Interactions between Three Tropical Soils and Municipal Solid Waste Landfill Leachate

Three tropical soils from Ghana, West Africa, were investigated in the laboratory for their potential as liners for waste containment. The key characteristic evaluated was the impact of municipal solid waste landfill leachate on the geotechnical, mineralogical, sorptive and diffusive properties and hydraulic conductivity of the soils. The observed unique characteristics of the soils included their silica: sesquioxide ratios that allowed them to be classified as either lateritic (ratio of 1.33–2.0) or nonlateritic (ratio greater than 2.0). After 5–11 pore volumes of leachate permeation through the soils, the specific surface of each soil decreased due to mineralogical transformations, while the cation exchange capacity of the soils increased. ${\mathrm{Na}}^{+}$ and ${\mathrm{K}}^{+}$ present at the exchange sites of the soils increased at the expense of desorbed ${\mathrm{Ca}}^{2+}$ . The effective diffusion coefficient, $D_e$ , obtained for potassium was $1.3–2.0\times {10}^{-10}{\mathrm{m}}^2∕\mathrm{s}$ while that of sodium was $7.3–14\times {10}^{-10}{\mathrm{m}}^2∕\mathrm{s}$ . New minerals formed in the soils included hydroxyapatite, pyromorphite, ferrihydrite, hydroxypyromorphite, and strengite. The mineralogical transformations, however, did not adversely alter the hydraulic conductivity of the soils. This finding, along with the observed relatively low $D_e$ values, suggests that the soils would be effective hydraulic barriers against the migration of potential contaminants in landfill leachate. The study also found that kaolinite and aluminum and iron oxyhydroxides with variable particle surface charge present in the soils allowed sorption of anions, such as, ${\mathrm{Cl}}^{-}$ , that are generally considered conservative (nonreactive) in liner-leachate compatibility studies on soils from temperate regions.