Chemistry Research

1. Colloids and Surfaces A: Physicochemical and Engineering Aspects 626 (2021) 127069
Available online 24 June 2021
0927-7757/© 2021 Elsevier B.V. All rights reserved.
Easy-handling carbon nanotubes decorated poly(arylene ether nitrile)
@tannic acid/carboxylated chitosan nanofibrous composite absorbent for
efficient removal of methylene blue and congo red
Shumei Zhao a
, Yingqing Zhan a,b,c,*
, Qingying Feng a
, Wei Yang a
, Hongyu Dong a
, Ao Sun a
,
Xin Wen a
, Yu-Hsuan Chiao d,**
, Shirui Zhang a
a
Oil & Gas Field Applied Chemistry Key Laboratory of Sichuan Province, Southwest Petroleum University, Chengdu, Sichuan 610500, China
b
College of Chemistry and Chemical Engineering, Southwest Petroleum University, 8 Xindu Avenue, Chengdu, Sichuan 610500, China
c
State Key Lab of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, 8 Xindu Avenue, Chengdu, Sichuan 610500, China
d
Department of Chemical Engineering, University of Arkansas, Fayetteville, AR 72701, USA
H I G H L I G H T S G R A P H I C A L A B S T R A C T
• A novel polymer nanofibrous composite
absorbent was designed.
• Decoration of acidified MWCNTs on
PDA/PEN nanofibrous mat and codepo­
sition of tannic acid/carboxylated chi­
tosan was applied.
• The maximum adsorption capacities for
MeB and CR could reach 633 and 589
mg/g at the optimum pH values,
respectively.
• The nanofibrous composite absorbent
exhibited good regeneration ability and
easy-handling property.
A R T I C L E I N F O
Keywords:
Nanofiber
Poly(arylene ether nitrile)
Composite membrane
Adsorption
Dyes removal
A B S T R A C T
Fabrication of highly efficient adsorption and easy-handling absorbent is promising for removal of dyes from
wastewater, but it was still a big challenge. In this study, a novel polymer nanofibrous composite absorbent was
designed by the decoration of acidified MWCNTs on polydopamine-modified poly(arylene ether nitrile) nano­
fibrous membrane and following co-deposition of tannic acid/carboxylated chitosan. The structure and composi­
tion of as-prepared nanofibrous composite absorbent were confirmed by SEM, FTIR, XPS, and TGA. The congo red
(CR) and methylene blue (MeB) were applied as probe dyes to evaluate the adsorption capacity of nanofibrous
composite absorbent (With the average pore diameter of 19.21 nm) for cation and anion dyes. It was found that the
adsorption capacity of MeB and CR at the optimum pH could reach 633 and 589 mg/g in 300 min, respectively. The
adsorption process of MeB and CR followed the Langmuir adsorption model and pseudo-second-order model.
Moreover, the nanofibrous composite absorbent exhibited easy-handling property and good regeneration ability,
showing the potential application for the efficient adsorption of various dyes from wastewater.
* Corresponding author at: Oil & Gas Field Applied Chemistry Key Laboratory of Sichuan Province, Southwest Petroleum University, Chengdu, Sichuan 610500,
China.
** Corresponding author.
E-mail addresses: 201599010032@swpu.edu.cn (Y. Zhan), ychiao@uark.edu (Y.-H. Chiao).
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and
Engineering Aspects
journal homepage: www.elsevier.com/locate/colsurfa
https://doi.org/10.1016/j.colsurfa.2021.127069
Received 29 April 2021; Received in revised form 20 June 2021; Accepted 21 June 2021

2. Colloids and Surfaces A: Physicochemical and Engineering Aspects 626 (2021) 127069
2
1. Introduction
Nowadays, the large-scale discharge of organic dyes-containing
wastewater from leather, paper printing, and metallurgy industries
has caused huge impact on the human health, which also poses a great
challenge to the treatment technology of dye wastewater [1–4].
Recently, nanomaterials have shown potential candidate as absorbent
for the removal of dyes from wastewater due to their regular pore
structure, large specific surface, and tunable surface properties. Up to
now, various nanomaterials such as graphitic carbon nitride [5,6], metal
organic frameworks [7], and carbon-based materials (graphene and
carbon nanotubes) [8,9] were applied to prepare absorbents for dyes
removal.
Among them, carbon nanotubes were widely used for purchasing
promising absorbents by surface functionalization [10–12], due to their
high mechanical strength, relatively low price, strong interaction with
pollutant, etc. For instance, Ahamad and co-workers developed mag­
netic carbon nanotube hybrid absorbent by embedding the magnetic
Fe3O4 nanoparticles with amino functionalized multi-walled carbon
nanotubes, which showed maximum adsorption capacity of 178.5 mg/g
for methylene blue (MB) [13]. Mohamed and co-workers prepared the
anthracite/carbon nanotube (An/CNTs) composite absorbent, which
possessed the methyl orange (MO) adsorption capacity (qmax) of 416.7
mg/g [14]. Zhang and co-workers fabricated the polyethyleneimine
(PEI)-functionalized magnetic carbon nanotubes adsorbent, which
showed the maximum adsorption capacity of 196.08 mg/gfor Alizarin
Red S [15]. Although the efficient removal of organic dyes was achieved,
most studies focused on preparation of CNTs-based bulk or powder, the
agglomeration, difficult separation, and recycling problem greatly
restricted their practical application. Thus, incorporating CNTs into
membrane or mat is a convenient way to conquer those drawbacks.
