Synthesis and characterization of a novel fluorene-based covalent triazine framework as a chemical adsorbent for highly efficient dye removal

Article abstract of DOI:10.1016/j.polymer.2020.122430

Covalent organic frameworks (COFs) are known as emerging candidates for textile wastewater treatment due to their high porosity and regular channel structure. In the current paper, a Fluorene-based COF (FLU-COF) was synthesized by a facile method directin

Full text of DOI:10.1016/j.polymer.2020.122430

Polymer 195 (2020) 122430
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Polymer
journal homepage: http://www.elsevier.com/locate/polymer
Synthesis and characterization of a novel fluorene-based covalent triazine
framework as a chemical adsorbent for highly efficient dye removal
Nazanin Mokhtari, Mohaddeseh Afshari, Mohammad Dinari*
Department of Chemistry, Isfahan University of Technology, Isfahan, 84156-83111, Islamic Republic of Iran
A R T I C L E I N F O
A B S T R A C T
Keywords:
Covalent organic frameworks (COFs) are known as emerging candidates for textile wastewater treatment due to
their high porosity and regular channel structure. In the current paper, a Fluorene-based COF (FLU-COF) was
synthesized by a facile method directing a crystalline framework. After a full characterization of the FLU-COF
with different methods, its ability in removing Sky Blue A, widely used color in the textile industry, was
investigated. Optimizing the parameters effective on the adsorption process by using the Central Composition
Design, led to finding the optimal dye removal condition to be as pH 5; temperature: 45 �C; absorbent dosage:
0.005 g; and contact time: 60 min. In this condition, the removal efficiency has reached over 99% with a
maximum adsorption capacity of 299 mg. gÀ 1. The adsorption isotherm is consistent with the Langmuir model
representing monolayer adsorption. Also, the kinetic investigations show the pseudo-second-order kinetics which
means that the chemical interactions play an important role in dye removal. This is of important interest to make
a laboratory method into an in-field method. Thus, a low-cost, simple, and portable method was introduced for
investigation of the removal of Sky Blue A. In this regard, the ability of FLU-COF in removing dyes from aqueous
solutions was examined by the mobile phone colorimetry as an alternative to the time-consuming laboratory
methods. The comparison between the obtained results from mobile phone colorimetry and the traditional
UV–Vis spectroscopy showed the accuracy of the method.
Triazine-based covalent organic framework
Fluorene
Dye removal
1. Introduction
as the adsorbent for dye removal. Currently, AC is the most common
adsorbent for dye removal from aqueous solutions. However, its appli-
Water pollution caused by accelerated development of industriali-
zation, especially textile industries, has become the first global concern
in recent years. Organic dyes are the most common pollutants in
wastewaters. Azo dyes are one of the principal subdivisions of organic
dyes that pose severe problems to human health [1]. Natural anaerobic
degradation of azo compounds, and also other N-containing aromatics,
form potentially carcinogenic aromatic amines [2,3]. Thus, removing
these hazardous materials is necessarily required for the protection of
human health and environmental [4]. In this regard, several conven-
tional methods have been introduced, such as membrane filtration,
photo-degradation, ion exchange, chemical oxidation, and adsorption.
High efficiency, low process cost, and simplicity made adsorption the
most favorable method [5].
cability is faced with limitations due to the low adsorption capacity,
slow adsorption kinetics, high cost, and difficult restoration. In order to
address these problems, new porous materials such as MOFs have been
proposed as high-performance adsorbents [14]. Although the use of
MOFs due to their high adsorption capacity has attracted much attention
in dye removal, their poor chemical stability and high sensitivity to
moisture limit their practical application. New POPs by making strong
covalent bonds between light elements have overcome this challenge
and exhibiting high chemical and thermal stability under environmental
conditions [15–18].
Covalent organic frameworks (COFs) are a class of POPs with crys-
talline ordered structure and defined pores [19,20]. COFs consist of
covalently bonded light elements, such as C, N, H, O, and B, extended in
regular frameworks [21]. Since their discovery in 2005, different link-
ages (e.g., hydrazine, imine, boroxine, and boronate) with different
molecular geometry (e.g., hexagonal, kagome, and square) have been
introduced [19,22]. A precise selection of linker and vertex could help
Several porous materials with high surface areas including ordered
mesoporous carbons, activated carbon (AC) [6,7], zeolites [8], metal-
À organic frameworks (MOFs) [9,10], porous organic polymers (POPs)
[11,12], and functionalized mesoporous silica [13] have been reported
* Corresponding author.
E-mail addresses: dinari@iut.ac.ir, mdinary@gmail.com (M. Dinari).
https://doi.org/10.1016/j.polymer.2020.122430
Received 27 February 2020; Received in revised form 23 March 2020; Accepted 28 March 2020
Available online 30 March 2020
0032-3861/© 2020 Elsevier Ltd. All rights reserved.

N. Mokhtari et al.
Polymer 195 (2020) 122430
scientists to predict the size of the pores in the structure [23]. COFs’
unique properties such as high surface area, as well as the structural
diversity and physicochemical stability, led to receiving extensive
attention during the last decade [24]. They have been applied in
different branches of science ranging from catalysis and adsorption to
drug delivery and energy storage [19,25–31]. Although COFs properties
and features suggest potential advantages, few investigations have been
conducted for dye adsorption from aqueous media [32].
containing DAF (0.4 mmol, 0.08 g). Then, sonication was conducted for
30 min to obtain a homogeneous suspension. Thereafter, 2 mL acetic
acid (6 M) added to the mixture, and then the mixture autoclaved for
seven days at 85 �C. Subsequently, the autoclave was cooled to room
temperature and the product collected by centrifugation (6000 rpm, 10
min). In order to remove the oligomers and unreacted precursors, the
precipitate washed three times with tetrahydrofuran (THF), acetone,
dichloromethane, and ethyl acetate, respectively. Finally, the obtained
precipitate was dried at 90 �C for 24 h under vacuum.
