Analytical studies on the oxidative degradation of the reactive textile dye Uniblue A

Article abstract of DOI:10.1021/es0008665

Oxidative degradation process of organic materials in wastewater is an area of significant current interest. In the case of commercial textile dyes, little is known about the structures of the actual products that can form once a dye has been submitted to oxidative conditions. Here, a product analysis approach was applied to identify some of the major early degradation products of Uniblue A (UBA) when reacted with peroxydisulfate (PDS). UBA is the vinyl sulfone form and major wastewater constituent of the commercial anthraquinone textile dye C.I. Reactive Blue 19. Using NMR, LC-MS, and Raman, four reaction products could be identified, and possible reaction pathways are discussed.Oxidative degradation process of organic materials in wastewater is an area of significant current interest. In the case of commercial textile dyes, little is known about the structures of the actual products that can form once a dye has been submitted to oxidative conditions. Here, a product analysis approach was applied to identify some of the major early degradation products of Uniblue A (UBA) when reacted with peroxydisulfate (PDS). UBA is the vinyl sulfone form and major wastewater constituent of the commercial anthraquinone textile dye C.I. Reactive Blue 19. Using NMR, LC-MS, and Raman, four reaction products could be identified, and possible reaction pathways are discussed.

Full text of DOI:10.1021/es0008665

Environ. Sci. Technol. 2000, 34, 5157-5164
environmental interest to study its oxidative degradation
Analytical Studies on the Oxidative
Degradation of the Reactive Textile
Dye Uniblue A
J E R E M Y E . B . M C C A L L U M , †
S T E P H E N A . M A D I S O N , S I B E L A L K A N ,
R I C H A R D L . D E P I N T O , A N D
R O Y U . R O J A S W A H L *
chemistry. UBA has considerable structural complexity on
one hand (amino and sulfone groups, quinoid structure),
while the absence of tautomerism (as in the case of
R-hydroxyazo dyes) renders it feasible for detailed product
analysis and meaningful mechanistic interpretations. Much
of the published chemistry of C.I. Reactive Blue 19 has
concerned the vinyl sulfone reactive moiety of the dye and
is well-documented in the literature (5-8). On a different
note, the formation of UBA aggregates in aqueous solution
has been reported (9) but under neutral to alkaline conditions
only. Hence for mechanistic reasons, dye degradation studies
are best carried out under moderately acidic conditions
because of its higher solubility through possible protonation
of the amino groups.
Unilever Research U.S., 45 River Road,
Edgewater, New Jersey 07020
Peroxydisulfate (PDS). This strong oxidant (E ) +2.05 V)
has long been used industrially for the destruction of
hydraulic fluids in the petroleum industry as an industrial
bleach and reaction initiator. Recently, it was suggested to
be a useful reagent for the destruction of organics in
hazardous wastewaters through a process named Direct
Chemical Oxidation (DCO), where the oxidant could be
regenerated by electrolytic methods (10, 11). Even though
PDS can also undergo ionic reactions (12, 13), its main
application of interest is the thermal or photochemical
Oxidative degradation process of organic materials in
wastewater is an area of significant current interest. In
the case of commercial textile dyes, little is known about
the structures of the actual products that can form
once a dye has been submitted to oxidative conditions.
Here, a product analysis approach was applied to identify
some of the major early degradation products of Uniblue
A (UBA) when reacted with peroxydisulfate (PDS). UBA is
the vinyl sulfone form and major wastewater constituent
of the commercial anthraquinone textile dye C.I. Reactive
Blue 19. Using NMR, LC-MS, and Raman, four reaction
products could be identified, and possible reaction pathways
are discussed.
•
-
generation of the sulfate radical anion SO
4
through ho-
molysis of the peroxide bond. At pH > 3, this is the only
-
reaction, while at pH < 3, HS
2
O
8
is formed, which decays
•-
4
to give sulfur tetroxide and bisulfate (14). At pH > 8.5, SO
is known to produce HO upon reaction with HO
Therefore, unequivocal mechanistic studies with SO
•
6
(15).
•
-
4
are
best carried out between pH 3 and pH 8.5. In recent work
by Salem et al. (16), the oxidation of coumarin-1, a laser dye,
by PDS was reported. A nonradical reaction mechanism
where the dye acts as a nucleophile and PDS is the electrophile
was proposed, based on the evidence that the radical trap
allyl acetate did not affect the dye fading kinetics.