In recent years, electrospun polymer nanofibrous mat and membrane
with easy-handling feature, tunable pore size, and high specific surface
area have received growing attentions in the area of water treatment
[16–18]. Especially, as a special engineering polymer, poly(arylene
ether nitrile) (PEN) with high heat-resistance, high mechanical strength,
and anti-corrosion performance shows promising candidate for prepar­
ing advanced polymer nanofibrous membrane via electro-spinning
technique [19,20]. In our previous work, we developed PEN poly­
pyrrole core/shell nanofibrous mat through electrospinning and in situ
polymerization of polypyrrole, which exhibited high adsorption capac­
ity for Cr (VI) (165.3 mg/g) [21]. More importantly, the PEN poly­
pyrrole core/shell nanofibrous mat presented a flexible membrane
morphology, which endowed the absorbent with easy-handling property
after adsorption of soluble pollutants. Therefore, considering the high
adsorption capacity for organic dyes and easy operation, this inspired us
to design nanofibrous composite membrane by introduction of CNTs and
further surface modification, which is expected to exhibit improved
facile recyclable ability, adsorption capacity, and rate.
In this work, we developed a facile method to fabricate the CNTs
decorated polymer nanofibrous membrane@tannic acid/carboxylated
chitosan absorbent for efficiently removing the dyes from aqueous so­
lutions (Scheme 1). For this purpose, the following three steps were
taken: (1) Fabrication of polydopamine modified PEN nanofibrous
membrane; (2) Decoration of CNTs onto the PEN nanofibers; (3) Co-
deposition of tannic acid/carboxylated chitosan. Tannic acid (TA) as a
cheap polyphenol has been widely used for surface functionalization of
various substrates [22–24]. On the one hand, the abundant -OH groups
on the surface of tannic acid can form stable intermolecular hydrogen
bonds with -COOH in carboxylated chitosan [25]. On the other hand, the
tannic acid can react with carboxylated chitosan via Michael addition
and Schiff base reactions. Therefore, the deposition of TA/CC contrib­
uted to the following advantages: (1) The multiple interfacial interaction
would enhance the mechanical strength and durability of nanofibrous
composite membrane; (2) The abundant -OH and -COOH groups of
TA/CC coating could enhance the active adsorption sites and show
excellent adsorption performance for dyes through electrostatic inter­
action and hydrogen bonding. As a result, the obtained easy-handling
PEN nanofibrous composite membrane exhibited fast and efficient
adsorption for both cationic and anionic dyes by adjusting the pH
values, showing potential application for the removal of various dyes
from wastewater.
2. Experimental
2.1. Materials
The multi-walled carbon nanotubes (MWCNTs, outside diameter:
10–30 nm, length: 10–30 µm, >95%) were supplied by Chengdu
Organic Chemicals Co. Ltd. N, N-dimethylformamide (DMF), hydro­
chloric acid (HCl, 37%), copper sulfate(CuSO4), hydrogen peroxide
(H2O2, 30%), tannic acid (TA), carboxylated chitosan (CC), and sodium
hydroxide (NaOH, 98%) were purchased from Aladdin (Shanghai,
China). Ethanol, methylene blue, and congo red were supplied by
Kelong Chemical Reagent Factory (Chengdu,China). PEN was synthe­
sized in the lab based on our previous work [26,27]. The acidified
MWCNTs were obtained according to the previous work [28].
2.2. Preparation of PDA-modified PEN nanofibrous membrane
In this work, the electrospinning technique was used to prepare PDA-
modified PEN nanofibrous membrane. Firstly, 3 g of dopamine and was
dispersed in 17 mL DMF. After 1.5 g of PEN was dissolved in the solution
and magnetically stirred at 70 ◦
C for 12 h, the mixed solution was
collected into a syringe. Then, the PEN/DA nanofibers were electrospun
onto aluminum foil with voltage of 20 kV for 4 h. In this process, the
feeding speed was 1 mL/h and the spinning distance was 15 cm. In order
to gain PDA-modified PEN nanofibrous membrane (p-PEN), the above
nanofibrous mat was soaked into the coagulation bath at room tem­
perature for 2 h (CuSO4 (5 mM), H2O2 (20 mM), Tris-HCl (10 mM)).
After washing by DI water for three times and drying in an oven at 60 ◦
C
overnight, the p-PEN was obtained.
Scheme 1. Schematic diagram for the preparation procedure of CNTs deco­
rated PEN nanofibrous membrane@tannic acid/carboxylated chito­
san absorbent.
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3. Colloids and Surfaces A: Physicochemical and Engineering Aspects 626 (2021) 127069
3
2.3. Preparation of p-PEN/a-CNTs@TA/CC nanofibrous composite
membrane
First of all, a certain amount of acidified MWCNTs (1 mg/mL) were
dispersed in water/ethanol mixed solution with the volume ratio of 1:4.
Then, the p-PEN nanofibrous membrane was immersed into above sus­
pension in an ice-water bath, followed by sonication for 30 min.
