Study On The Bioavailability Of Lead And Cadmium In Vegetable-growing Soils By The Method Of Isotope Dilution

Heavy metals in agricultural soil environment affect the growth quality of crops and thefood safety. The availability of heavy metals is not only closely related to the amounts heavymetals in soils, but also to their chemical forms. The forms of heavy metals decide theirbioavailability in soils and reflect their pollution levels to the grown vegetables. In order tocontrol and reduce the contamination of heavy metals in vegetables, it is important to assess thebioavailability of heavy metals in soils rapidly and accurately. In this dissertation, a method ofmeasuring trace lead and cadmium in vegetable-growing soils by isotope dilution andinductively coupled plasma mass spectrometry （ICP-MS） has been established. In addition, twomethods of assessing the bioavailability of lead and cadmium in soils are established based onthe technique of stable isotope dilution.In chapter one, the present situation of lead and cadmium contamination in vegetables andvegetable-growing soils has been described. And the analytical methods used for determining theamounts and the different chemical forms of lead and cadmium in soils have been introduced. Inaddition, the methods for estimating the bioavailability of lead and cadmium in soils have alsobeen reviewed.In chapter two, a method of determining lead and cadmium contents in soils by ID-ICP-MSwas established. The concentrations of enriched reagents were measured by the method ofreverse phase isotope dilution. The adding amounts of enriched isotope reagents (¹¹²Cd and²⁰⁶Pb), the mass bias, the dead time as well as the interferences of isobaric and polyatomic ionswere discussed and optimized. Ag and Tl（109Ag/107Ag and205Tl/203Tl）were chosen as theinternal standards. Cadmium and lead contents in a standard reference soil（GBW07423）wasdetermined by the established method and the recoveries were97.6%for Pb and94.9%for Cd,respectively. Lead and cadmium contents in vegetable-growing soils were measured in the sameexperimental conditions, and the results showed that lead and cadmium contents in most of thesoils excessed of the limits of “Farmland Environmental Quality Evaluation Standards for EdibleAgricultural Products （HJ332-2006）”.In chapter three, a method of measuring the amounts of labile Pb and Cd （E values） invegetable-growing soils based on isotope dilution was established. After adding the enriched isotope reagents (¹¹²Cd and²⁰⁶Pb) in soil suspension, the isotope ratios of²⁰⁸Pb/²⁰⁶Pb and¹¹²Cd/¹¹¹Cd at different equilibrium time were monitored. Etvalues were calculated with theformula of isotope dilution method at each equilibrium points. In addition, Etvalues werecompared with the amounts of Pb and Cd extracted with single chemical agents and sequentialextraction procedure of BCR approach. The results shows that the E_1minvalues of Pb in three soilsamples （Xiamen Tong’an and Putian Hanjiang） were2.15,6.64and94.1μg/g; while the E_1minvalues of Cd were0.0710,0.243and0.0790μg/g. The E_1minvalues were larger than thoseextracted with0.01mol/L NaNO₃and0.43mol/L CH₃COOH, but were less than the valuesextracted with0.05mol/L EDTA, and were also larger than the amounts of acid-soluble formsbut less than the amounts of reducible forms extracted by BCR approach. Even though thecomparatively large E values, which might be owing to the fixation phenomenon in soils, theE_1mincan be used to rapidly reflected the bioavailability of Pb and Cd in vegetable-growing soils.In chapter four, a method of measuring the bioavailability of Pb and Cd in soils by isotopedilution coupled with pot experiments was established. After adding the enriched isotopereagents (²⁰⁶Pb and¹¹²Cd) into the soils, the pot experiment of cultivating vegetables was carriedout. L values of Pb and Cd in soils were calculated by determining the contents of Pb and Cd inthe edible and inedible parts of vegetables by isotope dilution method. The isotope ratios(²⁰⁸Pb/²⁰⁶Pb and¹¹²Cd/¹¹¹Cd)of different chemical forms of Pb and Cd in soils extracted by BCRsequential extraction procedure were also determined durning the pot experiment. The resultsshowed that the isotope ratios(²⁰⁸Pb/²⁰⁶Pb and¹¹²Cd/¹¹¹Cd)were distinctly changed in soils afteradding the enriched isotope reagents (¹¹²Cd and²⁰⁶Pb) in soils. The L values of Pb in XMTAwere0.715μg/g and6.16μg/g，and Cd in PTHJ were0.086μg/g and0.180μg/g according to themeasuring results of Pb and Cd in edible and inedible vegetable parts. L value of Pb in soildetermined by inedible vegetable parts was larger than E_1minvalues, while L value of cadmiumwas less than E_1minvalues. In conclusion, L values can truly reflect the bioavailability of Pb andCd in vegetable-growing soils.