Eggshell-Derived CaO for Organic Pollutant Degradation

Journal of Cleaner Production 270 (2020) 122294
Contents lists available at ScienceDirect
Journal of Cleaner Production
journal homepage: www.elsevier.com/locate/jclepro
Calcium oxide a sustainable photocatalyst derived from eggshell for
efficient photo-degradation of organic pollutants
G. Vanthana Sree a, P. Nagaraaj a, *, K. Kalanidhi a, C.A. Aswathy a, P. Rajasekaran b
a
b
Department of Chemistry, Anna University (CEG Campus), Chennai, 25, India
Graduate School of Science and Technology, Shizuoka University, 3-5-1 Johoku, Naka-Ku, Hamamatsu, 432-8011, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 October 2019
Received in revised form
8 May 2020
Accepted 14 May 2020
Available online 22 May 2020
Combined application of nanotechnology and waste reutilisation process resulting in effective and cheap
nano-photocatalyst have better dye degradation. Calcium oxide nanoparticles (CaO NPs) were synthesized from waste Eggshells (ES) by calcination process and characterized utilizing the analytical techniques such as XRD, SEM, EDAX, XPS and BET analysis. The synthesized CaO NPs were examined for
photocatalytic dye degradation of two model dyes such as Methylene blue (MB) and Toluidine blue (TB)
in aqueous medium. From the outcomes, it is clear that CaO effectively degraded both the dyes within
15 min (MB in 15 min and TB in 10 min). The studies include optimisation of parameter which found to
be 7 pH, 20 ppm dye concentration and 50 mg of catalyst loading were effective operation condition. The
kinetics of dye degradation follows pseudo first order, catalyst recycled for 7 cycles and chemical oxygen
demand (COD) removal efficiency also analysed for degraded samples.
© 2020 Elsevier Ltd. All rights reserved.
Handling editor Jun Bi
Keywords:
Eggshell
Calcination
Calcium oxide
Sustainable
Photodegradation
1. Introduction
Water plays an important role in living system, but modern
human civilization has resulted in the contamination of water resources by releasing the waste materials. Industrial effluent, mining
effluent, agricultural runoff water, household waste water etc.
Contribute extensively towards the pollution of water resources. . In
industrial waste effluent, 20% of effluent is produced by textile
industries at various processing stages. Also, textile industries utilise excess of water for dying process and 50% of water is released as
dye effluents. These effluents contain reactive dyes and chemicals
which are non-biodegradable, toxic and create several environmental hazards (Matos et al., 2011). Textile effluent is a highly toxic
mixture which contains sulphur, vat dyes, nitrates, naphthol and
heavy metals. Various non-destructive processes such as precipitation, ozone treatment, ultrafiltration, membrane treatment,
reverse osmosis, flocculation and adsorption can be used for
removal of colour from effluents. But major drawback of nondestructive methods is that, they are simply concentrating the
* Corresponding author. Department of Chemistry, Anna University, Chennai, 25,
India.
E-mail address: nagaiitd@gmail.com (P. Nagaraaj).
https://doi.org/10.1016/j.jclepro.2020.122294
0959-6526/© 2020 Elsevier Ltd. All rights reserved.
reactive chemical into sludge, which require further treatment
(Pawar et al., 2015). These post treatment process will not be
required for destructive method where toxic pollutants will be
broken into non-toxic smaller molecules and released into environment. Advanced oxidation processes (AOPs) is one of the
destructive methods which has increased the interest for complete
degradation of dyes. AOPs include photocatalysis, photo-Fenton,
and photo-ozonation to degrade pollutants. The OH radical has
high oxidation potential (2.8 eV) which reacts with organic contaminants and mineralizes them to non-toxic compounds (Oveisi
et al., 2019). AOP degrade the pollutants by the generation of hydroxyl radicals and reactive oxygen species (from photo-catalyst)
that are important agents which react with pollutants and result
in complete decomposition (Kamat et al., 1999). Efficiency of AOP,
that depends on properties of catalyst bandgap, surface charge etc,
is also affected by nature of reaction medium and environmental
conditions. For instance, summer season will be favourable for
photo-degradation, as solar light intensity is high resulting in faster
and shorter time for dye degradation, whereas in other climates
light intensity is less and hence, the degradation of dyes can be
slower with increased reaction time. The pH of the reaction mixture
is also an important parameter because it controls the surface
charge, aggregation of materials and interaction with charged
organic materials. Application of nanomaterials in AOP process
2
G. Vanthana Sree et al. / Journal of Cleaner Production 270 (2020) 122294
improves the efficiency of dye degradation than bulk material due
higher surface area. Higher surface area of nanoparticles results in
production of large number of reactive species and thereby increases the degradation efficiency. Heterogeneous semiconductor
nanomaterials such as oxides and sulphides of metals (Cd, Zn, Ti, Ta,
Sn and W) have been efficiently utilized for photocatalytic degradation of several organic pollutants and dyes (Kamat et al., 1999,
Salim et al., 2015). Though many nanoparticles have been reported
for effective dye degradation, only few reports are available with
regard to photocatalyst synthesized from waste materials. Titanium
oxide and zinc oxide nanoparticles are well known for photocatalytic dye degradation individually and in composite form. In
this report, calcium oxide (CaO) nanoparticles derived from
eggshell is used as photocatalyst for effective dye degradation,
which is reutilisation of kitchen waste (eggshell) in a sustained
manner and is a cheaper catalyst than previously reported
materials.
In general, Calcium oxide (CaO as quicklime) is a white, caustic,
alkaline, crystalline powder made from the thermal decomposition
of limestone or shells containing calcium carbonate (CaCO3).
Chemically synthesized CaO is used as catalyst in many industries
in the form of toxic waste remediation agent, bactericide, adsorbent
and binder in paints etc. (Annam Renita et al., 2016; Tangboriboon
et al., 2012; Niju et al., 2014). Chemically synthesized CaO is used as
photo-catalyst in degradation of organic dyes such as methylene
blue (Rameshwar et al., 2014), Congo red (Anantharaman et al.,
2016), Malachite green (Bathla et al., 2019), Indico carmine
(Devarahosahalli Veeranna et al., 2014), Violet GL2B
(Madhusudhana et al., 2012), crystal violet (Sawant et al., 2015).
