further oxidize species in solution is called a non-active anode. These processes can
be pictured in Fig. 4.
We can see from this figure that in both types of anodes, the initial reaction is the
generation of hydroxyl radicals that will be adsorbed onto the surface. The active
anode will strongly interact with the adsorbed hydroxyl radicals, and, depending on
the conditions, it can form chemisorbed active oxygen (2), which has a lower
oxidizing capacity but is able to degrade organic compounds, R, oxidizing them to
RO (3).
M ÁOH
ð
Þ ! MO þ H
þ
þ e
À
ð2Þ
MO þ R ! M þ RO
ð3Þ
On the contrary, non-active anodes interact weakly with the hydroxyl radicals.
This enables them to completely degrade the organic matter, producing carbon
dioxide and water. Furthermore, both anodes are able to directly oxidize the organic
compounds by electron transfer, and, also, depending on the solutions employed,
other weak oxidizing species can be formed, such as peroxodisulfates and chlorine
compounds [70].
Fig. 4 Mechanism of oxidation of organic matter in active and non-active anodes
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