**3.1 Kinetic study of the Cd2+ adsorption**

To determine the time required to reach the Cd2+ adsorption equilibrium, we monitored the change in Cd2+ concentration as a function of time. The results are shown in **Figure 3**.

This study shows that equilibrium is established rapidly for all adsorbents (**Figure 3**). It reaches its maximum for 30 and 20 minutes, respectively for Ccra and Ccre, while the adsorption kinetics of Cd2+ on Clan is relatively slow.

From these curves, it is also found that 50% of Cd2+ is adsorbed on the Ccre before the equilibrium reached 20 minutes. The equilibrium times (teq) are

**Figure 3.** *Kinetic study of Cd (II) adsorption on raw chitin.*

classified in the following order: teq Ccre < teq Ccra <teq Clan. This kinetics is very fast in comparison with other works cited in the literature concerning the adsorption of Cd2+ on mineral surfaces, which require a very large equilibrium time. For example, for the adsorption of Cd2+ on the illite, the equilibrium is reached after 54 days [1] and in the case of a natural clay [55, 56], the equilibrium is reached after 50 days. Melchor [39] having studied the adsorption of Cd2+ on chitin, mentions a time of equilibrium (40 minutes) comparable to our result.