Therefore, the MWCNTs decorated p-PEN nanofibrous composite
membrane was obtained after rinsing with ethanol for several times and
drying in an oven at 60 ◦
C for 12 h.
For the deposition of TA/CC onto the surface of PEN nanofibers, TA
(0.4 mg/mL) was dissolved in deionized water (pH = 3), followed by
addition of a certain amount of CC (From 0.2–0.8 mg/mL) under me­
chanical stirring. Then, the MWCNTs decorated p-PEN nanofibrous
composite membrane was added into the TA/CC solutions and immersed
for 12 h. After washing with DI water for several times and drying in an
oven at 60 ◦
C for 12 h, the carbon nanotubes decorated poly(arylene
ether nitrile)@tannic acid/carboxylated chitosan nanofibrous compos­
ite membrane (p-PEN/a-CNTs@TA/CC) was obtained. In this study, the
p-PEN/a-CNTs@TA/CC-X was used to denote the nanofibrous composite
membrane, in which X represented the ratio of TA to CC. Unless
otherwise specified, the specimen used for the characterization and dye
adsorption was p-PEN/a-CNTs@TA/CC-1.
2.4. Characterization
The morphology of PEN p-PEN, p-PEN/a-CNTs, and p-PEN/a-CNTs
@TA/CC-X nanofibrous composite membranes were examined by
scanning electron microscopy (SEM, JSM-7500F, JEOL, Tokyo, Japan).
The thermal stability of p-PEN and p-PEN/a-CNTs@TA/CC nanofibrous
composite membranes was tested by TGA instrument (DSC823 TGA/
SDTA85/e Switzerland) under nitrogen atmosphere (40–800 ◦
C, 20 ◦
C/
min). The chemical composition of p-PEN, p-PEN/a-CNTs, and p-PEN/a-
CNTs @TA/CC nanofibrous composite membranes were examined by
Fourier transform infrared spectroscopy (FT-IR, Nicolet iS10, Thermo
Scientific, USA) and X-ray photoelectron spectroscopy (XPS, Escalab
250Xi, Thermo Scientific, USA).
2.5. Adsorption experiments
The typical anionic dye MeB and cationic dye CR were selected to
evaluate the adsorption capacity of p-PEN/a-CNTs@TA/CC nanofibrous
composite absorbent. For this purpose, 10 mg absorbent was added into
50 mL of 50 mg/L MeB and CR aqueous solution, and the pH values were
adjusted to 3, 5, 7, 9, and 11 by 1.0 M NaOH and 1.0 M HCl aqueous
solutions. Then, the above solutions were shaken in a water bath ther­
mostatic oscillator for 300 min at room temperature. After complete
adsorption, the absorbent was separated by a tweezers. The absorbance of
MeB and CR solutions with different pH values at the maximum absorp­
tion wavelength was scanned by an ultraviolet-visible spectrophotometer
(UV-6000PC, Metash Instruments Co., Ltd, Shanghai, China) for accurate
determination of the dye concentration after adsorption equilibrium. The
impact of ionic strength on adsorption performance of nanofibrous
composite membrane was studied by changing the salt concentration in
the NaCl medium (0.0–1.0 M). The adsorption capacity of adsorbent for
different dyes was calculated using the Eq. (1):
qe =
(C0 − Ce) × V
m
(1)
Where C0 and Ce are the initial and equilibrium concentration of dye,
respectively (mg/L). m represents the mass of adsorbent. qe and V
represent the adsorption capacity at equilibrium state and solution
volume, respectively.
To investigate the effect of time on dye adsorption, 10 mg absorbent
was added into 50 mL of 100 mg/L MeB and CR aqueous solution, and
the pH value were fixed at 11 for MeB and 3 for CR, respectively. After
shaking in a water bath thermostatic oscillator at 25 ◦
C, the remaining
concentration was determined at each time interval (Ct). The adsorption
capacity at each time interval (qt) was calculated according to Eq. (2):
qt =
(C0 − Ct) × V
m
(2)
Fig. 1. (a) Digital photograph of DA/PEN nanofibrous membrane; SEM images of (b)DA/PEN, (c) p-PEN, and (d) p-PEN/a-CNTs nanofibrous composite membranes.
S. Zhao et al.

4. Colloids and Surfaces A: Physicochemical and Engineering Aspects 626 (2021) 127069
4
Fig. 2. SEM images of PEN nanofibrous composite membranes with various mass ratios of TA to CC: (a) 1:0 (b) 2:1 (c) 1:1 and (d)1:2.
Fig. 3. (a) FT-IR spectra of p-PEN and p-PEN/a-CNTs@TA/CC nanofibrous composite membranes; (b) XPS spectra of p-PEN/a-CNTs@TA/CC nanofibrous composite
membrane; (c) C1s spectra of p-PEN/a-CNTs@TA/CC nanofibrous composite membrane; (d) TGA curves of PEN and p-PEN/a-CNTs@TA/CC nanofibrous compos­
ite membranes.
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5. Colloids and Surfaces A: Physicochemical and Engineering Aspects 626 (2021) 127069
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The adsorption isotherm measurements were conducted by mixing
10 mg absorbent and 50 mL of MeB and CR aqueous solution with
different concentrations (100–600 mg/L). In this process, the pH value
were adjusted at 11 and 3, respectively. After that, the above solutions
were shaken in a water bath thermostatic oscillator at 25, 35, and 45 ◦
C
for 300 min, respectively.
