Serendipitous discovery of super adsorbent properties of sonochemically synthesized nano BaWO4

Article abstract of DOI:10.1039/c3ra44350g

The superior adsorbent properties of BaWO₄ nanostructures have been reported for the first time. Flower shaped aggregates (～250 nm) of BaWO₄ nanoparticles, having an average size of ～10-15 nm with a high surface area of ～148.0 ± 0.2 m² g^-1, have been synthesized sonochemically and used for the adsorption of various cationic dyes from aqueous solutions. The sonochemically synthesized BaWO₄ have been characterized by scanning electron microscopy (SEM), selected area electron diffraction (SAED), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Fourier transform infrared spectra (FTIR). The adsorbent capacity of this tungstate is much higher than that reported for other nanomaterials like Fe₂O₃, MnO₂, WO₃, etc. Complete removal of dyes like rhodamine B and methylene blue was possible within a short time span of 10-15 minutes. The adsorption process was followed using UV-Visible spectroscopy, while the material before and after adsorption has been characterized using physicochemical and spectroscopic techniques. Various isotherms have been used to fit the data, and kinetic parameters were evaluated. Moreover, the adsorbed dyes could be desorbed completely from nanoparticle surfaces by annealing at moderate temperature and were found to be efficient for multicyclic use. Thus this sonochemically synthesized nano BaWO₄ has great significance in treatment of dye industry effluents as a promising adsorbent for cationic dyes from aqueous solution. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra44350g

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Serendipitous discovery of super adsorbent properties
of sonochemically synthesized nano BaWO4
Cite this: RSC Adv., 2013, 3, 22580
a
b
Anamika Singh, Dimple P. Dutta, Jayshree Ramkumar,c Kaustav Bhattacharya,b
*
Avesh Kumar Tyagib and M. H. Fulekarad
The superior adsorbent properties of BaWO4 nanostructures have been reported for the first time. Flower
shaped aggregates (ꢀ250 nm) of BaWO4 nanoparticles, having an average size of ꢀ10–15 nm with a high
surface area of ꢀ148.0 ꢁ 0.2 m2 gꢂ1, have been synthesized sonochemically and used for the adsorption of
various cationic dyes from aqueous solutions. The sonochemically synthesized BaWO4 have been
characterized by scanning electron microscopy (SEM), selected area electron diffraction (SAED),
transmission electron microscopy (TEM), X-ray diffraction (XRD) and Fourier transform infrared spectra
(FTIR). The adsorbent capacity of this tungstate is much higher than that reported for other
nanomaterials like Fe2O3, MnO2, WO3, etc. Complete removal of dyes like rhodamine B and methylene
blue was possible within a short time span of 10–15 minutes. The adsorption process was followed
using UV-Visible spectroscopy, while the material before and after adsorption has been characterized
using physicochemical and spectroscopic techniques. Various isotherms have been used to fit the data,
and kinetic parameters were evaluated. Moreover, the adsorbed dyes could be desorbed completely
from nanoparticle surfaces by annealing at moderate temperature and were found to be efficient for
multicyclic use. Thus this sonochemically synthesized nano BaWO4 has great significance in treatment of
dye industry effluents as a promising adsorbent for cationic dyes from aqueous solution.
Received 14th August 2013
Accepted 24th September 2013
DOI: 10.1039/c3ra44350g
www.rsc.org/advances
Organic dyes which nd application in textile, paper, leather, since it is simpler, cheaper and more efficient. Consequently,
cosmetics and various other industries are considered to be a researchers have developed many commercially available
major source of water pollution. The effluents from these adsorbents like activated carbon, silica gel, activated alumina,
industries contain these organic dyes which are mostly water and polymeric porous materials/frameworks.7–12 However, most
soluble and pose a serious threat to aquatic life by increasing of them suffer from drawbacks of limited adsorption capacity,
the chemical oxygen demand (COD) and reducing light pene- inefficient extraction and reusability.
tration and visibility in the water body.1,2 A large fraction of
Nanomaterials, due to their high surface area and energy have
these dyes are also toxic and carcinogenic posing serious threat the potential to develop into high performance adsorbents as
to the environment.3 Since most of them are stable and resistant they can attract dyes more efficiently and also as photocatalysts
to bio/photodegradation, the removal of these discharged dyes due to their size dependent band gap and catalytic properties.
has been an active area of research. Over the years, methods like Hence, there is a quest for new nanomaterials for application in
photocatalysis and direct adsorption have emerged as the dye removal from water effluent from industries. Recently BaWO4
frontrunners in this eld.4–6 A photocatalysis based process can has been demonstrated to be efficient photocatalysts under UV
easily eliminate some organic dyes via degradation, but it is light irradiation, as a result of its high degree of open structure
severely restricted by availability, materials choice and perfor- which intensies the vibrability or movability of atoms.13 Due to
mance/price ratio of the catalysts. In sharp contrast, the this the charge exchange or transfer via [W–O]N network become
adsorption methods for dye removal are much more popular more spatially available in case of BaWO4. High photocatalytic
efficiency was reported for the bonbon-like BaWO4 microcrystals
in the degradation of the RhB dye under UV-light.14 However,
aDepartment of Life Sciences, University of Mumbai, Santacruz (E), Mumbai 400 098,
India
bChemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India. E-mail:
dimpled@barc.gov.in; Fax: +912225505151; Tel: +912225592308
cAnalytical Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085,
India
there are very few reports on the synthesis of nanoparticles of
BaWO4 and none on its photocatalytic activity.15–18 Hence, it was
of interest to synthesize BaWO4 nanoparticles and study its
applicability for photodegradation of various dyes.
