International Journal of Phytoremediation

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Microwave enhanced sorption of methylene blue
dye onto bio-synthesized iron oxide nanoparticles:
kinetics, isotherms, and thermodynamics studies
Shymaa M. Shalaby, Fedekar F. Madkour, Hala Y. El-Kassas, Adel A.
Mohamed & Ahmed M. Elgarahy
To cite this article: Shymaa M. Shalaby, Fedekar F. Madkour, Hala Y. El-Kassas, Adel A.
Mohamed & Ahmed M. Elgarahy (2022) Microwave enhanced sorption of methylene blue dye
onto bio-synthesized iron oxide nanoparticles: kinetics, isotherms, and thermodynamics studies,
International Journal of Phytoremediation, 24:9, 902-918, DOI: 10.1080/15226514.2021.1984389
To link to this article: https://doi.org/10.1080/15226514.2021.1984389
View supplementary material Published online: 07 Oct 2021.
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Microwave enhanced sorption of methylene blue dye onto bio-synthesized iron
oxide nanoparticles: kinetics, isotherms, and thermodynamics studies
Shymaa M. Shalabya
, Fedekar F. Madkoura
, Hala Y. El-Kassasb
, Adel A. Mohamedc
, and
Ahmed M. Elgarahyd,e
a
Marine Science Department, Faculty of Science, Port-Said University, Port Said, Egypt; b
Marine Hydrobiology Department, National Institute
of Oceanography and Fisheries, Alexandria, Egypt; c
Marine Chemistry Department, National Institute of Oceanography and Fisheries, Suez,
Egypt; d
Environmental Science Department, Faculty of Science, Port-Said University, Port Said, Egypt; e
Production Department, Egyptian
Propylene and Polypropylene Company (EPPC), Port Said, Egypt
ABSTRACT
To adequately address the grave human health risks and environmental damage caused by the
uncontrolled utilization of organic dyes, we greenly synthesized iron oxide nanoparticles (IONPs)
using Spirulina platensis micro-algae for sequestration of cationic methylene blue (MB) dye from
an aqueous solution. The nano-engineered sorbent was thoroughly scrutinized by different spec-
tral analyses of; FT-IR, SEM, EDX, BET surface area, TEM, VSM, UV/Vis spectroscopy, and PHPZC
measurement. The adsorption of MB was methodically carried out in a batch process to investi-
gate the effects of initial pH (2.210.4), adsorbent concentration (0.55.0 g L–1
), initial dye concen-
tration (101000 mg L–1
), contact time (0230min), and adsorption temperature (298K, 308K,
318K, and 328 K). The outlined results inferred that the maximum adsorption capacity of MB dye
by IONPs (surface area of 134.003 m2
/g, a total pore volume of 0.3715cc/g, and average pore size
of 5.54 nm) was 312.5 mg g–1
under the optimized pH value (i.e., pH ¼ 10.4). Collectively, the
adsorption kinetics profile showed that the experimental data were in good agreement with the
PSORE model, and the equilibrium adsorption isotherm data were quantitatively dominated by
the Langmuir model. The thermodynamic findings conformed to the endothermic nature of the
adsorption process. Interestingly, the proposed microwave scenario enhanced the adsorption rate
and the equilibrium was attained in a very short time (only 1 min), compared with the normal
sorption conditions (70 min). Repeatability of the spent sorbent was successfully emphasized for
5 times of adsorption/desorption cycles using 0.5 M of HCl. The productive adsorbent admirably
sequestered MB dye from spiked real specimens (83%). These results demonstrated that IONPs
can be considered as a cost-efficient adsorbent in practical applications such as wastewater
purification.
HIGHLIGHTS
Facile biosynthesis of IONPs ( 10 nm size) using Spirulina platensis micro-algae.
The prepared material shows an excellent adsorptive capacity for MB dye (Qmax ¼
312.5 mg g–1
).
Efficient modeling of kinetics, isotherms, and thermodynamics parameters.
Accelerated fast kinetics of microwave-assisted sorption (equilibrium1 min).
KEYWORDS
Bio-synthesized iron oxide
nanoparticles; methylene
blue removal; microwave
enhanced sorption; sorbent
repeatability; treatment of
real specimen
CONTACT Ahmed M. Elgarahy ahmedgarahy88@yahoo.com; ahmed.gamal@sci.psu.edu.eg Environmental Science Department, Faculty of Science, Port
Said University, Port Said, Egypt.
Supplemental data for this article can be accessed at publisher’s website.
ß 2021 Taylor Francis Group, LLC
INTERNATIONAL JOURNAL OF PHYTOREMEDIATION
2022, VOL. 24, NO. 9, 902–918
https://doi.org/10.1080/15226514.2021.1984389

IONPs can efficiently reuse up to 5 times of sorption/desorption cycles.
More than 83 % removal efficiency for MB from spiked real specimens.
Introduction
Nowadays, the wastewater disposal rate is immensely
increasing owing to our irresponsible behaviors. The grow-
ing industrial advancement and massive technological
expansion have resulted in the availability of multiple water
pollutants such as dyes, heavy metals, pesticides, insecticides,
aromatic hydrocarbons, and pharmaceuticals in water bodies
(Vigneshwaran et al. 2021). For decades, the fast develop-
ment in the dye industrial sector’s (i.e., cosmetology, medi-
cine, plastic, paper, printing, pharmaceutical industries,
perfumery, food processing, leather, varnishes, textile, and
so on) has resulted in severe environmental contamination
(Sellaoui et al. 2021). Indeed, it has been reported that the
commercial multifarious dyes are produced with a yield of
more than 7 105
tons, annually, with a global market of
$42 billion by 2021(Lin et al. 2021). The discharge of dyes-
laden wastewater into the aquatic environment is a rigorous
contributor to water pollution (Bensalah et al. 2021). The
presence of these ejected substances, even at trace concentra-
tions could be unsafe for all living creatures considering
their non-biodegradable, recalcitrant, mutagenic, and car-
cinogenic nature (Mittal et al. 2021). Moreover, the persist-
ence of the disposed of dyes and/or their decomposition
products in the water bodies seriously disturbs the ecosys-
tem. They change the chemical oxygen demand (COD), bio-
logical oxygen demand (BOD), total suspended solids (TSS),
and total dissolved solids (TDS) contents in the aquatic sys-
tems and ultimately inhibit the solar light penetration into
the water, thus retarding photosynthetic processes in the
aquatic biota (Melnyk et al. 2021).
Methylene blue (MB) (3,7-Bis(dimethylamino) phenothia-
zin-5-ium chloride, is a heterocyclic aromatic compound
with a chemical formula of C16H18N3SCl. It is one of the
most common cationic dyes, widely used in cosmetics, tem-
porary hair coloring, dyeing of cotton, silk, wool, plastics,
paper, and for medical purposes (Siciliano et al. 2021).
Considering its complex aromatic structure, it is difficult to
degrade by conventional methods. However, MB is not
highly poisonous; it is associated with several diseases in
humans and animals when exposed to a higher dosage
(5 mg kg–1
). Indeed, the contact with MB causes eye
burns, while when ingested it produces nausea, vomiting,
and diarrhea; furthermore, the inhalation route, may give
rise to dyspnea and tachycardia (Seera et al. 2021).
Moreover, it causes adverse effects such as coronary vaso-
constriction, renal and mesenteric blood flow, respiratory
distress, hemolytic anemia, painful micturition, change in
coloration of urine to greenish-blue, change in coloration of
stool, and methemoglobinemia. Besides, it also has adverse
effects of neurotoxicity on the central nervous system
(Aparicio et al. 2021).
Globally, the treatment of dye-containing effluents has
become an indispensable task. In view of that, a variety of
techniques have been employed for the removal of organic
dyes from wastewater such as coagulation (Janu
ario et al.
2021), ion exchange (Pan et al. 2019), photocatalytic degrad-
ation (Wu et al. 2021), and membrane separation (Xue et al.
2021). Among these strategies, the adsorption technique is
an effective and comfortable method for water purification
from dyes. Recently numerous adsorbents have been utilized
to capture MB dye such as sodium alginate-kaolin beads
(Marzban et al. 2021), hierarchical flower-like Na2Ti3O7
structures (Reyes-Miranda et al. 2021), thermo plasma
expanded graphite (Siciliano et al. 2021b), and polyaniline
doped citric acid- magnetic iron oxide hybrids (Alves et al.
2021). Today, the synthesis of nanoparticles (NPs) grabs
increasing attention due to their admirable optical, physical,
and chemical properties, high reactivity, and large surface
area compared with other adsorbents. Like traditional meth-
ods, several physical and chemical pathways have been used
to prepare NPs such as sol–gel method, micro-emulsion,
liquid-phase reduction, etc. Regrettably, the hazardous
chemicals reagents consumption in the mentioned methods
in addition to their high cost represents a problematic issue.
Meanwhile, the generation of toxic by-products poses a
potential hazard to the environment (Xiao et al. 2020).
In reality, to deal with the previous drawbacks during the
traditional NPs synthesis processes, innovative and emerging
green technologies conforming to the cleaner production
concepts are currently targeted to mitigate the potential
harmful impacts on the natural environment. The green
synthesis of NPs using plants parts extracts (i.e., leaf and
fruit) or biological organisms (bacteria, seaweed, yeast, and
fungi) are evolved nowadays due to its low cost, high pro-
duction yield, and environmentally benign nature (Paiva-
Santos et al. 2021). These biogenic materials contain a vast
of bioactive constituents (i.e., amino acids, alkaloids, carbo-
hydrates, polyphenols, steroids, saponins, flavonoids, terpe-
noids, proteins, vitamins, organic acids, and reducing
sugars), employed as reducing, capping, and stabilizing
agents during the NPs synthesis process (Puthukkara et
al. 2021).
This will, in turn, encompass the proper managing of the
abandoned biogenic available materials by converting them
into a valuable product to create a livable planet. The bio-
synthesis strategy of NPs is a promising route as it is in
accord with the general principles of green chemistry.
Comprehensively, employing the bio-synthesized NPs in
wastewater treatment is an emerging pathway, opens the
horizons toward removing waste by waste in a greenway
through large-scale application. The application of iron
oxide nanoparticles (IONPs) as adsorbents for different tex-
tile dyes such as Congo red and Alizarin red S dyes was suc-
cessfully reported (Koohi et al. 2021; Nodehi et al. 2021).
INTERNATIONAL JOURNAL OF PHYTOREMEDIATION 903

