Influence of Pb Bonding and Speciation on the Pyrolysis Products of Contaminated Biomass

Phytoremediation is receiving increasing attention as an environmentally friendly remediation technique for contaminated soils, as it can target different contaminants, such as heavy metals. Phytoremediation processes produce large volumes of contaminated biomass that must be disposed of and possibly valorized. Among the possible treatments for heavy metal-contaminated biomass, a promising approach is to pyrolyze these biomasses. However, it must be considered that heavy metals can interact with the biomass pyrolysis decomposition pathways, resulting in variations in the yields and properties of pyrolysis products. In this work, the effects of lead (Pb) contamination on poplar biomass during slow pyrolysis were investigated. In particular, the focus of this paper is on the effect of the type of bonding of Pb with the biomass tissue, which is specific to the contamination type (authigenic or detrital), and on the effect of the chemical speciation of Pb. To study these aspects, poplar biomass was opportunely doped with lead acetate (Pb-(CH₃COO)₂) following different procedures aimed at simulating different bonds between Pb and biomass tissues. Moreover, to study the effect of Pb chemical speciation, poplar biomass was also doped with lead nitrate (Pb-(NO₃)₂). All the doped feedstocks, together with the parent biomass, were pyrolyzed under slow pyrolysis conditions at two pyrolysis temperatures (465 and 600 °C), and the obtained products, namely, biochar, bio-oil, and pyrolysis gases, were analyzed thoroughly. The obtained results show that the presence of Pb can indeed modify the pyrolysis pathways of lignocellulosic biomasses. The different bonding of Pb with biomass causes modifications in the yield of the liquid products. On the other hand, changing the Pb chemical speciation cause variations in the properties of all pyrolysis products. However, the extent of many Pb effects seems to be affected by the presence of inherent inorganics, such as alkali and alkali-earth metals (AAEMs).