Column-leaching and pilot-scale experiments were conducted to evaluate the use of biosolids (sewage sludges) to control the mobilization of metals from contaminated soils with smelting slags. The pilot-scale experiments using amended soils showed that Cu, Pb and Sb were retained, decreasing their concentrations from 250 mg/L, 80 mg/L and 6 mg/L, respectively in the leachates of contaminated soils, to <20 mg/L, 40 mg/L and 4 mg/L, respectively, in the amended material. Hydrogeochemical modeling of the leachates using Minteq revealed that the degree of complexation of Cu rose 56.3% and 57.6% in leachates of amended soils. Moreover, Cu may be immobilized by biosolids, possibly via adsorption by oxyhydroxides of Fe or sorption by organic matter. The partial retention of Pb coincides with the possible precipitation of chloropyromorphite, which is the most stable mineral phase in the pH-Eh conditions of the leachates from the amended material. The retention of Sb may be associated with the precipitation of Sb2
, which is the most stable mineral phase in the experimental conditions. The organic amendments used in this study increased some metal and metalloid concentrations in the leachates (Fe, Mn, Ni, As and Se), which suggests that the organic amendments could be used with caution to remediate metal contaminated areas.