## 4 Conclusions  
The present paper reports the results of an investigation of the possible use of surface-modified phillipsite- and clinoptiollite-rich tuffs as alternative ion exchangers for removing As from wastewater. Several surfactant-modified natural zeolite (SMNZ) exchangers were prepared by contacting tuff samples with HDTMA-Br or HDTMA-Cl solutions under continuous flow. On the obtained samples, the estimation of the anionic exchange capacity together with a kinetic and thermodynamic characterization was performed in batch conditions, while a fixed-bed test was also performed in order to characterize their behaviour under severe dynamic conditions.  
The influence of pH on As removal was preliminarily investigated, showing that the best performances were obtained for neutral pH values. For each tuff, modification with HDTMA-Br produced samples with an exchange capacity higher than those modified with HDTMA-CI, suggesting the formation of a complete and compact micellar structure, opposite to the patchy bilayer obtained on the samples modified with HDTMA-Cl. Kinetic tests demonstrated that As removal is in all cases reasonably fast: it reached 80% in 1 h and 100% in 2 h. Data modeling results suggested that the ion exchange process follows a pseudo-second order kinetic. The thermodynamic tests were satisfactorily modeled with the Langmuir equation, confirming the better performance of the Br-modified samples over the Cl-modified ones: the maximum amount of removable As was 7.86 mg g\({}^{-1}\) for CLI-Br sample.  
Moreover, the dynamic tests showed that every sample was able to treat at least 22 bed volumes: the CLI-Br sample showed the best performance (30 bed volumes), even if the PHI-Cl sample showed slightly higher efficiency and selectivity. Such results suggest a possible use of SMNZs as ion exchangers for the removal of As from wastewaters.  
Lastly, as also verified in previous papers [32], there is a different selectivity on SMNZs for various anions present in waters. Therefore, it is important to study the effects of these possible interfering cations. However, in this first study, it was intended to verify only the removal efficiency of the As (V) by our modified zeolite supports (mainly as regards phillipsite rich-tuff). Future research, which have already begun, will be directed at verifying (with appropriate kinetic tests) the effect of the presence of other anionic species; this will certainly give us more complete information on the removal efficiency of our SMNZs.  
B.d.G.: Conceptualization, supervision, validation. P.A.: methodology and, formal analysis. B.L.: visualization and writing--original draft. B.G.: investigation and data curation. Antonio Peluso: investigation and, resources. D.C.: project administration and writing--review and editing. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
The authors wish to acknowledge the European Union and the Campania Region for the financial support in the frame of the POR-FESR 2014-2020 Programme (IDRICA project--CUP B63D18000310007 SURF 18031 BP00000001).  
The authors declare no conflict of interest.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **k, L/min/mg** & **q, mg/g** & \(\mathbf{R}^{2}\) \\ \hline CLI-Br & 2.52E-04 & 1.12 & 0.94 \\ CLI-Cl & 2.83E-04 & 0.94 & 0.99 \\ PHI-Br & 3.78E-04 & 0.79 & 0.95 \\ PHI-Cl & 5.53E-04 & 0.86 & 0.92 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Thomas parameters from breakthrough curves.