Green Chemistry: Advancing Planetary Phosphorus Sustainability through the Synergy of Graphene Oxide Modified with Magnetic Nanoparticles (M@GO) for Extracting Tertiary Effluent Phosphorus in Sewage Treatment Plants.

Abstract

Securing the enduring sustainability of global phosphorus (P) utilization has become a key societal priority. The application of green chemistry and green engineering presents an opportunity to mitigate these challenges...

Full text

rsc.li/es-nano
Environmental
Science
Nano
rsc.li/es-nano
ISSN 2051-8153
PAPER
Leanne M. Gilbertson et al.
Methodology for quantifying engineered nanomaterial release
from diverse product matrices under outdoor weathering
conditions and implications for life cycle assessment
Volume 4
Number 9
September 2017
Pages 1756-1912
Environmental
Science
Nano
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This article can be cited before page numbers have been issued, to do this please use: A. Muñoz-Garcia,
P. Montoro Leal, M. M. López Guerrero, C. Vereda-Alonso and E. Vereda Alonso, Environ. Sci.: Nano,
2024, DOI: 10.1039/D3EN00859B.

From a circular economy perspective, wastewater treatment plants (WWTPs) play a crucial role
in the management of phosphorus. Therefore, the removal and recovery of phosphate is
incredibly attractive, proposing itself as a sustainable approach to deal with the challenge of lack
of renewable P deposits and reduce the environmental impact of P in water.
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Environmental Science: Nano Accepted Manuscript
Open Access Article. Published on 12 April 2024. Downloaded on 4/13/2024 1:50:51 PM.
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DOI: 10.1039/D3EN00859B

