Journal of Colloid and Interface Science 425 (2014) 75–82
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Composite nanofibers prepared from metallic iron nanoparticles and polyaniline: High performance for water treatment applications Madhumita Bhaumik a,⇑, Hyoung J. Choi b, Rob I. McCrindle a,⇑, Arjun Maity c,d,⇑ a
Department of Chemistry, Tshwane University of Technology, Private Bag X680, Pretoria, South Africa Department of Polymer Science and Engineering, Inha University, Incheon 402-751, South Korea c Smart Polymers Group, Polymers and Composites, Council for Scientific and Industrial Research (CSIR), Pretoria, South Africa d Department of Civil and Chemical Engineering, University of South Africa (UNISA), South Africa b
a r t i c l e
i n f o
Article history: Received 4 December 2013 Accepted 12 March 2014 Available online 20 March 2014 Keywords: Polyaniline Composites Nanofibers Chromium(VI) Arsenic(V) Congo red Adsorption Isotherm Kinetics
a b s t r a c t Presented here is a simple preparation of metallic iron nanoparticles, supported on polyaniline nanofibers at room temperature. The preparation is based on polymerization of interconnected nanofibers by rapid mixing of the aniline monomer with Fe(III) chloride as the oxidant, followed by reductive deposition of Fe0 nanoparticles, using the polymerization by-products as the Fe precursor. The morphology and other physico-chemical properties of the resulting composite were characterized by scanning and transmission electron microscopy, Brunauer–Emmett–Teller method, X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and vibrating-sample magnetometry. The composite fibers were 80–150 nm in diameter and exhibited the expected ferromagnetic behavior. The composite rapidly and efficiently removed As(V), Cr(VI), and also Congo red dye, from aqueous solutions suggesting their usefulness for removal of toxic materials from wastewater. The composite fibers have high capacity for toxin removal: 42.37 mg/g of As(V), 434.78 mg/g of Cr(VI), and 243.9 mg/g of Congo red. The fibers are easily recovered from fluids by exploiting their ferromagnetic properties. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction In recent years, conducting polymers nanostructures have attracted special attention in the fields of nanoscience and nanotechnology due to their unique combination of the electronic properties of conductive polymers and the large surface area of nanomaterials [1]. Specifically, the synthesis and applications of one dimensional (1D) nanostructured conducting polymeric materials with controllable morphology that includes nanotubes, nanowires and nanofibers, have been explored over the past few years [2–5]. Among 1D conducting polymer nanostructures, nanofibers of polyaniline (PANI) have stimulated research interest as they can be easily synthesized in bulk quantities and have a large surface area to volume ratio, with good redox properties, high conductivity and excellent environmental stability [6]. This has resulted in
⇑ Corresponding authors. Address: Smart Polymers Group, Polymers and Composites, Council for Scientific and Industrial Research (CSIR), Pretoria, South Africa. Fax: +27 128413553 (A. Maity). Fax: +27 123826286 (R.I. McCrindle and M. Bhaumik). E-mail addresses: [email protected] (M. Bhaumik), [email protected] (R.I. McCrindle), [email protected], [email protected] (A. Maity). http://dx.doi.org/10.1016/j.jcis.2014.03.031 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
potential applications in sensors, actuators, super capacitors, field effect transistors and in separation or purification systems [7–11]. Moreover, to improve the functionality of 1D conducting polymer nanostructures metal nanoparticles (with unique electronic, catalytic, magnetic and optical properties) were incorporated [12], as a secondary component, to form 1D nanocomposites. This incorporation can be considered as a constructive technique because the combination of metal particles with a conducting polymer offers an attractive route to reinforce the polymer as well as introduce electronic properties, based on morphological modification or electronic interaction between the two components [13]. Consequently, efforts have been devoted to attach various metal nanoparticles onto 1D conducting polymer supporting matrices and manufacturing multifunctional 1D composite nanostructures with enhanced physico-chemical properties. In particular, a series of noble metal nanoparticles including Ag, Au, Pt and Pd were produced inside or on the surface of PANI nanofibers via in situ redox reactions and/or by reduction processes in the presence of other reducing agents such as ethylene glycol or HCOOH, respectively [14–18]. Although noble metal nanoparticle supported on 1D PANI nanofibers have been studied extensively, magnetic nanoparticles such as zero valent iron (Fe0), with increased reactivity, supported on PANI nanofibers have not been widely investigated.
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In this work, a simple and facile chemical strategy to synthesize Fe nanoparticles supported on PANI nanofibers (PANI/Fe0) at room temperature is demonstrated. A simple synthesis protocol based on rapid-mixing polymerization method was employed first for the production of PANI nanofibers using ferric chloride (FeCl3) as oxidant [19]. After completion of the polymerization reaction, the PANI nanofibers and reaction by products (FeCl2 or any unreacted FeCl3) could be found in the polymerization solution. After addition of sodium borohydride (NaBH4) solution, as an external reducing agent, Fe0 nanoparticles were grown on the PANI nanofibers matrix. The present synthesis strategy represents a new type of 1D conducting polymer nanocomposite with several advantages. Firstly, the synthesis method is very simple and executed at room temperature without employing an external means such as template or surfactant. Secondly, no external metal precursor is used as the source of metal nanoparticles; rather a polymerization by-product is used as the precursor of Fe0 nanoparticles. Finally the physicochemical properties of the PANI/Fe0 composite nanofibers can easily be tuned by varying the Fe0 nanoparticle-loading in the PANI nanofibers. The as-synthesized PANI/Fe0 composite nanofibers are expected to display enhanced physico-chemical properties obtained synergistically from both components of the composite nanofibers. They could be used as multifunctional materials in various fields of nanotechnology and for environmental remediation. It is already well established that because of the presence of large amounts of amine and imine functional groups, PANI has interactions with some heavy metal ions and toxic dye molecules that have strong affinity to nitrogen. Consequently, the application of PANI for the removal of heavy metals and dyes has been studied intensively [11,20,21]. In addition, Fe0 nanoparticles with excellent electron donating capacity have been effectively used over the past few years to remediate surface and ground water contaminated with inorganic ions and organic compounds [22]. The large specific surface area 0
and greater density of the highly reactive surface sites of Fe0 nanoparticles have been proposed to result in a rapid and cost-effective water treatment system compared to conventional iron-based technologies [23]. Therefore, due to enhanced surface area, highly reactive surface sites and greater electron donor property, derived synergistically from both components of the PANI/Fe0 composite nanofibers, these are expected to perform efficiently in the removal of contaminants from water. Herein, the contaminant removal behavior of the obtained PANI/Fe0 nanofibers is studied by considering two heavy metal pollutants, arsenic(V) [As(V)], one of the most toxic ground water pollutants, chromium(VI) [Cr(VI], the most common heavy metal pollutant in industrial wastewater and one organic pollutant, Congo red (CR), a carcinogenic anionic azo dye released into waste effluents of different dying industries. Due to the toxic and carcinogenic effects of As(V), Cr(VI) and CR on human health and on the environment [24–26], their removal from water is important. Evaluation of the performance of the PANI/Fe0 composite nanofibers toward contaminant removal from water exhibited enhanced removal performance compared to the PANI nanofibers counterpart. 2. Experimental section 2.1. Materials Aniline (ANI, 99% acquired from Sigma–Aldrich, USA) was purified using vacuum distillation. Distilled ANI was stored in a refrigerator prior to use for polymerization. Anhydrous iron (III) chloride (FeCl3), sodium borohydride (NaBH4), potassium dichromate (K2Cr2O7), sodium arsenate heptahydrate (Na2HAsO4, 7H2O) and Congo red (CR) were purchased from Sigma–Aldrich, USA. Ultrapure water (type-2, resistivity-17.4 MX cm1), collected from an EASYpureÒ II, UV-ultrapure water system, was used for polymerization media and preparation of all aqueous solutions. All other chemicals used were of reagent grade.
Fig. 1. FE-SEM images of (a) PANI nanofibers and (b) PANI/Fe0 composite nanofibers and HR-TEM images of (c and d) PANI/Fe0 composite nanofibers at two different magnifications.
