Effect of co-pyrolysis of different plastics with sewage sludge on heavy metals in the biochar

The experiments of sewage sludge (SS) co-pyrolysis with four kinds of plastics (PE, PP, PS and PVC) were carried out in a high temperature tubular furnace to obtain four kinds of biochar (SSC_PE, SSC_PP, SSC_PS and SSC_PVC), respectively. The contents, residual rates, BCR speciation, leaching toxicity and potential ecological risk assessment of heavy metals (Cr, Mn, Ni, Cu, Zn, As, Cd and Pb) in biochar were studied. The results show that the residues of heavy metals except Cd are reduced by adding different kinds of plastics during SS pyrolysis. Compared with the biochar (SSC) obtained by SS pyrolysis, the addition of PE, PP and PS can promote the transformation of heavy metals speciation in biochar to more stable fractions (F3+F4) and achieve the immobilization of heavy metals. The addition of PVC only promotes the immobilization of Cr and As in biochar, while exhibiting an obvious activation effect on other heavy metals. The concentrations of leaching heavy metals in four kinds of biochar are lower than the limit value of the identification standard for extraction (GB5085.3-2007), and the potential ecological risks of the four kinds of biochar are all in a slight level. This work provides a good theoretical support for the process of the cooperative disposal of SS and waste plastics.