| Heavy metals such as Pb and Cd have serious biological toxicity to soil and edaphon and they are hard to be removed, too. In this study,we selected EDTA, citric acid, oxalic acid, tartaric acid and other organic acids to investigate the activation of Pb, Cd in brown soil, fluvo-aquic soil and cinnamon soil, so that we can improve the phytoremediation technology and provide theoretical basis for enhancing remediation efficiency. In addition, we selected earthworm as the research object which is the main soil fauna. By studying the cycle activity and metabolism of earthworm, we investigated the effect of earthworm to the availability Pb and Cd and the self-accumulation of earthworm to Pb and Cd. Meanwhile, this study used rape as an indicator of the activity of Pb and Cd. The main results are listed below:1. Three low-molecular organic acids and EDTA have similar effect on Pb and Cd activity. In brown soil, the order of Pb activating ability is citric acid> tartric aicd> oxalic acid, with the maximum activity efficiency 5.4%, 3.4% and 0.5% respectively; and the order of Cd activating ability is citric acid> tartric aicd> oxalic acid, with the maximum activity efficiency 47%, 32% and 21% respectively. In fluvo-aquic soil and cinnamon soil, the order of Pb activating ability is citric acid>oxalic acid>tartric aicd, the maximum activating ability of citric acid is 2.7% and 0.2% respectively, and oxalic acid and tartric aicd do not demonstrate obvious activating ability; the order of Cd activating ability is citric acid> tartric aicd > Oxalic acid, with maximum activity efficiency of 28%, 7.6%, 1.9% in fluvo-aquic soil and 25%, 12%, 3% in cinnamon soil respectively.2. Citric acid, tartaric acid and oxalic acid have inconspicuous activating ability at the concentration level 1mM, which may be related to the absorption of soil organic and clay. In brown soil, fluvo-aquic soil and cinnamon soil, the activating ability for Pb and Cd of citric acid rise with the increasing concentrations, while oxalic acid and tartaric acid have no significant activating ability in fluvo-aquic soil and cinnamon soil.3. Organic acid proves to have higher activating ability in brown soil than in fluvo-aquic soil and cinnamon soil. The results illustrate that Pb and Cd are more easily activated in brown soil. The phenomenon may caused by the different soil characteristics, which lead to the different form of Pb and Cd.4. The extraction state and reducible state concentrations of Pb and Cd increase along with the severity of their pollution condition. However, the oxidable sate concentrations of Pb and Cd do not change significantly. Earthworm can prominently enhance the extraction sate concentrations and reduce reducible state concentrations of Pb and Cd, and they can not obviously impact oxidable state concentrations.5. Earthworm has stronger Cd enrichment capability than Pb, and the amount of its enrichment elevates as the extension of hatch time.The range of enrichment coefficient K is from 1.6 to 49.2. Earthworm has some absorption ability to Pb and generally, the absorption amounts add along with the increasing concentration of Pb. However, there is no significant regularity between adsorption amounts and the range of absorption coefficient K is from 0.09 to 0.71.6. The extractable state concentration of acid Pb and Cd in earthworm excrement has a positive correlation with the concentration of Pb and Cd concentrations in soil. The concentrations of Pb and Cd in earthworm excrement are much higher than in soil, that is 2.9 times and 2.7 times respectively, and in addition, Cd proves to have significant accumulation in earthworm excrement. 7. The acid extractable state concentration of Pb and Cd in rape rhizosphere soil significantly increased than in control group, illuminating that the activity of earthworm increases the available state concentrations of Pb and Cd.8. The activity of earthworm notably increased Pb concentration in upper-earth part of rape, enhancing the transportation of Pb from rhizosphere to upper-earth, and the transportation rate accelerates as the increasing Pb concentration in rape rhizosphere. However, the range of absorption coefficient K is from 0.0055 to 0.063 for upper-earth part and from 0.16 to 0.78 for rhizosphere. Meanwhile, rape proves to have great Cd enrich capability, the range of enrichment coefficient K is from 1.34 to 16.26 for upper-earth and from 1.77 to 20.1 for rhizosphere. The result suggests that rape can be used for Cd polluted soil in remediation engineering. |
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