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2.2. Synthesis of PANI nanofibers Interconnected PANI nanofibers were synthesized via rapidmixing chemical oxidative polymerization at room temperature. In a typical polymerization process, 6 g of FeCl3 as oxidant was dissolved in 80 mL of ultrapure water in a 250 mL conical flask. Aniline (ANI) monomer (0.8 mL) was added all at a time to the oxidant solution with sufficient (600 rpm) magnetic stirring to evenly distribute the oxidant and monomer molecules thus preventing secondary growth of PANI. Stirring was continued for 5 min. The reaction mixture was then left without stirring for 2 days. The precipitated polymer was filtered, washed with water and acetone, and finally dried at 60 °C.
experiments were conducted by adding 0.01 g of adsorbents for Cr(VI) and CR and 0.02 g for As(V) under same experimental conditions for 24 hrs. Experimental solutions with different concentrations of As(V), Cr(VI) and CR were adjusted to pH 7.0 for As(V), pH 2.0 for Cr(VI) and pH 7.5 for CR, respectively. After a specified time, the adsorbents were separated from the solution and the residual concentrations were determined using an inductively coupled plasma-optical emission spectrometer (ICP-OES, Spectro-Arcos) for As(V) and a UV–vis spectrophotometer (Perkin Elmer – Lamda 35) for Cr(VI) and CR. The equilibrium sorption capacity was determined using Eq. (1):
qe ¼
ðC 0 C e Þ V m
ð1Þ
0
2.3. Synthesis of PANI/Fe composite nanofibers 0
The PANI/Fe composite nanofibers were synthesized using prepared PANI nanofibers (without removing them from the polymerization medium) as support and polymerization by-products FeCl2 and any remaining FeCl3 as the source of the Fe0 nanoparticles. To support the Fe0 nanoparticles on the PANI nanofibers, the polymerization mixture (PANI nanofibers and FeCl2/FeCl3) was stirred mechanically under a nitrogen atmosphere. Freshly prepared 0.5 M sodium borohydride (NaBH4) solution (100 mL) was added drop wise to the mixture containing PANI nanofibers, which resulted in the attachment of Fe0 nanoparticles onto the PANI nanofibers matrix. The mixture was stirred another 20 min for completion of reduction reaction. The product (PANI/Fe0) was filtered, washed with water and ethanol and dried at 60 °C.
where qe is the equilibrium amount of adsorbed per unit mass of adsorbent (mg/g), C0 and Ce are the initial and equilibrium adsorbate concentration, respectively, in mg/L, V is the sample volume (L) and m is the adsorbent mass in g. 3. Results and discussion 3.1. Morphology and physico-chemical characterization of the PANI/ Fe0 composite nanofibers The morphology of the PANI nanofibers and PANI/Fe0 composite nanofibers was investigated using scanning electron microscopy 350
(a)
2.4. Characterization of nanofibers
300
The morphology and size of the PANI and PANI/Fe0 nanofibers were investigated using field emission scanning electron microscope (FE-SEM), LEO Zeiss SEM and a high resolution transmission electron microscope (HR-TEM), JEOL JEM-2100 instrument with a LAB6 filament all operated at 200 kV, respectively. X-ray diffraction patterns were measured on a PANalytical X’Pert PRO-diffractometer. The Brunauer–Emmett–Teller (BET) surface area analyses of the Fe0 nanoparticles (synthesized in our laboratory using the same method described in Section 2.3, but in the absence of PANI nanofibers), PANI nanofibers and PANI/Fe0 composite nanofibers were performed using a low temperature N2 adsorption–desorption technique with a Micromeritics ASAP 2020 gas adsorption apparatus (USA). The IR spectrum of the PANI/Fe0 nanofibers was recorded with an attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectrometer (Perkin–Elmer Spectrum 100 spectrometer), equipped with an ATR accessory and a germanium crystal. Elemental mapping of the nanocomposite was performed using X-ray photoelectron spectroscopy (XPS) on a Kratos Axis Ultra device, with an Al monochromatic X-ray source (1486.6 eV). The isoelectric point (IEC) of the PANI/Fe0 composite nanofibers was measured using a Zeta-Sizer, Malvern Ltd., UK. Magnetic properties of the PANI/Fe0 composite nanofibers were recorded using a vibrating sample magnetometer (VSM, Lakeshore-7307, USA) with a maximum magnetic field of 400 kA/ m in powder form at room temperature.
250
2.5. Adsorption experiments
Intensity (cps)
(110)
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(020) (200)
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0 10
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2θ (degree)
Transmittance (%)
60
(b)
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1595
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1215 1164
To perform adsorption experiments, stock solutions (1000 mg/L) of As(V), Cr(VI) and CR were prepared by dissolving appropriate amount of Na2HAsO4, 7H2O, K2Cr2O7 and CR in 1 L ultrapure water. The kinetic experiments were carried out by adding 0.02 g of adsorbents (PANI or PANI/Fe0) to 20 mL of As(V), Cr(VI) and CR samples in 100 mL glass bottles placed in a thermostatic shaker, agitated at 200 rpm, for a predetermined time interval. The equilibrium
20
813
1284 1493
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Wavenumber (cm-1) Fig. 2. (a) XRD pattern of the PANI/Fe0 composite nanofibers and (b) ATR-FTIR spectrum of the PANI/Fe0 composite nanofibers.
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(SEM) and transmission electron microscopy (TEM). The SEM image of the pure PANI nanofibers prepared via rapid-mixing reaction of aniline monomer and FeCl3 oxidant is shown in Fig. 1a. The SEM image indicates the formation of smooth surface PANI nanofibers, 50–80 nm in diameter, with a tendency to agglomerate into an interconnected nanofiber network. After loading with Fe0 nanoparticles, the diameters of the PANI nanofibers increased to about 80– 150 nm (Fig. 1b) and assembled in nanofiber-like morphology. Typical TEM images of the prepared PANI/Fe0 composite nanofibers at three different magnifications are presented in Fig. 1c and d. It can be seen (Fig. 1c) that the composite nanofibers have a rougher surface than pure PANI form. The magnified TEM image (inset in Fig. 1c) revealed that Fe0 nanoparticles are facilely deposited/ embedded onto the PANI nanofibers matrix. The clear lattice fringes with inter planer spacing of 0.21 nm, corresponded to body centered cubic Fe (1 1 0) planes, suggesting a highly crystalline structure of the Fe0 nanoparticles (Fig. 1d). Rapid reaction of Fe0 nanoparticles with surrounding media, including moisture and air oxygen, results in a reduction of its chemical reactivity due to the formation of iron oxide. Protection of the Fe0 nanoparticles from oxide formation could effectively be achieved by supporting them on the polymer matrix. During
synthesis of Fe0 nanoparticles the presence of PANI nanofibers with high surface areas, could create an in situ surround for the generated Fe0 nuclei, to protect them from oxidation. This fact was confirmed by the X-ray diffraction (XRD) pattern of PANI/Fe0 composite nanofibers. The strong sharp diffraction peak (Fig. 2a) at 2h, of 44.79° and a small peak at 65.15° are in good agreement with the (1 1 0) and (2 0 0) planes of Fe0 nanoparticles [27]. This implies that Fe0 nanoparticles are successfully supported on the PANI nanofibers matrix. The average grain size of the supported Fe0 particles is 14.27 nm, as calculated from the broadening of the (1 1 0) diffraction peak using Scherrer’s formula. The Brunauer–Emmett–Teller specific surface area (SBET) of the PANI nanofibers, Fe0 nanoparticles and PANI/Fe0 composite nanofibers was determined from N2 adsorption–desorption curves and is found to be 38.99 m2/g, 7.22 m2/g, and 42.58 m2/g, respectively. The enhanced surface area of the PANI/Fe0 composite nanofibers compared to pure PANI nanofibers and Fe0 nanoparticles would suggest improved performance in removing contaminant from water. The formation of PANI/Fe0 composite nanofibers was also characterized using attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy
10
(a) 140000
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q (mg/g)
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Magnetic field (kA/m) Fig. 3. (a) XPS survey spectrum and N 1s spectrum (inset) of the PANI/Fe0 composite nanofibers and (b) room temperature hysteresis loop of PANI/Fe0 composite nanofibers.
0
0
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Time (min) Fig. 4. Adsorption rate of (a) As(V) removal onto PANI nanofibers and PANI/Fe0 composite nanofibers and (b) Cr(VI) removal onto PANI nanofibers and PANI/Fe0 composite nanofibers.
