Enhanced photocatalytic degradation of Amaranth dye on mesoporous anatase TiO2: Evidence of C-N, NN bond cleavage and identification of new intermediates

Article abstract of DOI:10.1039/c7pp00090a

The photocatalytic degradation mechanism of Amaranth, a recalcitrant carcinogenic azo dye, was investigated using mesoporous anatase TiO₂ under sunlight. Mesoporous anatase TiO₂ of a high photocatalytic activity has been synthesized using a sol-gel method and its photocatalytic activity for the degradation of Amaranth dye has been evaluated with respect to Degussa P25. The effect of bi-dentate complexing agents like oxalic acid, ethylene glycol and urea on the surface properties of TiO₂ catalyst has been investigated using TG-DTA, FTIR, HR-TEM, SAED, PXRD, EDS, UV-DRS, PL, BET N₂ adsorption-desorption isotherm studies and BJH analysis. The influence of catalyst properties such as the mesoporous network, pore volume and surface area on the kinetics of degradation of Amaranth as a function of irradiation time under natural sunlight has been monitored using UV-Vis spectroscopy. The highest rate constant value of 0.069 min^-1 was obtained for the photocatalytic degradation of Amaranth using TiO₂ synthesized via a urea assisted sol-gel synthesis method. The effect of the reaction conditions such as pH, TiO₂ concentration and Amaranth concentration on the photodegradation rate has been investigated. The enhanced photocatalytic activity of synthesized TiO₂ in comparison with P25 is attributed to the mesoporous nature of the catalyst leading to increased pore diameter, pore volume, surface area and enhanced charge carrier separation efficiency. New intermediates of photocatalytic degradation of Amaranth, namely, sodium-3-hydroxynaphthalene-2,7-disulphonate, 3-hydroxynaphthalene, sodium-4-aminonaphthalenesulphonate and sodium-4-aminobenzenesulphonate have been identified using LC-ESI-MS for the very first time, providing direct evidence for simultaneous bond cleavage pathways (-C-N-) and (-NN-). A new plausible mechanism of TiO₂ catalysed photodegradation of Amaranth along with the comparison of its toxicity to that of its degradation intermediates and products is proposed.

Full text of DOI:10.1039/c7pp00090a

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Enhanced photocatalytic degradation of Amaranth dye on
mesoporous anatase TiO2: evidence of C-N, N=N bond
cleavage and identification of new intermediates
Received 00th January 20xx,
Accepted 00th January 20xx
Amarja P. Naika, Akshay V. Salkara, Mahesh S. Majika and Pranay P. Morajkara*
DOI: 10.1039/x0xx00000x
Photocatalytic degradation mechanism of Amaranth,
a recalcitrant carcinogenic azo dye, was investigated using
www.rsc.org/
mesoporous anatase TiO2 under sunlight. Mesoporous anatase TiO2 of high photocatalytic activity has been synthesized
using sol-gel method and its photocatalytic activity for degradation of Amaranth dye has been evaluated with respect to
Degussa P25. Effect of bi-dentate complexing agents like oxalic acid, ethylene glycol and urea on surface properties of TiO2
catalyst has been investigated using TG-DTA, FTIR, HR-TEM, SAED, PXRD, EDS, UV-DRS, PL, BET N2 adsorption-desorption
isotherm studies and BJH analysis. The influence of catalyst properties such as mesoporous network, pore volume and
surface area on kinetics of degradation of Amaranth as a function of irradiation time under natural sunlight has been
monitored using UV-Vis spectroscopy. The highest rate constant value of 0.069 min-1 is obtained for the photo catalytic
degradation of Amaranth using TiO2 synthesized via urea assisted sol-gel synthesis method. The effect of reaction
conditions such as pH, TiO2 concentration and Amaranth concentration on photo-degradation rate has been investigated.
The enhanced photocatalytic activity of synthesized TiO2 in comparison to P25 has been attributed to mesoporous nature
of the catalyst leading to, increased pore diameter, pore volume, surface area and enhanced charge carrier separation
efficiency. New intermediates of photo catalytic degradation of Amaranth namely, sodium-3-hydroxynaphthalene-2,7-
disulphonate, 3-hydroxynaphthalene, sodium-4-aminonaphthalenesulphonate and sodium-4-aminobezenesulphonate
have been identified using LC-ESI-MS for the very first time, providing direct evidence for simultaneous bond cleavage
pathways (-C-N-) and (-N=N-). A new plausible mechanism of TiO2 catalysed photo-degradation of Amaranth along with
the comparison of its toxicity to that of its degradation intermediates and products is proposed.
as compared to rutile due to increased rate of photo generated
electrons and holes. Induction of mesoporosity in crystalline TiO2
1 Introduction
Organic azo dyes are one of the most common persistent organic
pollutants, directly emitted to the environment by industries such
as textile, leather, food and cosmetic 1,2. It is now established that,
these untreated dyes pose a great hazard to environment as their
complex aromatic structure, stability and low-biodegradability
results in their accumulation in natural water bodies and causes
biochemical and morphological effects on biological systems3.
Amaranth is one such widely used azo dye, used in food and
cosmetic industry 4. Food additives and contaminants committee
has restricted the use of fifteen synthetic dyes out of which,
Amaranth is one of the most toxic, mutagenic and carcinogenic in
nature 5,6.Owing to its harmful effect on biological systems, many
conventional and biological methods like coagulation, membrane
separation, adsorption etc. have been investigated for removal of
Amaranth from water bodies 3. However, due to its high solubility in
water, these methods are ineffective for complete separation or
mineralization. The photocatalytic dye degradation using TiO2 has
emerged as one of the most promising route, due to its low band
gap, inertness, economic viability, insolubility in water, stability to
photo corrosion and its effectiveness towards the degradation of
results in improved photocatalytic efficiency due to availability of
greater specific surface area for effective dye adsorption and mass
transport to the reactive sites, leading to enhancement in photo
degradation rate. Various methods such as hydrothermal,
combustion, co-precipitation, sol - gel etc.8-11 have been proposed,
to prepare crystalline mesoporous anatase TiO2 with very high
surface area of 234 m2g-1. When compared to former methods of
synthesis, sol-gel method has been found to be the most promising,
due to its low processing temperature, homogeneity and
stoichiometric synthesis control 11. In most of these synthesis
methods, the surface morphologies of TiO2 are usually modified
using expensive long chain complexing agents or surfactants via a
modified sol-gel process for better photo degradation efficiency.
