Journal of Colloid and Interface Science 284 (2005) 383–392 www.elsevier.com/locate/jcis Surface complexation modeling of the sorption of 2-, 3-, and 4-aminopyridine by montmorillonite Jaslin Ikhsan a,b , Michael J. Angove a , John D. Wells a,∗ , Bruce B. Johnson a a Colloid and Environmental Chemistry Laboratory, La Trobe University, P.O. Box 199, Bendigo, Victoria 3552, Australia b Kimia FMIPA Universitas Negeri Yogyakarta, Karangmalang, Yogyakarta 55281, Indonesia Received 2 July 2004; accepted 20 October 2004 Available online 24 December 2004 Abstract The sorption of 2-, 3-, and 4-aminopyridine on K-saturated Wyoming (SWy-K) and Texas (STx-K) and Ca-enriched Texas (STx-Ca) montmorillonite was measured at 25 ◦ C with 10 mM KNO 3 or 3.3 mM Ca(NO 3 ) 2 as the background electrolyte. The aminopyridines adsorbed to montmorillonite at low pH, but not at high pH. Extended constant capacitance surface complexation models (ECCMs) and attenuated total reflectance-FTIR data indicate that aminopyridines sorb to the silica-like faces by cation exchange, forming outer-sphere complexes between aminopyridinium ions and permanent negatively charged surface sites (X − ). X-ray diffraction data and sorption kinetics suggest that sorption occurs not only at external X − sites but also at those in the interlayer spaces. Differences in the sorption behaviors of 2-, 3-, and 4-aminopyridine result from differences in their pK a s. The extent of sorption of aminopyridines by the montmorillonite samples (SWy-K > STx-K > STx-Ca) results from the higher cation-exchange capacity of SWy-K, and from the fact that Ca 2+ is much more effective than K + in competing with protonated aminopyridines for the X − sites.  2004 Elsevier Inc. All rights reserved. Keywords: Montmorillonite; Kaolin; Kaolinite; 2-Aminopyridine; 3-Aminopyridine; 4-Aminopyridine; Extended constant capacitance model; ATR-FTIR; XRD 1. Introduction Compounds containing pyridine rings are widely distrib- uted in nature and have several important applications, in- cluding analytical reagents, drugs, dyes, pesticides, and al- kaloids. Aminopyridines have two nitrogen atoms, each of which has a lone pair of electrons. The ring nitrogen atom is known to be more basic than the amino nitrogen [1] and con- sequently protonation of aminopyridine is expected to take place on the ring nitrogen atom. It has been suggested [2,3] that both 2- and 3-aminopyridine sorb to clay minerals by coordinating to Lewis acid sites or exchangeable cations, with the ring nitrogen atom involved rather than amino ni- trogen. On the other hand, Busca et al. [4] proposed that 2-aminopyridine was held to iron oxide by multiple bonding * Corresponding author. Fax: +61-3-54447476. E-mail address: j.wells@latrobe.edu.au (J.D. Wells). through both the amino group and the ring nitrogen, while 3- and 4-aminopyridine were bound only through the ring N. They compared the IR spectra of adsorbed 2-aminopyridine with that for adsorbed aniline and found similar changes in bands corresponding to NH stretching and deformation frequencies supporting the idea that the amino nitrogen of 2-aminopyridine plays an important part in the binding to iron oxide layers. Montmorillonite crystals have two different regions which have been implicated in sorption [5,6]. The external regions (faces plus edges) carry both permanent negatively charged sites and sites with pH-dependent charges. The interlayer re- gion has permanent negatively charged sites resulting from isomorphous substitution, with the surface charge compen- sated by hydrated interlayer cations. In principle aminopyridines can sorb either to external active sites or in the interlayer space of montmorillonite. In- tercalation of cations or organic species into the interlayer 0021-9797/$ – see front matter  2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.10.027