Developing portable methods for on-site analysis has attracted sig-
nificant attention in recent years. The sample-handling and contami-
nation problems, as well as the high cost and time-consumption of in-
laboratory methods, increase interest in using the mobile phone color-
imetric methods [33,34]. Although the in-laboratory methods are more
reliable, the simplicity, portability, and capability of mobile phone
colorimetric methods in-field analysis make them good candidates for
replacing conventional methods [35,36].
2.4. Characterization
The structure of FLU-COF was studied by thermogravimetric analysis
(TGA), single-point BET specific surface area analysis, field-emission
electron microscopy (FE-SEM), powdered X-ray diffraction (PXRD),
and Fourier transform infrared (FT-IR) spectroscopy, as described in the
Supporting Information (SI).
In this manuscript, a Fluorene-based COF (FLU-COF) was designed
and synthesized using 4,4’,4”-((1,3,5-triazine-2,4,6-triyl)tris(oxy))tri-
benzaldehyde (TOB) as the vertex point and 2,7-diaminofluorene (DAF)
as the linker by a modified hydrothermal method. The prepared FLU-
COF was used to efficiently remove the highly water-soluble color Sky
blue A. This dye is the main color in the textile industry, leather and
jeans coloring, and paper-making. In order to find the optimal param-
eters of dye removal, the central composite design (CCD) was per-
formed. Then, a portable colorimetric platform was designed using a
mobile phone. The comparison between the results obtained from the
smart-phone and the UV–Vis spectrometer shows the reliability of the
smart-phone results and the capability of using this platform in-field
analysis.
2.5. Adsorption experiments
Sky Blue A (Acid blue 9, C37H42N4O9S3, MW: 787.90 g.molÀ 1), an
organic pigment with high water solubility (50 g.LÀ 1 in 90 �C) which is
mainly used for wool, silk, and nylon dyeing and exist in industrial
wastewaters, is considered as a practical candidate for adsorption. A
stock solution of Sky Blue A (1000 ppm) was prepared by dissolving an
appropriate amount of solid Sky Blue A in distilled water. Different so-
lutions with concentration ranges from 1.5 to 120 ppm were prepared by
simple dilution of the stock solution and then used for the calibration
curve (Fig. S2) by measuring their absorbance at 637 nm. In a general
batch experiment, 10 mg of FLU-COF were taken into 5 mL of dye so-
lution with different initial concentrations. After sufficient time, the
adsorbent was centrifuged (6000 rpm, 10 min), and the supernatant
subjected to a UV–Vis spectrophotometer. The parameters useful on the
adsorption, such as contact time, pH, and adsorbent dosage, were
investigated entirely. According to spectroscopic data, the removal ef-
ficiency (Re) and equilibrium adsorption capacity (qe) can be calculated
as follow:
2. Materials and methods
2.1. Reagents
2,7-Diaminofluorene (DAF), 2,4,6-trichloro-1,3,5-triazine (TCT),
tetrabutylammonium bromide (TBAB), p-hydroxybenzaldehyde, and
acetic acid (glacial) were purchased from Merck (Hohenbrunn, Ger-
many) chemical company. All the other materials were obtained from
Aldrich Chemical Co and used without further purification.
ðC0 À CeÞ
%Re ¼
� 100
(1)
(2)
C0
2.2. Preparation of 4,4’,4”-((1,3,5-triazine-2,4,6-triyl)tris(oxy))
V
m
tribenzaldehyde (TOB)
qe ¼ ðC0 À CeÞ � 100
TOB was synthesized by a modified procedure according to the
literature [37,38]. In a typical procedure, 50 mL aqueous solution of
NaOH (0.04 mol, 1.6 g) and p-hydroxybenzaldehyde (0.04 mol, 4.88 g)
were prepared. Then, another 50 mL solution of TCT (0.01 mol, 1.84 g)
in dichloromethane containing TBAB (6.5 � 10À 5 mol, 0.020 g) was
prepared separately. These solutions were mixed to obtain a biphasic
mixture, which is stirred for 24 h at room temperature. After sufficient
time, the organic phase separated and was washed with NaOH (10%
w/v) and distilled water. In the end, the organic phase was dried using
anhydrous sodium sulfate and evaporation under reduced pressure to
obtain a fluffy white product. The product was further purified by
recrystallization in ethyl acetate. Yield (%) ¼ 89; m.p. (�C) ¼ 174;
1HNMR (CDCl3):7.310 (d, 6H, J ¼ 6.8 Hz), 7.903 (d, 6H, J ¼ 6.8 Hz),
9.978 (s, 3H); elemental analysis for C24H15N3O6 (calcd./found): C
65.31/65.03, H 3.41/3.74, N 9.59/9.42, O 21.75/21.81.
where C0 (mg.LÀ 1) stands for the initial concentration, and Ce (mg.LÀ 1
)
is the concentration of Sky Blue A at the equilibrium. Moreover, V (L) is
the solution volume, and m (g) is the mass of adsorbent.
All the kinetic models used in the manuscript were presented in
detail in the supporting information.
2.6. Mobile phone colorimetric method
A mobile phone (HTC m8 eye), quartz cell, and a 10 W LED lamp was
used to prepare a simple colorimeter platform to measure the dye con-
centration in solutions. The ratio of red, green, and blue color value in
the sample image was examined using the Color Grab software (version
3.6.1). The software output shows a different value for each color. Thus,
the following equation was used to calculate the color ratio (CR) for the
samples [39]:
Rs
Gs
Bs
2.3. Synthesis of fluorene-based COF (FLU-COF)
þ
þ
Rr
Gr
Br
CR ¼
(3)
3
Although the sealed Pyrex-tube method was used to prepare FLU-
COF earlier, the crystallinity of the product was not satisfactory. Here-
in, we present a modified hydrothermal method to prepare the FLU-COF
with higher crystallinity. To prepare FLU-COF, TOB (0.6 mmol, 0.26 g)
was added to 15 mL solution of o-dichlorobenzene/n-butanol (1:1 v/v)
where Rs, Gs, and Bs stand for the red, green, and blue color value of the
sample, respectively. Also, Rr, Gr, and Br are attributed to the red, green,
and blue color value of the reference solution (distilled water), respec-
tively. In this regard, the concentrate of stock solutions was ranged from
2

N. Mokhtari et al.
Polymer 195 (2020) 122430
0.5 mg. LÀ 1 to 50 mg. LÀ 1 for finding the calibration curve (Fig. S4).
