Sustainable fabrication of hematite (α-Fe2O3) nanoparticles using biomolecules of Punica granatum seed extract for unconventional solar-light-driven photocatalytic remediation of organic dyes

Article abstract of DOI:10.1016/j.molliq.2021.116729

Seeking an effective strategy to develop inexpensive and highly stable photocatalyst materials for dye degradation is of great significance to resolve the global environmental problems caused by dye abuse. Herein, we have reported the environmental-friendly synthesis of exceptionally efficient and highly sustainable hematite nanoparticles (α-Fe₂O₃ NPs) using Punica granatum seed extract biomolecules. The morphology and phase purity of the synthesized α-Fe₂O₃ NPs were executed through a series of characterization techniques. FESEM and TEM analysis exhibited that the synthesized α-Fe₂O₃ NPs were irregular-shaped with an average size of 26.53 nm, while the BET surface area of nanoparticles was estimated to be 31.52 m²/g with an average pore diameter of 5.54 nm. α-Fe₂O₃ NPs exhibited typical ferromagnetic behavior at room temperature. The oxidation states of elements in α-Fe₂O₃ NPs were confirmed through X-ray photoelectron spectroscopy (XPS) analysis, while the charge on α-Fe₂O₃ NPs was found to be ?10.6 mV. The as-prepared α-Fe₂O₃ NPs were utilized in solar-light driven photocatalytic degradation of anionic Congo red (CR) and Bromophenol blue (BPB) dye. Experimental results indicated the remarkable photocatalytic activity of α-Fe₂O₃ NPs, and approximately 89.42% of CR and 87.96% of BPB was degraded in 240 min under sunlight irradiation at ambient conditions. The photocatalytic degradation experiments were also performed under different reaction conditions which followed the pseudo-first-order kinetics. The presence of reactive species generated in the reaction system liable for the CR and BPB degradation were corroborated through radical quenching experiments and further confirmed by EPR analysis. The excellent photocatalytic potential of α-Fe₂O₃ NPs suggested that the synthesized nanoparticles could effectually be applied to remove organic dyes from the wastewater.

Full text of DOI:10.1016/j.molliq.2021.116729

Journal of Molecular Liquids 339 (2021) 116729
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Sustainable fabrication of hematite (a-Fe2O3) nanoparticles using
biomolecules of Punica granatum seed extract for unconventional
solar-light-driven photocatalytic remediation of organic dyes
Adeel Ahmed a, Muhammad Usman a, Bing Yu a,b, Youqing Shen a, Hailin Cong a,b,
⇑
a Institute of Biomedical Materials and Engineering, College of Materials Science and Engineering, College of Chemistry and Chemical Engineering, Qingdao University, Qingdao
266071, China
b State Key Laboratory of Bio-Fibers and Eco-Textiles, Qingdao University, Qingdao 266071, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 February 2021
Revised 7 May 2021
Accepted 11 June 2021
Available online 15 June 2021
Seeking an effective strategy to develop inexpensive and highly stable photocatalyst materials for dye
degradation is of great significance to resolve the global environmental problems caused by dye abuse.
Herein, we have reported the environmental-friendly synthesis of exceptionally efficient and highly sus-
tainable hematite nanoparticles (
morphology and phase purity of the synthesized
terization techniques. FESEM and TEM analysis exhibited that the synthesized
irregular-shaped with an average size of 26.53 nm, while the BET surface area of nanoparticles was esti-
mated to be 31.52 m2/g with an average pore diameter of 5.54 nm.
-Fe2O3 NPs exhibited typical ferro-
magnetic behavior at room temperature. The oxidation states of elements in -Fe2O3 NPs were confirmed
through X-ray photoelectron spectroscopy (XPS) analysis, while the charge on -Fe2O3 NPs was found to
be ꢀ10.6 mV. The as-prepared -Fe2O3 NPs were utilized in solar-light driven photocatalytic degradation
of anionic Congo red (CR) and Bromophenol blue (BPB) dye. Experimental results indicated the remark-
able photocatalytic activity of -Fe2O3 NPs, and approximately 89.42% of CR and 87.96% of BPB was
a
-Fe2O3 NPs) using Punica granatum seed extract biomolecules. The
-Fe2O3 NPs were executed through a series of charac-
-Fe2O3 NPs were
a
a
Keywords:
Hematite nanoparticles
Biomolecules
Photocatalytic degradation
Sunlight irradiation
Congo red
a
a
a
a
Bromophenol blue
a
degraded in 240 min under sunlight irradiation at ambient conditions. The photocatalytic degradation
experiments were also performed under different reaction conditions which followed the pseudo-first-
order kinetics. The presence of reactive species generated in the reaction system liable for the CR and
BPB degradation were corroborated through radical quenching experiments and further confirmed by
EPR analysis. The excellent photocatalytic potential of
nanoparticles could effectually be applied to remove organic dyes from the wastewater.
Ó 2021 Elsevier B.V. All rights reserved.
a-Fe2O3 NPs suggested that the synthesized
1. Introduction
death of aquatic life by disturbing their metabolism through
several biochemical changes [6]. They also produced severe disor-
ders in human beings, including lung cancer, skin irritation, and
many other congenital and respiratory diseases [7,8]. Therefore,
these dyes-based effluents discharge directly into main water
streams from different industries should be eradicated before their
expulsion into the environment.
Scientific research communities have developed various
physico-chemical techniques such as adsorption, advanced chemi-
cal oxidation, chemical coagulation, electrocoagulation, photolysis,
chemical precipitation, photocatalysis, and membrane filtration for
the treatment of hazardous organic dyes [9,10,11,12]. The most
effective and non-destructive technique to remove the toxic dyes
from wastewater is photocatalysis due to simple operations and
high efficiency. In typical photocatalytic processes, dye degrada-
tion is usually executed under sunlight irradiation using the
The increasing highly toxic and stable pollutants from different
industries into the water system has become a crucial environmen-
tal issue that cannot be neglected due to their severe impacts on
human health [1,2]. The pollutants discharged from industries
are mainly consisting of synthetic dyes that are exceptionally con-
straint towards biodegradation due to their complex structure,
high molecular weight, a higher degree of aromaticity, and long-
lasting stability [3,4]. A small amount of these dyes adsorbed in
water systems worsen the water crisis and pose adverse effects
on aquatic life and human beings [5]. These dyes alleviated the
solar light permeation into the aquatic system and causing the
⇑
Corresponding author.
E-mail address: conghailin@qdu.edu.cn (H. Cong).
https://doi.org/10.1016/j.molliq.2021.116729
0167-7322/Ó 2021 Elsevier B.V. All rights reserved.

