Mechanisms of lead immobilization in treated soils

We investigated the leachability and immobilization mechanisms of lead in quicklime- and quicklime-fly-ash-treated artificial soils and lead-contaminated field soils. Artificially prepared clay-sand mixtures with two different types of clays at different clay-sand ratios enabled us to Investigate mineralogical and surface area effects. Batch extraction results indicate that Pb leachability in the montmorillonite-sand mixtures was much lower than in the kaolinite-sand mixtures. The effectiveness of the lime-based treatment was tested with different extraction tests and was compared to other effective immobilization techniques such as cement- and phosphate-based systems. The pH affects lead immobilization, and the apparent pH range of immobilization increases with increasing surface area (addition of fly ash). Lead immobilization was caused either by precipitation, adsorption, or chemical inclusion. However, the solubility of possible Pb precipitates (e.g. carbonate, sulphates, etc.), is much higher than the leachable Pb from both artificial and natural soils. Moreover, chemical extraction at pH 4 showed that much of the Pb is not adsorbed, while quicklim-fly-ash treatment in the artificial soils converted more than 70% of the total Pb to a strongly immobilized form. Therefore, the mechanism controlling the immobilization of Pb in lime-treated contaminated soils seems to be chemical inclusion in the newly formed pozzolanic products.