Thermal conversion of phytoremediation plant waste into biochar was performed as a safe approach for organic waste management. In this work, a promising phytoremediation plant-Silphium perfoliatum L. was pyrolyzed for biochar production at 350, 550 and 750 degrees C. The long-term leaching risk of potentially toxic metals (PTMs) in the derived biochars and the oxidation resistance of the biochars were investigated. The results showed that PTMs in the biochar could transform into more stable and less toxic forms with the elevated pyrolysis temperature. PTMs leaching behavior in the 350 degrees C biochar was considerably sensitive to the leaching while that in the 750 degrees C biochar was not affected. The biochar produced at a higher temperature possessed a greater oxidation resistance with little amount of C loss, and the stronger oxidation resistance would prevent the release of metals bound to the organic matter. The findings of this study demonstrated that high-temperature pyrolysis was able to reduce the potential risk of PTMs and ensure carbon stability. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.