Polypyrrole nanofibers as a high-efficient adsorbent for the removal of methyl orange from aqueous solution

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JECE 684 1–11 Journal of Environmental Chemical Engineering xxx (2015) xxx–xxx

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Polypyrrole nanofibers as a high-efficient adsorbent for the removal of methyl orange from aqueous solution Qianqian Xina , Jianwei Fua,* , Zhonghui Chena , Shujun Liua , Ya Yana , Jianan Zhanga , Qun Xua,* a

School of Materials Science and Engineering, Zhengzhou University, 75 Daxue Road, Zhengzhou 450052, PR China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 4 April 2015 Accepted 13 June 2015

The polypyrrole (PPy) nanofibers were synthesized by an oxidative template assembly route and evaluated as an adsorbent for the removal of a typical anionic dye (methyl orange, MO) from aqueous solution. The as-synthesized PPy nanofibers were characterized by scanning electron microscopy (SEM), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and N2 sorption. The effects of contact time, initial MO concentration, temperature, and initial solution pH on the adsorption of MO onto PPy nanofibers have been investigated. Experimental results showed that the PPy nanofibers possessed excellent adsorption capacity for MO (169.55 mg/g) at 25
C. The pseudo-second-order model could fit to the experimental data better comparing with the pseudo-first-order kinetic adsorption model. And isotherm data fitted well to the Langmuir isotherm model. Thermodynamic study revealed that the adsorption process was endothermic and spontaneous in nature. In addition, the possible adsorption mechanism was proposed based on the experimental results. And the selective adsorption behaviors of the polypyrrole nanofibers towards different organic dyes were explored and the re-use of adsorbate was tested. ã2015 Published by Elsevier Ltd.

Keywords: Methyl orange Polypyrrole nanofibers Kinetic model Isotherm Adsorption

6

Introduction

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A considerable amount of colored wastewater is generated due to the use of dyes or pigments in industries such as textiles, leather, paper, printing, cosmetics and pharmaceuticals to color their products [1]. Discharge of sewage containing even a small amount of dyes is a serious matter of environment because of harmful effects to the aquatic organism as well as carcinogenic and mutagenic effects to human beings [2]. Therefore, it is urgent to remove the dyes from effluents before disposal into natural water bodies. Among amounts of dyes, methyl orange (MO) is one of typical anionic azo dyes, and has been widely used in various industrial fields. Because of its high chromaticity, complex organic components, stable chemical quality and difficult to microbial degradation, MO-containing wastewater becomes more difficult to treat. To date, several physico-chemical technologies, including coagulation/flocculation, membrane separation, ion exchange, chemical oxidation, electrochemical techniques, adsorption and photocatalysts, have been investigated for removal of MO from waste effluents. In comparison with other techniques, adsorption

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* Corresponding authors. Fax: +86 371 67767827. E-mail addresses: [email protected] (J. Fu), [email protected] (Q. Xu).

and photocatalysts are presently considered as promising candidate ways [3–8]. Especially, the adsorption technology has attracted great interests from researchers owing to its simplicity in operation, high treatment efficiency without discharging any harmful by-products to treated water and easy scaling from laboratory scale to field scale [8]. The adsorbent is one of the key factors during the course of adsorption, determining the effectiveness of adsorption processes, so it has become an important research direction to find a suitable adsorbent for the removal of dyes. For MO, up to now, various natural and synthetic adsorbents have been studied for its removal from effluents, such as activated alumina, banana peel, carbon nanotubes, ZnLa0.02Fe1.98O4/PPy, Q3 g-Fe2O3/chitosan composite films, g-Fe2O3/MWCNTs/chitosan, surfactant modified silkworm exuviae [9–15]. But the adsorption capacities of these adsorbents are not high. Therefore, it is imperative to develop a new high-efficient adsorbent for removal of MO from wastewater. Since Shirakawa et al. reported the synthesis of electrically Q4 conducting polyacetylene for the first time [16], conducting polymers have formed a new interdisciplinary field of research and exhibited a broad application prospects in chemical and biological sensors [17–19], electromagnetic shielding [20,21], supercapacitors [22], adsorbent for heavy metal and so on [23]. In the past decades, various conducting polymer-based

http://dx.doi.org/10.1016/j.jece.2015.06.012 2213-3437/ ã 2015 Published by Elsevier Ltd.

