Chemical forms of heavy metals in pyrolytic char of heavy metal-implanted sewage sludge and their impacts on leaching behaviors

• Cu implantation inhibited the formation of Cu-immobilized speciation.
• Implantation of 0.5 wt% Cu contributed to stabilize Cr, Ni and Pb.
• More immobile speciations of Cr, Ni, Pb and Zn such as the residual forms are generated at high temperature.
• The optimal pyrolysis condition was 0.5% implanting concentration at 600 °C.
• Contents of leachable heavy metals were related to the residual fractions.

In this paper, we introduced a new sewage sludge (SS) recycle method, in which heavy metals were implanted into sewage sludge and then they underwent co-pyrolysis. Three-step sequential extraction procedure (BCR) was conducted to evaluate the four chemical forms of heavy metals in pyrolytic char, namely, acid-soluble fraction, reducible fractions, oxidizable fraction and residual fraction; and the fractionation results were compared with the leaching results of the concerned metals to explore the heavy metal speciations formed during co-pyrolysis. Experimental results indicated that Cu implanted into sewage sludge could influence the speciation distribution of Cr, Ni, Pb and Zn in pyrolytic char, as a result the leaching of heavy metals could be influenced. The sum of oxidizable fraction and residual fraction of Cr was deceased by ∼10%; the sum of Ni was increased by at least 13%; the sum of Zn was almost unchanged; the sum of Pb showed an overall increase trend. At the same time, the contents of Cu in oxidizable and residual forms decreased with Cu implantation. Pyrolysis temperature also affected the distribution of heavy metal speciation. High temperature contributed to the immobilization of Pb and Zn by significantly increasing the sum of oxidizable fraction and residual fraction. However, high temperature showed the less effect on the stabilization of Cr and Ni, and even had the negative impact on the suppression of Cu release. The leaching levels of Cu and Zn were related to the percentages of residual fractions, suggesting that the acid-soluble fraction, reducible fractions, and oxidizable fraction were unstable and prone to be extracted.