C. XING et al.: ADSORPTION BEHAVIOR OF POLYANILINE MICRO/NANOSTRUCTURES FOR METHYL ORANGE
529–534
ADSORPTION BEHAVIOR OF POLYANILINE
MICRO/NANOSTRUCTURES FOR METHYL ORANGE
ADSORPCIJA POLIANILNIH MIKRO- IN NANOSTRUKTUR ZA
METILORAN@
Cuijuan Xing, Aiqing Xia, Ling Yu, Yongchao Hao, Lili Dong, Zhiju Zhao,
Guiquan Guo
*
, Yu Wang, Limin Tian, Mengze Sun
School of Chemistry and Chemical Engineering, Xingtai University, No. 88 Quanbei East Street, Qiaodong District, Xingtai 054001, China
Prejem rokopisa – received: 2019-11-06; sprejem za objavo – accepted for publication: 2020-03-01
doi:10.17222/mit. 2019.270
Polyaniline (PANI) nanofibers were successfully synthesized with ammonium persulfate as an oxidant. The adsorption behavior
of methyl orange (MO) on PANI at different parameters was investigated. Equilibrium studies demonstrated the use of the
Langmuir adsorption model and the maximum adsorption capacity (Qe) was calculated to be 315.3 mg·g
–1
at 313 K. A kinetic
study revealed that the MO adsorption on PANI followed the pseudo-second-order kinetic model. Experimental results suggest
that PANI nanofibers have the potential to be used as low-cost and efficient adsorbent materials for the removal of organic
pollutants from water.
Keywords: PANI micro/nanostructures, sorption, methyl orange (MO)
Avtorji tega prispevka opisujejo uspe{no sintezo polianilnih (PANI) nanovlaken s pomo~jo uporabe amonijevega persulfata kot
oksidanta. Raziskovali so obna{anje metiloran`a (MO) med adsorpcijo na polianilnih nanovlaknih pri razli~nih parametrih.
Ravnote`ne {tudije so pri uporabi Langmuirjevega adsorpcijskega modela dale vrednost maksimalne adsorpcijske kapacitete
(Qe) 315,3 mg·g
–1
pri 313 K. Kineti~ne {tudije pa so pokazale, da adsorpcija metiloran`a na polianilnih vlaknih sledi
pseudokineti~nemu modelu drugega reda. Eksperimentalni rezultati ka`ejo, da bi bila polianilna nanovlakna lahko uporabna kot
u~inkovit nizko cenovni adsorpcijski material (adsorbent) za odstranitev organskih ne~isto~ iz vode.
Klju~ne besede: polianilne mikro- in nanostrukture, sorpcija, metiloran`
1 INTRODUCTION
Due to the indiscriminate disposal of organic
pollutants, water pollution has been a rising worldwide
environmental concern.
1–2
In particular, dyes, which are
the main organic contaminants from the dye manufact-
uring and textile branches, are necessary to be removed
from the waste water.
3
The removal of organic dyes from
natural water resources does not only protect the
environment itself, but also stops the toxic-dye transfer
within food chains. Traditional techniques for treating
organic dyes include membrane filtration, ion exchange,
precipitation, adsorption and coagulation.
4–6
However, it
is noteworthy to mention that adsorption is the most
widely used method because of its ease of operation and
relatively low cost.
7
Several adsorbents were studied for
dye removal including hybrid xerogel, caly, activated
carbon, sepiolite and pansil.
8–10
The development of
adsorbents with a high adsorption capacity and effec-
tivity is attracting attention of scientists.
Amino groups, one of the most effective chelating
functional groups, have attracted special attention with
regard to the enrichment of various contaminants from
aqueous solutions due to their high nucleophilicity and
reactivity.
11
Polyaniline (PANI), a well-known conduct-
ing polymer, has received a great deal of interest due to
its simple preparation, controllable electrical conduct-
ivity, environmental stability, good redox reversibility
and low cost. Recently, PANI and its composites have
been used effectively for the removal of inorganic and
organic pollutants. They possess a large amount of amine
and imine groups, which can interact with heavy metals
and organic pollutants, making them suitable for pollu-
tant adsorption from aqueous solutions. Previous studies
suggested that PANI composites are promising ad-
sorbents for Cr (IV),
12–14
Hg (II),
15
Pb (II)
16
and tannic
acid.
