## 3 Results and Discussion
### Ion Exchange Runs
#### 3.2.3 Thermodynamics of Anion Exchange  
Figure 6 shows the As species exchange isotherms in SMNZ-Br and SMNZ-Cl, obtained at pH = 7. The graphs show the equilibrium concentration of the ingoing cation in the solid phase (mg g\({}^{-1}\)) as a function of its equilibrium concentration in the liquid phase (mg L\({}^{-1}\)). Isotherms data allow us to state that:  
1. Samples obtained with the use of HDTMA-Br perform better than those obtained with HDTMA-Cl: concerning the phillipsite-rich tuff, maximum exchange is of 5.2 mg g\({}^{-1}\) for PHI-Br and 1.2 mg g\({}^{-1}\) for PHI-Cl, while for clinoptilolite-rich tuff, maximum exchange is 7.9 mg g\({}^{-1}\) for CLT-Br and 4.6 mg g\({}^{-1}\) for CLT-Cl. This is once again justified by what was previously verified about the micelle formation on the zeolitic surface: if the counterion that balances the positive charge of \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sample** & **Mathematical Model** & \multicolumn{4}{c}{**Parameters**} \\ \cline{3-6}  & & **K\({}_{1}\)** & **K\({}_{2}\)** & **S\({}_{m}\)** & **R\({}^{2}\)** \\  & & **(min\({}^{-1}\))** & **(g mg\({}^{-1}\) min\({}^{-1}\))** & **(mg g\({}^{-1}\))** & **R\({}^{2}\)** \\ \hline \multirow{2}{*}{CLI-Br} & Pseudo-first order & 0.143 & & 7.7 & 0.995 \\  & Pseudo-second order & & 0.025 & 8.2 & 0.995 \\ \hline \multirow{2}{*}{CLI-Cl} & Pseudo-first order & 0.309 & & 4.2 & 0.956 \\  & Pseudo-second order & & 0.116 & 4.4 & 0.991 \\ \hline \multirow{2}{*}{PHI-Br} & Pseudo-first order & 0.068 & & 4.1 & 0.984 \\  & Pseudo-second order & & 0.018 & 4.6 & 0.981 \\ \hline \multirow{2}{*}{PHI-Cl} & Pseudo-first order & 0.192 & & 1.2 & 0.967 \\  & Pseudo-second order & & 0.248 & 1.3 & 0.999 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Kinetic parameters. Figure 6: As exchange isotherms for SMNZ samples at a T = 25 \({}^{\circ}\)C.  
the nitrogenous head of HDTMA\({}^{+}\) is Br\({}^{-}\), the micellar structure that is obtained on the zeolite surface is complete and compact (the bilayer has an anion exchange capacity, AEC, equal to its ECEC). If Cl\({}^{-}\) is the counterion, the micellar structure that is generated is like a patchy bilayer (partial micelle structure), which has obviously less exchangeable anions with respect to the previous case. 2. Clinoptilolite modified samples perform better than phillipsite modified samples. This can be due to the different density of HDTMA in the micelle, that in turn depends on the Al on the surface of the zeolite: the higher (4.90) Si/Al ratio of clinoptilolite will produce a micelle sparser than that produced on the phillipsite surface (Si/Al ratio = 2.38). This allows a higher amount of complex anions such as HAsO\({}_{4}\)\({}^{2-}\) and H\({}_{2}\)AsO\({}_{4}\)\({}^{-}\) to enter the micellar structure, as the micelle can better rearrange itself to accommodate them. 3. The As anion can only partially exploit the surface anionic exchange capacity of both SMNZs investigated. This is a direct consequence of what was stated earlier: the uptake of a complex anion forces a rearrangement of the micellar structure, which probably displaces some exchange sites and makes them unavailable for further exchange. The equilibrium data were modeled with Langmuir, Sips and Toth equations--already described in the Materials and Method section (see Section 2.4.2). Figure 7 shows the results of these modelings, while Table 4 shows the significant parameters determined from each best fitting curve. The modeling results show that the Toth model seems to be the one that made the experimental thermodynamic data dense; this is also confirmed by the values of R\({}^{2}\), which are always higher for the Toth model. In any case, since the Langmuir isotherm is a two-parameter model that assumes adsorption on a defined number of sites that have the same affinity for the adsorbate, which forms a monolayer, and considering that the exchange of ions should not give multilayer formation, we will consider the Langmuir isotherm model as the most appropriate for our system.