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 applica- tions 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 next- generation 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 environ- mental 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 inhibi- tors. 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 environ- mental pollution because they are nonbiodegradable. 16 Many methods such as flocculation, 17 reverse osmosis, 18 and activated carbon adsorption 19 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 TiO 2 , doped TiO 2 , 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[2- methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (MEH- PPV) as photocatalysts for the degradation of various textile dyes. 22 The application of conducting polymer nanocomposites for photocatalysis has recently been studied. The nanocomposite of polypyrrole and TiO 2 nanoparticles was reported to exhibit higher photoctalytic activity than a suspension of TiO 2 nano- particles. 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 em- eraldine 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, Colour- Chem 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 emer- aldine 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 * To whom correspondence should be addressed. Tel.: +91-80- 22932651. Fax:+91-80-23601310. E-mail: satish@sscu.iisc.ernet.in. † Solid State and Structural Chemistry Unit, Indian Institute of Science. ‡ Department of Chemical Engineering, Indian Institute of Science. § National Chemical Laboratory. J. Phys. Chem. B 2008, 112, 10153–10157 10153 10.1021/jp803903x CCC: $40.75  2008 American Chemical Society Published on Web 07/30/2008