Please cite this article in press as: Q. Xin, et al., Polypyrrole nanofibers as a high-efficient adsorbent for the removal of methyl orange from aqueous solution, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.06.012

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nanomaterials or nanocomposites have been reported, such as graphene/polyaniline nanostructures, polyaniline/carbon nanotube sheet nanocomposites, polypyrrole-silver composite nanowire arrays, and so on [24–32]. Among the family of conducting polymers, PPy was regarded as one of the most valuable materials for its high electrical conductivity, facile synthesis in large scale, non-toxicity, long-term environment stability, excellent redox property and the existence of positively charged nitrogen atoms in the polymeric backbone [33–36]. And the application researches of PPy in many fields such as removal of Cr(VI) ions in aqueous solution, supercapacitor, ammonia sensor and lithium ion batteries have achieved great successes. However, nanostructured PPy with a high surface area have not been fully investigated as a potential adsorbent for organic dyes, especially MO. In this study, we test to explore the potential of the PPy nanofibers as dye adsorbent. In this study, the PPy nanofibers was synthesized based on an oxidative template assembly route [37], and it verified the PPy nanofibers could be evaluated as an effective adsorbent to remove MO from aqueous solution. The adsorption capacity of the PPy nanofibers for MO at 25
C could reach up to 169.55 mg/g, which was larger than that of the reported adsorbents [9–15]. Also, the PPy sorbent has two typical characteristics: easy recovery/removal from treated water completely and non-toxicity [38], so it will not cause secondary pollution. Besides, we further investigated the effects of initial solution pH, temperature, initial adsorbate solution concentration and contact time on the adsorption process. In order to further understand the characteristics of the adsorption process, the kinetics of MO adsorption on the PPy nanofibers were investigated by using pseudo-first-order, pseudo-second-order kinetic models and intra-particle diffusion model. Thermodynamics studies have also been performed and related parameters (DG
, DH
and DS
) were calculated. In addition, the electrostatic interaction and p–p stacking interactions mechanism were suggested to explain the high-efficient adsorption of MO by PPy nanofibers.

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Experimental

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Materials

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Hexadecyl trimethyl ammonium bromide (CTAB), ammonium persulphate (APS) and pyrrole (Py) was purchased from Aladdin Industrial Corporation. Methyl orange (MO) was supplied from Sinopharm Chemical Reagent Co., Ltd. All chemical reagents used in the experiment were of analytical reagent grade. All solutions were prepared with deionized water.

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Synthesis of the PPy nanofibers

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PPy nanofibers were synthesized according to a modified oxidative template assembly route [37]. Typically, 0.218 g CTAB was dissolved in HCl solution (60 ml, 1 mol/l) in an ice bath. Then 0.4119 g APS was added, and a white reactive template was formed immediately. After being magnetically stirred for 0.5 h and cooling to 0–5
C, pyrrole monomer (0.374 ml) was added into the asformed reactive template solution. After 24 h, a black precipitate was obtained. The resulting black precipitate was centrifuged, and washed several times with deionized water. Finally, the products were dried overnight at 80
C in an oven to yield PPy nanofibers.

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Adsorption experiments

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All adsorption experiments were carried out on a TQZ312 platform constant shaking incubator at a shaking speed of 150 rpm. Typically, a 20 ml solution of known dye concentration and 0.02 g of PPy nanofibers were added into 50 ml glass flasks and then

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shook under predefined temperature in the dark. After adsorption test at the completion of preset time intervals, the sorbent was separated through centrifugation at 4000 rpm for 10 min., and the concentration of MO left in supernatant was measured by a UV–vis spectrophotometer. The amount of MO adsorbed per unit mass of PPy nanofibers (q, mg g1) and the dye removal efficiency (R) were calculated by the following equations, respectively [39]: q¼V

C0  Ct m

C0  Ct R ¼ 100 m

108 109 110 111 112 113 114

(1)

(2)

where C0 and Ct (mg/l) are the initial and final (after adsorption) concentration of MO solution respectively, m (g) is the weight of PPy nanofibers, and V (L) is the initial volume of MO solution.

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Characterization

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The morphology and size of PPy nanofibers were investigated by scanning electron microscopy (SEM) with a JEOL JSM-7401F fieldemission microscope at an acceleration voltage of 5.0 kV and transmission electron microscopy (TEM) with a JEOL JEM-100CX microscope operated at 200 kV. The specific surface area and pore size distribution was measured on an ASAP 2020 adsorption apparatus. The Fourier transform infrared (FTIR) spectroscopy was measured on a PerkinElmer Paragon 1000 Fourier transform spectrometer at room temperature. The X-ray diffraction (XRD) pattern was recorded on a powder sample using a Bruker D8 Advance instrument with CuKa radiation. The adsorption experiments were conducted on a TQZ-312 platform constant shaking incubator which can regulate the temperature and oscillation rate accurately. A UV– vis spectrophotometer (Electrical and Instrument Analysis Instrument Co., Ltd., Shanghai, 752 N) was employed to determine the dye concentration in aqueous solutions.

