ferrite (KFeO 2 ) and potassium oxides. After it is cooled down to room temperature,
KFeO 2 becomes unstable and reacts with air H 2 O and CO 2 with the formation of
Fe 2 O 3 nanoparticles and KHCO 3 carbonate (Machala et al. 2015a). Conventional
experimental techniques confirmed the presence of neither Fe(V) nor Fe
(IV) intermediates during the thermal decomposition of ferrates. A later study
(Machala et al. 2015b) employing nuclear forward scattering of the synchrotron
radiation proved the presence of several percent of Fe(IV) and Fe(V) during the
decomposition of potassium ferrate(VI).
From the point of view of storage and handling of ferrates, the issue of stability of
potassium ferrate(VI) in air at room temperature is crucial as well. In the study of
Machala et al. (2009), the kinetics of solid-state transformation (aging) of potassium
ferrate(VI) under various air-humidity conditions (55–95% relative humidity) at
room temperature were studied by in situ
57
Fe Mössbauer spectroscopy. The kinetic
data showed a significant increase in the decomposition rate with increasing air
humidity (Fig. 8.4). Crystals of KHCO 3 and amorphous Fe(OH) 3 nanoparticles were
identified as the transformation products. The decomposition kinetics was very
unusual with two almost linear decay steps in case of lower humidity levels
(55–70%). The first slow decay was probably observed due to the formation of the
narrow compact layer of nanoparticulate Fe(OH) 3 reaction product. This layer limits
the access of both H 2 O and CO 2 participating in the reaction as the gaseous reactants.
The second decay with a much faster rate showed a nearly positive linear relationship with the humidity.
8.5 Stability of Ferrates in Aqueous Solution
For applications of ferrates to water treatment, an aqueous solution of dissolved
ferrate at a certain concentration should be usually prepared in advance. Therefore, it
is necessary to know the kinetics and mechanism of ferrates’ self-decay in water
under different conditions. Stability of ferrates in water depends on many factors
such as ferrate concentrations, pH, temperature, or presence of inorganic ions.
Generally, ferrates tend to reduce to Fe(III) and/or charge disproportionation takes
place. Concerning ferrate(VI), after it is dissolved in water, 5–10% of the ferrate are
decomposed to Fe(III) immediately (during 1–2 s). From the literature (Lee et al.
2014), it is known that ferrate(VI) undergoes a dimerization process with Fe(IV) as
an intermediate state; however, this reaction is probably too fast to identify Fe(IV) by
conventional experimental techniques. The reduction of ferrate(VI) is accompanied
by evolution of oxygen and increasing of pH up to 13. During the several-minutelong ferrate(VI) transformation in water, the pH value decreases slowly to approx.
10 and the content of ferrate(VI) slightly decreases by approx. 3%. Just before its
application to water treatment, it is necessary to decrease pH in order to have almost
neutral conditions along with enhancing the reactivity of the ferrate. The transformation of potassium ferrate(VI) in water can be described by chemical equation
8 Ferrates as Powerful Oxidants in Water Treatment Technologies
183
KFeO 2 becomes unstable and reacts with air H 2 O and CO 2 with the formation of
Fe 2 O 3 nanoparticles and KHCO 3 carbonate (Machala et al. 2015a). Conventional
experimental techniques confirmed the presence of neither Fe(V) nor Fe
(IV) intermediates during the thermal decomposition of ferrates. A later study
(Machala et al. 2015b) employing nuclear forward scattering of the synchrotron
radiation proved the presence of several percent of Fe(IV) and Fe(V) during the
decomposition of potassium ferrate(VI).
From the point of view of storage and handling of ferrates, the issue of stability of
potassium ferrate(VI) in air at room temperature is crucial as well. In the study of
Machala et al. (2009), the kinetics of solid-state transformation (aging) of potassium
ferrate(VI) under various air-humidity conditions (55–95% relative humidity) at
room temperature were studied by in situ
57
Fe Mössbauer spectroscopy. The kinetic
data showed a significant increase in the decomposition rate with increasing air
humidity (Fig. 8.4). Crystals of KHCO 3 and amorphous Fe(OH) 3 nanoparticles were
identified as the transformation products. The decomposition kinetics was very
unusual with two almost linear decay steps in case of lower humidity levels
(55–70%). The first slow decay was probably observed due to the formation of the
narrow compact layer of nanoparticulate Fe(OH) 3 reaction product. This layer limits
the access of both H 2 O and CO 2 participating in the reaction as the gaseous reactants.
The second decay with a much faster rate showed a nearly positive linear relationship with the humidity.
8.5 Stability of Ferrates in Aqueous Solution
For applications of ferrates to water treatment, an aqueous solution of dissolved
ferrate at a certain concentration should be usually prepared in advance. Therefore, it
is necessary to know the kinetics and mechanism of ferrates’ self-decay in water
under different conditions. Stability of ferrates in water depends on many factors
such as ferrate concentrations, pH, temperature, or presence of inorganic ions.
Generally, ferrates tend to reduce to Fe(III) and/or charge disproportionation takes
place. Concerning ferrate(VI), after it is dissolved in water, 5–10% of the ferrate are
decomposed to Fe(III) immediately (during 1–2 s). From the literature (Lee et al.
2014), it is known that ferrate(VI) undergoes a dimerization process with Fe(IV) as
an intermediate state; however, this reaction is probably too fast to identify Fe(IV) by
conventional experimental techniques. The reduction of ferrate(VI) is accompanied
by evolution of oxygen and increasing of pH up to 13. During the several-minutelong ferrate(VI) transformation in water, the pH value decreases slowly to approx.
10 and the content of ferrate(VI) slightly decreases by approx. 3%. Just before its
application to water treatment, it is necessary to decrease pH in order to have almost
neutral conditions along with enhancing the reactivity of the ferrate. The transformation of potassium ferrate(VI) in water can be described by chemical equation
8 Ferrates as Powerful Oxidants in Water Treatment Technologies
183