## 3 Results and Discussion
### Photocatalytic Reaction
#### 3.2.5 Reaction Pathway of Ethofencarb Degradation  
Since the active radicals that were found to lead this ethofencarb catalytic degradation are known to be non-selective, many degradation products can be formed. SPME-GC/MS analysis was carried out to identify the intermediate products of this process. Table 2 and Figure S2 present the identified intermediates, their retention times, and the mass of the main ions. Using an extensive library search, compound **1** was identified as 2-[(ethylsulfonyl)methyl]phenol with a fit value of 93%. Precisely, the peak at \(m/z=200\) observed in its mass spectrum corresponds to the molecular ion [M]\({}^{+}\) and the peak at \(m/z=107\) represents the characteristic ion [C\({}_{6}\)H\({}_{4}\)CH\({}_{2}\)OH]\({}^{+}\). Compound **2** presented main peaks at \(m/z\geq 120\) and \(m/z\geq 92\), corresponding to [C\({}_{6}\)H\({}_{4}\)OCO]\({}^{+}\) and [C\({}_{6}\)H\({}_{4}\)O]\({}^{+}\); its molecular mass was 177. It was, thus, identified as 3-methylbenzo[\(e\)][1,3]oxazine-2,4-dione, based on previous reports of ethofencarb degradation in other solvents [49; 50]. Compound **3** was identified as 3-methylbenzo[\(e\)][1,3]oxazine-2-one. The compound presented characteristic peaks at \(m/z\) 163 and 106, corresponding to molecular ion and [C\({}_{6}\)H\({}_{4}\)CH\({}_{2}\)O]\({}^{+}\), respectively. Compound **4** was shown to be 2-[(ethylsulfonyl)methyl]phenol by a library search with a fit value of 92%. The molecular ion peak [M]\({}^{+}\) appeared at \(m/z=168\), and the characteristic ion at \(m/z=107\) corresponds to [C\({}_{6}\)H\({}_{4}\)CH\({}_{2}\)OH]\({}^{+}\) group. Compound **5** showed a peak at \(m/z=166\) ([M]\({}^{+}\)) and a characteristic ion at \(m/z=107\) ([C\({}_{6}\)H\({}_{4}\)CH\({}_{2}\)OH]\({}^{+}\)), and it was identified as 2-methylbenzo[\(e\)][1,3]oxathiane, as previously described in ethofencarb degradation in non-aqueous media [51]. Compound **6** with \(m/z=124\) ([M]\({}^{+}\)) and 106 ([C\({}_{6}\)H\({}_{4}\)CH\({}_{2}\)O]\({}^{+}\)) was identified as 2-hydroxybenzyl alcohol. Compound **7** (\(m/z=122\) as [M]\({}^{+}\) and 93 as C\({}_{6}\)H\({}_{4}\)OH]\({}^{+}\)) was identified as 2-hydroxybenzaldehyde by a library search with a fit value of 98%. Compound **8** was a good match of diethyl disulfide (84% fit value). Its retention time was 11.48 min, conforming well to that of pure Et\({}_{2}\)S\({}_{2}\) in methanol obtained from commercial sources (\(\sim\)11.4 min). Et\({}_{2}\)S\({}_{2}\) is an interesting intermediate because it is most likely formed by dimerization of ethanethoid (a molecular segment of ethofencarb's side chain). Compound **9** was identified as ethanethiol by a library search with a fit value of 88%. Its molecular ion [M]\({}^{+}\) was \(m/z=62\) and characteristic ions were \(m/z=47\) and \(m/z=29\), corresponding to [CH\({}_{2}\)=SH]\({}^{+}\) and [CH\({}_{3}\)CH\({}_{2}\)]\({}^{+}\), respectively. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Peaks** & **Photodegradation Intermediates** & **R.T. (min)** & **MS Peaks (m/z)** \\ \hline
1 & & 26.36 & 200, 107, 77 \\
2 & & 25.07 & 177, 120, 92 \\
3 & & 23.88 & 163, 106, 78 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Identification of the intermediates from the photodegradation of ethofencarb by GC/MS. Figure 9: DMPO spin-trapping EPR spectra for (**a**) DMPO-O\({}_{2}\)\({}^{-}\) and (**b**) DMPO-OH under visible light irradiation with SnIn\({}_{4}\)S\({}_{8}\) catalyst. A plausible degradation mechanism is depicted in Figure 10 based on the detected intermediate products and the two possible reaction sites on the ethiofencarb molecule. The mechanism consists of two main routes. Route **A** involves oxidative cleavage of the CH\({}_{2}\)-S bond of ethiofencarb proceeded by a cyclization reaction to produce the cyclic adducts **2**-**3**. The photocleavage of the C-S bond has been well reported and is in accordance with its weak bond strength (272 kJ mol\({}^{-1}\)) [52].