17
PANI exhibits a high adsorption capacity for
tannic acid. Wan observed a significant improvement in
the humic-acid adsorption capacity when PANI was en-
capsulated on the surface of ATP.
18
An enhanced HA
adsorption was associated with electrostatic interactions
between the amine and imine groups of the adsorbents
and HA molecules in the solution.
Based on the premise that nitrogen atoms in PANI
can interact with organic pollutants in an aqueous
solution, PANI is expected to be an efficient and eco-
nomic adsorbent for removing dyes. Herein, PANI was
synthesized with a chemical oxidation to remove MO
from an aqueous solution. By employing batch experi-
ments, the removal of MO on PANI was investigated
Materiali in tehnologije / Materials and technology 54 (2020) 4, 529–534 529
UDK 620.18:66.021.3.081.3:547.556 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 54(4)529(2020)
*Corresponding author's e-mail:
guoguiquan1979@163.com (Guiquan Guo)

across a range of solution-chemistry conditions, includ-
ing time, temperature and ionic strength. The adsorption
kinetics and isotherms were investigated for a possible
mechanism. This study provides new insights into the
MO removal from aqueous solutions using PANI, which
can broaden the applicability of PANI to wastewater
cleanup.
2 EXPERIMENTAL PART
2.1 Materials
Aniline monomer (ANI, Beijing Chem. Co.) was
distilled under vacuum. Ammonium persulfate (APS),
methyl orange (MO) and other reagents were purchased
from Beijing Chem. Co. and used without further
purification.
2.2 Polymerization
Polyaniline was prepared by polymerizing ANI at
room temperature in an aqueous solution. Briefly, ANI
(10 mmol) was dissolved in 25 mL of distilled water
with supersonic stirring. Then an aqueous solution of
APS (2.3 g, 10 mmol), dissolved in 25 mL of distilled
water was dropped into the above solution and vigor-
ously stirred at room temperature. After having been
stirred for about 12 h, the product was collected with
vacuum filtration, and then washed with water, ethanol
and ether several times, respectively. Finally, the product
was being dried under vacuum at room temperature for
24 h.
2.3 Characterization
The morphology of the PANI was characterized with
field-emission scanning electron microscopy (FESEM,
JSM-6700F). FTIR and UV-Vis spectroscopy were
carried out to study the molecular structures. The FTIR
spectra of PANI in KBr pellets were recorded using an
IFS-113 V instrument. The resolution of the measure-
ments and the number of scans were 4 cm
–1
and 16,
respectively. UV-Vis spectroscopy of PANI in m-cresol
was measured with a Hitachi UV3100. The crystal
structures of the resulting polymer were characterized
with X-ray diffraction (a Micscience Model M18XHF
diffractometer). The concentration of MO in the solution
was analyzed using a UV-Vis spectrum instrument
(UV-2200, Beijing Beifen-Ruili Analytical Instrument
CO., Ltd).
2.4 Sorption experiments
2.4.1 Kinetic-sorption experiments
The sorption experiments were performed using a
batch technique. Briefly, samples of 0.1000±0.0001 g of
PANI were added into a series of 100 mL MO solution at
298 K in a temperature-controlled shaker (SHY-2A,
Danyang Appliance Co., Jiangsu). The agitation speed
was 150 min
–1
. The samples were withdrawn at appro-
priate time intervals. Then the mixture was filtered
through a 0.2-μm pore membrane. The supernatant of
each sample was analyzed using UV spectrometry based
on the law of Lambert-Beer, identified as the equilibrium
concentration of MO. The MO adsorbed on a PANI
sample was calculated from the difference between the
initial and equilibrium MO concentrations. These
experiments were carried out under the same conditions
at three levels, and the relative error was <5 %. The
adsorption capacity (Q
e
mg·g
–1
) of the MO adsorbed onto
PANI was obtained from Equation (1) and used for a
further adsorption-isotherm analysis:
Q
CCV
m
e
0 e
=
− ()
(1)
where C
0
and C
e
are the initial and equilibrium concen-
trations of MO. V is the volume of the MO solution (L)
and m is the weight of the PANI adsorbent (g).
