## 3 Results and Discussion
### Adsorption Isotherms
This higher heterogeneity for the PILC compared to the NC could be related to the new adsorption sites generated by the pillars presence and their porous structures. The results showed that for all the PILC materials, the systems for CPX have higher adsorption capacities than for TC and are also more heterogeneous. This can be related to the molecule size, due to the fact that the CPX size is lower than the TC size, allowing more access for CPX to the porous structure and resulting in higher adsorptions and affinities than the TC species. Finally, regarding the adsorption of TC on Si-PILC, the highest adsorption capacity was obtained for Si-PILC 25, and the best fit was obtained with the Freundlich model, suggesting that for this material the adsorption occurs on heterogeneous sites. This last result could be related to the lower amount of micropores in the Si-PILC 25 structure which could allow more access of the TCH\({}^{-}\) species to the adsorption sites. The Scatchard plots obtained for all the materials and both antibiotics are shown in Figure 10. From these plots, the \(R^{2}\) and binding constant (\(k_{d}\)) values can be obtained. The first could be related to the presence of nonspecific or multitype interactions, while the second indicates the adsorbate affinity \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & & \multicolumn{4}{c}{**TC**} & \multicolumn{4}{c}{**CPX**} \\ \hline  & & **NC** & **Si-PILC** & **Si-PILC** & **Si-PILC** & **NC** & **Si-PILC** & **Si-PILC** & **Si-PILC** \\  & & & **25** & **50** & **75** & **NC** & **25** & **50** & **75** \\ \hline \multirow{3}{*}{Freundlich} & \(k_{F}\) & \multirow{3}{*}{3.26} & 4.81 & 9.43 & 8.47 & 6.98 & 30.7 & 31.6 & 28.9 \\  & & & 2.28 & 2.68 & 4.36 & 4.16 & 2.75 & 5.52 & 5.69 & 5.49 \\  & & \(R^{2}\) & 0.84 & 0.99 & 0.94 & 0.95 & 0.96 & 0.91 & 0.97 & 0.98 \\ \hline \multirow{3}{*}{Langmuir} & \(\begin{array}{c}a_{m}\ (\text{mg.g}^{-1})\\ k\ (\text{(Lmg)}^{-1})\\ \end{array}\) & — & 57.4 & 40.4 & 36.7 & 75.7 & 74.5 & 61.9 & 74.1 \\  & & \(R^{2}\) & — & 0.01 & 0.04 & 0.04 & 0.01 & 0.43 & 1.23 & 0.19 \\ \hline \multirow{3}{*}{Sips} & \(\begin{array}{c}a_{m}\ (\text{mg.g}^{-1})\\ b\ (\text{Lmg}^{-1})\\ n\end{array}\) & 45.5 & — & 39.5 & 36.92 & 80.82 & 77.35 & 73.33 & 100.6 \\  & & \(b\ (\text{Lmg}^{-1})\\ n\end{array}\) & 0.01 & — & 0.04 & 0.04 & 0.01 & 0.37 & 0.59 & 0.07 \\  & & \(R^{2}\) & 0.45 & — & 0.96 & 1.02 & 1.13 & 1.24 & 1.89 & 2.56 \\  & & \(R^{2}\) & 0.97 & — & 0.99 & 0.99 & 0.99 & 0.99 & 0.99 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Freundlich and Langmuir parameters for CPX and TC adsorption on natural and pillared clay minerals. Figure 9: Experimental isotherms (symbols) and their best equations adjustments for the equilibrium adsorption data of TC and CPX on natural and pillared clays. for the adsorption sites. The \(R^{2}\) values obtained for the TC adsorption on four materials were 0.04, 0.78, 0.99, and 0.94 for the NC, Si-PILC 25, 50, and 75, respectively. These values suggest that the nonspecific interactions in the adsorption of TC on pillared clays are considerably higher than for the NC. Similarly, the \(k_{d}\) values obtained were 0.001, 0.02, 0.04, and 0.03 for the NC, Si-PILC 25, 50, and 75, respectively. These values evidence a poor affinity of the TCH\({}^{-}\) species for the adsorption sites in the four materials; the lowest affinity was obtained for the NC. In the case of CPX adsorption the values obtained were 0.91, 0.97, 0.93, and 0.74 (\(R^{2}\)) and 0.015, 0.45, 1.36, and 3.07 (\(k_{d}\)) for NC, Si-PILC 25, 50, and 75, respectively. As it can be observed, the presence of multitype interactions is lower for this molecule than for the TC and the \(k_{d}\) values obtained are higher than those obtained for adsorption of TC on all the materials. This could be due to the lower molecular size of CPX favors the interaction with the pillar for the PILC materials. Additionally, the Scatchard plot shapes can be related to different adsorption mechanisms.