Chlorpyrifos hydrolysis kinetics

Recently reported results for the hydrolysis kinetics of chlorpyrifos (7 ) suggest that equation 2 may not be a valid representation of alkaline hydrolysis kinetics for at least one class of pesticides (organophosphorothioates). In short, kg may be pH dependent. However, disappearance kinetics for such molecules are still adequately described at fixed pH by pseudo first-order kinetics. 

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. 

Alkaline Hydrolysis Studies. Alkaline catalyzed hydrolysis kinetics in sediment/water systems have been investigated for chlorpyrifos and the methyl and n-octyl esters of 2,4-dichlorophenoxyacetic acid (2,4-D). 

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. 

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). 

Chlorpyrifos is stable to hydrolysis in the pH range of 5-6 (Mortland and Raman, 1967). However, in the presence of a Cu(lf) salt (i.e., cupric chloride) or when present as the exchangeable Cu(II) cation in montmorillonite clays, chlorpyrifos is completely hydrolyzed via first-order kinetics in <24 h at 20 °C. It was suggested that chlorpyrifos decomposition in the presence of Cu(II) was a result of coordination of molecules to the copper atom with subsequent cleavage of the side chain containing the phosphorus atom forming 3,5,6-trichloro-2-pyridinol and 0,0-ethyl-0-phosphorothioate (Mortland and Raman, 1967). 

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. 

First, the computer calculated uncertainties shown for the calculated values of kj, k and kg are an indication that the model has considerable validity for describing the kinetics of the system, at least over one half-life in the disappearance of chlorpyrifos. Second, the values of k and kj are all similar and their magnitude indicates that in this case the assumption of rapid sorption/desorption kinetics compared to hydrolysis is valid. 

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. 

The results of these experiments in PONl knockout mice prompted a reexamination of the tVr vitro catalytic efficiencies of the two human PON 1 alloforms under more physiological salt concentrations (lower NaCl concentration). Results from kinetic analysis of substrate hydrolysis by purified human alloforms provided an explanation for the in v ivo finding. In the case of chlorpyrifos-oxon, the catalytic efficiency of both PONl alloforms was very high, and it was higher for the PON1rhj2 alloform (Table 1). Catalytic efficiency was still high in the case of diazoxon,... 

Blanche , P.-F., and A. St. George. 1982. Kinetics of chemical degradation of organophosphorus pesticides hydrolysis of chlorpyrifos and chlorpyrifos-methyl in the presence of copper(II), Pesticide Sci., 13 85-91. 

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