Zero-valent iron (ZVI) has been successfully used for the degradation of a wide range of organic contaminants in groundwaters in recent years. The rate of degradation of contaminants by ZVI may be enhanced by use of nanoscale zero valent iron (nZVI) particles which possess higher surface area than the more widely used granular materials. However, the most widely used method of producing nZVI involves the reduction of FeIII by sodium borohydride is expensive. Dithionite can be used to reduce Fe(II) and produce cost effective nZVI under conditions of high pH and in the absence of oxygen. The efficiency of trichloroethylene (TCE) degradation using dithionite nZVI particles (nZVIS2O4) is similar to that of the conventional borohydride particles (nZVIBH4). Oxidation of benzoic acid using the nZVIS2O4 particles results in different byproducts than those produced when nZVIBH4 particles are used. The high concentration of phenol compared to hydroxybenzoic acids suggests that OH addition is not the primary oxidation pathway when one is using the nZVIS2O4 particles. It is proposed that sulfate radicals (SO4−) are produced as a result of hydroxyl radical attack on the sulfite matrix surrounding the nZVIS2O4 particles, with these radicals oxidizing benzoic acid via electron transfer reactions rather than addition reactions. Low yields of oxidants limit the application of nZVI. It has recently been demonstrated that nZVI oxidative efficiency can be enhanced in presence of ethlylendiaminetetraacetic acid (EDTA). Additional insight into the nZVI-mediated process has been obtained from comparative studies of degradation of benzoic acid by nZVI particles and Fenton reagents in the absence and presence of EDTA at different pH. The efficiency of nZVI degradation is significantly hindered by the rapid aggregation of the iron nanoparticles, which may result in a decrease in available reactive surface area. These effects of aggregation can be overcome by surface modification through adsorption of capping agents which provide steric and electrosteric repulsive interactions between particles. Several high molecular weight (HMW) organic polymers have been used for preventing agglomeration of nZVI particles, such as water soluble starch, sodium carboxymethyl cellulose (CMC) and alginate. The degradation capabilities of different functionalized nZVIS2O4 particle were investigated. Iron-based bimetallic particles in which metals such as Pd and Ni have been combined with Fe, have been found to both enhance rates of halogenated organic contaminants reduction and generate more fully dehalogenated products relative to unamended iron. The results presented in this thesis demonstrate that formation of bimetallic particles with nZVI formed from the more cost effective dithionite reduction of ferrous salts also results in dramatic enhancement in reducing ability. The oxidising ability of nZVIBH4 particles can be enhanced dramatically by addition of polyoxometallates (POMs), redox catalysts which result in enhanced production of hydrogen peroxide. The extent of enhancement is quantified by examination of the oxidation of formic acid (to CO2) and kinetic modelling of the results obtained used to investigate the mechanism of the POM-mediated oxidation process.
| Identifer | oai:union.ndltd.org:ADTP/258216 |
| Date | January 2009 |
| Creators | Sun, Quan, Civil & Environmental Engineering, Faculty of Engineering, UNSW |
| Publisher | Publisher:University of New South Wales. Civil & Environmental Engineering |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://unsworks.unsw.edu.au/copyright, http://unsworks.unsw.edu.au/copyright |
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