Species-specific benefits in bioremediation of oil refinery sludge via large-scale vermicomposting process: a detailed comparison between Eisenia fetida and Eudrilus eugeniae

Bioremediation of oil refinery sludge (ORS) with earthworms offers a sustainable prospect in its pollution mitigation, however species-specific detoxification of ORS remains understudied. We conducted a 90-day mesocosm experiment to study the efficacy of two species, Eisenia fetida (EF) and Eudrilus eugeniae (EE), in ORS valorisation. Changes in physicochemical properties, heavy metal speciation (in gut and compost), and carbon fractions of the vermi-treated ORS composts were evaluated. EE yielded a 1.5-fold higher N increment than EF. EE reduced Ni's bioavailable fraction and residual fraction by 55.9 % and 39.4 %, respectively. EE showed an 11 % higher reduction in labile C pool than EF, with 1.95 % increment in its humic acid fraction. The gut bioaccumulation was 20-47 % higher in EE than EF across heavy metals. Moreover, the removal efficiency of heavy metals with EE was 24 % higher than EF. The higher vermiremediation benefit index (VBI) for EE (1.68) compared to EF (1.45) confirmed EE's efficient NPK mineralization and metal detoxification. Principal component analysis attributed earthworm growth, gut accumulation, and metal reduction to EE's higher VBI. Our findings underline the species-specific potential of E. eugeniae over E. fetida in ORS detoxification, while addressing the need to assess the long-term stability of vermiremediated ORS compost for safer agricultural use.