Shalaby2021 article green_synthesis of recyclable iron nanoparticles

RESEARCH ARTICLE
Green synthesis of recyclable iron oxide nanoparticles
using Spirulina platensis microalgae for adsorptive removal
of cationic and anionic dyes
Shymaa M. Shalaby1
& Fedekar F. Madkour1
& Hala Y El-Kassas2
& Adel A. Mohamed3
& Ahmed M. Elgarahy4,5
Received: 11 May 2021 /Accepted: 16 July 2021
# The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2021
Abstract
Globally, organic dyes are major constituents in wastewater effluents due to their large-scale industrial applications. These
persistent pollutants adversely impact the public health of different living entities. Thus, wastewater remediation has become
an indispensable necessity. Herein, we greenly synthesized iron oxide nanoparticles (SP-IONPs) using Spirulina platensis
microalgae to remove cationic crystal violet (CV) and anionic methyl orange (MO) dyes from their aqueous solution. The
engineered sorbent was thoroughly scrutinized by different characterization techniques of FT-IR, BET surface area, SEM,
EDX, TEM, VSM, UV/Vis spectroscopy, and pHPZC measurement. The proficiency of SP-IONPs was methodically appraised
for its sorptive performance towards the target CV and MO dyes under variable technological parameters (batch scenario).
Collectively, the outlined results inferred an amazing efficacy characterized to the SP-IONPs sorbent for the expulsion of relevant
dyes from the aqueous media. Regarding the dynamic static sorption data, the kinetics profile was ascribed to the pseudo-second
order model, whereas sorption isotherm was quantitatively dominated by the Langmuir theory with maximum sorption capacities
of 256.4 mg g-1
and 270.2 mg g-1
for CV and MO, respectively. Thermodynamics findings conformed the endothermic nature of
sorption process. Repeatability of the spent sorbent was successfully emphasized for 5 times of sorption/desorption cycles. The
productive sorbent admirably sequestered CV and MO dyes from spiked tap water. The potency of SP-IONPs as color collecting
material from real dyeing effluents was achieved.
Highlights •Green synthesis of iron oxide nanoparticles for efficient
sorption of crystal violet (CV) and methyl orange (MO) dyes.
•Maximum sorption capacity close to 256.4 mg g-1
and 270.2 mg g-1
for
CVand MO, respectively, fitted by Langmuir equation.
•Fast kinetics (equilibrium~60 min), fitted by pseudo-second order kinet-
ics model.
•Efficient modeling of thermodynamics parameters (endothermic nature).
•Sorbent stability over 5 sorption/desorption cycles; poorly affected by
system complexity (good efficiency in tap water and industrial
wastewater).
Responsible Editor: Tito Roberto Cadaval Jr
* Ahmed M. Elgarahy
ahmedgarahy88@yahoo.com; ahmed.gamal@sci.psu.edu.eg
Shymaa M. Shalaby
Shymaashalaby77@yahoo.com
Fedekar F. Madkour
fedekarmadkour@ymail.com
Hala Y El-Kassas
halayassin12@yahoo.com
Adel A. Mohamed
aaregal@gmail.com
1
Marine Science Department, Faculty of Science, Port-Said
University, Port-Said, Egypt
2
Marine Hydrobiology Department, National Institute of
Oceanography and Fisheries, Alexandria, Egypt
3
Marine Chemistry Department, National Institute of Oceanography
and Fisheries, Suez, Egypt
4
Environmental Science Department, Faculty of Science, Port-Said
University, Port-Said, Egypt
5
Egyptian Propylene and Polypropylene Company (EPPC),
Port-Said, Egypt
https://doi.org/10.1007/s11356-021-15544-4
/ Published online: 28 July 2021
Environmental Science and Pollution Research (2021) 28:65549–65572

Keywords Spirulina platensis; . Iron oxide nanoparticles; . Dyes removal; . Uptake kinetics; . Sorption isotherms; . Sorbent
repeatability; . Treatment of real effluents
Introduction
Water is conceived as one of the basic, strategic, and irreplace-
able key factors to conserve the sustainable development of
human society on our planet. Throughout the last few decades,
water and wastewater contamination associated with the ex-
plosive industrial growth has become one of the worst thorny
environmental issues, aroused a significance global concern,
because of their serious effects on variable living entities (i.e.,
agricultural, commercial, domestic and industrial) (Sellaoui
et al. 2021). Colloquially, among multitude of water polluters,
dyes are defined as colored aromatic compounds, currently
fulfilling innumerable purposes in diverse industrial applica-
tions (i.e., cosmetology, medicine, plastic, paper, printing,
pharmaceutical industries, perfumery, leather, varnishes, tex-
tile and so on) with estimated global consumption rate of 107
kg/year (Franco et al. 2021). Typically, they are categorized
into cationic (positively charged), anionic (negatively
charged), and nonionic species considering their dissolution
behaviors in aqueous medium (Elwakeel et al. 2020).
According to the literature, it was reported that more than 1
× 105
types of commercial synthesized dyes are enormously
consumed in different industries with a yield of > 7 × 105
/year
(Lin et al. 2021). The global colorant market has been expect-
ed to reach up to $42 billion by the coming of this year (Anwer
et al. 2019).
Regrettably, the uncontrolled utilization of synthetic dyes as-
sociated with the broaden industries, has dramatically led to
dumping of considerable quantities of dyes laden-wastewater
into the aquatic environment (Drumm et al. 2021a). Persistence
of these released dyed effluents and/or their decomposition prod-
ucts in the water bodies seriously endanger the health of different
living creatures. They deteriorate water quality (specification) by
changing the content of COD, BOD, TDS, and TSS, which
ultimately result in diminishment of photosynthesis efficiency
due to an increase in the water turbidity associated with reduction
of light penetration (Li et al. 2020). Additionally, they have se-
vere health risks on the human health because of displaying
carcinogenic, mutagenic and teratogenic characters (Drumm
et al. 2021b). In view of the above, a plenty of obsolete
physico-chemical scenarios such as ion exchange (Kaur and
Jindal 2019), chemical precipitation (Hisada et al. 2019), coagu-
lation (Badawi and Zaher 2021), ozonation (Bakht Shokouhi
et al. 2020), membrane separation (Mansor et al. 2020), and
photocatalytic degradation (Chatterjee et al. 2021) were designed
to purify water effluents (decolorizaton) from toxic dyestuffs.
Some worrisome restrictions associated with implementation of
the mentioned sophisticated strategies were faced such as high
operational cost, inefficiency in low polluter's concentrations,
toxic intermediates products, reagents consumption and sewage
sludge formation. Alternatively, adsorption is deemed as a green,
versatile, and emergent approach for wastewater treatment (He
et al. 2021).
In the last decades, there has been a major rise in interest in
nanotechnology and nanoparticles (NPs) with nanoscale dimen-
sions attributing to their admirable physicochemical properties,
high reactivity, and large surface area comparing with other
counterparts (Wong et al. 2019). Among a variety of physical
and chemicals protocols (i.e., sol-gel method, micro-emulsion,
electrochemical and etc.) applied for the synthesis of NPs, the
biocompatible synthesis of NPs in term of green nanotechnology
using plants parts extracts (i.e., leaf and fruit) or biological or-
ganisms (bacteria, seaweed, yeast and fungi) are evolved nowa-
days due to its low-cost, high production yield, and environmen-
tally benign nature (Paiva-Santos et al. 2021). These biogenic
materials contain a vast of bioactive constituents (i.e., amino
acids, alkaloids, carbohydrates, polyphenols, steroids, saponins,
flavonoids, terpenoids, proteins, vitamins, organic acids, and re-
ducing sugars), employed as reducing, capping, and stabilizing
agents during the synthesis process of NPs (Puthukkara et al.
2020). In particular, the microalgae species are widely employed
attributing to their unique bioactive constituents such as polyphe-
nols, polyunsaturated fatty acids, sterols, and sulfated polysac-
charides. The cultivation (culturing) of microalgae in wastewater
has numerous merits of production of valuable algal biomass
products and wastes remediation. Among a variety of microalgae
species, the cyanobacterium (blue-green alga) Spirulina platensis
is a microscopic filamentous prokaryotes that is cultivated and
commercialized worldwide. They can grow phototrophically,
heterotrophically or (iii) mixotrophically in different environ-
ments such as freshwater, marine water, and pond water with
severe living conditions of low light conditions and in the pres-
ence of organic matter and contaminants (Rahim et al. 2021).
Structurally, they are a rich source of minerals, vitamins, proteins,
zeaxanthin, polyunsaturated, fatty acids, myxoxanthophyll,
hycobiliproteins, carotenoids, ascorbic acid and phenolic com-
pounds. Based on these premises and considering the mentioned
benefits of Spirulina platensis, they made it very convenient to
be used in different fields (i.e., dietary supplements for humans/
animals, pharmaceutical applications, biofuels industries and
wastewater remediation (Moradi et al. 2021). Furthermore, the
rapid increase in demand for a balance between natural phenom-
ena and ecology in bio-environment to create a livable planet has
urged a significant need for continuous development of new
technologies that consume the abandoned biogenic available ma-
terials to produce high-value products. From the economic point
65550 Environ Sci Pollut Res (2021) 28:65549–65572