Introduction
Purpose and Scope of the Work. Wastewater management
is a major concern for many industries but in particular for
the textile dyeing industry (1). Here, reactive dyes pose a
particular problem because, among other reasons, they are
relatively expensive, tend to hydrolyze in the dyeing bath,
and up to 60% of the applied reactive dye can end up in the
wastewater (2). Commonly, oxidative processes are used for
treatment of such effluents. Often, this involves the use of
reactive oxygen species, which can generate free radicals at
the structural backbone of the dyes. For most dyes, little is
known about the actual reaction pathways that occur after
the initial attack. Similarly, dye degradation product analysis
has seldom been reported when using industrially important
textile dyes, even though product analysis can lead to
considerable mechanistic understanding. Undoubtedly, this
insight could be useful in the effective control of undesired
reactions in industrial dye degradation processes. In this
context, we describe here the detailed analysis of some of
the major, early degradation products of Uniblue A (UBA)
from its reaction with peroxydisulfate (PDS) and discuss
possible reaction pathways.
Experimental Section
Materials. All chemicals, unless otherwise noted, were
purchased from Aldrich and used as received. Uniblue A was
9
0% pure and purified with a semipreparative column as
described below. Potassium peroxydisulfate was purchased
from Fluka. Quinizarin-2-sulfonic acid was purchased from
the Sigma Library of Rare Chemicals. The water used in all
experiments was distilled and passed through a Milli-Q
deionization system.
Chrom atography. HPLC measurements were performed
on a Waters LC module 1, using a Waters 996 PDA detector.
A Nucleosil C18 (250 × 4.6 mm) 100 Å 5 µm column was used
for all analytical separations while the corresponding cor-
responding semipreparative column, C18 Nucleosil (250 ×
1
0 mm) 100 Å 5 µm, was used for large-scale separations. A
mobile phase consisting of 70:30, methanol:10 mM phosphate
buffer, pH 6.7, was utilized at a flow rate of 0.5 mL/ min for
analytical separation. The same mobile phase was used with
the semi-prep LC at a flow rate of 2.3 mL/ min for the
preparative separation of isolated fractions. A 103-mL sample
of a 4 mM Uniblue A solution (initial reaction concentration)
was injected using a 5-mL injection loop. Isolated fractions
were collected and stored under nitrogen at 0 °C. Relative
product percentages of total dye decomposition products
are based on amount of material collected and the degree
of dye decomposition under standard reaction conditions.
The further separation of peaks 3 and 4 (see Figure 1) was
accomplished at a flow rate of 0.7 mL/ min (on the analytical
Uniblue A (UBA). This dye is the vinyl sulfone form of the
anthraquinone dye C.I. Reactive Blue 19, which is used
industrially to dye cellulosic substrates including cotton
fabrics and which is one of the highest volume reactive dyes
on the market. UBA has been reported to be the predominant
constituent of textile wastewaters where C.I. Reactive Blue
1
9 is used in the dyeing process (3, 4). Hence, it is of immediate
*
Corresponding author phone: (201)840-2430; fax: (201)840-8262;
e-mail: roy.rojas-wahl@unilever.com.
†
Present address: UCLA, Department of Chemistry and Bio-
chemistry, Box 951569, Los Angeles, CA 90095-9569.
1
0.1021/es0008665 CCC: $19.00

2000 Am erican Chem ical Society
VOL. 34, NO. 24, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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5 1 5 7
Published on Web 11/17/2000

FIGURE 1. Chromatograms of UBA reactions with PDS (λ ) 254 nm). The numbers 1-4 correspond to the products shown in Figure 2.
A) 1 mM UBA + 2 mM peroxydisulfate at 65 °C for 7 h. (B) 0.1 mM UBA + 0.2 mM peroxydisulfate at 65 °C for 7 h.
(
column; 3.3 mL/ min on the preparative column) with a
mobile phase of 50:50, methanol:10 mM phopshate buffer,
pH 6.7.