Photocatalytic degradation of Amaranth using TiO2 in UV light
in presence of strong oxidizing agents such as H2O2, enhances
the rate of formation of oxidants i.e. holes, OH and HO2
radicals which leads to effective degradation (Gupta et al.13).
However, efficiency and safety in handling and implementing
such a process for practical applications depends on complete
knowledge of dye degradation intermediates products and
aqueous organic contaminants6,7
particular, has been found to exhibit higher photo catalytic activity
. Crystalline anatase TiO2 in
their toxicity evaluation4,5,12,13
.
No such analysis on
intermediates products have been provided in the work of
Gupta et al.13 Moreover, due to handling problem with bulk
H2O2, more focus has been directed towards enhancing the
photocatalytic properties of TiO2. Researchers have also
investigated the rate of degradation of Amaranth by doping
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transition metals like Cu, Ag on TiO2 surface4. Jain et al.
investigated the enhancement of photocatalytic degradation
of Amaranth by doping N and S onto TiO2 using thiourea as
surface modifier. However, these studies use methods such as
hydrothermal and combustion synthesis which lack practical
application for bulk catalyst synthesis. Moreover the metal
loaded catalysis leads to leaching of active metals from the
surface of catalyst which in turn leads to decrease in catalytic
efficiency. Therefore facile methods of synthesis, use of low
cost templates or complexing agents which are cost effective,
economical and provide appreciable photocatalytic activity of
TiO2 are desired.
14
2 Experimental
2.1 Materials and methods
Amaranth dye (85%) was purchased from LobaChem and has been
used without any further purification. Other chemicals such as
titanium isopropoxide (98%), urea (99.5%), oxalic acid (99.5%),
ethylene glycol (99.5%) and 10% aqueous ammonia solution were
obtained from Spectrochem and were used without any further
purification.
2.2 Catalyst synthesis
Mesoporous TiO2 catalyst has been synthesized by using sol gel
method. To 50 mL of Titanium isopropoxide-ethanol mixture at
8˚C, 10% ammonia solution (200 mL) was added for the
formation of metal hydroxide (at pH = 8). The metal hydroxide
solution was then peptized using 1N HNO3 followed by
addition of 16 g of structure directing agents (SDA) such as
urea, oxalic acid and ethylene glycol under constant stirring.
The mixtures were stirred for 1 hour, aged and dried at 100 ˚C
to form dry solid/semi solid. These as-synthesized mixtures
were subjected to calcination in air at 550˚C for 5 hours to
obtain mesoporous TiO2 catalyst. The synthesized pale yellow
coloured catalysts were labelled as: TU= TiO2 using urea, TOA=
TiO2 with oxalic acid, TEG= TiO2 using ethylene glycol.
While the research on development of cost effective and
efficient synthesis of TiO2 or supported TiO2 has gained large
interest, less focus has been channelled towards identification
of degradation products of Amaranth, various possible
degradation pathways and mechanisms. According to the
studies of Karkmaz et al.15 degradation of aqueous Amaranth
dye solution is assumed to occur via the C-N bond cleavage
leading to direct mineralization of azo group to N2 gas, while
the ring rupture process under mild acidic condition leads to
formation of products such as organic acids which further
mineralize to CO2. On the contrary, in a sono-electrochemical
experiment of Steter et al.16, the degradation of Amaranth is
suggested to be initiated by the N=N (azo bond) cleavage
leading to sodium-4-aminonaphthalenesulfonate and sodium-
2.3 Catalyst characterization
4-imino-3-hydroxynaphthalene-2,7-disulfonate.
Further
The thermal decomposition behaviour of as-prepared mixtures
was analysed using TG-DTA thermal analyser (NETSTA409 PC
Luxx). Phase and structure of as-synthesized TiO2 catalyst were
investigated by powder X-ray diffraction (PXRD, Philips PW-
1840) with Cu K alpha (k= 0.1540 nm) and nickel as filter over
the 2θ range of 10-80°. The surface functionalities of the
synthesized TiO2 were evaluated using FTIR spectroscopy
(Shimadzu IR- Prestige-21). The light absorption properties of
the photo catalyst were investigated using UV-diffuse
reflectance spectroscopy and solid state photoluminescence
spectroscopy (PL). The morphology of the catalyst was
determined by using Transmission Electron Microscopy (TEM,
Microscopy JEOL). The porosity measurements using N2
adsorption - desorption isotherm studies (Autosorb IQ QUA
211011) and BET, BJH analysis methods were used for
determination of surface area, pore size distribution and total
pore volume.
conversion of sodium-4-imino-3-hydroxynaphthalene-2,7-
disulfonate into 1-aminonaphthalene-2-ol, Sodium-4-amino-1-
naphthalenesulfonate which further convert to organic acids is
proposed and not via direct mineralization to N2 pathway.
However, in both the above studies, the primary intermediates
of C-N and N=N cleavage pathways namely, sodium-3-
hydroxynaphthalene-2,7-disulphonate,sodium-4-
aminonaphthalenesulphonate
hydroxynaphthalene-2,7-disulfonate
and
sodium-4-imino-3-
respectively, which
provide direct evidence for the two independent cleavages
have not been measured. Hence, the exact degradation
mechanism of Amaranth is yet to be fully understood.
Moreover, 1-aminonaphthalene-2-ol is
a common stable
undesired carcinogenic product of degradation of various dyes
such as Yellow 14, Solvent Orange 7, Solvent Red 24 and
Pigment Red 3 including Amaranth. Identification of such
intermediates and products of photo catalysed dye
degradation would provide
a better insight into the
2.4 Photocatalytic activity
degradation mechanism, which is extremely essential for the
photo catalysis technology implementation to safely treat
waste water polluted with Amaranth dye.
