Adsorption kinetics and thermodynamics of an anionic dye onto sepiolite Mahir Alkan, O ¨ zkan Demirbas ß * , Mehmet Dog ˘an Balikesir University, Faculty of Science and Literature, Department of Chemistry, 10145 Balikesir, Turkey Received 18 July 2006; received in revised form 2 December 2006; accepted 4 December 2006 Available online 23 January 2007 Abstract The adsorption kinetics of CI reactive blue 221 (RB221), an anionic dye, onto sepiolite was investigated in aqueous solution in a batch system with respect to stirring speed, contact time, initial dye concentration, pH, and temperature. Experimental results have shown that the acidic pH, increasing initial dye concentration and temperature favours the adsorption. Experimental data related to the adsorption of RB221 on sepiolite under diﬀerent conditions were applied to the pseudo-ﬁrst-order equation, the pseudo-second-order equation and the intra-particle diﬀusion equation, and the rate constants of ﬁrst-order adsorption (k 1 ), the rate constants of second-order adsorption (k 2 ) and intra-particle diﬀusion rate constants (k int ) were calculated, respectively. The experimental data ﬁtted very well the pseudo-sec- ond-order kinetic model and also followed the intra-particle diﬀusion model up to 20 min, whereas diﬀusion is not only the rate control- ling step. The activation energy of system (E a ) was calculated as 7.73 kJ mol 1 . The thermodynamics parameters of activation such as Gibbs free energy, enthalpy, entropy were also evaluated and found that DG * , DH * , and DS * are 47.9 (49.4, 50.0, 51.3) kJ mol 1 , 5.30 (5.21, 5.13, 5.05) kJ mol 1 , and 145.3 (148.8, 142.7, 143.1) J K 1 mol 1 at 20 (30, 40, 50) °C, respectively. The results indicate that sepiolite could be employed as a low-cost material for the removal of textile dyes from eﬄuents. Ó 2006 Elsevier Inc. All rights reserved. Keywords: Adsorption; Sepiolite; CI reactive blue 221; Dye; Kinetic models 1. Introduction The treatment of textile waste comprising of dyestuﬀs and other non-biodegradable organics and inorganic poses considerable problems in the wastewater treatment indus- try. Conventional primary and secondary processes are insuﬃcient in removing these contaminants, with the majority of the research in this ﬁeld conducted using ter- tiary treatments [1]. Reactive dyes are the most common dyes used due to their advantages, such as bright colours, excellent colourfastness and ease of application [2,3]. They exhibit a wide range of diﬀerent chemical structures, pri- marily based on substituted aromatic and heterocyclic groups. A large number of reactive dyes are azo com- pounds that are linked by an azo bridge [4]. Many reactive dyes are toxic to some organisms and may cause direct destruction of creatures in water [5]. In addition, since reac- tive dyes are highly soluble in water, their removal from eﬄuent is diﬃcult by conventional physicochemical and biological treatment methods [6,7]. In general, there are ﬁve main methods of reducing colour in textile eﬄuent streams: adsorption, oxidation–ozonation, biological treatment, coagulation–ﬂocculation, and membrane processes [8,9]. Adsorption phenomenon in solution systems plays a vital role in many areas of practical environmental technol- ogy, which are mainly in water and wastewater treatment due to several advantages such as high eﬃciency, simple operation and easy recovery/reuse of adsorbent [1]. Amongst the numerous techniques of dye removal, adsorp- tion is a procedure of choice for the removal of dissolved organic compounds from wastewater. Adsorption has a speciﬁc advantage of removing the complete dye molecule, 1387-1811/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2006.12.007 * Corresponding author. Tel.: +90 266 612 1278; fax: +90 266 612 1215. E-mail addresses: malkan@balikesir.edu.tr (M. Alkan), ozkan@bali- kesir.edu.tr (O ¨ . Demirbas ß), mdogan@balikesir.edu.tr (M. Dog ˘an). www.elsevier.com/locate/micromeso Microporous and Mesoporous Materials 101 (2007) 388–396