Properties and Uses of Chlorpyrifos in the United States - Reviews of Environmental Contamination and Toxicology - page 16

Fig. 2 Schematic diagram of
a two-compartment kinetic
model for chlorpyrifos (CPY)
degradation
kinetics (de Vette and Schoonmade 2001 ). Nonetheless, some of the half-lives
reported in SI Table A-2 that have been derived from 1st-order degradation kinetics
might overestimate the persistence of CPY in the environment.
There have been several approaches to calculate rate constants of degradation for
this biphasic degradation of CPY. The DT 50 values reported by Bidlack were calcu-
lated using the Hamaker two-compartment kinetic model (Nash 1988 ), but details
of the goodness of fit were not provided and the DT 50 values do not correspond to
degradation rate constants (Bidlack 1979 ). Also, bi-phasic degradation, described
by use of the double first-order parallel (DFOP) model, best characterized the data
from three dissipation studies performed in terrestrial environments (Yon 2011 ).
To obtain the biphasic rate constants for the available aerobic soil degradation
results, a dissipation model was structured with two compartments for the parent
compound; one adsorbed in such a manner that was not available for biological
degradation or abiotic hydrolysis, and the other in which these processes can occur
(Fig. 2 ). The initial thought was to consider these as adsorbed and dissolved com-
partments, respectively. However, it is known that partitioning of CPY between soil
and soil pore water reaches equilibrium within hours (Racke 1993 ), whereas the
biphasic degradation process observed for CPY occurs over a period of several
days. The two compartments were identified as Labile CPY and Adsorbed CPY .
Reversible movement of parent CPY between these compartments was represented
as two simple first-order processes shown by arrows F 1 and F 2 in Fig. 2 , with rate
constants k ads and k des . This model has advantages over older two-compartment
models in that simple first-order equations are used and the rate constants are not
concentration-dependent as they are in the Hamaker kinetic equations (Nash 1988 ).
Since the reported concentrations of CPY include both compartments, the model
was configured so that measured values are entered as the sum of the amounts in
these two compartments at each time point (Fig. 2 ). The sum of processes that
degrade CPY was also described as a first-order kinetic process F 3 , but was non-
reversible. The rate constant for this process was designated k m . The resulting set of
three first-order equations was integrated numerically using Model-Maker Version
4.0 software from Cherwell Scientific Software Ltd. UK. Metabolism data from 11
soils reported in two studies (Bidlack 1979 ; de Vette and Schoonmade 2001 ) were
it to this model. It was assumed that the CPY was entirely in the labile compartment
at time-zero, and the rate of degradation was determined by k m and the concentration