Changes in metal availability during sediment oxidation and the correlation with the immobilization potential.

Fractionation of metals (Pb, Cd, Ni, Zn, Cu and Cr) in severely contaminated sediment has been investigated to determine its speciation and eco-toxic potential at the beginning of the experiment and after 18 months of sediment oxidation. Pb, Ni and Zn showed a high risk, while Cu, Cd and Cr showed low to medium risk at the beginning. Oxidation yielded an increased mobility of all metals apart from cadmium. The ratio of the simultaneously extracted metals (SEM) and acid volatile sulfides (AVS) was found to be >1. Semi-dynamic and toxicity characteristic leaching tests were conducted to assess the effectiveness of solidification/stabilization (S/S) thermal treatment with clay and long-term leaching behavior of these metals. A diffusion-based model was used to elucidate the controlling leaching mechanisms. Applied S/S thermal treatment was effective in immobilizing metals, irrespective of their different availability in the untreated samples. The controlling leaching mechanism appeared to be diffusion.