| Summary: | 碩士 === 國立屏東科技大學 === 環境工程與科學系 === 94 === Organic chlorinated solvents are widely used in manufacturing processes. They are not easy to be degraded or decomposed in the environment owing to their special physical, chemical and biological characterization. If those solvents are not proper handled or disposal, they may possibly leaked into soil and further pollute the aquifer. Among the remediation technology, permeable reactive barrier (PRB) packed with zero iron is non-destructive and safe. Furthermore the particle sizes of iron can be nanoized to increase the reaction rate that enhances the degradation rate of organic solvents from groundwater.
The objective of this study is to investigate the effects of nanosacle iron PRB on the trichloroethylene (TCE) degradation efficiency enhanced by surfactant solubilization and electrolysis. The experiments were divided into two stages. Experiments in the first stage were designed and performed in a lab-scale sand box to determine the optimal parameters such as potential, iron barrier site, iron amount) of electrolysis-enhanced nanosacle iron PRB (ENIPRB) degradation of TCE. The second stage was experiments of solutes′ transport through porous media in a bench-scale sand box. The results from preliminary study showed that the average particle size and BET specific surface area of lab synthesized nanosacle iron were 189.7 nm and 30.0 m2/g.
The iron component of particles was detected through X-ray powder diffraction (XRD) examination at 2θ=44.74°. The results from experiments of the first stage showed that TCE removal efficiency was only 54﹪by addition of 5 grams of nano iron (Fe) and enhanced by electrolysis to 96% As the potential gradient increased to 1 V/cm, removal efficiency of TCE in the outlet of sand box could reach to 96﹪after 14 hour of operation. TCE removal efficiency was enhanced by the potential gradient set at 2V/cm and reach to 100% after 8 hour of operation. TCE removal efficiency of the potential gradient set at 0.5V/cm was approximately the same as that of non- electrolysis. Although the TCE removal efficiency increased with higher potential gradient, some precipitates deposited on the iron surface and could block the pores of reactive barrier as potential gradient set at 2V/cm that was not suitable for a long-term operation. Furthermore, Surface normalized reaction rate constants (Ksa) increased with the iron amount that raised TCE removal efficiency. The site of iron barrier located behind anode was better for TCE removal.
The results from experiments of the second stage showed that breakthrough time of solutes was slightly slower than that of water flow owing to the influence of advection and dispersion. The recovery rate of TX-100 could reach to 92％.The concentration of TCE ranged from 250 mg/L to 270 mg/L in the absence of TX-100 while increased to 700 mg/L with addition of 0.07 grams of TX-100 that indicated the solubility and mobility of TCE was significantly enhanced by surfactant. The concentration of TCE at sampling point D located behind iron wall reduced from 320 mg/L to 180 mg/L with addition of 5 grams of iron while conductivity and concentration of chlorine and ferrous increased. After 20 hours of operation, the TCE concentration raised again due to the increase of pH that resulted in precipitation of iron hydroxide and formation of non-reactive sites on the surface of nanosacle iron. As the voltage was applied, TCE concentration reduced from 220 mg/L to 135 mg/L because H+ released from anode by water electrolysis could wash away the deposits on the surface of nanosacle iron to enhanced the reactivity of iron surface. The potential difference would accelerate the release of electrons from iron near anode that induced TCE reduction. From the results of the first and second addition of TX-100 into sand box through injection well showed the mobility of TCE was not significantly different. This study shows TX-100 combined with electrolysis-enhanced nanosacle iron reactive barrier can effectively degrade TCE in groundwater and this technique can be referred as an alternative for in-situ remediation of organic chlorinated solvents in aquifer.
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