UMR

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KINETIC MODELS
UMR Œnologie-Ampélologie Université Victor
Segalen Bordeaux 2
351 cours de la Libération, TALENCE Cedex
France
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Where does first-order come from ? Reason 1 the
experience Experience shows that many biotic and
abiotic processes in environmental compartments
such as soil effectively follow single first
order kinetics (exponential decay)
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Reason 2 pragmatism     The equation is simple
and has only two parameters     It is easy to
fit the equation to experimental data      DT50
and DT90 values are easy to calculate
     Parameters are theoretically independent
of concentration and time and appropriate for
use as input for pesticide leaching models.
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Reason 3 scientific justifications     abiotic
hydrolytic processes often follow first-order
reaction kinetics     biotic degradation
processes may be approximated by first-order
reaction ex. when responsible microbial agents
(or enzymes) are in excess compared to the
chemical (pseudo first order reaction kinetics).
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A phylogeny for the disappearance models The
 Metabolism  case
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Reason 1 heterogeneity The Gustafsson and Holden
assumption The soil can be divided into a large
number of independent compartments whith
distributed first order rate constants. If pdf
Gamma distribution
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Single first order kinetics (SFO)
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But First-order reaction kinetics may not be
obeyed
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The bi-phasic Gustafson Holden model (FOMC)

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Reason 2 limited availability. In the soil,
pesticides are distributed between a solid phase
and a liquid phase where they are available for
degradation. This partition induces a bi-phasic
pattern of degradation
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The bi-phasic Hockey Stick model (HS)

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The bi-phasic bi-exponential model (DFOP)

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Reason 3 microbial behaviour ? Different
environmental factors affect the activity of the
microbial degraders. ? Respective
substrate concentration and cell density may
induce very different degradation patterns
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True lag phase The logistic model
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Lag phase The Hockey stick model (HS)
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