| Summary: | 碩士 === 國立中正大學 === 應用地球物理研究所 === 104 === Constructed wetland (CW) is one of the mostly used technologies in wastewater treatment, which is low cost and has acceptable efficiency. By means of the biochemical interactions among water, microscopic organism, aquatic plant and sediments in natural environment, CW can remove biochemical oxygen demand (BOD), ammoniacal nitrogen (NH3-N), suspended solid (SS) and heavy metals. The studied CW, Hebao Island free water surface CW, is located in Chiayi County, south Taiwan. The CW is composed of 5 cells designed for removing SS and BOD due to the pollution from livestock farms in the upstream area. However, the removal of SS was not effective. In some cases, the SS concentration of inflow is even higher than that of outflow. That the sediments on the slope were flushed into the CW was considered as the main problem. After all the refinement, the issue has not improved yet. In this study, rare earth elements (REEs), which is a common tool of the studies of earth sciences, were used as a natural tracer for the study of suspended solid (SS) in the constructed wetland.
REEs generally have a unique source and would fractionate in certain regular patterns during biochemical reactions due to lanthanide contraction. They can be an excellent natural tracer in the environmental researches. After normalized by North American Shale Composite, the REEs pattern for the samples with the total of dissolved and suspended matters is characterized by a middle REE (MREE) enrichment and light REE depletion. According to the previous theoretical studies, the MREE enrichment could be achieved by a selected adsorption of MREEs by organic matters, which is generally humic substance in natural surface water. It is suggested that the refinement of removal efficiency of SS should focus on organic matters but not sediments. On the contrary, the REE pattern of dissolved matters is flat and shows no MREE enrichment, which means the major source of dissolved REEs is possibly sediments. In addition, a distinct europium positive anomaly can be observed. It reconfirms that the dissolution of feldspar in sediments was strongly involved. The results of principal components analysis show that one of the removals of pollutants is depended on precipitation of iron hydroxide. The other one is depending on the fluctuation of water quality in this constructed wetland.
Keywords: Constructed wetland; Suspended solid; Rare earth elements
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