New insights into the selective adsorption mechanism of cationic and anionic dyes using MIL-101(Fe) metal-organic framework Modeling and interpretation of physicochemical parameters.pdf
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New insights into the selective adsorption mechanism of cationic and anionic dyes using MIL-101(Fe) metal-organic framework Modeling and interpretation of physicochemical parameters.pdf
2. Journal of Contaminant Hydrology 247 (2022) 103977
2
nanoparticles–based adsorbents that have different active adsorption
sites and great surface areas are very promising for water purification
(Ahmadijokani et al., 2020).
Metal-organic frameworks (MOFs), a new group of porous crystalline
materials, consist of organic ligands and the desired metal clusters
(Younas et al., 2020). Recently, MOFs have been employed in various
applications as catalysis, storage of hydrogen, separation of gases, drug
delivery, CO2 capture, sensing, and wastewater purification (Younas
et al., 2020). The suitability of MOFs for different applications is due to
the appropriate pore size, high surface area, simplicity of preparation,
and the high stability of these porous coordination polymers (Mandal
et al., 2018; Jhung et al., 2012; Cavka et al., 2008). Moreover, the
simplicity for modification of MOFs structure can change their textural
properties thus contributing to increase and widen the acceptability of
these materials in numerous fields. Generally, MOFs are fabricated via
bridging metal ion clusters with metal ions using organic linkers and the
experimental parameters as reaction time, pressure, temperature, solu
tion pH, and the used solvent are considered (Safaei et al., 2019). Defects
in the preparation conditions (i.e., metal ion, organic ligand, metal-
ligand coordination geometry, pore surface hydrophobicity) have
contributed to decrease the stability of MOFs for industrial applications
(Adegoke et al., 2020; Burtch et al., 2014; Wang et al., 2016; Yuan et al.,
2018). A water–stable MOFs type, MIL-101 (MIL: Material Institute
Lavoisier), was combined with different metal precursors as chromium
(Cr) (MIL-101-Cr), aluminum (Al) (MIL-101-Al), and iron (Fe) (MIL-101-
Fe) and finally used in water purification (Li et al., 2019a; Zhao et al.,
2018).
Different varieties of MOFs such as an amino-functionalized Zr-based
MOFs (Lv et al., 2019), RGO/ MOFs (Liu et al., 2019), MIL-101-Cr, MIL-
53-Al and ZIF-8 (Zhang et al., 2020), Cu-BTC-1 (Mantasha et al., 2020),
Cu-BTC (Kaur et al., 2019), MOF-235 (Haque et al., 2011), UiO-66
(Molavi et al., 2018), and Fe-MIL-101 (Konik et al., 2019) were used
in the adsorption of MB and MO. In several adsorption systems, the
experimental data modeling was conducted with the Langmuir and
Freundlich equations. The fundaments and hypothesis of these classical
models cannot provide a scientific meaning and reliable explanation for
the influence of operating parameters as the adsorbate concentration
and temperature in the removal mechanism (Ramadan et al., 2021;
Sharib et al., 2021). To obtain a better understanding of the MO and MB
removal mechanisms by MIL-101(Fe), it is mandatory to use the
Fig. 1. Characterization results of MIL-101(Fe) adsorbent: (a) XRD pattern, (b, c) SEM images, and (d) FTIR spectrum.
M. Shakly et al.
3. Journal of Contaminant Hydrology 247 (2022) 103977
3
advanced adsorption models based on the statistical physics theory
(Mohamed et al., 2020; Seliem et al., 2020). This theory is appropriate to
provide additional steric and energetic parameters for the analysis and
explanation of the adsorption mechanism. The interpretation of the
physicochemical factors associated to MB and MO adsorption onto MIL-
101 (Fe) can contribute to clarify the adsorption mechanisms at the
molecular level (Ramadan et al., 2021; Sharib et al., 2021; Mohamed
et al., 2020). The main objectives of the current study were to (a)
characterize MIL-101(Fe) metal organic framework prepared through a
facile solvothermal method, (b) determine the removal performance of
the prepared MIL-101(Fe) for MB and MO dyes in single and binary
systems, (c) use different models (i.e., kinetics and isotherms) to
describe the adsorption processes, (d) provide new results and a deeper
understanding of the interactions between dyes molecules and MIL-101
(Fe) active sites via statistical physics calculations, and (e) evaluate the
possibility of the multiple use of MIL-101(Fe) for the adsorption of
cationic and anionic dyes. Overall, the results of this study contributed
with new insights into MO and MB adsorption performance and mech
anism using MIL-101(Fe) as an adsorbent.
2. Materials used and methods
2.1. Materials
Ferric chloride hexahydrate (FeCl3.6H2O: Alpha Chemica India),
terephthalic acid (Benzene-1,4-dicarboxylic acid BDC: Merck KGaA
Germany) and N, N-dimethyl formamide (DMF: Techno Pharm chem
India) were used in this study in the preparation of MIL-101(Fe)
adsorbent. MO and MB dyes and absolute ethyl alcohol (99.9%) were
supplied from Merck KGaA, Germany and the two dyes were tested as
adsorbates.
