A montmorillonite clay (M) sample from the Amazon region, Brazil, was intercalated with pyridine (Py), dimethyl sulfoxide (DS) and 3-aminopropyltriethoxysilane (APS). The chemically modified montmorillonite (M(P/APS)) sample showed modification of its physical-chemical properties including: specific area 41.39 m(2) g(-1) (M) to 198.45 m(2) g(-1) (M(P/APS)). Solid-state (29)Si CPMAS/NMR of the silylated montmorillonite samples showed Q(2) and Q(3) signals as well as T(2) and T(3) Signals. The appearance of T(2) and T(3) signals can be attributed to the grafting of APS to the interlayer surface silanol groups. The natural and modified clays were used for mercury cation adsorption from aqueous solutions at room temperature and pH 3.0. The energetic effects (Delta(int)H degrees, Delta(int)G degrees and Delta(int)S degrees) caused by mercury cation adsorption were determined through calorimetric titrations. (C) 2008 Published by Elsevier Ltd.