Therefore, the main objective of the present work was to
provide sustainable development and comprehensive utiliza-
tion scenario of natural resources of economical and com-
mercially affordable Spirulina platensis micro-algal powder
(i.e., US $5–7/ kg) to greenly synthesize IONPs and further
assess its efficacy to remove methylene blue (MB) dye as a
model adsorbate of cationic dyes from its aqueous solution.
In the first part, the as-prepared nano-engineered adsorbent
was systematically scrutinized by multiple spectral analyses
of; Fourier-transform infrared spectrometry (FT-IR),
Scanning electron microscopy (SEM), Energy dispersive X-
ray analysis (EDX), Brunauer Emmett and Teller (BET) sur-
face area, Transmission electron microscopy (TEM),
Vibrating-sample magnetometry (VSM), Ultraviolet/visible
(UV/VIS) spectroscopy and Zeta potential measurement
(PHPZC). Secondly, the adsorption behavior of IONPs sor-
bent toward the MB dye was assayed under various oper-
ational conditions. The effect of microwave heating (MWH)
technology as an attractive and rapid approach for speeding
up the MB adsorption process onto IONPs and upgrading
its selectivity was evaluated. The adsorption kinetics, iso-
therms, thermodynamics, and adsorption mechanism were
discussed in detail. In addition, the repeatability of IONPs
for consecutive adsorption, desorption, and re-adsorption
cycles was researched. The effectiveness of the IONPs for
the removal of MB from spiked real specimens
was researched.
Materials and methods
Materials
The as-used chemical reagents throughout the present work
were of standard analytical grade and were employed dir-
ectly without any further purification. Micro-algal powder of
Spirulina platensis (99.5%) was provided by National
Research Center (NRC), Cairo, Egypt. Ferric chloride hexa-
hydrate (FeCl3.6H2O, 98%), methanol (CH3OH, 99.8%),
and ethanol (C2H5OH, 99.8%) were purchased by Merck
(Germany). Methylene blue (MB) dye (100%) was provided
by Sigma-Aldrich (Darmastadt, Germany). Deionized (DI)
water (0.05 lS cm1
) was utilized for the preparation of dif-
ferent standard and working solutions. The pH values of all
dye working solutions were controlled by using 0.8 M of
diluted HCl and/or NaOH.
Green synthesis of iron oxide nanoparticle (IONPs)
Preparation of Spirulina platensis micro-algal extract
Initially, the collected micro-algal powder (MALGP) of
Spirulina platensis was thoroughly washed with running tap
water (TW) to remove fine dust particles, followed by
repeated rinsing with DI water. The MALGP powder was
air-dried for 72 h at ambient temperature (i.e., 25 ± 1
C).
Afterwards, it was placed in an oven (Gallenkamp BS Model
OV-160, Loughborough (LE), UK) at a temperature of 60
C
for 2–3 h to remove all the moisture. To prepare MALGP
extract, about 12.0 g of the MALGP was heated in 120.0 mL
of DI water with continuously stirring (150 rpm) using a
reciprocal agitator (Rota bit, J.P. Selecta, Spain) at 75
C for
1 h, until the color of the aqueous extract solution changed
to pale green. The resulting solution was cooled to room
temperature, filtered through a Whatman filter paper (diam.
45 mm), and stored in the refrigerator for further use.
Preparation of iron oxide nanoparticles (IONPs)
Typically, 30 mL of freshly prepared 0.6 M of iron (III) of
FeCl3.6H2O was slowly added to the greenish colored
MALGP extract in 1:1 volume ratio at room temperature.
The immediate color change of the solution from green to
intense black revealed the successful formation of iron oxide
nanoparticles (IONPs). The corresponding solution was left
under stirring for another 2 h and the resultant solution was
then heated to dryness on a hot plate till the formation of
black solid precipitate was noticed. The produced dark
IONPs (solid) were magnetically collected from the solution
using a neodymium magnet, rinsed four times with DI
water, and dehydrated at 70
C for 6 h before
characterization.
Preparation of MB dye solution
Stock standard solution of MB dye (1000 mg L–1
) was pre-
pared for different sorption experiments. This was obtained
by dispersing its salt in a suitable amount of DI water. The
salt mixture was left to stir (100 rpm) for approximately
20 min to ensure effective dissolution. The batch pollutant
sorption was conducted by further stepwise dilution of
stock solution.
Physical characterization of IONPs sorbent
The as-fabricated IONPs sorbent material was scrutinized
using various analytical and spectral techniques. FTIR of
IONPs was recorded using a Nicolet IS10 FT-IR (Thermo
Fischer Scientific, Waltham, MA, USA) model. The sample
was prepared by grinding the powdered nanoparticles with
KBr at a 1:100 ratio to obtain a thin film. The analyses were
carried out in the range of 400 cm1
4000 cm1
. BET sur-
face area, pore volume and pore size analyses characterized
to IONPs sorbent were determined by a Quantachrome
NOVA 3200e. The degassing was performed at 160
C for
4 h under vacuum; ramp rate was 10 deg min1
; samples
adsorbed N2 at liquid N2 temperature (77 K). Analysis of
obtained data was done using NovaWin software (v11.0)
(Quantachrome Instruments, Boynton Beach, FL, USA). To
analyze the structure of IONPs particles, SEM coupled with
EDX analysis system (Jeol Ltd.; JSM-6510LV, Tokyo, Japan)
was proceeded to monitor the morphological features and
elemental content of IONPs sorbent. TEM analysis (TEM-
2100HR, JEOL, Tokyo, Japan) was used to perform the
ultrahigh-resolution scrutinizing of the biosynthesized
IONPs. Magnetization behavior examination of IONPs sor-
bent was employed by using the VSM tool (VSM, PMC
MicroMag 3900 model, Princeton, NJ, USA). The Surface
904 S. M. SHALABY ET AL.

