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7.2 Heavy Metal Sequestration and Remediation
by Prussian Blue Analogs
The peculiar structure of Prussian blue analogs combined with the presence of
metallic sites (at least two) makes them useful in a wide range of applications. In
this section, however, the utilization of Prussian blue analogs as adsorbent for the
heavy metal removal is highlighted. We address first the basis of the ion exchange
characteristics of the Prussian blue analogs, which is closely linked to the development of potentiometric sensors. Then, we introduce the notion of electrochemically
switched ion exchange as a friendly method for ion separation on the next subsection. The second subsection includes the cesium removal from wastewaters, the
remediation of the thallium intoxication followed eventually by an original application where metal hexacyanoferrates was utilized for rare-earth elements recovery.
7.2.1 Ion Exchange, Potentiometry,
and Voltammetric Response
Among the structural characteristics of the Prussian Blue analogs reported in the
Fig. 7.1, the presence of cavities and free space inside the cube offers the possibility
of accommodating several ions and molecules. Metal ions, for instance, can be shuttled inside and outside the structure, and thus their recovery is a strong potential
application. A key factor is represented by the metal dimension compared to the
available size of the cavities that may affect the selectivity and the activity of the
Prussian blue analogs toward different ions. A relevant study was performed by Tani
et al. (1998) who suggested that the effective dimension of the hydrated cation plays
a key role. Cations with small hydrated radius are reported to be allocated easily
into the Prussian blue like structure but the opposite holds true for large cations. In
other words, there is a selectivity order for the metal ions. The observed selectivity
coefficients for alkali metal ions of a copper hexacyanoferrate electrode were in the
order cesium ion > rubidium ion > potassium ion > sodium ion > lithium ion. This
order follows their dehydration energies, as the ion must be partially dehydrated for
the insertion into the given structural site. Interestingly, the highest selectivity coefficient was observed for cesium, highlighting the perspective of metal
hexacyanoferrate- based technology for its sequestration. Giorgetti et al. (2001)
observed the same selectivity order in the nickel derivatives.
Following these ion-exchange possibilities offered by the Prussian blue analogs,
the different abilities of the Prussian blue analogs were tested in real and complex
matrices, such as seawater. The figure of merit in this case is quantitatively represented by the Nernst partition law that describes the partition coefficient. A metal
7 Metal Hexacyanoferrate Absorbents for Heavy Metal Removal
7.2 Heavy Metal Sequestration and Remediation
by Prussian Blue Analogs
The peculiar structure of Prussian blue analogs combined with the presence of
metallic sites (at least two) makes them useful in a wide range of applications. In
this section, however, the utilization of Prussian blue analogs as adsorbent for the
heavy metal removal is highlighted. We address first the basis of the ion exchange
characteristics of the Prussian blue analogs, which is closely linked to the development of potentiometric sensors. Then, we introduce the notion of electrochemically
switched ion exchange as a friendly method for ion separation on the next subsection. The second subsection includes the cesium removal from wastewaters, the
remediation of the thallium intoxication followed eventually by an original application where metal hexacyanoferrates was utilized for rare-earth elements recovery.
7.2.1 Ion Exchange, Potentiometry,
and Voltammetric Response
Among the structural characteristics of the Prussian Blue analogs reported in the
Fig. 7.1, the presence of cavities and free space inside the cube offers the possibility
of accommodating several ions and molecules. Metal ions, for instance, can be shuttled inside and outside the structure, and thus their recovery is a strong potential
application. A key factor is represented by the metal dimension compared to the
available size of the cavities that may affect the selectivity and the activity of the
Prussian blue analogs toward different ions. A relevant study was performed by Tani
et al. (1998) who suggested that the effective dimension of the hydrated cation plays
a key role. Cations with small hydrated radius are reported to be allocated easily
into the Prussian blue like structure but the opposite holds true for large cations. In
other words, there is a selectivity order for the metal ions. The observed selectivity
coefficients for alkali metal ions of a copper hexacyanoferrate electrode were in the
order cesium ion > rubidium ion > potassium ion > sodium ion > lithium ion. This
order follows their dehydration energies, as the ion must be partially dehydrated for
the insertion into the given structural site. Interestingly, the highest selectivity coefficient was observed for cesium, highlighting the perspective of metal
hexacyanoferrate- based technology for its sequestration. Giorgetti et al. (2001)
observed the same selectivity order in the nickel derivatives.
Following these ion-exchange possibilities offered by the Prussian blue analogs,
the different abilities of the Prussian blue analogs were tested in real and complex
matrices, such as seawater. The figure of merit in this case is quantitatively represented by the Nernst partition law that describes the partition coefficient. A metal
7 Metal Hexacyanoferrate Absorbents for Heavy Metal Removal