2.2. Synthesis of MIL-101(Fe)
MIL-101(Fe) was synthesized by a hydrothermal method following a
similar procedure reported by (Hu et al., 2019). Ferric chloride hexa
hydrate (FeCl3.6H2O) and Benzene-1,4-dicarboxylic acid was dissolved
in DMF with 2:1 ratio, the suspended solution was sonicated for 20 min
until was fully homogenous, and then it was transferred into a Teflon-
lined stainless-steel autoclave and heated at 110 ◦
C for 20 h. The light
orange product was separated via centrifugation and purified by
washing with DMF for 3 h followed by hot ethanol for 2 h. This step was
repeated for three times in order to remove the unreacted BDC. The
purified product was dried at 70 ◦
C for 30 min, and then activated at
150 ◦
C for 10 h before carrying out the dye adsorption studies.
2.3. Characterization of MIL-101 (Fe)
MIL-101 (Fe) was characterized using different techniques to inves
tigate its physical and chemical properties. X–ray diffraction (XRD)
pattern of this adsorbent was recorded in the 2θ range from 5.02–79.98◦
using a Panalytical Philips diffractometer operated at 40 kV and 35 mA
under Cu–kα radiation (λ = 0.154 nm). The surface morphology of MIL-
101 (Fe) was observed using (Zeiss Sigma 500 VP analytical FE-SEM).
The samples were coated with gold before analysis to enhance the sur
face conductivity. The functional groups of MOF surface were investi
gated with FTIR spectrum (Bruker optics -vertex 70 equipment) at
constant ambient temperature of 25 ◦
C by accumulating 10 scans at 1
cm− 1
resolution in the 4000–400 cm− 1
region. The Iso-ionic point and
the particles size range of MIL-101 (Fe) were obtained from Zeta po
tential and sizer (Malevern Zeta sizer-nano series ZS 90). The ther
mogravimetric analysis (TGA) and differential scanning calorimetry
(DSC) were performed to analyze the thermal stability of MIL-10(Fe) in
the temperature range of 25–1200 ◦
C with a heating rate of 10 ◦
C/min
(Labsys evo Setaram, France).
2.4. Adsorption experiments of MB and MO dyes
Stock solutions (1000 mg/L of MB and 2000 mg of MO) were pre
pared, and the desired initial concentrations of the tested dyes were
obtained using dilutions with high purity distilled water. To avoid any
possible reactions associated to the photo catalytic process, all adsorp
tion experiments were conducted in dark glasses in the absence of light.
After each adsorption experiment, the solutions containing MO and MB
dyes were separated from MIL-101 (Fe) adsorbent using syringe filters.
The concentrations of the studied dyes in the filtrated solutions were
quantified using a UV double beam spectrophotometer at 664 and 464
nm for MB and MO dyes, respectively. All MO and MB removal experi
ments were performed in triplicates and the results were averaged for
Fig. 2. TGA (a) and DSC (b) analysis of MIL-101(Fe).
Fig. 3. Effect of pH on the removal of MO and MB dyes by MIL-101(Fe)
adsorbent. The error bars were obtained from triplicate experiments.
M. Shakly et al.
4. Journal of Contaminant Hydrology 247 (2022) 103977
4
data evaluation obtaining standard deviations below 5.2%.
2.5. MO and MB adsorption kinetics
To study the adsorption kinetics, the experiments were performed at
the following conditions: 0.15 g of MIL-101 (Fe) mass, pH 4.0 for MO
and 9.5 for MB, initial concentrations of 30 mg/L for MB and 100 mg/L
for MO, adsorption temperature of 20 ◦
C, and different contact times
ranging from 15 min to 14 h. The adsorbed amounts (qt) of MB and MO
after each time interval were calculated using Eq. (1).
qt (mg/g) = (C0–Ct)
V
m
(1)
Where C0 (mg/L) is the initial concentrations of MO and MB, Ct (mg/
Fig. 4. The adsorption kinetics of MB and MO onto MIL-101(Fe) adsorbent at 20 ◦
C. (a, b) contact time effect, (c, d) Pseudo-First Order, (e, f) Pseudo-Second Order
and (g, h) intra-particle diffusion.
M. Shakly et al.
5. Journal of Contaminant Hydrology 247 (2022) 103977
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L) is the final concentration of the tested dyes after time (t), V (L) is the
solution volume, and m (g) is the mass of MIL-101 (Fe) adsorbent (g). To
study the MO and MB adsorption onto MIL-101(Fe) and to identify the
possible rate–controlling step, the kinetic results were fitted to the
pseudo-first order (Lagergren, 1898), the pseudo-second order (Ho and
McKay, 1999), and intra-particle diffusion (Weber and Morris, 1962)
models as given below:
ln(qe − qt) = lnqe − k1t Pseudo − first − order (2)
t
/
qt =
(
1
/
k2q2
e
)
+ t
/
qe Pseudo − second − order (3)
qt = kp t1/2
+ C Intra − particle diffusion (4)
where k1(min− 1
) and k2(g/mg min) indicate the adsorption rate
constants of the first order and the second-order kinetics, respectively.
For the intra-particle diffusion equation, kp(mg/g min) and C (mg/g)
represent the rate constant and the intercept value of this model,
respectively.