Eggshell (ES) also acts as a source of calcium oxide which contains
94% of calcium carbonate. Egg is an important nutrient source for
humans as it contains excess of protein, which is consumed up to
million metric tons per year. Simultaneously, ES produces a significant amount of waste which is disposed into landfill without
any pre-treatment, resulting in the odours by biodegradation and
also changes the quality of soil. These ES derived CaO NPs have been
utilized in many fields namely biodiesel production, adsorption of
pollutants and heavy metals, dielectrics, catalysts, biomaterials,
fuel cell applications and fillers. (Annam Renita et al., 2016;
Tangboriboon et al., 2012; Niju et al., 2014). Though chemically
synthesized CaO is applied as photocatalyst, eggshell derived CaO
has to be analysed and the same is discussed in this article. CaO
synthesis is a simple and easy process which does not need any
solvent or chemicals for processing and results in eco-friendly and
sustained material. In the present study, CaO NPs were synthesized
from waste Eggshells by calcination process and experiments were
carried with two cationic dyes namely Methylene blue and Toluidine blue with their drawback as mentioned below.
Methylene blue (MB) is a cationic dye which contains conjugated aromatic moiety. It was the earliest synthesized antimalarial
drug used from the 19th century. Also, it is used for falciparum
malaria in combination with amodiaquine (Bountogo et al., 2010).
Though, it is used as medicine, the immune system and reproductive systems are strongly affected by MB and also leads to
carcinogenic and genotoxic effects. It also has negative impact on
environment and is not easily degraded by microbes because of its
complex aromatic structures, hydrophilic property, light and temperature stability etc. Thus, these toxic dyes must be removed from
wastewater to prevent the environment. The photocatalytic
degradation of MB in water medium is reported with regard to
many nano-materials and the way they are composited, is tabulated with their efficiency in Table 1. Molecular structure of MB and
Toluidine blue are given below in Fig. 1. Toluidine blue(TB) is a
phenothiazine class of dye used in various fields like medicine,
textile and biotechnology. Inspite of large application potential, TB
has a mutagenic effect due to the toxic interaction with DNA and
RNA (Salim et al., 2015). The photocatalytic degradation of TB in
water medium has been reported with many nano-materials and
the composited are tabulated in Table 2.
The synthesized CaO NPs were examined for photocatalytic dye
degradation of two model dyes such as Methylene blue and Toluidine blue in aqueous medium. The studies include effect of pH, dye
concentration, catalyst loading, kinetics of dye degradation, catalyst
recycling and COD removal. In order to find out the optimized
condition for degradation, computer stimulation was also studied
using RSM and ANNOVA method which helps to correlate the
experimentally and computationally determined rate constant and
optimized condition. Response surface methodology (RSM) is a
statistical and mathematical method used for understanding the
functional interaction between various parameters. It is used for
the modelling and investigation of multivariable systems to evaluate the individual and interactive effect. In photo-degradation,
influence of variables such as effect of pH, effect of dye concentration, irradiation time and catalyst loading on the percentage
degradation were analysed by RSM and ANNOVA to reduce the
number of experiments and the manual errors which are complex
in nature (Karchiyappan T, 2017). Thus, the main objective of the
present work is to report the photocatalytic dye degradation efficiency of CaO (preferred less due to its higher band gap energy)
NPs, that expected to have better efficiency than previous reports
due to size down effect and precursor properties. In extended
search for faster degradation of pollutants, ECaO was expected to
achieve the target which confirmed by testing against the cationic
dyes. Additionally, optimise the reaction parameters by both
experimentally and statistically using RSM analysis.
2. Experimental sections
2.1. Materials
Methylene blue, Toluidene blue, Rhodamine B, Ethyl acetate,
chitosan, acetic acid, Ammonium oxide, Benzoquinone, isopropanol, sulphuric acid, sodium hydroxide were purchased from
Sigma-Aldrich Chemical Co, (India) in analytical grade and were
used without further purification. Tap water was used for photocatalysis experiment.
2.2. Instruments
The completion of calcination reaction, structure, crystallinity,
crystal size and purity of the CaO NPs at various temperature and
time were examined using powder X-ray diffraction (XRD) analysis
with spectrum range of 10 e80 using XPERTPRO diffractometer
having Cu Ka radiation (I ¼ 30 mA, V ¼ 40 kV). For CaCO3 and CaO
NPs, X-Ray photoelectron spectroscopy (XPS) studies was carried
out to determine the binding energy and a PHI 5000 VERSAPROBE II
SCANNING ESCA MICROPROBE system was used to confirm the
purity. The surface morphology and composition of CaO NPs were
analysed by scanning electron microscopy (SEM) VEGA 3 TESCAN,
and energy dispersive X-ray spectroscopy (EDS) Hitachi instrument. From the brunauer emmett teller (BET) analysis, specific
surface area of the synthesized CaO NPs was calculated on the basis
of nitrogen adsorption desorption isotherms measured at 77 K
using a quantachrome instrument version 3.0 for which the sample
was degassed at 150 C before nitrogen adsorption. The UVeVisible
absorption spectra of model dyes with various time intervals were
observed using Elico SL-159 UVeVis spectrophotometer. Initial
confirmation experiments were carried out in a HEBER multi-lamp
Photo-reactor (Model e HML-MP88) at room temperature
39 C(~aveg).
G. Vanthana Sree et al. / Journal of Cleaner Production 270 (2020) 122294
3
Table 1
Previously reported MB photocatalyst with their efficiency.
