# Uses of Nanoclays and Adsorbents for Dye Recovery: A Textile Industry Review
## 2 Sewage Treatment
### With Nanotechnologies
#### 2.2.3 Zeolite  
Natural zeolite is a mineral widely used to treat wastewater. For the dehydrated zeolite, its chemical formula is [Al\({}_{12}\)Si\({}_{12}\)O\({}_{48}\)]12-[129]. Its different specific characteristics (e.g., porosity, surface area, permeability, CEC) give it a very particular behavior [109; 130; 131]. Its high specific surface areas is around 200-860 m\({}^{2}\) g\({}^{-1}\)[132]. Having processed natural zeolite, its particle size lies between 1 mm and 12 mm. Its density is approximately double that of water, while its specific surface vastly varies and can be substantially increased by means of modifications [133]. For example, if the aim is to increase its surface, clay can be treated with nitric acid, which removes part of the aluminum from its structure to make it less crystalline and, hence, the specific surface increases [134] (Figure 3).  
When analyzing its electric charge, both the total electric charge and the amount of cations in the zeolite skeleton remain quite stable in aqueous solution during the proton exchange process. Its CEC is around 563 cmol\(\cdot\)kg\({}^{-1}\)[135]. However, zeolite possesses excellent absorption capacity because the number of exchanged ions during this process differs, and the electric field in the crystal changes [136]. Zeolite's characteristic channels and pores favor ion transport by promoting their transfer and capacity to adsorption [109; 137].  
The main form by which zeolites are able to clean wastewater is by filtration and adsorption. When water pollutants pass through zeolite, they are captured, while water molecules pass through the pores [138]. Those pollutants trapped in zeolite in the previous phase are decomposed of microorganisms. After their decomposition, several plants adsorb them. In this way, the substratum is completely released from any previous impurity and can once again perform its function [139].  
The zeolites used are either natural zeolites, which previous paragraphs have mentioned, or they are synthetic zeolites. When analyzing the size of component crystals, it is stressed that natural zeolites have a non-uniform crystal size because they contain other impurities, while synthetic zeolites are extremely pure, and their crystals are uniform in size. Zeolites also form differently because natural zeolites can take days or years to form, while they can be synthesized in a laboratory in just a few hours or days; their crystal size is extremely uniform, and their pore size can also be controlled by designing them according to given required specifications [140].  
Synthetic zeolites can be obtained and used as artificial and natural elements. However, this does not imply that, from a cost perspective, any element is suitable for synthetization. The most widely used methods are hydrothermal [141], alkali-leaching [142], or sol-gel [143]. Each method is employed according to the desired zeolite type to be obtained. Evidently, it is necessary to bear in mind each method's limitations and advantages. In addition, certain limitations with these nanoclays should be considered, such as their non-reverse adsorption and secondary elements' steric blockage. Their porosity characteristics make high-molecule adsorption difficult [144].  
The authors in [145] demonstrated that zeolite in basic or neutral medium is able to adsorb up to 99% Ni (II), which is common in pigments. In [146], the authors bound zeolite to TiO\({}_{2}\) to adsorb up to 99% Cd (II) in any pH medium. Figure 4 shows the process of adsorption of dye by zeolite.