Amaranth stock solution of 30 ppm was prepared. Calibration
plot for the dye solution has been obtained by varying the
concentration from 10 - 100 ppm. 50 mg of the catalyst was
dispersed in 200 mL (30 ppm) dye solution and the pH of the
solution was recorded (pH = 5.7). The mixture was exposed to
natural sunlight (12:00-2:00 pm, IST) under constant stirring
condition for 90 minutes. The decrease in absorbance at 520
nm with increase in time was noted. The influence of various
experimental parameters such as pH, dye concentration and
catalyst concentration on the rate of photo degradation of
Amaranth were investigated. The preliminary products of
photo degradation of Amaranth were identified using LC-ESI-
MS (Agilent 6520). The HPLC analysis (Agilent 1200) were
performed using PhenomenexKinetex XB C18 column (length
30 mm, internal diameter 3.0 mm). Impurity profiling of
In the present study, we use a modified sol-gel method to
prepare highly active mesoporous anatase TiO2 nanocatalyst
using Ti-isopropoxide precursor and cost effective bi-dentate
complexing agents such as urea, ethylene glycol and oxalic
acid. A comparative study of photo catalytic activity of
synthesized TiO2 catalyst for degradation of Amaranth dye
under various experimental conditions of pH, TiO2
concentration and Amaranth concentration in comparison to
Degussa P25 is investigated. The identification of new
degradation products, evidence for C-N and N=N bond
cleavages along with a plausible mechanism of Amaranth
degradation has been presented.
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Amaranth stock solution was performed using LC-ESI-MS and
UPLC-ESI-MS technique (Acquity SDS Mass Spectrometer from
Waters). The eluent gradient conditions used for separation
are as listed in table ST1 in supplementary material. The mass
parameters were optimized using a dual spray (ESI) with
positive/negative polarity, operated under full scan mode with
gas temperature of 350 oC and a flow rate of 10 Lmin-1.
101
200
105,211
004
205
116
220
TEG
3 Results and discussion
3.1 TG-DTA and FTIR analysis
The representative TG-DTA curves of as-synthesized mixture of
titanium isopropoxide with urea (TU), ethylene glycol (TEG) and
oxalic acid (TOA) as a function of calcination temperature are
presented in SF1 in supplimentary material. In brief, all as
synthesized mixtures showed initial weight loss below 150 °C
attributed to the loss of physically adsorbed water. With increase in
calcination temperature, the decomposition of organic moety with
loss of CO/CO2, NOx occurs untill final transformation into stable
oxide of titanium at 550°C. Samples calcined at higher temperatures
resulted in decrease in the intensity of the IR vibrational peaks of
NH, CH, C=O. Complete elimination of NH, CH, C=O and appearance
of intense and broad absorption band due to Ti-O-Ti stretching and
bending vibration modes in the region 800-400 cm-1 confims
complete removal of organic moeity and formation of oxide of
Titanium19 at 550 °C, in good agreement with TG studies see SF2 in
suplementary materials.
TOA
TU
10 20 30 40 50 60 70 80 90
2θ (Degree)
Figure 1: Powder X-ray diffraction pattern of synthesized TiO2
catalyst with different SDA.
3.3 Optical properties
3.2 PXRD analysis
The powder X-ray diffraction pattern of all the three synthesized
catalysts (Figure 1) shows the formation of pure anatase phase of
TiO2 with peaks at 2θ value 25.3°, 37.81°, 48.0°, 53.9°, 55.06°, 62.7°,
68.7° and 70.2° confirmed by comparing with the standard pattern
(JCPDS number, 84-1286). The presence of sharp diffraction peaks
indicates high crystallinity of the catalyst. Diffractogram of TU, TOA
and TEG did not present any peak at 2θ = 27.3° which indicates
absence of rutile phase in the as-synthesized sample, in good
agreement with the reports by Munner et al. 20. The crystallite size
of the synthesized nanoparticles has been calculated by using
Scherer equation. Average crystallite size of the TiO2 catalyst was
estimated to be in the range of 10 -20 nm (see Table 1).
TU
TOA
TEG
B
Catalyst Crystallite Band
size
BET BJH
Pore
Pore
Gap Surface Surface volume diameter
area area
nm
eV
m2g-1 m2g-1
ccg-1
nm
3.2 3.3 3.4 3.5 3.6
eV
DEG
TU
10.0
17.6
3.20
3.21
50 ---
---
---
77.5 69.0
51.7 40.0
30.0 23.7
0.20
12.2
TEG
TOA
18.3
15.0
3.27
3.32
0.09
0.08
10.1
3.0
340
360
380
400
Wavelength (nm)
Table 1: Comparison of properties of synthesized TiO2 to that of
Degussa P25
Figure 2: A) PL spectra of TU and DEG, B) UV-DRS spectra of as-
synthesized TiO2 catalysts.
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Figure 2 B shows UV-DRS plots of synthesized TiO2 catalyst using The selected area electron diffraction pattern of TU, TEG and TOA,
different structure directing agents. The band gap of TiO2 has been is shown as an inset in Figure 3. The intensity and brightness of the
determined by plotting F∞2 v/s eV (inset), along with absorbance polymorphic rings in the case of TEG and TOA was found to be weak
v/s wavelength plot. Using the wavelength of absorption (λ), the whereas in the case of TU bright spots were observed which
value of E (eV) has been calculated using the following equation:
indicates enhanced poly crystallinity of TU catalyst. The ‘d’ values
obtained from the electron diffraction pattern, 0.351 nm, 0.233 nm,
0.186 nm and 0.170 nm, corresponds to the planes (101), (004),
(200) and (105). The ‘d’ spacing at 0.351 nm corresponding to (101)
plane is the highest intensity peak in XRD at 2θ = 25° and is the
characteristic peak of anatase phase of TiO2. EDS surface analysis of
TU, TOA and TEG (Figure 3) showed peaks corresponding to Ti and
O with no other impurity peaks within the EDS detection limit. The
1240
ꢀ ꢁꢂꢃꢄ ꢅ
ꢆ
The band gap of the synthesized TiO2 samples were found to be in
the range of 3.20-3.33eV (see Table 1) with the lowest band gap of
TU and the highest of TOA. In earlier studies, it has been reported
that TiO2 synthesized using urea generally leads to N doping in TiO2,
leading to a decrease in band gap and a shift in absorption band to
visible region above 400 nm 21, 22. However, the obtained band gap
in case of TU is similar to Degussa P25. In order to evaluate the
possibility of N-doping and the origin of pale yellow colour in TU,
photoluminescence measurement on DEG and TU were performed
(see Figure 2 A). The PL emission spectrum of TiO2 was recorded
at an excitation wavelength of 220 nm (high absorption
region). The PL emission spectra of both P25 and TU were
found to be similar with major peaks at 361, 421, 461, 491,
535 and 604 nm. The 361 nm peak (lower than the band edge
emission) is due to band to band transition 23. Emission peaks
at longer wavelengths in the visible region 421, 461, 491, 535
and 604 nm are attributed to surface state emissions which
are located within the band gap of TiO224. The oxygen
vacancies and surface hydroxyl groups present on the TiO2
surface are known to act as excellent traps for charge carries,
reducing the electron-hole recombination rate 23,24,25,26,27. The
major difference between the PL spectra of P25 and TU is
observed in the emission intensity at 460 nm. The ratio of
emission intensities at 460 to 421 nm decreases to 0.73 in TU
compared to 0.97 in P25. This is because of the presence of
oxygen vacancies that form shallow- trap states near the
absorption band edge and acts as efficient electron trap
centres or colour centers26,27. The PL emission intensity
decreases with increase in magnitude of F centres. On the
contrary, N-doping on TiO2 surface generally acts as a deep
trap state for positive charge carriers (holes) and the PL
XRD, SAED and EDS analysis are in good agreement and confirms
30, 31
the formation of high purity anatase TiO2
.