1
Green Chemistry: Advancing Planetary Phosphorus Sustainability through the Synergy of
Graphene Oxide Modified with Magnetic Nanoparticles (M@GO) for Extracting Tertiary Effluent
Phosphorus in Sewage Treatment Plants.
Andrea Muñoz-Garcia1,, Pablo Montoro-Leal1,, María del Mar López Guerrero1,*, Carlos Vereda-
Alonso2,, and Elisa Vereda Alonso1
1- Department of Analytical Chemistry, University of Malaga, 29071 Malaga, Spain
2- Department of Chemical Engineering, University of Malaga, 29071 Malaga, Spain
* Correspondence: mmlopez@uma.es
 Equal contribution
Abstract
Securing the enduring sustainability of global phosphorus (P) utilization has become a key societal priority.
The application of green chemistry and green engineering presents an opportunity to mitigate these
challenges and contribute to the sustainable closure of the global phosphorus cycle by addressing the
extraction of phosphorus from waste and subsequent reuse.
In this manuscript the feasibility of a novel magnetic graphene oxide for wastewater phosphorus
recovery/removal is described. The primary technical benefit of this solid adsorbent lies in its easy
separation from treated water through magnetic field application. The key factors affecting the sorption
efficiency (contact time, pH, and adsorbent dosage) are studied. During the first 30 min, at pH 8 and with
a dosage of 0.8 g L-1, 25 % of the initial concentration is reduced. Among the 3 thermodynamic models
proposed, the Langmuir isotherm provides the best fit to the experimental results, with a maximum
adsorption capacity of 2.69 mg g-1. Four kinetic models are evaluated to describe the adsorption of
phosphorus on this magnetic graphene oxide for different initial adsorbate concentrations and adsorbent
dosages. Among them, Langmuir kinetics provide the best fit to the experimental data. The adsorption rate
constant is 0.72 L mg-1 h-1, and the desorption rate is 0.58 h-1, in accordance with the identified Langmuir
isotherm. Parameter values calculated from a mass transfer kinetic model indicate that the mass transfer of
phosphorus between the bulk liquid and the solid surface is not the rate-limiting step of the adsorption
process. Following the separation of this magnetic solid from the treated wastewater, an ammonia aqueous
solution can recover the phosphorus from the solid adsorbent. Preliminary results show absorbed
phosphorus recovery yields above 99% with a solid-liquid ratio up to 5 times higher than that used in the
adsorption process.
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Environmental Science: Nano Accepted Manuscript
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Keywords
Magnetic nanoparticles, Graphene oxide, Phosphorus adsorption, Wastewater, Adsorption kinetics models,
Phosphorus recovery.
Introduction
Phosphorus is an element of vital importance in the development of industry and agriculture. Phosphorus
is used in detergents, toothpastes, fireworks, and in the heads of matches.1
Phosphorus (P) together with nitrogen (N) and potassium (K) are biolimiting nutrients for plants, being
included as chemical constituents of fertilizers.2 Phosphate is necessary for plant growth. Phosphorus is not
found as a free element on earth due to its high reactivity. As a consequence of the high consumption of
phosphorus, more and more reserves of phosphate minerals are being exploited (1 g kg-1),3,4 being found
mainly as phosphate. Thus, P is a relatively restricted resource. Additionally, it is classified by the European
Union as a strategically important material, as indicated in its list of critical raw materials (CRMs).5 The
cost of phosphate rock (PR) utilized in fertilizer production impacts the expenses associated with food,
particularly in the case of cereals, and subsequently, it contributes to global hunger. As a result, numerous
countries are addressing this issue to tackle the challenges arising from the abrupt rise in the cost of
phosphate rock.6 From a circular economy perspective, wastewater treatment plants (WWTPs) play a
crucial role in the management of phosphorus. Therefore, the removal and recovery of phosphate is
incredibly attractive, proposing itself as a sustainable approach to deal with the challenge of lack of
renewable P deposits and reduce the environmental impact of P in water.7
On the other hand, anthropogenic phosphorus cycles are at the center of two major challenges, food
safety and environmental protection. Phosphate compounds, such as orthophosphates and polyphosphates,
are potential contributors to eutrophication. Therefore, their removal from industrial and domestic
wastewater is imperative prior to discharge. Certainly, the extraction and recuperation of phosphorus from
wastewater treatment plants (WWTPs) have the dual benefits of diminishing eutrophication and fulfilling
the need for phosphorus-based fertilizers.6 However, wastewater treatment and phosphorus removal from
wastewater is frequently an expensive process8 and so far, the results of phosphorus recovery from
wastewater for the production of fertilizers fall within a range from 10–25% to 70–90% of the phosphorus.
Effluent quality standards are under continuous update. Currently, in Europe, the allowable values for
discharges from urban wastewater treatment plants (UWWTP) to sensitive areas that are prone to
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Environmental Science: Nano Accepted Manuscript
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3
eutrophication are within the range of 1 to 2 mg L-1 for phosphorus.9 These values are even lower in the
US, being within the range 0.05 to 0.1 mgL-1 P.10
There are many methods available for the removal of phosphorus present in water. The most widely
used methods for phosphate removal are chemical precipitation using iron coagulants11–13 and biological
processes.14,15 However, these methods do not provide the removal rates required to gather the discharge
regulations. Chemical precipitation typically achieves removal rates between 50% and 80%, while
biological processes achieve removal rates ranging from 10% to 25%. Although chemical precipitation
remains a well-established and highly effective method for phosphorus removal, widely used in numerous
wastewater treatment plants (WWTPs), ongoing research is dedicated to significantly improving in
phosphorus removal to minimize the consumption of chemicals, reduce treatment costs and ensure a more
sustainable overall process. As a result, the recovery of P from wastewater is evidently a trending.
Sorption is a simple, economical, and environmentally friendly method of removing phosphorus from
water. Many studies have reported the use of different materials for phosphorus removal via sorption
(limestone, shale, slag, iron-rich gravel, zeolite, marble dust, etc.).16,17 Recently, the development and
application of nanoscale adsorbents have emerged as a promising methodology for remediating a diverse
range of contaminants from wastewater.18–21 Among available sorbents is found graphene oxide (GO). This
sorbent has a large surface area with a high density of polar oxygen, presenting different functional groups
such as epoxy, carboxylic acid, carbonyl and hydroxyl groups. Additionally, it features a rich system of
delocalized HH electrons that strongly interacts with organic compounds.22 GO can be modified with
magnetic nanoparticles (MNPs)23,24 for use in magnetic solid phase extraction (MSPE). In this work, a novel
patented magnetic graphene oxide25 was synthesized through double coupling between iron oxide MNPs
and GO, and was called M@GO. The double coupling confers greater stability and durability to the
material. A notable advantage of this adsorbent lies in its straightforward recoverability from treated water
through the application of a magnetic field.
To date, M@GO has been effectively employed for the chemical analysis of metals in aqueous media
as an on-line adsorbent.26–28 However, its application on a larger scale in the field of environmental
engineering remains unstudied. The objective of the current study was to assess the viability of M@GO as
a phosphorus sorbent, for both its removal and recovery from a real wastewater. This type of study requires
an understanding of the thermodynamics and kinetics associated with the adsorption process. Therefore,
the most important parameters affecting sorption performance (contact time, pH and adsorbent dosage) are
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Environmental Science: Nano Accepted Manuscript
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4
optimized. In only 30 min, with a dosage of 0.8 g L-1, the initial concentration of phosphate (expressed as
phosphorus) is reduced in a 25 %. These results are similar to or better than those reported in the
bibliography and to the best of the authors’ knowledge, the one making the recovery/removal using green
chemistry, avoiding the use of hazardous reagents and/or great volumes of those. In addition, M@GO can
be reused, thus the decontamination is performed without generating contaminated sludges, which are
difficult to manage.
Different thermodynamic and kinetic models are proposed to describe the phosphorus adsorption on
M@GO, indicating the rate-limiting step of the process. The surface morphology of M@GO, with and
without adsorbed phosphorus, was studied by X-ray photoelectron spectroscopy (XPS), and energy
dispersive X-ray spectroscopy-transmission electron microscopy (EDX-TEM). Additionally, this study
explores beyond mere removal and delved into the field of phosphorus recovery from M@GO, aiming to
convert waste into a valuable resource suitable for recycling as fertilizer.
Materials and methods
Wastewater and M@GO
The wastewater stock was collected from the tertiary treatment outlet of the Peñón del Cuervo UWWTP
(Málaga, Spain) managed by EMASA. The pH of the wastewater was 8.3 and the phosphate concentration
was 2.8 mg L-1 PO4-P. Microbiological parameters indicate the absence of bacteria, and the turbidity was
7.7 NTU. This wastewater stock was stored in 2 L plastic bottles in a freezer at -20 °C until use. It should
be noted that this UWWTP is not affected by the phosphorus levels required by the EU since it does not
discharge into sensitive areas.
The adsorbent nanomaterial, M@GO, was synthesized in the laboratory following the procedure
indicated in the bibliography 25.
Batch sorption experiments
The removal of phosphorus from the wastewater was studied by batch experiments. These experiments
were carried out by mixing a known mass of M@GO with 25 mL of wastewater within a 50 mL
polypropylene tube for different contact times. The temperature was maintained at 25 °C and mixing was
carried out on an end-over-end shaker at 40 rpm. Afterward, the supernatant was decanted, while the
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5
particles were retained within the polypropylene tube using a neodymium magnet. This method obviated
the need for a centrifugation step. All experiments were performed in triplicate.
The concentrations of phosphorus in the supernatants were determined by the Watanabe and Olsen
method 29, based on the ammonium molybdate spectrophotometric method. This method is based on the
formation of phosphomolybdate ion complexes (PO43-•12MoO4) and it is used for estimation of PO43-.
Therefore, this is the phosphorus species studied in this work, expressed as PO4-P.
The effect of the initial water pH on PO4-P removal was evaluated for a M@GO dosage of 0.8 g L-1.
Sodium phosphate solutions in deionized water buffered at different pHs (2 to 12) was used for these
experiments. The pH was set in each case using: diluted HCl (pH=2), acetic acid-sodium acetate buffers
(3<pH<5), boric acid-borax buffers (5<pH<10), and diluted NaOH (pH>10). In these experiments, the
contact time was 10 min and mixing was performed in an ultrasonic bath. The separation of the M@GO
and the determination of PO4-P in the supernatants were performed following the same procedure as
described above.
The effect of M@GO dosage on the PO4-P removal was studied by varying the solid to liquid ratio (S/L)
from 0.4g LQ to 1.6g LQ for the initial PO4-P concentration of the wastewater and under the optimized
initial pH.
The kinetic experiments were carried out for 3 different initial PO4-P s concentrations: The original
concentration (2.8 mg L-1 PO4-P), a higher one (5.4 mg L-1 PO4-P) and a lower one (1.2 mg L-1 PO4-P). The
highest concentration was achieved by spiking the necessary amount of sodium phosphate to the original
wastewater. The lower concentration was obtained by diluting the original wastewater with distilled water.
All those experiments were performed with a M@GO dosage of 0.8 g L-1. Additionally, kinetic experiments
were also conducted for the original wastewater using M@GO dosage of 0.4 g L-1 and 1.6 g L-1. The contact
times used were: 10 min, 20 min, 30 min, 1 h, 2 h, 3h, and 5 h.
The adsorption isotherm at 25 °C was obtained from the batch experiments results obtained at a M@GO
dosage of 0.8 g L-1 and at contact time of 5 h. The PO4-P initial concentration was varied from 1.2 mg L-1
PO4-P to 5.4 mg L-1 PO4-P. Additionally, the adsorption equilibrium was also studied at M@GO dosages
of 0.4 g L-1 and 1.8 g L-1 to assess the dosage effect on equilibrium.
The PO4-P sorption capacity of M@GO and the percentage of PO4-P removed from water were
obtained, at a contact time , as:

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
=

0




=
(

0


)


(1)
%

=

0



0
·100
(2)
where (mg g-1 PO4-P) is the amount of phosphate expressed as phosphorus retained by 1 g of M@GO

at time , (mg L-1 PO4-P) is the initial concentration of the phosphate expressed as phosphorus in the