2.6. MO and MB adsorption isotherms
Equilibrium studies related to MO and MB adsorption onto MIL-101
(Fe) were performed at the next conditions: Initial dye concentrations
(25–500 mg/L for MB and 25–2000 mg/L), three temperatures (20, 40
and 60 ◦
C), 0.01 g of MIL-101 (Fe), and solution pH 4 for MO and 9.5 for
MB. In all these experiments, 10 mg of the tested adsorbent was mixed
with 30 mL of each dye solution at 300 rpm using digital orbital shaker
(SHO–2D, Germany). The liquid phases containing MO and MB were
Table 1
Parameters of Kinetic Models for the Adsorption of MO and MB onto MIL101-Fe.
Co (mg/L) K1 (min 1
) qe exp (mg/g) qe cal (mg/g) R2
Dye Pseudo-first order
MO 100 0.316 80.238 0.909
MB 28 0.073 2.5 0.903
Dye Pseudo-second order
MO 100 0.01 104 108.7 0.993
MB 28 4.538 27.1 27.1 1
Fig. 5. Modeling of adsorption isotherms of MB and MO dyes on MIL-101 (Fe) MOF using Langmuir and Freundlich equations.
M. Shakly et al.
6. Journal of Contaminant Hydrology 247 (2022) 103977
6
separated and the concentrations of these dyes in solutions were
determined. The equilibrium adsorption quantities of MO and MB dyes
(qe) were calculated using Eq. (1) assuming that t corresponded to the
equilibrium time.
2.6.1. Classical modeling analysis
The experimental data of MB and MO were fitted to the Langmuir
(1916) and Freundlich (Freundlich, 1906) equations to find the suitable
classical model that can describe the adsorption of dyes onto MIL-101
(Fe) at equilibrium. The non-linear relations defined the Langmuir
(Eq. (5)) and Freundlich (Eq. (6)) isotherm models are given below:
qe =
qmax KLCe
(1 + KLCe )
(Langmuir model) (5)
qe = KF Ce
1/n
(Freundlich model) (6)
where qmax (mg/g) is the maximum adsorption capacity and KL (L/
mg) represents the constant of the Langmuir model. On the other hand,
KF((mg/g)(mg/L)− 1/n
)) signifies the adsorption capacity and n is the
heterogeneity factor of the Freundlich model.
2.6.2. Advanced modeling analysis
Herein, three advanced statistical physics models (i.e., single layer,
double layer, and multilayer) were employed to investigate the dye–
MIL-101 (Fe) interaction (Ramadan et al., 2021; Sharib et al., 2021).
• Firstly, the single layer model assumes that MO and MB removal by
MIL-101 (Fe) was in the form of one layer directed by a distinctive
adsorption energy (ΔE) (i.e., the adsorption of each tested dye mol
ecules via MIL-101 (Fe) active sites is nearly governed by a constant
energy). The mathematical expression of this advanced single layer
adsorption model is given by Eq. (7) (Sharib et al., 2021).
qe =
nDM
1 +
(
c1/2
c
)n (7)
where n is the number of MO and MB dye molecules captured by active
site of MIL-101(Fe) adsorbent, C1/2 signifies the concentration at half-
saturation related to the formed adsorbate layer, and DM is the density
of adsorption sites.
• Secondly, the double layer model suggests that these adsorption
systems are characterized by the formation of two layers of dyes
molecules (MB or MO) with two dissimilar adsorption energies (i.e.,
ΔE1 for dye– MIL-101 (Fe) interaction and ΔE2 for MB–MB or
MO–MO interaction). The mathematical expression of the double
layer model is denoted by Eq. (8) (Sharib et al., 2021).
qe = nDM
(
c
c1
)n
+ 2
(
c
c2
)2n
1 +
(
c
c1
)n
+
(
c
c2
)2n
(8)
where C1 and C2 describe the two concentrations at half saturation
attributed to the first and second removed dye layers, respectively.
• Finally, the multilayer model suggests that the MO and MB adsorp
tion onto MIL-101(Fe) results in the formation of an adaptable but
limited number of adsorbed dye (i.e., MO or MB) layers. Conse
quently, the total adsorbed layers number (1 + N2) of each tested dye
is related to a single layer (N2 = 0), double layer (N2 = 1), and
multilayer process (N2 > 1) (Mohamed et al., 2020). Similar to the
double layer model, two energies (ΔE1 and ΔE2) were associated to
this multilayer model (i.e., dye–MIL-101(Fe) and dye-dye
Table 2
Parameters of isotherm equations for the MB adsorption on MIL-101 (Fe) MOF.
Isotherm Model 20 ◦
C 40 ◦
C 60 ◦
C
Langmuir
qmax (mg/g) 402.00 391.67 352.96
kL (L/mg) 0.0365 0.0227 0.0165
R2
0.9613 0.9618 0.9660
Freundlich
kF ((mg/g)(mg/L)− 1/n
) 75.51 64.76 49.26
1/n 0.2881 0.3617 0.3283
R2
0.9408 0.9443 0.9554
Multiple layer
N2 3.12 3.08 2.49
Qsat (mg/g) 373.74 348.07 325.08
R2
0.9742 0.9962 0.9844
Table 3
Parameters of isotherm equations for the MO adsorption on MIL-101 (Fe) MOF.
