Biofabrication of Fe nanoparticles in aqueous extract of Hibiscus sabdariffa with enhanced photocatalytic activities

Article abstract of DOI:10.1039/c7ra01251a

Hibiscus sabdariffa is a strongly basic dye with a large number of medicinal applications and is used for various diagnostic purposes. The biofabrication of Fe nanoparticles using Hibiscus sabdariffa (Roselle) flower extract is an eco-friendly and cost-effective protocol reported for the first time in the present investigation. The natural plant extract acts as a non-toxic, biodegradable and economical reducing agent in the biosynthesis of the Fe nanoparticles. Different well-known complementary characterization techniques such as UV-vis spectroscopy, Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), X-ray diffraction (XRD), dynamic light scattering (DLS) and thermogravimetric analysis-differential scanning calorimetry (TGA-DSC) were used to investigate the morphological, structural and optical properties of the as-synthesised Fe nanoparticles. The resulting nanoparticles were then utilized as a photocatalyst for the degradation of the toxic organic dye Congo red (CR). UV-visible spectroscopy was used to determine the photocatalytic performance of the Fe nanoparticles under UV irradiation. The results of the photocatalytic degradation of Congo red in aqueous solution under UV light showed a higher photocatalytic activity. A reasonable mechanism was proposed for the enhanced photocatalytic activity of the Fe nanoparticles.

Full text of DOI:10.1039/c7ra01251a

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Biofabrication of Fe nanoparticles in aqueous
extract of Hibiscus sabdariffa with enhanced
Cite this: RSC Adv., 2017, 7, 25149
photocatalytic activities
Abdulmohsen Alshehri, Maqsood Ahmad Malik, *a Zaheer Khan,a
a
a
b
Shaeel Ahmed Al-Thabaiti and Nazim Hasan
Hibiscus sabdariffa is a strongly basic dye with a large number of medicinal applications and is used for
various diagnostic purposes. The biofabrication of Fe nanoparticles using Hibiscus sabdariffa (Roselle)
flower extract is an eco-friendly and cost-effective protocol reported for the first time in the present
investigation. The natural plant extract acts as a non-toxic, biodegradable and economical reducing
agent in the biosynthesis of the Fe nanoparticles. Different well-known complementary characterization
techniques such as UV-vis spectroscopy, Fourier transform infrared spectroscopy (FTIR), transmission
electron microscopy (TEM), X-ray diffraction (XRD), dynamic light scattering (DLS) and thermogravimetric
analysis-differential scanning calorimetry (TGA-DSC) were used to investigate the morphological,
structural and optical properties of the as-synthesised Fe nanoparticles. The resulting nanoparticles were
then utilized as a photocatalyst for the degradation of the toxic organic dye Congo red (CR). UV-visible
spectroscopy was used to determine the photocatalytic performance of the Fe nanoparticles under UV
irradiation. The results of the photocatalytic degradation of Congo red in aqueous solution under UV
light showed a higher photocatalytic activity. A reasonable mechanism was proposed for the enhanced
photocatalytic activity of the Fe nanoparticles.
Received 30th January 2017
Accepted 12th April 2017
DOI: 10.1039/c7ra01251a
rsc.li/rsc-advances
Fe nanoparticles have been synthesized by different chem-
ical and physical methods such as electrochemical processes,
1
. Introduction
18
1
9
20
21
Nanotechnology is an interdisciplinary area of research that has photochemical reduction, thermal evaporation, etc. In
22
undergone extensive development and attracted attention general, sodium borohydride (NaBH
4
), hydrazine, sodium
worldwide during the last few years and has made an impact in dodecyl sulphate, etc., are used in the presence of organic
1,2
23–25
different areas of human life. Metal nanoparticles have surfactants or with the use of capping agents.
These
attracted considerable attention from researchers because of reducing agents have proven fatal to the natural environment;
their unusual chemical and physical properties. They enable an in addition, they produce harmful by-products that damage
26
array of potential applications in materials science, electronics, human health. However, the majority of these techniques
optics, medicine, biomedical science and engineering and involve hazardous chemicals, require special equipment,
3–5
catalysis.
Among magnetic metallic nanomaterials, iron employ large amounts of energy, are generally expensive and
nanoparticles have gained considerable attention for site require complex cleansing processes. For this reason, there is
remediation and the treatment of water polluted by organic and a growing requirement to create environment-friendly and
inorganic contaminants because of the higher inherent reac- sustainable protocols for the synthesis of Fe nanoparticles. To
6
tivity of their surface sites. Iron nanoparticles have unique overcome these technical complications, natural principles
characteristics that include suitability for environmental have been employed in recent years and materials science
7
remediation, high efficiency for the degradation of aqueous researchers are attempting to understand, mimic and exploit
8
,9
10
11
12
27
organic solutes, catalytic activity, and optical, electronic,
and antibacterial properties. Thus, they nd a role in a wide still demands an economical, commercially feasible and
natural processes with this aim. The eld of nanotechnology
13
14
28
range of applications such as cosmetics, biomedicine,
environment-friendly method for synthesizing nanoparticles.