0
aqueous phase, (mg L-1 PO4-P) is the concentration of phosphate expressed as phosphorus in the aqueous

phase at time , (L) is the volume of the aqueous phase, (g) is the mass of M@GO, and rSL (g L-1) is



the solid-liquid ratio.
Adsorption models
Adsorption isotherm
Three of the most widely used isotherm models were fitted to the results for the adsorption isotherm
experiments: Freundlich, Langmuir and Dubinin-Radushkevich. An extensive bibliography 30–35 can be
found on these models applied to different situations: gas-solid and liquid-solid adsorptions, adsorption on
heterogeneous surfaces, ion exchange, etc., and also in the phosphorus adsorption from the aqueous phase
on different adsorbents 36,37.
The Freundlich isotherm equation can be expressed as:


=



(
1

)

(3)
where qe (mg gQ PO4-P) is the amount of phosphate expressed as phosphorus adsorbed per unit mass
of M@GO at equilibrium, Ce (mg LQ PO4-P) is the phosphate expressed as phosphorus concentration in
the aqueous phase at the equilibrium, and Kf (mg1Q82 L1/n gQ), and n (dimensionless) are constants specific
for the adsorbate-adsorbent pair at a given temperature.
The Langmuir isotherm equation can be expressed as:


=


_





1
+




(4)
where qmax_L is the maximum concentration of phosphate expressed as phosphorus that can be adsorbed
on M@GO (mg gQ PO4-P) and KL is the Langmuir equilibrium constant (L (mg PO4-P)Q).
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7
The Dubinin-Radushkevich (DR) isotherm model is based on the potential theory of adsorption in which
the distribution of adsorption space is a function of adsorption potentials 30. This model is used to describe
the adsorption on a heterogeneous surface and allow the determination of the energy of adsorption 38.


=


_

exp
(

$
%
2
)
(5)
where qmax_DR is the maximum concentration of phosphate expressed as phosphorus that can be adsorbed
on M@GO (mg gQ PO4-P), S is a coefficient related to mean adsorption potential, and T is the Polanyi
potential, which is expressed as:
%
=
'
ln
(
1
+
1


)
(6)
where is the universal gas constant (J mol-1 K-1)
The value of the adsorption energy, E (J mol-1) is obtained from the following equation.
*
=
1
2
$
(7)
Kinetic models
The results from the batch kinetic experiments were fitted to the following four kinetic models:
Pseudo-first order equation
The “empirical kinetic equation proposed by Largergren in 1898” 39 is “the most widely used rate equation
for sorption of a solute from a liquid solution” 40. The integral form of this rate equation is:

=


(
1

exp
(

+
1

)
)
(8)
where q (mg gQ) is the concentration of adsorbate on the solid at time t, qe (mg gQ) is the
concentration of solute adsorbed on the solid at infinite time, and k1 (h-1) is the first-order rate constant.
Pseudo-second order equation
This model would be equivalent to the pseudo-first order but proposing that the adsorbate reacts with two
adsorption sites. The integral form of this rate equation is:

=

2

+
2

1
+


+
2

(9)
where k2 (g mg-1 h-1) is the second-order rate constant.
However, these empirical models can be derived from different theoretical approaches based on
fundamental theories of sorption kinetics 34,39.
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8
Kinetics approach to Langmuir isotherm
A kinetic derivation of the Langmuir isotherm can be achieved by assuming an elementary adsorption and
desorption mechanism whose rate is expressed by:
,
+

-.

#
,

/
/
=
+


(


_



)



+
/



(10)
where A is the adsorbate in the liquid phase, S# is an available site of the solid adsorbent for adsorption,
S#A represents the site of sorbent occupied by the adsorbate, ka (L mg-1 h-1) and kd (h-1) are the adsorption
and desorption rate constants, respectively, and rSL (g L-1) is the solid-liquid ratio. The difference (qmax_L -
q) is the concentration of free sites for adsorption, assuming a single adsorbate. Logically, the expression
for the Langmuir isotherm is obtained when the sorption rate is zero (equilibrium), where the ratio of the
kinetic constants is the Langmuir equilibrium constant (KL).

=


_

+

+
/

1
+
+

+
/

0


=
+

+
/
(11)
Solving the mass balance Eq. (1) for (q·rSL) yields

·


=
(

0


)
(12)
Substituting q·rSL into Eq. (10) and rearranging, the sorption rate is

/
/
=
+


2
+
(


_





0
+
1


)
+



+




0
(13)
Separating variables and integrating with the limits C=C0 when t=0:

1

0
+

/
=
1


0
/

2
+
(


_





0
+
1


)


1



0
=
1


0
/

2
+
2

+
3
(14)
The integral form of this rate equation is:

=
(
2
+
4
)
[
2

0
+
2

4
2

0
+
2
+
4
exp
(

+

4

)
]

(
2

4
)
2
(
1

[
2

0
+
2

4
2

0
+
2
+
4
exp
(

+

4

)
]
)
(15)
where the parameters b, c, and  are given by the following expressions:
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9
2
=


_





0
+
1


;
3
=


0


; 4
=
2
2

4
3
(16)
Mass transfer
This mechanistic approach is used when the overall rate of adsorption is limited by the rate of mass-transfer
of adsorbate between the bulk liquid and the surface of the adsorbent. The rate of mass transfer across an
interface that separates two phases may be expressed by:
/
/
=

+
3

(



9
)
=

+


(

9


)
(17)
where Ci (mg L-1) and qi (mg g-1) are the concentrations of the adsorbate at the interface, kc (dm h-1) is
the liquid-phase mass-transfer coefficient when using concentration gradient in the liquid phase, kq (g dm-
2 h-1) is the solid-phase mass-transfer coefficient when using concentration gradient at the solid phase, and
a (m-1) is the interfacial area per unit of total volume. The product of a mass-transfer coefficient and the
interfacial area per unit of volume is known as the volumetric mass-transfer coefficient: kca (h-1) and kqa (g
dm-3 h-1).
It is generally assumed that liquid and solid concentrations at the interface are in equilibrium. Thus, if
equilibrium were described by a Langmuir isotherm, the concentrations at the interface could be obtained
by simultaneously solving the following equations.
+
3
+

=
(

9


)
(



9
)

9
=


_




9
1
+



9
(18)
Solving the system Eq. (18) for the solute concentration at the liquid interface yields:

9
=

1
2
(
+

+
3
(


_



)
+
1




)
+
+
1
2
[
(
+

+
3
(


_



)
+
1




)
2
+
4
(
+

+
3



+



)
]
0.5
(19
)
The sorption rate as a function of the concentration of the adsorbate in the liquid phase is obtained by
elimination of q into Eq. (19) using the mass balance Eq. (1) and then substituting Ci into equation Eq. (17):
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10
/
/
=

+
3

(

+
1
2
(
+

+
3
(


_



0




)
+
1




)


1
2
[
(
+

+
3
(


_



0




)
+
1




)
2
+
+
4
(
+

+
3
(

0


)




+



)
]
0.5
)
(20)
To the best of authors’ knowledge, an integral form of (20) cannot be obtained, so in this case, that
integration is performed using the fourth-order Runge-Kutta integration method 41.
Model validation
The capability of the models presented above to describe the adsorption process has been assessed by fitting
those models to experimental data. The validation is performed by testing the goodness-of-fit of each
model. Here, the coefficient of determination (R2) and the average relative error (ARE) are used as
validation methods.