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Results and discussion

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Characterization of PPy nanofibers

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Fig. 1a–d exhibits the SEM and TEM images of as-synthesized PPy nanofibers. As shown in Fig. 1a and b, the as-synthesized PPy was flexible and owned homogeneous fibrous morphology with an average diameter of 60–80 nm. To deeply investigate the nanofiber microstructure, the sample was characterized by transmission electron microscopy as shown in Fig. 1c and d. Results revealed that the solid PPy nanofibers had rough surface, which could increase their specific surface area and active adsorption sites. Thus, the rough surface was favorable for dye removal from aqueous solution. XRD characterization shows that the as-synthesized PPy nanofibers are amorphous with a broad peak at about 26
, as shown in Fig. 1e. Fig. 1f shows the nitrogen adsorption–desorption isotherms of as-synthesized PPy nanofibers. It could be seen that the isotherm plot of the PPy nanofibers was type III with a steep adsorption above the partial pressure of 0.8, suggesting the existence of mesopores and macropores on the samples, which could also be confirmed from the pore size distribution curve (inset of Fig. 1f. Obviously, the pore diameter for the PPy nanofibers was basically fixed at 20–100 nm. These pores might result from the interstitial spaces among the PPy matrix. Such pore structure is advantageous for applications that require rapid mass transport and effective adsorption/desorption of dye molecules. According to the standard Brunauer–Emmett–Teller (BET) method, the specific surface area of the PPy nanofibers was calculated to be 86 m2/g, which was far greater than that of HCl dedoped polyaniline

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Fig. 1. (a, b) SEM images of the PPy nanofibers. (c, d) TEM images of the PPy nanofibers. (e) XRD pattern of the as-synthesized PPy nanofibers. (f) Nitrogen adsorption– desorption isotherm and pore size distribution (inset) of the PPy nanofibers. 163

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nanofibers (average diameter = 30 nm, 54.6 m2/g) with relative smooth surface reported by Huang et al. [40]. The relatively high specific surface area could benefit the contact between the adsorbate molecules and the PPy nanofibers.

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Effects of contact time and initial MO solution concentration

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The effects of contact time and initial MO solution concentration on the adsorption capacities of the PPy nanofibers were conducted at room temperature by fixing the solution pH (approaching 7) and varying the initial MO concentrations from 30 to 200 mg/l. Fig. 2a presents several plots of adsorption capacity (qt) of MO onto PPy nanofibers versus contact time at different initial concentrations (30, 50, 100, 150 and 200 mg/l). Obviously, the removal rate of MO on the PPy nanofibers was very rapid at low initial concentrations (30, 50, and 100 mg/l) and it had reached equilibrium after 30 min. To the high initial concentration of 150 and 200 mg/l, the adsorption MO on the adsorbent was found to be rapid during the initial stage (30 min) and then became slow with the increase in contact time (30 to 120 min), and nearly reached a plateau after 120 min. of the experiment. The phenomenon could be ascribed to the fact that there were numbers

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of vacant surface sites available for adsorption during the initial stage, and the adsorption was initially rapid. As the adsorption process proceeded, the remaining vacant surface sites decreased obviously and many of them were difficult to occupy because of the repulsive forces between the solute molecules on the solid and bulk phases [41]. At this point, the adsorption process reached equilibrium. Fig. 2b shows the effect of the initial MO solution concentration on the equilibrium adsorption capacity. It could be easily observed that the equilibrium adsorption capacities of MO at different concentrations presented an increasing trend (from 29.01 to 169.55 mg/g), since initial MO concentration gradient could provide a driving force to overcome the mass transfer resistance of the dye. But the removal efficiency of MO by PPy nanofibers exhibited a trend of increasing at low concentrations and then decreasing at high concentrations. It could be accounted for that the certain amount of PPy nanofibers had limited adsorption capacity. In consideration of both adsorption capacities and removal efficiency at different concentrations, 150 mg/l is chosen as the optimum concentration of MO in the following experiments. At this concentration, the removal efficiency and the adsorption capacity of MO on the PPy nanofibers were all high. Fig. 2c shows the photographs of MO solutions before (right) and

Please cite this article in press as: Q. Xin, et al., Polypyrrole nanofibers as a high-efficient adsorbent for the removal of methyl orange from aqueous solution, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.06.012

