Adsorption and Desorption Kinetics of Anionic Dyes on Doped

Feb 5, 2009 - Debajyoti Mahanta, Giridhar Madras, S. Radhakrishnan and Satish Patil* ... J. Phys. Chem. B , 2009, 113 (8), pp 2293–2299. DOI: 10.102...
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J. Phys. Chem. B 2009, 113, 2293–2299

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Adsorption and Desorption Kinetics of Anionic Dyes on Doped Polyaniline 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, India, 560012, and Polymer Science and Engineering, National Chemical Laboratory, Pune, India, 411008 ReceiVed: NoVember 6, 2008; ReVised Manuscript ReceiVed: December 18, 2008

In this study, we report an approach for the adsorption and desorption of anionic (sulfonated) dyes from aqueous solution by doped polyaniline. In this study, we have synthesized PANI with two dopants, namely, p-toluenesulfonic acid (PTSA) and camphorsulfonic acid (CSA), and used these to adsorb various dyes. It was found that the doped PANI selectively adsorbs anionic dyes and does not adsorb cationic dyes. The adsorption of anionic dyes causes the variation in electrical conductivity of PANI, indicating its potential as a conductometric sensor for these dyes at very low concentration. The adsorbed dyes were desorbed from the polymer by using a basic aqueous solution. The adsorption and desorption kinetics of the dye in the presence of doped PANI were also determined. Introduction The discovery of electrical conductivity in π-conjugated polymers was one of the major scientific achievements due to theirexcitingelectrochemical,optical,andconductingproperties.1-4 Initial interest for these polymers was because they have potential applications in electronic devices.5-7 Over the past few decades, these materials are extensively used in organic electronics such as organic solar cells,8 light-emitting diodes,9 and field effect transistors.10 Among the conducting polymers, polyaniline (PANI) has received increasing attention owing to its easy synthesis, high conductivity, and excellent environmental stability.11,12 PANI is used in plastic batteries, optical storage lithography, harmonic generators, display devices, magneticrecording,solidstatesensors,andcorrosioninhibitors.13-15 PANI doped with Pd, Cu, and Pd/Cu has been used for the electrochemical oxidation of methanol and formic acid.16 The major practical disadvantage of PANI is its insolubility in most organic solvents. The processibility of polyaniline can be improved by doping with a functionalized protonic acid such as camphorsulfonic acid (CSA) and p-toluenesulfonic acid (PTSA).17,18 Water contamination due to dyes from the textile sources is a major environmental concern. In addition to their unwanted colors, some of these dyes may degrade to produce carcinogens and toxic products.19 Many methods such as flocculation,20 reverse osmosis,21 and adsorption by different materials have been used in wastewater treatment.22 Among them, adsorption is an effective technique for the treatment of wastewater containing dyes. Many adsorbents have been tested for the possibility of lowering the dye concentration from aqueous solutions, such as orange peel,23 acid activated red mud,24 fly ash,25 and other materials.26-30 However, none of the above materials show selective adsorption of dyes or organics. Development of such materials would be useful in making sensors for the identification of certain * To whom correspondence should be addressed. Telephone: +91-8022932651. 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.

classes of dyes even at ppm levels. The emeraldine salt (HCl, doped) of PANI has been used successfully for the selective removal of anionic dyes from aqueous solution.31,32 The interaction between the negatively charged anion of the dye and the positively charged PANI backbone is responsible for the anionic dye adsorption by PANI emeraldine salt from aqueous dye solution. In our earlier study,31 we investigated the adsorption of dyes (mainly containing sulfonic groups) in the presence of HCl doped PANI. In this study, we report the adsorption of different types of anionic dyes by p-toluenesulfonic acid (PTSA) and camphorsulfonic acid (CSA) doped PANI i.e., sulfonic groups are incorporated in the polymer. In addition, we investigate the effect of pH and polar solvents on adsorption and desorption, and on the kinetics. Experimental Section Materials. Aniline (S.D. Fine Chemicals Ltd., Mumbai, India) was purified by distillation before use. Ammonium persulfate (APS), p-toluenesulfonic acid (PTSA), and D-10-camphorsulfonic acid (CSA) were obtained from S.D. Fine Chemicals Ltd., Mumbai, India. The dyes Orange-G (OG), Methylene Blue (MB), Rhodamine-B (RB), Malachite Green (MG) (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), and Remazol Brilliant Blue R (RBBR; Color Chem Ltd., Ahmedabad, India) were used as received. The structures of the dyes are given in the Supporting Information. Adsorption and Desorption Experiments. PTSA and CSA doped polyanilines were synthesized by chemical oxidative polymerization method in the aqueous medium, as described in detail elsewhere.33 For adsorption experiments, 100 mg of doped PANI was added to 100 mL of different dye solutions of different initial concentrations (100-600 ppm) and stirred for 2 h. Samples were collected at different time intervals (0, 2, 10, 20, 30, 60, and 120 min, respectively), and the concentration of dye was determined by UV-vis spectroscopy. The dye concentrations were calibrated with the Beer-Lambert law at λmax values of 480, 664, 640, 555, 591, 617, and 554 nm for

