J. Phys. Chem. B 2008, 112, 10153–10157
10153
Adsorption of Sulfonated Dyes by Polyaniline Emeraldine Salt and Its Kinetics Debajyoti Mahanta,† Giridhar Madras,‡ S. Radhakrishnan,§ and Satish Patil*,† Solid State and Structural Chemistry Unit and Department of Chemical Engineering, Indian Institute of Science, Bangalore 560012, India, and Polymer Science and Engineering, National Chemical Laboratory, Pune 411008, India ReceiVed: May 2, 2008; ReVised Manuscript ReceiVed: May 31, 2008
A method for the removal of anionic (sulfonated) dyes from aqueous dye solutions using the chemical interaction of dye molecules with polyaniline is reported. Polyaniline (PANI) emeraldine salt was synthesized by chemical oxidation. Sulfonated dyes undergo chemical interactions with the charged backbone of PANI, leading to significant adsorption of the dyes. This phenomenon of selective adsorption of the dyes by PANI is reported for the first time and promises a green method for removal of sulfonated organics from wastewater. The experimental observations from UV-vis spectroscopy, X-ray diffraction, and conductivity measurements rule out the possibility of secondary doping of polyaniline salt by sulfonated dye molecules. A possible mechanism for the chemical interaction between the polymer and the sulfonated dye molecules is proposed. The kinetic parameters for the adsorption of sulfonated dyes on PANI are also reported. Introduction In the past two decades, there has been growing interest in the field of conducting polymers in both academia and industry.1–4 Conducting polymers have a wide range of attractive applications in optoelectronic devices such as light-emitting diodes,5 field-effect transistors,6 and organic solar cells.7 Because of their low cost of synthesis and easy processability, these polymers are becoming the most promising new materials for nextgeneration electronic devices.8 Polyaniline (PANI) is one of the most extensively used and studied conducting polymers.9–11 The major practical advantages of polyaniline are its high environmental stability, high electrical conductivity, and easy synthesis. Polyaniline has versatile applications in plastic batteries, optical storage lithography, harmonic generators, display devices, magnetic recording, solid-state sensors, and corrosion inhibitors.12–14 PANI doped with Pd, Cu, and Pd/Cu has been used for the electrochemical oxidation of methanol and formic acid.15 The dyes from textile sources are major sources of environmental pollution because they are nonbiodegradable.16 Many methods such as flocculation,17 reverse osmosis,18 and activated carbon adsorption19 have been used in wastewater treatment. However, photocatalysis is often used as a technique to degrade dyes because it is simple and cost-effective.20 Conventionally, inorganic semiconductors are used as photocatalysts, and TiO2, doped TiO2, and ZnO are extensively used for the degradation of dyes in wastewater.21 Conducting polymers have band gaps in the same range as inorganic semiconductors. Further, the band gaps of these polymers can be tuned by chemical manipulation of the backbone. Therefore, conducting polymers with suitable band gaps could, in principle, act as photocatalysts for dye degradation. Recently, we reported the application of conducting polymers such as poly(3-hexylthiophene) (P3HT) and poly[2methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (MEHPPV) as photocatalysts for the degradation of various textile * To whom correspondence should be addressed. Tel.: +91-80- 22932651. Fax:+91-80-23601310. E-mail:
[email protected]. † Solid State and Structural Chemistry Unit, Indian Institute of Science. ‡ Department of Chemical Engineering, Indian Institute of Science. § National Chemical Laboratory.
