以超過濾法探討嘉南平原地下水砷之錯合物

Identification of Arsenic Complex Species in Groundwater of the Chianan Plain, SW Taiwan, Using Ultrafiltration Method

李治平(Chih-Ping Lee)

指導教授 ： 劉聰桂

共同指導教授 ： 溫良碩(Liang-Saw Wen)

國立臺灣大學/理學院/地質科學研究所/碩士(2010年)

https://doi.org/10.6342/NTU.2010.03128

摘要

前人對台灣嘉南沿海一帶地下水研究結果，發現地下水中有機物與砷濃度呈正相關，並推論砷與水中腐植物質以錯合物形式存在。由於超過濾法能針對溶質之分子大小進行篩選，故適用於腐植物質與金屬錯合物之研究。本研究利用超過濾法，探討嘉南平原地下水，包括（1）砷是否與腐植物質錯合，（2）砷物種，（3）有機物如何影響砷在地下水之釋出。
本研究採用六個嘉南平原地下水樣本進行超過濾實驗。選用兩種超過濾膜，篩選之孔徑同為1000 Daltons，材質分別為醋酸纖維（Cellulose acetate，CA）與再生纖維（Regenerated cellulose，RC）。在地下水樣品超過濾分析前，先進行砷與溶解性有機碳（Dissolved organic carbon，DOC）之質量平衡實驗。地下水樣品之超過濾分析時，量測濾液砷與DOC濃度，除與原水樣濃度比較外，並比較兩種超過濾膜結果。此外，也進行三價砷（H3AsO30）與五價砷（HAsO42-）水溶液之超過濾實驗，及三價砷和五價砷添加於十份（II）水樣之實驗。十份（II）之濾液更以液相層析─氫化─原子螢光光譜儀（LC-HG-AFS）分析三價砷（H3AsO30）與五價砷（HAsO42-）濃度，確認砷物種相對含量。
質量平衡實驗顯示，不論基質存在與否，砷或有機物均未被CA或RC兩種超過濾膜吸附。嘉南平原地下水樣品超過濾分析結果，顯示該兩種超過濾膜均能有效攔阻樣本中大於1000 Daltons之有機物，但對砷之攔阻效果卻差異甚大： CA膜可攔阻一半以上的砷，但RC膜卻無法攔阻大部分的砷。由兩過濾膜對地下水砷攔阻率之差異，反映地下水中的砷主要並非和腐植物質錯合。以1 mg/L之三價砷與五價砷水溶液，分別測試濾膜對砷物種之攔阻效率，發現三價砷可穿透濾膜，但五價砷則否，且CA膜攔阻五價砷之能力勝於RC膜，此現象可歸因於Donnan電荷排斥效應。添加實驗結果顯示，地下水基質不影響CA膜與RC膜對三價砷之過濾，但降低兩種濾膜對五價砷之攔阻率，為地下水基質之陽離子中和濾膜之負電所導致。以十份（II）之超過濾濾液分析砷物種，確認濾液之砷多為五價。
比較地下水樣本之超過濾結果，推論嘉南平原地下水中的砷與腐植物質，並非以砷與腐植物質之錯合物為主；且五價砷為地下水主要之砷物種。五價砷存於地下水中，可能由於有機物氧化三價砷，或地下水硫化物氧化三價砷以形成五價砷硫化物所致。地下水有機物對於砷之遷移與富集所扮演之角色，可能是促進微生物進行鐵（氫）氧化物之還原溶解、使沈積物的砷脫附，或阻擋水中的溶解砷，使其無法再被吸附於沈積物表面。

關鍵字

砷 ； 腐植物質 ； 嘉南平原 ； 地下水 ； 超過濾

並列摘要

Prior studies on the groundwater geochemistry of Chianan Plain, SW Taiwan, showed that the arsenic concentration was positively correlated with the content of organic matters in groundwater. Furthermore, it was proposed that arsenic complexing with humic substances in groundwater is the major arsenic species. However, little evidence has presented to confirm the existence of the complex in groundwater. On the other hand, ultrafiltration is a technique capable of separating molecules based on molecular sizes. Therefore, ultrafiltration is suitable for confirming whether the arsenic-humic substances complex exists in groundwater. In order to confirm if the arsenic-humic substances complex is predominant in the groundwater of Chianan Plain and to identify the dominant arsenic species, the ultrafiltration experiments are conducted on arsenic aqueous solutions and six groundwater samples. In addition, the relationship between arsenic and organic matters are discussed and the roles of organic matters to arsenic mobilization in the groundwater are explored.
The ultrafiltration experiments recruit two kinds of membrane materials for comparison, including cellulose acetate (CA) and regenerated cellulose (RC). The molecular weight cut-off (MWCO) of the membranes is 1000 Daltons. The mass balance experiments were conducted to identify whether arsenic or natural organic matters are adsorbed onto the membranes. Arsenic and dissolved organic carbon (DOC) concentrations in permeates were analyzed and compared with the concentrations in groundwater. Ultrafiltration were also applied to As(III) and As(V) aqueous solutions and the arsenic-spiked Shefeng-2 (SHF-2) samples to figure out the effects of oxidation states of arsenic and the impacts of the matrix. The permeates of SHF-2 sample was further analyzed the As(III) and As(V) oxyanions by LC-HG-AFS to evaluate the contents of inorganic arsenic species.
The results of mass balance experiments showed that neither arsenic nor organic matters were adsorbed on both of the membranes. The results of groundwater ultrafiltration demonstrated that the DOC concentrations in permeates while using different membranes were comparable to each other; however, the arsenic concentrations in permeates differ significantly between the two membranes. Ultrafiltration with CA membranes rejects more arsenic than ultrafiltration with RC membranes. The difference in arsenic concentrations in permeates suggests that arsenic-humic substances complex is not predominant in the groundwater of Chianan Plain. Ultrafiltration of As(III) and As(V) aqueous solutions respectively showed that, in the ultrafiltration conditions used in this study, there was no rejection to As(III) but strong rejection to As(V) by both CA and RC membranes. Moreover, CA membranes rejected more As(V) than RC membranes. The difference in rejection between As(III) and As(V) results from the influence of Donnan exclusion. The ultrafiltration of the spike sample showed that there is no matrix effect on As(III) but on As(V). The less rejection of As(V) in groundwater samples results from neutralization of membrane charges by matrix. The arsenic speciation of the SHF-2 sample confirmed that As(V) is the predominant arsenic species in the groundwater.
As revealed by the ultrafiltration of groundwater and arsenic aqueous solutions, it is thus suggested that arsenic-humic substances complex is not predominant in the groundwater of the Chianan Plain. The presence of high amount of As(V) may derived from oxidation of As(III) by natural organic matters or by sulfide ions to form thioarsenates. The roles of organic matters to arsenic mobilization and enrichment in groundwater are acting as substrates for microbes to reductively dissolve Fe(III) oxyhydroxide and make arsenic released, desorbing arsenic directly from sediments, and impeding arsenic from re-adsorbing onto the surfaces of sediments.

並列關鍵字

arsenic ； humic substances ； Chianan Plain ； groundwater ； ultrafiltration

參考文獻

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