3. Results and discussion
3.1. Synthesis and characterization of FLU-COF
The TOB, a trialdehyde compound, was synthesized through a simple
displacement reaction between TCT and p-hydroxybenzaldehyde in the
presence of NaOH in a biphasic solution of dichloromethane and water.
In the second step, FLU-COF was prepared by a reaction between TOB
and DAF in hydrothermal conditions at 85 �C. The dark green product is
found to be insoluble in usual organic solvents such as N, N-dime-
thylformamide (DMF), dimethyl sulfoxide (DMSO), acetonitrile, chlo-
roform, acetone, and ethanol. FLU-COF was thoroughly characterized,
and its ability to remove organic dyes from wastewaters was investi-
gated (Scheme 1).
The FT-IR spectra of TOB, DAF, and FLU-COF were presented in
Fig. 1. The presence of peaks at 2700-2830 cmÀ 1 is attributed to the
vibration of aldehyde C-H in the TOB structure. Moreover, the band that
appeared at 1699 cmÀ 1 is attributed to the C¼O vibration. The phenolic
C-O bond vibration appeared at 1211 cmÀ 1. The bands shown at 1372-
1565 cmÀ 1 are attributed to the C-N and C¼N bonds in the triazine ring
(Fig. 1a). The presence of two bands at 3350-3410 cmÀ 1 is related to the
amine group presented in the DAF (Fig. 1b) [40]. Evidence of the for-
mation of FLU-COF is the disappearance of the C¼O stretching fre-
quency at 1699 cmÀ 1 and the formation of new imine bonds, which are
presented at 1571 cmÀ 1 (Fig. 1c).
The X-ray diffraction pattern of FLU-COF is presented in Fig. 2(a).
The diffraction peaks existed in 3.2�, and 5.8� were assigned to (1 0 0)
and (2 1 0) plates, which are a result of the formation of hexagonal
structure. The presence of (1 0 0) facet at 3.2� suggested that the FLU-
COF can be in both eclipsed and staggered form. For a better compari-
son, the predicted XRD patterns of FLU-COF were shown in Fig. 2(b and
c) [41].
Fig. 1. FT-IR spectra of (a) TOB, (b) DAF, and (c) FLU-COF.
The stability of the FLU-COF was investigated by using TGA (Fig. 3).
The FLU-COF shows high thermal stability with a char yield of 57% (at
Scheme 1. Schematic view of the preparation of FLU-COF.
3

N. Mokhtari et al.
Polymer 195 (2020) 122430
Fig. 2. XRD pattern of (a) FLU-COF, (b) predicted eclipsed form, (c) predicted staggered form, (d) eclipsed form of FLU-COF, and (e) staggered form of FLU-COF.
800 �C, Ar atmosphere). The physisorbed moisture led to a small mass
Table 1
loss below 100 �C. The decomposition temperature of 5% wt. (T5%) and
Thermal properties of the FLU-COF.
10% wt. (T10%) were 210 �C and 317 �C, respectively. Furthermore, in a
%5
%10
Samples
T
(�C)
T
(�C)
Char yield (%)
57
LOI (%)
40.3
dec
dec
halogen-free structure, there is a correlation between the char yield and
the limiting oxygen index (LOI) described by Van Krevelen and Hoftyzer
[42]. Thus, the LOI of FLU-COF is found to be 40.3. The data obtained
from the TGA curve was summarized in Table 1. Hence, the FLU-COF
can be classified in self-extinguishing material. The DTA also sub-
stantiates the results obtained from the TGA.
FLU-COF
210
317
3.2. Dye adsorption
3.2.1. Optimization strategy
The surface morphology of the FLU-COF was investigated by FESEM,
EDX, and elemental mapping. The FESEM images (Fig. 4a) exhibit a
flake-like structure, in contrast to the amorphous structure presented by
Konavarapu [43]. Moreover, the surface homogeneity was studied by
elemental mapping analysis and EDX (Fig. 4b and c). The single point
Optimization of adsorption condition is one of the critical steps to
achieve the maximum adsorption capacity by FLU- COF. The effect of
operating parameters, such as pH, temperature, contact time, and
adsorbent dosage, should be investigated for optimization; one way is
studying the effect of different values of each variable in invariant value
for other variables that known as on factor at a time. Another way is the
investigation of the relationship between variables specified by using
statistical experimental design. In the design of the experiment method
(DOE), a set of pre-designed experiments will be done [44]. The ad-
vantages of this method are reducing the test number, less consumption
of time and material, finding the real optimize condition by the multi-
variable interaction, and discover a mathematical model that shows the
relation of the experiment parameter with test results [45].
BET analysis showed a specific surface area of 264.6 m2. g À 1
.
In this study, the level of useful parameters was optimized by using
the central composition design (CCD). The CCD consist of a fractional
factorial design, a star design that can estimate the mathematical model
includes constant and linear terms, interactions, and a quadratic term.
The experiment was done in one block. The main parameter and their
levels are shown in Table 2. The designed matrix and the experiment
results are shown in Table 2.
3.2.2. ANOVA analysis and model fitting
The precision and accuracy of the designed method were examined
using the analysis of variance method (ANOVA). The results are pre-
sented in Table S1. The result of ANOVA Interpretation and the model
Fig. 3. TGA and DTA curve of FLU-COF.
