2017), Fe 2 O 3 (Wang et al. 2013; Peng et al. 2015; Sklari et al. 2015), FeOOH (Zhang
et al. 2012, 2015a), Fe 3 O 4 (Chen et al. 2016b; Tian et al. 2016b; Zhang et al. 2017),
γ-Fe 2 O 3 /Fe 3 O 4 (Plakas et al. 2016), Fe-Cu (Garrido-Ramírez et al. 2016; Zhao et al.
2016, 2018a), CoFe hydroxide (Ganiyu et al. 2017), and Fe 3 S 4 /Fe 7 S 8 (Choe et al.
2018). In these processes, no soluble iron catalyst is added to the solution since
Fenton’s reaction (Eq. 11.2) occurs between Fe(II) adhered to the cathode surface
and H 2 O 2 produced at the same surface. Furthermore, leaching of iron ions from the
catalyst can contribute to conventional homogeneous Fenton’s reaction. The main
aim when using heterogeneous catalysts is to make the application of water treatments without pH regulation feasible, i.e., at natural pH, which is usually
circumneutral. These materials minimize the formation of iron hydroxide, with the
consequent reduction of costs associated to sludge management. A potential drawback of such catalysts is their gradual solubilization upon the use and the reuse,
which limits their lifetime, especially under acidic conditions. Therefore, it is crucial
to test their stability in consecutive degradation cycles. Selected degradation and
mineralization results for some organic pollutants treated by hetero-EF and heteroPEF are summarized in Table 11.2. A good performance can be observed for both
processes within the pH range 2–7, although it is slightly better at pH 3.0, which is in
agreement with the optimum conditions of Fenton’s reaction (Eq. 11.2). The data
corroborate the viability of the heterogeneous treatments at neutral pH, showing
acceptable effectiveness. A clear superiority of hetero-PEF over hetero-EF, leading
to faster removal of the target pollutant and TOC, can also be observed under
comparable conditions, which can be accounted for by the additional generation of
•
OH from photoreaction (Eq. 11.4) and the photolysis of intermediates such as final
Fe(III)-carboxylate complexes via the reaction in Eq. 11.5.
As an example, Fig. 11.4a shows that magnetite (Fe 3 O 4 ) presents a relatively
uniform and spherical shape, with an average diameter of 40–50 nm, whereas
Fig. 11.4b confirms its successful loading on the activated carbon acting as GDE
(Zhang et al. 2017). Figure 11.4c illustrates the hetero-EF mechanism proposed to
destroy tetracycline upon the attack of different reactive oxygen species (ROS)
including: (i) the superoxide radical (O 2
•- ) produced from O 2 via the reaction in
Eq. 11.7, (ii) H 2 O 2 formed from O 2 via the reaction in Eq. 11.1, and (iii)
•
OH
produced from heterogeneous Fenton’s reaction (Eq. 11.8), with the subsequent
reduction of Fe
III to Fe
II via the reaction in Eq. 11.9
O 2 g
ð Þ þ e
À
! O 2
• À
ð11:7Þ
Fe
II
þ H 2 O 2 ! Fe
III
þ
• OH þ OH
À
ð11:8Þ
Fe
III
þ e
À
! Fe
II
ð11:9Þ
where means the Fe 3 O 4 surface. On the other hand, Fig. 11.4d schematizes the
generation of
•
OH to attack Acid Orange 7, using a CoFe hydroxide/carbon felt
cathode (Ganiyu et al. 2017).
• OH is formed homogeneously from the reactions
in Eqs. 11.2 and 11.6 in acidic medium, owing to Fe
2+ and Co
2+ leaching from the
268
I. Sirés and E. Brillas
et al. 2012, 2015a), Fe 3 O 4 (Chen et al. 2016b; Tian et al. 2016b; Zhang et al. 2017),
γ-Fe 2 O 3 /Fe 3 O 4 (Plakas et al. 2016), Fe-Cu (Garrido-Ramírez et al. 2016; Zhao et al.
2016, 2018a), CoFe hydroxide (Ganiyu et al. 2017), and Fe 3 S 4 /Fe 7 S 8 (Choe et al.
2018). In these processes, no soluble iron catalyst is added to the solution since
Fenton’s reaction (Eq. 11.2) occurs between Fe(II) adhered to the cathode surface
and H 2 O 2 produced at the same surface. Furthermore, leaching of iron ions from the
catalyst can contribute to conventional homogeneous Fenton’s reaction. The main
aim when using heterogeneous catalysts is to make the application of water treatments without pH regulation feasible, i.e., at natural pH, which is usually
circumneutral. These materials minimize the formation of iron hydroxide, with the
consequent reduction of costs associated to sludge management. A potential drawback of such catalysts is their gradual solubilization upon the use and the reuse,
which limits their lifetime, especially under acidic conditions. Therefore, it is crucial
to test their stability in consecutive degradation cycles. Selected degradation and
mineralization results for some organic pollutants treated by hetero-EF and heteroPEF are summarized in Table 11.2. A good performance can be observed for both
processes within the pH range 2–7, although it is slightly better at pH 3.0, which is in
agreement with the optimum conditions of Fenton’s reaction (Eq. 11.2). The data
corroborate the viability of the heterogeneous treatments at neutral pH, showing
acceptable effectiveness. A clear superiority of hetero-PEF over hetero-EF, leading
to faster removal of the target pollutant and TOC, can also be observed under
comparable conditions, which can be accounted for by the additional generation of
•
OH from photoreaction (Eq. 11.4) and the photolysis of intermediates such as final
Fe(III)-carboxylate complexes via the reaction in Eq. 11.5.
As an example, Fig. 11.4a shows that magnetite (Fe 3 O 4 ) presents a relatively
uniform and spherical shape, with an average diameter of 40–50 nm, whereas
Fig. 11.4b confirms its successful loading on the activated carbon acting as GDE
(Zhang et al. 2017). Figure 11.4c illustrates the hetero-EF mechanism proposed to
destroy tetracycline upon the attack of different reactive oxygen species (ROS)
including: (i) the superoxide radical (O 2
•- ) produced from O 2 via the reaction in
Eq. 11.7, (ii) H 2 O 2 formed from O 2 via the reaction in Eq. 11.1, and (iii)
•
OH
produced from heterogeneous Fenton’s reaction (Eq. 11.8), with the subsequent
reduction of Fe
III to Fe
II via the reaction in Eq. 11.9
O 2 g
ð Þ þ e
À
! O 2
• À
ð11:7Þ
Fe
II
þ H 2 O 2 ! Fe
III
þ
• OH þ OH
À
ð11:8Þ
Fe
III
þ e
À
! Fe
II
ð11:9Þ
where means the Fe 3 O 4 surface. On the other hand, Fig. 11.4d schematizes the
generation of
•
OH to attack Acid Orange 7, using a CoFe hydroxide/carbon felt
cathode (Ganiyu et al. 2017).
• OH is formed homogeneously from the reactions
in Eqs. 11.2 and 11.6 in acidic medium, owing to Fe
2+ and Co
2+ leaching from the
268
I. Sirés and E. Brillas