Photocatalytic degradation of commercial dye, CI Reactive Red 35 in aqueous suspension: Degradation pathway and identification of intermediates by LC/MS

Article abstract of DOI:10.1016/j.molcata.2013.03.018

The photodegradation of Reactive Red 35 dye by artificial UV light was investigated in the presence of titania P-25. Six reaction intermediates were identified and separated by LC/MS, giving insight into mechanistic details and degradation pathways. The d

Full text of DOI:10.1016/j.molcata.2013.03.018

Journal of Molecular Catalysis A: Chemical 374–375 (2013) 66–72
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Journal of Molecular Catalysis A: Chemical
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Photocatalytic degradation of commercial dye, CI Reactive Red 35 in aqueous
suspension: Degradation pathway and identification of intermediates by LC/MS
Priti Bansal, Dhiraj Sud∗
Department of Chemistry, Sant Longowal Institute of Engineering and Technology (Deemed to be University), Longowal 148106, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The photodegradation of Reactive Red 35 dye by artificial UV light was investigated in the presence
of titania P-25. Six reaction intermediates were identified and separated by LC/MS, giving insight into
mechanistic details and degradation pathways. The degradation process takes place through competitive
reactions such as removal of chromophoric group, hydroxylation of the aromatic ring, substitution on
aromatic ring, devinylsulphonation, C N bond cleavage, decarboxylation and further ring opening to
give aliphatic compounds. The probable degradation pathways were proposed and discussed.
© 2013 Elsevier B.V. All rights reserved.
Received 22 October 2012
Received in revised form 16 March 2013
Accepted 18 March 2013
Available online 26 March 2013
Keywords:
Reactive dye
Photocatalysis
CI Reactive Red 35
Azo dye
LC/MS
1. Introduction
of wastewater in the textile finishing industry is physicochemi-
cal flocculation in combination with the biological treatment [7].
On a global scale, over 0.7 million tons of organic synthetic dyes
are manufactured each year mainly for use in textile, leather goods,
industrial painting, food, plastics, cosmetics and consumer elec-
tronics sector [1]. Unfortunately, exact data on the quantity of dyes
discharged in the environment are not available. It is assumed
that a loss of 1–2% in production and 1–10% loss in use are a fair
estimate [2]. Due to large-scale production and extensive appli-
cation and their direct discharge into aquatic streams, synthetic
dyes can cause considerable environmental damage and related
health problems. Reactive dyes constitute approximately 12% of
the worldwide production of the commercialized synthetic dyes
and are extensively used in the textile industry [3]. Reactive dyes
contain a reactive anchor (e.g. vinylsulphone, chlorotriazine) that
bonds covalently with the fiber during the dyeing process [4].
Some hydrolyzed reactive dyes, which undergo side-reactions of
nucleophilic addition with water, have little affinity for the fab-
ric in the dyeing process [5]. The level of unexhausted reactive
dyes typically remain at 0.06 g L−1, but possibly even as high as
0.6–0.8 g L−1, in dyehouse effluents [6] can lead to severe organic
and colour pollution in the water environment. Strict environmen-
tal regulations, water scarcity and sustainable approach have forced
the industrial sector to adopt the practice of recycling and reuse
of treated wastewater. The common method for the treatment
The incapability of conventional treatment to effectively remove
many bio-recalcitrant coloured pollutants leads to explore the new
efficient treatment systems which are effective for complete degra-
dation and mineralization of these compounds. In recent years,
semiconductor-assisted photocatalysis has been extensively inves-
tigated, mainly due to its capacity to degrade a high number of
recalcitrant chemicals in gaseous or aqueous systems, through rel-
atively inexpensive procedures. Titanium dioxide (TiO2) is found to
be more efficient catalyst for photocatalytic degradation of pollut-
ants due to faster electron transfer to molecular oxygen [8–10].
