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 degrees C with 10 mM KNO3 or 3.3 mM Ca(NO3)(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-aininopyridine result from differences in their pK(a)s. The extent of sorption of arninopyridines by the montrnorillonite samples (SWy-K > STx-K > STx-Ca) results from the higher cation-exchange capacity of SWy-K, and from the fact that Ca2+ is much more effective than K+ in competing with protonated aminopyridines for the X- sites. (c) 2004 Elsevier Inc. All rights reserved.