Plasmon Resonances (SPR) of IONPs was measured by UV-
VIS double-beam (JENWAY 6800 UV/VIS) at a wavelength
range of 350–800 nm. Zeta potential measurement (pHPZC)
of IONPs sorbent was recorded using the pH-drift method-
ology. Proper amounts of the sorbent were blended with
0.1 M of NaCl solutions with previously adjusted initial pH
(pHi) values (i.e., from 1 to 11). After 24 h, the equilibrium
pH (pHeq) values were notated. The subtracting results
between pHi and pHf values (DpH) were graphically charted
against pHi values. The pH value of point zero charges
(pHPZC) was computed from the intersection dot of the rep-
resented curve at which equals zero.
Sorption assay experiments
A batch sorption scenario was conducted in singe MB dye
solutions to assess the impact of different operational
parameters on the sorption of MB by IONPs. Sorption
experiments were performed by changing one of the set
parameters and keeping the others fixed. A set of experi-
mental runs were carried out in stoppered Erlenmeyer
flasks (50 mL).
To specify the impact of initial solution pH (pHi) on the
sorption process, 0.03 g of IONPs sorbent was immersed in
a given concentration of 20 mL of MB solutions (C0: 100 mg
L–1
) with pre-adjusted pH values ranging from 2.210.4
using 0.8 M of diluted HCl and/or NaOH. The examined
solutions were continuously stirred at the following pre-
adjusted conditions of temperature (T) ¼ 25 ± 1
C, contact
time (t) ¼ 100 min, and stirring speed (SS) ¼ 200 rpm. The
pH was not maintained during the sorption process but the
equilibrium pH (pHeq) was systematically registered using a
pH meter (Aqualytic AL15). The value of pHPZC can be esti-
mated by plotting DpH (pHeq – pHi) versus pHi. Hereafter,
the MB-laded sorbent was magnetically gathered from the
solution and 5 mL of the withdrawn supernatants were ana-
lyzed for the residual MB concentration at kmax of 665 nm,
using a Palintest 7100 spectrophotometer (Palintest, Ltd.,
Gateshead, UK) (Ayouch et al. 2021). The sorbed amount of
MB dye per IONPs mass at time t, qt (mg g–1
), equilibrium
time, qe (mg g–1
), and the removal efficiency (R%) was
investigated from Eq. (1), Eq. (2) and Eq. (3), respectively.
The influence of sorbent concentration (solid: liquid
ratio) was explored by altering the IONPs concentration
from 0.01 to 0.1 g with 20 mL of MB dye solutions at (C0 ¼
100 mg L–1
, T¼25
C ± 1, t¼100 min, and SS¼200 rpm).
After equilibrium, the remaining MB dye concentrations
were spectrophotometry quantified.
The influence of contact time was proceeded by mixing
0.3 g of IONPs with 200 mL of MB solutions (C0: 100 mg
L1
, T¼25 ± 1
C, t¼230 min, and SS¼200 rpm). At stipu-
lated time intervals, samples (5 mL) of MB dye solutions
were periodically withdrawn and the MB concentrations
C(t)(i) (mg L–1
) was determined, considering the decrement
in the solution volume.
The influence of primary MB concentration was carried
out by contacting 0.03 g of IONPs with 20 mL of MB solu-
tions of various primary concentrations (C0: 101000 mg
L–1
) at (T¼25 ± 1
C, t¼100 min, and SS¼200 rpm).
Utilization of sorption sites (UOS%) throughout the sorp-
tion process can be determined by the given Eq. (4).
The influence of temperature was studied by blending
0.03 g of IONPs with 20 mL of MB solutions (C0 ¼ 100 mg
L–1
, t¼100 min, and SS¼200 rpm) at different environmental
temperatures (i.e., 298 K, 308 K, 318 K, and 328 K) in a shak-
ing incubator (LSI-3016R, LabTech S.r.l., Sorisole
(BG), Italy).
The influence of competitors ions (herein NaCl) as a cor-
responding effect on the sorption process was implemented
by adding various concentrations of NaCl (545 g L–1
) into
20 mL of MB solutions (C0: 100 mg L1
) in the presence of
0.03 g of IONPs sorbent at (T¼25 ± 1
C, t¼100 min,
and SS¼200 rpm).
The influence of microwave heating (MWH) radiation on
the MB sorption process was tested using a domestic MW
apparatus (CLATRONIC MWG 756 E) with technical speci-
fications of 800 watts, 2.45 GHz, and 20 L for designed
power supply, frequency, and cavity volume, respectively.
Firstly, the catalytic degradation of MB dye solution under
individual MWH radiation was studied by subjecting 20 mL
of dye solution (C0: 100 mg L1
) to MWH radiation without
as-prepared IONPs for 1 min (to avoid water gasification) at
maximum rated power supply. The degradation efficiency
(%) of dye was calculated using Eq. (5).
The effect of MWH power supply intensity on the sorp-
tion process was studied by contacting 0.03 g of IONPs with
20 mL of MB solutions at different intensities from 144–800
watts for 1 min. After equilibrium, the remaining MB dye
concentrations were spectrophotometry quantified.
The influence of MWH time as a function of kinetics on
the MB sorption process was proceeded by mixing 0.03 g of
IONPs with 20 mL of MB solutions (C0: 100 mg L1
) at dif-
ferent pre-set periods from 5 to 60 s.
qt ¼ C0 Ct
ð Þ
V
m
(1)
qe ¼ C0 Ce
ð Þ
V
m
(2)
R ¼
C0 Ce
C0
100 (3)
UOS% ¼
qexp
qcal
100 (4)
Degradationefficiency %
ð Þ ¼
Co Ce
ð Þ
Co
100 (5)
C0: initial MB dye concentration (mg L–1
),
Ct: MB dye concentration at time t (mg L–1
),
Ce: equilibrium MB dye concentration (mg L–1
),
V: volume of MB dye solution (L),
M: IONPs adsorbent mass (g).
qexp: the experimental maximum adsorption capacity
(mg g–1
),

qcal: the calculated maximum adsorption capacity
(mg g–1
).
Kinetics modeling analysis
The kinetic premise (time-dependent) process is very valu-
able for the perception of the whole sorption process scen-
ario, necessary for the modeling of wastewater treatment
plants. Plainly, they depict insights about the sorption
pathway, sorption rate, equilibrium time, and potential
rate-controlling step during solid and liquid interface. In
light of this, four commonly applied kinetics models
including; pseudo first-order rate equation (PFORE),
pseudo second-order rate equation (PSORE), intra-particle
diffusion (Weber and Morris model; WM), and Elovich
model were used to disclose the best-fitted kinetics model
describing the adsorption process (Kalhori et al. 2017;
Mohebali et al. 2018; Nodehi et al. 2020).
Isothermal modeling analysis
Isothermal studies are beneficial tools to clarify the nature
of the interaction between sorbent and sorbate. They con-
clude sorbent’s affinity, maximum sorption capacity, sorp-
tion shape (monolayer/multilayer), and isotherm type. Three
prominent models, namely Langmuir (LAM), Freundlich
(FR), and Temkin (TK) were mathematically compared to
match the experimental outcomes (Ali et al. 2019; Mohebali
et al. 2019). LAM model hypothesizes that sorbate homoge-
nously distributes on the sorbent’s receptors sites as each
site can individually accommodate one sorbate (monolayer
coverage). Once all receptor sites get saturated with sorbate,
no longer substantial competition or interaction occurs
between the sorbed species (Huang et al. 2021).
Furthermore, a dimensionless separation parameter, RL, is
an indicative factor to comprehend the sorption process
favorability. It can be defined by Eq. (6):
RL ¼
1
1 þ KLC0
(6)
RL: factor signifies the nature of sorption process,
KL: the LAM equilibrium constant.
The nature of adsorption process depends mainly on the
calculated RL value; linear (RL¼ 1), favorable (0 RL 1),
unfavorable (RL 1), and irreversible (RL¼ 0).
Contrarily, the FR model infers the multilayer coverage
and describes the occurrence of the non-uniform heat sorp-
tion process onto heterogeneous surfaces resulting from the
dropping in binding strength associated with an enhance-
ment in the occupation degree of receptors sites (steric hin-
drance) (Rathika and Raghavan 2021). Finally, the TK
model is another isothermal model that mainly suggests the
linear decrement of sorption energy over the exponential
decline as stated by the FR model. Additionally, sorbent
exhaustion is also taken into account, after the completion
of the sorption process (Ngabura et al. 2018).
Linearized and non-linearized forms of the kinetics and
isothermal models used in the present study are listed in
Table 1.
Thermodynamics analysis
Thermodynamic considerations are essential tools for the
plausible and conspicuous understanding of the adsorption
process. Detailed insights related to sorption energetic
changes can be illustrated regarding thermodynamic param-
eters (Kasperiski et al. 2018). Change in free energy of sorp-
tion (DG), change in Gibbs free energy (DGo
), change in
entropy (DSo
), and change in enthalpy (DHo
) were explored
by using the given equations, from Eq. (7) to Eq. (12). The
values of different thermodynamics functions for MB sorp-
tion on IONPs sorbent were determined by plotting ln Kc
against 1/T.
Table 1. Linear and non-linear forms of adsorption kinetics and isotherms models used in this study (Shayesteh et al. 2020).
Kinetic models Non-linear form Linear form Parameter Definition
Pseudo-first order qt ¼ qe ½1 e k1 t
logðqe qtÞ ¼ logqe k1
2:303