1
5
16
17
bioremediation, clinical materials, and engineering.
Shahwan et al. used green tea leaves to fabricate Fe nano-
particles. They utilized the resulting materials as a Fenton-like
catalyst for the degradation of methylene blue and methyl
orange. Nadagouda et al. reported the synthesis of iron
nanoparticles utilizing green tea leaf and sorghum bran
a
Chemistry Department, Faculty of Science, King Abdulaziz University, P.O. Box 80203,
Jeddah 21589, Saudi Arabia. E-mail: maqsoodchem@gmail.com
29
b
Department of Chemistry, Faculty of Science, Jazan University, P.O. Box 2097, Jazan
30
extracts. Maaza et al., Mishra et al., and Kalita and Ganguli
8
2621, Saudi Arabia
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used Hibiscus sabdariffa ower extract as a reducing and stabi- anthocyanin compounds such as cyanidin 3-rutinoside, del-
lizing agent for the synthesis of cerium oxide, gold, and silver phinidin 3-sambubioside, cyanidin 3-sambubioside, cyanidin 3-
nanoparticles, respectively, for the characterization of the main glucoside and delphinidin 3-glucoside are also present in this
physical properties, the evaluation of cytotoxicity against U87 plant. Maganha et al. also reported that delphinidin, delphini-
glioblastoma cells under hyperglycemic conditions, and the din 3-glucoxyloside, delphinidin 3-monoglucoside and cyanidin
31–33
39
catalytic reduction of 4-nitrophenol.
However, it is an 3,5-diglucoside are major anthocyanin components in HS.
essential environmental aspect of wastewater remediation to However, anthocyanin as a pigment is present in large amounts
develop suitable eco-friendly semiconductor photocatalysts. in the calyces and is responsible for their bright red color.
Because light-assisted photoinitiated redox reactions, which are Gradinaru et al. reported two predominant anthocyanins (del-
involved in the degradation of carcinogenic and neurotoxic dyes phinidin 3-sambubioside and cyanidin 3-sambubioside) and
in industry, are tedious, photocatalysts have been proven to be two minor anthocyanins (cyanidin 3-glucoside and delphinidin
40
effective reaction inducers involved in reducing environmental 3-glucoside) in HS. Fig. 1 shows the structures of some of these
pollution. Likewise, water pollution caused by industrial waste HS anthocyanins.
34
leads to serious problems throughout the world. Moreover,
some dyes, which are the by-products of textile industries, prove 2.2. Preparation of Hibiscus sabdariffa extract
fatal to ecosystems and cause various severe problems such as
Fresh samples of red petals of HS were collected from a local
carcinogenicity in humans, eutrophication in water bodies and
market in Jeddah, Saudi Arabia and were air-dried at room
35
damage to living organisms. Photocatalytic degradation under
visible-light irradiation has gained widespread attention
because it is rapid, highly efficient, and economical and causes
temperature. Hibiscus samples were stored in glass jars and
ushed with nitrogen. Using a domestic blender, dried owers
of HS were ground to a ne powder. In a typical procedure,
0.0 g clean HS ower petals were weighed in a beaker and

36
no secondary pollution. The problems associated with
conventional methods (the use of toxic stabilizers, solvents, and
surfactants) encouraged us to synthesise biocompatible Fe
nanoparticles using Hibiscus sabdariffa (HS) red ower extract
as an effective stabilizing agent without the addition of a stabi-
lizer or capping agent for the rst time. In addition, the as-
prepared Fe nanoparticles were used as a catalyst for the
degradation of CR.
1
added to 400 mL distilled water. Our main objective was to carry
out non-thermal extraction; therefore, the aqueous extraction
solution was kept at room temperature for 25 h to obtain a high
concentration of anthocyanins. The obtained red extract solu-
tion was then vacuum-ltered twice to eliminate any residual
solids, and the red extract was collected for further experiments.
It has been reported that aqueous extracts of HS have stronger
antioxidant properties in comparison to extracts obtained with
41
2. Experimental
methanol.