Figure 3: HR-TEM images with selected area diffraction
pattern of TiO2 catalyst synthesized using Urea (TU), Ethylene
glycol (TEG) and oxalic acid (TOA) after calcinations in air at
emission intensity increases with increase in the amount of N- 550˚C
doping 28,29. Hence, the PL measurements indicate that the N-
doping has probably not occurred or is insignificant in the
3.5 N2 adsorption-desorption
synthesized TU and the pale yellow colour of the synthesized
TiO2 powders is therefore attributed to the presence of F
centers (F or F1+ or F2+) in TU.
Mesoporous TiO2 prepared using sol gel method displayed type IV
isotherms with hysteresis loop, typical of mesoporous adsorbents
as shown in Figure 4. The initial part of the isotherm (low pressure)
represents the monolayer-multilayer adsorption on to the
mesoporous walls followed by capillary condensation/pore filling
mechanism32. A pronounced effect of different SDA on the
mesoporous structure of the synthesized catalyst was observed.
Catalyst TU displayed H2a hysteresis with a characteristic step in
desorption curve indicating the existence of complex pore structure
with interconnected pore network. Such a phenomenon occurs
when the wide pores have to access the external surface only
through narrow necks.
3.4 HR-TEM, SAED and EDS analysis
For the synthesized TiO2 catalyst, the surface morphology, particle
size and shape has been investigated by using HR-TEM analysis. The
HR-TEM images in Figure 3 are of TiO2 samples obtained using urea,
ethyleneglycol and oxalic acid after calcination in air at 550°C,
which illustrates the formation of nano size TiO2 particles. The
average particle size was around 30 nm. HR-TEM images indicate
the formation of aggregates of cuboidal shaped nanoparticles.
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282 and 330 nm due to the π electron excitation on the
naphthalene rings substituted with SO3- and OH groups. The dye
degradation reactions were performed under following reaction
conditions: (i) Adsorption in dark with catalyst (ii) photolysis
without catalyst and (iii) photo-catalytic degradation using
synthesized TiO2 catalysts. The initial reaction conditions were set
to [Amaranth] = 30 ppm, pH = 5.6, TiO2 = 50 mg at 30 ˚C and
reaction time of 90 mins. The change in absorbance as a function of
adsorption time or photocatalytic reaction time was noted using
UV-Vis spectrometer at 520 nm.
TU
TEG
TOA
0.4
120
60
0
0.2
0.0
0 5 10 15 20 25 30 35 40 45
Poresize(nm)
During the photo degradation experiment in natural sunlight
without catalyst, an insignificant decrease in the absorbance of <4%
from the initial value was observed after 90 minutes of irradiation
time. The corresponding change in the dye concentration was
estimated by establishing the calibration plot of absorbance versus
dye concentration. On the contrary, the addition of TiO2 catalyst
and irradiation in sunlight resulted in continuous decrease in
intensity of absorption bands of Amaranth without appearance of
new absorption bands. The colour of the solution changed from an
initial pink to colourless at the end of 90 minutes. Accordingly, %
degradation was calculated. Typical UV-visible spectra depicting the
decrease in absorbance of the dye as a function of irradiation time
using TU catalyst is presented in Figure 5. It was observed that the
pH of the dye solution after the photocatalytic reaction decreases
to 4.51 from an initial pH of 5.6, the change in the absorbance due
to this pH change was accounted and % degradation due to
photocatalytic reaction was corrected, more detailed in section
3.10. Among the various structure directing agents used to obtain
TiO2, the photo-catalyst TU showed the highest decrease in dye
concentration (82.6%) after 90 minutes of reaction time.The lowest
decrease in dye concentration was observed (62.3%) for TOA as
shown in Figure 6A. In order to account for the change in photon
flux of the solar radiation during a typical reaction run of 90
minutes on different days, every set of dye degradation experiment
was performed with reference to the degradation experiment using
commercially available Degussa P25 catalyst. The corresponding
relative variation in the percentage conversion of the synthesized
TiO2 catalyst was accordingly recalculated.
0.0
0.5
1.0
RelativepressureP/P
0
Figure 4 : N2 adsorption-desorption isotherms and corresponding
pore size distribution of mesoporous TiO2 catalyst synthesized
using 1)Urea (TU), 2) Ethylene glycol (TEG) and 3)Oxalic acid (TOA)
Hence, desorption through large pores is not possible until the
condensate from narrow pores is desorbed, resulting in a H2a
hysteresis. This is understandable, as urea acts as a fuel during heat
treatment of the as-synthesized samples, resulting in rapid
liberation of gases such as CO/CO2, NOx etc. This may result in the
observed complex pore structures with interconnected network of
pores. On the other hand, the H1 hysteresis observed in the case of
TEG indicates the formation of cylindrical or tubular uniform pores.
The hysteresis loop observed in case of TOA is more similar to H3
hysteresis with the only difference being the extension of hysteresis
even to low pressure region, indicating the presence of
microporous-mesoporous structure 32. Both TU and TEG displayed a
narrow pore size distribution in the range of 5-30 nm. The average
pore diameter and pore volume for all three catalysts are as listed
in Table 1. The highest surface area, pore size and pore volume
were recorded for catalyst TU.
