Mirex, photoreduction

Mirex photoreduction in air-saturated distilled water was considerably slower (20-400 days). Finally, Zepp et al. (44) reported that the photoinduced dehydrochlorination of l,l,l-trichIoro-2,2-bis(p-methoxyphenyl)ethane (me-thoxychlor) is greatly accelerated in natural waters with high DOM concentrations. 

The degradation of mirex in water occurs primarily by photolysis. During the photodecomposition of mirex, the chlorine atoms are replaced by hydrogen atoms. The primary photoreduction product of... 

The half-life was reduced sixfold in the two former cases and twofold in the latter. Benzamide and reduction photoproducts (mono- and dichlorinated derivatives) could be characterized by GC-MS. Chlorothalonil, which bears several electron-withdrawing substituents, could be only oxidized by a strong oxidant and the involvement of hydroxyl radicals was very likely. Photoreduction also occurred. Since chlorothalonil is very poorly soluble in water, it might sorb to DOM and react with solvated electrons produced within the hydrophobic zones of DOM in a similar way to Mirex. 

In addition to the chloroacetates, as shown by comparisons with the estimates in Table V, the half-lives for both lindane and mirex in the natural water samples would have been considerably longer than those observed, had reaction with solvated electrons in bulk solution been the dominant mechanism for photoreaction. The higher efficiency of these halocarbon reactions may be attributable to sorption of the chloroacetates on the NOM, which permits more facile electron capture. Other possible pathways for reactions of sorbed halocarbons include direct photoreduction by excited states of the NOM, which, like solvated electrons, also are quenched by oxygen. Alternatively, the enhancement may involve other direct electron-transfer mechanisms such as amine-halomethane reactions. These alternative possibilities are examined in the following section. 

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