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Topics in Current Chemistry (2020) 378:6
2 Ferrite Photocatalysts for Environmental Applications: Water
Detoxification and Air Depollution
2.1 Introduction
Photocatalysis as an advanced oxidation process has been applied for water treatment and for air depollution. These processes are based on the formation of highly
oxidative species that can convert recalcitrant organic pollutants into mineral compounds, i.e. CO 2 , H 2 O and mineral acids. Ferrites are among the catalysts that can
be employed. The main properties of those materials are their high stability and the
good mobility of the oxygen in the network and so the ability to form vacancies
and to stabilize metals with unusual oxidation states [13, 16]. In this scenario, ferrites have been employed in both fields (water and air treatment), taking advantage
of their activation by visible light to generate the species that carry out the photocatalytic process [44]. Applications of ferrites in environmental remediation include
not only their use as photocatalysts but also as catalysts in intensified processes by
simultaneously applying single processes with the purpose of obtaining a higher
efficiency by reducing the inherent drawbacks of each single treatment [31, 37].
2.2 Ferrite Photocatalysts for Water Detoxification
There are several works dealing with the use of ferrite catalysts for the treatment
of water by photocatalysis (Table 2). The first ones were reported in 2015 by Zaharieva et al. [85] and Mahto et al. [86]. In the first case, the authors synthesized
Cu 0.25 Fe 2.75 O 4 by a co-precipitation method with a further thermal treatment and
applied it for the degradation of malachite green. They obtained better activity than
the reference TiO 2 -P25. In the second case, Fe 3 O 4 , MnFe 2 O 4 and ZnFe 2 O 4 , also
obtained by co-precipitation, were employed to degrade azo dyes in aqueous solution. Materials were functionalized with citric acid to improve the photoactivity.
Recently, other methods, as mentioned in “Ferrites: Structure, Synthesis and
Properties”, have been employed for the synthesis of ferrites such as combustion
route, electrospinning and sol–gel [34, 87, 88]. Ansari et al. synthesized a cobalt
hexaferrite, CoFe 12 O 19 , by sol–gel method and using a natural agent as reducing
agent: maltose and fructose. In order to increase the activity, the nanoparticles were
supported on carbon nanotubes or graphene sheets. The effect of the reducing agent
as well as the calcination temperature was evaluated. The bandgap value of 3.0 eV
allowed their use in the photocatalytic degradation of methyl orange under visible
light. The authors obtained 75% of methyl orange degradation in 50 min of reaction
as well as a higher activity when the ferrite was supported owing to an improved
charge separation and a lower charge recombination.
CaFe 2 O 4 , with spinel structure, was obtained by electrospinning by El-Rafei’s
group [87]. The material showed good magnetization, a key parameter for recovery
purposes in water treatment, as well as photoactivity under simulated sunlight. The
study focused on the effect of heat-treatment temperature and showed the need for
treatment above 800 °C to get a well-formed ferrite phase.
121
Reprinted from the journal
Topics in Current Chemistry (2020) 378:6
2 Ferrite Photocatalysts for Environmental Applications: Water
Detoxification and Air Depollution
2.1 Introduction
Photocatalysis as an advanced oxidation process has been applied for water treatment and for air depollution. These processes are based on the formation of highly
oxidative species that can convert recalcitrant organic pollutants into mineral compounds, i.e. CO 2 , H 2 O and mineral acids. Ferrites are among the catalysts that can
be employed. The main properties of those materials are their high stability and the
good mobility of the oxygen in the network and so the ability to form vacancies
and to stabilize metals with unusual oxidation states [13, 16]. In this scenario, ferrites have been employed in both fields (water and air treatment), taking advantage
of their activation by visible light to generate the species that carry out the photocatalytic process [44]. Applications of ferrites in environmental remediation include
not only their use as photocatalysts but also as catalysts in intensified processes by
simultaneously applying single processes with the purpose of obtaining a higher
efficiency by reducing the inherent drawbacks of each single treatment [31, 37].
2.2 Ferrite Photocatalysts for Water Detoxification
There are several works dealing with the use of ferrite catalysts for the treatment
of water by photocatalysis (Table 2). The first ones were reported in 2015 by Zaharieva et al. [85] and Mahto et al. [86]. In the first case, the authors synthesized
Cu 0.25 Fe 2.75 O 4 by a co-precipitation method with a further thermal treatment and
applied it for the degradation of malachite green. They obtained better activity than
the reference TiO 2 -P25. In the second case, Fe 3 O 4 , MnFe 2 O 4 and ZnFe 2 O 4 , also
obtained by co-precipitation, were employed to degrade azo dyes in aqueous solution. Materials were functionalized with citric acid to improve the photoactivity.
Recently, other methods, as mentioned in “Ferrites: Structure, Synthesis and
Properties”, have been employed for the synthesis of ferrites such as combustion
route, electrospinning and sol–gel [34, 87, 88]. Ansari et al. synthesized a cobalt
hexaferrite, CoFe 12 O 19 , by sol–gel method and using a natural agent as reducing
agent: maltose and fructose. In order to increase the activity, the nanoparticles were
supported on carbon nanotubes or graphene sheets. The effect of the reducing agent
as well as the calcination temperature was evaluated. The bandgap value of 3.0 eV
allowed their use in the photocatalytic degradation of methyl orange under visible
light. The authors obtained 75% of methyl orange degradation in 50 min of reaction
as well as a higher activity when the ferrite was supported owing to an improved
charge separation and a lower charge recombination.
CaFe 2 O 4 , with spinel structure, was obtained by electrospinning by El-Rafei’s
group [87]. The material showed good magnetization, a key parameter for recovery
purposes in water treatment, as well as photoactivity under simulated sunlight. The
study focused on the effect of heat-treatment temperature and showed the need for
treatment above 800 °C to get a well-formed ferrite phase.
121
Reprinted from the journal