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(XPS). The ATR-FTIR spectrum of PANI/Fe0 (Fig. 2b) represents the bands at 1595 cm1 and 1493 cm1 which are assigned to the stretching vibration of quinonoide (Q) and benzenoid (B) rings of PANI nanofibers [28]. The stretching bands at 1288 cm1 and 1215 cm1 correspond to CAN vibrational frequencies in Q–B–Q and B unites and those at 1167 cm1 and 813 cm1 are associated with BANH+@Q stretching and aromatic CAH deformation vibration of linear PANI backbone, respectively [29]. The XPS survey spectrum (Fig. 3a) of the PANI/Fe0 demonstrates C 1s, N 1s, Cl 2p and Fe 2p peaks which are associated with the binding energies of C, N, doped Cl and Fe, respectively. These results suggest the presence of Fe0 nanoparticles on the surface of PANI nanofibers. From the quantitative XPS analysis, the atomic percentage of Fe0 nanoparticles on the composite nanofibers is found to be 26.89. Moreover, the N 1s (inset in Fig. 2c) core spectrum of the PANI/Fe0 could be deconvoluted into three peaks arising from three different electronic states which include quinonoid imine (@NA) with binding energy centered at 398.2 eV, benzenoid amine (ANHA) with binding energy centered at 399.1 eV and doped imine (ANH+) with binding energy centered at 400.1 eV [30]. The isoelectric point (IEC) i.e. the pH value at the point of zero
50
(a)
charge of the prepared PANI/Fe0 composite nanofibers in 0.1 M NaCl was also measured from the zeta potential versus pH plot (supporting Fig. S1) and is found to be at pH 8.4. Magnetic properties of the PANI/Fe0 were investigated to test the possibility of magnetic separation and regeneration of the material after contaminant removal. The room temperature magnetic hysteresis loop of the PANI/Fe0 is presented in Fig. 3b. The nonlinear hysteresis loops with 22.6 emu/g saturation magnetization (Ms) and nonzero remnant magnetization (Mr) show well pronounced ferromagnetic properties of the PANI/Fe0 nanofibers which are favorable for their easy separation after contaminant removal from water by the application of a simple magnetic field.
3.2. Removal of arsenic(V), chromium(VI) and Congo red by PANI nanofibers and PANI/Fe0 composite nanofibers The contaminant removal behaviors of the as-synthesized PANI nanofibers and PANI/Fe0 composite nanofibers were examined in batch adsorption mode. Fig. 4a and b demonstrates the sorption capacity (qt) of the PANI/Fe0 for the removal of 10 mg/L As(V) and 100 mg/L Cr(VI), at different time intervals. It can be observed that adsorption rate is very rapid and almost complete removal is achieved within a very short period (10 min) at room temperature. The sorption rates for the removal of As(V) and Cr(VI), under
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Fig. 5. (a) Adsorption isotherm of As(V) removal onto PANI nanofibers and PANI/Fe composite nanofibers and (b) linear Langmuir isotherm fit (initial conc. 5–100 mg/ L).
Fig. 6. Adsorption isotherm of Cr(VI) removal onto PANI nanofibers and PANI/Fe0 composite nanofibers and (b) linear Langmuir isotherm fit (initial conc. 75–250 mg/ L).
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similar experimental conditions, using pure PANI nanofibers, are depicted in Fig. 4a and b. This reveals the relatively slower removal rate compared to the PANI/Fe0 composite nanofibers. The rapid sorption of the PANI/Fe0 can be explained in terms of its large surface area and highly reactive exposed surface sites, which are easily accessible for the removal of target contaminants. In the PANI/Fe0 composite nanofibers treatment system, the mechanism for the removal of As(V) at pH 7.0 is considered to proceed through Coulombic interaction [31] as the negatively charged 2 H2AsO 4 or HAsO4 (predominant As(V) species between pH 2 and 7) are attracted to the positively charged (below isoelectric point at pH 8.4) surface sites of the PANI/Fe0. In addition to the Coulombic interaction, exchange of doped Cl ions of PANI with H2AsO 4 or HAsO2 could also be regarded as an associated As(V) removal 4 mechanism. The mechanism describing the removal of Cr(VI) by the PANI/ Fe0 composite nanofibers system involves the ion exchange properties of the doped (Cl) polyaniline, as well as the electron donating properties derived from both components of the composite nanofibers, which reduce the Cr(VI) species to Cr(III) species and subsequent partial adsorption as Cr(III) oxides/hydroxides [32,33]. Equilibrium data are a prerequisite to evaluate the adsorption capacity of a material and to design and operate an adsorption process efficiently. Figs. 5a and 6a represent the adsorption isotherms for the removal of As(V) and Cr(VI) at 25 °C using PANI/Fe0 and PANI nanofibers, respectively. Enhanced sorption capacities for As(V) and Cr(VI) are observed by PANI/Fe0 nanofibers compared to the PANI nanofibers counterpart. For the determination of maximum adsorption capacity of the PANI/Fe0 composite nanofibers, adsorption equilibrium data were fitted with Langmuir and Freundlich isotherm models. The linearized Langmuir (Eq. (2)) and Freundlich (Eq. (3)) models are mathematically expressed as follows:
Ce 1 Ce ¼ þ qe Q m b Q m ln qe ¼ ln K F þ
ð2Þ
1 ln C e n
ð3Þ
where Qm is the maximum adsorption capacity (mg/g); b is the free energy of adsorption (L/mg); KF and n are the Freundlich isotherm parameters related to adsorption capacity (mg/g) and intensity of adsorption, respectively. The linearized Langmuir and Freundlich isotherms are shown in Figs. 5b and S2(a) (supporting information) for As(V) and Figs. 6b and S2(b) (supporting information) for Cr(VI), respectively. From the figures, the isotherm parameters for PANI nanofibers and PANI/Fe0 composite nanofibers were obtained and are presented in Table 1. Based on the higher values of correlation coefficients (R2) for the Langmuir model compared to the Freundlich model, adsorption data are better described by the Langmuir model. Therefore, it suggests that the adsorption of As(V) and Cr(VI) on the surface of PANI nanofibers and PANI/Fe0 composite nanofibers occur by monolayer formation. The maximum adsorption capacities are calculated as 42.37 mg/g for As(V) and 434.78 mg/g for Cr(VI), whereas under the same experimental condition, the maximum sorption capacities of the PANI nanofibers are found to be as 31.84 mg/g for As(V) and 344.82 mg/g for Cr(VI). The enhanced capacities for the removal of As(V) and Cr(VI) by the PANI/Fe0 nanofibers are attributed to the high surface area of the PANI nanofibers and greater reactivity of the supported Fe0 nanoparticles. Table 2 summarizes the maximum adsorption capacities for As(V) and Cr(VI) removal using PANI/Fe0 composite nanofibers and some nanomaterials reported previously. The maximum adsorption capacity of the PANI/Fe0 nanofibers is much higher for Cr(VI) and is comparable for As(V) removal than those of previously reported nanomaterials [26,34–38]. It should be mentioned for all those reported nanomaterials adsorption may be the only removal mechanisms for As(V) and Cr(VI), whereas for the PANI/Fe0 composite nanofibers adsorption as well as reduction are found to be the leading removal mechanisms, which could facilitate high performance removal of As(V) and Cr(VI) from water. Besides highly efficient removal of toxic inorganic ions, including As(V) and Cr(VI), PANI/Fe0 composite nanofibers could also be applied for decolorization of organic azo dyes. In this regard, the decolorization efficiency for the removal of anionic azo dye CR by PANI/Fe0 was investigated and compared with pure PANI nanofibers. The efficiency of the decolorization of a 50 mg/L CR solution
Table 1 Langmuir, Freundlich isotherm constants for As(V), Cr(VI) and CR adsorption onto PANI nanofibers and PANI/Fe0 composite nanofibers. Adsorbate
Adsorbent
Langmuir constants Qm (mg/g)
Freundlich constants b (L/mg)
R2
KF (mg/g)
1/n
R2
As(V)
PANI PANI/Fe0
42.37 31.84
0.5820 0.0398
0.9999 0.9992
20.72 2.19
0.1867 0.5944
0.9178 0.9823
Cr(VI)
PANI PANI/Fe0
344.82 443.78
0.4603 0.8518
0.9998 0.9998
162.09 238.91
0.1901 0.1625
0.8270 0.9547
CR
PANI PANI/Fe0
136.98 243.90
0.2579 0.3306
0.9992 0.9995
79.58 139.21
0.1112 0.1191
0.9577 0.9814
Table 2 Comparison of the adsorption capacities for As(V), Cr(VI) and CR removal using PANI/Fe0 composite nanofibers with reported nanomaterials. Adsorbent
Maximum As(V) removal capacity (mg/g)
Maximum Cr(VI) removal capacity (mg/g)
Maximum CR removal capacity (mg/g)
Ref.