Isotherm Model 20 ◦
C 40 ◦
C 60 ◦
C
Langmuir
qmax (mg/g) 1066.96 934.17 830.97
kL (L
mg
) 0.0077 0.0071 0.0068
R2
0.9449 0.9447 0.9398
Freundlich
kF ((mg/g)(mg/L)− 1/n
) 67.60 53.36 45.27
1/n 0.4019 0.4141 0.4190
R2
0.8834 0.8700 0.8537
Multiple layer
N2 5.36 5.01 3.76
Qsat (mg/g) 908.39 777.01 677.92
R2
0.9631 0.9655 0.9632
Table 4
Comparison of the adsorption capacities of several Metal Organic Frameworks
for the removal of MB & MO dyes.
Adsorbent Dye
adsorbate
Adsorption
capacity (mg/
g)
Optimum
pH
T
(◦
C)
Ref
Amine-
MOF-Fe
MB 312 9 20 (Paiman
et al.,
2020)
UiO-66 MB 90 5.5 25 (Chen
et al.,
2015)
NH2-UiO-
66
MB 96 5.5 25 (Luo et al.,
2017)
ZIF-67 MB 57.14 10 – (Haque
et al.,
2010)
MIL-101
(Fe)
MB 149 9 20 (Paiman
et al.,
2020)
MIL-101
(Fe)
MB 402 9.5 20 This study
MIL-101 MO 140 3.5 45 (Haque
et al.,
2010)
UiO-66 MO 39 5.5 25 (Chen
et al.,
2015)
NH2-UiO-
66
MO 28 5.5 25 (Chen
et al.,
2015)
Cu-BDC MO 86.7 4 25 (Salama
et al.,
2018)
ZIF-67 MO 75.5 4 – (Luo et al.,
2017)
MIL-101
(Fe)
MO 1067 4 20 This study
M. Shakly et al.
7. Journal of Contaminant Hydrology 247 (2022) 103977
7
interactions). Generally, the adsorption energy (ΔE1) was related to
the interaction between the first adsorbed dye layer (unchanging
number and equal to unity) and MIL-101(Fe) adsorption sites, while
the other energy (ΔE2) was linked to the dye–dye interaction and
thus, ΔE1 is found to be greater than ΔE2. The multilayer adsorption
model is given by Mohamed et al. (2020); Li et al. (2019b):
qe = n DM
F1(c) + F2(c) + F3(c) + F4(c)
G(c)
(9)
F1(c) = −
2
(
c
c1
)2n
1 −
(
c
c1
)n +
(
c
c1
)n
(
1 −
(
c
c1
)2n
)
(
1 −
(
c
c1
)n )2
, (10)
F2(c) =
2
(
c
c1
)n(
c
c2
)n
(
1 −
(
c
c2
)n N2
)
1 −
(
c
c2
)n , (11)
F3(c) = − N2
(
c
c1
)n(
c
c2
)n(
c
c2
)n N2
1 −
(
c
c2
)n , (12)
F4(c) =
(
c
c1
)n(
c
c2
)2n
(
1 −
(
c
c2
)n N2
)
(
1 −
(
c
c2
)n )2
, (13)
Fig. 6. Modeling of adsorption isotherms of MO and MB dyes on MIL-101 (Fe) MOF using a multilayer statistical physics model.
M. Shakly et al.
8. Journal of Contaminant Hydrology 247 (2022) 103977
8
G(c) =
(
1 −
(
c
c1
)2n
)
1 −
(
c
c1
)n +
(
c
c1
)n(
c
c2
)n
(
1 −
(
c
c2
)n N2
)
(
1 −
(
c
c2
)n )2
, (14)
where N2 is the layers of removed dye molecules, C1 is the half saturation
concentration connected to the first layer, and C2 is the corresponding
one correlated to N2 adsorbed dye layers.
2.7. Regeneration of MIL-101(Fe) adsorbent
Recycling and utilization of the MIL-101(Fe) adsorbent were inves
tigated via the solvent method using ethanol as an eluent. The MIL-101
(Fe) loaded with the tested dyes (MB and MO) was continuously agitated
on a rotatory shaker at 200 rpm for 4 h at 25 ◦
C to achieve the complete
removal of the adsorbed dyes. This adsorption/desorption test was
repeated five times under the same conditions. At the end of each
desorption round, the MIL-101(Fe) was washed several times by distilled
water and dried at 65 ◦
C for 12 h before the next desorption cycle.
3. Results and discussion
3.1. Characterization of MIL-101 (Fe) adsorbent
X-ray diffraction (XRD) was carried out to identify the crystallinity
and structure of the synthesized MIL-101 (Fe) adsorbent. Different peaks
related to the MIL-101 (Fe) were observed at 2 thetas = 5.3◦
, 8.82◦
,
9.18◦
and 18.5◦
, see Fig. 1a. These detected peaks were comparable with
the previous studies (Hu et al., 2019; Wang et al., 2018; Liu et al., 2018a;
Li et al., 2016).
