臭氧化降解除草剂2,4-D的机理研究(Ⅱ):降解路径

为了考察除草剂2，4-D在臭氧化反应过程中的降解机理，在前文分析中间产物的基础上，进一步跟踪反应溶液中几个主要中间产物和氯离子的浓度变化趋势，由此提出可能的降解路径。结果说明，2，4-D臭氧化降解过程主要是羟基自由基的间接氧化起作用，臭氧分子的直接氧化所占比重很小。对前文分析出的主要中间产物进行分类，并通过判断反应溶液中中间产物和氯离子浓度随反应时间变化的趋势，可知2，4-D在羟基自由基作用下，主要先产生了2，4-二氯苯酚等含氯的酚类，再经过脱氯，形成无氯的芳香族化合物。苯环结构开裂后，形成了大部分有机酸，另一小部分有机酸在2，4-D初步降解为含氯芳香族化合物时即可产生。脱氯过程有一个初始较慢后来较快的趋势，这说明2，4-D直接脱氯较少，大部分脱氯过程发生在形成2，4-DCP等含氯芳香族中间产物后，此时，脱氯产生无氯的芳香族中间产物。在最后的深度氧化阶段，经过一系列中间产物反应之后，部分2，4-D能降解为水、二氧化碳等终产物。

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.