Urchin-like a-FeOOH 3D flower like iron oxide nanostructure Hollow CeO2 nanosphere Fe3O4/BN nanotubes Flower like CeO2 Urchin-like-iron oxide nanostructure Polypyrrole–polyaniline nanofibers Hierarchical hollow MnO2 nanostructure PANI nanofibers PANI/Fe0 nanofibers
58 7.6 22.4 32.1 15 39.6 –
5.4 – 15.4 – 6.8 35.0 227
275 – – –
31.2 42.37
344.82 434.78
[26] [34] [35] [36] [37] [38] [32,39] [41] Present study Present study
109.2 222.2 60 136.98 243.90
M. Bhaumik et al. / Journal of Colloid and Interface Science 425 (2014) 75–82
by PANI/Fe0 and PANI nanofibers at different time intervals is illustrated in Fig. 7. Rapid decolorization of CR is observed for PANI/Fe0. Within 10 min, 99.81% decolorization is achieved by 1 g/L PANI/ Fe0, whereas for the same reaction time only 49.52% decolorization is obtained using the same amount of PANI nanofibers. Decolorization of CR by PANI/Fe0 composite nanofibers at pH 7.5 involves the combined effects of adsorptive removal of anionic CR by positively charged surface sites of PANI nanofibers, together with the reductive degradation of CR by supported Fe0 nanoparticles. It is established that Fe0 nanoparticles, after reaction with H2O or H+, can generate hydrogen atoms that induce cleavage of azo (AN@NA) bonds, thereby damaging the chromophore group and conjugated system of CR molecules [39,40]. The adsorption isotherms for CR removal using PANI and PANI/Fe0 and their fitting with linear Langmuir and Freundlich models are presented in Fig. 8a and b and S2(c) (supporting information). The maximum CR adsorption capacities of 243.9 mg/g for PANI/Fe0 and 136.98 mg/g for PANI nanofibers are obtained from the linear Langmuir model as represented in Table 1. This result indicates that the CR removal performance of PANI nanofibers could be greatly enhanced by effective supporting of the Fe0 nanoparticles. Table 2 compares the CR adsorption capacities of PANI/Fe0 composites nanofibers and some nanomaterials reported previously [26,38,39,41]. It appears that the adsorption capacity of CR using the as-synthesized PANI/Fe0 is significantly higher than most of the reported nanomaterials except urchin-like a-FeOOH.
Decolorization efficiency (%)
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Time (min) Fig. 7. Decolorization efficiency at different time intervals.
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4. Conclusions
qe (mg/g)
200
A successful and very simple strategy was developed to synthesize PANI/Fe0 composite nanofibers without prior modification of PANI nanofibers at room temperature. Effective supporting of the Fe0 nanoparticles on the PANI nanofibers surface was accomplished using borohydride reduction of the unreacted FeCl3 oxidant and polymerization of the by-product, FeCl2. The synthesized PANI/ Fe0 composite nanofibers demonstrated significantly higher performance for the removal of inorganic and organic contaminants from water due to the 1D fiber like structure of PANI and greater reactivity of Fe0 nanoparticles. The PANI/Fe0 composite nanofibers exhibited superior capacity of 42.37 mg/g and 434.7 mg/g for the removal of As(V) and Cr(VI), respectively. Moreover, the enhanced decolorization efficiency of the PANI/Fe0 was also observed for the removal of anionic CR dye. These results suggest that the composite nanofibers of PANI/Fe0 have great potential to decontaminate water containing inorganic heavy metals and organic pollutants. Furthermore, the PANI/Fe0 composite nanofibers are expected to have significant applications as sensors, nanoelectrocatalysts and in nanoelectronics. Finally, using this synthesis protocol, a variety of nanostructured conducting polymers supported metals nanoparticles could be prepared for applications in diverse fields of nanotechnology.
150
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Acknowledgments
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The financial support from the Tshwane University of Technology (TUT) and National Research Foundation (NRF), South Africa are gratefully acknowledged. The Council for Scientific and Industrial Research (CSIR), South Africa, is thanked for providing the research infrastructure to this project.
PANI PANI/Fe
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Appendix A. Supplementary material
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Fig. 8. (a) Adsorption isotherm of CR removal onto PANI nanofibers and PANI/Fe composite nanofibers and (b) linear Langmuir isotherm fit (initial conc. 100– 400 mg/L).
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2014.03.031.
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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
X. Lu, W. Zhang, C. Wang, T.C. Wen, Y. Wei, Prog. Polym. Sci. 36 (2011) 671. X.Y. Zhang, S.K. Manohar, J. Am. Chem. Soc. 127 (2005) 14156. D. Zhang, Y. Wang, Mater. Sci. Eng. 134 (2006) 9. M. Wan, Adv. Mater. 20 (2008) 2926. H.D. Tran, D. Li, R.B. Kaner, Adv. Mater. 21 (2009) 1487. D. Li, J. Huang, R.B. Kaner, Acc. Chem. Res. 42 (2009) 135. F. Gu, L. Zhang, X. Yin, L. Tong, Nano Lett. 8 (2008) 2757. C.O. Baker, B. Shedd, P.C. Innis, P.G. Whitten, G.M. Spinks, G.G. Wallace, R.B. Kaner, Adv. Mater. 20 (2008) 155. K. Wang, J.Y. Huang, Z.X. Wei, J. Phys. Chem. C 114 (2010) 8062. S.Y. Lee, G.R. Choi, H. Lim, K.M. Lee, S.K. Lee, Appl. Phys. Lett. 95 (2009) 079901. X. Guo, G.T. Fei, H. Su, L.D. Zhang, J. Phys. Chem. C 115 (2011) 1608. Y. Xia, Y. Xiong, B. Lim, S.E. Skrabalak, Angew. Chem. Int. Ed. 48 (2008) 60. K.E. Hinda, R.P. Socha, G.D. Sulka, J. Phys. Chem. C 117 (2013) 19382. S.K. Pillalamarri, F.D. Blum, A.T. Tokuhiro, M.F. Bertino, Chem. Mater. 17 (2005) 5941. R.J. Tseng, J. Huang, J. Ouyang, R.B. Kaner, Y. Yang, Nano Lett. 5 (2005) 1077. Z. Chen, L. Xu, W. Li, M. Waje, Y. Yan, Nanotechnology 17 (2006) 5254. B.J. Gallon, R.W. Kojima, R.B. Kaner, P.L. Diaconescu, Angew. Chem. Int. Ed. 46 (2007) 7251. S. Guo, S. Dong, E. Wang, Small 5 (2009) 1869. J. Huang, R.B. Kaner, Angew. Chem. Int. Ed. 43 (2004) 5817. J. Wang, B. Deng, H. Chen, X. Wang, J. Zheng, Environ. Sci. Technol. 43 (2009) 5223. M.M. Ayad, A.A. El-Nasr, J. Phys. Chem. C 114 (2010) 14377.
[22] W.X. Zhang, J. Nanopart. Res. 5 (2003) 323. [23] R.A. Crane, T.B. Scott, J. Hazard. Mater. 211–212 (2012) 112. [24] L.C. Roberts, S.J. Hug, T. Ruettimann, M. Billah, A.W. Khan, M.T. Rahman, Environ. Sci. Technol. 38 (2004) 307–315. [25] A. Zhitkovich, Chem. Res. Toxicol. 24 (2011) 1617. [26] B. Wang, H. Wu, L. Yu, R. Xu, T.T. Lim, X.W. Lou, Adv. Mater. 24 (2012) 1111. [27] R. Dev, N. Mukherjee, S. Ahammed, B.C. Ranu, Chem. Commun. 48 (2012) 7982. [28] Q.S. Gao, S.N. Wang, Y. Tang, C. Giordano, Chem. Commun. 48 (2012) 260. [29] J. Yin, X. Xiang, L. Xiang, X. Zhao, J. Mater. Chem. 20 (2010) 7096. [30] E.T. Kang, K.G. Neoh, K.L. Tan, Prog. Polym. Sci. 23 (1998) 277. [31] S.R. Kanel, J.M. Greneche, H. Choi, Environ. Sci. Technol. 40 (2006) 2045. [32] M. Bhaumik, A. Maity, V.V. Srinivasu, M.S. Onyango, Chem. Eng. J. 181–182 (2012) 323. [33] T. Liu, D.C.W. Tsang, I.M.C. Lo, Environ. Sci. Technol. 42 (2008) 2092. [34] L.S. Zhong, J.S. Hu, H.P. Liang, A.M. Cao, W.G. Song, L.J. Wan, Adv. Mater. 18 (2006) 2426. [35] C.Y. Cao, Z.M. Cui, C.Q. Chen, W.G. Song, W. Cai, J. Phys. Chem. C 114 (2010) 9865. [36] R. Chen, C. Zhi, H. Yang, Y. Bando, Z. Zhang, N. Sugiur, G. Golberg, J. Colloid Interface Sci. 359 (2011) 261. [37] R. Srivastava, J. Colloid Interface Sci. 348 (2010) 600. [38] H.U. Lee, S.C. Lee, Y.C. Lee, S. Vrtnik, C. Kim, S. Lee, Y.B. Lee, B. Nam, J.W. Lee, S.Y. Park, J. Hazard. Mater. 262 (2013) 130. [39] M. Bhaumik, R. McCrindle, A. Maity, Chem. Eng. J. 228 (2013) 506. [40] J. Fan, Y. Guo, J. Wang, M. Fan, J. Hazard. Mater. 166 (2009) 904. [41] J.B. Fei, Y. Cui, X.H. Yan, W. Qi, Y. Yang, K.W. Wang, Q. He, J.B. Li, Adv. Mater. 20 (2008) 452.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Composite nanofibers prepared from metallic iron nanoparticles and polyaniline: High performance for water treatment applications Madhumita Bhaumik a,⇑, Hyoung J. Choi b, Rob I. McCrindle a,⇑, Arjun Maity c,d,⇑ a
Department of Chemistry, Tshwane University of Technology, Private Bag X680, Pretoria, South Africa Department of Polymer Science and Engineering, Inha University, Incheon 402-751, South Korea c Smart Polymers Group, Polymers and Composites, Council for Scientific and Industrial Research (CSIR), Pretoria, South Africa d Department of Civil and Chemical Engineering, University of South Africa (UNISA), South Africa b
