## 3 Results and Discussion  
### Effects of the UV Light Source and TiO\({}_{2}\) Photocatalysis on Degradation of Pesticides  
VUV was more effective than UVC for pesticide degradation in single solutions (20 ppm) of each pesticide (pyracolostrobin, boscalid, fluidoxonil, and azoxystrobin) (Figure 2). Degradation percentage of boscalid, fluidoxonil, and azoxystrobin were 28%, 87%, and 53% under VUV, and 3%, 58%, and 48% under UVC, respectively, after 180 min of treatment. Pyracolostrobin was eliminated after almost 60 min under both VUV and UVC. Figure 2: Degradation in individual pesticide solutions under different treatments. (**a**) pyracolostrobin; (**b**) boscalid; (**c**) fluidoxonil; and, (**d**) azoxystrobin. The concentration of each single solution was 20 ppm (_a/v_). Sample solutions were continuously stirred with a magnetic stirrer at 300 rpm during treatment. UVC = UVC (254 nm) photolytic treatment; VUV = VUV (254 nm and 185 nm) photolytic treatment; UVC-TiO\({}_{2}\) = UVC-assisted TiO\({}_{2}\) photocatalytic treatment; VUV-TiO\({}_{2}\) = VUV-assisted TiO\({}_{2}\) photocatalytic treatment. Results represent the mean of three measurements \(\pm\) standard deviation. Pesticides generally have several functional parts possibly participating in photolysis. Based on a chemical class or a functional moiety basis, mechanisms in the direct photolysis of pesticides include photo-induced cleavage of a specific bond, intramolecular bond formation, rearrangement, and oxidation/reduction [14]. It was suggested that the photodegradation of pyracolostrobin was obtained by substituting chloride for an -OH group followed by the scission of the 4-hydroxyphenyl and pyrazol bond (N-C bond) [30]. For boscalid, it was proposed that the transformation pathways under UV irradiation was caused by ionizations at the site of double bonds in the pyridine ring as well as at the oxygen and the nitrogen atoms of the amide function [31]. Direct photolysis of fluidoxonil was proposed to proceed through an initial radical cation intermediate resulting from photoionization from a singlet excited state [32]. Furthermore, geometrical isomerization such as the twisted structure around the C=C bond was theoretically proposed for photo-transformation pathways of azoxystrobin [33]. In particular, boscalid was hardly degraded by UVC (254 nm). Usually, boscalid is hydrolvtically and photolytically stable in soil and water [34]. According to one report of Food Safety Commission (2004), 99.4-99.5% of the initial amounts of boscalid in buffer solution was detected after 30 days at 25 \({}^{\circ}\)C. In addition, 94.4% and 99.6% was detected under Xenon light (315-400 nm) after 15 days in buffer solution and under Xenon light (290-800 nm) after 120 h in sterilized water, respectively [35]. However, the result of the present study showed that VUV (185 nm) treatment was somewhat effective in degrading boscalid in an aqueous solution. A study also reported the photolytic degradation of boscalid under UV-visible light (200-650 nm) [31]. It can be considered that shorter wavelength can accelerate the rate of photolytic degradation. However, incidentally, maximum absorption wavelength of boscalid is 207 and 228 nm [36], which is shorter than UVC wavelength (254 nm) and longer than VUV wavelength (185 nm). It can be assumed that the degradation rate of photolysis is faster when the wavelength of the photon is shorter than the maximum absorption wavelength of the substance. Furthermore, VUV at 185 nm can produce energetic photons, leading to mainly direct photolysis of target compounds as well as indirect decomposition through the formation of hydroxyl radicals and other strong oxidants, such as ozone and hydrogen peroxide [23]. There was no significant difference between rate constants for UVC and UVC-TiO\({}_{2}\) treatments and between rate constants for VUV and VUV-TiO\({}_{2}\) treatments, attributed to a filtering effect of TiO\({}_{2}\) particles on the surface of the quartz tube that weakened the intensity of UV light.