water with various pollutants has been recognized as a global problem that needs
cost-effective and environmentally friendly solution(s). One of the ways to overcome the above-mentioned problem is to use ferrate(VI) that offers desirable chemical properties, such as extremely high redox potential (+2.2 V in acidic and + 0.7 V
in alkaline solution, respectively) and benign reaction product(s). Thanks to these
unique properties, ferrate(VI) is able to eliminate microorganisms, viruses, inorganic, and organic (both natural and synthetic) compounds (Jiang and Lloyd 2002;
Kubiňáková et al. 2017; Mackul’ak et al. 2016; Rai et al. 2018).
The most common oxidation states in which iron can be usually found are Fe(0),
Fe(II), and Fe(III). Other, unusually high oxidation states, e.g., +IV, +V, and +VI,
have also been observed [Fe
VI O 4 ]
2À . The first mention of ferrates(VI) dates back to
1702. Later on, in 1715, K 2 FeO 4 was prepared by oxidation of iron in molten KNO 3 .
During the smelting of iron ore with potassium carbonate and after the dissolution of
residues in alkaline solution, the violet coloration was observed. Some researchers
assumed that the violet color of this product was caused by the presence of Fe
VI
O 3
–II
compound (Mácová et al. 2009). In an aqueous environment, ferrates(VI) possess
dark violet color and the ion [Fe
VI O 4 ]
2À has tetrahedral structure (Jiang and Lloyd
2002).
A lot of new information has emerged in the literature over the past years relating
to the preparation of ferrate(VI), its application to the removal of various organic,
inorganic compounds, radionuclides, and the mechanism of ferrate action (Feng
et al. 2016; Híveš et al. 2016; Jiang and Lloyd 2002; Jiang 2014; Kralchevska et al.
2016a; Kubiňáková et al. 2015a, b; Mackul’ak et al. 2016; Rai et al. 2018; Sharma
et al. 2016a, b). In this chapter, we summarize the current knowledge of a ferrate
synthesis using various reaction routes, ferrate stability, and applicability to the
elimination of broad range of inorganic and organic compounds. The disinfection
properties of ferrates are also mentioned.
8.2 Synthesis of Ferrates(IV, V, VI)
There are several approaches to synthetizing potassium ferrate(VI) (K 2 FeO 4 ), potassium ferrate(V) (K 3 FeO 4 ), and sodium ferrate(IV) (Na 4 FeO 4 ) (Mura et al. 2017;
Sharma et al. 2015). Particularly wet chemical methods, electrochemical methods,
and thermal processes (Fig. 8.1) are used. When selecting a suitable method of
synthesis, it is necessary to consider its experimental difficulty and compromise
between purity and the amount of the as-prepared ferrate. Generally, wet chemical
synthesis and electrochemical synthesis result in a high-purity ferrate sample, but the
amounts of the synthesized material are relatively low. Therefore, such an approach
is not suitable for large-scale production, i.e., application of ferrates to water
treatment. On the other hand, thermal syntheses enable to prepare much higher
amounts of ferrates (up to kilograms per one synthetic cycle), but their purity is
relatively low (approx. 30–50 wt.% of K 2 FeO 4 in sample) mainly due to selfdecomposition of ferrates at high temperatures.
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L. Machala et al.
cost-effective and environmentally friendly solution(s). One of the ways to overcome the above-mentioned problem is to use ferrate(VI) that offers desirable chemical properties, such as extremely high redox potential (+2.2 V in acidic and + 0.7 V
in alkaline solution, respectively) and benign reaction product(s). Thanks to these
unique properties, ferrate(VI) is able to eliminate microorganisms, viruses, inorganic, and organic (both natural and synthetic) compounds (Jiang and Lloyd 2002;
Kubiňáková et al. 2017; Mackul’ak et al. 2016; Rai et al. 2018).
The most common oxidation states in which iron can be usually found are Fe(0),
Fe(II), and Fe(III). Other, unusually high oxidation states, e.g., +IV, +V, and +VI,
have also been observed [Fe
VI O 4 ]
2À . The first mention of ferrates(VI) dates back to
1702. Later on, in 1715, K 2 FeO 4 was prepared by oxidation of iron in molten KNO 3 .
During the smelting of iron ore with potassium carbonate and after the dissolution of
residues in alkaline solution, the violet coloration was observed. Some researchers
assumed that the violet color of this product was caused by the presence of Fe
VI
O 3
–II
compound (Mácová et al. 2009). In an aqueous environment, ferrates(VI) possess
dark violet color and the ion [Fe
VI O 4 ]
2À has tetrahedral structure (Jiang and Lloyd
2002).
A lot of new information has emerged in the literature over the past years relating
to the preparation of ferrate(VI), its application to the removal of various organic,
inorganic compounds, radionuclides, and the mechanism of ferrate action (Feng
et al. 2016; Híveš et al. 2016; Jiang and Lloyd 2002; Jiang 2014; Kralchevska et al.
2016a; Kubiňáková et al. 2015a, b; Mackul’ak et al. 2016; Rai et al. 2018; Sharma
et al. 2016a, b). In this chapter, we summarize the current knowledge of a ferrate
synthesis using various reaction routes, ferrate stability, and applicability to the
elimination of broad range of inorganic and organic compounds. The disinfection
properties of ferrates are also mentioned.
8.2 Synthesis of Ferrates(IV, V, VI)
There are several approaches to synthetizing potassium ferrate(VI) (K 2 FeO 4 ), potassium ferrate(V) (K 3 FeO 4 ), and sodium ferrate(IV) (Na 4 FeO 4 ) (Mura et al. 2017;
Sharma et al. 2015). Particularly wet chemical methods, electrochemical methods,
and thermal processes (Fig. 8.1) are used. When selecting a suitable method of
synthesis, it is necessary to consider its experimental difficulty and compromise
between purity and the amount of the as-prepared ferrate. Generally, wet chemical
synthesis and electrochemical synthesis result in a high-purity ferrate sample, but the
amounts of the synthesized material are relatively low. Therefore, such an approach
is not suitable for large-scale production, i.e., application of ferrates to water
treatment. On the other hand, thermal syntheses enable to prepare much higher
amounts of ferrates (up to kilograms per one synthetic cycle), but their purity is
relatively low (approx. 30–50 wt.% of K 2 FeO 4 in sample) mainly due to selfdecomposition of ferrates at high temperatures.
178
L. Machala et al.