Influencing Factors, Kinetics, and Pathways of Pesticide Degradation by Chlorine Dioxide and Ozone: A Comparative Review

Abstract

In agricultural production, pesticides play an important role in increasing crop yields. However, pesticide residues are caused by improper handling by users during the production process. Chlorine dioxide and ozone, as strong oxidants with similarity in spatial structure, effectively degrade pesticide residues and are widely used in water treatment and the food industry. In order to better understand the mechanism of chlorine dioxide and ozone on pesticides, the properties of chlorine dioxide and ozone are introduced in this review. Herbicides, insecticides, and fungicides were selected for this study, and the influencing factors, kinetics, and degradation pathways of degraded pesticides are presented. The degradation of pesticides by chlorine dioxide follows the second-order kinetic model, reacting with functional groups with high electron density in pesticides by electron transfer. Ozone selectively undergoes electrophilic reactions with pesticides in solution. In addition, when the reaction system is alkaline, ozone accelerates the decomposition to form hydroxyl radicals ($·\mathrm{O}\mathrm{H}$), which react with pesticides. Ozone degradation of pesticides satisfies the pseudo-first-order kinetic model. By comparing the mechanism of pesticide degradation by chlorine dioxide and ozone, this paper provides a theoretical basis for solving the problem of pesticide residues in the food industry and water treatment in the future.

1. Introduction

According to the definition in the document of the International Code of Conduct on Pesticide Management of the Food and Agriculture Organization of the United Nations (FAO), pesticides are chemical and biological substances or mixtures thereof that serve the function of repelling, destroying, and controlling pests and regulating plant growth [1]. Pesticides have played an important role in modern agricultural production, and according to their characteristics, they can improve crop yield and quality, making a huge contribution to the development of the entire green revolution [2]. There are various types of pesticides, which are used in agricultural production, such as acaricides, insecticides, herbicides, fungicides, etc. It has a wide range of applications globally. According to FAO’s database, global agricultural pesticide use will total 3.7 million tonnes in 2022, an increase of 4 percent from 2021 and 13 percent from a decade ago. Global pesticide use in the last decade has increased by 121% for herbicides, 54% for fungicides, and 48% for insecticides. The top five countries in terms of global pesticide use are Brazil (801,000 tonnes), the United States (468,000 tonnes), Indonesia (295,000 tonnes), Argentina (263,000 tonnes), and China (236,000 tonnes) [3]. Improper use of pesticides can cause pesticide residues in water and food. This study showed that about 113 pesticide residues were found in drinking water samples from 31 countries around the world. Of these, 61 insecticide, 31 herbicide, and 21 fungicide residues were found [4]. Pesticide residues are also common in food. The report on pesticide residues in food published by the European Union showed that of the 87,863 samples analyzed, 3.9% exceeded the MRLs, of which 2.5% were non-compliant [5]. In China, more than 40,000 batches of 135 types of fruits and vegetables collected from 45 key cities were analyzed for pesticide residues. Among them, 81.6 percent of the samples were detected with pesticide residues, and 532 pesticides were detected 115,981 times [6].

Pesticide residues can enter the ecosystem through surface water and groundwater and pollute water bodies. In water bodies, pesticides negatively affect algae, protozoa, fungi, and bacterial species; chlorpyrifos, carbofuran, and DDT pesticides inhibit algal growth and photosynthesis; diuron inhibits bacterial metabolism at high concentrations; and propiconazole significantly inhibits mycelial and spore development [7]. Exposure to pesticide residues poses a serious threat to human health. Human exposure to triazine herbicides can cause endocrine disorder [8], heart disease [9], and other symptoms [10]. Human exposure to sulfonylurea herbicides can induce hypoglycemia [11] and cardiovascular disease [12]. Human exposure to organophosphorus insecticides may cause general malaise, fatigue, headache, poor memory, concentration, anorexia, nausea, and other symptoms [13,14,15]. Human exposure to organochlorine pesticides can produce symptoms such as blindness, confusion, convulsions, and chronic bronchitis [16,17,18].

In recent years, there have been physical, chemical, and biological methods to solve the problem of pesticide residues.

Table 1. Advantages and disadvantages of some methods of dealing with pesticide residues.

