materials and reduction of phosphate concentrations in sewage treatment. Reactions
are very fast (seconds to minutes). Ferrate can be used to treat emerging
micropollutants, radionuclides, pharmaceuticals, endocrine disruptors, dyes and
persistent organic pollutants (POPs). It can also remove metals (arsenite, arsenate,
and metals in MÀcyanide complexes, with M ¼ Cd, Cu, and Zn), and nonmetals
(Sharma et al. 2015; Jiang 2014; Rai et al. 2018; Jiang et al. 2018). Ferrate
(VI) exhibits several additional desirable properties including the formation of few
byproducts, the absence of mutagenic or carcinogenic byproducts, a high oxidizing
power over the entire pH range, and the ability to produce stable salts. There is also
a tremendous interest in exploiting potassium ferrate(VI) as a coagulant to deactivate
harmful microorganisms (bacteria and viruses).
One of the drawbacks of ferrate is its preparation. Various strategies, including
wet chemical, electrochemical, and thermal approaches have been proposed and
optimized. The most common way is dry oxidation by heating/melting of various
iron oxide-containing minerals under strong alkaline and oxygen flow conditions.
Electrochemical methods that employ anodic oxidation using iron or an alloy as the
anode and NaOH or KOH as the electrolyte, and wet oxidation of an Fe(III) salt
under strong alkaline conditions using hypochlorite or chlorine as the oxidant are
other alternatives. However, each method has its advantages and disadvantages, but
an effort to improve the ways of the synthesis has been made recently, which can be
consulted in Rai et al. (2018).
Numerous reactions occur in the ferrate reaction systems: (i) the generation of Fe
(V) and Fe(IV) through one-electron and two-electron transfer processes,
(ii) production of radical species that can also generate Fe(V) and Fe(IV) species,
(iii) further reactions of Fe(V) and Fe(IV) with contaminants, (iv) selfdecompositions of Fe(VI), Fe(V), and Fe(IV) species, (v) reactions of ferrates with
ROS, O 2
•– and H 2 O 2 , produced from self-decompositions. Among the inorganic
contaminants, cyanides and sulfide react via a one-electron transfer step, while
oxy-compounds of sulfur, selenium, arsenic, and nitrogen probably go through an
initial two-electron transfer step. Regarding oxidation of organic compounds by Fe
(VI), a range of electron equivalents per Fe(VI) (i.e., oxidation capacity) is reported.
More aspects and examples can be found in Sharma et al. (2015).
In conclusion, Fe(VI) provides different oxidation capacities to oxidize contaminants while Fe(VI) is reduced to Fe(III) or Fe(II). In some cases, as POPs of
fluorocompounds, Fe(VI) cannot degrade the contaminant, but Fe(V) and Fe(IV),
using solid compounds such as K 3 FeO 4 or Na 4 FeO 4 present higher reactivity to
degrade these molecules. As a consequence, ferrates are highly promising and
environmentally friendly agents exhibiting multimodal activity due to their high
oxidation capacity simultaneously combined with disinfection and coagulation.
However, some aspects such as the high pH of the water being treated with an
alkaline ferrate solution remains to be a great concern. Some problems such as an
easier ferrate synthesis, stability control, mechanisms of action, large-scale production of Fe(VI), and large-scale applications have yet to be solved (Jiang et al. 2018).
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M. I. Litter
are very fast (seconds to minutes). Ferrate can be used to treat emerging
micropollutants, radionuclides, pharmaceuticals, endocrine disruptors, dyes and
persistent organic pollutants (POPs). It can also remove metals (arsenite, arsenate,
and metals in MÀcyanide complexes, with M ¼ Cd, Cu, and Zn), and nonmetals
(Sharma et al. 2015; Jiang 2014; Rai et al. 2018; Jiang et al. 2018). Ferrate
(VI) exhibits several additional desirable properties including the formation of few
byproducts, the absence of mutagenic or carcinogenic byproducts, a high oxidizing
power over the entire pH range, and the ability to produce stable salts. There is also
a tremendous interest in exploiting potassium ferrate(VI) as a coagulant to deactivate
harmful microorganisms (bacteria and viruses).
One of the drawbacks of ferrate is its preparation. Various strategies, including
wet chemical, electrochemical, and thermal approaches have been proposed and
optimized. The most common way is dry oxidation by heating/melting of various
iron oxide-containing minerals under strong alkaline and oxygen flow conditions.
Electrochemical methods that employ anodic oxidation using iron or an alloy as the
anode and NaOH or KOH as the electrolyte, and wet oxidation of an Fe(III) salt
under strong alkaline conditions using hypochlorite or chlorine as the oxidant are
other alternatives. However, each method has its advantages and disadvantages, but
an effort to improve the ways of the synthesis has been made recently, which can be
consulted in Rai et al. (2018).
Numerous reactions occur in the ferrate reaction systems: (i) the generation of Fe
(V) and Fe(IV) through one-electron and two-electron transfer processes,
(ii) production of radical species that can also generate Fe(V) and Fe(IV) species,
(iii) further reactions of Fe(V) and Fe(IV) with contaminants, (iv) selfdecompositions of Fe(VI), Fe(V), and Fe(IV) species, (v) reactions of ferrates with
ROS, O 2
•– and H 2 O 2 , produced from self-decompositions. Among the inorganic
contaminants, cyanides and sulfide react via a one-electron transfer step, while
oxy-compounds of sulfur, selenium, arsenic, and nitrogen probably go through an
initial two-electron transfer step. Regarding oxidation of organic compounds by Fe
(VI), a range of electron equivalents per Fe(VI) (i.e., oxidation capacity) is reported.
More aspects and examples can be found in Sharma et al. (2015).
In conclusion, Fe(VI) provides different oxidation capacities to oxidize contaminants while Fe(VI) is reduced to Fe(III) or Fe(II). In some cases, as POPs of
fluorocompounds, Fe(VI) cannot degrade the contaminant, but Fe(V) and Fe(IV),
using solid compounds such as K 3 FeO 4 or Na 4 FeO 4 present higher reactivity to
degrade these molecules. As a consequence, ferrates are highly promising and
environmentally friendly agents exhibiting multimodal activity due to their high
oxidation capacity simultaneously combined with disinfection and coagulation.
However, some aspects such as the high pH of the water being treated with an
alkaline ferrate solution remains to be a great concern. Some problems such as an
easier ferrate synthesis, stability control, mechanisms of action, large-scale production of Fe(VI), and large-scale applications have yet to be solved (Jiang et al. 2018).
144
M. I. Litter