FATE OF HEAVY METALS DURING SLOW PYROLYSIS OF CONTAMINATED BIOMASSES

The contaminated biomasses from phytoremediation require a safe disposal because of the risk of release of the accumulated potentially toxic elements (PTEs) into the environment. Pyrolysis can be a good candidate as post-processing treatment because it would allow the reduction of the mass of the contaminated biomass, the concentration of the PTEs in the solid product and the combined production of a vapor phase heavy metal free energy carrier. The purpose of this work is to investigate the fate of heavy metals during the slow steam assisted pyrolysis with the aim to find an optimal temperature that could be a compromise between the weight reduction of the contaminated material, the concentration of the PTEs in the solid residue and good structural quality and chemical characteristics. Two species have been selected: Populus nigra L. and Arundo donax L. cultivated on disused industrial sites contaminated with Cd, Pb, Zn and Cu. For each species different parts of the plant have been tested in slow pyrolysis conditions (heating rate = 5 K/min) in the temperature range 653-873 K. The yield of the pyrolysis products and the concentration of the heavy metals in the chars have been studied as function of temperature. All the detected heavy metals start to devolatilize at about 653 K. Cd is almost absent in the chars produced at temperature higher than 673 K, whereas a significant fraction of Zn, Cu and Pb, about 85wt%, is retained in the char even at higher temperature. In view of further applications of the chars their internal specific surface area (BET surface) has been evaluated. Results demonstrate that BET surface increases with final temperature and is affected by the chemistry and the structure of the initial feedstock. Char produced at 873 K from Populus nigra branches has the highest BET surface, namely 325 m2/gr.