Polycyclic aromatic hydrocarbon (PAH) remediation during vermicomposting and composting: Mechanistic insights through PAH-budgeting

Knowledge on the mechanism of earthworm-induced removal of polycyclic aromatic hydrocarbons (PAH) in vermicomposting systems and interaction with nutrient mineralization and microbial growth is scarce in literature. Moreover, the PAH accumulation capacity of Eudrilus eugeniae has not been studied. This research, therefore, investigates the apportionment dynamics of 13 PAH compounds in aerobic composting and vermicomposting (Eisenia fetida and E. eugeniae) systems using novel budget equations. The PAH removal efficiency of vermicomposting was significantly higher (2–threefold) than composting with concurrent microbial augmentation (p < 0.01). However, the 4–6 ring compounds reduced more significantly (30–50%) than the 3-ring PAHs (p < 0.01), and E. eugeniae was an equally competitive PAH-accumulator compared to E. fetida. The budget equations revealed that although the bioaccumulation capabilities of earthworms were retarded due to PAH exposure, earthworms facilitated PAH-immobilization in decomposed feedstock. A marked increase in bacterial, fungal, and actinomycetes proliferation in PAH-spiked vermibeds with parallel removal of the PAHs indicated that earthworm-induced microbial enrichment plays a vital role in PAH detoxification during vermicomposting. Correlation analyses strongly implied that earthworm-driven mineralization-humification balancing and microbial enrichment could be the critical mechanism of PAH remediation under vermicomposting.