Thermal Treatment of Hyperaccumulator Biomass: Heavy Metal Emissions and Ash Characteristics from Sedum plumbizincicola in a Fluidized Bed Reactor

Thermal conversion of biomass derived from phytoremediation—particularly Sedum plumbizincicola —offers a promising route for resource recovery but raises concerns regarding heavy metal (HM) emissions. Here, we systematically investigate the speciation, volatility, and partitioning behavior of Cd, Pb, and Zn during fluidized bed combustion of HM-laden biomass across a temperature range of 600–900°C. Volatility follows the order Cd ≫ Pb > Zn, with Cd exhibiting peak emission at 850°C due to its low boiling point and high chloride affinity. Sequential ash sampling reveals that > 85% of Cd and ~ 70% of Pb are captured in fine particulate matter (< 2.5 µm) collected by bag filters, whereas Zn predominantly remains in coarse bottom and cyclone ashes. Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS) confirms surface enrichment of HMs on fine ash particles, consistent with vaporization–condensation mechanisms. Notably, although Zn is not regulated under China’s GB 13271 − 2014 boiler emission standard, its high residual fraction in coarse ash presents a viable feedstock for secondary resource recovery. Our findings demonstrate that integrating optimized combustion temperature control with high-efficiency particulate filtration can simultaneously minimize atmospheric HM release and enable safe ash valorization—supporting circular strategies for phytoremediation biomass management in alignment with global soil protection frameworks (e.g., China’s Soil Action Plan, EU Soil Thematic Strategy).