## 3 Results and Discussion
### XPS Measurements of the Spent Adsorbents  
The examination of the chemical states of Fe and C elements in the samples by XPS provides additional information about the chemical composition at the surface and the chemical state of the elements, after the adsorption process. Table 8 shows the binding energy (in eV) of the selected photoemissions and in the case of the C _1s_ and O _1s_ signals, the % of each area of the different contributions of the deconvoluted spectra are also included (in brackets). The survey XPS spectrum of the Acid Red 66 dye shows the presence of the C, O, S, Na, Cl and N elements. The C 1s core level spectrum can be deconvoluted into four components corresponding to -C=C- and adventitious carbon (284.8 eV (70%)), C-OH (285.7 eV (9%)), C-N=N (286.7 eV (17%)) and C-SO3- (288.2 eV(4%)) [59; 60]. The S 2\(p\) core level spectrum shows a doublet S 2\(p\)3/2 and S 2\(p\)1/2 at 168.1 and 169.3 eV owing to the sulfate groups of the dye. Interestingly, the corresponding peak position on the residual sodium appears with BE at 1071.8 eV pointing to charge compensating the sulphate group. Obviously, the nitrogen to nitrogen bond of the dye is observed at 399.9 eV. Furthermore, The O 1s core level spectrum shows two contributions at 531.8 eV (47%) and 533.1 eV (53%) assigned to -SO3- and C-OH groups, respectively [59]. According to the data from XPS, the solids after dye adsorption contain Fe, C, O and N elements. In the case of Fe sample, the binding energies (BE) located at about 710.5-711.0 and 724.2 eV are ascribed to Fe 2\(p\)3/2 and Fe 2\(p\)1/2, respectively. This indicates that Fe species exist as Fe3+ and Fe3+ coming from the \(\gamma\)-Fe2O3 and Fe3O4 besides that of \(\alpha\)-Fe2O3 phases [22]. Moreover, the shake satellite peak at 718.8 eV is associated with the \(\gamma\)-Fe2O3. The O 1\(s\) core level spectra exhibit contributions at 530.0 and 531.5 eV. While the first contribution is assigned to lattice oxygen from iron oxides, the latter is due to surface hydroxyl groups and oxygen bonded to carbon [22]. These results confirm that the samples do not suffer from phase transformation and the \(\alpha\)-Fe2O3, \(\gamma\)-Fe2O3 and Fe3O4 phases are preserved, after the adsorption process. The C 1s core level spectrum depicts three contributions with BE values at 284.9 eV (82%), 286.4 eV (8%) and 288.6 eV (10%) being assigned to C=C and adventitious carbon, C-O and sulfonate species, from the adsorbed dye on solid surface. Accordingly, the atomic concentration of the elements for spent Fe sample is C: 23.3%, N: 0.33%, O: 51.75%, Fe: 24.3% and 0.30% of sulfur. Judging from the fact that the dye possesses all these chemical entities, a strong adsorption of the dye on solid surface is likely. This is in accordance with adsorption studies results that suggest the chemisorption of the dye onto the sample Also, a limited amount of carbon species on surface is found, as a consequence of the saturation of the Fe adsorbent with the dye. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{8}{c}{**Binding energy (eV)**} \\ \cline{2-7}  & **Fe 2\(p\)3/2 & Fe 2\(p\)1/2 & S 2\(p\)3/2 & Na 1s & Cl 2\(p\)3/2 & N 1s & C 1s & O 1s \\ \hline \multirow{3}{*}{Fe} & 710.8 & 724.2 & & & 399.6 & 284.9(82) & 530.0(68) \\  & & & & & & 286.4 (8) & 531.5(32) \\  & & & & & & 288.6 (4) & \\ \hline \multirow{3}{*}{FeC} & 710.9 & 724.3 & 168.1 & & 399.7 & 285.0(45) & 529.9(44) \\  & & & & & & 286.1(43) & 531.4(36) \\  & & & & & & 288.0(12) & 532.8(20) \\ \hline \multirow{3}{*}{Fe(FeC)} & 710.6 & 724.3 & 168.1 & & 399.7 & 285.0(50) & 530.4(48) \\  & & & & & & 286.2(38) & 531.5(35) \\  & & & & & & 288.1(12) & 532.9(17) \\ \hline \multirow{3}{*}{Acid Red dye} & & 168.1 & 1071.8 & 198.2 & 399.9 & 284.8(70) & \\  & & & & & & 285.9 (9) & \\  & & & & & & 286.7(17) & 531.8(47) \\ \cline{1-1}  & & & & & & 288.3 (4) & 533.2(53) \\ \hline \end{tabular}
\end{table} 
Table 8: XPS data obtained from the spectra of the solids.