# Cationic and Anionic Dye Adsorption on a Natural Clayey Composite
## 3 Results and Discussion
### Solution pH Effect  
The CNB dye removal was higher than 96% at a low pH and decreased around 75% in a solution at pH higher than 8 (Figure 4a). However, ADY removal was around 90% at a low pH and decreased to lower than 30% at a pH higher than 8 (Figure 4b). The RC particles are charged positively. Similar charges on the CNB and the RC's surface cause repulsion between the CNB and the RC. However, results in Figure 4a suggest that the CNB participates in breaking and making bonds. These interactions are described as inner-sphere complexes due to CNB's and the surface's ability to reach each other, enabling the orbitals' overlapping responsible for the bonding. [PERSON] [44] concluded that this bonding process is significant when the inorganic surface exposes metals like iron or aluminum to utilize organic adsorbate as one of the ligands. This result agrees that the RC sample is a mixture of kaolinite clay particles (KPs); Na, K-rich laumontite particles (LPs), and iron-containing particles (IPs). We hypothesize that the IPs expose iron on their surfaces, which forms covalent bonds with the CNB dye molecules, and show affinity by the CNB molecules. In a high-pH solution, the CNB removal decreases to around 75% (Figure 4a). The RC particles are charged negatively. The OH\({}^{-}\) neutralizes the CNB. Opposite charges on KPs and CNB dye molecules cause the emergence of electrostatic attraction between CNB and KPs. Nevertheless, the result suggests no affinity between the CNB and the IPs at high pH. Thus, the removal of CNB molecules at a high pH decreases. Therefore, the results suggest that in a low-pH solution, the mechanism of CNB removal could be due to chemisorption. At high pH, the removal mechanism could be due to physisorption. The adsorption mechanism can be broadly divided into three categories: physisorption, chemisorption, and ion exchange [45]. The physisorption mechanism refers to surface adsorption without disrupting the adsorbent's electronic orbitals and adsorbate. It could involve van der Waals interactions, electrostatic interactions, hydrogen bonding, diffusion, and hydrophobic interaction (\(\pi\)-\(\pi\) interaction and Yoshida interaction [46]. Physisorption is a reversible and exothermal phenomenon [47]. In contrast, the chemisorption _mechanism_ involves electronic orbital and valence forces between absorbent and adsorbate. It generates a chemical bond to the surface of the adsorbent and is irreversible. The mechanism of the chemisorption process can involve complex formation, chelation, covalent bonding, redox \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample Constituent** & **Fundamental, cm\({}^{-1}\)** &
\begin{tabular}{c} **Vis–NIR** \\ **Wavelength, nm** \\ \end{tabular} & **Vis–NIR Mode** \\ \hline Fe oxides (Hematite) & & 404, 444,529 650, 884 & Electronic transitions \\ \hline \multirow{4}{*}{Water} & \(\
u_{1}\): O-H: 3278 cm\({}^{-1}\) & 1915 & \(\
u_{1}+\
u_{2}\) \\  & \(\
u_{2}\): O-H: 1645 cm\({}^{-1}\) & 1455 & \(2\
u_{2}+\
u_{3}\) \\  & \(\
u_{3}\): O-H: 3484 cm\({}^{-1}\) & 1380 & \(\
u_{1}+\
u_{3}\) \\  & & 1135 & \(\
u_{1}+\
u_{2}+\
u_{3}\) \\  & & 940 & \(2\
u_{1}+\
u_{3}\) \\ \hline \multirow{4}{*}{Hydroxyl} & \(\
u_{1}\): O-H: 3575 cm\({}^{-1}\) & 1400 & \(2\
u_{1}\) \\  & \(\
u_{1}\): O-H: 3575 cm\({}^{-1}\) & 930 & \(3\
u_{1}\) \\  & & 700 & \(4\
u_{1}\) \\ \hline \multirow{4}{*}{Kaolin doublet} & \(\
u_{1a}\): O-H: 3695 cm\({}^{-1}\) & 1395 & \(2\
u_{1a}\) \\  & \(\
u_{1b}\): O-H: 3620 cm\({}^{-1}\) & 1415 & \(2\
u_{1b}\) \\  & \(\delta\): O-H: 915 cm\({}^{-1}\) & 2160 & \(2\
u_{1a}+\delta\) \\  & & 2208 & \(2\
u_{1b}+\delta\) \\ \hline \multirow{4}{*}{Kaolin doublet} & \(\
u\): Fe-H: 3508 cm\({}^{-1}\) & 1425 & \(2\
u\) \\  & \(\delta\): Fe-OH: 853 cm\({}^{-1}\) & 2293 & \(\
u+\delta\)[43] \\ \hline Magnesiochloritoid & \(\delta\): Mg-OH: 1090 cm\({}^{-1}\) & 2322 & \(\
u+\delta\)[42,43] \\ \hline \hline \end{tabular}
\end{table}