## 3 Results and Discussion  
### Advanced Oxidation Process UV/H\({}_{2}\)O\({}_{2}\)  
#### 3.1.1 Malathion and TOC Removal  
Figures 4 and 5 show the behavior of malathion and TOC concentration in the AOP. For the three H\({}_{2}\)O\({}_{2}\) doses, malathion removal reaches its highest level, 95 \(\pm\) 3.2%, with an asymptotic tendency after 90 min of irradiation. However, TOC removal around 50%-60% at 90 and 120 min of irradiation, with H\({}_{2}\)O\({}_{2}\) doses of 82 and 65 mM, respectively, were obtained. The tendency of high malathion removal but low TOC removal (22%-40%) was also observed for the 49 mM H\({}_{2}\)O\({}_{2}\).  
Figure 4: Removal of malathion by UV/H\({}_{2}\)O\({}_{2}\)-advanced oxidation process at three different H\({}_{2}\)O\({}_{2}\) dosage. The values represent the mean values of five replicates with their standard deviation. Ce = malathion concentration at time t (0, 10, 30, 60, 90, 120 or 150 min), C\({}_{0}\) = initial malathion concentration (In this research = 22 mg/L).  
The differences between malathion and TOC removal behavior is related to the degradation of pesticide to less complex organic molecules, in terms of chemical structure, but together contributes to the same organic carbon concentration [31]. Regarding TOC removal, it is only observed if the degradation product is CO\({}_{2}\), equivalent to mineralization. Results showed that the malathion molecule was practically not detected (Figure 4) at 90 min for 65 and 82 mM H\({}_{2}\)O\({}_{2}\) doses, but the degradation byproducts were quantified as 40 \(\pm\) 3.2% of TOC initial concentration (Figure 5). Organic byproducts of malathion degradation by AOP are malaxon, malathion dicarboxylate, dimethyl dithiophosphate, dimethyl thiophosphate and methyl thiophosphate, which are quantified as TOC [32].  
The formation of the aforementioned byproducts depends on the hydroxyl radicals (\(\bullet\)OH), which in photolysis are correlated to the initial concentration of the oxidant reagent, in this case H\({}_{2}\)O\({}_{2}\)[33]. According to the results shown in Figures 4 and 5 the removal tendency for malathion and TOC is remarkably similar for 65 and 82 mM H\({}_{2}\)O\({}_{2}\) (\(p\) = 0.09), particularly, after 90 min of irradiation. The results for both doses of H\({}_{2}\)O\({}_{2}\) did not show significant differences (\(p\) = 0.13).  
Results from the lower dose tested of 49 mM H\({}_{2}\)O\({}_{2}\) demonstrated that 100% of malathion removal can be achieved but it requires 150 min of irradiation (Figure 4). However, this condition exhibited the highest TOC removal at 120 min (\(p\) = 0.009), which is associated to H\({}_{2}\)O\({}_{2}\) depletion, since the production of \(\bullet\)OH is limited when consumed completely.  
The UV/ H\({}_{2}\)O\({}_{2}\) system achieves its highest removal efficiency at 130 min of irradiation, suggesting consumption of most of the reagent; therefore, a longer HRT does not increase its removal rate (Figures 4 and 5).  
The optimal H\({}_{2}\)O\({}_{2}\) dose is in a range 65-82 mM. Thus, addition of H\({}_{2}\)O\({}_{2}\) generates \(\bullet\)OH that enhances the efficiency of dissolved organic matter oxidation [34; 35]. Nevertheless, increase reagent amount may affect degradation, since excess peroxide molecules cause competing reactions with organic matter by \(\bullet\)OH, and thus their depletion [36].  
Figure 5: Total organic carbon (TOC) behavior during malathion removal by UV/H\({}_{2}\)O\({}_{2}\)-advanced oxidation process at three different H\({}_{2}\)O\({}_{2}\) dosage. The values represent the mean values of five replicates with their standard deviation. Ce = TOC concentration at time t (0, 10, 30, 60, 90, 120 or 150 min), C\({}_{0}\) = initial TOC concentration (In this research = 41.8 mg/L).