The post-treatment of biomass from phytoremediation is not yet a well-established practice due to the risk induced by the presence of Potentially Toxic Elements (PTEs). Pyrolysis is a thermochemical treatment that reduces the volume and weight of contaminated matter producing a combustible vapor phase and a solid residue (char). A key factor enhancing the economic and the environmental sustainability of biomass valorization through pyrolysis is the production of a market value char. A proper choice of the pyrolysis operating conditions should take into account the effect of final temperature on PTEs release, on the char physicochemical properties as well as on the mobility of retained PTEs. In particular, in this work the influence of both the temperature and the plant parts is discussed (branches and leaves of Populus Nigra L. and rhizomes and culms of Arundo donax L.) on the release of Cd, Pb, Cu, and Zn in the temperature range 653-873 K under steam assisted slow pyrolysis conditions. The mobility of the heavy metals retained in the chars was also studied as well as the product yields, the gas composition and char porosity. The results suggested that in presence of Cd it is necessary to operate at low-temperature (lower than 703 K) to obtain a heavy metals free vapor phase fuel, whereas in presence of one or more metals among Pb, Cu, and Zn, it is possible to conduct a pyrolytic treatment at higher temperatures, thus obtaining a char with high BET surface area and lower metals mobility.