Generally, solid materials allow the use of higher pH conditions, so their separation
from the waste stream is possible.
Fe(II) catalyzes O 3 degradation giving the ferryl intermediate, which can directly
oxidize the organic pollutant or evolve to HO
• (Litter 2005 and references therein):
Fe
2þ
þ O 3 ! FeO
2þ
þ O 2
ð7:36Þ
FeO
2þ
þ H 2 O ! Fe
3þ
þ HO
•
þ HO À
ð7:37Þ
7.2.5 Electrochemical Oxidation
The application of electrical current (2–20 A) between two suitable electrodes
(Domènech et al. 2004; Brillas et al. 1998; Zhou et al. 2018) immersed in water
produces primary chemical reactions, with HO
• generation, which can be used to
oxidize pollutants:
H 2 O ! HO
•
þ H
þ
þ e À
ð7:38Þ
O 2 þ 2H
þ
þ 2e À ! H 2 O 2
ð7:39Þ
The efficiency of the system can be improved by adding Fe(II), and this process is
known then as electro-Fenton (EF) (Nidheesh et al. 2018). If an Fe sacrificial anode
is used, it provides stoichiometric amounts of Fe for the Fenton reaction, and the
process is known as electrocoagulation (EC) (or peroxicoagulation) (Zhou et al.
2018; Fernandes et al. 2015 and references therein).
EF processes can be homogeneous or heterogeneous. Homogeneous EF processes use soluble forms of iron; the most common ones are salts such as ferrous
sulfate, ferric chloride, etc., which generate ferrous or ferric ions in water and
undergo Fenton reaction with the in situ generated H 2 O 2 . The heterogeneous EF
process uses solid iron catalysts, generally very slightly soluble or insoluble in water;
the most common ones are iron oxides. Generated H 2 O 2 reacts with ferrous ions,
resulting in the generation of HO
• as in Eq. 7.22. The generated ferric ions undergo
cathodic reduction (Eq. 7.40) and regenerate Fe
2+ ; conventional Fenton chain
reactions also take place (Nidheesh et al. 2018).
Fe
3þ
þ e À ! Fe
2þ
ð7:40Þ
In the EF process, the increase in solution pH is counterbalanced by the generation of protons coming from the water oxidation at the anode (Eq. 7.41), and from
the generation of carboxylic acids coming from the degradation of the pollutants.
7 Introduction to Oxidative Technologies for Water Treatment
131
from the waste stream is possible.
Fe(II) catalyzes O 3 degradation giving the ferryl intermediate, which can directly
oxidize the organic pollutant or evolve to HO
• (Litter 2005 and references therein):
Fe
2þ
þ O 3 ! FeO
2þ
þ O 2
ð7:36Þ
FeO
2þ
þ H 2 O ! Fe
3þ
þ HO
•
þ HO À
ð7:37Þ
7.2.5 Electrochemical Oxidation
The application of electrical current (2–20 A) between two suitable electrodes
(Domènech et al. 2004; Brillas et al. 1998; Zhou et al. 2018) immersed in water
produces primary chemical reactions, with HO
• generation, which can be used to
oxidize pollutants:
H 2 O ! HO
•
þ H
þ
þ e À
ð7:38Þ
O 2 þ 2H
þ
þ 2e À ! H 2 O 2
ð7:39Þ
The efficiency of the system can be improved by adding Fe(II), and this process is
known then as electro-Fenton (EF) (Nidheesh et al. 2018). If an Fe sacrificial anode
is used, it provides stoichiometric amounts of Fe for the Fenton reaction, and the
process is known as electrocoagulation (EC) (or peroxicoagulation) (Zhou et al.
2018; Fernandes et al. 2015 and references therein).
EF processes can be homogeneous or heterogeneous. Homogeneous EF processes use soluble forms of iron; the most common ones are salts such as ferrous
sulfate, ferric chloride, etc., which generate ferrous or ferric ions in water and
undergo Fenton reaction with the in situ generated H 2 O 2 . The heterogeneous EF
process uses solid iron catalysts, generally very slightly soluble or insoluble in water;
the most common ones are iron oxides. Generated H 2 O 2 reacts with ferrous ions,
resulting in the generation of HO
• as in Eq. 7.22. The generated ferric ions undergo
cathodic reduction (Eq. 7.40) and regenerate Fe
2+ ; conventional Fenton chain
reactions also take place (Nidheesh et al. 2018).
Fe
3þ
þ e À ! Fe
2þ
ð7:40Þ
In the EF process, the increase in solution pH is counterbalanced by the generation of protons coming from the water oxidation at the anode (Eq. 7.41), and from
the generation of carboxylic acids coming from the degradation of the pollutants.
7 Introduction to Oxidative Technologies for Water Treatment
131