| Summary: | 博士 === 國立臺灣大學 === 環境工程學研究所 === 99 === With the findings of Dense Non-Aqueous Phase Liquids (DNAPLs), which are easily formed from the release of chlorinated hydrocarbons into groundwater aquifers, the challenge to clean up the contaminated sites have been arduous since the 1980’s. Cost-effective and reliable technologies are sought after to treat DNAPLs contamination. The use of nanoscale metal particles to treat DNAPLs or contaminants released from them is a novel scheme developed in recent years, involving the utilization of nanoscale zero valent iron (NZVI) or bimetallic nanoparticles in reducing chlorinated hydrocarbons. This research takes an in-depth look at three aspects: 1. Manufacturing stabilized NZVI. The proximity of nanoparticles makes them aggregate easily and harder to disperse. Surface modifiers such surfactant or polymer are needed to enhance stability and practicality. 2. Researching the in-situ application. In-lab studies of the effectiveness of NZVI in treating chlorinated hydrocarbons have been proven, but the in-situ applications are scarce. 3. Researching the transport of NZVI. The deploying range of NZVI after injection affects directly the amount of NZVI reagent used the number of injection wells applied. With the development of a model to predict the transport of NZVI, the design would allow the delivering of NZVI to be more applicable and affordable. .
With regard to the first aspect, the biodegradable nonionic-surfactant modified NZVI suspension was developed and deployed. These approaches are effective on handling dissolved contaminants from DNAPLs, however, are not effective in dealing with the main pollutant sources, residual saturation or DNAPLs pools. In this respect, this study has also included the preparation of hydrophobic NZVI suspension for environmental restoration. These agents are used to enhance the destruction of chlorinated DNAPLs in source zones by creating better contact between the DNAPLs and reducing NZVI.
A field investigation on the influence of NZVI on geochemical properties of groundwater and contaminants degradation was also conducted. A 200 m2 pilot-scale field test successfully demonstrated the effective remediation of groundwater contaminated with chlorinated organic compounds in Taiwan within six months by using NZVI. Both commercially available and on-site synthesized NZVI were used. A monitoring system allowing the collection of three-dimensional spatial data from 13 nested multi-level monitoring wells was established to monitor geochemical parameters in groundwater. The degradation efficiency of vinyl chloride (VC) measured at most of the monitoring points was 50-99%. A decrease in oxidation-reduction potential (ORP) values from about -100 to -400 mV after NZVI injection was observed. This revealed that NZVI is an effective means of achieving highly reducing conditions in the subsurface environment. Both VC degradation efficiency and ORP showed a correlative tendency as an increase in VC degradation efficiency corresponded to a decrease of ORP. This is in agreement with the previous studies suggesting that ORP can serve as an indicator for the NZVI reactivity. VC was degraded by NZVI quickly, with the process supposedly abiotic, while the 1,2-DCA degradation was relatively sluggish within three months. Nevertheless, as 1,2-DCA is known to resist chemical reduction by NZVI, the observation of 1,2-DCA degradation and hydrocarbons production suggests that biological processes have been involved. The bioremediation may be attributed to the production of hydrogen as electron donor from the corrosion of ZVI in the presence of water or the added biodegradable surfactant serving as the carbon source as well as electron donor to stimulate the microbial growth.
To have insight of the NZVI transport, a new trajectory simulation algorithm was developed to describe the efficiency of a single collector (pore) to catch submicrometer particles moving through saturated porous media. A constricted-tube model incorporating the deterministic (interception, hydrodynamic retardation, van der Waals force and gravitational sedimentation), stochastic (Brownian diffusion) and thermodynamic (electrostatic and steric repulsion force) mechanisms was established to predict the transport and deposition of surface modified NZVI particles by applying Lagrangian trajectory analytical approach. The simulation results show good agreement with the results predicted by existing energy-barrier-free models except for the particle size less than 100 nm at low approach velocity. The number of realizations per starting location could be decreased down to one hundred with the simulations still exhibiting acceptable relative standard deviation for engineering purposes. The correlation equations with dimensionless parameters fitting the simulation results were proposed to provide a quick estimation of the collection efficiency. With the consideration of energy barriers, the model successfully describes the breakthrough curve of polymer-modified NZVI in a bench-top soil column as well. The novel simulation scheme can be a useful tool for predicting the behavior of the nanoscale colloidal particles moving through filter beds or saturated soil columns under conditions with repulsion and attraction forces among surfaces.
|
|---|
