The sorption of cationic dyes onto kaolin: Kinetic, isotherm and thermodynamic studies A.R. Tehrani-Bagha a, ⁎, H. Nikkar a , N.M. Mahmoodi a , M. Markazi b , F.M. Menger c a Department of Environmental Research, Institute for Color Science and Technology, Tehran, Iran b Department of Inorganic Pigments and Glazes , Institute for Color Science and Technology, Tehran, Iran c Chemistry Department, Emory University, Atlanta 30322, Georgia abstract article info Article history: Received 12 April 2010 Received in revised form 25 August 2010 Accepted 26 August 2010 Keywords: Dye removal Cationic dye Kaolin Kinetic Isotherm Thermodynamic This study investigated the potential use of a low-cost inorganic powder (Persian Kaolin) for removal of Basic Yellow 28 (BY28), Methylene Blue (MB) and Malachite Green (MG) from aqueous solution. Kaolin was characterized via a laser particle size analyzer (PSA), scanning electron microscope (SEM), X-ray diffraction (XRD) and X-ray fluorescence (XRF) techniques. The effect of adsorbent dosage, dye concentration, initial pH and the presence of various electrolytes was studied. The isotherm data of dyes were correlated reasonably well by the Langmuir adsorption isotherm. The values of the adsorption capacity of kaolin towards the cationic dyes ranged from 16 mg/g to 52 mg/g, being probably dependent on the geometry of the dye molecules. The resulting degrees of dye removal were 65–99% for initial dye concentration of 10 mg/L and for kaolin loadings of 0.8–2.5 g/L. It was found that the adsorption of dyes on kaolin followed a pseudo-second order equation. The recovery and reuse of kaolin for a second and third time have also been studied. The thermodynamic studies showed that the dye adsorption onto kaolin is a spontaneous, endothermic and a physical reaction. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The use of different synthetic dyes in various industries such as textile, leather, paint, etc. produces the high volume colored wastewater. Some of these dyes are toxic and must be removed or degraded prior to environmental discharge. Several wastewater treatment methods have been used during the past years. These methods include chemical oxidation[1–5], filtration[6,7], biological [8,9], and adsorption [10–14]. Each of these methods has their own advantages and disadvantages which has been reviewed in several papers [15–20]. Among the mentioned methods, adsorption processes are attractive due to their high efficiency, low-cost, availability, and easy design. However, it should be noted that solid waste disposal is more costly and unfavorable than liquid treatment [21–25]. Although adsorption using activated carbon is attractive, owing to its high efficiency, it is a high-cost adsorbent. Recently, the natural inorganic and agricultural adsorbents have attained significant appeal [15–17,26]. Kaolin is one of the common low-cost natural clay adsorbents with a Al 2 Si 2 O 5 (OH) 4 or Al 2 O 3 ·2SiO 2 ·2H 2 O chemical structure. Kaolinite is an aluminosilicate consisting of chemical composition SiO 2 : 40 mol%, Al 2 O 3 : 20 mol% and H 2 O:40 mol%. It has a two-layer triclinic crystal structure consisting of stacked pairs of tetrahedral silica sheets and octahedral alumina sheets. Each pair of sheets is bound together through common oxygen atoms, while successive pairs are held together by hydrogen bonding between silica–oxygen and aluminum- hydroxyl groups. Hence, the resulting crystal contains a silica face of SiO 2 , an alumina face carrying AlOH groups, and edges that possess both SiOH and AlOH sites. The surfaces of kaolinite are believed to carry a constant structural negative charge due to the isomorphous substitution of Si 4+ by Al 3+ in the silica layer, whereas the charge on the alumina face and on the edges due to the protonation/ deprotonation of exposed hydroxyl groups depends on the pH of the solution [23,27]. Literature review shows that kaolinite has been used in some research works for effluent remediation and to remove of cationic dyes from aqueous solution[28–31]. In the present study, experiments were done to remove basic dyes, Basic Yellow 28 (BY28), Methylene Blue (MB) and Malachite Green (MG) from aqueous solutions by an adsorption technique using a Persian Kaolin as an adsorbent. Characterization of kaolin was studied by measuring scanning electron microscopy (SEM), particle size distribution, X-ray diffraction (XRD). Effects of adsorbent dosage, initial dye concentration, and solution pH, were studied. The equilibrium data were tested with the Langmuir and Freundlich isotherm models. Different kinetic models for the adsorption of dye are presented. In addition, the recovery and reuse of Persian Kaolin were investigated with the aim of minimizing the economic burden of the method. Desalination 266 (2011) 274–280 ⁎ Corresponding author. Tel.: + 98 21 22956126; fax: + 98 21 22947537. E-mail address: tehrani@icrc.ac.ir (A.R. Tehrani-Bagha). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.08.036 Contents lists available at ScienceDirect Desalination journal homepage: www.elsevier.com/locate/desal