零價矽結合重金屬離子降解四氯乙烯還原脫氯反應之研究

Reductive Dechlorination of Tetrachloroethylene by Zerovalent Silicon in the Presence of Heavy Metal Ions

李俊錡(Chun-Chi Lee)

指導教授 ： 董瑞安

國立清華大學/原子科學院/生醫工程與環境科學系/博士(2010年)

摘要

氯化有機污染物如四氯化碳、四氯乙烯及三氯乙烯為地下水中常見之污染物質。以零價鐵為填充材料的滲透性反應牆為一種利用所充填活性金屬還原降解氯化有機物之方法，此方法可有效且長時效的分解環境中常見之氯化有機污染物質。然而，在無氧的情況下長期使用零價鐵的結果，不可避免的會使得系統的pH上升，進而導致零價鐵表面形成不具反應性之鐵氧化物，使得去除污染物之效果大幅降低。零價矽被發現可以與零價鐵金屬結合，在無氧環境中可減緩pH的上升。然而，利用零價矽及其相關之雙金屬材料在不同的環境條件下分解氯化有機物質之機轉與操作條件仍少有相關文獻報導。
研究的主要目的是評估利用零價矽還原降解氯化有機物之可行性，並且瞭解在不同環境因子下包括pH、界面活性劑及共污染物對零價矽的活性影響以釐清重金屬離子銅及鎳離子對於氯化有機物降解速率及反應機制之可能效應。研究中發現，利用零價矽對氯化有機物之降解速率會隨著系統中pH而增加，在pH 7.2時，四氯化碳之擬一階反應速率常數(kobs)為0.5 h-1，然而在pH 9.5時，其降解速率可增加至2.11 h-1。研究中亦發現，較難分解之烯類氯化有機物如四氯乙烯亦可被零價矽還原，然而降解之產物依舊為含氯之三氯乙烯，且反應半衰期長達8.5天。利用環境中常見之重金屬污染物質包括鎳離子及銅離子搭配零價矽具則有協同效應可加速氯化有機物之降解速率及產物分佈比率。在含有1.5 wt% 鎳離子的情況下，零價矽降解四氯乙烯的之能力及反應速度均有大幅度的提升，從原先之3.4 × 10-3 h-1提升至5.2 × 10-2 h-1。 利用X-光光電子能譜儀(XPS) 來鑑定鎳離子於零價矽表面之價態，發現鎳離子可以有效的被零價矽還原成鎳零價金屬並沈積於其表面。電子微探分析儀 (EPMA)也被應用於鑑定零價鎳沈積在零價矽表面之分佈情形。研究中，成功的建立利用零價矽搭配不同濃度之鎳離子降解四氯乙烯之反應速率、分解產物分佈及其表面特性間之交互關係。
零價及一價銅為另一種具有催化零價鐵加速還原氯化有機物的重金屬。在零價矽系統中，從XPS的結果得知零價矽可還原所吸附之銅離子而形成銅-矽雙金屬材料，然而，在零價矽系統中添加3 wt %之銅離子卻會降低其對氯化有機物的降解能力，其擬一階反應速率常數從原先的3.4 × 10-3 降至1.7 × 10-3 h-1，其主要的原因為銅離子在零價矽之最佳化pH情況下，會形成不溶性之氫氧化銅並覆蓋於零價矽表面，進而抑制了零價矽之反應能力。降低所添加之銅離子濃度至0.06 wt %發現可有效降低氫氧化銅之形成，擬一階反應速率常數研究提升至2.8 × 10-2 h-1。利用聚乙二醇（PEG）進行零價矽表面修飾為另一種可行之方法，當零價矽表面修飾PEG搭配0.15 wt % 銅離子時可將四氯乙烯有效的分解，其擬一階反應速率常數由原先之0.36 提升至0.56 h-1。然而，當添加之銅離子濃度高於0.15 wt % 時，四氯乙烯的降解速率將被抑制。從EPMA分析結果搭配理論計算，當銅離子添加量為0.16 wt % 時對PEG表面修飾之零價矽的表面的覆蓋率為100%。因此，再增加銅離子濃度於系統中則會降低對四氯乙烯的降解速率。
研究中也發現界面活性劑可以有效地加速零價矽對四氯乙烯之反應速率。當系統中添加SDS以及Tween 80 時，並無助於四氯乙烯降解速率的提升。然而，添加CTAB以及PEG於零價矽系統中則可以有效地加速零價矽降解四氯乙烯之能力。添加0.2 M之PEG於零價矽系統中，其反應速率常數可由原先之3.4 × 10-3 增加至0.36 h-1。所添加之PEG濃度與其相對應之反應速率常數也呈現一線性關係。此外，其反應速率常數相對於其四氯乙烯反應起始濃度之關係，可利用Langmuir-Hinshelwood方程式解釋，表面添加PEG對零價矽的反應仍為表面催化反應。由XPS之結果可以得知零價矽表面修飾PEG可以有效降低SiO2之生成，而使得零價矽表面修飾PEG系統相對於零價矽具有較高之反應性。
由本研究中所得到的結果可以清楚地呈現零價矽不僅具有強還原能力可有效降解氯化有機物及吸附重金屬，所吸附的重金屬能被還原成零價金屬，進而形成雙金屬系統，加速對氯化有機物的分解能力及改變產物分佈結構。零價矽反應後產生之二氧化矽並不會使溶液的pH上升，且為環境友善的物質，不會對環境造成二次危害，而零價矽表面修飾PEG可以抑制二氧化矽的生成。零價矽為一環境友善材料並可對有機及無機污染物進行處理，在實際應用上可為滲透性反應牆系統提供另一種有效材料之選擇。

