Meta-analysis of the fate and transformation of heavy metals during pyrolysis process via data-mining the literature and machine learning

Abstract

Pyrolysis is attracting increasing attention as a promising waste disposal technology. However, the major challenge for the safe application of biomass pyrolysis products is the presence of heavy metals (HMs) in some biomass. This global meta-analysis was performed by compiling 21029 individual data points from 146 published articles. It elucidated the effects of pyrolysis temperature, feedstock characteristics on the migrations and transformations of HMs during pyrolysis process. Results showed the remaining ratios of Cd, Pb, Zn and Cr were decreased with increasing pyrolysis temperature, whereas increased with higher ash content in feedstock, indicating opposite effects of these parameters. The F1 and F2 of Cd, Pb, Cu, Cr, Zn, As and Ni were predominantly concentrated within 0–10 % at 600°C pyrolysis temperature, especially for As, Cd and Pb which were near zero. The proportions of the F3 fractions for Cd, Zn, and Ni increased first and then decreased with increasing pyrolysis temperature, attaining maximum values at 605.11°C, 513.39°C and 473.58°C, respectively. The remaining ratios of Cd, Pb and Zn were decreased with increasing C, H, O content in biomass. Ash content in biomass exhibited positive correlations with the proportion of Cd_F4, Pb_F4, Cr_F4 and Ni_F3 and negative correlations with Cd_F2, Pb_F2, Pb_F3, Zn_F1 and Mn_F1. S content exhibited positive correlations with Mn_F2 and Cd_F4 and negative correlations with Pb_F3, Ni_F4, Cd_F2, and Zn_F4. Random forest model identified pyrolysis temperature played dominant roles in the change of chemical speciation of HMs, followed by S and ash content in biomass.