dyes.22 The application of conducting polymer nanocomposites for photocatalysis has recently been studied. The nanocomposite of polypyrrole and TiO2 nanoparticles was reported to exhibit higher photoctalytic activity than a suspension of TiO2 nanoparticles.23 However, in this study, we report that polyaniline can remove anionic (sulfonated) dyes without the application of UV and visible light through a chemical interaction with the sulfonated dyes. Thus, the objective of the article is to examine the use of polyaniline for the removal of various sulfonated dyes from aqueous solutions and propose a mechanism of chemical interactions between the sulfonated dyes and polyaniline emeraldine salt. Experimental Section Materials. Aniline (S.D. Fine Chemicals Ltd., Mumbai, India) was purified by distillation before use. Ammonium persulfate and hydrochloric acid were obtained from S.D. Fine Chemicals Ltd., Mumbai, India, and used without any further purification. The dyes Orange G (OG), Methylene Blue (MB), Rhodamine B (RB) (all from S.D. Fine Chemicals Ltd., Mumbai, India), Alizarine Cyanine Green (AG, Rolex Laboratory Reagents, Mumbai, India), Coomassie Brilliant Blue R-250 (CBB, Merck, Mumbai, India), Remazol Brilliant Blue R (RBBR, ColourChem Ltd., Ahmedabad, India) were also used as received. Adsorption Reactions. Emeraldine salt (ES) of PANI was synthesized by the conventional route using a 1:1 molar ratio of aniline to oxidant in an acidic medium (0.1 M HCl). After filtration and washing, the product was further doped with hydrochloric acid to achieve the maximum doping. The emeraldine base (EB) was obtained by treatment of the emeraldine salt with ammonium hydroxide. For adsorption experiments, 0.1 g of the desired material (ES or EB) was added to 100 mL of different dye solutions having different concentrations ranging from 50 to 500 ppm. These solutions were stirred for 2 h. During this process, samples were collected from the reaction beaker at different time intervals, and the concentration of the dye was determined by UV-vis absorption spectroscopy. The dye concentrations were calibrated with the Beer-Lambert law at
10.1021/jp803903x CCC: $40.75 2008 American Chemical Society Published on Web 07/30/2008
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Figure 1. Concentration profiles of sulfonated and nonsulfonated dyes in the presence of PANI emeraldine salt.
Figure 3. UV-vis spectra of Orange G, PANI emeraldine salt, and PANI emeraldine salt containing Orange G (50 and 100 ppm).
Figure 2. Concentration profiles of Orange G in the presence of polyaniline emeraldine salt. The inset shows the change in color of Orange G in the presence of PANI emeraldine salt in aqueous solution.
Figure 4. Concentration profiles of various sulfonated dyes in the presence of PANI emeraldine base.
λmax values of 480, 664, 640, 555, 591, and 554 nm for OG, MB, AG, CBB, RBBR, and RB, respectively. After adsorption of the dye, the PANI samples were washed with distilled water and dried. These samples were then characterized by UV-vis spectroscopy, X-ray diffraction (XRD), and conductivity. The XRD patterns were recorded on a Philips X’pert Pro Diffractometer with Cu KR radiation in the 2θ range from 20° to 80° at a scanning rate of 1°/min. The conductivity measurements were carried out by a two-probe technique with a Keithley model 614 electrometer. The UV-vis measurements were carried out with a Perkin-Elmer Lambda 35 UV-visible spectrophotometer using a cell with a 1-cm optical path length. The spectrum was recorded in the wavelength range of 300-900 nm. Results and Discussions Experiments were conducted with a wide range of dyes, encompassing various classes of dyes including heteropolyaromatic (MB), azo (OG), xanthane (RB), and anionic sulfonated (OG, CBB, RBBR, AG) dyes. Figure 1 shows the variation of the dye concentration with time in the presence of doped PANI. All experiments were carried out for 2 h, but data are shown only for 1 h because there was no significant adsorption after this time. There was no significant adsorption of the nonsulfonated (cationic) dyes such as Methylene Blue and Rhodamine B, suggesting possible chemical interactions between the polyaniline salt and sulfonated dyes, as discussed later.
