## 3 Results and Discussion
### Degradation of Reactive Black 5
#### 3.2.3 Effect of Contact Time
For the NGO support, the spectrum shows bands at 1110, 1228, 1370, 1460, and 1580 cm\({}^{-1}\) that may correspond to alkoxy, epoxy and carboxyl groups, respectively [54]. The spectrum of NGO-Mn\({}_{3}\)O\({}_{4}\) nanocomposite, revealed that the carboxyl bands of NGO were not visible, may be due to their participation as nucleation centers and/or as a consequence of the thermal treatment during the synthesis. The broadness of the band at about 1200 cm\({}^{-1}\), attributed to C-O bonds, may illustrate the presence of -OH groups on the surface of NGO. The peaks at 1450 cm\({}^{-1}\) are indicative of stable forms of chemisorbed oxygen, while the bands at 621 and 592 cm\({}^{-1}\) are characteristic of the Mn-O stretching modes of the manganese oxide nanoparticles and reveal the successful impregnation of manganese oxide into the carbon matrix. \begin{table}
\begin{tabular}{c c c} \hline \hline  & \multicolumn{2}{c}{**Mn \% Leaching in the Solution**} \\ \cline{2-3}  & & **+ H\({}_{2}\)O\({}_{2}\)** \\ \hline Mn\({}_{3}\)O\({}_{4}\) & 8.5 & 6.6 \\ NGO–Mn\({}_{3}\)O\({}_{4}\) & 1.6 & 3.3 \\ MnO\({}_{2}\) & 8.4 & 9.5 \\ NGO–MnO\({}_{2}\) & 1.0 & 4.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Leaching of manganese in the solution after RB5 degradation. Figure 9: Recovery efficiency of the NGO—MnO\({}_{2}\) and NGO—Mn\({}_{3}\)O\({}_{4}\) after 5 cycles of adsorption—desorption (Co = 100 mg/L, pH = 3, m = 0.01 g, V = 0.02 L). The spectra of the nanocomposites after the degradation of RB5, reveals bands that may be attributed to reaction products formed after the reaction between the dye and their functional groups. The shoulder appeared at \(\sim\)1574 cm\({}^{-1}\) and the bands at \(\sim\)1226 cm\({}^{-1}\) could be attributed to N-H bending and C-N bond respectively, as a result of the cleavage of the -N=N- bond of the dye that resulted to the formation of primary amines. The bands at \(\sim\)1390 cm\({}^{-1}\) revealed the presence of surface species with sulfone and/or sulfonate groups indicative of RB5 degradation. Similar results have been presented by [PERSON] et al. after degradation of RB5 by Mn\({}_{85}\)Al\({}_{15}\) particles [55] as well as by [PERSON] et al., [56] after electrochemical treatment of RB5 solution. In order to further investigate the nature of nanocomposites and the alterations of the surface chemistry after the degradation of RB5, differential thermal gravimetric measurements (DTG) were implied between 25 and 1000 \({}^{\circ}\)C in nitrogen (Figure 11). For all samples, the peaks appeared at 100 and 200 \({}^{\circ}\)C represented the removal of physically adsorbed water and the dehydration of crystal water interlayer water in the nanostructure of manganese oxide phase/and or NGO, [57] respectively. In Figure 11, the peaks representing the changes for the NGO-Mn\({}_{3}\)O\({}_{4}\) nanocomposite are presented. The peak at about 430 \({}^{\circ}\)C could be attributed to reduction of Mn\({}_{3}\)O\({}_{4}\) to Mn(II). Mn\({}_{3}\)O\({}_{4}\) (Mn\({}_{1}\) +II, Mn\({}_{2}\) +III) can be transformed into Mn\({}_{5}\)O\({}_{8}\) (Mn\({}_{2}\) +II, Mn\({}_{3}\) +IV) in the temperature range of 450-500 \({}^{\circ}\)C [58]. After release of oxygen, due to the reduction of Mn\({}_{5}\)O\({}_{8}\) to Mn\({}_{2}\)O\({}_{3}\) occurs and Mn(II) is finally reduced to Mn\({}^{\circ}\) at about 740 \({}^{\circ}\)C. For the modified graphene oxide supported nanocomposites, the patterns appeared complex since the graphene phase is expected to participate in the reduction. In Figure 11a, the changes for the NGO-Mn\({}_{3}\)O\({}_{4}\) nanocomposite, after the degradation of RB5, are also shown. After the degradation of RB5, the relative intensity of the peaks corresponding to Mn\({}_{3}\)O\({}_{4}\) and Mn(II) was different than that for the initial sample. Also, a peak corresponding to the reduction of Mn(II) to Mn\({}^{\circ}\) was detected. Concerning the nanocomposite NGO-MnO\({}_{2}\), a peak at the 800-840 \({}^{\circ}\)C, related to Mn(II), is shown.