Enrichment characteristics and environmental risk assessment of heavy metals in municipal sludge pyrolysis biochar

Pyrolysis treatment of municipal sludge is a promising sludge treatment technology. However, due to the presence of heavy metals in sludge, the practical application of sludge pyrolysis products, especially biochar, is greatly affected. In this paper, the enrichment characteristics of different heavy metals in municipal sludge and its pyrolysis biochar and their leaching characteristics in the environment were studied. The environmental risks of municipal sludge and biochar were also assessed. The experimental results revealed that pyrolysis would promote the enrichment of different heavy metals in sludge in pyrolysis biochar. And with higher pyrolysis temperature, the enrichment is stronger. But for metals with high volatility such as Hg, Cd, Se, increasing the pyrolysis temperature will reduce their enrichment. Pyrolysis can inhibit the heavy metals in sludge leaching, and the effect of pyrolysis temperature on different kinds of heavy metals is different. The leaching rates of Se, As and Cr first increased and then decreased as the temperature of pyrolysis increased. The leaching rate of Cd increased as the temperature of pyrolysis increased. Considering the leaching rate of each heavy metal, the leaching toxicity of heavy metals in pyrolysis biochar is the lowest at the pyrolysis temperature of 400 °C. The risk assessment of municipal sludge and biochar revealed that the most harmful heavy metals in the original sludge were Hg, Cd and Se, followed by Cu and Zn. Sludge pyrolysis into biochar can reduce the content of Hg, Cd and Se. The use of copper and zinc water pipes should be reduced for sewage pipe laying to decrease the contents of Cu and Zn in municipal sludge.