Chlorpyrifos systems

In 1985, Berteau and Mengle (1985) of the California Department of Health Services and Maddy of the Department of Food and Agriculture conducted a preliminary review of pesticides used indoors. They noted several cases (six) from the Pesticide Illness Surveillance system in which illness was reported after structural pest control. Hypothetical exposure estimates for infants, children, and adults following label use for propoxur, DDVP, and chlorpyrifos were sometimes greater than toxic levels. In 1987, Berteau et al. (1989) reiterated the concern about the potential magnitude of indoor exposures, particularly for children. 

Rawlings, N.C., S.J. Cook, and D. Waldbillig. 1998. Effects of the pesticides carbofuran, chlorpyrifos, dimethoate, lindane, triallate, trifluralin, 2,4-D, and pentachlorophenol on the metabolic endocrine and reproductive endocrine system in ewes. Jour. Toxicol. Environ. Health 54A 21-36. 

Chlorpyrifos inhibits substrate-borne reception and emission of sex pheromone in Tri-chogramma brassicae, an entomophagus insect massively used as a biological control agent of com borers, among survivors of an LC20 dose. Inhibition was probably due to nervous system effects and was not specific to pheromone communication (Delpuech et al. 1998). 

Holladay, S.D., S.A. Smith, H. El-Habback, and T. Caceci. 1996. Influence of chlorpyrifos, an organophosphate, on the immune system of Nile tilapia. Jour. Aquat. Anim. Health 8 104-110. 

Menconi, M. and A. Paul. 1994. Hazard Assessment of the Insecticide Chlorpyrifos to Aquatic Organisms in the Sacramento-San Joaquin River System. Calif. Dept. Fish Game, Environ. Serv. Div., Admin. Rep. 94-1. 74 pp. 

There is one final observation using the bandwidth information The data of Tables IX and X suggest that the flame photometric detector (chlorpyrifos) produces more consistent data than the electron capture detector. The chlorpyrifos data clearly had the narrowest bandwidth yet both the range and sample size of this set were comparable to the others studied. The range of chlorpyrifos was 500 to 1 whereas those of fenvalerate and chlorothalonil were 2000 to 1 and 1000 to 1, resp. Chlorpyrifos had 30 samples whereas the other two had 36 and 30, resp. Chlorpyrifos had 5 analysis levels while the other two had 6 each. More data of this sort is needed to compare various detector systems. 

Biological. Using the experimentally determined first-order biotic and abiotic rate constants of chlorpyrifos in estuarine water and sediment/water systems, the estimated biodegradation half-lives were 3.5-41 and 11.9-51.4 d, respectively (Walker et al, 1988). 

The fact that sorptive equilibrium can be approached quite slowly is illustrated dramatically by data for the system in which chlorpyrifos is sorbed to EPA-14, one of a group of sediments collected and characterized for the U. S. 

Environmental Protection Agency (22). Figure 1 is a plot of the sediment/aqueous concentration ratio versus time for this system. It is characterized by a rapid sorption process and a much slower sorption process which does not reach equilibrium until about 10 days after initial mixing of the sediment and chlorpyrifos solution. 

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. 

The data from a representative study of the disappearance of chlorpyrifos from an EPA-14 sediment/water system (p=0.20, fraction sorbed = 0.94) is illustrated in Figure 3. Comparison with Figure 1 shows that once sorptive equilibrium is achieved (t>14,000 minutes) the disappearance rate is first order for both the water and sediment phases. Also, the aqueous disappearance rate constant calculated from the slope of the linear portion of the natural log aqueous concentration versus time plot is 0.5 0.2 x 10 min, which is similar to the values measured in sediment-free EPA-14 supernatant (Table II). A plot summarizing two experiments using EPA-23 sediment is shown in Figure 4. The value of calculated from the... 

Data from these studies were analyzed by a computer using equations 8 based on our simple kinetic model for the sediment/water systems (eqn. 7). The computer program (23) uses concentrations of chlorpyrifos in the water and sediment phases and product concentrations (obtained by difference) as a... 

Figure 3. Chlorpyrifos disappearance from an EPA-14 sediment/ water system, P= 0.20, t = 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. 

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. 

Several features of the PCBB experiments are different than those for chlorpyrifos. The hydrolysis reaction proceeds via a different mechanism. The rate enhancements observed for chlorpyrifos in natural waters and the aqueous phases of the sediment/water systems (as compared to sterile distilled water) are not observed for PCBB. The values of kj and k calculated for PCBB are slower than those for chlorpyrifos anS similar in magnitude to the hydrolysis rates. 

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. 

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). 

Experimental and Calculated Values of the Rate Constants for the Alkaline Hydrolysis of Chlorpyrifos in Sediment/Water Systems ... 

Esters of 2,4-D. Studies of the alkaline hydrolysis of the methyl and n-octyl esters of 2,4-D in sediment/water systems (24), though less detailed than the chlorpyrifos studies, show similar effects. Results from Investigations using EPA-13 at pH s near 10 for the methyl and octyl esters of 2,4-D are summarized in Figure 7. Under the conditons in these experiments, the fractions of the methyl and octyl esters which are sorbed to the sediment are 0.10 and 0.87, respectively. The aqueous hydrolysis half-lives of the methyl and octyl esters at pH=10 are 3.6 and 27 minutes, respectively. In the sediment/water system, the methyl ester, which is mainly in the dissolved phase, hydrolyzes at a rate similar to that expected for the sediment-free system at the same pH. The octyl ester, on the other hand, hydrolyses at a rate which is considerably retarded (and non-first-order) when compared to the expected aqueous phase rate. Though the data are less detailed and do not permit calculations similar to those conducted for chlorpyrifos, it is clear that the effect of sorption is to considerably slow the alkaline hydrolysis rate. 

Studies of the disappearance of the octyl ester at pH 9.8 in sediment/water systems aged 3 days prior to pH adjustment are summarized in Figure 8. For the systems with p=0.013 and 0.005 (fractions sorbed =. 978 and. 945) the rate is pseudo first order, but the rate constant is 10 times smaller than the aqueous value (1.6x10 min ) at this pH. As was suggested for chlorpyrifos, this k value may be characteristic of the actual value of k. At p=0.001, (fraction sorbed = 0.78), the disappearance kinetics is not first order, but shows rapid disappearance of the aqueous ester, followed by disappearance of the sorbed ester at a rate similar to the studies with higher sediment to water ratios. 

G. Jeanty, Ch. Ghommidh and J.L. Marty, Automated detection of chlorpyrifos and its metabolites by a continuous flow system-based enzyme sensor, Anal. Chim. Acta, 436(1) (2001) 119-128. 

---