10.1021/jp809796e CCC: $40.75  2009 American Chemical Society Published on Web 02/05/2009

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Figure 1. Concentration profiles of various dyes in the presence of (a) PTSA doped and (b) CSA doped PANI. (c) Comparison of adsorption of OG dye of 400 ppm initial concentration by HCl, PTSA, and CSA doped PANI.

Figure 2. Concentration profiles of Orange-G in the presence of (a) PTSA doped and (b) CSA doped PANI.

OG, MB, AG, CBB, RBBR, MG, and RB, respectively. After adsorption of dye, the PANI samples were removed by centrifugation and washed with distilled water several times and finally dried under vacuum. The desorption experiments were conducted with different initial concentrations of the dye solution mixed with 100 mg of doped PANI for 12 h. The dye-loaded samples were separated by centrifuge and washed with distilled water. The samples were dried completely and used for desorption experiments. The dyeloaded samples of PTSA and CSA doped PANI were mixed with 100 mL of basic buffer solution (pH 9.2). After the desorption of the dyes, the dyes were adsorbed again onto doped PANI. The adsorption efficiency did not significantly decrease by repeated adsorption and desorption.

These samples were then characterized by UV-vis spectroscopy, X-ray diffraction (XRD), and conductivity. The XRD patterns were recorded on a Phillips X’pert Pro Diffractometer with Cu KR radiation in the 2θ range from 5° to 45° at the scanning rate 1°/min. The conductivity measurements were carried out by a two-probe technique by a Keithley Model 614 electrometer. The UV-vis measurements were carried out with a Perkin-Elmer Lambda 35 spectrometer using a cell with a 1 cm path length. The spectra were recorded in the wavelength range 300-900 nm. Results and Discussion Various classes of dyes such as heteropolyaromatic (MB), azo (OG), xanthane (RB), anthraquinonic (AG), triarylmethane

Anionic Dyes on Doped PANI

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Figure 3. (a): Concentration profile of AG and OG in the presence of PTSA doped PANI from a dye mixture. (b) Concentration profiles of OG dye in the presence of PTSA doped PANI at different pH values.

Figure 4. Variation of equilibrium amount adsorbed, q, with equilibrium dye concentration for (a) PTSA and (b) CSA doped PANI. The inset shows the linear variation of Ce/qe with Ce.

Figure 5. Pseudo-second-order kinetic plots for the removal of various sulfonated dyes of initial concentration of 400 ppm by (a) PTSA doped PANI and (b) CSA doped PANI.

(CBB), and chlorinated triarylmethane (MG) were used in the adsorption experiments. Among these dyes, OG, AG, CBB, and RBBR are sulfonated anionic dyes while the other three dyes, MB, RB, and MG, are cationic dyes. Figure 1a,b shows the variation of concentration of various dyes (initial concentration of 400 ppm) in the presence of 100 mg of PTSA doped and CSA doped PANI. It was observed that there was no significant adsorption of nonsulfonated cationic dyes by PTSA and CSA doped PANI samples, while a significant adsorption was observed in the case of sulfonated anionic dyes. This suggests possible chemical interactions

between the negatively charged sulfonated groups with the positively charged sites in PANI backbone. Figure 1c shows that the maximum amount of dyes adsorbed by 100 mg of CSA doped PANI was higher than that adsorbed by 100 mg of PTSA doped PANI and both are significantly higher than that of HCl doped PANI, as reported in our previous paper.31 This indicates the effect of dopant on the dye adsorption. It was found that the adsorption of dye increased with the bulkiness of the counterion contributed from the dopant protonic acids. As the size of the counterion increases, the interchain distance also increases, and this results in the easier diffusion

