## 3 Results and Discussion
### Adsorption Isotherms  
The batch adsorption experiments were performed in the conditions mentioned above varying the initial concentrations between 40-610 mg.L\({}^{-1}\) for the TC and 18-500 mg.L\({}^{-1}\) for the CPX. The contact time was set to a period of 24 h and the contact solutions were set at pH 8.5 and 10 for TC and CPX, respectively. These values were chosen based on the results previously shown and in order to evaluate the behavior of the clays adsorption when the species with a net charge of -1 are present (CPX\({}^{-}\) and TCH\({}^{-}\)). The adsorption isotherms obtained for the antibiotics on four materials and their best adjustments are shown in Figure 9. Taking into account the [PERSON] et al. classification [46], different behaviors can be associated with the isotherm shapes obtained. For the adsorption of TC on the natural clay, the isotherm can be classified as sigmoidal type (S type) which is considered to be a result of two opposite mechanism. In first place the TCH\({}^{-}\) species has a low affinity towards the negative surface of the clay but once some molecules are adsorbed, they can become new adsorption sites favoring subsequent adsorptions. This phenomenon is called cooperative adsorption and the isotherm saturation point occurs when no vacant sites remain [46; 47]. The isotherms obtained for the adsorption of TC on pillared clays showed some differences among them. The Si-PILC 50 and 75 isotherms can be classified as high affinity type (H-type) and the one obtained for the Si-PILC 25 was a Langmuir type (L-type). Both types of isotherms are obtained when the saturation of solid sites is progressive until reaching its limited adsorption capacity [46; 47]. The H-type isotherm is usually associated with the adsorption of ionic solutes, where the competition between adsorptive and solvent  
Figure 8: Intraparticle diffusion model for the adsorption of TC (\(\bullet\)) and CPX (O) on Si-PILC 75. \begin{table}
\begin{tabular}{c c c c} \hline  & & **TC** & **CPX** \\ \hline  & & **pH 8.5** & **pH 10** \\ \hline  & \(q_{e}\) (mg.g\({}^{-1}\)) & 30.8 & 35.2 \\
**Pseudo first order** & \(k_{1}\) (1 min\({}^{-1}\)) & 1.10 & 1.86 \\  & \(R^{2}\) & 0.93 & 0.93 \\ \hline  & \(q_{e}\) (mg.g\({}^{-1}\)) & 33.9 & 3.21 \\
**Pseudo second order** & \(k_{2}\) (g(mg.min)\({}^{-1}\)) & 0.04 & 0.07 \\  & \(R^{2}\) & 0.98 & 0.98 \\ \hline  & \(k^{{}^{\prime}}\) (mg.(g.min\({}^{0.5}\))\({}^{-1}\)) & 1.91 & 0.92 \\
**Intraparticle** & \(k^{{}^{\prime\prime}}\) (mg.(g.min\({}^{0.5}\))\({}^{-1}\)) & 0.40 & 0.13 \\  & \(R^{2}\) & 0.97 & 0.95 \\ \hline \end{tabular}
\end{table} 
Table 3: Pseudo-first-, -second-order, and intraparticle rate parameters for TC and CPX on Si-PILC 75.  
molecules is low. This could be the result of the higher hydrophobicity exhibited by the pillared clays in contrast with the natural clay. Also, these results evidenced that pillared clays have more affinity for the TCH\({}^{-}\) species present in solution than natural clay. The high affinity shown suggests that the adsorption occurs by a different mechanism and it could be due to the presence of new adsorption sites on the surface of the pillared clays associated with the pillars. Additionally, Si-PILC 25 was the pillared clay with the lowest affinity but the highest adsorption capacity and this could be explained considering that it was the material with the lowest ratio Si/ clay. This could implicate that the Si-PILC 25 interlayer region has probably lower pillars density resulting in a porous structure that favors the interaction between the TC species and the pillars. The adsorption isotherms obtained for CPX can be classified as H-type for Si-PILC 50 and 25 and L-type for Si-PILC 75 and NC. Their analysis is similar to the one done for TC adsorption, all the pillared clays showed a higher affinity towards the CPX\({}^{-}\) than the natural clay. The natural clay isotherm suggests a lower affinity of the anionic CPX species present towards the more negatively charged clay than the one observed for the PILC.