FE-SEM images (Fig. 1b and c) display the morphological features of
the MIL-101 (Fe) surface. It was observed that the MIL-101(Fe) had a
special octahedral structure with diameters from 0.7 to 2 μm, which was
consistent to the other reported studies (Hu et al., 2019; Wang et al.,
2018).
FTIR spectrum (Fig. 1 d) shows a strong band at 1574 cm− 1
that
indicated (C=O) of the carboxylate group (Hu et al., 2019). The broad
band detected at 3376 cm− 1
corresponded to the hydroxyl group of the
adsorbed water molecules (Li et al., 2019a). The strong band at 1392
cm− 1
was attributed to the vibration of (C–C) in benzene ring; however,
the parent bands at 545 and 748 cm− 1
were related to (Fe-OH) vibration
MB dye MO dye
n
DM
(mg/g)
Qsat
(mg/g)
20 °C 40 °C 60 °C 20 °C 40 °C 60 °C
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0
10
20
30
40
50
60
70
80
90
0
20
40
60
80
100
120
140
160
0
50
100
150
200
250
300
350
400
0
100
200
300
400
500
600
700
800
900
1,000
Fig. 7. Statistical physics parameters for the MB and MO adsorption on MIL-101 (Fe) MOF.
M. Shakly et al.
9. Journal of Contaminant Hydrology 247 (2022) 103977
9
(Wang et al., 2018). These FTIR results also agreed to the earlier studies
(Li et al., 2019a; Hu et al., 2019; Wang et al., 2018) and thus confirmed
the preparation of MIL-101(Fe).
Particles size range of MIL-101(Fe) was obtained using zetasizer
measurement and results indicated that it ranged from 470 to 603 nm.
Also, the surface charge value of the prepared MIL-101(Fe) was − 3 at
pH 7.5.
TGA analysis displayed the presence of two main weight losses
within the temperature range of 25–650 ◦
C (Fig. 2a). The first weight
loss (about 16%) below 400 ◦
C could be related to the elimination of
water molecules and free terephthalates in the pores of MIL-101(Fe). In
addition, the significant second weight loss (38%) from 400 to 650 ◦
C
could be due to the decomposition of organic ligands that result in the
collapse of MIL-101(Fe). TGA result of this study is similar to that of
other UIO-66 and MIL-101(Cr) metal organic frameworks (Hu et al.,
2015; Yang and Yan, 2011). Furthermore, the DCS result displays the
existence of two sharp endothermic peaks (Fig. 2b). The first endo
thermic sharp peak observed at 250–300 ◦
C could be attributed to the
evaporation of H2O molecules and free terephthalate in pores of the MIL-
101(Fe), while the second one at 600–700 ◦
C could be associated to the
decomposition of MIL-101(Fe). Consequently, TGA and DSC results
indicated that MIL-101(Fe) is characterized by high thermal stability up
to 650 ◦
C.
3.2. pH effect on the adsorption of MB and MO onto MIL-101(Fe)
Adsorption experiments were conducted at a wide range of solution
pH (i.e., pH 2–12) to find the optimum value of this parameter for the
removal of MB and MO (100 mg/L) using 0.01 g of MIL-101(Fe) at 20 ◦
C.
Fig. 3 displays that the removal efficiency for MB increased with
improving pH value where the highest efficiency was achieved at pH
9.5. This behavior for MB adsorption was related to the electrostatic
attraction between the positively charged MB molecule and negatively
charged MIL-101(Fe) surface, whose pH at zero point of charge was
pHPZC = 7.1. Thus, the surface of MIL-101 (Fe) was negatively charged
at pH > 7.1 and it was more effective to attractive and remove the
cationic MB dye. On the contrary case, the maximum percentage of MO
adsorption was attained at pH 4 due to the electrostatic attraction force
caused by the negatively charged anionic MO dye. This indicated that
the MIL-101(Fe) was found to be an excellent adsorbent for the tested
dyes and thus, all adsorption experiments were conducted at pH 9.5 and
pH 4 for MB and MO, respectively.
3.3. Contact time effect on MB and MO adsorption using MIL-101(Fe)
Very rapid adsorption of both MB and MO dyes were observed in the
first 15 min with removal percentages of 52.7 and 76% for MO and MB
respectively, see Fig. 4a and b. It was assumed that the adsorption
process of MB and MO molecules was very fast at the beginning due to
the availability of numerous active sites on the MIL-101(Fe) surface.
After that, the interaction time between dyes and MIL-101(Fe) started to
be slower (i.e., a gentle slope was observed) due to the limitation of the
available adsorption sites. Finally, the removed amounts of both dyes
were nearly constant after contact times of 2 h for MB and 10 h for MO,
which indicated that equilibrium was achieved after the saturation of
active sites of the MIL-101(Fe) surface.
3.4. MB and MO adsorption kinetics
As indicated, the kinetics of dyes adsorption were investigated using
different models as the Pseudo-first order (Lagergren, 1898), Pseudo
-second order (Ho and McKay, 1999), and intra-particle diffusion
models (Weber and Morris, 1962). The Pseudo-first order considers that
the adsorption process is strongly depends on the diffusion, while the
Pseudo-second order assumes that a chemical adsorption can be
involved in the removal process (Ahmadijokani et al., 2020). The
experimental data of MO and MB were fitted to these different kinetic
models and the best fit model was identified via the R2
value, see Fig. 4.