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TABLE 1: qe Values for Adsorption of OG Dye by PTSA and CSA Doped PANI qe, mg of dye/mg of PANI dye

PTSA doped

CSA doped

OG CBB RBBR AG

0.342 0.207 0.171 0.095

0.400 0.231 0.254 0.151

of the negatively charged sulfonated dye anions to the positively charged sites on the polymer backbone. The counterion in CSA doped PANI is bigger than that of PTSA doped PANI; therefore, CSA doped PANI shows higher dye adsorption. To investigate this phenomenon further, the adsorption equilibria of an azo dye, Orange-G (OG), in the presence of PTSA doped PANI and CSA doped PANI were examined. The experiments were conducted with different initial concentrations of OG ranging from 100 to 600 ppm in the presence of 100 mg of PTSA and CSA doped PANI. When the initial concentration was less than 300 ppm, the dye was completely adsorbed in 2 h. At higher concentrations of dye, some amount of the dye was adsorbed while the rest remained in solution. The variation of the concentration of the dye in the solution with time in the presence of PTSA and CSA doped PANI is shown in Figure 2a and Figure 2b, respectively. Since the conductivity of doped PANI changes with dye adsorption, it can be used as sensor for detecting dyes. Therefore, it is important to determine whether PANI can be used for a specific dye in the presence of a mixture. It is also important to determine whether the adsorption of one dye is significantly affected by the presence of the other dye. Therefore, the adsorption of a mixture of AG and OG from a 100 mL solution of initial concentrations 400 and 500 ppm, respectively, by 100 mg of PTSA doped PANI was investigated (Figure 3a). It was observed that 100 mg of PTSA doped PANI adsorbs 50 ppm AG and 320 ppm OG (relative ratio of 6.4 for OG compared to AG) from the dye mixture. On the other hand, 100 of mg PTSA doped PANI can adsorb around 95 ppm AG and 350 ppm OG from their individual solutions (relative ratio of 3.7 for OG compared to AG). Thus the adsorption of OG dye is only slightly affected by the presence of the other dye (AG). This confirms the greater affinity of OG dye anions than of the AG dye anions toward the positive sites of the PANI backbone, giving high selectivity for possible sensing applications. Often industrial wastewater contains a mixture of pollutants and organic solvents such as alcohols,34,35 so the influence of ethanol and methanol on adsorption of OG was investigated. The concentration profiles showed that the adsorption of OG is not significantly affected in the presence of ethanol and methanol (not shown). The absence of the influence of polar groups on adsorption indicates that the adsorption is controlled not by surface forces but by chemical interaction. It is well-known that pH plays a significant role in adsorption of dyes. Therefore, 100 mg of PTSA doped PANI was added to 100 mL of 500 ppm OG solution of different pH values. The pH was measured after the addition of PANI. It was observed that in acidic pH PANI adsorbs dye, while in basic pH the adsorption is negligible (Figure 3b). In acidic pH, PANI exists in the doped state and the dye molecules are also in the ionic state. Therefore, the interaction of the dye anion and the positively charged sites of PANI backbone is possible. However, in a basic solution when PTSA doped PANI is added, PANI is dedoped and the positively charged sites are no longer available in the backbone. The interaction between the dye and PANI is

not possible in this case, which results in lower adsorption of the dye on the polymer. This clearly brings out the importance of the dopant as well as the doping level of PANI in its dye adsorption characteristics. All further experiments were conducted at pH 3.9, at which maximum dye adsorption has been observed for doped PANI. Adsorption Kinetics. The percentage removal of dye was calculated by using the formula

percentage removal ) 100

C0 - Ce C0

(1)

where C0 is the initial concentration of the dye solution and Ce is the equilibrium concentration of the dye solution in mg/L. The percentage removal was 100% for C0 e 300 ppm, while it was 60% and 65% in the presence of PTSA doped and CSA doped PANI, respectively, for C0 ) 600 ppm. It may be pointed out here that, in the case of HCl doped PANI, the percentage removal was only 40% at 500 ppm dye concentration.31 This confirms the superiority of PTSA and CSA doped PANI over HCl doped PANI. The equilibrium uptake was calculated as

qe )

(C0 - Ce)V W

(2)

where qe is the amount of dye adsorbed by PANI at equilibrium. V is the volume of the solution in liters and W is the mass of PANI in milligrams taken for the experiments. Figure 4a and Figure 4b show the amount adsorbed, qe, as a function of equilibrium concentrations of the dye in solution by PTSA and CSA doped PANI, respectively. The Langmuir isotherm indicates that