Reactive dyes consist of a chromophoric system (e.g. azo,
anthraquinone, phthalocyanin), the anchor groups and the group
which increase the water solubility (mostly SO3−). The decolour-
ization of reactive dyes results in destroying the chromophoric
system; however, the formation of toxic/nontoxic intermediates
or byproducts may take place. The requirements for recycling the
decolourized water are: (i) it should have lower toxicity than
untreated waste water and (ii) no intermediate/byproduct formed
during degradation process should be present that negatively affect
the dyeing process. Therefore, keeping these facts in consideration,
the study of reactive intermediate formed during degradation pro-
cess seems to be imperative. Until now, however, there have been
only a few investigations on the identification of intermediates
formed during the degradation process of reactive dyes. Degrada-
tion mechanism involving biological treatment of reactive Black
was revealed by using LC–MS/MS [11]. McCallum et al. [12] ana-
lyzed only four early degradation products of reactive blue 19
∗
Corresponding author. Tel.: +91 9463067541; fax: +91 1672280057.
E-mail address: suddhiraj@yahoo.com (D. Sud).
1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.03.018

P. Bansal, D. Sud / Journal of Molecular Catalysis A: Chemical 374–375 (2013) 66–72
67
2.3. Procedure
The degradation experiments were carried out by adding
100 mg of photocatalyst (TiO2 Degussa) to 100 ml of dye solution
and suspension was subjected to irradiation under UV light. The
aqueous suspension was magnetically stirred and aerated through-
out the experiment. At different time intervals aliquot was taken
out with the help of syringe and then filtered through Millipore
syringe filter of 0.45 ␮m. The absorption spectra were recorded
at ꢀmax 536 nm. The rate of degradation was studied in terms of
changes in absorption spectra. The decolourization efficiency (%)
has been calculated as:
Fig. 1. Structure of Reactive Red 35.
by using NMR, LC/MS and Raman spectroscopy. The formation of
highly oxidized products formed during ozonation of Reactive yel-
low 84 such as salts (NO3−, SO42−, Cl−) and short chained carboxylic
acids (oxalic acid, formic acid, etc.) was observed using HPIC [13].
With the help of LC–mass spectrometry polar components in sur-
face water were reported on the basis of accurate masses [14].
Degradation products of CI Reactive Orange 16 were identified by
GC/MS [15]. In our earlier work [16,17] identification of intermedi-
ates of CI acid orange 7 by LC/MS and CI reactive blue 160 by GC/MS
for establishing the mechanism of photocatalytic degradation was
carried out.
CI Reactive Red 35 (RR35) is a monoazo commercial dye, com-
monly used for dyeing cotton, viscose, flex and jute but not suitable
for silk, wool and polyester. These dyes suitable for white and colour
resistant printing and tinting of white ground in printing and Reac-
tive Red 35 is reasonably photostable to sunlight. However, only a
few studies on their degradation have been reported in the litera-
ture [18] and no study on TiO2-assisted photocatalytic degradation
pathway of the RR35 dye under UV-irradiation has been reported.
Keeping in view, widespread use in industries and thereby its pos-
sibility of release in the wastewater RR35 has been selected for
present work. In continuation of our previous work on establishing
the reaction pathway for photocatalytic degradation of synthetic
dyes [16,17], the present research focuses on identification of the
reaction intermediates formed during photodegradation of RR35
dye in the UV/TiO2 process by using liquid chromatography–mass
spectrometry (LC/MS) and ion chromatography (IC) for proposing
the probable mechanistic pathway of the dye.
C0 − C
Efficiency (%) =
× 100
C0
where C0 is the initial concentration of dye and C is the concen-
tration of dye after photo-irradiation. Similar experiments were
carried out by varying the pH of the solution, concentration of dye
and dose of photocatalysts.
To evaluate formation and degradation of the reaction inter-
mediates and to assess the mineralization, the photocatalytic
experiments were carried out up to 8 h under optimized conditions.
For COD analysis 2 ml of test solution was pipette into the
standard amount of potassium dichromate oxidizing mixture and
digested at 150 ◦C for 2 h. Then COD was measured using COD
meter.