(proton) time domain, 2000 data points, spectral width 2500
Hz, 32 scans; F1 (proton) time domain, 256 rows. HMQC:
F2 (proton) time domain, 2000 data points, spectral width
2500 Hz, number of scans 128 and 256; F1 (carbon-13) time
domain, 256 rows, spectral width 7500 Hz. HMBC: F2
(proton) time domain, 2000 data points, spectral width 2500
Hz, number of scans 128 and 256; F1 (carbon-13) time
domain, 256 rows, spectral width 14 kHz.
13C. The spectrum was measured in a 5-mm NMR tube
with 90o 13C pulse for a 9.5 µs. Ten thousand transients were
accumulated with a 5-s relaxation delay between pulses.
Spectral width: 22 kHz.
NMR. One-dimensional proton NMR data and two-
dimensional homonuclear and heteronuclear chemical shift
correlation NMR data were collected on a Bruker DMX-500
1
spectrometer using a 5-mm H triple axes inverse (TXI) probe
with triple axes gradients. Proton resonances were used for
the identification of decomposition products of UBA. To
obtain more detailed structural information, the proton
1
chemical shifts were correlated to the H chemical shifts using
homonuclear correlation spectroscopy (COSY) and correlated
1
3
to the C chemical shifts using heteronuclear multiple
quantum coherence spectroscopy (HMQC) and hetero-
nuclear multiple bond correlation spectroscopy (HMBC).
NMR samples were run on the total mass of each of the
DEPT-135. The spectrum was measured in a 5-mm NMR
1
tube with H decoupler pulses of 90° pulse for 5.5 µs, 180°
pulse for 11 µs, and 135° pulse for 10 µs. Five thousand
transients were accumulated with a 10-s relaxation delay
between pulses. Spectral width: 27 kHz.
products collected in dimethyl sulfoxide-d
6
(DMSO). Carbon-
1
4
3 NMR data were collected at 100.6 MHz on a Bruker DMX-
00 spectrometer using 5 mm C broad band observe probe.
MS/LC-MS. These experiments were conducted on a
Finnigan MAT LCQ Ion Trap mass spectrometer using
negative ion electrospray in the full-scan mode. Typical
source operating conditions were 3.5 kV spray voltage, heated
capillary temperature of 200 °C, and sheath and auxiliary gas
flows of 60 and 20 au, respectively. The UBA reaction mixtures
were separated with a Nucleosil C18 100/ 5 µm 150 × 2.1 mm
column using 65/ 35, methanol:10 mM ammonium acetate,
as the mobile phase flowing at 0.2 mL/ min. All fraction
collected peaks (from semi-prep) were analyzed by direct
infusion. Tandem MS experiments were conducted by direct
infusion into the mass spectrometer.
1
3
Distortionless enhancement by polarization transfer (DEPT-
35) experiment was acquired at 125.8 MHz on a Bruker
DMX-500 spectrometer using a broad band observation probe
for distinguishing CH and CH groups. The 1D and 2D NMR
1
2
data sets were processed with the Bruker XWINNMR software,
operating on UNIX workstations. All NMR experiments were
performed at 300 K. DMSO was used as a solvent for all NMR
experiments.
1
H. The spectrum was measured in a 5-mm NMR tube
1
with 90° H pulse for 7.5 µs. One thousand transients were
accumulated with a 3-s relaxation delay between pulses. Total
experiment time: 1 h. Spectral width: 2500 Hz. COSY: F2
IR/Ram an. Infrared. The spectrum was acquired on a
Bio-Rad FTS-40 spectrometer with a DTGS detector. The dye
5
1 5 8
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 24, 2000

SCHEME 1. Major Decomposition Products Resulting from the Degradation of UBA (4 mM) with PDS (8 mM) at 65 °C
was mixed and ground with KBr in ca. 1/ 10 ratio and analyzed
by diffuse reflectance (DRIFTS). Thirty-two scans were
Radical Scavenging Experim ents. A sample of 1 mM allyl
alcohol was used in the standard dye reaction of 1 mM UBA
and 2 mM K S O at 65 °C.
2 2 8
-
1
coadded and averaged at a resolution of 8 cm
.
Raman. The same dye/ KBr mixture described above was
pressed into a pellet. The Nicolet Raman 950 FT-Raman
spectrometer was used. A near-IR laser (1064 nm) excited
the Raman scattering. A power of 0.26 W was incident on the
sample. A total of 1024 scans was coadded and averaged.