2
.1. Materials and chemical constituents of Hibiscus
2.3. Biogenic preparation of Fe nanoparticles
sabdariffa owers
In a typical experiment, a 0.01 M solution of FeCl was prepared
3
3 2
Iron(III) chloride hexahydrate (FeCl $6H O, purity > 99%) was
with double-distilled water and freshly prepared HS extract was
used as a reducing agent and stabilizer. For the biogenic
synthesis of Fe nanoparticles, 5 mL HS extract was added to 100
purchased from Sigma-Aldrich. The water used as a solvent
throughout this experiment was double-distilled water with
ꢁ
6
ꢁ1
a conductivity of 1.05 ꢀ 10 S cm . Hibiscus sabdariffa, which
has the common name Roselle, belongs to the family Malvaceae
and produces red owers in the form of calyces. These eshy
calyces (sepals) are important raw materials for the food
industry and have commercial importance for the production of
beverages, juices, jams, and syrups. Furthermore, these calyces
possess high pigment contents and are used as a good source of
natural food colorants. In addition to the use of Roselle in the
beverage and food industries, it is additionally consumed as
animal feed and in nutraceuticals, cosmetics and pharmaceu-
mL of the aqueous solution of FeCl
3
and stirring was carried out
ꢂ
for 2 min at 25 C to ensure that a homogeneous reaction
occurred within the solution. It was observed that reduction
took place rapidly, as indicated by a colour change. The entire
reaction system was covered to avoid any photoinduced
phenomena. The colloids were found to be stable for about one
month and underwent no precipitation or colour change. The
Fe sol solution was centrifuged to obtain a dried powder of Fe
nanoparticles for further analyses and characterization.
37
ticals. Numerous studies of the different constituents of this
38
2.4. Characterization of Fe nanoparticles
plant have been reported in the literature. Among these are
3
+
0
a notable (and underreported) group of phenolic compounds, The progress of the reaction and the reduction of Fe to Fe
which have recently obtained a great amount of research were monitored by recording the UV-vis spectra of the nano-
interest owing to their vital role in the promotion of human reaction mixture. The UV-visible spectra of the pure red extract
health and hygiene. The chemical constituents present in this of HS and the FeCl solution were also recorded for comparison
3
plant are phenolic compounds, avonoids with their respective with the spectrum of the Fe sol solution. UV-visible spectral
glycosides and anthocyanins. The compounds present in this analysis was carried out using a Shimadzu UV-260 spectropho-
plant include organic and phenolic acids such as hydroxycitric tometer with 1 cm quartz cuvettes in the range from 200 to
acid, citric acid, protocatechuic acid and hibiscus acid. Flavo- 700 nm. X-ray diffraction (XRD) was used to examine the
noids such as gossypetin, quercetin, and luteolin and structure of the synthesised Fe nanoparticles. This examination
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Fig. 1 Chemical structures of some anthocyanin compounds found in Hibiscus sabdariffa.
ꢂ
ꢁ1
˚
was carried out with Ni-ltered Cu Ka radiation (l ¼ 1.54056 A) a nitrogen atmosphere at a heating rate of 10 C min in an
using a Rigaku X-ray diffractometer operating at 40 kV and 150 inert atmosphere to determine the presence of organic
ꢂ
ꢂ
mA at a scanning rate of 0.02 per step in the 2q range of 10 # compounds from the HS ower extract.
q # 80 . FTIR analysis was carried out to determine the func-
ꢂ
2
tional groups present in HS ower extract and their possible
involvement in the synthesis of Fe nanoparticles. The FTIR
2.5. Photocatalytic activity
spectra of the synthesised Fe nanoparticles was also recorded in The photocatalytic degradation efficiency of the biofabricated
ꢁ
1
transmission mode (4000–650 cm ) using a Bruker Tensor II Fe nanoparticles was measured by performing the degradation
FTIR spectrometer. The biofabricated Fe nanoparticles were of CR dye under UV light irradiation. The photocatalytic activity
characterized by dynamic light scattering (DLS) with the use of of the Fe nanoparticles was determined by the decolourization
a Laser Spectroscatter 201 instrument (RiNA GmbH, Berlin, of an aqueous solution of CR under UV irradiation. A stock
ꢁ
3
ꢁ1
Germany). The polydispersity and particle size were determined solution of CR [2.0 ꢀ 10 M] [MW ¼ 619.66 mg L CR] was
by DLS measurements. The DLS measurements were performed prepared by dissolving CR in an appropriate amount of deion-
using a beam of He–Ne laser at 660 nm, and Dispersion Tech- ized water. Further dilution of the stock solution was performed
nology Soware (DTS) was used to t the curves. The surface to obtain the desired experimental solutions used for degrada-
morphology of the Fe nanoparticles was examined by TEM tion reactions. The photocatalytic experiments were performed
