催化性雙金屬還原水中硝酸鹽之研究

Reduction of Nitrate by Catalytic Bimetallic Particles

曾文裕(Wen-Yu Tseng)

指導教授 ： 駱尚廉

國立臺灣大學/工學院/環境工程學研究所/碩士(2006年)

https://doi.org/10.6342/NTU.2006.00449

摘要

硝酸鹽本身不具毒性，低濃度時有高度的穩定性，在人體中易還原成亞硝酸鹽而阻礙血液載氧的功能，嚴重者會有死亡的危險。現行的脫硝技術包括有物化脫硝及生物脫硝，傳統的脫硝方式常會產生大量濃縮廢液，亦或需要經常性的更換設備，利用微生物進行脫硝反應又必須視環境及生物特性的不同而難以掌握去除效率，故本研究利用化學上常運用之氧化還原反應來去除水中所含之硝酸鹽。由於零價金屬具強大之還原能力，且價格較為低廉，因此被運用在去除水中存在之毒物，尤以零價鐵的使用最為廣泛。故利用其還原特性可將水中硝酸鹽降解減少對於人體的威脅。
過去的研究指出加入少量的第二金屬會加快零價鐵粉對於硝酸鹽的降解效率，第二金屬通常是貴金屬如：鈀、鉑等，透過兩種不同金屬之電位差形成加凡尼電池(galvanic couple)以加速反應。本實驗採用較容易取得的銅做為第二金屬，氯化銅溶液中放置鐵粉進行氧化還原反應，將離子態的銅還原成金屬銅並負載於鐵粉表面上，實驗證明在鐵粉有負載銅時，其降解效率大於沒有負載銅的情況，由於降解效率與pH值高低成反比，故在高pH時更能看出其效果。
文獻中對於硝酸鹽降解的產物多所描述，為了要求降解效率通常將反應環境之pH值降低，但產物為氨居多；為去除此二次污染物，實驗運用貴金屬（如：鈀、金、鉑等）負載於銅表面上，製成催化性雙金屬，讓硝酸鹽先由銅鐵雙金屬降解為亞硝酸鹽後，再還原成無害的氮氣。惟在pH值較高的反應環境中，才會產生較多氮氣，而高pH值又會令降解效率變慢，故實驗中添加HEPES做為緩衝溶液，以防止pH上升過快。
實驗結果顯示貴金屬中以鈀效果最佳，且當pH=8.3時對氮氣選擇率最高，最適加藥比例則為鐵：銅：鈀=100%：0.5%：0.3%。添加貴金屬時，反應速率較只有銅存在時為慢，但最終皆會反應完全，其用意在於防止藥品尚未到達欲處理之地點時已反應完畢，導致處理效果不佳。

關鍵字

硝酸鹽 ； 零價鐵 ； 催化性雙金屬 ； 氮氣

並列摘要

Nitrate has no toxicity itself and is very stable at a low concentration. It is very easy to reduce into nitrite which hinders the function of blood carrying oxygen. The current technologies of denitrification are not economic and generate secondary brine wastes, and the efficiency of biological dentrification is influenced by the environment and the characteristic of microorganisms. Because of the above-mentioned defect our research focuses on the oxidation-reduction reaction to reduce nitrate. Zero-valent metal is easy available has powerful ability of reduction. Especially, zero-valent iron (ZVI) is widely used in the previous researches.
Previous researches indicated that it accelerates the efficiency of denitrification of ZVI by adding a few secondary metal. By the different electric potential of the two metals, they become a galvanic couple and accelerate the reaction. In this study, we add copper chloride into iron powder such that the iron powder will be coated by the metallic copper. This experiment proved that the efficiency of denitrification of iron powder coated by copper is faster than iron powder without copper.
Many researches described that the product of degradation of nitrate for the most part is ammonium. We need to raise the pH value and add a noble metal for forming nitrogen. Furthermore, we use HEPES to be buffer to prevent the pH value raising too fast.
The result shows that palladium is the best selection, and when pH is equal to 8.3 the selectivity of nitrogen is highest. The best ratio to iron, copper and palladium is 100%:0.5%:0.3%. Besides, when a noble metal existed, the rate of denitrification is slower that the rate without a noble metal, but it will react completely finally. The purpose above is preventing the medicament is over without arriving at the groundwater.

並列關鍵字

nitrate ； zero-valent iron ； catalytic bimetal ； nitrogen

參考文獻

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