A. Ahmed, M. Usman, B. Yu et al.
Journal of Molecular Liquids 339 (2021) 116729
photocatalysts [13,14]. The sunlight irradiations with energy equal
to or more than the photocatalyst bandgap can be absorbed on the
photocatalyst surface and produce electron-hole pairs at the pho-
tocatalyst surface due to the electron transitions between two
energy bands [15,16]. These electron-hole pairs reacted with water
and molecular oxygen and converted into strong oxidizing species
(ÅOH, HOÅ2, and OÅ2–) [17]. These radicals (ÅOH) produced in this way
could quickly degrade the toxic dyes into innocuous and less toxic
end-products [18].
a
-Fe2O3 NPs, and their solar-light driven photocatalytic activity
was appraised against the degradation of model Congo red (CR)
and Bromophenol blue (BPB) dyes. The effect of different operating
parameters on catalytic applications of
ied. The synthesized -Fe2O3 NPs proved to be efficient photocata-
lysts for better dye degradation and could be employed to purify
dye polluted wastewater.
a-Fe2O3 NPs was also stud-
a
2. Experimental procedure
In recent years, metal and their oxide based nanoparticles have
attracted researchers’ attention due to their vast and surprising
applications in the field of catalysis [19]. Different methods, such
as sol–gel, micro-emulsion, electrochemical, sonochemical,
hydrothermal, chemical reduction and biological have been
adopted to fabricate nanoparticles for photocatalytic applications
[20,21,22,23,24,25]. Recently, Zinatloo-Ajabshir et al. synthesized
the Ag2WO4 nanoparticles via sonochemical method and effec-
tively utilized them for visible-light-driven photocatalytic degra-
dation of Acid red 14, eriochrome cyanine R, and Rhodamine B
[26]. A. Alharbi et al. adopted the combustion method for the fab-
rication of hematite nanoparticles from Egyptian iron waste using
L-arginine and examined their photocatalytic activity against
malachite green degradation [27]. Similarly, Khalid et al. Ag3PO4/
N-TiO2 heterostructure via co-precipitation method and studied
their photocatalytic activity against RhB degradation [28]. These
methods efficiently degraded the dyes; however, they have high
operating costs, require high energy, and generate toxic by-
products, making the process less feasible [29,30]. Therefore, a
growing need to develop clean, cost-effective, viable, and biocom-
patible procedures for the synthesis of nanoparticles [31]. For this
purpose, plant-mediated green synthesis has emerged as a highly
attractive and alternative synthesis route for the synthesis of
nanoparticles [32,33]. The plant extract contains several biomole-
cules that could be used as both reducing and capping agents in
the synthesis of nanoparticles [34,35].
Several metal-based nanoparticles using plant extracts have
been synthesized and effectively utilized for catalytic applications
[36,37,38,39]]. Kaur et al. fabricated the Fe2O3 nanoparticles using
R. indica leaf extracts and evaluated their photocatalytic activity
against Reactive Yellow-86 dye, which revealed that 98% of dye
was degraded within 60 min under solar-light irradiation [40].
Al-Zaqri et al. adopted the biological method and synthesized the
ZrO2 nanoparticles using Wrightia tinctoria leaf extract and
assessed their photocatalytic activity against Reactive Yellow dye
[41]. Also, Hamidian et al. demonstrated the synthesis of Co doped
CeO2 NPs using Salvadoral persica extract and examined their activ-
ity against the Acid Orange 7 degradation [42]. Among these,
2.1. Plant material and chemicals
Ferric chloride hexahydrate (FeCl3ꢁ6H2O, 98%), sodium hydrox-
ide (NaOH, 97%), hydrochloric acid (HCl), congo red (CR, C32H22N6-
Na2O6S2, greater than98%), bromophenol blue (BPB, C19H10Br4O5S,
ACS reagent) and 5,5-Dimethyl-1-pyrroline N-oxide (DMPO,
C6H11NO, 97%) were purchased from Aladdin Industrial Co. Ltd.,
China. 1,4-benzoquinone (BQ, C6H4O2, ꢂ98%), tert-butyl alcohol
(TBA, C4H10O, ꢂ99.5%), silver nitrate (AgNO3, 99.8%) and ethylene-
diaminetetraacetic acid (EDTA, C10H16N2O8, 98%) were obtained
from Sinopharm Chemical Reagent Co. Ltd., China. All the chemi-
cals were of high purity and used without further purification. P.
granatum seeds were collected from the local market of Qingdao,
China. All glassware was washed with water followed by ethanol
to eliminate the dust particles and other impurities and vacuum
dried before use. The double deionized (DI) water with a resistivity
of 18.2 MXcm was used throughout the experiments.
2.2. Extract preparation
Fresh P. granatum seeds were collected and carefully washed
with DI water to eradicate the impurities. After drying at room
temperature, 25 g of seeds were added to 150 mL of DI water
and ground in an electric grinder to make a homogeneous mixture.
The obtained mixture was heated at 70 °C for 45 min along with
continuous magnetic stirring. The resultant composition was then
cooled down and centrifuged at 10000 ꢃ g for 8 min to remove any
solid material. The composition was filtered by using filter paper,
and the supernatant was stored at 4 °C and further used for the
preparation of
2.3. Synthesis of
For the fabrication of
ous FeCl3ꢁ6H2O solution was mixed with 50 mL of P. granatum seed
extract and magnetically stirred at 75 °C for 15 min. On heating,
the color of the solution was changed from dark brown to black
a
-Fe2O3 NPs.
a
-Fe2O3 NPs
a
-Fe2O3 NPs, 150 mL of 2 ꢃ 10ꢀ3 M aque-
Hematite (a-Fe2O3) nanoparticles are considered the most promi-
nent material due to their high biocompatibility and unique cat-
alytic properties, making them auspicious candidates for their
after 15 min of reaction, which indicated the formation of
Fe2O3 NPs. The resulting solution was allowed to settle down for
2 h, and -Fe2O3 NPs suspensions thus obtained were centrifuged
at 8000 ꢃ g for 10 min. Then, -Fe2O3 NPs were isolated from the
a-
a
applications in photocatalysis [43].
a-Fe2O3 NPs have a proper
a
bandgap energy (2.1 eV); hence they could be used as photocata-
lysts and other environmental applications [44]. Additionally, the
supernatant by applying an external magnetic field and washed
three times with DI water followed by ethanol to eradicate the
unreacted material. The final product was vacuum dried at
100 °C for 24 h and finely pulverized, and stored in a stoppered
bottle for photocatalytic activity evaluation.
large surface area, high reactivity, and paramagnetic nature of
a-
Fe2O3 NPs also proved them effective photocatalysts for the degra-
dation of dyes [45,46].
In the present study, we have adopted the green chemistry
principles and synthesized the a-Fe2O3 NPs by using the biomole-
cules of Punica granatum seed extract as a reducing agent. P. grana-
tum has various biological molecules, including caffeic acid, ellagic
acid, gallic acid, apigenin, punicalin, punicalagin, and catechin,
along with small amounts of other biomolecules as displayed in
Fig. S1 [47,48]. These plant-derived biomolecules are used as
2.4. Characterizations of a-Fe2O3 NPs
The synthesized
analytical techniques. The morphology of
a
-Fe2O3 NPs were characterized using different
-Fe2O3 NPs was exe-
a
cuted using Field emission scanning electron microscopy (FESEM,
JEM-6700F). The quantitative assessment of elements was exam-
ined through Energy-dispersive X-ray spectroscopy (EDS, JEOL
Japan) analysis. The crystalline phases and purity of as-prepared
reducing and capping agents in the synthesis process of
a-Fe2O3
NPs and could limit the growth of NPs by preventing them from
aggregations. Advanced techniques characterized the as-prepared
2