2.4.2 Equilibrium sorption experiments
A batch method was also used to obtain sorption
isotherms. Isotherm experiments were conducted in
distilled water with various initial concentrations of MO
C. XING et al.: ADSORPTION BEHAVIOR OF POLYANILINE MICRO/NANOSTRUCTURES FOR METHYL ORANGE
530 Materiali in tehnologije / Materials and technology 54 (2020) 4, 529–534
Figure 1: SEM images of PANI micro/nanostructures

(5-250 mg·dm
–1
). An amount of 0.1000±0.0001 g of
PANI was added to 250-mL glass-stoppered flasks. Each
flask contained 100 mL of solution. The flasks were then
agitated at a constant speed of 150 min
–1
in a tempera-
ture-controlled shaker. After equilibrium was estab-
lished, the supernatants were separated. The residual MO
concentrations were analyzed and the amount of sorption
onto PANI was determined as in the kinetic experiments.
3 RESULTS AND DISCUSSION
3.1 Surface morphology of the PANI products
Scanning-electron-microscopy (SEM) images of
PANI are shown in Figure 1. It can be seen from the
figure that nanofibers with an average diameter of 220
nm were obtained. There are protuberances on the
surfaces of nanofibers, providing a good platform for the
MO adsorption.
3.2 Structural and chemical characterization of the
PANI products
FTIR and UV-vis spectroscopy were used to charac-
terize the molecular structure of PANI. Typical FTIR
spectra (Figure 2a) showed that the characteristic ab-
sorption bands for the PANI fibers are similar to those of
PANI.
19
For example, the C=C stretching deformation of
the quinoid (1581 cm
–1
) and benzenoid rings
(1501 cm
–1
), the C-N stretching of the secondary
aromatic amine (1302 cm
–1
), the aromatic C-H in-plane
bending (1142 cm
–1
), and the out-of-plane deformation
of C-H in the 1,4-disubstituted benzene ring (820 cm
–1
)
are observed. The UV-vis absorbance spectra of the
PANI sample dissolved in m-cresol is shown in Fig-
ure 2b. Bands at 430 nm and >800 nm were observed for
PANI, and they were assigned to polaron bands of the
emeraldine salt of PANI.
20
The band at 430 nm
corresponds to the partially oxidized form of PANI, most
likely in the intermediate state between the leucoemeral-
dine form containing benzenoid rings and the emeraldine
form containing conjugated quinoid rings in the main
polymer chain. The emeraldine form transformed into
the fully oxidized pernigraniline form, characterized by a
wide band at around 800 nm.
Figure 2c shows XRD patterns for PANI. PANI
exhibits a broad featureless XRD pattern centered around
2 = 20–30°, indicating that the sample is probably
amorphous. Some additional sharp peaks centered at
2 = 6.5°, 18.5°, 20.4° and 25.5° are observed and can be
attributed to the periodicity perpendicular and parallel to
the polymer chain, respectively.
21
3.3 Adsorption performance of PANI
3.3.1 Sorption kinetics
In order to determine the MO sorption equilibration
time and investigate the kinetic characteristics of the
sorption, experiments were carried out with there initial
MO concentrations of 100 mg·L
–1
, 200 mg·L
–1
and
250 mg·L
–1
. Figure 3 shows the plots of Q
t
-t at 298 K.
The results indicate that adsorption equilibrium was
reached within 120 min as the initial MO concentration
is 100 mg·L
–1
. As a comparison, the initial MO concen-
tration increased to 200 mg·L
–1
and 250 mg·L
–1
, whilethe
adsorption time before reaching the equilibrium state
was 300 min. The sorption equilibration time was
therefore determined to be 360 min to ensure a complete
equilibration.
Pseudo-second-order Equation (2) was tried to inter-
pret the kinetic curves and it can be expressed as:
C. XING et al.: ADSORPTION BEHAVIOR OF POLYANILINE MICRO/NANOSTRUCTURES FOR METHYL ORANGE
Materiali in tehnologije / Materials and technology 54 (2020) 4, 529–534 531
Figure 3: Sorption-kinetic curves of MO on the PANI micro/nano-
structure (line: pseudo-second-order model fitting)
Figure 2: a) FTIR spectra, b) UV-vis absorptionand c) XRD patterns of PANI micro/nanostructures

d
d
t
et
Q
t
kQQ =−
2
2
() (2)
where Q
e
and Q
t
(mg·g
–1
) are the sorption amounts of
the sorbate at equilibrium and various times t (min),
while k
2
(g·mg
–1
·min
–1
) is the rate constant.