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Fig. 2. (a) Effect of contact time on adsorption capacity of MO on PPy nanofibers under the condition of mass of adsorbent (20 mg), MO concentration (30, 50, 100, 150, 200 mg/l, 20 ml) and temperature (298 K). (b) Effect of initial MO solution concentration on adsorption capacity of MO on PPy nanofibers under the condition of mass of adsorbent (20 mg), MO solution (20 ml) and temperature (298 K). (c) The photographs of MO solutions before (left) and after (right) adsorption by PPy nanofibers. 205 206 207 208 209 210 211 212

after (left) adsorption by PPy nanofibers. Obviously, MO solution became almost colorless after adsorption, indicating that the PPy nanofibers were a high-effective adsorbent for the removal of MO from the aqueous solution. Effect of temperature To evaluate the effect of temperature on the adsorption capacities of the PPy nanofibers for MO, batch adsorption studies were carried out in the temperature range of 10–40
C. As shown in

Fig. 3a, the adsorption capacities of the PPy nanofibers for MO increased slightly with the increase of system temperature. The reason could be ascribed to the following reasons. The thermal motion of MO molecules became more frequent at higher temperature and more molecules could interact with the active sites on PPy nanofibers, which was favorable for more MO molecules to be adsorbed by PPy. In addition, the rate of diffusion of dye molecules increased along with the increasing of temperature, owing to the decrease in the viscosity of the solution [41]. It should be noted that the adsorption capacities of PPy nanofibers

Fig. 3. (a) Effects of temperature on the adsorption of MO onto the PPy nanofibers under the condition of mass of adsorbent (20 mg) and MO solution (150 mg/l, 20 ml). (b) Effects of initial pH on the adsorption of MO onto the PPy nanofibers under the condition of mass of adsorbent (20 mg), MO solution (150 mg/l, 20 ml) and temperature (298 K). (c) The chemical structure of MO under acid and base condition.

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for MO remained at a relative high value (above 145 mg/g) in the full temperature range, indicating that the PPy nanofibers were a promising adsorbent for the removal of MO from aqueous solution.

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Effect of solution initial pH

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The solution pH can affect the surface charge of the adsorbent, the degree of ionization of different adsorbates, the dissociation of functional groups on the active sites of the adsorbent as well as the structure of the dye. So the effect of pH on the removal of MO by PPy nanofibers was studied systemically. Fig. 3b shows the effect of initial pH on the removal of MO onto PPy. Obviously, the adsorption capacities of PPy nanofibers for MO were relatively high, which maintained above 130 mg/g over the pH range of 2–10. Moreover, with the increase of pH from 2 to 7, the adsorption capacities increased from 130.11 to 147.24 mg/g, while with the pH further increase (pH < 10), the adsorption capacities would decrease slightly and the adsorption capacities had a sharp decrease when pH > 10. Thus, it was concluded that the neutral solutions benefited the removal of MO by PPy nanofibers. The reasons could be as follows. Generally, the surface of the PPy nanofibers is positively charged due to the protonation of the nitrogen atoms of the polymer matrix in the presence of adequate H+ ions [42] in a certain pH range (pH < 10). MO is generally considered to be an anionic dye with SO3 group and has two different chemical structures, as can be shown in Fig. 3c. The charge state of MO is usually dependent on the solution pH. When the solution pH was lower than 7.0, the MO ion could gradually take on a positive charge on one of the nitrogens. Thus, the electrostatic attraction between PPy and MO would be weak, and the adsorption capacities could decrease with the decrease of the solution pH. When the pH was higher than 7.0 (pH < 10), the deprotonation of the nitrogen atoms of the polymer PPy matrix and the competitive interaction between excess OHand anions of the MO dye molecule on the PPy surfaces could occur in the solution, leading to decrease in adsorption capacity at higher pH values. It should be noted that the pH value for a zero charge of the PPy nanofibers is about 10 [42]. When the pH was higher than 10, the charge state of PPy nanofibers was electronegative. And the adsorption capacities of PPy nanofibers for MO presented a sharp decrease but not zero. This suggested that the electrostatic interaction played the predominant role in adsorption but is not the only adsorption mechanism between anionic dyes and PPy nanofibers. And other mechanisms like p–p stacking interactions between bulk p systems on PPy surfaces and MO molecules with C¼C or benzene rings could also make a contribution to the adsorption. The adsorption capacity of MO onto PPy nanofibers is higher than that of many other reported adsorbents. A comparison of the adsorption capacities of MO onto various adsorbents are summarized in Table 1, which indicates that the PPy nanofiber adsorbents hold great potential for MO removal from aqueous solutions.