A. Ahmed, M. Usman, B. Yu et al.
Journal of Molecular Liquids 339 (2021) 116729
a
-Fe2O3 NPs were confirmed through X-ray diffraction (XRD, Bru-
ker D8) analysis with Cu K radiation (k = 1.54 Å) between 10
and 80° and the scanning speed of 0.02° per min was used. The
functional groups involved in reducing and capping of -Fe2O3
having irregular shapes. The aggregation in a-Fe2O3 NPs was due
a
to the electrostatic interaction among nanoparticles and the bio-
molecules [35]. The average diameter of nanoparticles was mea-
sured using ImageJ software and found to be 26.53 nm.
Furthermore, the detailed investigation of structure, morphology
a
NPs were executed through Fourier transform infrared spectropho-
tometer (FTIR Nicolet Nexus 470) in the wavelength range of
and size of
HRTEM micrograph of a
a
-Fe2O3 NPs was done through HRTEM analysis. The
4000–400 cmꢀ1. The specific surface area (SBET) of
a-Fe2O3 NPs
-Fe2O3 NPs showed the interplanar spacing
was measured with Micromeritics ASAP 2020 sorptometer by
Brunauer-Emmett-Teller (BET) method using N2 adsorption–des-
of lattice fringes was 0.32 nm.
The quantitative assessment of synthesized
a-Fe2O3 NPs was
orption isotherms. The magnetic properties of
measured through Vibrating sample magnetometer (VSM, Lake-
Shore 7404) at room temperature. The oxidation states of elements
a
-Fe2O3 NPs were
executed through EDS, as presented in Fig. 1e and f. The presence
of intense peaks of Fe and O in the spectrum indicate that the pre-
pared sample mainly consisted of Fe and O that proved the suc-
present on the surface of
a
-Fe2O3 NPs were confirmed through X-
cessful formation of a-Fe2O3 NPs. The appearance of small peaks
ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI)
analysis. Dynamic light scattering (DLS) measurements were exe-
for C, Pt, Cl, Si, and S was also observed. The C, S, and Cl peaks were
generally ascribed to polyphenolic groups of the extract, while the
additional peaks of Si and Pt were attributed to the impurities [50].
The prepared sample’s quantitative assessment revealed that
43.73% of samples consisted of Fe, 34.06% of O, 9.46% of C, 7.22%
of Pt, 4.69% of Cl, and 0.54% of Si, and 0.3% of S by weight. The
NPs prepared from green chemistry route was of different size
and shapes and had a large surface area and reactive sites which
improved their potential for use in catalytic applications [31].
cuted to measure the size and charge of the
a-Fe2O3 NPs. The
leaching concentration of metal was assessed by an Inductively
coupled plasma optical emission spectrometer (ICP-OES, Varian
720).
2.5. Evaluation of photocatalytic activity
The crystallinity and phase purity of as-prepared
was determined by carrying out the XRD analysis as shown in
Fig. 2a. The synthesized -Fe2O3 NPs shows the sharp diffraction
peaks at 2 theta values of 24.13(012), 33.15(104), 35.61(110),
40.85(113), 49.47(024), 54.08(116), 57.58(018), 62.44(214),
63.98(300), 72.26(119), and 75.42(220) respectively. All the
a-Fe2O3 NPs
The photocatalytic potential of biosynthesized a-Fe2O3 NPs was
assessed through the degradation of model CR and BPB dye under
sunlight irradiation. The photodegradation experiments were exe-
cuted in the presence of sunlight, having an average intensity of
647 W/m2. The typical degradation experiment involved mixing
50 mL aqueous solution of 50 ppm of each CR and BPB dye with
a
diffraction peaks of as-prepared
ment with XRD patterns of standard hexagonal hematite
(JCPDS card no. 33-0664) [51]. There were not any other additional
peaks that indicated the high crystalline purity of as-prepared
a
-Fe2O3 NPs were in good agree-
0.05 g/L of the
a-Fe2O3 catalyst separately. The reaction mixtures
a
-Fe2O3
were placed in the dark and stirred continuously for 30 min to
attain the adsorption–desorption equilibrium. After this, both the
mixtures were exposed to sunlight for 240 min. The concentration
of CR and BPB dye was determined by taking out the samples
(2 mL) at regular time intervals and analyzing them through a
UV–Vis spectrophotometer (Perkin Elmer, USA). The degradation
percentage of dyes was appraised using Eq. (1).
a-
Fe2O3 NPs. These XRD results were in substantial agreement with
the data reported elsewhere by Guo et al. [52] and prove the suc-
cessful formation of
a-Fe2O3 NPs from its precursor salt solution.
The average crystallite size of
a-Fe2O3 NPs was measured using
Debye Scherrer equation and found to be 40.67 nm.
ꢀ
ꢁ
ðCo ꢀ CÞ
The FTIR spectroscopic analysis was executed to ascertain the
involvement of different biological molecules present in the P.
granatum seeds extract that are liable for the formation and stabi-
Degradationð%Þ ¼
ꢃ 100
ð1Þ
Co
here, Co represents the dye concentration at the initial state while C
is the dye concentration after degradation at the time, t.
lization of
as-prepared
a
-Fe2O3 NPs. The FTIR spectrum of seeds extracts and
-Fe2O3 NPs are displayed in Fig. 2b. As shown in
a
Fig. 2b, the distinct absorption bands for seed extract were
observed at 3462, 3418, 1731, 1610, 1416, 1242, 1095, 998, and
956 cmꢀ1 correspond to the various biomolecules. The absorption
peaks at 3462 and 3418 cmꢀ1 are ascribed to OAH stretching, and
in-plane bending vibrations of hydroxyl groups correspond to the
phenolic compounds in seeds extract [53]. Similarly, the bands at
1731, 1610, and 1416 cmꢀ1 are allocated to C@O stretching vibra-
tions in dimerized saturated aliphatic acids, stretching vibrations
of CC@C in aromatic rings bending vibrations of OAH, CAN in aro-
matic amines, respectively. The absorption peaks observed at 1242
and 1095 cmꢀ1 are ascribed to the stretching vibration of CAN and
3. Result and discussion
3.1. Characterization
Several studies have widely publicized that the biomolecules
present in P. granatum seeds extract can effectively reduce and sta-
bilize the metal ions into metal NPs. They can also limit particles’
growth and tend to bind with metal atoms through chelation and
ruled out the process of aggregations [49]. P. granatum seed extract
has an appropriate number of bioactive molecules that can reduce
and mediate the fabrication of
solution without the involvement of any surfactant.
a
-Fe2O3 NPs from FeCl3ꢁ6H2O salt
CAH in amino acids [54]. The as-prepared
slight shift in absorption bands due to the binding effects of
Fe2O3 NPs with the seed extract’s functional groups. The absorp-
a-Fe2O3 NPs showed a
a-
To characterize the shape and distribution of
a-Fe2O3 NPs,
FESEM studies were carried out, as depicted in Fig. 1. The SEM
tion bands for a-Fe2O3 NPs were observed at 3447, 3328, 1741,
images of -Fe2O3 NPs were taken at different resolutions revealed
a
1633, 1523, 1456, 1032, 795, and 525 cmꢀ1. The peaks that
that the as-prepared NPs were irregular in shape and highly aggre-
gated (Fig. 1a and b). The aggregations of NPs were mainly due to
the hydrogen bonding and electrostatic interaction among biomo-
lecules. Moreover, the aggregation in the particles was ascribed to
the nature of biomolecules present in seed extract. The morphol-
ogy and size distribution of as-prepared nanoparticles was further
analyzed through TEM and HRTEM analysis (Fig. 1c and d). It can
appeared at 3447 and 3328 cmꢀ1 correspond to OAH groups’
stretching vibration in polyphenols that bound the
a-Fe2O3 NPs
surface. The peaks in the wavelength range of 500–1000 cmꢀ1
were mainly due to bonding between metal and oxygen groups
that confirms the formation of
a-Fe2O3 NPs. Moreover, the charac-
teristic band that appeared at 525 cmꢀ1 is ascribed to the stretch-
ing vibrations of Fe–O among
formation [55].
a-Fe2O3 NPs that reinforced their
be seen from Fig. 1c that the
a-Fe2O3 NPs were aggregated and
3

