cleavage of dioxygen and water molecules happens forming O, OH, and O 2 H
radicles (Oturan and Aaron 2014; Deng and Zhao 2015).
Recently, the sonochemical AOPs have been used broadly in wastewater, sludge,
and water treatment to degrade organic pollutants as reported by Zouaghi et al.
(2011); Ma (2012), and Dalhatou et al. (2015). Insufficient formation of OH radicles
is the major drawback of this process; thus to overcome this problem it is being
combined with other AOPs such as UV irradiation or by adding other oxidizing
agents (O 2 , H 2 O 2 ) or Fenton reagent. These hybrid processes have led to the
improvement of organic pollutants’ degradation (Oturan and Aaron, 2014), and
several studies have been conducted to investigate the efficiency of these hybrid
processes such as Neppolian et al. (2012), Badmus et al. (2018), and Barik and
Gogate (2018).
8.6.4.4 Electrochemical AOPs
Electrochemistry is considered as an efficient, effective, and environmental method
that includes the use of electric energy and based on electron transfer. It can produce
sufficient OH radicles in the effluent to degrade the organic contaminants effectively.
These radicles can be generated electrochemically through electrically inducing
Fenton’s reagent (H 2 O 2 + Fe ions) and this process is known as electro-Fenton
process (EF process). In situ electrogeneration of H 2 O 2 is occurred in the presence of
iron ions by reduction of oxygen dissolved (O 2 ) in an acidic medium (Brillas et al.
2009). EF process is one of the environmentally friendly electrochemical AOPs
which consists of electrically aided Fenton process. The process can be run either by
adding Fenton reagent directly in the reactor along with an inert anode or by adding
only hydrogen peroxide and providing Fe
2+ ions through cast iron anode. The
accumulation or transport of hydrogen peroxide can be prevented if its continuous
supply to contaminated effluent is being ensured through two-electron redox reaction in acidic medium (Nidheesh and Gandhimathi 2012; Ribeiro et al. 2015).
The process can take place in either combined or separate electrochemical cells,
though combined cells are more beneficial as oxidation reactions occurring on both
cathode and anode can add up to the efficiency of the process to degrade/decompose
organic pollutants. Additionally, the efficiency of the process can also be improved
by coupling other AOPs with the EF process such as photolysis (photo-electroFenton), peroxi-coagulation, and solar-photo-electro-Fenton (Pipi et al. 2014;
Ribeiro et al. 2015). These electrochemical Fenton processes have been used for
degradation of several POPs including atrazine (Oturan et al. 2012), endosulfans
(Errami et al. 2012), diruron (Pipi et al. 2014), pentachlorophenol (Govindan et al.
2014), and hexachlorocyclohexanes (HCHs) such as lindane (Wacławek et al. 2016;
Dominguez et al. 2018a, b).
8 Persistent Organic Pollutants (POPs): Sources, Types, Impacts, and Their. . .
235
radicles (Oturan and Aaron 2014; Deng and Zhao 2015).
Recently, the sonochemical AOPs have been used broadly in wastewater, sludge,
and water treatment to degrade organic pollutants as reported by Zouaghi et al.
(2011); Ma (2012), and Dalhatou et al. (2015). Insufficient formation of OH radicles
is the major drawback of this process; thus to overcome this problem it is being
combined with other AOPs such as UV irradiation or by adding other oxidizing
agents (O 2 , H 2 O 2 ) or Fenton reagent. These hybrid processes have led to the
improvement of organic pollutants’ degradation (Oturan and Aaron, 2014), and
several studies have been conducted to investigate the efficiency of these hybrid
processes such as Neppolian et al. (2012), Badmus et al. (2018), and Barik and
Gogate (2018).
8.6.4.4 Electrochemical AOPs
Electrochemistry is considered as an efficient, effective, and environmental method
that includes the use of electric energy and based on electron transfer. It can produce
sufficient OH radicles in the effluent to degrade the organic contaminants effectively.
These radicles can be generated electrochemically through electrically inducing
Fenton’s reagent (H 2 O 2 + Fe ions) and this process is known as electro-Fenton
process (EF process). In situ electrogeneration of H 2 O 2 is occurred in the presence of
iron ions by reduction of oxygen dissolved (O 2 ) in an acidic medium (Brillas et al.
2009). EF process is one of the environmentally friendly electrochemical AOPs
which consists of electrically aided Fenton process. The process can be run either by
adding Fenton reagent directly in the reactor along with an inert anode or by adding
only hydrogen peroxide and providing Fe
2+ ions through cast iron anode. The
accumulation or transport of hydrogen peroxide can be prevented if its continuous
supply to contaminated effluent is being ensured through two-electron redox reaction in acidic medium (Nidheesh and Gandhimathi 2012; Ribeiro et al. 2015).
The process can take place in either combined or separate electrochemical cells,
though combined cells are more beneficial as oxidation reactions occurring on both
cathode and anode can add up to the efficiency of the process to degrade/decompose
organic pollutants. Additionally, the efficiency of the process can also be improved
by coupling other AOPs with the EF process such as photolysis (photo-electroFenton), peroxi-coagulation, and solar-photo-electro-Fenton (Pipi et al. 2014;
Ribeiro et al. 2015). These electrochemical Fenton processes have been used for
degradation of several POPs including atrazine (Oturan et al. 2012), endosulfans
(Errami et al. 2012), diruron (Pipi et al. 2014), pentachlorophenol (Govindan et al.
2014), and hexachlorocyclohexanes (HCHs) such as lindane (Wacławek et al. 2016;
Dominguez et al. 2018a, b).
8 Persistent Organic Pollutants (POPs): Sources, Types, Impacts, and Their. . .
235