Evaluation of flash and slow pyrolysis applied on heavy metal contaminated Sorghum bicolor shoots resulting from phytoremediation

Treatment and/or disposal of metal contaminated biomass are still an unsolved problem. Knowledge of the metal distribution is of prime importance concerning the application of pyrolysis product streams. Sorghum bicolor (L.) Moench was cultivated in a semi-hydroponic system to assess its potential use in phytoremediation and biomass production. Plants were grown in a greenhouse using perlite as substrate, half-strength Hoaglandʹs solution as control (CTR) and the same solution supplemented with a mixture of Ni and Zn (CTM; 10 g m−3 each). Shoot and root biomass were determined and analyzed for their metals content. Flash and slow pyrolysis were performed on S. bicolor shoots at 450 °C. Biomass and pyrolysis products were analyzed focusing on metal distribution. Mass and energy balances were determined. S. bicolor delivered good shoot biomass with relatively moderate concentrations of Ni and Zn. Metal concentrations in the pyrolysis oils were below detection limits and almost all metals accumulated in the char. In fact, 99% of Ni and 98% of Zn were recovered in the char when a slow pyrolysis process was applied, while in flash pyrolysis conditions both metals were found back in the char and in the heating transfer medium. Furthermore, the percentages of char and oil were higher in slow pyrolysis compared to flash pyrolysis. Energy recovery in the char from slow pyrolysis was higher than flash pyrolysis. Flash and slow pyrolysis can likely offer a valuable processing method for metal contaminated biomass, thus limiting the waste disposal problem associated with phytoremediation of metals.