coupled with an energy-dispersive X-ray spectrometer (EDX) in a 500 mL jacketed glass beaker equipped with a magnetic
(JSM-7001F, JEOL, Tokyo, Japan). The elemental contents of the stirrer under UV light at 100 W (maximum emission at 365 nm)
synthesised Fe nanoparticles were determined at different (Fig. 2). In a typical experiment, biologically synthesised Fe
magnications by sprinkling the Fe nanoparticles onto nanoparticles were dispersed in 250 mL of an aqueous solution
a metallic carbon disk. Transmission electron microscopy of CR with continuous stirring. Before illumination, the
(TEM) images were acquired to observe the morphology of the suspension was stirred for 30 min in dark conditions to achieve
Fe nanoparticles with a JEOL JEM-2010F microscope (Tokyo, adsorption/desorption equilibrium between the Fe nano-
Japan) operating at 120 kV. Nanoparticle samples were soni- photocatalyst and CR. All dye degradation experiments were
ꢂ
cated in analytical-grade methanol for 30 s and mounted on performed at 30 C (room temperature) by continuous circula-
carbon-coated copper grids. Thermogravimetric analysis (TGA) tion of water to the outer jacket of the beaker (with the reactor in
and differential scanning calorimetry (DSC) were also con- an aqueous medium) to ensure a constant temperature
ducted using an EXSTAR TGA/DTA 6000 analyzer under throughout the experiment. Then, the experimental mixture
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Fig. 2 Schematic of experimental setup for visible-light photodegradation of Congo red.
was exposed to UV irradiation with continuous stirring. At the initial concentration of CR, and C represents the CR
specic intervals, 5 mL samples of the experimental solution concentration at a certain reaction time (min). The degradation
were withdrawn and centrifuged to separate the catalyst parti- kinetics of CR was investigated under the optimum experi-
ꢁ
5
cles. The photocatalytic degradation of CR was monitored by mental conditions of [CR] ¼ 1.0 ꢀ 10 M, Fe nanocatalyst ¼ 15
measuring its absorbance with a UV-vis spectrophotometer. The mg/250 mL Congo red dye solution and pH ¼ 4.
reaction was monitored with a Shimadzu UV-260 spectropho-
tometer and the decrease in the absorbance intensity of CR at
3
. Results and discussion
l
¼ 497 nm was recorded at intervals of 10 min. The effects of
max
the Fe photocatalyst concentration were investigated with 3.1. Characterization of biofabricated Fe nanoparticles
loadings of 5.0, 10.0, 15.0, and 20.0 mg in 250 mL CR solution
0
3+
The formation of Fe by the reduction of Fe was monitored by
recording the UV-vis spectrum of the Fe nanoparticle solution in
ꢁ5
(
1.0 ꢀ 10 M). One more set of experiments was carried out, in
ꢁ5 ꢁ5
which initial CR concentrations of 1.0 ꢀ 10 M, 2.0 ꢀ 10 M,
3+
the range of 200–800 nm. The reduction of Fe to Fe nano-
particles was evident from the colour change (to a yellowish-
brown colour) when the red extract of HS was mixed with
ꢁ5
ꢁ5
3
.0 ꢀ 10 M, and 4.0 ꢀ 10 M were used with a xed loading
of the Fe nanophotocatalyst of 15 mg in 250 mL dye solution.
The optimum conditions that enable high photocatalytic
degradation efficiency, such as the amount of catalyst and
initial concentration of CR, and their inuence on photo-
catalytic degradation were studied. The following equation was
used to calculate the photocatalytic decolourization efficiency of
3
FeCl . UV-visible spectroscopy is a useful technique for studying
the kinetics of the formation of Fe nanoparticles. Fig. 3 shows
the UV-visible spectrum of Fe nanoparticles stabilized by the
organic constituents of HS extract. Studies of the UV-visible
absorption spectra provided strong evidence of the formation
of Fe nanoparticles and their growth. The absorption spectra of
42
the Fe nanoparticles for CR:
A
o
ꢁ A
the FeCl
in Fig. 3. The peaks for the pure HS extract at 287, 322 and
19 nm correspond to the presence of various natural moieties
in the extract. As shown in Fig. 3, the peaks for the HS extract at
87, 322 and 519 nm completely disappeared aer the reaction
3
solution and the extract of HS were recorded, as shown
Degradation ð%Þ ¼
ꢀ 100
ꢀ 100
A
o
(1)
5
C
o
ꢁ C
¼
C
o
2
where A and A are the initial absorbance of the CR solution and
the absorbance aer a certain time, respectively. C represents
o
with the iron ions, and the colour of the reaction mixture
changed from red to dark yellow. More importantly, a strong
o
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absorption peak for the Fe nanoparticles synthesized by the HS
extract was observed at 335 nm. This indicates that various
components present in the extract acted as reducing and
capping agents for the synthesis of the Fe nanoparticles. The
hydroxyl groups present in biologically active molecules rst
3
+
bound with Fe to give ferric hydroxide, which was partially
reduced by other bioactive materials to form Fe nanoparticles.