All the kinetics parameters were calculated and compared with the
experimental data as shown in Table 1. The values of R2
for MO
adsorption on MIL-101(Fe) indicating that the best fit model was the
Pseudo-Second-order equation (R2
= 0.993) in comparison to the
Pseudo-First order equation (R2
= 0.909). Also, the best fitted model (R2
= 1) for MB was the Pseudo-Second order kinetics equation followed by
the Pseudo-first order (R2
= 0.903). These results indicated that the
adsorption kinetics of both MB and MO on MIL-101 (Fe) were described
by the Pseudo-Second-order model and thus, the adsorption of MO and
MB could be related to chemisorption process. The intra-particle diffu
sion analysis attributed to each dye molecule displayed dissimilar linear
stages over the whole-time range (Fig. 4). The initial sharp stage can be
associated to the external mass transfer of MB and MO molecules from
the solution to the outer surface of the MIL-101(Fe) adsorbent. The
second and third linear sections characterized the control of pore-
diffusion and equilibrium stages, respectively. Consequently, the
adsorption of MB and MO was mostly directed by the film diffusion
during the first stage, followed by pore-diffusion and equilibrium at
subsequent stages.
3.5. Classical modeling of MB and MO adsorption isotherms
Modeling of MB and MO adsorption data at equilibrium with the
applied traditional models (Langmuir and Freundlich equations) are
displayed in Fig. 5 and the corresponding adjusted parameters of these
models are listed in Tables 2 and 3. Langmuir model was the best
alternative to fit MB and MO adsorption data where their R2
values were
highest at all tested temperatures. Therefore, the removal of these dyes
molecules was related to homogenous adsorption sites of MIL-101(Fe)
and the adsorbates–adsorbent interaction resulted in the formation of
a single dye layer. The maximum adsorption capacities (qmax) for MB
E
(kJ/mol)
E
(kJ/mol)
20 °C 40 °C 60 °C
0
5
10
15
20
25
30
35
40
E1
E2
0
2
4
6
8
10
12
14
16
18
20
E1
E2
MB
MO
Fig. 8. Adsorption energies for MB and MO adsorption on MOF MIL-101
(Fe) MOF.
M. Shakly et al.
10. Journal of Contaminant Hydrology 247 (2022) 103977
10
were 402, 391.67, and 352.96 mg/g and 1066.96, 934.17, and 830.97
mg/g for MO at 20, 40, and 60 ◦
C, as given in Table 2. The decrease of
qmax values with the increment of solution temperature suggested that
the adsorption of MB and MO molecules by MIL-101(Fe) was exothermic
(i.e., the dyes–adsorbent interaction was more energetic at low tem
perature) (Sharib et al., 2021; Mohamed et al., 2020). Moreover, the
adsorption capacities associated to MO molecules were higher than
those of MB dye, which indicated the selectivity of the as–synthesized
Fig. 9. Adsorption selectivity of MIL-101(Fe) for the adsorption of MB and MO dyes in binary systems.
Fig. 10. MB and MO removal after the regeneration of MIL-101 (Fe) adsorbent.
M. Shakly et al.
11. Journal of Contaminant Hydrology 247 (2022) 103977
11
MIL-101(Fe) active sites for this azo dye. This variation in the removal
efficiency of MIL-101(Fe) could be related to the chemical structures (i.
e., C14H14N3NaO3S for MO and C16H18ClN3S for MB) and the molecular
size (i.e., 1.2 nm for MO and 1.38 nm for MB) of these dye molecules
(Seliem et al., 2020). The molecular characteristics of MO and MB
played a relevant role for transfer phenomena in the interface between
these dyes and MIL-101(Fe) adsorbent surface. Furthermore, the
decrease of KF values from 75.51 to 49.26 for MB and from 67.60 to
45.27 (mg/g)(mg/L)− 1/n
with temperature also supported the
exothermic adsorption of these dyes. Calculated 1/n values by the
Freundlich equation were lower than unity thus signifying an effective
dye–MIL-101(Fe) interaction even at low concentrations of these
organic pollutants (Sharib et al., 2021; Mohamed et al., 2020). The
maximum adsorption capacities for MO and MB by the synthetic MIL-
101(Fe) and other MOFs are listed in Table 4. It can be observed that
the qmax values of MIL-101(Fe) were higher than those reported for
different MOFs. Therefore, it can be concluded that the MIL-101(Fe) is a
promising adsorbent for removal of dyes-bearing solutions.
3.6. Advanced statistical modeling of MB and MO adsorption isotherms
The multilayer model (Fig. 6) presented the highest R2
values
compared to the single- and double-layer models at all solution tem
peratures (i.e., 20, 40, and 60 ◦
C). Therefore, the steric (e.g., n, DM, and
Qsat) and energetic (ΔE) parameters of this multilayer adsorption model
were deeply analyzed to obtain a better understanding of the adsorption
mechanism of MB and MO dyes on MIL-101(Fe).
