Polyaniline nanofibers and nanocomposites: Preparation, characterization, and application for Cr(VI) and phosphate ions removal from aqueous solution

2.1 Instrumentation

Water bath shaker type Lab. Companion BS-11, digital scale KERN-ABS, UV–visible spectrometer, CARY 100 Conc, pH meter type Trans BP 300, Scanning Electron Microscope (SEM) model Philips XL series 30, Shimadzu 8400 FTIR and Shimadzu-XRD 6000 were employed in this work.

2.2 Materials

Ammonium peroxydisulphate (APS), hydrochloric acid, sodium hydroxide and potassium dihydrogen phosphate were of analytical grade and used as received, aniline was double distilled under vacuum pressure; deionized water was used throughout this work. Powdered PET was obtained by grinding waste bottles in a small grinder after shredding to a small pieces, the PET powder was washed several times with deionized water then acetone. PET powder was sieved with ⩽500 μm. Natural silica (S) of particle size 150–200 μm was washed with deionized water and dried, acid treated natural silica (SA) was prepared by treating dried natural silica with 0.1 M HCl for overnight then washed with deionized water and dried, fiber glass was obtained by grinding chopped strand fiber glass, then treated with acetone for 3 h and dried.

2.3 Preparation of PANI nanofiber

PANI nanofibers were prepared by two methods: interfacial and rapid mixing polymerizations (Jiazing and Richard, 2004). In the former toluene was used as an organic phase, in which aniline was dissolved, and aqueous phase containing APS and the doping acid (hydrochloric acid 1 M). The aniline was polymerized at the interface between the two phases. In the latter the polymerization was performed by rapid mixing of two solutions, aniline in acidic deionized water and APS in acidic deionized water into a beaker and the mixture was stirred moderately with a magnetic bar at 5–10 °C. In both methods the polymerization lasted for 3 h, the doped PANI was filtered and washed with plenty of distilled water then with ethanol and acetone to remove all unreacted aniline, oligomers and impurities, then dried.

2.4 Preparation of nanocomposites

The polyaniline nanofiber composites were prepared by in situ polymerization of aniline in the presence of S, SA, FG and PET in hydrochloric acid solution by rapid addition of a specified amount of APS in 1 M HCl to the mixture then was moderately stirred using magnetic bar. After 3 h, the product was filtered and washed with distilled water then ethanol and acetone to remove unreacted aniline, oligomers and impurities then dried in an oven at 80 °C for 6 h and stored in a sealed container.

2.5 Adsorption experiments

A stock solution of potassium dichromate and potassium dihydrogen phosphate of 1000 mg/L was prepared separately. From these stock solutions different concentrations of Cr(VI) and phosphate were prepared by dilution of specified volume of the stock solution. All batch experiments were carried out by mixing 50 ml of certain concentration of Cr(VI) or phosphate with certain amount of nanocomposite in a water bath shaker for definite time interval. At the end of each adsorption experiment the solution was made alkaline at pH ⩾ 12, and the residual Cr(VI) was determined using UV–Visible spectrophotometer (Ansari, 2006), residual phosphate was also determined spectrophotometrically after complexation with ammonium molybdate and sodium sulfide in concentrated sulfuric acid. For calibrating the UV–Visible spectrophotometer, standard samples of dichromate and phosphate were prepared and the corresponding absorption for each concentration was measured at λ_max 372 nm and λ_max 715 nm, respectively. The linear regression curve was drawn between the absorption and concentration for each sample, Figs. 2 and 3. The adsorption capacity of the nanocomposites for Cr(VI) and phosphate was calculated by the following equation:

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Figure 2

Calibration curve between the concentration of dichromate and absorbance at λ_max 372 nm.

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Figure 3

Calibration curve between the concentration of phosphate ion and absorbance at λ_max 715 nm.

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3.1 Characterization of PANI nanofiber

Pure polyaniline nanofibers were prepared by interfacial and rapid mixing methods, whereas, the PANI nanocomposites were prepared by rapid mixing method. Analysis of the prepared nanofibers was carried out by Scanning Electron Microscope, X-ray diffraction and FTIR. The presence of the characteristic absorption peaks of PANI in the FTIR spectrum indicates the successful polymerization of aniline via interfacial or rapid mixing methods and nanocomposites, as shown in Fig. 4 and Table 1.

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Figure 4

FTIR spectrum of PANI nanofiber by interfacial polymerization.

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The X-ray diffraction confirms the formation of PANI nanofiber, two characteristic peaks around 2θ = 20° and 2θ = 25° presented in the XRD pattern of the PANI nanofiber Fig. 5, the same XRD pattern was obtained by other researchers (Yangyong and Xinli, 2007). According to the studies reported by (Lunzy and Banka, 2000; Banka and Lunzy, 1999) and (Chaudhari and Kelkar, 1996) the nanofibers are in partial crystallinity. The partial crystallinity of polyaniline nanofiber may be due to the amine and imine groups in the structure of doped PANI, which can form stronger intermolecular and intramolecular hydrogen bonds. SEM images of PANI nanofiber after completion of polymerization by interfacial and rapid-mixing techniques as well as nanocomposites, are clearly shown in Fig. 6, with PANI nanofibers of diameter of 30–80 nm.

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Figure 5

XRD of polyaniline nanofiber.

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Figure 6

SEM images of PANI nanofiber (A) interfacial (B) rapid mixing (C) PANIFG (D) PANIPET (E) PANISA (F) SA.