of view, the present work aimed to get the utmost benefit from
the renewable Spirulina platensis microalgae powder (i.e., US
$5–7/ kg), commercially available to greenly synthesize an iron
oxide nanoparticle sorbent (SP-IONPs) and systematically inves-
tigated its efficacy to remove crystal violet (CV) and methyl
orange (MO) dyes from their aqueous solutions as emblematic
examples for cationic and anionic dyes, respectively. The first
part of this study concentrates on different structural characteri-
zation analyses (physico-chemical properties) of the as-formed
SP-IONPs sorbent; Fourier-transform infrared spectrometry (FT-
IR), scanning electron microscopy (SEM), Brunauer Emmett
and Teller (BET) surface area, energy-dispersive X-ray analysis
(EDX), transmission electron microscopy (TEM), vibrating-
sample magnetometry (VSM), ultraviolet/visible (UV/VIS) spec-
troscopy, and Zeta potential measurement (pHPZC). After that,
the adsorptive performance of SP-IONPs sorbent towards two
common CV and MO dyes under causative operational param-
eters (i.e., solution pH, sorbent concentration, contact time, pri-
mary polluter concentration and temperature) was inspected to
obtain a mechanistic understanding of dyes-sorbent interaction
system, which is indispensable for large scale application, includ-
ing the selection of a suitable adsorbent, customization of as-
synthesized adsorbents, and defining of optimal eluent. The re-
peatability as well as susceptibility of SP-IONPs to capture CV
and MO dyes from spiked real samples (i.e. tap water and indus-
trial wastewater) was successfully researched.
Materials and methods
Materials
The as-used chemical reagents throughout the present work
were of standard analytical grade and were employed directly
without any further purification. Micro-algal powder of
Spirulina platensis was provided by National Research
Center (NRC), Cairo, Egypt. Ferric chloride hexahydrate
(FeCl3.6H2O), methanol (CH3OH), and ethanol (C2H5OH)
were purchased by Merck (Germany). Crystal violet (CV)
and methyl orange (MO) dyes were provided by Sigma-
Aldrich (Darmastadt, Germany). Table 1 presents the main
characteristics of CV and MO dyes. Deionized (DI) water
was utilized for the preparation of different working solutions.
The pH values of all dyes working solutions were controlled
by using 0.8 M of diluted HCl and/or NaOH.
Green synthesis of iron oxide nanoparticle (SP-IONPs)
Preparation of Spirulina platensis micro-algal supernatant
The obtained micro-algal powder (MALGP) of Spirulina
platensis was initially washed throughout running tap water
(TW). Afterwards, they were rinsed three times with DI water
to take away any adhered impurities particles. The rewashed
MALGP was naturally air-dried for 72 h at ambient tempera-
ture (i.e., 25 ±1 °C). The dried MALGP (12 g) was dunked in
a 500-mL round-bottomed flask contacting 120 mL of DI
water and boiled with continous stirring (~ 150 rpm) using a
reciprocal agitator (Rota bit, J.P. Selecta, Spain) for 1 h at
75°
C to maximize the release of the contained phytochemi-
cals. After allowing the homogenized solution to naturally
cool at 25 ±1 °C, the MALGP was screened through a
Whatman filter paper (diam. 45 mm) and the supernatant clear
solution was collected in polypropylene tubes and kept for the
preparation of Spirulina platensis iron oxide nanoparticles
(SP-IONPs).
Preparation of Spirulina platensis iron oxide nanoparticles
(SP-IONPs)
Indeed, optimization of the SP-IONPs synthesis conditions
were attempted by six trials of varying the initial concen-
trations of iron (III) of FeCl3.6H2O from 0.1 to 0.6 M with
95 mL of MALGP supernatant to refine the synthesis con-
ditions. Table S1 (see Supplementary Materials) presents
the different SP-IONPs formation conditions with their
maximum absorbance values. The best synthesis condi-
tions (utmost absorbance) were continued during the pres-
ent work. In a typical procedure, 0.6 M of iron (III) of
FeCl3.6H2O was dispersed in 95 mL of DI water and ag-
itated at ~ 200 rpm for 1 h to ensure its complete dissolu-
tion. Then, about 95 mL of MALGP supernatant was gent-
ly introduced into the iron (III) suspension (95 mL) with a
MALGP supernatant: Fe (III) volume ratio of 1:1.
Immediately, the mixture color turned from yellow to in-
tense brown which affirmed the successful synthesis of
SP-IONPs. The corresponding solution was left under stir-
ring for another 2 h and the produced homogenous solu-
tion was transferred to a hot air oven (Gallenkamp BS
Model OV-160, Loughborough (LE), UK) at 75°C for 24
h. The resulting dark SP-IONPs (solid) was magnetically
collected from the solution using neodymium magnet,
rinsed four times with DI water and dehydrated at 70°C
for 6 h before characterization.
Preparation of dyes solutions
Stock standard solutions of CV and MO dyes (1000 mg L-1
)
were prepared for different sorption experiments. This was
obtained by dispersing their salts in suitable amounts of DI
water. The salts mixtures were left to stir (~ 100 rpm) for
approximately 20 min to ensure effective dissolution. The
batch sorption polluter’s solutions were conducted by further
stepwise dilution of stock solutions.
65551
Environ Sci Pollut Res (2021) 28:65549–65572

Physical characterization of SP-IONPs sorbent
FT-IR spectroscopy analysis was executed to analyze the
functionalities present on the surface of synthesized SP-
IONPs sorbent. The FT-IR spectra of the examined samples
were registered on a Nicolet IS10 FT-IR (Thermo Fischer
Scientific, Waltham, MA, USA) model in the range of 400–
4000 cm−1
. BET surface area, pore volume and pore size
analyses characterized to SP-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 min−1
; 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). SEM coupled with EDX analysis system
(Jeol Ltd.; JSM-6510LV, Tokyo, Japan) was proceeded to
monitor the morphological features and elemental content of
SP-IONPs sorbent. TEM analysis (TEM-2100HR, JEOL,
Tokyo, Japan) was used to perform the ultrahigh resolution
scrutinizing of the biosynthesized SP-IONPs. Magnetization
Table 1 The main characteristics of CV and MO dyes
Crystal Violet Methyl Orange
Empirical Formula C25N3H30Cl C14H14N3NaO3S
Molecular Weight
(g/mol)
407.98 327.34
λmax 590 464
Chemical structure
CAS Number 548-62-9 547-58-0
pKa values 5.31 - 8.64 3.7

