- 學位論文
零價鐵表面改質對水中硝酸鹽還原脫硝反應之影響
Effects of zero-valent iron surface modification on aqueous nitrate reduction
摘要
零價鐵還原破壞水中污染物質已引起廣泛研究與運用，多種含氯烷類、烯類與農藥、放射性物質、重金屬以及氧酸根離子的實驗室與模場研究建立了基礎的反應機制與動力學的數據。目前研究重點除了有效提高污染物降解速率外，另一關注的焦點在於降低毒性產物的生成與累積，藉由金屬表面特性的改質以提升反應活性並減少毒性產物的生成，將可使此一還原性材料深具實用價值。 本研究利用氫氣前處理與粒徑奈米化提供高活性零價鐵，接續地披覆催化性金屬改變反應途徑，以硝酸鹽為目標污染物探討反應活性、中間及最終產物的產率與反應機制。本研究分為三個部分，第一部分為材料的製備與特性分析；第二部分為金屬脫硝反應的動力學與產物分析；第三部分為反應機制的探討。第一部分的研究內容有金屬顆粒的表面積與氧化態變化以及披覆催化性金屬顆粒的型態與分散性；第二部分為各種表面活化技術對金屬脫硝反應的速率、中間產物與最終產物產率之影響；第三部分研究內容為反應機制的探討。 氫氣前處理可有效還原商業鐵粉表面的惰性氧化層，因此避免反應初期的活性遲滯現象並增加反應速率(2倍)。以0.01 M氯化鐵可製得粒徑 10 – 30 nm(比表面積 56 m2 g-1) 的奈米鐵，在無酸化或加入緩衝溶液的環境下其反應速率為商業鐵粉的7倍，由此可知，氫氣前處理與金屬粒徑奈米化均可提供高活性的鐵表面。披覆催化性金屬於鐵粉表面的實驗發現Cu/Fe降解中性硝酸鹽溶液的速率遠高於Fe、Pd/Fe與Pt/Fe等，最佳覆載量為0.25-0.50% (w/w)，其降解速率為商用鐵粉的7倍。硝酸鹽吸附在零價鐵表面，接受電子直到完全還原為最高還原態的氨氮(N(-III)) 後才釋出，因此反應過程中無亞硝酸鹽生成且可偵測之最終產物為氨氮。雙金屬Cu/Fe系統中，催化性金屬Cu表面產生高活性的吸附態氫原子氫化硝酸鹽，由於中間產物-亞硝酸鹽吸附於銅表面的親和力遠低於硝酸鹽，導致出流水殘餘約20 - 40%氮質量平衡的亞硝酸鹽量，因此，本研究以自行開發的依序置換法披覆第二催化性金屬Pd於Cu/Fe表面，利用第二催化性金屬對亞硝酸鹽的高親和力接續提供還原能力，提高最終產物氮氣的產率至30%左右。本研究提供有效活化鐵金屬表面的處理方式，不僅大大提高反應活性還可持續地活性再生；催化性雙金屬的披覆提高脫氮反應之氮氣選擇率，因此，研究結果將有助於開發高選擇性轉化硝酸鹽為氮氣之零價金屬材料。
並列摘要
Extensive studies over the past 15 years have demonstrated that chemical reduction of many substances in the environment, such as halogenated organic compounds, heavy metals and oxo-anions can be coupled to Fe0 oxidation. Early investigations have gained insight concerning the mechanism and kinetics of the electron transfer process through batch and column experiments. However, production and accumulation of intermediates and by-products, which are more toxic than the parent compounds, have been observed. Thus, not only reaction rate but also benign products yields attracts interesting. Both surface treatments, H2-reducing pretreatment at 400℃ and reducing particle size into nanoscale, were attempted to enhance the removal of nitrate (40 mg-N L-1) using zerovalent iron in a HEPES buffered solution at a pH of between 6.5 and 7.5. After the iron surface was pretreated, the deposition of noble metal onto freshly pretreated iron promoted nitrate degradation. The aims of this work are to develop an effective reductive material for removing nitrate at neutral pH and an effective surface treatment to activate and restore the reactivity of this material. Additionally, attention was also given to the reaction mechanisms through both the investigation of kinetic control and the identification of the reaction products and intermediates. After H2-reducing pretreatment, the removal of the passive oxide layers that covered the iron was indicated by the decline in the oxygen fraction (energy dispersive X-ray analysis) and the overlap of the cyclic polarization curves. The reaction rate was doubled, and the lag of the early period disappeared. Then, the deposition of copper onto freshly pretreated iron promoted nitrate degradation more effectively than that onto a nonpretreated iron surface, because of the high dispersion and small size of the copper particles. An optimum of 0.25-0.5% (w/w) Cu/Fe accelerated the rate by more than 7 times that of the nonpretreated iron. The aged 0.5% (w/w) Cu/Fe with continual dipping in nitrate solution for 20 days completely restored its reactivity by regeneration process with H2 reduction. Both high surface area materials, Fe0 and Cu/Fe nanoparticles, were employed for the denitrification in unbuffered 40 mg-N L-1 nitrate solutions at initial neutral pH. As compared with microscale Fe0 (<100 mesh), the efficiency and rate of nitrate removal using Fe0 and Cu/Fe nanoparticles were highly promoted, accompanied by relative high rates of pH rise. Reactions with Fe0 nanoparticles, ammonium as the end product accounted for 95+% of the reduced nitrate and no detectable amount of nitrite was observed at the final pH 10. Though nitrate was complete transformed using 5.0% Cu/Fe nanoparticles in 60 minutes, nitrite as much as 40% of nitrogen mass balance persisted in the reactor. Nitrate was proposed to remain sorbed to the Fe0 surface until the formation of ammonia was achieved. For reactions with bimetallic Cu/Fe nanoparticles, however, the stepwise reduction by atomic hydrogen on the copper surface dominated the nitrate removal; and the accumulation of nitrite resulted from its less affinity to copper surface and its negligible removal rate at alkaline conditions. Finally, a method with sequent coating Cu and Pd onto Fe surface was used to fabricate a catalytic reductive material (Pd/Cu/Fe) that yields about 30% N2 selectivity. Pd/Cu/Fe is a potentially reductive material for nitrate reduction with high N2 selectivity. Heating the aged reductive material in flowing H2/N2 at 400℃ for 3 h almost completely restored its reactivity. Both surface treatments facilitated the development of a process that could effectively remove nitrate and conveniently regenerate the reductive medium.