S.No Material
Source
Efficiency with time
1
UV light
2
cadmium selenide nanoparticles in uncapped and capped form
with 50% PVP
palladium-doped TiO2
2
3
Silver nanoparticles in sodium borohydride
Bentonites with TiO2
Solar light
UV source
4
5
Zinc oxide/Nickel Ferrite nanocomposite
modified TiO2/BiVO4 nanocomposite
UV source
Visible light
6
ZnO supported montmorillonite
UV source
7
SiO2 supported bimetallic heterogeneous Photo-Fenton catalyst
Visible light
source
8
Copper nanoparticles supported on montmor-illonite
31% efficiency for uncapped form and 48% for capped form Chepape et al.
in 120 min
(2017)
Maximuim removal efficiency at 80min
Nguyen et al.
(2018)
92.06% within 14 min.
Fairuzi et al. (2018)
99% removal in 90min
Rossetto et al.
(2010)
98% removal efficiency in 70%
160 min for complete decolourisation
Samsudin et al.
(2018)
99% removal in 150 min
Fatimah et al.
(2011)
86% of removal in solar light and 76% of removal in reactor Ahmed et al.
(2016)
100% degradation within 120 min
Mekewi et al.
(2016).
UV light
Reference
Fig. 1. Molecular structure of a) methylene blue and b) Toluidine blue. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of
this article.)
Table 2
Previous reports fro TB degradation with various photocatalyst with their efficiency.
S.No
Material
Source
Efficiency with time
Reference
1
2
3
4
cadmium sulphide
Magnesium oxide
BiPO4/Bi2S3-HKUST-1-MOF nanocomposite
Zinc ferrite (ZnFe2O4)
UV source
UV radiation
Visible light
visible light
90% removal in 170 min
Complete removal in 70 min
Degraded in 65min
Reaction completes in 90 min at pH 9.5
Neelakandeswari et al. (2011)
(Salim et al., 2015)
Mosleh et al. (2016)
(Parsoya and Ameta, 2016,Parsoya and Ameta, 2016)
2.3. Synthesis of calcium oxide nanoparticles (CaO NPs)
Eggshell was collected from kitchen waste and washed several
times using tap water to remove the inner membrane and dried
overnight in hot air oven at 80e100 C. After the drying process, it
was powdered using mortar and pestle, which resulted in slight
pale yellow granular powder. 5 g of ES powder (CaCO3) was
calcined at three different temperatures with various incubation
time in box furnace. Each sample was labelled as per the calcination
temperature (time of incubation) such as 700(1), 700(2), 700(3),
800(1), 800(2), 800(3), 900(1). After calcination, the obtained white
coloured CaO nanoparticles were stored in the desiccator to prevent the adsorption of moisture from environment (Tangboriboon
et al., 2012).
2.4. Photocatalytic experiments
The photocatalytic activity of synthesized CaO NPs was evaluated by photo-degradation of aqueous MB and TB textile dyes. Stock
solution (100 ppm) of dye was prepared by dissolving 10 mg of dye
molecules in 100 mL of distilled water which was diluted into 1:9
ratio of dye to water for 10 ppm working solution. The catalytic
experiments were carried out with 50 mL of MB (10 ppm/mL) and
50 mg of CaO NPs under aeration to prevent settling of catalyst. The
catalyst and MB mixture were sonicated for 5 min for the uniform
distribution of the catalyst in the beaker and the mixture was kept
in dark environment for 1 h (Nithya et al., 2018). About 3 mL of the
reaction mixture from the dye degradation tube was withdrawn at
regular time intervals (5 min), centrifuged and absorbance values
were measured using spectrophotometer. Further optimisation
experiments were carried out in the open environment in the
presence of direct sunlight from 10.30 a.m. to 3.30 p.m. . The
samples were collected at regular intervals such as 0, 5, 10, 15 and
20 min and spectral data was also collected. The degradation reaction was monitored by recording the UV spectrum. For effect of
catalyst dosage, the experiment was repeated in solar environment
with different catalyst loading such as 25 mg, 50 mg, 75 mg and
100 mg and constant dye concentration (10 ppm). For optimizing
the efficiency of pH, the dye degradation was carried out by taking 5
individual 50 mL of dye solution in a separate beaker and the pH
(pH-3, pH-5, pH-7, pH-9 and pH-12) of the dye solutions were
adjusted using 0.1M HCL and 0.1M NaOH solution, with fixed
catalyst loading (50 mg for MB and 25 mg for TB), dye concentration (10 ppm/mL) and time (Nithya et al., 2018). Further, the optimum dye concentration was also determined. The photo-removal
efficiency percentage was calculated from equation (1).
% Photo-removal efficiency ¼ (C0 Ct)/C0 100 —————————(1)
4
G. Vanthana Sree et al. / Journal of Cleaner Production 270 (2020) 122294
where C0 is the initial concentration of dye and Ct is the concentration of dye at the time of photo-irradiation (final). In order to
understand the reaction kinetics of photo-degradation, the logarithmic plot of dye concentration as a function of irradiation was
time plotted in which the apparent rate constant k, was calculated
from equation (3). k value for each sampling time was calculated
and average range of rate constant was compared with other reaction conditions.
-ln(Ct/C0) ¼ kt ————————
(2)
k ¼ -ln(Ct/C0)/t————————
(3)
The value of k can also be predicted from the slope of a plot of
ln(C0/Ct) versus t (Nguyen et al., 2018).
2.5. CaO NPs incorporated chitosan beads preparation
About 50 mL of 1% chitosan solution was prepared by dissolving
0.5 g of chitosan in 50 mL of 0.1% acetic acid solution. After the clear
and homogenous solution formation, 50 mg of CaO NPs was added
slowly under vigorous stirring with 0.1% cross linker namely glycerol (50ml/50 mL) and citric acid (0.05 g/50 mL). Following the
formation of homogenous mixture, the solution was poured
dropwise into 2M NaOH solution using the pipette. Droplets were
solidified into beads which incubated for 24 h in NaOH solution and
then neutralised using water wash. Beads stability was tested by
overnight stirring in medium speed under water medium (SafaeiGhomi et al., 2013).
sample weight from 5g to 2.7 g(46% loss) thereby indicating that
99% of calcium carbonate was decomposed to CaO and CO2. These
were correlated with results of research work on duck shell derived
calcium oxide synthesis carried under various temperatures
(Tangboriboon et al., 2012).
