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ARTICLE pubs.acs.org/JPCC

High-Performance and Reproducible Polyaniline Nanowire/Tubes for Removal of Cr(VI) in Aqueous Solution Xiao Guo, Guang Tao Fei,* Hao Su, and Li De Zhang Key Laboratory of Materials Physics and Anhui Key Laboratory of Nanomaterials and Nanostructures, Institute of Solid State Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, P.O. Box 1129, Hefei 230031, People’s Republic of China ABSTRACT: In this Article, bulk-quantity one-dimensional polyaniline (1D PANI) nanowire/tubes with rough surface were prepared by a simple chemical oxidation method. This kind of PANI nanostructure can not only remove Cr(VI) rapidly and effectively in one step from aqueous solution by reducing Cr(VI) to Cr(III) as well as adsorbing the reduced Cr(III) simultaneously, but also be easily regenerated for reuse. During the removal of Cr(VI) process, the as-synthesized PANI was oxidized from emeraldine salt to pernigraniline, and pernigraniline could be reconverted into emeraldine salt by acid treatment. In addition, the morphology of the PANI was not changed after used for Cr(VI) removal. This study not only provides a facile way to fabricate bulk-quantity 1D PANI nanostructure, but also shows a reproducible material for removal of toxic Cr(VI) from wastewater.

1. INTRODUCTION Huge quantity of chromium is discharged into the environment for its wide applications in chromium plating, leather tanning, textile dyeing, printing inks, wood preservation, paints, and pigments.1,2 Usually, chromium exists in hexavalent [Cr(VI)] and trivalent [Cr(III)] forms in aqueous environment. Cr(VI) is highly toxic, carcinogenic, mutagenic, teratogenic to living organisms, and extremely mobile.3,4 In contrast, Cr(III) is less toxic and can be easily removed by adsorption or precipitation.4 Therefore, the strategy based on Cr(VI) reduction and consequent precipitation was often used for Cr(VI) removal. However, in the Cr(VI) reduction process, great quantities of reductants, such as ferrous sulfate or sulfur dioxide, are consumed and difficult to be cycled back for reuse. Besides, these reductants always generate large amounts of secondary waste products.5 On the other hand, in the Cr(III) removal process, large amounts of acids or bases have to be used to adjust the solution pH for forming Cr(OH)3 precipitation. Thus, on the basis of the viewpoint of practical application, an environment-friendly material that can fast and effectively remove Cr(VI) in a facile way and is easy to regenerate is desired. Since Rajeshwar et al.6 reported for the first time that the electrosynthesized polypyrrole films could reduce Cr(VI) effectively in 1993, conducting polymers used for removing Cr(VI) have been intensively studied due to the possibility of recycling and poor solubility.7-9 Ruotolo et al.10 proved that polyaniline (PANI) was superior to polypyrrole and more suitable for Cr(VI) removal because of its higher reaction rate and better stability. Consequently, there is increasing concern over the use of PANI r 2011 American Chemical Society

for Cr(VI) removal.3,5,10,11 However, most of the previous works focus on conducting polymer films, which are difficult to prepare in large amount in a short time. On the other hand, films always have small specific surface areas, which may result in the relatively low activity because the reaction mainly occurs at the surface. PANI nanostructure with rough surface may be a good candidate for large-scale removal of Cr(VI) because of its large specific surface area, low-cost, and easy bulk production.12-15 In addition, the removal of Cr(III) as the product of Cr(VI) reduction was not studied. In this Article, we prepared bulk-quantity onedimensional (1D) PANI nanowire/tubes with rough surface and demonstrated that this PANI nanostructure can not only remove Cr(VI) rapidly and effectively in one step from aqueous solution by reducing Cr(VI) to Cr(III) as well as adsorbing the reduced Cr(III) simultaneously, but also be easily regenerated for reuse.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Analytical grade aniline, ammonium persulfate (APS), tartaric acid, and potassium dichromate were obtained commercially and used without further purification. 2.2. Preparation and Characterization of PANI Nanowire/ Tubes. The bulk-quantity 1D PANI nanowire/tubes were prepared as follows: analytical grade aniline (4 mmol) and tartaric Received: September 25, 2010 Revised: December 14, 2010 Published: January 7, 2011 1608

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The Journal of Physical Chemistry C