2.4. LC/MS analysis
For the identification of degradation products, the samples
were analyzed by LC/MS (Water Alliance 2795 LC). A capillary
column Terra C-18 (5 ␮m × 100 mm length) was used for separa-
tion of product intermediates. The mobile phase was a mixture of
acetonitrile–water (70/30 (v/v)) filtered through Millipore syringe
filter of 0.22 ␮m. The flow rate of elute was 0.08 mL min−1 and the
injection volume was 20 ␮L. The eluent from the chromatographic
column successively enter the UV–vis diode array detector, the ESI
interface and the quadruple ion trap mass analyzer. MS analysis
in the positive ions mode was performed on a mass spectrometer
equipped with an ESI ion source. The ESI probe tip and capillary
potentials were set at 2.5 kV and 25 V, respectively. The mass range
was 50–400 m/z. The heated capillary was set to 200 ◦C.
2. Experimental
2.1. Materials
Titania P-25 (surface area 50 m2/g) was obtained from Degussa.
Commercially available Reactive Red 35 (RR35) was obtained from
Nahar Fabrics, Lalru, India and was used without further purifica-
tion. Structure of the dye is shown in Fig. 1. Double distilled water
was used for preparation of various solutions. pH of the solutions
was adjusted with 1 M HCl or 1 M NaOH.
2.5. Ion chromatography
The concentration of NO3−, SO4
ions and low molecular
2−
weight aliphatic acids (formic, acetic and oxalic acid) in solution
was determined by ion chromatography on a model 761 Compact
IC (Metrohm) using a MetrosepASupp5-250 column and carbonate
as standard eluent.
2.2. Instruments
3. Results and discussion
Photochemical degradation experiments were carried out in
specially designed reaction vessels (diameter 0.08 m, volume
500 ml) in the photoreactor equipped with 4 UV tubes each of 30 W
(Philips). The intensity of UV light was 2.4 × 10−6 einstein/min mea-
sured by chemical method, i.e. potassium ferrioxalate actinometry
[19]. The experimental set-up was reported earlier [20]. Constant
stirring of solution was ensured by using magnetic stirrers and aer-
ation was done with the help of aquarium aerator. The spectra were
taken with UV–VIS Spectrophotometer (Shimadzu 1650); pH meter
(Thermo Orion 920A) was used to adjust the pH of the solution. COD
analysis was carried with Thermo Orion Aqua Fast II AQ 2040 COD
meter.
3.1. UV–vis spectra
Time-dependent UV–vis spectrum of RR35 dye during pho-
toirradiation with TiO2 is shown in Fig. 2. The spectra of RR35
show peaks at 379, 310 and 234 nm in UV region and a main
band with a maximum at 536 nm in visible region. The rate of
decolourization was recorded with respect to the change in the
intensity of absorption peak in visible region. The prominent peak
was observed at ꢀmax, i.e. 536 nm which decreased gradually and
finally disappeared indicating that the dye had been decolour-
ized.

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P. Bansal, D. Sud / Journal of Molecular Catalysis A: Chemical 374–375 (2013) 66–72
Fig. 2. Time-dependent UV–vis absorption spectra for decolourization of RR35 using
Fig. 4. Effect of catalyst dose on decolourization rate of RR 35 dye (dye initial con-
TiO2.
centration – 25 mg/L, pH – 7).
3.2. Photolysis/photocatalysis of Reactive Red 35
TiO2 powders (commercial photocatalysts) have scattering albedo
(ω) higher than 0.5 [23], which increases the radiation scattering
results in reducing the number of photons absorbed; hence further
increase in catalyst dose decreases the decolourization efficiency.
Therefore, the catalyst dose 1.5 g/L was fixed for degradation of
RR35.
Decolourization of RR35 was investigated under different exper-
imental conditions, viz. UV light alone, UV/TiO2, and Dark/TiO2.