With this aim, BaWO4 was synthesized using the sono-
chemical method. The nanoparticles so obtained were charac-
terized using powder XRD and TEM. For the photodegradation
dSchool of Environment & Sustainable Development, Central University of Gujarat,
Gandhinagar, Gujarat 382 030, India
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studies, common dyes like rhodamine B and methylene blue equation from the FWHM (full width half maxima) of XRD
were selected. It was during the photodegradation studies that peaks was found to be ꢀ15 nm. On heating the as prepared
the superior adsorbent property of this nano BaWO4 material product at 500 ꢃC for 2 h, there was no change in the XRD
was observed. The as-prepared products are expected to be new pattern which indicates that the tetragonal phase is quite stable
promising materials for industrial dye effluent remediation. (Fig. 1b). There was a slight increase in the average crystal size
The details of this work are discussed herein.
of the sample aer heating.
A simple sonochemical method was used for the synthesis of
To examine the actual size and morphology of the particles,
barium tungstate nanostructures. All the reactions were carried the samples were systematically analyzed by SEM, TEM and
out under ambient conditions. High purity barium nitrate HRTEM. SEM micrographs were recorded using Seron Inc.
[Ba(NO3)2] and sodium tungstate [Na2WO4$2H2O] were make model AIS 2100 scanning electron microscope at 20 keV.
obtained from commercial sources. In a typical reaction, an The samples were prepared for SEM by putting the drop of
aqueous solution [Na2WO4$2H2O] (1.92 g, 5.81 mmol) was ultrasonicated barium titanate nanopowder dispersed in
added dropwise to an aqueous solution of [Ba(NO3)2] (1.52 g, methanol over a mirror polished Si single crystal to reduce the
5.81 mmol). The system was then irradiated with high intensity affect of agglomeration. Conventional TEM micrographs were
(100 W cmꢂ2) ultrasonic radiation (Oscar Ultrasonics) operating recorded on a Libra 120 keV Electron Microscope (Carl Zeiss).
at 40 kHz, under air for 1.5 h. The titanium horn was inserted to The particulates so obtained, were dispersed in methanol
a depth of 6 cm in the solution. The reaction cell was not solution and then deposited on the carbon coated copper grids
ꢃ
thermostated and the nal temperature was 45 C. Aer soni- for TEM/SAED studies. The low magnication SEM image of as
cation, the precipitate was washed with water and centrifuged. prepared BaWO4 shows spherical aggregates with dimensions
The white precipitate obtained was dried in air. Sonochemical around 200–250 nm (Fig. 2a). The TEM image of BaWO4 shows
synthesis of BaWO4 using ethylene glycol as solvent has been formation of ower shaped structures with dimensions of ꢀ250
reported earlier.15 However, in our case, the approach is simpler nm (Fig. 2b). Each individual “petal” of the ower consists of
and faster since the reaction is done in aqueous medium and is many closely attached nanoparticles. This close attachment can
completed in 1.5 h compared to 5 h in the previous case.
be observed in high magnication TEM image (Fig. 2c), where
X-Ray diffraction (XRD) measurements were carried out on a outline of the individual nanoparticles are visible. The SAED
Philips Instrument, operating with monochromatized Cu-Ka pattern of BaWO4 conrmed the crystalline nature of the
ꢂ1
ꢃ
˚
radiation (l ¼ 1.5406 A) and employing a scan rate of 0.02 s
particles (Fig. 2d). The indexing of the concentric rings corre-
in the scattering angular range (2q) of 10ꢃ to 80ꢃ. Silicon was sponds to tetragonal crystal structure. Fig. 2e shows a repre-
used as an external standard for correction due to instrumental sentative HRTEM image of a single BaWO4 nanoparticle in the
line broadening. The average crystallite size was calculated from
the diffraction line width based on Scherrer's relation: d ¼ 0.9l/
b cos q, where l denotes the wavelength of X-rays and b is the
corrected full width at half maxima (FWHM). The XRD pattern
of the as prepared BaWO4 sample is shown in Fig. 1a, which
reveals the crystalline nature of the product. The XRD patterns
depict scheelite structure of BaWO4 with characteristic peaks
which are consistent with standard data for tetragonal crystal
system with I41/a space-group symmetry (PCPDF le no.
˚
43-0646). The calculated lattice parameters of a ¼ b ¼ 5.614 A,
˚
c ¼ 12.712 A are very close to that of the reported values. The
average crystallite size calculated using the Debye–Scherrer
Fig. 2 (a) SEM, (b) low magnification TEM, (c) high magnification TEM, (d) SAED
Fig. 1 Powder XRD of (a) as prepared and (b) thermally treated BaWO4.
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pattern and (e) HRTEM image of as prepared unheated BaWO4.
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F(R) ¼ A(hn ꢂ Eg)m/2
(1)
ower shaped aggregate. The size of the nanoparticle was seen
to be ꢀ12 nm. The measured spacing of the crystallographic
planes is 0.21 nm, corresponding to the (204) lattice fringe of
tetragonal BaWO4. The sizes of the nanoparticles obtained from
HRTEM images are consistent with the calculated values
obtained from the XRD data.
