Chlorpyrifos neutral hydrolysis

Rates of hydrolysis may be influenced by the presence of dissolved organic carbon, or organic components of soil and sediment. The magnitude of the effect is determined by the structure of the compound and by the kinetics of its association with these components. For example, whereas the neutral hydrolysis of chlorpyrifos was unaffected by sorption to sediments, the rate of alkaline hydrolysis was considerably slower (Macalady and Wolf 1985) humic acid also reduced the rate of alkaline hydrolysis of 1-octyl 2,4-dichlo-rophenoxyacetate (Perdue and Wolfe 1982). Conversely, sediment sorption had no effect on the neutral hydrolysis of 4-chlorostilbene oxide, although the rate below pH 5 where acid hydrolysis dominates was reduced (Metwally and Wolfe 1990). 

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. 

Chlorpyrifos, 0-0-diethyl 0-(3,5,6-trichloro-2-pyridyl) phosphorothioate, is the compound for which the most exhaustive kinetic investigations were conducted (10). The kinetics of the hydrolysis as a function of pH in distilled and buffered distilled water systems is summarized by the pH-rate profile shown in Figure 2 (7j. The value, k jg=(6.22 0.09) x 10 min is the neutral hydrolysis rate constant for chlorpyrifos in distilled water at 25°C. 

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. 

Chlorpyrifos. As was the case for the neutral hydrolysis studies, the most detailed kinetic investigations of alkaline hydrolysis kinetics in sediment/water systems have been conducted using chlorpyrifos (10). As can be seen from Figure 2, alkaline hydrolysis of chlorpyrifos is not second-order, so the value selected for k cannot be calculated from the pH and a second-order rate constant. Nevertheless, since aqueous kinetics at alkaline pH s for chlorpyrifos was always pseudo-first order, careful pH measurements and Figure 2 can be used to select accurate values for k at any pH. 

Two types of investigations of the alkaline hydrolysis of chlorpyrifos in sediment/water systems were made, all at pH s between 10.6 and 10.8. First, studies were conducted in which the pH was adjusted (using a carbonate buffer) immediately upon mixing the sediments (EPA-23 and EPA-26) with the chlorpyrifos solution. Second, a study using EPA-26 was made in which the alkaline buffer was not added until three days after mixing the sediment with the chlorpyrifos solution. Three days represents a time which is long with respect to the achievement of sediment-water equilibrium for this system, yet short compared to the neutral hydrolysis half life (-50 days). 

---