Mechanism of Herbicide 2,4-D Degradation by Ozonation(II):Degradation Pathway

In order to study the mechanism of herbicide 2,4-D degradation through ozonation,the concentration of intermediate products and chloride ion was tracked during the experiments.A possible degradation pathway was subsequently proposed.The experimental results showed that the indirect oxidation of hydroxyl radical played the crucial role in degradation of 2,4-D.The direct oxidation of O3 molecule was not the major aspect.From the tendency of concentration of intermediate products and chloride ion during the reaction,many reaction steps were deduced.2,4-D was attacked at the aromatic ring by hydroxyl radical to produce 2,4-dichlophenol.Chloric aromatics,such as dichlorophenol,chlorophenol,were the first intermediate products produced from the hydroxyl radical attack.After dechlorination from chloric aromatics,dechlorinated aromatics were formed.Then aromatic ring was broken to produce a large quantity of organic acids.In addition,a small quantity of organic acids were the byproducts of the first step of 2,4-D degradation to form chloric aromatic.The process of dechlorination in the first reaction time was rather slow,but subsequently the concentration of chloride ion in the reaction solution increased rapidly,which showed that the direct dechlorination of 2,4-D structure was rather less.Dechlorination mostly occurred on the chloric aromatic structure.The dechlorination reaction from chloric aromatics to produce dechlorinated aromatics was rather fast.During the final deep oxidation,part of 2,4-D was degraded to form water,carbon dioxide and other end products after a series of reaction between intermediate products.