Removal of C.I. Basic Green 4 (Malachite Green) from aqueous solutions by adsorption using cyclodextrin-based adsorbent: Kinetic and equilibrium studies

Abstract

Batch sorption experiments were carried out for the removal of C.I. Basic Green 4 (Malachite Green), a cationic dye from its aqueous solution using cyclodextrin-based material (CD/CMC material) as adsorbent. The operating variables studied were adsorbent mass, particle size, agitation speed, solution pH, contact time and initial dye concentration. Adsorption experiments indicated that the adsorption capacity was dependent of operating variables and the process was strongly pH-dependent. Kinetic measurements showed that the process was uniform and rapid. Sorption of dye reached equilibrium in 120 min. In order to investigate the mechanism of sorption, adsorption data were modeled using the pseudo-first-order and pseudo-second-order kinetic equations, and intraparticle diffusion model. On the basis of the non-linear chi-square test, it was found that the adsorption kinetics followed a pseudo-second-order model for the dye concentration range studied in the present work, suggesting that the rate-limiting step may be chemisorption. Equilibrium isotherm was analyzed using the Langmuir and the Freundlich isotherms. The characteristic parameters for each model have been determined. The Langmuir model yields a much better fit than the Freundlich model. The maximum sorption capacity was 91.9 mg/g at 25 °C and the negative value of free energy change indicated the spontaneous nature of adsorption.

Introduction

The removal of pollutants from wastewaters is a matter of great interest in the field of water pollution. Amongst the numerous techniques of pollutant removal, adsorption is an effective and useful process. Adsorption techniques employing solid sorbents are widely used to remove certain classes of chemical pollutants from waters, especially those that are not easily biodegradable. Dyes represent one of the problematic groups. Currently, a combination of biological treatment and adsorption on activated carbon is becoming more common for removal of dyes from wastewater. Although commercial activated carbon is a preferred adsorbent for color removal, its widespread use is restricted due to high cost. For this reason, alternative non-conventional sorbents have been investigated. It is well known that natural materials, waste materials from industry and agriculture and biosorbents can be obtained and employed as inexpensive sorbents. Recently, an extensive list of sorbent literature for dye removal has been compiled by Crini [1].

C.I. Basic Green 4 (BG 4, Malachite Green) is a N-methylated diaminotriphenylmethane dye which is most widely used for coloring purpose, amongst all other dyes of its category [2]. This cationic dye is generally used for the dyeing of cotton, wool, silk, paper, leather and also in distilleries. It is also used as therapeutic agent (fungicide, ectoparasiticide) and as antiseptic, but only for external applications on the wounds and ulcers. However, BG 4 has now become a highly controversial compound due to its reported toxic effects [3], [4]. It is known to be highly toxic to mammalian cells and to act as a liver tumour promoter [5], [6]. Its oral consumption is also hazardous and carcinogenic [3], [4]. However, despite the large amount of data on its toxic effects, BG 4 is still used in aquaculture and other industries [3].

A number of non-conventional sorbents has been studied in the literature for their capacity to remove BG 4 from aqueous solutions, such as sugarcane dust [7], [8], algae [9], [10], sawdust [11], [12], [13], [14], bottom ash [15], fly ash [16], de-oiled soya [17], maize cob [18], peat [19], iron humate [20], [21], mixed sorbents [22], microbial biomass [23], activated slag [24], waste product from agriculture [25], bentonite [26], magnetic nano-particles [27], coal [28], [29] and carbons prepared from agricultural wastes and industrial by-products [16], [30], [31], [32], [33], [34], [35], [36], [37]. Cyclodextrins have received less consideration as a source material for the preparation of adsorbents [1], [38].

Cyclodextrins (CDs) are torus-shaped cyclic oligosaccharides with an internal hydrophobic cavity containing 6–12 glucose units joined together by α-1,4-glycosidic linkages. The most common and well-studied CDs are alpha-CD (α-CD), beta-CD (β-CD) and gamma-CD (γ-CD), which consist of 6, 7 and 8 glucopyranose units, respectively [39]. CDs are natural molecules derived from starch. Beta-cyclodextrins are available commercially at a low cost. Over 30 years, CDs and CD-based materials have been greatly developed owing to CDs’ unique character which can form inclusion complex with other compounds through host–guest interactions. The cavity of a CD provides a hydrophobic space in which a guest can be sequestered in an aqueous medium. CDs are known to form stable complexes with a wide range of compounds, including dyes. They can be widely used in many fields such as pharmaceutical industry, organic and polymer synthesis, catalysis, chromatography and foods. They also have a potential use in wastewater treatment as a result of their abilities to form inclusion complexes with many pollutants.

Recently, in our group, we prepared and characterized new organic sorbents containing β-CD molecules for the removal of aromatic pollutants from aqueous solutions [38], [40]. Water-insoluble CD materials can be prepared in the form of gels or beads. We proposed the use of these materials as new adsorbents to adsorb benzene and phenol derivatives. The results showed that they exhibited interesting sorption properties. It was shown that the network structure of the material played a role in the adsorption mechanism by the presence of both physical adsorption (surface mass transfer) due to the polymer and chemical adsorption due to cyclodextin molecules. Pursuing our interest in such materials, we reported, here, the use of cyclodextrin-based sorbent (sample CD/CMC) for the sorption of C.I. Basic Green 4 (BG 4). Studies concerning the effects of operating variables (i.e. the effects of adsorbent mass, particle size, agitation speed, pH, contact time and initial dye concentration) on the sorption capacity were evaluated using the batch method. In order to investigate the sorption mechanism, the constants of sorption and intraparticle diffusion were determined using equations of Lagergren [41], Ho and McKay [42], [43], and Weber and Morris [44]. The equilibrium data have been analyzed using Freundlich and Langmuir isotherms and the characteristics parameters for each isotherm have been determined.