3.6 Percentage Purity and Impurity profiling of Amaranth dye
The LC-ESI-MS analysis of Amaranth stock solution showed major
peak on chromatogram due to Amaranth dye with minor
unresolved impurity bands which were below the mass detection
limit (SF5 in supplementary material). Hence, further analysis were
performed with Ultra Performance LC-ESI-MS under positive scan
mode. Two impurity peaks at retention time 0.18 mins (11%) and
1.27 (7%) corresponding to m/z peaks of 144.2 and 301.3
respectively were detected (see SF6 and SF7 in supplementary
material). The former impurity was identified to be α-
naphthylamine, however, no particular structure could be assigned
to the second impurity.
0.6
0minutes
15 minutes
30 minutes
45 minutes
60 minutes
75 minutes
90 minutes
0.4
0.2
0.0
200
300
400
500
600
700
800
3.7 Photocatalytic activity of synthesized TiO2 catalysts
Wavelength(nm)
Figure 5 : UV-Visible spectra depicting a decrease in absorbance of
Amaranth dye as a function of photo-catalytic reaction time in
presence of sunlight.
Amaranth belongs to the azo family of dyes with typical absorption
peaks at 520 nm due to the π electron delocalization in the
chromophore (azo group) and four absorption bands at 216, 242,
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earlier in degradation experiments in sunlight. The measured
adsorption efficiency along with adsorption rate constant at 30 0C is
presented in Table 2. IR investigations were performed on pure
Amaranth and Amaranth adsorbed on TiO2 in order to extract
information about mode of interaction of Amaranth with the
catalyst during the adsorption. Amaranth (in absence of TiO2) shows
several characteristic absorption bands especially in the region
1800 -1000 cm-1 as seen in Figure 7A,B.
A
30
25
20
15
10
5
Blank
DEG
TU
TEG
TOA
The bands at 1195, 1168 and 1040 cm-1 are due to asymmetric
-
stretching vibration in SO3 Na+ and the coupling between benzene
mode and ν(SO3)33 as seen in Figure 7B. The peak at 1340cm-1 is due
to O-H bending vibrations or it may also be attributed to SO2-O
fingerprint15. The two intense peaks at 1496 and 1370 cm-1 were
attributed to –N=N- bond and the aromatic ring vibrations sensitive
to the azo group 33. The band at 1614 cm-1 are linked to C=C
aromatic skeletal vibrations. It is evident from Figure 7B that,the
ratio of intensities of the peaks at 1195 cm-1 to that of 1496 cm-1 is
0.95 in solid Amaranth, while the ratio decreases to 0.89 in case of
Amaranth adsorbed on TiO2, indicating that Amaranth undergoes
adsorption on TiO2 surface via interaction through SO3- group.
0
0
20
40
60
80
100
120
Time(minutes)
4.0
B
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
B
DEG
TU
TEG
TOA
A
0
20
40
60
80
100
B
Time (minutes)
Figure 6 : A) Kinetics profiles of photocatalytic degradation of
amaranth over TiO2 catalyst, B) Linear fits to the logarithmic ratio
of initial concentration of Amaranth at time ‘0’ to concentration at
time ‘t’. [Amaranth]0 = 30 ppm, TiO2 = 50 mg, solution pH = 5.7 at
30˚C.
4000 3500 3000 2500 2000 1500 1000 500
Previous studies on photo degradation of Amaranth dye have
Wavenumber (cm-1)
2,
reported pseudo first order reaction kinetics
4. A similar
conclusion was drawn from our studies as the photo degradation
kinetics of Amaranth were found to be best described by pseudo
B
first
order
kinetic
model,
given
A
by
plot
equation
of
(ln[Amaranth]0/[Amaranth]t=kt).
ln[Amaranth]0/[Amaranth]t gave a straight line fit, from the slope of
which the rate constant k was calculated (see Figure 6B). Table 2
presents a list of calculated rate constants along with degradation
efficiency of each of the synthesized TiO2 catalyst. The TiO2 catalyst
synthesized using urea showed the highest photo degradation
efficiency of 82.6% after 90 minutes of reaction time with a pseudo
first order rate constant of 0.042 min-1. The decreasing order of
degradation efficiency of TiO2 catalyst is as follows,
TU>DEG>TEG>TOA.
1370
1341
1168
1496
1041
1195
1750
1500
1250
1000
3.8 Adsorption efficiency
Wavenumber (cm-1)
As compared to the photo degradation efficiency of the synthesized
catalyst, the adsorption efficiencies measured in dark for a
comparable time period showed ~30% adsorption on TU. The
decreasing adsorption efficiency trend was similar to that observed
Figure 7 : IR spectra depicting the adsorption of Amaranth dye on
to the surface of TiO2 catalyst.
6 | J. Name., 2012, 00, 1-3
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Since the average adsorption capacity in the dark is ~30% after 90 could be due to less penetration of light due to the turbidity formed
minutes for TU, the observed 82.6 % decrease in Amaranth from increased amount of catalyst34. The corresponding pseudo
concentration at pH=5.6 for a similar irradiation time period has to first order rate constant also shows almost linear increase with
be the synergistic effect of the presence of interconnected porous increase in catalyst amount up to 110 mg. An optimum value of k =
network in TU with average pore diameter of 12.2 nm, large pore 0.051 mins-1 at [TU] = 110 mg at pH= 5.6 after 60 minutes of
volume of 0.20 ccg-1 leading to significantly large surface area of irradiation in sunlight of catalyst has been estimated. The above
77.5 m2g-1 and greater dye adsorption efficiency, as well as low estimated value is ~2 times greater than the reported value by
band gap which facilitates rapid degradation of dye and easy Karmaz et. al (k= 0.027 mins-1 using 375 mg of TiO2 and similar
desorption of degraded products from the mesopores, thus Amaranth concentration) at pH= 5.6
regenerating the active surface for new dye molecules to be
.
adsorbed. The apparent rate constant (kapp) for degradation of dye
was found to be ~10 times greater than that of adsorption rate
constant as listed in Table 2
90
80
70
60
50
.