3.6.1. The n parameter
The determination of steric n parameter provides information asso
ciated to the removal mechanism of the tested adsorbates (MB and MO
dyes) on MIL-101(Fe) especially in terms of adsorption orientation and
molecular aggregation phenomenon. In particular, the adsorption ge
ometry (vertical or horizontal) of the removed MB and MO molecules on
the adsorbent can be analyzed using this parameter (Ramadan et al.,
2021; Mohamed et al., 2020; Barakat et al., 2020). Based on the value of
n parameter, two main scenarios can be recognized to describe the ge
ometry and mechanism of dyes adsorption onto MIL-101(Fe) surfaces
(Sharib et al., 2021; Barakat et al., 2020).
• The first scenario (n > 1): The adsorption of MO and MB molecules
can take place in a vertical position through multi–interactions
mechanism (i.e., one active site of MIL-101(Fe) can capture several
dye molecules).
• The second scenario (n < 1): MO and MB adsorption occurs in a
horizontal location via a multi–docking mechanism (i.e., numerous
MIL-101(Fe) functional groups can remove one dye molecule).
Fig. 7 exhibits the values of n parameter related to MB and MO
adsorption on MIL-101(Fe) as a function of solution temperature. For
MO dye, the n parameter values were 0.95, 1.02, and 1.33 at 20, 40, and
60 ◦
C, respectively. As a result, horizontal position and multi–docking
mechanism could be expected for MO adsorption at 20 ◦
C, while vertical
position and multi–interactions mechanism could occur at 40 and 60 ◦
C.
The n values were higher than unity at 40 and 60 ◦
C and this result
indicated the effect of aggregation phenomenon with increasing tem
perature. On the other hand, the n values associated to MB adsorption
were 1.17, 1.68, and 1.78 and thus, vertical orientation and multi
–interactions mechanism were involved in the removal of this dye by the
MIL-101(Fe) active sites at all temperatures. Consequently, the aggre
gation of MB molecules (i.e., MB-MB binding) in solution was identified
at all tested temperatures and this phenomenon resulted in the existence
of the non-parallel adsorption geometry and a multi–molecular mech
anism. The high degree of MB and MO aggregations with temperature
suggested that these adsorption processes were energetically activated
in solutions before the interaction between dyes and MIL-101(Fe)
surface (Li et al., 2020).
3.6.2. The DM parameter
Fig. 7 displays the number of MIL-101(Fe) active sites (i.e., DM
parameter) with respect the adsorption temperature. From 20 to 60 ◦
C,
the DM parameter decreased from 77.33 to 52.38 mg/g for MB and from
149.91 to 107.07 mg/g for MO dye. The decrease of DM value with
temperature displayed a reverse trend as compared to the steric n pa
rameters (i.e., the increment of n parameter caused a reduction of the
occupied active sites number of the MIL-101(Fe) surface). Thus, the
trend of DM parameter assessed the accumulation of the removed dyes
molecules at high temperature. Moreover, the aggregated MO and MB
molecules could be more selective to definite functional groups of the as-
synthesized MIL-101(Fe) (Mobarak et al., 2019). Also, it can be noticed
that the DM values associated to MO dye were higher than those of MB
dye at all temperatures, which could be considered as an important
variable of the enhanced adsorption capacity of MIL-101(Fe) for MO
dye.
3.6.3. Total number of the adsorbed MB and MO layers (Nt = 1 + N2)
To understand the adsorption mechanism of MB and MO molecules
on MIL-101(Fe), the determination of the total layers of adsorbed dyes is
necessary (Barakat et al., 2020). The calculated Nt values were 3.12,
3.08, and 2.49 for MB and 5.36, 5.03, and 3.76 for MO at 20, 40, and
60 ◦
C, respectively, see Fig. 7. The reduction of Nt value with temper
ature increments could be related to the influence of thermal agitation,
which resulted in a disordered movement of the adsorbed MB and MO
molecules. The disturbed state associated to dye-dye interaction caused
a decrease of the adsorbed dye layers (Seliem et al., 2020; Barakat et al.,
2020). Like the DM parameter, the Nt values associated to MO dye were
higher than those of MB dye at all adsorption temperatures (i.e., the N2
parameter presented a significant impact to improve the MO adsorption
capacity).
3.6.4. The adsorbed MB and MO quantities at saturation (Qsat = n. DM. Nt)
To estimate the maximum MIL-101(Fe) performance for the
adsorption of MO and MB dyes, the calculation of Qsat values was done.
Fig. 7 exhibits the trend of Qsat versus temperature, and the values of this
parameter were 373.74, 348.07, and 325.08 mg/g for MB and 908.39,
777.01, and 677.92 mg/g for MO at 20, 40, and 60 ◦
C, respectively.
These Qsat values confirmed the exothermic interactions associated to
MB and MO adsorption on MIL-101(Fe) surface. For the investigated
dyes, it was identified that the DM and Nt parameters preserved the same
trend of the Qsat parameter with respect to improving temperature.