Ce Ce 1 ) + qe qm k2qm

(3)

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 insets of Figure 4. Figure 1a and Figure 1b show the concentration profiles of various dyes with initial concentration of 400 ppm in the presence of 100 mg of PTSA and CSA doped PANI, respec-

dqt ) ks(qe - qt)2 dt

(4)

tively. A second-order model for adsorption indicates36 where ks is the rate constant in mg of PANI/[(mg of dye) min] 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

t 1 1 ) + t qt qe ksqe2

(5)

condition qt ) 0 at t ) 0: Thus according to eq 5, a plot of t/qt versus t should be linear for various dyes as shown in Figure 5a and Figure 5b for PTSA and CSA doped PANI, respectively. The values of qe

Anionic Dyes on Doped PANI

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Figure 6. Desorption profile of OG dye from (a) PTSA doped PANI and (b) CSA doped PANI.

Figure 7. Pseudo-second-order kinetic plots for desorption of OG dye from OG loaded (a) PTSA doped PANI and (b) CSA doped PANI.

determined from the slope and intercept of the plot are reported in Table 1. Desorption Experiments. As discussed in the Experimental Section, 100 mL of OG dye solution of different initial concentrations of 400, 500, and 600 ppm were mixed with 100 mg of PTSA and CSA doped PANI and stirred for 12 h. These samples were then removed and used for desorption experiments. At pH 9.2, the PANI salts were dedoped to emeraldine base form and the positively charged sites were no longer available in the PANI backbone; therefore, the desorption of dye molecules occurs from PANI powder dispersed in the buffer solution. The concentration profiles of dye desorption for PTSA doped and CSA doped PANI are shown in Figure 6a and Figure 6b, respectively.

dqt ) kd(qe - qt)2 dt

(6)

Desorption Kinetics. In order to determine the desorption kinetics, the results shown in Figure 6 are modeled by a secondorder desorption model.36 where qt is the amount desorbed at time t in mg of dye/mg of

t 1 1 ) + t 2 qt q kdqe e

(7)

PANI and kd is the desorption rate constant in mg/L min. The above equation can be integrated with the initial condition of qt ) 0 at t ) 0:

The values of qe and kd were calculated from the linear plot of t/qt with time (Figure 7, based on eq 7). The qe and kd values are 0.26 mg of dye/mg of PANI and 0.78 mg/L min, and 0.30 mg of dye/mg of PANI and 1.90 mg/L min for PTSA doped and CSA doped PANI, respectively. Electrical Conductivities. The electrical conductivity of both PTSA doped PANI and CSA doped PANI after exposure to dye solution decreases considerably as shown in Figure 8. This result is in good agreement with our previous results obtained for HCl doped PANI.31 The electrical conductivity was measured for the samples made by compaction of PTSA doped PANI and CSA doped PANI powder. This suggests that the dye adsorbed on the surface of the PANI particles gives rise to a thin layer of low-doped material because of the removal of the counterions. It is also observed that crystallinity is not changed after the addition of dyes to the PTSA and CSA doped PANI samples. It supports the argument that the dye does not penetrate fully into the PANI particles. Otherwise, a reduction of crystallinity would have been observed as reported in the case of dodecylbenzenesulfonic acid doped PANI.37 A wide range of anionic dyes have been used in the present study and they do not form donor/acceptor complexes with PANI; hence the possibility of obtaining high conductivity after their adsorption appears to be remote. It is interesting to note that the amount of maximum dye uptake was found follow the same order, PANI-CSA > PANI-PTSA > PANI-HCl, as that of the drop in electrical conductivity. This suggests that the amount of dye adsorbed depends on the extent of doped species still remaining after exposure to dye solution. If the charge on the doped PANI reduces drastically at low dye concentration, there is no scope for further adsorption of the dye. An interesting possibility arises for the use of conducting

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Figure 8. Conductivity profiles of PTSA and CSA doped PANI containing Orange-G.

Figure 10. UV-vis spectra of Orange-G, (a) PTSA doped PANI and dye containing PTSA doped PANI samples, and (b) CSA doped PANI and dye containing CSA doped PANI samples.

Figure 9. XRD pattern of (a) PTSA doped and (b) CSA doped PANI before and after interaction with OG dye.

