## 3 Results and Discussion
### Degradation of Reactive Black 5
#### 3.2.3 Effect of Contact Time
The complex peaks between 300 and 600 \({}^{\circ}\)C may present the reduction of Mn(IV) to Mn(II) with loss of oxygen [59]. MnO\({}_{2}\) is reduced to Mn\({}_{2}\)O\({}_{3}\) and then to Mn\({}_{3}\)O\({}_{4}\) and MnO. Then, Mn(II) is reduced between 750 and 800 \({}^{\circ}\)C. There is no peak at about 200-250 \({}^{\circ}\)C indicative to the mass loss from the removal of the oxygen- containing functional surface groups. Figure 10: FTIR patterns of raw (black line) and after the RB5 degradation (red line) of NGO (**a**); NGO–Mn\({}_{3}\)O\({}_{4}\) (**b**) and NGO–MnO\({}_{2}\) (**c**) in presence of H\({}_{2}\)O\({}_{2}\). The degradation activity of the NGO-MnO nanocomposites was also investigated spectroscopically, by monitoring their decolorization efficiency with the UV-Vis spectra (250-750 nm) of the solution. Figure 12 presents the UV-vis spectra of the RB5 aqueous solutions after 2 h contact time with NGO-MnO\({}_{2}\) and NGO-Mn\({}_{3}\)O\({}_{4}\) nanocomposites, at pH 3, for initial solutions of 5, 50 and 100 mg/L RB5. From the Figure, it is seen that for the pure RB5 solution, the maximum of absorbance was presented at \(\lambda_{\rm max}\sim\) 593 nm, in the visible region, that can be attributed to the -N=N- azo bond of the dye [60]. The intensity of this peak is relative to the azo dye concentration in the solution and the blue color of the dye that is attributed to the n\(\rightarrow\)p transition of the non-bonding electrons to the anti-banding pi-group orbital of the double bond system. Absorbances were also detected at \(\lambda\) = 230 and 318 nm that are due to the benzene and naphthalene aromatic rings of RB5, respectively [61]. With the decrease of the initial concentration, the UV-vis spectra indicated a decrease of the band, due to the decolorization of the solution and the possible cleavage of the azo bonds for the formation of -NH\({}_{2}\) groups [62], while after degradation different peaks were observed (Figure 13) at 238, 278 and 318 nm attributed to degradation products. Figure 11: DTG curves for (**a**) NGO–Mn\({}_{3}\)O\({}_{4}\) and (**b**) NGO–MnO\({}_{2}\) modified composites in absence and presence of RB5. Recently, as concluded from the FTIR spectra based on the disappearance of the band at 1497 cm\({}^{-1}\) indicative of the cleavage of the -N=N- bond and the appearance of the band at 1640 cm\({}^{-1}\) indicative of the presence of a primary amine as a result of RB5 degradation [38], the non-radical mechanism of RB5 degradation in an acidic environment could be described by the following equations [55]:  
\[\mathrm{Mn}\rightarrow\mathrm{Mn}^{2+}+2\mathrm{e}^{-} \tag{3}\]  
\[2\mathrm{H}_{2}\mathrm{O}\rightarrow\mathrm{H}_{3}\mathrm{O}^{+}+\mathrm{OH}^{-} \tag{4}\]  
Figure 12: UV-Vis curves for RB5 on NGO–Mn\({}_{3}\)O\({}_{4}\) in 3 different RB5 concentrations (pH = 3, m = 0.01 g, V = 0.02 L). Figure 13: UV-Vis curves for RB5 (**a**,**b**) after degradation on NGO, MnO\({}_{2}\), Mn\({}_{3}\)O\({}_{4}\), NGO–Mn\({}_{3}\)O\({}_{4}\) and NGO–MnO\({}_{2}\) nanocomposites for Co = 50 mg/L (pH = 3, m = 0.01 g, V = 0.02 L). \[2\mathrm{H_{3}O^{+}}+2\mathrm{e^{-}}\rightarrow\mathrm{H_{2}}+2\mathrm{H_{2}O} \tag{5}\]  
\[\mathrm{R-N=N-R+H_{2}}\rightarrow\mathrm{R-NH-NH-R} \tag{6}\]  
Figure 9:0  
In acidic environment the favorable \(\mathrm{H_{3}O^{+}}\) concentration increases and facilitate the degradation of RB5 by forming active \(\mathrm{H_{2}}\) and therefore the cleavage of the -N=N- bond of the azo-dye. The formation of hydrogen was testified by the appearance of bubbles in the solution during the redox reaction (inset photo in Figure 6). Hydroxyl groups are able to promote catalytic activity [37]. Besides, surface hydroxyl groups of the carbonaceous support facilitate the adsorption of RB5 through hydrogen bonds as well as the RB5 degradation through the intermediate oxidation. Modified graphene oxide, as manganese oxides' support, possesses a sufficient amount of hydroxylic groups due to the NaOH modification that leads to an increased RB5 degradation.