奈米級零價鐵及銅鐵雙金屬還原水中硝酸鹽之研究

Abstract

硝酸鹽(NO3-)為一自然界中常見污染物，研究已證明過量攝取將對人體造成致癌的風險，而現行的物化處理技術，僅將硝酸鹽作相的轉移並無法破壞去除，並造成高濃度之廢液，需另外處理增加成本。而生物處理技術的操作維護成本高，並且需要長時間作用以及廣大的用地，於使用上有相當大之限制。故本實驗著眼於化學反應技術上，尋求更有效經濟之方法。而零價金屬為最常使用之反應材料，其擁有強大還原能力，可用來去除去水中硝酸鹽(NO3-)及含鹵有機物(HOCs)，目前又以零價金屬鐵研究最為廣泛。利用鐵的還原性質可將硝酸鹽氮降解成氨氮(NH4+)，減少對人體之危害。 為了提升處理效能，本研究以奈米技術合成奈米金屬顆粒，利用硼氫根(BH4-)還原溶液中之鐵離子(Fe3+)，合成高活性之奈米零價金屬鐵(nano Fe0)，由於製備技術的不同，粒徑由數個奈米至數百奈米(1~103nm)不等，其高活性來自奈米顆粒之高比表面積(56.6 m2/g)。相較於市售之微米等級鐵粉，對去除污染物的反應速率可以大幅提升。由於奈米微粒特殊性質，可將質量傳輸的限制因子降至最低，提高利用率，使用少量藥劑可處理更多之污染物。 在實際地下水中仍然含有許多背景物種存在，本實驗以批次實驗個別探討六種陰離子(氟離子、氯離子、溴離子、碳酸根、硫酸根、磷酸根等離子)，對以零價金屬鐵去除硝酸鹽氮之影響，此外，將改變反應系統溫度，探討溫度反應之影響，並在此兩個實驗下佐以雙金屬技術，將鐵表面附著少量銅金屬來提升反應速率，降低環境因子對於反應之影響，並尋求動力模式來預測反應結果。 本實驗結果顯示，溫度對於零價金屬降解硝酸鹽反應有重大之影響，溫度升高降解越快，溫度降低反應則延遲。此外，水中之陰離子會同樣對降解反應會造成干擾。氟、氯、溴三離子會對新鮮鐵表面有腐蝕效應，使得可反應位置(reactive site)被消耗，造成反應速率降低。而碳酸根、硫酸根、磷酸根三離子則是會與鐵離子在表面形成鈍化層，阻斷硝酸鹽與鐵表面的接觸，使反應速率降低。而使用銅金屬作為第二金屬時，於各批次實驗中皆能有效的提升反應效率，並且使最終產物產生較少量的氨氮。

Nitrate is a common pollutant in the environment and pose potential health risk to cause cancer. Current technologies such as ion exchanges and reverses osmosis do not destroy and generate secondary brine wastes. Biological denitrification is unfavorable because of it requires intensive maintenance and high cost. Our research focuses on chemical reduction technology, and finds a more powerful and economic process to deal with nitrate pollution. Zero-valent iron (Fe0) is the most widely used material in the previous researches of the chemical reduction technology. Fe0 transforms nitrate to ammonia in a great amount, and decreases the health risk of nitrate to nitrite. In our experiment, we use a strong reductant (NaBH4) to produce high active nano-Fe0 which has a small diameter within the range of only 1~103nm, and yet a large surface area of 56.6m2/g. In comparison with commercial iron powder, the reduction capacity of nano-Fe0 is considered relatively large. In fact, there are many kinds of negative ion other than nitrate in real groundwater. We use batch test to find the reaction of nano-Fe0 with nitrate when negative ions presents. There are six common ions is discussed：Fluoride, chloride, bromide, carbonate, sulfate, and phosphate. We also control reaction temperature from 10℃ to 60℃ to see how the reaction rate changes in response. In other hand, we synthesize nano-Cu/Fe bimetallic particle to enhance the capability of reducing pollutants, and test in all situations the same with nano-Fe0. According to the experimental results, there are great effects to reducing nitrate by controlling temperature. The reaction rate raise with temperature increase. The presence of negative ions also causes the nitrate reaction rate slow down. The one-valent negative ions (Fluoride, Chloride, Bromide) interferes the nitrate reduction by increasing the rate of corroding iron surface and consuming the reactive site. The others negative ions (Carbonate, Sulfate, and Phosphate) restrain the reaction by forming complex compounds and precipitating on the particle. This phenomenon would cover the reactive site and block the contact of nitrate and iron surface.