Method	Advantages	Disadvantages	Reference
cold plasma	high efficiency, non-thermal nature, and wide range of applicability	high cost of equipment and immature technology	[24]
ultrasound	fast, efficient, and non-polluting	device immaturity, energy consumption, and noise hazard	[25]
irradiation	non-polluting and highly efficient	high capital investment and high facility requirements	[26]
pulsed electric field	short processing time	high costs and inability to handle solids	[27]
Fenton	low energy consumption, low cost, and high efficiency	iron sludge production and catalyst wastage	[28]
photocatalysis	non-toxic and chemically stable	catalyst selectivity and high cost of efficient photocatalysts	[29]
electrochemical	wide degradation range and no toxic by-products	electric power systems are not practical for large-scale application	[30]
bioremediation	green, non-toxic, and completely degradable	microorganisms promote growth under harsh conditions; immature process for large-scale use	[31,32]

shows a comparison of the different methods of treating pesticide residues. Chlorine dioxide and ozone are typical green oxidants that react with organic matter with high efficiency and do not produce harmful substances such as trihalomethanes [19,20]. In addition, chlorine dioxide and ozone can react with organics in both gaseous and solution forms compared to traditional chemical oxidation techniques. Chlorine dioxide and ozone have low treatment costs and are widely used in the food and wastewater industries [21,22,23].

In the past, there have been many studies on the degradation of pesticides by chlorine dioxide and ozone. The reaction between chlorine dioxide and ozone with pesticides is influenced by factors such as pH value, temperature, and time. The reaction between chlorine dioxide and pesticides follows a second-order kinetic model, and the reaction rate constant increases with the increase in pH [33,34]. Chlorine dioxide undergoes electron transfer with the sulfide ether in methiocarb, removing hypochlorite and chlorite to produce by-products such as methiocarb sulfone and methiocarb sulfoxide [35]. The same reaction of chlorine dioxide with sulfide ether exists in the reaction of chlorine dioxide with azamethiphos [36]. The reaction between ozone and pesticides can be divided into two processes: direct reaction and indirect reaction [37]. These two processes follow a pseudo-first-order kinetic model [38]. As the pH increases, ozone decomposes to produce hydroxyl radicals, promoting indirect reactions and increasing the kinetic constants of indirect reactions. Ozone reacts with the electron-rich double bonds in dichlorvos, and the direct reaction of ozone produces an oxygen-containing cyclic structure, and the indirect reaction reacts with the double bonds in an addition reaction to produce a hydroxyl structure [39].

At present, studies on the degradation mechanism of pesticides by chlorine dioxide and ozone have been reported, but fewer articles systematically summarize the mechanism of pesticide degradation by chlorine dioxide and ozone for pesticides as a pollutant. Therefore, from the structure of chlorine dioxide and ozone as a starting point, we focused on the study and comparison of the mechanism of these two advanced oxidation technologies in the food and water treatment industries to deal with pesticide residues and to clarify the applicable conditions of these two oxidants. It is hoped that this will provide a theoretical basis for future research on pesticide degradation residues and that the two oxidants, chlorine dioxide and ozone, can be used rationally when facing different types of pesticides.

2. Methods

A systematic approach based on preferred reporting items for systematic reviews and meta-analyses statement entry lists was used to analyze the literature for this review (Figure 1). Web of Science is a multidisciplinary database covering thousands of scholarly journals in numerous disciplines. The original research paper was searched on the Web of Science using an advanced subject search with keyword combinations of (chlorine dioxide or ClO₂) and (pesticides or herbicides or insecticides or fungicides); (ozone or O₃) and (pesticides or herbicides or insecticides or fungicides).

An advanced search of Web of Science identified 502 original papers in English. In order to screen the papers, selection criteria were set to compare the two technologies, chlorine dioxide and ozone, for the treatment of pesticide residues in food and water, excluding articles that were not relevant to the topic. Literature was manually screened on the basis of title and abstract, then the full text of the literature was obtained, and 150 papers were assessed to be screened as potentially meeting the criteria, then 70 papers were referenced for review after further assessment of the full text. Based on the performance of chlorine dioxide and ozone, the two technologies were compared in terms of (1) properties and spatial structure, (2) factors affecting the treatment of pesticides, (3) kinetics, and (4) pesticide degradation pathways. These aspects are discussed in the following sections.

