# Decolorization of Orange-G Aqueous Solutions over C\({}_{60}\)/Mcm-41 Photocatalysts
## 3 Results and Discussion
### Decolorization by Adsorption  
The electronic spectra of OG, OG-AA, and AA aqueous solutions are seen in Figure 1. The peaks at 475 and 410 nm, responsible for the color, are attributed respectively to the azo-bonds of hydrocarbon and azo form and the peaks at 250 and 330 nm are assigned to benzene and naphthalene rings of OG,respectively [35; 36]. It should be noted that ascorbic acid absorption peak (265 nm) interferes with the benzene ring peak of azo dye located at 250 nm. On the other hand, we were obliged to doubly dilute the initial AA aqueous solution (Figure 1) in order to record correctly its spectrum. This was not necessary in the case of a solution containing both substances indicating that an interaction takes place between them in the solution. Figure 2 shows the electronic spectra of an OG-AA aqueous solution in contact with 3C\({}_{60}\)/MCM-41 catalyst recorded in the range 350-600 nm at various contact times in the dark. We observe that the absorbance in the above range decreases significantly up to 60 min and afterwards the decrease is slowing down and absorbance remains almost constant for the period of 120-180 min. The kinetics of decolorization in the absence of light over two samples with different C\({}_{60}\) loadings (3 and 9 wt % C\({}_{60}\)) are illustrated in Figure 3 (left part). The absorbance at 475 nm has been taken  
Figure 1: Electronic spectra of aqueous solutions of OG (25 \(\upmu\)M), OG and AA (25 \(\upmu\)M OG & 0.5 mM AA), and AA (0.25 mM). Figure 2: Absorbance of an aqueous solution containing OG and AA recorded at various contact times with the photocatalyst 3C\({}_{60}\)/MCM-41 in the dark. as a measure of decolorization. We observe that the extent of decolorization due to the adsorption depends on the photocatalyst composition. To study the adsorption kinetics, the pseudo-first-order and pseudo-second-order kinetic models were both examined according to Equations (1) and (2), respectively [37]. \[ln(q_{e}-q_{t})=ln(q_{e})-k_{A1}t \tag{1}\]  
\[\frac{t}{q_{t}}=\frac{1}{k_{A2}q_{e}^{2}}+\frac{t}{q_{e}} \tag{2}\]  
where, qe and qt (mg g\({}^{-1}\)) symbolize the amount of OG absorbed per gram of photocatalyst at adsorption equilibrium and each time t (min), respectively. kA1 (min\({}^{-1}\)) and kA2 (g mg\({}^{-1}\) min\({}^{-1}\)) represent pseudo-first-order and pseudo-second-order rate constant, respectively. The aforementioned kinetic analysis of OG adsorption experiments showed that a pseudo second order kinetic law (Equation (2)) describes much better the experimental data (R\({}^{2}>0.99\)). This finding has been also reported for the adsorption kinetics of OG on the surface of other adsorbents [37]. The calculated kA2 values are given in Table 1. Inspection of this table shows that the adsorption rate constant values decrease as the C\({}_{60}\) loading of the photocatalysts increases. This observation evidenced that the adsorption of OG takes place on the bare surface of MCM-41. In order to further study this point, we have determined the specific adsorption of orange G (SA: expressed in umol of OG adsorbed per gram of catalyst) on the surfaces of the samples studied after equilibration for 180 min in the dark. The values obtained are illustrated in Table 1.