## 3 Results and Discussion
### Adsorption Studies
#### 3.5.3 Adsorption Kinetics  
Plot of the equilibrium of the dye adsorption versus the contact time (Figure 9) for distinct concentration of the Acid Red 66 dye is shown. The Figure 9a depicts that dye adsorption increase remarkably during first initial minutes until a certain concentration point and then, slowly rises over the course of 120 min. This is an indicative of the dye molecules are absorbed during the first minutes of reaction; a further occupation of the adsorption sites of the solids is seen with the consequent decrease of the interactions between the adsorbent and azo dye, as the reaction proceeds. Considering the influence of the solids on the dye adsorption, the Fe sample exhibits the lowest discoloration capacity, among the solids studied. Accordingly, the extent of Acid Red 66 removal over the time demonstrates that the Fe sample has 52.6% of the removal efficiency, while FeC gives nearly two times more. Also, the Fe/FeC holds 98.4% from the equilibrium percent removal, as clearly demonstrated in Table 6.  
To illustrate the abovementioned results, the UV-Vis profiles of Fe/FeC sample depict absorbance spectrum after the adsorption at different reaction times (Figure 10). The absorbance bands at 510, 350 and 278 nm are observed for the red glow tone of the Acid Red 66 dye solution (Figure 10, inset). These bands are completely vanished from the spectra at 120 min of dye decolorization. Indeed, the solution became clear and the adsorbent is subsequently removed from the solution by using a conventional ferromagnet bar. This indicates that decolorization of Acid Red 66 occurs quickly because of the deconstruction of the conjugated aromatic structures of the dye. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**adsorbent**} & \multirow{2}{*}{\
\begin{tabular}{c} **Dye Removal** \\ **(\%)** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} \(q_{t}\) \\ **(mg g-1)** \\ \end{tabular} } & \multicolumn{3}{c}{**Pseudo 1 st Order**} & \multicolumn{3}{c}{**Pseudo 2 nd Order**} \\ \cline{5-10}  & & & \(k_{1}\) & \(q_{t1}\) & \(k_{2}\) & \(SD\) & \(k_{2}\) & \(q_{t2}\) & \(SD\) \\  & & & \((\text{min}^{-1})\) & \((\text{mg}\cdot\text{g}^{-1})\) & \(\text{R}^{2}\) & \(SD\) & \((\text{g}-\text{g}^{-1}\text{min}^{-1})\) & \((\text{mg}\cdot\text{g}^{-1})\) & \(\text{R}^{2}\) & \(SD\) \\ \hline Fe & 52.6 & 4.001 & 0.014 & 3.720 & 0.976 & 0.055 & 0.0094 & 4.249 & 0.994 & 0.024 \\ FeC & 73.3 & 12.13 & 0.022 & 11.37 & 0.968 & 0.067 & 0.0132 & 12.40 & 0.987 & 0.043 \\ FeFeC & 98.4 & 13.31 & 0.024 & 12.82 & 0.985 & 0.045 & 0.0134 & 13.85 & 0.997 & 0.022 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Kinetic parameters at 25 \({}^{\circ}\)C for Acid Red 66 adsorption on the solids. Figure 9: Kinetics analyses of the adsorption of Acid Red 66 on the solids studied: (**a**) Non-linear plots for pseudo1 st order kinetics, pseudo 2 nd order kinetics and intraparticle diffusion model. (**b**) Linear plot model for intraparticle diffusion. The initial concentration of the dye is 30 mg\(\cdot\)L\({}^{-1}\), pH = 6.8, T = 25 \({}^{\circ}\)C and dosage of the adsorbent of 2 g\(\cdot\)L\({}^{-1}\). One key factor governing the efficiency of adsorption could be attributed to structural properties of the solid sample, although the literature reports indicate that the adsorption of anionic dyes onto \(\alpha\)-Fe\({}_{3}\)O\({}_{3}\), \(\gamma\)-Fe\({}_{3}\)O\({}_{3}\) and \(\alpha\)-FeO(OH) occurs as isolated phases [49; 54]. However, the results obtained demonstrate that the mixture of the abovementioned phases in Fe sample does not display good adsorption capacities. Judging from the fact that about 70% of Fe\({}_{3}\)O\({}_{4}\) is present in Fe sample (XRD, Mossbauer and EDS results), the magnetite phase is likely prevalent for the adsorption phenomena. Apparently, the anionic azo dye adsorption takes place primarily in the OH groups responsible for the negative charge of Fe from the accessible cubic Fe\({}_{3}\)O\({}_{4}\) structure through the sulfate groups of the dye.