Research paper Interactions of the dye, Rhodamine B with kaolinite and montmorillonite in water Krishna G. Bhattacharyya a, ⁎, Susmita SenGupta b , Gautam Kumar Sarma a a Department of Chemistry, Gauhati University, Guwahati 781014, Assam, India b Department of Chemistry, B N College, Dhubri 783324, Assam, India abstract article info Article history: Received 29 July 2013 Received in revised form 5 July 2014 Accepted 12 July 2014 Available online xxxx Keywords: Kaolinite Montmorillonite Acid-treatment Rhodamine B Adsorption Two of the most common clay minerals, kaolinite and montmorillonite, were allowed to interact with the cationic dye, Rhodamine B in aqueous solution. The clay minerals were used along with those modified by treatment with 0.25 and 0.50 M H 2 SO 4 . The influences of pH, interaction time, clay mineral amount, dye concentration and temperature on the adsorption process were monitored and explained on the basis of physico-chemical characteristics of the clay minerals and the behaviour of the dye molecules. The optimum pH for Rhodamine B adsorption was 4.0. The interactions attained equilibrium in ~180 min and the rate processes followed second order kinetics. It was observed that montmorillonite had a Langmuir monolayer capacity of more than eight times that of kaolinite. The modified clay minerals showed slightly higher adsorption capacity than the raw clay minerals. Adsorption was endothermic with enthalpy of adsorption increasing with rise in temperature and the endothermic interactions were driven by increase in entropy and decrease in Gibbs energy. The results suggested that the dye can be separated from aqueous solution by both clay minerals. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Clay minerals consist of extremely fine particles (size b 2 μm) and are characterized by a layered structure composed of a tetrahedral (silica) and an octahedral sheet (alumina). Kaolinite, Al 2 Si 2 O 5 (OH) 4 , has a 1:1 layer (or T\O) structure with one tetrahedral and one octahedral sheets as the repeating unit and montmorillonite, (M y + ·nH 2 O)(Al 2 - y 3+ Mg y 2+ ) Si 4 4+ O 10 (OH) 2 has a 2:1 layer (or T\O\T) structure with one octahedral sheet sandwiched between two tetrahedral sheets as the repeating unit (Brigatti et al., 2006). The surface properties of the clay minerals are determined by the actual chemical composition, nature of surface atoms (mostly oxygen and hydrogen), type and extent of defect sites, layer charge and the type of exchangeable cations present (Schoonheydt and Johnston, 2006). When isomorphous substitution is absent and there are few defect sites, O atoms in the surface occur in Si\O bonds, which have considerable covalent character and impart hydrophobicity to the surface. Isomorphous substitution introduces hydrophilic exchangeable cations that polarize the surface O-atoms (Nulens et al., 1996). The presence of OH groups on the surface (as in kaolinite) and defect sites also gives rise to hydrophilicity. In general, large surface area, considerable cation exchange capacity (CEC) and the presence of both Bronsted and Lewis acidity make the clay minerals model materials for adsorption study. Further chemical modification of the clay minerals could be brought about by treating with a mineral acid like HCl or H 2 SO 4 . In this process, protons replace the exchangeable cation like ferric, ferrous, aluminium and magnesium ions and the tetrahedral and octahedral sheets are partially dissolved creating new acid sites and making the material more porous. Kaolinite is resistant towards an acid as the aluminium cations are shielded by the Si\O network and the presence of Al\O\Si bonds and the dissolution rate of kaolinite in sulphuric acid is three times higher than that in hydrochloric acid of equivalent concentration (Komadel and Madejová, 2006; Panda et al., 2010). Clay minerals have been successfully used to remove dyes from aqueous solutions. For example, thermal and acid-treated Moroccan clay, bentonite, organo-modified montmorillonite, acid-activated bentonite, natural and acid-treated agalmatolite and kaolinite, natural and acid-activated montmorillonite are tested for adsorptive removal of the dyes, methylene blue (Mouzdahir et al., 2010), Congo red (Toor and Jin, 2012), Remazol Brilliant Blue R (Silva et al., 2012), Gentian Violet (Bellir et al., 2012), Coomassie Blue (Sales et al., 2013), and Reactive Blue 21 (Vanaamudan et al., 2014). The use of clay nano- adsorbents for dyes in aqueous solutions or suspensions has been reviewed by Liu and Zhang (2007). Rhodamine B is a cationic xanthene dye (Fig. 1) used in printing, textile, and photographic industries (Guo et al., 2011) and as a water tracer fluorescent material (Richardson et al., 2004). Rhodamine B causes irritation of the skin, eyes and respiratory tract and is implicated in causing carcinogenicity, reproductive and developmental toxicity, Applied Clay Science 99 (2014) 7–17 ⁎ Corresponding author. Tel.: +91 9864031987; fax: +91 361 2570599. E-mail address: kgbhattacharyya@gmail.com (K.G. Bhattacharyya). http://dx.doi.org/10.1016/j.clay.2014.07.012 0169-1317/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Applied Clay Science journal homepage: www.elsevier.com/locate/clay