| Cadmium-contaminated soil can cause significant damage to human by food chain, and need risk assessment followed by remediation. The bioavailability of chemicals to earthworms can be modified dramatically by soil physical/chemical characteristics, yet expressing exposure as total chemical concentrations does not address this problem. The main objective of this research is to:(1) access if bioavailable Cd concentrations could be used in the expression of exposure; (2) provide a scientific basis for ecological security threshold of Cd-contaminated soils based on the bioavailability; (3) improve the traditional initial screening assessment of contaminated soil samples by total concentrations and guideline values only.In this study, the bioavailable Cd concentrations of selected three Chinese soils (black, yellow-brown and red soils) were extracted using four extracting methods:water,0.01M CaCl2 solution,0.11M HAc solution and Diffusive Gradient in Thin film (DGT) technique. The added Cd concentration ranged from 0.1 to 40 mg per kilogram soil. And the chronic toxic effect of cadmium (Cd) to earthworm (Eisenia fetida) in three soils was studied using pot experiments. The production of reactive oxygen species (ROS), activities of antioxidation enzymes (SOD and GST), contents of glutathione (GSH), lipid peroxidation product malonaldehyde (MDA), protein carbonyl (PCO) and metallothionein (MT) were monitored as biochemical responses of earthworms to Cd exposure, which contributed to the decision of bioavailable Cd security threshold. The relationship between bioavailable Cd concentrations and biochemical responses were gained by fitting adsorption model. Furthermore, the laboratory tests results were verified by the field toxicity tests based on exposure estimates utilizing techniques that measure the bioavailable fraction of Cd.The major results were summarized as follows:1. The bioavailable Cd concentrations given by CaCl2, HAc, soil solution and DGT measurement were found to be better correlated with bioaccumulation and effects in earthworms than total Cd concentrations.2. Cd could significantly induce the accumulation of hydroxyl radical (·OH), the massive accretion of GSSG, SOD, GST and MT, and the oxidative damage which was evidenced by the significant increase of MDA.·OH, MT, MDA and PCO were sensitive and stable to Cd stress which can be as a set of potential earthworm biomarkers.3. Good dose-response relationships were found between the bioavailable Cd and the biochemical responses of earthworms. It was suggested that the bioavailable Cd concentrations could be more adequate for expressing the Cd exposure of earthworms in soils.4. The laboratory tests results showed that the toxicity threshold of Cd concentrations was found to be 0.1-0.5 mg/kg in the studied soils based on the biochemical responses of the earthworms to Cd exposure, corresponding to extractable Cd concentration in a smaller variation range:DGT-Cd 1.65-2.00 μg/L, soil solution-Cd 2.01-3.88 μg/L, HAc-Cd 0.146-0.813 mg/kg and CaCl2-Cd 0.0292-0.0327 mg/kg. And in the field tests, the toxicity threshold of Cd concentrations were in DGT-Cd 2.03 μg/L, soil solution-Cd 1.92 μg/L, HAc-Cd 0.745 mg/kg and CaCl2-Cd 0.0340 mg/kg, which were nearly inaccordance with laboratory test results. Based on that, our results suggested that the applications of bioavailability may serve as a toxic diagnosis method for the heavy metal risk assessment in soils.5. The risk of 16 field soils in YiXing using the Cd concentrations in soils, toxicity effect of earthworm, and the Cd concentrations in grains were integrated assessed. The results indicated that the ecological risk was not exist for the S15,S6,S16,S14. The moderate and considerable ecological risk was for the S1, S7, S13. The others were suffering very high ecological risk. |
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