behavior examination of SP-IONPs sorbent was employed by
using VSM tool (VSM, PMC MicroMag 3900 model,
Princeton, NJ, USA). The Surface Plasmon Resonances
(SPR) of SP-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 SP-IONPs sor-
bent was recorded using the pH-drift methodology. 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 (ΔpH) were graphically charted against pHi
values. The pH value of point zero charge (pHPZC) was com-
puted from the intersection dot of the represented curve at
which equals zero.
Sorption assay experiments
The sorption performance of SP-IONPs towards CV and MO
dyes was conducted by batch equilibrium scenario. For all
sorption studies, the process was comprehensively run by
varying one of the affecting operational parameter, while
keeping the other environmental conditions constant. With a
view to investigate the impact of initial solution pH (pHi),
0.03 g of SP-IONPs was separately immersed in stoppered
Erlenmeyer flasks (50 mL) with a series of 20 mL of MO
and CV (C0: 20 mg L-1
) with altered pHi values ranging from
2.2 to 10.4 attained with the utilization of 0.8 M of diluted HCl
and/or NaOH to modulate the acidity and basicity nature of
the working system, respectively. The tested solutions were
continuously agitated under maintained conditions of temper-
ature (T) = 25 ± 1 °C, contact time (t) = 100 min and stirring
speed (SS) = 200 rpm. The pH was not regulated during the
sorption but the equilibrium pH (pHeq) was systematically
noted using an Aqualytic AL15 pH-meter (Aqualytic GmbH
& Co, Dortmund, Germany). After that, the MO and CV load-
ed sorbents were magnetically gathered from the solution and
the remaining supernatants were estimated for the residual
MO and CV concentrations at λmax of 464 nm and 590 nm,
respectively, using a Palintest 7100 spectrophotometer
(Palintest, Ltd., Gateshead, UK) (Tran et al. 2020; Samrot
et al. 2021). The sorbed amount of dyes per SP-IONPs mass
at equilibrium stage, qe (mg g-1
) and the removal efficiency (R
%) was investigated as displayed in Eq. 1 and 2, Table S2 (see
Supplementary Material).
The influence of sorbent concentration (solid: liquid ratio)
was explored by altering the SP-IONPs concentration from
0.5 to 5.0 g L-1
with 20 mL of MO and CV dyes solutions at
(C0 = 20 mg L-1
, T = 25°
C ±1, t = 100 min and SS = 200 rpm).
After equilibrium, the remaining MO or CV dyes concentra-
tions were spectrophotometry quantified.
Kinetics experiments were proceeded as function of con-
tact time by mixing 1.5 g of SP-IONPs with 1000 mL of MO
and CV solutions (C0: 20 mg L−1
, T = 25 ± 1 °C, t = 180 min
and SS = 200 rpm). At stipulated time intervals, samples (5
mL) of MO and CV dyes solutions were periodically with-
drawn and the MO and CV concentrations C(t)(i) (mg L-1
) was
determined by using the given Eq. 3 in Table S2 (see
Supplementary Material).
The isothermal studies were carried out by contacting
0.03 g of SP-IONPs with 20 mL of MO and CV solutions of
various primary concentrations (C0: 10–1000 mg L-1
) at (T =
25 ± 1 °C, t = 100 min and SS = 200 rpm). Utilization of
sorption sites (UOS %) throughout sorption process can be
determined by the Eq. 4 in Table S2 (see Supplementary
Material).
Thermodynamics parameters were studied by blending
0.03 g of SP-IONPs with 20 mL of MO and CV solutions
(C0 = 20 mg L-1
, t = 100 min and SS = 200 rpm) at different
environmental temperatures (i.e., 25 °C, 35 °C, 45 °C, and 55
°C) in a shaking incubator (LSI-3016R, LabTech S.r.l.,
Sorisole (BG), Italy).
Evaluation of interfering ions (herein NaCl) as a corre-
sponding effect on sorption process was implemented by
adding various concentrations of NaCl (5–45 g L-1
) into 20
mL of MO and CV solutions (C0: 20 mg L−1
) in the presence
of 0.03 g of SP-IONPs sorbent at (T = 25 ± 1 °C, t = 100 min
and SS = 200 rpm).
Kinetics modeling analysis
From the viewpoint of sorption system design, sorption kinet-
ics studies are essential to gain a full visualization
(knowledge) about the pathway of sorption reaction, equilib-
rium (saturation) time as well as determine all the steps poten-
tially controlling the sorbate and sorbent interaction involved
in the liquid–solid interface. They are helpful for researchers
as fundamentals for laying-out of the industrial sorption sys-
tem. To disclose the nearest fitted kinetics model describing
the kinetics sorption process, the outcome data were handled
using four commonly applied kinetics models including;
pseudo first order rate equation (PFORE) (Ho 2004),
pseudo-second-order rate equation (PSORE) (Ho and
McKay 1999), intra-particle diffusion (Weber and Morris
model; W&M) and Elovich model (Chien and Clayton
1980). The linearized & non-linearized expressions of the
mentioned models are listed in Table S3 (see Supplementary
Material).
Isothermal modeling analysis
It is prerequisite tool to analyze the distribution of sorbate
from liquid phase to solid phase up to equilibrium stage under
the controlled (fixed) conditions. They were adopted to eluci-
date insights about type of isotherm, sorption mechanism,
sorbent affinity, reaction nature whether monolayer or
65553

multilayer sorption and maximum sorption capacity. Three
prominent models, namely Langmuir (LAM) (Langmuir
1918), Freundlich (FR) (Freundlich 2017) and Temkin (TK)
(Ostrovskii 1989) were mathematically compared to match
with the experimental outcomes. The isothermal linearized
and non-linearized expressions are displayed in the Table S4
(see Supplementary Material). LAM model theory is built on
three assumptions; monolayer sorption, identical sorption sites
and sorption of any molecule on the sorbent active sites is
independent of occupancy characterized to its neighboring
sites (Huang et al. 2021). A further investigation of the
LAM equation can be computed using a dimensionless con-
stant separation factor, RL. It clarifies the feasibility and favor-
ability of sorption process. It can be assessed by the Eq. 5 in
Table S2 (see Supplementary Material). RL factor signifies the
nature of sorption process; linear (RL= 1), favorable (0 < RL<
1), unfavorable (RL >1) and irreversible (RL= 0).
Contrarily, FR model is an empirical equation supposes the
heterogeneous nature of sorbent's active sites as well as the
possibility of multilayer accumulation existence (Rathika and
Raghavan 2021). TK model is another isothermal model
mainly suggests the linear decrement of sorption energy over
the exponential decline as stated by FR model. Additionally,
the sorbent exhaustion is also taken into account, after the
completion of sorption process (Ngabura et al. 2018).
Thermodynamics analysis
Detailed and useful insights in term of thermodynamics char-
acteristics such as spontaneity, feasibility and reaction nature
(i.e., exothermic or endothermic) can be easily investigated by
accounting the determined values of thermodynamics param-
eters (Kasperiski et al. 2018). Change in free energy of sorp-
tion (ΔG), change in Gibbs free energy (ΔGo
), change in en-
tropy (ΔSo
), and change in enthalpy (ΔHo
) were explored by
using the given Eqs. 6–8 in Table S2 (see Supplementary
Material).
The values of different thermodynamics functions for MO
and CV sorption on SP-IONPs sorbent were determined by
plotting ln Kc against 1/T.
Repeatability (recovery) studies
Stability is of great significance to be tested up for judging the
sorbent repeatability for commercial applications. To guarantee
long-term sorption capacity of the SP-IONPs sorbent, the loaded
MO and CV dyes have to be efficiently eliminated from the
sorbent surface. The experimental procedures of desorption pro-
cess were executed in a similar pattern of sorption process. The
loaded SP-IONPs with MO and CV collected from the sorption
study was gently rinsed with DI water, followed by an alkaline/
acidic regeneration method, conducting by placing the washed
sorbent in contact with 9 mL of 1 M of NaOH and HCl
(desorbent agents), respectively, for 30 min. The suspension
was centrifuged to separate the supernatant and subsequently it
was analyzed for the residual concentrations of MO and CV. The
separated sorbent was repeatedly rinsed with DI water up to pH
value of 7. The regenerated SP-IONPs sorbent was dried for 2 h
at 40 °C in an oven and subjected for 5 consecutive biosorption–
desorption cycles. The capacity of desorption (DES, %) for the
SP-IONPs sorbent was investigated as:
DES %
ð Þ ¼
Amount of desorbed dye mg
ð Þ into the elution solution
Amount of sorbed dye mg
ð Þ
100
ð1Þ
Potential implementation of SP-IONPs sorbent for
decontamination of CV and MO dyes from real
samples
Although eliminating of target dyes polluters from simulated
wastewater using as-used sorbent presents its potential to be
exploited in wastewater treatment, testing its susceptibility on real
environment should be explored in order to determine the true
capacity characterized to the sorbent. To evaluate the impact of
the solution complexity on the sorption process, the sorption ex-
periments were performed on spiked TW samples collected from
Water Supply facilities at Port-Said, Egypt. Briefly, a given
amount of SP-IONPs sorbent (0.03 g) was individually immersed
in 20 mL of MO and CV spiked TW samples (concentrations of
dyes was varied between 5 and 20 mg L-1
) at (T = 25 ± 1 °C, t =
180minandSS=200rpm).Thesampleswerefilteredandfiltrates
were measured for the residual MO and CV concentrations.
The performance of SP-IONPs adsorbent for treatment of real
effluents specimen was tested in order to explore its susceptibility
for industrial wastewater treatment application. To achieve this,
textile dyeing wastewater specimen was collected from the outlet
of the local dyeing facility at industrial Zone, Port-Said, Egypt.
The potential application of SP-IONPs in adsorptive color re-
moval was systematically inspected by simply mixing 0.03 g
of SP-IONPs sorbent with 20 mL of wastewater sample at con-
ditions of (T = 25°C ±1, t = 100 min and SS = 200 rpm). The
change in COD and color were monitored before and after ad-
sorption process. All sorption tests were performed in triplicate,
and the averages were recorded. The limit of experimental errors
on triplicates was systematically below 5%.
Results and discussions
Green synthesis mechanism of SP-IONPs using MALGP
supernatant
In general, the green synthesis of SP-IONPs is considered as
an environmentally sustainable approach, can be achieved by