To investigate this phenomenon further, the adsorption equilibria of the azo sulfonated dye, Orange G, in the presence of doped PANI was examined. For this end, experiments were conducted with different initial concentrations of OG in the presence of 100 mg of PANI (HCldoped emeraldine salt) for 2 h. When the initial concentration of the dye was 100 ppm, the dye was completely adsorbed in 1 h. At higher concentrations of the dyes, the dye was not completely adsorbed, indicating that there is a saturation limit for the polymer above which it does not remove the dyes. The variation of the concentration of the dye in the solution with time was measured, as shown in Figure 2. These experiments were extended to some other sulfonated dyes, namely, Alizarine Cyanine Green, Coomassie Brilliant Blue, and Remazol Brilliant Blue R, and it was found that the polymer adsorbs the respective dyes from the solution in all of these cases. The presence of the dye in the polyaniline was confirmed from the UV-vis spectra of the polyaniline samples after the reactions. Figure 3 compares the UV-vis spectra of the polyaniline in formic acid before and after reaction with 50 and 250 ppm Orange G dye solutions. A typical UV-vis spectrum of pristine PANI (ES) has three distinct absorption bands in the ranges of 300-330, 400-430, and 780-826 nm corresponding to a π-π* transition and polaronic and bipolaronic bands. In addition to these peaks, we observed a new feature at 511 nm. The peak at 511 nm could be due to the dye molecules that have chemically interacted with polyaniline. We also observed the shift in bipolaron band at 763 nm in pristine polyaniline to 753 and 746 nm, respectively, for the samples after reaction with 100 and 250
Adsorption of Sulfonated Dyes by PANI Emeraldine Salt
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SCHEME 1: Dissociation of Orange G
ppm Orange G dye solutions. Thus, we observed a blue shift in the polaron band with increasing dye loading in the polymer samples. This suggests that PANI has a lower delocalization of electrons in the polymer chains and/or a lower doping level after the interaction with the dye molecules. When the reaction of polyaniline emeraldine base was attempted for the removal of the dyes, it was found that the activity of emeraldine base toward the removal of the dyes from solution was considerably less than that of the salt form, as shown in Figure 4. Mechanism. It is well-known that the degree of ionization of a dye molecule depends on the pH of the aqueous medium. Orange G contains two sulfonated groups (sSO3Na) and one hydroxyl group (sOH). In acidic aqueous solutions, the functional group of OG (sSO3Na) gets ionized, and the dye exists in anionic form as shown in Scheme 1. When PANI emeraldine salt is added to water, the pH of the water becomes acidic (pH ) 3.9) and does not change during the course of adsorption. The addition of dyes with sulfonated functional groups to this aqueous solution results in the dissociation of the functional group into its anionic form. The sSO3- group on the dye could lead to chemical interactions with the positively charged backbone of PANI emeraldine salt, and Na+ ions interact with the chloride ions that are invariably present in doped PANI. This will lead to the adsorption of various sulfonated dyes on the emeraldine salt of PANI. In basic aqueous solutions (dispersion of emeraldine base in water), the dissociation of the functional group of the dye would be inhibited, and no chemical interaction with the PANI backbone would be expected. When the emeraldine base of polyaniline is used instead of the emeraldine salt for the above experiments, we found that the base form cannot remove the dye from solution. This indicates that the positively charged backbone and chloride ions that are invariably present in the
ES form are the possible sites for chemical interactions with the sulfonated dye molecules. After the dye was adsorbed on PANI, a base (ammonium hydroxide) was added to the solution. This base dedoped PANI and resulted in the desorption of the dye. The XRD pattern obtained for the Orange G-containing polyaniline is not significantly different from that for pristine polyaniline (Figure 5). This indicates that the crystallinity of polyaniline is not significantly affected by the dye, which ignores the possibility of secondary doping by the dye molecules in the polymer. The structure of PANI is unaltered after the adsorption and desorption of the dye. The above studies seem to indicate that the adsorption of the dye was due to chemical interactions. To verify this hypothesis, adsorption kinetics studies were conducted to determine the rate parameters. Kinetics of Adsorption. The percentage removal of dyes was calculated as
percentage removal ) 100
(C0 - Ce) C0
The equilibrium uptake was calculated as
qe )
(C0 - Ce)V W
where qe is the amount adsorbed at equilibrium, C0 is the initial concentration of the dye, Ce is the equilibrium concentration of the dye solution, V is the volume of the solution, and W is the mass of PANI taken for the experiment. Figure 6 shows the percentage removal of the dye OG for different initial dye concentrations. It is obvious that the removal is complete (100%) at lower OG concentrations, whereas less than 50% is adsorbed at very high OG concentrations.