Fig. 3 depicts the photocatalytic decolourization of RR35 under
these experimental conditions. The decolourization rate was
recorded in terms of change in intensity of characteristic peak at
536 nm. Initially blank experiments were performed under UV irra-
diation without addition of any catalyst (UV alone) and only 4.55%
decolourization was observed. The adsorption of the dye was also
observed with TiO2 catalyst, i.e. dark/TiO2. Only 7% adsorption of
the dye was seen with TiO2 under dark conditions. After that pho-
tocatalytic experiments were carried out using catalyst at fixed
dye concentration (25 mg/L), pH 7 and catalyst loading of 1 g/L
(UV/TiO2). The complete decolourization of dye was achieved after
3 h with TiO2 as a photocatalyst (UV/TiO2) under these experimen-
tal conditions.
Fig. 5 shows the colour removal efficiency of photocatalysts as
a function of pH. The results reveal that the lesser degradation of
dye occurs in basic solution and higher in acidic region with TiO2.
The interpretation of pH factor on the efficiency of photocatalytic
degradation process can be explained on the basis of acid–base
property of metal oxide surface and the ionization state of ioniz-
able organic molecule. The point zero charge (pzc) for TiO2 (Degussa
P25) is 6.8 [23]. TiO2 surface is positively charged in acidic media
(pH < 6.8) whereas it is negatively charged under alkaline condition
(pH > 6.8). RR35 is an anionic dye in aqueous solution. For TiO2, rate
of photodecolourization increased with decrease in pH, exhibiting
maximum efficiency (100%) at pH 4. Findings of others [24,25] also
show that degradation of anionic dyes is more in acidic medium
because at pH higher than pzc of titania, its surface becomes nega-
tively charged, so adsorption will be less.
After optimizing the experimental conditions, the photocat-
alytic discolouration of RR35 was carried out by varying the initial
concentration of the dye from 10 to 100 ppm. As the concentra-
tion of the dye is increased, the rate of photodegradation decreases
indicating either to increase the catalyst dose or time span has to be
increased for the complete removal. Fig. 6 clearly shows that with
increase in initial concentration of dye percentage decolourization
of RR35 decreases with TiO2.
3.3. Effect of process parameters on degradation of RR35
In order to optimize the process parameters, i.e. pH and dose of
catalyst, the experiments were performed by varying catalyst con-
centration from 0.5 g/L to 2.0 g/L and pH of the dye (2–10). The graph
plotted (Fig. 4) between amount of catalyst used and percentage
degradation reveals that with an increase in catalyst dose degra-
dation efficiency increases up to 1.5 g/L. Thus with the increase
of catalyst dosage, total active surface area increases, hence the
availability of more active sites on catalyst surface [21]. It has
also been reported that the catalyst amount has both positive and
negative impacts on the photodecomposition rate [22]. Moreover,
Fig. 7 shows the kinetics of disappearance of RR35 for an initial
concentration of 25 mg/L under optimized conditions with TiO2.
The results show that the photocatalytic decolourization of the
dye can be described by the first order kinetic model, ln(C0/C) = kt,
Fig. 3. Photocatalytic decolourization of RR35 dye (dye initial concentration –
Fig. 5. Effect of pH on decolourization rate of RR35 dye (dye initial concentration –
25 mg/L, pH – 7, catalyst dose – 1 g/L).
25 mg/L, catalyst dose – 1.5 g/L, time – 3 h).

P. Bansal, D. Sud / Journal of Molecular Catalysis A: Chemical 374–375 (2013) 66–72
69
Fig. 6. Effect of initial concentration of RR35 dye on percentage decolourization
under optimized conditions (catalyst dose – 1.5 g/L, pH – 4).
where C0 is the initial concentration and C is the concentration at
any time, t. The semilogarithmic plots of the concentration data
gave a straight line. The correlation constant for the fitted line was
calculated to be R2 = 0.909 for TiO2. The rate constant for degra-
dation of RR35 using TiO2 was calculated to be 0.029 min−1. This
constant is a function of many operational conditions and will vary
with reactor configuration unless the effect of the photon absorp-
tion is included in the rate equation [26].