3.9 Effect of catalyst concentration
The study was carried out using catalyst amount of 30, 50, 80, 110,
140, 170 mg at pH = 5.6. Figure 8 shows the percentage
degradation efficiency of Amaranth dye with varying catalyst
amount. It can be observed from the graph that the degradation
efficiency of the catalyst increases with increase in catalyst amount
up to 110 mg. This could be due to increase in number of active
sites and available total surface area for adsorption and light
induced degradation. At this catalyst amount almost 99.1% of the
Amaranth dye is decolourized within 60 minutes. When the loading
of catalyst was less than the optimal value, the generation of •OH
and •O2‾ superoxide radical anions is proportionally decreased
which results in low catalytic activity. With further increase in
catalyst above the optimum value, either leads to saturation or has
a slight negative effect on the degradation efficiency which
40 60 80 100 120 140 160 180
Amount of catalyst (mg)
Figure 8 : Variation in % degradation efficiency as a function
of catalyst loading.
#
*Catalyst Degradation Adsorption
%
Ads
%
%
Rate
constant
min-1
Rate
constant
min-1
Degd Degd
corrected
for pH
kapp
kads
change
100
80
60
40
20
Blank
DEG
---
---
---
3.7
---
`0.031
0.0033
22.4
92.3
76.61
TU
0.042
0.0043
30.1
99.1
82.60
TEG
TOA
0.021
0.015
0.0014
0.0015
9.0
83.3
75.0
69.14
62.25
11.0
0
20 30 40 50 60 70 80 90 100 110
*Catalyst amount = 50 mg, [Amaranth] = 30 ppm and total reaction time =
[Amaranth] (ppm)
#
90 minutes at pH = 5.6 % Degradation is corrected of the change in
absorbance due to pH change at the end of photocatalytic reaction. Ads =
Adsorption, Degd = Degradation.
Figure 9 : Effect of initial dye concentration on photo catalytic
degradation rate
Table 2 : Comparative analysis of degradation efficiency and
degradation rate constants of synthesized TiO2 vs Degussa P25.
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3.10 Effect of dye concentration
6.0
1.0
pH change during photocatalytic reaction
Absorbance of pH adjusted solution (in dark)
Absorbance during photocatalytic reactiion
It has been observed that lower the initial concentration of the dye,
faster is the degradation rate (see Figure 9). This is because, with
optimum catalyst amount available, the dye molecules easily
adsorb on the catalyst surface. However as the dye concentration
increases, the rate of dye adsorption overrides the rate of photo
degradation. With further increase in dye concentration, the dye
itself acts as an internal filter for incident light and hence the light
photons cannot reach the catalyst surface which in turn reduces the
formation of OH and O2 radicals 13, 28. (Kinetics plots along with
rate constant and % degradation using TU catalyst are presented in
SF4 and ST2 in supplementary material).
0.8
0.6
0.4
0.2
0.0
5.7
5.4
5.1
4.8
4.5
•
•-
A
3.11 Effect of pH on degradation rate
0
20
40
60
80
100
Time (minutes)
The effect of pH on the rate of degradation of Amaranth was
investigated in the pH range 2.0 - 11.0. In the first step, the initial
absorbance of the dye solution (in dark, without catalyst) at
different initial pH (2, 3, 5.6, 7, 8, 9 and 11) was recorded (marked
as ‘A’ in Table 3). It was observed that the pH of the solution after
photocatalytic reaction (in sunlight) decreases (see Figure 10A) to
lower values (marked as ‘B’ in Table 3) as compared to the initial
pH, possibly due to the formation of organic/inorganic acids. A
typical plot of change in pH during the photocatalytic reaction and
the corresponding change in absorbance is presented in Figure 10A.
In order to account for the change in absorbance due to this pH
change, the pH of the dye stock solution was adjusted to the values
listed as ‘B’ in Table 3 and the corresponding decrease in
absorbance was noted (marked as ‘C’ in Table 3 and presented in
Figure 10B). The % degradation due to photocatalytic reaction was
calculated as [(C/A)-(B/A)]×100. The estimated values of % rate
constants at each initial reaction pH are listed in Table 3.
After photocataytic
reaction (in light)
Absorbance for adjusted
pH (in dark)
0.9
0.6
0.3
0.0
Initial absorbance (in dark)
.
B
2
4
6
8
10 12 14
pH
It can be seen that the highest degradation efficiency of 97.14%
with an optimum rate constant of 0.069 mins-1 has been estimated
with 110 mg of catalyst TU at pH = 2 and 30 ppm dye solution under
sunlight. On the contrary at pH 11, % degradation due to
photocatalytic reaction is almost negligible. This could be due to
fact that with the increase in pH the TiO2 band edge positions shifts
to more negative values, thereby resulting in the decrease in
oxidation potential. Furthermore, the dissociation of TiOH into H+
and TiO- results in decrease in adsorbed hydroxyl groups and hence
a decrease in hydroxyl radical concentration. In addition to this,
under extreme basic conditions, the TiO2 surface becomes
negatively charged and hence would repel the negatively charged
Figure 10 : Efffect of pH on photo catalytic degradation rate.
3.13 Photocatalytic degradation mechanism
TiO2 + h
ν
→ e− + h+
(O2) ads + e−→ O2
•−
H2O + h+→•OH + H+
15,
Two possible mechanisms have been suggested in the literature
16. According to the studies of Karkmaz et al. 15, under acidic
conditions, the surface of TiO2 is positively charged,
-
dye (R-SO3 ) leading to lower adsorption rate and reduced
degradation efficiency
Ti-OH + H+↔ Ti-(OH2) +.