a r t i c l e
i n f o
Article history: Received 4 December 2013 Accepted 12 March 2014 Available online 20 March 2014 Keywords: Polyaniline Composites Nanofibers Chromium(VI) Arsenic(V) Congo red Adsorption Isotherm Kinetics
a b s t r a c t Presented here is a simple preparation of metallic iron nanoparticles, supported on polyaniline nanofibers at room temperature. The preparation is based on polymerization of interconnected nanofibers by rapid mixing of the aniline monomer with Fe(III) chloride as the oxidant, followed by reductive deposition of Fe0 nanoparticles, using the polymerization by-products as the Fe precursor. The morphology and other physico-chemical properties of the resulting composite were characterized by scanning and transmission electron microscopy, Brunauer–Emmett–Teller method, X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and vibrating-sample magnetometry. The composite fibers were 80–150 nm in diameter and exhibited the expected ferromagnetic behavior. The composite rapidly and efficiently removed As(V), Cr(VI), and also Congo red dye, from aqueous solutions suggesting their usefulness for removal of toxic materials from wastewater. The composite fibers have high capacity for toxin removal: 42.37 mg/g of As(V), 434.78 mg/g of Cr(VI), and 243.9 mg/g of Congo red. The fibers are easily recovered from fluids by exploiting their ferromagnetic properties. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction In recent years, conducting polymers nanostructures have attracted special attention in the fields of nanoscience and nanotechnology due to their unique combination of the electronic properties of conductive polymers and the large surface area of nanomaterials [1]. Specifically, the synthesis and applications of one dimensional (1D) nanostructured conducting polymeric materials with controllable morphology that includes nanotubes, nanowires and nanofibers, have been explored over the past few years [2–5]. Among 1D conducting polymer nanostructures, nanofibers of polyaniline (PANI) have stimulated research interest as they can be easily synthesized in bulk quantities and have a large surface area to volume ratio, with good redox properties, high conductivity and excellent environmental stability [6]. This has resulted in
⇑ Corresponding authors. Address: Smart Polymers Group, Polymers and Composites, Council for Scientific and Industrial Research (CSIR), Pretoria, South Africa. Fax: +27 128413553 (A. Maity). Fax: +27 123826286 (R.I. McCrindle and M. Bhaumik). E-mail addresses: [email protected] (M. Bhaumik), [email protected] (R.I. McCrindle), [email protected], [email protected] (A. Maity). http://dx.doi.org/10.1016/j.jcis.2014.03.031 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
potential applications in sensors, actuators, super capacitors, field effect transistors and in separation or purification systems [7–11]. Moreover, to improve the functionality of 1D conducting polymer nanostructures metal nanoparticles (with unique electronic, catalytic, magnetic and optical properties) were incorporated [12], as a secondary component, to form 1D nanocomposites. This incorporation can be considered as a constructive technique because the combination of metal particles with a conducting polymer offers an attractive route to reinforce the polymer as well as introduce electronic properties, based on morphological modification or electronic interaction between the two components [13]. Consequently, efforts have been devoted to attach various metal nanoparticles onto 1D conducting polymer supporting matrices and manufacturing multifunctional 1D composite nanostructures with enhanced physico-chemical properties. In particular, a series of noble metal nanoparticles including Ag, Au, Pt and Pd were produced inside or on the surface of PANI nanofibers via in situ redox reactions and/or by reduction processes in the presence of other reducing agents such as ethylene glycol or HCOOH, respectively [14–18]. Although noble metal nanoparticle supported on 1D PANI nanofibers have been studied extensively, magnetic nanoparticles such as zero valent iron (Fe0), with increased reactivity, supported on PANI nanofibers have not been widely investigated.
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In this work, a simple and facile chemical strategy to synthesize Fe nanoparticles supported on PANI nanofibers (PANI/Fe0) at room temperature is demonstrated. A simple synthesis protocol based on rapid-mixing polymerization method was employed first for the production of PANI nanofibers using ferric chloride (FeCl3) as oxidant [19]. After completion of the polymerization reaction, the PANI nanofibers and reaction by products (FeCl2 or any unreacted FeCl3) could be found in the polymerization solution. After addition of sodium borohydride (NaBH4) solution, as an external reducing agent, Fe0 nanoparticles were grown on the PANI nanofibers matrix. The present synthesis strategy represents a new type of 1D conducting polymer nanocomposite with several advantages. Firstly, the synthesis method is very simple and executed at room temperature without employing an external means such as template or surfactant. Secondly, no external metal precursor is used as the source of metal nanoparticles; rather a polymerization by-product is used as the precursor of Fe0 nanoparticles. Finally the physicochemical properties of the PANI/Fe0 composite nanofibers can easily be tuned by varying the Fe0 nanoparticle-loading in the PANI nanofibers. The as-synthesized PANI/Fe0 composite nanofibers are expected to display enhanced physico-chemical properties obtained synergistically from both components of the composite nanofibers. They could be used as multifunctional materials in various fields of nanotechnology and for environmental remediation. It is already well established that because of the presence of large amounts of amine and imine functional groups, PANI has interactions with some heavy metal ions and toxic dye molecules that have strong affinity to nitrogen. Consequently, the application of PANI for the removal of heavy metals and dyes has been studied intensively [11,20,21]. In addition, Fe0 nanoparticles with excellent electron donating capacity have been effectively used over the past few years to remediate surface and ground water contaminated with inorganic ions and organic compounds [22]. The large specific surface area 0
and greater density of the highly reactive surface sites of Fe0 nanoparticles have been proposed to result in a rapid and cost-effective water treatment system compared to conventional iron-based technologies [23]. Therefore, due to enhanced surface area, highly reactive surface sites and greater electron donor property, derived synergistically from both components of the PANI/Fe0 composite nanofibers, these are expected to perform efficiently in the removal of contaminants from water. Herein, the contaminant removal behavior of the obtained PANI/Fe0 nanofibers is studied by considering two heavy metal pollutants, arsenic(V) [As(V)], one of the most toxic ground water pollutants, chromium(VI) [Cr(VI], the most common heavy metal pollutant in industrial wastewater and one organic pollutant, Congo red (CR), a carcinogenic anionic azo dye released into waste effluents of different dying industries. Due to the toxic and carcinogenic effects of As(V), Cr(VI) and CR on human health and on the environment [24–26], their removal from water is important. Evaluation of the performance of the PANI/Fe0 composite nanofibers toward contaminant removal from water exhibited enhanced removal performance compared to the PANI nanofibers counterpart. 2. Experimental section 2.1. Materials Aniline (ANI, 99% acquired from Sigma–Aldrich, USA) was purified using vacuum distillation. Distilled ANI was stored in a refrigerator prior to use for polymerization. Anhydrous iron (III) chloride (FeCl3), sodium borohydride (NaBH4), potassium dichromate (K2Cr2O7), sodium arsenate heptahydrate (Na2HAsO4, 7H2O) and Congo red (CR) were purchased from Sigma–Aldrich, USA. Ultrapure water (type-2, resistivity-17.4 MX cm1), collected from an EASYpureÒ II, UV-ultrapure water system, was used for polymerization media and preparation of all aqueous solutions. All other chemicals used were of reagent grade.
Fig. 1. FE-SEM images of (a) PANI nanofibers and (b) PANI/Fe0 composite nanofibers and HR-TEM images of (c and d) PANI/Fe0 composite nanofibers at two different magnifications.
