# Cationic and Anionic Dye Adsorption on a Natural Clayey Composite
## 3 Results and Discussion
### Vis-NIR Analysis  
As the energy quantum is directly related to photon frequency (\(v\)) and inversely related to its wavelength (\(\lambda\)), the resulting absorption spectrum exhibits characteristic shapes useful for analytical purposes. The photon wavelength at which radiation is absorbed appears as a reduced signal of reflected radiation and is displayed in % reflectance (\(R\)), which can then be transformed to apparent absorbance: \(A=\log(1/R)\). The wavelength at which the absorption occurs (i.e., the energy quantum size) also depends on the chemical matrix and environmental factors such as neighboring functional groups and temperature. This wavelength allows the detection of a range of molecules that contain the same type of bonds [34].  
The RC's DR spectrum (Figure 3) shows variations in the absorbance, a wavelength associated with functional groups of iron oxides, water, hydroxyl, and clay minerals (Table 3). Iron oxides are spectrally active across the Vis-NIR region based on the electronic transition of iron cations Fe\({}^{3+}\), Fe\({}^{2+}\), as summarized by [PERSON] and [PERSON] [35]. The RC's DR spectrum displays the steep red slope between the 400 nm and 700 nm  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Card** & **Compound** & **Chemical Formula** & **wt\%** \\ \hline
00-155-0598 & Kaolinite & Al\({}_{2}\)H\({}_{4}\)O\({}_{5}\)Si\({}_{2}\) & 48.6 \\
00-900-6106 & Magnesiochlorotid & Al\({}_{2}\)Fe\({}_{0.35}\)H\({}_{2}\)Mg\({}_{0.65}\)O\({}_{7}\)Si & 22.4 \\
00-154-8100 & Na, K-rich laumontite & Al\({}_{2}\)Ca\({}_{0.68}\)K\({}_{0.26}\)Na\({}_{0.36}\)Cl\({}_{5.63}\)Si\({}_{4}\) & 23.3 \\
00-900-9782 & Hematite & \(\alpha\)Fe\({}_{2}\)O\({}_{3}\) & 5.8 \\ \hline \hline \end{tabular}
\end{table}
Table 2: The red clay diffractogram qualitative analysis.  
Figure 2: Diffractogram of the red clay sample. Kaolinite (K); hematite (H); magnesiochlorotid (M); and Na, K-rich laumontite (L).  
characteristic of Fe\({}^{3+}\). A broad minimum is around 650 nm and 884 nm attributable to iron oxide minerals such as hematite (Table 3). Clay-sized hematite crystals can arise as a secondary mineral formed by weathering processes in soil. Thus, it would be responsible for the reddish color found in the RC samples. In contrast, samples with goethite would show a yellowish-brown color [7; 35; 36; 37; 38].  
The minima around 1400 nm and 1900 nm are commonly associated with tones and overtones of H\({}_{2}\)O and OH- (Table 3) [38]. The RC's DR spectrum displays one or more small minima between around 2150 and 2400 nm. These minima would be due to Si-OH and cation-OH bonds in phyllosilicate minerals (e.g., kaolinite) [37; 38]. Previous investigations confirm the kaolin doublet's observation around 1400 nm and 2200 nm (Figure 3 and Table 3), corresponding to the -OH stretch vibration of water in the crystal lattice of silicates and Al-OH bending modes [39]. The minimum around 2310-2330 nm could be assigned to magnesium chloritoid [40; 41]. The RC's DR spectrum shows features associated with functional groups of Fe oxides (hematite), water, hydroxyl, kaolinite, and magnesiochloritoid. Therefore, untreated red clay's characterization suggests a clayey composite mainly constituted by a mineral clay (kaolinite), laumontite, and secondary minerals (iron-containing particles).  
Figure 3: The red clay diffuse reflectance spectrum and its first derivative.