The morphology of the heated sample was also studied using
TEM. On heating, clover shaped structures of BaWO4 was
obtained with dimensions of ꢀ2.5 mm (Fig. 3a). The distance
from the apex of each petal to the center of the clover is about
1.2 mm. Each petal of the clover was actually an agglomeration
of nanoparticles with sizes ranging between 10 and 20 nm
(Fig. 3b). The agglomeration process is attributed to Van der
Waals forces. The primary ower shaped aggregates exhibit a
tendency to form bigger agglomerates, in a bid to minimize the
surface free energy. The change from ower to clover like
structure is a result of an abnormal/discontinuous grain growth
where, some grains grow faster than others with increasing
sintering temperature. BaWO4 has a high anisotropy in inter-
facial energy and this promotes the grain growth.
The specic surface area of barium tungstate nanoparticles
was determined using a Bel Japan Inc., Belsorp II surface area
analyzer by measuring N2 adsorption and adopting the well-
known BET procedure. It is determined by physical adsorption
of a gas on the surface of the solid and by measuring the
amount of adsorbed gas corresponding to a monomolecular
layer on the surface.19–21 From the BET analysis, we found that
the as prepared BaWO4 nanoparticles have an average specic
surface area of 148.0 ꢁ 0.2 m2 gꢂ1 whereas in case of the heated
sample it reduces to 61 ꢁ 0.5 m2 gꢂ1. This can be explained
since heating results in the formation of larger micron sized
agglomerates of the BaWO4 nanoparticles leading to less
specic surface area.
The optical absorption property of a semiconductor depends
on its energy band structure, which is a key factor in deter-
mining its photocatalytic activity. UV-Vis diffuse reectance
spectra of the samples were obtained on a DR-UV-Vis spectro-
photometer (JASCO-V670-NIR) with BaSO4 as reference and
using the integrating sphere upto the near IR range. Diffuse
reectance spectra (DRS) of the as prepared BaWO4 as well as
heated BaWO4 samples were recorded wherefrom their band
absorption edges were estimated. An estimate of the optical
band gap (Eg) of a crystalline material may be obtained using
the Kubelka Munk function F(R) as given below:
where hn ¼ photon energy and m is a constant that depends on
the nature of optical transition.22 For direct transition, m is
equal to 1 (if allowed) or 3 (if forbidden), while for indirect
allowed transition, m is equal to 4 (if allowed) or 6 (if forbidden).
For a weakly absorbing semiconductor, F(R) is proportional to
the absorbance A. Hence, the energy intercept of a plot of A2 (for
direct allowed transition) and A1/2 (for indirect allowed transi-
tion) versus hn yields the respective optical band gap Eg, when
the linear regions are extrapolated to the zero ordinate.23
Tungstates (AWO4; A ¼ Ca, Sr, Ba) have been reported to
demonstrate an optical absorption spectrum governed by direct
electronic transitions.24 Fig. 4 shows a plot of the square of the
absorption coefficient as a function of photon energy for both
the unheated and heated BaWO4 samples. It has been reported
that bulk BaWO4 synthesized via solid state route has a band
gap of 3.8 eV. However, our sonochemically synthesized as
prepared and heated BaWO4 samples exhibits band gap of
4.87 eV and 4.98 eV, respectively. The increase in the band gap
energy in case of our sonochemically synthesized nano BaWO4
can be attributed partially to quantum connement effects. As
depicted in the Table 1, it can be clearly seen that the process of
synthesis and the morphology of the material affects the band
gap values of BaWO4 to a large extent. Heat treatment of the as
prepared BaWO4 sample increases the size of the nanoparticles
agglomerates considerably but exhibits a blue shi in the band
gap. This may be due to the elimination of defect sites in the
nanocrystals as a result of heating. Both as prepared and heated
BaWO4 have an identical scheelite structure of tetragonal crystal
system with I41/a space-group symmetry as has been depicted
by the XRD results above. Here the W ions are placed within
tetrahedral O-ion cages and are isolated from each other, while
Ba ions are surrounded by eight oxygen ions.25 From the liter-
ature reports (Table 1) we nd that there is almost no change in
the crystal structure or symmetry of these materials, with the
Fig. 3 (a) low magnification TEM and (b) high magnification TEM of thermally
Fig. 4 The DR-UV-Vis spectra of as prepared BaWO4 (unheated BaWO4) and BTH
(BaWO4 heated at 500 ꢃC for 2 h).
treated BaWO4.
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Table 1 Comparison of BaWO4 synthesized using various techniques
Crystal
structure
Calcination
Band gap
(eV)
Method
Formal shape
temperature (ꢃC)
Reference
Molten ux
Czocharski
Czocharski
Solid state reaction
Polymeric precursor
Solid state metathetic
Co-precipitation
Co-precipitation
Co-precipitation
Sonochemical
Octahedrons-like microcrystals
Monocrystals
Monocrystals
Bulk ceramics
Nanopowders
Rod-like microcrystals
Bonbon-like microcrystals
Spindle-like microcrystals
Rice-like microcrystals
Flower shaped microcrystal agglomerate
Clover shaped microcrystal
Scheelite
Scheelite
Scheelite
Scheelite
Scheelite
Scheelite
Scheelite
Scheelite
Scheelite
Scheelite
Scheelite
1000
1000
1200
1400
600
3.8
4.8
4.9
3.8
4.1
4.8
4.9
4.0
3.8
4.87
4.98
42
39
27
43
26
44
14
14
—
25
25
25
500
14
Our work
Our work
Sonochemical heating
different synthesis procedures. Therefore the alteration in the 5 mL of the above solution in a quartz cuvette in the dark.
