ni has strong correlation,therefore this study results showed that the extractable Cd and Ni may have induced apoptosis by activating inflammation and mitochondrial apoptotic pathway-related factors.Moreover,soil extracts can increase the risk of field soil heavy metals to human body health.