using the biocompatible biological sources (i.e., algae, bacte-
ria, yeast, plants and fungi). They are rich in a vast of bioactive
compounds majorly contributing in the reductions of iron
ions (Prasad et al, 2017). These phytochemicals have a dual
role by simultaneously proceed as reducing and capping
(stabilizing) agents during the SP-IONPs synthesis process.
Firstly, metal ions are produced by treating the iron precursor
with the biological constituent (reduction). This is followed by
creation of a nucleation center which consequently sequesters
the rest of metal ions and integrates the neighboring nucle-
ation site. The end product of the mentioned reaction is the
SP-IONPs. The size, growth as well as morphology of SP-
IONPs can be controlled considering the nature of bio-active
components (Vasantharaj et al. 2019).
Morphological and structural characterization of SP-
IONPs sorbent
Generally, the FT-IR spectrum is employed to differentiate the
involved bindings groups in the sorption process. The FT-IR
spectral data of Spirulina platensis, biosynthesized bare SP-
IONPs and loaded sorbent with CV and MO dyes is displayed
as overlay graph in Fig. 1. The broad characteristic absorption
band 3606.92 cm−1
is originally linked with stretching vibra-
tion of O-H for alcohol (-OH) or carboxylic (-COOH) found
in polysaccharides, proteins or polyphenols. A Small signal
band around 2923.12 cm−1
belongs to stretching frequencies
of C-H in aliphatic acids. The detected peak at 1713.22 cm−1
is assigned to C=O stretching vibration of (-COOH). A weak
peak at 1107.9 cm−1
is related to N-H stretching of aliphatic
amines and 1055.66 cm−1
is associated with C-O-C stretch
group. Vibration peak at 876.48 cm-1
associates with =C-H
group (Asghar et al. 2018; Pan et al. 2019). Numerous weak
peaks in the range of 400 – 850 cm-1
(i.e. 783.92 cm−1
, 723.1
cm−1
, 623.8 cm−1
, 556.3 cm−1
, and 496.5 cm−1
) confirms the
synthesis of SP-IONPs sorbent attributed to stretching of the
metal–oxygen (Fe-O) group. These findings are similar to
those recorded by (Bishnoi et al. 2018; Rahmani et al.
2020). After loading of SP-IONPs with CV dye, new peak
appeared at 1588.21 cm−1
is parallel to stretching vibration
of C=C characterized to aromatic ring, while the other peak
at 1364.74 cm−1
refers to stretching vibration of C–N featured
to aromatic tertiary amine. These findings confirmed the sorp-
tion of CV onto SP-IONPs sorbent (Elgarahy et al. 2019).
Whereas, the sorption of MO dye onto SP-IONPs was sup-
ported by the occurrence of new peaks at 1597.30 cm−1
and
1512.63 cm−1
, 1493.82 cm−1
, and 1055.79 cm−1
belong to
benzene ring, azo bond, stretching vibration of benzenoid/
quinonoid and stretching frequency of C-N in MO dye, re-
spectively (Prasad and Joseph 2017; Li et al. 2018). Figure 2
presents the nitrogen adsorption-desorption isotherms curve
of SP-IONPs sorbent. In general, the adsorption process wide-
ly depends on the adsorbent’s surface area. Large surface area
and porosity offer many sorptive active sites encouraging the
facile accessibility of sorbate to adsorbent surface. In this
study, the BET specific surface area of 134.003 m2
/g and a
total pore volume of 0.3715 cc/g were found more consider-
able than the other green synthesized iron oxide nanoparticles,
as shown in Table S5 (see Supplementary Material).
Moreover, the average pore size of 5.54 (nm) indicates the
mesoporous nature of SP-IONPs sorbent, considering the
International Union of Pure and Applied Chemistry
(IUPAC) classification of sorption isotherms. These structural
characters perfectly promote the sorption of CV and MO dyes
onto SP-IONPs sorbent and thereafter the current produced
SP-IONPs can be explicably used to remove dyes contami-
nants from wastewaters.
SEM images of native SP-IONPs sorbent and after sorption
processes of crystal violet and methyl orange dyes with dif-
ferent magnifications of 10,000×, 15,000×, 20,000×, and
30,000× are displayed in Fig. 3. It revealed that SP-IONPs
are agglomerated and tend to form a non-regular (non-
uniform) surface in nature and almost distinctive. Basically,
the aggregation of SP-IONPs particles can be attributed to the
magnetic interactions (dipole-dipole) between the iron spe-
cies. Additionally, presence of different bioactive reducing
agents (i.e., polyphenols) in the MALGP supernatant, can
greatly influence the final morphology and size of the iron
nanoparticles. Some big clumped clusters were formed due
to assemblage of tiny building blocks of various bioactive
compounds (Aksu Demirezen et al. 2019; Pai et al. 2021).
After CV and MO dyes sorption, an organized fashion of
dye molecules crumpling was homogenously noted on the
sorbent’s surface, affirmed their sorption onto SP-IONPs sur-
face. Regarding to the CHNS analysis (EDX analysis), the
native sorbent mainly consists of Fe, O, Cl which is in line
with the expected chemical composition of as-formed SP-
IONPs. Whereas, the appearance of other recognizable peaks
related to other elements (i.e., C and N) and (i.e., C, N, Na and
S) for CV and MO dyes, respectively, are in accord with the
chemical structures of the mentioned dyes, and largely sup-
ports their sorption onto SP-IONPs sorbent (Fig. 4).
To further check the resultant SP-IONPs with high accura-
cy, TEM analysis was employed as seen in Fig. 5(a). It was
possible to identify dark (black) and tiny clusters of SP-IONPs
particles with some agglomerations dispersed in the dense
layer (matrix) of bioactive compounds. This was reported by
similar investigations of FeNPs synthesis via green pathway
(Carvalho and Carvalho 2017; Plachtová et al. 2018).
Moreover, in the dimensional view, SP-IONPs particles pre-
sented a slightly irregular and rounded shape with variable
particle sizes in the nanometer range ( 10 nm), may possibly
be related to different bioactive constituents (functional
groups) in the MALGP supernatant, further interact with each
other via different intermolecular forces. These aggregates
may point out to the H-bonding between hydroxyl groups
65555

