Search for a command to run...
The role of pyrolysis of lignocellulosic biomass within the context of a progressive shift from fossil to renewable sources of energy and materials has grown in the past few years. RED II European directive supports the use of low-impact indirect land use change (ILUC) sources such as biomasses from marginal and degraded soils. However, these biomasses are often contaminated with heavy metals, whose presence poses environmental concerns due to their possible devolatilization during pyrolysis. In this work, the fate of lead (Pb) during pyrolysis was investigated focusing on two aspects: the effect of the contamination type, Pb being chemically bound to the biomass tissues or deposited as mineral on the biomass outer surface, and the effect of the initial chemical speciation. Poplar was opportunely doped to mimic these two types of contaminatation: first, an appropriate doping procedure with lead acetate (Pb(CH3COO)2) was applied to polar to study the effect of Pb contamination type. Then, poplar was doped with lead acetate and lead nitrate (Pb(NO3)2)) to investigate the effect of Pb chemical speciation. The treated and untreated poplar samples were pyrolyzed at different temperatures (465, 600, and 800 °C), and the produced biochars were analyzed for Pb content and characteristics. The obtained results highlight that the contamination type affects mostly Pb recovery, defined as the amount of Pb retained in the biochar after pyrolysis. In particular, the dry mixing doping procedure results in a considerably higher recovery at 800 °C than any other analyzed doping procedure (25% and about 0%, respectively). On the other hand, both Pb speciation and contamination type determine the chemical transformations that the heavy metal undergoes during pyrolysis and, consequently, the Pb mobility in the biochar. Moreover, Pb mobility, which is tied to the amount of Pb in soluble and exchangeable chemical forms, is strictly correlated with the presence of PbO in the produced biochars. The obtained results demonstrate that both the initial speciation of Pb and the type of contamination play relevant roles in the devolatilization of Pb during pyrolysis as well as in the mobility of the Pb retained in the biochar.