## 5 Kinetics of Pesticide Degradation  
### Degradation of Pesticides by Chlorine Dioxide  
In the study of the degradation of pesticides by chlorine dioxide, the chemical equation (Equation (10)) between chlorine dioxide and pesticides is as follows:  
\[aCIO_{2}+bPesticide\to Products \tag{10}\]  
where \(a\) and \(b\) are the stoichiometric numbers of chlorine dioxide and pesticides.  
According to the law of mass action, the reaction rate equation can be expressed in the form of the power product of the concentration of each reactant. According to studies, the reaction of chlorine dioxide with pesticides can be well described by a second-order kinetic model [88], as shown in Equation (11):  
\[\frac{d[Pesticide]}{dt}=-k_{0}[Pesticide][ClO_{2}] \tag{11}\]  
where \(k_{0}\) is the second-order reaction rate constant (L\(\cdot\)min\({}^{-1}\cdot\)mol\({}^{-1}\)); \([ClO_{2}]\) is the concentration of \(ClO_{2}\) (mol\(\cdot\)L\({}^{-1}\)); and [Pesticide] indicates the concentration of pesticide (mol\(\cdot\)L\({}^{-1}\)).  
In the process of research, in order to facilitate this study, when the concentration difference between the two reactants is more than one order of magnitude and the stoichiometric number is very similar, the difference is not large, and the second-order reaction rate equation can be rewritten into the expression form of the first-order reaction [89; 90]. Thus, Equation (12) can be rewritten as follows:  
\[\frac{d[Pesticide]}{dt}=-k_{1}[Pesticide] \tag{12}\]  
where \(k_{1}\) is a pseudo first-order reaction rate constant (min\({}^{-1}\)), \(k_{1}=k_{0}[ClO_{2}]\).  
During the reaction of chlorine dioxide with pesticides, there are many factors that affect the rate of chemical reaction. In the study of chlorine dioxide with the phenylurea herbicide fenuron, the secondary reaction rate constant \(k\) was found to be 10.37 (L\(\cdot\)min\({}^{-1}\cdot\)mol\({}^{-1}\)) at a pH of 7 and a temperature of 10 \({}^{\circ}\)C, and it was found that the value of \(k\) gradually became larger as the temperature increased (\(k_{40^{\circ}}\)C = 94.42 \(>k_{30^{\circ}}\)C = 42.85 \(>k_{20^{\circ}}\)C = 17.87 L\(\cdot\)min\({}^{-1}\cdot\)mol\({}^{-1}\)) in accordance with the Arrhenius equation [91]. In the reaction between chlorine dioxide and the carbamate insecticide methiocarb, it was also found that under the given pH value of 8, the second-order reaction rate constant conforms to the Arrhenius equation. In addition, this study also investigated the effect of pH on the reaction rate, and it was shown that the reaction rate constant increased as the pH value increased. Chlorine dioxide exists in molecular form in aqueous solution, and methiocarb is neutralin aqueous solution [92]. It was concluded that chlorine dioxide reacts more readily with methiocarb under alkaline conditions [35]. In the reaction of organophosphorus acaricide phorate and organophosphorus insecticide diazinon with chlorine dioxide, it was also found that the second-order reaction constants were higher under neutral and alkaline conditions than under acidic conditions (\(k_{10.7}=0.0213>k_{7}=0.0137>k_{4.6}=0.0047\)) [33]. Light is also one of the factors influencing the rate of chemical reactions. In a study of the reaction of the triazine herbicide atrazine with light and chlorine dioxide, the pseudo first-order reaction rate constant of sunlight combined with chlorine dioxide is relatively small (\(k=0.067\) min\({}^{-1}\)), while the pseudo first-order reaction rate constant of ultraviolet combined with chlorine dioxide is relatively large (\(k_{\text{LV}}=0.095\) min\({}^{-1}\)) [80]. In pesticide degradation, reactant concentration also determines the magnitude of the reaction rate. In the reaction of the herbicide dinoseb with chlorine dioxide, a good linear relationship was found between the pseudo first-order reaction rate constants and the initial concentrations of chlorine dioxide and dinoseb [93].