FATE OF HEAVY METALS DURING SLOW PYROLYSIS OF CONTAMINATED BIOMASSES

Restoration of soils contaminated by potentially toxic elements (PTEs) can be carried out through phytoremediation technique. Pyrolysis has been studied as post-processing treatment with the aim to reduce the mass of the contaminated biomass, concentrate the PTEs in the solid product and produce 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 and, at the same time, to follow the evolution of the structural characteristics of the solid residue in view of a further application. In this work Arundo donax L., (rhizomes and culms), cultivated on dismissed industrial sites contaminated with Cd, Pb, Zn and Cu, were pyrolyzed under slow pyrolysis conditions (heating rate = 5 K/min) up to a final temperature varying in the range 703-873K. The products yield and the distribution of the heavy metals in the chars were studied and compared with the results obtained by a previous experimental campaign conducted on Populus nigra L in the temperature range 653-873. All the heavy metals detected in Arundo sample start to devolatilize at about 703 K. Nevertheless, Cd is almost absent in the chars produced at temperature higher than 703 K, whereas a significant fraction of Zn, Cu and Pb is retained in the char even at higher temperature. In view of the further utilization of the obtained chars their internal specific surface area (BET surface) has been measured. Results demonstrate that BET surface increases with final temperature and is affected by the chemistry and the structure of the initial feedstock. Char obtained from Populus nigra L. branches has the highest BET surface at 873 K.