3. Properties and Structure of Chlorine Dioxide and Ozone

3.1. Similarities

The spatial structure and electron dot diagram of chlorine dioxide are shown in Figure 2a,b. Chlorine dioxide is a symmetric nonlinear triatomic molecule. The chlorine–oxygen bond is formed with a bond angle of 117.5° and a bond length of 1.47 Å [40]. The chlorine–oxygen bond shows obvious double-bond characteristics. Chlorine dioxide has a conjugated system as the 3d orbital of chlorine is conjugated to the p-orbital of oxygen to form a $\mathsf{\pi }$ bond [41]. Chlorine dioxide possesses an odd number of electrons and selectively reacts with electron-rich groups (aniline, phenols, etc.) [42]. In an aqueous solution, chlorine dioxide reacts with organic substances by electron transfer [43]. In the reaction, chlorine dioxide gains electrons to be reduced to the chlorite anion (Equation (1)) [34].

$$Cl{O}_2+e^{-}\to Cl{O}_2^{-}$$

(1)

Ozone is a symmetric nonlinear triatomic molecule. The spatial structure of ozone and the electron dot diagram of the two-radical mode are shown in Figure 2c,d. One oxygen atom is the central atom, and the other two oxygen atoms are connected to the central atom through covalent bonds. The bond angle of ozone is 116°49′, and the bond length is 1.278 Å [44]. The three oxygen atoms are in sp² hybridization. The electrons of the central oxygen atom form a $\mathsf{\pi }$ bond with the electrons of the two terminal oxygen atoms [45,46]. Due to the molecular structure of ozone, there are three types of reactions when reacting with aromatic compounds: ozone–oxygen substitution, electron transfer, and Criegee ozonation [47]. In the reaction of ozone with phenol, electron transfer may have occurred to produce a superoxide anion radical (Equation (2)) [48].

$$C_6{H}_5{O}^{-}\stackrel{O_3}{\to }{C}_6{H}_5{O}^{•}+O_3^{•-}$$

(2)

Chlorine dioxide and ozone have a similar spatial structure and chemical properties. In their spatial structure, both chlorine dioxide and ozone show symmetrical and nonlinear triatomic molecules with $\mathsf{\pi }$ bonds in the molecular structure. During the reaction with pesticides, chlorine dioxide and ozone react with some organic compounds by electron transfer.

3.2. Differences

Chlorine dioxide is a greenish yellow gas, which is one of the few compounds in nature that exists in the form of a single free radical [43]. Chlorine dioxide is an unstable gas that is prone to decomposition. In solution, chlorine dioxide is a dissolved gas [49]. At 25 °C, the concentration of chlorine dioxide is approximately 23 times greater than it would be if it were at equilibrium in the gas phase [50]. In water, chlorine dioxide has a relatively long half-life and does not undergo hydrolysis reactions, existing in molecular form. The stability of chlorine dioxide depends on the pH value of the solution. Unlike chlorine, which is rapidly and reversibly hydrolyzed in water at any pH, chlorine dioxide is quite stable in weakly acidic solutions and less stable in neutral solutions [51]. In alkaline (pH > 8) solutions, a disproportionation reaction occurs to form chlorite anion and chlorate anion (Equation (3)) [52]. These two ions are by-products of chlorine dioxide formation in water treatment and pose a threat to human health [53,54]. In solution, the chlorite anion may gain four electrons to be reduced to a chloride ion (Equation (4)) [43]. Generally, it is difficult for the chlorite anion to gain four electrons for the reduction reaction, but in water treatment, the reduction in chlorite anions readily occurs under acidic conditions (Equation (5)) [55].

$$2Cl{O}_2+2O{H}^{-}\to Cl{O}_2^{-}+Cl{O}_3^{-}+H_{2}O$$

(3)

$$Cl{O}_2^{-}+2H_{2}O+4e^{-}\to C{l}^{-}+4O{H}^{-}$$

(4)

$$Cl{O}_2^{-}+4H^{+}+4e^{-}\to C{l}^{-}+2H_{2}O$$

(5)

Ozone is a colorless gas with an irritating odor [56]. When the concentration of ozone is high, it appears blue [57]. In practical applications, ozone is not too concentrated and appears colorless. At room temperature, ozone is an unstable gas with a half-life of 20–50 min, which is higher for gaseous ozone than for ozone in solution [21,58,59]. In addition, ozone decomposes into oxygen and does not cause secondary pollution [21]. In solution, temperature affects the solubility of ozone, which is almost insoluble in water when the temperature reaches 60 °C. In addition, ozone has a high oxidation potential in water. The half-life of ozone is affected by pH value. When the pH value is greater than 7.5, ozone will accelerate decomposition, undergoing a chain reaction that produces $·\mathrm{OH}$ in water with a stronger oxidation potential than ozone (Equations (6)–(9)) [60].