2
=
1

<

9
=
1
(

9


=
9
)
2
<

9
=
1
(

9


=
=
=
)
2
,*
=
1


<
9
=
1
|

9


=
9

9
|
(21)
where is the concentration value predicted by the model for the experimental value of , is the
=
9

9
=
average of the experimental concentrations, and n is the number of experimental points obtained in the
experiment.
Although many of the proposed models can be expressed in a linearized form, the fits are performed by
non-linear regression, as recommended by the literature 42–44. Linear modeling entails the transformation of
data usually at different scales, which limits the comparison of the parameters chosen for model validation.
Model assessment should be done at the original scale of the data 43.
For the adsorption isotherm models, non-linear two-parameter fits were performed by minimizing the
sum of square error (SSE):
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11
*
=

<
9
=
1
(

9


=
9
)
2
(22)
Once the adsorption isotherm model is chosen, non-linear one-parameter fits were performed for the
proposed kinetics models since the values of isotherm parameters have been previously obtained (kinetics
must be consistent with equilibrium). Thus, the only one unknown parameter is the corresponding rate
constant. However, non-linear two-parameter fit had to be performed for the mass transfer model, since the
two mass-transfer coefficients are unknowns. According to the proposed models, it is expected that the
kinetic parameters are independent of the M@GO dosage and of the initial concentration of PO4-P
(


)
. Thus, the fit of each model was performed simultaneously to all the experimental data obtained in the
(

0
)
5 kinetic experiments (3 dosages and 3 initial concentrations). The standard deviations in those experiments
are not the same for all experimental points, so the fits were performed by minimizing the sum of the
squares of the errors divided by their corresponding standard deviation 45:
(
?
2
)
?
2
=

<
9
=
1
(

9


=
9
)
2
@
2
9
(23)
Characterization
For the characterization of M@GO before and after phosphate adsorption, several instrument were used:
XPS analysis was conducted using a Physical Electronics ESCA 5701 instrument (Chanhassen, MN, USA),
the binding energies (BE) were assigned based on the position of the C 1s peak at 284.8 eV, the residual
pressure in the analysis chamber was maintained below 3 x 10-9 torr during data acquisition; the surface
morphology was studied by TEM-EDX images using a JEOL JEM-1400 instrument (Peabody, MA, USA);
and N2 adsorption isotherms were registered using a Micromeritics ASAP 2020 V4.02 instrument
(Norcross, GA, USA).
Desorption study
Preliminary tests indicated that ammonia solutions at pH values above 9.5 could serve as effective eluents
for extracting PO4-P from M@GO in the form of ammonium phosphate. Then, three parameters were
considered for the optimization of PO4-P desorption: ammonium hydroxide concentration, solution volume,
and contact time. A central composite design (CCD) was used to optimize the desorption conditions. This
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13
from 40% of PO4-P, for its highest initial concentration, to 60% for its lowest one. These removals are
practically identical to those obtained at a contact time of 24 h, therefore, for subsequent experiments,
equilibrium is considered to be reached after 5 h.
0
1
2
3
4
5
6
0 2 4 6 8 24
C (mg L-1)
t (h)
5.6 mg L¹
2.8 mg L¹
1.2 mg L¹
Fig. 2 Effect of contact time on aqueous PO4-P concentration for a M@GO dosage of 0.8 g L-1 and at three different initial PO4-P
concentrations
In addition, the PO4-P concentration of the original wastewater (2.8 mg L-1) is reduced in 30 min of
treatment to levels below those required by the EU for discharges in sensitive areas from UWWTP with an
equivalent population (p.e.) of between 10 000 - 100 000. Although this is sufficient to comply with the
EU criteria, which require adherence to either concentration values or percentage reduction, it should be
noted that if the percentage reduction criterion is chosen, the minimum value of 80% reduction would not
be achieved.
Adsorption isotherm
Fig. 3 shows the results of the adsorption experiments carried out until the equilibrium. The error bars
represent 2 times the standard deviation of the concentrations, both in the liquid (the horizontal ones) and
in the solid (the vertical ones), of triplicate experiments. This graph also differentiates the tests carried out
at different dosages of M@GO, making the effect of dosage on equilibrium visible as well. As can be seen,
there are no significant differences between the results obtained at dosages of 0.4 g L-1 and 0.8 g L-1.
However, the points obtained for a dosage of 1.6 g L-1 clearly deviate from the trend followed by the results
at lower dosages. This effect may be due to the agglomeration of the M@GO nanoparticles at high dosages,
which would reduce the free surface area per unit mass available for PO4-P adsorption.
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14
0
0.5
1
1.5
2
2.5
0 1 2 3 4
q (mg g-1)
C (mg L-1)
0.4 g L¹
0.8 g L¹
1.6 g L¹
Freundlich
Langmuir
DR
Fig. 3 Adsorption isotherm of PO4-P on M@GO at different M@GO dosages
Thus, the 3 isotherm models proposed (Freundlich, Langmuir and Dubinin-Radushkevich) were fitted
to the experimental results obtained at M@GO dosages below 1.6 g L-1. The Solver tool of Microsoft Excel
was used to perform a non-linear two-parameter fit for each model minimizing the SSE. The results of these
fits are shown in Fig. 3 and in Table 1. As can be seen, the best fit corresponds to the Langmuir isotherm,
based on both its higher R2 value and its lower ARE value. The second-best fit corresponds to Dubinin-
Radushkevich isotherm, whose graphical representation is almost indistinguishable from that of Langmuir.
The interest of the DR model is the adsorption energy, whose value here is lower than 8 kJ mol-1. This could
indicate that the process corresponds to a physical adsorption. 46
The value of qmax_L =2.69 mg g-1(Table 1) obtained for M@GO is similar or higher than others found in
the literature for other forms and types of carbon in real wastewater samples.47
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15
For comparative purposes, data of analogous methods reported in the literature are registered in Table
2. The direct comparison is difficult due to the different experimental conditions. Nonetheless, the removal
for the target element using the developed method were similar or better to those reported in the
bibliography and to the best of the authors’ knowledge, the one making the recovery/removal using green
chemistry not using hazardous reagents and/or great volumes of those; as well as the appearance of
contaminated sludge that must be disposed.
Table 1 Fits of the isotherm models to the experimental
equilibrium results.
Isotherm model
Parameters
KF (L1/n mg(1-1/n) g-1) =
1.46
n (-) =
2.75
R2 =
0.9821
Freundlich
ARE (%) =
5.44%
KL (L mg-1) =
1.25
qmax_L (mg g-1) =
2.69
R2 =
0.9983
Langmuir
ARE (%) =
2.23%
 (mol2 J-2) =
1.06 10-8
qmax_DB (mg g-1) =
3.21
E (kJ mol-1) =
7.04
R2 =
0.9931
Dubinin-Radushkevich
ARE (%) =
3.56%
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16
CPR Chemical phosphorus removal
EBPR Enhanced biological P removal
Kinetic study
Once it has been proven that the Langmuir isotherm adequately describes the adsorption equilibrium, the
next step is to find a kinetic model consistent with the equilibrium model. Thus, as indicated in section
Model validation, the parameters of the Langmuir isotherm calculated in the previous section will be
included in all proposed kinetic models. Therefore, the only unknowns are the kinetic parameters involved
in those models.
Table 2. Methods reported in the literature
Treatment process
Samples
P removed
Reference
Digested sludge
67%
Digested sludge
60%
CPR with FeSO4
addition
Dewatered sludge
47%
48
CPR with FeClSO4
addition
Liquid sludge
Dried granules
80%
49
EBPR
Activated sludge
9-26 %
50
AB technology with
Fe
addition
A-stage surplus
sludge
B-stage surplus
sludge
Digested sludge
52-55%
37-43%
47-59%
50
Not mentioned
Excess and digested
sludge
70-90%
51
CPR Activated carbon
Nitric-acid treatment,
thermal modification
treatment and
nitric-acid thermal
treatment
Groundwater and
wastewater
80%
52
CPR Laterite soil
Thermal treatment
and activated carbon
Thermal treatment
Synthetic wastewater
59-63%
68-69%
53
CPR with M@GO
Wastewater Malaga
75%
This work
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17
For pseudo-first and pseudo-second order models, this entails obtaining an expression for qe by
substituting the mass balance Eq. (1) in the Langmuir isotherm equation Eq. (4), which yields:


