## 3 Results and Discussion
### Characterization of MWCNTs Layer  
The carbon nanotube layers were characterized by means of a thermal analysis to study the method of dye adsorption. TG and DTA analyses were carried out at different heights of the MWCNT layers to define the stratigraphic trend of adsorption inside each column. The MWCNTs related to the last cycle before the color change were chosen for the analyses. The samples number was considered as a function of height of the MWCNT layer. Figure 7 shows the samples taken in the different columns. A comparison between the different TG curves of samples was possible. A TG curve of the pure RB5 dye is presented in Figure 8. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **TC [mg/L]** & **IC [mg/L]** & **TOC [mg/L]** \\ \hline RB5 & 9.539 & 0.247 & 9.292 \\ \hline C1-01 & 13.190 & 7.333 & 5.859 \\ C1-15 & 3.522 & 1.508 & 2.014 \\ C1-24 & 3.508 & 1.909 & 1.600 \\ C1-36 & 2.864 & 1.237 & 1.627 \\ C1-37 & 3.079 & 1.123 & 1.956 \\ C1-38 & 3.532 & 1.342 & 2.190 \\ C1-39 & 3.749 & 1.395 & 2.354 \\ C1-41 & 5.458 & 1.380 & 4.078 \\ \hline C2-01 & 3.890 & 3.292 & 0.599 \\ C2-09 & 3.421 & 1.815 & 1.606 \\ C2-10 & 3.890 & 1.970 & 1.920 \\ \hline C3-01 & 4.836 & 2.953 & 1.883 \\ C3-06 & 5.463 & 1.374 & 4.089 \\ C3-07 & 4.260 & 0.713 & 3.548 \\ \hline C4-01 & 2.018 & 1.228 & 0.790 \\ C4-02 & 3.150 & 1.006 & 2.144 \\ \hline C5-01 & 5.391 & 0.589 & 4.812 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: TOC, IC, and TC of permeate in the adsorption columns after different cycles. Cx-yy: x is the name of the column, and yy is the number of cycles. Figure 7: Samples taken in the different layers of MWCNTs. From the graphs shown in Figure 8, it is possible to observe that the lower layers tended to have a smaller weight loss. This trend is most noticeable in the columns that had the highest total layer of MWCNTs. In fact, in column C1, which had a total layer of MWCNTs equal to 4.5 cm, the difference between the first and last layer was equal to 11.96%, while in column C4, which had a total layer equal to 0.4 cm, the difference was equal to 0.13% (Table 6). Figure 8: TG curves of samples as a function of height of the MWCNT layer. To confirm this inhomogeneity of the layers, Figure 9 shows the DTA thermal analyses for the C1 column only, as it is characterized by a higher nanotube layer, which allowed a more effective distinction of the three layers. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Layer** & **Total Weight Loss [\%]** & \
\begin{tabular}{c} **Height of Total MWCNT** \\ **Layer in the Column [cm]** \\ \end{tabular} &
\begin{tabular}{c} **Difference Weight** \\ **Loss between High** \\ **and Low Layer (\%)** \\ \end{tabular} \\ \hline C1-Low & 82.05 & & \\ C1-Medium & 90.43 & 4.50 & 11.96 \\ C1-High & 94.01 & & \\ \hline C2-Low & 86.58 & & \\ C2-Medium & 87.25 & 2.25 & 7.88 \\ C2-High & 94.46 & & \\ \hline C3-Low & 89.51 & & 0.24 \\ C3-High & 89.75 & 1.4 & \\ \hline C4-Low & 92.12 & & 0.13 \\ C4-High & 92.25 & & \\ \hline C5 & 92.42 & 0.4 & — \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Relative weight losses of TG curves. Figure 9: DTA curves of C1 column for different height of layers (high, medium, and low). DTA curves for the C1 column show two exothermal peaks at ca. 446 \({}^{\circ}\)C and 580 \({}^{\circ}\)C, very close to those of RB5 at 413 \({}^{\circ}\)C and 617 \({}^{\circ}\)C, respectively (Figure 9). The thermal analysis of the middle part of the MWCNT layer presented the same thermal behavior but the intensities of the exothermal peak of DTA curve were lower. The deeper layer presented exothermal peaks of DTA curve that were very low, almost invisible. This could be due to a smaller amount of dye adsorbed at this MWCNT layer. This observation was the same for all of the analyzed columns. Figure 10 shows a zoom of the DTA peaks at ca. 446 \({}^{\circ}\)C, comparing the several heights for each column to highlight better the variation of exothermal peaks for each column.