$$O_3+O{H}^{-}\to H{O}_2^{•}+O_2^{•-}$$

(6)

$$O_3+O_2^{•-}\to O_2+O_3^{•-}$$

(7)

$$O_3^{•-}+H^{+}\to H{O}_3^{•-}$$

(8)

$$H{O}_3^{•-}\to O{H}^{•}+O_2$$

(9)

4. Factors Affecting Pesticide Degradation

4.1. pH Value

The effectiveness of chlorine dioxide in degrading pesticides is affected by the pH of the reaction system (Figure 3). According to studies, chlorine dioxide is more active under neutral and alkaline conditions and less active under acidic conditions [61]. The oxidation of chlorine becomes stronger with increasing pH. In addition, the pH value in the solution affects the degree of ionization of ionizable organic compounds [62]. For non-ionizable organic compounds, an increase in pH may affect the hydrogen bonding of organic compounds [35]. In the study of chlorine dioxide with the organophosphorus insecticides dimethoate and azamethiphos, complete degradation of azamethiphos was found at pH 9, and the degradation rate of dimethoate under pH 7 was 98.22% [36]. In the reaction system of organophosphorus acaricide phorate with chlorine dioxide, the phorate was completely degraded at pH 10.7 [33].

The increase in pH value also has a significant impact on the ozone degradation system (Figure 4). The pH value can affect the solubility of ozone in water [63]. As the pH value increases, it promotes the decomposition of ozone to produce $·\mathrm{O}\mathrm{H}$, and some organic compounds react better with $·\mathrm{O}\mathrm{H}$, improving the degradation rate [64]. In the system where ozone reacted with five pesticides—alachlor, carbendazim, diuron, pyrimethanil, and tebuconazole—the degradation rate increased with increasing pH [65]. In the degradation of prometryne, a triazine herbicide commonly used in aquaculture, it was found that the best degradation of prometryne by ozone was achieved at pH 7, with a degradation rate of 77.63% [66].

In general, the pH value has a great influence on the degradation of pesticides by ozone and chlorine dioxide. In the reaction system of chlorine dioxide and ozone with pesticides, both showed that the degradation rate of pesticides was higher in neutral and alkaline conditions than in acidic conditions. However, the mechanism of the effect of pH on these two oxidants is different. For chlorine dioxide, the increase in pH increased the activity of chlorine dioxide. For ozone, the increase in pH promoted the decomposition of ozone, producing more $·\mathrm{OH}$.

4.2. Concentrations of Chlorine Dioxide and Ozone

An appropriate increase in the concentration of chlorine dioxide will promote the progression of the reaction, thus increasing the degradation rate of pesticides. In the study of chlorine dioxide degradation of sulfonylurea herbicides nicosulfuron and thifensulfuron in water, when the concentration of chlorine dioxide was changed from 5 mg/L to 10 mg/L, the degradation rate of nicosulfuron was changed from 52.55% to 64.17%, and the degradation rate of thifensulfuron was changed from 34.38% to 46.75% [67]. However, in water treatment, excessive chlorine dioxide delivery can produce large quantities of chlorite and chlorate [68]. These chlorites and chlorates can have an impact on human health. [53] A high concentration of chlorine dioxide has bleaching properties and is commonly used for paper bleaching [69]. In the process of food application, it will affect the sensory changes in food [70,71].

The concentration of ozone promotes the reaction of ozone directly with pesticides, and at the same time, it also increases the concentration of $·\mathrm{OH}$ produced by ozone, which promotes the reaction of $·\mathrm{OH}$ with pesticides and increases the degradation rate of pesticides. In the study of pyrethroid insecticide permethrin, benzoylurea insecticide chlorfluazuron, and organochlorine fungicide chlorothalonil on Chinese white cabbage, it was found that the degradation rate of these three pesticides increased with the concentration of ozone [72]. Similarly, in the degradation of the triazole fungicide difenoconazole and the substituted urea herbicide linuron on treated carrots, the degradation rate of the pesticides increased with the concentration of ozone [73]. However, in the food industry, excessive ozone concentrations can cause some negative effects on the quality of produce, such as morphological changes, spoilage, discoloration, weight loss, and unpleasant odors [74].