The UV-visible spectra of the as-synthesised Fe nanoparticles
were also recorded as a function of time to determine their
stability. We did not observe any signicant changes in the
shape of the UV-visible spectra (the position of the wavelength
maxima and the absorbance remained constant with an increase
in the reaction time; Fig. 3), which indicated that the resulting
nanoparticles were stable under our experimental conditions
and ruled out the possibility of their aggregation.
The shape, size, and morphology of the Fe nanoparticles
were analyzed using transmission electron microscopy.
Aliquots of the Fe nanoparticle solution were placed on
a carbon-coated copper grid and allowed to dry under ambient
conditions. A TEM image of the Fe nanoparticles is shown in
3
Fig. 3 UV-vis spectra of the FeCl solution, the aqueous extract of red
petals of Hibiscus sabdariffa and the Fe nanoparticles at different time
intervals after synthesis.
Fig. 4 (a) Transmission electron microscope image (inset – selected area electron diffraction (SAED) image of Fe-nanoparticle), (b) particle size
measurement of by dynamic light scattering, (c) XRD pattern and (d) EDX spectrum of Fe-nanoparticles (inset – HR-TEM micrograph of
a spherical Fe-nanoparticle).
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Fig. 4(a), which reveals the successful biological formation and presence of Fe and O elements in the composite. The spectrum
stabilization of the Fe nanoparticles. From the transmission contained intense peaks of C, O and Fe, which conrmed the
electron microscopy (TEM) analysis, it is clear that the particles presence of iron. The presence of Cu and C was due to the
are mainly spherical with a size of 100 nm. The particles are carbon-coated grid that was used for the preparation of the
observed to have a mixture of spherical morphologies and are samples. Fe nanoparticles have reactive surfaces and are highly
well separated from each other. It can be seen that the one-pot liable to be oxidised, and hence their particle size increases;
reaction yielded nearly monodisperse Fe nanoparticles. The size nevertheless, anthocyanins stabilize the surfaces of iron nano-
distribution and the average size of the biologically synthesised particles and limit surface oxidation because of the biogenic
Fe nanoparticles were measured by dynamic light scattering molecular environment.
(DLS). The hydrodynamic radius RH was calculated by the
Analysis by Fourier transform infrared spectroscopy (FTIR)
Stokes–Einstein equation, and the diameter was found to be in was carried out to identify the presence of biomolecules in the
the range of 10–100 nm (Fig. 4(b)). The size distribution of the HS petal extract that were responsible for the bioreduction and
Fe NPs was probably due to the fact that non-thermal extracts of stabilization of the Fe nanoparticles. The FTIR spectra of the HS
HS in water contain various naturally derived compounds with extract and biologically synthesised Fe nanoparticles are shown
43
different reducing properties. Anthocyanin compounds found in Fig. 5(a) and (b). In Fig. 5(b), two strong absorption bands are
ꢁ1
ꢁ1
in the HS petal extract probably played a crucial role in observed at around 638 cm and 575 cm , which correspond
controlling the oxidation and aggregation of the Fe nano- to Fe–O bonds, whereas these bands are not observed in the
48
particles to enhance their dispersion by acting as surface spectrum of the HS extract (Fig. 5(a)). We conrmed the
capping agents. Delie et al. in their pioneering review suggested presence of anthocyanin compounds in the Fe NPs: the FTIR
ꢁ
1
that the determination of the size and polydispersity index (PDI) spectrum displays bands at 1629 cm due to C]C stretching
4
4
ꢁ1
of nanoparticles is not straightforward and is totally depen- vibrations in aromatic rings and 1066 cm due to C–O–C and
49
dent on the experimental techniques (different physical theo- O–H absorption peaks, respectively. The same pattern is also
ries and data processing methods), mean sizes of particles, observed in Fig. 5(a) as a result of direct analysis of the HS
ꢁ
1
ꢁ1
variations in distribution, refractive index of the solvent used, extract, with bands at 1657 cm
and nature of the dispersive medium.
and 1417 cm . Strong
45,46
ꢁ1
The polydispersity of asymmetric stretching of vinyl alkyl ethers at 1220 cm is
the as-prepared Fe nanoparticles was calculated using the observed in Fig. 5(b); on the other hand, vibrational bands of
4
7
following relation:
aliphatic hydrocarbons, which correspond to –CH and –CH
2
ꢁ
1
ꢁ1
groups, are observed at 2854 cm and 2935 cm . The intense
ꢁ
1
ꢁ1
PDI ¼ s/R
d
(2)
absorption bands at 3426 cm and 3438 cm in Fig. 5(b) and
a), respectively, are attributed to O–H stretching vibrations. HS
(
where PDI, s, and R
dispersity index (ca. 0.182) indicates the narrow size distribu-
tion of the spherical Fe nanoparticles.
