RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

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 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-aminopyridine result from differences in their pKas. 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 Ca²⁺ is much more effective than K⁺ in competing with protonated aminopyridines for the X⁻ sites.