- 學位論文
負載奈米銅顆粒之陽離子交換樹脂還原破壞水中四氯化碳之研究
Degradation of Aqueous Carbon Tetrachloride by Nanoscale Copper Particles Supported on a Cation-Resin
摘要
零價金屬自發性釋放電子還原水中含氯有機物已引起廣泛研究與運用，過去十五年，對於反應機制、途徑與動力學已有不少的數據與討論，毒性副產物的生成與累積是目前關注的焦點。四氯化碳是常見的地下水含氯污染物，但四氯化碳無法與金屬d軌域形成π鍵吸附，導致60-80%轉化為含氯中間產物(三氯化碳與二氯化碳)累積在處理系統中，所以降低毒性產物的產率應當是選擇零價金屬的重要考量。 本研究導入離子交換樹脂為載體，負載奈米金屬顆粒(nano-M0)兼具回收水溶性金屬離子(Mn+)的功能，接續地，中高溫的氫氣轉化Mn+為nano-M0提供反應活性的再生。就還原方法而言:氣態還原的成本遠低於液相還原法(NaBH4或N2H4為還原劑)且熱處理可提高nano-M0在載體上的機械穩定度;就功能而言:以離子交換樹脂為載體可侷限Mn+成核結晶的程度，降低nano-M0的粒徑大小，同時可避免nano-M0在反應過程中發生團聚作用，此外，樹脂的陽離子交換功能可吸附反應產物- Mn+，避免出流水金屬濃度高於法規標準，同時釋出強酸離子(H+)降低反應系統pH值; 就實用性而言: 質量輕、可塑性高的高分子載體具有安裝與更新簡便的特性。 本研究目的在開發有效、方便裝卸且可再生的還原性材料，以利現地處理系統的運用，同時探討反應動力學與機制。本研究分為三個部分，第一部分為材料的製備與特性分析；第二部分為含氯有機物的破壞行為；第三部分為反應機制探討。第一部分的研究內容有載體上金屬顆粒的粒徑分佈與表面積、金屬氧化態變化以及載體對金屬離子與目標污染物的吸脫附行為；第二部分研究內容為水中四氯化碳的降解動力學反應、水質條件的干擾與活性再生方法；第三部分研究內容為金屬銅降解四氯化碳的反應機制探討。 零價銅顆粒奈米化程度對反應活性有相當大的影響，當銅顆粒平均粒徑為33 nm，一階的比表面積反應速率常數(surface-area-normalized rate coefficient, kSA)為粉末狀銅元素(0.04mm)的19倍; 銅顆粒平均粒徑為8 nm，kSA遽增為125倍。利用正戊烷萃取殘存在水樣與載體中四氯化碳的量，分辨材料吸附與破壞四氯化碳的作用，其中破壞的反應階數為0.91-0.94相當接近一階反應，反應速率常數(ka)是0.8 h-1、吸附常數是0.41 h-1、脫附常數是0.3 h-1，由活化能的大小(Ea=25.4 kJ/mol)推論水中四氯化碳消失的速率近似物理程序所控制。水中溶氧與硫化物對nano-Cu/resin降解四氯化碳反應有負影響。強酸型陽離子交換樹脂以H+交換水中反應產物Cu2+，因此反應後水溶液pH約3-4、殘餘銅離子濃度低於0.1 mg/L。銅離子的回收率會因沉澱物與錯合物的形成而減少，主要是受水中硫化物、氨氮與磷酸根影響。 貴金屬Pd、Pt、Au、Ru的添加均可大幅提升降解速率，且ka與貴金屬材料的交換電流密度(exchange current density, io)成正相關(ka=5.69log(i0)+30.15, R2=0.986)，因此第二金屬表面氫原子濃度(H․)是控制雙金屬系統降解四氯化碳的主要因子。水中溶氧與硫化物分別因為競爭第二金屬表面氫原子與毒化作用導致反應速率下降，本研究的製備程序可當作再生方法且幾近100%回收反應活性。 比較不同金屬(Fe、Zn、Sn、Al、Si、Cu)降解四氯化碳的產物，發現含氯中間產物的產率與材料的i0與自由電子數比值成正相關，只有銅的系統不產生含氯中間產物且最終產物為一氧化碳，故推論降解途徑中單電子與雙電子傳遞的選擇性主要受金屬表面H․的濃度與自由電子的供給的影響，最終產物為甲烷或一氧化碳是與金屬形成氧化物與氯化物的生成熱有關。 本研究創新的材料型態擴展了使用零價金屬的選擇性，同時提供安裝與更換便捷的系統設計。創新的參數：交換電流密度與自由電子數成功地說明各類零價金屬降解四氯化碳的途徑選擇性。負載型奈米銅完全轉化四氯化碳為一氧化碳，同時兼具銅離子濃度的控制與回收，有利於一個高效率、可再生的去毒化系統的設置。
並列摘要
The use of zero valent metal as a reductant of organohalides has been extensively investigated over the last 15 years. The disappearance of contaminants is attributed to a corrosion-like process, in which the metal donates electron to reduce target pollutants, accompanied by the dissociation of water. Early investigations have gained insight concerning the mechanism and kinetics of the electron transfer process through batch and column experiments. The degradation of carbon tetrachloride by iron, the most commonly used reactive material, yields chlorinated intermediates at a level of 60-80% of the degraded carbon tetrachloride for lack of a stronglyπbond formed between carbon tetrachloride and d-orbit. Thus, not only reaction rate but also chlorinated intermediates yields attracts interesting. Nanoscale zerovalent copper supported on a cation resin was successfully synthesized to enhance the removal of carbon tetrachloride (CCl4) from contaminated water. The use of the cation resin as a support prevents the reduction of surface area due to agglomeration of nanoscale zero valent copper particles. Moreover, the cation resin recycles the copper ions resulting from the reaction between CCl4 and Cu0 by simultaneous ion exchange. The decline in the amount of CCl4 in aqueous solution results from the combined effects of degradation by nanoscale zero valent copper and sorption by the cation resin; thus the amount of CCl4 both in aqueous solution and sorbed onto the resin were measured. The orders of degradation reaction (Na) with respect to aqueous CCl4 concentration were 0.94 and 0.91 for 0.2 and 0.5g Cu0/resin loading per vial, respectively (both values were close to unity). Reducing the size of a Cu0 particle on the resin to approximately close to 10 nm would sharply increase kSA by a factor of 112-125.The pseudo-first-order rate constant normalized by the surface-area and the mass concentration of nanoscale zero valent copper (kSA) was 2.1 0.1 L h-1 m-2, approximately twenty times that of commercial powdered zero valent copper (0.04 mm). Due to the exchange between Cu2+ and the strongly acidic ions (H+ or Na+), the pH was between 3 and 4 in unbuffered solution and Cu2+ at the concentration of less than 0.1 mg L-1 was measured after the dechlorination reaction. Bimetallic particles are extremely interesting in accelerating the dechlorination of chlorinated organics. Four noble metals (Pd, Pt, Ru and Au), separately deposited onto the copper surface through a spontaneous redox process, promoted the CCl4 dechlorination rate, and the catalytic activity of the noble metal followed the order of Pd>> Ru> Pt> Au. This order was found to be dependent on the concentrations of adsorbed atomic hydrogen, indicating that the initial reaction was cathodically controlled. Resin as a support extends the choice of zero valent metals and provides easy emplacement and replacement. Both exchange current density and the number of free electron successful elucidate the selectivity of single-electron and two-electron transfer. Nano-metal/resin would facilitate the development of a process that could be designed for convenient emplacement and regeneration of porous reductive medium.