Preparation and characterization of xanthan gum-cl-poly(acrylic acid)/o-MWCNTs hydrogel nanocomposite as highly effective re-usable adsorbent for removal of methylene blue from aqueous solutions

Abstract

In this work, xanthan gum-cl-poly acrylic acid (XG-cl-pAA) hydrogel and xanthan gum-cl-poly acrylic acid/oxidized MWCNTs (XG-cl-pAA/o-MWCNTs) hydrogel nanocomposite was successfully surface modify by microwave assisted copolymerization, in which N, N′-methylenebisacrylamide (MBA) was used as a cross-linking agent. A copolymerization of acrylic acid (AA) onto xanthan gum (XG) initiated by microwave radiation method. Different weight percentages of oxidized MWCNTs were incorporated into the hydrogel matrix during the grafting reaction. An optimum hydrogel based on maximum swelling capacity further incorporated with oxidized MWCNTs to form XG-cl-pAA/o-MWCNTs. The structure, thermal stability, wettability and morphology of XG-cl-PAA and XG-cl-PAA/o-MWCNTs were characterized by fourier transform infrared (FTIR), Raman, X-ray diffraction (XRD), Thermogravimetric analysis (TGA), Contact angle, and scanning electron microscope (SEM). The effects of pH, contact time and equilibrium concentration on the MB dye adsorption were investigated batch wise. Optimal conditions were obtained at pH ⩾ 6 due to the generation of negatively charged groups (COO⁻) in the adsorbent, which can strongly interact with the positive charges from MB and time of adsorption equilibrium was achieved in 30 min. The XG-cl-PAA/o-MWCNTs hydrogel nanocomposite exhibited a very high adsorption potential, and its adsorption capacities calculated based on the Langmuir isotherm for MB was 521.0 mg/g at 30 °C. The dye adsorption data fitted well to the pseudo-first-order model and Langmuir model. The adsorption-desorption cycle of hydrogel nanocomposite was repeated several times without significant loss of adsorption capacity.

Introduction

Organic dyes are one of the most hazardous materials in industrial effluents that are discharged from various industries such as textiles, leather, food, pharmaceuticals, cosmetics and paper. These pollutants contaminate the environment by imparting undesirable color into water sources and impede light penetration of sunlight into water and they also slow down photosynthetic activity [1]. Consequently, the remove toxic organic dyes found in wastewaters have become an urgent issue in the field of water remediation. Moreover, many of the synthetic dyes are toxic, even at very low concentrations, carcinogenic and adversely affect the human health [2]. For that reason, environmental regulations demand the removal of these contaminants before discharging industrial wastewaters into water bodies [3]. Methylene Blue (MB) is an important cationic dye with broad used in textile and paper industries [4], [5]. MB has been shown to cause eye burns, heart rate increases, cyanosis, jaundice, tissue necrosis and mental confusion [6], [7]. In this study, MB was selected as a model cationic dye.

A variety of traditional methods have been employed for removal of synthetic dyes from industry effluent/wastewater. However most of these techniques possess certain drawbacks such as high operational cost, generation of toxic residues causing secondary pollution, or partial removal of pollutants [8]. The adsorption is globally recognized as the most promising method for the removal of synthetic dyes and metal ions from wastewater [9], [10], [11], [12], [13], [14].

In this concern, biopolymers are preferred due to their special features, including biocompatibility, low cost and ready availability. However, the use of biopolymers as adsorbents has inherent drawbacks such as poor specific surface area, poor mechanical properties, and their solubility in water. To overcome these shortcomings, it is important to develop modified polysaccharides as adsorbents with these desired properties [15], [16], [17], [18], [19], [20], [21].

Biopolymer-based hydrogels have been paid special attention as an ideal adsorption material. Hydrogels are used in various applications including health, drug delivery, beauty care products, farming and wastewater treatment. The use of hydrogels for the adsorption of synthetic dyes from contaminated water is of interest owing to their high adsorption limits, recovery capacities and regeneration for repeated recycles [22].

Recently, a significant amount of attention has focused on developing hydrogels nanocomposite because of their enhanced mechanical/thermal stability and swelling properties when compared with the hydrogels without additives. In this direction, the engineering of hydrogel nanocomposites have become an interesting area of research globally. Numerous approaches have been studied for the development of hydrogel nanocomposite, with a wide range of nanoparticles, such as carbon-based, montmorillonite, polymeric, ceramic, and metallic nanomaterials [22], [23], [24], [25], [26]. Multi-walled carbon nanotubes (MWCNTs) have attracted great attention because of their excellent mechanical, electrical and thermal properties [27]. Furthermore, their large specific surface areas, hollow and layered structures make them serve as excellent adsorbents [28], [29]. However, the practical use of MWCNTs as adsorbents has been overshadowed by their poor dispersibility in water [29], [30]. To overcome this shortcoming, some pre-treatments, such as functionalization with strong acid or surface modification with surfactants, polymers or proteins, are ideal to improve the dispersion of MWCNTs in water [31]. Ma et al. prepared Alkali-Activated MWCNTs for the adsorption of Methyl Orange (MO) and MB from aqueous solution [32]. The use of MWCNTs has been extended in the matrices of hydrogels for effective adsorption of various contaminants. Very recently [33], reviewed various applications devoted to MWCNTs-based hydrogel nanocomposite. A new generation of MWCNTs embedded hydrogel has been developed for various applications purposes among which effluent treatment could be mentioned [33]. Hosseinzadeh et al. prepared carrageenan/MWCNTs hybrid hydrogel nanocomposite for adsorption of crystal violet (CV) from aqueous solution [34]. The outcome of their study indicated that the adsorption capacity of the adsorbent was found to vary with MWCNT concentration, agitation time and initial pH of solution. Sun et al., investigated the synthesis of inorganic–organic composite hydrogel based on hemicellulose and MWCNTs for the removal of MB dye from aqueous solution [35]. Chatterjee et al., focused on the development of MWCNT-impregnated chitosan hydrogel beads for the removal of Congo red (CR) [36]. However, a direct incorporation of MWCNTs in the matrices of polymeric hydrogels has inherent disadvantages in terms of dispersion. To obviate these inconveniences, oxidation of MWCNTs is recommended in order to improve their hydrophilicity and dispersibility in aqueous solutions.

Herein, we report on the microwave-assisted synthesis, characterizations and applications of XG-cl-pAA and XG-cl-pAA/o-MWCNTs as an adsorbent to remove MB from aqueous solution. To establish and examine sorption isotherm equations for the dye (adsorbate) sorption based on the use of the XG-cl-pAA/o-MWCNTs (adsorbent) as well as to investigate and study the kinetic parameters of the adsorption process. The schematic representation of the present work were shown in Scheme 1.