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Bioavailability of Cadmium and Zin to Two Earthworm Species in High-metal Soils

  • Author(s): Liu, Ying
  • Advisor(s): Parker, David R
  • et al.
Abstract

It was generally recognized that the bioavailability, other than total content of heavy metals in soil, is crucial to predict metals' toxic effect on environment. Stable Isotope Dilution technique (SID) equipped with inductively coupled plasma-mass spectrometry (ICP-MS) is a good way for measuring the potential bioavailable portion of metals in soils. In Chapter 2, we build method to accurately determined Cd (111Cd/114Cd) and the ratio of Zn (66Zn/68Zn) in soil suspension and then applied this to stable isotope dilution technique to successfully obtain the labile fraction of Cd and Zn in polluted soils.

In Chapter 3, it was found when compared to chemical extraction methods, the stable isotope dilution technique is the better way to assess bioavailability of Cd and Zn in soil to the earthworms Eisenia fetida and Lumbricus terrestris. Besides, the result showed on average, the fraction of labile Cd (44.7%) was significantly higher than that of Zn (25.4%), indicating that Cd is more chemically reactive in soil, which may explain the larger BSAFs for Cd in earthworms. In addition, it was found the accumulation of heavy metals was not only element-specific, but also species-specific.

In Chapter 4, the uptake and elimination kinetics of Zn and Cd by E. fetida and L. terrestris were investigated then. The kinetic rates were calculated using a first-order, one-compartment, toxico-kinetic model. Pronounced differences were found on the earthworms' uptake and elimination kinetics between Zn and Cd (p<0.01).

Lastly in Chapter 5, it was found earthworms' activity increased the pH value and decreased organic carbon content of soils. In addition, the results showed that the Cd, Zn contents in the casts was significantly decreased, while the percentage of 0.01M Ca(NO3)2, 1M CaCl2-extractable Cd and Zn, as well as the fraction of labile Cd and Zn in the casts were significantly increased, suggesting that the earthworms effectively improved heavy metals' lability.

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