4

N. Mokhtari et al.
Polymer 195 (2020) 122430
Fig. 4. (a) FESEM images of FLU-COF, (b) elemental mapping, and (c) EDX pattern of FLU-COF.
unit (g), can be calculated with the adsorption isotherms. According to
the literature [5], the adsorption capacity (qm) obtained from the
adsorption isotherms can be used to indicate the adsorbent character-
istics. Fig. 6a shows the adsorption isotherm of Sky Blue A over
Table 2
Independent variables, their symbols, and levels for central composition design.
Variable
Effect symbol
Variable levels
FLU-COF. The presence of aromatic rings and the formation of
π-π
À 1
þ1
stacking forces between the adsorbent and the Sky Blue A dye structure,
as well as the presence of amine and imine functional groups and for-
mation of hydrogen bonds between FLU-COF and dye molecules, lead to
prepare an excellent condition for removing highly steric organic dye.
Regarding this information, different isotherms (Langmuir, Freundlich,
Temkin, and Dubinin-Radushkevich) [46] have been examined, and
their results were summarized in Table .3 and Fig. S3. The intra-particle
diffusion model was studied to gain more information about the
adsorption process. A quick look at the Weber-Morris plot (Fig. S3 f)
shows initial fast adsorption followed by a linear plateau. Thus, it could
be said that the adsorption process consists of two steps. At first, the dye
molecules were diffused through the solution to the external surface of
FLU-COF indicating a mass transfer from the solution to the external
surface of the adsorbent. In the second step, the adsorption-desorption
process led to establishing an equilibrium which is shown in Fig. S3 f
as a linear plateau [47–49]. Also, according to the correlation coefficient
(R2), the adsorption data were best fitted to the Langmuir model
corroborating the monolayer adsorption of Sky Blue A on the exterior
surface of FLU-COF [50]. Thus, dye molecules were not penetrating into
the inner layers of absorbent. The adsorption energy emphasizes this
point that completed adsorption is only obtained by the formation of a
monolayer of adsorbate on the FLU-COF surface [51,52].
Temperature (�C)
pH
A
B
C
D
5
45
9
3
Adsorbent Dosage (mg)
Time (min)
1.5
1
10
90
equation also presented in Supplementary material (Table S2). The F-
value of the designed model found to be 12.70, representing that the
model is significant. The probability of having a large F-value with this
magnitude for a model by noise is only 0.01%.
Values of “Prob > F” less than 0.0500 indicate model terms are
significant. In this case, B, C, D, C2, D2 are significant model terms.
Values higher than 0.1000 indicate the model terms are not significant.
The “Lack of Fit F-value” of 2.63 implies the Lack of Fit is not sig-
nificant relative to the pure error. There is a 14.85% chance that a “Lack
of Fit F-value” this large could occur due to noise. Final Equation in
Terms of Coded Factors is found to be as follow:
Rem oval efficiency ¼ þ89.24 þ 2.06A À 3.87B þ 13.41C þ 3.65D À 3.03AB
À 2.04AC À 0.60AD þ3.24BC À 0.39BD À 0.33CD þ1.96A2 þ0.71B2 -5.90C2
-3.85D2
(4)
Here, Y is (%) of the color removal.
The influence of different factors such as contact time, pH, adsorbent
dosage, and temperature on the removal efficiency was studied indi-
vidually by using the CCD model, and their corresponding three-
dimensional plots were shown in Fig. 5. The removal efficiency was
increased by increasing the contact time until it reaches a maximum of
299.7 mg. g À 1. The removal efficiency insignificantly increased by
temperature due to the endothermicity of the adsorption process.
Regarding the results obtained from the CCD model, the optimal con-
dition was found as contact time of 60 min; pH of 5; adsorbent dosage of
0.005 g, at 45 �C.
3.2.4. Adsorption kinetic study
To investigating the adsorption kinetics, the adsorption isotherm at
optimal condition was fitted to the Elovich, Intra-particle diffusion,
pseudo-first-order, and pseudo-second-order kinetic models. The results
were summarized in Table 4, and the corresponding diagrams were
presented in Fig. S3. The Pseudo-second-order model is most consistent
with the experimental data, which means that the chemical interactions
play an essential role in the Sky Blue A adsorption. According to the
theory of classical physical chemistry, the adsorption is a surface effect
that can be divided into physical adsorption and chemical adsorption.
The kinetic studies show that the experimental data has followed the
3.2.3. Adsorption at the equilibrium
The maximum value of the dye (mg) adsorbed per adsorbent mass
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N. Mokhtari et al.
Polymer 195 (2020) 122430
Fig. 5. Three dimensional (3d) plots of (a) contact time and adsorbent dosage, (b) adsorbent dosage and pH, (c) contact time and pH, and (d) contact time and
adsorbent dosage.
Fig. 6. (a) Adsorption isotherm for Sky Blue A over FLU-COF at optimal condition, (b) Langmuir plot of adsorption, (c) adsorption curve vs. contact time, and (d)
pseudo-second-order kinetic model (condition: contact time of 60 min; pH of 5; adsorbent dosage of 0.005 g, at 45 �C). (For interpretation of the references to color in
this figure legend, the reader is referred to the Web version of this article.)
pseudo-second-order model. Regarding the information in the literature
[47,53,54], this model explains the presence of chemical interaction
between the adsorbate and adsorbent. High nitrogen content of the
FLU-COF with free lone pair electrons can cause the formation of
hydrogen bonds between the adsorbent and the Sky Blue A. This process
can reach equilibrium by the formation of π-π stacking forces. There
could be some interlayer hydrogen bonding forces in the FLU-COF, but
the distance between the H-bonding donor and acceptor prevents the
formation of intramolecular interactions (Scheme 1). According to
previous studies [33,55], the presence of hydrogen bonding, would
6

N. Mokhtari et al.
Polymer 195 (2020) 122430
Table 3
Isotherms coefficients for Sky Blue A adsorption over FLU-COF (condition: contact time of 60 min; pH of 5; adsorbent dosage of 0.005 g, at 45 �C).
