## 3 Results and Discussion
### Degradation of Reactive Black 5
#### 3.2.1 Effect of pH  
The effect of solution pH on RB5 removal/degradation was examined in the pH range of 3-9 and the results are illustrated in Figure 3. From the Figure it is seen that the degradation efficiency decreased with the pH increase (89% at pH 3.0 to 43% at pH 9.0 for MnO\({}_{2}\)), indicating that the degradation of RB5 by the under-examination materials is facilitated in acidic solutions. Oxidation of organic pollutants by Mn\({}_{3}\)O\({}_{4}\) and MnO\({}_{2}\) is usually pH-dependent, because of the pH-dependent redox potential of MnO\({}_{2}\)/Mn\({}^{2+}\) and the speciation of organic pollutants [52]. This dependence may be attributed to the fact that MnO\({}_{2}\) requires the involvement of protons to be reduced to Mn\({}^{2+}\) and/or to the fact that the  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Samples** & **S\({}_{\text{BET}}\) (m\({}^{2}\)/g)** & **V\({}_{\text{t}}\) (cm\({}^{3}\)/g)** & **V\({}_{\text{meso}}\) (cm\({}^{3}\)/g)** & **V\({}_{\text{mic}}\) (cm\({}^{3}\)/g)** \\ \hline NGO & 9.10 & 0.080 & 0.080 & 0 \\ Mn\({}_{3}\)O\({}_{4}\) & 2.38 & 0.014 & 0.014 & 0 \\ MnO\({}_{2}\) & 38.50 & 0.336 & 0.336 & 0 \\ NGO–Mn\({}_{3}\)O\({}_{4}\) & 7.52 & 0.056 & 0.056 & 0 \\ NGO–MnO\({}_{2}\) & 118.12 & 0.231 & 0.231 & 0 \\ \hline \end{tabular}
\end{table}
Table 1: Textural parameters of the manganese oxides, of NGO and of NGO–Mn\({}_{3}\)O\({}_{4}\) and NGO–MnO\({}_{2}\) nanocomposites.  
Figure 2: N\({}_{2}\) adsorption isotherms and pore diameter distribution (inset) for NGO–Mn\({}_{3}\)O\({}_{4}\) and NGO–MnO\({}_{2}\) modified composites.  
RB5 oxidation is considered a surface reaction connected to the surface chemistry and charge of the materials involved [53].  
The importance of the solution pH is due to the fact that it impacts the surface charge of the supporting material that acts as adsorbent (NGO). The pKa values of RB5 found to be almost close to 3.8 and 6.9. The pKa of the sodium sulfonate groups (-SO\({}_{3}\)Na i. e. sulfonic groups) attached to the dye molecules are known to be very low and can reach negative values, while the pKa value of 3.8 is close to the pKa of aniline (i.e., 4.6) and can be attributed to the -NH\({}_{2}\) groups of the dye molecules. This means that at pH lower than 3.8 and higher than the pKa of the sulfonate groups, the dye molecules are in their anionic forms and have got the highest negative charge. From the potentiometric titrations results the nanocomposites' surface is positively charged at pH lower than this value [40]. Thus, in acidic medium, the electrostatic attractions between the anionic dyes and the positively charged surface of the nanocomposites are maximized, so that the pH-dependent adsorption isotherms of the dye show nanocomposites' surface by \(\pi\)-\(\pi\) and hydrophobic interactions as well as by electrostatic attractions. Since the maximum decolorization occurred at pH 3, equilibrium and kinetic experiments were performed at this pH.  
Figure 3: (**a**) Effect of initial pH on RB5 adsorption/degradation onto NGO, MnO\({}_{2}\), Mn\({}_{3}\)O\({}_{4}\), NGO–Mn\({}_{3}\)O\({}_{4}\) and NGO–MnO\({}_{2}\) nanocomposites (Co = 100 mg/L, m = 0.01 g, V = 0.02 L); (**b**) potentiometric titration results for the nanocomposite catalysts.