Sorption parameters were obtained by fitting the
experimental data using a non-linear regression method,
as shown in Table 1. In the pseudo-second-order kine-
tics, the calculated Q
e
values were nearly the same as the
experimental values. Based on the correlation coeffi-
cients, r
2
, the pseudo-second-order equation describes
the curves well.
Table 1: Kinetic parameters of the pseudo-second-order models for
MO sorption on PANI micro/nanostructures at different initial
concentrations (298 K)
C
0
/mg·dm
–1
Q
e
(cal)/mg·g
–1
k
2
/g·mg
–1
·min
–1
r
2
100 42.10 0.003090 0.9227
200 102.0 0.0002619 0.9175
250 127.9 0.0001828 0.9375
3.3.2 Sorption isotherms
It is well established that temperature is an important
factor influencing any adsorption process. Figure 4
shows the adsorption isotherms of MO at four tempe-
ratures (293, 298, 303 and 313) K. The experimental
data for MO on PANI are analyzed using the Langmuir
adsorption isotherm model, which is applicable to highly
heterogeneous surfaces. Equation (3) is given as:
Q
QKC
KC
e
m L e
L e
=
+ 1
(3)
where Q
e
is the adsorption amount of MO on the
adsorbent (mg·g
–1
) at an equilibrium state, Q
m
is the
adsorption capacity of MO on the adsorbent (mg·g
–1
),
C
e
is the equilibrium concentration of MO (mg·L
–1
), and
K
L
is the Langmuir adsorption constant, related to the
adsorption energy. It can be seen from Table 2 that the
Langmuir model shows good agreement with the
experimental data. As the temperature increases, MO
molecules are more favorably adsorbed on PANI. The
adsorption capacity of MO is 131.1 mg·g
–1
and
139.9 mg·g
–1
at 293 K and 298 K. As the temperature
increases to 303 K and 313 K, the adsorption capacity
of MO increases to 157.6 mg·g
–1
and 315.3 mg·g
–1
,
respectively. A temperature increase is known to in-
crease the rate of diffusion of adsorbate molecules
across the external boundary layer and in the internal
pores of the adsorbent.
Table 2: Langmuir constants for the adsorption of MO onto PANI
micro/nanostructures at different temperatures
T/K Q
m
/mg·g
–1
K
L
/L·mg
–1
r
2
293 131.1 0.01834 0.9124
298 139.9 0.02101 0.9218
303 157.6 0.02338 0.9302
313 315.3 0.03800 0.9252
3.3.3 Thermodynamic parameters
Thermodynamic parameters of the MO sorption were
evaluated, including the standard free-energy change
( G
 ), standard enthalpy change ( H
 ) and standard
entropy change ( S
 ), which is important for under-
standing the sorption process. The parameters were
calculated using Equations (4) and (5):
ΔGR TK
 =− ln (4)
lnK
H
RT
S
R
=− +
ΔΔ

(5)
R is the universal gas constant and T is the Kelvin
temperature; K is the thermodynamic equilibrium con-
stant for the adsorption process. In our study, we used
the constant derived from the Langmuir model (K
L
). The
values of H
 and S
 were obtained from the plots of ln
K versus 1/T. The values of these thermodynamic para-
meters are listed in Table 3.
The positive value of  H
 suggests that the sorption
was an endothermic process, indicating that a higher
temperature is beneficial to the sorption process. The
positive value of S
 demonstrates increased randomness
at the solid-solution interface during the adsorption of
MO on PANI, indicating a strong tendency toward the
MO adsorption. The free-energy  G
 values were nega-
tive over the entire temperature range, indicating that the
MO adsorption onto PANI was thermodynamically
feasible and could occur spontaneously.