+
Adsorption of Amaranth on the surface of Ti-(OH2) occurs via
3.12 Regeneration of catalyst
interaction of sulphonyl group in the ortho position with respect to
the OH group on the naphthalene ring. The formation of OH
radicals and h+ on the catalyst surface are believed to initiate the
After every run of degradation experiment, the catalyst was kept in
hot boiling deionised water under constant stirring for 1 hour and
washed with deionised water before reactivation at 400˚C. The
catalytic efficiency of the activated catalyst was re-measured. This
procedure was repeated 4 times. The observed decrease for every
cycle is as follows: C1= 96.8%, C2= 96.7%, C3 = 96.3%, C4 = 96.3%.
radical degradation of Amaranth according to following reactions
:
8 | J. Name., 2012, 00, 1-3
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Initial pH of Initial dye
Final pH after
photocatalytic
reaction
Abs at final pH
after
Initial dye pH
Abs at pH adjusted
to B value
% Degd due to
photocatalytic
reaction
Degd
Rate constant
min-1
the dye
solution
#Abs
adjusted to B value
photocatalytic
reaction
(light)
(dark)
(dark)
C
(dark)
(dark)
A
(light)
[(C/A)-(B/A)]x100
kapp
B
2
3
0.70
0.66
0.75
0.60
0.57
0.56
0.55
1.78
2.89
4.51
6.71
6.50
5.92
10.76
0.01
0.01
0.01
0.15
0.17
0.28
0.43
1.78
2.89
4.51
6.71
6.50
5.92
10.76
0.69
0.65
0.63
0.58
0.55
0.55
0.46
97.14
96.96
82.66
71.66
66.66
48.21
5.5
0.069
0.051
0.040
0.035
0.030
5.6
7
8
9
0.020
0.009
11
*Catalyst = TU, Catalyst amount = 110 mg, [Amaranth] =30 ppm and total
reaction time = 60 minutes. #Abs= Absorbance
(marked as cleavage-A) and N=N cleavage (marked as
cleavage-B and B’) respectively in Figure 11, are considered,
(mass spectra are presented in supplementary material).
Cleavage-B leads to formation of two primary intermediates
B1 and B’1 as explained earlier. B’1 transforms into 1-
aminonaphthalen-2-ol (m/z = 216) via loss of two SO3- groups,
which further converts into phenol (m/z = 117) and malic acid
Table 3 : Effect of pH on degradation efficiency of TU.
2−
R-SO3− + •OH → R• + HSO4− (→ H+ + SO4
)
−
•
R-SO3 + h+→ R-SO3
(m/z
=
135) via ring rupture mechanism of 1,2-
191), in
dihydroxnaphthalene-1,2-diol tautomer (m/z
=
agreement with Steter et al. 16. However, the formation of
phenol can also be explained via intermediate B1 which
transforms in to sodium-4-aminobenzenesulphonate (B2, m/z
= 172) that undergoes loss of SO3- and NH2 groups via oxidative
OH addition leading to phenol. Moreover, C-N cleavage
-
Where R-SO3 represents Amaranth dye. The radical reactions
of R• initiates dye degradation process via ring rupture, leads
to formation of organic acids such as lactic acid and malic acid
both of which transform to formic acid before converting to
gaseous CO2 along with direct mineralization of azo group to
(cleavage-A)
hydroxynaphthalene-2, 7-disulfonate (A1, m/z = 365), which
leads
to
formation
of
sodium-3-
16
gaseous N2. On the contrary, Steter et al. performed sono-
electrochemical degradation of Amaranth and product
identification by LC-MS and suggested that, the Amaranth
degradation occurs by cleavage of the –N=N- (pathway B’ in
Figure 11) leading to sodium-4-amino naphthalenesulphonate
-
losses two SO3 groups to form 3-hydroxynaphthalene (m/z =
162).3-hydroxynaphthalene
may
undergoing
rupture
mechanism to form phenol. It is important to note here that
this is the first study on identification of four probable new
intermediates namely, sodium-3-hydroxynaphthalene-2,7-
disulphonate (A1), 3-hydroxynaphthalene (A2), sodium-4-
(B1)
and
sodium-4-imino-3-hydroxynaphthalene-2,7-
disulfonate (B’1) and not via direct mineralization to N2 gas.
Further conversion of (B’1) into 1-aminonaphthanlene-2-ol
(B’2) which converts to naphthalene-1,2-diol tautomer (B’3) is
assumed to form salicylic acid via ring rupture, followed by
conversion to phenol, oxalic acid and malic acid etc. Hence,
different intermediates and products in these individual
studies suggest contradictory bond cleavage pathways.
Moreover, the primary intermediates A1, B1 and B’1 of C-N
and N=N bond cleavage steps respectively, have not been
detected so far, to the best of our knowledge.
aminonaphthalene
sulphonate
(B1)
and
sodium-4-
aminobezenesulphonate (B2) formed during photocatalytic
degradation of Amaranth to the best of our knowledge. Two
new plausible pathways A and B leading to formation of
phenol are presented in Figure 11. Moreover, there were some
other fragmentation patterns observed in mass spectra which
were complex to assign any particular structure. The two
impurities detected in Amaranth stock solution in UPLC-ESI-
MS, will be negligible and unresolved in LC-ESI-MS as discussed
earlier and does not influence the proposed mechanism of
Amaranth degradation. The above discussion is therefore
restricted to qualitative identification only. Nonetheless, the
degradation kinetics and identification of degradation
intermediates and products provide a better insight into the
various possible degradation pathways simultaneously
operating on TiO2surface.
The present study reports qualitative identification of primary
intermediates and products using LC-ESI-MS technique. The
detected m/z values and their comparison with the proposed
mechanisms by Steter et al. and Karkmaz et al. leads to an
interesting observation. The various products detected using
LC-ESI-MS in our study, can be explained only if two
simultaneous bond cleavage pathways, i.e. C-N bond cleavage
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SO3Na
A1
SO Na B'1
C-N
cleavage
pathway
SO3Na
OH
HO
HN
3
HO
OH/h+
O2
B'
A
N
N
A
B
SO3Na
SO3Na
N=N
cleavage
pathway
NaO3S
SO3Na
m/z = 365
-2SO3
-2SO3
B
Amaranth
Ring
B2
NH B1
B'2
OH
NH2
A2
2
H2N
HO
rupture
NaO3S
m/z =244
SO3Na
m/z =172
m/z = 216
m/z =162
+OH -NH2
+OH -NH2
Ring rupture
COOH
OH
B'3
OH
O
OH
Ring
OH
O
rupture
-CO2
m/z = 191
m/z = 117
O
HO
OH
O OH
m/z = 135
Figure 11 : Plausible mechanism of Amaranth degradation
comparisons it can be concluded that, photocatalytic
degradation of amaranth using TiO2 under natural sunlight can
be safely implemented as a pre-treatment procedure to
decolourize waste water contaminated with Amaranth.
However, for complete water purification, total mineralization
of Amaranth is desired.
3.14 Toxicity of Amaranth versus photocatalytic degradation
products.