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Table 1 Comparison of the adsorption capacities of MO onto various adsorbents. Adsorbents

Adsorption capacity (mg/ g)

References

Activated alumina Banana peel Carbon nanotubes ZnLa0.02Fe1.98O4/PPy g-Fe2O3/chitosan composite films g-Fe2O3/MWCNTs/chitosan Surfactant modified silkworm exuviae PPy nanofibers

9.8 21 35.4–64.7 74.34 29.41 60.5–66.1 77.68

[9] [10] [11] [12] [13] [14] [15]

169.55

This work

5

Kinetic analysis

272

Adsorption is a physico-chemical process that involves mass transfer of a solute from liquid phase to the adsorbent surface. In order to understand the characteristics of the adsorption process, the kinetics of MO adsorption on the PPy nanofibers were investigated by using pseudo-first-order and pseudo-second-order kinetic models. The two kinetic models can be expressed in linear form as Eqs. (3) and (4) [43,44].

273

lnðqe  qt Þ ¼ lnðqe Þ  k1 t

(3)

t 1 t ¼ þ qt k2 q2e qe

(4)

where qe (mg/g) and qt (mg/g) were the amounts of MO adsorbed at equilibrium and any time t (min), respectively; k1 (min1) and k2 (g mg1/min) were the rate constant of pseudo-first-order kinetic model and pseudo-second-order kinetic model, respectively. The kinetic parameters including correlation coefficients (R2), k1, k2 and calculated qe,cal values were determined by linear regression and were given in Table 2. It could be easily observed that the R2 value (0.9999) of the pseudo-second-order kinetic model was much higher than that of pseudo-first-order kinetic model. Moreover, the values of qe,cal of the pseudo-second-order kinetic model were close to the experimental ones (qe,exp). Hence, the pseudo-secondorder kinetic model is more appropriate to describe the adsorption behavior of MO onto PPy nanofibers. Fig. 4a and b was the linear plots of the above discussed models. Obviously, the pseudo-second-order kinetic model was more valid to describe the adsorption behavior of MO onto PPy nanofibers, which was consistent with Table 2. The pseudo-second-order kinetic model includes all the steps of adsorption including external film diffusion, intraparticle diffusion and surface adsorption. The experimentally observed adsorption rate is the overall rate of the whole process. Therefore, it is necessary to predict the rate-limiting step of the adsorption process. Generally, the intraparticle diffusion is more likely the rate-limiting step for a batch reactor [45]. To test the possibility of intraparticle diffusion as rate-limiting step for the adsorption behavior of MO on the PPy nanofibers, the Weber–Morris equation was applied and expressed by the Eq. (5) below [46]. qt ¼ ki t1=2 þ C

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(5) 1

where ki (mg g /min ) was the intraparticle diffusion rate constant and C (mg/g) was the intercept related to the thickness of the boundary layer. Fig. 4c shows the plots of qt against t1/2, and the corresponding kinetic parameters were listed in Table 2. It was clear that the regression of qt versus t1/2 was inclined to be linear and the plots did not pass through the origin, suggesting that the intraparticle diffusion is not the only rate-controlling step and the film diffusion maybe also significant in the rate-controlling step because of the large intercepts of linear portion of the plots [44]. So the overall adsorption process maybe jointly controlled by film diffusion and intraparticle diffusion, and intraparticle diffusion played a predominant role in controlling the adsorption process.

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Adsorption isotherms

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The equilibrium adsorption isotherm is essential in describing the interaction behavior between adsorbate and adsorbent. Herein, two important isotherms, i.e., Langmuir isotherm and Freundlich isotherm were selected in this study. The Langmuir isotherm model is based on the assumption of a monolayer adsorption, where all the sorption sites are identical and energetically

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Table 2 Kinetic models for the adsorption of MO onto PPy nanofibers at 298 K. C0 (mg/l)

qe

exp

(mg/g)

k1 (min 30 100 150

329 330 331 332

29.50 99.22 147.24

1 lnðqe Þ ¼ lnðK F Þ þ lnðC e Þ n

336 337 338 339 340 341 342 343 344 345 346

0.024 0.020 0.031

1

)

qe

cal

0.675 3.535 1.120

(mg/g) 0.9297 0.8117 0.9853

R2

Pseudo-second-order k2 (g/(mg min))

qe

0.048 0.010 0.001

29.533 99.305 147.929

equivalent. While the Freundlich isotherm is based on multilayer adsorption on heterogeneous surface and there are interactions between the adsorbed molecules. The linear forms of the two isotherms were given by the following Eqs. (6) and (7). Ce 1 1 ¼ K L þ Ce q0 q0 qt