t qe, (mg g–1
) Adsorption capacity at equilibrium time.
k1 (L mg1
) Rate constant of the pseudo-first-order
kinetic model.
Pseudo-second order qt ¼ k2 t
1 þ k2 qe t
t
qt
¼ 1
k
2
q2
e
þ 1
qe

t qe (mg g–1
) Adsorption capacity at equilibrium time.
K2 (g mg1
min1
) Rate constant of pseudo-second-order
kinetic model.
Intraparticle diffusion – qt ¼ kit0.5
þ X Ki (mg g1
min–0.5
) The intraparticle diffusion constant.
X (mg g1
) Thickness of boundary layer.
Elovich equation dqt
dt ¼ aebq
qt ¼ 1
b lnab þ 1
b lnt a (mg g1
min1
) The initial sorption rate.
b (g mg1
) The desorption constant.
Isotherm models Non-linear form Linear form
Langmuir qe ¼
qm;L KL Ce
1 þ KL Ce
Ce
qe
¼ Ce
qm;L
þ 1
KLqm;L
qm (mg g1
) Langmuir maximum sorption capacity.
KL (L mg1
) Langmuir constant related to the
energy of adsorption
Freundlich qe ¼ KF C1=n
e lnqe ¼ lnKf þ
1
n
lnCe
Kf (mg g1
) (L mg1
)1/n
Freundlich constant relative sorption capacity.
n Freundlich constants related to the sorption
capacity and intensity.
Temkin
qe ¼
RT
bT
½lnðAT CeÞ qe ¼
RT
bT

lnAT þ
RT
bT

lnCe
B (kJ mol1
) Temkin constant related to heat of sorption.
A (L g1
) Temkin equilibrium isotherm constant.

Kc ¼
CS
Ce
(7)
DG ¼ DGo
þ RTlnKc atconstantT
ð Þ (8)
DGo
¼ RTlnKc atequilibriumstage
ð Þ (9)
lnKc ¼
DG8
RT
(10)
DGo
¼ DHo
TDSo
atconstantT
ð Þ (11)
lnKC ¼
DH8
RT
þ
DS8
R
(12)
Cs: the sorbed equilibrium concentrations of MB on the
IONPs surface,
Kc: the equilibrium constant,
T: working temperature,
DGo
: change in Gibbs free energy,
DSo
: change in entropy,
DHo
: change in enthalpy.
Reusability studies
In general, sorbent stability and reusability are extremely
crucial features as they delineate the possibility of sorbent to
be undergone with recovery process throughout the long-
term utilization in industrial application. In a view of this,
the MB sorption/desorption cycles were employed for 5
cycles. Typically, 0.03 g of IONPs was immersed in a given
concentration of 20 mL of MB solution (C0: 20 mg L1
)
under constant stirring (200 rpm) at 25 ± 1
C for 100 min.
After solid/liquid separation, the MB concentration in super-
natant liquid was monitored. The spent MB-loaded sorbent
was rinsed with DI water to remove any unsorbed MB dye
and then purged with an acidic desorbing agent of 0.5 M
HCl (9 mL) for 30 min. Finally, the regenerated sorbent was
rinsed again with DI water and dried at 40
C for 2 h to pro-
ceed in the next cycle. Repeatedly, 5 runs of sorption/
desorption were performed and the desorption efficacy
(DE%) was computed using the following formula as pre-
sented in Eq. (13):
DES %
ð Þ ¼
AmountofdesorbeddyeðmgÞintotheelutionsolution
AmountofsorbeddyeðmgÞ
100
(13)
Feasibility of IONPs for disposal of MB dye from
real samples
The discerned extrapolation of IONPs to be admitted in
real wastewater treatment plants is a critical aspect for
judging its real sorption capacity. The existence of several
constituents (i.e., inorganic and organic matter) can affect
on the adsorption process. Applicability and selectivity of
IONPs adsorbent to remove MB dye from real TW sam-
ples was attained. Four samples were collected from
Water Supply facilities at Port-Said, Egypt. The physico-
chemical characterization of TW samples was registered
in (Table S1, see Supplementary Material). Briefly, the
tested samples were spiked with a gradient concentration
of MB dye and sorption experiments were achieved by
individually immersing with 0.03 g of IONPs adsorbent
with 20 mL of the tested samples at (T¼25 ± 1
C,
t¼100 min, and SS¼200 rpm). Tested samples were centri-
fuged and 5 mL of supernatants were analyzed for the
residual MB concentrations. All sorption tests were per-
formed in triplicate, and the averages were recorded. The
limit of experimental errors on triplicates was systematic-
ally below 5%.
Results and discussions
Green synthesis mechanism of IONPs using MALGP
In general, the green synthesis of IONPs is considered an
environmentally sustainable approach, can be achieved by
using biocompatible biological sources (i.e., algae, bacteria,
yeast, plants, and fungi). They are rich in a vast of bioactive
compounds majorly contributing to the reductions of iron
ions. These phytochemicals have a dual role by simultan-
eously proceeding as reducing and capping (stabilizing)
agents during the IONPs synthesis process. Firstly, metal
ions are produced by treating the iron precursor with the
biological constituent (reduction). This is followed by the
creation of a nucleation center which consequently seques-
ters the rest of metal ions and integrates the neighboring
nucleation site. The end product of the mentioned reaction
is the IONPs. The size, growth, and morphology of IONPs
can be controlled considering the nature of bio-active com-
ponents (Vasantharaj et al. 2019). Table S2 (see
Supplementary Materials) presents the different IONPs for-
mation conditions with their maximum absorbance values.
The best synthesis conditions (utmost absorbance) were con-
tinued during the present work.
Figure 1. FT-IR spectra of (a) iron oxide nanoparticles (IONPs) sorbent before
sorption of MB dye, and (b) after MB sorption.

Morphological and structural characterization of
IONPs sorbent
A detailed characterization of the adsorbent was already
published (Shalaby et al. 2021). Complementary information
is presented in the supplementary material section (sec-
tion I).
Generally, the FT-IR spectrum is employed to identify
the involved functional groups in the sorption process.
Normally, the molecules are vibrating and once they absorb
photons from an appropriate energy source, and thus
change their vibration (Khodabandehloo et al. 2017). The
FT-IR spectral data of Spirulina platensis, biosynthesized
bare IONPs and loaded sorbent with MB dye is displayed as
an overlay graph in Figure 1. The broad characteristic
absorption band 3606.92 cm1
is originally linked with O-H
stretching vibration of alcohol, phenols, and carboxylic acids
found in polysaccharides, proteins or polyphenols. A Small
signal band around 2923.12 cm1
belongs to C-H stretching
bond in the methyl group (Khodabandehloo et al. 2017).
The detected peak at 1713.22 cm1
is assigned to C¼O
stretching vibration of (-COOH). A weak peak at
1107.9 cm1
is related to N–H stretching of aliphatic amines
and 1055.66 cm1
is associated with C–O–C stretching of
cellulose. Vibration peak at 876.48 cm-1
associates
with¼C–H group (Asghar et al. 2018; Z. Pan et al. 2019).
Numerous weak peaks in the range of 400850 cm-1
(i.e.,
783.92 cm1
, 723.1 cm1
, 623.8 cm1
, 556.3 cm1
, and
496.5 cm1
) confirm the synthesis of IONPs sorbent attrib-
uted to stretching of the metal–oxygen (Fe-O) group
(Shayesteh et al. 2017). These findings are similar to those
recorded by (Bishnoi et al. 2018; Rahmani et al. 2020).
After loading of IONPs with MB dye, a new peak appeared
at 1600.0 cm1
, 1394.3 cm1
, and 1336.21 cm1
are parallel
to stretching vibrations of –C¼C–, –C¼N–, and –C–N–,
respectively, while the other peak at 884.3 cm1
refers to
out-of-plane ring (¼CH) (Sakin Omer et al. 2018).
Confirmatory, this reveals the richness of IONPs adsorbent
with numerous functional groups involved in the MB
adsorption process.
The surface morphology details of IONPs sorbent before
and after MB dye sorption was illustrated in Figure 2. It can
be seen that its original surface relatively exhibits a distinct
irregular geometry, rugged and full of abundant protrusions.
A large number of folding and grooves forms characterized
to sorbent surface can be observed provides an enhanced
surface area and offers facile accessibility for the MB mole-
cules to interact with IONPs sorbent surface. This improved
shape agreed with the expected structure allows accelerated
and greater sorption of MB dye molecules onto the IONPs
surface. After MB dye sorption, an organized fashion of dye
molecules crumpling was homogenously noted on the sorb-
ent’s surface without any observable agglomeration.
Elemental survey conjecture (EDX spectrum) of IONPs sor-
bent was conducted to affirm its successive synthesizing pro-
cess. The quantitative findings presented that its main
constituents are Fe, O, Cl, C, and N as portrayed in
Figure 2. This agreed with the structural composition of the
sorbent’s precursors. Whereas, the elevated characteristics
peaks of C and N elements besides the appearance of new
recognizable peaks of Na and S elements, is in accord with
the chemical structures of MB dye, and largely supports its
sorption onto IONPs sorbent.
Figure 2. SEM – EDX analyses of (a) IONPs before sorption of MB dye, and (b) after MB sorption.