In the reaction system with pesticides, as the concentration of chlorine dioxide and ozone increases, it increases the rate of reaction with pesticides and accelerates the reaction process. Under the same conditions, the degradation rate of pesticides increases with increasing concentrations of chlorine dioxide and ozone. However, excessive concentrations of chlorine dioxide and ozone also have adverse effects. Excessive concentrations of chlorine dioxide and ozone in treating pesticides on the surface of fruits and vegetables can affect the quality of fruits and vegetables [75]. In addition, excessive chlorine dioxide can produce by-products such as chlorite and chlorate when treating pesticides in water [76].

4.3. Light

Chlorine dioxide has good absorption of light in the wavelength range of 220~240 nm and 300~420 nm, and when chlorine dioxide absorbs the energy of light, it undergoes the breaking of the Cl-O bond, generating chlorine–oxygen radicals ($\mathrm{ClO}·$), trilinear oxygen atoms ($\mathrm{O}\left(3\mathrm{p}\right)$), and chlorine radicals ($\mathrm{Cl}·$) [77,78,79]. These produce reactive substances that can react with a number of pollutants (Figure 5). In the degradation reaction of pesticides, some appropriate increase in light can improve the degradation rate of pesticides. In the degradation with the organophosphorus insecticides azamethiphos and dimethoate, studies have shown that the degradation efficiency is higher under light conditions than under dark conditions [36]. In addition, in a study using light activation of chlorine dioxide in the reaction with the triazine herbicide atrazine, the results showed that the degradation rate of atrazine was significantly enhanced under light activation and that $·\mathrm{OH}$ and chlorine radicals ($\mathrm{Cl}·$) generated by chlorine dioxide after absorbing light played a dominant role in the degradation of atrazine [80].

The technology of ozone combined with ultraviolet radiation is a common advanced oxidation technique, which is applied to degrade difficult-to-degrade organic compounds in wastewater. This technology can also degrade disinfection by-products [81]. In the solution, ozone undergoes photolysis under UV irradiation to produce oxygen and hydrogen peroxide, and hydrogen peroxide photolysis produces $·\mathrm{O}\mathrm{H}$ (Figure 6) [82,83]. In addition, ozone in solution also produces $·\mathrm{O}\mathrm{H}$. In the degradation with the carbamate insecticide carbofuran, ozone combined with UV was more effective than ozone [84]. In the degradation study with the benzimidazole fungicide carbendazim, UV combined with ozone gave better degradation with 98% degradation [85].

The action of light enables chlorine dioxide and ozone to decompose and produce decomposition products. These decomposition products play a major role in the degradation process with pesticides, increasing the reaction rate with pesticides and improving the degradation rate of pesticides. However, there are differences in the wavelengths of light exposure for chlorine dioxide and ozone, which produce different degradation products.

4.4. Temperature

Temperature is an important factor in pesticide degradation reactions, which changes the rate of reaction and thus affects the rate of degradation. In the degradation of chlorine dioxide with the carbamate insecticide methiocarb, it was found that as the temperature of the solution increased from 7 °C to 35 °C, the residual amount of methiocarb became less [35].

The solubility of ozone is 12.07 mg/L at 20 °C. According to Henry’s law, the solubility of ozone decreases with increasing temperature [86]. Ozone decomposes into $·\mathrm{OH}$ in water. As the temperature increases, the decomposition rate of ozone becomes faster. In the degradation of the organophosphorus insecticide fenitrothion on lettuce and cherries treated with the ozone microbubble technique, it was found that the residual amount of fenitrothion decreased as the solution temperature increased from 15 °C to 30 °C [87].

5. Kinetics of Pesticide Degradation

5.1. 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:

$$aCl{O}_2+bPesticide\to Products$$

(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):

$${\displaystyle \frac{d[Pesticide]}{dt}}=-k_0[Pesticide][Cl{O}_2]$$

(11)

where k₀ is the second-order reaction rate constant ($\mathrm{L}·{\min }^{-1}·{\mathrm{mol}}^{-1}$); [ClO₂] is the concentration of ClO₂ ($\mathrm{mol}·{\mathrm{L}}^{-1}$); and [Pesticide] indicates the concentration of pesticide ($\mathrm{mol}·{\mathrm{L}}^{-1}$).