and other moieties in the structure of phenolic compounds
(Bibi et al. 2019). Similar particle sizes have been recorded
via green synthesis of various biomaterials (Aksu Demirezen
et al. 2019). Magnetization performance of SP-IONPs was
demonstrated as shown in Fig. 5(b), whereas M (emu/g) is
the function of the applied magnetic field H(Oe), Ms is the
saturation magnetization, Mr is the remnantmagnetization,
Hci is the intrinsic coercivity and Hc (χ) is the magnetic sus-
ceptibility. As listed in Table 2, the measured (determination)
values present that SP-IONPs is ferromagnetic material with a
narrow hysteresis loop which is matched with the values of
121.34 Oe, 168.84 ×10-6
emu/(g.Oe) and 20.68 × 10-3
emu/g
for Hci, Hc (χ) and Mr, respectively. The small Msvalue of
0.2705 emu/g can be evidenced by the pronounced surface
effects on NPs at smaller sizes. Briefly, the surface of NPs
consists of some canted or disordered spins that prevent the
core spins from aligning along the field direction resulting in
decrease of the saturation magnetization of the small sized
nanoparticles.
UV-VIS spectroscopy is considered as a beneficial charac-
terization technique to study the optical properties of the ma-
terials in the UV-Visible spectral region. Figure 6 displays
shifting of characteristic absorption peak of ferric (III) chlo-
ride from 360 nm to 405 nm. The new shifted UV-Vis spec-
trum is an indicative of SP-IONPs formation using Spirulina
platensis, which similarly agreed (harmony), with other stud-
ies reported by (Jagathesan and Rajiv 2018; Madubuonu et al.
2019). The zetametric measurements of SP-IONPs sorbent
Fig. 1 FT-IR spectra of (a) iron
oxide nanoparticles (SP-IONPs)
sorbent before sorption of CV/
MO dyes, (b) after CV sorption,
and (c) after MO sorption
Fig. 2 Nitrogen adsorption–
desorption isotherms curve of SP-
IONPs sorbent

manifested that pHPZC is close to 7.2: sorbent is protonated at
pH pHPZC and vice versa (deprotonated) at pHPZC pH.
Impact of variable operational parameters
Influence of initial medium pH
Undoubtedly, the medium pH is regarded as one of the most
significant operational parameters impacting on the overall
sorption process. It drastically effects on chemical speciation
(ionization) of sorbate, surface charge of sorbent in addition to
diffusion rate of sorbate from the working medium onto the
interior structure of sorbent (Rigueto et al. 2020). To explore
the influence of initial pH medium on CV and MO separation
using SP-IONPs sorbent, sorption experiments was investigat-
ed in the pHi range from 2.2 to 10.4. The strong dependency
of CV and MO sorption onto SP-IONPs sorbent can be illus-
trated as shown in Fig. 7(a). For CV dye, the equilibrium
sorption capacity was relatively low 7.8 mg g-1
(R% =
58.5%) in the acidic medium (i.e., pHi = 2.2) and gradually
increased along with an enhancement in the pHi up to
achieved 12.74 mg g-1
(R% = 95.5%) at pHi of 10.4. The
diminished sorption capacity at low pH values can be ascribed
by the intensive competition between the great number of
hydrogen ions (H+
) and CV dye molecules to be sorbed on
the sorbent active sites (unfavorable sorption). In addition, the
protonated sorbent surface in the mentioned conditions re-
stricted the sorption of CV molecules on its surface (interionic
repulsive forces). Whereas, the extent of CV sorption degree
greatly raised in alkaline environment (favorable sorption) can
be hypothesized to deprotonation of SP-IONPs surface which
effectively facilitated the sorption of CV on the negatively
charged sorbent surface via electrostatic attraction forces
(Mittal et al. 2021). Contrarily, for MO dye, sorption capacity
of SP-IONPs reached to a maximum value of 13.1 mg g-1
(R%
= 98.2%) at pHi of 2.2, and then sharply diminished up to
9.0 mg g-1
(R% = 67.5%) at pHi of 10.4. The elevated sorption
capacity at acidic environment can be attributed to protonation
of sorbent’s surface that in turn preferably contributing in an
establishment of electrostatic interaction between positively
charged sorbent surface (beneficial) to adsorb anionic MO
dye molecules. Moreover, with continuous enhancement in
the pHi (alkaline region), a noticeable reversible sorption per-
formance was induced. The sharply decrement in sorption
capacity degree can be deduced by shifting the charge of sor-
bent surface from positive to negative (deprotonation of
Fig. 3 SEM of (a) SP-IONPs before sorption of CV/MO dyes, (b) after CV sorption, and (c) after MO sorption at different magnifications of 10,000×,
15,000×, 20,000×, and 30,000×
65557

functional groups) that hinders the attraction of MO dye mol-
ecules onto SP-IONPs sorbent. Additionally, the decline in
sorption capacity can be illustrated by the generated competi-
tion between MO dye molecules and higher quantity of OH-
to
be sorbed on the SP-IONPs surface (Raza et al. 2021). These
findings matched with the recorded acid dissociation constant
(pKa) values of CV (MWt = 407.97 g/mol; pKa = 5.31 and
8.64) and the other pKa value of MO (MWt = 327.33 g/mol;
pKa = 3.7) which affirms the tendency of CV and MO dyes to
be sorbed onto SP-IONPS at alkaline and acidic environment,
respectively (Hou et al. 2007; Abdi et al. 2020). The influence
of medium pH on CV and MO elimination can also be dem-
onstrated on the pHZPC value (7.2) of SP-IONPs sorbent as a
persuasive factor for clarifying CV and MO sorption mecha-
nism (Fig. 7b) (Xiao et al. 2020). Notably, the admirable ap-
titude of SP-IONPs sorbent towards CV and MO under unfa-
vorable conditions probably means that other mechanisms are
involved in the binding of CV and MO on the SP-IONPs
sorbent. The sorption of both CV and MO dyes onto SP-
IONPs may be guided by electrostatic attraction forces as
follow:
SP‐IONPs þ OH‐
basic region
ð Þ→SP‐IONPs‐‐‐OH‐ ð2Þ
SP‐IONPs‐‐‐OH‐
þ CVþ
→SP‐IONPs‐‐‐OH‐
‐‐‐CVþ
ð3Þ
SP‐IONPs þ Hþ
acidic region
ð Þ→SP‐IONPs‐‐‐Hþ
ð4Þ
SP‐IONPs‐‐‐Hþ
þ MO‐
→SP‐IONPs‐‐‐Hþ
‐‐‐MO‐
ð5Þ
The possible sorption mechanisms of CV and MO dyes
onto SP-IONPs sorbent will be discussed in detail later in
the “Sorption mechanisms of CV and MO dyes onto SP-
IONPs sorbent” section.
Influence of sorbent concentration
Critically, sorbent concentration plays a substantial role in
whole sorption process assessment. Economically, a promis-
ing sorbent has to be able to eliminate considerable amounts
of sorbate at low doses to minimize operational costs.
Accessibility of sorbate molecules to the sorbent's active sites
is closely associated with the availability of sorbent surface
area. Ten different amounts of SP-IONPs sorbent ranging
from 0.5 to 5 g L-1
were conducted to investigate the influence
of sorbent dose increment on CV and MO dyes separation.
Figure S1 (see Supplementary Material) exposed the graphical
representation of the corresponding qe (mg g-1) and R% of
CV and MO against sorbent dose of SP-IONPs. The obtained
data unveiled that the CV and MO sorption efficiencies were
Fig. 3 continued.