Model
Langmuir
qmax
Freundlich
KF
Temkin
KT
Dubinin-Radushkevich
Adsorbate
Sky Blue A
KL
R2
n
R2
bT
R2
qd
B
R2
909
2.45
0.999
361.16
4.65
0.942
18.81
21.85
0.985
647.84
1.4˟10À 7
0.676
Table 4
Kinetic coefficients for the adsorption of Sky Blue A by FLU-COF from its solutions (300 mg. LÀ 1, qe exp.:299.74, condition: contact time of 60 min; pH of 5; adsorbent
dosage of 0.005 g, at 45 �C).
Model
Pseudo- first order
Pseudo-second order
Intra-particle
Elovich
Adsorbate
qe
K1
R2
qe
K2
R2
Kid
I
R2
α
β
R2
Sky Blue A
4.035
0.036
0.53
303.03
0.018
1
0.736
292.97
0.447
7.46˟1063
1.97
0.529
accelerate the adsorption process [56].
Intellectual property
We confirm that we have given due consideration to the protection of
intellectual property associated with this work and that there are no
impediments to publication, including the timing of publication, with
respect to intellectual property. In so doing we confirm that we have
followed the regulations of our institutions concerning intellectual
property.
3.3. Mobile phone colorimetric method
To investigating the reliability of the method, adsorption isotherms
and kinetic models were also studied and showed a complete consis-
tency with the UV–Vis spectroscopy results (Figs. S5 and S6). Similarly,
the Langmuir isotherm and the pseudo-second-order kinetic model were
best fitted to the experimental data as it is summarized in Tables 5 and 6.
Scheme 2 shows an illustration of the designed device for mobile phone
colorimetry.
Research ethics
We further confirm that any aspect of the work covered in this
manuscript that has involved human patients has been conducted with
the ethical approval of all relevant bodies and that such approvals are
acknowledged within the manuscript.
3.4. Comparison of FLU-COF adsorption capacity with recent studies on
dye removal
The ability of FLU-COF in removing organic dyes was compared with
recent studies and presented in Table 7. The impressive performance of
FLU-COF within 60 min and adsorption capacity of 299 mg. gÀ 1 in
comparison to MOFs (entries 1 and 3), and POPs (entry 2), and different
nanocomposites (entries 4–8) corroborates the application of COFs in
removing hazardous organic materials from water and wastewaters.
IRB approval was obtained (required for studies and series of 3
or more cases)
Written consent to publish potentially identifying information, such
as details or the case and photographs, was obtained from the patient(s)
or their legal guardian(s).
We confirm that the manuscript has been read and approved by all
named authors.
4. Conclusion
We confirm that the order of authors listed in the manuscript has
been approved by all named authors.
In conclusion, a fluorene-based covalent organic framework was
synthesized following a condensation reaction between 4,4’,4”-((1,3,5-
triazine-2,4,6-triyl)tris(oxy))tribenzaldehyde and 2,7-diaminofluorene,
and used for the adsorption of Sky Blue A from aqueous solutions. The
maximum adsorption capacity of 299 mg. gÀ 1 was obtained, and its
application on a mobile phone was investigated. The adsorption
isotherm obeys the Langmuir equation representing monolayer
adsorption, and the adsorption kinetic data follows the pseudo-second-
order equation due to the presence of chemical interactions. The mo-
bile phone colorimetric tests represent an excellent agreement with the
experimental UV–Vis data. The ability of FLU-COF in colorimetry with
portable devices can become an alternative to the conventional methods
in field-analysis.
Declaration of competing interest
No conflict of interest exists.
CRediT authorship contribution statement
Nazanin Mokhtari: Software, Visualization, Writing - original draft,
Formal analysis, Investigation. Mohaddeseh Afshari: Software, Formal
analysis, Writing - original draft. Mohammad Dinari: Supervision,
Project administration, Conceptualization, Validation, Investigation,
Resources, Writing - review & editing.
Funding
No funding was received for this work.
Table 5
Isotherms coefficients for Sky Blue A adsorption by smartphone colorimetric technique (condition: contact time of 90 min; pH of 5; adsorbent dosage of 0.005 g, at 45
�C).
Model
Langmuir
qmax
Freundlich
KF
Temkin
KT
Dubinin-Radushkevich
Adsorbate
Sky Blue A
KL
R2
n
R2
bT
R2
qd
B
R2
666
0.0021
0.972
0.018
2.07
0.949
18.81
16.81
0.97
437.95
5.2˟10À 3
0.913
7

N. Mokhtari et al.
Polymer 195 (2020) 122430
Table 6
Kinetic coefficients for the adsorption of Sky Blue A by smartphone colorimetric technique (condition: contact time of 90 min; pH of 5; adsorbent dosage of 0.005 g, at
45 �C).
Model
Pseudo- first order
Pseudo-second order
Intra-particle
Elovich
Adsorbate
qe
K1
R2
qe
K2
R2
Kid
I
R2
α
β
R2
Sky Blue A
100.7
0.035
0.82
270.3
0.001
0.998
12.35
155.66
0.854
4.37
28.75
0.975
[2] D. Brown, P. Laboureur, The degradation of dyestuffs: part I primary
biodegradation under anaerobic conditions, Chemosphere 12 (3) (1983) 397–404.
[3] J. McCann, E. Choi, E. Yamasaki, B.N. Ames, Detection of carcinogens as mutagens
in the Salmonella/microsome test: assay of 300 chemicals, Proc. Natl. Acad. Sci.
Unit. States Am. 72 (12) (1975) 5135–5139.
