Colloids and Surfaces A: Physicochem. Eng. Aspects 398 (2012) 69–75 Contents lists available at SciVerse ScienceDirect Colloids and Surfaces A: Physicochemical and Engineering Aspects jo ur nal homep a ge: www.elsevier.com/locate/colsurfa Modeling sorption of fluoride on to iron rich laterite Meththika Vithanage a,∗ , Lakmal Jayarathna a , Anushka Upamali Rajapaksha a , C.B. Dissanayake a , M.S. Bootharaju b , T. Pradeep b a Chemical and Environmental Systems Modeling Research Group, Institute of Fundamental Studies, Hantana Road, Kandy 20000, Sri Lanka b DST Unit of Nanoscience, Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India a r t i c l e i n f o Article history: Received 28 December 2011 Received in revised form 11 February 2012 Accepted 15 February 2012 Available online 22 February 2012 Keywords: Diffuse double layer Chemisorption Monodentate mononuclear XPS FT-IR Inner-sphere a b s t r a c t The efficacy and the interface interactions of fluoride on laterite were investigated using batch methods; under various ionic strengths, pH, fluoride loading and diverse spectroscopy along with surface com- plexation modeling. The laterite used in this study was rich in iron (40%) and aluminum (30%). Proton binding sites were characterized by potentiometric titrations yielding pH ZPC around pH 8.72. Adsorp- tion of fluoride on laterite is strongly pH dependent showing a maximum adsorption at pH <5, though not affected by the electrolyte concentration. Experimental data were quantified with a 2pK general- ized diffused layer model considering two different surface binding sites for both protons and anions, using reaction stoichiometries. Surface complexation modeling showed that both Fe and Al sites of the laterite surface contributes to fluoride adsorption via inner-sphere complexation forming monodentate mononuclear interaction with laterite. Fluoride adsorption followed the Freundlich isotherm, indicat- ing multi-site complexation on the laterite surface. FT-IR spectroscopic data provides an evidence for increased hydrogen bonding, indicated by the broadening of the OH stretch features around 3300 cm -1 . © 2012 Elsevier B.V. All rights reserved. 1. Introduction In the natural environment, fluoride occurs as the fluoride ion, F - , which is considered as the most reactive electronegative ion. Weathering, dissolution and other pedogenic processes can release fluoride into groundwater. Fluoride contamination prob- lem is being recorded in several countries of the world such as Pakistan, India, Sri Lanka, Germany, Sweden, Netherlands and Japan [1–6]. Although the maximum World Health Organization (WHO) recommended level is 1.5 mg/L in drinking water, recent reports showed very high concentrations of fluoride around 5–10 mg/L around some localized areas in the Dry Zone of Sri Lanka [7]. Though fluoride is considered as an essential element for human health, especially for the strengthening of tooth enamel, excessive doses can be harmful [8,9]. Presence of fluoride in water does not impart any colour, odor or taste. Therefore it acts as a dose-dependent invisible poison such as arsenic in groundwater. Among various technologies used for fluoride removal [10–13], adsorption is considered as the most convenient, effective, and economical. Iron-based sorbents such as hydrous ferric oxide [14], brick powder [15], laterite [16], goethite [17] and aluminum based sorbents such as alumina [18], activated alumina [19], bauxite [20] have been found to be an effective and environmentally friendly ∗ Corresponding author. Tel.: +94 812232002; fax: +94 812232131. E-mail address: meththikavithanage@gmail.com (M. Vithanage). sorbent for the removal of fluoride. Later, iron and aluminum based mixed hydroxides showed good adsorption capacities for fluoride removal [21]. Long lasting weathering, intensified by high temper- ature and rainfall in the tropics produces laterite from many diverse rocks such as schists, gneisses, migmatites, granites through the laterization process where iron is enriched with highly soluble alkali and alkaline earths and less silica. The chemical composition may vary from conakryte (Fe 2 O 3 ·H 2 O·Al 2 O 3 ), ferricrete (Fe 2 O 3 ·SiO 2 ·Al 2 O 3 ·H 2 O), ortho–meta bauxite (H 2 O·Al 2 O 3 ·Fe 2 O 3 –Al 2 O 3 ·Fe 2 O 3 ) to latosols (SiO 2 ·Al 2 O 3 ·H 2 O·Fe 2 O 3 ) [22]. However, based on the parental rock composition, depth and climatic conditions, laterites show divergent chemical compositions [23]. Mineralogically, laterite is essentially a mixture of varying proportions of goethite, hematite, kaolin and gibbsite due to chemical alterations in weathering [22,23]. Being abundant in the tropics, laterite is commonly found in Sri Lanka and consists of higher concentrations of iron than in the Indian laterites [24]. Understanding the interactions between fluoride and laterite, will assist in predicting its adsorptive properties and provide an accurate description of fluoride mobility in the environment. Sarkar et al. [16] has studied the capability of laterite on fluoride adsorption. However, the surface complexation or mechanisms of adsorption have not been considered in any detail [16]. Also very few studies have attempted surface complexation modeling of the fluoride interaction with clay [25] but restricted to kaolinite and no 0927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2012.02.011