A. Ahmed, M. Usman, B. Yu et al.
Journal of Molecular Liquids 339 (2021) 116729
Fig. 1. (a,b) FESEM image of a-Fe2O3 NPs at different magnification, (c) EDS spectrum, inset showed the weight percentage, (d) EDS mappings of a-Fe2O3 NPs.
The surface area and pore diameter of synthesized
was examined from N2 adsorption–desorption isotherms, as illus-
trated in Fig. 2c. The BET isotherm of -Fe2O3 NPs exhibited a type
IV isotherm with four different stages. The specific surface area of
-Fe2O3 NPs was evaluated from BET isotherm and it found to be
31.52 m2/g. While, the average pore diameter of synthesized
a
-Fe2O3 NPs
(Ms), remanence (Mr), and coercivity (Hc), were also determined
-Fe2O3 NPs exhibited typ-
ical ferromagnetic behavior with Ms and Mr values of 2.067 and
0.52 emu/g, respectively, while the Hc of the sample was found
to be 722 Oe.
from the MꢀH loop. It can be seen that
a
a
a
a
-
The element composition and the oxidation state of synthesized
Fe2O3 NPs was determined using Barette-Joyner-Halenda (BJH)
technique and found to be 5.54 nm. The BET isotherm of nanopar-
ticles exhibited the large surface area and narrow pore size that
provides more catalytic sites on catalyst surface thus could be
effectually utilized for the photodegradation of dyes.
a
-Fe2O3 NPs were confirmed through XPS analysis, as presented in
Fig. 3. The XPS spectrum survey in Fig. 3a indicated the core exis-
tence of iron and oxygen in -Fe2O3 NPs, while the carbon peak
was due to the instrument background.
The Fe 2p spectra (Fig. 3b) of -Fe2O3 NPs was fitted into bind-
a
a
The magnetic properties of the material are directly related to
their recovery and reuse in practical applications [56]. Therefore,
ing energy peaks at 710.72 eV for Fe2+ 2p3/2, 712.34 eV for Fe3+
2p3/2, 724.38 eV for Fe2+ 2p1/2, 726.47 eV for Fe3+ 2p1/2 along with
several satellite peaks [57]. Fe 2p3/2 peak was quite narrow com-
pared to Fe 2p1/2 peak and covered more area due to the degener-
acy of four states of 2p3/2, whereas 2p1/2 had only two states spin–
orbit (j-j) coupling [58]. The satellite peak that appeared at
the magnetic properties of
a-Fe2O3 NPs were studied under the
applied magnetic field of ꢀ15000 to +15000 Oe, and the corre-
sponding magnetic hysteresis (MꢀH) loop is depicted in Fig. 2d.
Various magnetic parameters, such as saturation magnetization
4

A. Ahmed, M. Usman, B. Yu et al.
Journal of Molecular Liquids 339 (2021) 116729
Fig. 2. (a) XRD pattern of
loop of -Fe2O3 NPs.
a-Fe2O3 NPs, (b) FTIR spectrum of seed extract and a-Fe2O3 NPs, (c) N2 adsorption–desorption surface area of a-Fe2O3 NPs, (d) Magnetic hysteresis
a
Fig. 3. XPS spectra of a-Fe2O3 NPs; (a) full-range scan of the a-Fe2O3 NPs, (b) Fe 2p core level, (c) O 1s core level, (d) C 1s core level.
5

A. Ahmed, M. Usman, B. Yu et al.
Journal of Molecular Liquids 339 (2021) 116729
718.55 eV was due to Fe3+ ions in
a-Fe2O3 NPs. Similarly, The O 1s
The potential of the valence band (EVB) and conduction band
spectra (Fig. 3c) of
a
-Fe2O3 were fitted into two peaks attributed to
(ECB) for a-Fe2O3 NPs was calculated using Eqs. (3) and (4),
lattice oxygen within the metal oxide (Fe-O, 529.68 eV) and the
respectively.
oxygen within hydroxyl groups (Fe-OH, 531.3 eV) on the catalyst
EVB ¼ X ꢀ Ee þ 0:5Eg
ð3Þ
ð4Þ
surface, respectively [59]. The C 1s XPS spectra of
a-Fe2O3 NPs
(Fig. 3d) were deconvoluted to three characteristic peaks with
binding energy at 284.76 eV, 286.38 eV, and 288.67 eV correspond
to CAC, CAOH, and OAC@O, respectively. The results obtained
from XPS analysis proved the existence of Fe as Fe3+ and oxygen
ECB ¼ EVB ꢀ 0:5Eg
where, X represents the absolute electronegativity of semiconduc-
tor which equals 5.87 eV
as O2ꢀ which confirmed the formation of
mation of
analysis, which already confirmed the crystalline phase formed
as -Fe2O3.
The Zeta potential analysis was executed to measure the stabil-
ity and surface charge of -Fe2O3 NPs. The zeta potential of -Fe2O3
a
-Fe2O3 NPs. The confir-
and Ee is the energy of free electrons on the hydrogen scale
(~4.5 eV vs NHE). The Eg, EVB, and ECB of a-Fe2O3 NPs was calculated
to be 2.04, 2.46, and 0.42 eV respectively.
a
-Fe2O3 NPs by XPS studies was best fitted with XRD
a
3.2. Possible mechanism for the formation of a-Fe2O3 NPs
a
a
Phytochemical analysis of P. granatum seeds extracts as pre-
sented in Fig. S1 revealed the presence of various biomolecules
including caffeic acid, ellagic acid, gallic acid, apigenin, punicalin,
punicalagin, and catechin and coincides with previously reported
studies [47,49]. Among these biomolecules, catechins are polyphe-
nolic compounds that have high antioxidant properties, making
them capable of forming chelates with Fe3+ species. Fe3+ ions form
octahedral hexa aqua iron (III) complex ions ([Fe(H2O)6]3+) with
water molecules when the precursor solution is heated. The aqua
complex decomposes and deprotonates during the reaction, result-
ing in Fe3+–R species ([Fe(OH)(H2O)]2+), where R is ((OH)(H2O).
Through their ortho-dihydroxy benzene groups, the catechin in
the aqueous extract form chelates with the Fe3+–R. After formula-
tion of the chelate, the Fe3+–R species undergoes a phase transition
NPs showed a charge of ꢀ10.6 mV as shown in Fig. 4, which spec-
ified that the as-prepared nanoparticles were highly stable under
ordinary conditions because the large positive (more than
30 mV) and negative (less than ꢀ30 mV) Zeta potential values
are ascribed to higher stability. The negative value of zeta potential
indicated that the surface of
a-Fe2O3 NPs was capped and stabi-
lized by negatively charged biomolecules present in P. granatum
seeds extract. Moreover, particles’ negative charge also creates
repulsive forces among NPs, preventing them from aggregation
and providing more stability [60].
The size of synthesized
a-Fe2O3 NPs was measured using the
DLS technique and presented in Fig. 5a. The average size of
a
-
Fe2O3 NPs was found to be 39.82 nm, which was coincides with
the results from XRD.
to
a-Fe2O3 [64]. After adding the extract, the pH of the solution
UV–Vis spectroscopy was used to investigate the photoabsorp-
varies from acidic to basic. The basic compounds in the extract
tion property of synthesized
of -Fe2O3 NPs were recorded in the UV–Vis range from 300 to
800 nm as presented in Fig. 5b. The -Fe2O3 NPs exhibited wide
absorption peaks in the wavelength region of 300–550 nm [61].
The UV–Vis spectrum of -Fe2O3 NPs indicates the semiconducting
nature of -Fe2O3 NPs thus, could be effectively utilized as photo-
catalyst material. The optical bandgap of -Fe2O3 NPs was calcu-
a-Fe2O3 NPs. The absorbance spectra
effectively abstract the H+ ions produced during the deprotonation
a
reactions, allowing the polyphenolic compounds to form
a-Fe2O3
a
NPs (Fig. S2). Metal oxide nanoparticles also have large surface
energy and tend to accumulate in aqueous systems. However,
the steric repulsion between the capping biomolecules stabilizes
the synthesized metal oxide nanoparticles in the current study
a
a
a
[65]. The reactions that lead to the synthesis of
be described as follows (Eq. (5)–(8)):
a
-Fe2O3 NPs can
lated from the UV–Vis absorption spectra using the Tauc Plot
relationship given in Eq. (2) [62].
ꢄ
ꢅ
3þ
ꢂ
ꢃ
Fe3þ þ H2O ! FeðH2OÞ
ð5Þ
ð6Þ
n
ðah
m
Þ ¼ A h
m
ꢀ Eg
ð2Þ
6
ꢄ
ꢅ
3þ
2þ
where ‘
a
’ is the absorption coefficient, ‘h’ represents the Planck’s
FeðH2OÞ6
! ½FeðOHÞðH2OÞꢄ
constant, ‘ʋ’ represents the frequency, ‘A’ is the is the energy inde-
pendent constant, ‘Eg’ represents the direct bandgap energy, and ‘n’
is the nature of transitions.
The bandgap energy of prepared
extrapolating the straight-line fit between (
estimated bandgap energy of -Fe2O3 NPs was calculated to be
2.04 eV and well-matched with the literature [63]. The lower band-
gap energy of -Fe2O3 NPs may be due to the presence of defect
levels within the bandgap in the nanoparticles.
ðdecomposition and deprotonationÞ
½FeðOHÞðH2OÞꢄ2þ þ Y ! Y
a-Fe2O3 NPs was evaluated by
2
ahm) versus hm. The
ꢀ Fe3þðOHÞðH2OÞðchelate formationÞ
a
ð7Þ
ð8Þ
a
where Y is catechin biomolecule
Fe3þðOHÞðH2OÞ !
a
ꢀ Fe2O3ðphase transition on annealingÞ
3.3. Photocatalytic activity
The photocatalytic activity of the
a-Fe2O3 catalyst was executed
by carrying out the degradation of model CR and BPB dye under
sunlight irradiation. Before performing the photodegradation
experiments, the reaction mixture containing dye solution and cat-
alyst was set aside in the dark to accomplish the adsorption–des-
orption equilibrium. Then the resultant mixture was exposed to
sunlight under ordinary conditions, and the concentration of dye
solution was determined after regular time intervals to measure
Fig. 4. Zeta potential measurements of a-Fe2O3 NPs.
6