[4] S. Young, L. Balluz, J. Malilay, Natural and technologic hazardous material releases
during and after natural disasters: a review, Sci. Total Environ. 322 (1–3) (2004)
3–20.
[5] B. Chen, X. Wang, C. Wang, W. Jiang, S. Li, Degradation of azo dye direct sky blue
5B by sonication combined with zero-valent iron, Ultrason. Sonochem. 18 (5)
(2011) 1091–1096.
[6] A. Roghanizad, M.K. Abdolmaleki, S.M. Ghoreishi, M. Dinari, One-pot synthesis of
functionalized mesoporous fibrous silica nanospheres for dye adsorption: isotherm,
kinetic, and thermodynamic studies, J. Mol. Liq. 300 (2020), 112367.
^
[7] A.F. Streit, L.N. Cortes, S.P. Druzian, M. Godinho, G.C. Collazzo, D. Perondi, G.
L. Dotto, Development of high quality activated carbon from biological sludge and
its application for dyes removal from aqueous solutions, Sci. Total Environ. 660
(2019) 277–287.
[8] H. Mittal, R. Babu, A.A. Dabbawala, S. Stephen, S.M. Alhassan, Zeolite-Y
incorporated karaya gum hydrogel composites for highly effective removal of
cationic dyes, Colloid. Surface. Physicochem. Eng. Aspect. 586 (2020), 124161.
[9] M. Hasanzadeh, A. Simchi, H.S. Far, Nanoporous composites of activated carbon-
metal organic frameworks for organic dye adsorption: synthesis, adsorption
mechanism and kinetics studies, J. Ind. Eng. Chem. 81 (2020) 405–414.
[10] E. Wu, Y. Li, Q. Huang, Z. Yang, A. Wei, Q. Hu, Laccase immobilization on amino-
functionalized magnetic metal organic framework for phenolic compound removal,
Chemosphere 233 (2019) 327–335.
Scheme 2. Schematic view of smartphone colorimeter to estimate the dye
concentration.
[11] M. Li, H. Zhao, Z.-Y. Lu, Porphyrin-based porous organic polymer, Py-POP, as a
multifunctional platform for efficient selective adsorption and photocatalytic
degradation of cationic dyes, Microporous Mesoporous Mater. 292 (2020), 109774.
[12] R. Soltani, A. Shahvar, H. Gordan, M. Dinari, M. Saraji, Covalent triazine
framework-decorated phenyl-functionalised SBA-15: its synthesis and application
as a novel nanoporous adsorbent, New J. Chem. 43 (2019) 13058–13067.
[13] W. Zhao, J. Sun, Q. Tang, H. Kong, Y. Fu, B. Jiang, Y. Zhang, Q. Ji, Atomic
intercalation of magnesium in mesoporous silica hollow spheres and its effect for
removal of dyes, Appl. Surf. Sci. 507 (2020), 144919.
Table 7
A comparison of FLU-COF with recent adsorbents on dye removal.
Entry
Adsorbent
Adsorbate
Ci
Time
qe
Reference
(mg.
(min)
(mg.
LÀ 1
)
LÀ 1
)
a
[14] N.M. Mahmoodi, M. Oveisi, A. Panahdar, B. Hayati, K. Nasiri, Synthesis of porous
metal-organic framework composite adsorbents and pollutant removal from
multicomponent systems, Mater. Chem. Phys. 243 (2020), 122572.
1
2
3
4
5
6
7
8
9
Zr-based MOF
TPOP–SO3H
Cd-MOF1
Acid Blue
92
40
40
40.0
97.1
105.0
32.0
43.9
9.6
[57]
[58]
[59]
[60]
[61]
[62]
[62]
[63]
[15] W. Gao, J. Tian, Y. Fang, T. Liu, X. Zhang, X. Xu, X. Zhang, Visible-light-driven
photo-Fenton degradation of organic pollutants by a novel porphyrin-based porous
organic polymer at neutral pH, Chemosphere 243 (2020), 125334.
Methylene
blue
300
10
150
30
[16] M. Huang, L. Yang, X. Li, G. Chang, An indole-derived porous organic polymer for
efficiently visual colorimetric capture of iodine in aqueous media via the
Methylene
blue
synergistic effects of cation-π and electrostatic forces, Chem. Commun. 56 (2020)
GO–Fe3O4
Methylene
blue
20
3
1401–1404.
[17] Z.-W. Liu, C.-X. Cao, B.-H. Han, A cationic porous organic polymer for high-
capacity, fast, and selective capture of anionic pollutants, J. Hazard Mater. 367
(2019) 348–355.
D–GOÀ COOH–
(PEI–PAA)8
PVA–PAA–GO-
COOH–PDA
PVA–PAA–GO-
COOH–PDA
Magnetic
Methylene
blue
10
120
240
240
360
60
Congo red
25
[18] X. Zhang, J. Wang, X.-X. Dong, Y.-K. Lv, Functionalized metal-organic frameworks
for photocatalytic degradation of organic pollutants in environment, Chemosphere
(2019), 125144.
Methylene
blue
10
26.9
11.9
299.0
[19] S.-Y. Ding, W. Wang, Covalent organic frameworks (COFs): from design to
applications, Chem. Soc. Rev. 42 (2) (2013) 548–568.
Methylene
blue
3.74
300
MWCNT
[20] A. Shahvar, R. Soltani, M. Saraji, M. Dinari, S. Alijani, Covalent triazine-based
framework for micro solid-phase extraction of parabens, J. Chromatogr. A 1565
(2018) 48–56.
FLU-COF
Sky Blue A
Present
study
[21] A.P. Cote, A.I. Benin, N.W. Ockwig, M. O’Keeffe, A.J. Matzger, O.M. Yaghi, Porous,
crystalline, covalent organic frameworks, Science 310 (5751) (2005) 1166–1170.
[22] S.D. Brucks, D.N. Bunck, W.R. Dichtel, Functionalization of 3D covalent organic
frameworks using monofunctional boronic acids, Polymer 55 (1) (2014) 330–334.
