## 3 Results and Discussion
### Degradation of Reactive Black 5
#### 3.2.3 Effect of Contact Time
The maximum desorption capacity for NGO-Mn\({}_{3}\)O\({}_{4}\) and NGO-MnO\({}_{2}\) that was found about 4.2% and 6.1% of the removed RB5 respectively, was noticed at pH 9.0, where the RB5 retention forces were deteriorated, conflicting to adsorption that found to be more effective at pH 3.0. The organic solvents that were also tested for RB5 elution from the nanocomposites, found to be less effective than deionized water at pH = 9.0, with methanol to be the optimal eluent. The low desorption capacity led to the conclusion that the amount of RB5 eluted from the used nanocomposites are in agreement with the total non-degradated dye. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Samples**} & \multirow{2}{*}{**Co**} & \multicolumn{2}{c}{**Pseudo-First Order**} & \multicolumn{2}{c}{**Pseudo-Second Order**} \\ \cline{3-6}  & & \(\mathbf{K_{1}}\) & \(\mathbf{R^{2}}\) & \(\mathbf{K_{2}}\) & \(\mathbf{R^{2}}\) \\ \hline \multirow{2}{*}{NGO} & 100 & \(1.92\times 10^{-2}\) & 0 & \(3.60\times 10^{-4}\) & 0.393 \\ \cline{2-6}  & 100 + H\({}_{2}\)O\({}_{2}\) & \(1.85\times 10^{-1}\) & 0.857 & \(3.63\times 10^{-3}\) & 0.961 \\ \hline \multirow{2}{*}{MnO\({}_{2}\)} & 100 & \(1.91\times 10^{-2}\) & 0.371 & \(2.10\times 10^{-4}\) & 0.691 \\ \cline{2-6}  & 100 + H\({}_{2}\)O\({}_{2}\) & \(3.73\times 10^{-1}\) & 0.998 & \(3.09\times 10^{-2}\) & 0.999 \\ \hline \multirow{2}{*}{Mn\({}_{3}\)O\({}_{4}\)} & 100 & \(4.30\times 10^{-3}\) & 0.504 & \(5.00\times 10^{-5}\) & 0.66 \\ \cline{2-6}  & 100 + H\({}_{2}\)O\({}_{2}\) & \(8.97\times 10^{-2}\) & 0.972 & \(2.10\times 10^{-3}\) & 0.924 \\ \hline \multirow{2}{*}{NGO-MnO\({}_{2}\)} & 100 & \(7.60\times 10^{-2}\) & 0.982 & \(1.60\times 10^{-3}\) & 0.999 \\ \cline{2-6}  & 100+ H\({}_{2}\)O\({}_{2}\) & \(2.80\times 10^{-1}\) & 0.999 & \(2.08\times 10^{-2}\) & 0.999 \\ \cline{2-6}  & 40 & \(4.78\times 10^{-1}\) & 0.939 & \(2.58\times 10^{-2}\) & 0.991 \\ \cline{2-6}  & 40 + H\({}_{2}\)O\({}_{2}\) & \(1.47\times 10^{-1}\) & 0.984 & \(0.61\times 10^{-2}\) & 0.983 \\ \hline \multirow{2}{*}{NGO-Mn\({}_{3}\)O\({}_{4}\)} & 100 & \(1.79\times 10^{-2}\) & 0.977 & \(2.70\times 10^{-4}\) & 0.999 \\ \cline{2-6}  & 100 + H\({}_{2}\)O\({}_{2}\) & \(3.20\times 10^{-1}\) & 0.998 & \(2.66\times 10^{-2}\) & 0.999 \\ \cline{2-6}  & 40 & \(1.14\times 10^{-1}\) & 0.950 & \(0.52\times 10^{-2}\) & 0.993 \\ \cline{2-6}  & 40 + H\({}_{2}\)O\({}_{2}\) & \(0.67\times 10^{-1}\) & 0.590 & \(0.29\times 10^{-2}\) & 0.898 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Kinetic parameters. Figure 8: Elution experiments by (**a**) deionized water (**b**) ethanol, methanol and acetonitrile of the loaded the NGO–MnO\({}_{2}\) and NGO–Mn\({}_{3}\)O\({}_{4}\) nanocomposites (Co = 100 mg/L, m = 0.01 g, V = 0.02 L). For the examination of the reuse of the nanocomposites, the spent materials were eluted with methanol and reused for further application for 5 cycles. The results are presented in Figure 9. The decrease of their gradual degradation ability during the 5 cycles may be attributed either to the elution of Mn in the solution during the degradation that resulted to the reduction of their efficiency or to the oxidation of the RB5 into smaller molecules that were adsorbed on the nanocomposites' surface, having as a consequence their pore blocking and the decrease of their further adsorption ability. For the examination of the materials' stability the leaching of manganese in the solution, after RB5 degradation for 2 h, was examined and the measurements of the manganese in the reaction solutions were performed by ICP. From the results, presented in Table 3, it is shown the low percentage of the eluted manganese in the solution compared to the Mn concentration in the material, as well as the stability of the materials and the beneficial effect of the NGO support during the RB5 degradation. The FT-IR spectra of NGO support and of the nanocomposites prepared, presented in Figure 10, provide information about their surface chemistry before (black line) and after the degradation of RB5 (red line).