選擇性光輔助雙金屬還原水中硝酸鹽為氮氣之研究

Abstract

本研究以成本低廉、反應迅速且效果良好的零價金屬還原法作為基礎，結合光還原之技術，將硝酸鹽還原為氮氣。鈀鋅雙金屬負載比例實驗中，最佳材料為20 wt.% Pd/Zn，其氮氣選擇性從酸洗後鋅粉之29.59%大幅提升至78.53%。然而貴金屬價錢昂貴、金屬材料污泥處理繁複，且副產物仍未完全符合法規標準，故本研究結合光還原技術，解決中間產物殘留之問題，並提升氮氣的選擇性。紫外光降解亞硝酸鹽先導實驗中，最佳反應條件為──紫外光波長254 nm，加入甲酸且不調整酸鹼值，得到近100%的亞硝酸根降解率，氮氣選擇性則為85%，氨氮產生量幾乎為零，顯現該技術降解亞硝酸根之潛力。本研究將上述兩種技術結合為──光輔助雙金屬還原硝酸鹽。在重量負載比例實驗中，得到Zn及1 wt.% Pd/Zn為最佳材料，相較於單純使用雙金屬，光輔助系統巨幅地降低貴金屬負載量且提高氮氣選擇性約14%，並將兩種材料用於後續實驗，得到以下結論：（1）兩步驟並在合適時間加甲酸，能有效地將〖∙CO〗_2^-作用於亞硝酸根上，提高氮氣選擇性；（2）甲酸添加量與〖∙CO〗_2^-產量成正比，然而過多甲酸對降解效果並沒有顯著幫助，甚至對於鈀鋅雙金屬有負面影響；（3）當材料添加量由12.30 g/L降至3.08 g/L時，雖然反應所需時間延長，但氮氣選擇性並不受影響，顯示光輔助可有效降低材料使用量，進而減少材料之污泥殘留，甚至能避免金屬離子溶於水中之風險；（4）相較於氮氣，曝氬氣更能有效地去除水中溶氧，而溶氧會使〖∙CO〗_2^-降解效果變差，因此曝氬氣能提高氮氣選擇性。最終Zn及1 wt.% Pd/Zn皆得到近乎100%硝酸鹽降解率，而氮氣選擇性則分別為92.29%及92.34%。零價鋅取得和鈀鋅雙金屬相同的效果，由於其價格便宜且製程簡單，故成為較佳的材料選擇。相較於單純使用雙金屬，本研究在光輔助下大幅降低貴金屬負載20 wt.%，且提高氮氣選擇性14%，並能在減少材料使用量四分之三的情況下，達到相同的硝酸鹽降解率及氮氣選擇性。

This research applied the low-cost, fast-reacting, and effective zero-valent metal reduction method, combined with photo-reduction technologies, to convert nitrate to N2.The best material, 20 wt.% Pd/Zn, greatly increased the N2 selectivity from 29.59% of pretreated Zn to 78.53%, in the experiment of Pd/Zn bimetallic loading ratio. However, precious metals are expensive, metal material sludge treatment is complicated, and the by-products still do not fully meet the regulatory standards. Therefore, this research combines photo-reduction technology to solve the problem of intermediate product residues and improve N2 selectivity. Nearly 100% conversion rate of nitrite is obtained, N2 selectivity is 85%, and NH4+ production is almost zero, from UV pilot experimtent, showing the potential of this technology to degrade nitrite. The best reaction conditions are: UV wavelength of 254 nm, adding formic acid without adjusting the pH value. Photoassisted bimetallic reduction is a combination of the above two technologies. Zn and 1 wt.% Pd/Zn are obtained as the best materials, in the weight loading ratio experiment. The photo-assisted system greatly 'reduces the precious metal loading' and increases the N2 selectivity about 14%, compared to use of bimetal alone. Applying the above two materials to subsequent experiments, the following conclusions are obtained: (1) Two steps and adding acid at a suitable time can effectively make 〖∙CO〗_2^- act on nitrite and improve N2 selectivity; (2) The amount of formic acid added is directly proportional to the yield of 〖∙CO〗_2^-. However, too much formic acid does not significantly improve the degradation effect, and even has a negative effect on the Pd/Zn; (3) When the amount of materials are reduced from 12.30 g/L to 3.08 g/L, although the reaction time is prolonged, the N2 selectivity is not affected. It shows that photoassisted can effectively reduce the amount of material used, thereby reducing 'metal material sludge' and even avoid the 'risk of dissolving metal in water'; (4) Ar aeration can remove dissolved oxygen in water more effectively, compared with N2. Dissolved oxygen will make the effect of 〖∙CO〗_2^- worse, so Ar aeration can improve N2 selectivity. The results shows that 100% nitrate removal efficiency and about 92% N2 selectivity were achieved when using Zn or 1 wt.% Pd/Zn. Compared with the use of bimetal alone, under photoassisted systems, this study greatly reduces the precious metal loading by 20 wt.%, and increases the N2 selectivity by 14%, and can achieve the same effect while reducing the material usage by three-quarters.