- 學位論文
利用稀土元素研究人工濕地內懸浮固體物之特性
Study of Suspended Solid in Constructed Wetland Using Rare Earth Elements
摘要
人工溼地是現今廣泛使用於汙水整治的系統之一，在自然環境中透過土壤、水、微生物和水生植物間的作用，內部包含了物理、化學及生物的反應，加上建造時人為設計與控制環境基本參數，最後達到淨化水質的目的。其中水質淨化的主要目標分別有以下三者:生物需氧量、氨氮和懸浮固體物的去除。本研究的人工溼地位置為嘉義縣的荷苞嶼人工濕地，是表面水自由流動式的人工溼地，即水流大多在地表流動，藉由沉澱、植物吸收或細菌分解移除懸浮固體物。在前人之調查報告中顯示，此濕地入流水之懸浮固體物濃度有長期偏高的趨勢，且出流水的懸浮固體物濃度大於入流水，過去均認為是溼地邊坡土壤所致，故選取此場址進行研究。在本研究中將使用稀土元素對人工濕地內之懸浮固體物進行示蹤。 近年來，稀土元素已被廣泛應用於地球科學領域中，由於稀土元素在天然水體及土壤中來源較單一，在環境中受化學反應影響時呈趨勢性變化，方便研判其來源與化學反應過程，且稀土元素受到鑭系收縮現象影響，其化學性質上有一定的規律和特性。將分析結果數據利用北美頁岩(NASC)進行正規化後，觀察稀土元素圖譜的趨勢顯示，各樣本點中懸浮固體物粒徑較大者呈現中稀土元素富集、輕稀土貧乏的情況，據前人的研究，中稀土富集可能肇因於有機物的選擇性吸附，而非過去認為的邊坡土壤所造成。而溶解態的稀土元素則無中稀土富集現象，取而代之的是明顯的銪正異常，且整體上呈水平趨勢，代表溶解態的稀土元素來源與北美頁岩近似，而銪正異常更證明沉積岩中長石的溶解作用為溶解態稀土元素的主因。最後，透過元素分析及主成分分析後之結果，表明了此人工溼地汙染物去除主要為以下兩種機制，鐵錳鋁水合物的吸附沉降及人工濕地內部之水質變化。 關鍵字：人工溼地；懸浮固體物；稀土元素
並列摘要
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
參考文獻
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