A. Ahmed, M. Usman, B. Yu et al.
Journal of Molecular Liquids 339 (2021) 116729
Fig. 5. (a) DLS size measurements of a-Fe2O3 NPs, (b) UV–Vis spectra a-Fe2O3 NPs.
constant (L mgꢀ1). While C and Ci indicate the dye and intermedi-
the rate of degradation. After 240 min of sunlight irradiation, the
dye solutions showed an intense change in concentration because
many dye molecules adsorbed on the catalyst surface and trans-
formed into innocuous end products. The intensity peak of CR
and BPB dye at kmax = 497 nm and 590 nm, respectively, was
reduced due to the degradation of dye molecules, as shown in
Fig. 6a and d. The percentage of CR and BPB degradation was mea-
sured from the concentration of control and degraded dye using Eq.
(1) and displayed in Fig. 6b and e. The results have shown that
89.42% of CR and 87.96% of BPB dye degraded after 240 min of sun-
ates concentration (mg Lꢀ1), correspondingly.
P
KC and KiCi in the denominator are tantamount to KCo; thus,
Eq. (9) can be written as
dc
kKC
dt 1 þ KC0
here Co stands for the initial dye concentration.
ꢀ
¼
ð10Þ
At low initial dye concentration, the product of KCo is less than
1; thus, it could be neglected. Therefore, the photocatalytic dye
degradation followed the pseudo-first-order under Langmuir-
Hinshelwood kinetic at different reaction parameters.
light irradiation using the
The reaction kinetics for the degradation of CR and BPB using
the -Fe2O3 catalyst was determined using the Langmuir-
a-Fe2O3 catalyst.
a
dc
Hinshelwood model. This kinetic model is the most applicable
and widely used model to explain the photocatalytic degradation
of dyes and can be measured from Eq. (9).
ꢀ
¼ kKC ¼ kapp:C
ð11Þ
dt
By integrating the equation at C = Co and t = 0, the following
relation is obtained
ꢆ
ꢇ
C
C0
dc
kKC
P
ꢀln
¼ kapp:
t
ð12Þ
ꢀ
¼
ð9Þ
dt 1 þ KC þ KiCi
In this equation, kapp is the apparent rate constant (minꢀ1), C
and Co denotes the concentration of dyes at the initial state and
after degradation at the time, t respectively.
here, k is the reaction rate constant (mgLꢀ1 minꢀ1) that includes dif-
ferent parameters, K and Ki represent the adsorption equilibrium
Fig. 6. Photocatalytic degradation of dyes at different irradiation time; (a) absorption spectra of CR, (b) degradation rate of CR, (c) kinetic plot for CR, (d) absorption spectra of
BPB, (e) degradation rate of BPB, (f) kinetic plot for BPB. Conditions; ([Dye] = 50 ppm; [catalyst] = 0.05 g/L; pH = 5.8; T = 25 °C).
7

