Recovery of Metal Ions by Chitosan: Sorption Mechanisms and Influence of Metal Speciation

Chitosan is characterized by a high affinity for metal ions due to its high content of amine groups. The sorption mechanism depends on both the protonation of these amine groups and the speciation of metal ions. Metal cations may be adsorbed at pH close to neutrality by chelation mechanism while metal anions can be adsorbed in acidic solutions through ionic interactions with protonated amine groups. Several examples are considered. The first example focuses on Cd sorption, which proceeds by a chelation mechanism on free non‐protonated amine groups in neutral media. In acidic solutions the protonation of amine groups limits the ability of amine groups to complex Cd. The cross‐linking of chitosan with glutaraldehyde also results in a dramatic decrease of sorption properties due to the decrease in the density of complexation sites available for sorption. The sorption of vanadium(V) and molybdenum(VI) illustrates the high capacity of chitosan for the sorption of oxo‐anions. They are very efficiently sorbed in acidic solutions by ionic interactions. The correlation of sorption capacities with the distribution of metal species shows that the sorbent has a greater affinity for highly charged anionic species. The sorption of complex anionic species such as chloro‐complexes of Pd and Pt; and that of copper complexed with organic ligands have also been studied. The optimum conditions for sorption are obtained when anionic complexes predominate in the solution. The chemical modification of chitosan, obtained by grafting of sulfur compounds, allows modifying the sorption mechanism: the ion‐exchange polymer is transformed to a dual ion‐exchange and chelating polymer.

Copper sorption isotherm in presence of sodium citrate (0.004 M ) (? RNH: fraction of protonated amine groups; ACuC: total fraction of anionic copper complexes; Cu‐FAL: total fraction of copper‐free anionic ligands).