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2.2. Synthesis of PANI nanofibers Interconnected PANI nanofibers were synthesized via rapidmixing chemical oxidative polymerization at room temperature. In a typical polymerization process, 6 g of FeCl3 as oxidant was dissolved in 80 mL of ultrapure water in a 250 mL conical flask. Aniline (ANI) monomer (0.8 mL) was added all at a time to the oxidant solution with sufficient (600 rpm) magnetic stirring to evenly distribute the oxidant and monomer molecules thus preventing secondary growth of PANI. Stirring was continued for 5 min. The reaction mixture was then left without stirring for 2 days. The precipitated polymer was filtered, washed with water and acetone, and finally dried at 60 °C.
experiments were conducted by adding 0.01 g of adsorbents for Cr(VI) and CR and 0.02 g for As(V) under same experimental conditions for 24 hrs. Experimental solutions with different concentrations of As(V), Cr(VI) and CR were adjusted to pH 7.0 for As(V), pH 2.0 for Cr(VI) and pH 7.5 for CR, respectively. After a specified time, the adsorbents were separated from the solution and the residual concentrations were determined using an inductively coupled plasma-optical emission spectrometer (ICP-OES, Spectro-Arcos) for As(V) and a UV–vis spectrophotometer (Perkin Elmer – Lamda 35) for Cr(VI) and CR. The equilibrium sorption capacity was determined using Eq. (1):
qe ¼
ðC 0 C e Þ V m
ð1Þ
0
2.3. Synthesis of PANI/Fe composite nanofibers 0
The PANI/Fe composite nanofibers were synthesized using prepared PANI nanofibers (without removing them from the polymerization medium) as support and polymerization by-products FeCl2 and any remaining FeCl3 as the source of the Fe0 nanoparticles. To support the Fe0 nanoparticles on the PANI nanofibers, the polymerization mixture (PANI nanofibers and FeCl2/FeCl3) was stirred mechanically under a nitrogen atmosphere. Freshly prepared 0.5 M sodium borohydride (NaBH4) solution (100 mL) was added drop wise to the mixture containing PANI nanofibers, which resulted in the attachment of Fe0 nanoparticles onto the PANI nanofibers matrix. The mixture was stirred another 20 min for completion of reduction reaction. The product (PANI/Fe0) was filtered, washed with water and ethanol and dried at 60 °C.
where qe is the equilibrium amount of adsorbed per unit mass of adsorbent (mg/g), C0 and Ce are the initial and equilibrium adsorbate concentration, respectively, in mg/L, V is the sample volume (L) and m is the adsorbent mass in g. 3. Results and discussion 3.1. Morphology and physico-chemical characterization of the PANI/ Fe0 composite nanofibers The morphology of the PANI nanofibers and PANI/Fe0 composite nanofibers was investigated using scanning electron microscopy 350
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2.4. Characterization of nanofibers
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The morphology and size of the PANI and PANI/Fe0 nanofibers were investigated using field emission scanning electron microscope (FE-SEM), LEO Zeiss SEM and a high resolution transmission electron microscope (HR-TEM), JEOL JEM-2100 instrument with a LAB6 filament all operated at 200 kV, respectively. X-ray diffraction patterns were measured on a PANalytical X’Pert PRO-diffractometer. The Brunauer–Emmett–Teller (BET) surface area analyses of the Fe0 nanoparticles (synthesized in our laboratory using the same method described in Section 2.3, but in the absence of PANI nanofibers), PANI nanofibers and PANI/Fe0 composite nanofibers were performed using a low temperature N2 adsorption–desorption technique with a Micromeritics ASAP 2020 gas adsorption apparatus (USA). The IR spectrum of the PANI/Fe0 nanofibers was recorded with an attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectrometer (Perkin–Elmer Spectrum 100 spectrometer), equipped with an ATR accessory and a germanium crystal. Elemental mapping of the nanocomposite was performed using X-ray photoelectron spectroscopy (XPS) on a Kratos Axis Ultra device, with an Al monochromatic X-ray source (1486.6 eV). The isoelectric point (IEC) of the PANI/Fe0 composite nanofibers was measured using a Zeta-Sizer, Malvern Ltd., UK. Magnetic properties of the PANI/Fe0 composite nanofibers were recorded using a vibrating sample magnetometer (VSM, Lakeshore-7307, USA) with a maximum magnetic field of 400 kA/ m in powder form at room temperature.
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To perform adsorption experiments, stock solutions (1000 mg/L) of As(V), Cr(VI) and CR were prepared by dissolving appropriate amount of Na2HAsO4, 7H2O, K2Cr2O7 and CR in 1 L ultrapure water. The kinetic experiments were carried out by adding 0.02 g of adsorbents (PANI or PANI/Fe0) to 20 mL of As(V), Cr(VI) and CR samples in 100 mL glass bottles placed in a thermostatic shaker, agitated at 200 rpm, for a predetermined time interval. The equilibrium
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(SEM) and transmission electron microscopy (TEM). The SEM image of the pure PANI nanofibers prepared via rapid-mixing reaction of aniline monomer and FeCl3 oxidant is shown in Fig. 1a. The SEM image indicates the formation of smooth surface PANI nanofibers, 50–80 nm in diameter, with a tendency to agglomerate into an interconnected nanofiber network. After loading with Fe0 nanoparticles, the diameters of the PANI nanofibers increased to about 80– 150 nm (Fig. 1b) and assembled in nanofiber-like morphology. Typical TEM images of the prepared PANI/Fe0 composite nanofibers at three different magnifications are presented in Fig. 1c and d. It can be seen (Fig. 1c) that the composite nanofibers have a rougher surface than pure PANI form. The magnified TEM image (inset in Fig. 1c) revealed that Fe0 nanoparticles are facilely deposited/ embedded onto the PANI nanofibers matrix. The clear lattice fringes with inter planer spacing of 0.21 nm, corresponded to body centered cubic Fe (1 1 0) planes, suggesting a highly crystalline structure of the Fe0 nanoparticles (Fig. 1d). Rapid reaction of Fe0 nanoparticles with surrounding media, including moisture and air oxygen, results in a reduction of its chemical reactivity due to the formation of iron oxide. Protection of the Fe0 nanoparticles from oxide formation could effectively be achieved by supporting them on the polymer matrix. During
synthesis of Fe0 nanoparticles the presence of PANI nanofibers with high surface areas, could create an in situ surround for the generated Fe0 nuclei, to protect them from oxidation. This fact was confirmed by the X-ray diffraction (XRD) pattern of PANI/Fe0 composite nanofibers. The strong sharp diffraction peak (Fig. 2a) at 2h, of 44.79° and a small peak at 65.15° are in good agreement with the (1 1 0) and (2 0 0) planes of Fe0 nanoparticles [27]. This implies that Fe0 nanoparticles are successfully supported on the PANI nanofibers matrix. The average grain size of the supported Fe0 particles is 14.27 nm, as calculated from the broadening of the (1 1 0) diffraction peak using Scherrer’s formula. The Brunauer–Emmett–Teller specific surface area (SBET) of the PANI nanofibers, Fe0 nanoparticles and PANI/Fe0 composite nanofibers was determined from N2 adsorption–desorption curves and is found to be 38.99 m2/g, 7.22 m2/g, and 42.58 m2/g, respectively. The enhanced surface area of the PANI/Fe0 composite nanofibers compared to pure PANI nanofibers and Fe0 nanoparticles would suggest improved performance in removing contaminant from water. The formation of PANI/Fe0 composite nanofibers was also characterized using attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy
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Time (min) Fig. 4. Adsorption rate of (a) As(V) removal onto PANI nanofibers and PANI/Fe0 composite nanofibers and (b) Cr(VI) removal onto PANI nanofibers and PANI/Fe0 composite nanofibers.
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(XPS). The ATR-FTIR spectrum of PANI/Fe0 (Fig. 2b) represents the bands at 1595 cm1 and 1493 cm1 which are assigned to the stretching vibration of quinonoide (Q) and benzenoid (B) rings of PANI nanofibers [28]. The stretching bands at 1288 cm1 and 1215 cm1 correspond to CAN vibrational frequencies in Q–B–Q and B unites and those at 1167 cm1 and 813 cm1 are associated with BANH+@Q stretching and aromatic CAH deformation vibration of linear PANI backbone, respectively [29]. The XPS survey spectrum (Fig. 3a) of the PANI/Fe0 demonstrates C 1s, N 1s, Cl 2p and Fe 2p peaks which are associated with the binding energies of C, N, doped Cl and Fe, respectively. These results suggest the presence of Fe0 nanoparticles on the surface of PANI nanofibers. From the quantitative XPS analysis, the atomic percentage of Fe0 nanoparticles on the composite nanofibers is found to be 26.89. Moreover, the N 1s (inset in Fig. 2c) core spectrum of the PANI/Fe0 could be deconvoluted into three peaks arising from three different electronic states which include quinonoid imine (@NA) with binding energy centered at 398.2 eV, benzenoid amine (ANHA) with binding energy centered at 399.1 eV and doped imine (ANH+) with binding energy centered at 400.1 eV [30]. The isoelectric point (IEC) i.e. the pH value at the point of zero
50
(a)
charge of the prepared PANI/Fe0 composite nanofibers in 0.1 M NaCl was also measured from the zeta potential versus pH plot (supporting Fig. S1) and is found to be at pH 8.4. Magnetic properties of the PANI/Fe0 were investigated to test the possibility of magnetic separation and regeneration of the material after contaminant removal. The room temperature magnetic hysteresis loop of the PANI/Fe0 is presented in Fig. 3b. The nonlinear hysteresis loops with 22.6 emu/g saturation magnetization (Ms) and nonzero remnant magnetization (Mr) show well pronounced ferromagnetic properties of the PANI/Fe0 nanofibers which are favorable for their easy separation after contaminant removal from water by the application of a simple magnetic field.