=
(
1
+



0
+


_





)
2






[
(
1
+



0
+


_





)
2

4

2





_


0
]
0.5
2




(24
)
And then, substituting Eq. (24) into the pseudo-first and pseudo-second order kinetic expressions, Eq.
(8) and Eq. (9) respectively.
For the MT model, it should be considered that the M@GO dosage affects the values of the volumetric
mass-transfer coefficients through the interfacial area per unit of liquid volume. Thus, if the two coefficients
for a dosage of 0.8 g L-1 are used as fitting parameters, those for a dosage of 0.4 g L-1 should be half of
those for a dosage of 0.8 g L-1. Analogously, the coefficients for 1.6 g L-1 should be twice those for 0.8 gL-1.
Furthermore, since it is expected that both initial PO4-P concentration and M@GO dosage do not affect
those kinetic parameters, the fit of each kinetic model is performed simultaneously to all the experimental
data obtained in 4 of the 5 kinetic experiments. The results obtained at a M@GO dosage of 1.6 g L-1 are
excluded from these fits for the same reason that they were excluded in the adsorption isotherm fits: M@GO
dosage does affect the equilibrium at values above 0.8 g L-1. However, once those fits are done, the results
of those models for a dosage of 1.6 g L-1 are also presented in the following figures and tables. The Solver
tool of Microsoft Excel was used to perform a non-linear one-parameter fit for each model (two-parameters
fit for MT model) minimizing the Chi-squared .
(
?
2
)
Fig. 4a shows the results of the kinetic experiments carried out with a M@GO dosage of 0.8 g L-1 for
different initial PO4-P concentrations. The dashed lines represent the fit of the proposed models to the
experimental data. At first glance, the pseudo first order (PS1), Langmuir kinetics (LK), and mass-transfer
(MT) models are able to adequately reproduce the experimental results, and therefore, also the effect of the
initial concentration on the adsorption process. The pseudo second order (PS2) model fails at the first 30
min of the experiment for an initial concentration of 2.8 mg L-1.
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0
1
2
3
4
5
6
0 1 2 3 4 5
C (mg L-1)
Time (h)
5.6 mg L¹ PS1
2.8 mg L¹ PS2
1.2 mg L¹ LK
MT
(a)
0
1
2
3
012345
C (mg L-1)
Time (h)
0.4 g L¹ PS1
0.8 g L¹ PS2
1.6 g L¹ LK
MT
(b)
Fig. 4 Effect of initial PO4-P concentration on adsorption kinetics for a M@GO dosage of 0.8 g L-1 (a). Effect of M@GO dosage on
adsorption kinetics for an initial concentration of 2.8 mg L-1 PO4-P (b). Experimental and model results
Similarly, Fig. 4b shows the results of the kinetic experiments carried out with an initial phosphorous
concentration of 2.8 mg L-1 for different M@GO dosage. As can be seen, again the PS1, LK and MT models
reproduce sufficiently well the effect of increasing the M@GO dosage from 0.4 g L-1 to 0.8 g L-1. However,
none of those models adequately reproduce the results obtained at a M@GO dosage of 1.6 g L-1, whose
results were excluded from these fits for the reasons already indicated above.
Table 3 shows the values of the parameters used to assess the goodness-of-fit of each model, as well as
the calculated values of the kinetic parameters. The column labeled "All" contains the R2 and ARE values
calculated with all results of the 4 kinetic experiments performed at M@GO dosages below 1.6 g L-1. The
rest of the columns present the values of those same parameters calculated individually for each kinetic
experiment, which yield from the same global fit.
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Table 3 Fits of the kinetics models to the experimental kinetic results.
C0 (mg L-1)
2.8
5.4
1.2
2.8
Model
Parameters
rSL (g L-1)
All
0.8
0.8
0.8
0.4
R2
0.993
0.957
0.916
0.940
0.959
PS1
k1 (h-1) =
2.87
ARE
5.1%
4.7%
4.2%
9.3%
2.3%
R2
0.989
0.858
0.893
0.990
0.949
PS2
k2 (g mg-1 h-1) =
5.17
ARE
4.9%
8.2%
4.8%
3.9%
2.6%
R2
0.993
0.968
0.897
0.959
0.944
LK
ka (L mg-1 h-1) =
0.72
ARE
4.8%
4.1%
4.5%
7.6%
2.9%
kca (h-1)* =
1.96
R2
0.993
0.953
0.913
0.950
0.947
MT
kqa (g L-1 h-1)* =
2.96
ARE
5.0%
4.9%
4.1%
8.6%
2.7%
* Values of the volumetric mass-transfer coefficient for a M@GO dosage of 0.8 g L-1.
As shown in Table 3, the best fit according to ARE corresponds to the LK model (4.8%), followed in
ARE increasing order by PS2, MT and PS1 models. If R2 is selected as validation criterion, no significant
differences are observed between the PS1, LK and MT, with PS2 presenting the worst value. Thus, the LK
model is the best model to simulate the adsorption process. Additionally, its theoretical basis is an advantage
over empirical models whose theoretical basis is not so clear.
Finally, the values of the volumetric mass-transfer coefficients obtained from the fit of the MT model
can indicate if mass-transfer is limiting the overall rate of the adsorption process. If M@GO nanoparticles
are assumed to be spheres about 100 nm in diameter (D), the mass-transfer surface area (interface) per unit
liquid volume can be estimated as

