Vegetal biomaterial can be viewed as a natural ion-exchange material that primarily contains weak acidic and basic groups on its surface. One of the common procedures to investigate whether ion-exchange is the mechanism responsible for metal sorption is to determine the concentration of alkaline and alkaline-earth metal ions or protons (when the sorbent is pretreated with acid) released from the sorbent to the solution after metal uptake. The determination of the concentration of ions released into the solution (M: Na+, K+, Ca2+, Mg2+, H+) allows the balance of the concentration of the absorbed toxic metal ion (M*), through a charge balance, not explicitly reported in equation (2).

R-M + M* 5 R-M* + M (2)

On the solid material the appearance of the sorbed metals, associated with the disappearance of alkaline and alkaline-earth metal ions, can be followed by Scanning Electron Microscopy (SEM) coupled with energy dispersive X-ray analysis (EDAX). This technique greatly contributes to indicate that ion exchange takes place between alkaline and alkaline-earth metal ions on the sorbent and the toxic metal ions in the solution.

1.2 Spectroscopic analysis

Useful information on the role of functional groups on metal sorption can be reached by non-destructive spectroscopic methods, observing the modifications induced by the metal on the spectra of the pure adsorbent.