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:

$${\displaystyle \frac{d[Pesticide]}{dt}}=-k_1[Pesticide]$$

(12)

where k₁ is a pseudo first-order reaction rate constant (min⁻¹), k₁ = k₀[ClO₂].

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·min⁻¹·mol⁻¹) at a pH of 7 and a temperature of 10 °C, and it was found that the value of k gradually became larger as the temperature increased (k_40°C = 94.42 > k_30°C = 42.85 > k_20°C = 17.87 L·min⁻¹·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 neutral in 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 > k₇ = 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_UV = 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].

5.2. 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 $·\mathrm{OH}$ 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).

$$-{\displaystyle \frac{d[Pesticide]}{dt}}=k_{O_3}[O_3][Pesticide]+k_{·OH}[·OH][Pesticide]$$

(13)

where ${\mathrm{k}}_{{\mathrm{o}}_3}$ is the second-order reaction rate constant of direct ozone oxidation ($\mathrm{L}·{\mathrm{s}}^{-1}·{\mathrm{mol}}^{-1}$); ${\mathrm{k}}_{·\mathrm{OH}}$ is the second-order reaction rate constant of indirect oxidation of $·\mathrm{OH}$ ($\mathrm{L}·{\mathrm{s}}^{-1}·{\mathrm{mol}}^{-1}$); [O₃] is the concentration of O₃ ($\mathrm{mol}·{\mathrm{L}}^{-1}$); [$·OH$] is the concentration of $·\mathrm{OH}$ ($\mathrm{mol}·{\mathrm{L}}^{-1}$); and [Pesticide] represents the concentration of pesticide ($\mathrm{mol}·{\mathrm{L}}^{-1}$).

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].

$$-{\displaystyle \frac{d[Pesticide]}{dt}}=k_{O_3}^0[Pesticide]$$

(14)

where ${\mathrm{k}}_{{\mathrm{O}}_3}^{°}$ is the pseudo first-order reaction rate constant (min⁻¹), ${\mathrm{k}}_{{\mathrm{O}}_3}^{°}$=${\mathrm{k}}_{{\mathrm{O}}_3}$[O₃].

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.

$$C_t=C_0\exp (-kt)$$

(15)

Taking logarithms of both sides of Equation (15), the first-order kinetic model can be rewritten as follows:

$$\ln C_t-\ln C_0=-kt$$

(16)

where C_t is the concentration of the pesticide at moment t, C₀ 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 pirimiphos-methyl on maize, the study used zero-order, first-order, and second-order kinetic models to simulate the reaction of ozone with pirimiphos-methyl. The results showed that the first-order kinetic model simulated ozone degradation better than the other two models (R²(1) = 0.94 > R²(0) = 0.90 > R²(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].

Table 2. Reaction rate constants of ozone and common pesticides in solution.

Name	Pesticide Type	Reaction Conditions	Reaction Rate Constant	Reference
Thiamethoxam	Nicotine insecticides	tertiary butyl alcohol, pH = 7	$k_{{\mathrm{O}}_3}$$=15.4; k_{·\mathrm{OH}}$ = 3.9 × 109	[100]
Isoprothiolane	Organic sulfur fungicides	tertiary butyl alcohol, pH = 7.5	$k_{O_3 }$= 255.8	[101]
Clothianidin	Nicotine insecticides	tertiary butyl alcohol, pH = 7	$k_{O_3}$$=102.64; k_{·\mathrm{OH}}$= 3.7 × 109	[102]
Fluopyram	Pyridinylethylbenzamide fungicides	pH = 6.5	K$=k_{O_3}$$+k_{·\mathrm{OH}}$= 1.002 × 108	[103]
2-Chloro-N-(2,6-dimethylphenyl) acetamide	Chloroacetamide herbicides	60 °C	k = 2.40 × 104	[104]
Dichlorvos	Organophosphorus insecticide	tertiary butyl alcohol, pH = 7, 20 °C	$k_{O_3}$= 590	[39]
Alachlor	Amide herbicides	pH = 9.75	$k_{O_3}$$=2.8; k_{·\mathrm{OH}}$= 3.2 × 1010	[105]
Tembotrione	Triterpenoid herbicides	tertiary butyl alcohol, pH = 7	$k_{O_3}$= 8.9 × 105	[106]
Sulcotrione	Triterpenoid herbicides	tertiary butyl alcohol, pH = 7	$k_{O_3}$$=6.7 \text{×} {10}^5$	[106]

Table 2. Reaction rate constants of ozone and common pesticides in solution.

