## 5 Kinetics of Pesticide Degradation
### Degradation of Pesticides by Ozone  
There are two ways in which ozone reacts with pesticides in solution, and these two ways occur simultaneously. These two methods are direct oxidation and indirect oxidation. Ozone direct oxidation is the reaction between ozone and electron-rich functional groups in pesticides. The indirect oxidation of ozone is the reaction between the intermediate product -OH produced by ozone in solution and pesticides [94,95]. According to the law of mass action, the kinetic equation for ozone and pesticides is shown in Equation (13).  
\[-\frac{d[\textit{Pesticide}]}{dt}=k_{O_{3}}[O_{3}][\textit{Pesticide}]+k_{ OH}[\cdot\textit{OH}][\textit{Pesticide}] \tag{13}\]  
where k\({}_{O_{3}}\) is the second-order reaction rate constant of direct ozone oxidation (L-s\({}^{-1}\cdot\)mol\({}^{-1}\)); k\({}_{OH}\) is the second-order reaction rate constant of indirect oxidation of -OH (L-s\({}^{-1}\cdot\)mol\({}^{-1}\)); [O\({}_{3}\)] is the concentration of O\({}_{3}\) (mol\(\cdot\)L\({}^{-1}\)); [-OH] is the concentration of -OH (mol\(\cdot\)L\({}^{-1}\)); and [_Pesticide_] represents the concentration of pesticide (mol\(\cdot\)L\({}^{-1}\)).  
In direct degradation experiments of pesticides, for the sake of simplicity, when the concentrations of two reactants differ significantly by more than one order of magnitude, the second-order reaction rate equation can be rewritten as the expression form of the first-order reaction [96].  
\[-\frac{d[\textit{Pesticide}]}{dt}=k_{O_{3}}^{0}[\textit{Pesticide}] \tag{14}\]  
where k\({}_{O_{3}}^{\circ}\) is the pseudo first-order reaction rate constant (min\({}^{-1}\)), k\({}_{O_{3}}^{\circ}=\) k\({}_{O_{3}}[O_{3}]\).  
A number of studies have investigated the reaction of ozone with pesticides under solution conditions (Table 2). In addition to liquid-phase conditions, there are many applications for the reaction of ozone with pesticides under gas conditions. According to studies, higher ozone concentrations and longer reaction times under gas conditions can be effective in degrading pesticides, and under these conditions, it can be assumed that ozone reacts with pesticides in accordance with a first-order kinetic model.  
\[C_{t}=C_{0}\exp(-kt) \tag{15}\]  
Taking logarithms of both sides of Equation (15), the first-order kinetic model can be rewritten as follows:  
\[\ln C_{t}-\ln C_{0}=-kt \tag{16}\]  
where \(C_{t}\) is the concentration of the pesticide at moment t, \(C_{0}\) is the concentration of the pesticide at moment 0, and \(k\) is the first-order reaction rate constant (min\({}^{-1}\)).  
Degradation of pesticides by ozone gas usually addresses the problem of pesticide residues on food products. In the study of the degradation of organophosphorus pesticides malathion, chlorpyrifos, profenofos, and ethion on chili peppers, it was found that the reaction of pesticides on chili peppers fumigated with ozone satisfies the first-order kinetic model. The degradation rate constants after ozone fumigation were found to be about 10,000 times different in order of magnitude than the degradation rate constants without fumigation by using a blank control [97]. In the degradation study with the organophosphorus insecticide pirimihos-methyl on maize, the study used zero-order, first-order, and second-order kinetic models to simulate the reaction of ozone with pirimihos-methyl. The results showed that the first-order kinetic model simulated ozone degradation better than the other two models (R\({}^{2}\)(1) = 0.94 \(>\) R\({}^{2}\)(0) = 0.90 \(>\) R\({}^{2}\)(2) = 0.88) [98]. In the study of the ozone treatment of lettuce, it was also demonstrated that the reaction of ozone with pesticides satisfies the first-order kinetic model [99].