Colloids and Surfaces A: Physicochemical and Engineering Aspects
Adsorption of Acid Blue 193 from aqueous solutions onto BTMA-bentonite
Introduction
Colored dyes are important water pollutants which are generally present in the effluents of the textile and other industries. The high level of production and extensive use of dyes generates colored wastewater which produces toxicological and technical problems and environmental pollution. Some dyes, for instance, are reported to cause allergy, dermatitis, skin irritation, cancer and mutation in human. Thus, the removal of color dyes from wastewater before they are contacted with unpolluted natural water bodies is important. Although several traditional chemical and biological processes exist for dye removal, the application of these techniques has been restricted due to the essentially nonbiodegradable nature of dyes, which are stable to light and oxidation [1], [2], [3].
Adsorption is one of the effective methods to remove colored textile contaminants from wastewaters. Adsorption phenomenon in solution systems plays a vital role in many areas of practical environmental technology, which are mainly in water and wastewater treatment due to several advantages such as high efficiency, simple operation and easy recovery/reuse of adsorbent [4], [5], [6]. Even though the most promising adsorbent for adsorption is activated carbon, which has a high surface area and a high adsorption capacity, it is very expensive, has high operation costs and there is a need for regeneration after each adsorption cycle [7], [8], [9]. Therefore, there is a growing need to find low cost and efficient, locally available materials for the removal of dyes. Some clays such as sepiolite [10], kaolinite [11], montmorillonite [12], smectite [13], bentonite [3], [14] and alunite [15] have been investigated for this purpose. These kinds of clays have a variety of surface and structural properties, high chemical stability, high specific surface area and high adsorption capacity and hence they can be used to remove dye from effluents.
The water-soluble anionic dyes are used to dye fabrics, such as wool, nylon and silk. Because of the weak interactions between the negatively charged surface in clays and anionic dyes, a few studies on the adsorption of acid dyes have been carried out using bentonite as an adsorbent [3], [16], [17], [18], but none of them has investigated adsorption of Acid Blue 193 (AB193) dye onto BTMA-bentonite. In addition, this kind of clay is mainly used as an emulsifying agent for aspaltic and resinous substances, as an adhesive agent in horticultural sprays and insecticides, in concrete mixtures, and as a plasticizer in ceramic materials. It is also used in refining oils and fats, drilling mud, foundry sands, in some detergents, cosmetics, pharmaceuticals, thickeners and extenders for paints, coating and filling of paper [19], [20]. Bentonite is natural clay which contains montmorillonite. The inner layer is consisted of an octahedral sheet situated between two tetrahedral sheets. Substitutions within the lattice structure of trivalent aluminum for quadrivalent silicon in the tetrahedral sheet and of ions of lower valence, especially magnesium, for trivalent aluminum in the octahedral sheet result in unbalanced charges in the structural units of clays. The above factors generally cause a good adsorbent for the removal of dye in aqueous solutions [3], [21].
The surface properties of bentonite may be greatly modified with a surfactant by simple ion-exchange reactions to lead van der Waals interaction between organic surfactant cations and adsorbate. The modification of clay surface with surfactant is called as organoclay to cause to transform organophobic to strongly organophilic and therefore the adsorption capacity increases [22]. This kind of surfactant-modified organobentonite has been used extensively for a wide variety of environmental applications [23].
The characteristics of the adsorption behavior are usually understood by means of both equilibrium isotherm and adsorption kinetics. The adsorption isotherm is also an inevitable tool for the theoretical evaluation and interpretation of thermodynamic parameters including changes in Gibbs free energy, entropy and enthalpy. In this type of study, the construction of an adsorption isotherm plays an important role in understanding the adsorption mechanism. For adsorption kinetics, temporal variations of the amount of adsorption are measured and thus the obtained experimental data are used to develop a proper kinetic model [5], [6].
The present paper is to investigate the possibility of BTMA-bentonite as an adsorbent for removal of an anionic acid dye, which is, namely Acid Blue 193, from aqueous solution by adsorption method. The adsorption capacity of AB193 onto BTMA-bentonite was carried out using various kinetic models. The experimental data were fitted into Langmuir, Freundlich and Dubinin–Radushkevich (D–R) equations to determine which isotherm gives the best correlation to experimental data. The calculated thermodynamic parameters from the Langmuir isotherm constant were also used to explain the nature of adsorption.
Section snippets
Materials
A commercial textile dye AB193 (Isolan Dark Blue 2-SGL) was obtained from Dystar, Turkey, and used without further purification. The chemical structure of AB193 is depicted in Fig. 1. Bentonite was provided from Çanakkale, Turkey. It was crushed, ground, sieved through a 63-μm sieve and dried at 110 °C in an oven for 2 h prior to use. The determined cation exchange capacity (CEC) and the surface area of the natural bentonite by the methylene blue method [24] were 980 mmol kg−1 and 767 m2 g−1,
Chemical composition of bentonite
The chemical composition of bentonite obtained by using EDX analysis, given in Table 1, indicates the presence of silica and alumina as major constituents along with traces of sodium, potassium, iron, magnesium, calcium and titanium oxides in the form of impurities. XRD results combined with EDX analysis show that most of the aluminum is in the form of bentonite. XRD also indicated the presence of free quartz in bentonite. It is, thus, expected that the adsorbate species will be removed mainly
Conclusion
From the foregoing experiments, the reasonable concludes that BTMA-bentonite is an effective adsorbent for removing AB193 from aqueous solution, and it can be represented as a suitable adsorbent and environmentally clean utilization of wastewater.
The kinetics of adsorption of AB193 onto BTMA-bentonite is studied on the basis of the pseudo-first-order, pseudo-second-order and intraparticle kinetic models under several different initial dye concentrations, temperatures and pH. A
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