Name	Pesticide Type	Reaction Conditions	Reaction Rate Constant	Reference
Thiamethoxam	Nicotine insecticides	tertiary butyl alcohol, pH = 7	$k_{{\mathrm{O}}_3}$$=15.4; k_{·\mathrm{OH}}$ = 3.9 × 109	[100]
Isoprothiolane	Organic sulfur fungicides	tertiary butyl alcohol, pH = 7.5	$k_{O_3 }$= 255.8	[101]
Clothianidin	Nicotine insecticides	tertiary butyl alcohol, pH = 7	$k_{O_3}$$=102.64; k_{·\mathrm{OH}}$= 3.7 × 109	[102]
Fluopyram	Pyridinylethylbenzamide fungicides	pH = 6.5	K$=k_{O_3}$$+k_{·\mathrm{OH}}$= 1.002 × 108	[103]
2-Chloro-N-(2,6-dimethylphenyl) acetamide	Chloroacetamide herbicides	60 °C	k = 2.40 × 104	[104]
Dichlorvos	Organophosphorus insecticide	tertiary butyl alcohol, pH = 7, 20 °C	$k_{O_3}$= 590	[39]
Alachlor	Amide herbicides	pH = 9.75	$k_{O_3}$$=2.8; k_{·\mathrm{OH}}$= 3.2 × 1010	[105]
Tembotrione	Triterpenoid herbicides	tertiary butyl alcohol, pH = 7	$k_{O_3}$= 8.9 × 105	[106]
Sulcotrione	Triterpenoid herbicides	tertiary butyl alcohol, pH = 7	$k_{O_3}$$=6.7 \text{×} {10}^5$	[106]

6. Pesticide Degradation Pathways

6.1. Herbicides

There are various types of herbicides with different compositions. According to the different structures and functional groups in herbicides, chlorine dioxide and ozone will undergo different reactions. For example, the reaction between triazine herbicides, ozone, and chlorine dioxide mainly involves the substitution of hydroxyl groups and dealkylation reactions (Figure 7(1)) [80,107]. In the degradation of phenylurea herbicides, processes are involved in which the hydroxyl group replaces the hydrogen on the nitrogen element of the benzene ring that is attached to the urea structure, the hydroxyl group complexes with the benzene ring, and the cleavage of the urea structure (Figure 7(2)) [108,109]. In addition, the thioether bond in the triazine herbicide ametryn reacts with chlorine dioxide to form a sulfonyl group (Figure 8(1)) [110]. The degradation pathway of chlorine dioxide with sulfonylurea herbicides involves the cleavage of the urea structure and the cleavage of the sulfonyl group (Figure 8(2)) [67]. Ozone degradation of the triazine herbicide metribuzin occurs as a process of triazine ring oxidation, forming an acylurea structure, which reacts with the urea structure and ultimately opens the ring to a small molecule (Figure 8(3)) [111].

6.2. Insecticides

Insecticides have a variety of structures and degradation pathways. The degradation of organophosphorus insecticides by chlorine dioxide and ozone involves desulfurization of the thiophosphoryl group, dealkylation, and hydroxyl dehydration (Figure 9(1–3)) [36,112,113]. The degradation of carbamate insecticides involves the breaking of the carbamate group (Figure 9(4)) [113,114]. In addition, the thioether bond in the carbamate insecticide methiocarb reacts with chlorine dioxide to form intermediate adducts, which gradually oxidize into sulfonyl and sulfonyl groups (Figure 10(1)) [35]. The organophosphorus insecticide pirimiphos-methyl reacts with ozone in two pathways that result in dealkylated products, acetaldehyde, hydrogen peroxide, and carbonyl products (Figure 10(2)) [115]. The reaction of the organochlorine pesticides lindane and endosulfan with ozone under alkaline conditions involves dechlorination, ring opening, and demethylation (Figure 10(3,4)) [116].

