Effects of pyrolysis temperature and time on the speciation and bioaccumulation of heavy metals derived from sludge

Objective To reduce the environmental risk of heavy metals in land utilization of sewage sludge.

Method The speciation and concentration of heavy metals were investigated in a batch system by considering the effects of various parameters including carbonizing temperature and time. A pot experiment was conducted to investigate effects of different sludge biochar on the growth and heavy metal accumulation of maize plant.

Result and conclusion The pyrolysis temperature was a key factor to affect the speciation and concentration of heavy metals in biochar. Increasing pyrolysis temperature might cause the increase of the total metal contents in biochar. There were the highest increases in the concentrations of Cu, Zn, Pb and Ni at 350-500℃, while Cd and Cr at 200℃; The ratio of exchangeable and carbonatebound heavy metals in carbonized products, which had high environmental risk, significantly reduced when increasing pyrolysis temperature, but the concentration was independent. The optimal pyrolysis temperature was 350℃ for passivating Cu, Zn, Cd and Ni, but 500℃ for passivating Pb. The pot experiment results exhibited that application of pyrolytic sludge significantly promoted maize plant growth. The higher pyrolysis temperature and longer time under a certain condition, the more significant the effect was. Compared to raw sludge, appropriate carbonized sludge can significantly reduce accumulation of heavy metals of maize plant, which reduces the risk of heavy metals into the food chain.