- 學位論文
在超臨界二氧化碳中以零價鐵還原多氯聯苯
Reductive Dechlorination of Polychlorinated Biphenyls by Zerovalent Iron in Supercritical Carbon Dioxide
摘要
多氯聯苯(Polychlorinated Biphenyls，簡稱PCBs)多年前在台灣曾引起相當嚴重的公害事件，而它的安定性也使得它難以被破壞分解。我的研究內容主要是結合超臨界流體技術、零價鐵和連線方式。利用超臨界流體優秀的萃取能力，結合零價鐵的還原能力，將多氯聯苯萃取並還原成較無毒性的聯苯，輔以連線方式增加效率，最後使用GC-MS做為偵測儀器。 實驗方式是將多氯聯苯注入裝滿零價鐵粉的反應槽中，加熱至指定溫度後反應1到4小時，再利用超臨界二氧化碳將它萃取出來。研究結果顯示：在溫度190℃以上時，反應始有較明顯的作用；而當溫度升到300℃時，4小時的反應時間即可將95%的多氯聯苯完全還原成聯苯。 而使用連線法的實驗方式是將多氯聯苯注入海砂中，反應槽中裝滿零價鐵粉，將萃取槽和反應槽串連，以萃取-還原-萃取的過程將多氯聯苯還原成聯苯。研究結果表示：在300℃、反應時間4小時、壓力200 atm 和反應槽長度20公分的情況下，有71%的多氯聯苯完全還原成聯苯。決定最後聯苯濃度的是反應槽長度和超臨界二氧化碳的壓力，它的主要損失是出現在萃取過程。 最後探討零價鐵還原多氯聯苯和水份的關係，根據文獻記載，零價鐵要還原多氯聯苯，必須有溶劑來提供氫離子，文獻中多以水來做為溶劑，但在吾人的實驗過程中並未加入水和任何溶劑，卻擁有相當好的脫氯還原效果。後來以熱重分析儀-紅外線光譜儀(TGA-IR)來分析零價鐵粉，推測零價鐵粉本身即附有水份，鐵粉上附著的水份便足以提供還原所需的氫離子。
並列摘要
Polychlorinated biphenyls (PCBs) caused very serious public hazard incidents in Taiwan many years ago, and their chemical stabilities make them difficult to decompose. My research objective is to combine supercritical fluid techniques and zero-valent iron powder to chemically reduce these toxic substances to non-toxic ones. Supercritical carbon dioxide is chosen for its outstanding extraction ability, while zero-valent iron is chosen for its low cost, low toxicity, and universal availability. The experiment was carried out by first spiking polychlorinated biphenyl standards onto the iron powder that was pre-filled into a reduction cell, followed by filling the cell with liquid carbon dioxide. The oven was then heated to the desired temperature set point, whence the carbon dioxide was converted to supercritical state. After the desired amount of time has lapsed (1 to 4 hours), extraction of the reduction products was done by using more supercritical carbon dioxide. The result of this study shows that reduction of PCBs does not proceed below 190℃, and that at 300℃ and 4 hours reaction time, 95% of the PCBs was converted to biphenyl. The online experiment was performed by spiking the PCB standard onto sea sand placed in an extraction cell and connecting the extraction cell in series with a reduction cell downstream that was completely filled with iron powder. The PCB was extracted with supercritical carbon dioxide from the extraction cell and then delivered to the reduction cell for dechlorination. The results show that at 300 ℃ and 4 hours reaction time, about 70% of the PCB was converted to biphenyl. The role that water plays as a hydrogen donor in reduction of polychlorinated biphenyls with zero-valent iron was also investigated. In the former experiments, despite the fact that water was not added, reduction of PCBs proceeded almost to completion, indicating that a hydrogen donor was present after all. Analyzing the raw iron powder with thermogravimetric analysis-infrared spectrometry (TGA-IR), it was found that about 0.13 wt. % of water adsorbed on the surface of the iron powder, which was more than 100 times the mole ratio of the spiked PCBs. It is postulated that these adsorbed water molecules serve as the hydrogen source required for the reduction of the PCBs.