2.6. Desorption and reusability experiments
The desorption and reusability experiments of the absorbent were
performed for five successive cycles. In each cycle, 10 mg absorbent was
added into 50 mL of MeB and CR solution with initial concentration of
50 mg/L at pH = 11 and pH = 3, respectively. After shaking in a water
bath thermostatic oscillator for 300 min at 25 ◦
C, the adsorbent was
washed by absolute ethanol, followed by drying in an oven at 60 ◦
C for
next cycle.
3. Results and discussion
3.1. The structure and morphology of the nanofibrous composite
absorbent
The morphologies of the DA/PEN, p-PEN, and p-PEN/a-CNTs nano­
fibrous membranes were characterized by SEM, as shown in Fig. 1. As
could be seen from Fig. 1a, the DA/PEN presented a white and flexible
membrane-like morphology, due to the physical stacking of PEN nano­
fibers. From the SEM image in Fig. 1b, the nanofibers of DA/PEN were
relatively smooth. After treating in the Tris-HCl solution, there were
many rough granular structures on the surface of the nanofibers
(Fig. 1c), which was due to the self-polymerization of dopamine. With
the further ultrasonic treatment in the a-CNTs dispersion, the carbon
nanotubes were decorated and entangled together on the surface of p-
PEN nanofibers (Fig. 1d).
The SEM images of nanofibrous composite membranes prepared
with different proportions of tannic acid and carboxylated chitosan (TA:
CC) were shown in Fig. 2. When only TA participated in the reaction,
many aggregates appeared on the surface of the membrane (Fig. 2a).
With the addition of CC, the surface of the nanofibers became relatively
smooth, and a certain layer of polymer coating appeared on the nano­
fibers, indicating the chemical crosslinking between TA and CC
(Fig. 2b). As the ratio of TA:CC increased to 1:1, the nanofibrous
membrane structure was completely formed, and the organic functional
layer was uniformly loaded on the surface of p-PEN/a-CNTs, as shown in
Fig. 2c. However, further increasing the ratio of CC: TA would cause the
serious crosslinking and make the membrane become dense (Fig. 2d),
which had a negative effect on the adsorption capacity. Therefore, the p-
PEN/a-CNTs@TA/CC nanofibrous composite membrane with the TA:CC
ratio of 1:1 was selected for the batch adsorption tests. Moreover, from
Fig. S1 of pore size distribution, the average pore diameter of the
nanofibrous composite absorbent (With the TA:CC ratio of 1:1) is
19.21 nm, which is beneficial to the adsorption of dyes.
The chemical structure of composite membranes was examined by
FT-IR and XPS shown in Fig. 3a and b. Typically, the absorption band at
2230 cm− 1
could be observed for p-PEN and p-PEN/a-CNTs@TA/CC,
which was assigned to the -CN characteristic peak of PEN [29]. After
modification by TA/CC, the characteristic absorption peak of C-N
appeared at 1640 cm− 1
, and C–
–O appeared at 1720 cm− 1
. Besides, a
wide absorption band at 3100–3650 cm− 1
corresponded to amino and
hydroxyl groups of TA/CC. Moreover, the C, N, and O elements could be
found in the XPS spectra (Fig. 3b). From Fig. 3c, the C1s peak of
p-PEN/a-CNTs@TA/CC was divided into four peaks including C-H/C-C
(284.3 eV), C-OH/C-N (285.0 eV), C–
–O (286.4 eV), and O-C–
–O
(289.1 eV). Furthermore, from the TGA curves (Fig. 3d), the mass loss of
~9 wt% in the range of 250–500 ◦
C was attributed to the decomposition
of TA/CC, which further confirmed the successful surface modification.
3.2. Batch adsorption tests
3.2.1. Effect of the ratio of TA to CC
In this article, the MeB and CR were employed as model pollutants
for evaluating the adsorption capacities of nanofibrous composite
membrane. Typically, the adsorption was conducted in neutral condi­
tions (Room temperature). As could be seen from Fig. 4, the pristine PEN
nanofibrous membrane exhibited low adsorption ability toward MeB
and CR. Because the pristine PEN nanofibers lacked active adsorption
sites, the adsorption capacity for MeB and CR were only 12 and 12 mg/g,
respectively. While, for p-PEN and p-PEN/a-CNTs, the adsorption ca­
pacity towards MeB and CR increased to 52, 68 and 54, 63 mg/g,
respectively. These results were attributed to the π-π interaction between
organic dyes and polydopamine. However, it was worth noting that the
p-PEN/a-CNTs exhibited higher adsorption capacity for MeB than CR,
due to the fact that the combination of a-CNTs and PDA occupied a
certain amount of adsorption sites under neutral conditions. In this case,
the functional group (-COOH) on the surface of carbon nanotubes
adsorbed the MeB by effective electrostatic attraction. With the
following deposition of TA/CC, the adsorption capacity toward MeB and
CR was further enhanced. Moreover, the adsorption capacities increased
with the increasing rate of CC: TA and then decreased. The maximum
adsorption capacity was achieved with the ratio of 1:1 (165 mg/g for
MeB and 185 mg/g for CR). However, further increasing rate of CC: TA
lead to the decline in adsorption capacity. As shown in Fig. 2, the surface
of p-PEN/a-CNTs@TA/CC-X nanofibers was too thick, making the
nanofibrous composite membrane sticky. Therefore, the porosity was
reduced and the dye molecules was difficult to enter the membrane.