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acid (1 mmol) were dispersed in 10 mL of deionized water at room temperature with magnetic stirring for 15 min. Next, the aniline solution was cooled in an ice-water bath for 5 min. After that, 10 mL of oxidant aqueous solution containing 4 mmol of APS precooled in the ice-water bath for 5 min was poured into the aniline solution, and the polymerization reaction was carried out in the ice-water bath without agitation. After 12 h, the resulting dark green PANI was washed with deionized water and ethanol several times and was dried in a dynamic oven at 60 °C. The morphology and molecular structure of the as-synthesized PANI were characterized by field-emission scanning electron microscopy (FESEM, Sirion 200), transmission electron microscopy (TEM, JEOL-2010), and Fourier transform infrared spectroscopy (FTIR, JASCO FT-IR 410 spectrophotometer). 2.3. Applications of the As-Synthesized PANI for Cr(VI) Removal. The Cr(VI) solution was prepared by dissolving potassium dichromate (K2Cr2O7) into deionized water. The pH values of solution were well adjusted by KOH and H2SO4 with a pH meter (Mettler Toledo SG2-ELK). For the kinetics experiment, the as-synthesized PANI (18 mg) was ultrasonically dispersed into 30 mL of Cr(VI) solution at pH 5. Next, an appropriate amount of the reaction solution was taken out at predetermined intervals and centrifuged quickly. The supernatant liquids were used for analyzing the Cr concentration by inductively coupled plasma emission spectrometer (ICP, Thermo Icap 6300). For the effect of pH values and Cr(VI) concentration of solution on the removal capacity experiment, the as-synthesized PANI powder (6 mg) was ultrasonically dispersed into 10 mL of Cr(VI) solution with different pH values and concentrations. After reaction for 1 h, the reaction solution was centrifuged, and the supernatant liquid was used for Cr concentration analysis by ICP. The previous reports showed that Cr(III) can be removed by adjusting solution pH in the range of 7-9 to form Cr(OH)3 precipitation.16,17 In contrast, Cr(VI) species are soluble over the entire pH range.4 So in this study before the ICP examination the pH of the appropriate amount of obtained supernatant liquid was adjusted in the range of 7.5-8. After pH adjustment, the Cr concentration of the remaining supernatant liquid was examined by ICP, which was close to the Cr(VI) concentration of the reaction solution. Also, the Cr concentration of the supernatant liquid without pH adjustment is the sum of Cr(VI) and Cr(III) concentrations, that is, the total Cr concentration. Therefore, the Cr(VI) removal and reduction capacities of the as-synthesized PANI can be quantified and calculated using the equation shown as follows: Q ¼

ðC o - C e ÞV m

ð1Þ

where V (L) is the volume of Cr(VI) solution, m (g) is the mass of the as-synthesized PANI, Co (mmol/L) is the initial Cr(VI) concentration, and Ce is the total Cr or Cr(VI) concentration of the solution after reaction 1 h, corresponding to Q (mmol/g) for the removal or reduction capacity, respectively. Finally, the used PANI nanostructure was separated and rinsed several times with deionized water and dried for X-ray photoelectron spectroscopy (XPS, ESCALAB 250) analysis to ascertain the oxidation state of chromium adsorbed on PANI nanostructure.

3. RESULTS AND DISCUSSION Figure 1a shows that the as-synthesized PANI is composed of uniform 1D nanostructures with an average diameter of about

Figure 1. (a) SEM and (b) TEM images of the as-synthesized PANI.

Figure 2. FTIR spectrum of the as-synthesized PANI.

180 nm, and Figure 1b reveals that these 1D nanostructures consist of both nanowires and nanotubes with very rough surface constituted by nanoaciculaes, which indicates that large surface area is possessed by this kind of PANI nanowire/tubes. The molecular structure of the as-synthesized PANI was characterized with FTIR shown in Figure 2. The strong absorptions at 1577, 1496, 1300, and 1137 cm-1 are assigned to the stretching vibrations of C-N in NdQdN, N-B-N, B-NH-B, and B-NHþdQ (Q, quinoid ring; B, benzenoid ring), respectively. The relative absorption intensities at 1577 and 1137 cm-1 correspond to the extent of oxidation and protonation, respectively.18,19 These prove that the as-synthesized PANI is in emeraldine salt form, which is the protonated intermediate oxidation state of PANI. The as-synthesized PANI was directly used to remove Cr(VI) in aqueous medium with initial Cr(VI) concentration of 0.8 mmol/L. Figure 3 shows the changes of the remaining total Cr and Cr(VI) concentrations in the solution at different times. It is obvious that both the remaining total Cr and Cr(VI) concentrations decrease sharply from 0.8 to about 0.1 mmol/L in 5 min and then decrease slightly. In addition, the fact that the concentrations of Cr(VI) are slightly lower than that of the total chromium indicates that there exists a little Cr(III) in the solution. These reveal that the as-synthesized PANI can effectively and quickly remove Cr(VI) in aqueous solution in one step. The fast removal kinetics is possibly attributed to the large surface area of this kind of PANI 1609