strongly dependent on the SP-IONPs sorbent concentration.
Firstly, the adsorption of CV and MO increased from 88.7 to
94.3% and from 90.9 to 96.7%, respectively, with an incre-
ment in adsorbent dose from 0.5 to 4.5 g L-1
. While, further
increasing dosages of SP-IONPs, R% was slightly decreased
from 94.3 to 94.2% and from 96.7 to 96.6% for CV and MO
dyes, respectively. Increasing the adsorbent quantity im-
proved (favored) the CV and MO separation efficiency pro-
cess by enhancing the sorptive active site available for adsorp-
tion of the studied pollutants onto SP-IONPs (Marrakchi et al.
2020). While on increasing the adsorbent concentration be-
yond a certain limit may result in its agglomeration (over-
crowding), which makes the particle size larger and reduces
its active surface area. This can be interpreted by the screen
effect phenomenon coming down from the slight blocking of a
certain number of sorption sites. A similar explanation for CV
dye adsorption using Parthenium iron nanoparticles (Pa-INPs)
was conducted by (Rawat et al. 2021) who reported a decrease
in adsorption of dye on an increasing adsorbent dose above a
certain limit. Contrarily, the sorption capacities (qe) of SP-
IONPs fell sharply from 35.5 to 3.7 mg g-1
and from 36.3 to
3.8 mg g-1
for CV and MO dyes, respectively, with an incre-
ment of the SP-IONPs dosage, since the adsorbent dosage and
qe (mg g-1
) are inversely proportional. A similar investigation
was reported for tartrazine and Bordeaux red dyes removal
using greenly synthesized iron oxide nanoparticles (De Lima
Barizão et al. 2020). Thus, optimizing the relation between
adsorbent mass, qe and R% should be achieved.
Sorption kinetics analysis
Elucidation of sorption kinetics (time-dependent variations of
pollutant removal) is of practical importance for defining the
numerical sorption characteristics such as residence time,
sorption rate, and rate-controlling step. Investigating the effect
of operational time on the sorption of CV and MO using SP-
IONPs sorbent is helpful to realize the nature of sorption pro-
cess. Besides that, the derived findings based on the sorbent-
sorbate interaction are required for optimization of the process
design. As seen in Fig. 8, under selected experimental condi-
tions, sorption profiles of CV and MO by the as-used sorbent
preferentially exhibited an abrupt accelerated during the first
few minutes (initial phase). Presumably, the relative rapid
sorption rate (~ 60% at first 10 min) could be clarified by
the great availability of the unoccupied sorptive sites that
can be easily occupied as a result of high concentration gradi-
ent between them from one hand and CV and MO dyes mol-
ecules from the other hand (Prajapati and Mondal 2021).
Fig. 3 continued.
65559

Notably, with going on with the operational time, the sorption
rates became slower until the equilibrium was attained (pla-
teau condition). The last crowding diffusion (tapering off in
the sorption rate) could be stemmed from decline of concen-
tration gradient in addition to deficit of active binding sites
(Bhowmik et al. 2019). The kinetics outcome data derived
from CV and MO batch experiments were subjected to
PFORE, PSORE, WM and Elovich models, respectively
(a)
(b)
(c)
Fig. 4 EDX analyses of (a) SP-
IONPs before sorption of CV/MO
dyes, (b) after CV sorption, and
(c) after MO sorption
Table 2 Magnetic properties of SP-IONPs sorbent
Parameter Unit Value
Intrinsic coercivity (Hci) Oe 121.34
Saturation magnetization (Ms) emu/g 0.27075
Remnant magnetization (Mr) emu/g 20.68 × 10-3
Magnetic susceptibility (χ) emu/(g.Oe) 168.84 ×10-6

Fig. 5 (a) TEM images of SP-
IONPs dried in vacuum at 65o
C at
magnification of 10 nm and (b)
magnetization performance of
SP-IONPs
65561

(Jabli et al. 2020). PFORE model is a well-known model
assuming that the change of sorbate separation rate in term
of time is directly proportional to the difference in saturation
concentrations (Rigueto et al. 2020). Whereas PSORE theory
stated that the sorption controlling stage is disclosed by chem-
ical reaction (chemisorption) (Deniz and Kepekci 2016). The
matched graphs are depicted in (Figures S2a, b, see
Supplementary Material). The linearized fitting method was
employed to simulate the kinetics results and the R2
dynamic
parameter was used to assess the suitability of each model for
portraying the sorption process. Generally, the higher R2
, the
better the tested model fits. The calculated data are displayed
in Table 3. The resultant values of K2, R2
were 0.01103 g
mg-1
min-1
, 0.9987 and 0.0128 g mg-1
min-1
, 0.9996 for CV,
MO dyes, respectively, indicated that the sorption rate mainly
correlated with the PSORE model (chemisorption).
Theoretically, at solid-solution interface, the sorption path-
way consists of three main stages involving (i) film diffusion,
(ii) intra-particle diffusion, and (iii) interaction between sor-
bate molecules and sorbent active sites. In details, displace-
ment of sorbate from the bulk solution to the external bound-
ary layer surrounding sorbent surface firstly occurs. Then, a
reduction in the sorbate concentration in the examined solu-
tion is attributed to surpassing the boundary layer (external
mass–transfer resistance). Finally, after completion of the sor-
bate diffusion onward the sorbent interior surface, intense
interlinkage among sorbate molecules and superficial binding
groups on sorbent surface happens (Melo et al. 2018). The
overall kinetics sorption process of sorbate migration from
studied solutions onto sorbent surface may be governed by
the diffusional process and the surface chemical reaction. To
explore the governing sorption step, WM and Elovich
(a)
(b)
Fig. 6 Ultraviolet/visible (UV/
Vis) spectroscopy measurements
of Spirulina platensis extract with
different concentrations of FeCl3
solution

theories were widely tested. As summarized in the Figure S2c
(see Supplementary Material), the graphical drawings of CV
and MO sorption didn't pass through the origin, suggesting
that intraparticle diffusion step wasn't the sole controlling step
and the boundary layer may had a dominant role in the sorp-
tion process. The X values parameter values acquired from
fitting results gave an idea about the great contribution of
boundary layer on CV and MO sorption processes (Table 3).
Moreover, Elovich kinetics model was also studied. Its as-
sumption is built on the variations in sorption energies of
sorptive sites because of the heterogeneous nature of these
binding sites (Elwakeel et al. 2020). As derived from
Figure S2d (see Supplementary Material), the initial sorption
rate (α) and desorption constant (β) were 4.628 mg g-1
min-1
and 0.502 g mg-1
for CV dye in addition to 5.659 mg g-1
min-1
and 0.509 g mg-1
for MO dye respectively, reflecting the high
affinity of SP-IONPs towards CV and MO dyes, compared
with their values of 2.049 mg g-1
mi-1
and 0.632 g mg-1
as well
as 0.817 mg g-1
min-1
and 0.829 g mg-1
, reported for the
sorption of CV onto Citrus limon activated carbon and MO
dye onto cork powder adsorbents, respectively (Krika and el
Farouk Benlahbib 2015; Foroutan et al. 2021).
Sorption isotherm analysis
In particular, the isothermal analysis is necessary to deliver
important information characterized to sorbent surface prop-
erties; it's affinity to the examined pollutant, maximum sor-
bent capacity and sorption mechanism. As seen in Fig. 9, it
was presented that uptake trend (sorption rate) of CV and MO
onto SP-IONPs sorbent possessed an ascending performance
of mass transfer rate with an enhancement in the initial
(starting) concentration. Possibly, this is referred to the fact
of high concentration gradient between the plethora of sorbate
molecules and definitive sorptive sites which acting as a driv-
ing force to surpass the mass transfer resistance during CV and
MO sorption onto SP-IONPs (Noreen et al. 2020). This sig-
nificantly accelerates the sorption rate up to the equilibrium
state. Similar behavior was observed during the sorption of
CV and MO onto gum arabic-cl-poly(acrylamide)
nanohydrogel and polyaniline-kapok fiber nanocomposite
sorbents, respectively (Sharma et al. 2018; Gapusan and
Balela 2020). Fundamentally, LAM, FR, and TK isothermal
models were analyzed to manifest the most appropriate model
elaborating the mechanism of sorption reaction process. Their
Fig. 7 (a) Sorption capacities of
(mg g-1
) Removal % of SP-
IONPs for CV/MO dyes as a
function of pHi, and (b) Graph of
ΔpH (pHf – pHi) against initial pH
(pHi) from CV and MO sorption
(C0: 20 mg L-1
; T: 25 ± 1 o
C; t:
100 min; m: 1.5 g L−1
; V: 20 mL
and SS of 200 rpm)
65563