=


A
B
@
DE
6
(25)
where is the particle density, whose estimated value is . Thus,
A
BCDE
(
+G


3
)
2 700 +G


3
according to the value of shown in the Table 2, the value of the liquid mass-transfer coefficient would
+
3

be: . If the specific surface of particles measured by N2 adsorption/desorption isotherm
+
3
=
3·
10

8

K

1
(47.04 m2 g-1) were used instead of that of a sphere, the value of the liquid mass-transfer coefficient would
be: . Both estimates are well below the usual mass-transfer coefficients for complete
+
3
=
1.5·
10

8

K

1
suspension of solids in water, which are in the range of about to 54. This
2.7·
10

5

K

1
1.5·
10

4

K

1
difference of at least 3 orders of magnitude below the usual values in systems controlled by mass transfer
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indicates that the overall rate of the adsorption process is limited by the adsorption step and not by the mass-
transfer between the bulk liquid and the solid surface. That is, the very low mass-transfer coefficients
obtained from the MT model actually correspond to kinetic constants of the adsorption step.
Characterization
Fig. 5 shows a TEM image of the morphology of the M@GO surface (a), in which it can be observed that
the Fe3O4 nanoparticles, with a diameter between 12 and 20 nm, are randomly fixed between the graphene
oxide sheets. The amplification of the phosphorus region in the XPS spectrum shows the corresponding
peak (b). This figure also shows the EDX spectroscopy of M@GO before (c) and after (d) the phosphorus
adsorption. As can be seen, the adsorbed phosphorus (green dots) is mainly distributed on the iron nanopar-
ticles (pink color) and, to a lesser extent, also between the graphene sheets (blue color).
Fig. 5 TEM image of M@GO (a), XPS spectra P region (b), EDX images of M@GO before (c) and after (d) phosphorus adsorption
(a)
(c)
(d)
(b)
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From the N2 adsorption-desorption experiment for M@GO without P, a type IV isotherm was obtained,
which is typical of mesoporous materials. The nanomaterial presents a pore size and specific surface of
93 Å and 47.04 ± 0.16 m2 g-1.
Desorption study
The results of the 17 CCD experiments indicated that the parameter contact time was statistically significant
at the 95 % confidence level (p < 0.05), as can be seen in the pareto chart, Fig. 6a. The response surface
obtained is showed in Fig. 6b. Using the optimized parameters, 2 mL NH4OH 5% w/w, and a contact time
of 130 min, the absorbed PO4-P recovery yields (95 ± 4) % with a solid-liquid ratio up to 5 greater than the
ratio used in the adsorption process.
(a)
(b)
Fig. 6 CCD Results (a) Standardized pareto chart. (b) Response surface. The P desorption percentage is referred to phosphate
species.
The standardized pareto chart (Fig. 6a) illustrates that the only significant factor affecting phosphate
desorption is the contact time, with a p-value = 0.037 < 0.05, ammonia concentration (A) has a positive
influence on the response function (% P desorption) and ammonia volume (B) has a negative influence on
the response function, though both parameters are not significative on the response function. The central
composite factorial design results reveal a crossed interaction between the two factors under investigation
(ammonia concentration and volume of ammonia solution) leading to the curvature of the response surface.
As can be seen in Fig. 6b, the recovery of adsorbed PO4-P increases with ammonia concentration when the
volume of ammonia solution is low. However, contrary to expectations, this behavior is not observed for
higher volumes of ammonia solution. Similarly, the effect of ammonia solution volume follows the same
pattern. It would be expected higher PO4-P recovery for lower solution volumes (resulting in a higher solid
mass to solution volume ratio); however, this behavior is only evident for high ammonia concentrations.
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This crossed interaction between ammonia concentration and volume of ammonia solution likely arises
from the involvement of multiple equilibrium reactions in the overall recovery process. These preliminary
results do not allow for further conclusions regarding the recovery mechanism.
Conclusion
The new sorbent material (M@GO) can be used for the recovery/removal of phosphate from a real tertiary
effluent in sewage treatment plant. The adsorption process makes it possible to reduce the PO4-P
concentration to 75% of the least restrictive value allowed by European legislation for UWWTP discharges
in sensitive areas.
Langmuir isotherm model adequately describes the adsorption equilibrium, resulting in a maximum
adsorption capacity of 2.69 mg g-1. The adsorption energy value calculated by fitting the Dubinin-
Radushkevich isotherm (7.04 kJ mol-1) indicates that the process corresponds to a physical adsorption.
Among the four models studied, the Langmuir kinetic model best reproduces the PO4-P adsorption kinetics,
yielding an adsorption rate constant of 0.72 L mg-1 h-1.. The value of the liquid mass-transfer coefficient
resulting from the fit of the mass transfer model indicates that the rate-limiting
+
3
=
1.5·
10

