## 3 Results and Discussion
### Effects of the UV Light Source and TiO\({}_{2}\) Photocatalysis on Degradation of Pesticides
Therefore, no matter what oxidant was produced by TiO\({}_{2}\) photocatalysis, the direct UV photolysis effect was decreased due to a reduction in UV intensity that was caused by the TiO\({}_{2}\) coating. Several studies reported high formation rates of hydroxyl radicals (OH-) produced by UV-TiO\({}_{2}\) photocatalysis probably affected degradation of the pesticides and mineralization of pesticide intermediates [37; 38]. However, further supporting investigation, such as TOC (total organic carbon) observation, will be needed in order to validate the TiO\({}_{2}\) photocatalytic degradation of the pesticides. The degradation efficiency of an individual pesticide (5 ppm) in a cocktail solution (20 ppm) was not equal to the degradation efficiency in a single pesticide solution containing 20 ppm of each pesticide under VUV-TiO\({}_{2}\). After 180 min of each treatment, degradation percentages of pyracolostrobin, boscalid, and fluidoxonil in the cocktail solution were significantly lower versus for single solutions (94.3% vs. 100%, 5.2% vs. 30.8%, 17.3% vs. 84.8%, respectively) (Figure 3). However, the degradation of azoxystrobin in a cocktail solution and in a single solution did not differ significantly during the whole treatment time. This was probably due to an inhibitory effect of each pesticide, its oxidative intermediates, and emulsifier additives in commercial pesticide brands. The TiO\({}_{2}\) photocatalytic process under VUV illumination relies primarily on a non-selective reaction of the strong oxidation potential of reactive oxygen species (ROS), such as hydroxyl radicals (OH-) and superoxide radical ion (O\({}_{2}\)-), produced on the TiO\({}_{2}\) surface. The higher adsorption affinity of non-target compounds onto the TiO\({}_{2}\) surface can scavenge produced ROS and adsorb to the TiO\({}_{2}\) photocatalyst surface where they both displace the adsorption of target contaminants and interfere with the ROS production process [39]. In this study, azoxystrobin and its oxidative intermediates were probably degraded more by the VUV-TiO\({}_{2}\) photocatalytic treatment in competition, as compared with the other pesticides [40]. Table 2 shows the degradation rate constants of pyracolostrobin, boscalid, fluidoxonil, and azoxystrobin. In single solutions, degradation rates of pyracolostrobin and boscalid were fitted to pseudo-first order kinetics, the rate of fluidoxonil was fitted to pseudo zero order kinetics, and the rate of azoxystrobin was fitted to pseudo second order kinetics. Generally, pseudo-first order kinetics of experiments involving TiO2 photocatalysis are an indicator of the Langmuir-Hinshelwood mechanism, a widely reported surface reaction mechanism [41]. The pseudo-second order kinetic model implies that the oxidation rate rapidly slowed during the course of the reaction as a result of accumulation of recalcitrant oxidation intermediates. The pseudo-zero order kinetic model of photolysis/photocatalysis indicated that there was no accumulation of intermediates resistant to further oxidation [19]. In the cocktail solution, the order of the degradation rates of pyracolostrobin, boscalid, and fluidoxonil were changed to pseudo-second order kinetics, which indicated that the presence of azoxystrobin was the causative factor that decreased the degradation rates of pyracolostrobin, boscalid, and fluidoxonil in the cocktail solution due to the accumulation of oxidative intermediates of azoxystrobin [42]. Figure 3: Degradation efficiencies of the VUV-assisted TiO2 photocatalytic treatment (VUV-TiO2) in a cocktail solution of pesticides. (**a**) pyracolostrobin (Pyr), (**b**) boscalid (Bos), (**c**) fluidoxonil (Flu), and (**d**) azoxystrobin (Azo). The pesticide concentration of each single solution was 20 ppm (_w/v_). The concentration of each pesticide in a cocktail solution was 5 ppm (_w/v_) and the total pesticide concentration of the cocktail solution was 20 ppm (_w/v_). All of the treatments were performed under magnetic stirring at 300 rpm.