Impact of variable operational parameters
Influence of initial solution pH
Solution pH has a remarkable role throughout the sorption
process as it significantly governs the acidity and/or alka-
linity of the sorption medium, sorbent surface charge, and
the chemical speciation’s of pollutants present in aqueous
solution (Rigueto et al. 2020). The pH dependence of MB
sorption process onto IONPs sorbent was monitored in a
wide tolerated range of pHi ranging from 2.2 to 10.4. The
strong dependency of MB sorption onto IONPs sorbent
can be illustrated as shown in Figure 3(a). Distinctly, the
equilibrium sorption capacity was relatively low 38.6 mg
g–1
(R% ¼ 58.0%) in the acidic medium (i.e., pHi ¼ 2.2)
and gradually increased along with an enhancement in the
pHi up to 62.0 mg g–1
(R% ¼ 93.0%) at pHi of 10.4. It is
clear that the poor sorption capacity of IONPs sorbent
toward the MB dye at acidic conditions can be attributed
to an increment in the concentration of hydrogen ions
(Hþ
), which compete with MB dye molecules to be sorbed
on the sorbent active sites (unfavorable sorption). In add-
ition, the protonated sorbent surface in the mentioned con-
ditions restricted the sorption of MB molecules on its
surface (interionic repulsive forces). More specifically, MB
dye has different protonated species in the range of pH
from 0 to 6, as stated by Sousa et al. (2019) (Sousa et al.
2019). At pH 1, MB affords three cationic charges on
the N atom (2 lateral groups and thiazole ring). In the pH
range of 12, trivalent cations can be found side by side
with two divalent cations (2 lateral N and thiazole/lateral N
group). Between pH of 23, divalent cationic species pre-
dominately (2 lateral N groups) presents in the medium
with a noticeable surge of monovalent cationic species (one
lateral N group protonated). At pH 4, the monovalent
cationic species uniquely exist in the solution (Elwakeel et
al. 2021). Comprehensively, the decline in cationic charges
held by the dye with an increase in the solution pH directly
results in an enhancement in the IONPs sorption capacity.
Contrarily, with going up in the solution pH, the sorption
capacity of IONPs increased (favorable sorption), which
can be hypothesized to deprotonation of IONPs surface
which effectively facilitated the sorption of MB on the
negatively charged sorbent surface via electrostatic attrac-
tion forces (Rahman-Setayesh et al. 2019). Meanwhile,
sorption of MB onto as-used sorbent is strongly associated
with pHPZC value. Commonly, it is defined as a value at
which the amounts of positive and negative charges are the
same (neutral charge). The acidity/basicity of sorbent sur-
face is very relative to the pHPZC value whereas it is posi-
tively charged when pH pHPZC and vice versa. As
illustrated in Figure 3(b), it can be concluded that pHPZC
of IONPs is nearly to be 7.2, which attracts the cationic
MB to its surface (electrostatic interaction) (Xiao et al.
2020). Notably, the considerable affinity of IONPs sorbent
toward MB in unfavorable conditions probably means that
other mechanisms may be involved in the MB adsorption
process, as discussed later in the section “Sorption mecha-
nisms of MB dye onto IONPs sorbent.”
Influence of sorbent dosage
Investigating the influence of applied sorbent dosage is a
potential parameter to be considered during the evaluation
of sorption technology. To identify the optimal sorbent
dose, a series of IONPs different doses ranging from 0.5 g
L–1
up to 5 g L–1
were conducted to eliminate MB dye from
an aqueous solution. As presented in Figure S5 (see
Supplementary Material), the uptake profile of IONPs
toward MB dye clearly declined from 166.4 mg g–1
to
20.3 mg g–1
with an intensifying in the applied IONPs dos-
age from 0.5 g L–1
up to 4.5 g L–1
and then slightly declined
to 18.32 mg g–1
with further increment in sorbent dose to
5 g L–1
. Whereas, a sharp climb (improvement) in R% of
MB dye from 83.2% to 91.7% was facilely accompanied with
an augmentation in sorbent dose. This can be assigned by
providing (supplying) a greater surface area which relates to
an enrichment in the existent adsorptive receptor sites avail-
able to combine with MB dye molecules (Shayesteh et al.
2016). Furthermore, a palpable inhibition in the R% of MB
dye with excessive sorbent dose (beyond 4.5 g L–1
) is rela-
tively recognized because of the unavailability of adsorptive
receptor sites resulting from their agglomeration (overlap-
ping) and thus constrains MB molecules from reaching the
sorbent receptor sites (Marrakchi et al. 2020). Thus, it is
noteworthy that the supposed sorbent dose of 1.5 g L–1
was
sufficient to improve the MB sorption process in line with
the lowest manufacturing cost and highest productivity.
Figure 3. (a) Sorption capacities (mg g–1
) removal% of IONPs for MB dye as a
function of pHi, and (b) Graph of DpH (pHf – pHi) against initial pH (pHi) from
MB sorption (pHi ¼ 2.2–10.4, C0 ¼ 100 mg L–1
, T¼25 ± 1
C, t¼100 min,
m¼0.03 g, V¼20 mL, and SS¼200 rpm).

