# Cationic and Anionic Dye Adsorption on a Natural Clayey Composite
## 3 Results and Discussion
### Adsorption Isotherms  
The results of fitting the Langmuir, Freundlich, Tempkin, and Dubinin-Radushkevich isotherms models to the CNB adsorption data are shown in Table 5. The Langmuir isotherm model fits very well with CNB experimental data (\(R^{2}>0.96\) in all adsorption experiments). This result supports the hypothesis that CNB removal proceeds by forming a chemisorbed monolayer on IPs. Thus, this result is in agreement with the chemisorption mechanism proposed above. The maximum monolayer adsorption capacities were 10.614, 6.146, and 6.013 mg/g for 0.25, 0.50, and 0.75 g/50 mL of adsorbent, respectively. The \(R_{L}\) values are less than 1. It agrees with the CNB molecules' favorable adsorption by the IPs [55]. The TM model shows the best fit between the isotherm function and the ADY dye-adsorption data (Table 6). The R\({}^{2}\) values were 0.913, 0.906, and 0.670 for 0.25 g/50 mL, 0.50 g/50 mL, and 0.75 g/50 mL of RC, respectively. This result agrees with the suggested mechanism for ADY removal by monolayer formation throughout ADY physisorption on KPs. However, the TM considers the interaction between the absorbent and the absorbate while ignoring large and low concentrations. Thus, the TM is valid for an intermediate concentration range [22]. The low R\({}^{2}\) values at 0.75 for the TM suggest that the TM becomes less suitable for experimental data representation. The system is becoming more complex. Numerous factors, including the pH, temperature, and surface chemistry of the absorbent, among others, can influence the ADY adsorption. Since the complex phenomenon involved in liquid-phase adsorption is not considered by this equation [56], the DRM is not necessarily significantly worse than TM (Table 6). However, the results suggest that the TM is statistically more suitable to describe the isotherms' adsorption  
Figure 8: Effect of adsorbent dosage on adsorption capacity of (**a**) Basic Navy Blue 2 RN and (**b**) Drimaren Yellow CL-2R. Adsorbate concentration 50 mg/L. pH (cationic) = 6, pH (anionic) = 2.  
of the ADY dye onto the clayed subtract. However, both models lead to low adsorption energy that suggests physic adsorption. In agreement with the Temkin model, the adsorption of ADY molecules on the KPs causes a linear decrease in adsorption heat (\(\Delta H\)) [57]. The enthalpy variation in the system will be negative (\(\Delta H=H_{l}-H_{0}\) ; \(H_{l}<H_{0}\)); then, energy is lost [57; 58; 59]. By the second principle of thermodynamics, there will be an increase in the system's entropy. Therefore, at the equilibrium state, the system entropy is higher accordingly with an exothermic adsorption process. A small experiment supported this interpretation. It was verified that the temperature of 50 mL of ADY dye solution in agitation with 0.5 g of RC particles increased from 22.2 \({}^{\circ}\)C to 25.5 \({}^{\circ}\)C in 60 min. Three solutions were placed in agitation in a container with thermal insulating walls. One contained only water, the second [water + clay], and the third [water + clay + dye]. After 60 minutes, we measured the temperature