## 3 Results and Discussion
### Degradation of Reactive Black 5
#### 3.2.3 Effect of Contact Time  
The kinetic experiments were carried out in aqueous solutions of RB5 for two different initial concentrations (40 and 100 mg/L) in absence (non radical oxidation) and presence (radical oxidation) of H\({}_{2}\)O\({}_{2}\). The appropriate for complete mineralization concentration of H\({}_{2}\)O\({}_{2}\) found to be 0.15 M, estimated from the equation:  
\[2\mathrm{C}_{26}\mathrm{H}_{21}\mathrm{O}_{19}\mathrm{N}_{5}\mathrm{S}_{6} \mathrm{Na}_{4}+87\mathrm{H}_{2}\mathrm{O}_{2}\to 52\mathrm{CO}_{2}+108 \mathrm{H}_{2}\mathrm{O}+8\mathrm{Na}+10\mathrm{N}+12\mathrm{S} \tag{2}\]  
From the experimental results shown in Figures 5 and 6, equilibrium was attained at 60 min for all samples. The nanocomposites presented higher removal than the pure manganese oxides, indicating an improved decolorization, may be due to the synergetic effect of adsorption-oxidation. The pseudo-first order (dashed lines) and pseudo-second order (compact lines) kinetic models were fitted to the experimental results and the kinetic parameters, as well as the goodness of the fit, are presented in Table 2. From the Table, it is concluded that pseudo-second order kinetic model presented a better fit and R\({}^{2}\) indicative of a synergetic adsorption-degradation mechanism and not a neat degradation. Figure 4: Effect of initial concentration of RB5 (pH = 3, m = 0.01 g, V = 0.02 L). Figure 5: (**a**,**b**) Effect of contact time on the degradation of RB5 on NGO, MnO\({}_{2}\), Mn\({}_{3}\)O\({}_{4}\), NGO–Mn\({}_{3}\)O\({}_{4}\) and NGO–MnO\({}_{2}\) nanocomposites for initial RB5 concentration of 100 mg/L (**a**) in presence and (**b**) absence of H\({}_{2}\)O\({}_{2}\) (Co = 100 mg/L, pH = 3, m = 0.01 g, V = 0.02 L). (inset: decolorization after 60 min for Mn\({}_{3}\)O\({}_{4}\) in the first row and NGO–MnO\({}_{2}\) in the second row). Figure 6: Effect of contact time on the degradation of RB5 on NGO–Mn\({}_{3}\)O\({}_{4}\) and NGO–MnO\({}_{2}\) nanocomposites for initial RB5 concentration of 40 mg/L and 100 mg/L in presence (radical) and absence (non-radical) of H\({}_{2}\)O\({}_{2}\) (pH = 3, m = 0.01 g, V = 0.02 L). As presented in Figure 5b the presence of H\({}_{2}\)O\({}_{2}\) increased the RB5 degradation. This could be attributed to the effect of \(\bullet\)OH radicals that could be enhanced by the increase concentration of H\({}_{2}\)O\({}_{2}\). For this reason, the influence of different doses of H\({}_{2}\)O\({}_{2}\) concentration on RB5 oxidative degradation was investigated, and the results are presented in Figure 7a. From this Figure, it is concluded that with the increase of H\({}_{2}\)O\({}_{2}\) dose from 0.05 mM until the concentration of 0.15 mM, the RB5 degradation increases. In non-radical oxidation, the nanocomposites presented a better performance than the respective manganese oxides while in radical oxidation their performance was similar (Figure 5b). The contribution of OH radicals to RB5 degradation was examined by adding methanol to the suspension as scavenger. In Figure 7b the influence of different concentrations of methanol as a quenching agent is presented. As seen from the Figure, the increase of methanol volume results to a decrease of RB5 degradation. It can be concluded that OH\(\bullet\) radicals are generated during the reaction, react rapidly with methanol, resulting in the decrease of RB5 oxidation. Both nanocomposites showed similar decrease. The effect of the scavenging leads to the conclusion that surface-bound OH\(\bullet\) play the dominant role in the RB5 degradation. The possible reuse of the loaded materials was studied, in order to assess the environmental safety of the composites, for this reason, the elution ability was tested for the nanocomposites and the results are presented in Figure 8. Deionized water, at a range of pH values (Figure 8a), as well as ethanol, methanol and acetonitrile (Figure 8b) were used as eluents for RB5 elution from loaded NGO-Mn\({}_{3}\)O\({}_{4}\) and NGO-MnO\({}_{2}\) nanocomposites.