Sorption kinetics (conventional microwave heat-
ing) analyses
Generally, kinetics models enable the explication of adsorp-
tion pathways such as residence time, sorption rate, and
rate-controlling step. Exploring the impact of exposure (resi-
dence) time on the sorption of MB dye using IONPs is help-
ful to realize the nature of the sorption process. As declared
in Figure 4(a), the sorption profile of MB dye tempestuously
increased up to 40% of the whole sorption process within
only 10 min and then it gradually slow down till reach to
equilibrium stage with prolonged exposure time up to
230 min and no more significant sorption of MB was
noticed beyond the mentioned saturation time. The quick
increment of MB sorption rate in the first stage can be
assigned to the availability of more vacant receptor sites and
a high concentration gradient between sorbent receptor sites
and bulk solution (Prajapati and Mondal 2021). Whereas,
the recounted immovable MB sorption behavior in the
second stage is probably credited to the regular occupation
of IONPs sorbent by the captured MB dye until achieving
the complete equilibration. Additionally, the tardy saturation
issue can be interpreted by the created electrostatic static
repulsive forces between the sorbed MB dye molecules on
IONPs sorbent and dye molecules present in the aqueous
solution (Shayesteh et al. 2021).
For better manifesting of MB sorption dynamics consid-
ering sorption time, PFORE, PSORE, WM, and Elovich
models were adopted to fit the kinetic data and hence clarify
the regulation of MB onto IONPs sorbent (Jabli et al. 2020).
PFORE expresses that the change in pollutant’s concentra-
tion is conjunctionally governed with sorption reaction time.
Whereas, PSORE assumes that the chemisorption theory is
the main controller of the sorption process, whereby occurs
by sharing/ exchange of electrons (valency forces) between
sorbent and sorbate in form of covalent forces/ion exchange,
respectively (Rigueto et al. 2020). The fitted graphs are
declared in (Figure S6(a), and 6(b), see Supplementary
Material), whereas the respective parameters were declared
in Table 2. By considering the measured R2
values, the
studied MB sorption could be well described by PSORE.
Moreover, the closeness between experimental and estimated
sorption capacities values majorly elucidates that the MB
sorption process seems to be driven by the chemisorp-
tion mechanism.
More specifically, the entire sorption process comprises
of four stages; (i) external (film) diffusion - the sorbate
(molecules/ions) transport from the bulk solution to sorbent
surface, (ii) inward (intra-particle) pore diffusion – the sor-
bate diffuses from the external surface of sorbent to its
interior pores, and (iii) surface chemical interaction – the
sorbate binds with the sorbent’s receptor sites (Melo et al.
2018). In order to develop an envisioning related to the
mechanism of the sorption process and its governing step,
the intraparticle diffusion model was implemented as dem-
onstrated in (Figure S6(c), see Supplementary Material). As
the sorption data plot didn’t exhibit a straight line passing
through the origin, this summarized that the intraparticle
diffusion step isn’t the sole governing step, maybe others
(film diffusion and/or surface chemical interaction) steps
can be suggested. Boundary layer thickness (X) plays a dom-
inant role in the sorption process. The determined X values
reflect their great impact on the sorption process as
Figure 4. Uptake kinetics of MB dye sorption onto IONPs under; (a) conven-
tional heating conditions (pHi ¼ 10.4, C0 ¼ 100 mg L–1
, T¼25 ± 1
C,
t¼0–230 min, m¼0. 3 g, V¼200 mL, and SS¼200 rpm), and (b) microwave heat-
ing conditions (pHi ¼ 10.4, C0 ¼ 100 mg L–1
, T¼25 ± 1
C, t¼0–60 s, m¼0. 03 g,
and V¼20 mL).
Table 2. Uptake kinetics parameters for the sorption of MB dye onto IONPs
sorbent under normal conditions (pHi ¼ 10.4, C0 ¼ 100 mg L1
, T¼25 ± 1
C,
t¼0–230 min, V¼200 mL, m¼0.3 g, and SS¼200 rpm).
Kinetic models MB dye
Pseudo first order model
k1(min1
) 0.03915
R2
0.9767
Pseudo second-order model
k2 (g mg1
min1
) 0.001035
qe (mg g1
) 59.52
R2
0.9979
Intraparticle diffusion model
Ki (mg g1
min0.5
) 8.325
6.323
0.4350
X (mg g1
) 5.426
1.605
48.50
R2
0.9599
0.9903
0.7734
Elovich model
a (mg g1
min1
) 10.91
b (g mg1
) 0.09228
R2
0.9556

presented in Table 2. Finally, the Elovich model states that
the heterogeneous nature of sorbent’s receptors’ sites has led
to noticeable variations in their sorption energies. A linear
plot of qt versus lnt was depicted as appeared in
(Figure S6(d), see Supplementary Material). The ascribed
results of initial sorption rates (a) and desorption constants
(b) were 10.91 mg g–1
min–1
and 0.09228 g mg–1
, respect-
ively, corroborates the high affinity of IONPs sorbent
toward MB dye. Among the conventional heating (CH)
methods, the microwave heating (MWH) approach is a
promising, unique, and simple separation process that repre-
sents faster methodology by multi-folding the pace of chem-
ical reactions. Particularly, three effects of (i) thermal, (ii)
specific excitation, and (iii) non-thermal are generated inside
the working specimen as a result of exposure to MWH radi-
ation (Elgarahy et al. 2019). The first and second ones play
a prominent role in water decontamination by producing
hydroxyl radicals (OH) which are efficacious in pollutants
degradation. The last one can concurrently enhance the
motional levels of molecules, debilitating the chemical stabil-
ity of pollutants and easily overcoming diffusional limits
throughout the treatment process. Other effects associated
with MWH radiation such as superheating, polarization,
spin alignment and nuclear spin rotation strongly support
the efficiency of the removal process (Mahmoud et al.
2018a). As seen in Figure 4(b), 1 min was sufficient to
achieve the equilibrium state by robustly enforces MB mole-
cules (additional energy) toward the IONPs sorbent, com-
paring with the CH technology. The MWH kinetics in terms
of PFORE, PSORE, WM, and Elovich models was studied
as presented in (Figure S7, see Supplementary Material). By
analyzing the data derived from the MWH scenario (Table
3), the resultant values of K2, R2
was 0.046 g mg–1
min–1
,
0.982, indicated that the sorption rate mainly correlated
with the PSORE model (chemisorption). Plainly, the half-
sorption time (t1/2) highlights the powerful effects of MW
radiation on the sorption process. Simply, it is the amount
of time retained by the sorbent to attain 50% of saturation.
The t1/2 was found to be about 0.26 s and 15.46 min for MW
and CH heating methods, respectively.
Catalytic degradation of MB under MWH technology
The individual effect of MWH radiation on the extent of
MB decolorization was investigated. Mostly, the degradation
efficiency (%) of MB dye can be measured by comparing the
differences in MB concentrations prior and post to exposure
to MW radiation. The degradation efficiency (%) of 9.29%
was monitored as an utmost value for MB catalytic degrad-
ation of MB dye. Primarily, the degradation process relies
on the produced (OH) resulting from MWH radiation and
their posterior interaction with MB molecules (Elwakeel et
al. 2020). The depressed MB degradation efficiency com-
pared to its counterpart in the presence of IONPs sorbent
revealed that MB sorption is mainly dependent on the struc-
tural features of IONPs sorbent.
Sorption isotherm analysis
Apparently, sorption isotherm studies is necessary to deliver
information relating to the surface characters of sorbent, its
affinity toward the studied polluters, maximum sorption
capacity and the proposed sorption mechanism. As dis-
played in Figure 5, it was observed that the sorption rate of
MB onto IONPs sorbent dramatically escalated with an
expanding up in the density of MB dye molecules. At lower
MB concentration, the sorption sites of sorbent’s surface
weren’t fully interacted with MB dye molecules, whereas
they gradually saturated with an increase in the primary
concentration of MB up to the saturation point. The
recorded ascending adsorptive capacities at elevated primary
MB concentration are severely anticipated to the developed
collisions and the accelerated interfacial mass transfer
(migration) between the higher MB concentrations and the
definitive receptor sites on IONPs sorbent IONPs (Noreen
et al. 2020). This adsorptive performance complies with the
obtained findings of the sorption of MB onto biochar
derived from the fruit seed of Cedrela odorata L (Subratti et
al. 2021).
Table 3. Uptake kinetics parameters for the sorption of MB dye onto IONPs
sorbent under microwave conditions (pHi ¼ 10.4, C0 ¼ 100 mg L–1
,
T¼25 ± 1
C, t¼0–60 s, m¼0. 03 g and V¼20 mL).
Kinetic models MB dye
Pseudo first-order model
k1(min1
) 5.185
R2
0.9448
Pseudo second-order model
k2 (g mg1
min1
) 0.04669
qe (mg g1
) 79.36
R2
0.9825
Intraparticle diffusion model
Ki (mg g1
min0.5
) 118.6
42.47
11.50
X (mg g1
) –18.23
21.79
45.67
R2
0.9556
0.9633
1.000
Elovich model
a (mg g1
min1
) 596.5
b (g mg1
) 0.05433
R2
0.9810
Figure 5. Sorption isotherm of MB dye sorption onto IONPs (pHi ¼ 10.4, C0 ¼
10–100 mg L–1
, T¼25 ± 1
C, t¼100 min, m¼0.03 g, V¼20 mL and SS¼200 rpm).

