| Summary: | 碩士 === 國立暨南國際大學 === 土木工程學系 === 100 === Abstract
Contamination of soil and groundwater by pesticides, organic solvents, petroleum hydrocarbons, and other organic compounds has become a serious problem. Nanoscale zero-valent iron (nZVI) is widely used for the treatment of chlorinated compounds due to its high reactivity and strong reducing power. Recently, many studies focus on the enhancement of stabilization, dechlorination capacity, and transport of nZVI. However, information on the effects of nZVI application on the intrinsic biodegradation is lacking.
In this study, biodegradability of dispersants and their effects on the intrinsic biodegradation of trichloroethylene (TCE) were evaluated. Results of microcosm study show that polyacrylic acid (PAA) was not degraded by in situ microorganisms under both aerobic and anaerobic conditions. Removal of total organic carbon (TOC) of polyvinyl alcohol-co-vinyl acetate-co-itaconic acid (PV3A) and polyoxyethylene (20) sorbitan monolaurate (Tween 20) reached 100 and 37%, respectively, under aerobic conditions during 140 days of reaction. Complete degradation of PV3A was achieved while degradation of Tween 20 was quite slow under anaerobic conditions during a 130-day incubation. Natural biodegradation of TCE was observed under both aerobic and anaerobic conditions during 130 days of reaction. TCE may be removed by cometabolism using soil organic matter under aerobic conditions. Under anaerobic conditions, microbial reductive dechlorination of TCE occurred with the production of less chlorinated byproducts. No significant effects were observed on the intrinsic biodegradation of TCE under aerobic and anaerobic conditions with the presence of the three selected dispersants. However, more notable accumulation of TCE biodegrading byproducts was detected with the addition of PV3A and Tween 20, probably due to the preference of native microbes for more biodegradable and less toxic substances.
Results of polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) analysis show that the diversity of the microbial community was enhanced with the presence of more biodegradable dispersants, PV3A and Tween 20, compared to the less biodegradable dispersant, PAA. Results of this study indicate that it is necessary to select an appropriate dispersant based on the biodegradability of the dispersant and the effects of the dispersant on the intrinsic biodegradation of TCE before the application of nZVI for site remediation to prevent the accumulation of TCE biodegrading byproducts and the residual of the dispersant in the subsurface.
Keywords: Trichloroethylene (TCE); nanoscale zero-valent iron (nZVI); dispersant; intrinsic biodegradation; polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE)
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