On the compatibility and theoretical equations for mixtures of tropical soils and bentonite for barrier purposes.

Abstract

Vast regions of the globe are covered by tropical soils, increasing the perspective of using them for barrier design at solid/liquid disposal facilities. They comprise diverse pedological taxonomies, but have common features, specially the dominant mineral composition of quartz, kaolinite clay and Fe-Al-Mn oxides/hydroxides. Despite the vast occurrence, sometimes tropical soil samples may not be suitable for barrier purposes, which can be overcome using bentonite as additive, provide that technical evaluations are adopted. An important technical issue to be addressed is the compatibility between the barrier materials and the liquids to be disposed. Compatibility studies usually involve laboratory testing using direct and indirect methods, which were involved in this research. The indirect studies involved modified Atterberg limits and the direct approach was developed using direct permeation. The tropical soil samples were selected to be in different weathering stages, considering the laterization degree. The chosen bentonite content to compose the mixtures was 3, 6, 9 and 12% on a dry weight basis. The elected chemical solutions for these studies were: calcium chloride (CaCl2), nitric acid (HNO3), sodium hydroxide (NaOH), ethanol (C2 H5OH) and tap water. As a summary, the main research objectives were: (1) comparison between direct and indirect methods for compatibility evaluation; (2) assessment of the influence of the bentonite content on the compatibility of tropical soil samples; (3) investigation of the influence of the laterization degree on compatibility and (4) evaluation of theoretical equations to predict the compatibility of these samples through back analysis. The results indicate that: indirect and direct methods are similar from a qualitative point of view; bentonite is more sensitive to chemicals than the natural soil components; the laterization degree is not so influent on the compatibility and good compatibility predictions were obtained with the proposed theoretical equations.