Thermal conversion of a promising phytoremediation plant (Symphytum officinale L.) into biochar: Dynamic of potentially toxic elements and environmental acceptability assessment of the biochar.

Symphytum officinale L., as a hyperaccumulator, was pyrolyzed into biochar at 350, 550, and 750 °C, respectively. PTEs could be enriched in biochars except Cd volatilized greatly at 750 °C. In order to evaluate the environmental acceptability of biochars, a series of sequential and single extractions and biochar oxidation procedures were performed for simulating different environmental conditions. There was a sharp decline in PTEs release under various conditions when the temperature above 550 °C, indicating PTEs might transform into more stable forms at higher temperature. Thus, increasing the pyrolysis temperature is helpful for reducing biochar phytotoxicity, suppressing biochar leaching and improving biochar environmental safety. Moreover, the economic feasibility analysis of the biochar confirmed the practicability of it. Findings from this work illustrated that biochars pyrolyzed from Symphytum officinale L. at the temperature higher than 550 °C might be environmental acceptable, which is beneficial for biochar application.