4 th Int. Congress on Environmental Geotechnics, Rio de Janeiro, 11-15 de agosto de 2002; Vol. 1, pp.439-446; Ed. Balkema. 1 INTRODUCTION The compacted clay liners are usually constructed to prevent subsoil contamination at solid and liquid wastes disposal sites. The clay liner is a passive method of protection, based on the concept of interruption or at least minimization for a long period of time of the chemical flux from the waste deposit into the surrounding soil and water (both surficial and ground waters). In Brazil, the most used materials for construction of low permeability liners are the tropical soils such as the lateritic clays and/or the saprolitic residual soils, because they are more abundant and present appropriate geotechnical properties. The compacted clay soil layer is designed to work as physical barrier to the chemical species migration through the liner (low hydraulic conductivity and diffusion coefficients) and also as a filter, retaining the leachate constituents in the solid phase, mostly by sorption mechanisms. The clay soils retention capacity is well known, but a variety of mechanisms may contribute to the process. The most active constituents of the soil are the clay minerals and the soil organic matter. Tropical soils are composed primarily by kaolinite and hydroxides of Fe and Al, and all of them present a pH dependent particle surface charge. As reported in the literature (Grim 1968; Bohn et al. 1985; and others), both negative and positive charges do exist on the same mineral, and therefore the clay soil has the ability to adsorb both positive (cations) and negative ions (anions). The major sources of negative charges in clay particles are the isomorphous ion substitutions within the clay lattice and the ionization of hydroxyl groups attached to silicon of broken bonds on particle edges. These mechanisms are common to all clay minerals, but the relative importance of each of them to the clay overall ion exchange capacity varies from mineral to mineral. Tropical soils have therefore the capacity of retaining both cations and anions, and which one will prevail depends on the existing pH relative to the soil Zero Point of Charge pH value (ZPC): below the ZPC pH, the soil will retain more anions than cations, and above the ZPC pH, the opposite will occur. The ionization of functional groups in the soil particle surfaces are pH dependent, and thus generates a pH variable charge. This mechanism is a significant source of charge for the 1:1 clay minerals such as kaolinite (50% of total charge or more), and the only source for soil allophane and hydrous oxides such as hematite, goethite and gibbsite. In consequence, soil compositions dominated by these minerals have their net charge highly dependent on the solution pH value (Castro 1989; Evaluation of ions retention capacity of a residual soil of Rio de Janeiro, Brazil R.M.G. Mendonça COPPE-UFRJ, Rio de Janeiro, Brazil M.C. Barbosa & F.J.C.O. Castro COPPE-UFRJ, Rio de Janeiro, Brazil ABSTRACT: The paper presents the results of laboratory experiments investigating the cation and anion sorption parameters of a saprolitic residual soil from the State of Rio de Janeiro. The soil is a sandy silt with 58% of fines (%<#200 ASTM sieve), and a plasticity index Ip of 29%. The soil clay fraction is constituted by kaolinite, goethite and traces of vermiculite, and the soil is representative of the materials used for construction of liners at various industrial and domestic wastes disposal sites in Rio de Janeiro. The soil was completely characterized, including the determination of the physical and geotechnical index properties, mineralogical composition, and physico-chemical parameters such as the Cation Exchange Capacity (CEC), the soil natural pH and the Zero Point of Charge pH (ZPC), the exchangeable complex composition and the organic matter content. Distinct series of Batch tests were performed to investigate the soil sorption behaviour relative to the ions K + , Cl - and phosphate (occurring primarily as HPO 4 2- ), at two pH levels, 2.5 and 6.5. The soil Zero Point of Charge is around 3.5, close to the kaolinite typical value. The residual soil sorption behaviour was also compared to pure kaolinite, goethite and hematite samples measured sorption patterns for the phosphate ion at the same pH conditions.