3.2.2. Effect of solution pH
In general, the pH values have an important influence on the
adsorption capacity for cationic MeB and anionic CR. With the pH varied
from 3 to 11, the adsorption capacity of MeB increased from 67 to
384 mg/g (Fig. 5). While CR had an opposite trend, the membrane to­
ward CR showed high adsorption capacity of 396 mg/g at pH = 3 and
low adsorption capacity of 59 mg/g at pH = 9. The different adsorption
behaviors in acidic and alkaline conditions could be explained as follow.
Under alkaline condition, due to multiple carboxyl groups in CC, the
strong electrostatic interaction between the CC and MeB would be one of
the principle pathways for MeB adsorption. Under acidic condition, the
-OH on the adsorbent would be protonated, and the lower pH could lead
to the higher degree of protonation. Consequently, there was strong
Fig. 4. Comparison of MeB and CR adsorption capacities among different
nanofibrous composite membranes. M0: pristine PEN nanofibrous membrane;
M1: p-PEN nanofibrous composite membrane; M2: p-PEN/a-CNTs nanofibrous
composite membrane; M3: p-PEN/a-CNTs@TA nanofibrous composite mem­
brane; M4: p-PEN/a-CNTs@TA/CC-0.5 nanofibrous composite membrane; M5:
p-PEN/a-CNTs@TA/CC-1 nanofibrous composite membrane; M6: p-PEN/a-
CNTs@TA/CC-2 nanofibrous composite membrane.
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6. Colloids and Surfaces A: Physicochemical and Engineering Aspects 626 (2021) 127069
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electrostatic repulsion between the MeB and adsorbent, thus leading to
the decrease of adsorption capacity. For CR, the -OH and -NH2 groups of
CR were protonated under acidic condition. Meanwhile, the -SO3Na of
adsorbent dissociated into -SO3
-
and Na+
, which was beneficial to
enhance the electrostatic interaction between CR and the adsorbent.
To further explore the different adsorption process for these two
dyes, the pH dependence of zeta potential for p-PEN/a-CNTs@TA and p-
PEN/a-CNTs@TA/CC was shown in Fig. 5b. As the pH value increased,
the zeta potential of p-PEN/a-CNTs@TA surface decreased from 8.5 mV
at pH = 3 to − 21.1 mV at pH = 11. In contrast, the zeta potential of p-
PEN/a-CNTs@TA/CC exhibited a larger variation range and more
negative value within the same pH range (20 mV at pH = 2 and − 23 mV
at pH = 11). Meanwhile, compared with p-PEN/a-CNTs@TA, the PZC
(point of zero charge) of p-PEN/a-CNTs@TA shifted from pH = 5.2 to
pH = 7.3. The apparent surface charge properties of the p-PEN/a-
CNTs@TA/CC mainly depended on the deprotonation and protonation
of the TA/CC coating functional group. In the case of strong acidity, the
phenolic -OH group in TA and the -NH2 group of CC tended to be pro­
tonated, which could enhance the positive charge number of composite
nanofibrous membrane and capture more negatively charged adsorbates
under acidic condition. On the contrary, the carboxyl group of CC and
phenolic hydroxyl group of TA were easily deprotonated in a wide pH
range. Under alkaline condition, the surface of composite nanofibrous
membrane adsorbed OH-
, which made the surface of membrane nega­
tively charged. This was why our composite absorbent possessed
different adsorption capacity under acidic and alkaline conditions.
Therefore, based on the above discussion, the adsorption tests on CR and
MeB were carried out under the conditions of pH = 3 and pH = 11,
respectively.
3.2.3. Effect of ionic strength
In the practical application, the dye wastewater is often accompanied
by inorganic ions, which have huge impact on the adsorption perfor­
mance of absorbent. In this work, the effect of ionic strength on
adsorption capacity for MeB and CR was investigated by changing the
concentration of NaCl in the dye solutions. It obvious from Fig. 6 that, as
the concentration of NaCl increased from 0 to 0.1 M, the adsorption
capacity of p-PEN/a-CNTs@TA nanofibrous composite membrane for
MeB and CR was slightly decreased. This was due to the fact that the
electrostatic interaction between p-PEN/a-CNTs@TA/CC and dye mol­
ecules could be interfered by the negative charges of Cl-
. In addition, the
negative Cl-
could participate in competitive adsorption, which would
occupy the available adsorption sites and hinder the adsorption of dyes.
Nonetheless, as the concentration of NaCl reached 0.1 M, adsorption
capacity of p-PEN/a-CNTs@TA/CC nanofibrous composite membrane
for MeB and CR remained at 355 and 340 mg/g, respectively. Therefore,
the above results indicated that our nanofibrous composite membrane
possessed stable adsorption performance under salty condition.
3.3. Adsorption kinetics
Fig. 7a displayed the effect of adsorption time on the adsorption
capacity of p-PEN/a-CNTs@TA/CC nanofibrous composite membrane.
