pB 2+ABSTRACT: Two smectites with different layer charge localization were used to characterize the chemical and structural aspects of minerals treated with 1.9 x 10 .2 M and 1 M heavy metal solutions (Zn 2 § and pb2+). Natural and treated smectites were analysed by: (1) X-ray powder diffraction first at 25~ and relative humidity of 60% and then at increasing temperature up to 400~ (2) thermogravimetric and differential thermal analysis; (3) X-ray fluorescence; (4) atomic absorption and UV visible spectrophotometries. The sorption of both metals depends on 2:1 layer features and is enhanced when the total layer charge is mostly due to substitution in the octahedral sheet. The variability between Zn 2+-and pb2+-treated smectites in the interlayer water content depends both upon cation electrostatic features and upon smectite layer charge localization. The greatest water content was observed in Cation exchange between metals from interstitial solutions and 2:1 phyllosilicates is a major process affecting the retention of toxic contaminants in the vadose zone and influencing both plant growth and water quality.Recently, following Pearson's HSAB theory (1963, 1968), Xu & Harsh (1992) demonstrated that the alkali cation selectivity of the 2:1 clays could be quantitatively predicted from the net tetrahedral and octahedral charge of the mineral, which behaves as ligand, and from the absolute electronegativity and softness of the exchangeable cations. Cations and ligands are generally Lewis acids and bases, respectively, of different strengths: for example, alkali metal cations are hard, Zn 2+ and Pb 2+ are borderline, and Hg 2+ and Cd 2+ are soft acids, while OH-and H20 are hard, NO~ and SO~-are borderline, and CN-and CO are soft bases. Hard acids prefer to bind to hard bases, while soft acids prefer to bind to soft bases. The location of layer charge (X 1+) in smectite determines the strength of the clay ligand and can affect the selectivity of metals: smectite behaves as a soft base when the layer charge is located in the octahedral sites, while it behaves as a hard base