In order to elaborate the sorption behavior of MB onto
IONPs, LAM, FR and TK isotherm models were employed
to predict the mechanism of the sorption process. Linear
plots of the mentioned as-used isotherm models are sum-
marized in (Figure S8, Supplementary Material), whereas
their corresponding parameters were listed in Table 4.
Referring to the derived outcomes, the justifiable MB sorp-
tion process perfectly tended to be described by with LAM
model. The maximum sorption capacity of IONPs was
reported to be 312.5 mg g–1
. Additionally, the calculated val-
ues of RL for MB dye were in the range of 0.040.8, sug-
gesting their favorability to be sorbed onto IONPs sorbent
obeyed the mentioned model. This was strongly supported
by the calculated value of UOS% (98%) for IONPs toward
the MB dye.
Sorption thermodynamics
The effect of temperature domain (i.e., 298 K, 308 K, 318 K,
and 328 K) on the interaction between MB dye and IONPs
was investigated to comprehend the nature of the adsorption
process by keeping the other operational parameters of pH,
adsorbent dosage, and MB concentration constant.
Generally, the outlined results revealed that the adsorption
capacity of IONPs increased with temperature. This implied
that the adsorption of MB dye onto IONPs is more feasible
at higher temperatures, which is correlated to a decrease in
the solution viscosity and an increase in the kinetic energy
of MB molecules, which in turn enhances the diffusion of
dye molecules into the IONPs adsorbent’s pores (Rajumon
et al. 2019).
The various thermodynamic parameters such as DGo
,
DSo
, and DHo
were also evaluated for the adsorption process
of MB onto IONPs (Table 5). Their values were determined
by plotting ln Kc against 1/T (Figure 6). The negative mag-
nitudes of DGo
(i.e., 4.840, 5.201, 5.563, and 5.924 kJ
mol–1
) at different running temperatures seriously meant
that MB sorption onto IONPs sorbent was spontaneous in
nature. The gradual decrease in the DGo
with a rise in tem-
perature indicates an acceleration in the MB adsorption pro-
cess from 298 to 328 K. This trend also depicts the feasibility
of adsorption at higher temperatures. Moreover, the positive
value of DHo
(i.e., 5.933 kJ mol–1
) signifies that the reaction
is endothermic. Similarly, the positive signs of DSo
(i.e.,
0.03615 kJ mol–1
K–1
) justified the enhancement in the sys-
tem randomness during the solid-liquid interface (Chen et
al. 2019).
Influence of MW power intensity
It is critical to investigate the influence of various micro-
wave power intensities on the sorption process. It was obvi-
ous that there is a direct relationship between the sorbent
capacity and MW power intensity as displayed in Figure 7.
With an increase in the MW power intensity from 144 watts
(20%) to 800 watts (100%), the sorbent capacity enhanced
from 57.5 mg g–1
(86.3%) to 60.6 mg g–1
(90.9%). This
improvement in the sorption capacity can be attributed to
the synergistic effects between interior and volumetric
Table 4. Isothermal parameters for the sorption of MB dye onto IONPs sor-
bent (pHi ¼ 10.4, C0 ¼ 10–1000 mg L–1
, T¼25 ± 1
C, t¼100 min, m¼0.03 g,
V¼20 mL, and SS¼200 rpm).
Isothermal models MB dye
Langmuir
KL(L mg1
) 0.01982
qexp (mg L–1
) 306.6
qm (mg L1
) 312.5
R2
0.9840
Freundlich
n 1.887
Kf (mg g1
) (L mg1
)1/n 13.30
R2
0.9695
Temkin
A (L mg1
) 0.5425
B (J mol1
) 52.06
R2
0.9464
Figure 6. Van’t Hoff plots for MB dye sorption onto IONPs (pHi ¼ 10.4, C0 ¼
100 mg L–1
, T¼25–55
C, t¼100 min, m¼0.03 g, V¼20 mL, and SS¼200 rpm).
Figure 7. Sorption capacities (mg g–1
) Removal% of IONPs for MB dye as a
function of MW power intensity (MWP ¼ 144–800 watt, pHi ¼ 10.4, C0 ¼
100 mg L–1
, T¼25–55
C, t¼100 min, m¼0.03 g, and V¼20 mL).
Table 5. Thermodynamics parameters for the sorption of MB dye onto IONPs
sorbent (pHi ¼ 10.4, C0 ¼ 100 mg L–1
, T¼298 k–328 k, t¼100 min, m¼0.03 g,
V¼20 mL, and SS¼200 rpm).
Dye
DHo
(kJ mol1
)
DSo
(kJ mol1
K1
) R2
DGo
(kJ mol1
)
298 K 308 K 318 K 328 K
MB 5.933 0.03615 0.9993 –4.840 –5.201 –5.563 –5.924

heating, which enlarges the sorbent surface area and config-
ures new surface structural pores (Mahmoud et al. 2018b).
This, of course, facilitates the process of rushing of MB dye
molecules inside the IONPs sorbent and consequently
evolves the sorption process.
Influence of competitors ions (NaCl addition)
In reality, plenty of cations (i.e., Naþ
, Kþ
, Mg2þ
and Ca2þ
)
and anions (i.e., Cl–
, NO3–
, SO4
2–
and CO3
2–
) are normally
present in industrial and natural wastewater. Coexisting of
these competitors ions in aqueous effluents can interfere
with MB considering their competitive impact. The as-sued
sorbent should possess an anti-interference character during
the sorption process (Zheng et al. 2020). Notably, under
extreme conditions of ionic strength, counteraction phenom-
enon can be observed on the capturing of MB using IONPs
as displayed in (Figure S9, see Supplementary Material).
Sorption efficacy of sorbent decreased from 58.53 (R% ¼
87.8%) to 52.86 mg g–1
(R% ¼ 79.3%) with gradual enhance-
ment of NaCl concentration from 5.0 to 45.0 g L–1
. The
poor movement (adverse inhibition effect) of MB toward
IONPs sorbent at higher concentration of NaCl (salting out)
is explored regarding four aspects as follow: (i) the enhanced
NaCl concentration aggravated the competition between the
Naþ
ions and MBþ
dye molecules which consequently con-
sumed some of the available sorption sites on the IONPs,
(ii) the enhanced NaCl concentration increased the shield
(screen) effect between MB dye molecules and IONPs and
hence weakened the electrostatic interaction between sorbate
and sorbent surface, and (iii) the ionic strength impacted on
the activity coefficient of MB and therefore hindered (inhib-
ited) their transfer to the surface of IONPs too. This was
consistent with the investigation of the ionic strength effect
on the sorption of MB dye onto Archidendron jiringa seed
shells (Hurairah et al. 2020).
Repeatability performance
Economically, sorbent reusability is an indispensable prop-
erty considering its practicality. It firmly implies the ten-
dency of exhausted sorbent to reuse through recurring
sorption/desorption cycles after undergoing the appropriate
recovery procedures (Cechinel et al. 2018). In the present
work, an acidic treatment appeared as a potential method-
ology to effectively desorb the adsorbed MB from the satu-
rated sorbent surface. As clarified in Table 6, experimental
results presented that IONPs still exhibited a high elimin-
ation percentage nearly 85.4% toward MB dye up to the 5th
repeated adsorption run scenario. This is owned to the
structural stability of IONPs as well as the efficacy of eluent
to suppress the attachment of MB from the surface of
IONPs converting the receptor’s sites into their native forms
and hence reviving them after each cyclic repetition.
Accordingly, the proven results successfully emphasize the
satisfactory performance of IONPs to be reused more than
one time.
Comparison of IONPs sorbent with other sorbents for the
sorption of MB dye
To highlight the better adsorption capacity of synthesized
IONPs as compared to other adsorbents reported in the lit-
erature, a comparison table is also presented (Table 7).
Although it is not possible to compare the performance of
any adsorbent as it is largely dependent on numerous
experimental conditions such as synthesis procedures, con-
stituent units, solution pH, sorbent dose, primary pollutant
concentration, residence time, temperature, and many
others. So, here, we have provided comparison data in
which the maximum adsorption capacities are considered. It
generalized the superior adsorption capacity of the synthe-
sized IONPs over the other counterparts for the removal of
MB dye. The unique physicochemical characters of IONPs
may have endorsed it with adsorption capacity. Therefore,
the cost-efficient IONPs could be considered as a promising
adsorbent for removing cationic MB dye from wastewater.
Feasibility of IONPs sorbent for decontamination of MB
dye from real samples
To further judge the usefulness (activity) criterion of devel-
oped IONPs to decontaminate MB dye, the sorption process
was carried out in real water samples, as it is believed to be
more realistic than the simulated one. Indeed, the least R%
of IONPs toward MB in real spiked water samples was
83.7% (Figure 8). Obviously, from the ascertained results
Table 6. Sorption and desorption findings of sorbed MB dye from IONPs sur-
face after 5 times of sorption/desorption cycles (C0 ¼ 100 mg L1
,
T¼25 ± 1
C, m¼0.03 g, V of HCL ¼ 9 mL (0.5 M), and des t¼30 min).
Sorption/desorption cycle
MB dye
Amount sorbet Removal
DES (%)
(mg g1
) (%)
First sorption operation 59.20 88.8 –
Cycle 1 58.73 88.1 99.21
Cycle 2 58.26 87.4 98.42
Cycle 3 57.86 86.8 97.74
Cycle 4 56.93 85.4 96.17
Table 7. Comparison of sorption performance for MB with various sorbents in the present and other studies.
Biosorbent Biosorption capacities of MB (mg g1
) References
Kaolin 52.76 Mouni et al. 2018
Zeolite/nickel ferrite/sodium alginate bionanocomposite 54.05 Bayat et al. 2018
Hydrogel beads of poly(vinyl alcohol)-sodium alginate-chitosan-montmorillonite 137.2 Wang et al. 2018
Porous cellulose-derived carbon/montmorillonite nanocomposites 138.1 Tong et al. 2018
EDTA-modified bentonite 160.00 De Castro et al. 2018
Polyvinyl alcohol/carboxymethyl cellulose hydrogels reinforced with graphene oxide and bentonite 172.00 Dai et al. 2018
Biochar-derived date palm fronds waste 206.61 Zubair et al. 2020
Mesoporous activated carbon-alginate beads 230.00 Nasrullah et al. 2018
Bio-synthesized Iron oxide nanoparticles (IONPs) 312.5 Present study

