Effects of pyrolysis temperature on properties of swine manure biochar and its environmental risks of heavy metals

Converting swine manure into biochar has increasingly been considered a useful method to reduce heavy metal (HM) availability; however, the associated environmental risks need further exploration. In this study, swine manure was pyrolyzed at diff;erent temperatures ranging from 200 to 700 degrees C; this was aimed at studying the effect of temperature on the swine manure biochar properties, while elucidating the environmental risks of HMs. The aromaticity and oxidation-resistance stability of biochar improved with an increase in the pyrolysis temperature. Cu and Zn concentration in biochar (except 200 degrees C) were significantly higher than manure (maximum increase of 91.9 % for Cu and 85.1 % for Zn). The residual ratios (R) of Cu and Zn gradually decreased as the temperature increased from 200 to 500 degrees C and increased over 500 degrees C. The R of Cd decreased dramatically as the temperature increased from 200 to 700 degrees C (79.1 % at 200 degrees C versus 1.34 % at 700 degrees C). The HM speciation in biochar was affected by the temperature. High temperatures caused HMs to transition from unstable to stable fractions, thereby reducing the leaching toxicity and environmental risks. There was also a significant correlation between the potential ecological risk index and the O/C ratio (Regression coefficients (R-2) = 0.98, P-value of a hypothesis test (p) <0.01), indicating that the O/C index is a good tool to assess the environmental risks of HMs in biochar. Based on the environmental risk assessment value, this study highlights that pyrolysis at 600 degrees C is probably the most optimal temperature condition for treating livestock manure with high concentrations of Cu, Cd, and Zn.