11.1 Introduction
The growing imbalance between water demand and water resources throughout the
world, along with the scarcity of fresh water to sustain all human activities, has
stimulated the development of a large variety of physical, chemical, photochemical,
and electrochemical methods that show great efficiency in treating urban and
industrial wastewater, aiming at its further reuse. The electro-Fenton (EF) process,
initially called electrogenerated Fenton’s reagent, was first used for wastewater
treatment in the mid ‘80s (Brillas et al. 2009). EF is considered an electrochemical
advanced oxidation process (EAOP) since it consists in the generation of homogeneous hydroxyl radical (
•
OH). This is achieved upon the occurrence of Fenton’s
reaction between Fe
2+ and electrogenerated H 2 O 2 (Brillas et al. 2009; Oturan and
Aaron 2014; Sirés et al. 2014).
• OH is the second strongest oxidizing species known
after fluorine, showing a large ability to mineralize most organic pollutants in water.
The most characteristic feature of EF is that H 2 O 2 is continuously dosed to the
reaction medium from the two-electron reduction of O 2 . This gas can be either
directly fed as pure O 2 or air into the wastewater or pumped through a gas-diffusion
device, further accepting the electrons typically supplied to a carbonaceous cathode.
Thanks to the production of H 2 O 2 on site, expensive and dangerous steps like
industrial synthesis, transportation, storage, and handling can be avoided. Conventional homogeneous EF involves the addition of Fe
2+ to the wastewater, although
Fe
3+ can be used alternatively because it is reduced to the former ion at the cathode
surface (Oturan and Aaron 2014; Martínez-Huitle et al. 2015). The continuous Fe
2+
and H 2 O 2 (re)generation represents a significant advantage over the classical chemical Fenton process. In addition, the use of an undivided cell can accelerate the
decontamination because of the production of heterogeneous M(
•
OH) from water
discharge at the surface of the anode M. On the other hand, one important drawback
of EF when applied to organics removal is the production of Fe(III)-carboxylate
complexes as final byproducts, which are quite refractory to
• OH attack. This
phenomenon also occurs in the dark Fenton. To overcome this problem, the
photoelectro-Fenton (PEF) process was developed in the mid ‘90s (Brillas et al.
2009). It consists in the simultaneous irradiation of the solution with artificial UV
light or, more recently, sunlight, thus promoting the photolysis of intermediates like
the Fe(III)-carboxylate species. As a result, a larger mineralization can be usually
attained. The application of EF and PEF, along with the related processes, to the
treatment of synthetic and real wastewater has been discussed in several authoritative
reviews (Brillas et al. 2009; Oturan and Aaron 2014; Sirés et al. 2014; MartínezHuitle et al. 2015; Moreira et al. 2017). Recently, the fundamentals, reactions, and
applications of these EAOPs have been summarized in a comprehensive book edited
by Zhou et al. (2018).
Other drawbacks of homogeneous EF and PEF include the limited pH range, i.e.,
~3, to avoid the catalyst loss by precipitation, and the increase of soluble iron content
in the treated effluent (Martínez-Huitle et al. 2015). Over the last decade, many
efforts have been made to overcome such limitations, thus re-enforcing the EF and
258
I. Sirés and E. Brillas
The growing imbalance between water demand and water resources throughout the
world, along with the scarcity of fresh water to sustain all human activities, has
stimulated the development of a large variety of physical, chemical, photochemical,
and electrochemical methods that show great efficiency in treating urban and
industrial wastewater, aiming at its further reuse. The electro-Fenton (EF) process,
initially called electrogenerated Fenton’s reagent, was first used for wastewater
treatment in the mid ‘80s (Brillas et al. 2009). EF is considered an electrochemical
advanced oxidation process (EAOP) since it consists in the generation of homogeneous hydroxyl radical (
•
OH). This is achieved upon the occurrence of Fenton’s
reaction between Fe
2+ and electrogenerated H 2 O 2 (Brillas et al. 2009; Oturan and
Aaron 2014; Sirés et al. 2014).
• OH is the second strongest oxidizing species known
after fluorine, showing a large ability to mineralize most organic pollutants in water.
The most characteristic feature of EF is that H 2 O 2 is continuously dosed to the
reaction medium from the two-electron reduction of O 2 . This gas can be either
directly fed as pure O 2 or air into the wastewater or pumped through a gas-diffusion
device, further accepting the electrons typically supplied to a carbonaceous cathode.
Thanks to the production of H 2 O 2 on site, expensive and dangerous steps like
industrial synthesis, transportation, storage, and handling can be avoided. Conventional homogeneous EF involves the addition of Fe
2+ to the wastewater, although
Fe
3+ can be used alternatively because it is reduced to the former ion at the cathode
surface (Oturan and Aaron 2014; Martínez-Huitle et al. 2015). The continuous Fe
2+
and H 2 O 2 (re)generation represents a significant advantage over the classical chemical Fenton process. In addition, the use of an undivided cell can accelerate the
decontamination because of the production of heterogeneous M(
•
OH) from water
discharge at the surface of the anode M. On the other hand, one important drawback
of EF when applied to organics removal is the production of Fe(III)-carboxylate
complexes as final byproducts, which are quite refractory to
• OH attack. This
phenomenon also occurs in the dark Fenton. To overcome this problem, the
photoelectro-Fenton (PEF) process was developed in the mid ‘90s (Brillas et al.
2009). It consists in the simultaneous irradiation of the solution with artificial UV
light or, more recently, sunlight, thus promoting the photolysis of intermediates like
the Fe(III)-carboxylate species. As a result, a larger mineralization can be usually
attained. The application of EF and PEF, along with the related processes, to the
treatment of synthetic and real wastewater has been discussed in several authoritative
reviews (Brillas et al. 2009; Oturan and Aaron 2014; Sirés et al. 2014; MartínezHuitle et al. 2015; Moreira et al. 2017). Recently, the fundamentals, reactions, and
applications of these EAOPs have been summarized in a comprehensive book edited
by Zhou et al. (2018).
Other drawbacks of homogeneous EF and PEF include the limited pH range, i.e.,
~3, to avoid the catalyst loss by precipitation, and the increase of soluble iron content
in the treated effluent (Martínez-Huitle et al. 2015). Over the last decade, many
efforts have been made to overcome such limitations, thus re-enforcing the EF and
258
I. Sirés and E. Brillas