## 1 Introduction
The documented characteristics of affinity of natural zeolites toward several heavy metal cations and their modest cost, due to the wide availability, make the ion-exchange an advantageous procedure, suitable for pollution removal [29; 30]. In order to make a zeolite able to exchange anions, the cations found on the raw zeolite surface can be replaced by long chained cationic surfactants such as hexadecyltrimethylammonium (HDTMA), by an ion exchange process; due to the large dimensions of such molecules, the exchange takes place on the zeolite grain surface only. If the formation of a double layer (micelle formation) of organic molecules is induced by ion-exchange and tail-tail interaction among the surfactant hydrophobic chains, the charge on the zeolite surface changes from negative to positive [31; 32]. The positive surface charge provides sites for the exchange of anions such as chromate and arsenate. In this way, surfactant-modified zeolites (SMZ) can act as both cation and anion inorganic exchangers. Figure 1 shows the mechanism that leads to the formation first of the monolayer by surface ion exchange and later of the patchy or complete bilayers by interaction of the hydrophobic tails of the surfactant molecules. Recently, natural zeolites, particularly clinoptiolite, modified with cationic surfactant, were used to remove different types of contaminants from water: nitrates [9], chromates [33; 34], cesium, lead, zinc [34], perchloroethylene [35], sulfates [36], NSAIDs [37] and antibiotics [38]. Regarding the As(V) removal, some researchers examined the behavior of both HDTMA-modified, mordenite and clinoptiollite rich tuff, in the elimination of this polluting oxyanion from aqueous solutions [39]. The authors, although basing on the results of thermodynamic runs, showed that the two samples removed arsenic with different kinetics. Moreover, in agreement with an earlier study, they established that surfactant surface coverage plays a key role in sorption; in particular, when the HDTMA loading level on zeolite exceeds monolayer coverage, a substantial increase in arsenate sorption capacity could be attained. Li et al. [40] also carried out a comparison between a surface-modified clinoptiolite and kaolinite minerals in the arsenic removal process. For this aim, the authors performed some equilibrium runs by contacting the modified systems with arsenate or arsenite solution at 0.1-2.0 mM in tubes, under continuous stirring for 24 h. The outcomes exhibited a notable increase in arsenate sorption capacity when the loading level of HDTMA on zeolite and kaolinite surfaces overrun monolayer coverage up to the bilayer formation (200% of the ECEC). Furthermore, Mendoza-Barron et al. [41] emphasized the effect of pH on the As(V) removal process through a clinoptiolite sample modified with HDTMA-Br. In aforementioned work, after the surface modification of a clinoptiollite-rich rock to improve its anionic capacity, adsorption equilibrium runs of As(V) from water solutions were carried out. Based on the results, it was proved that the affinity of the modified zeolite for As(V) was extremely dependent on the form of arsenic species implicated in the solution at different pH. The adsorption capacity increased with solution pH, from pH 5 to pH 7. This implied that, among the different As forms, the affinity of the HAsO4\({}^{2-}\) toward the SMZ was higher than that of the H\({}_{2}\)AsO4\({}^{-}\). However, most of the published research has mainly considered the study of the As removal mechanism through the execution of thermodynamic test. On the contrary, few are the data concerning kinetic analysis in batch or in dynamic conditions to better simulate the effects and the feasibility of a more realistic As abatement process such as the pump and treat column. Moreover, clinoptiolite was the main zeolite sample investigated.