A. Ahmed, M. Usman, B. Yu et al.
Journal of Molecular Liquids 339 (2021) 116729
The pseudo-first-order kinetic plot for the photocatalytic degra-
dation of CR and BPB dye using the a-Fe2O3 catalyst under different
90.14% was observed for CR dye at the initial concentration of
60 ppm, while BPB dye showed an ultimate degradation of
89.27% at 70 ppm. This increase in the catalyst’s degradation effi-
ciency was only up to certain limits. It showed a gradual decrease
as the initial concentration of dye increased from 60 to 100 ppm for
CR and 70 to 100 ppm in BPB dye. When the dye concentration
increases beyond certain limits, the active sites on the catalyst sur-
face are being covered with more dye molecules. As a result, the
active sites on the catalyst surface decreases, which consecutively
decreases the degradation efficiency.
irradiation times is shown in Fig. 6c and f, respectively. The rate
constant for photodegradation reactions of CR and BPB dye can
be obtained by plotting the ꢀln(C/Co) versus time and gives the
straight line. The value of the rate constant for CR and BPB dye
was observed at 0.0092 minꢀ1 and 0.0071 minꢀ1 correspondingly.
The photocatalytic degradation efficiency of CR and BPB dye
was monitored by performing the COD experiment using the acidic
dichromate method. The COD values were recorded up to 240 min
exposure to solar light at predetermined time intervals as pre-
sented in Fig. S3. A gradual reduction in COD level was observed
with time due to ROS generation at the catalyst surface [66]. The
correlation of COD reduction with dye degradation efficiency was
also assessed and presented in Table S1 and Table S2. The reduc-
tion in COD level increased the degradation efficiency of dyes. It
can be seen from the results in Fig. S1a that the COD level declined
from 1593 mg/L to 445 mg/L in CR, and 91.27% degradation was
achieved. While in the case of BPB dye, the COD value was reduced
from 1372 mg/L to 412 mg/L with a degradation efficiency of
88.47%. The overall COD reduction was found to be 72.06 and
69.97% for CR and BPB dye, respectively. The COD analysis specifi-
cally revealed that COD amounts decreased as the reactions pro-
gressed, which leads to the complete mineralization of dyes. The
upsurge in degradation efficiency was also confirmed by UV–Vis
spectral analysis.
The kinetics of the reaction under different initial concentra-
tions of dye were also studied, and results are interpreted in
Fig. 7b and d. The rate constant values for CR and BPB degradation
were varied from 0.0014 to 0.0103 minꢀ1
, and 0.0003 to
0.0048 minꢀ1 as the initial concentration of dye increased from
10 to 100 ppm. The values of the rate constant and regression coef-
ficients (R2) of the reaction are listed in Table 1.
3.4.2. Effect of catalyst dosage
The catalyst dosage is the most influential parameter that can
substantially affect the photodegradation of dyes. The impact of
applied catalyst dosage on photocatalytic degradation of dyes
was studied by varying the concentrations of catalysts from 0.01
to 0.05 g/L for a fixed concentration of dye (50 ppm), and the con-
sequences are interpreted in Fig. 8.
It can be seen that the solution with less amount of catalyst
showed less color removal while the color removal was substan-
tially high when the catalyst dosage was high (Fig. 8a and c).
Results had shown that the rate of dye degradation increases from
51.98 to 87.31% and 30.27 to 86.57% for CR and BPB dye when the
amount of catalyst increased from 0.01 to 0.05 g/L. The increase in
mineralization efficiency of dye was due to the reason that the
solution with higher catalyst dosage has more availability of cat-
alytic sites for the adsorption of dye molecules at surface [67]. Con-
sequently, more dye molecules are adsorbed on the catalyst
surface and decomposed into colorless and harmless by-products.
The pseudo-first-order kinetics of the reaction for different cat-
alyst loadings was determined by plotting the ꢀln(C/Co) versus
time for CR and BPB dye as depicted in Fig. 8b and d, respectively.
The rate constant values increased in a similar pattern from 0.0032
to 0.0079 minꢀ1 and 0.0013 to 0.0077 minꢀ1 for CR and BPB,
respectively (Table 2). The increases in rate constant value in a
comparable manner indicated the increase in degradation rate
and were the best fit into the pseudo-first-order Langmuir-
Hinshelwood kinetic model.
The adsorption performance of
a-Fe2O3 NPs for CR and BPB
degradation was also assessed under dark conditions, and the
results are interpreted in Fig. S4. In an experimental setup,
0.05 g/L of catalyst was immersed in 50 mL aqueous solution of
each CR (50 ppm), and BPB (50 ppm) separately and continuously
stirred at room temperature for 60 min. After predetermined time
intervals, 2 mL aliquot was taken out and centrifuged to remove
the Supernatants. Subsequently, the Supernatants were analyzed
through UV–vis spectrophotometer while the residual dye concen-
trations were determined from the UV–vis absorption spectra. The
absorption peaks of CR and BPB at 497 nm and 590 nm were found
to be decreasing with increasing the time that confirmed the
adsorption removal of dyes (Fig. S1). It can be seen from the results
in Fig. S1 that the 35.61 and 31.4% removal of CR and BPB dye was
achieved with
does not show good adsorption behavior; therefore, it can be con-
cluded that the -Fe2O3 catalyst cannot be used for the degrada-
tion of dyes under dark. Hence, the degradation experiments of
a-Fe2O3 catalyst after 60 min. The a-Fe2O3 catalyst
a
dyes were performed with the
of light.
a-Fe2O3 catalyst in the presence
The influence of different reaction parameters including initial
dye concentration, catalyst loading, pH, and temperature on photo-
3.4.3. Effect of pH
catalytic activity of the
the degradation of CR and BPB dye.
a
-Fe2O3 catalyst was also examined against
The effect of pH is the most crucial and noticeable dynamic as it
governs the charges on the catalyst surface and consequently
affects the adsorption rate of dye molecules on the catalyst surface.
Also, the solution’s pH has a substantial impact on the catalyst’s
size and aggregation, which influences the degradation of dyes.
Therefore, the resilient effect of pH on degradation of CR and BPB
dye was executed using a predetermined amount of catalyst
(0.05 g/L) and dye (50 ppm) over a wide range of pH varying from
4 to 11. The pH values of dye solutions were adjusted from 4 to 11
by adding different HCl and NaOH amounts to the solutions. The
dye solutions were exposed to sunlight, and their degradation effi-
ciency was measured using UV–Vis absorption spectra. The charge
on the catalyst surface was determined using Zeta potential analy-
3.4. Effect of operating parameters on photocatalytic degradation
3.4.1. Effect of initial dye concentration
The catalyst’s degradation efficiency is strongly affected by the
initial concentration of the dye molecule in the reaction mixture.
The influence of different initial concentrations of dyes was exe-
cuted by altering the concentration from 10 to 100 ppm for a fixed
catalyst loading (0.05 g/L) whereas the pH of the dye solution was
maintained at 5.8. The consequences of varying initial concentra-
tions of dye on photocatalytic degradation efficiency of the
Fe2O3 catalyst are depicted in Fig. 7.
a-
sis as presented in Fig. S5(a-e). The isoelectric point (IEP) of the a-
As can be seen from the results in Fig. 7a and c, the increase in
the initial concentration of CR and BPB dyes up to certain limits
showed a continuous rise in percentage degradation, and beyond
certain limits, it tends to decrease. The maximum degradation of
Fe2O3 catalyst was determined from Zeta potential values and
observed at a pH value of 8.46 (Fig. S6). The catalyst surface carried
positive charges at pH < pH(IEP) while it carried negative charges at
pH > pH(IEP) [35].
8

A. Ahmed, M. Usman, B. Yu et al.
Journal of Molecular Liquids 339 (2021) 116729
Fig. 7. Photocatalytic degradation of dyes at different initial dye concentration; (a) degradation rate of CR, (b) kinetic plot for CR, (c) degradation rate of BPB, (d) kinetic plot
for BPB. Conditions; ([Dye] = 10–100 ppm; [catalyst] = 0.05 g/L; pH = 5.8; T = 25 °C).
Table 1
strong electrostatic interaction among the positively charged cata-
lyst surface and anionic dyes resulting in higher degradation of
The kinetic parameters for CR and BPB dye degradation under the effect of different
initial concentrations using
a
-Fe2O3 catalysts.
dyes [68]. Similarly, at pH > pH(IEP), the electrostatic repulsion
among negatively charged catalyst surface and anionic dye mole-
cules suppressed the mineralization efficiency of dyes [69].
The photocatalytic degradation of dyes under different pH con-
ditions followed the pseudo-first-order kinetics as presented in
Fig. 9b and d for CR and BPB dye, respectively. The rate constant
values for CR and BPB dye varied irrespective from 0.0028 to
0.0092 minꢀ1 and 0.0035 to 0.0073 minꢀ1 as the pH surge from 4
to 11 (Table 3). The highest rate constant value for CR and BPB
dye was observed at pH 4 and pH 6, respectively, demonstrating
their maximum removal efficiency.
Dye concentration
(ppm)
Degradation
(%)
Rate constant, k
R2
(minꢀ1
)
CR
10
20
30
40
50
60
70
100
BPB
10
20
30
40
50
60
70
100
29.80
44.25
54.06
66.02
79.32
90.14
87.19
83.29
0.0014
0.0026
0.0035
0.0046
0.0066
0.0103
0.0089
0.0081
0.9892
0.9925
0.9931
0.9971
0.9973
0.9996
0.9983
0.9980
33.17
46.15
56.13
62.84
72.50
79.42
89.27
85.91
0.0003
0.0007
0.0013
0.0020
0.0025
0.0033
0.0048
0.0037
0.9802
0.9918
0.9960
0.9972
0.9981
0.9988
0.9996
0.9995
3.4.4. Effect of temperature
The impact of temperature is a crucial factor that can influence
the photodegradation of dyes because it can change dye adsorption
rate on the catalyst surface. It is worth examining the effect of tem-
perature on dye degradation because it can help determine the
nature of the reaction that it is either endothermic or exothermic.
The temperature effect was executed by altering the temperature
in the range of 25 to 65 °C for a predetermined dye concentration
(50 ppm) and catalyst (0.05 g/L), as displayed in Fig. 10. It was
observed that the degradation was relatively high for CR dye at
55 °C, and it tends to decrease with further increase in tempera-
ture. As shown in Fig. 10a, 93.55% of dye was removed at 55 °C,
and the mineralization efficiency was slightly reduced up to
89.07% with further increase in temperature at 65 °C. The slight
decline in mineralization efficiency with increasing temperature
infer that the photocatalytic degradation of CR dye is endothermic.
The decline in degradation efficiency at higher temperatures is
may be due to the rapid decomposition of H2O2 [70]. However, in
Fig. 10c, BPB dye showed a continuous increase in degradation
To evaluate the influence of catalyst surface on the photocat-
alytic degradation of CR and BPB under different pH conditions,
we measured the dye concentration changes using UV–vis spec-
troscopy as presented in Fig. 9. The results have shown that CR
dye showed maximum degradation under slightly acidic condi-
tions, and 88.29% of dye was removed at pH 4, and it tends to
decrease under weak acidic and alkaline conditions from 75.19 to
47.38%, as shown in Fig. 9a. While BPB showed maximum degrada-
tion under weakly acidic conditions and 88.61% of dye was
degraded at pH 6; however, it showed a gradual decrease under
mild acidic and alkaline conditions Fig. 9c. At pH pH < pH(IEP), a
9

