Photochemical oxidation reactions pesticides

In addition, several naturally occurring metal oxides exhibit semiconducting properties that may catalyze the photochemical production of hydroxyl and hydroperoxyl radicals in aqueous solution— species which, as noted earher, can react with pesticide compounds (Zepp and Wolfe, 1987). Chemical structures located at the surfaces of natural solids may also participate in pesticide transformation reactions as Brpnsted bases, oxidants (e.g., Voudrias and Reinhard, 1986), reductants (e.g., Wolfe et al., 1986), hydrolysis catalysts (Wei et al, 2001), complexing agents (e.g.. Torrents and Stone, 1991), or sources of protons for hydrogen bonding (e.g., Armstrong and Chesters, 1968). 

Automobile exhaust is another source of 2,4- and 2,6-DNPs in air (Nojima et al. 1983). 2,4-DNP is also used as an insecticide, acaricide, and fungicide (HSDB 1994). Therefore, application of this type of pesticide could be a source of 2,4-DNP in air. Photochemical reactions of benzene with nitrogen oxides in air also produce dinitrophenols in the atmosphere (Nojima et al. 1983). Dinitrophenols have been detected in emissions from hazardous waste combustion (James et al. 1984). Dinitrophenols may be present in the aerosol or vapor phase near hazardous waste disposal sites. It has been suggested that the most important origin of dinitrophenols is their formation by photochemical reactions in the atmosphere (Nojima et al. 1983). 

It is evident from the scheme shown in Fig. 3.21 that the chemical structures of pesticides are quite diverse they undergo various physico-chemical effects in the environment after application (solar radiation, heat, air, soil, water) as well as being subjected to various metabolic transformations in plants, microorganisms, insect and animals. Common metabolic transformations are schematically surveyed in Table 3.20, which shows that primarily oxidation, hydrogenation, reduction and hydrolytic reactions are concerned. Also among the individual chemical compounds mutual chemical reactions take place. Some examples of photochemical reactions of pesticides in water are presented in Figs 3.22 to 3.26. Biochemical reactions of DDT and DDE are shown in Fig. 3.27. The number of individual chemical species is hence significantly multiplied in the hydrosphere due... 

Unfortunately, research studies that address environmentally relevant atmospheric fate processes of pesticides are relatively few in comparison to studies that measure transformations on land surfaces and in water. This scarcity of fate information is related to the difficulty in attaining relevant tropospheric photochemical and oxidative information under both environment and controlled laboratory conditions. Only a limited number of studies exist that have measured airborne pesticide reactivity under actual sunlight conditions (d, 7,8), These studies enq)loyed photochemically stable tracer confounds of similar volatility and atmospheric mobility to con5)ensate for physical dilution. The examined airborne sunlight-exposed pesticides in these limited studies had to react quickly to provide environmentally measurable reaction rate constants. The field examination of tropospheric reaction rates for the vast majority of agricultural pesticides is impractical since reaction rates for many of these compounds are probably too slow to yield reliable rate constant information. 

Of the field and laboratory air studies that have been performed, sunlight-induced chemical oxidations and photochemical reaction pathways usually render pesticide residues less toxic, more polar, and more susceptible to being washed-out of the air mass (J1,12,13,14,15), Field and laboratory atmospheric pesticide fate studies have also reported the formation of photooxidation products that can have equal or higher toxicity and/or greater environmental persistence than the parent pesticide 16,17,18,19), Because of the limited number of atten ted atmospheric fate studies, there remains a substantial degree of uncertainty in regards to the mechanistic behavior and possible fate of many pesticide groups that can reside in the lower atmosphere. 

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