TiO
2
SPECIATION PRECLUDING MINERALIZATION OF 4-
TERT-BUTYLPYRIDINE. ACCELERATED
MINERALIZATION VIA FENTON PHOTO-ASSISTED
REACTION
A. NEDOLOUJKO and J. KIWI*
M
Institute of Physical Chemistry II, Swiss Federal Institute of Technology (EPFL), 1015, Lausanne,
Switzerland
(First received 1 April 1999; accepted in revised form 1 August 1999)
AbstractÐTiO
2
mediated photodegradation of 4-tert-butylpyridine (from now on tBP) in the presence
of H
2
O
2
was absent for all practical purposes, because only 3% of the stoichiometric amount of CO
2
was generated under mercury lamp irradiation (120 W). This study has the objective of rationalizing
why in the presence of TiO
2
and light the tBP molecule does not degrade, but it readily undergoes
mineralization in homogeneous solution in the presence of Fe
3+
/H
2
O
2
. The adsorption of tBP on TiO
2
was observed to be 1% and was also found to be a function of the suspension pH. The low
adsorption observed was due to the lack of electrostatic attraction between the ionic species of tBP and
on the TiO
2
surface in well de®ned pH regions. To explain this eect, the speciation of the two
reactants was modeled as a function of pH.
Fenton photo-enhanced reactions led to full mineralization of tBP in relatively short times following
an induction period. No complex intermediate was observed between tBP and Fe
3+
. In a typical run
tBP (7.4 mM) in the presence of H
2
O
2
(5.8 Â 10
À2
M) and Fe
3+
(1.5 Â 10
À2
M) in aqueous solution
was seen to mineralize in a little more than 2 h. The same solution irradiated in the presence of TiO
2
instead of Fe
3+
was seen to generate only about 3% of the stoichiometric amount of CO
2
. The
mineralization products of tBP were identi®ed as CO
2
, NH
4
and NO
À
3
X The timing of the oxidant
addition during the degradation process was shown to either reduce the time of tBP degradation or
minimize the required amount of H
2
O
2
. Aqueous solutions of tBP generate CO
2
in the presence of
H
2
O
2
and Fe
3+
in the dark and under light after an induction period. The mineralization due to
Fenton reactions in homogeneous media was observed to follow pseudo-®rst order decay kinetics. This
work describes and explains the absence of reaction limitations during the photodegradation of tBP in
Fenton photo-assisted reactions as compared to TiO
2
photocatalytically mediated processes. 7 2000
Elsevier Science Ltd. All rights reserved
Key wordsÐTiO
2
, surface speciation modeling, photo-Fenton assisted reactions, 4-tert-butylpyridine
mineralization, eect of light intensity
INTRODUCTION
Pyridines are toxic compounds causing anorexia,
nausea and other disorders (Manahan, 1994). Var-
ious pyridine bases have been found in waste waters
from the meat processing industry and the current
methods to remove pyridines are based on adsorp-
tion and not on destructive techniques to suppress
this pollutant (Bockris, 1977).
It is of interest therefore to devise new routes for
the mineralization of pyridine and its derivatives
either by homogeneous or heterogeneous methods.
Photocatalytic mineralization through TiO
2
par-
ticles or homogeneous methods may be suitable in
this respect and are the object of this study. Recent
studies have reported that pyridine and substituted
pyridines do not decompose or decompose very
slowly in irradiated suspensions of TiO
2
(Hidaka et
al., 1994; Maillard-Dupuy et al., 1994).
In the present study we report on the photode-
gradation of 4-tert-butylpyridine (from now on
tBP) by two dierent methods. The results obtained
with TiO
2
in the presence of H
2
O
2
will be compared
to those obtained by the use of the photo-Fenton
assisted reaction. The detailed study of the adsorp-
tion of organic compounds on the sensitizer TiO
2
under light irradiation as a precursor step during
the mineralization of the latter compounds has been
sparsely explored (Stone et al., 1993; Ollis and Al-
Ekabi, 1993). In the case of pyridine no study is
available, considering in a detailed way the adsorp-
Wat. Res. Vol. 34, No. 13, pp. 3277±3284, 2000
7 2000 Elsevier Science Ltd. All rights reserved
Printed in Great Britain
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*Author to whom all correspondence should be addressed.
Tel.: +41-21-693-3621; fax: +41-21-693-4111; e-mail:
john.kiwi@ep¯.ch