Figure 5. XRD patterns of PANI emeraldine salt before and after interacting with OG dye.
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Figure 6. Percentage dye removal as a function of initial dye (OG) concentration.
Figure 8. Concentration profiles of various sulfonated dyes in the presence of PANI emeraldine salt.
Figure 7. Variation of the equilibrium amount adsorbed with equilibrium concentration. The inset shows the linear variation of Ce/qe with Ce.
Figure 9. Pseudo-second-order kinetic plots for the removal by PANI of various sulfonated dyes with an initial concentration of 175 ppm.
Figure 7 shows the amount adsorbed, qe, as a function of the equilibrium concentration of the dye in solution. For example, when 392 ppm (mg/L) of OG dye was taken in 100 mL of solution with 100 mg PANI and stirred, the amount remaining in the solution after 2 h of stirring was 173 ppm. Thus, the amount adsorbed was 219 ppm. Therefore, qe ) (392 - 173)0.1/ 100 ) 0.22 mg of adsorbate (OG)/mg of adsorbent (PANI). To verify whether the system followed the Langmuir-Hinshelwood mechanism, kinetics experiments were carried out with different initial concentrations of the dye (OG), as shown in Figure 2. The Langmuir isotherm indicates that
Ce Ce 1 ) + qe qm K2qm where K2 is the Langmuir adsorption constant in L/mg and qm is the adsorption capacity in mg of dye/mg of PANI. Thus, a plot of Ce/qe versus Ce should be linear, as shown in the inset of Figure 7. The values of K2 and qm determined from the slope and the intercept of the linear plot are 1.5 L/mg and 0.22 mg of dye/mg of PANI, respectively. A rapid uptake of the dye by the adsorbent indicates maximum efficiency, and this occurs in physical adsorption or strong chemisorption. Figure 8 shows the concentration profiles of various sulfonated dyes with initial concentrations of 175 ppm in the presence of 100 mg of PANI. A second-order model for adsorption indicates
dqt ) ks(qe - qt)2 dt where ks is the rate constant in mg of PANI/[(mg of dye)min]
TABLE 1: Kinetic Parameters for the Removal of Various Sulfonated Dyes by PANI dye OG CBB RBBR AG
qe ksqe2 (mg of dye/mg of PANI) {mg of dye/[(mg of PANI) min]} 0.175 0.129 0.100 0.056
0.062 0.059 0.043 0.027
and qt is the amount adsorbed at time t in mg of dye/mg of PANI. The above equation can be integrated with the initial condition qt ) 0 at t ) 0 to give
t 1 1 ) + t qt k q 2 qe s e
Thus, a plot of t/qt versus t should be linear for various dyes, as shown in Figure 9. The values of ksqe2 and qe, determined from the slope and intercept of the plot, respectively, are reported in the Table 1. The initial adsorption rate, as t f 0, is ksqe2, and this is clearly the highest for OG, followed by CBB, RBBR, and AG. The equilibrium adsorption capacity, qe, also follows the same order. Electrical Conductivity. The electrical conductivity of PANI after exposure to the dye solution decreases considerably (Figure 10). This is rather surprising, especially because PANI is known to be doped by sulfonated compounds such as camphor sulfonic acid, dodecyl benzene sulfonic acid, and so on.24 It should be pointed out that the electrical conductivity was measured for the samples made by compaction of PANI powder. This suggests that the dye adsorbed on the surface of the PANI particles gives
Adsorption of Sulfonated Dyes by PANI Emeraldine Salt
J. Phys. Chem. B, Vol. 112, No. 33, 2008 10157 Acknowledgment. We thank the Department of Science and Technology, India, for financial support. References and Notes
Figure 10. Conductivity profile of PANI (emeraldine salt) containing Orange G.
