P A P E R
Competitive oxidative cleavage and epoxidation of
4,40-diaminostilbene-2,20-disulfonic acid by Fe(III) aqua complexes
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Pascal Wong-Wah-Chung,* Gilles Mailhot, Jean-Franc¸ois Pilichowski and Michele Bolte
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Laboratoire de Photochimie Moleculaire et Macromoleculaire (CNRS UMR 6505),
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Universite Blaise Pascal, 24 avenue des Landais, 63177, Aubiere cedex, France.
E-mail: pascal.wong-wah-chung@univ-bpclermont.fr; Fax: þ33 4 73 40 77 00;
Tel: þ33 4 73 40 71 72
Received (in Toulouse, France) 29th July 2003, Accepted 27th November 2003
First published as an Advance Article on the web 24th February 2004
4,40-Diaminostilbene-2,20-disulfonic acid (DSD) has been investigated as a model compound of fluorescent
whitening agents. The reaction of DSD with Fe(III) aqua complexes has been studied in the dark at room
temperature: a fast degradation of DSD was observed under our experimental conditions. HPLC analysis was
used to follow the kinetics of the redox reaction. The process involves epoxidation, leading to DSD epoxide (1),
and the oxidative cleavage of the double bond, leading to 5-amino-2-formyl-benzenesulfonic acid (2) together
with the reduction of Fe(III) into Fe(II). The dependence of the redox process on [Fe(H2O)5(OH)]2þ
concentration indicates that the dominant reaction pathway is a reaction between DSD and the Fe(III)
monomeric species, [Fe(H2O)5(OH)]2þ. The complete degradation of DSD is observed with an excess of
oxidant; the ratio [Fe(H2O)5(OH)]2þ:[DSD] has to be higher than 2. A mechanism giving rise to the two
degradation products is proposed.
dark degradation of DSD in the presence of Fe(III) at room
Introduction
temperature. With the aim of elucidating the reaction and
the Fe(III) species involved in the process, we studied the beha-
viour of different DSD and Fe(III) mixtures. This paper
deals with a complete investigation on the Fe(III) induced
degradation of DSD.
4,40-Diaminostilbene-2,20-disulfonic acid (DSD) is an impor-
tant intermediate in the synthesis of a great number of fluores-
cent whitening agents. As a result, it can be discharged in non
negligible amounts into the aquatic environment. Stilbene deri-
vatives have been investigated during the last few decades,1–3
because they are difficult to remove through the usual biologi-
cal wastewater treatments and because of their inefficient
photodegradation,4 that is, poor degradability either by micro-
organisms or by sunlight in surface waters. The development
of new techniques to eliminate such compounds from water
becomes, therefore, important. In addition, DSD is a conveni-
ent model molecule of stilbenic fluorescent whitening agents
for such studies.
Among the photodegradation processes, Fenton’s method,
which leads to reactive OH radicals, strong oxidising agents,
has been widely used to eliminate these types of compounds.5
Moreover, So¨rensen and Frimmel showed that DSD in water
can be eliminated significantly in the UV/H2O2 process.6 More
recently, Yu et al. and Zhu et al. improved the degradability of
DSD by a pre-treatment combining ferrous hydrogen per-
oxide oxidation and ozonation or coagulation-flocculation
processes.7,8
In our laboratory, a method based on the Fe(III) aqua com-
plexes appeared to be very efficient in the photoinduced degra-
dation of different pollutants: 4-chlorophenol,9 4-octylphenol10
and 2,6-dimethylphenol,11 for example. The reaction involves
hydroxyl radicals (ꢀOH) formed during the photolysis of Fe(III)
aqua complexes.12,13 In all the investigated systems, the mono-
meric species [Fe(H2O)5(OH)]2þ, so-called Fe(OH)2þ, appears
to be the most photoactive species.
This method has been applied to eliminate this compound.
The work on the fate of DSD was separated into two different
parts: (i) the direct photolysis of DSD, this compound absorb-
ing up to 380 nm and therefore undergoing a photoreaction
under solar light,14 and (ii) the Fe(III) photoinduced degrada-
tion.15 However, it appears that there is also an immediate
Experimental
Reagents
4,40-Diaminostilbene-2,20-disulfonic acid (trans-DSD) was an
Acros product (95%) used without further purification.
Sodium perchlorate was a Prolabo product (99%) and perchlo-
ric acid was a Merck product. Ferric perchlorate nonahydrate
[Fe(ClO4)3ꢁ9H2O; 97%] was a Fluka product kept in a dessica-
tor. The Fe(III) solutions were prepared by diluting a stock
solution [2.0 ꢂ 10ꢃ3 mol Lꢃ1 in Fe(ClO4)3ꢁ9H2O] to the appro-
priate Fe(III) concentration. All the solutions were prepared
with ultrapure aerated water (Millipore aQ, resistivity ¼ 18.2
MO cm). The pH was adjusted with HClO4 and pH measure-
ments were carried out with an Orion pH meter to 0.01 unit.
The ionic strength was not controlled. Deoxygenated solutions
were obtained by bubbling with argon for 20 min at room
temperature.
A chemical method,16 shortly described here, was used to
separate the products obtained from the reaction of DSD with
Fe(III) (3.4 ꢂ 10ꢃ5 and 3.0 ꢂ 10ꢃ4 mol Lꢃ1, respectively).
Firstly, the solution was concentrated by vacuum evaporation
of most of the solvent. Afterwards, an aqueous solution of
NaOH was added dropwise until the pH was around 9.5.
The solution was filtered to eliminate the iron hydroxide pre-
cipitate. Separation was then achieved by elution through a
silica gel column (silica gel 60) using AcOEt–EtOH–H2O
(j ¼ 70:18:12) as eluent. The purification of the products
was achieved by passing through an Amberlite column (Hþ/
Naþ). The final products were dried by lyophilisation.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 4 5 1 – 4 5 6
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