The effects of biochar addition to compost on change characteristics and passivation effect of heavy metals (Cd, Pb, Cu, Zn, Ni) were investigated during the process of sludge composting with two different composts (group A:with biochar; group B:without biochar) and land application of compost. The results indicated that the total amount of heavy metals (except Ni) did not change significantly during the process of sludge composting and land application of compost. Additionally, biochar addition had little effect on the total amount of heavy metals. During the sludge composting process, five heavy metals (Cd, Pb, Cu, Zn, Ni) were passivated. Sludge composting with the addition of biochar can decrease the available contents of heavy metals, and the passivation effect of heavy metals was significant (P<0.05). The passivation rate of the five examined heavy metals (Cd, Pb, Cu, Zn, Ni)ranged from 16.39%-43.10%, and the passivation effect for Zn and Ni was more significant. However, the passivation effect was not significant in the sludge composting process without the addition of biochar (P>0.05). The concentrations of heavy metals in soil increased with the application of sewage sludge compost products. In the short term, biochar had a certain passivation effect on the available heavy metals in soils with sludge compost application, but the effect was not significant.
Keywords: available heavy metals; biochar; land use; passivation; sludge compost.