Journal of Hazardous Materials 165 (2009) 1142–1151 Contents lists available at ScienceDirect Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat Research Article Adsorption kinetics of maxilon yellow 4GL and maxilon red GRL dyes on kaolinite Mehmet Do˘ gan a,∗ , M. Hamdi Karao˘ glu b , Mahir Alkan a a Balikesir University, Faculty of Science and Literature, Department of Chemistry, 10145 Balikesir, Turkey b Mu˘ gla University, Faculty of Science and Literature, Department of Chemistry, Mu˘ gla, Turkey article info Article history: Received 21 July 2008 Received in revised form 9 September 2008 Accepted 27 October 2008 Available online 5 November 2008 Keywords: Kaolinite Dyes Kinetics Activation parameters abstract Kaolinite, a low-costly material, is the most abundant phyllosilicate mineral in highly weathered soils. In this work, the adsorption kinetics of maxilon yellow 4GL (MY 4GL) and maxilon red GRL (MR GRL) dyes on kaolinite from aqueous solutions was investigated using the parameters such as contact time, stir- ring speed, initial dye concentration, initial pH, ionic strength, acid-activation, calcination and solution temperature. The equilibrium time was 150min for both dyes. The results showed that alkaline pH was favorable for the adsorption of MY 4GL and MR GRL dyes and physisorption seemed to play a major role in the adsorption process. It was found that the rate of adsorption decreases with increasing temperature and the process is exothermic. The adsorption kinetics followed the pseudo-second-order equation for both dyes investigated in this work with the k 2 values lying in the region of 1.79 × 10 4 to 107.87 × 10 4 g/mol min for MY 4GL and 3.44 × 10 4 to 72.09 × 10 4 g/mol min for MR GRL. The diffusion coefficient values calcu- lated for the dyes were in the range of 3.76 × 10 -9 to 62.50 × 10 -9 cm 2 /s for MY 4GL and 1.98 × 10 -9 to 44.00 × 10 -9 cm 2 /s for MR GRL, and are compatible with other studies reported in the literature. The ther- modynamic activation parameters such as the enthalpy, entropy and free energy were determined. The obtained results confirmed the applicability of this clay as an efficient adsorbent for cationic dyes. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Various kinds of synthetic dyestuffs appear in the effluents of wastewater in some industries such as dyestuff, textiles, leather, paper, plastics, etc. [1]. Discharge of dye-bearing wastewater into natural streams and rivers from textile, paper, carpet, leather, dis- tillery, and printing industries poses severe problems because dyes impart toxicity to the aquatic life and cause damage to the aesthetic nature of the environment [2]. However, wastewater containing dyes is very difficult to treat, since the dyes are recalcitrant organic molecules, resistant to aerobic digestion, and are stable to light, heat and oxidizing agents [3]. Such effluents contain a number of contaminants, including acid or caustic, dissolved solids, toxic compounds and color [4]. Considering both volume-discharged and effluent combustion, the wastewater from the textile industry is rated as the most polluting among all industrial sectors [2]. Color is the first contaminant to be recognized in wastewater. The pres- ence of very small amounts of dyes in water is highly visible and undesirable [5]. During the past three decades, several physical, chemical and biological decolorization methods such as aerobic and anaero- bic microbial degradation, coagulation, and chemical oxidation, ∗ Corresponding author. Tel.: +90 266 612 10 00 fax: +90 266 612 12 15. E-mail address: mdogan@balikesir.edu.tr (M. Do˘ gan). membrane separation process, electrochemical, dilution, filtration, flotation, softening, and reverse osmosis, have been proposed [2]. However, all of these methods suffered with one or another limita- tion, and none of these were successful in removing color from the wastewater completely. Amongst the numerous techniques of dye removal, the adsorp- tion process is one of the effective techniques that have been successfully employed for color removal from wastewater [1]. Almost all the work related to adsorption techniques for color removal from industrial effluents was based on studies using acti- vated carbon. However, although activated carbon is a preferred sorbent, its widespread use is restricted due to high cost. In order to decrease the cost of treatment, attempts have been made to find inexpensive alternative adsorbents. There is growing interest in using low cost, commercially available materials for the adsorption of dyes. Various low-cost materials have been used for the removal of dyes. Such materials range from industrial waste to agricultural products. Clays have been used as promising low-cost adsorbents. Kaolin- ite is the most abundant phyllosilicate mineral in highly weathered soils. It is a 1:1 aluminosilicate comprising a tetrahedral silica sheet bonded to an octahedral sheet through the sharing of oxygen atoms between silicon and aluminium atoms in adjacent sheets. The tetra- hedral sheet carries a small permanent negative charge due to isomorphous substitution of Si 4+ by Al 3+ , leaving a single-negative charge for each substitution [6]. Both the octahedral sheet and 0304-3894/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2008.10.101