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 Quality Standard (GB 3838-2002). Our results agreed with previously published results , confirming that CWs are effective at removing HMs from contaminated wastewater, and that the RRs are different for different metals. Moreover, it could be seen from the present results that the removal efficiency of CW on HMs would be differernt due to the status of the water pollution in the YR as well as the growth cycle of the cattails. That is, during stages III-V, the removal efficiency was relatively lower than stages I-II, this was attributed to the relative slowly growth of the cattailss which would in turn produce less biomass. This has been confirmed by previous studies (Rousseaua et al. 2008; Si et al. 2011).
The Cd, Cu, Pb, and Zn concentrations in the wetland substrate samples from the YR and CW are shown in Table 2. The HM concentrations were significantly higher in the YR sludge samples than in the CW substrate samples, except for Pb, and moreover, no significant variations in the YR water concentrations were found at any time, indicating that the HM concentrations in the YR were relatively stable. 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 . 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...