Adsorption of Anionic Dyes on a Reversibly Swelling Cationic Superabsorbent Polymer Neelesh Bharti Shukla, Giridhar Madras Department of Chemical Engineering, Indian Institute of Science, Bangalore 560012, India Correspondence to: G. Madras (E-mail: giridhar@chemeng.iisc.ernet.in) ABSTRACT: A cationic monomer [2-(methacryloyloxy)ethyl]trimethylammonium chloride was polymerized using N,N 0 -methylenebisa- crylamide as the crosslinker to obtain a cationic superabsorbent polymer (SAP). This SAP was characterized by Fourier transform- infrared spectroscopy, and the equilibrium swelling capacity was determined by swelling in water. The SAP was subjected to cyclic swelling/deswelling in water and NaCl solution. The conductivity of the swelling medium was monitored during the swelling/deswel- ling and was related to the swelling/deswelling characteristics of the SAP. The adsorption of five anionic dyes of different classes on the SAP was carried out and was found to follow the first-order kinetics. The Langmuir adsorption isotherms were found to fit the equilibrium adsorption data. The dye adsorption capacity of the SAP synthesized in this study was higher than that obtained for other hydrogels reported in the literature. V C 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 000: 000–000, 2012 KEYWORDS: superabsorbent; hydrogels; swelling; deswelling; adsorption Received 18 January 2012; accepted 20 April 2012; published online DOI: 10.1002/app.37929 INTRODUCTION The three-dimensional networks of polymers that are able to absorb and retain more than 100 times of liquid of their own weight are called superabsorbent polymers (SAPs). 1 The SAPs contain ionic groups in the three-dimensional networks, which results in the superabsorbent characteristics, and allows them to adsorb oppositely charged materials from the swelling medium. The presence of ionic groups results in a very high osmotic pressure difference between the network and surrounding me- dium. 2 The osmotic pressure is balanced by the inflow of large amount of water resulting in the high degree of swelling of the SAPs. The superabsorbent nature of these SAPs makes them an obvious material of choice for personal hygiene products, such as diapers and sanitary napkins. 3 They are also used as contact lenses, 4 controlled drug release devices, 5,6 tissue engineering scaffolds, 6 and wound-healing dressing 7 in the field of biomedi- cal engineering. Although most of the commercially used SAPs are anionic based on acrylic acid monomer, 8 cationic SAPs may be useful in the field of water purification for the removal of anionic impurities. Cationic hydrogels of poly(N,N-diethylamino ethyl methacry- late) (PDEAEMA) crosslinked with ethylene glycol dimethacry- late, 9 epichlorohydrin crosslinked hydroxyl propyl cellulose hydrogel, 10 and poly(vinyl alcohol) and carboxymethyl cellulose copolymer hydrogels synthesized by electron beam irradiation 11 have been used for the adsorption of anionic dyes. The copoly- mer of acrylamide and [2-(methacryloyloxy)ethyl]trimethylam- monium chloride (METAC) was used for the removal of tolui- dine blue. 12 METAC has been grafted onto sawdust and used for the removal of anionic dyes. 13 A cationic magnetic hydrogel based on (3-acrylamidopropyl)trimethylammonium chloride has been used for the removal of natural organic matter. 14 Cationic hydrogels have also been used for the removal of Cr(VI) from ground water and soil. 15 Selective adsorption of Hg(II) has also been carried out by METAC based water-insoluble resin. 16 Compared to the significant literature available on the synthesis of anionic superabsorbents, there are relatively few studies on cationic superabsorbents. In this study, a cationic SAP of METAC was synthesized by solution polymerization using N,N 0 - methylenebisacrylamide (MBA) as crosslinker. The superabsorb- ent was subjected to cyclic swelling/deswelling in deionized (DI) water and NaCl solution. The SAP was found to follow first- order kinetics during the swelling/deswelling cycles. The equilib- rium swelling capacities, limiting swelling capacities, and the rate constant of swelling/deswelling were determined. To the best of our knowledge, this is the first study where the conductivity of the medium was monitored during the V C 2012 Wiley Periodicals, Inc. WWW.MATERIALSVIEWS.COM WILEYONLINELIBRARY.COM/APP J. APPL. POLYM. SCI. 2012, DOI: 10.1002/APP.37929 1