Fermi energy is not foreseen. However the change in the band Removal percent and adsorbed amount of each dye was deter-
gap energies with local structural imperfections leads us to mined spectrophotometrically by measuring the absorbance of
think that rather than the long range ordering of the BaWO4, the sample solutions before and aer removing process at
the short range disorders plays a more important role in appropriate wavelengths corresponding to the maximum
determining their band gap values.26
absorbance of each dye, which was 546 and 668 nm for RhB and
The theoretical study shows that the VB is mainly formed by MB, respectively. All tests were performed in triplicate to ensure
p-orbitals of oxygen with very little contribution from the the repeatability of the results; the mean of the three
tungsten d states. The CB however is formed mainly from the measurements was then reported. All the experiments were
tungsten d states with small contributions from p states of performed at room temperature. The sorption studies were
oxygen. The energy levels participating in optical transitions carried out using the batch mode. A weighed amount of the
therefore arise from O-p states to W-d states.27 The W(VI) are unheated nano barium tungstate was equilibrated with a solu-
strong second order Jahn Teller (SOJT) distorters and therefore tion containing a known concentration of the dye (rhodamine B
highly symmetric local environment for them is not forseen.28 (RhB)/methylene blue (MB)) maintained at a particular pH for a
Consequently, there is a large variation in the band gap values known period of time. The change in the absorbance value of
of BaWO4 depending on its size, morphology and method of the dye solution with respect to the original solution was
synthesis as manifested in Table 1. It can be seen that in our measured as a function of time, pH and initial concentration of
case, the direct energy band gap values are very close to the the dye. Fig. 5a is a schematic representation of the dye sorption
bonbon like BaWO4 and pure BaWO4 monocrystals.
and the optical photographs showing the various stages of
The increment in the band gap with calcinations is similar to sorption are shown in Fig. 5b. The different parameters were
that observed by Santos et al.26 Wood and Tauc associated the monitored to get maximum uptake. The heated BaWO4 samples
decrease of the gap energy value with the existence of localized on the contrary did not exhibit any sorption characteristics.
states in the band gap due to presence of the structural This proves that this superior adsorbent property manifested by
defects.29 This signies that the amount of localized states in the sonochemically synthesized as prepared BaWO4 samples, is
the band gap for the heated sample is lower than that in the as primarily due to its morphology and high surface area in the
prepared sample. This is basically due to the stronger short nano dimensions.
range disorder of Ba at lower temperature. The effect of the
The time of equilibration is one of an important parameter
stronger short range disorder than the long range disorder was which determines the kinetics of the reaction. For the time
also seen in the PL and Raman experiments by Santos et al.26 variation studies, a known amount of the nanotungstate (7 mg)
However there is nonlinearity in this trend of having a quantum is equilibrated with a known concentration of dye (20 ppm) at a
connement or defect states with the different synthesis particular pH. The results of these studies are shown in Fig. 6.
process and hence it still remains an open question.
Fig. 6a shows a typical curve in which the change in the
The sorption experiments were conducted individually for absorbance of RhB dye solution was monitored over a period of
each of the dyes, but the same procedure was used for both of time on 7 mg unheated BaWO4 nanoparticles. It is seen that the
them, as detailed below. The solution pH, concentration of kinetics of the reaction is very fast and the sorption is
BaWO4 nanoparticles, and initial dye concentration were the completed within 10–15 min. The amount of nanotungstate
variables in this study. Solutions containing different concen- added was optimized aer doing several experiments where we
trations of rhodamine B (RhB) and methylene blue (MB) were added different amounts of BaWO4 to 20 ppm of dye in the
prepared. The pH values of the solutions were adjusted cuvette (5 mL). The results indicated that 7 mg was the
according to need by using HCl or ammonia solution. Subse- optimum amount which leads to complete adsorption of
quently, various amounts of adsorbent sample were added to the dye within 10–15 min. Less than this amount resulted in
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Fig. 5 (a) Schematic representation of the sorption process and (b) photograph depicting the sorption of dyes on BaWO4 NPs.
Fig. 6 Effect of time of equilibration on (a) on absorbance of RhB maintained at a particular pH with a known amount of dosage and (b) amount of RhB and
methylene blue taken up by barium tungstate nanoparticles.
longer time for complete removal of the dye. Fig. 6b gives a the dye was around 10–15 minutes for the concentration range
correlation between the amount of dye taken up (mg gꢂ1) as a of 10–40 mg Lꢂ1. However, in the same period, the sorption
function of time. It is seen that both RhB and MB are adsorbed efficiency for a 50 mg Lꢂ1 concentration was around 70% and it
to the maximum limit within 15 and 20 min, respectively. Since required about 50 minutes for complete sorption. The steady
rhodamine B and methylene blue are cationic dyes, it is state adsorption isotherm of dye sorption for the initial dye
expected that the pH of the dye solution can affect the sorption concentration of 10 to 50 mg Lꢂ1, by nanoparticles is presented
process.30 In order to study this effect, a known amount of the in Fig. 8c. From this gure it is seen that the amount taken up
sorbent (7 mg) was equilibrated with the RhB and MB dye by the sorbent keeps on increasing and it is not saturated. This
solutions of known concentration (20 ppm) maintained at indicates that the maximum amount that can be sorbed on the
varying pHs for different equilibration periods. The results are nano material is not reached within the concentration range
shown in Fig. 7. It is seen that the kinetics of sorption depends studied. Compared to other reported sorbent materials for
on the pH of the external solution. The optimum pH for effec- cationic dyes like Fe3O4@C and carbon based materials like
tive and fast sorption was 3 for both the dyes and hence all the graphene and graphene oxide, our sonochemically synthesized
further sorption studies were carried out at this pH. The pH nano BaWO4 possess very high sorption capacity and shows
dependence of the sorption gives an idea that the process considerably faster adsorption kinetics.31,32 Based on the opti-
follows an ion exchange mechanism. The effect of initial mization of various experimental factors, the possible mecha-
concentration of the dye on the sorption process was studied by nism for sorption of dye has been explained as follows. The
varying the dye concentrations from 10 to 50 mg Lꢂ1 in solution surface complexation via electrostatic interaction is the
maintained at pH 3 and equilibrated for 10 minutes. The results predominant mode of interaction and sorption of these dyes is
of this study are shown in Fig. 8. Fig. 8a and b shows the time possible due to the presence of surface hydroxyl groups.