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The Journal of Physical Chemistry C nanostructure as the Cr removal action mainly occurs on the surface of the PANI nanostructure. The element mapping analysis (Figure 4a) of the PANI after reaction with Cr(VI) solution at pH 5 exhibits that the used PANI contains Cr element besides C and N elements, which directly demonstrated that the lost Cr in solution was adsorbed

Figure 3. Changes of the Cr concentration at different times (initial Cr(VI) concentration: 0.8 mmol/L; solution pH at 5).

Figure 4. (a) Element mapping and (b) Cr 2p XPS spectrum of the PANI after reaction with Cr(VI) for 1 h (initial Cr(VI) concentration: 0.8 mmol/L; solution pH at 5).

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by PANI. Figure 4b shows the XPS spectrum of the used PANI. The fact that the binding energy of the Cr 2p3/2 locates at 577.3 eV confirms that the adsorbed Cr is in Cr(III) form.20,21 All of these imply that the as-synthesized PANI is a good candidate for Cr(VI) removal in one step by Cr(VI) reduction coupled with Cr(III) adsorption. The effect of solution pH on the Cr(VI) removal and reduction capacities of the as-synthesized PANI was also investigated. Figure 5a shows the changes of the total Cr and Cr(VI) concentrations in solution after reaction for 1 h at different pH values. The corresponding removal and reduction capacities were shown in Figure 5b, which were calculated from the values in Figure 5a by eq 1. The result displays that both the Cr(VI) reduction capacity and the reduced Cr(III) absorption capacity (removal capacity) of PANI are pH-dependent. The obvious increase of Cr(VI) concentration with the increase of pH from 1 to 12 (Figure 5a) shows that the reduction capacity of the assynthesized PANI decreases with the decrease of acidity (Figure 5b). However, it is worth noting that the PANI still presents good reduction efficiency when the pH increases up to 9; that is, the assynthesized PANI is suitable for Cr(VI) reduction in a wide pH range. Contrarily, the sharp decrease of total Cr concentration with pH increasing from 1 to 5 (Figure 5a) shows that the Cr(III) absorption capacity of PANI increases with pH increasing (Figure 5b). Above pH 5, the concentration of total Cr has an increase tendency similar to that of Cr(VI), and the difference between them is slight, suggesting that most of the reduced Cr(III) was removed from solution. To further understand the effect of pH on the Cr(VI) removal performance, the FTIR spectra of PANI treated under different conditions were characterized as shown in Figure 6. The relative absorption intensity at 1577 cm-1 in spectrum a of Figure 6 is higher than that of the as-synthesized PANI shown in Figure 2, indicating that the oxidation extent of PANI increased after used for removing Cr(VI) at pH 5. It is also much greater than the absorption intensity at 1496 cm-1, suggesting that some PANI has been oxidized from the emeraldine salt form to pernigraniline form (the highest oxidation state of PANI) during the Cr(VI) removal process.22 In addition, the acid chromate (HCrO4-) form is the predominating species of Cr(VI) in the pH range of 2-6.23 Therefore, in the Cr(VI) solution at pH 5, the main reaction proceeding between Cr(VI) and the as-synthesized PANI can be proposed as follows: 2HCrO4 - þ 14Hþ þ 3PANIo f 2Cr3þ þ 3PANI2þ þ 8H2 O ð2Þ

Figure 5. Effect of solution pH on the changes of the Cr concentration (a) and the corresponding removal and reduction capacities (b) after reaction for 1 h (initial Cr(VI) concentration: 0.8 mmol/L). 1610



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absorption intensity at 1577 cm-1 and the increase of Cr(VI) reduction capacity with pH decreasing. This may be the reason why the PANI synthesized by chemical oxidation of aniline with APS is always in emeraldine salt form,15 although the potential of APS (2.05 V) is much higher than that of Cr(VI) (1.33 V), because a large amount of sulfuric acid was produced in the polymerization process. This further demonstrates that the pernigraniline of PANI can be reduced to emeraldine salt in acidic solution, as shown:

Figure 6. FTIR spectra of the PANI treated under different conditions. Spectrum a, after reducing Cr(VI) in solution at pH 5; spectrum b, after reducing Cr(VI) at pH 3; spectrum c, after reducing Cr(VI) at pH 1; and spectrum d, after reducing Cr(VI) at pH 5 and further treating with 1 M H2SO4 aqueous solution.

