請用此 Handle URI 來引用此文件: http://hdl.handle.net/11455/94759
標題: 維生素C還原降解四氯化碳
作者: 林雅婷
關鍵字: 維生素C
四氯化碳
還原
鐵礦
鹼性
現地化學還原法
Vitamin C
carbon tetrachloride
reduction
iron mineral
alkaline
in situ chemical reduction
出版社: 環境工程學系所
摘要: 維生素C (即抗壞血酸, Ascorbic acid, AA)為可釋放兩電子之還原劑,因此,本研究探討AA還原降解四氯化碳(Carbon tetrachloride, CCl4)之可行性,並評估不同pH、AA濃度、CCl4濃度、鐵礦存在、土壤存在及緩衝pH系統對AA還原降解CCl4之影響,亦進一步推估AA及CCl4之降解途徑。由不同pH條件下之實驗結果可知,於pH 13之條件下,AA可有效還原降解CCl4,亦可觀察到氯仿(Chloroform, CHCl3)之生成及降解,進一步由AA及CCl4濃度影響試驗,可得知AA與CCl4之反應為二階反應,其反應速率常數為0.253 ± 0.018 M-1 s-1。此外,鐵礦之存在可提高AA還原降解CCl4之速率,由CCl4之假一階降解速率常數(k1)及鐵礦之比表面積可換算求得CCl4降解之比表面積反應速率常數(kSA, L d-1 m-2),不同鐵礦於pH 13及AA存在下CCl4還原降解之kSA 依序為針鐵礦 (4.171 × 10-3) >黃鐵礦 (3.634 × 10-3) >赤鐵礦 (3.222 × 10-3) >磁鐵礦 (1.661 × 10-3) >零價鐵 (0.157 × 10-3)。藉由CCl4轉換CHCl3之分率(α值)小於1,可知鹼性pH下AA藉由單電子及兩電子轉移降解CCl4。於pH 13之條件下,AA之可能最終分解產物為蘇糖酸及草酸。當pH 13及土壤存在之環境,AA可還原降解CCl4約90% (7天反應時間),相較於相同條件無土壤存在之組別,CCl4約降解80%,可知土壤之存在並不會抑制AA還原降解CCl4之速率。此外,於磷酸緩衝系統(pH 7 ~ 13),僅於緩衝pH為13時,可觀察到CCl4之降解,然而磷酸存在會影響AA還原降解CCl4之效果,但存在赤鐵礦、針鐵礦及黃鐵礦時,可提高AA還原降解CCl4之能力。進一步採用天然地下水進行試驗,於pH 13之條件下,無論鐵礦/土壤存在與否,因地下水中所含之物質,對AA還原降解CCl4造成之影響不顯著,因此,於此些環境下,AA仍可有效還原降解CCl4。 英文摘要 Vitamin C (a.k.a. ascorbic acid, AA) is a two-electron reductant with a redox potential of - 0.06 V, which can potentially reduce organic contaminants. In this laboratory study, reduction of dissolved carbon tetrachloride (CCl4) by AA was investigated under a variety of experimental conditions. The parameters evaluated included effects of pH, AA concentration, CCl4 concentration, presence of iron mineral, presence of soil and pH buffer system on reductive degradation of CCl4 by AA. The results indicate that CCl4 was reduced by AA under pH 13 and chloroform (CHCl3) was a transformation byproduct of CCl4. When CCl4 levels were reduced to near complete disappearance, the decrease of CHCl3 was then observed. Analysis of reaction kinetics between CCl4 and AA revealed an overall second-order reaction with a rate constant of 0.253 ± 0.018 M-1 s-1. Furthermore, the reduction rate of CCl4 by AA at pH of 13 could be enhanced with the presence of iron minerals. Based on CCl4 degradation rate constant (k1) and specific surface area of an iron mineral, the surface area normalized rate constants (kSA, L d-1 m-2) can be determined. An order of effect of iron minerals on kSA of CCl4 degradations was FeOOH (4.171 × 10-3) > FeS2 (3.634 × 10-3) > Fe2O3 (3.222 × 10-3) > Fe3O4 (1.661 × 10-3) > Fe0 (0.157 × 10-3). In the absence or presence of iron minerals, the fraction of CCl4 transformed to CHCl3 (α) was less than 1, indicating simultaneous one- and two-electron transfer processes. The end-products of AA at pH of 13 included threonic acid and oxalic acid. Besides, CCl4 can be degraded by AA in the soil slurry system at pH 13. The observed extent of CCl4 degradation was around 90% in the soil slurry system (7-d reaction) was analogous to 80% observed in the aqueous system. The presence of soils appeared not to inhibit CCl4 degradation by AA. Further experiments were carried out in the phosphate pH buffer system (pH from 7 to 13). Results revealed that CCl4 was reduced by AA under pH 13 in the presence of phosphate buffer. Also, these results indicated that phosphate buffers would hinder AA reductive reaction rates. However, CCl4 degradation rate by AA could be enhanced with the presence of Fe2O3, FeOOH and FeS2 in the phosphate pH buffer system. In addition, experiments were conducted to assess the effect of groundwater constituents under pH 13 in the presence or absence of soils and/or iron minerals, which exhibited minor influence on degradation of CCl4. Therefore, CCl4 can be degraded by AA at pH 13 under natural environments.
URI: http://hdl.handle.net/11455/94759
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