Investigation Of Chemical Immobilization On The Bioavailability Of Lead And Cadmium In Vegetable-growing Soils By The Method Of Isotope Labeling

With the increase of agricultural, anthropogenic and industrial activities, the quality andsafety of vegetable-growing soil turned out to be threatened. There has been a great concernon environmental issues arising from the presence of heavy metals, especially of lead (Pb) andcadmium (Cd). Due to the speciality of Pb and Cd, it is generally recognized that it is thelabile fraction rather than the total soil Pb and Cd content which is critical when assessingtheir availability in soils. Therefore, we our human being should pay more efforts to controland remediate heavy metals in soil, especially concerning the bioavailability in both Pb andCd. In-situ immobilization is a desirable strategy for reducing metal bioavailability throughprecipitation or adsorption by adding chemical amendments to contaminated soils.The thesis contains four chapters.Chapter1, the contamination of Pb and Cd in vegetable-growing soils and theinvestigating methods of bioavailability of Pb and Cd in soils are extensively reviewed.Additionally, techniques concerning immobilization of heavy metal’s bioavailability are alsodiscussed.Chapter2focuses on the immobilizing efficiencies of various amendments on thepotential availability of Pb and Cd. Different kinds of chemical amendments were added tothe soils, which were incubated in closed plastic containers for30d. Humic acid presented themost effect on reducing both of water soluble and exchangeable Pb fractions. For Cd, rapepowder provided the strongest immobilizing efficitiveness on the water soluble and exchangefractions.Chapter3, isotope labeling method is established and applied to the investigation ofdistribution of Pb and Cd in vegetable-growing soils. The remediation efficiencies on theavailability of Pb and Cd are estimated using stable isotope labeling method. Results showedthat actived carbon and rice hull presented the highest immobilizing efficiency on watersoluble Cd by96%, and zeolite combining with chitosan also reduced the largest amount ofexchangeable Cd by59%. For Pb, the mixture of coconut husk and lime reduced watersoluble and exchangeable fractions by24%and73%, respectively, which were the highest immobilizing capacity.Chapter4focuses on the investigation of the in-situ immobilization on thephytoavailability of Pb and Cd using the pot experiment combined with isotope labelingmethod. Results showed that lime and coconut husk reduced the largest amount of edible partuptake of Pb by53%, and zeolite and chitosan presented the most immobilizing efficiency onCd uptake by edible part at56%. For Cd, actived carbon and rice hull immobilized watersoluble fraction at the largest extent by93%, and zeolite combining with chitosan reducedexchangeable Cd by35%, which was the highest efficiency. In addition, Cd provided thestrongest correlations between metal bioavailability and root concentrations, with CDGTgivingthe strongest relationship for Cd concentrations in roots (R2>0.89). This suggests that thesolubility of Ni is highly indicative of shoot concentrations and that isotope labeling is apromising measure of heavy metal’s bioavailability.Overall, the addition of amendments provide large capacity in reducing bioavailable Pband Cd in vegetable-growing soils, and results indicated that stable isotope labeling can be arobust approach for evaluating metal bioavailability.