# Uses of Nanoclays and Adsorbents for Dye Recovery: A Textile Industry Review
## 2 Sewage Treatment
### With Nanotechnologies
#### 2.2.2 Montmorillonite (MMT)  
Montmorillonite (MMT) (Figure 2) is the most abundant mineral in the smectites group. The chemical composition for montmorillonite is 80% SiO\({}_{2}\), 13% Al\({}_{2}\)O\({}_{3}\), and 3% Fe\({}_{2}\)O\({}_{3}\)[111]. It is a naturally abundant and cheap clay that offers efficient adsorbance. Its structure [112] is formed by two layers: a tetrahedric silicon one and an octahedric aluminum oxide one, which create a 2:1 diachetrahedral layer characterized by having a wide dehydroxylation temperature range of 500-700 \({}^{\circ}\)C [79; 113; 114; 115; 116; 117]. MMT particle size can range between 2 \(\upmu\)m and 0.1 \(\upmu\)m, with a mean of around 0.5 \(\upmu\)m, although its particle size can be smaller when exfoliated in solvent. Particles are groups of not genuine crystal silicate laminate and their specific surface area (SSA) is approximately 31.26-57.19 m\({}^{2}\cdot\)g\({}^{-1}\)[118]. MMT possesses permanent loads thanks to the isomorphic substitutions of silicate layers, but with no homogeneous distributions [119].  
The porosity and ion adsorption of MMT are both well-studied characteristics. The purpose of many research works has been to achieve wider basal spacing [120]. Thanks to MMT's cation exchange capacity (CEC), whose values are approximately 90-145 cmol\(\cdot\)kg\({}^{-1}\)[121], scientists have improved MMT's adsorption and hydration capacities. Moreover, cation exchange alters clay porosity. All this makes MMT a potential element to be used as a purifier and filter. X-ray diffraction techniques have been applied to establish a direct  
Figure 2: Montmorillonite (MMT) SEM.  
relation between basal spacing and adsorption capacity, and its pore size [122]. Other studies have also verified that basal spacing is affected by both load type and the position of clays. Those with loads on tetrahedric layers have narrower basal spacing, while those with loads on octahedric layers possess wider basal spacing [123].  
It is worth pointing out that isomorphic substitutions of trivalent aluminum for divalent magnesium or iron (II) leave clay crystals with a negative charge, which has to be compensated with surface cations. Clay's own nature determines its swelling and ion exchange capacities in adsorption phases. Much interest has been shown to this exchange control in recent decades [124]. With MMT, this control takes place by variation in pH, and by the interlaminar space where ion exchange occurs.  
In the nanoclay basal space, three different scenarios exist: when the basal space is minimum and the adsorption between the laminar nanoclay is minimum or null; when the basal space widens; and when nanoclay is totally exfoliated. This leads to a drawback: that of needing intermediate steps to improve MMT's adsorption capacity by taking into account large size-formed molecular dyes [112].  
How pH affects the quantity of methylene blue dye, which clay can adsorb, has been studied. Indeed, MMT treated with acid interchanges leuko-derived dyes, and the stability of that obtained is greater when dye is placed between nanoclay layers. Some authors have concluded that treating clay with acid can allow more adsorbed dye [114; 115; 116; 117; 125]. In another work, [PERSON] and [PERSON] prepared MMT by carbon hybridization to adsorb Congo Red dye [37]. MMT's hydrated carbon nanolayers were processed by saccharose calcining that, once formed, was able to adsorb 84% of the dye.  
Studies have also been found that have demonstrated how the bath's ionic strength increases or decreases nanoclay adsorption capacity. Organic and inorganic salts influence the dispersion of nanoclay properties by conferring stable dispersion, coagulation, or even gel generation. As explained above, the basal space is significant for achieving the adsorption phenomenon [126; 127; 128].