Lignin depolymerization as a source of polycyclic aromatic hydrocarbons in agricultural waste composting

To clarify if lignin microbial depolymerization contributes to polycyclic aromatic hydrocarbons (PAHs) in composting systems, this study analyzed aerobic composts of corn stover, rice straw, and lawn waste via FTIR, Two-Dimensional Correlation Spectroscopy, ¹³C NMR, and HPLC, focusing on lignin depolymerization, PAH patterns, and their correlation. Results showed lignin content decreased overall in all systems. FTIR confirmed systematic lignin degradation via side-chain cleavage, aromatic ring fragmentation, and bond breaking, with distinct depolymerization pathways among feedstocks. PAH profiles differed by feedstock: straw-based composts contained acenaphthene and benzo[a]pyrene, whereas lawn-waste compost contained acenaphthene, fluoranthene, benzo[ghi]perylene, and newly formed indeno[1,2,3-cd]pyrene. Two-Dimensional Correlation Spectroscopy analysis revealed that the difference in the transformation sequence of lignin functional groups among different feedstocks is the core mechanism leading to the variation in PAH formation patterns. Furthermore, the characteristic peak of bridgehead aromatic carbons was detected in the late-stage compost samples by ¹³C NMR, directly confirming the formation of fused aromatic ring structures during lignin depolymerization, providing structural evidence that lignin is a potential source of PAHs in composting systems. This study elucidates their molecular correlation, aiding safe agricultural waste composting and risk control.