CaCO3(s) / CaO(s) þ CO2(g)
3.1.2. X-ray diffraction analysis
Fig. 3 shows the X-ray diffraction spectra of pure eggshell, its
various temperature and time calcined one such as 700 C (3 h),
800 C (1,2, and 3 h) and 900 C (1h) (Bathla et al., 2019). From
Fig. 3, it is clear that eggshell and 700 C (1e3) hour samples clearly
match with calcium carbonate XRD pattern of JCPDS no 85e1908.
Samples from 800 C, 1e2 h exhibit that XRD patterns did not
match with CaCO3 or CaO, thus it may be the intermediate product
and this is evident that 800 C with short time (1 & 2 h) is not
enough for CaO formation. But 800 C with 3 h calcined sample
XRD pattern exactly matched with calcium oxide nanoparticles
having face centred cubic structure JCPDS No:74e1226. It has major
peaks 2q at 32 , 37, 54 , 64 , 67 similar to CaO peaks with h k l
plane of 32 (1 1 1), 37 (2 0 0), 54 (2 2 0), 64 (3 1 1), 67 (2 2 2).
Similarly for CaO NPs XRD pattern was observed at 900 C calcined
eggshell. The optimized temperature of CaO NPs synthesis from
eggshell is 800 C with 3 h. The crystallite size (D) could be
calculated using the DebyeeScherrer equation (D ¼ Kl/bcosq),
3. Results and discussion
3.1. Calcium oxide characterisation
3.1.1. Effect of calcination temperature
The chicken eggshell powder was calcined in the furnace at
different temperatures and time namely 700(1), 700(2), 700(3),
800(1), 800(2), 800(3), and 900 C(1) resulting in fine powder form
with different colour and texture compared to the ES powder.
Calcined temperatures of 700(1) and 700(2) produced powders in
black and dark grey colours due to incomplete CO2 emission,
whereas at 700(3) slight grey powder was produced. At 800(1) and
800(2) calcination obtained was partially white solid, but they
contained tiny solid particles similar to ES powder. Samples from
800(3) and 900(1) resulted in fine CaO NPs with 46% weight loss
due to complete CO2 removal from CaCO3. As the calcination temperature was high, the quantity of metal oxide formed was high as
indicated by the colour changes of the chicken ES powder, where
pale yellow became white with fine powder form as shown in Fig. 2.
The calcination temperature at 800(3) and 900(1) reduced the
Fig. 2. CaO formation from ES.
Fig. 3. (A) XRD spectra for ES, and all samples.
G. Vanthana Sree et al. / Journal of Cleaner Production 270 (2020) 122294
5
nanoparticles and the elemental composition of synthesized CaO
NPs was found to be 64 wt% for Ca and 34 wt% for O (Fig. 5B inserted
table).
3.1.5. BET analysis of CaO NPs
From the bet analysis, CaO from the ES was found to be in
mesoporous structure with pore size of 2.475 nm in diameter.
Surface area was 13.130 m2/g and pore volume to be 0.035 cc/g. The
adsorption desorption isotherm graph given below in Fig. 6 confirms that CaO follows the type 4 isotherm. It clearly evidences the
formation of multilayer on catalyst surface during the reaction.
Fig. 4. XPS spectra for ES and CaO product.
where b (full-width at half-maximum or half-width) is in radian
and q is the position of the maximum diffraction peak in radian, K is
the constant value of about 0.9, and l is the X-ray wavelength
(1.5406 Å for Cu Ka) (Devarahosahalli Veeranna et al., 2014). The
crystallite size of CaO has been found for each individual peak as
tabulated below and average size was found to be 26.04 nm.
At 900(1), moisture can be absorbed in higher percentage by
CaO from the environment (due to higher surface energy) followed
by calcium hydroxide formation. This can be proved by the
appearance of new peak at 34 which is considered to be a major
peak for Ca(OH)2 of JCPDS No: 72e0156 with hexagonal structure.
3.1.3. XPS analysis of CaO NPs
The wide XPS spectra of ES and CaO nanoparticles are shown in
Fig. 4. The binding energy is related to the kinetic energy by the
equation BE ¼ hv - KE, where hv is photon energy (typically
1486.6 eV for Al ka radiation) and KE is the emitted electrons kinetic energy. The major components of eggshell and CaO NPs
contain C, O, Ca. Generally, binding energy of C 1S (OeC]O) appears at 288.5 eV, where C]O individually have binding energy at
289 eV and CeO have separate peak at 286 eV (Fig. 4). Oxygen also
showed peak from 527.2 eV to 532 eV corresponding to all oxygen
bonds namely C]O, CeO, CaeO in the form of a combined narrow
and intense peak. Calcium showed 2p 3/2 and 2p 1/2 peak at 343.87
and 347 eV as binding energy which can be compared with the
previous results of XPS spectra of CaCO3(Raliya et al., 2016). CaO
spectra show only two major peaks that correspond to calcium and
oxygen. Carbon peaks disappeared and intensity of oxygen peak
was reduced due to the removal of CO2. Single oxygen generated
the peak at same 529 eV range due to Ca]O binding and calcium
peak showed slight negative shifting.
3.1.4. SEM and EDAX analysis of CaO NPs
The morphology of synthesized CaO NPs was examined utilizing
the scanning electron microscopy instrument as shown in Fig. 5A.
The 10 mm magnification showed the clusters of flack like structure
distributed uniformly (Fig. 5A), whereas 1 mm and 500 nm
magnification showed spongy porous arrangement as depicted in
Fig. 5A.
