**5.2 Study of Zn2+ adsorption isothermals on raw chitin**

Monitoring the amount of Zn2+ adsorbed as a function of the residual concentration (**Figure 8a**) shows that the adsorption phenomenon obeys Henry's law. For Ccra and Clan, isotherms are straight with high slopes. This time, Zn2+ is strongly retained by the Ccra than by the Ccre, the concentration decreases from 100 to 1.85 mg/l by adsorption on the Ccra, which proves a particular affinity of the Zn (II) to the Ccra. This affinity is stronger than that exhibited by mineral surfaces such as illite, montmorillonite, and kaolinite [2]. **Figure 8b** shows the calculation of the average removal percentages for six adsorption tests.

**Figure 7.** *Kinetic study of the Zn (II) adsorption on the raw chitin.*

#### **Figure 8.**

*(a) Experimental isotherms of Zn (II) adsorption on raw chitin (b) Effect of the origin of raw chitin on the elimination efficiency of Zn.*


#### **Table 6.**

*Values of Freundlich parameters deduced from Zn2+ adsorption isotherms on the raw chitin.*

Elimination percentages reach 98% for Ccra and 97% for Clan. These results are compared to those found for mineral surfaces such as Al2O3, zeolite, and albeit whose percentages of elimination do not go beyond 30% [10].

Linear adsorption transforms according to the Freundlich model—**Table 6** show that Zn has a great affinity for hard shells. The Freundlich model makes it possible to describe the adsorption of Zn: R<sup>2</sup> > 0.95. In this case, the adsorption is hydrophobic in nature through Van Der Waals interaction and hydrogen bonds.

The perfectly applicable Freundlich model shows that the adsorption capacity of Zn2+ on the Ccra is twice as large as that on the Ccre (**Table 6**).

According to the isotherms of **Figure 8a**, the percentage of adsorption decreases when the concentration of Zn2+ increases, the same remark was recorded by Spark and Bourg [17, 62] during the study of Zn adsorption on kaolinite and clay mineral.

## **6. Study of the lead adsorption**