關鍵字

零價矽 ； 四氯乙烯 ； 滲透性反應牆 ； 還原降解

並列摘要

Chlorinated hydrocarbons such as carbon tetrachloride (CT), trichloroethylene (TCE), and tetrachloroethylene (PCE) are the most often found toxic organic pollutants in the contaminated groundwater. Permeable reactive barrier (PRB), a developed chemical reduction technology filled with zerovalent iron as the reductive material, is an effcetive method which can longevously dechlorinate the chlorinated contaminants in groundwater. However, the increased pH and formation of iron oxides are inevitable when zerovalent iron is applied for the long-term remediation under anoxic conditions. The combination of zerovalent silicon with iron has been found to maintain the solution pH during dechlorination processes. In addition, zerovalent silicon is also a strong reductive material that can apply to dechlorinate the chlorinate hydrocarbons. However, the application of zerovalent silicon and the bimetallic system for dechlorination under various conditions remains unclear.
The main purpose of this study was to evaluate the feasibility of using zerovalent silicon as the reductive material applied in the dechlorination of chlorinated compound and elucidate the parameters can influence the reactivity of the silicon system when applied in environment. For this purpose, pH value effect, co-contaminant effect of inorganic metal ion such as Fe(II), Ni(II), Cu(II) and in the presence of amphiphiles compounds were selected as the parameter to elucidate the interaction mechanism in each parameters to the reactivity of zerovalent silicon. The dechlorination efficiency and rate of chlorinated hydrocarbons by zerovalent silicon increased upon increasing pH from 7.2 to 9.5. The dechlorination followed the pseudo-first-order kinetics and the rate constant (kobs) for CT dechlorination increased from 0.5 h-1 at pH 7.2 to 2.11 h-1 at pH 9.5. In addition, PCE could also be dechlorinated by zerovalent silicon. However, the half life of PCE dechlorination by zerovalent silicon was 8.5 d and the complete transform to non-toxic hydrocarbon was rare. The synergistic effect of Ni(II) and Cu(II) ions which are commonly found heavy metal contaminants in groundwater on the dechlorination rate as well as mechanism by zerovalent silicon was investigated. The dechlorination efficiency and rate of PCE can be significantly enhanced in the presence of Ni(II). The kobs for PCE dechlorination increased from 3.4 x 10-3 to 5.2 x 10-2 h-1 when the loading of Ni(II) increased from 0 to 1.5 wt%. X-ray photoelectron spectroscopy (XPS) were used to characterize and confirmed that the added Ni(II) was reduced to zerovalent Ni by the reduction of zerovalent silicon. Electron probe micro-analyzer (EPMA) was used to characterize the particle size and distribution of reduced Ni species on the silicon surface. The relationship was established and clearly identified between the change in kobs value and the by-products distribution in silicon system with each loading of Ni(II) and its related variable change in physical morphology of Ni distribution on silicon surface.
Zerovalent and monovalent copper species was found as the catalyst can enhance the dechlorination rate when combined to reductive metal. Although the formation of zerovalent copper in the silicon system was observed and characterized by XPS, the dechlorination was inhibited. The kobs value was decreased from 3.4 x 10-3 to 1.7 x 10-3 when 3 wt % of Cu(II) was amended, presumably attributed to the formation of insoluble Cu(OH)2 at the optimized pH value in silicon system. The limitation of Cu loading in silicon system can be minimized by decreasing the loading of Cu(II). Dechlorination results gave the fully support that the kobs value was increased to 2.8 x 10-2 h-1 when the loaded Cu(II) was decreased to 0.03 wt %.
The synergistic effect of Cu(II) ion on PCE dechlorination by PEG-coated zerovalent silicon was achieved. With the addition of 0-0.15 wt% of Cu(II) in PEG-coated zerovalent silicon, the kobs for PCE dechlorination increased from 0.35 to 0.56 h-1. However, the dechlorination ability of PEG-coated zerovalent silicon decreased when the loading of Cu(II) was higher than 0.15 wt %. The EPMA results and theoretical calculation indicate that the surface coverage of Cu species on the zerovalent silicon surface is responsible for the change in kobs for PCE dechlorination in the presence of various concentrations of Cu(II). The calculated 100 % surface coverage of Cu onto the silicon surface is located at 0.16 wt %. The dechlorination efficiency and rate of PCE by zerovalent silicon was significantly inhibited when the zerovalent silicon surface was completely covered with Cu atom at 0.17 wt% loading of Cu(II) which gave the full support to the theorical calculation and the observation from EPMA results.
The dechlorination of PCE by zerovalent silicon can also be enhanced by the addition of surfactant. The surfactants including were selected for comparison. Addition of SDS and Tween 80 had little effect on enhancement of PCE dechlorination, while CTAB and PEG could significantly enhance the dechlorination efficiency of PCE by zerovalent silicon, and a nearly complete dechlorination was observed. The kobs value of PCE dechlorination by zerovalent silicon would be enhanced from 0.0034 to 0.36 h-1 when the loading of PEG increased from 0 to 0.2 M. A linear relationship between the PEG concentration and kobs for PCE dechlorination was established. Moreover, the kobs for PCE dechlorination was dependent on the initial pollutant concentration and followed the Langmuir-Hinshelwood relationship. The XPS results indicate that addition of PEG can prevent the formation of SiO2, and is the major plausible reason to dramatically enhance the reactivity of the zerovalent silicon system.
The results obtained in the study clearly shows that the zerovalent silicon was not only a pH adjuster in zerovalent iron system but also can act as an alternative reductive material to dechlorinate and remove organic and inorganic pollutants.

並列關鍵字

Silicon ； Tetrachloroethylene ； Permeable reactive barrier ； Reductive dechlorination

參考文獻

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