| 摘要: | 地下水為台灣重要用水來源之一，近年來地下水中硝酸鹽的污染有惡化的趨勢。硝酸鹽本身並不具毒性，攝入人體內易還原成亞硝酸鹽而阻礙血液載氧的弁遄A嚴重時會有致命的危險；而亞硝酸鹽在人體內也易轉變成亞硝胺成為致癌因子。
現行的脫硝技術中，物化脫硝不僅維護及操作費用高，亦會產生高濃度廢水；生物脫硝則須重視環境及生物特性的不同而難以掌握其去除效率。近年來已逐漸將零價金屬廣泛運用於水中硝酸鹽之還原處理。
過去幾年中，釵h研究為提高零價金屬對污染物之去除效果，分別添加大量零價金屬、緩衝溶液或控制起始pH值於酸性條件，導致其成本及操作手續的增加，亦產生二次污染。故本研究於提高處理效能中，考慮成本及操作問題，選擇較易取得之鐵、銅、銀金屬，以化學還原法（利用離子態金屬溶液，添加強還原劑溶液（硼氫化鈉）進行還原）自行合成奈米級零價金屬顆粒。本研究以奈米零價鐵做為第一金屬，再添加第二金屬（銅），合成奈米零價鐵銅雙金屬，最後再添加銀金屬合成出奈米零價鐵銅銀三金屬。本研究係以批次實驗進行，在未控制水體環境條件下，進行奈米零價金屬對水體中硝酸鹽之去除效率之探討。此外，本研究也以不同組合及比例之奈米級複合三金屬在不同硝酸鹽氮起始濃度和不同起始pH值條件下，探討其對硝酸鹽之去除效果，並推算其反應速率常數及半衰期等相關動力參數。
實驗結果發現，一起合成之奈米級複合金屬對硝酸鹽去除效果較佳。此外，比較不同組合之奈米級零價金屬（Fe0、Ni0、Cu0、Fe0/Ni0、Fe0/Cu0、Fe0/Cu0/Ni0、Fe0/Cu0/Ag0）對硝酸鹽氮去除效果時，在控制起始pH值於酸性、鹼性及未控酸條件下，發現奈米級零價複合三金屬（Fe0/Cu0/Ag0）對硝酸鹽去除效果最佳，優於其他各種奈米級零價金屬組合。
在自行合成之Fe0/Cu0對硝酸鹽去除效果的實驗中發現，當第二金屬Cu0所佔比例越少時對硝酸鹽去除效率越好，其最佳比例為19：1。在自行合成之Fe0/Cu0/Ag0對硝酸鹽去除效果實驗中發現，當第三金屬Ag0所佔比例越少時對硝酸鹽去除效果越好，其最佳比例為19：1：0.1。此外，並發現一起合成之Fe0/Cu0、Fe0/Cu0/Ag0與硝酸鹽溶液進行反應時，於起始pH值控制在2 ~13範圍中，皆有良好去除效果。
本研究中也發現Fe2+須在反應環境為酸性條件下才能形成，當pH值超過5.7時，Fe2+濃度則趨近於零。
本研究於氮的質量平衡探討中發現，自行合成之Fe0/Cu0/Ag0在pH值13的強鹼條件下，不僅對硝酸鹽有高去除效果，氨氮產率也較低。
Groundwater is one of the important water resources in Taiwan. Recently, it has deteriorated by the pollution of nitrate. Nitrate has no toxicity itself and is very easy to reduce to form nitrite, which hinders the blood function of carrying oxygen. Nitrites also can react with secondary amines to form N-nitrosamines, which are the well-known mutagen and carcinogen.
Current denitrification technologies such as physical and chemical treatments not only require intensive maintenance and high cost, but also might generate highly polluted wastewater. Biological denitrification effects might be limited by the environment and the characteristics of microorganisms. Recently, zero-valent metal has widespread uses on the reduction treatment of nitrate.
Previous researches indicate that it can enhance the efficiency of denitrification by adding high dosage of zero-valent metal, buffering solution or controlling pH-stat within the acidic condition, which might result in spending high cost and generating secondary pollution.
For promoting the treatment efficiency, the strong reductant (NaBH4) was used to synthesize nano-Fe0, nano-Cu0 and nano-Ag0 in this study. Nano-Fe0 was chosen as the main metal, and then nano-Cu0 was added as secondary metal to synthesize the nanoscale Fe0/Cu0 bimetallic particles. Finally, nano-Ag0 was added as minor metal for the synthesis of nanoscale Fe0/Cu0/Ag0 trimetallic particles.
The aim of this study is to realize the removal efficiency and kinetics of aqueous nitrates treated by nanoscale Fe0/Cu0/Ag0 under uncontrolled water environment. The influencing factors, including initial pH, varying composition and ratios of trimetallic particles, initial nitrate concentration would be operated by batch tests on the reaction of nanoscale trimetallic particles with nitrates and also to find the kinetic coefficients such as rate constants and half-life.
Results reveal that the nanoscale bimetallic particles synthesized simultaneously had better removing effects on nitrate than those synthesized separately. In addition, nanoscale trimetallic particles had the best removing effects than other synthesized particles on nitrate under acidic, alkaline and uncontrolled pH conditions.
Results also show that the removal efficiencies of nitrates in the pH-stat 2-13 are all significant by adding nanoscale Fe0/Cu0/Ag0 synthesized simultaneously, and better than the others. In addition, the higher of the ratio of Fe0 in bimetallic Fe0/Cu0 can attain the better removal efficiency on nitrates, especially the ratio of 19/1, whereas the lower of the ratio of Ag0 in trimetallic Fe0/Cu0/Ag0 can attain the better removal efficiency on nitrates, especially the ratio of 19/1/0.1.
Fe2+ could be found after reaction under the acidic condition. However, if the solution pH exceeded 5.7, Fe2+ would disappear gradually.
From the exploration of nitrogen mass balance, it can be noted that the nanoscale synthesized Fe0/Cu0/Ag0 had higher removing potential on nitrate under highly alkaline condition. Meanwhile, the reaction product of ammonia nitrogen was less. |
