| The fate of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) in the environment is of great interest due to its high quantity usage and its chlorinated aromatic nature. It is highly susceptible to oxidative degradation in aqueous solutions, producing a variety of chlorinated aromatic intermediates. Mechanistic and kinetic details of 2,4-D oxidation have been elucidated using the advanced oxidation processes of radiolysis, sonolysis and photocatalysis. The main intermediate is 2,4-dichlorophenol (2,4-DCP), which, in some systems, displays a degree of resistance to further oxidative degradation. This intermediate, along with 2-(2,4-dichlorophenoxy)propionic acid (2,4-DP), 2,4-D methyl ester and 2,4,6-trichlorophenol (2,4,6-TCP) were tested in various advanced oxidation studies.; Gamma radiolysis experiments supporting •OH conditions were performed on the chlorinated aromatic compounds. Intermediates were identified and degradation rates were determined. Experiments that utilized both 18O-labeled water and unlabeled water in γ-radiolysis yielded mechanistic data on the reactivity of the hydroxyl radical with 2,4-D. This was complemented by computational and pulse radiolysis studies of the •OH attack on both 2,4-D and 2,4-DCP. Theoretical calculations involved the compilation of data for both gas phase and water solvent. In the pulse radiolysis work, both •OH oxidations and electron transfer oxidations were studied.; The kinetically controlled attack ipso to the ether functionality was established as the main, selective pathway between •OH and 2,4-D. The •OH-mediated breakdown of 2,4-DCP is pH dependent and less selective. At low pH, 2,4-DCP reacts via • OH-addition to the aromatic ring, most likely at all sites, while the 2,4-dichlorophenoxide ion may follow two different reaction pathways in its reactivity.; The oxidations of the chlorinated aromatic compounds were studied using the advanced oxidation technologies of sonolysis and photocatalysis. While the chlorinated phenols were easily oxidized in the sonolysis experiments, they were relatively long-lived using photocatalysis. Oxidations using TiO 2 photocatalysis successfully mineralized the organic compounds, which was not possible in sonolysis. By simultaneously carrying out high frequency sonolysis and photocatalysis, fast degradation rates and complete mineralization with no build up of toxic intermediates even at very low catalyst loadings were achieved. |