The scattering was detected with a liquid nitrogen-cooled
Ge detector.
Product Identification
Four products could be separated and identified from the
reaction of UBA and PDS. Typical LCs are shown in Figure
1. The structural assignments are summarized in Scheme 1
and are discussed below.
Uniblue A. To structurally identify the dye decomposition
products, it was first necessary to understand the NMR
characteristics of the dye itself. Its structure is shown in
Scheme 1 (including labels); the 1H NMR and the COSY
(homonuclear correlation spectroscopy) are depicted in
Figure S1 (see Supporting Information). The protons that
belong to the same spin system were sorted out by COSY.
The assignments including 13C and HMBC (heteronuclear
multiple bond correlation spectroscopy) are displayed in
Table 1 (spectra not shown). The short- and long-range
molecular connectivity were used to map out the entire
molecular structure.
Product 1. The material corresponding to peak 1 was
identified as 1, as shown in Scheme 1. Typically, 12.4 mg
(0.018 mmol) of this product was collected from prep-LC
runs and represents roughly 8% of the total dye decomposi-
tion products from the standard degradation reaction at an
Standard Reaction Conditions and Im pact on Products
Identified. Degradation reactions of UBA (0.1 and 1 mM)
were carried out with PDS (0.2 and 2 mM K S O , respectively)
2 2 8
at 65 °C for 7 h and left open to air, unless otherwise
mentioned. The initial pH of the solutions was approximately
5
.5 and decreased gradually over time to pH 3 for the 1 mM
UBA reactions and to pH 5.2 for the 0.1 mM UBA reactions.
No buffer was used to facilitate MS analysis. Additionally,
control reactions were run at constant pH values of 5.5 and
3
2 4
.0 (preadjusted with H SO and kept constant during the
reaction by stepwise addition of diluted NaOH solution).
Also, for the purpose of product isolation only, the experi-
ments were repeated at higher concentrations of UBA (2 and
4
mM) and PDS (4 and 8 mM, respectively). In all cases the
same major decomposition products could be identified by
LC and LC-MS, except for the run at 0.1 mM UBA (0.2 mM
PDS) concentration, where no peak for 1 could be detected.
1
initial dye concentration of 4 mM. With respect to UBA, H
VOL. 34, NO. 24, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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5 1 5 9

TABLE 1. Assignments of NMR Resonances of Uniblue A
1Ha
δb
δc
COSY
HMBC
a, b 8.28 (q, J 1 13.75 Hz, 126.9
J 2 7.47 Hz)
Hc, Hd Ha, Cc, Hb, Cd
e
8.04 (s)
123.6
133.8, 134.5 Ha, Hb Hc, Ca, Hd, Cb
c, d 7.89 (m )
i
7.73 (s)
121.0
131.9
127.5
122.6
Hi, Cg, Ch
Hf, Hh Hg, Ci, Ch
g
h
f
7.69 (t, J 7.78 Hz)
7.62 (d, J 7.98 Hz)
7.59 (d, J 7.75 Hz)
Hg
Hh
Hh, Cg, Ci
Hf, Ch
j
7.16 (q, J 1 16.45 Hz, 139.1
J 2 9.85 Hz)
Hk, Hl Hj, Ck, Cl
k
l
6.40 (d, J 1 16.44 Hz) 130.2
6.25 (d, J 1 9.83 Hz) 130.2
Hj
Hj
Hk, Cj
Hl, Cj
FIGURE 2. Fragmentation pattern of 1.
a
Letters refer to arom atic and vinyl protons. Blanks are due to singlet
or unobserved correlation. b Proton chem ical shift values (δ) are
followed by the apparent splitting patterns (s, d, t, or q) and apparent
coupling constants (Hz). c Carbon-13 chem ical shift values (δ) obtained
from HMQC.
troscopy indicate the presence of the azo linkage, with a
-1
peak at 1460 cm , where there was no detection of a peak
in the IR (Figure S5, see Supporting Information). It has been
reported that the azo linkage in azobenzene has been detected
-
1
at 1510 cm for the cis isomer and from 1460 to 1380 for the
trans isomer (17-19), which is therefore the most likely
structure in our case as well.