Linearized experimental data were graphically represented as
appeared in Figure S3 (see Supplementary Material) and their
calculated parameters were displayed in Table 4. Considering
the goodness of the utilized models by evaluating the obtained
R2
values, the analyzed results listed in Table 4 indicated that
sorption behavior of CV and MO onto SP-IONPs sorbent was
best fitted by LAM isotherm model rather than other models.
Maximum sorption capacities of SP-IONPs were reported to
be 256.41 mg g-1
and 270.27 mg g-1
for CV and MO dyes
respectively. Additionally, the calculated values of RL for CV
and MO dyes were in the range of 0.04–0.8, suggesting their
favorability to be sorbed onto SP-IONPs sorbent obeyed the
mentioned model. This was strongly supported by the calcu-
lated values of UOS % of SP-IONPs (~ 98%) for the two dyes.
Variations in sorption capacities between CV and MO dyes
were chiefly controlled by their different sorption rates onto
SP-IONPs sorbent. Since the molecular weight (MWt) of MO
is smaller that of CV, the sorption rate of MO is greater than
CV and therefore MO is preferentially to be sorbed on the
surface of SP-IONPs sorbent rather than CV. Moreover, by
comparing with CV, MO has less steric hindrance, so MO has
a higher UOS% for the active sites of SP-IONPs sorbent.
Sorption thermodynamics
Certainly, temperature plays a significant role in CV and MO
sorption performance onto SP-IONPs sorbent by influencing
on the diffusion rate of CV and MO molecules through exte-
rior boundary layer framing the SP-IONPs surface. The reli-
ance of sorption process on the working temperature was im-
plied. A perusal of Fig. 10 exposed the linear plot of van’t
Hoff equation for CV and MO sorption. The outlined results
revealed that the initial increase in temperature favored the
Table 3 Uptake kinetics parameters for the sorption of CV and MO
dyes onto SP-IONPs sorbent
Kinetic models CV MO
Pseudo first order model k1(min−1
) 0.046 0.045
qe (mg g−1
) 7.492 7.935
R2
0.9866 0.9901
Pseudo second order model k2 (g mg−1
min−1
) 0.01103 0.0128
qe (mg g−1
) 11.668 11.876
R2
0.9987 0.9996
Intraparticle diffusion model Ki (mg g−1
min0.5
) 2.721 2.467
0.813 0.786
0.034 0.046
X (mg g−1
) -1.661 -0.814
4.586 5.031
10.608 10.773
R2
0.957 0.972
0.969 0.968
0.857 0.893
Elovich model α (mg g−1
min−1
) 4.628 5.659
β (g mg−1
) 0.502 0.509
R2
0.950 0.955
Fig. 8 Uptake kinetics of (a) CV and (b) MO dyes sorption onto SP-IONPs (C0: 20 mg L−1
; T: 25 ± 1 o
C; t = 180 min; V: 200 mL; m: 0.3 g and SS of 200
rpm)

sorption of CV and MO onto the active sites of SP-IONPs
sorbent. This increase in sorption efficacy of sorbent may be
well explained by diminution in the solution viscosity and an
increase in the sorption spots number which has led to accel-
erate the mobility of CV and MO molecules towards the sor-
bent surface (Rajumon et al. 2019). An enhancement in the
diffusion rate of CV and MO molecules into SP-IONPs sor-
bent and strengthening in the physical bond between dyes
molecules and sorbent surface are considered as the major
reasons for exegesis the stronger sorption at high temperature
ranges (Pugazhendhi et al. 2018). Overall, sorption thermody-
namics analysis helps in comprehending the dependence of
sorption process on the environmental temperature, identify
optimum sorption parameters and explore the sorption mech-
anism. The computed main findings of ΔGo
, ΔSo
, and ΔHo
values were presented in Table 5. As listed in Table 5, the
negative magnitudes of ΔGo
at different running temperatures
seriously meant that CV and MO sorption onto SP-IONPs
sorbent was spontaneous in nature. Decrease in the values of
ΔGo
s with increasing temperature affirmed the favorability of
sorption process at higher temperatures and vice versa. The
positive values of ΔHo
signified the endothermic nature of
sorption process. Similarly, the positive signs of ΔSo
justified
an enhancement in the system randomness during sorption
pattern (Chen et al. 2019).
Influence of interfering ions (NaCl addition)
Background impurities such as cations (i.e., Na+
) and anions
(i.e., Cl-
) are usually employed as mordants in the dyeing
process, so they are often released along with the dyeing ef-
fluents. To accomplish dyes sorption from wastewater, the as-
used sorbent should simultaneously has an anti-interference
ability and good selectivity towards the target dye during the
extraction process (Zheng et al. 2020). As shown in Figure S4
(see Supplementary Material), the purification processes of
CV and MO dyes molecules by SP-IONPs sorbent were fun-
damentally dependent on the concentration of other compet-
ing (co-existing) ions present in the working environment.
With an increase in the concentration of NaCl, the capture
capacities of SP-IONPs sorbent towards CV and MO dyes
slightly dropped from 11.98 mg g-1
(R% = 89.9%) to
9.57 mg g-1
(R% = 71.8%) and from 12.32 mg g-1
(R% =
92.4%) to 9.99 mg g-1
(R% = 74.9%) for CV and MO dyes,
respectively. This phenomenon can be attributed to three as-
pects as follow: (i) the enhanced NaCl concentration aggra-
vated the competition between the Na+
, Cl-
ions and CV+
,
MO-
dyes molecules which consequently consumed some of
available sorption sites on the SP-IONPs; (ii) the enhanced
NaCl concentration increased the shield (screen) effect be-
tween CV and MO dyes molecules and SP-IONPs and hence
weakened the electrostatic interaction between sorbate and
sorbent surface; (iii) the ionic strength impacted on the activity
coefficient of CV and MO and therefore hindered (inhibited)
their transfer to the surface of SP-IONPs too. This was con-
sistent with the investigation of ionic strength effect on the
sorption of CV and MO onto Rhizophora mucronata stem-
barks and cationic surfactants modified coffee waste, respec-
tively (Lafi and Hafiane 2016; Oloo et al. 2020).
Repeatability performance
Irrefutably, the spent sorbent should exhibit a high recyclabil-
ity (reutilization) to save the gross of sorption process as well
as enhance its durability for practical applications.
Additionally, an excellent desorption process can open up
the prospect for the recovery of desorbed pollutants by numer-
ous techniques (i.e., an electrolysis process) and accordingly
insert them as a raw materials in numerous industries
(Cechinel et al. 2018). In the present work, multiple
sorption/desorption cycles experiments were consecutively
performed, the equilibrated sorption capacities of SP-IONPs
sorbent for CV and MO dyes and their decolorization
(recovery) efficiencies were mentioned. As noticed in
Table 6, the SP-IONPs still maintained a high R % (more than
94%) up to the fifth cycle of the initial use. Hence, regenera-
tion results obviously implied the high chelation (capture)
capacities of beneficial HCl and NaOH to desorb CV and
MO dyes molecules from the exhausted sorbent. Last but
not the least; the as-synthesized SP-IONPs has a good reus-
ability as proved above.
Comparison of SP-IONPs sorbent with other sorbents
for the sorption of CV and MO dyes
To prove the effectiveness of the developed SP-IONPs sor-
bent, comparing its sorption capacities towards CV and MO
dyes with those characterized to other sorbents is desirable. In
this context, Tables S6 S7 (see Supplementary Material)
Table 4 Isothermal parameters for the sorption of CV and MO dyes
onto SP-IONPs sorbent
Isothermal models CV MO
Langmuir KL(L mg−1
) 0.0203 0.0235
qexp (mg L-1
) 251.33 266.0
qm (mg L−1
) 256.41 270.27
R2
0.9874 0.9915
Freundlich n 2.0112 2.0157
Kf (mg g−1
) (L mg−1
)1/n 12.5572 13.9363
R2
0.963 0.959
Temkin A (L mg−1
) 0.5324 0.6108
B (J mol−1
) 62.96 60.00
R2
0.9559 0.9662
65565

highlight the maximum sorption capacities of numerous sor-
bents towards CV and MO dyes. It can be summarized that as-
prepared sorbent distinctly exhibited superior CV and MO
dyes separation efficiencies over the previously reported sor-
bents in the mentioned tables. Herein, the as-designed SP-
IONPs sorbent can be counted as a highly convenient and
efficient candidate for the purification of CV and MO dyes
from aqueous environment.
Potential implementation of SP-IONPs sorbent
for decontamination of CV and MO dyes from real samples
Indeed, evaluating the sorption performance of SP-IONPs sor-
bent towards CV and MO dyes from real water is considered
to be more realistic rather than simulated ones. It represents a
key criterion for judging the applicability of the designed SP-
IONPs as a color collecting material for CV and MO dyes. In
Fig. 9 Sorption isotherm of (a)
CV and (b) MO dyes sorption
onto SP-IONPs (C0: 10–1000 mg
L-1
; T: 25 ± 1 o
C; t = 100 min; m:
1.5 g L−1
; V: 20 mL and SS of 200
rpm)
Table 5 Thermodynamics parameters for the sorption of CV and MO dyes onto SP-IONPs sorbent
Dye ΔHo
(kJ mol−1
)
ΔSo
(kJ mol−1
K−1
)
R2
ΔGo
(kJ mol−1
)
298 K 308 K 318 K 328 K
CV 12.848 0.0568 0.989 - 4.083 - 4.652 - 5.220 - 5.788
MO 14.284 0.064 0.952 - 4.892 - 5.536 - 6.179 - 6.823

