Diazinon neutral hydrolysis

Neutral Hydrolysis Studies. Investigations of neutral (pH-independent) hydrolysis kinetics in sediment/water systems were conducted for three organophosphorothioate insecticides (chlorpyrifos, diazinon and Ronnel), 4-(p-chlorophenoxy)butyl bromide, benzyl chloride, and hexachlorocyclopentadiene. 

Diazinon and Ronnel. The conclusion that neutral hydrolysis of sorbed chlorpyrifos is characterized by a first-order rate constant similar to the aqueous phase value is strengthened and made more general by the results for diazinon, 0,0-diethyl 0-(2-iso-propyl-4-methyl-6-pyrimidyl) phosphorothioate, and Ronnel, 0,0-dimethyl 0-(2,4,5-trichlorophenyl) phosphorothioate (10). The results for the pH independent hydrolysis at 35°C for these compounds in an EPA-26 sediment/water system (p=0.040) are summarized in Table IV. Because the aqueous (distilled) values of k for diazinon and Ronnel are similar in magnitude to the value for chlorpyrifos, and because these values were shown by the chlorpyrifos study to be slow compared to sorption/desorption kinetics, computer calculations of were not deemed necessary and were not made for these data. 

Thus, for chlorpyrifos, diazinon, Ronnel (and by extension, other organophosphorothioate pesticides), neutral hydrolysis proceeds at similar rates in both the aqueous and sediment phases of sediment/water systems. 

Dr. P. Jeffers at the State University of New York at Cortland is gathering information to determine the persistence of organophosphorus compounds in groundwater and the effects of various soils on the degradation and transport of these compounds. Both neutral and base hydrolysis processes will be evaluated. Transport studies in soil columns will be conducted to determine the mobility of diazinon in soils. 

Diazinon persists for six months or longer in nearly neutral unflooded soil (I, 8). Its life is shorter in acid soils (9) because of the rapid hydrolysis under acid conditions (10,13,14). Diazinon applied to a nearly neutral flooded soil persists only for 2 months (2). When applied to an acid soil (Luisiana clay with an initial pH 4.7) immediately after submergence, its persistence is shortened considerably (2). In such an acid soil if the application of diazinon is delayed to two or three weeks after submergence, the insecticide is not degraded rapidly because soil pH rises by about two units after reduction in waterlogged soils. In fact, diazinon controlled rice pests for 15 days after it was applied to Luisiana clay that was kept submerged for four weeks before application (15). 

Chemical/Physical. In water, diazinon is hydrolyzed following first-order kinetics forming 2-isopropyl-4-methyl-6-hydroxypyrimidine and diethyl thiophosphoric acid or diethyl phosphoric acid in the pH range 3.1 to 10.4. At pH values of 7.4 and 10.4, the persistence of diazinon is 185 and <6 days, respectively (Gomaa et al., 1969). Cowart et al. (1971) reported a half-life of approximately 2-3 weeks in a neutral solution at room temperature. Chapman and Cole (1982) reported the following hydrolysis half-lives of diazinon in a sterile 1% ethanol/water solution at 25°C 0.45, 2.0, 7.8, 10 and 7.7 weeks at pH values of 4.5,5.0,6.0,7.0 and 8.0, respectively. Diazoxon was also found infogwater collected near Parlier, CA (Glotfelty et al., 1990). 

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