## 1 Introduction  
Arsenic is considered as one of the most toxic pollutants due to its mutagenic and carcinogenic effect on human beings. The continuous industrialization of developing countries will probably lead to an increase in arsenic release in the environment, which will cause potentially serious problems throughout the world [1]. The presence of arsenic in water is due to the dissolution of minerals from subterranean strata or from anthropogenic activities such as the leaching of manmade arsenic compounds deriving from smelting of metal ores, agricultural pesticides, desiccants and wood preservatives production [2]. In particular, the presence of arsenic in water supplies has been indicated as responsible for arsenical dermatitis, enlargement of liver, neurological effects, heart disease and several forms of cancer [3]. Arsenic is sensitive to mobilization at pH values typical of natural waters (pH 6.5-8.5) under both oxidizing and reducing conditions; while it can occur in the environment in several oxidation states (-3, 0, +3 and +5), in natural waters it is mostly found in inorganic forms as oxyanions of trivalent arsenite [As(III)] or pentavalent arsenate [As(V)] [4]. According to the EPA (Environmental Protection Agency) the acceptable level of As in drinking water is fixed at 0.010 mg/L [5]. The oxidation-reduction conditions affect the speciation of arsenic: in oxygen-rich environments, where aerobic conditions persist, arsenate [As(V)] is prevalent and exists as a monovalent (H\({}_{2}\)AsO\({}_{4}\)-) or divalent (HAsO\({}_{4}\)\({}^{2-}\)) anion, whereas arsenite [As(III)] exists as acid (H\({}_{3}\)AsO\({}_{3}\)) and anionic (H\({}_{2}\)AsO\({}_{3}\)-) species in a moderately reducing environment, where anoxic conditions persist [6; 7]. Between the two forms, As(III) is more \"mobile\" and at the same time much more toxic than As(V) which is easily fixed onto soil particles [8]. Several techniques have been developed to remove arsenic from water: coagulation, reverse osmosis, adsorption and anion exchange processes are some of the commonly used [9; 10]. Due to both their low cost and a relatively operational ease, there is now a raising interest on sorption methods that seem to be the most promising way to remove arsenic from water sources [11]. Several materials have already been studied for the adsorption of arsenic from water, including metal-loaded coral limestone [12; 13] hematite and feldspar, sandy soils [14], activated carbon [15; 16; 17; 18], activated alumina [19; 20], lanthanum-impregnated silica gel [21], hydrous zirconium oxide [22] and hydrous iron oxide [23] and zero valent iron [24; 25]. Ohki et al. [13] found an adsorption capability of 150 ug/g As(V) on coral limestone, while Xu et al. [14] investigated the adsorption capacity of drinking water treatment solids (DWTS) in As(V) removal processes: at its best, DWTS adsorption capability was found to be 170 mg/g, although heavily affected by pH conditions. Recently, activated carbons (both commercial and synthetized at lab scale) were heavily tested to remove arsenic from water sources: Pattanayak et al. [26] found that char carbon can remove 89 mg/g of As(III) and 34.46 mg/g of As(V) from water under strong acidic conditions (pH = 2). Coconut husk carbon showed also high performances: this material was tested by Manju et al. [27] on arsenic removal from industrial wastewater, and they found that it was able to adsorb 146.3 mg/g of As(III). In recent years, other materials with anion exchange properties gained attention as potentially good candidates to remove arsenic from water sources, since their anion exchange capacity can be useful in the removal process. Among the possible candidates, zeolites seem to be quite promising. Zeolites are inorganic cation-exchangers of natural and synthetic origin. Many deposits of natural zeolites of sedimentary origin, commonly referred to as zeolite tuffs, are diffused all over the globe [28].