The formation of Fe(VI) during the transformation of ferrate(V) in water has an
important application aspect. As it has been already mentioned, potassium ferrate
(V) can be synthesized by the solid-state method on a large scale (contrary to ferrate
(VI)). But after ferrate(V) is dissolved in water, the final oxidizing agent is Fe(VI).
Concerning sodium ferrate(IV), after it has been dissolved in water, immediate
transformation to Fe(III) takes place. The process is accompanied by rapid increase
of pH up to 13 and evolution of oxygen. The corresponding chemical equation
describing the transformation can be written as
Na 4 FeO 4 þ 3=2H 2 O ! NaFeO 2 þ 3NaOH þ 1=4O 2
ð8:6Þ
It is worth mentioning that due to this rapid decay, the applicability of sodium
ferrate(IV) to water treatment is very limited. This was confirmed, e.g., in the study
by Machalová Šišková et al. (2016), where very low efficiency in the degradation of
estrogenic hormones was observed in case of sodium ferrate(IV), in comparison with
potassium ferrates(V) or (VI).
8.6 Effect of Buffering Inorganic Ions on Stability
of Ferrates
The study by Kolář et al. (2016) provided a systematic kinetics investigation of the
decay of ferrate(VI) in the presence of inorganic buffering ions (borate, phosphate,
and carbonate) at a pH range from 6.0 to 9.0. Detailed kinetic analysis revealed that
carbonate anion enhanced the Fe(VI) transformation rate, compared to phosphate
and borate ions. The order of the Fe(VI) decay rate in neutral solution condition was
carbonate > phosphate ! borate. In alkaline solution, the decay rates of Fe(VI) were
similar for the studied buffering ions. The results indicated that carbonate ions
induced the formation of ultrasmall iron(III) oxyhydroxide nanoparticles (<5 nm),
which contributed to the increased decay of Fe(VI) because of their larger specific
surface area, where the catalytic reactions took place. On the other hand, the
observed low reactivity of borate with Fe(VI) demonstrates that borate is the least
reactive buffer for studies of Fe(VI) reactivity in neutral solutions.
8.7 Degradation of Organic Pollutants by Ferrate
One of the fresh challenges in water treatment is to develop new methods for
elimination of organic pollutants, especially for persistent compounds. Various
conventional methods such as adsorption, filtration, sedimentation, oxidation processes, and photocatalytical methods require relatively high operational costs and
yield inadequate efficiency. Moreover, currently the emphasis is put on green
8 Ferrates as Powerful Oxidants in Water Treatment Technologies
185
important application aspect. As it has been already mentioned, potassium ferrate
(V) can be synthesized by the solid-state method on a large scale (contrary to ferrate
(VI)). But after ferrate(V) is dissolved in water, the final oxidizing agent is Fe(VI).
Concerning sodium ferrate(IV), after it has been dissolved in water, immediate
transformation to Fe(III) takes place. The process is accompanied by rapid increase
of pH up to 13 and evolution of oxygen. The corresponding chemical equation
describing the transformation can be written as
Na 4 FeO 4 þ 3=2H 2 O ! NaFeO 2 þ 3NaOH þ 1=4O 2
ð8:6Þ
It is worth mentioning that due to this rapid decay, the applicability of sodium
ferrate(IV) to water treatment is very limited. This was confirmed, e.g., in the study
by Machalová Šišková et al. (2016), where very low efficiency in the degradation of
estrogenic hormones was observed in case of sodium ferrate(IV), in comparison with
potassium ferrates(V) or (VI).
8.6 Effect of Buffering Inorganic Ions on Stability
of Ferrates
The study by Kolář et al. (2016) provided a systematic kinetics investigation of the
decay of ferrate(VI) in the presence of inorganic buffering ions (borate, phosphate,
and carbonate) at a pH range from 6.0 to 9.0. Detailed kinetic analysis revealed that
carbonate anion enhanced the Fe(VI) transformation rate, compared to phosphate
and borate ions. The order of the Fe(VI) decay rate in neutral solution condition was
carbonate > phosphate ! borate. In alkaline solution, the decay rates of Fe(VI) were
similar for the studied buffering ions. The results indicated that carbonate ions
induced the formation of ultrasmall iron(III) oxyhydroxide nanoparticles (<5 nm),
which contributed to the increased decay of Fe(VI) because of their larger specific
surface area, where the catalytic reactions took place. On the other hand, the
observed low reactivity of borate with Fe(VI) demonstrates that borate is the least
reactive buffer for studies of Fe(VI) reactivity in neutral solutions.
8.7 Degradation of Organic Pollutants by Ferrate
One of the fresh challenges in water treatment is to develop new methods for
elimination of organic pollutants, especially for persistent compounds. Various
conventional methods such as adsorption, filtration, sedimentation, oxidation processes, and photocatalytical methods require relatively high operational costs and
yield inadequate efficiency. Moreover, currently the emphasis is put on green
8 Ferrates as Powerful Oxidants in Water Treatment Technologies
185