II International Symposium on Carbon for Catalysis ABSTRACTS

PP-<strong>II</strong>-9
selective path (i.e. leading to carbon dioxide and water) seems to take place
preferentially (see the below Figure concerning the abatement of the pharmaceutical
Ofloxacin) 5 .
• Corrosion effects are unfortunately observed for this electrode as a consequence of
high operating potentials and long electrolysis time that come from the ability of
carbon to readily adsorb oxygen and desorb oxides of carbon renders it of limited use
in high oxidizing environmental.
An enhancement of the demolition oxidative kinetics can though be achieved with the use of
three dimensional electrodes consisting of a fixed bed of activated carbon pellets (3D GAC).
This electrode configuration is rather attractive and particularly appropriate to treat low
concentration solutions 6 . Most of the contributions to the enhancement of the mass transfer
coefficient and the limiting current density in the three-dimensional materials come from the
increase of the specific surface area (i.e. a more extensive interfacial electrode surface is
guaranteed for the electrochemical reaction) and from the induced static mixing. However,
after a specific investigation campaign, it was possible to conclude that not all the specific
area is useful for the electro-oxidation, supposing that the oxidation kinetics is equal to that of
the graphite electrode, the utilized area only a little fraction of the formally available in the
whole set of the GAC pellets. This suggests that the process possibly occurs only over the
external activated carbon sites of the pellet surface. The reaction between C and O would
indeed result in an accumulation of CO + CO 2 at the internal porosity and the occurrence of
further reaction steps requires that molecules get rapidly at the reaction front and the products
diffuse away from it through the carbon porosity. However, the diffusion rate of organic
original molecules and products is smaller than the reaction rate of carbon with oxygen.
Moreover, the structural stability of activated carbon bed is rather low, when working at
relative high potentials as in the case of graphite electrode, as testified by the dark colour of
the treated solution caused by suspended carbon fine particles, and by a loss of the bed
cohesion also due to the oxidation to carbon monoxide/dioxide formation. The measured CO 2
amounts formed from activated carbon pellets oxidation inside the reactor anodic
compartment, are proportional to the electric charge passed and are equal to about 12 mg CO 2
/Ah. Conversely, one of the most promising electrode for the electrochemical waste water
treatment is the boron-doped diamond, which represents an exciting new carbon material.
However, a long way to go is still in front of us to fully understand the relationship between
241