## 1 Introduction  
Zeolites are hydrous and crystalline alumino-silicates minerals with a regular and microporous structure (pore size from 3.0 to 10 A) [1]. Their frameworks are comprised of tetrahedral building units TO\({}_{4}\) (T = Si or Al), connected together by sharing one oxygen atom at the corners to form complex three-dimensional open networks of channels or cavities and cages (0.2-1.2 nm) occupied by H\({}_{2}\)O molecules and exchangeable cations [2]. The partial substitution of Si\({}^{4+}\) by Al\({}^{3+}\) in the zeolite framework produces a negative charge that is often neutralized by the exchangeable alkali and/or alkaline earth cations (Na\({}^{+}\), K\({}^{+}\), Ca\({}^{2+}\), Sr\({}^{2+}\), or Ba\({}^{2+}\)) [3] or by H\({}^{+}\)[4] for acidic zeolite in order to maintain electrical neutrality throughout the structure. Natural zeolites exhibit interesting structural properties such as a large specific surface, flexibility, and well-defined micropores in molecular dimensions, as well as high chemical and physical stability (e.g., thermal and mechanical) [5], and sorption characteristics obtained by combined ion-exchange and molecular-sieve properties [6]. These outstanding properties allow these materials to offer promising applications in many sustainable processes such as wastewater treatment [7] and petroleum refining [8] as well in agriculture and construction [9]. Over the last decades, many researches have focused on clinoptilolite due to its attractive selectivity for toxic metal cations and organic substances [6; 10; 11; 12; 13; 14]. It was reported that the ion-exchange process in natural zeolites depends on factors related to adsorbate and adsorbent characteristics such as pH and temperature of the solution, ion concentration, crystal structure, and chemical composition of the zeolitic material [10; 15]. The cation exchange capacity (CEC) of a zeolite is directly dependent on the Si/Al ratio [16]. This important property of the natural zeolites can be used to remove heavy metals form polluted water. For instance, the lead (Pb\({}^{2+}\)) is a toxic metallic specie which is necessary to eliminate from the environment [17].  
It was also reported that a variation in the SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) ratio of the natural zeolites affects the hydrophilicity/hydrophobicity of this material, which is correlated with the adsorption, catalysis, and ion exchange properties [18; 19]. Zeolite dealumination using acid as a chemical agent allows zeolites with a higher SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) ratio to be obtained, which are usually used for the removal of organic hydrophobic contaminants in water [20] and volatile organic compounds in air [21]; whereas alkali treated zeolite using organic hydroxides [22] or sodium hydrate [16; 23] can achieve materials with a lower SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) ratio for the removal of ammonium [24] and heavy metals [6; 25].  
In previous studies [16; 18; 19; 22; 23; 25; 26; 27; 28; 29; 30], the researches focused mainly on the effect of the SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) ratio on the structural properties of zeolites. Moreover, these studies used synthesized zeolites [19; 22; 26; 27; 28; 30], whereas the only raw zeolites used being mordenite and clinoptilolite. [PERSON] et al. [31] reported the Pb(II) removal from an acid media. However, there was no information in the literature about the adsorption behavior of this heavy metal using acid modified natural zeolites. Therefore, the main objective of this research was to know the correlation between the Si/Al ratio and the Pb(II) removal capacity from aqueous solutions using non- and acid-modified clinoptilolite-rich tuffs. In addition, the structural and morphological properties of the materials were studied by XRD, SEM, and IR techniques. This study was aimed to evaluate the influence of contact time, initial pH, adsorbent dosage, and initial concentration on the removal of Pb(II) from aqueous solutions.