Heavy Metal Removal in a Constructed Wetland and Benefits for the Development of the Toad Bufo Raddei

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Results and discussion
3.1 Effect of the CW on HM removal from the YR water
A summary of the concentrations of the major contaminations in the water samples from the YR and CW is given in Table 1. The concentration of the HMs in the water from the CW system were much lower than that from the YR throughout the experimental period. The average removal rates (RRs) in the CW were 86.2%, 61.7%, 82.9%, and 76.7% for Cd, Cu, Pb, and Zn, respectively, (that is, in the order Cd>Pb>Zn>Cu), showing that the CW removed the HMs from the YR water efficiently. Importantly, the concentrations of all of the HMs studied were decreased by the CW to below the permissible limits defined in the level III guidelines in the Chinese Surface Water Environment
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However, in Table 1, large differences were found in the concentrations of the HMs in the water from the YR during differernt periods due to the changes of pollutant emissions from the metal smelting factories and other pollution sources located in the upper reaches of the YR. Hence, we concluded that the HM concentrations in the YR sludge had reached equilibrium after a long period of accumulation, and would not be easily changed by the water quality in the YR.
The accumulation of HM in sediment and the absorption by plants in the CW were two important mechanisms for reducing the HM concentrations in CW water [30]. In the present study, the Cu and Pb concentrations in the CW sediment changed little over time throughout the periods in our experiment, except between stage I and stage II, wheras the Cd and Zn concentrations increased significantly with time (P<0.05), indicating the rapid enrichment of these HMs in the CW sediment. Meanwhile, the HMs accumulation in the cattails were the same as our previous studies. By comparing the HM concentrations in the YR and CW water samples (Table 1), it can be illustrated that the CW system in the present study was quite effective at removing HMs from the contaminated YR water.
3.2 Beneficial effects of the CW on the development of B. raddei
The DNA damage levels, HM enrichment factors, liver coefficients, oxidative stress levels, and physiological indices of the toads living in the CW were recorded and compared with the

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