Table 3: Thermodynamic parameters for MO adsorption onto PANI
micro/nanostructures
T/K
K
L
×10
–6
/
mL·g
–1
 G
 /
kJ·mol
–1
 H
 /
kJ·mol
–1
 S
 /
J·mol
–1
·K
–1
293 0.01834 –23.91
27.00 58.61
298 0.02101 –24.61
303 0.02338 –25.34
313 0.03800 –27.44
C. XING et al.: ADSORPTION BEHAVIOR OF POLYANILINE MICRO/NANOSTRUCTURES FOR METHYL ORANGE
532 Materiali in tehnologije / Materials and technology 54 (2020) 4, 529–534
Figure 4: Adsorption isotherms of MO on PANI micro/nanostructures
at different temperatures

The sorption isotherms of MO in NaCl solutions,
having concentrations of 0 mol·L
–1
, 0.5 mol·L
–1
and 1.0
mol·L
–1
were investigated in our study. As shown in
Figure 6, sorption isotherms of MO on PANI accorded
well with the Langmuir model. The constants of the
Langmuir sorption equation and the correlation coeffi-
cients are listed in Table 4. This table shows that the Q
m
values increased from 139.9 mg·g
–1
to 210.8 mg·g
–1
when
the concentration of NaCl increased from 0 to
0.5 mol·L
–1
. While the concentration of NaCl increased
to 1.0 mol·L
–1
, the Q
m
values decreased to 181.0 mg·g
–1
.
These results conclusively demonstrate that NaCl had a
strong influence on the sorption capacity of PANI. The
reasons can be explained with two aspects. Firstly, for
hydrophobic organic chemicals such as MO, its water
solubility plays an important role in the sorption be-
havior.
22
A solution of NaCl contains inorganic ionic
species of Na
+
and Cl
–
, which can push organic com-
pounds to the surface of the adsorbent, hence reducing
the water solubility of organic compounds. The reason
for this is attributed to "salting out". Meanwhile, the
inorganic ionic species of Na
+
and Cl
–
make the ioni-
zation equilibrium of the MO adsorbed to PANI. On the
other hand, the Na
+
and Cl
–
in a solution can increase the
doping level and decease efficient activity sites. There-
fore, it is reasonable to expect that a higher concentration
of NaCl (1.0 mol·L
–1
) might go against the adsorption of
MO on PANI.
Table 4: Langmuir constants for the adsorption of MO onto PANI
micro/nanostructures in the presence of various concentrations of
NaCl
C
NaCl
/mol·dm
–1
Q
m
/mg·g
–1
K
L
/L·mg
–1
r
2
0 139.9 0.02101 0.9218
0.5 210.8 0.02425 0.9779
1.0 181.0 0.02243 0.9534
4 CONCLUSIONS
In this study, PANI was successfully prepared via a
facile-solution route, using ammonium peroxydisulfate
as the initiator. The sorption of MO on PANI under
various conditions was investigated to evaluate its kinetic
and thermodynamic behavior. The results obtained are as
follows:
1) The sorption equilibrium was achieved within 5 h.
The kinetics of the process can be described with a
pseudo-second-order kinetic-rate equation. The initial
concentration of MO influenced the sorption rate.
With an increase in the initial concentration from
100 mg·L
–1
to 250 mg·L
–1
, the rate constant decreased
from 3.890 ×10
–3
to 3.125×10
–4
g·mg
–1
·min
–1
.
2) Sorption isotherms of MO on PANI can be described
well with the Langmuir equation. Thermodynamic
parameters of the sorption were determined.  G
 (298 K) was found to be 24.61 kJ·mol
–1
. This value
indicated that the process was spontaneous.  H
 and
 S
 were 27.00 kJ·mol
–1
and 58.61 J·mol
–1
·K
–1
.
Electrostatic interactions between positively charged
polymer chains and anionic dye were responsible for
the high adsorption capacity for MO.
3) The NaCl concentration and the temperature affected
the sorption behavior. A higher sorption capacity was
found for a higher temperature. With an increase in
the NaCl concentration from 0 to 0.5 mol·L
–1
, the Q
m
increased from 139.9 mg·g
–1
to 210.8 mg·g
–1
.H o w -
ever, the Q
m
decreased to 181.0 mg·g
–1
when the
concentration of NaCl increased to 1.0 mol·L
–1
.
Acknowledgments
We acknowledge the support of the National Science
Foundation for Young Scientists of the Hebei Province
(Grant Nos. E2018108011 and B2017108031) and
Municipal People’s Livelihood Science and Technology
Security Special Project of the Xingtai City in 2018
(2018ZZ18).
C. XING et al.: ADSORPTION BEHAVIOR OF POLYANILINE MICRO/NANOSTRUCTURES FOR METHYL ORANGE
Materiali in tehnologije / Materials and technology 54 (2020) 4, 529–534 533
Figure 6: Adsorption isotherms of MO on PANI micro/nanostructures
in different media
Figure 5: Van’t Hoff plot for the MO sorption onto PANI micro/nano-
structures

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