Mechanistic insights into the migration and speciation evolution of N, P, K and metals during hydrothermal liquefaction of food waste: A multi-phase distribution analysis

Hydrothermal liquefaction of food waste is a promising route for high-value utilization of organic wastes. However, the complex migration and transformation behaviors of heteroatoms and heavy metals among multiphase products remain a major barrier to practical application. This study investigates the fate and speciation evolution of nitrogen, phosphorus, potassium, and representative heavy metals in bio-oil, solid, and aqueous phases over a temperature range of 250–370 °C. Results show that increasing temperature enhances nitrogen enrichment in bio-oil, rising from 29.08% to 40.83%, accompanied by a transformation from protein-bound nitrogen to more stable aromatic heterocyclic forms, with pyridinic nitrogen reaching 34.84% at 370 °C. Phosphorus is predominantly retained in the solid phase, with recovery efficiencies of 68.00–88.50%, and elevated temperatures promote the conversion of non-apatite phosphorus into stable apatite minerals. Potassium exhibits strong hydrophilicity and preferentially migrates to the aqueous phase, where its partitioning ratio reaches up to 87.94%. Meanwhile, hydrothermally induced structural reorganization drives heavy metals to transform from unstable organically bound forms to stable lattice-bound residual fractions, substantially reducing their environmental bioavailability in the resulting biochar. This study reveals the intrinsic role of reaction temperature in regulating product polarity partitioning and elemental stabilization. An optimal HTL temperature window of 310–340 °C is identified, balancing bio-oil denitrogenation, efficient nutrient recovery, and safe utilization of biochar. The marked decrease in GCF highlights the effectiveness of HTL in mitigating heavy metal environmental risks. These findings provide theoretical guidance for targeted quality control and resource recovery in food waste HTL systems.