Degradation of prochloraz
The percentage of PCZ degradation using thermal-activated persulfate at two different concentrations of 1.4 mM and 4.2 mM are shown in Fig. 1. As expected, faster degradation of PCZ was achieved by increasing the persulfate concentration. This is attributed to the higher production of sulfate radicles(Taha et al. 2016). Although a higher PS concentration of 10 mM was previously studied for pesticide degradation(Sajjadi et al. 2019), lower PS concentrations were used in this study to slow down the degradation rate of the tested pesticides and their DPds. Furthermore, such a slowdown in the degradation rate permits the accumulation of the DPds and helps in their accurate identification. Consequently, it takes only 30 and 15 min to degrade 97% and 100% of PCZ at 50°C using 1.4 mM and 4.2 mM persulfate, respectively.
Simultaneous degradation of prochloraz and tebuconazole
As elucidated, the used lower concentration of PS enables an efficient degradation of PCZ and TBZ with their DPds, at their single solutions, only with slower rates compared to the usage of higher PS concentration. Consequently, the degradation of the PCZ/ TBZ mixture solution with their DPds was tested also at the lower concentration of PS. It was elucidated that, the degradation percentage of PCZ in case it coexists with TBZ was lowered to 52% (Fig. 3) compared to 87% in case of its single presence in water (Fig. 1.), under the same degradation conditions of 50 ºC, 15 min, and PS of 1.4 mM. This is mainly attributed to the direct relationship between the amount of available sulfate radicles and the total concentration of the organic pollutant. The same degradation percentage results were obtained for TBZ which lowered to 31% (a mixture of PCZ/ TBZ, Fig. 3) from 82% (a single solution of TBZ, Fig. 2). Almost a complete degradation of the PCZ/TBZ mixture was obtained only after one hr at 50 ºC and PS of 1.4 mM. Consequently, efficient and faster degradation of PCZ/TBZ mixture require a higher cost based on the longer degradation time or higher PS concentrations.
Identification and confirmation of the main degradation products
Peaks related to the DPds were detected by comparing the total ion chromatograms (TICs) of these experiments with those of control samples. Masses (m/z) of these DPds were detected also based on the presence of their isotopic molecular masses (Taha et al. 2016).
Figure 4 shows that there were two peaks eluted with the retention times (tR) of 8.67 and 8.70 min of the following molecular masses (m/z); 352 and 324 Da. These m/z with their isotopic masses are shown in Fig. 4 (the related mass spectra). The structure of these DPds was confirmed to be N′-Formyl-N-propyl-N-[2-(2,4,6-trichlorophenoxy) ethyl]urea (m/z = 352 Da) and (1-propyl-1-[2-(2,4,6 trichlorophenoxy) ethyl]urea) (m/z = 324 Da) using the LC-MS/MS MRM method, Fig. 6. These DPds were obtained by the cleavage of the imidazole ring of PCZ(Chu et al. 2020), which is further degraded into trichlorophenol, Scheme 1A.
N′-Formyl-N-propyl-N-[2-(2,4,6-trichlorophenoxy)ethyl]urea and (1-propyl-1-[2-(2,4,6 trichlorophenoxy) ethyl] urea) were eluted very close to the related peak of PCZ, tR 8.71 as a result of using a short run time of 15 min. A DPd of m/z equal 346 Da was also detected and expected based on the related mass spectrum that refers to a presence of an isotopic chlorine atom, Fig. 8. However, the accurate structure related to this mass wasn’t confirmed using several suggested MRMs. On the other hand, the negative scan analyses (not shown) of PCZ samples indicate the presence of trichlorophenol as a Dpd.
Figure 5 shows that there were three peaks eluted with tR of 7.77, 7.90, and 8.28 min of the following m/z; 249, 263, and 321 Da which are expected to be the main DPds of TBZ. The structures related to m/z of 321 and 249 Da were confirmed to be 1-(4-chlorophenyl)-3-hydroxy-4-4-dimethyl-3(1,2,4-triazol-1-ylmethyl)pentan-1-one and 4-(4-chlorophenyl)-1-(1,2,4-triazol-1-yl)Butan-2-one, respectively, using at least two MRMs per each compund, Fig. 7. These DPds were obtained by direct hydroxylation of TBZ and the cleavage of the ter-butyl chain(Stamatis, Antonopoulou, and Konstantinou 2015). However, in the current study, a di-ketone structure (m/z = 263 Da) was also confirmed, Fig. 7 and Scheme 1B. A ketone and hydroxyl structure of 265 Da was also previously reported during the degradation of TBZ using a combined vacuum UV and UVC(Del Puerto et al. 2022).
TBZ and all DPds were completely degraded using 4.2 mM Persulfate and a temperature of 50 ºC after 15 and 60 min respectively. There is no detection of any further DPds by the negative scan analysis of TBZ samples.
In the current study, persulfate adducts with PCZ and TBZ weren’t detected as previously reported for the degradation of boscalid(Taha et al. 2016). This is may be attributed to the low tendency of the DPds of PCZ and TBZ to lose a proton and interact with persulfate ions. Finally, identifying the DPds (using LC-MS/Scan methods) and confirming their DPds with the target pesticides (using LC-MS/MS methods) increase the confidentiality of the current overall degradation processes without fear of the presence of any highly stable DPd.