Effect of Feedstock and Temperature on the Distribution of Heavy Metals in Char from Slow Steam Pyrolysis of Contaminated Biomasses

Restoration of soils contaminated by potentially toxic elements (PTEs) can be carried out through phytoremediation technique able to reduce bioavailable PTEs by translocation them from the soil to the different organs of the plants. The European LIFE ECOREMED project is now implementing an eco-compatible protocol for soil restoration through phytoremediation in Litorale Domitio - Agro Aversano NIPS (South Italy, Campania region). Populus nigra (P. nigra) is among the fastest-growing trees among the short rotation coppice with an annual dry matter production of 17.8 Mg ha-1 y-1. Poplar trees, though not hyperaccumulators, are effective in uptaking PTEs from a very deep layer due to the efficiency of their root systems. However, after soil restoration, the problem of a safe disposal of the contaminated biomasses should be faced. Pyrolysis could be an eco-sustainable post-processing treatment capable to reduce the mass of the contaminated biomass, concentrate the PTEs in the residual char and produce a vapor phase heavy metal free energy carrier. In addition there are many possibilities for valorizing char as fuel in traditional and advanced power generation facilities, in the agronomic field for carbon sequestration and as fertilizer, as contaminant adsorbent in wastewater and soil, as adsorbent or catalyst for gas cleaning, as catalyst for syngas conversion to liquid hydrocarbons and biodiesel production, as raw material for supercapacitors and, finally, as filler in wood and polymer composites. The stability of PTEs and the physico-chemical properties of the produced chars are relevant to assess if such material could be converted into a valuable resource. Steam assisted pyrolysis tests of P. nigra, contaminated by Cd, Cu, Pb and Zn, have been conducted under slow pyrolysis conditions. The effect of the pyrolysis final temperature has been investigated in the temperature range 653-873 K in order to study the distribution of heavy metals in the pyrolysis products. The results show that the concentration of the metals in the chars along the temperature has a non-monotonous trend for P. nigra branches. The maximum concentration of Cd and Pb is measured in the chars produced at 653 K and 753 K, respectively. In chars derived from leaves Cd concentration shows the same trend as in branches, whereas the concentration of Cu, Pb, Zn increases in the whole temperature range. The metals recovery in the chars slightly decreases with temperature in all the examined cases, except for Cd that show a quite complete devolatilization even at temperature higher than 753 K.