TABLE 2. Assignments of NMR Resonances of Product 1
1Ha
δb
δc
COSY
HMBC
Product 2. The material corresponding to peak 2 was
identified as 2, as shown in Scheme 1. Under standard
reaction conditions at an initial dye concentration of 4 mM,
s
a
r
e
p
b
d
c
i
g
o
h
f
8.23 (s)
8.22 (d)
8.16 (d,J 7.94 Hz)
8.12 (s)
120.73
Hs, Cr,
Ha, Cb, Cd
Hr, Co
d
126.40 Ha, Hc
128.14 Hr, Hp
119.07
129.32
126.48
4
4
8.8 mg (0.096 mmol) of 2 was collected, representing roughly
0% of the decomposition products. Product 2 was formed
8.10 (d, J 7.93 Hz)
d
7.98 (d)
7.94 (t)
7.93 (t)
Hb, Ca
Hd, Ca
Hc, Cd
Hi, Cg
Hg, Ci
Ho, Cr
Hh, Cf
Hf, Ch
Hl, Ck
Hk, Cl
Hu, Ct
Hv, Ct
by the oxidation of the primary aromatic amino group on
the anthraquinone ring. NMR data (not shown) are nearly
identical to those of the dye. Mass spectrometry data (Figure
S6, see Supporting Information) further support the assigned
structure by showing a [M - H] one mass unit higher than
that of the dye (m/ z 484 vs m/ z 483). The change in mass to
an even number indicates that there is an odd number of
nitrogens on the molecule (original dye contains two
nitrogens). The MSMS experiments afforded the same
fragmentation pattern as UBA itself (not shown).
e
130.76
e
130.76 Hc, Hb
121.43 Hi, Hf
134.17 Hg, Hi
128.03
131.11
131.08
129.26 Hl, Hj
129.26 Hk, Hj
130.72 Hu, Ht
130.72 Hv, Ht
7.88 (s)
e
7.86 (t)
7.77 (t)
e
d
7.75 (d)
7.70 (d)
d
l
6.25 (d, J 9.92 Hz)
6.39 (d, J 16.33 Hz)
6.31 (d, J 9.92 Hz)
6.46 (d, J 16.48 Hz)
7.15 (q, J 19.77, J 2 16.48 Hz) 136.51 Hj, Hl,k Hj, Cl,k
7.24 (q, J 19.76, J 2 16.48 Hz) 136.53 Ht, Hu,v Ht, Cu,v
k
u
v
j
Product 3. Peak 3 of the LC chromatogram is of a vinyl-
hydrolyzed form of 2 and 1-amino-4-hydroxyanthraquinone-
t
2
-sulfonic acid 3, as identified by LC-MS (m/ z 318, not
a
Letters refer to arom atic and vinyl protons. Blanks are due to singlet
shown). Other minor components are also found in peak 3,
but they remain to be identified.
or unobserved correlation. b Proton chem ical shift values (δ) are
followed by the apparent splitting patterns (s, d, t, or q) and apparent
c
Product 4. The main component of peak 4 has been
identified to be quinizarin-2-sulfonic acid (1,4-dihydroxy-
anthraquinone-2-sulfonic acid), as shown in Scheme 1. Figure
coupling constants (Hz). Carbon-13 chem ical shift values (δ) obtained
from HMQC. Unresolved doublets. e Unresolved triplets.
d
3
shows a comparison of the UV-vis of 4 and commercial
NMR indicates the presence of an additional vinyl sulfone
group and approximately four aromatic protons, as seen in
Figure S2 (see Supporting Information). Table 2 summarizes
the data obtained on this reaction product. Figure S3 (see
Supporting Information) shows the HMBC spectrum of the
aromatic region. HMBC represents the connectivity between
the nonprotonated carbonyl resonances of the quinone ring
quinizarin-2-sulfonic acid and the LC separation of the two
products. These products exhibited the exact same retention
time, and a co-injection resulted in only one single peak. MS
and NMR experiments of the isolated fraction and com-
mercial source have further confirmed the identity of 4 to be
quinizarin-2-sulfonic acid (spectra not shown). The ap-
proximately 4 mg of quinizarin-2-sulfonic acid collected was
found to represent approximately 5% of the total dye
decomposition products.
and the protons H
a
and H
b
. Also the assignments of the
protons H and H
d
c
were derived from their corresponding
carbons, which showed that the unfunctionalized part of the
anthraquinone moiety remained intact. The proposed struc-
1
3
Mechanistic Studies
ture of 1 is further supported by a C and a DEPT-135 NMR
experiment (spectra not shown). Most importantly, mass
spectrometry experiments give an ion m/ z 662 for peak 1.