EDAX analysis confirmed the formation of pure CaO
3.1.6. Thermal analysis
Thermal stability of eggshell, optimum calcination temperature
for synthesis of CaO and stability of CaO were studied from Thermogravimetric analysis. In Fig. 7, thermal decomposition of
eggshell shows initial mass loss of 0.1% at 100e200 C due to
moisture loss. Secondly, mass reduction in range of 500e600 C
occurred due to the removal of organic materials in egg shell,
whereas major mass reduction appeared at 700e820 C due to the
removal of CO2 from CaCO3 and formation of CaO. This evidences
that 800 OC was optimum calcination temperature for synthesis of
CaO NPs from eggshell (Witoon et al., 2011). Thermal stability of
CaO NPs remains stable up to 400 C and mass loss of 21% was due
to the conversion of calcium hydroxide Ca(OH)2 to CaO. Further loss
occurred at the range of 580e610 C due to conversion of CaCO3
into CaO which is due to the presence of unconverted eggshell in
trace amount.
3.2. Photocatalytic studies
Initially photocatalytic efficiency of the CaO was studied for MB,
TB and Rhodamine B dyes were tested which absorbs in the visible
region of light at 667 nm, 630 nm and 550 nm, respectively (Fig. 8).
The reaction mixture of catalyst and Dye solution were sonicated
and then kept in the dark condition for adsorption test. After 1 h,
samples were centrifuged and UV spectral data was analysed where
MB concentration remained the same. These were clear evidence
that catalyst does not adsorb MB. In contrast, TB resulted in
adsorption from 35 to 60% of dye molecules based on catalyst
concentration in 1 h slowly after which becomes constant. After the
adsorption studies, degradation experiment was carried out in
reactor in order to find whether the catalyst could degrade the dye
molecule and also to calculate the overall reaction time. Samples
were collected from reactor at half an hour interval at which both
MB and TB became colourless solutions in the first cycle itself and
Rhodamine B remained coloured even after 4 h. In UV spectra, the
intensity of the absorption peaks decreases to minimum for MB and
TB which was mainly due to the result of photocatalytic reaction.
For Rhodamine B, peak remained constant which indirectly state
that CaO does not have the efficiency to degrade this dye. Under the
light irradiation, MB and TB turned to colourless solution within
15 min, it concludes that CaO have higher photo-degradation efficiency on MB and TB. Thus, CaO acts as adsorbent followed by
photocatalyst on Tb whereas act directly as photocatalyst with MB.
After the confirmation of photocatalytic activity on MB and TB,
experiments were extended to solar environment and spectral
analysis was taken for each sample at 5min interval. Spectral data
clearly indicates the successive decrease in absorption within
15 min for MB and 10 min for TB. This short time degradation was
achieved because the smaller size of the CaO NPs provides larger
surface area for photon adsorption, more free radical production
and faster degradation of the dye. The LangmuireHinshelwood
model was chosen to investigate the kinetics of MB and TB
photo-degradation. The logarithmic plot of MB and TB
6
G. Vanthana Sree et al. / Journal of Cleaner Production 270 (2020) 122294
Fig. 5. A: SEM image of CaO at 10 mm magnification(a), 1 mm magnification(b), 500 nm magnification(c).
5B: Elemental analysis of CaO.
Fig. 7. Thermal analysis of eggshell and CaO.
Fig. 6. Adsorption desorption isotherm of CaO.
concentration as a function of irradiation time confirms that
degradation of MB and TB follows pseudo-first-order kinetics.
3.2.1. Effect of catalyst loading
The catalyst dose is an important parameter in photocatalytic
degradation of organic dyes. Increase in the photocatalyst dosage
will increase the dye removal efficiency, as a result of multiplication
of the active sites thereby increasing the active species concentration in the reaction mixture. Positive increment will occur up to
certain concentration, beyond which efficiency becomes constant
or reduced. This is due to the blocking of light penetration with
G. Vanthana Sree et al. / Journal of Cleaner Production 270 (2020) 122294
7
Fig. 8. Degradation of (a)MB, (b)TB and (c) Rhodamine B using CaO in reactor.
increasing photocatalyst amount. It is clear from Fig. 9a that increase in CaO NPs dosage (25 mg, 50 mg, 75 mg and 100 mg) increases the removal efficiency. Efficiency varies from 92.99% to
96.19% when catalyst concentration increases from 25 mg to 50 mg.
But further increase in catalyst concentration does not show much
difference in degradation. So, 50 mg/50 mL was optimum catalyst
loading for methylene blue dye degradation.
Kinetic study of MB degradation is shown in Fig. 9c. The logarithmic plot of MB concentration as a function of irradiation time
for all the concentrations confirms that degradation of MB follows
pseudo-first-order kinetics by having R2 below and slope represents the rate constant which are tabulated below in Table 3 with
calculated k values. Rate constant increased from 0.1645 to 0.2501
min1 by increasing the catalyst dosage from 25 mg to 100 mg.
Major increase in rate constant was seen between 25 mg and 50 mg
and moderate increase from 50 mg to 100 mg (see Table 3).
Similarly, effect of catalyst CaO NPs (25 mg, 50 mg, 75 mg and
100 mg) loading on TB dye degradation (Fig. 10a) and within 10 min
complete degradation was observed. TB was adsorbed by CaO
which increases with increase in catalyst loading. Due to adsorption
and large pollution, higher number of dye molecules present on
surface results in reduction of reactive species production. Thereby
degradation efficiency decreased with increase in catalyst concentration as shown in Fig. 10b. Reduction in efficiency was due to
decreased reactive species production and. From Fig. 10c, the reaction follows the first order kinetics and has a rate constant of
0.2445 min1 and the optimum condition of TB dye degradation
was found to be 25 mg/50 mL (Table 4).
Table 3
Kinetics of catalyst concentration variation on MB degradation.