and Table 8, it is concluded that IONPs are an ideal adsorb-
ent for removing MB dye from effluents.
Sorption mechanisms of MB dye onto IONPs sorbent
From the mechanistic point of view, the presence of numer-
ous reactive groups on the sorbent’s skeleton serving as a
major controller for underlying the mechanism of any sorp-
tion process. For instance, the larger density of heteroatoms
reactive group, the preferable reciprocity between sorbent
and target pollutant. Noteworthy, it is necessary to realize
that the sorption mechanism usually doesn’t involve only
one route but a combination of direct and/or indirect path-
ways (i.e., physisorption, ion exchange, chelation, coordin-
ation, and so on) depending on sorbent’s functionality,
target pollutant type, and solution’s ionic environmental
state. Herein, the functional groups present on the IONPs
surface can reasonably present a cooperative influence and
justify high adsorption of MB dye molecules by multiple
plausible adsorptive removal mechanisms namely; (i) elec-
trostatic interaction, (ii) H-bonding, (iii) ion exchange, (IV)
Lewis acid-base interaction, (v) p-p stacking, and (VI)
reduction process. MB is a cationic dye, holds a positive
charge on its surface when dissolved in water, which
strongly favors the electrostatic interaction between the
negatively charged functional groups (i.e., –COOH and
–OH) on the IONPs (pHPZC ¼ 7.2) adsorbent and MB dye
molecules. Besides, the formation of H-bonding between the
Table 8. Sorption of MB dye from spiked real effluents using IONPs sorbent
(C0 ¼ 5–20 mg L–1
, T¼25 ± 1
C, t¼100 min, m¼0.03 g, V¼20 mL,
and SS¼200 rpm).
Spiked samples Spiked tap water
Dye concentration (mg L1
) 5 10 15 20
MB
Removal% 90.8 88.3 86.2 83.7
Sorption capacity (mg g1
) 3.026 5.886 8.620 11.16
Figure 8. Removal (%) of IONPs toward MB dye from spiked TW specimens
(concentrations of dye were varied between 5 and 20 mg L–1
) (C0 ¼ 5–20 mg
L–1
, T¼25 ± 1
C, t¼100 min, m¼0.03 g, V¼20 mL, and SS¼200 rpm).
Scheme 1. Suggested mechanisms of MB dye sorption onto IONPs sorbent.

carboxyl groups (H-donors) on IONPs and H-acceptors pre-
sent on the characteristic rings of organic MB dye should be
counted. Moreover, ion exchange may contribute to the
removal of MB by IONPs adsorbent because of the presence
of hydroxyl (OH–
) and¼O functional groups on the adsorb-
ent surface (Aragaw et al. 2021). Indeed, the ion exchange
tendencies of IONPs synthesized by green strategies toward
different organic contaminants (i.e., dyes, pesticides, and
other emerging organic contaminants) was successfully
reported (Biftu et al. 2020; Hassan et al. 2020). So that, it
can be introduced as one of the proposed mechanisms inter-
preting the adsorption of MB dye molecules onto IONPs.
Otherwise, the nitrogen atoms characterized for MB chem-
ical structure can act as Lewis base interacts with Fe3þ
; thus,
Lewis acid-base interaction occurs (Fadillah et al. 2020). In
addition, p–p stacking is proven to participate in the MB
adsorption process as the adsorbed MB dye molecules affect-
ing on the electronic structure of IONPs via p–p conjuga-
tion interactions (p region in MB molecule) (Li et al. 2021).
Furthermore, the corresponding synthesis conditions (i.e.,
nitrogen or oxygen medium) considerably affect the activity
of as-produced INOPs adsorbent. The resultant Fe0 pro-
duced from the IONPs synthesis process directly interacts
with H2O and release electrons (e–
), which are thereafter
consumed by Hþ
to produce active hydrogen with a strong
reducibility property (Xiao et al. 2020). The possible adsorp-
tion mechanisms of MB dye using IONPs were speculated
and illustrated in Scheme 1.
Conclusion
Effective and sustainable removal of organic dyes from
aquatic systems is very important considering its negative
impact on living creatures. In this study, we successfully
synthesized iron oxide nanoparticles (IONPs) by an in-situ
eco-friendly preparation process using Spirulina platensis
micro-algae. The structure, composition, and properties of
the prepared IONPs were investigated by several physico-
chemical techniques of FT-IR, SEM, EDX, BET surface area,
TEM, VSM, UV/Vis spectroscopy, and PHPZC measurement.
The adsorption capacity of IONPs toward MB dye was eval-
uated under different experimental conditions of; initial pH
(2.210.4), adsorbent dosage (0.55.0 g L–1
), initial dye con-
centration (101000 mg L–1
), residence time (0230 min),
and temperature domain (298 K, 308 K, 318 K, and 328 K).
The findings revealed that the IONPs exhibited a high sur-
face area of 134.003 m2
/g, a total pore volume of 0.3715 cc/g,
and an average pore size of 5.54 nm. The adsorption cap-
acity of the IONPs for MB at 298 K was high as 312.5 mg
g–1
under the optimized pH value (i.e., pH ¼ 10.4). The
adsorption isotherm fitted with the Langmuir equation well
indicated that this is a homogenous adsorption process.
Thermodynamic parameters showed that the adsorption of
dye is spontaneous endothermic in nature. Admirably, the
proposed microwave technique shortened the equilibrium
time up only 1 min. Moreover, the recyclability test of
IONPs was efficiently conducted up to 5 times of adsorp-
tion/desorption cycles using the desorbing agent of 0.5 M of
HCl. The IONPs adsorbent introduced a great potential to
remove MB dye from spiked real water samples (83%).
These results reported that IONPs can be conceived as a
strong candidate in practical applications such as wastewater
purification.
Acknowledgments
This work was performed at the Faculty of Science, Port-Said
University, Port-Said, Egypt. The authors; therefore, acknowledge with
thanks the University’s technical support.
Ethical approval
This study didn’t use any kind of human participants or human data,
which require any kind of approval.
Consent for publication
Our study didn’t use any kind of individual data such as video
and images.
Disclosure statement
We wish to confirm that there are no known conflicts of interest asso-
ciated with this work and there has been no significant financial sup-
port for this work that could have influenced its outcome. We confirm
that the manuscript has been read and approved by all named authors
and that there are no other persons who satisfied the criteria for
authorship but are not listed. We further confirm that the order of
authors listed in the manuscript has been approved by all of us.
ORCID
Shymaa M. Shalaby http://orcid.org/0000-0001-6429-0252
Fedekar F. Madkour http://orcid.org/0000-0001-8871-9877
Adel A. Mohamed http://orcid.org/0000-0001-6506-5663
Ahmed M. Elgarahy http://orcid.org/0000-0003-4959-2652
Data availability statement
All data generated or analyzed during this study were included in the
submitted article. In addition, the datasets used or analyzed during the
current study were available from the corresponding author on reason-
able request.
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