conformity with the studied operational parameters, reliability
of the as-prepared sorbent to remove CV and MO dyes from
real environment was achieved (Fig. 11). The physico-
chemical parameters of the collected TW samples were sum-
marized in Table S8 (see Supplementary Material). Under the
optimum conditions, the minimal R % of CV and MO dyes
was above 82% (Table 7). This can be logically clarified by
the complicated composition nature (in organic compounds)
characterized to real water which compete with CV and MO
dyes molecules to occupy the vacant sites of the SP-IONPs
sorbent.
For the treatment of textile dying wastewater real speci-
men, the treated wastewater was found to be slightly colorless
(~ 98% of color removal) and the measured COD value de-
creased from 340.0 mg L-1
(Table S9, see Supplementary
Materials) to 27 mg L-1
. These results demonstrated the great
potential of SP-IONPs sorbent as a color collecting material to
safeguard the public health of different biota.
Sorption mechanisms of CV and MO dyes onto SP-IONPs
sorbent
Typically, the sorption process of any sorbate onto sorbent surface
is led by several mechanisms. The binding groups present on the
SP-IONPs surface extremely play a dominant role in the sorption
process. After examining the desorption process, it is concluded
that there is still somewhat physical interaction between CV/MO
dyes molecules and the SP-IONPs as, after the desorption process,
some CV/MO dyes molecules remained on the surface of the SP-
IONPs sorbent. The decolorization potency of SP-IONPs towards
CV and MO dyes can be interpreted by (i) electrostatic attraction,
(ii) H-bonding, (iii) Lewis acid-base interaction, (iv) π- π interac-
tion, and (v) reduction process. Studying the influence of initial
solution pH on the sorption of two dyes by SP-IONPs proved the
contribution of the electrostatic phenomenon in pulling CV and
MO dyes molecules towards SP-IONPs sorbent as manifested by
(Eqs. 2–5). Besides, the presence of hydroxyl groups (OH-
) on the
Fig. 10 Van’t Hoff plots for CV
and MO dyes sorption onto SP-
IONPs (C0: 10–1000 mg L-1
; T:
25-55 o
C; t = 100 min; m: 1.5 g
L−1
; V: 20 mL and SS of 200 rpm)
Table 6 Desorption findings of sorbed CV and MO dyes from SP-IONPs surface after 5 times of sorption/desorption cycles
Sorption/desorption cycle CV MO
Amount sorbet
(mg g−1
)
Removal (%) DES (%) Amount sorbet
(mg g−1
)
Removal
(%)
DES (%)
First sorption operation 12.24 91.8 - 12.41 93.1 -
Cycle 1 12.05 90.4 98.42 12.20 91.5 98.33
Cycle 2 11.9 89.2 97.16 12.04 90.3 96.99
Cycle 3 11.73 88.0 95.80 11.88 89.15 95.75
Cycle 4 11.51 86.3 94.01 11.74 88.1 94.62
65567

SP-IONPs surface smoothly participates in its interaction with
organic dye molecules. Mostly, the possibility of H-bonding
configuration between characteristic rings of organic dyes and
-OH groups on the SP-IONPs should be counted (Saha et al.
2011). Additionally, the nitrogen atoms characterized for both
dye’s chemical structures acting as Lewis base interacts with
Fe3+
; thus, Lewis acid-base interaction occurs (Fadillah et al.
2020). Otherwise, the formed layers of CV (triarylmethane
category) or MO (azo category) dyes resulting from their
sorption onto SP-IONPs can easily facilitate their interaction
with their counterparts in the solutions via π-π interaction
between the benzene rings of free and captured dyes mole-
cules (Rawat et al. 2021). Moreover, the environmental syn-
thesis conditions (i.e., nitrogen or oxygen atmosphere) majorly
influence the efficacy of as-formed SP-IONPs. The produced
Fe0 resulting from the synthesis of SP-IONPs can react with
H2O and release electrons (e-
) which can be further consumed
by H+
to produce active hydrogen with a strong reducibility
character. The liberated (e-) from active hydrogen can be
endorsed with CV and MO dyes molecules and form –
C=N- and –C=C- attributing to benzene ring cleavage (Xiao
et al. 2020). The phenomena of CV and MO sorption by SP-
IONPs are schematically represented in Scheme 1.
Conclusion
In conclusion, the current study provides insights into the
manipulation of toxic crystal violet (CV) and methyl or-
ange dyes using greenly synthesized iron oxide nanoparti-
cles (SP-IONPs). The produced SP-IONPs sorbent was
characterized using FT-IR, BET surface area, SEM,
EDX, TEM, VSM, UV/VIS spectroscopy, and pHPZC.
The outcome data experimentally symbolized that the
sorption efficiency of SP-IONPs towards CV and MO
dyes was above 95% under optimized operational param-
eters. Furthermore, various kinetics and isotherm hypothe-
ses were adopted to fit the sorption data of CV and MO
dyes onto SP-IONPs. The modeling findings of kinetics
studies revealed that the sorption of both CV and MO
dyes was consistent with pseudo-second-order model.
The isotherm investigations conformed that CV and MO
sorption obeyed the Langmuir model with maximum sorp-
tion capacities of 256.41 mg g-1
and 270.27 mg g-1
for
CV and MO, respectively. Surprisingly, the productive SP-
IONPs were able to tackle more than 82% of the relevant
dyes from spiked tap water samples. To sum up, the re-
cyclable SP-IONPs sorbent can be considered as freestand-
ing candidate for the expulsion of CV and MO dyes lad-
en-wastewater.
Table 7 Sorption of CV and MO dyes from spiked real effluents using
SP-IONPs sorbent
Spiked tap water Spiked samples
Dyes concentration ( mg L−1
) 5 10 15 20
CV Removal % 87.1 86.0 84.6 82.0
Sorption capacity (mg g−1
) 2.91 5.73 8.46 10.93
MO Removal % 91.6 89.7 87.8 86.4
Sorption capacity (mg g−1
) 3.05 5.98 8.78 11.66
Fig. 11 Removal (%) of SP-
IONPs towards CV and MO from
spiked TW samples (concentra-
tions of dyes were varied between
5 and 20 mg L-1
)

Supplementary Information The online version contains supplementary
material available at https://doi.org/10.1007/s11356-021-15544-4.
Acknowledgements This work was performed at Faculty of Science,
Port-Said University, Port-Said, Egypt. The authors; therefore, acknowl-
edge with thanks the University technical support. Also, the authors
would like to thank Prof. Dr. Khaild Zaki Elwakeel, professor of
Environmental Chemistry, Environmental Science Department, Faculty
of Science, Port Said University, Egypt, for his cooperation and helpful
guidance in preparing the revised version.
Author contribution Shymaa M. Shalaby: conceptualization, method-
ology, investigation, and writing original draft.
Fedekar F. Madkour: conceptualization, methodology, and
investigation.
Hala Y El-Kassas: conceptualization, methodology, and
investigation.
Adel A. Mohamed: conceptualization, methodology, and
investigation.
Ahmed M. Elgarahy: conceptualization, investigation, data curation,
writing, reviewing, and editing.
Data availability All data generated or analyzed during this study were
included in the submitted article. In addition, the datasets used or ana-
lyzed during the current study were available from the corresponding
author on reasonable request.
Declarations
Ethics approval and consent to participate This study did not use any
kind of human participants or human data, which require any kind of
approval.
Consent for publication Our study did not use any kind of individual
data such as video and images.
Competing interests The authors declare no competing interests.
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