Fig. 8. Absorbance changes of spectral peaks (234, 310 and 536 nm) of RR35 at as
function of irradiation time in aqueous solution.
decreases with time and disappears after 3 h and 6 h, respectively,
resulting in complete decolourization of the solution and degrada-
tion of intermediates containing naphthalene ring. The absorbance
at 234 nm first increases and then decreases up to 2 h, after that no
regular trend is observed and even after 8 h it does not disappear;
these changes can be attributed to the formation of intermediates
resulting from the photodegradation of the azo dye and intermedi-
ate containing benzene ring.
3.4. Mineralization of Reactive Red 35
As the reduction of chemical oxygen demand (COD) reflects
the extent of degradation or mineralization of an organic species,
the percentage change in COD was studied for dye sample (initial
concentration 25 mg/L) under optimized conditions (catalyst dose
1.5 g/L, pH 4, time 3 h). The COD reduction is lesser (74.52%) in 3 h
than percentage decolourization (100%) which may be due to the
formation of smaller uncoloured products. Therefore, it seems that
to achieve complete mineralization of dyes, longer irradiation time
is required.
3.5.2. LC/MS analysis
The retention time in the liquid chromatograms of various inter-
mediates formed during photocatalytic degradation of RR35 at
different time interval (40 min to 8 h) is shown in Table 1. The com-
ponents eluted having different retention time were subjected to
mass spectrometry and identified by interpretation of their frag-
ment ions in the mass spectra. The chromatographs suggest that
the entire components except E disappear after 8 h of photocat-
alytic treatment. The mass spectra of intermediates are shown in
Fig. 9.
3.5. Analysis of intermediates formed after photocatalytic
degradation of RR354
Six main compounds detected in the solution are presented
in Table 2. The six intermediates of CI Reactive Red 35 appeared
during 8 h of irradiation are: sodium 4-amino-5-hydroxy-6-
(phenylediazenyl)naphthalene-2-sulphonate (intermediate A),
3.5.1. Spectrophotometric analysis
The spectra of RR35 show peaks at 379, 310 and 234 nm in UV
region and a main band with maximum at 586 nm in visible region.
The absorbance peak at 234 and 310 nm is due to the benzene and
naphthalene rings [27], and the absorbance peak at 536 nm may
be due to the azo linkage of RR35. To study the fate of colourless
intermediates formed photocatalytic reaction was carried up to 8 h.
The change in absorbance value of CI Reactive Red at 234, 310 and
536 nm as a function of irradiation time in aqueous solutions is
shown in Fig. 8. It is observed that absorption at 536 and 310 nm
sodium
phenolate
3-((1-hydroxynaphthalen-2-yl)diazenyl)-4-methoxy-
(intermediate B), N-(7-diazenyl-3,8-dihydroxy-
naphthalen-1-yl)acetamide (intermediate C), 3-diazenyl-4-
hydroxybenzenesulphonic acid (intermediate D), phthalic acid
(intermediate E) and N-(m-tolyl)acetamide (intermediate F).
Phthalic acid has already been reported during degradation of
various dyes [15,27–30].
3.5.3. IC analysis
2−
The formation of NO3−, SO4
(tR = 17.4 min and 25.5 min,
respectively) and smaller aliphatic acids (acetate, formate,
oxalate ions having tR = 6.2 min, 7.1 min, 30 min, respectively) was
Table 1
Retention time of intermediates obtained formed photocatalytic degradation of
RR35 identified by LC/MS.
Product
tR (min)
A
B
C
D
E
F
19.45
12.30
5.13
7.42
5.05
4.98
Fig. 7. Kinetics analysis for RR35 dye under optimized conditions.

70
P. Bansal, D. Sud / Journal of Molecular Catalysis A: Chemical 374–375 (2013) 66–72
Fig. 9. Mass spectra of intermediates A–F.
confirmed by the ion chromatography (IC) by their retention time
3.6.1. Generation of •OH radical
(tR) and also during fragmentation pattern.
When the catalyst is exposed to UV radiation with light energy
greater than its band gap energy (Eg 3.14 eV) electrons are pro-
moted from the valence band to conduction band and resulted in
formation of an electron–hole pair [31].
