Novel stimuli responsive gellan gum-graft-poly(DMAEMA) hydrogel as adsorbent for anionic dye

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Highlights

  • pH and temperature responsive gellan gum-graft-poly((2-dimethylaminoethyl) methacrylate) gels have been made.

  • Suitability of the material as adsorbent for anionic dyes has been confirmed with methyl orange.

  • Maximum adsorption is achieved at pH 3.0.

Abstract

In this study, gellan gum-grafted-poly((2-dimethylamino) ethyl methacrylate) (GG-g-poly(DMAEMA)) hydrogel was made by free radical polymerization in aqueous media employing microwave irradiation technique. Ammonium persulfate (APS)/N,N,N′,N′-tetramethylethylenediamine (TMEDA) were used as initiator–accelerator pair. N,N′-methylenebisacrylamide (MBA) has been used as crosslinker. The gel was characterized by FTIR, XRD, TGA, DSC and SEM techniques. The characteristic peaks at 1724, 2630, 1147, 1650 and 1535 cm−1 in the IR spectrum confirms grafting and gel formation. The TGA data reveals that synthesized gels were thermally more stable than gellan gum. The XRD studies confirm the crystalline nature of the synthesized material. Swelling behaviour of the hydrogel under different temperatures and pH conditions was investigated. The results indicated drastic changes in swelling around pH 7.0 and 50 °C. The gels were evaluated as an adsorbent to remove an anionic dye, methyl orange (MO), from aqueous solution. The pH conditions for maximum adsorption were optimized, the adsorption data is observed to fit best to the Freundlich isotherm model and the maximum adsorption capacity was found to be 25.8 mg g−1. The kinetic analysis revealed a second-order adsorption process. The thermodynamic parameters showed the adsorption to be exothermic and non-spontaneous at high temperatures.

Introduction

In recent years polymeric adsorbents have been increasingly used to remove and recover organic pollutants from waste waters [1], [2], [3]. Coskun and Delibas [4] studied removal of methylene blue (MB) from aqueous solution by poly(AMPS-co-IA) hydrogels by batch equilibration technique. Patel and Patel [5] prepared novel cationic poly[AAm/NVP/DAPB] hydrogels for removal of some textile anionic dyes such as reactive golden yellow, orange-II and acid yellow from aqueous solution. Huang et al. [6] modified chitosan by crosslinking and protonation for use as adsorbent to remove amido black 10B from aqueous solution. Abdel-Aziz and Siyam [7] reported the radiation synthesis of poly(acrylamide-acrylic acid-dimethylaminoethylmethacrylate) resin and its use for binding anionic dyes such as indigo carmine and eriochrome black-T.

Modification of polysaccharides for development of effective adsorbent materials is a topic of current research interest. The insertion of new functional groups on the polysaccharide backbone results in an increase of polarity and hydrophilicity of the material which in turn enhances the adsorption of polar adsorbates and improves the adsorption selectivity for the target pollutant. The grafting of carboxyl groups [8], amine functions [9] has been regarded as an interesting method for these purposes. Salama et al. [10] synthesized carboxymethylcellulose-g-poly(2-(dimethylamino) ethyl methacrylate) hydrogel and studied its use for removal of methyl orange from aqueous solution. The hydrogels resulting from the modification of polysaccharides find applications also in diverse fields such as controlled drug delivery [11], as artificial organs in biotechnology [12], as biosensors [13], as antibacterial agents [14].

The microwave irradiation is one technique, which is used for preparation of the hydrogels in several studies. Due to the microwave heating process has high temperatures for attack the solution with relatively short times and thus creates reactions faster than under conventional thermal conditions [15]. The hydrogels obtained by microwave irradiation exhibited higher the equilibrium swelling ratios [16]. Recently, Sharma et al. [17] investigated water retention and dye adsorption behaviour of gum ghatti-cl-poly(acrylic acid-aniline) conducting hydrogel synthesized under microwave irradiation. Mittal et al. [18] synthesized biodegradable hydrogel of gum ghatti (Gg) with a co-polymer mixture of acrylamide and methacrylic acid (Gg-cl-P(AAm-co-MAA)) by microwave-assisted free radical graft co-polymerization technique.

In our previous study we reported grafting of tertiary amine functionalized 2-(dimethylamino)ethyl methacrylate (DMAEMA) on gellan gum (GG) [19]. Until now, as far as we could ascertain, no data are available concerning the removal of water-soluble anionic dyes by gellan gum derivatives. Hence we have carried out detailed investigation on crosslinked graft copolymer of GG and poly(dimethylamino)ethyl methacrylate (PDMAEMA) as an adsorbent material for anionic dyes, considering methyl orange (MO) as a representative dye molecule. The objectives of this study were to (i) synthesize GG-g-poly(DMAEMA) gels employing microwave radiation and characterize the structure; (ii) study the pH and temperature responsiveness; (iii) investigate the adsorption efficiency for anionic dyes considering MO as a model dye and (iv) study the thermodynamic and kinetic aspects of adsorption process.

Section snippets

Materials

GG, DMAEMA and TMEDA were obtained from Aldrich Chemical Company, India. APS, MBA were obtained from Spectrochem Pvt. Ltd. Mumbai, India. The anionic dye, MO was obtained from S.D. Fine Chemicals, Mumbai, India. DMAEMA was purified by removal of inhibitor using 0.5% NaOH, followed by washing with distilled water. All other chemicals were used as received. Double distilled water was used in the preparation of gels and for swelling studies.

Microwave assisted synthesis of gellan gum-graft-poly(2-(dimethylamino) ethylmethacrylate) gels: (GG-g-poly(DMAEMA))

The gels derived from DMAEMA and GG was prepared by

Synthesis of GG-g-poly(DMAEMA) gels by microwave assisted method

The mechanism of formation of GG-g-poly(DMAEMA) gels is shown in Fig. 1. The redox initiator such as APS and accelerator TMEDA produce ions and their presence enhance the ability of the aqueous reaction mixture to convert the microwave energy to heat energy [20], [21], [22]. Under the influence of microwave dielectric heating, the generation of free radicals from the initiators facilitates the grafting reactions [23]. The acrylic monomer (DMAEMA) polymerizes on the radical sites generated on GG

Conclusion

Gels containing GG and different amounts of poly(DMAEMA) could be made by simultaneous graft copolymerization and crosslinking by microwave irradiation technique. The presence of poly(DMAEMA) imparts pH and temperature responsive character to the gel. Drastic increase in swelling is observed around pH 7.0. A representative sample, GG-g-poly(DMAEMA)-1 has been studied for removal of MO from aqueous solution and maximum adsorption of 25.8 mg g−1 was attained at pH 3. Experimental dye adsorption

Acknowledgement

One of the authors, KJS, thanks Department of Science and Technology, Government of India, New Dehli, for a Research Fellowship through PURSE grants.

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