Catalyst concentration
25 mg
50 mg
75 mg
100 mg
R2 value
K ¼ rate constant (from graph)
K ¼ rate constant (calculated)
0.9651
0.1645
0.170711
0.9915
0.2089
0.210856
0.9911
0.2403
0.241833
0.9968
0.2501
0.25958
3.2.2. Effect of dye concentration
The amount of OH radicals formation on the catalyst surface
and interacted with the dye molecules represent the rate of
degradation. Increase in dye concentration will reduce the reactive
species concentration that will result in decrease in degradation
efficiency and rate of reaction. Once, the amount of dye molecules
increases, number of dye molecules for excitation and contact with
the catalyst surface also increases. Thus, the quantity light penetrates into the solution, photon reach the surface of the catalyst will
be reduced and also more amount of light absorbed by the dye than
the catalyst. Also, increase in dye concentration will result in the
requirement of catalyst surface needed for better degradation. If
reaction time and quantity of catalyst remain constant, then the
OH radical (primary oxidant) formed on the catalyst surface also
remains constant. Thus the number of reactive groups cleaving the
dye molecules decreases with increasing amount of the dye. Fig. 11a
shows the influence of dye concentration on MB degradation efficiency of CaO NPs. Degradation efficiency decreased from 10 ppm
(98.26%) to 20 ppm (90.34%) with reaction time increasing from
15 min to 20 min. But efficiency reduced drastically at 30 ppm
Fig. 9. Effect of varying the catalyst concentration on MB degradation and its kinetic studies.
8
G. Vanthana Sree et al. / Journal of Cleaner Production 270 (2020) 122294
Fig. 10. Effect of varying the catalyst concentration on TB degradation and its kinetic studies.
Table 5
Kinetics of dye concentration variation on MB degradation.
Table 4
Kinetics of catalyst concentration variation on TB degradation.
Catalyst concentration
2
R value
K ¼ rate constant (from graph)
K ¼ rate constant (calculated)
25 mg
0.9817
0.2445
0.241129
50 mg
0.9517
0.2499
0.270858
75 mg
0.9413
0.2442
0.269004
100 mg
0.9553
0.2238
0.244738
concentration at which efficiency lowered to 77.74% and reaction
time increased to 30 min for complete degradation. Thus, the optimum MB dye concentration was found to be 20 ppm (Table 5).
Similarly, the studies was also carried out in TB dye (Fig. 12aed),
10 and 20 ppm dye concentration have similar efficiency (94.199%
and 93.5%) like MB and 30 ppm dye concentration efficiency was
reduced to 87% due to the reduction of rate constant from 0.1895 to
0.1106 min1. The optimum TB dye concentration was found to be
20 ppm/mL for the further studies (see Table 6).
3.2.3. Effect of pH
Both Methylene blue and Toluidine blue are cationic dyes in
aqueous solution and interaction of its cationic configuration with
catalyst was examined between the pH ranges 3e12. Point of zero
charge (PZC) is the pH point at which surface charge becomes zero
and pH below the PZC value, surface will be positive charge
whereas in above pH negative charge. Most of the cationic dyes
showed better efficiency at basic pH due to strong electrostatic
interactions with the negative catalyst surface. The photocatalytic
efficiency decreased with decreasing pH because of the
Dye concentration
2
R value
K ¼ rate constant (from graph)
K ¼ rate constant (calculated)
10 ppm
20 ppm
30 ppm
0.9817
0.2445
0.241129
0.9985
0.207
0.204325
0.99999
0.1471
0.153443
electrostatic repulsion between the dye and the catalyst. In MB, at
pH 3degradation does not occur where in pH 5 degradation occurs
slowly with an efficiency of 86.92% after 15 min. In neutral pH,
degradation efficiency was 96.52% and increased to 98.32% at pH 9
and pH 12 as shown in Fig. 13. Rate constant increased from 0.2051
min1 to 0.241 min1 as tabulated in Table 7.
Likewise, TB also showed negative degradation efficiency at
acidic medium (see Table 8). At pH 3 and pH 5, TB solution does not
undergo any degradation whereas in MB, pH 5 showed degradation
up to 86%. In neutral pH, efficiency was 94.2% which increased to
95.2% at pH 9 with increase in rate constant from 0.061 min1 to
0.2435 min1. But further increase to pH 12 was reduced to 88.9%
(Fig. 14b) with reduction of rate constant to 0.2000 min1.
3.2.4. Wastewater treatment
In order to calculate the maximum degradation efficiency of
catalyst, reaction was extended to real wastewater collected from
lake contaminated by industrial effluents. Initially, waste water was
filtered to remove solid particles and 50 mL of solution with 50 mg
of catalyst was taken for reaction. In spite of adsorption, CaO had
Fig. 11. Effect of varying the dye concentration on MB degradation and its kinetic studies.
G. Vanthana Sree et al. / Journal of Cleaner Production 270 (2020) 122294
9
Fig. 12. Effect of varying the dye concentration on TB degradation and its kinetic studies.
Table 7
Kinetics of pH variation on MB degradation.
Table 6
Kinetics of dye concentration variation on TB degradation.
Dye concentration
10 ppm
20 ppm
30 ppm
pH
5
7
9
12
R2 value
K ¼ rate constant (from graph)
K ¼ rate constant (calculated)
0.99058
0.1895
0.200593
0.99492
0.1557
0.18949
0.9528
0.1106
0.153443
R2 value
K ¼ rate constant (from graph)
K ¼ rate constant (calculated)
0.9839
0.15
0.155678
0.9896
0.2051
0.222317
0.9823
0.2175
0.24375
0.9753
0.241
0.269713
better photocatalytic efficiency which was evidenced by reduction
of absorbance from 1 to 0.3 within 1 h, in sunlight irradiation
(Fig. 15). But in the dark reaction, absorbance was reduced to 0.6
even after 24 h that means 40% of materials being adsorbed by CaO
after 24 h. Overall, catalyst showed 37% efficiency on adsorption
removal at 24 h but removal efficiency of 70% could be achieved
within 1 h of light irradiation.