3.2. Removal of arsenic(V), chromium(VI) and Congo red by PANI nanofibers and PANI/Fe0 composite nanofibers The contaminant removal behaviors of the as-synthesized PANI nanofibers and PANI/Fe0 composite nanofibers were examined in batch adsorption mode. Fig. 4a and b demonstrates the sorption capacity (qt) of the PANI/Fe0 for the removal of 10 mg/L As(V) and 100 mg/L Cr(VI), at different time intervals. It can be observed that adsorption rate is very rapid and almost complete removal is achieved within a very short period (10 min) at room temperature. The sorption rates for the removal of As(V) and Cr(VI), under
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Fig. 5. (a) Adsorption isotherm of As(V) removal onto PANI nanofibers and PANI/Fe composite nanofibers and (b) linear Langmuir isotherm fit (initial conc. 5–100 mg/ L).
Fig. 6. Adsorption isotherm of Cr(VI) removal onto PANI nanofibers and PANI/Fe0 composite nanofibers and (b) linear Langmuir isotherm fit (initial conc. 75–250 mg/ L).
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similar experimental conditions, using pure PANI nanofibers, are depicted in Fig. 4a and b. This reveals the relatively slower removal rate compared to the PANI/Fe0 composite nanofibers. The rapid sorption of the PANI/Fe0 can be explained in terms of its large surface area and highly reactive exposed surface sites, which are easily accessible for the removal of target contaminants. In the PANI/Fe0 composite nanofibers treatment system, the mechanism for the removal of As(V) at pH 7.0 is considered to proceed through Coulombic interaction [31] as the negatively charged 2 H2AsO 4 or HAsO4 (predominant As(V) species between pH 2 and 7) are attracted to the positively charged (below isoelectric point at pH 8.4) surface sites of the PANI/Fe0. In addition to the Coulombic interaction, exchange of doped Cl ions of PANI with H2AsO 4 or HAsO2 could also be regarded as an associated As(V) removal 4 mechanism. The mechanism describing the removal of Cr(VI) by the PANI/ Fe0 composite nanofibers system involves the ion exchange properties of the doped (Cl) polyaniline, as well as the electron donating properties derived from both components of the composite nanofibers, which reduce the Cr(VI) species to Cr(III) species and subsequent partial adsorption as Cr(III) oxides/hydroxides [32,33]. Equilibrium data are a prerequisite to evaluate the adsorption capacity of a material and to design and operate an adsorption process efficiently. Figs. 5a and 6a represent the adsorption isotherms for the removal of As(V) and Cr(VI) at 25 °C using PANI/Fe0 and PANI nanofibers, respectively. Enhanced sorption capacities for As(V) and Cr(VI) are observed by PANI/Fe0 nanofibers compared to the PANI nanofibers counterpart. For the determination of maximum adsorption capacity of the PANI/Fe0 composite nanofibers, adsorption equilibrium data were fitted with Langmuir and Freundlich isotherm models. The linearized Langmuir (Eq. (2)) and Freundlich (Eq. (3)) models are mathematically expressed as follows:
Ce 1 Ce ¼ þ qe Q m b Q m ln qe ¼ ln K F þ
ð2Þ
1 ln C e n
ð3Þ
where Qm is the maximum adsorption capacity (mg/g); b is the free energy of adsorption (L/mg); KF and n are the Freundlich isotherm parameters related to adsorption capacity (mg/g) and intensity of adsorption, respectively. The linearized Langmuir and Freundlich isotherms are shown in Figs. 5b and S2(a) (supporting information) for As(V) and Figs. 6b and S2(b) (supporting information) for Cr(VI), respectively. From the figures, the isotherm parameters for PANI nanofibers and PANI/Fe0 composite nanofibers were obtained and are presented in Table 1. Based on the higher values of correlation coefficients (R2) for the Langmuir model compared to the Freundlich model, adsorption data are better described by the Langmuir model. Therefore, it suggests that the adsorption of As(V) and Cr(VI) on the surface of PANI nanofibers and PANI/Fe0 composite nanofibers occur by monolayer formation. The maximum adsorption capacities are calculated as 42.37 mg/g for As(V) and 434.78 mg/g for Cr(VI), whereas under the same experimental condition, the maximum sorption capacities of the PANI nanofibers are found to be as 31.84 mg/g for As(V) and 344.82 mg/g for Cr(VI). The enhanced capacities for the removal of As(V) and Cr(VI) by the PANI/Fe0 nanofibers are attributed to the high surface area of the PANI nanofibers and greater reactivity of the supported Fe0 nanoparticles. Table 2 summarizes the maximum adsorption capacities for As(V) and Cr(VI) removal using PANI/Fe0 composite nanofibers and some nanomaterials reported previously. The maximum adsorption capacity of the PANI/Fe0 nanofibers is much higher for Cr(VI) and is comparable for As(V) removal than those of previously reported nanomaterials [26,34–38]. It should be mentioned for all those reported nanomaterials adsorption may be the only removal mechanisms for As(V) and Cr(VI), whereas for the PANI/Fe0 composite nanofibers adsorption as well as reduction are found to be the leading removal mechanisms, which could facilitate high performance removal of As(V) and Cr(VI) from water. Besides highly efficient removal of toxic inorganic ions, including As(V) and Cr(VI), PANI/Fe0 composite nanofibers could also be applied for decolorization of organic azo dyes. In this regard, the decolorization efficiency for the removal of anionic azo dye CR by PANI/Fe0 was investigated and compared with pure PANI nanofibers. The efficiency of the decolorization of a 50 mg/L CR solution
Table 1 Langmuir, Freundlich isotherm constants for As(V), Cr(VI) and CR adsorption onto PANI nanofibers and PANI/Fe0 composite nanofibers. Adsorbate
Adsorbent
Langmuir constants Qm (mg/g)
Freundlich constants b (L/mg)
R2
KF (mg/g)
1/n
R2
As(V)
PANI PANI/Fe0
42.37 31.84
0.5820 0.0398
0.9999 0.9992
20.72 2.19
0.1867 0.5944
0.9178 0.9823
Cr(VI)
PANI PANI/Fe0
344.82 443.78
0.4603 0.8518
0.9998 0.9998
162.09 238.91
0.1901 0.1625
0.8270 0.9547
CR
PANI PANI/Fe0
136.98 243.90
0.2579 0.3306
0.9992 0.9995
79.58 139.21
0.1112 0.1191
0.9577 0.9814
Table 2 Comparison of the adsorption capacities for As(V), Cr(VI) and CR removal using PANI/Fe0 composite nanofibers with reported nanomaterials. Adsorbent
Maximum As(V) removal capacity (mg/g)
Maximum Cr(VI) removal capacity (mg/g)
Maximum CR removal capacity (mg/g)
Ref.
Urchin-like a-FeOOH 3D flower like iron oxide nanostructure Hollow CeO2 nanosphere Fe3O4/BN nanotubes Flower like CeO2 Urchin-like-iron oxide nanostructure Polypyrrole–polyaniline nanofibers Hierarchical hollow MnO2 nanostructure PANI nanofibers PANI/Fe0 nanofibers
58 7.6 22.4 32.1 15 39.6 –
5.4 – 15.4 – 6.8 35.0 227
275 – – –
31.2 42.37
344.82 434.78
[26] [34] [35] [36] [37] [38] [32,39] [41] Present study Present study
109.2 222.2 60 136.98 243.90
M. Bhaumik et al. / Journal of Colloid and Interface Science 425 (2014) 75–82
by PANI/Fe0 and PANI nanofibers at different time intervals is illustrated in Fig. 7. Rapid decolorization of CR is observed for PANI/Fe0. Within 10 min, 99.81% decolorization is achieved by 1 g/L PANI/ Fe0, whereas for the same reaction time only 49.52% decolorization is obtained using the same amount of PANI nanofibers. Decolorization of CR by PANI/Fe0 composite nanofibers at pH 7.5 involves the combined effects of adsorptive removal of anionic CR by positively charged surface sites of PANI nanofibers, together with the reductive degradation of CR by supported Fe0 nanoparticles. It is established that Fe0 nanoparticles, after reaction with H2O or H+, can generate hydrogen atoms that induce cleavage of azo (AN@NA) bonds, thereby damaging the chromophore group and conjugated system of CR molecules [39,40]. The adsorption isotherms for CR removal using PANI and PANI/Fe0 and their fitting with linear Langmuir and Freundlich models are presented in Fig. 8a and b and S2(c) (supporting information). The maximum CR adsorption capacities of 243.9 mg/g for PANI/Fe0 and 136.98 mg/g for PANI nanofibers are obtained from the linear Langmuir model as represented in Table 1. This result indicates that the CR removal performance of PANI nanofibers could be greatly enhanced by effective supporting of the Fe0 nanoparticles. Table 2 compares the CR adsorption capacities of PANI/Fe0 composites nanofibers and some nanomaterials reported previously [26,38,39,41]. It appears that the adsorption capacity of CR using the as-synthesized PANI/Fe0 is significantly higher than most of the reported nanomaterials except urchin-like a-FeOOH.