needed for the complete removal of rhodamine B and methy-
The sorption data were tted to both equilibrium and kinetic
lene blue, respectively for different concentrations. It is seen modelling to get an overall idea of the sorption process. Equi-
from these gures that the time needed for complete removal of librium modelling was carried out using the most fundamental
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Fig. 7 Effect of pH on uptake of (a) rhodamine B and (b) methylene Blue.
Fig. 8 Effect of initial concentration of the dye on the time needed for the complete removal of (a) rhodamine B and (b) methylene blue and (c) amount taken up by
the sorbent.
models of Langmuir and Freundlich using the linearised on multilayer adsorption on heterogeneous surface, holds the
equations for these isotherms.33 The main consideration of the assumption that the adsorption sites are distributed exponen-
Langmuir isotherm is sorption that occurs at specic homoge- tially with respect to the heat of adsorption. The results of these
neous sites within the adsorbent, indicating a monolayer modelling are given in Fig. 9 and the data calculated from these
adsorption process (constant heat of adsorption for all sites). models are given in Table 2. The experimental results match
The Freundlich adsorption isotherm, which is generally based slightly better with the Freundlich isotherm as compared to
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Fig. 9 Equilibrium modelling of sorption of rhodamine B and methylene blue using (a) Langmuir and (b) Freundlich Isotherms.
Table 2 Calculated values of parameters of dye sorption by barium tungstate
the Freundlich model also and using this model we can conrm
that the process is favoured as given by the value of n which lies
within 1 to 10.
nanoparticles, using different equilibrium models
Langmuir
Freundlich
The kinetic modelling was carried out using the pseudo rst
and second order kinetic models and the results are given in
Fig. 10.34 From the rst order modelling, the values of K1ads for
Qo (mg gꢂ1
)
b
R2
Kf
n
R2
Rhodamine B
Methylene blue 52
50
0.003 0.9825 0.23
0.002 0.9875 0.1
1
1
0.9939
0.9989
rhodamine B and methylene blue are 0.16 and 0.13 minꢂ1
,
respectively (Table 3). The values of the amount of rhodamine B
and methylene blue sorbed onto the nano sorbent at equilib-
rium were found to be 12 and 16 mg gꢂ1, respectively. The
second order kinetics modelling gave a value of sorbed amount
of 25 mg gꢂ1 for both rhodamine B and methylene blue at
Langmuir isotherm. There is not much difference in the values
of R2 for both the models as can be seen in Table 2. This indi-
cated that the sorption rst occurs at monolayer hence the
Langmuir isotherm is applicable. Once the monolayer is
covered, further sorption occurs and thus the Freundlich model
becomes applicable. The fact that sorption is not restricted to
monolayer is probably the reason for the high uptake capacity.
The fact that Freundlich model is applicable indicates that
probably the sorption energy for each molecule may be different
due to the variations in the surface property of the nano-
structures. From the Langmuir isotherm model, it is found out
Table 3 Calculated values of parameters of dye sorption by barium tungstate
nanoparticles, using different kinetic models
First order
Second order
K1ads
qe
K2ads
qe
(minꢂ1
)
(mg gꢂ1
)
R2
(g mgꢂ1
)
(mg gꢂ1
)
R2
that the maximum sorption capacity of the nanotungstate for Rhodamine B 0.16
12
16
0.9946 0.01
0.9983 0.009
25
25
0.9894
0.9844
both rhodamine B and methylene blue was about 50 mg gꢂ1
This sorption capacity is quite high. The sorption process obeys
.
Methylene
blue
0.13
Fig. 10 Kinetic modelling of sorption of rhodamine B and methylene blue using pseudo (a) first and (b) second order models.
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equilibrium and the second order rate constant K2ads was
calculated and found to be around 0.01 g mgꢂ1. In the
concentration range studied, it is observed that both rst and
second order kinetics are applicable and therefore it is again an
indication that the sorbent capacity has not reached saturation
within the concentration range studied in this particular work.
