陰離子在電化學處理含重金屬廢水程序之影響研究

Effect of Anions on Electrochemical Coagulation for Cadmium Removal

黃建宏(Chien-Hung Huang)

指導教授 ： 陳俊成

淡江大學/工學院/水資源及環境工程學系博士班/博士(2009年)

https://doi.org/10.6846/TKU.2009.01000

摘要

為使含重金屬過量之有害事業廢棄物降低其含量以通過TCLP測試成為一般事業廢棄物以降低其處置成本，研究者經常使用稀釋過後之硝酸、硫酸及鹽酸來萃取重金屬。而其萃取液與其他含重金屬之廢水問題相同需要加以妥善處裡，而電化學混凝法為常被選用之處理技術。故本研究以硝酸根離子(NO3-)、硫酸根離子(SO42-)及氯離子(Cl-)為對象，分別及混合調配水溶液，以金屬鋁為陽極、石墨為陰極，探討陰離子對電化學混凝程序中陽極腐蝕、電流、電流效率、導電度、溶液pH變化、膠羽形成及重金屬鎘移除之影響。

在固定電壓6伏特、反應時間10分鐘的操作條件下，實驗結果顯示，硝酸根離子能令電解槽維持穩定之電流，故於陽極溶出鋁離子、於陰極分解水並使溶液pH上升，但由於pH上升過高，導致Al(OH)4-成為主要物種，故無法觀察到膠羽。在硫酸鹽溶液中，由於陽極鋁板表面迅速形成惰性氧化層導致電解槽幾乎無電流產生，故亦無電混凝反應產生。在氯化鈉溶液中，氯離子提供較高之導電度並令電解槽維持相對較高之電流，鋁離子溶出、溶液pH隨時間呈現先升高後下降然後維持在9〜10之間，故可形成並觀察到明顯之氫氧化鋁膠羽、產生之膠羽將鎘從溶液中有效混凝移除。當硝酸鹽及硫酸鹽混合存在於溶液中，少量之硝酸根離子即可主導硫酸鹽溶液之電化學程序，雖然並無膠羽形成以產生混凝作用但經由鎘之還原程序可將溶液中約50%之鎘移除。當硫酸鹽溶液中存在微量之氯鹽時，氯離子即可穿透鋁板之惰性氧化層而導致電解槽電流明顯產生，此時溶液中之鎘於先於陰極被還原並接續由產生之膠羽所混凝移除，這個機制對鎘之移除相當有效，在反應之10分鐘內可移除超過99.5%之鎘。

關鍵字

電化學混凝 ； 硫酸鹽 ； 硝酸鹽 ； 氯鹽 ； 陽極溶解 ； 鎘

並列摘要

Solutions of nitrate, sulfate, chloride and their combination were prepared and tested in a parallel plate electrochemical cell to study their effect on electrochemical process. Among the parameters studied were conductivity, current, current efficiency, pH change, coagulant formation, and cadmium removal. Nitrate ions were able to sustain a healthy current in the cell. The current dissolved aluminum from the anode, dissociated water molecules on the cathode, and increased pH in the solution. Due to excessive pH, Al(OH)4- became the dominating species and therefore no coagulant was observed. In sodium sulfate solutions, the aluminum anode quickly built up an inert film that prevented current from crossing the cell. Due to the lack of current, no reaction was observed throughout the 10-min treatment. In sodium chloride solutions, chloride ions provided electrolytic conductivity to sustain a current in the cell. The current dissolved aluminum, generated hydroxyl groups, increased pH, produced coagulant, and removed cadmium from the solution.

In nitrate-sulfate mixtures, a small amount of nitrate ions dominated the performance of the sulfate solution in an electrochemical process. Although no coagulant was produced, the process removed 50% of the cadmium from the solution through cathodic reduction. In a solution of both sulfate and chloride, chloride ions were able to penetrate the inert film formed in a sulfate solution and drew a healthy current across the cell. Cadmium was initially removed by cathodic reduction followed by coagulation. This mechanism was very effective in cadmium removal. A 10-min treatment removed more than 99.5% of the cadmium, even with a substantial amount of sulfate in the solution.

並列關鍵字

Electrochemical coagulation ； Sulfate ； Nitrate ； Chloride ； Anodic dissolution ； Cadmium

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