Electrochemical Incineration in the Presence of Halides
C. A. Martínez-Huitle, S. Ferro, and A. De Battisti
z
Department of Chemistry, University of Ferrara, 44100 Ferrara, Italy
The influence of different halides on electrochemical incineration of oxalic acid was studied. Experiments were initially carried out
at a Pt electrode in alkaline media, in which case, in the absence of mediating effects, a slow electroxidation was observed.
However, with halides, the rate of the electrochemical incineration was increased significantly, following the order F
-
Br
-
Cl
-
, and the rate of the mediated process was practically independent of applied current density. An explanation has been
attempted for the role of halides, in relation with the oxygen evolution reaction, concomitant with the electrochemical incineration.
© 2005 The Electrochemical Society. DOI: 10.1149/1.2042628 All rights reserved.
Manuscript submitted June 16, 2005; revised manuscript received July 11, 2005. Available electronically August 25, 2005.
The so-called electrochemical incineration of organic substrates
in aqueous media has been thoroughly discussed during the last two
decades, as an effective method for the detoxification of wastewaters
containing biorefractory pollutants.
1-7
As thoroughly demonstrated
in literature, although the electrochemical approach requires rela-
tively complex equipment and specific handling skills, it leads to
the in situ generation of highly reactive species, which may be more
effective in the destruction of organics.
8
In practice, the minimization of reaction by-products and of dan-
gerous chemical transportation, coupled with a fast elimination of
pollutants, make electrochemical incineration an interesting alterna-
tive to more traditional chemical attacks.
6-8
As an example, the elec-
trogeneration of strong oxidants, like CeIV, AgII, CoIII, can be
an interesting alternative to less environmentally friendly thermal
methods in the destruction of highly toxic organic process by-
products. The high strong-acid concentration needed to achieve sig-
nificant concentrations of the above species makes this method more
suitable for the treatment of low-water-content sludges, with a main
process occurring essentially in two steps.
5-7
The oxidative attack of organics in aquatic media does not follow
a conceptually different path. We can speak formally of direct and
indirect mediated processes. Into the latter group we can include
the above reported “metal-ion-assisted” oxidations, as well as
active-chlorine and ozone-mediated attacks, where the main reaction
stages take place in the solution bulk. The former group would in-
clude those processes whose main stages take place at the electrode
surface, through adsorption of reactants and intermediates, the
strong oxidant being essentially the hydroxyl radical. Considering
the mechanism recently proposed by Comninellis,
9
who assumed the
action of electrogenerated OH
•
extended to a reaction cage in the
vicinity of the electrode surface, rather than limited at the surface
itself, the distinction between direct and mediated incineration be-
comes even less clearcut.
10
A particularly interesting case is that of
active-chlorine mediation, which has been dealt with in several
papers,
11-16
and whose practical applications have been described in
a number of patents.
17,18
The interest for this path to electrochemical incineration is due to
i the ubiquitous presence of chloride ions in a number of effluents
and also of natural waters, which makes the involvement of active-
chlorine, in anodic processes in these media, inevitable; ii the
chemistry and electrochemistry of chlorine higher oxidation states at
pH not far from neutrality, which make its use as an incineration
mediator of some practical interest. The complexity of active-
chlorine reactivity in the presence of organics deserves further thor-
ough insights. The well-known formation of organic chlorinated
compounds, common intermediates in the chloride-mediated oxida-
tion of aromatic substrates,
12
but also in the oxidation of aliphatic
molecules, which can yield chloroform
13
through the haloformic
reaction
19,20
requires understanding of the conditions under which
these phenomena take place. Within this frame, the electrochemical
approach may supply a considerable improvement, due to the more
careful control of process variables.
In earlier works, it has been shown that at different pH in the
alkaline region, a complex substrate like glucose, can be incinerated
in a wide anodic current density range, especially in the presence of
chlorides.
14,15
In these articles, the experimental data were ac-
counted for by the assumption that the main part of the incineration
process is a surface reaction, based on adsorbed oxychloro-radicals.
The possibility that some role could be played by the anodic shift of
the oxygen evolution, caused by Cl
-
ions in solution has also been
taken into consideration. The latter aspect has been discussed earlier,
mainly in relation with the electrosynthesis of strong oxidants.
21-23
As a further development of the investigation carried out on the role
of chlorides in glucose electro-oxidation,
15
and also on the basis of
other evidence,
15,21-23
in this work research was extended to the
anodic oxidation of oxalic acid OA, in the presence of different
halides: F
-
, Cl
-
and Br
-
, to better focus on volume and surface
effects in indirect electrochemical incineration. OA has been consid-
ered an interesting substrate, because of its often discussed insuffi-
cient reactivity toward anodic oxidation, which also results in an
incomplete mineralization of more complex organic frames.
24
Experimental
Bulk electrolyses were carried out in a single-body, thermostated
Pyrex glass cell of 250 mL; the anode was either a Ti/Pt plate De
Nora, Milan, Italy or a Pt cylindrical grid, both with a geometrical
area of 15 cm
2
, and the test solution volume was 200 mL. A larger
cylindrical platinum grid was used as the counter electrode when the
cylindrical Pt anode was investigated, while a Zr cathode was used
for the Ti/Pt plate. Experiments were performed at 25°C, investigat-
ing the role of applied current density J and mediators; the J range
was established at 300 and 600 A m
-2
. Solutions were vigorously
stirred, by a magnetic stirrer. For the quasi-steady polarization curve
measurements, a small Pt wire was used, with a real surface area of
about 4.7 mm
2
, as determined by the evaluation of hydrogen
accommodation.
25
Constant current experiments and electrochemical analyses were
performed with an Amel model 553 galvanostat, and an Autolab
PGSTAT20 EcoChemie, The Netherlands, respectively. Quasi-
steady polarization curves were carried out at a scan rate of
0.5 mV s
-1
and with a 0.45 mV step potential, in solutions of NaCl,
NaBr, or NaF at different concentrations, in alkaline media and us-
ing Na
2
SO
4
to further increase the electrolyte conductivity. Potential
values were referred against a double-walled, saturated calomel
electrode SCE, with an intermediate saturated NaNO
3
solution.
Fresh 0.1 M solutions of oxalic acid Fluka, dihydrate salt were
prepared for use in 0.25 M NaOH Riedel-de-Haën and 0.5 M
Na
2
SO
4
Baker analyzed, using distilled water. NaCl, NaBr, and
NaF Fluka were used as received, without further purification.
During the electrolysis, the oxalic acid residual content was de-
termined by a conventional titration method with KMnO
4
as re-
ported in Ref. 26. The instantaneous current efficiency ICE for the
anodic oxidation was calculated, using the following relation
13
z
E-mail: dbtcll@unife.it
Electrochemical and Solid-State Letters, 8 11 D35-D39 2005
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