Literature reports on toxicological impact of Amaranth dye
tested on albino rats confirm that Amaranth at 10 times ADI
(Acceptable Daily Intake) leads to fatal abnormalities and
doses of 47 mg/kg by weight of Amaranth could impair hepatic
function35. Meanwhile, the literature studies on the
degradation products and intermediates of Amaranth dye
detected in this study such as, sodium-3-hydroxynaphthalene-
4 Conclusions
2, 7-disulphonate,
aminonaphalenesulphonate,
3-hydroxynaphthalene, sodium-4-
sodium-4-
Nano sized mesoporous anatase TiO2 catalysts were
synthesized using a sol gel method. A pronounced effect of
SDA on porous nature of synthesized TiO2 catalyst leading to
high surface area of 77.5 m2g-1 (using urea) was observed.
Amaranth, one of the food dyes which is prohibited due to its
carcinogenic effect, was effectively decolorized to less
hazardous products using synthesized mesoporous TiO2
catalysts under natural sunlight in water. Mesoporous TiO2
obtained using urea showed better photo catalytic activity
with highest rate constant of 0.069 min-1 for the degradation
of Amaranth as compared to the rest of SDA’s and
commercially available Degussa P-25, which was attributed to
the synergistic effect of mesoporous nature of TiO2 leading to
reduction in electron–hole recombination rate, increased dye
adsorption rate, efficient photo degradation on TiO2 due to
low band gap and ease of desorption of degradation products
from mesopores, leading to regeneration of active surface.
New intermediates of photocatalytic degradation of Amaranth
namely, sodium-3-hydroxynaphthalene-2,7-disulphonate, 3-
aminobezenesulphonate, phenol, malic acid indicate that they
are comparatively far less damaging. Tests performed by
Safety Assessment Laboratory, Panapharm Laboratories Co.
Ltd, report sodium-3-hydroxynaphthalene-2,7-disulphonate to
be non-mutagenic and non-toxic. 1-aminonaphthalene-2-ol is
reported to be a carcinogen when tested on rats 36,37. 1-
aminonaphthalene-2-ol is observed to be the stable end
product of chemical reduction of many dyes such as Solvent
Yellow 14, Solvent Orange 7, Solvent Red 24 and Pigment Red
3 38. It is also the product of anaerobic metabolism of azo dyes
such as Sudan I, Sudan II, Sudan III, Sudan IV and Para
Red39.However, in our study, 1-aminonaphthalene-2-ol is only
an intermediate which is transformed into less toxic products
such as malic acid, phenol etc. Phenol is known to be non-
genotoxic and thus it is not considered to be direct cancer
risk40.No reports on carcinogenicity of malic acid is available in
the literature to the best of our knowledge. From the above
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hydroxynaphthalene, sodium-4-aminonaphalenesulphonate 13. V. K. Gupta, R. Jain, A. Mittal, T. A. Saleh, A. Nayak, S.
and sodium-4-aminobezenesulphonate were identified for the
very first time, providing the evidence for two simultaneous
bond cleavage pathways (-C-N-) and (-N=N-) leading to
Agarwal and S. Sikarwar, Photo-catalytic degradation of
toxic dye amaranth on TiO2/UV in aqueous suspensions,
Mater. Sci. Eng., 2012, 32, 12–17.
degradation of Amaranth. The toxicity comparison between 14. A.K. Jain, S. Sharma, and R. Ameta, Enhanced photocatalytic
Amaranth dye and the identified degradation products suggest
that, mesoporous TiO2 nano catalyst can be used effectively
and safely to pre-treat coloured waste water contaminated 15. M. Karkmaz, E. Puzenat, C. Guillard, and J. M. Herrmann,
activity of N, S-doped titania for degradation of Amaranth,
Merit Res. J. Environ. Sci. Toxicol., 2015, , 25-30.
3
with Amaranth dye.
Photocatalytic degradation of the alimentary azo dye
amaranth Mineralization of the azo group to nitrogen
Appl. Catal. B, 2004, 51, 183-194.
,
Acknowledgements
16. J. R. Steter, W. R. P. Barros, M. R. V. Lanza, and A. J.
Motheo, Degradation route for amaranth dye by
This work has been funded by the UGC-BSR start-up research
grant project No. F.30-102/2015 and SERB ECR award project
No. ECR/2015/000047. Authors would like to thank NCCR, IITM 17. N. Hafizah and L. Sopyan, Nanosized TiO2 photocatalyst
sonoelectrochemical
process
using
BDD
anode,
Chemosphere, 2014, 117, 200-207.
for extending facilities for catalyst characterization. We owe our
sincere gratitude to Prof. B. Viswanathan (NCCR, IITM-India),
Prof. J. B. Fernandes, Prof. K. Priolkar and Dr. P. Samant (Goa 18. D.C.L. Vasconcelos, V.C. Costa, E. H. M. Nunes , A. C. S.
powder via sol-gel method: Effect of hydrolysis degree on
powder properties, Int. J. Photoenergy, 2009, 1-8.
University-India) for their guidance, valuable suggestions and
constant encouragement throughout the work.
Sabioni and W. L. Vasconcelos, Optical characterization of
316L stainless steel coated with sol–gel titania, Journal of
non crystalline solids, 2012, 358, 3042-3047
19. R. Sharmila Devi, R. Venckatesh and R. Sivaraj, Synthesis of
Titanium Dioxide Nanoparticles by Sol-Gel Technique, Int. J.
Innov. res. ci. eng. Technol., 2014, 3(8), 15206-15211.
20. M. M. Ba-Abbad, A. A. H. Kadhum, A. B. Mohamad, M. S.
Takriff and K. Sopian, Synthesis and catalytic activity of
TiO2 nanoparticles for photochemical oxidation of
concentrated chlorophenols under direct solar radiation,
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DOI: 10.1039/C7PP00090A
Graphical Abstract
Highlights
ꢀ Urea assisted sol-gel synthesis of mesoporous anatase TiO2 of high photo catalytic activity
ꢀ Highest photocatalytic Amaranth degradation rate constant of 0.069 min-1
ꢀ Identification of novel intermediates viz sodium-3-hydroxynaphthalene-2,7-disulphonate,
3-hydroxynaphthalene, sodium-4-aminonaphthalenesulphonate and sodium-4-aminobezenesulphonate
ꢀ Evidence of C-N and N=N bond cleavages and new plausible mechanism of Amaranth degradation.
1