PANI as conductometric sensors for these dyes at very low concentration. Further, there appears to be good selectivity for OG even in the presence of other dyes. XRD Studies. In Figures 9a and 9b, the powder XRD patterns of the polymer before and after OG dye adsorption are shown for PTSA doped PANI and CSA doped PANI, respectively. These indicate that there is no significant change in crystallinity for both samples before and after dye adsorption. The crystallinity of PANI is not affected by the dye molecules. This observation rules out the possibility of secondary doping. UV-Visible Spectroscopy. The presence of OG dye in the PTSA and CSA doped PANI samples after the adsorption reactions was confirmed by UV/vis spectroscopy as shown in Figure 10. All the samples of PTSA and CSA doped PANI along with the dye-loaded samples were characterized by UV/vis spectroscopy using m-cresol as solvent. The PTSA doped PANI

exhibits peaks at 320 and 420 nm with an extended free carrier tail characteristic of an extended coil conformation with increasing absorption at 850 nm.33 OG dye has its absorption maxima at 487 nm. The dye-loaded PTSA doped samples show a peak at 513 nm which is due to the interaction of OG dye molecules with the polymer backbone. The shift in the peak position of dye in the polymer represents the chemical interaction of the dye with the polymer backbone. Similarly pristine CSA doped PANI in m-cresol shows its characteristic peak at 440 nm corresponding to transition from polaron band to π* band along with an extended free carrier tail representing the extended coil structure of the polymer in m-cresol.38 Here also the OG peak position shifts from 487 to 502 nm, representing the chemical interaction of the dye with the polymer backbone. Conclusions We have successfully utilized polyaniline doped with functionalized protonic acids (CSA and PTSA) for removal of anionic dyes from water. It has been demonstrated that, by changing dopants in polyaniline, it is possible to enhance its dye adsorption characteristics. There is very good selectivity for OG dye, and this together with the change in conductivity can lead to the development of an excellent material that can be used as a sensor. The maximum dye adsorption/removal by PANI clearly depends on the nature of the dopant (the order being CSA > PTSA > HCl), and this is associated with the extent of charge neutralization on PANI after dye adsorption.

Anionic Dyes on Doped PANI Acknowledgment. We thank the Department of Science and Technology, India, for financial support. The authors thank Ms. S. Aarthi for help in measuring the adsorption. Supporting Information Available: The structures of the dyes used in this study. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Shirakawa, H.; Louis, E. J.; MacDiarmid, A. G.; Chiang, C. K.; Heeger, A. J. J. Chem. Soc., Chem. Commun. 1977, 578. (2) Rudge, A.; Raistrick, I.; Gottesfeld, S.; Ferraris, J. P. Electrochim. Acta 1994, 39, 273. (3) Patil, A. O.; Heeger, A. J.; Wudl, F. Chem. ReV. 1988, 88, 183. (4) Heeger, A. J.; Kivelson, S.; Schrieffer, J. R.; Su, W. P. ReV. Mod. Phys. 1988, 60, 781. (5) Burroughes, J. H.; Jones, C. A.; Friend, R. H. Nature 1988, 335, 137. (6) Leeuw, D. M.; Simenon, M. M. J.; Brown, A. R.; Einerhand, R. E. F. Synth. Met. 1997, 87, 53. (7) Segawa, H.; Wu, F. P.; Nakayama, N.; Maruyama, H.; Sagisaka, S.; Higuchi, N.; Fujitsuka, M.; Shimidzu, T. Synth. Met. 1995, 71, 2151. (8) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. AdV. Funct. Mater. 2001, 11, 15. (9) Gross, M.; Muller, D. C.; Nothofer, H. G.; Scherf, U.; Neher, D.; Brauchle, C.; Meerholz, K. Nature 2000, 405, 661. (10) Stutzmann, N.; Friend, R. H.; Sirringhaus, H. Science 2003, 299, 1881. (11) Yue, J.; Epstein, A. J. J. Am. Chem. Soc. 1990, 112, 2800. (12) Lee, K.; Cho, S.; Park, S. H.; Heeger, A. J.; Lee, C. W.; Lee, S. H. Nature 2006, 441, 65. (13) Scrosati, B. Polym. Int. 1998, 47, 50. (14) Xie, D.; Jiang, Y.; Pan, W.; Li, D.; Wu, Z.; Li, Y. Sens. Actuators, B 2002, 81, 158.

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