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R. Sukanya Devi et al.
of these radicals is observed to be entirely different. In the case of active electrode
(Class I electrodes), the generated hydroxyl radical reacts with the surface of the
electrode enabling the oxidation of organic pollutants by direct transfer of electrons
from the electron surface. The oxidation in direct electrolysis happens due to the
transfer of electrons and chemical oxidation of the pollutant at the anode surface
and not with the reaction of hydroxyl radicals generated. The conversion of organic
pollutant to different simpler compounds or species is dependent on the electrodes
used. For example, if graphite is used as electrode, it results in electrode burning,
resulting in very low removal efficiencies. In case of certain electrodes like Platinum,
Titanium, Iridium dioxide electrodes, results in formation of the oxides which in turn
chemically attack the organic pollutant resulting in different simpler compounds and
efficiency. Apart from breaking down of dyes into simpler compounds it is observed
that there is a decrease in the Total Dissolved Soild (TDS) and Chemical Oxygen
Demand (COD) values of the treated effluent using different electrodes [46].
In bulk electrolysis, the non-active electrodes (Class 2 electrodes) and the hydroxyl
radicals generated do not react at the anodic surface instead it reacts directly with
the organic pollutants in the electrochemical cell and very near to the electrode, as
the span of hydroxyl radical is short. Since the reaction zone is very small, very
often researchers consider it as direct electrolysis reaction, although it is a mediated
electrochemical oxidation reaction.
In direct electrolysis, it is important to consider the processes that happen on the
electrode surface and how the mass transfer of organic pollutants from the solution
happens near the electrode surface and their reactions take place. The following
stages should be completed for effective oxidation at the electrode surface to take
place.
• Transfer of the organic pollutants from the bulk solution to the electrode surface
• Mediated oxidation by hydroxyl radicals at the electrode surface
• From the electrode surface, oxidized product has to be transferred to the bulk
solution [47].
Figure 5 [47] explains the above-mentioned stages.
During direct electrode oxidation, the main point of concern is the formation of
polymer layer on the anodic surface. Moreover, the direct electrochemical oxidation
is fixed based on the anodic potential, before the O 2 evolution results in decrease
in catalytic activity also known as poisoning effect. This effect is dependent on the
nature of the organic pollutants, their adsorption capacity, concentration of aromatic
organic compounds like phenols, chlorophenols, etc. [48].
3.3.2 Indirect Electrochemical Oxidation Process
In indirect electrochemical oxidation process, the hydroxyl radicals are formed at the
bulk solution. In indirect electrochemical oxidation, a mediator is generated at the
anode or at the cathode, which is responsible for the reaction to happen [49]. Chlorine,
hydrogen peroxide are the mostly used mediators; however, any salt present in the
R. Sukanya Devi et al.
of these radicals is observed to be entirely different. In the case of active electrode
(Class I electrodes), the generated hydroxyl radical reacts with the surface of the
electrode enabling the oxidation of organic pollutants by direct transfer of electrons
from the electron surface. The oxidation in direct electrolysis happens due to the
transfer of electrons and chemical oxidation of the pollutant at the anode surface
and not with the reaction of hydroxyl radicals generated. The conversion of organic
pollutant to different simpler compounds or species is dependent on the electrodes
used. For example, if graphite is used as electrode, it results in electrode burning,
resulting in very low removal efficiencies. In case of certain electrodes like Platinum,
Titanium, Iridium dioxide electrodes, results in formation of the oxides which in turn
chemically attack the organic pollutant resulting in different simpler compounds and
efficiency. Apart from breaking down of dyes into simpler compounds it is observed
that there is a decrease in the Total Dissolved Soild (TDS) and Chemical Oxygen
Demand (COD) values of the treated effluent using different electrodes [46].
In bulk electrolysis, the non-active electrodes (Class 2 electrodes) and the hydroxyl
radicals generated do not react at the anodic surface instead it reacts directly with
the organic pollutants in the electrochemical cell and very near to the electrode, as
the span of hydroxyl radical is short. Since the reaction zone is very small, very
often researchers consider it as direct electrolysis reaction, although it is a mediated
electrochemical oxidation reaction.
In direct electrolysis, it is important to consider the processes that happen on the
electrode surface and how the mass transfer of organic pollutants from the solution
happens near the electrode surface and their reactions take place. The following
stages should be completed for effective oxidation at the electrode surface to take
place.
• Transfer of the organic pollutants from the bulk solution to the electrode surface
• Mediated oxidation by hydroxyl radicals at the electrode surface
• From the electrode surface, oxidized product has to be transferred to the bulk
solution [47].
Figure 5 [47] explains the above-mentioned stages.
During direct electrode oxidation, the main point of concern is the formation of
polymer layer on the anodic surface. Moreover, the direct electrochemical oxidation
is fixed based on the anodic potential, before the O 2 evolution results in decrease
in catalytic activity also known as poisoning effect. This effect is dependent on the
nature of the organic pollutants, their adsorption capacity, concentration of aromatic
organic compounds like phenols, chlorophenols, etc. [48].
3.3.2 Indirect Electrochemical Oxidation Process
In indirect electrochemical oxidation process, the hydroxyl radicals are formed at the
bulk solution. In indirect electrochemical oxidation, a mediator is generated at the
anode or at the cathode, which is responsible for the reaction to happen [49]. Chlorine,
hydrogen peroxide are the mostly used mediators; however, any salt present in the