IJUP08 - Universidade do Porto

Azo-dye Orange II degradation by Fenton-like reaction using as
catalysts transition metals in carbon supports
Filipa Duarte a , F.J. Maldonado-Hódar b , Luis M. Madeira a
a LEPAE, Departamento de Engenharia Química, Faculdade de Engenharia da Universidade do
Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal
b Departamento de Química Inorgánica, Facultad de Ciencias, Universidad de Granada, 18071
Granada, Spain
Pollution of water caused by industries is a serious problem in the developed countries and
its elimination is a great challenge. For instance, to destroy synthetic dyes is very hard
because they are usually stable compounds and often non-biodegradable, formed by long
organic molecules. Advanced chemical oxidation methods can be effective for their
degradation as they can result in almost complete mineralization in a cheap and easy way.
One of the most well-known methods is the oxidation with Fenton’s reagent, where
hydrogen peroxide reacts with iron (or with another transition metal catalyst) in acidic
medium, leading to the production of powerful radicals (HO • ), which are able to attack the
organic compounds.
This reaction can be carried on in a homogeneous or heterogeneous system. The first
process is not adequate because it leads to a very high metal concentration in solution, ca.
50-80 ppm, while European directives don’t allow more than 2 ppm of Fe, for instance, in
discharged water [1]. Thus, heterogeneous reaction has been carried out using transition
metals (Fe, Co, Ni, Cu, Mn, W) as active phase deposited on carbon aerogels. Two
approaches were used to prepared the catalysts: in the first case, the metal precursors were
introduced in the starting solution of the polymer precursors, resulting in metal-doped
carbon aerogels; in the second case, metals were added by impregnation on two carbon
aerogels previously prepared, one mesoporous (support M) and another microporous
(support AS-3). The porosity and dispersion of the catalysts was evaluated by different
techniques: N2 adsorption, mercury porosimetry, TG, XRD, HRTEM and SEM. The dye
degradation experiments were performed in a slurry batch reactor, using a 0.1 mM orange
II (model compound) solution, as well as 6 mM of hydrogen peroxide, at 30 ºC and pH =
3. The absorbance was continuously read by a spectrophotometer and the metal leaching
quantified by atomic absorption.
The best performance was obtained with the Fe, Co and Ni catalysts, although the latter
was not so active and the second provided high leaching values. It is noteworthy that the
Fe-doped catalyst showed the lowest leaching. When employing impregnated catalysts, the
use of mesoporous supports leads to more active catalysts. The AS-3 support has a very
high adsorption capacity, not being relevant for catalytic purposes. Comparing doping and
impregnation techniques, it is observed that impregnation is more efficient probably
because metals (Fe or Co) are more accessible to the dye molecules, in spite the high
porosity of metal-doped carbon aerogels. The combination of the mesoporous support M
with a highly active metal as Fe provided an efficient and promising catalyst for practical
applications, with complete decolourisation of the solution in 2-3h employing soft
oxidation conditions, with leaching values below 2.5%.
References:
[1] Sabhi, S. and Kiwi, J. (2001), Degradation of 2,4-dichlorophenol by immobilized iron catalysts,
Water Research, 35 (8), 1994-2002.
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