The UV–Vis spectra for the adsorption experiments of MeB and CR was
shown in Fig. S2. Obviously, the adsorption capacity increased rapidly in
a short time and then reached a platform. The high adsorption rate was
attributed to the abundant adsorption sites on the absorbent and the
high dye concentration. With the time going on, the concentration of dye
reduced and the adsorption on the nanofibers were saturated. The
pseudo-first-order and pseudo-second-order adsorption models[30]
were adopted for evaluating the adsorption kinetics of absorbent toward
MeB and CR:
ln(qe − qt) = ln qe − k1t (3)
t
qt
=
1
k2q2
e
+
t
qe
(4)
Herein, the k1 and k2 represented the adsorption rate constants of
these two models, respectively. Fig. 7b and c showed the ln(qe-qt) vs. t
and t/qt vs.t plots. Besides, the calculated data was presented in
Table S1. Normally, the higher regression factor (R2
) suggests better
kinetic model for describing the adsorption behavior. Obviously, the
Fig. 5. (a) Effect of solution pH on the adsorption capacity toward different dyes and (b) Zeta potential of p-PEN/a-CNTs@TA/CC and p-PEN/a-CNTs@TA nano­
fibrous composite membranes.
Fig. 6. The influence of ionic strength on the removal efficiency of MeB
and CR.
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7. Colloids and Surfaces A: Physicochemical and Engineering Aspects 626 (2021) 127069
7
pseudo-second-order model of MeB and CR showed larger R2
(MeB, R2
= 0.999; CR, R2
= 0.999), indicating that it was more suitable to
describe the adsorption of MeB and CR. Also, it suggested that the
adsorption rate was limited by the chemical adsorption [31],which was
achieved by sharing/exchanging electrons between adsorbents and
dyes. Combined with the zeta potential analysis in Fig. 5b, the strong
interaction between positively charged dyes and negatively charged
adsorbent played a significant role in the adsorption process.
In order to further investigate the rate control step for the adsorption
of dyes, the intraparticle diffusion model was applied [32], as shown in
Eq. (5).
qt = kpt1/2
(5)
As seen from Fig. 7d, the obtained curves could be divided into two
stages. The adsorbate molecules quickly entered the large pores and
wider mesopores, and then slowly penetrated into the smaller meso­
pores. In addition, the simulated parameters were presented in Table S1.
Owing to the rapid film diffusion and surface adsorption, the first stage
(ki1 = 26.921, and 25.391 for MeB and CR, respectively) exhibited the
larger slope. It meant that the liquid film of adsorbent was the main way
for dye diffusion from solution to the outer surface of adsorbent, which
could further interact with the active adsorption sites via electrostatic
attraction and chemisorption. However, the lower ki2 indicated that the
diffusion of dye molecules within the particles in the small pores was the
rate-limiting step in the adsorption process [33].
3.4. Adsorption isotherms
To better understand the adsorption process of the nanofibers, the
effects of different initial dye concentrations were investigated at
T = 298.15 K, and the pH of MeB and CR were at 11 and 3, respectively.
As the dye concentration increased, the equilibrium adsorption capacity
was continuously enhanced (Fig. 8a). With the initial concentration of
500 mg/L, the maximum adsorption capacity of MeB and CR were 633
and 589 mg/g, respectively. In this work, the adsorption mechanism
was explained by the Langmuir and Freundlich isotherms models [34]:
Ce
qe
=
1
qmKL
+
Ce
qm
(6)
ln qe = ln KF +
1
n
ln Ce (7)
Herein, the Ce and qm refer to the equilibrium concentration of the
MeB and the maximum adsorption capacity, respectively. KL and KF
represent the constants of the Langmuir and Freundlich, respectively. 1/
n is the intensity of the adsorption. Besides, in order to examine the
feasibility of Langmuir isotherm, the RL (dimensional constant separa­
tion factor) was applied [35]:
RL =
1
1 + KLC0
(8)
According to the above equation, the isotherm could be divided into
following four situations: (1) RL = 0, irreversible; (2) RL = 1, linear; (3)
0 < RL < 1, favorable; and (4) RL > 1, unfavorable. Fig. 8b and c dis­
played the plots of Ce/qe vs. Ce and lnqe vs. lnCe, respectively. In
addition, the related parameters were presented in Table S2. Obviously,
both the fitting curves of Langmuir model had higher correlation coef­
ficient (R2
>0.99) than the Freundlich model, indicating that the
Langmuir model was more suitable to explain the dye adsorption of
membrane toward MeB and CR. Besides, the RL values for MeB and CR
were in the range of 0–1, implying that these adsorption process was
Fig. 7. (a) Effect of time on the adsorption of MeB and CR. Adsorption kinetic curves of MeB and CR by (b) pseudo-first-order kinetic model and (c) pseudo-second-
order kinetic model. (d) Intraparticle diffusion.
S. Zhao et al.

8. Colloids and Surfaces A: Physicochemical and Engineering Aspects 626 (2021) 127069
8
easy. In general, the Langmuir model supposes that the adsorption is
mono-layer and the surface is uniform, and the adsorption capacity of
each position is the same [36]. That was to say that the negative charges
with high density were evenly distributed on nanofiber surface and the
monolayer adsorption was the key factor for the adsorption of dyes in
the solution. Therefore, the p-PEN/a-CNTs@TA/CC nanofibrous com­
posite membrane exhibited efficient removal of CR and MeB from water.
