Leaching of polycyclic aromatic hydrocarbons (PAHs) from sewage sludge-derived biochar

Polycyclic aromatic hydrocarbons (PAHs), a family of persistent organic pollutants with various negative health effects, are inherently formed during biochar pyrolysis. However, the knowledge regarding the leaching potentials and mechanisms of PAHs remains limited for biochar implementation to soil. In this study we evaluated the leaching behaviors of PAHs from sewage sludge-derived biochar pyrolyzed at different temperatures (300-700 degrees C) using the protocol of Synthetic Precipitation Leaching Procedure (SPLP) with deionized water. Leachate concentrations of sixteen U. S. Environmental Protection Agency PAHs increased with the increasing pyrolysis temperature, exhibiting an opposite pyrolytic temperature dependence with their concentrations in biochar. The total leachate PAH concentration peaked at 700 degrees C with 11.75 mu g/L, corresponding to 15.9% of total PAHs present in biochar. PAH leaching was associated with the release of hydrophobic organic compounds (HOCs) that created a mobile phase to facilitate the mobilization of PAHs into water. The enhanced release of calcium, aluminum, and barium from the biochars with pyrolysis temperature could also favor the leaching of biochar PAHs, due to the destruction of HOCs-(metal ions)-mineral linkages, which improved the release of HOCs and HOCs-bound PAHs; and because the extent of metal cross-linking in biochar is reduced, enabling better diffusion of PAHs through the inner matrix and thus accelerating their desorption.