An evaluation of lead leachability from stabilized/solidified soils under modified semi-dynamic leaching conditions

Semi-dynamic leaching tests were conducted for artificial soils contaminated with lead oxide (PbO) in order to assess the long-term leaching behavior of lead (Pb). In order to simulate “worst case” leaching conditions, the semi-dynamic leaching test was modified using 0.014 N acetic acid (pH = 3.25) instead of distilled water. Lead contaminated artificial soils were prepared by mixing kaolinite or montmorillonite with fine quartz sand. The contaminated soil was then subjected to stabilization/solidification (S/S) treatment using quicklime, fly ash, or quicklime–fly ash combination. Fly ash was added in order to provide sources of aluminum, calcium, or silicate for the formation of precipitates and pozzolanic reaction products. The effectiveness of S/S treatment was evaluated by determining diffusion coefficients (De) and leachability indices (LX). A model developed by de Groot and van der Sloot [de Groot, G.J., van der Sloot, H.A., 1992. Determination of leaching characteristics of waste materials leading to environmental product certification. Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes, 2, STP 1123, T.M. Gilliam and C.C. Wiles, eds., ASTM, Philadelphia, 149–170.] was used in order to elucidate the controlling leaching mechanisms. Slurry tests were also performed by mixing PbO with quicklime and fly ash, in order to study the immobilization mechanisms of Pb. The resulting reaction products were identified using X-ray diffraction (XRD) analyses. Overall, the test results indicated that S/S treatment was effective in immobilizing Pb and the treated soils can be considered acceptable for “controlled utilization” based on LX values. The controlling leaching mechanism was found to be diffusion, in most quicklime-treated samples. Precipitation was identified as the most likely Pb immobilization mechanism in quicklime–fly ash-treated slurries. Lead silicate, Pb2SiO4 (which is highly insoluble) is the most probable precipitate that can be associated with the decrease in Pb leachability. No evidence of pozzolanic reaction products such as calcium silicate hydrates (CSH) and calcium aluminate hydrates (CAH) was identified by XRD.