## 1 Introduction  
Clays and clay minerals are of significant interest because of their interesting characteristics that make them suitable for industrial applications. They have been accepted as one of the most appropriate low-cost adsorbents and common component in different industrial applications [1; 2; 3; 4; 5; 6; 7]. Among the various types of clays, bentonite is more popular. Its physicochemical and adsorption properties depend on the montmorillonite content, the crystalline structure of the clay minerals that constitute them and the nature of the inter-layer cations [8; 9]. In order to eliminate impurities and different exchangeable cations from clay minerals and to prepare a well-defined material for use as catalyst and adsorbent, various activations have been used, most often with inorganic acids [10; 11; 12; 13]. Indeed, the treatment with minerals acids significantly changed their textural characteristics. This can expand their applications to include environmental protection [14; 15], catalytic cracking of petroleum [16], catalyst supports in many chemical processes and detergent in paper industries and textile fabrications [17; 18; 19]. In fact, the most applications of these clays are bleaching, purification and stabilization of vegetable oils.  
Acid-activated clays have attracted much current interest as adsorbents and catalysts for a range of acid mediate processes [20; 21; 22]. Nowadays, considerable progress is achieved in the synthesis of many new inorganic mesoporous molecular sieves based on solid acids. However, modified clay minerals remain one of the widely and mostly used in the industry as mesoporous solid catalysts and adsorbents [23]. The acid attack on clay minerals has additional effects on the mineralogical composition of the raw material and related properties; that is, organic matter is leached out and feldspar can be partly attacked (or dissolved). This process causes increase in surface area and surface acidity and introduces permanent mesoporous [18; 24; 25; 26; 27] and also removes metal ions from the crystal interlayer, which partially delaminated the clay.  
The acid treatment strips octahedral ions such as Fe, Al, Mg and removes tetrahedral ions from the clay minerals due to the isomorphous substitution in the crystal lattices [28]. The extent to which these ions are removed depends on the relevant factors. Several factors may contribute when a clay undergoes an acid activation such as; the nature of acid, the mass percentage of acid, the temperature of activation reaction, contact time of solid-to-liquid, the stirring rate of mixture, the size of the solid particle and the liquid-to-solid mass ratio. Four parameters are known to strongly influence the structural properties and can affect significantly the required results: the temperature, the contact time, the solid-to-liquid ratio and mass percentage of acid. These parameters are the main factors that delimit the extent of acid-activation and thus, the induced physicochemical and structural changes [13; 14; 15]. As a consequence of these structural changes, acid-treatment can extend the fields of application of these porous solids such as in the control of atmospheric pollution [15]. This work is a continuation of numerous studies carried out in the field of surface and colloid science and dealing with clay minerals [15; 29; 30; 31; 32; 33; 34; 35]. Natural green clay mineral collected from south of Tunisia was treated with sulfuric acid in order to prepare suitable adsorbent for several organic compounds. The present work aims to prepare, characterize and investigate the effect of acid treatment factors on some physicochemical and bleaching properties of the raw material. Removal of cationic dye, methylene blue used as textile dye in industries, by the batch adsorption technique was studied.