According to the calculated steric parameters, it can be concluded that
the removal efficiency of MIL-101(Fe) for MO and MB dyes was
controlled by the DM and Nt parameters. Furthermore, the high values of
the Qsat for MO as compared to that of MB confirmed the important roles
of DM and Nt parameters to determine the preference of MIL-101(Fe) for
the adsorption of this azo dye.
3.7. Energetic parameters
To obtain an appropriate interpretation for the interactions between
MB and MO molecules and MIL-101(Fe) active sites, the adsorption
energies were calculated (Seliem et al., 2020; Barakat et al., 2020) These
adsorption energies (ΔE) were estimated as follows (Barakat et al.,
2020).
C1 = Cse−
ΔE1
RT (15)
C2 = Cse−
ΔE2
RT (16)
where c1and c2 are the half-saturation concentrations and cs is the sol
ubility of the tested MB and MO adsorbates.
Fig. 8 shows the ΔE values as a function of the adsorption
M. Shakly et al.
12. Journal of Contaminant Hydrology 247 (2022) 103977
12
temperature. The negative ΔE values confirmed the exothermic in
teractions between the dyes molecules and the MIL-101(Fe) adsorbent.
This performance agreed with the influence of solution temperature to
reduce the MO and MB adsorption capacities. Besides, the ΔE values of
MO and MB adsorption were < 40 kJ/mol at 20, 40, and 60 ◦
C, which
suggested physical adsorption processes (Sharib et al., 2021; Barakat
et al., 2020). ΔE1 was associated to MIL-101(Fe)–dye (MB or MO)
interaction, while ΔE2 described the dye–dye interface. Therefore, ΔE1
values were higher than ΔE2 all tested temperatures. Although the ΔE
values associated to MB adsorption were higher than those of MO at all
temperature, the adsorbed MB dye quantities were low and, conse
quently, the selectivity of MIL-101(Fe) was not governed by the ΔE
parameter.
3.8. Selectivity of MIL-101 (Fe) for the adsorption of MB and MO dyes in
binary systems
The adsorption selectivity is an important parameter for studying the
behavior of an adsorbent for the removal of water pollutants (Liu et al.,
2018b). For instance, graphene like metal organic framework (BUC-17)
was found to be more selective to anionic dyes from an organic dye
mixture (Li et al., 2017). Moreover, Eu-based MOF (BUC-88) displayed
high selectivity to many pharmaceuticals and personal care products
(Wang et al., 2021). In this study, the adsorption selectivity of the MIL-
101(Fe) for the dye removal was investigated using a mixture of MB and
MO dyes at different pH values, keeping the other operating parameters
constant. Fig. 9 shows that MIL-101 (Fe) surface was selective to MO and
MB dyes at pH 4 and 9.5, respectively. Besides, the adsorption capacities
of the tested dyes were slightly decreased in the binary system as
compared to the single-component one at optimum pH values due to the
competition between MO and MB molecules to interact with MIL-101
(Fe) active sites (i.e., weak antagonism interaction). At pH 7, both
dyes were slightly adsorbed without a significant preference for any dye
molecule, and the adsorption processes could be associated to the π- π
stacking effect. These results suggested that the MIL-101 (Fe) MOF could
be a more selective adsorbent for anionic and cationic dyes depending
on the solution pH value.
3.9. Reusability study
Fig. 10 shows the removal efficiency of MIL-101(Fe) after five
regeneration cycles for both MB and MO dyes. It is clear that MIL-101
(Fe) presented more than 95% for MB removal and 90% for MO
removal after all regeneration rounds. For MB dye, increment of the
removal efficiency in the last cycles (i.e., from cycle 3 to 5) could be
attributed to the high surface area of MIL-101(Fe)-MB interaction,
which required frequent washing and drying under vacuum to achieve
the complete desorption of the attached MB molecules (Paiman et al.,
2020). Based on the regeneration results, the investigated adsorbent can
be reutilized numerous times to remove the tested dyes without a sig
nificant loss of its removal efficiency, thus suggesting an economic
benefit and high stability of this adsorbent in the purification of dyes-
bearing water.
4. Conclusions
MIL-101(Fe) (MOF) was successfully prepared through a facile sol
vothermal method, which was characterized and employed as an
adsorbent for MO and MB at different experimental conditions. In single
and binary adsorption systems, MIL-101(Fe) was more selective to MO
dye as compared to MB dye. The adsorption of both MO and MB fol
lowed the pseudo-second order and the Langmuir equations. A multi
layer model from statistical physics theory was utilized to understand
the MO and MB adsorption mechanisms at a molecular level. MO
adsorption was governed by multi–docking and multi–interactions
mechanisms, while MB adsorption was controlled only by
multi–interactions mechanism. The density of MIL-101(Fe) active sites
and the total adsorbed dye layers formed on MOF surface were the main
parameters that determined the adsorption capacity of MO dye. The
adsorption of MB and MO molecules by MIL-101(Fe) was mainly caused
by physical interactions at all solution temperatures. The reusability
study demonstrated that the MIL-101(Fe) can maintain an outstanding
performance after four adsorption/desorption cycles. The current study
clearly proved that the MIL-101(Fe) can be utilized as an adsorbent for
efficient removal of anionic and cationic dyes from polluted solutions.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
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