模擬於暴雨中受到農業逕流影響所造成底泥硫化鉛之溶解與轉變

Abstract

在厭氧環境的底泥裡，重金屬以穩定的硫化金屬形式存在。然而，近年來因氣候變遷造成暴雨事件頻繁增加，原本處於厭氧環境的底泥受到物理性擾動後，可能使水中溶氧提升，進而導致硫化金屬受到氧化溶解，提高重金屬被人體攝入之風險。此外，含磷之農業廢水亦會受到暴雨沖刷而進入自然水體，磷酸鹽可能會與重金屬離子產生沉澱，使重金屬再度以穩定形式存在於環境中。本研究利用連續曝氣裝置，模擬因暴雨事件引起之溶氧量改變，造成不同硫化鉛溶解狀況，研究結果顯示不同溶氧量對於硫化鉛之溶解並無顯著影響，pH值才是造成硫化鉛溶解速率有所差異的主因。在水中加入不同濃度的磷酸鹽後，水中鉛濃度驟降至低於方法偵測極限，且添加越高濃度的磷酸鹽，鉛濃度從低於偵測極限到再度被檢出的時間也將延長。含磷污染源與水體具有不同距離，經由暴雨沖刷進入水體之時間也不同，故磷酸鹽在不同的時間被加入實驗溶液以模擬上述情形，但鉛濃度可被測得的時間並未受到不同磷酸鹽加入時間影響。本研究亦透過掃描式電子顯微鏡觀察是否有磷酸－金屬沉澱物產生，結果顯示確實有磷氯鉛礦(Pb5(PO4)3Cl)或氫氧磷鉛石(Pb5(PO4)3OH)之結構，其尺寸隨加入的磷酸鹽濃度和加入磷酸鹽時間增加而加大。

In natural water systems, heavy metals are fixed by metal sulfides in anoxic sediments. After physical disturbances, metal sulfides can be suspended into oxic water column and facilitate their oxidative dissolution. The remobilization of lead could be affected by water chemistry and constituents such as phosphate in runoff or wastewater discharge. In this study, a batch reactor system with continual aeration was used to simulate the re-suspension of PbS during a storm event with different dissolved oxygen (DO), pH and phosphate addition. The results demonstrated that the rate of PbS dissolution was mainly controlled by pH instead of DO. After phosphate addition, Pb(II) decreased immediately to not detected (ND) and the period of ND was longer when a higher concentration of phosphate was added. To simulate sources of pollution which have different distances from the river, different time of phosphate addition was conducted. The results showed that the period of ND Pb(II) was the same no matter when phosphate was added. The presence of soluble lead which is toxic to aquatic life was hindered by phosphate due to the formation of chloropyromorphite (Pb5(PO4)3Cl) hydroxypyromorphite (Pb5(PO4)3OH) as observed by scanning electron microscope. The SEM images showed that larger size of Pb5(PO4)3Cl crystals were formed if a higher concentration of phosphate and a later time of phosphate was introduced.