Scheme 1. Schematic Illustration of the Conversion between Emeraldine Salt and Pernigraniline of PANI

where PANIo and PANI2þ represent the tetramers of emeraldine salt form and pernigraniline form of PANI, respectively. Also, the oxidation of PANI from emeraldine salt form to pernigraniline form by Cr(VI) can be shown as (A- is the counterion):

With the decrease of solution pH, the relative absorption intensity at 1577 cm-1 decreased obviously; see spectra b and c in Figure 6. This contradicts with the decrease of the concentration of Cr(VI) shown in Figure 5a with pH decreasing, which indicated more PANI should be oxidized to pernigraniline form; that is, this relative absorption intensity should increase. Besides, comparing spectrum c in Figure 6 with the spectrum in Figure 2, it is surprising that the FTIR spectrum of PANI after reaction with Cr(VI) at pH 1 is similar to that of the as-synthesized PANI, implying that they have a similar molecular structure. In addition, the FTIR spectrum d in Figure 6 of the PANI used for Cr(VI) removal at pH 5 and further treated with 1 M H2SO4 is also similar to that of Figure 2. This gives an indication that the used PANI could be easily regenerated by acid treatment. So it can be deduced that in the strong acidic solution of Cr(VI), for example, at pH 1, the Cr(VI) removal process might be accompanied by a chemical reduction process of PANI from pernigraniline form to emeraldine salt form simultaneously, which could well explain the mentioned contradiction between the decrease of relative

which is a reverse process of eq 3. This regeneration phenomenon of PANI from pernigraniline form to emeraldine salt form in acidic aqueous solution quite agrees well with the previous reports that the pernigraniline is unstable under ambient conditions.24 Especially, MacDiarmid et al.25 proved that the treatment of pernigraniline with aqueous 1 M HCl resulted in its reduction to the emeraldine oxidation state with concomitant ring chlorination. So we think that the conversion between the emeraldine salt and pernigraniline in this study can be shown as Scheme 1. From spectrum a to spectrum c in Figure 6, the obvious increase of the relative absorption intensity at 1137 cm-1 indicates that the protonation extent of the used PANI increased when the pH decreased from 5 to 1. Meanwhile, the reduced Cr(III) dominantly existing in monovalent Cr(OH)2þ form at neutral pH range (pH 5.5-7) is gradually changed into divalent Cr(OH)2þ (pH 4-5.5) and Cr3þ (pH below 4).17 Hence, the electrostatic repulsion between the used PANI and the reduced Cr(III) increased significantly with pH decreasing, resulting in the decrease of adsorption of the reduced Cr(III). This agrees well with the larger adsorption of the reduced Cr(III) at higher pH in acidic solution. However, from the viewpoint of electrostatics, the positively charged Cr(III) species are still unfavorable to be absorbed on the PANI. So we think that the chelation interaction should be the factor that overcomes the electrostatic repulsion between the Cr(III) species and PANI at pH below 7. Furthermore, the oxidizing power of Cr(VI) in solution is also determined by its concentration apart from the solution pH values.5 So it is also important to investigate the effect of Cr(VI) concentration on its removal by the as-synthesized PANI. The experiment was carried out at pH 5 with various initial concentrations of Cr(VI) because Cr(VI) can be well removed by the assynthesized PANI at this pH value as shown in Figure 5. Figure 7a shows the changes of the remaining total Cr and Cr(VI) concentrations versus initial Cr(VI) concentration (Co). The nonlinear variations of the remaining total Cr and Cr(VI) concentrations with Co suggest that the Cr(VI) concentration would have an important effect on the removal and reduction capacities of the as-synthesized PANI. To better understand the effect of initial Cr(VI) concentration on the removal performance, the removal and reduction capacities (Q) as a function of Co are shown in Figure 7b, which are calculated from the values in Figure 7a by eq 1. It can be clearly seen that the removal and reduction capacities have a similar variation tendency and increase with Co. However, when Co increases to 1.7 mmol/L, the removal and reduction 1611