This mass accounts for an addition of 179 mass units,
corresponding to the anilino vinyl sulfone side chain addition
to a radical of the dye itself. Figure S4 (see Supporting
Information) depicts the fragmentation pattern of m/ z 662
constructed from tandem MS data. The fragmentations are
consistent with that of the dye (loss of 64 and 91) (Figure 2).
Oxygen? The UBA and PDS reaction was carried out both
open to air and under an inert atmosphere and a degassed
reaction solution. The then-obtained LCs are in both cases
identical to those in Figure 1. Apparently, oxygen does not
play a role in the formation of the identified reaction products.
Radical or Nonradical? For comparative purposes, PDS
self-decomposition was followed in the presence of 1 mM
NaCl to probe the effect of ionic strength. From Figure 4, it
is apparent that ionic strength does not play a major role in
the PDS decomposition. Additionally, it is evident that PDS
decomposes at a faster rate when in the presence of UBA,
2
The loss of 28 is indicative of N . This is substantiated by the
fact that the change from m/ z 662 to m/ z 634 is a neutral loss
and an odd number of nitrogens is present. Raman spec-
5
1 6 0
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 24, 2000

FIGURE 3. Comparison of LC behavior and absorbance of 4 with quinizarin. (A) LC separation of 4 and quinizarin. (B) UV spectra of 4
and quinizarin.
•
-
indicating that the dye does not react solely with free SO
4
reaction of UBA and PDS was performed. Nothing was
detected by GC, while a blank experiment with benzene (2
mM in solution, 10 mL of headspace volume injected) added
to the reaction mixture gave a clear signal.
2
-
but with PDS directly (S
2
O
8
+ UBA f products) or with a
, which may be in equilibrium with PDS itself
•
-
caged SO
4
2
-
•-
(
S
2
O
8
h 2SO
4
f products). To know whether the mech-
anism involves radicals or not, a sulfate radical scavenger
was added to the reaction mixture to see if a decrease in dye
decomposition could be detected. With the addition of 1
Proposed Reaction Pathways. Figure 5 contains three
possible UBA dye fading pathways leading to the formation
of the major product 2. As indicated above, we believe PDS
•-
mM allyl alcohol, which has been reported to react with SO
4
•-
acts in a radical process through SO
4
. The latter species is
8
-1
-1
with a rate constant of 7.4 × 10 mol
s
(20), dye
generally accepted to react primarily as an one electron
oxidant (21), especially with aromatic compounds (22-24),
and only in a few instances have hydrogen abstraction (25)
or addition reactions (23) been observed. In contrast, the
less selective hydroxyl radical has been shown to react more
or less randomly by multiple mechanisms through all three
types of mechanism, and the formation of major amounts
(poly)hydroxylated aromatic products is very characteristic
for the latter species (26). In our study with PDS and UBA,
the formation of 2 is best explained by proposing the radical
degradation was completely inhibited. Hence, the second
possibility is the most likely one. After approximately 4 h, the
rate of UBA consumption decreases while the decomposition
of PDS continues. This indicates that PDS is oxidizing UBA
decomposition products forming secondary oxidation prod-
ucts.
Volatiles? Additionally, the possibility of almost total
degradation of the dye into volatiles such as benzene, for
one, was explored. Analysis of the headspace of a sealed
VOL. 34, NO. 24, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
5 1 6 1

FIGURE 4. Decomposition study of PDS and UBA.
