Adsorption enhancement of methylene blue dye at kaolinite clay–water interface influenced by electrolyte solutions† Khushi Mukherjee, Ankit Kedia, K. Jagajjanani Rao, Satarupa Dhir and Santanu Paria * The contamination of surface water by dyes released from the effluent of textile industries is a major environmental concern. Adsorption is a cheap and easy separation technique to remove dyes from the effluent water. In this study, the adsorption behaviors of a widely used cationic dye, methylene blue (MB), on kaolinite clay surface in the presence and absence of electrolytes have been reported. The adsorption isotherms of MB in the absence of electrolytes follow the Langmuir model, however, in the presence of electrolytes they follow the Freundlich model. At a constant dye concentration (below the saturation equilibrium concentration), the dye adsorption increases linearly along with the increasing ionic strength of the electrolyte solutions. Among the four electrolytes (NaCl, CaCl 2 , Na 2 SO 4 , Na 2 HPO 4 ) studied here, Na 2 HPO 4 has the highest adsorption enhancement ability, 127% with respect to that of pure MB at a 2 mM initial concentration. This study shows that the adsorption capacity of kaolinite clay can be enhanced significantly by the use of electrolytes, which is very useful for the remediation of dye contaminated waste water. 1. Introduction Effluents from different industries such as textile, leather, paper, and printing are highly contaminated with dyes, and are becoming a major concern to the environment due to their potential toxic effects to humans and aquatic life. 1,2 Many traditional techniques for controlling environmental pollution such as occulation, chemical oxidation, and membrane sepa- ration have proved to be less effective processes for water soluble dyes. However, adsorption is considered to be a reliable and potential technique for removal of dyes from contaminated effluents. 3–5 The adsorption of dyes has been performed using several adsorbents such as activated charcoal, 6,7 polymeric resins, 8 sugarcane bagasse, 9 biological molecules, 10 and so on, which make the process economic, eco-friendly, and simple. Because of the anionic nature of the clay particles in aqueous media, a number of studies have been reported using clay mineral for removing dye from waste water. 11 Since clays are available in nature abundantly, clay based adsorption processes are in general economic, to treat a huge amount of industrial effluents. Among different clays, kaolinite is a widely used clay for different applications for its easy availability and inexpen- sive. Kaolinite clay structurally consists of two layers, tetrahedral Si layer and octahedral Al layer which are inter- linked. 12 The charged layers are neutralized by the presence of Na and K ions and the interlayer space also contain some amount of water. 12,13 The water present in the interlayer space of the kaolinite molecule can be removed by annealing the clay at high temperature, which may increase the dye adsorption capacity 13 but the processing cost will increase signicantly. Methylene blue adsorption from aqueous media has been reported using different clay adsorbents such as montmoril- lonite, nontronite, 14 laponite, 15 sepiolite, 16 Egyptian smectitic, 11 montmorillonite and nontronite 14 etc. The change in rheolog- ical 16 and other properties of clay aer adsorption of dye was also studied. The adsorption of methylene blue is also used to calculate the percentage of bentonite mud present in the dril- ling circulation uid, determination of cation exchange capacity, and surface area of clay. 17,18 The effects of different parameters on dye adsorption capacity such as pH, tempera- ture, adsorbent dose, cation exchange capacity, etc. were reported in the literature. 19,20 In a recent study it has been reported that, the acid treatment (HCl) of bentonite clay signicantly (95%) enhances the adsorption capacity of Reactive Red 223 dye onto bentonite clay. 21 In recent years some researchers including our group have reported that the addition of electrolytes signicantly enhances the adsorption capacity of surfactants on different solid surfaces, which in turn leads to decrease the consumption of surfactants. 22–27 In the present study, we report the effect of electrolytes of different valances on the methylene blue adsorption onto the kaolinite clay–water interface. The adsorption isotherms in the Interfaces and Nanomaterials Laboratory, Department of Chemical Engineering, National Institute of Technology, Rourkela 769008, Orissa, India. E-mail: santanuparia@yahoo.com; sparia@nitrkl.ac.in; Fax: +91 661 246 2999 † Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra03534a Cite this: RSC Adv. , 2015, 5, 30654 Received 26th February 2015 Accepted 26th March 2015 DOI: 10.1039/c5ra03534a www.rsc.org/advances 30654 | RSC Adv. , 2015, 5, 30654–30659 This journal is © The Royal Society of Chemistry 2015 RSC Advances PAPER