3.2.5. Chemical oxygen demand (COD) analysis
COD analysis would also help us to understand deeply that the
dye molecules were not only degraded, it was sized down to ecofriendly by-products. COD of dye solutions before and after reaction was analysed and COD was measured for both samples. COD
values were reduced from 823 mg/l to 109 mg/l for MB and TB,
initial COD was 789 mg/l whereas final sample was 149 mg/l. In the
waste water, catalyst did not show colour modification but in terms
of COD, it had better reduction from 804 mg/l to 250 mg/l. Therefore overall efficiency was found to be 86.76% for MB, 81.15% for TB
and 68.91% for real waste water sample (Fig. 18). Lower COD
removal efficiency for waste water was due to the presence of more
chemical components which can’t be degraded by CaO or by-
products itself accumulated in water.
3.3. Mechanism of photo catalysis
Once the electron gets the sufficient energy (equivalent to band
gap energy), it jumps from valence band to conduction band with
the formation of hole in valence band. Electron reacts with the
dissolved oxygen (O2) in aqueous solution which also acts as the
electron scavenger results in the formation active free radicals OH,
O
2 , etc. Simultaneously, holes will get electron from electron donors (H2O), resulting in OH free radicals formation. After the free
radical formation, dye molecules were attacked by active species
(OH, O
2 etc.)and ended in the de-coloration and opening-ring
reactions.
Dye þ hv /dye* (Absorption of photons and excitation of dye
molecules)
hv þ CaO/ e(CB) þ H*(VB) (Electron-Hole pair formation)
e(CB) þ O2 / O2 ˉ (Oxygen ionosorption)
Fig. 13. Effect of varying the pH on MB degradation and its kinetic studies.
10
G. Vanthana Sree et al. / Journal of Cleaner Production 270 (2020) 122294
Fig. 14. Effect of pH on TB degradation and its kinetic studies.
2O2ˉþ 2H2O/ H2O2 þ 2OH þ O2 (O
2 Neutralization by protons)
Table 8
Kinetics of pH variation on TB degradation.
pH
5
7
9
12
R2 value
K ¼ rate constant (from graph)
K ¼ rate constant (calculated)
0.9499
0.0051
0.005634
0.9956
0.0061
0.222186
0.9827
0.2435
0.256858
0.9797
0.2000
0.224033
H2O2þe(CB) /-OH þOH (Decomposition of H2O2 and OH
formation)
hþ þ H2O / Hþ þ OH (water splitting by photo-hole to produce
OH radicals)
OH / O
2 þdye / Dye degradation (Electrophilic attack on Dye
molecules)
During the degradation reaction, photo-holes remain inactive in
the initial step due to the cationic nature of reactant. So, the OH/
þ
O
2 radicals initiate the reaction by attacking the ReS ¼ R functional group in both MB and TB, due to the direct interaction with
catalyst surface. The electrophilic attack of radicals cleaves the
double bond of heteroatom S and leads the reaction from CeSþ¼C
to CeS(¼O)eC. It will be the result of ring opening mechanism that
occurs at the aromatic ring in middle which contains both heteroatoms, S and N to conserve the double bond conjugation. Thus,
MB and TB degradation starts with the cleavage of the double
bonds at the ReSþ ¼ R functional group into ReS(¼O)eR, and
further OH/O
2 radical attack results in sequential degradation
from ReSO2eR, ReSO3HeR to final product SO2,
4 CO2 and phenol.
Fig. 15. Absorbance spectra of waste water at initial, absorption and photodegradation.
3.3.1. Scavenging analysis
þ
Superoxide radical (O
2 ), photogenerated hole (h ), and OH
radicals are effective attacking groups which involve in photo-
Fig. 16. COD reduction and removal % of CaO on various samples.
G. Vanthana Sree et al. / Journal of Cleaner Production 270 (2020) 122294
11
catalytic degradation of pollutants. In order to find out which
reactive species were important in the dye degradation, MB was
taken as model dye with CaO NPs as catalyst. Series of control experiments with scavengers namely isopropanol (IP) as OH radical
scavenger, benzoquinone (BQ) as O
2 scavenger and ammonium
oxalate (AO) as hþ scavenger were separately added into the reaction mixture and experiments were conducted as normal in solar
condition. As shown in Fig. 17, the presence of BQ results in the
absence of superoxide radical causes a prominent decrease in the
degradation efficiency. It represents that the O
2 is formed by the
reaction of dissolved O2 with photo induced electrons upon visible
light irradiation, involving predominantly in degradation. In
contrast, the addition of IP and AO causes a slight decrease in the
degradation efficiency (Fig. 17). These results represent that O
2 is
the primary active group in the photodegradation of methylene
blue dye. The comparison of overall efficiency of dye degradation
after 15 min is shown in Fig. 17c.
3.4. Reusability studies
Fig. 18. Recycling efficiency of catalyst.
After the dye degradation experiment, the catalyst was separated through centrifugation. The catalyst was washed for three to
four times with distilled water followed by acetone wash and dried
in an oven at 80 C for 12 h. Then the catalyst was calcined at 800 C
for 2 h and utilized for the next run which was tested for 7 cycles
and efficiency as shown in Fig. 18. The efficiency of dye degradation
was reduced gradually for each cycle as a result of catalyst loss by
leaching and blocking of poreswhich indicated in Table 9. Catalyst is
in the form of a fine powder and could not be removed completely
from reaction mixture. Tracer amount of particles remain in liquid
form even after centrifuge. Reduction in efficiency in each cycle will
be due to material lose.
3.5. Photocatalytic activity of beads
Chitosan-CaO beads with two cross-linkers namely glycerol and
citric acid were prepared and subjected for dye degradation. Beads
showed lower dye degradation efficiency with increased time as
compared to powder form of CaO NPs as a result of lower active site
availability. In MB model dye, after 15 min glycerol beads showed
56.78% and citric acid beads showed 62.12% of degradation. Similarly, TB model dye, glycerol beads showed 60.98% degradation
after 10 min whereas citric acid beads exhibited 72.4% of removal
efficiency. Glycerol beads showed lower efficiency than citric acid
Table 9
Percentage of reduction of reused catalyst in each reaction.
