The sorption of metal ions by biomass occurs via functional groups on its surface by one or more mechanisms. All the sorbents derived from different by-products of agriculture share a common network of lignin and cellulose, and differ for the presence of functional groups which characterize each single biomass. As said before, identification of the functional groups is crucial for understanding the mechanism that governs the sorption process. Indeed, each functional group presents its own coordinating abilities toward the different metal ions. These coordinating abilities can be rationalized in term of the hard/soft character both of the binding group and of the metal ion. In order to highlight the importance of each different binding group in the mechanism of metal ion adsorption, the percent incidence drawn out from 1997 to nowadays literature is presented in Fig. 1.

Fig. 1. Incidence of the different binding groups on biomass surface involved in metal ion complexation.

Potentiometric titrations, chemical treatments of the sorbent, alkaline and alkaline-earth metal ion release and spectroscopic techniques are the procedures widely followed to reveal the binding groups. A brief survey of these methods is presented in the next sections.