Immobilization of Metal Hexacyanoferrate Ion-Exchangers for the Synthesis of Metal Ion Sorbents--A Mini-Review

Abstract

Metal hexacyanoferrates are very efficient sorbents for the recovery of alkali and base metal ions (including radionuclides such as Cs). Generally produced by the direct reaction of metal salts with potassium hexacyanoferrate (the precursors), they are characterized by ion-exchange and structural properties that make then particularly selective for Cs(I), Rb(I) and Tl(I) recovery (based on their hydrated ionic radius consistent with the size of the ion-exchanger cage), though they can bind also base metals. The major drawback of these materials is associated to their nanometer or micrometer size that makes them difficult to recover in large-size continuous systems. For this reason many techniques have been designed for immobilizing these ion-exchangers in suitable matrices that can be organic (mainly polymers and biopolymers) or inorganic (mineral supports), carbon-based matrices. This immobilization may proceed by in situ synthesis or by entrapment/encapsulation. This mini-review reports some examples of hybrid materials synthesized for the immobilization of metal hexacyanoferrate, the different conditionings of these composite materials and, briefly, the parameters to take into account for their optimal design and facilitated use.

Keywords: biopolymers; composite materials; encapsulation; in situ synthesis; ion-exchanger; metal hexacyanoferrate; polymers; porous mineral supports.

Figure 1

“In-drop” synthesis of Prussian Blue.

Figure 2

TEM photographs of bulk particles of metal hexacyanoferrates (reprinted with permission from Royal Society of Chemistry [18]).

Figure 3

SEM-EDX analyses of cross-sections of hybrid Prussian Blue/alginate capsules before and after Tl(I) sorption (reprinted with permission from Elsevier [164]).

Figure 4

Effect of increasing concentration of salts (of alkali and alkaline-earth metal salts) on Tl(I) sorption capacity (reprinted with permission from Elsevier, [164]).

Figure 5

Schematic route for the synthesis of chitin sponge composites.

Figure 6

Textural properties of chitin sponges: effect of chitosan concentration and freezing temperature (reprinted with permission from Elsevier [153].

Figure 7

SEM analysis of cross-section of hybrid metal hexacyanoferrate/chitin beads (reprinted with permission from Royal Society of Chemistry [18]).