The biofabricated Fe nanoparticles were further investigated
by XRD, as shown in Fig. 4(c). A series of X-ray diffraction
patterns were recorded in the range of 20–70 . As can be clearly
d
have the usual meanings. The low poly-
has antioxidant properties that induce the reduction of the iron
precursor salt, and nanoparticles stabilized by anthocyanin
moieties are the resulting product, as conrmed by FTIR data.
It has been established that the weight loss of metal nano-
ꢂ
ꢂ
particles at lower (#200 C) and higher temperatures ($200 C)
ꢂ
might be due to the evaporation of adsorbed water and the
seen from Fig. 4(c), characteristic diffraction peaks were
observed, which were assigned to the (220), (311), (400), (422),
(
511), and (440) Bragg reections. Furthermore, these patterns
were also comparable to the standard XRD pattern of magnetite
JCPDS card no. 89-0950). The purity of the synthesised material
(
was observed from the wide-angle XRD pattern, and peaks could
easily be identied and corresponded to the pure cubic phase of
Fe nanoparticles. However, no additional peaks (impurity
peaks) were observed, which conrms that high-purity crystal-
line Fe nanoparticles were successfully synthesised. The crys-
talline nature of the Fe-nanoparticles was analyzed by selected
area electron diffraction (SAED) (Fig. 4(a) inset). The selected
area electron diffraction (SAED) pattern has been recorded from
an area where large number of nanoparticles was found and
these biofabricated Fe-nanoparticles were observed to be crys-
talline. The HR-TEM (Fig. 4(d) inset) result also reveals that
spherical Fe-nanoparticles were well dispersed and capped with
water soluble anthocyanin compounds present in the HS petal
extract. To identify the elemental composition of the Fe nano-
particles, energy-dispersive X-ray spectroscopy (EDS) (Fig. 4(d)
Fig. 5 FTIR spectra of (a) aqueous extract of HS and (b) synthesised Fe
nanoparticles.
(inset – HRTEM)) was carried out, which clearly revealed the
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has a wide range of uses in plastics, paper, rubber, textile
industries and microbiology. Environmentally, CR has some
adverse effects on the human body such as respiratory tract
irritation, eye irritation, and central nervous system depression
if direct contact or inhalation occurs. As-prepared Fe nano-
particles that were reduced and stabilized by HS were
investigated for the photocatalytic degradation of CR under
visible-light irradiation. Fig. 7(a) shows the effect of the Fe
nanoparticles on the oxidation of CR under visible-light irra-
diation. The main absorbance peaks of CR with the Fe nano-
particles were at wavelengths of 497 nm (p / p*), 350 nm (n /
p*) and 235 nm (n / n*) because of the azo functional group.
The intensity of the peaks gradually decreased as the irradiation
time increased. The UV-visible spectrum shows that the Fe
nanoparticles displayed rapid photocatalytic activity on the
surface, which is stabilized by biological moieties, as depicted
in Fig. 7(b), which shows the percentage degradation of CR at
different exposure times under UV light irradiation in the
presence of Fe nanoparticles. Fig. 7(c) shows the degradation
efficiency of CR under optimum experimental conditions, and it
is clearly indicated in the plot that CR was completely degraded.
The percentage degradation of the dye was calculated using eqn
Fig. 6 TGA and DSC curves of Fe nanoparticles obtained from
aqueous extract of HS.
50,51
breakdown of the organic skeleton, respectively.
The
decomposition pathway of Fe nanoparticles was studied by TGA
and DSC analysis. Fig. 6 shows that the weight loss of the Fe
(1). Within 30 minutes the Fe nanoparticles degraded more
ꢂ
nanoparticles started at ca. 130 C, which might be due to the
than 55% of CR. This is explained on the basis that the nano-
particles were stabilized by anthocyanin moieties from the HS
extract, which enhanced the immobilization of dye molecules
on the nanoparticle surface, and photocatalytic activity
occurred immediately under UV light. As ultraviolet light was
incident on the Fe nanoparticles, photogenerated electron–hole
pairs were generated and migrated to the nanoparticle surface
to induce redox reactions of aqueous organic substrates in the
presence of oxygen.
