Journal of Photochemistry and Photobiology A: Chemistry 152 (2002) 175–181
Near-UV–VIS light induced acid orange 7 bleaching in the
presence of SiW
12
O
40
4-
catalyst
Idil Arslan-Alaton
a,∗
, John L. Ferry
b
a
Department of Environmental Engineering, Faculty of Civil Engineering, Istanbul Technical University, Maslak, Istanbul 80626, Turkey
b
Department of Chemistry and Biochemistry, Graduate Sciences Research Center (GSRC), University of South Carolina,
631 Sumter Street, Columbia, SC 29208, USA
Received 7 May 2002; accepted 13 May 2002
Abstract
The azo dye acid orange 7 (AO7) was decolourized by SiW
12
O
40
5-
, the one-electron photo-chemically reduced form of SiW
12
O
40
4-
catalyst, in aqueous solutions of aerated and de-aerated sacrificial electron donor isopropanol. Under our experimental conditions, the AO7
decay rate depended upon the competition between the azo dye and molecular oxygen for SiW
12
O
40
5-
. The colour removal rate constant
was found as 5.2 × 10
4
M
-1
s
-1
by applying AO7–O
2
competition kinetics based on a simple kinetic reaction scheme.
© 2002 Elsevier Science B.V. All rights reserved.
Keywords: Acid orange 7; Textile azo dyes; Competition kinetics; Heteropolyblue; Polyoxometalates; Photochemical treatment; SiW
12
O
40
4-
catalyst
1. Introduction
Environmental issues associated with residual colour in
waste streams of the dye-manufacturing and dye-consuming
industries have gained more attention in the recent years
due to the increasing public awareness in environmental
issues and the introduction of stricter wastewater regula-
tions worldwide. Dyes are required to exhibit a high degree
of photolytic and microbial stability to fulfill the fastness
requirements of textile consumers. As a consequence, com-
mercial dyes are not readily degradable by conventional
chemical and biochemical treatment processes [1,2]. Though
fixation and biodegradability of new commercial dyes have
been improved significantly, still significant losses occur
during manufacturing and processing of dye products that
end up in publicly owned treatment plants.
Within the overall category of dyestuffs, azo dyes con-
stitute a significant portion and probably have the least de-
sirable consequence in terms of environmental impact [3].
They are readily reduced under anoxic conditions to poten-
tially hazardous aromatic amines and are carcinogenic in
their own right [4].
Most methods practiced for dye removal include physi-
cal adsorption or coagulation–flocculation steps, biological
∗
Corresponding author. Tel.: +90-212-285-65-76;
fax: +90-212-285-65-87.
E-mail address: arslanid@itu.edu.tr (I. Arslan-Alaton).
treatment as well as destructive chemical oxidation (i.e.
ozonation and advanced oxidation processes) and reduction
[2,5]. More recently, research has focused on alternative
methods of degrading textile azo dyes via heterogeneous
photocatalysis [6–12] as well as uncatalyzed and catalyzed
[13–15] wet air oxidation processes. Both approaches in-
volve the formation of active oxygen species, such as
hydroxyl (OH
•
) and hydroperoxyl (HO
2
•
) radicals [16]
that initiate fast catalytic reactions resulting in the com-
plete colour removal. From the ecological point of view,
destructive treatment processes are preferred to phase trans-
fer methods to prevent a secondary contamination of the
residual effluent by the treatment agents.
The ability of polyoxometalates (heteropolyacids and
their salts; abbreviated herein as POM) and their transition-
metal substituted derivatives to undergo reversible one or
multi-electron transfer with retaining their original struc-
ture, has rendered these compounds attractive acid and
redox catalysts in a variety of industrial catalytic applica-
tions [17,18]. For instance alternative K
5
SiW
11
VO
40
and
-Na
x
H
5-x
PMo
10
V
2
O
40
-mediated catalytic processes have
been developed that proceed at higher rates and consume
significantly less chemicals than conventional technologies
[18,19]. As such, they pose less threat to the environment.
Particularly the polyoxotungstates PW
12
O
40
3-
, SiW
12
O
40
4-
and W
10
O
32
4-
undergo fast, step-wise and re-
versible redox reactions, acting as facile multi-electron
and dioxygen relays [19,20]. Absorption of near-UV light
1010-6030/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII:S1010-6030(02)00185-5