# Decolorization of Orange-G Aqueous Solutions over C\({}_{60}\)/Mcm-41 Photocatalysts
## 3 Results and Discussion
### Decolorization by Adsorption
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **C\({}_{60}\)** & **SSA\({}_{\text{BET}}\)** & **SA** & **k\({}_{\text{A2}}\times 10^{3}\)** & **k\({}_{\text{R}}\times 10^{2}\)** & **Decolorization** \\  & **wt \%** & **(m\({}^{2}\)g\({}^{-1}\))** & **(umol g\({}^{-1}\))** & **(g min\({}^{-1}\) mg\({}^{-1}\))** & **(min\({}^{-1}\)g\({}^{-1}\))** & **(\%)\({}^{*}\)** \\ \hline
1C\({}_{60}\)/MCM-41 & 1 & 851 & 37.2 & 6.1 & 0.56 & 41 \\
3C\({}_{60}\)/MCM-41 & 3 & 831 & 36.5 & 5.1 & 1.38 & 74.9 \\
6C\({}_{60}\)/MCM-41 & 6 & 813 & 11.2 & 0.5 & 0.63 & 39.5 \\
9C\({}_{60}\)/MCM-41 & 9 & 777 & 10.4 & 0.2 & 0.54 & 40.8 \\
12C\({}_{60}\)/MCM-41 & 12 & 760 & 9.6 & 0.2 & 0.48 & 44.3 \\ \hline \hline \end{tabular}  
* Determined at the end of the combined experiment in the dark and then under irradiation for 135 min.  
\end{table} 
Table 1: Values of specific surface area (SSA\({}_{\text{BET}}\)), specific adsorption (SA), rate constants of adsorption (kA2) and photocatalytic decomposition (kR) of Orange G and percentage of decolorization of the aqueous solutions  
Figure 3: Change of absorbance determined at 475 nm for OG -AA aqueous solutions with the time in the presence of the 3C\({}_{60}\)/MCM-41 and 9C\({}_{60}\)/MCM-41 photo-catalysts as well as for the OG aqueous solution in the presence of the 3C\({}_{60}\)/MCM-41 photo-catalyst. The values on the \(y\)-axis are normalized over the absorbance at zero time. Inspection of Table 1 shows that the extent of adsorption follows similar trend to those of specific surface area of the catalysts and the adsorption kinetic constant, as well. However, the decrease in the specific adsorption is abrupt from the second to the third sample (36.5 to 11.2 umol g-1) whereas the corresponding decrease in the specific surface area is not so large (831 to 813 m2g-1). This suggests an additional reason for the decrease of the adsorption. One may propose that the decrease in the specific adsorption is also due to coverage of adsorption sites, situated on the surface of MCM-41, by the supported C60 clusters as the concentration of the latter is duplicated from the second to the third sample. The relatively low dispersion of the C60 supported clusters in the samples 9C60/MCM-41 and 12C60/MCM-41 [31; 32] leaves uncovered sufficient portion of MCM-41 surface, justifying the relatively small additional decrease in the specific adsorption observed from the third to the fifth sample. Therefore, we consider that the decolorization through adsorption is taking place mainly on the bare surface of the carrier. In a previous work [10], studying the mechanism of OG adsorption on the surface of TiO2, we had concluded that the adsorption of OG occurs mainly through the one of the two -SO3- groups of its structure, presumably due to steric constraints, on the positively charged surface sites. This has been also confirmed by [PERSON] et al. [37], who studied the selective adsorption of anionic dyes on silver bromide-based nanomaterials and concluded that electrostatic attraction of -SO3- groups by positively charged surface sites is the dominating contribution to the adsorption of these dyes. It should be noticed that the above mentioned nanomaterials exhibited higher SA (-58 umol/g) than that achieved over the best of our photocatalyst (3C60/MCM-41). This can be explained taking into account that although C60/MCM-41 photocatalysts are characterized by too high SSABET values, the silica nature of their support implies low surface density of positively charged sites at pH >-3 [38], where the adsorption and photocatalytic experiments took place.