8

K

1
)
step is the adsorption step and not the mass transfer between the bulk liquid and the solid surface.
The desorption results indicate that the adsorption process is reversible. Desorption using ammonia
solution yields PO4-P recoveries of (95 ± 4) % of that retained in the M@GO with a much higher solid-
liquid ratio than that used in the adsorption from the wastewater.
The results obtained after optimizing the most important parameters of the adsorption process (pH,
contact time, M@GO dosage), suggest that a continuous stirred tank adsorption process is a promising
application to investigate and for scaling up to an urban wastewater treatment plant. Thus, further research
should focus on assessing the monetary benefits and environmental risks, including the potential effects of
reusing the adsorbent for multiple adsorption/desorption cycles on phosphate removal and recovery
efficiencies.
Author Contributions
Andrea Muñoz-Garcia: Investigation, Methodology, Data curation. Pablo Montoro-Leal: Investigation,
Methodology, Data curation. María del Mar López-Guerrero: Investigation, Methodology, Supervision,
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Writing – review & editing. Carlos Vereda-Alonso: Software, Writing – original draft, Writing – review &
editing, Funding acquisition. Elisa Vereda Alonso: Conceptualization, Writing – review & editing, Funding
acquisition.
Conflict of interest
The authors have no relevant financial or non-financial interests to disclose.
Acknowledgements
Authors acknowledge all the collaboration from EMASA.
Funding
This work was supported by the Spanish Ministry of Science and Innovation [Research Project PID2021-
126794OB-100] and the University of Málaga [Support for Prototypes E3/05/21], [II Plan Propio (B1-
2022_20 and B4-2023-19, Universidad de Málaga] and CEI MAR funds. Muñoz-Garcia and Montoro-Leal,
also acknowledge the collaboration grant and fellowship FPU18/05371, respectively, obtained from the
Spanish Ministry of Science and Innovation. Funding for open access charge: Universidad de
Malaga/CBUA.
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References
1 S. Dey, A. Sreenivasulu, G. T. N. Veerendra, A. V. Phani Manoj and N. Haripavan, Synthesis and
characterization of mango leaves biosorbents for removal of iron and phosphorous from contaminated
water, Appl. Surf. Sci. Adv., 2022, 11, 100292.
2 F.W. Wellmer and R. W. Scholz, The Right to Know the Geopotential of Minerals for Ensuring Food
Supply Security: The Case of Phosphorus, J. Ind. Ecol., 2015, 19, 3–6.
3 N. Boujelben, J. Bouzid, Z. Elouear, M. Feki, F. Jamoussi and A. Montiel, Phosphorus removal from
aqueous solution using iron coated natural and engineered sorbents, J. Hazard. Mater., 2008, 151, 103–
110.
4 J.W. Choi, S.-Y. Lee, K.-Y. Park, K.-B. Lee, D.-J. Kim and S.-H. Lee, Investigation of phosphorous
removal from wastewater through ion exchange of mesostructure based on inorganic material,
Desalination, 2011, 266, 281–285.
5 G. A. Blengini, C. EL Latunussa, U. Eynard, C. Torres de Matos, D. Wittmer, K. Georgitzikis, C. Pavel,
S. Carrara, L. Mancini, M. Unguru, D. Blagoeva, F. Mathieux and D. Pennington, Study on the EU’s
list of Critical Raw Materials - Final Repor, Directorate-General for Internal Market, Industry,
Entrepreneurship and SMEs (European Commission), 2020.
6 F. Di Capua, S. de Sario, A. Ferraro, A. Petrella, M. Race, F. Pirozzi, U. Fratino and D. Spasiano,
Phosphorous removal and recovery from urban wastewater: Current practices and new directions, Sci.
Total Environ., 2022, 823, 153750.
7 P. J. A. Withers, J. J. Elser, J. Hilton, H. Ohtake, W. J. Schipper and K. C. van Dijk, Greening the global
phosphorus cycle: how green chemistry can help achieve planetary P sustainability, Green Chem., 2015,
17, 2087–2099.
8 A. Das Gupta, S. Sarkar, P. Ghosh, T. Saha and A. K. Sil, Phosphorous dynamics of the aquatic system
constitutes an important axis for waste water purification in natural treatment pond(s) in East Kolkata
Wetlands, Ecol. Eng., 2016, 90, 63–67.
9 The Council of the European communities, 2014.
10 J. López, M. Reig, E. Licon, C. Valderrama, O. Gibert and J. L. Cortina, Evaluating the integration of
nanofiltration membranes in advanced water reclamation schemes using synthetic solutions: From
phosphorous removal to phosphorous circularity, Sep. Purif. Technol., 2022, 290, 120914.
11 M. I. Aguilar, J. Sáez, M. Lloréns, A. Soler and J. F. Ortuño, Nutrient removal and sludge production
in the coagulation–flocculation process, Water Res., 2002, 36, 2910–2919.
12 A. H. Caravelli, E. M. Contreras and N. E. Zaritzky, Phosphorous removal in batch systems using ferric
chloride in the presence of activated sludges, J. Hazard. Mater., 2010, 177, 199–208.
13 Y. Zhou, X.H. Xing, Z. Liu, L. Cui, A. Yu, Q. Feng and H. Yang, Enhanced coagulation of ferric
chloride aided by tannic acid for phosphorus removal from wastewater, Chemosphere, 2008, 72, 290–
298.
14 T. Mino, M. C. M. van Loosdrecht and J. J. Heijnen, Microbiology and biochemistry of the enhanced
biological phosphate removal process, Water Res., 1998, 32, 3193–3207.
15 D. Mulkerrins, A. D. W. Dobson and E. Colleran, Parameters affecting biological phosphate removal
from wastewaters, Environ. Int., 2004, 30, 249–259.
16 E. Osama, O. Junya and H. Kazuaki, Removal of Phosphorus from Water Using Marble Dust as Sorbent
Material, J. Environ. Prot., , DOI:10.4236/jep.2012.38084.
17 C. A. Prochaska and A. I. Zouboulis, Removal of phosphates by pilot vertical-flow constructed wetlands
using a mixture of sand and dolomite as substrate, Ecol. Eng., 2006, 26, 293–303.
18 M. El-Kammah, E. Elkhatib, S. Gouveia, C. Cameselle and E. Aboukila, Enhanced removal of
Thiamethoxam from wastewater using waste-derived nanoparticles: Adsorption performance and
mechanisms, Environ. Technol. Innov., 2022, 28, 102713.
19 A. Gulyás, S. Genç, Z. S. Can and N. Semerci, Phosphate recovery from sewage sludge supernatants
using magnetic nanoparticles, J. Water Process Eng., 2021, 40, 101843.
20 M. Kaur, M. Kaur, D. Singh, M. Feng and V. K. Sharma, Magnesium ferrite-nitrogen–doped graphene
oxide nanocomposite: effective adsorptive removal of lead(II) and arsenic(III), Environ. Sci. Pollut.
Res., 2022, 29, 48260–48275.
21 M. Verma, R. Borah, A. Kumar, S.-H. Chae, S.-Y. Pan, V. Kumar, M. S. Vlaskin and H. Kim, Capturing
of inorganic and organic pollutants simultaneously from complex wastewater using recyclable
magnetically chitosan functionalized with EDTA adsorbent, Process Saf. Environ. Prot., 2022, 167, 56–
66.
22 D. R. Dreyer, A. D. Todd and C. W. Bielawski, Harnessing the chemistry of graphene oxide, Chem.
Soc. Rev., 2014, 43, 5288–5301.
Page 25 of 27 Environmental Science: Nano