When sorption of dye occurs at the surface of the nano-
particles, there exists a possibility of dye aggregation which can
affect the data modelling using different isotherms. Therefore a
separate study was carried out to conrm whether there was
aggregation of the dye on the surface of the nanoparticles. The
nanoparticles were loaded with different concentrations of the
dye and the results of the UV-Visible spectroscopy shown in
Fig. 11 were explained to understand the phenomena. The UV-
Vis prole of the RhB dye is also given for comparison. We nd
that upon adsorption there is a shi in the peak of the UV-Vis Fig. 12 UV-Vis absorption spectra of (a) 50 ppm rhodamine B adsorbed on as
prepared BaWO4, (b) 50 ppm rhodamine B adsorbed on as prepared BaWO4
heated to 200 ꢃC and (c) as prepared BaWO4.
absorption prole of the dye. The RhB dye has one band with a
maximum absorption centered at lmax ¼ 552 nm whereas for
the adsorbed RhB on BaWO4 this peak shis to a higher value of
565 nm. The FWHM of this shied peak at 565 nm is increased
two fold. Along with it there are appearance two new peaks at proles of as prepared BaWO4, 50 ppm RhB dye adsorbed on it,
ꢃ
401 and 425 nm respectively. The formation of the new peaks, and also of the 200 C thermally treated 50 ppm dye adsorbed
peak shi to higher entity and the increment in the FWHM BaWO4 sample. It can be clearly seen that upon thermal treat-
value shows that there is denitely a very strong electronic ment there is a complete removal of the dye from the BaWO4
interaction between the BaWO4 surface and the RhB dye. As the surface and also there is no decomposition of the dye on this
525 nm peak upon adsorption shis to a higher value we can surface. Therefore it can be inferred that the process of
easily say that there is no de-ethylation upon adsorption. adsorption has not led to any reactive chemical adsorption,
Therefore this process of adsorption of the RhB on BaWO4 is since if that was the case, it would have been far harder to
that of strong electronic interaction rather than that of phys- remove the dye from the surface completely. The sorption
isorption, however it is not to the extent of reactive chemi- capacity of the thermally regenerated BaWO4 was equivalent to
sorption. It is seen from Fig. 11, with increasing concentration that of the starting material, i.e. as prepared BaWO4 and 7 mg of
of dye sorbed on the nanoparticles, there is no change in the the product was able to remove 40 ppm of RhB dye in 15
peak at 565 nm, thus indicating the absence of any dye aggre- minutes. We have tested the sorption capacity of the regen-
gation due to increase in dye concentration.
erated sample upto four consecutive cycles and did not observe
The 50 ppm dye loaded unheated BaWO4 was heated in a any noticeable difference in its performance which indicates the
furnace at 200 ꢃC to check whether it was possible to thermally stability of the BaWO4 sorbent.
regenerate the adsorbent. Fig. 12 shows the UV-Vis absorption
The dye removal experiments were also carried out using the
photochemical source. The irradiation source was a 400 W
medium-pressure mercury lamp (SAIC) located inside a quartz
tubule, situated horizontally in the reactor with 170 mW cmꢂ2
ux. The photocatalytic irradiations were carried out in a glass
petridish with 80 mg of catalyst having rhodamine-B dye
adsorbed on it. It was irradiated and then DR-UV measurement
was taken for the dye and catalyst from time to time. The extent
of reaction (dye degradation) was monitored by measuring the
decrease in the absorbance value at 565 nm using a JASCO V-670
UV-Visible-NIR spectrometer. The amount of degraded dye was
estimated using the formula (A0 ꢂ Ai)/A0, where Ai is the
absorbance of a rhodamine-B dye adsorbed on the catalyst
surface measured at different time intervals during the photo-
degradation process and A0 is the absorbance of the initial
rhodamine B dye adsorbed on the catalyst surface (50 ppm –
rhodamine dye on the as prepared BaWO4 catalyst). The irra-
diator itself has a ow of water to cut off the IR radiations and
this ensures that the effect of these degradations is exclusively
due to UV rays. The photocatalytic efficacy of the sonochemi-
cally synthesized BaWO4 was determined by studying the dye
Fig. 11 The UV-Vis spectra of (a) rhodamine B, (b) 30 ppm, (c) 40 ppm and (d) 50
ppm rhodamine B adsorbed on as prepared BaWO4.
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degradation of RhB that was adsorbed on its surface. The
The group theory calculations on scheelite-type tetragonal
central idea was to understand how fast the photochemical tungstates, predicts a C64h point group and I41/a space group.36,37
process would degrade the dye and the surface would be clean. According to the character table, the C4h point group has 26
The dye degradation prole of the adsorbed dye is shown in the different vibrational modes which are as represented below.38
Fig. 13. We nd that with time there is a decrease in the
G(vib) ¼ 3Ag + 5Au + 5Bg + 3Bu + 5Eg + 5Eu
(2)
intensity of the adsorbed dye and in 240 min about 56% of
the dye is degraded. Post 240 min of irradiation there was no
further lowering in the intensity of the adsorbed RhB, indi-
cating that saturation for the photodegradation of RhB was
reached. Recent research reported in the literature shows that
each defect on the crystal surface can act as active site for the
degradation of Rh-B dyes.14,35 In the photocatalytic mechanisms
reported for tungstates, the defects on the crystal surface and
electronic structure ([WO4]and [WO4] clusters) play an impor-
tant role in producing OH_and O2ꢂ radicals, which are the most
