Stabilization of heavy metals in biochar pyrolyzed from phytoremediated giant reed (Arundo donax) biomass

The pyrolysis of phytoremediated giant reed (Arundo donax) biomass could cause secondary pollution of heavy metals. The stabilization of heavy metals in the pyrolysis process with external materials such as Al2O3, CaCO3, FeCl3 and NaOH, was studied. The results showed that 37% As and 97% Cd in biochar were stabilized when giant reed powder was pyrolyzed at 250 °C with 5% Al2O3 for 2 h. Furthermore, 59% Pb in biochar was stabilized at 400 °C with 5% CaCO3 for 1 h. Under biochar produced in optimized pyrolysis conditions, Cd mainly existed in a residual fraction, while Pb and As mainly existed in oxidizable fraction in BCR analysis. In XRD analysis, As was only found in Ca2As2O7; Cd in biochar mainly existed in Cd (AlCl4)2, CdPbO3 or CdSO3; and Pb mainly existed as Pb3O2SO4.