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R2

Pseudo-first-order

RL ¼

where Ce (mg/l) was the equilibrium concentration, qe (mg/g) was the amounts of MO adsorbed at equilibrium time, q0 (mg/g) was the Langmuir constants related to adsorption capacity and KL (l/ mg) was the adsorption rate. KF and n were Freundlich constants. Fig. 5a and b shows the Langmuir isotherm and Freundlich isotherm for adsorption of MO onto PPy nanofibers, respectively. KL, q0, KF and n could be determined from these two isotherms and their values are summarized in Table 3. It could be seen that the correlation coefficient (R2) for Langmuir isotherm model was higher than 0.99, and the adsorption capacity (q0 = 173.01 mg/g) calculated from the model was close to the experimental data (169.55 mg/g). This indicated that the adsorption feature of PPy nanofibers could be well described by the Langmuir isotherm.

(mg/g) 0.9999 0.9999 0.9999

ki (mg/(g min))

C (mg/g)

0.031 0.271 1.799

28.908 94.462 115.176

R2

0.9356 0.5960 0.7760

In addition, the important features of Langmuir adsorption isotherm could also be expressed by a dimensionless constant called separation factor or equilibrium parameter (RL), which could evaluate the feasibility of adsorption on adsorbent. It can be calculated by Eq. (8) [47]:

(6)

(7)

cal

Intraparticle diffusion

1 1 þ K L C0

347 348 349 350 351

(8)

where KL was the Langmuir equilibrium constant and C0 (mg/l) was initial dye concentration. The value of RL indicated the type of the isotherm to be either irreversible (RL = 0), favorable (0 < RL < 1), unfavorable (RL > 1) or linear (RL = 1). The calculated values of RL were in the range of 0–1 in this study, indicating the adsorption of MO onto PPy nanofibers were favorable, and the PPy nanofibers are an excellent adsorbent material for MO removal from aqueous solution.

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Thermodynamic analyses

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Thermodynamics parameters including Gibbs free energy change (DG
), enthalpy change (DH
) and entropy change (DS
) can evaluate the effect of temperature on the adsorption of MO onto PPy nanofibers and provide in-depth information regarding the inherent energetic changes associated with the adsorption

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Fig. 4. (a) Pseudo-first-order model, (b) pseudo-second-order model, and (c) intraparticle diffusion model for the adsorption of MO onto the PPy nanofibers.

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Fig. 5. (a) Langmuir and (b) Freundlich isotherms for the adsorption of MO onto the PPy nanofibers. (c) Regression of Van’t Hoff for thermodynamic parameters.

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process. They could be determined using the following equations:

DG0 ¼ RTlnðK L Þ

lnðK L Þ ¼ 369 368 370 371 372 373 374 375 376 377 378 379 380 381 382 383

DS R






DH

(9)




(10)

RT

where KL (l/mol) is the Langmuir equilibrium constant at different temperature; R (8.314 J/(mol K)) is the universal gas constant, and T (K) is the system temperature. DG
could be easily determined from Eq. (9). DS
and DH
could be calculated according to the intercept and slope of the Van’t Hoff plots of ln (KL) versus 1/T, as shown in Fig. 5c. The thermodynamic parameters at various temperatures were presented in Table 4. The negative values of DG
implied that the adsorption of MO onto PPy nanofibers was spontaneous and thermodynamically favorable. Moreover, the decrease of DG
with the increase of temperature suggested that the adsorption was more spontaneous at high temperature. The positive value of DH
(3.49 kJ/mol) indicated that the adsorption process was endothermic in nature. Moreover, the positive value of DS
(0.015 kJ/(mol K)) meant that the randomness increased at the solid–liquid interface during the adsorption of MO in the aqueous solution on the PPy nanofibers.

Table 3 Isotherm parameters for the adsorption of MO onto PPy nanofibers at 298 K. Langmuir

Freundich

KL (l/mg)

q0 (mg/g)