[23] N. Huang, P. Wang, D. Jiang, Covalent organic frameworks: a materials platform
for structural and functional designs, Nat. Rev. Mater. 1 (10) (2016) 1–19.
[24] M. Dinari, M. Hatami, Novel N-riched crystalline covalent organic framework as a
highly porous adsorbent for effective cadmium removal, J. Environ. Chem. Eng. 7
(2019), 102907.
a
Initial concentration.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.polymer.2020.122430.
[25] J.W. Crowe, L.A. Baldwin, P.L. McGrier, Luminescent covalent organic frameworks
containing a homogeneous and heterogeneous distribution of
dehydrobenzoannulene vertex units, J. Am. Chem. Soc. 138 (32) (2016)
10120–10123.
References
[1] Y. Zhang, G. Huang, C. An, X. Xin, X. Liu, M. Raman, Y. Yao, W. Wang, M. Doble,
Transport of anionic azo dyes from aqueous solution to gemini surfactant-modified
wheat bran: synchrotron infrared, molecular interaction and adsorption studies,
Sci. Total Environ. 595 (2017) 723–732.
[26] H. Furukawa, O.M. Yaghi, Storage of hydrogen, methane, and carbon dioxide in
highly porous covalent organic frameworks for clean energy applications, J. Am.
Chem. Soc. 131 (25) (2009) 8875–8883.
8

N. Mokhtari et al.
Polymer 195 (2020) 122430
[27] N. Mokhtari, S. Taymouri, M. Mirian, M. Dinari, Covalent triazine-based polyimine
framework as a biocompatible pH-dependent sustained-release nanocarrier for
sorafenib: an in vitro approach, J. Mol. Liq. 297 (2020), 111898.
[28] L. Valentino, M. Matsumoto, W.R. Dichtel, B.J. Marineas, Development and
performance characterization of a polyimine covalent organic framework thin-film
composite nanofiltration membrane, Environ. Sci. Technol. 51 (24) (2017)
14352–14359.
[47] H.A. Ahsaine, M. Zbair, Z. Anfar, Y. Naciri, N. El Alem, M. Ezahri, Cationic dyes
adsorption onto high surface area ‘almond shell’activated carbon: kinetics,
equilibrium isotherms and surface statistical modeling, Mater. Today Chem. 8
(2018) 121–132.
[48] H.A. Ahsaine, M. Zbair, R. El Haouti, Mesoporous treated sewage sludge as
outstanding low-cost adsorbent for cadmium removal, Desalination Water Treat.
85 (2017) 330–338.
[29] Y. Zeng, R. Zou, Y. Zhao, Covalent organic frameworks for CO2 capture, Adv.
Mater. 28 (15) (2016) 2855–2873.
[49] S. Aoudj, A. Khelifa, N. Drouiche, R. Belkada, D. Miroud, Simultaneous removal of
chromium (VI) and fluoride by electrocoagulation–electroflotation: application of a
hybrid Fe-Al anode, Chem. Eng. J. 267 (2015) 153–162.
[30] M. Dinari, N. Mokhtari, S. Taymouri, M. Arshadi, A. Abbaspourrad, Covalent
polybenzimidazole-based triazine frameworks: a robust carrier for non-steroidal
anti-inflammatory drugs, Mater. Sci. Eng. C 108 (2020), 110482.
[31] S. Gopi, M. Kathiresan, 1, 4-Phenylenediamine based covalent triazine framework
as an electro catalyst, Polymer 109 (2017) 315–320.
[50] S. Sun, C. Wang, S. Han, T. Jiao, R. Wang, J. Yin, Q. Li, Y. Wang, L. Geng, X. Yu,
Interfacial nanostructures and acidichromism behaviors in self-assembled
terpyridine derivatives Langmuir-Blodgett films, Colloid. Surface. Physicochem.
Eng. Aspect. 564 (2019) 1–9.
[32] S.-B. Yu, H. Lyu, J. Tian, H. Wang, D.-W. Zhang, Y. Liu, Z.-T. Li, A polycationic
covalent organic framework: a robust adsorbent for anionic dye pollutants, Polym.
Chem. 7 (20) (2016) 3392–3397.
[51] H. Baseri, S. Tizro, Treatment of nickel ions from contaminated water by magnetite
based nanocomposite adsorbents: effects of thermodynamic and kinetic parameters
and modeling with Langmuir and Freundlich isotherms, Process Saf. Environ.
Protect. 109 (2017) 465–477.
[33] M. Afshari, M. Dinari, Synthesis of new imine-linked covalent organic framework
as high efficient absorbent and monitoring the removal of direct fast scarlet 4BS
textile dye based on mobile phone colorimetric platform, J. Hazard Mater. 385
(2020), 121514.
�
[52] C.S. Araújo, I.L. Almeida, H.C. Rezende, S.M. Marcionilio, J.J. Leon, T.N. de Matos,
Elucidation of mechanism involved in adsorption of Pb (II) onto lobeira fruit
(Solanum lycocarpum) using Langmuir, Freundlich and Temkin isotherms,
Microchem. J. 137 (2018) 348–354.
[34] T.-S. Lai, T.-C. Chang, S.-C. Wang, Gold nanoparticle-based colorimetric methods
to determine protein contents in artificial urine using membrane micro-
concentrators and mobile phone camera, Sensor. Actuator. B Chem. 239 (2017)
9–16.
[53] H. Han, W. Wei, Z. Jiang, J. Lu, J. Zhu, J. Xie, Removal of cationic dyes from
aqueous solution by adsorption onto hydrophobic/hydrophilic silica aerogel,
Colloid. Surface. Physicochem. Eng. Aspect. 509 (2016) 539–549.
