ISSN 1203-8407 © 2007 Science & Technology Network, Inc. J. Adv. Oxid. Technol. Vol. 10, No. 2, 2007 219
Simple Models for the Control of Photo-Fenton by Monitoring H
2
O
2
Jordi Bacardit*
, 1
, Isabel Oller
2
, Manuel I. Maldonado
2
, Esther Chamarro
1
, Sixto Malato
2
, and
Santiago Esplugas
1
1
Department of Chemical Engineering, University of Barcelona. Martí i Franquès 1, 08028-Barcelona, Spain
2
Plataforma Solar de Almería-CIEMAT, Carretera Senés km 4, 04200-Tabernas (Almería), Spain
Abstract: It has been demonstrated that Photo-Fenton process is feasible for the remediation of wastewater
containing multiple organic pollutants. In order to promote its industrialization, control strategies have to be
defined and tested. The objective of the present work is to describe a mathematical equation that fits the amount
of COD abated regarding the dose of H
2
O
2
. UVA Laboratory-scaled and solar up-scaled experiments are
compared. Also two H
2
O
2
supply strategies are compared; one in which H
2
O
2
is added at the beginning of an
experiment; the second, H
2
O
2
is added in doses through an experiment. According to the results, simple
equations have been found to fit experimental COD degraded depending on the amount of H
2
O
2
supplied. There
is also a ratio of COD degraded per amount of H
2
O
2
consumed that fits a wide range of operating conditions.
The ratio is estimated to be 0.51 (mg O
2
/mg H
2
O
2
) which can be considered as a stoichiometry of Photo-Fenton
process in the current conditions.
Introduction
Advanced Oxidation Processes (AOPs) are
promising methods for the remediation of con-
taminated wastewaters containing non-biodegradable
organic pollutants (1). They are mostly based on the
production of hydroxyl radical HO
•
in water, which is
the most powerful oxidising species after fluorine
(2.80 V and 3.03 V respectively) (2). The capability
of exploiting the high reactivity of HO
•
radicals in
driving oxidation processes is suitable for achieving
the complete abatement and mineralization of the
pollutants through even less reactive species (3).
Some of these AOPs, such as Photo-Fenton or
Photocatalysis, are based on the interaction of
chemical reagents or catalysts with a light source.
The present study deals with Photo-Fenton (or photo-
assisted Fenton) process (Ph-F). As it is classically
described, hydroxyl radicals are produced in this
technique by decomposition of hydrogen peroxide
when reacting with ferrous ions (Fenton reaction) (4)
(Reaction 1). The irradiation with sunlight or an
artificial light source of wavelength 180-400 (5) nm
or even in the visible spectra (6), increases the rate of
contaminant degradation mainly by stimulating the
reduction of Fe
3+
to Fe
2+
(5) (Reaction 2).
• − + +
+ + → + HO HO Fe Fe O H
3 2
2 2
(1)
• + + +
+ + → + HO H Fe O H Fe
2 hν
2
3
(2)
Keywords: Photo-Fenton, COD, efficiency, control, modeling,
hydrogen peroxide
*Corresponding author; E-mail: bacardit@angel.qui.ub.es
As it is generally accepted, hydroxyl radicals
formed by these reactions, oxidize the organic species
(Reaction 3), generating organic-radical species, which
undergo oxidation up to mineralization, dimerize
(Reaction 4), or are reduced (Reaction 5) (4).
• •
+ → +
i 2 i
R O H H R HO (3)
(dimer) product 2R
i
→
•
(4)
H R Fe Fe R
i
3 H 2
i
+ → +
+ + •
+
(5)
A modern interpretation of Fenton (and photo-
Fenton) mechanism, assumes that other oxidizing
intermediates such as highly valent iron-complexes
(Fe
4+
) are formed during the oxidation of Fe
2+
to Fe
3+
(Reaction 6 and 7) (7).
[ ]
+ + +
→ − → +
4
aq 2 2
2
2 2
2
aq
Fe O H Fe O H Fe (6)
− • + +
+ + → + HO HO Fe O H Fe
3
aq 2
4
aq
(7)
Fenton and Photo-Fenton processes are quite
complex, and are still matter of discussion. Their
mechanisms imply numerous reactions, some of them
highly undesirable (8) because they produce an excess
of reagent consumption. Some side reactions are
commonly represented by Eq. 8 to 11.
• •
+ → +
2 2 2 2
HO O H O H HO (8)
2 2
O H HO HO → +
• •
(9)
− + + •
+ → + OH Fe Fe HO
3 2
(10)
+ − + •
+ → +
3 2
Fe R Fe R (11)
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