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3.2 Adsorption of Cr(VI) and phosphate

The adsorption is the affinity of interaction between the adsorbent active sites on its surface and the adsorbate molecules. The fillers S, SA, PET powder, and FG powder were used to prepare the PANI nanocomposites. The pH of the adsorption medium is an important factor that controlled the adsorption process; therefore, the effect of pH on adsorption was investigated. It was observed that maximum removal of Cr(VI) was obtained at pH 2 onto all nanocomposites, while, the maximum removal of phosphate was obtained at pH 1 onto PANIS and PANISA. Generally the removal percentage decreased with increasing pH for adsorption of both adsorbates onto nanocomposites. The removal percentage of Cr(VI) onto PANIS was reduced from 95% at pH 2 to 34% at pH 6, from 99% at pH 2 to 55% at pH 6 onto PANISA, from 84% at pH 2 to 36% at pH 6 onto PANIFG and from 84% at pH 2 to 34% at pH 6 onto PANIPET. Whereas, the removal percentage of phosphate by PANIS was reduced from 21% at pH 1 to 7% at pH 6 and from 40% at pH 1 to 16% at pH 6 by PANISA. The higher removal percentage of Cr(VI) onto PANISA is attributed to the pores and channels present in natural silica that were opened after treating with acid, Fig. 6F, which gave the chance for PANI nanofiber to enter these pores and channels and in turn, increase the surface area of PANI nanofiber, to whom the higher adsorption capacity belongs. S, SA, and PET waste were also explored as adsorbent for Cr(VI) and phosphate ions, it was found that there was a very low adsorption of these adsorbates onto the surfaces of S, SA, and PET, while pure PANI nanofiber alone exhibits more than 99% removal for Cr(VI). These experiments gave an indication that the process occurred on the PANI surfaces only.

It seems that the mechanism of Cr(VI) and phosphate ions’ sorption onto nanocomposites mostly occurred via anion exchange process. When the HCl doped PANI is treated with an aqueous solution of Cr(VI) or phosphate ( ${\text{PO}}_4^{-3}$ ) ions in acidic media, the chloride ion (mobile dopant anion) in the polymer is readily exchanged for chromate or phosphate anions, so the removal of Cr(VI) or phosphate results. At pH ⩽ 2 the dichromate is converted to chromate ${\text{HCrO}}_4^{-}$ ions as follows: $${\text{Cr}}_2{\text{O}}_4^{-2}+2{\text{H}}^{+}⟶2{\text{HCrO}}_4^{-}$$ Then the nanocomposite surfaces can remove chromate anions as follows: $${\text{PANI}}^{+}{\text{Cl}}^{-}(\text{polymer})+{\text{HCrO}}_4^{-}(\text{solution})⟶{\text{PANI}}^{+}{\text{HCrO}}_4^{-}(\text{polymer})+{\text{Cl}}^{-}(\text{solution})$$ Exchangeable anions Cl⁻ in the PANI exist only in the acid doped state, so undoped PANI does not adsorb Cr(VI) or phosphate ions, Fig. 1.

The phosphate uptake obviously increased with the decrease of pH. When the pH is low the positive charge on the surface of PANI is concentrated leading to static electricity force between positive PANI surfaces and negative phosphate ions, then the mechanism of phosphate ion removal may be due to electrostatic force besides interaction of ion-exchange (Long et al., 2011; Kanwal et al., 2012). At low pH the phosphate anions can be present in solution as ${\text{HPO}}_4^{-2}$ or ${\text{H}}_2{\text{PO}}_4^{-}$ as follows: $${\text{PO}}_4^{-3}+{\text{H}}^{+}⟶{\text{HPO}}_4^{-2}$$ $${\text{HPO}}_4^{-2}+{\text{H}}^{+}⟶{\text{H}}_2{\text{PO}}_4^{-}$$ The low removal percentage of phosphate ion ${\text{H}}_2{\text{PO}}_4^{-2}$ onto nanocomposites in comparison to chromate anion ${\text{HCrO}}_4^{-}$ can be attributed to the difficulty in replacing monovalent anion Cl^- for divalent anion ${\text{H}}_2{\text{PO}}_4^{-2}$ unless more doped PANI is required, as follows: $${\text{PANI}}^{+}{\text{Cl}}^{-}(\text{polymer})+{\text{HPO}}_4^{-2}(\text{solution})⟶{({\text{PANI}}^{+})}_2{\text{HPO}}_4^{-2}(\text{polymer})+{\text{Cl}}^{-}(\text{solution})$$ The effect of contact time and nanocomposite weight for Cr(VI) and phosphate adsorption were also investigated. All nanocomposites take about 90 min to reach equilibrium and concentration of 10 g/L for Cr(VI) adsorption, except PANISA, where the adsorption was very rapid, and takes less than 2 min and 2 g/L of adsorbent to attain equilibrium Table 2. This behavior is attributed to the large surface area of PANISA, due to, an increase in number of pores and channels in the structure of PANISA after treatment with acid, these pores and channels are clearly shown in Fig. 6F. The weight and time effect on the adsorption of phosphate ions are presented in Table 3. The effect of adsorbent weight was studied by varying the nanocomposite weight and keeping all the other experimental variables constant.

3.3 Adsorption kinetics

In order to obtain further insight into the mechanism of the adsorption of Cr(VI) and phosphate onto nanocomposites, the experimental data were regressed against the Pseudo-first order model represented by the following linear equation (Long et al., 2011):

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Figure 7

Second-order plot of adsorption of phosphate onto nanocomposites.

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Figure 8

Second-order plot of adsorption of Cr(VI) onto nanocomposites.

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