FIGURE 5. Possible reaction pathways for the decomposition of UBA with PDS yielding the major product 2.
cation of the dye as the first intermediate. In fact, when UBA
is treated with one-electron oxidizing reagents, convincing
evidence for the formation of a UBA radical cation has been
reported already (27-29). Subsequently, there are three
possible pathways leading to 2: (a) further oxidation to a
bis-cationic quinoid structure followed by aromatic substi-
tution, where subsequently two reducing equivalents could
come from other dye molecules; (b) adduct formation of the
•
-
dye radical cation with SO
4
followed by double hydrolysis;
or (c) direct aromatic substitution followed by one-electron
reduction and hydrolysis. Even when using simple model
compounds such as aromatic amines (30), no pathway can
unequivocally be confirmed or ruled out, and some specula-
tion remains unavoidable at this time. Also the formation of
5
1 6 2
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 24, 2000

SCHEME 2. Proposed Pathway for Formation of Reaction Product 1
3
3
and 4 can easily be rationalized in a similar way: Product
might arise from reaction of UBA at the vinylsulfonylphenyl-
reaction mechanism of potassium peroxydisulfate with
anthraquinone dyes. Three possible reaction pathways, all
occurring at the 1,4-diamino moiety of UBA and based on
initial one-electron oxidation of the dye, are proposed to
explain the formation of 2-4, while 1 arises most likely
through radical coupling mechanism and subsequent oxida-
tion of the so-formed hydrazine to the corresponding azo
species. Because peroxydisulfate is currently being used in
an emerging, alternative oxidation technology called direct
chemical oxidation (DCO) (37) and because UBA is an
important commercial dye, it would be interesting to degrade
UBA under DCO conditions. Especially the possibility of the
formation of a product like 1, which contains an electron-
withdrawing and hence oxidation potential-increasing azo
group might represent a challenge to any oxidative technique.
For example, in oxidative activated sludge processes, only 3
out of 18 azo dyes were biodegraded (38).
substituted nitrogen. Product 4 may arise from further
oxidation of 2 or 3. Thus, for the formation of 2-4, the sulfate
radical anion most likely reacts through one-electron oxida-
tion of the dye. To the best of our knowledge, there is no
reasonable way to explain the formation of these products
through a hydrogen abstraction or aromatic addition process.
Clearly, at UBA concentrations of 1 mM and higher,
however, the pathway for formation of 1 provides a distinctive,
different look into the dye degradation process. Here we
believe that either direct hydrogen abstraction at the -NH
2
group by PDS or radical cation formation of UBA with
subsequent aminyl proton loss take place. The former
mechanism was suggested by the work of Gupta and
Srivastava using simple aromatic amines (31). Under acidic
conditions, similar to our study, they reported azo bond
formation from the oxidation of 4-chloro-2-methylaniline
by PDS through hydrogen abstraction at the -NH
2
groups
Acknowledgments
and subsequent coupling of the so-formed anilinoradicals
to yield the corresponding hydrazine species, which was then
further oxidized by PDS to give the azo compound. Using
various aromatic amines, many results have been obtained
that led to similar mechanistic conclusions (32-36). On the
basis of this general reaction scheme, we propose in Scheme
We thank Prof. A. G. M. Barrett (Imperial College, London)
and Drs. P. Sarojini and L. van Gorkum (Unilever Research
U.S.) as well as Dr. J. Oakes (Unilever Research Port Sunlight,
U.K.) for helpful discussion. Mr. D. Drutis (Unilever Research
U.S.) is thanked for kindly providing the Raman spectral
analysis. Dr. J. F. Cooper (Lawrence Livermore National
Laboratory, Berkeley, CA) is thanked for the kind supply of
his manuscript. We also thank the Editor and the referees for
helpful comments.
2
a comparable pathway for the formation of 1. The ani-
linovinyl sulfone radical could be formed from the further
degradation of 2, where the anilinovinyl sulfone part is
liberated in the process of the formation of 4. Here, the free
anilinovinyl sulfone could then again react with the sulfate
radical anion through hydrogen abstraction or radical cation
formation followed by proton loss, both of which would yield
the anilinovinyl sulfone radical species required for the
formation of1. Similarly, the anilinovinyl sulfone radical could
be formed during the formation of 3 from UBA itself.
Finally, for those cases where UBA concentrations of 1
mM as well as 2 and 4 mM were used, the last two of which
were run for product collection purposes only, possible
heterogeneous effects cannot be entirely ruled out (9), but
our chosen acidic reaction conditions render this possibility
unlikely (31).
Supporting Information Available
Figures S1-S6 showing the NMR spectrum of UBA; the NMR,
MS, Raman, and IR spectra of 1, and the MS spectrum of 2
(
6 pages). This material is available free of charge via the
Internet at http:/ / pubs.acs.org.
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