Kinetic analysis is one of the most signicant methods of
dening reaction pathways. The kinetic parameters of the
visible-light degradation of aqueous CR dye by Fe nanoparticles
were investigated under optimum conditions to understand the
reaction characteristics of the anionic CR dye under UV irradi-
ation (Fig. 7(d)). A plot of the degradation of CR with respect to
removal of water chemisorbed on the crystal lattice. The second
exothermic peak at ca. 275 C was due to the combustion of
ꢂ
organic constituents present on the surface of the Fe nano-
ꢂ
particles. Some residues were le (ca. 40%) above 600 C, which
ꢂ
were stable up to 900 C. The DSC curve displays two main
ꢂ
peaks (one exothermic peak at 680 C and another endothermic
ꢂ
peak at 720 C; Fig. 6). We did not observe any mass loss at
higher temperatures. Thus, we may state condently that pure
zero-valent Fe nanoparticles were formed under our experi-
mental conditions.
It is well known that HS petal extracts contain various
organic acids (L-ascorbic, citric, malic, stearic, and proto-
catechuic), anthocyanins, saccharides (galactose, mucopoly-
52
saccharide, and polysaccharide), carotene, pectin, and wax.
The literature includes multiple investigations of the use of
ascorbic acid as a reductant for carrying out the syntheses of
various metal nanoparticles (Ag, Au, Cu, and Fe). It has been
established that the presence of a capping agent, which hinders
the unlimited growth of nanoparticles, is essential for the
synthesis of perfectly transparent sols of these metal nano-
o
the irradiation time (plot of ln(C /C) vs. irradiation time) follows
a linear relationship and thus exhibits pseudo-rst-order
58
kinetics (eqn (3)):
ꢀ
ꢁ
C
o
ln
¼ kapp
t
(3)
C
53–56
particles.
In the present study, the Hibiscus sabdariffa extract
ꢁ1
where kapp (min ) is the apparent reaction rate constant of
acted as a reducing agent as well as a capping agent. Thus, we
may state condently that the presence of a stabilizer is not
mandatory for the synthesis of Fe nanoparticles using a green
chemical reduction method.
decolorization calculated from the slope determined by plotting
ꢁ1
ln(C
o
/C) versus t, and was found to be 0.042 min . C
o
is the
initial concentration of the dye and C is the concentration at
2
time t. Furthermore, the regression coefficient (R ) of 0.98
conrmed that the photodegradation of CR dye tted the
Langmuir–Hinshelwood kinetic model.
3
.2. Photocatalytic activity of biofabricated Fe nanoparticles
0
0
CR (1-naphthalene sulphonic acid, 3,3 -(4,4 -biphenylenebis-
azo))bis(4-amino-) disodium salt) is
anionic azo dye, which was selected as a model pollutant and has been thoroughly studied. Throughout the experiment,
In photocatalytic processes, an important factor is the
(
a
benzidine-based amount of catalyst, which determines the rate of degradation
ꢁ5
because it is associated with carcinogenicity and causes great a xed dye concentration (1.0 ꢀ 10 M) and an irradiation time
harm to aquatic biota. CR has an aromatic structure that of 70 min were employed with different loadings of Fe nano-
57
provides physicochemical, thermal and optical stability. CR particles (5.0, 10.0, 15.0, and 20.0 mg in 250 mL CR solution).
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Fig. 7 (a) UV-vis spectra of the degradation of CR, (b) percentage degradation of CR dye with respect to irradiation time, (c) degradation
efficiency of CR, and (d) pseudo-first-order kinetics of degradation of CR.
Fig. 8(a) shows the effect of the amount of catalyst on the of excess catalyst and to ensure optimum absorption efficiency
degradation efficiency of Congo red dye. At a dose of Fe nano- of photons. This may be attributed to unfavourable light scat-
particles of 15.0 mg the maximum efficiency was attained in tering and hindered penetration of light into the solution in the
comparison to the other doses. It is known that in heteroge- event of excess photocatalyst loading. It can be explained on the
neous photocatalytic reactions there is an increase in photo- basis of the active sites and surface availability of the catalyst, as
59,60
degradation in proportion with the catalyst loading.
In well as the penetration of UV light into the suspension. The
general, for a given photocatalytic application it is necessary to maximum efficiency of the Fe nanoparticles was achieved at
determine the optimum catalyst concentration to avoid the use a dose of 15.0 mg, and the efficiency decreased when the dose
Fig. 8 The effects of the catalyst concentration (a), the initial dye concentration (b), and the pH (c) on the photodegradation of CR.
