Investigation Of Chemical Immobilization Of Pb In Vegetable-growing Soil And The Mechanisms Of Soil Remediation

There has been a great concern on environmental issues arising from the presence of heavy metals,especially of lead（Pb）in vegetable-growing soil.The toxic effects on plant of Pb depend not only on the total soil Pb concentration,but relying on its various chemical species.In-situ immobilization is a cost-effective method for remediation of metal contaminated soil.It is a desirable strategy for reducing metal bioavailability via precipitation or sorption by adding chemical amendments to contaminated soils.The dissertation aims to assess the effects on the bioavailability of Pb after amendments,and to discuss the mechanism of Pb immobilization in soil using isotope labeling method,X-ray diffractin（XRD）and model Visual MINTEQ.The dissertation contains four chapters.In chapter one,the contamination,the hazards and the bioavailability of Pb in agricultural soils were summarized based on reading a lot of literatures.Techniques concerning immobilization of Pb and its bioavailability were also discussed.Chapter two focused on the immobilizing efficiencies of different kinds of amendments on the immobilization of Pb contaminated in soils.Different kinds of chemical amendments were added to the soils and the amounts of chemical active Pb in soils were estimated with the method of single chemical extractant.On the basis of single amendment tests,three amendments which had the better amendment efficient were chosen to be a compound amendment.The ratio of the composition of the compound amendment was optimized by the approach of response surface analysis.Results showed that bone meal（BM）,diammonium phosphate（DAP）and hydroxyapatite（HA）presented the most effects on reducing Pb contents in acid soluble fractions with the reduction values of 60.3%,75.6%and 80.5%,respectively.The compound amendment with a optimum ratio of 10.00 g/kg_（soil）BM,15.00 g/kg_（soil）HA and 20.00g/kg_（soil）DAP could significantly reduce the Pb amount in acid soluble fraction by 98.8%。In chapter three,the distribution of Pb in vegetable-soils was studied by isotope labeling method.And the mechanism of Pb immobilization in soil was investigated by XRD and model Visual MINTEQ.Results showed that the four amendments could not only decrease Pb contents in the exchangeable and carbonate bound fractions but also increase its concentration in the residual fraction.Complex amendment and DAP presented the highest immobilizing efficiency on exchangeable fraction of Pb by 93.3%and 89.7%,respectively.The results of XRD and model Visual MINTEQ showed that the primary reason of Pb immobilization was the formation of pyromorphite（Pb₅（PO₄）₃Cl）compounds after amendments addition.Chapter four focused on the investigation of the in-situ immobilization on the phytoavailability of Pb using the pot experiment combined with isotope labeling method.Result showed that complex amendment and DAP reduced the largest amount of root uptake of Pb by57.1%and 50.9%,respectively.The subcellular distribution of Pb in various vegetable tissues showed that the Pb absorbed from the soil mainly deposited in the cell wall accounting for 67.1%-74.0%and the cytoplasmic protion was only about 6%of the total Pb,which suggested that the cell wall was Pb main accumulation area of vegetable.Overall,the results provided a theoretical basis for the control of vegetable pollution and improvement of food security.