rise to a thin layer of low-doped material because of the removal of the Cl- ions as described above. This is in agreement with the reduction in the intensity of the bipolaronic peak that is associated with doping level of PANI after treatment with the dye solution. The fact that the crystallinity also does not change much after exposure to the dye solution also supports the argument that the dye does not penetrate fully into the PANI particles. Otherwise, a reduction in crystallinity would have been observed, as reported in the case of PANI doped with dodecyl benzene sulfonic acid.25 The dyes used in the present study are only anionic and generally do not form donor/acceptor complexes, and hence, the possibility of obtaining high conductivity after their adsorption appears to be remote. Nonetheless, an interesting possibility arises for using conducting PANI as a conductometric sensor for these dyes at very low concentrations. Conclusion In this study, polyaniline emeraldine salt was synthesized by chemical oxidation and used for the adsorption of sulfonated dyes from water. A mechanism was proposed based on the chemical interaction of PANI with the sulfonate group of the dyes. This mechanism was supported by UV-vis, XRD, and conductivity results. The kinetics of adsorption and adsorption equilibrium parameters were also determined in this study.
(1) Shirakawa, H.; Louis, E. J.; MacDiarmid, A G.; Chiang, C. K.; Heeger, A. J. J. Chem. Soc., Chem. Commun. 1977, 578. (2) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 530. (3) Baughman, R. H. Synth. Met. 1996, 78, 339. (4) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science 1992, 258, 1474. (5) Greenham, N. C.; Moratti, S. C.; Bradley, D. D. C.; Friend, R. H.; Holmes, A. B. Nature 1993, 365, 628. (6) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14, 99. (7) Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. ReV. 2007, 107, 1324. (8) Wang, H. L.; MacDiarmid, A. G.; Wang, Y. Z.; Epstein, A. J. Synth Met. 1996, 78, 33. (9) Yue, J.; Epstein, A. J. J. Am. Chem. Soc. 1990, 112, 2800. (10) Liu, W.; Kumar, J.; Tripathy, S.; Sanecal, K. J.; Samuelson, L. J. Am. Chem. Soc. 1998, 121, 71. (11) Unde, S.; Ganu, J.; Radhakrishnan, S. AdV. Mater. Opt. Electron. 1996, 6, 151. (12) Kitani, A.; Kaya, M.; Sasaki, K. J. Electrochem. Soc. 1986, 133, 1069. (13) Huang, J.; Virji, S.; Weiller, B. H.; Kaner, R. B. J. Am. Chem. Soc. 2003, 125, 314. (14) Lu, W. K.; Elsenbaumer, R. L.; Wessling, B. Synth. Met. 1995, 71, 2163. (15) Athawale, A. A.; Bhagwat, S. V.; Katre, P. P. Sens. Actuators B: Chem. 2006, 114, 263. (16) Raffainer, I. I.; Rudolf von Rohr, P. Ind. Eng. Chem. Res. 2001, 40, 1083. (17) Golob, V.; Vinder, A.; Simonic, M. Dyes Pigm. 2005, 67, 93. (18) Al-Bastaki, N. Chem. Eng. Process. 2004, 43, 1561. (19) Namasivayam, C.; Kavitha, D. Dyes Pigm. 2002, 54, 47. (20) Kabra, K.; Chaudhary, R.; Sawhney, R. L. Ind. Eng. Chem. Res. 2004, 43, 7683. (21) Hoffman, M. R.; Martin, S. T.; Choi, W. T.; Bahnemann, D. W. Chem. ReV 1995, 95, 69. (22) Muktha, B.; Madras, G.; Guru Row, T. N.; Scherf, U.; Patil, S. J. Phys. Chem. B 2007, 111, 7994. (23) Chowdhury, D.; Paul, A.; Chattopadhyay, A. Langmuir 2005, 21, 4123. (24) Cau, Y.; Smith, P.; Heeger, A. J. Synth. Met. 1992, 48, 91. (25) Chang-Qing, J.; Su-moon, P. Synth. Met. 2001, 12, 443.
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