Bioremediation of a polycyclic aromatic hydrocarbon-contaminated urban soil: degradation dynamics and phytotransformation pathways

Purpose Bioremediation experiments of polycyclic aromatic hydrocarbon (PAHs) using ryegrass (Lolium perenne), a turfgrass in urban landscapes, were carried out aiming to explore the ryegrass-assisted degradation in an actual PAH-contaminated urban soil and the phytotransformation pathways of PAHs within plants. Methods Dynamics of plant-assisted degradation of 15 PAHs in pot soils were recorded over 100-day cultivation, and at the end, the accumulation of PAHs in ryegrass was investigated. Ryegrass with phenanthrene exposure was cultured hydroponically to investigate the phytotransformation intermediates using gas chromatography-mass spectrometry and ultra-performance liquid chromatography coupled to high-resolution mass spectrometry. Results Overall, dissipation of PAHs in soils exhibited an initial fast and subsequent slow degradation pattern. Eventually, on average, 55.7% and 47.1% of Sigma PAHs were removed in the planted and unplanted soils, respectively, with the high-molecular-weight PAHs highly eliminated particularly. Plant growth resulted in 4.8-11.1% more Sigma PAHs removed from soil compared to the plant-free controls. The PAHs all exhibited the bioconcentration and translocation factors below 1. The mass balance calculation showed that plant uptake accounted for < 1% of the dissipation increase while plant-promoted microbial biodegradation made the dominant contribution. A total of 7 metabolites were tentatively identified based on which possible transformation pathways were proposed. Conclusion The results indicated that ryegrass may act as a metabolic sink for aqueous PAHs while microbes play a dominant role in soil PAH dissipation via biodegradation. The results may pave the path for a scientific, safe, and effective implementation of landscape configuration to ameliorate urban soil pollution.