## 1 Introduction  
Industries such as dyes, textiles, paper, pharmaceuticals, and plastics generate a considerable amount of wastewater. Dyes are the primary contaminants recognized in wastewater due to their color. Approximately 15% of the total world production of dyes is lost during the dyeing process. Consequently, a large quantity of dyes is released into textile effluents [1]. Removing these contaminants from industrial wastewaters before effluents are discharged into the aquatic environment is obviously necessary. Various methods, such as biological processes [2], ultrasound [3], active carbon adsorption [4], ozone treatment [5], coagulation/ flocculation [6], and ion exchange resin [7, 8] and ion exchange membrane adsorption methods [9], have been investigated in an attempt to remediate dye-containing wastewaters. However, these methods are not always efficient and economical enough,so new alternative technologies that rely on low-cost materials are mandatory to solve the issues related to dye-containing wastewaters [10].  
In this sense, natural clays have been modified to prepare several types of materials, including nanotubes [11], metakaolins [12], and zeolites [12]. The final material depends on the parent clay and on the treatment to which the clay is submitted. Zeolites obtained from kaolin are highlighted as advantageous, inexpensive materials for this purpose. Zeolites can be prepared by treating kaolin with alkaline compounds. The reactivity of the clay increases if it is calcined at medium temperatures (ca. 600 \({}^{\circ}\)C), to form metakaolins in the intermediate step [13].  
Numerous researchers have revisited the presence of transition metal cations within the zeolite structure in recent years because these metal cations impart new properties to the solid, making them applicable for catalytic purposes or for environmental remediation through adsorption of various contaminants. For instance, Fe(III) cations, which very often exist as impurities in many zeolites, have been introduced into synthetic zeolites structurally or via cation exchange [14]. The presence of transition metal cations within the structure of a solid matrix can alter the crystallinity of the solid and even hinder clay conversion to zeolites in some cases [15].  
Zeolites synthesized from several natural sources, such as clays and fly ash, are very important porous and selective adsorbents and display high ion exchange capacities. Table 1 lists some literature results on the preparation methods and on the conventional uses of zeolites.  
Kaolin, which is very abundant in nature, is composed of kaolinite, a TO (tetrahedral-octahedral) clay mineral consisting of aluminum and silicon oxides; it also contains some iron, titanium, or manganese impurities as isomorphous substituents in both the octahedral and the tetrahedral layers or as extra framework phases, depending on where the clay originates. However, kaolin does not present good adsorption capacity due to its low cationic exchange capacity (CEC) and specific surface area (SSA) as compared to TOT (tetrahedral-octahedral-tetrahedral) clay minerals and other synthetic materials. Therefore, the synthesis of zeolites from clays such as kaolin is advantageous because zeolites have higher adsorption capacity [16; 17]. In this context, we aimed to synthesize zeolites starting from two natural clay fractions as precursors, white kaolin and red kaolin (with higher iron content), and to investigate the potential of the resulting zeolites to adsorb the cationic dyes depicted in Table 2.