Advanced Oxidation Processes for Removal of Dyes from Aqueous Media - Green Chemistry for Dyes Removal from Wastewater - page 101

Various anodes have been tested for dye-containing wastewater treat-
ment and boron-doped diamond has proven to be the most efficient for
dyes [133-136].
Generally, oxidation of organic matter by electrochemical treatment can
be classified as direct oxidation on the surface of the anode and indirect
oxidation distant from the anode surface.
In a direct anodic oxidation process, the pollutants are first absorbed on
the anode surface and then destroyed by the anodic electron transfer reac-
tion. Direct anodic oxidation yields rather poor removal [137].
In an indirect oxidation process, during electrolysis, the oxidants are
regenerated by the electrochemical reactions. The pollutants are then
destroyed in the bulk solution by the oxidation reaction of regenerated
oxidants. All the oxidants are generated in situ and are used immedi-
ately [137].
Some researchers have defined the steps which the energy supplied to an
electrode undergoes in four sections: Firstly, transportation of electroactive
particle from the bulk solution to the electrode surface is realized. Then
electroactive particle is adsorbed on the surface of the electrode. Electron
transfer between the bulk and the electrode is realized. And finally, the
reacted particle is either transported to the bulk solution (desorption) or
deposited on the electrode surface [138].
This method has been recently used for decolorizing and degrading
dyes from aqueous solutions [133-137]. The results of these investigations
indicated that the operating parameters such as cell voltage and pH play an
important role on the electrochemical oxidation of organic pollutants, and
controlling these parameters leads to an efficient treatment.
The cell voltage significantly influences both the electrochemical and
adsorption catalytic process. It is recommended to study at an optimum
cell voltage even though a higher cell voltage causes faster removal effi-
ciency. Because increasing cell voltage increases the overpotential required
for the generation of oxidants, the energy consumption becomes higher
as larger cell voltage is applied. At high cell voltage, the service life of elec-
trodes is shortened [139].
It has been reported that the influent pH is an important operating
factor influencing the performance of the electrochemical process [140].
Although the treatment performance depends on the nature of the pollut-
ants, with all pollutants the best removal efficiency has been found near a
pH of 7. The power consumption is, however, higher at neutral pH due to
the variation of conductivity. When conductivity is high, the pH effect is
not significant [140].