175
7.1.3 Metal Hexacyanoferrates or Prussian Blue Analogs
An extensive research was conducted based on Prussian blue, and therefore a wide
variety of Prussian blue analog compounds, fabricated by simply substitution of one
of the transition metals, were reported. Due to the strong Fe-C ligation, the easiest
structural modification concerns the N-coordinated site of the CN group; therefore
its replacement leads to the general class of metal hexacyanoferrate materials. More
generally, including the possibility of substituting also the C-end site, the class of
materials called Prussian blue analogs or metal hexacyanometallates would be
obtained, with general formula Z x M y [B(CN) 6 ]∙mH 2 O. In the formula here reported,
M and B are transition metals, while x and y stoichiometric coefficients, and Z is an
alkali ion. The atom B is iron for metal hexacyanoferrates. Structurally speaking the
metal hexacyanometallates feature the same extended and periodic structure of the
Prussian blue, i.e., the metal ions are placed in the corners of the cube structure,
alternatingly, and cyanide anions act as links between the metals (forming a -B-CN-M- linear fragment) along the three directions of space. Generally, structures
with different concentration of M(CN) 6
nion vacancies are possible, and thus both
images of the Fig. 7.1 still apply for the Prussian blue analogs. The space available
as large empty cavities (sometimes called zeolitic cavities) can be occupied by
alkali ions, and this is the motivation of their use as sorbents for metals in environmental pollution remediations.
Not all the Prussian blue analogs adopt the cubic structure, but even in the hexagonal lattice (Rodríguez-Hernández et al. 2007), the presence of open channels
and cavities ensures the ion-exchange characteristics. Eventually, the -B–CN–M–
NC–B– structural moiety can be tuned to define the size and the structure of metal
hexacyanoferrates, therefore modulating a series of physicochemical properties
described below.
7.1.4 Application of Prussian Blue Analogs, Uses,
and Characteristics
The peculiar structure of Prussian blue and Prussian blue analogs coupled with the
easy and scalable synthesis procedure has foster an intense research toward multiple
applications. The synthesis of the Prussian blue analogs is generally performed in
aqueous media by coprecipitation, and controlled temperature (Mullaliu et al.
2018a, b). The obtained precipitate can be easily recovered by filtration. Alternatively,
the material can be fabricated by electrochemical routes, in which a conductive
substrate of large surface area can be enriched by several suitable functional groups
(Mortimer and Rosseinsky 1983; Guadagnini et al. 2010). This device is well usable
as electrochemical sensor for a wide variety of analytes.
Neff (1978) first deposited thin films of Prussian blue by electrochemical means,
observing an electrochromic behavior of the material: a color change when the
7 Metal Hexacyanoferrate Absorbents for Heavy Metal Removal
7.1.3 Metal Hexacyanoferrates or Prussian Blue Analogs
An extensive research was conducted based on Prussian blue, and therefore a wide
variety of Prussian blue analog compounds, fabricated by simply substitution of one
of the transition metals, were reported. Due to the strong Fe-C ligation, the easiest
structural modification concerns the N-coordinated site of the CN group; therefore
its replacement leads to the general class of metal hexacyanoferrate materials. More
generally, including the possibility of substituting also the C-end site, the class of
materials called Prussian blue analogs or metal hexacyanometallates would be
obtained, with general formula Z x M y [B(CN) 6 ]∙mH 2 O. In the formula here reported,
M and B are transition metals, while x and y stoichiometric coefficients, and Z is an
alkali ion. The atom B is iron for metal hexacyanoferrates. Structurally speaking the
metal hexacyanometallates feature the same extended and periodic structure of the
Prussian blue, i.e., the metal ions are placed in the corners of the cube structure,
alternatingly, and cyanide anions act as links between the metals (forming a -B-CN-M- linear fragment) along the three directions of space. Generally, structures
with different concentration of M(CN) 6
nion vacancies are possible, and thus both
images of the Fig. 7.1 still apply for the Prussian blue analogs. The space available
as large empty cavities (sometimes called zeolitic cavities) can be occupied by
alkali ions, and this is the motivation of their use as sorbents for metals in environmental pollution remediations.
Not all the Prussian blue analogs adopt the cubic structure, but even in the hexagonal lattice (Rodríguez-Hernández et al. 2007), the presence of open channels
and cavities ensures the ion-exchange characteristics. Eventually, the -B–CN–M–
NC–B– structural moiety can be tuned to define the size and the structure of metal
hexacyanoferrates, therefore modulating a series of physicochemical properties
described below.
7.1.4 Application of Prussian Blue Analogs, Uses,
and Characteristics
The peculiar structure of Prussian blue and Prussian blue analogs coupled with the
easy and scalable synthesis procedure has foster an intense research toward multiple
applications. The synthesis of the Prussian blue analogs is generally performed in
aqueous media by coprecipitation, and controlled temperature (Mullaliu et al.
2018a, b). The obtained precipitate can be easily recovered by filtration. Alternatively,
the material can be fabricated by electrochemical routes, in which a conductive
substrate of large surface area can be enriched by several suitable functional groups
(Mortimer and Rosseinsky 1983; Guadagnini et al. 2010). This device is well usable
as electrochemical sensor for a wide variety of analytes.
Neff (1978) first deposited thin films of Prussian blue by electrochemical means,
observing an electrochromic behavior of the material: a color change when the
7 Metal Hexacyanoferrate Absorbents for Heavy Metal Removal