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Figure 7. (a) Changes of Cr concentration and (b) the corresponding removal and reduction capacities of the as-synthesized PANI versus initial Cr(VI) concentration after reaction for 1 h at pH 5.

method. This kind of PANI nanostructure can rapidly and effectively remove Cr(VI) in one step from aqueous solution over a suitable pH range, although the capacities of the Cr(VI) removal and reduction were highly pH-dependent. Besides, the concentration of Cr(VI) also has a significant effect on the removal and reduction capacities, the maxima of which are 1.63 and 1.67 mmol/g in the solution at pH 5, respectively. More importantly, the used PANI can be easily regenerated, maintaining almost the same Cr(VI) removal capacity and morphology. This study not only provides a facile way to fabricate bulk-quantity PANI nanostructure, but also shows a reproducible material for removal of toxic Cr(VI) from wastewater.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ86-551-5591453. Fax: þ86-551-5591434. E-mail: gtfei@ issp.ac.cn. Figure 8. (a) SEM and (b) TEM images of the PANI after being used for Cr(VI) removal at solution pH 5 for two cycles.

capacities, respectively, reach 1.63 and 1.67 mmol/g and change insignificantly when further increasing Co. This is possibly ascribed to the full oxidation of PANI from emeraldine salt to pernigraniline. Besides, the removal capacity is just slightly lower than the reduction capacity; that is, the reduced Cr(III) was almost adsorbed on the PANI. This further proves that the assynthesized PANI can be used for Cr(VI) removal in one step from aqueous solution. Additionally, the result of using the regenerated PANI to remove Cr(VI) at pH 5 with the initial concentration of 0.8 mmol/L shows that the Cr(VI) concentration could still be reduced to about 0.1 mmol/L and almost all the reduced Cr(III) was absorbed, which means the capacities of reduction and absorption of the regenerated PANI decreased insignificantly. The morphology of PANI after used two cycles is shown in Figure 8. The SEM and TEM images show that the used PANI still consists of uniform nanowires and nanotubes, which is similar to the as-synthesized PANI shown in Figure 1. These confirm that the synthesized PANI is a reproducible material for Cr(VI) removal.

4. CONCLUSIONS In summary, bulk-quantity PANI nanowire/tubes with rough surface were successfully fabricated by a facile chemical oxidation

’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 11074254), the Ministry of Science and Technology of China (No. 2005CB623603), the Hundred Talent Program of the Chinese Academy of Sciences, and the President Foundation of the Hefei Institute of Physical Sciences. ’ REFERENCES (1) Khezami, L.; Capart, R. J. Hazard. Mater. 2005, 123, 223–231. (2) Lakshmipathiraj, P.; Raju, G. B.; Basariya, M. R.; Parvathy, S.; Prabhakar, S. Sep. Purif. Technol. 2008, 60, 96–102. (3) Farrell, S. T.; Breslin, C. B. Environ. Sci. Technol. 2004, 38, 4671– 4676. (4) Lin, C. J.; Wang, S. L.; Huang, P. M.; Tzou, Y. M.; Liu, J. C.; Chen, C. C.; Chen, J. H.; Lin, C. Water Res. 2009, 43, 5015–5022. (5) Olad, A.; Nabavi, R. J. Hazard. Mater. 2007, 147, 845–851. (6) Wei, C.; German, S.; Basak, S.; Rajeshwar, K. J. Electrochem. Soc. 1993, 140, L60–L62. (7) Wang, Y.; Rajeshwar, K. J. Electroanal. Chem. 1997, 425, 183–189. (8) Alatorre, M. A.; Gutierrez, S.; Paramo, U.; Ibanez, J. G. J. Appl. Electrochem. 1998, 28, 551–557. (9) Rodriguez, F. J.; Gutierrez, S.; Ibanez, J. G.; Bravo, J. L.; Batina, N. Environ. Sci. Technol. 2000, 34, 2018–2023. (10) Ruotolo, L. A. M.; Liao, A.; Gubulin, J. C. J. Appl. Electrochem. 2004, 34, 1259–1263. (11) Ruotolo, L. A. M.; Gubulin, J. C. J. Appl. Electrochem. 2003, 33, 1217–1222. (12) Huang, J. X.; Kaner, R. B. J. Am. Chem. Soc. 2004, 126, 851–855. (13) Chiou, N. R.; Epstein, A. J. Adv. Mater. 2005, 17, 1679–1683. 1612



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