oxidizing species in this process. Moreover, it is believed that
other generated species such as [BaO8] clusters can participate
in the mechanism of photocatalysis.14 The low photocatalytic
activity exhibited by our BaWO4 samples may be due to
formation of lower number of defect sites on the crystal surface
and that of the [WO4] clusters and this probably can be attrib-
uted to the synthesis method adopted in this case. However,
usually the photocatalytic dye degradation processes are
studied in the aqueous medium where the rst monolayer of
the concerned photocatalyst is completely covered with –OH
Here A, B are degenerate modes whereas E is a doubly
degenerate mode; g, u are the parity terms. We know that the
total vibrational frequencies are composed of the Raman and
the infrared bands. C4h being a cyclic group will be centro-
symmetric and therefore will obey the “mutual exclusion prin-
ciple rule”. Hence, out of this 26 vibrational modes there are 13
Raman mode having following components:
G(Raman) ¼ 3Ag + 5Bg + 5Eg
(3)
Therefore, 13 IR active modes are present in this point group
which are as shown in eqn (4). However, in the infrared spectra,
1Au and 1Eu are acoustic infrared modes, and 3Bu are forbidden
infrared modes, therefore, they are considered to be infrared-
inactive modes.14
G(IR) ¼ 5Au + 3Bu + 5Eu
(4)
Hence, it can be inferred that there are only 8 infrared active
and H2O. The photo-irradiation therefore leads to the formation modes as represented below in eqn (5)
of a number of radicals. In our study we have checked the
photodegradation with only the adsorbed dye on the surface of
the catalyst. In this case, there will be only partial coverage of
G(IR)active ¼ 4Au + 4Eu
(5)
The Infrared spectra of our BaWO4 samples were taken in the
JASCO FTIR (V-610) spectrophotometer with KBr pellets. The IR
spectra for the different samples are given in Fig. 14. The IR
absorption ranging from (3200–3700) and (1350–1700) cmꢂ1 are
due to the water present in the sample.39 Therefore the peaks at
3348, 1730, 1630, 1441 and 1355 cmꢂ1 pertains to –OH groups
or the adsorbed water over the BaWO4 surface. With heating,
there is a distinct reduction in the intensity of these peaks. For
the unheated sample strong absorption bands with weak
the –OH, and H2O in the rst monolayer which results in
generation of lesser number of free radicals responsible for the
photocatalytic process. With lower number of generated free
radicals controlling the kinetics of the process, the extent of
photodegradation is low. Hence we can infer that the reason for
56% degradation of the dye under photochemical process is due
to the above reason and is not an indication on the number of
defect sites created due to the synthesis method adopted.
Fig. 13 The effect of photocatalytic irradiation with the UV light on the adsor-
bed process of rhodamine B dye on as prepared BaWO4 with time, (a) 0 min (b) 15
min (c) 60 min (d) 120 min (e) 180 min (f) 240 min.
Fig. 14 FT-IR spectra in the mid IR region of (a) as prepared BaWO4, (b) heat
treated BaWO4 and (c) adsorbed RhB on as prepared BaWO4.
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shoulders are found at 735/758 cmꢂ1 and 815/830 cmꢂ1. These 13 Z. Shan, Y. Wang, H. Ding and F. Huang, J. Mol. Catal. A:
two bands are related to n3[(1Eu) and (1Au)] internal modes Chem., 2009, 302, 54.
which are assigned to anti-symmetric stretching vibrations in 14 L. S. Cavalcante, F. M. C. Batista, M. A. P. Almeida,
[WO4]2ꢂ clusters.40 The band at 873 cmꢂ1 is also assigned to the
1Au mode.41 The mid IR region spectra help us to identify only 3
modes out of the total 8 modes that are found. All the other IR-
A. C. Rabelo, I. C. Nogueira, N. C. Batista, J. A. Varela,
M. R. M. C. Santos, E. Longo and M. Siu Li, RSC Adv.,
2012, 2, 6438.
active bands are in the far-IR region. In case of RhB loaded as 15 T. Thongtem, A. Phuruangrat and S. Thongtem, Appl. Surf.
prepared BaWO4, reductions in the peaks at 735/758 cmꢂ1 and
Sci., 2008, 254, 7581.
930 cmꢂ1 were seen, but the peak positions did not change. This 16 Z. J. Luo, H. M. Li, J. X. Xia, W. S. Zhu, J. X. Gao and
clearly indicates the adsorption of dye on the BaWO4 surface by
physical forces instead of chemical combination.
B. B. Zhang, Mater. Lett., 2007, 61, 1845.
17 A. Phuruangrat, T. Thongtem and S. Thongtem, Superlattices
Microstruct., 2012, 52, 78.
In summary, ower shaped aggregates of BaWO4 nano-
particles could be synthesized using a simple sonochemical 18 Y. Shen, W. Li and T. Li, Mater. Lett., 2011, 65, 2956.
process. The as-prepared BaWO4 showed very high specic 19 S. Brunauer, P. H. Emmett and E. Teller, J. Am. Chem. Soc.,
surface areas and an excellent ability to remove organic
pollutants from wastewater which was manifested in its uptake 20 K. S. Walton and R. Q. Snurr, J. Am. Chem. Soc., 2007, 129,
of various dyes like RhB and MB in aqueous solution. In the 8552.
concentration range studied, the adsorption kinetics of RhB 21 A. Ahami, M. Saber-Tehrani and H. Bagheri, J. Hazard.
and MB was fast and it is observed that both rst and second Mater., 2010, 181, 836.
order kinetics are applicable. The adsorption capacity was high 22 P. Kubelka and F. Munk-Aussig, Ein Zeit. Fur. Tech. Physik,
for both dyes at pH 1–3 and decreased with the further increase 1931, 12, 593.
in pH. Isotherm analysis reveals that multilayer adsorption may 23 K. V. Luca, S. Djajanti and R. F. Howe, J. Phys. Chem. B, 1998,
be involved, as both the Langmuir and Freundlich model was 102, 10650.
tted well for these cases. The binding of these dyes with 24 R. Lacomba-Perales, J. Ruiz-Fuertes, D. Errandonea,
1938, 60, 309.