Furthermore, compared with some other fiber-based adsorbents listed in
Table 1, p-PEN/a-CNTs@TA/CC nanofibrous composite membrane
exhibited much higher adsorption capacities, indicating a desirable
nanofibrous adsorbent for treating wastewater containing different kind
of dyes at different pH.
3.5. Adsorption thermodynamics
The effect of temperature on adsorption capacity of p-PEN/a-
CNTs@TA/CC nanofibrous composite membrane toward MeB and CR
was investigated. As shown in Fig. 8d, with the temperature increasing,
the adsorption capacity of MeB slightly decreased from 356 to 332 mg/
g, indicating the adsorption process is an exothermic reaction. More­
over, the thermodynamic parameters including ΔG◦
, ΔS◦
, and ΔH◦
were
calculated according to the following Equations:
ln kd =
ΔS0
R
−
ΔH0
RT
(9)
kd =
qe
Ce
(10)
ΔG0
= − RT ln kd (11)
Where kd represents the distribution coefficient for the adsorption, R is
the gas constant, T is the temperature. The plot of lnkd vs. T− 1
and the
values of ΔG◦
, ΔS◦
and ΔH◦
were shown in Fig. 8d and Table S3,
respectively. The negative values of ΔH◦
(ΔH◦
of MeB = − 8.904 kJ/mol
and ΔH◦
of CR = − 10.680 kJ/mol) reflected the exothermic nature of
both dyes adsorption. The positive value of ΔS◦
(ΔS◦
of MeB = 1.111 J/
mol K and ΔS◦
of CR = 0.518 J/mol K) confirmed that the adsorption of
p-PEN/a-CNTs@TA/CC nanofibrous composite membrane toward MeB
and CR was an entropy increasing reaction, and the degree of disorder in
the adsorption process increased. In addition, the negative value of the
ΔG◦
confirmed the adsorption of both dyes on the nanofibrous com­
posite membrane was spontaneous process.
Fig. 8. (a) Adsorption isotherms; (b) Langmuir isotherms; (c) Freundlich isotherms at 298.15 K and (d) Effect of temperature on the adsorption capacities of p-PEN/
a-CNTs@TA/CC nanofibrous composite membrane toward MeB and CR.
Table 1
Comparison of adsorption capacity of p-PEN/a-CNTs@TA/CC toward MeB and
CR with those of other fiber-based adsorbents reported previously.
Dyes Adsorbents qe (mg/
g)
References
MeB (fun-PAN/DS) 243.44 [37]
PVDF/PDA/PPY-3 370.4 [38]
PVA/starch nanofiber membrane 400 [39]
PAN/β-CD blend nanofiber membrane 108.66 [40]
Sericin/β-cyclodextrin/poly (vinyl alcohol)
composite nanofiber adsorbent
261.1 [41]
p-PEN/a-CNTs@TA/CC 666 This work
CR PVDF/PDA/PPY-3 384.6 [38]
Sepiolite-based organic modified nanofibers 539.71 [42]
PDA@DCA-COOH 175.98 [43]
Chitosan/PVA nanofibers modified with PANI/
SiO2 Nanoparticles
110.2 [44]
p-PEN/a-CNTs@TA/CC 617 This work
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9. Colloids and Surfaces A: Physicochemical and Engineering Aspects 626 (2021) 127069
9
3.6. Regeneration property and stability
The regeneration property and chemical stability of the nanofibrous
membranes were further investigated. First, the five cycles of adsorp­
tion/desorption process were carried out, as shown in Fig. 9a. With the
increment of cycle times, the adsorption amounts toward MeB and CR
slightly decreased from 393 and 382 mg/g to 340 and 330 mg/g,
respectively. These results indicated the good reusability of the p-PEN/
a-CNTs@TA/CC nanofibrous composite membrane. To further test the
chemical stability, the composite membrane was immersed in 0.1 M HCl
and 0.1 M NaOH solutions for 48 h, respectively. Obviously, the
adsorption capacity shown in Fig. 9b was basically unchanged.
Moreover, the SEM images of p-PEN/a-CNTs@TA/CC nanofibrous
composite membrane (Fig. 9c and d) indicated that morphologies were
not changed after treatments in HCl and NaOH conditions, indicating
the good alkali and acidic tolerance of the nanofibrous composite
membrane. Furthermore, from the mechanical properties shown in
Fig. S3, the composite membrane possessed high tensile strength of
11.5 MPa and high tensile modulus of 226.02 Mpa, which could enable
the absorbent to recycle freely while maintaining its structural integrity.
