Methylene blue removal by alginate–clay quasi-cryogel beads

https://doi.org/10.1016/j.reactfunctpolym.2016.07.001Get rights and content

Abstract

Nowadays, dyes constitute a large part of pollutants and have long been used in dyeing, paper and pulp, textiles, plastics, leather, cosmetics, and food industries. Among the conventional dye removal techniques, adsorption is prominent. Research challenges are on developing low-cost, biodegradable and efficient adsorbents. This study investigates polysaccharide–clay composite beads for the removal of methylene blue dye. Alginate–montmorillonite composite beads were prepared and then a novel cryogelation-like strategy was developed by deep-freezing the alginate beads at − 21 °C. This process changed the morphology of beads and improved surface area and adsorption capacity. The results of the batch adsorption experiments were modeled using isothermal, kinetic, and thermodynamic models. It is found that the adsorption is favorable and follows physical mechanism, with an endothermic process up to 40 °C. The prepared composite beads are candidates for effective adsorbents for the dye removal.

Introduction

Environmental pollution due to synthetic dyes is a concern in water politics. Since dyes are important constituents in consumer goods, numerous industries discharge tons of synthetic dyes in the environment. Synthetic dyes are in a wide spectrum of complex molecular structure and functionality and some of them are known as toxic to nature [1], [2], [3]. They cause some dangerous results such as affecting aquatic ecosystem [4] by blocking sunlight. Depending on their types, dyes and their metabolic breakdown products can be harmful to human health as well [5], [6]. Therefore, it is important to eliminate dyes and other by-products from water sources. The challenging point in dye removal is their resistance to light, water, and chemical agents [7]. Many conventional treatment technologies for dye removal include coagulation and flocculation, photodecomposition, ion exchange, oxidizing agents, membranes and electrochemical methods. Among them, adsorption is reported as an economical and effective technique [1], [8]. Biomaterials are environmentally green materials and are capable of removing dyes from wastewaters, bringing the advantages of being eco-friendly, highly effective, and cheap [1]. Alginate bio-copolymer is a polysaccharide consisting of 1-4 linked α-D-mannuronate (M) and β-L-guluronate (G) monomeric units. In the presence of divalent cations, alginate forms a crosslinked cation–alginate gel system. This material is often doped with additives in order to improve its network structure and functionality. Alginates are successfully used in numerous applications and wastewater treatment studies are among these [9], [10].

Methylene blue (MB) is a heterocyclic aromatic compound, finding uses in medicine, pharmaceutics, dye industry, and analytical chemistry. Although MB is not very toxic, it has several effects on human health like shock, cyanosis, jaundice, and tissue necrosis [11], [12]. Some properties of MB are given in Table 1. Several papers about removal of MB were reported, such as removal by commercial activated carbon [13], tea waste [14], graphene oxide/calcium alginate composites [15], and montmorillonite (MMT) clays [16], [17]. The advantage of incorporating MMT into polymer matrices is the ease in operation, since filtration of solely MMT powder from the system may cause another difficulty. On our previous studies, incorporation of montmorillonite clay improved drug encapsulation of calcium alginate beads [18], [19] and increased the fluoride adsorption capacity of aluminum alginate beads [20], [21].

In this paper, MB was selected as a model dye for the removal from aqueous solution using calcium alginate–montmorillonite hybrid beads. A novel strategy for preparation of alginate–montmorillonite hybrid beads was developed by a simple and efficient freezing step. Cryogelation is a technique which the crosslinking reactions take place temperatures below the freezing point of the reaction media. This results in the formation of frozen solvent crystals and unfrozen microchannels that host the crosslinking reaction, which leads to highly porous structure after melting [22], [23]. This strategy is already used for polysaccharides [24], [25], [26]. Since the liquid is frozen just after the crosslinking, it is called quasi-cryogel in this paper. In that way, the morphology of beads were changed without any chemical modification but a simple freezing step and adsorption efficiency increased compared to the beads prepared by common method. Results indicate that proposed novel material is an efficient, cheap, and natural material for dye removal from aqueous solutions.

Section snippets

Materials

Alginic acid sodium salt with a high viscosity (14,000 cP of 2% solution) and montmorillonite K10 (MMT) were supplied from Sigma–Aldrich (St. Louis, Missouri, USA). Methylene blue was obtained from Merck (Darmstadt, Germany). Calcium chloride was from J.T. Baker (Phillipsburg, New Jersey, USA).

Batch adsorption experiments were performed using a Nüve ST-402 shaking water bath (Ankara, Turkey). Determination of methylene blue in the samples was done using a Shimadzu UV-1800 spectrophotometer

Characterization of beads

Fig. 1 shows the SEM images of the beads. AC beads maintain some spherical shape after drying; however, FAC beads have more pellet like shape. Freezing the beads produced significant cracks and pores on the surface of the bead, resulting in an increased surface area without losing the bead integrity. This is due to the rapid volume increase during the freezing process at − 21 °C. Fractured surfaces of both beads are caused by the instant gelation of alginate with calcium ions, preventing

Conclusions

Quasi-cryogel FAC beads are shown to have higher capacity for MB, the endothermic physical adsorption of MB on FAC beads is found to be favorable and the system follows typical second-order kinetics. Freezing the beads led to a significant increase in adsorption behavior by increasing the adsorbent surface area, without damaging the bead integrity. Where most studies required an addition of another adsorbent and/or surface modification by chemicals, the advantage of the novel method is in its

Acknowledgments

This work is a part of Güler Uyar's MSc thesis and funded by Istanbul Technical University.

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