Removal of contaminant metals from fine grained soils, using agglomeration, chloride solutions and pile leaching techniques

Abstract

A leaching process based on the use of a HCl–CaCl 2 solution, with total chloride concentration 4 M, was investigated for the removal of contaminant metals from fine acidic soils. The possibility to apply this treatment on piles constructed on-site was also examined as a low cost treatment option. The soil sample used in the study was fine in texture, i.e. clay loam, acidic (pH 5.6), and contaminated mainly with Pb, up to 16 000 mg Pb/kg dry soil, due to past mining activities. The experimental work comprised all the treatment stages, including agglomeration of fine soil particles to increase the permeability of soil, leaching of the agglomerated soil in a laboratory column, removal of metals from the leachate, regeneration and recycling of the leaching solution and final washing of the treated soil. The initial agglomeration treatment resulted in the formation of coarse aggregates and the percolation of leaching solution through the soil column was maintained at high levels, i.e.75 ml/cm 2 per day, during the whole treatment. A low amount of HCl acid was required for the removal contaminants from this particular soil, i.e. 0.44 mol HCl/kg soil, due to the absence of acid consuming minerals. The extractions achieved were 94% for Pb, 78% for Zn and more than 70% for Cd. The co-dissolution of soil matrix was very limited, with a total weight loss about 3.5%. The final pH of the soil after washing was found to be 5.15, i.e. slightly lower compared to the initial pH of the soil. The results of this study indicate that chloride leaching, in combination with agglomeration and pile leaching techniques, can be a cost effective option for the removal of metal contaminants from fine acidic soils.