6.3. Fungicides

Based on the current study, we found that the reaction of the triazole fungicide tebuconazole with chlorine dioxide involves a dealkylation process (Figure 11(1)) [117]. There are many degradation paths of ozone and triazole fungicides, and the degradation products are more complex. The reaction of bitertanol involves the transformation of the benzene ring to benzoic acid and the transformation of the hydroxyl group to the carbonyl group (Figure 11(2)) [118]. The reaction of tebuconazole involves dechlorination, dehydroxylation, triazole ring cleavage, and removal of the benzene ring (Figure 11(3)) [119].

7. Conclusions and Outlook

This paper reviews the process of degradation of pesticide residues in food and water by two oxidants, ozone and chlorine dioxide, where the efficiency of pesticide degradation is affected by factors such as temperature, light, and initial concentration. Herbicides, insecticides, and fungicides are used in large quantities during agricultural production and are detected in water bodies and food. It was found by summarizing that chlorine dioxide reacts with pesticides by the phenomenon of antibiotic electron transfer with functional groups (hydroxyl, thioether, amino, etc.) that have high electron density. Ozone in the gaseous state and pesticides also undergo electron transfer, and high electron density functional groups have an electrophilic reaction. In solution, there will be the decomposition of ozone generated by the free radicals together.

Chlorine dioxide and ozone have strong oxidizing properties and are widely used in the food industry and water treatment industry. Chlorine dioxide and ozone have better degradation effects on pesticide residues, and the effects of chlorine dioxide and ozone on pesticide degradation are shown in

Table 3. Degradation of pesticide residues by chlorine dioxide and ozone.

Pesticide	Chlorine Dioxide	Ozone	Reference
atrazine	C(ClO2) = 100 μM, solar, 30 min, degradation 100%	C(O3/H2O2) = 20 mol/L pH = 7, degradation 92.59%	[80,120]
diuron	C(ClO2) = 94 μM, pH = 4, 2 min, degradation 97.8%	C(O3) = 34.8 mg/L pH = 3, 2 h, degradation 20%	[108,121]
nicosulfuron	C(ClO2) = 10 mg/L, pH = 3, 6 h, degradation 92.77%	none	[67]
dimethoate	C(ClO2) = 10 mg/L, 6 h, light, degradation 98.22%	C(O3) = 10mg/L, 15 min, degradation 60.3%	[36,122]
methiocarb	C(ClO2) = 63 μM, pH = 7.4, 6 h, 22 °C degradation 78.21%	C(O3) = 2.8 mg/L, pH = 7, 22 °C, degradation 100%	[35,114]
tebuconazole	0 °C, 24 d, degradation 31.29%	C(O3) = 0.4 ppm, 25 °C small degradation rate	[117,123]
lindane	none	57 mg/minO3, pH = 12, degradation 82%	[116]

. Chlorine dioxide can effectively degrade triazine herbicides, phenylurea herbicides, and sulfonylurea herbicides; has a good removal effect on organophosphorus insecticides, carbamate insecticides, pyrethroid insecticides, amide fungicides, and triazole fungicides; and a poor degradation effect on organochlorine pesticides. Ozone can degrade most of the pesticides, but ozone has problems such as a short half-life and extreme instability, which cannot effectively deal with pesticide residues. Therefore, these two oxidation technologies should be combined to design a set of efficient and low-cost programs according to the actual situation.

However, according to the available literature, there are a number of problems to be solved for the degradation of pesticides by chlorine dioxide and ozone:

(1)

Currently, there are many studies on the degradation of herbicides and insecticides by chlorine dioxide and ozone, but there is relatively little research on the degradation of fungicides. In the future, emphasis can be placed on the degradation of different types of fungicides.

(2)

Current research has focused on the mechanism of pesticide residues in water and on the surface of fruits and vegetables, and not enough attention has been paid to the toxicity of intermediate degradation products, which may be hazardous to human health. Future studies could focus on the toxicity of intermediate degradation products.

(3)

Chlorine dioxide and ozone, as oxidants, currently have less research on the degradation of pesticides in gaseous form compared to aqueous solutions. Gaseous chlorine dioxide and ozone are commonly used in the food industry. In the future, we can focus on the degradation of pesticides by gaseous chlorine dioxide and ozone.

(4)

Currently, the mechanism of pesticide degradation by chlorine dioxide and ozone requires further investigation. In the process of pesticide degradation, quantum chemical calculation is a means to study the degradation mechanism. In future research, the mechanism of pesticide degradation can be elaborated in more depth by using quantum chemical calculations.