CONTAMINANTS MOBILITY IN ACID-GENERATING WASTES SUBSOIL SYSTEMS/

Redox reactions and microbial activity are considered as the principal causes for acid generation in the majority of solid waste disposal si tes. The generated acid liberates hazardous elements from the solid ph ase by enhancing dissolution/desorption reactions. Addition of alkalin e materials to acid-generating wastes can help, at least temporarily, in the immobilization of these elements. Depletion of alkalinity in th e wastes, or in the underlying soils, may result in the migration of c ontaminants to ground aquifers. The alkaline capacity of fine till (a type of subsoil used to embank uranium tailings) was determined to be 0.02 g (as NaOH)/g of till. The tailings leachate was examined and fou nd to be acidic (pH 2.0), probably due to pyrite oxidation accelerated by T. ferrooxidans bacteria. The interaction of such metals-laden aci dic leachate with the fine till was also examined, by using Cd-109 as a tracer. For a liquid/solid ratio of 10, it was found that cadmium re tention on fine till was effected by precipitation and sorption. Cd re tention was characterized as being pH dependent and reversible. This r evealed some information about the weak linkages between sorbed Cd and the outer mineral surface, and Cd-precipitate redissolution under aci dic conditions. Upon examining cadmium speciation (at pH 6.0), it was found that 40% of Cd was being sorbed and 50% of it was being precipit ated.