[54] A. Maleki, U. Hamesadeghi, H. Daraei, B. Hayati, F. Najafi, G. McKay, R. Rezaee,
Amine functionalized multi-walled carbon nanotubes: single and binary systems
for high capacity dye removal, Chem. Eng. J. 313 (2017) 826–835.
[55] X. Zhu, S. An, Y. Liu, J. Hu, H. Liu, C. Tian, S. Dai, X. Yang, H. Wang, C.W. Abney,
Efficient removal of organic dye pollutants using covalent organic frameworks,
AIChE J. 63 (8) (2017) 3470–3478.
[35] N.J. Vickers, Animal communication: when I’m calling you, will you answer too?
Curr. Biol. 27 (14) (2017) R713–R715.
[36] A. Shahvar, M. Saraji, D. Shamsaei, Headspace single drop microextraction
combined with mobile phone-based on-drop sensing for the determination of
formaldehyde, Sensor. Actuator. B Chem. 273 (2018) 1474–1478.
[37] F.G. Khan, M.V. Yadav, A.D. Sagar, Synthesis, characterization, and antimicrobial
evaluation of novel trichalcones containing core s-triazine moiety, Med. Chem.
Res. 23 (5) (2014) 2633–2638.
[56] C. Muthukumaran, V.M. Sivakumar, M. Thirumarimurugan, Adsorption isotherms
and kinetic studies of crystal violet dye removal from aqueous solution using
surfactant modified magnetic nanoadsorbent, J. Taiwan Inst. Chem. Eng. 63 (2016)
354–362.
[38] N. Mokhtari, M.M. Khataei, M. Dinari, B.H. Monjezi, Y. Yamini, Imine-based
covalent triazine framework: synthesis, characterization, and evaluation its
adsorption, Mater. Lett. (2019), 127221.
[57] M. Hasanzadeh, A. Simchi, H.S. Far, Kinetics and adsorptive study of organic dye
removal using water-stable nanoscale metal organic frameworks, Mater. Chem.
Phys. 233 (2019) 267–275.
[39] S. Sumriddetchkajorn, K. Chaitavon, Y. Intaravanne, Mobile device-based self-
referencing colorimeter for monitoring chlorine concentration in water, Sensor.
Actuator. B Chem. 182 (2013) 592–597.
[58] Y. Li, H. Wang, W. Zhao, X. Wang, Y. Shi, H. Fan, H. Sun, L. Tan, Facile synthesis of
a triptycene-based porous organic polymer with a high efficiency and recyclable
adsorption for organic dyes, J. Appl. Polym. Sci. 136 (39) (2019), 47987.
[59] F.Y. Yi, J.P. Li, D. Wu, Z.M. Sun, A series of multifunctional metal–organic
frameworks showing excellent luminescent sensing, sensitization, and adsorbent
abilities, Chem. Eur. J. 21 (32) (2015) 11475–11482.
[40] N. Mokhtari, M. Dinari, O. Rahmanian, Novel porous organic triazine-based
polyimide with high nitrogen levels for highly efficient removal of Ni (II) from
aqueous solution, Polym. Int. 68 (6) (2019) 1178–1185.
[41] T. Ma, E.A. Kapustin, S.X. Yin, L. Liang, Z. Zhou, J. Niu, L.-H. Li, Y. Wang, J. Su,
J. Li, Single-crystal x-ray diffraction structures of covalent organic frameworks,
Science 361 (6397) (2018) 48–52.
[60] T. Jiao, Y. Liu, Y. Wu, Q. Zhang, X. Yan, F. Gao, A.J. Bauer, J. Liu, T. Zeng, B. Li,
Facile and scalable preparation of graphene oxide-based magnetic hybrids for fast
and highly efficient removal of organic dyes, Sci. Rep. 5 (2015), 12451.
[61] X. Zhao, K. Ma, T. Jiao, R. Xing, X. Ma, J. Hu, H. Huang, L. Zhang, X. Yan,
Fabrication of hierarchical layer-by-layer assembled diamond-based core-shell
nanocomposites as highly efficient dye absorbents for wastewater treatment, Sci.
Rep. 7 (2017), 44076.
[42] P.J. Hoftyzer, D. Van Krevelen, Properties of Polymers: Correlations with Chemical
Structure, Elsevier, Amsterdam, 1972.
[43] S.K. Konavarapu, K. Biradha, Luminescent triazene-based covalent organic
frameworks functionalized with imine and azine: N2 and H2 sorption and efficient
removal of organic dye pollutants, Cryst. Growth Des. 19 (1) (2018) 362–368.
[44] L. Mousavi, Z. Tamiji, M.R. Khoshayand, Applications and opportunities of
experimental design for the dispersive liquid–liquid microextraction method–A
review, Talanta 190 (2018) 335–356.
[62] R. Xing, W. Wang, T. Jiao, K. Ma, Q. Zhang, W. Hong, H. Qiu, J. Zhou, L. Zhang,
Q. Peng, Bioinspired polydopamine sheathed nanofibers containing carboxylate
graphene oxide nanosheet for high-efficient dyes scavenger, ACS Sustain. Chem.
Eng. 5 (6) (2017) 4948–4956.
[45] R. Leardi, Experimental design in chemistry: a tutorial, Anal. Chim. Acta 652 (1–2)
(2009) 161–172.
[46] M. Faraj, M. Erhayem, Langmuir, Freundlich, Temkin and dubinin–radushkevich
isotherm studies of equilibrium sorption of methylene blue from aqueous solution
onto mulberry tree (morus nigra L) roots powder, J. Pure Appl. Sci. (Ankara) 17 (1)
(2018).
[63] J.-L. Gong, B. Wang, G.-M. Zeng, C.-P. Yang, C.-G. Niu, Q.-Y. Niu, W.-J. Zhou,
Y. Liang, Removal of cationic dyes from aqueous solution using magnetic multi-
wall carbon nanotube nanocomposite as adsorbent, J. Hazard Mater. 164 (2–3)
(2009) 1517–1522.
9