Investigation of a basic dye adsorption from aqueous solution onto raw and pre-treated sepiolite surfaces E. Eren * , B. Afsin Ondokuz Mayis University, Arts and Science Faculty, Department of Chemistry, 55139 Kurupelit/Samsun, Turkey Received 4 March 2005; received in revised form 1 July 2005; accepted 7 November 2005 Available online 10 January 2006 Abstract The temperature, pH and ionic strength effects on the adsorption of crystal violet (CV) by raw, base activated and Al-saturated sepiolite samples were studied. Al-saturated sepiolite has relatively weak ability for CVadsorption compared to the raw clay, whereas the highest adsorp- tion observed for the base activated sepiolite may be due to the Na þ ions replaced part of Mg 2þ located at the edges of the channels. Different thermodynamic parameters for raw and pre-treated sepiolite samples indicate the adsorption of the dye component on a non-uniform surface. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Thermodynamics; Cationic dye; Charge reversal; Adsorption isotherm; Crystal violet; Sepiolite 1. Introduction Cationic dyes, commonly known as basic dyes, are widely used in acrylic, nylon, silk, and wool dyeing. These group dyes include a broad spectrum of different chemical structures, pri- marily based on substituted aromatic groups. Due to complex chemical structure of these dyes, they are resistant to break- down by chemical, physical and biological treatment. Further- more, any degradation by physical, chemical or biological treatments may produce small toxic and carcinogenic prod- ucts. Adsorption is known to be a promising technique, which has great importance due to the ease of operation and compa- rable low cost of application in decoloration process. Although activated carbons have been most widely used for the adsorp- tion of dyes, clay materials have been increasingly gaining attention because they are cheaper than activated carbons and their sheet-like structures also provide high specific sur- face area [1e3]. Sepiolite is a clay mineral with a unit cell formula Si 12 O 30 (OH,F) 4 (H 2 O) 4 $8H 2 O; a general structure formed by an alternation of blocks and tunnels [4]. In the inner blocks, all corners of silica tetrahedra are connected to adjacent blocks, but in outer blocks some of the corners contain Si atoms bound to hydroxyls (SieOH). These silanol groups at the external surface of the silicate, are usually accessible to organic species, and act as neutral adsorption sites. In addition to that, some isomorphic substitutions occur in the tetrahedral sheet of the lattice of the mineral form leading to negatively charged adsorption sites which are occupied by exchangeable cations. These characteristics of sepiolite make it a powerful sorbent for organic molecules and organic cations [5]. When a negatively charged particle, such as a sepiolite, is introduced into a solution containing cations, the cations in suspension are driven towards the surface of the particles through several mechanisms. (i) A neutral complex may be formed by binding a monovalent cation to a monovalent negative site on the sor- bent surface with electrostatic forces. (ii) Charged complexes arise from the binding of two monovalent organic cations to one monovalent site. (iii) Neutral sites occur at the external surface, a monovalently charged complex may form by the binding of one organic cation and a neutral site [6]. The aim of the present study is to investigate the adsorption behavior of a basic dye onto raw and pre-treated sepiolite sam- ples. Crystal violet (CV) was selected as the modeled basic * Corresponding author. Tel.: þ90 362 457 6020; fax: þ90 362 457 6081. E-mail address: erdalern@omu.edu.tr (E. Eren). 0143-7208/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.dyepig.2005.11.004 Dyes and Pigments 73 (2007) 162e167 www.elsevier.com/locate/dyepig