Research On Heavy Metal Removal From Stormwater Runoff In Constructed Wetlands With Media Of Zeolite

Runoff pollutions such as heavy metal, suspension solid and organic materials would have adverse effects on urban water environment, if drained into receiving water directly without treatment. Constructed wetlands as a representative in ecological cleaning technology with the advantages of efficient, ecological, less operational and maintenance costs, more tolerant to environmental conditions and pollutants shock loading have been increasingly applied in storm runoff treatment. Based on the remediation of stormwater runoff pollution, the experiments of clinoptilolite adsorption on heavy metals and the test of simulation constructed wetland have been conducted to study the competitive adsorption behavior and mechanism of lead(Pb2+), copper(Cu2+), zinc(Zn2+), chromium(Cr1+) on clinoptilolite and the mechanism of heavy metal removal in constructed wetland with media of clinoptilolite. In addition, the mechanism of heavy metal release from sediment have been focused on. The main contents are as follows:(1) To investigate the mechanism of Pb2+adsorption on natural clinoptilolite, a sequential desorption procedure by NaCl and HNO3was applied to zeolite samples which were in different sorption stages in both higher and lower initial concentrations, the adsorption processes and the dominate mechanism at different stages was studied. It was found that Pb2+can be absorbed by clinoptilolite by two mechanisms:ion exchange and physisorption. Physisorption play a leading role in absorbent process. According to the results, ion exchange occurs on the surface of the clinoptilolite and reaches equilibrium in5minitutes. The ion exchange capacity isn’t influenced by the initial concentrations. Moreover, the proportion of lead adsorbed by ion exchange is higher in low initial concentration condition than that in high initial concentration condition. On the other hand, physisorption process occurs on the surface of clinoptilolite at lower concentrations and reach equilibrium quickly, while it may diffuse to micropores at higher concentrations. The diffusion process is slow and takes a long time to reach equilibrium. The proportion of Pb2+absorbed by physisorption in high concentration condition is higher compared with that in low concentration condition.(2) Pb2+, Cu2+, Zn2+and Cr3+has been used as experimental subject to explore its adsorptive behavior in both single and binary components systems. For single components system the adsorptive capacity in an order of Cr3+> Cu2+> Pb2+> Zn2+which hydrolysis constant plays a fundamental role. However,in the binary adsorption, the research indicates that one ions displaced another ions on the surface of zeolite, the adsorption capacities of the competitive ions on zeolite had decreased by different degrees excepted Cr3+which is probably due to surface precipitation on the zeolite, the preference ranking of zeolite to four heavy metal ions is Cr3+Pb2+> Cu24> Zn2+which means not only governed by hydrolysis constant but also related to electronegativity and hydration energy.(3) Synthetic stormwater has been used as experimental subject to explore the removal mechanism for heavy metal, the mechanism of heavy metal release from wetlands have been focused on, heavy metal content and chemical speciation from release source was studied further. The results show that for Pb2+,Cu2+,Cr3+the mounts of heavy metal in an order of heavy metals in sediments> heavy metals adsorption on zeolite> heavy metals uptake by plants, for Zn2+in an order of heavy metals adsorption on zeolite> heavy metals in sediments> heavy metals uptake by plants. The average removal in the four constructed wetlands was100%for Pb,98%for Cu,75%for Cr and65%-72%for Zn, the removal efficiency was correlated to the mechanism of heavy metals removal in constructed wetland, the initial concentration of heavy metal and the adsorption capacity of heavy metals. Metals retained by sediments was release source which appear to be more susceptible to the external environment. The released capacity of heavy metals in sediment was determined by the percentage of the form of exchangeable and carbonates. In addition, plant roots stimulates the release of heavy metals from sediment by releasing oxygen.