Comparative Study On Bioavailability Of Heavy Metals In Farmland Soils With Anthropogenic Pollution And High Geological Background

Farmland pollution by heavy metals has received continuous attention and exploration in recent years.Heavy metal pollution in soil not only inhibit the growth of crops,but also degrade the quality of the soil.Long-term consumption of crops contaminated by heavy metals will cause severe harm to human health.The causes of heavy metal pollution in soil are mainly divided into two types,one is anthropogenic activities;the other is high geogenic background.The ecological hazards of heavy metal pollution in soil are not only related to its total concentration,but also to a greater extent determined by their species distribution.In this thesis,to study on bioavailability of heavy metals in farmland soils with anthropogenic pollution and high geological background comparatively.Three typical areas were selected: 1)anthropogenic pollution(Haiyan and Wenling,Zhejiang),2)high geogenic background of sulfide mining(Daxin,Guangxi)and Xijikeng lead-zinc mining(Guipin,Guangxi),and 3)high geogenic background of karst(Hengxian,Wuming,Guiping,et al,Guangxi).Soil and rice grain samples from these areas were collected to identify enrichment characteristics and ecological risks of heavy metals.In this study,0.01 MCaCl2 extraction method,0.05 M EDTA extraction method,0.43 M HNO3 extraction method and DGT(diffusive gradients in thin-films)technique were used to systematically study the bioavailability of heavy metals in these areas.The main conclusions are as follows:(1)The concentrations of heavy metals in soils of the three types of pollution areas were varied.In general,Cd was the most common and the most polluted metal in all areas,which poses a huge risk to agricultural ecology and food safety.The main factors controlling soil Cd content in anthropogenic contaminated areas are soil organic carbon and S content,while in sulfide mining areas are soil p H and the content of major elements such as S,Fe,Al.In karst area,soil Cd content are mainly controlled by soil p H and Ca O content.(2)For rice grains,the most cotaminated metal is Cd,followed by Ni and As.As expected,the correlation of the total concentration of heavy metals in soils with those in rice grains was not significant in polluted areas.The main controlling factors of Cd in rice grains were soil p H and bioavailable concentrations of Cd.(3)Regardless of the chemical extraction methods(0.01 MCaCl2,0.05 M EDTA,and 0.43 M HNO3),the extraction rate of Cd is the highest among heavy metal elements,so Cd is the most active heavy metal element.In typical sulfide mines(Xijikeng leadzinc mining),the extraction rate of Cd is the highest and the bioavailability is the strongest;while in areas with karst background(Daxin lead-zinc mining and Guangxi karst area),the extraction rate of Ca Cl2 is relatively low and the bioavailability lower.The desorption rate constant(k-1)and response time(Tc)estimated by the DIFS(DGTinduced fluxes in soils and sediments)model explained that Cd was more bioavailable than Ni.Cd was easier than Ni to desorb from soil solid phase into solution phase.(4)The extracted Cd content of 0.01 MCaCl2 has a strong correlation with the Cd content of rice grains in the three types of regions.Hence,0.01 MCaCl2 extraction is an effective method to study the bioavailability of Cd.(5)There was a generally negative correlation between the concentration of Ca Cl2-extractable heavy metals(such as Cd and Ni,except As)and soil p H,indicating that artificial regulation of soil p H would reduce the bioavailability of most heavy metals.