臺灣嘉南平原溪口鄉地下水層砷之礦物學特徵 及釋出機制

Mineralogy and Mobilization Mechanisms of Arsenic in Groundwater in Xikou township, Chianan Plain, Taiwan

林宜(Yi Lin)

指導教授 ： 王尚禮

國立臺灣大學/生物資源暨農學院/農業化學研究所/碩士(2015年)

https://doi.org/10.6342/NTU.2015.01908

摘要

臺灣嘉南平原地區地下水中含高濃度砷，但對於通氣帶 (vadose zone) 中砷之釋出機制仍不夠瞭解，因此本研究自嘉南平原選擇高砷地區，進行80公尺的岩芯鑽探，採集不同深度的岩芯和地下水，分析砷礦物學特徵組成和地下水溶液化學，以探討通氣帶中砷的釋出機制。所採集之地下水樣進行pH、Eh、砷物種，以及化學組成分析。岩芯則以連續萃取法和As K-edge X光吸收近緣結構 (XANES) 分析其砷礦物組成。在不同深度之地下水層中，砷的濃度範圍為8-355 μg L-1。以HPLC-ICP-MS分析地下水的砷物種，結果顯示無機態As(III) 為優勢物種，有機砷含量則低於偵測極限。地下水層分別於20-30公尺與50-70公尺處的砷濃度最高，而此兩個水層亦分別含有最高的硫酸鹽與鐵濃度，推論此兩個深度有不同的砷釋出機制。於As K-edge之XANES圖譜中發現，除了接近表層之岩芯 (深度6.2公尺) 以As(V) 為主之外，其他深度皆以As(III) 與As(V) 混合之形式存在；進一步以LCF分析發現，砷黃鐵礦 (Arsenopyrite, FeAsS) 主要分布於深度15-35公尺之岩芯。連續萃取法之結果顯示，在深度15-35公尺之岩芯中，砷主要為硫化物與有機型態。由於此層之地下水中亦富含硫酸鹽，推測此深度岩芯之砷釋出機制為含砷硫化物的氧化溶解。而深度約50公尺之岩芯的砷型態以無定形與結晶性良好鐵鋁氧化物結合相為主，對照此層地下水富含Fe2+，推測此層砷的釋出機制為鐵氧化物的還原溶解。基於上述結果並結合溶液化學之熱力學計算，進一步確認於深度20-30公尺與50-70公尺處的砷釋出機制分別為含砷之硫化物氧化溶解，以及含砷之鐵氧化物的還原溶解。

關鍵字

砷 ； 嘉南平原 ； 地下水水化學 ； 岩芯地質化學 ； X光吸收近緣結構

並列摘要

Chianan plain in southwestern Taiwan has long been well known for the prevalence of black-foot disease since 1920s, but the mobilization mechanism of As in the vadose zone in this area has not been well understood. To elucidate the mechanisms of As release in the vadose zone in Chianan plain, drilling of a borehole was conducted at a selected site and the groundwater and core samples were collected at different depths up to 80 m. The groundwater samples were analyzed for pH, Eh, As speciation, and elemental composition. The As-containing minerals in the core samples were determined using As K-edge X-ray absorption spectroscopy (XAS) in conjunction with sequential extraction. The As concentrations in the groundwater samples of different depths ranged from 9 to 370 ppb. As(III) was predominant in all the samples, while the concentration of organic As was negligible. The concentration of As was relatively high in the groundwater samples collected at the depths of 20-30 and 50-60 m, accompanying with the high concentrations of sulfate (41.1-51.1 mg L-1) and Fe (8.3-10.4 mg L-1), respectively. The As K-edge XANES spectra revealed that arsenopyrite (FeAsS) was predominantly present in the core at the depth of 15-35 m. The results of sequential extraction showed that the As in cores at the depths of 15-35 and 50-70 m were associated with sulfides and Fe hydroxides, respectively. Based on these results, it was therefore proposed that the predominant mechanisms of As release to groundwater at the depths of 20-30 and 50-60 m were the oxidative dissolution of As-containing sulfide minerals and the reductive dissolution of As-containing Fe minerals, respectively.

並列關鍵字

Arsenic ； Chianan plain ； Groundwater chemistry ； X-ray absorption spectroscopy ； Sequential extraction

參考文獻

劉麗純。2007。嘉南平原高砷地下水腐植物質的特性。國立臺灣大學地質科學研究所碩士論文。

賴志傑。2008。嘉南平原之水文地質環境中砷之分布與特徵：意涵砷之釋出過程。國立臺灣大學生物環境系統工程學研究所碩士論文。

吳文菁。2008。探討砷在關渡平原中水田與旱田土壤之分佈。國立中興大學土壤環境科學研究所碩士論文。

陳冠宇。2007。台灣西南海岸平原地下水中砷之遷移與富集。國立臺灣大學地質科學研究所博士論文。

黃泰祥。2013。利用土壤水分管理降低兩種砷污染土中糙米砷濃度。國立臺灣大學農業化學研究所碩士論文。