淡水河流域銀之物種變化與空間分布

Abstract

淡水河為北台灣第一大川，山高水急，侵蝕與搬運作用強烈，其三大支流橫貫擁有六百萬人口之大台北都會區。河水中之微量元素物種的存在形式、空間分布以及輸出量，除了受到自然風化的影響，更遭受人為活動的衝擊。本研究利用物理分離方法來分析淡水河中之Ag物種：總溶解態（total dissolved, ≤0.45 μm）、真溶解態（truly dissolved, ≤1 kDa）、膠體（colloidal, 1 kDa ~ 0.1 μm & 0.1 μm ~ 0.45 μm）與顆粒態（particulate, ≥0.45 μm），顆粒樣品更進一步地利用序列酸解法來分離出不同相的Ag物種。 總溶解態Ag在流域水體中的濃度範圍為1.6 ~ 300 pM，在河川上游區段的濃度變化相對較窄（1.6 ~ 7.5 pM），當河川進入都會區時，濃度快速上升至300 pM，反映出人為輸入的影響。Ag於河口區域表現出強烈的移除作用，與Fe的移除行為相當類似。溶解態Ag濃度約有70%以上是屬於1 kDa ~ 0.45 μm的部分，顯示著Ag物種主要以膠體形式存在。在河口地區，伴隨著鹽度上升，膠體所占之比重逐漸下降。顆粒態Ag在水體中的濃度範圍為0.05 ~ 10 μg/g，濃度高低之分佈與總溶解態Ag非常相似，在進入都會區時濃度均急劇攀高。序列酸解之結果顯示出顆粒態Ag幾乎沒有表面吸附相，而有超過80%為鐵錳載體相，礦物晶格相大多出現在溶氧值低之河段，暗示著有自生源硫化礦物生成。 分布係數log Kd約為5.7±0.4，顯示出對於顆粒體的高度親合力。淡水河輸出至海洋的總溶解態Ag通量為0.14 mol/day，自流域中輸入的溶解態Ag約有86%在河口地區被移除。

Spatial variations and export loading of Ag in the waters of the Danshuei Tributary were investigated. The Danshuei River is the largest estuary that flows through the city of Taipei, Taiwan. Physical filtration methods were used in this study to define Ag speciation operationally. The following forms were determined: total dissolved (≤0.45 μm), colloidal (1 kDa ~ 0.1 μm, 0.1 μm ~ 0.45 μm), truly dissolved (≤1 kDa), and particulate (≥0.45 μm) fractions. Particulate samples were further separated into different phases by sequential leaching methods. The concentrations of Ag in the total dissolved fraction (≤0.45 μm) ranged from 1.6 to 300 pM. In general, upstream concentrations fell within a fairly narrow range, 1.6 ~ 7.5 pM. Downstream Ag concentrations showed significant increases (~300 pM) as the rivers entering the urban area, indicating the strong impacts of Ag-laden discharge effluents. On average, more than 70% of the 0.45 μm filter-passing Ag was associated with colloidal matter (1 kDa ~ 0.45 μm), decreasing with increasing salinity. All fractions of dissolved Ag exhibited a non-conservative estuarine removal mixing pattern. A strong correlation was found between the colloidal Ag and colloidal Fe concentrations, implying that Fe controlled the removal behavior of Ag in the estuary. The concentrations of particulate Ag in waters ranged from 0.05 to 10 μg/g, and its spatial distribution was very similar to that of the total dissolved Ag. More than 80% of particulate Ag was associated with iron-manganese oxyhydroxide/sulfide phases. The refractory phase was elevated in hypoxic river waters, implying that reduced sulfur minerals such as pyrite were likely formed. The high affinity of Ag to suspended particulates was reflected by a high mean particle/water distribution coefficient of log Kd=5.7±0.4. The export loading flux of dissolved Ag was about 0.14 mol/day. About 86% of the Ag input from whole watershed was removed within the estuary.