A. Ahmed, M. Usman, B. Yu et al.
Journal of Molecular Liquids 339 (2021) 116729
Fig. 8. Photocatalytic degradation of dyes for different catalyst loadings; (a) degradation rate of CR, (b) kinetic plot for CR, (c) degradation rate of BPB, (d) kinetic plot for BPB.
Conditions; ([Dye] = 50 ppm; [catalyst] = 0.01–0.05 g/L; pH = 5.8; T = 25 °C).
Table 2
experiments using different scavengers and further verified
through EPR analysis. During quenching experiments, TBA was
The kinetic parameters for CR and BPB dye degradation under the effect of different
catalyst dosage using a-Fe2O3 catalysts.
used as quenching agent for hydroxyl radical (ÅOH) while BQ was
chosen as an effective radical scavenger for superoxide radical
(OÅ2ꢀ). Similarly, the electrons (eꢀ) holes (h+) generated during
degradation process were quenched using AgNO3 and EDTA
respectively [71]. As illustrated in Fig. 11a and b, TBA showed sub-
stantial impact on the dye degradation process and dramatically
suppress the mineralization efficiency of CR and BPB dye to
46.21% and 43.32% respectively. Furthermore, the addition of BQ
in the reaction system also suppress the degradation of CR and
BPB to 51.12% and 54.09% respectively. The substantial decrease
in CR and BPB mineralization efficiency using TBA and BQ as scav-
Catalyst dosage (g/L) Degradation (%) Rate constant, k (minꢀ1
)
R2
CR
0.01
0.02
0.03
0.04
0.05
51.98
58.60
64.32
74.56
87.31
0.0032
0.0038
0.0043
0.0057
0.0079
0.9973
0.9981
0.9985
0.9992
0.9994
BPB
0.01
0.02
0.03
0.04
0.05
30.27
49.91
64.99
82.28
86.57
0.0013
0.0025
0.0041
0.0065
0.0077
0.9758
0.9755
0.9907
0.9916
0.9964
Å
engers verified the major contribution of OH and OÅ2ꢀ radicals in
the degradation processes. On the other hand, the addition of
AgNO3 to the reaction system slightly declined the mineralization
efficiency CR and BPB to 81.52% and 72.47%, while in case of EDTA,
the mineralization efficiency of CR and BPB declined to 86.07% and
79.42% demonstrating the minor contribution of eꢀ and h+ towards
dye degradation.
from 53.12 to 86.41% as the temperature rise from 25 to 65 °C,
which is attributed to the higher stability of dye.
The photocatalytic degradation of CR and BPB dyes followed the
linear correlation of ꢀ ln(C/Co) versus time under the effect of dif-
ferent temperature conditions and best fitted to Langmuir-
Hinshelwood kinetic model as shown in Fig. 10b and d. The values
of reaction rate constant were observed between 0.0040 and
0.0093 minꢀ1 and 0.0009 to 0.0066 minꢀ1 for CR and BPB dye,
respectively, as the temperature increased from 25 to 65 °C
(Table 4). The CR showed the maximum rate constant value at
55 °C while that of BPB at 65 °C correspond to their higher degra-
dation efficiency.
The EPR analysis was executed to further corroborate the con-
Å
tribution of OH and OÅ2ꢀ radicals towards CR and BPB degradation
using
a-Fe2O3 catalyst as presented in Fig. 11c and d. DMPO was
employed as spin trapping agent to authenticate the involvement
Å
of OH and OÅ2ꢀ radicals through the EPR technique. As can be seen
from Fig. 11c, a four-line spectrum for DMPO-ÅOH (1:2:2:1) was
Å
noticed for CR and BPB that particularly belongs to OH radicals
[62]. In Fig. 11d, the signals of DMPO-OÅ2ꢀ adduct was detected in
the reaction system with six characteristic peaks that typically
belongs to OÅ2ꢀÅ radicals [72]. These consequences explicated the
generation of OH and OÅ2ꢀ in the reaction system as a dominant
reactive species that were responsible for the excellent catalytic
3.5. Identification of active species
The presence of main reactive species and their contribution
towards dye degradation were corroborated by radical quenching
performance of a-Fe2O3 towards CR and BPB degradation.
10

A. Ahmed, M. Usman, B. Yu et al.
Journal of Molecular Liquids 339 (2021) 116729
Fig. 9. Photocatalytic degradation of dyes at different pH conditions; (a) degradation rate of CR, (b) kinetic plot for CR, (c) degradation rate of BPB, (d) kinetic plot for BPB.
Conditions; ([Dye] = 50 ppm; [catalyst] = 0.05 g/L; pH = 4–11; T = 25 °C).
Table 3
produced from the
a-Fe2O3 surface acted as strong oxidizing
The kinetic parameters for CR and BPB dye degradation under the effect of different
pH conditions using -Fe2O3 catalysts.
agents and are liable for the complete photodegradation of CR
and BPB dye into harmless end-products, CO2 and H2O [76]. The
reactions involved in the generation of ÅOH and O2Åꢀ radicals on cat-
alyst surface are explained in Eq. (13)–(21).
a
pH
CR
Degradation (%)
Rate constant, k (minꢀ1
)
R2
4
6
8
9
11
BPB
4
6
88.29
75.19
61.11
55.85
47.38
0.0092
0.0060
0.0041
0.0036
0.0028
0.9987
0.9982
0.9967
0.9943
0.9931
-Fe2O3(e +hþ)
ð13Þ
ð14Þ
ð15Þ
ð16Þ
ð17Þ
ð18Þ
ð19Þ
ð20Þ
ꢀ
a
a
a
a
-Fe2O3 + h
m
ꢀ!
a
-Fe2O3(hþ) + H2Oꢀ!
a
-Fe2O3 + Hþ+ÅOH
a
-Fe2O3+ÅOH
84.88
88.61
75.34
66.96
62.23
0.0066
0.0073
0.0051
0.0040
0.0035
0.9993
0.9994
0.9984
0.9977
0.9988
-Fe2O3(hþ) + OH ꢀ!
ꢀ
8
9
11
ꢀ
Å
ꢀ
-Fe2O3(e ) + O2ꢀ!
a
-Fe2O3 + O2
O2 +Hþꢀ!HO2
Å
ꢀ
Å
3.6. Mechanism of dye degradation
Å
2HO2 ꢀ!H2O2 + O2
Based on results mentioned above, a probable mechanism was
assumed for the photodegradation of CR and BPB dye using
Å
ꢀ
Å
ꢀ
H2O2 + O2 ꢀ! OH + OH +O2
a
-
Fe2O3 NPs and well explained in Fig. 12. Under sunlight illumina-
tion, the photons are adsorbed on the catalyst surface and gener-
ated the electrons (eꢀ) and holes (h+) at the catalyst surface
[73,74]. As a result, the eꢀ generated in the conduction band will
react with oxygen (O2) and converted into superoxide anion radical
(OÅ2ꢀ) which on further protonation by H2O molecules converted
Å
ꢀ
Dye + O2 ꢀ!Degradation products
Dye + ÅOHꢀ!Degradation products
ð21Þ
It was observed from results that the synthesized
a
-Fe2O3 cata-
into hydroperoxyl radical (ÅOOH) radicals [75]. The OOH radicals
Å
lyst have promising catalytic potential towards dye degradation.
rearranged into H2O2 under sunlight irradiation, which on further
To prove the practical applicability of
catalytic performance of synthesized
pared with those of previously reported
a
a
-Fe2O3 catalyst, the photo-
-Fe2O3 catalyst was com-
-Fe2O3 catalyst in the
Å
reaction with OÅ2ꢀ produced OH radicals.
Likewise, the h+ created in the valence band will react with H2O
molecules and converted into ÅOH radicals. The ÅOH and OÅ2ꢀ radicals
a
literature and listed in Table 5.
11

