Chlorpyrifos disappearance rate constants

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... 

Table II. Ranges of Measured Rate Constants for the Disappearance of Chlorpyrifos from Natural Water Samples at 25°C...

In the first type of study, pseudo first-order kinetics were observed in both the sediment and aqueous phases from t=0 through two half-lives in overall chlorpyrifos disappearance (total time -8 days). For these studies, computer calculations using the model illustrated in equations 7 were again used to calculate values for kj, k and kg, assuming a value of k equal to the pseudo first-order rate constant in distilled water buffered to the same pH. Values were also calculated for Obfi assuming kg 0 (equation 10) for comparison to the experimental kg values. The results of these calculations are shown in Table VII. 

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. 

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