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

• Main Authors: Lee, Chun-Chi, 李俊錡
• Other Authors: Doong, Ruey-An
• Format: Others
• Language: en_US
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/21782826147109426248

博士 === 國立清華大學 === 生醫工程與環境科學系 === 99 === 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 ?嵱. 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.