3.6. Probable mechanistic photocatalytic degradation pathway of
Reactive Red 35 under UV light
On the basis of the experimentally identified intermediate
species a proposed degradation pathway of RR 35 (Fig. 10) is as
follows.
TiO2 + hꢁ(UV) → TiO2(eCB− + hVB
)
+

P. Bansal, D. Sud / Journal of Molecular Catalysis A: Chemical 374–375 (2013) 66–72
71
Table 2
Chemical structures of intermediates of RR35 identified by LC/MS analysis.
Name of compound
Chemical structure
M. Wt.
SO3Na
N
N
Fig. 9(A)
Sodium 4-amino-5-hydroxy-6-(phenylediazenyl) naphthalene-2-sulphonate
365.2
OH
NH
2
NaO
N
N
Fig. 9(B)
Fig. 9(C)
Sodium 3-((1-hydroxynaphthalen-2-yl)diazenyl)-4-methoxyphenolate
N-(7-diazenyl-3,8-dihydroxynaphthalen-1-yl)acetamide
316.2
245.2
OH
OH
OCH3
HN
N
NHCOCH3
NH
HO
HO3S
N
Fig. 9(D)
Fig. 9(E)
3-Diazenyl-4-hydroxybenzenesulphonic acid
202.2
166
OH
COOH
COOH
Phthalic acid
H3C
Fig. 9(F)
N-(m-tolyl)acetamide
149
CH3COHN
NaO S
3
O
NaO S
SO Na
3
3
O
S
N
N
O
OH
NHCOCH
3
OCH
3
C-N bond Cleavage
m/z 749.6
Devinylsulphonation
C-N bond Cleavage
Devinylsulphonation
HO S
3
N
NH
OH
SO Na
3
(D)
*m/z 202.2
OH
N
N
NaO
OH
NH
2
N
N
HN
N
*m/z 365.2
(A)
NHCOCH
HO
OH
OCH
3
3
(C)
*m/z 245
(B)
*m/z 316.2
COOH
H C
3
COOH
(E)
*m/z 166
CH COHN
3
(F)
*m/z 149
mineralization
Fig. 10. Proposed degradation pathway of RR35 dye with TiO2 under optimized conditions.

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P. Bansal, D. Sud / Journal of Molecular Catalysis A: Chemical 374–375 (2013) 66–72
−
+
as LC–ESI-TOF method, leading to additional information regarding
the elemental composition which can be calculated based on deter-
mined accurate mass.
where eCB and hVB are the electrons in the conduction band and
holes in the valance band respectively. Further both these entities
can migrate to the catalyst surface, where they can undergo redox
reaction via the formation of •OH radical [32,33].
Redox potential of •OH radical is very high (E0 +2.8 V); thus, it is
a powerful oxidizing agent and attacks dye present at or near the
surface of TiO2 to oxidize it. The hydroxylated dye may degrade via
devinylsulphonation and cleavage of C N bond.
Acknowledgement
Support from NIPER, Mohali is gratefully acknowledged.
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4. Conclusion
RR35 could be successfully decolourized and degraded by tita-
nia based photocatalysis. Sulphonated and polar intermediates are
identified using LC/MS technique as polar components cannot be
extracted efficiently in the solvents usually used in GC–MS analysis.
Smaller aliphatic acids and inorganic ions (SO42−and NO3−) were
detected using IC technique. A detailed reaction mechanism has
been established from the initial step of generation of •OH free rad-
ical to the final products. The formation of hydroxylated products,
C-N bond cleavage, devinylsulphonation, opening of naphthalene
ring and their subsequent degradation products were formed in
agreement with general rules as proposed for the degradation of
other complex molecules in water. Phthalic acid still remains after
8 h of degradation in very less quantity. LC/MS technique has been
successfully employed for the detection of intermediate which
cannot be detected by other techniques but form integral part of
degradation of the dye. In addition, MS spectra collected during
this work may be used as a spectral data bank for further studies
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