R2

KF (l/g)

n

R2

0.5553

173.01

0.9995

81.9484

3.4169

0.8459

Adsorption mechanism

384

In this study, the FTIR spectra of PPy nanofibers and PPy nanofibers/MO (after adsorption) were employed to gain insight into the adsorption mechanism, as shown in Fig. 6a. FTIR spectrum of pure PPy nanofibers showed the following characteristic adsorption peaks. The adsorption band at 3438 cm1 was ascribed to the stretching vibration of N H group for PPy [48]; the peaks at 1644 and 1558 cm1 were assigned to the antisymmetrical and symmetrical vibration bands of pyrrole rings [49]; the peaks located at 1172 cm1 were assigned to in-plane deformation vibration of C H; the band at 1380 cm1 was due to the C N stretching vibration; two weak peaks at 2914 and 2846 cm1 were attributed to the stretching vibration mode of CH [50]; the peak at 906 cm1 was attributed to the out-plane stretching vibration of C H [49]. After adsorption of MO onto PPy nanofibers, the FTIR spectrum of PPy nanofibers/MO exhibited obvious changes. The bands at 3438 cm1 presented on PPy nanofibers shifted to lower wavenumbers (3431 cm1), indicating N H group of PPy played an important role in the adsorption process. Furthermore, the peak at 1654 and 1556 cm1 those were assigned to pyrrole rings shifted to 1637 and 1546 cm1 after MO adsorption. These red shifts mentioned above can be attributed to the following reasons. (i) The

385

Table 4 Thermodynamic parameters for the adsorption of MO (150 mg/l) onto PPy nanofibers. T (K)

DG
(kJ/mol)

DH
(kJ/mol)

DS
(kJ/(mol K))

283 293 313

0.651 0.887 1.100

3.49

0.015

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386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405

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Fig. 6. (a) FTIR spectra of ppy nanofibers and PPy/MO. (b) UV–vis spectra of MO, PPy nanofibers before and after dye adsorption. (c) Adsorption schematic illustrations of MO on PPy nanofibers through (i) electrostatic interactions and (ii) p–p interactions. 406 407 408 409

positively charged surfaces of the adsorbent resulting from the existence of numerous N+H on PPy nanofibers can provide adsorption sites for electrostatic interaction with MO, a typical anionic dye. (ii) As MO is an ideally planar molecule with aromatic

backbone and PPy nanofibers also contain a number of aromatic rings, the p–p stacking interactions could occur between MO molecules and PPy nanofibers, resulting in the red shift of the peaks (1654 and 1556 cm1, pyrrole rings). Besides, the properties

Fig. 7. (a) The photographs of MO solutions reaching adsorption equilibrium before and after NaCl addition. (b) The UV–vis absorption spectra of MO solutions reaching adsorption equilibrium before and after NaCl addition. (c) Adsorption of MO using PPy nanofibers via replacement of Cl ions.

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410 411 412 413

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of nanometer scale, regular fibrous morphology and relatively high specific surface area can make PPy nanofibers contact and absorb MO molecules efficiently. These aspects mentioned above synergistically contribute to the high adsorption capacity and removal efficiency of PPy nanofibers for MO dye. Fig. 6b presents the UV–vis absorbance spectra of PPy, MO and PPy/MO (after adsorption). It can be observed that there is a distinct characteristic peak (wavelength = 463 nm) of MO on the PPy nanofibers after dye adsorption, suggesting that MO molecules have successfully been adsorbed by PPy nanofibers. The possible adsorption mechanism and the electrostatic interactions between PPy nanofibers and MO were schematically illustrated in Fig. 6c. To further confirm the existence of the electrostatic interactions between PPy nanofibers and MO, we conducted an additional experiment. It has been known that salt can interfere in electrostatic interaction by electrostatic screening effect [51–53]. Theoretically, when the electrostatic forces between the adsorbent surface and adsorbate ions are attractive, an increase in ionic strength will decrease the adsorption capacity. So we added a certain amount of NaCl, a salt, to the dye aqueous solution, in which the PPy nanofibers had reached adsorption equilibrium to MO. We found that the clear solution gradually recover the color of MO in several minutes, as shown in Fig. 7a. Then we used UV–vis spectra to investigate the possible changes of the dye solution concentration after the addition of NaCl. As shown in Fig. 7b, it was clear that the intensity of peak at maximum absorption wavelength after adding NaCl was far higher than that before adding NaCl into the equilibrium solution of MO. This result suggested that the addition of NaCl could produce an electrostatic screening effect, making the dye molecules be released from the adsorbent to the solution. The results showed electrostatic interaction dominated the adsorption

Fig. 8. The photographs of MB, MO, and MB/MO mixture in aqueous solution before (a) and after (b) desorption by PPy nanofibers.

9

process, which was consistent with Fig. 3b. The adsorption process is shown in Fig. 7c [54].