A. Ahmed, M. Usman, B. Yu et al.
Journal of Molecular Liquids 339 (2021) 116729
Fig. 10. Photocatalytic degradation of dyes under different temperature conditions; (a) degradation rate of CR, (b) kinetic plot for CR, (c) degradation rate of BPB, (d) kinetic
plot for BPB. Conditions; ([Dye] = 50 ppm; [catalyst] = 0.05 g/L; pH = 5.8; T = 25–65 °C).
Table 4
solution was obtained after the vigorous stirring of the catalyst
(0.01 g) in deionized water (100 mL) for 30 min. Following that,
The kinetic parameters for CR and BPB dye degradation under the effect of different
temperature conditions using
a-Fe2O3 catalysts.
a 10 mL solution was syringe-filtered and acidified with an equiv-
alent volume of 3% HNO3 for ICP-OES analysis. It can be seen from
Fig. S7 that the concentration of Fe leakage was increased from
0.16 to 0.24 mg/L and 0.18 to 0.29 mg/L respectively after four con-
secutive cyclic runs for CR and BPB dye degradation. The concen-
tration of Fe leakage was much less than the discharge standard
of dye wastewater (GB3838-2002, GB 25476–2010) that indicated
Temperature (°C)
Degradation (%)
Rate constant, k (minꢀ1
)
R2
CR
25
35
55
65
BPB
25
35
55
65
62.22
75.59
93.55
89.07
0.0040
0.0060
0.0127
0.0093
0.9946
0.9957
0.9995
0.9986
the better stability of the
BPB degradation efficiency declined, no obvious phase change
was observed in the -Fe2O3 catalyst after four consecutive cyclic
a-Fe2O3 catalyst [84]. Albeit the CR and
53.12
58.72
75.31
86.41
0.0009
0.0017
0.0038
0.0066
0.9690
0.9789
0.9840
0.9937
a
runs that was in strong agreement with XRD results (Fig. 13e).
Thus, the consequences of reusability analysis predict and support
the applicability of the
dation of dyes from wastewater.
a-Fe2O3 catalyst towards the photodegra-
3.7. Reusability analysis
The catalysts’ reusability and photostability are the critical
dynamics to assess their catalytic efficacy to make the process eco-
4. Conclusion
nomical [83]. To verify the stability of the
a-Fe2O3 catalyst, four
The hematite nanoparticles (a-Fe2O3 NPs) were fabricated using
biomolecules of P. granatum seed extract and characterized by var-
ious advanced techniques. FESEM and TEM analysis showed the
consecutive photodegradation experiments were performed at
fixed reaction conditions, and their outcomes are presented in
Fig. 13a-d.
formation of irregular-shaped and highly aggregated
with an average size of 26.53 nm measured from TEM analysis.
The synthesized
-Fe2O3 NPs have large surface area (31.52 m2/
g) and narrow pore size distribution with an average pore diameter
of 5.54 nm. -Fe2O3 NPs exhibited typical ferromagnetic behavior
a-Fe2O3 NPs
After four consecutive cyclic runs, the
a-Fe2O3 catalyst exhib-
ited a slight loss of 10.49% and 6.18% in degradation efficiency
for CR and BPB, specifying the exceptional stability of catalyst.
The certain loss in catalyst performance owed to the adsorption
of intermediate products on catalytic sites, destruction of catalytic
sites and the leaching concentration of metal.
a
a
with saturation magnetization and remanence of 2.067 and
0.52 emu/g, respectively, while the coercivity of the sample was
The leaching concentration of Fe ions in the reaction solution
found to be 722 Oe. The as-prepared
for photocatalytic degradation of CR and BPB dye from wastewater
under sunlight irradiation. When -Fe2O3 NPs were exposed to
a-Fe2O3 NPs were utilized
was determined to analyze whether
a-Fe2O3 catalyst posed a
potential risk to the environment. For this purpose, Fe leaching
a
12

A. Ahmed, M. Usman, B. Yu et al.
Journal of Molecular Liquids 339 (2021) 116729
Fig. 11. Effect of scavenger on (a) CR degradation, (b) BPB degradation using a .
-Fe2O3 NPs, and EPR analysis for (c) DMPO-ÅOH, (d) DMPO-O2Åꢀ
Fig. 12. Proposed mechanism of dye degradation using a-Fe2O3 catalyst.
sunlight, an electron-hole pair generated on the catalyst surface
that resulted the formation of highly reactive species (ÅOH and
OÅ2ꢀ) and degraded the dye into innocuous and less toxic end-
products. The synthesized a-Fe2O3 NPs proved their photocatalytic
efficiency by degrading 89.42% and 87.96% of CR and BPB dye,
temperature up to certain limits promotes the degradation pro-
cess, and it tends to decrease beyond certain limits. The acidic
pH of the solution and increase in catalyst dosage was more
favorable for dye degradation. The kinetic parameters for dye
degradation under different reaction conditions were also mea-
sured, and it followed the Langmuir–Hinshelwood (L–H)
respectively, in 240 min of irradiation time.
The influence of different operating parameters such as ini-
tial dye concentration, catalyst loading, pH, and the temperature
was scrutinized on photocatalytic degradation of dyes. It was
noted that the increase in the initial concentration of dye and
pseudo-first-order kinetics. The reusability study of a-Fe2O3
NPs after four successive runs showed a slight loss in degrada-
tion efficiency, which proved that the as-prepared NPs were
highly stable.
13

A. Ahmed, M. Usman, B. Yu et al.
Journal of Molecular Liquids 339 (2021) 116729
Table 5
Comparison of photocatalytic performance of
a-Fe2O3 catalyst with previously reported catalysts.
Catalyst
Synthesis method
Pollutant
Degradation efficiency
Ref.
a
a
a
a
a
a
-Fe2O3 NPs
-Fe2O3 NPs
-Fe2O3 NPs
-Fe2O3 nanoflowers
-Fe2O3 nanosheets
-Fe2O3 NPs
Co-precipitation
Hydrolysis
Combustion
Hydrothermal and calcination
Hydrogel-mediated dissolution–recrystallization
Hydrothermal
Biological
Combustion
Biological
Biological
Methylene blue
Methylene blue
Malachite green
Methyl orange
Bisphenol S
Rhodamine B
Reactive blue 4
Rhodamine B
Congo red
92% within 16 h
80% within 120 min
46.29% within 3 h
42% within 180 min
90% within 180 min
~79% within 140 min
95.08% within 56 min
57.39 within 3 h
[77]
[78]
[27]
[79]
[80]
[81]
[49]
[82]
Iron oxide NPs
a-Fe2O3 NPs
a-Fe2O3 NPs
a-Fe2O3 NPs
89.42% within 240 min
87.96% within 240 min
This study
This study
Bromophenol blue
Fig. 13. Reusability and photostability of
a-Fe2O3 catalyst for photocatalytic degradation of dyes; (a) photocatalytic degradation rate of CR, (b) rate constant plot for CR, (c)
photocatalytic degradation rate of BPB, (d) rate constant plot for BPB.
14

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CRediT authorship contribution statement
Adeel Ahmed: Conceptualization, Investigation, Validation,
Methodology, Data curation, Writing - original draft. Muhammad
Usman: Conceptualization, Methodology. Bing Yu: Methodology,
Investigation. Youqing Shen: Conceptualization, Writing - review
& editing. Hailin Cong: Conceptualization, Supervision, Writing -
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Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgments
This work is financially supported by the National Natural
Science Foundation of China (21675091, 21874078, 22074072),
the Taishan Young Scholar Program of Shandong Province
(tsqn20161027), the Major Science and Technology Innovation
Project of Shandong Province (2018CXGC1407), the Key Research
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