Abstract Vegetation has successfully been used for phytoremediation of heavy metal(loid) contaminated soils. Previous works found that the metal(loid)-enriched biomass can be converted into biofuels through pyrolysis. However, the potential emission of metal(loid)s at higher pyrolysis temperatures, the leaching potential of minerals in chars, and the quality of the products needs further consideration. In this work, the metal(loid)-enriched biomass was engineered by pre-mixing with magnesium carbonate to study the effect on pyrolytic product properties and metal(loid) deportment. Heavy metal contaminated mangrove grown in a land contaminated with a lead–zinc smelter slags was used as the biomass. The biomass and magnesium carbonate mixture as the feedstock was subjected to pyrolysis at temperatures from 300 to 900 °C under the heating rate of 10 °C/min. Results showed that the feedstock mainly decomposed at temperatures between 176 and 575 °C. Amongst the 10 studied metal(loid)s in this work, most elements exhibited more than 70% of elemental recovery in chars at pyrolysis temperatures up to 700 °C. Pyrolysis also enhanced heavy metal stability in chars produced at temperatures above 300 °C. This study indicated that co-pyrolysis of heavy metal contaminated biomass with magnesium carbonate enabled the pyrolysis temperature up to 700 °C with minimal environmental risks, providing a safe and value-added way of phytoremediation residual management.