Decolorization efficiency (%)
100
80
60
40
PANI
20
0
PANI/Fe 0
0
10
20
30
40
50
Time (min) Fig. 7. Decolorization efficiency at different time intervals.
(a)
250
4. Conclusions
qe (mg/g)
200
A successful and very simple strategy was developed to synthesize PANI/Fe0 composite nanofibers without prior modification of PANI nanofibers at room temperature. Effective supporting of the Fe0 nanoparticles on the PANI nanofibers surface was accomplished using borohydride reduction of the unreacted FeCl3 oxidant and polymerization of the by-product, FeCl2. The synthesized PANI/ Fe0 composite nanofibers demonstrated significantly higher performance for the removal of inorganic and organic contaminants from water due to the 1D fiber like structure of PANI and greater reactivity of Fe0 nanoparticles. The PANI/Fe0 composite nanofibers exhibited superior capacity of 42.37 mg/g and 434.7 mg/g for the removal of As(V) and Cr(VI), respectively. Moreover, the enhanced decolorization efficiency of the PANI/Fe0 was also observed for the removal of anionic CR dye. These results suggest that the composite nanofibers of PANI/Fe0 have great potential to decontaminate water containing inorganic heavy metals and organic pollutants. Furthermore, the PANI/Fe0 composite nanofibers are expected to have significant applications as sensors, nanoelectrocatalysts and in nanoelectronics. Finally, using this synthesis protocol, a variety of nanostructured conducting polymers supported metals nanoparticles could be prepared for applications in diverse fields of nanotechnology.
150
PANI (experimental) 100
0
PANI/Fe (experimental) Langmuir model fit Freundlich model fit
50
0
20
40
60
80
100
120
140
C (mg/L) e
(b)
1
e
0.8
0.6
e
C /q
81
Acknowledgments
0.4
The financial support from the Tshwane University of Technology (TUT) and National Research Foundation (NRF), South Africa are gratefully acknowledged. The Council for Scientific and Industrial Research (CSIR), South Africa, is thanked for providing the research infrastructure to this project.
PANI PANI/Fe
0.2
0
0
20
40
60
80
100
120
0
140
C (mg/L)
Appendix A. Supplementary material
e
0
Fig. 8. (a) Adsorption isotherm of CR removal onto PANI nanofibers and PANI/Fe composite nanofibers and (b) linear Langmuir isotherm fit (initial conc. 100– 400 mg/L).
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2014.03.031.
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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
X. Lu, W. Zhang, C. Wang, T.C. Wen, Y. Wei, Prog. Polym. Sci. 36 (2011) 671. X.Y. Zhang, S.K. Manohar, J. Am. Chem. Soc. 127 (2005) 14156. D. Zhang, Y. Wang, Mater. Sci. Eng. 134 (2006) 9. M. Wan, Adv. Mater. 20 (2008) 2926. H.D. Tran, D. Li, R.B. Kaner, Adv. Mater. 21 (2009) 1487. D. Li, J. Huang, R.B. Kaner, Acc. Chem. Res. 42 (2009) 135. F. Gu, L. Zhang, X. Yin, L. Tong, Nano Lett. 8 (2008) 2757. C.O. Baker, B. Shedd, P.C. Innis, P.G. Whitten, G.M. Spinks, G.G. Wallace, R.B. Kaner, Adv. Mater. 20 (2008) 155. K. Wang, J.Y. Huang, Z.X. Wei, J. Phys. Chem. C 114 (2010) 8062. S.Y. Lee, G.R. Choi, H. Lim, K.M. Lee, S.K. Lee, Appl. Phys. Lett. 95 (2009) 079901. X. Guo, G.T. Fei, H. Su, L.D. Zhang, J. Phys. Chem. C 115 (2011) 1608. Y. Xia, Y. Xiong, B. Lim, S.E. Skrabalak, Angew. Chem. Int. Ed. 48 (2008) 60. K.E. Hinda, R.P. Socha, G.D. Sulka, J. Phys. Chem. C 117 (2013) 19382. S.K. Pillalamarri, F.D. Blum, A.T. Tokuhiro, M.F. Bertino, Chem. Mater. 17 (2005) 5941. R.J. Tseng, J. Huang, J. Ouyang, R.B. Kaner, Y. Yang, Nano Lett. 5 (2005) 1077. Z. Chen, L. Xu, W. Li, M. Waje, Y. Yan, Nanotechnology 17 (2006) 5254. B.J. Gallon, R.W. Kojima, R.B. Kaner, P.L. Diaconescu, Angew. Chem. Int. Ed. 46 (2007) 7251. S. Guo, S. Dong, E. Wang, Small 5 (2009) 1869. J. Huang, R.B. Kaner, Angew. Chem. Int. Ed. 43 (2004) 5817. J. Wang, B. Deng, H. Chen, X. Wang, J. Zheng, Environ. Sci. Technol. 43 (2009) 5223. M.M. Ayad, A.A. El-Nasr, J. Phys. Chem. C 114 (2010) 14377.
[22] W.X. Zhang, J. Nanopart. Res. 5 (2003) 323. [23] R.A. Crane, T.B. Scott, J. Hazard. Mater. 211–212 (2012) 112. [24] L.C. Roberts, S.J. Hug, T. Ruettimann, M. Billah, A.W. Khan, M.T. Rahman, Environ. Sci. Technol. 38 (2004) 307–315. [25] A. Zhitkovich, Chem. Res. Toxicol. 24 (2011) 1617. [26] B. Wang, H. Wu, L. Yu, R. Xu, T.T. Lim, X.W. Lou, Adv. Mater. 24 (2012) 1111. [27] R. Dev, N. Mukherjee, S. Ahammed, B.C. Ranu, Chem. Commun. 48 (2012) 7982. [28] Q.S. Gao, S.N. Wang, Y. Tang, C. Giordano, Chem. Commun. 48 (2012) 260. [29] J. Yin, X. Xiang, L. Xiang, X. Zhao, J. Mater. Chem. 20 (2010) 7096. [30] E.T. Kang, K.G. Neoh, K.L. Tan, Prog. Polym. Sci. 23 (1998) 277. [31] S.R. Kanel, J.M. Greneche, H. Choi, Environ. Sci. Technol. 40 (2006) 2045. [32] M. Bhaumik, A. Maity, V.V. Srinivasu, M.S. Onyango, Chem. Eng. J. 181–182 (2012) 323. [33] T. Liu, D.C.W. Tsang, I.M.C. Lo, Environ. Sci. Technol. 42 (2008) 2092. [34] L.S. Zhong, J.S. Hu, H.P. Liang, A.M. Cao, W.G. Song, L.J. Wan, Adv. Mater. 18 (2006) 2426. [35] C.Y. Cao, Z.M. Cui, C.Q. Chen, W.G. Song, W. Cai, J. Phys. Chem. C 114 (2010) 9865. [36] R. Chen, C. Zhi, H. Yang, Y. Bando, Z. Zhang, N. Sugiur, G. Golberg, J. Colloid Interface Sci. 359 (2011) 261. [37] R. Srivastava, J. Colloid Interface Sci. 348 (2010) 600. [38] H.U. Lee, S.C. Lee, Y.C. Lee, S. Vrtnik, C. Kim, S. Lee, Y.B. Lee, B. Nam, J.W. Lee, S.Y. Park, J. Hazard. Mater. 262 (2013) 130. [39] M. Bhaumik, R. McCrindle, A. Maity, Chem. Eng. J. 228 (2013) 506. [40] J. Fan, Y. Guo, J. Wang, M. Fan, J. Hazard. Mater. 166 (2009) 904. [41] J.B. Fei, Y. Cui, X.H. Yan, W. Qi, Y. Yang, K.W. Wang, Q. He, J.B. Li, Adv. Mater. 20 (2008) 452.