S.No
Number of cycles
Degradation efficiency
Efficiency reduction %
1
2
3
4
5
6
7
1
2
3
4
5
6
7
96.19
92.43
89.52
86.45
83.29
79.31
76.58
0
3.90893
6.934193
10.12579
13.41096
17.5486
20.38673
beads in both MB and TB degradation (Fig. 19). In order to increase
the efficiency and reduce the reaction time, further optimisation of
beads formulation and formation need to be carried out in future.
3.6. RSM analysis
3.6.1. Statistical experimental design
In RSM, Box-Behnken design (BBD) was applied to investigate
the effects of experimental parameters including the four independent variables namely initial pH (A), dye concentration (B),
irradiation time (C) and catalyst loading (D). These variables were
Fig. 17. Effect of scavengers on MB degradation.
12
G. Vanthana Sree et al. / Journal of Cleaner Production 270 (2020) 122294
Fig. 19. Degradation efficiency of chitosan-CaO beads and its kinetics.
Table 10
Annova analysis of MB photodegradation reaction.
Source
Sum of Squares
Df
Mean
Square
F
Value
p-value
Prob > F
Model
A-time
B-pH
C-catalyst
D-dye
AB
AC
AD
BC
BD
CD
A2
B2
C2
D2
Residual
Lack of Fit
Pure Error
Cor Total
35600.58
24300.00
30.08
44.08
1633.33
2.25
6.25
100.00
12.25
0.25
0.25
8884.00
0.12
0.12
365.68
471.28
388.08
83.20
36071.86
14
1
1
1
1
1
1
1
1
1
1
1
1
1
1
14
10
4
28
2542.90
24300.00
30.08
44.08
1633.33
2.25
6.25
100.00
12.25
0.25
0.25
8884.00
0.12
0.12
365.68
33.66
38.81
20.80
75.54
721.86
0.89
1.31
48.52
0.067
0.19
2.97
0.36
7.427E-003
7.427E-003
263.91
3.426E-003
3.426E-003
10.86
<0.0001
<0.0001
0.3605
0.2717
<0.0001
0.7998
0.6731
0.1068
0.5560
0.9325
0.9325
<0.0001
0.9542
0.9542
0.0053
Significant
1.87
0.2872
not significant
examined using multiple regression analysis on the experimental
data; the model for the predicted response eff is as follows: where
eff is the degradation efficiency.
eff ¼ 84.60 þ 45.00 * Aþ1.58* Bþ1.92* C-11.67* D þ0.75*A*
Bþ1.25*A*C 5.00*A*D-1.75*B*C-0.25*B*Dþ0.25*C*D-37.01*A20.13*B2 -0.13*C2-7.51*D2
Where, A, B, C and D represent the initial pH, dye concentration,
irradiation time and catalyst loading, respectively. From the model,
the value of the determination coefficient (R2) was found to be
0.9869, which implied that 98.69% of the variations could be
explained by the fitted model. For a good statistical model, R2
adjusted value (0.9739) and R2 value (0.9869) must be closer to
each other. As shown in Table 10, R2 adjusted value is 0.9739, which
implies that small variations could not be explained by the model
and also confirms that a high degree of correlation exists between
the observed and predicted values. A relatively low value of C.V.
(coefficient of variation) (8.78%) indicates a better reliability of the
experiments values. The F-value (75.54) and p-value (<0.0001) (P
values less than 0.05) imply this model as a significant one.
From Fig. 20, it is clear that irradiation time, catalyst concentration, dye concentration and pH have important role in both individual and interrelated form. At lower concentration, dye
degradation efficiency increased with increasing time, but
increasing the dye concentration reduced the dye removal
efficiency even after a long time (Fig. 20a). Similarly, increase in the
catalyst loading increased the dye degradation (Fig. 20b). The influence of pH in dye degradation was also evidenced from Fig. 20c
and e. Increase in the pH of dye solution increased the dye degradation efficiency. In addition, lower catalyst loading can effectively
remove both the dyes in higher pH but least efficient in lower pH
(Fig. 20d).
4. Conclusions
CaO NPs were successfully synthesized from waste eggshells by
calcination process followed by appropriate characterisation
studies. The optimized conditions of dye degradation are pH-9,
20 ppm dye concentration (MB and TB) and catalyst dosage
50 mg in 50 mL for MB whereas 25 mg in 50 mL for TB dye. From the
outcomes, CaO effectively degraded both the dyes within 15 min
(MB is 15 min and TB is 10 min). The kinetics of both the dye
degradation follows the pseudo first order kinetics model. Oxygen
plays an important role in photo catalysis as confirmed in scavenger
studies. Chitosan-CaO beads with citric acid cross linker have more
efficiency than glycerol but reduced efficiency than free form of
catalyst. RSM and ANNOVA model were done for the theoretical
study and found to be significant models. Synthesized CaO NPs are
potential applicants for the effective removal of organic dyes from
waste matrix.
G. Vanthana Sree et al. / Journal of Cleaner Production 270 (2020) 122294
Fig. 20. RSM analysis of various parameters and their response in correlated condition.
13
14
G. Vanthana Sree et al. / Journal of Cleaner Production 270 (2020) 122294
CRediT authorship contribution statement
G. Vanthana Sree: Formal analysis, Writing - original draft. P.
Nagaraaj: Formal analysis, Writing - original draft. K. Kalanidhi:
Formal analysis, Writing - original draft. C.A. Aswathy: Formal
analysis, Writing - original draft. P. Rajasekaran: Formal analysis,
Writing - original draft.
Declaration of competing interest
All the authors declare that there was no financial interest the
authors have not received any funding.
Acknowledgements
The author thanks the UGC BSR start-up grant (F.30-368/2017
(BSR)) for the financial support for this research work. The author
G.V thanks Dr. Keerthi and Dr. Baskarlingam for providing laboratory facility during the research work.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jclepro.2020.122294.
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