Metal cations can be adsorbed by chelation on amine groups of chitosan in near neutral solutions. In the case of metal anions, the sorption proceeds by electrostatic attraction on protonated amine groups in acidic solutions. However, the presence of ligands and the pH strongly control sorption performance (sorption isotherm) and the uptake mechanism (changing the speciation of the metal may result in turning the chelation mechanism into the electrostatic attraction mechanism). Several examples are discussed with precious metals (Pd, Pt), oxo-anions (Mo, V) and heavy metals (Cu, Ag). Sorption performance (equilibrium uptake but also kinetics) is also strictly controlled by other structural parameters of the polymer (degree of deacetylation, crystallinity for example) that control swelling and diffusion properties of chitosan. The identification of the limiting steps of the sorption process helps in designing new derivatives of chitosan. Diffusion properties may be improved by physical modification of chitosan (manufacturing gel beads, decreasing crystallinity). Selectivity can be enhanced by chemical modification (grafting, for example, sulfur compounds). Several examples are discussed to demonstrate the versatility of the material. This versatility allows the polymer to be used under different forms (from water soluble form, to solid form, gels, fibers, hollow fibers ...) for polymer-enhanced ultrafiltration and sorption processes. These interactions of metal ions with chitosan can be used for the decontamination of effluents, for the recovery of valuable metals but also for the development of new materials or new processes involving metal-loaded chitosan. Several examples are cited in the design of new sorbing materials, the development of chitosan-supported catalysts, the manufacturing of new materials for opto-electronic applications or agriculture (plant disease treatment ...).
Chitosan is a well-known biopolymer, whose high nitrogen content
confers remarkable ability
for the sorption of metal ions from dilute effluents. However, its
sorption performance in both
equilibrium and kinetic terms is controlled by diffusion processes.
Gel bead formation allows
an expansion of the polymer network, which improves access to the
internal sorption sites and
enhances diffusion mechanisms. Molybdate and vanadate recovery
using glutaraldehyde cross-linked chitosan beads reaches uptake capacities as high as 7−8 mmol
g-1, depending on the
pH. The optimum pH (3−3.5) corresponded to the predominance
range of hydrolyzed polynuclear
metal forms and optimum electrostatic attraction. While for beads,
particle size does not
influence equilibrium, for flakes, increasing sorbent radius
significantly decreases uptake
capacities to 1.5 mmol g-1. Sorption
kinetics are mainly controlled by intraparticle diffusion
for beads, while for flakes the controlling mechanisms are both
external and intraparticle
diffusions. The gel conditioning increases the intraparticle
diffusivity by 3 orders of magnitude: intraparticle diffusivities range between
10-13 and 10-10
m2 min-1, depending on the
sorbent
size and the conditioning.
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