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
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59
60
Environmental Science: Nano Accepted Manuscript
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23 B. F. do Nascimento, C. M. B. de Araújo, D. del Carmen Pinto Osorio, L. F. O. Silva, G. L. Dotto, J. V.
F. L. Cavalcanti and M. A. da Motta Sobrinho, Adsorption of chloroquine, propranolol, and metformin
in aqueous solutions using magnetic graphene oxide nanocomposite, Environ. Sci. Pollut. Res., 2023,
30, 85344–85358.
24 M. K. Goswami, A. Srivastava, R. K. Dohare, A. K. Tiwari and A. Srivastav, Recent advances in
conducting polymer-based magnetic nanosorbents for dyes and heavy metal removal: fabrication,
applications, and perspective, Environ. Sci. Pollut. Res., 2023, 30, 73031–73060.
25 Oficina Española de Patentes y Marcas, EP 4 095 097 A1, 2023, EP 4 095 097 A1.
26 P. Montoro-Leal, J. C. García-Mesa, I. Morales-Benítez, L. Vázquez-Palomo, M. del M. López Guerrero
and E. I. Vereda Alonso, Synthesis of a novel magnetic nanomaterial for the development of a
multielemental speciation method of lead, mercury, and vanadium via HPLC-ICP MS, Microchim. Acta,
2023, 190, 296.
27 P. Montoro-Leal, J. C. García-Mesa, I. Morales-Benítez, A. García de Torres and E. I. Vereda Alonso,
Semiautomatic method for the ultra-trace arsenic speciation in environmental and biological samples
via magnetic solid phase extraction prior to HPLC-ICP-MS determination, Talanta, 2021, 235, 122769.
28 I. Morales-Benítez, P. Montoro-Leal, J. C. García-Mesa, M. M. López Guerrero and E. Vereda Alonso,
New magnetic chelating sorbent for chromium speciation by magnetic solid phase extraction on-line
with inductively coupled plasma optical emission spectrometry, Talanta, 2023, 256, 124262.
29 F. S. Watanabe and S. R. Olsen, Test of an Ascorbic Acid Method for Determining Phosphorus in Water
and NaHCO3 Extracts from Soil, Soil Sci. Soc. Am. J., 1965, 29, 677–678.
30 M. M. Dubinin, The Potential Theory of Adsorption of Gases and Vapors for Adsorbents with
Energetically Nonuniform Surfaces., Chem. Rev., 1960, 60, 235–241.
31 H. Freundlich, Über die Adsorption in Lösungen, Z. Für Phys. Chem., 1907, 57U, 385–470.
32 N. D. Hutson and R. T. Yang, Theoretical basis for the Dubinin-Radushkevitch (D-R) adsorption
isotherm equation, Adsorption, 1997, 3, 189–195.
33 I. Langmuir, The constitution and fundamental properties of solids and liquids. Part I. Solids, J. Am.
Chem. Soc., 1916, 38, 2221–2295.
34 Y. Liu and Y.-J. Liu, Biosorption isotherms, kinetics and thermodynamics, Sep. Purif. Technol., 2008,
61, 229–242.
35 M. Villen-Guzman, D. Gutierrez-Pinilla, C. Gomez-Lahoz, C. Vereda-Alonso, J. M. Rodriguez-Maroto
and B. Arhoun, Optimization of Ni (II) biosorption from aqueous solution on modified lemon peel,
Environ. Res., 2019, 179, 108849.
36 N. Boujelben, F. Bouhamed, Z. Elouear, J. Bouzid and M. Feki, Removal of phosphorus ions from
aqueous solutions using manganese-oxide-coated sand and brick, Desalination Water Treat., 2014, 52,
2282–2292.
37 U. A. Edet and A. O. Ifelebuegu, Kinetics, Isotherms, and Thermodynamic Modeling of the Adsorption
of Phosphates from Model Wastewater Using Recycled Brick Waste, Processes, 2020, 8, 665.
38 M. A. Al-Anber, Thermodynamics Approach in the Adsorption of Heavy Metals, IntechOpen, 2011.
39 W. Rudzinski and W. Plazinski, Kinetics of Solute Adsorption at Solid/Solution 7#-j A
Theoretical Development of the Empirical Pseudo-First and Pseudo-Second Order Kinetic Rate
Equations, Based on Applying the Statistical Rate Theory of Interfacial Transport, J. Phys. Chem. B,
2006, 110, 16514–16525.
40 Y. S. Ho, J. C. Y. Ng and G. McKay, Kinetics of Pollutant Sorption by Biosorbents: Review, Sep. Purif.
Methods, 2000, 29, 189–232.
41 S. Chapra and R. Canale, Numerical Methods for Engineers, McGraw-Hill Education, 2009.
42 E. C. Lima, F. Sher, A. Guleria, M. R. Saeb, I. Anastopoulos, H. N. Tran and A. Hosseini-Bandegharaei,
Is one performing the treatment data of adsorption kinetics correctly?, J. Environ. Chem. Eng., 2021, 9,
104813.
43 E. D. Revellame, D. L. Fortela, W. Sharp, R. Hernandez and M. E. Zappi, Adsorption kinetic modeling
using pseudo-first order and pseudo-second order rate laws: A review, Clean. Eng. Technol., 2020, 1,
100032.
44 J.-P. Simonin, On the comparison of pseudo-first order and pseudo-second order rate laws in the
modeling of adsorption kinetics, Chem. Eng. J., 2016, 300, 254–263.
45 W. H. Press, S. A. Teukolsky, W. T. Vetterling and B. P. Flannery, in Numerical Recipes in C: The Art
of Scientific Computing, Cambridge University Press, 2nd ed., 1992.
46 M. Chabani, A. Amrane and A. Bensmaili, Kinetic modelling of the adsorption of nitrates by ion
exchange resin, Chem. Eng. J., 2006, 125, 111–117.
47 X. Xiao, S. Liu, X. Zhang and S. Zheng, Phosphorus removal and recovery from secondary effluent in
sewage treatment plant by magnetite mineral microparticles, Powder Technol., 2017, 306, 68–73.
Page 26 of 27Environmental Science: Nano
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
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26
48 E. Frossard, J. P. Bauer and F. Lothe, Evidence of vivianite in FeSO4-flocculated sludges, Water Res.,
1997, 31, 2449–2454.
49 M. S. Poffet, K. Käser and T. A. Jenny, Thermal Runaway of Dried Sewage Sludge Granules in Storage
Tanks, CHIMIA, 2008, 62, 29–29.
50 P. Wilfert, A. Mandalidis, A. I. Dugulan, K. Goubitz, L. Korving, H. Temmink, G. J. Witkamp and M.
C. M. Van Loosdrecht, Vivianite as an important iron phosphate precipitate in sewage treatment
plants, Water Res., 2016, 104, 449–460.
51 P. Wilfert, A. I. Dugulan, K. Goubitz, L. Korving, G. J. Witkamp and M. C. M. Van Loosdrecht,
Vivianite as the main phosphate mineral in digested sewage sludge and its role for phosphate recovery,
Water Res., 2018, 144, 312–321.
52 A. M. E. Khalil, O. Eljamal, T. W. M. Amen, Y. Sugihara and N. Matsunaga, Optimized nano-scale
zero-valent iron supported on treated activated carbon for enhanced nitrate and phosphate removal from
water, Chem. Eng. J., 2017, 309, 349–365.
53 U. Sreekumar, A. K. Thalla and V. V. Nair, Comparative evaluation of phosphate abatement using
laterite soil and activated carbon, Int. J. Environ. Sci. Technol., 2019, 16, 4493–4502.
54 V. J. Inglezakis, M. Balsamo and F. Montagnaro, Liquid–Solid Mass Transfer in Adsorption Systems—
An Overlooked Resistance?, Ind. Eng. Chem. Res., 2020, 59, 22007–22016.
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Open Access Article. Published on 12 April 2024. Downloaded on 4/13/2024 1:50:51 PM.
This article is licensed under a
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DOI: 10.1039/D3EN00859B