## 3 Results and Discussion
### DAP Mineralization and Identification of Oxidation Intermediates  
The TOC removal is necessary for the complete destruction of the starting molecule and its transformation into CO2 and H2O. For this reason, the mineralization of DAP was followed by measuring the TOC changes during DAP degradation by Fenton oxidation of 100 mg/L DAP aqueous solution using 1000 mg/L H2O2 and 50 mg/L Fe2+ at pH = 3.2. Figure 8 presents the changes in the concentrations of DAP, TOC and the intermediates (in mg C/L) with time during the treatment. As it can be seen, the concentration of DAP and TOC were satisfactorily reduced. DAP was rapidly and completely removed by the Fenton oxidation after a reaction time of 120 min. However, a slower exponential decrease of TOC rather than DAP was observed. TOC removal reached about 95% after 360 min treatment. This indicates that the degradation of organic compounds is more rapid than their complete mineralization. In the first step of Fenton oxidation, DAP is degraded into intermediates without carbon dioxide formation. This is confirmed by the changes of the intermediates concentration (determined by the mass difference between TOC and DAP (mg C/L) showing a rapid increase in the intermediates concentration to reach a maximum at 60 min and then parallel decay than TOC. Figure 7: Competitive kinetic model for the determination of the absolute rate constant of the reaction of DAP and HO\({}^{\bullet}\) hydroxyl using benzoic acid as a reference compound. Experimental conditions: [DAP]0 = 100 mg/L; [BA]0 = 100 mg/L, [H2O2] = 1000 mg/L; [Fe2+] = 50 mg/L; pH = 3.2. On the basis of the results above, the oxidative degradation of DAP by the Fenton process was accompanied by the formation of aromatic and aliphatic intermediate products. For this reason, it is very important to identify the main intermediates. Their identification was carried out by HPLC analyses. Figure 9 presents the evolution of the concentration of the aromatic and aliphatic intermediates with time during the treatment of aqueous solution containing 100 mg/L DAP by the Fenton process using 1000 mg/L H\({}_{2}\)O\({}_{2}\) and 50 mg Fe\({}^{2+}\)/L and at pH 3.2. This figure shows that phthal acid (PA), 1,2-dihydroxybenzene (DHB) and 1,2,4-trihydroxybenzene (THB) were the main aromatic intermediates identified during the oxidation of DAP by the Fenton process. These three intermediates (PA, DHB and THB) are clearly formed from the beginning of the treatment of DAP (about 5 min). A maximum concentration of 4.3 ppm phthal acid (PA), 1.3 ppm 1,2,4-trihydroxybenzene (THB) and 10.6 ppm 1,2-dihydroxybenzene (DHB) were found, and then completely removed in 45, 60 and 120 min of treatment, respectively. This result indicated the opening of the aromatic ring and the generation of aliphatic intermediates. Maleic, formic and oxalic acids are the detectable aliphatic intermediates of DAP oxidation by the Fenton process (see Figure 8). The formation of maleic acid appeared after 30 min of treatment, attaining a plateau concentration of about 11 mg/L after around 45 min of treatment and then decreasing to very low values (2 mg/L) after 120 min treatment. Formic acid starts its formation at the beginning of treatment, reaches its maximum concentration of 10 mg/L and then completely disappears after 45 min. On the other hand, oxalic acid was, as expected, the major and most persistent carboxylic acid generated. The concentration of oxalic acid increased rapidly at the starting of treatment. The highest concentration of oxalic acid was quantified as 38 mg/L at 60 min of the treatment. After that time, the amount of oxalic acid gradually decreased to a value of 14 mg/L at the end of the treatment. Consequently, these six intermediates were more or less quickly oxidized into CO\({}_{2}\) and H\({}_{2}\)O, and quasi-completely mineralized after about 180 min of treatment. Figure 8: Changes in concentrations (mg C/L) of DAP, intermediates and TOC during Fenton oxidation of 100 mg/L DAP.