Assessment Of Heavy Metal Pollution In Vegetable-growing Soil And Investigation Of The Bioavailability Of Pb With Isotope Labelling Method

The migration, transformation and the toxic effect on plant of heavy metals in soil dependnot only on the metal’s total concentrations, but relying on their various chemical species.Therefore, the studies on heavy metals pollution assessment and analysis and the researches onthe bioavailability of heavy metals （Pb） have an important practical significance for theevaluation of agricultural soil environmental quality and food safety.This dissertation consists of four chapters.In chapter one, on the basis of access to a lot of literatures on heavy metal contamination,hazards and evaluation methods were summarized. The review was mainly focused on themethod of assessing the bioavailability of Pb in soils, as well as the rule of migration andtransformation about the Pb in the plant-soil system and the distribution of characteristics inthe plant.In chapter two, the concentrations of heavy metals in40soil samples were measured.And the contamination of heavy metals in soils was assessed with single factor index andNemerow comprehensive pollution index, geoaccumulation index （Igeo） and potentialecological risk index （RI）, respectively. The results showed that most of the vegetable growingsoil has different levels of heavy metal pollution, mainly for the Hg, Cd and Pb pollution, andZn、Cu、Ni、Cr and As with the characteristic associated pollution. Ecological risk indexassessment indicated that the SMNS, GWMS and HCDB heavy metals of potential ecologicalrisk index （RI） in soil samples respectively919,814and783, were more than strongecological risk criteria （RI≥600） are strong ecological harm, especially Hg and Cd haveparticularly serious pollution.In chapter three, Soil samples were collected in SMNS, SMHC and GWDD where is atdifferent Pb pollution level. The amounts of labile Pb in soils were estimated with differentsingle chemical extractants and a modified BCR approach compare with the isotope dilutionmethod （E value and L value）, and a reasonable and accurate method of assessing leadbioavailability in soils was established. The results showed that EDTA> NaOH> lowmolecular weight organic acids> CH3COOH> NaNO₃> CaCl₂. The fractions of Pb in the firstthree fractions in BCR sequential extraction procedure were summed up as∑BCR values. The∑BCR values ranged from30.0%to37.7%. Lbranged from25%to30%of the total Pbcontents. The labile pools by%E24hvalues ranged from53.2%to61.7%with an average57%in the three soil samples, which was significantly higher（p<0.05）than those estimated withsingle chemical extractants, a modified BCR approach and Lbvalue, respectively. A strongnegative correlation between E24hvalues and pH was found in the sampled soils （R2=0.984）.The results indicated that the pools of labile Pb in soils may be governed by soil pH, whichmay be very useful in further soil remediation for reducing the bioavailability of Pb incontaminated agricultural soils.In chapter four, the absorption mechanism of lead absorbed in vegetable-growing soils byplant was studied by isotope labeling method. In order to research the lead forms, lead forms,which labeled by²⁰⁶Pb in rhizosphere soils, were extracted through modified BCR sequentialextraction during the plant cultivation, and the isotope ratios of²⁰⁸Pb/²⁰⁶Pb in vegetablesdifferent tissues were measured. The subcellular distribution of Pb in vegetables differenttissues was researched through differential centrifugation. The results showed that, the isotoperatios of Pb (²⁰⁸Pb/²⁰⁶Pb) in acid soluble form changed the most obviously, which indicatedthis lead form was the most easily absorbed by plant. The isotope ratios of Pb (²⁰⁸Pb/²⁰⁶Pb) inresidual form remained unchanged, which suggested that this form was extremely stable andcould hardly be transformed into other forms. The²⁰⁸Pb/²⁰⁶Pb ratio changes in various parts ofthe vegetables followed the order: root> stem> leaf, which indicated that the root was themain part where plant absorbed lead from soil. The subcellular distribution of lead invegetable various tissues showed that, the Pb absorbed in the soil mainly deposited in the cellwall accounting for60-80%of the total Pb; followed by cell organelles, the ratio is about20%;finally, the cytoplasmic portion is only about10%of the total Pb, which suggested that the cellwall is Pb main accumulation area of vegetable absorbed from soil. The results provide atheoretical basis for the control of vegetables pollution and improvement of food security.