**5.1 Kinetic study of the Zn2+ adsorption**

As with other metals, the kinetic study shows that equilibrium is established rapidly. According to the curves of **Figure 7**, the contact times Ccre-Zn2+, Ccra-Zn2+, and Clan-Zn2+ required to reach equilibrium are around 40, 60, and 50 minutes, respectively. The times are very low in comparison with those necessary to reach equilibrium on mineral surfaces such as on the hydroxide of Fe (III), which requires 48 hours [8] and the goethite, which requires 42 days. Indeed, Balistrieri [58] having studied the adsorption of Zn2+ and Cd2+ on goethite, found that initially, there is a fast adsorption kinetics (a few hours) followed by a very slow diffusion controlling the penetration of the metal.

The tendency of surfaces to fix Zn2+ ions decreases with an increase in ion surface charge, since Zn2+ binds rapidly early with high affinity to the surface of the materials, and then slowly to with less activity. In addition, the increase in surface charge may induce some electrostatic repulsion. This effect on surface coverage is less pronounced with Cd2+ and Cu2+. The same remarks were recorded for the kinetics of Cu2+ adsorption on algae [60].
