Transformation characteristics of polycyclic aromatic hydrocarbons during hydrothermal liquefaction of sewage sludge

•PAHs are re-formed in hydrothermal liquefaction of sludge (7.3–13.8 mg/kg).•Formation of PAHs is inhibited under certain conditions (300 °C and 0.1 g/mL).•After liquefaction, higher than 89% of PAHs exist in bio-oil, and a few in biochar.•Migration of PAHs to bio-oil increases with increasing reaction temperature/time.•Content/toxicity of PAHs in bio-oil can be reduced by adding suitable catalysts.