# A Review of Adsorbents for Heavy Metal Decontamination: Growing Approach to Wastewater Treatment
## 6 Factors Affecting the Adsorption Process
### Types of Adsorbent
#### 6.6.6 Zeolites  
Zeolites are obtained from nature through silicate minerals and can be prepared by artificial methods. Zeolites are able to adsorb different sizes, shapes, and polarity adsorbates. Hence, zeolite is also known as a molecular sieve. Zeolites (clinoptiollite) showed strong attraction for lead and other toxic metals [18]. Zeolites were considered better adsorbents because of their ion exchangeable groups. Zeolites are three-dimensional aluminosilicates of potassium, calcium, and sodium arranged in a tetrahedral structure where aluminum and silicon are surrounded by oxygen atoms. These are basically used in the ion exchange process as well as adsorbents to adsorb different metals and impurities. [PERSON] showed that the total adsorption value for zeolites is approximately 155.4 mg of lead per gram of zeolite [113]. [PERSON] et al. reported zeolites that were obtained by the reactions of organic cations ethylenediylaminonium (EHDDMA) and cetylpyridinium (CETYL) for the removal of metal ions. The obtained zeolite contained positively charged ions which were used to bind chromate ions as well as in the anion exchange process. Results demonstrated that cetylpyridinium modified zeolite executes better performance than ethyl hexadecyl dimethyl ammonium. Extraction values were found to be 0.65 mg of chromium per gram of zeolite with cetylpyridinium and 0.42 mg of chromium per gram of zeolite with ethyl hexadecyl dimethyl ammonium group. Hence, a clinoptiollite type of material and zeolite appeared to be powerful and economical for the extraction of toxic metals [146]. [PERSON] et al. synthesised zeolites by the use of coal fly ash for the extraction of heavy metals like lead, cadmium, copper, nickel, and manganese from the aquatic system. Zeolite dose 1.0 g/L was used for extraction of single metal as well as multiple heavy metals having starting concentration of 100 ppm. The kinetics of the adsorption was also studied by mixing the zeolite in single-heavy metal and multi-heavy metal systems with the initial concentration of 100 mg/L. It was found that with an increase in adsorbent dose, removal efficiency gets increased. The results obtained from the experiments were in agreement with the Langmuir adsorption isotherm and the highest sorption value obtained for different metal ions Pb (II), Cu (II), Cd (II), Ni (II), and Mn (II) were 65.75, 56.06, 52.12, 34.40, and 30.89 mg/g and 45.28, 32.86, 26.93, 16.25, 14.63 mg/gfor single and multi-metal ions. Sorption of different metals on the adsorbent followed the trend Pb (II) > Cu (II) > Cd (II) > Ni (II) > Mn (II) [147]. Natural zeolites were synthesised by [PERSON] et al., for the extraction of Co (II), Cu (II), Zn (II), and Mn (II) from wastewater. The batch process was used for the removal of metal solutions having a concentration range from 100-400 ppm and the dose of adsorbent used was 10 g for 500 mL metal ions solutions. The adsorption capacity of different metal ions on the adsorbent was decreased with an increase in the concentration of metal ions solutions. In zeolites, both adsorptions, as well as ion-exchange phenomena were used for the extraction. The order of the exchange of metals was found in the following sequence Co (II) > Cu (II) > Zn (II) > Mn (II). The highest exchange value was found to be Cu (II)-66.10%, Co (II)-77.96%, Zn (II)-45.96%, and Mn (II)-9.84% [148]. [PERSON] et al. reported the use of synthetic mineral adsorbent for the removal of Cd (II) and Pb (II) ions from aqueous solutions by an ion-exchange method. They had shown that the adsorption process was pH-dependent and was found to follow the pseudo-second-order kinetic model and Freundlich isotherm model. They had explained the mechanism of Cd (II) and Pb (II) ions adsorption with the help of a schematic diagram Figure 7 [149].