445

Selective adsorption

447

From what has been discussed above, we knew that the surface charge of PPy nanofibers was positive, which was favorable for the removal of anionic dyes from aqueous solution, but unfavorable for the removal of cationic dyes. So we tried to explore the potential of PPy nanofibers as the selective adsorbent. Herein, the mixed aqueous solution of MO (anionic dye) and MB (cationic dye) was employed as the test model. As contrast experiments, the adsorption behavior of pure MB and MO onto PPy nanofibers was also performed, respectively. Fig. 8 shows the color changes of MB, MO, and MB/MO solutions before and after adsorption by PPy nanofibers, respectively. Obviously, the color change of MB solution before and after adsorption could not be observed clearly, indicating that MB dye could not be adsorbed effectively by PPy nanofibers. However, MO dye could be adsorbed thoroughly by PPy nanofibers, and the reasons have been discussed above. To the mixed aqueous solution of MO and MB, it was easily observed that after the absorption, the color of the mixed dye solution changed from black-green to blue, which was as same as the color of the aqueous solution of pure MB, indicating that MO was adsorbed preferentially by PPy nanofibers from aqueous solution, while MB was still left in the solution. The result suggested that the PPy nanofibers have great potential as selective adsorbent.

448

Desorption

470

Desorption is of importance to re-use the adsorbent or to recycle valuable adsorbates. In accordance with the effect of

471

Fig. 9. (a) Desorption of MO hardly occurs in distilled water (left). After the addition of NaOH, desorption occurs (right). (b) Occupation degree of adsorption sites with MO during three cycles of adsorption and desorption.

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449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469

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surface charge on the adsorption, it was considered that the alkali solution could be employed to realize desorption of the adsorbed MO. Sodium hydroxide with different concentrations (0.1–0.6 M) was applied to investigate the desorption performance. It was found that the desorption efficiencies rose up to 79.42% with the increase of the alkali concentration up to 0.2 M owe to the enhanced electrostatic repulsion. And the desorption efficiencies would not continue to increase sharply with the further increase of the alkali concentration. Thus, NaOH solution (0.2 mol/l) was available for desorption of MO. This was investigated with fresh, completely MO filled adsorbent in distilled water. The solution became only slightly orange, suggesting a minor desorption of MO. However, after the addition of NaOH (a certain amount) suddenly became orange indicating desorption of MO (as shown in Fig. 9a). Besides, the adsorption and desorption was investigated several times. The activated adsorbent was placed in neutral water containing MO and subsequently immersed in NaOH solution (0.2 M) to adsorb and desorb MO, respectively. The adsorption and desorption was quantified with UV–vis spectroscopy, which revealed that up to 79.42% of the MO was released in every cycle (Fig. 9b), indicating that this material possessed good adsorption capacity and stability and the adsorbent could be used many times.

495

Conclusions

474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493

496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528

529 530 531

In this study, we have successfully synthesized PPy nanofibers with high specific surface area, which have been proved to be an effective adsorbent for the removal of MO from aqueous solution. As suggested by our experimental data, the PPy nanofibers showed excellent adsorption capacity of 169.55 mg/g for MO, which can be attributed to the high specific surface area of the PPy nanofibers, and the electrostatic interactions and p–p stacking interactions between PPy nanofibers and MO molecules. The adsorption followed the pseudo-second-order kinetic model with correlation coefficients R2 (0.9999) and intraparticle diffusion model indicated the intraparticle diffusion was not the rate-limiting step. Thermodynamic studies indicated the adsorption process of MO by PPy nanofibers was endothermic and spontaneous in nature. Besides, the PPy nanofibers also exhibited a selective adsorption behavior towards anionic MO dye and cationic MB dye, which are very competitive with other promising adsorbents. The adsorbate can be released by base treatment, which makes it possible to the re-use of adsorbate or the recovery of valuable adsorbates. Therefore, it is expected that the PPy nanofibers could be used as an effective and selective adsorbent for adsorption, separation and purification purposes. Acknowledgements We are grateful to the National Natural Science Foundation of China (No. 51003098, 21101141), the Foundation of State Key Laboratory of Chemical Engineering (No. SKL-ChE-13A04), the National Science Foundation for Post-doctoral Scientists of China (No. 2014M550385), the Foundation of Henan Educational Committee for Key Program of Science and Technology (No. 12A430014, 14B430036), the Foundation of Zhengzhou General Science and Technology Project (No. 141PPTGG385), and the Q5 financial support from the Program for New Century Excellent Talents in Universities (NCET). References [1] A. Afkhami, R. Moosavi, Adsorptive removal of Congo red, a carcinogenic textile dye, from aqueous solutions by maghemite nanoparticles, J. Hazard. Mater. 174 (1–3) (2010) 398–403, doi:http://dx.doi.org/10.1016/j.jhazmat.2009.09.066. 19819070.

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