3.7. Adsorption mechanism
In order to deeply understand the adsorption mechanism, the p-PEN/
a-CNTs@TA/CC before and after adsorption toward CR were tested by
FT-IR and XPS. Compared with the FT-IR spectrum of p-PEN/a-
CNTs@TA/CC before adsorption (Fig. 10), two new peaks of p-PEN/a-
CNTs@TA/CC-CR were found at the absorption bands of 1650 and
1720 cm− 1
, which were attributed to the characteristic peaks of C–
–N
and C–
–O, respectively. This indicated that the adsorbent-dye compound
was formed. Furthermore, after adsorption, these two peaks blue shifted
to 1645 and 1415 cm− 1
, respectively. Besides, the nanofibrous com­
posite membrane exhibited a much narrower characteristic absorption
band at 3100–3650 cm− 1
. Therefore, the results of FTIR proved the
interaction between CR and p-PEN/a-CNTs@TA/CC during the
adsorption process, which could be further confirmed by XPS. The O1s
and N1s spectra of p-PEN/a-CNTs@TA/CC nanofibers before and after
adsorption were shown in Fig. 11 and Table S4. From the N1s spectra of
p-PEN/a-CNTs@TA/CC (Fig. 11a and b), three peaks observed at 399.2
(N-H), 401.2(C-N), and 402.2(-NH2) eV of p-PEN/a-CNTs@TA/CC
shifted to 399.5, 400.8 and 401.6 eV in the spectrum of p-PEN/a-
CNTs@TA/CC-CR, respectively. In addition, the contents of N-H, C-N,
and -NH2 decreased from 78.61%, 14.46%, and 6.93–91.92%, 1.56%,
and 6.51%, respectively. Furthermore, for the O1s spectra (Fig. 11c and
Fig. 9. (a) Regeneration property of p-PEN/a-CNTs@TA/CC nanofibrous composite membranes toward MeB and CR; (b)The adsorption capacity of p-PEN/a-
CNTs@TA/CC nanofibrous composite membranes in acidic and alkaline solutions after shaking for 12 h, respectively; (c) The SEM images of p-PEN/a-CNTs@TA/CC
nanofibrous composite membranes after treating in acidic and alkaline solutions.
Fig. 10. FT-IR spectra of p-PEN/a-CNTs@TA/CC nanofibrous composite
membrane before and after adsorption of CR.
S. Zhao et al.

10. Colloids and Surfaces A: Physicochemical and Engineering Aspects 626 (2021) 127069
10
d), the binding energies at 530.9 and 532.5 eV were assigned to the
C–
–O and C-OH, respectively. After the adsorption of CR, they shifted to
531.1 and 532.8 eV, respectively. Accordingly, the proportion of C-OH
decreased from 78.47% to 51.32%, and the proportion of C–
–O
increased from 21.53% to 48.77%. Therefore, these functional groups
participated in the adsorption of dyes. In fact, the N and O atoms could
provide their lone pair of electrons and had a strong affinity for H
protons. On the one hand, under acidic condition, the abundant -OH and
-NH2 of p-PEN/a-CNTs@TA/CC protonated to form -OH2
+
and -NH3
+
,
which would interact with functional groups the –SO- 3 of anionic dyes.
On the other hand, the -OH, N-H, -NH2, and -COOH of p-PEN/a-
CNTs@TA/CC would interact with functional groups of CR to form
various hydrogen bond, thus improving the adsorption capacity of
anionic dyes. Moreover, both dye molecules and adsorbents contain
Fig. 11. N1s and O1s XPS spectra of p-PEN/A-CNTs@TA/CC (a, b) and p-PEN/a-CNTs@TA/CC-CR (c, d).
Scheme 2. The adsorption mechanism of dyes onto p-PEN/a-CNTs@TA/CC nanofibrous composite membrane.
S. Zhao et al.

11. Colloids and Surfaces A: Physicochemical and Engineering Aspects 626 (2021) 127069
11
numerous benzene rings, which can adsorb dyes by forming π-π conju­
gates [30]. Based on the above analysis, the electrostatic interaction,
hydrogen bond, and π-π conjugates mainly contributed to the efficient
adsorption of anionic dyes (Scheme 2).
4. Conclusions
In summary, the novel PEN nanofibrous composite absorbent was
successfully constructed by the decoration of acidified MWCNTs and
following co-deposition of tannic acid/carboxylated chitosan. The hi­
erarchical core/shell structure and abundant active sites contributed to
the highly efficient removal of both anionic MeB and cationic CR by
regulating the pH value. It was found that the adsorption capacity of
MeB and CR at the optimum pH could reach 633 and 589 mg/g in
300 min, respectively. The adsorption process of two different dyes
followed the pseudo-second-order model and Langmuir adsorption
model. Furthermore, the easy-handling property and good regeneration
ability endowed the PEN nanofibrous composite absorbent with poten­
tial application for removing various dyes from wastewater.
CRediT authorship contribution statement
Shumei Zhao: Conceptualization, Methodology, Software. Qingy­
ing Feng: Data curation, Writing-Original draft preparation. Wei Yang
& Hongyu Dong: Visualization, Investigation. Yingqing Zhan & Yu-
Hsuan Chiao: Supervision. Xin Wen & Shirui Zhang: Software, Vali­
dation. Ao Sun: Writing- Reviewing and Editing.
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.
Acknowledgments
This work was financially supported by the National Natural Science
Foundation of China (51903215), Sichuan Province Sci-Tech Supported
Project (2020YJ0168), International Science and Technology Coopera­
tion Project from Chengdu Municipal Government (2020-GH02-00074-
HZ).
Appendix A. Supporting information
Supplementary data associated with this article can be found in the
online version at doi:10.1016/j.colsurfa.2021.127069.
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