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irradiation time and amount of photocatalyst were kept
constant, which led to the formation of dimers and higher-order
aggregates owing to hydrophobic interactions between aromatic
rings. Hence, the rate of formation of hydroxyl and superoxide
radical anions was dramatically reduced, and thereby the rate of
degradation decreased. The adsorbed dye molecules at high
concentrations could have exerted an inhibitory effect on the
photocatalytic degradation, which was possibly due to a lack of
direct contact between photogenerated holes, hydroxyl radicals
or superoxide radicals. Moreover, the presence of high
concentrations of the dye may have acted as an inner lter that
resulted in excitation by photons away from the surface of the
Fe nanoparticles. At high concentrations, most of the dye
molecules, aer being adsorbed on the surface of Fe nano-
particles, may block the active reaction sites and might result in
60
a decrease in the photodegradation efficiency.
The nature of the dye, the surface of the photocatalyst and
the pH are essential parameter that impose limits on the extent
of degradation and greatly inuence the photocatalytic degra-
dation of dyes. Therefore, it is necessary to optimize the pH for
the complete degradation of dyes. The surface charge properties
of the photocatalyst are related to the pH of the experimental
suspension. The effects of the pH on the degradation of CR were
investigated using the optimum amount (15.0 mg) of Fe nano-
Fig. 9 Recovery and reuse of Fe nanoparticles for the catalytic
degradation of CR under UV light irradiation.
was increased further to 20.0 mg. On the basis of observations
of the catalyst loading efficiency, the catalyst loading was kept at
15.0 mg in the degradation experiments to achieve the
ꢁ5
particles at a xed concentration of CR solution (1.0 ꢀ 10 M)
maximum performance.
in the pH range of 4.0 to 10.0 (Fig. 8(c)). As the pH increases the
surface of a photocatalyst becomes negatively charged and
causes the repulsion of anionic surfaces of the dye from the
The efficiency depended on the dye concentration, which is
worth considering from the viewpoint of applications. As shown
in Fig. 8(b), the maximum efficiency for dye degradation
was observed at a pH of 4 with 15 mg Fe nanoparticles. A
degradation rate of 96.1% was achieved at a dye concentration
catalyst surface, which thus results in a corresponding decline
61,62
in the degradation of CR.
Furthermore, the magnetic Fe
nanoparticles can be easily recovered by applying a magnetic
force aer the treatment of the dye and can be recycled for
multiple treatment procedures owing to their long-term
stability. Thus, photocatalysis by Fe nanoparticles could be
used as a practical and useful technique for the removal of
carcinogenic azo dyes from wastewater, which contributes to
environmental pollution.
ꢁ
5
ꢂ
of 1 ꢀ 10 M at room temperature (30 C). It was observed that
ꢁ5
ꢁ5
for aqueous concentrations of CR of 2.0 ꢀ 10 M, 3.0 ꢀ 10
M
ꢁ5
and 4.0 ꢀ 10 M the degradation rate decreased to 80%, 71%
and 60%, respectively. The reason for the observed changes
could be linked to the adsorption of an increasing number of
dye molecules on the surface of the Fe nanoparticles with an
increase in the initial dye concentration. The UV light
Fig. 10 (a) Structure of Congo red and (b) scheme of the photocatalytic mechanism of the Fe nanoparticles under UV light irradiation.
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In a typical experiment, Fe nanoparticles were recovered by nanoparticles is environmentally safe and robust, and the cata-
adding a magnetic bar to a decolorized reaction mixture con- lytic activity of these Fe nanoparticles for the degradation of CR
taining CR and Fe nanoparticles. The solution was decanted, was non-toxic, economically viable and environmentally benign.
the magnetic bar was taken out, and the particles were These ndings highlighted the above method for the production
dispersed in double-distilled water. The resulting black of excellent photocatalysts for the degradation of azo dyes.
dispersion of particles was centrifuged and the Fe nanoparticles
were separated. The recovered Fe nanoparticles were reused ve
times for the degradation of CR under similar experimental
Acknowledgements
conditions. Fig. 9 shows that the performance of the nano- This project was funded by the Saudi Basic Industries Corpo-
particles was slightly affected aer the fourth cycle, which ration (SABIC) and the Deanship of Scientic Research (DSR) at
indicated that the as-prepared Fe nanoparticles acted as an King Abdulaziz University, Jeddah, under grant no. (S-17-342-
excellent catalyst without the loss of catalytic activity.
37). The authors, therefore, acknowledge with thanks SABIC
On the basis of the above observed results, a suitable mecha- and DSR for technical and nancial support.
nistic pathway for the degradation of Congo red dye by Fe nano-
particles in aqueous solution can be proposed. A possible
photocatalytic mechanism of the use of Fe nanoparticles for the
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