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The analysis of variance of the results obtained from the anodic oxidation of the GES–EE2 mixture was performed after verifying the assumptions of normality, homoscedasticity, and independence. This analysis shows that both the current density as well as the pH and concentration of the supporting electrolyte have a significant influence on the oxidative process (α = 0.05, d.f. = 2, FNa2SO4 = 14.16, FCurrent = 41.71, FpH = 36.98, p ≤ 0.001 in the three cases). Moreover, it also allowed the observation that there is no interaction between the studied factors (α = 0.05, d.f. = 8, F = 26.8, p = 0.688). The COD reduction percentage, which is the parameter used for monitoring the GES–EE2 mixture oxidation ranged from 70.83 ± 5.87% to 92.11 ± 5.53% (Table 1). Treatments 6, 15, 24, and 25 showed mixture degradation percentages greater than 90% and, according to the Tukey media comparison test (level of significance α = 0.05), there is no significant difference among them. The anodic oxidation on non-active electrodes, such as the BDD electrodes, can be explained through the set of Equations (2)–(4) (Panizza & Cerisola 2009). The process initiates with the oxidation of the water molecule and the consequent generation of the OH free radical, this radical being the main oxidizing species present in the oxidative process. The OH establishes a weak interaction with the BDD anode surface, allowing an easy reaction with the organic compound present that normally ends in the mineralization of the molecule (Martínez & Ferro 2006).
formula
2
formula
3
formula
4
Figure 2 shows COD removal percentages through anodic oxidation. It can be observed that when the density of the current supplied to the system was 16 and 32 mA cm−2, a direct relationship between current density and the efficiency of the anodic oxidation was achieved, i.e., the greater the current density, the greater the percentage of COD removal. This is congruent with the observations made in similar systems studied by others (Frontistis et al. 2011; Sun et al. 2012). This direct relationship leads to thinking that, under these conditions, the process is controlled by current density which is below its limiting value and the observed behavior can be explained through Equations (2) and (3), where it is noticed that the greater the current supplied to the system, the greater the generation of [OH]ads (Equation (2)), and thus, the greater the percentage of removal of the organic compound (Equation (3)). With a current density of 48 mA cm−2, it was observed that the COD removal reached a maximum and then stabilized in some cases, while in other cases, it decreased compared to the results obtained with 32 mA cm−2. This can be attributed to the predominance of collateral or parasitic reactions (Souza et al. 2011), such as the oxygen evolution reaction (Equation (4)), that competes with Equation (1). On the other hand, the relationship between efficiency of the anodic oxidation and current density was not observed at 48 mA cm−2. This confirms that, under these conditions, the oxidative system efficiency is not controlled by current density, which indicates that this factor is above or at least close to its limiting value.
Table 1

Media comparison through the Tukey test with a level of significance α = 0.05

Treatment[Na2SO4] MCurrent mApHCOD removal % ± σ2Tukey p < 0.05
0.02 100 73.95 ± 3.15 AB 
0.02 100 76.82 ± 2.86 ABCD 
0.02 100 78.93 ± 1.51 ABCDEF 
0.02 200 84.66 ± 3.55 BCDEFGH 
0.02 200 89.90 ± 2.98 GH 
0.02 200 92.11 ± 5.53 
0.02 300 86.45 ± 2.85 CDEFGH 
0.02 300 83.31 ± 0.17 BCDEFGH 
0.02 300 82.67 ± 0.86 BCDEFGH 
10 0.05 100 70.83 ± 5.87 
11 0.05 100 78.19 ± 3.84 ABCDE 
12 0.05 100 89.01 ± 0.97 FGH 
13 0.05 200 75.80 ± 3.16 ABC 
14 0.05 200 85.82 ± 5.44 CDEFGH 
15 0.05 200 91.00 ± 1.22 
16 0.05 300 71.47 ± 0.83 
17 0.05 300 83.52 ± 5.21 BCDEFGH 
18 0.05 300 87.43 ± 1.96 DEFGH 
19 0.10 100 79.29 ± 6.83 ABCDEFG 
20 0.10 100 74.74 ± 4.91 AB 
21 0.10 100 89.71 ± 1.43 FGH 
22 0.10 200 88.39 ± 3.36 EFGH 
23 0.10 200 87.55 ± 4.04 DEFGH 
24 0.10 200 91.35 ± 0.92 
25 0.10 300 90.04 ± 0.97 GH 
26 0.10 300 88.07 ± 1.31 EFGH 
27 0.10 300 88.52 ± 1.58 EFGH 
Treatment[Na2SO4] MCurrent mApHCOD removal % ± σ2Tukey p < 0.05
0.02 100 73.95 ± 3.15 AB 
0.02 100 76.82 ± 2.86 ABCD 
0.02 100 78.93 ± 1.51 ABCDEF 
0.02 200 84.66 ± 3.55 BCDEFGH 
0.02 200 89.90 ± 2.98 GH 
0.02 200 92.11 ± 5.53 
0.02 300 86.45 ± 2.85 CDEFGH 
0.02 300 83.31 ± 0.17 BCDEFGH 
0.02 300 82.67 ± 0.86 BCDEFGH 
10 0.05 100 70.83 ± 5.87 
11 0.05 100 78.19 ± 3.84 ABCDE 
12 0.05 100 89.01 ± 0.97 FGH 
13 0.05 200 75.80 ± 3.16 ABC 
14 0.05 200 85.82 ± 5.44 CDEFGH 
15 0.05 200 91.00 ± 1.22 
16 0.05 300 71.47 ± 0.83 
17 0.05 300 83.52 ± 5.21 BCDEFGH 
18 0.05 300 87.43 ± 1.96 DEFGH 
19 0.10 100 79.29 ± 6.83 ABCDEFG 
20 0.10 100 74.74 ± 4.91 AB 
21 0.10 100 89.71 ± 1.43 FGH 
22 0.10 200 88.39 ± 3.36 EFGH 
23 0.10 200 87.55 ± 4.04 DEFGH 
24 0.10 200 91.35 ± 0.92 
25 0.10 300 90.04 ± 0.97 GH 
26 0.10 300 88.07 ± 1.31 EFGH 
27 0.10 300 88.52 ± 1.58 EFGH 
Figure 2

COD removal in the 27 initial treatments following the structure of a 33 factorial design. The initial average concentration was 713.72 mg L−1 of COD with a variability of 20.5 mg L−1 of COD.

Figure 2

COD removal in the 27 initial treatments following the structure of a 33 factorial design. The initial average concentration was 713.72 mg L−1 of COD with a variability of 20.5 mg L−1 of COD.

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