¨
´
´
tungstate surface mainly involves electronic interaction as
analyzed from the UV-Vis absorption and FT-IR spectra. More-
D. Martınez-Garcı and A. Segura, Europhys. Lett., 2008, 83,
37002.
¨
over, the adsorbed dyes could be desorbed from the BaWO4 25 E. Gurmen, E. Daniels and J. S. King, J. Chem. Phys., 1971, 55,
surface by annealing at 200 ꢃC and showed signicant multi-
cyclic adsorption efficiency.
1093.
26 M. Anicete-Santos, F. C. Picon, C. N. Alves, P. S. Pizani,
J. A. Varela and E. Longo, J. Phys. Chem. C, 2011, 115,
12180.
27 M. Tyagi, S. G. Singh, A. K. Chauhan and S. C. Gadkari,
Physica B, 2010, 405, 4530.
References
1 C. A. Murray and S. A. Parsons, Water Sci. Technol.: Water 28 M. Bacci, S. Porcinai, E. Mihokova, M. Nikl and K. Polak,
Supply, 2004, 4, 113.
2 E. Forgacs, T. Cserhati and G. Oros, Environ. Int., 2004, 30,
953.
3 M. T. Uddin, M. A. Islam, S. Mahmud and
M. Rukanuzzaman, J. Hazard. Mater., 2009, 164, 53.
4 L. Zhao, X. F. Chen, X. C. Wang, Y. J. Zhang, W. Wei,
Y. H. Sun, M. Antonietti and M. M. Titirici, Adv. Mater.,
2010, 22, 3317.
Phys. Rev. B: Condens. Matter Mater. Phys., 2001, 64,
104302, (6pp).
29 D. L. Wood and J. Tauc, Phys. Rev. B: Solid State, 1972, 5,
3144.
30 J. Ramkumar, S. Chandramouleeswaran, V. Sudarsan,
R. K. Vatsa, S. Shobha, V. K. Shrikhande, G. P. Kothiyal
and T. Mukherjee, J. Non-Cryst. Solids, 2010, 356,
2813.
5 L. L. Zhang, Z. G. Xiong and X. S. Zhao, ACS Nano, 2010, 4, 31 Z. Zhang and J. Kong, J. Hazard. Mater., 2011, 193,
7030.
325.
6 L. Zhou, C. Gao and W. J. Xu, ACS Appl. Mater. Interfaces, 32 G. K. Ramesha, A. Vijaya Kumara, H. B. Muralidhara and
2010, 2, 1483.
S. Sampath, J. Colloid Interface Sci., 2011, 361, 270.
33 R. Shukla, J. Ramkumar and A. K. Tyagi, Int. J. Nanotechnol.,
2010, 7, 989.
34 J. Ramkumar, S. Chandramouleeswaran, V. Sudarsan,
R. K. Mishra, C. P. Kaushik, K. Raj and A. K. Tyagi, J.
Hazard. Mater., 2009, 172(1), 457.
7 B. R. Muller, Carbon, 2010, 48, 3607.
8 A. J. Romero-Anaya, M. A. Lillo-Rodenas and A. Linares-
Solano, Carbon, 2010, 48, 2625.
9 P. Tzvetkova, P. Vassileva and R. Nickolov, J. Porous Mater.,
2010, 17, 459.
`
10 N. Kawasaki, F. Ogata and H. Tominaga, J. Hazard. Mater., 35 T. Saison, N. Chemin, C. Chaneac, O. Durupthy, V. Ruaux,
`
2010, 181, 574.
L. Mariey, F. Mauge, P. Beaunier and J. P. Jolivet, J. Phys.
11 L. M. Camacho, A. Torres, D. Saha and S. G. Deng, J. Colloid
Interface Sci., 2010, 349, 307.
12 J. A. Costanzo, C. A. Ober, R. Black, G. Carta and
E. J. Fernandez, Biomaterials, 2010, 31, 2857.
Chem. C, 2011, 115, 5657.
36 W. W. Ge, H. J. Zhang and J. Y. Wang, J. Cryst. Growth, 2004,
270, 582.
37 S. P. S. Porto and J. F. Scott, Phys. Rev., 1967, 157, 716.
This journal is ª The Royal Society of Chemistry 2013
RSC Adv., 2013, 3, 22580–22590 | 22589

View Article Online
RSC Advances
Paper
38 T. T. Basiev, A. A. Sobol, Y. K. Voronko and P. G. Zverev, Opt. 41 X. Wang, H. Xu, H. Wang and H. Yan, J. Cryst. Growth, 2005,
Mater., 2000, 15, 205. 284, 254.
39 D. Ran, H. Xia, S. Sun, P. Zhao, F. Liu, Z. Ling, W. Ge, 42 P. Afanasiev, Mater. Lett., 2007, 61, 4622.
H. Zhang and J. Wang, Cryst. Res. Technol., 2006, 41, 43 H. W. Eng, P. W. Barnes, B. M. Auer and P. M. Woodward,
1189.
J. Solid State Chem., 2003, 175, 94.
40 R. K. Khanna and E. R. Lippincott, Spectrochim. Acta, Part A, 44 P. Parhi, T. N. Karthik and V. Manivannan, J. Alloys Compd.,
1968, 24, 905.
2008, 465, 380.
22590 | RSC Adv., 2013, 3, 22580–22590
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