Low-temperature thermal-enhanced anoxic biodegradation of polycyclic aromatic hydrocarbons in aged subsurface soil

Abstract

Many deep soil environments are anoxic due to the scarcity of O2, wherein NO3–, SO42–, Fe3+, Mn4+, and HCO3– (CO2) can function as terminal electron acceptors (TEAs). Anoxic biodegradation of PAHs using NO3– and SO42– as TEAs has been the subject of extensive research. However, information related to the degradation of PAHs under methanogenic conditions using CO2 as TEA is poorly understood, although it is a critical pathway in elimination of PAHs from anoxic soils. However, the low degradation rate of PAHs under anoxic conditions is the primary bottleneck restricting the promotion and application of this technology. Therefore, in this study, a low-temperature (< 50 °C) thermal enhanced biodegradation microcosm experiment was conducted using three temperatures (15 °C, 30 °C, and 45 °C) under methanogenic conditions. The results revealed that the limited PAH removal was somewhat compensated for by elevated temperature (e.g., 45 °C) compared to lower temperature (e.g., 15 °C or 30 °C), and removal efficiency of 2-, 3- and 4-ring PAHs (except for benz(a)anthracene (BaA)) obeyed the order of 45 °C > 30 °C > 15 °C. Kinetic analysis and t-test confirmed this and indicated that the promotion of PAH removal is more significant when temperature is raised from 30 °C to 45 °C than from 15 °C to 30 °C. Soil microbial communities were significantly affected by both the incubation time and temperature, and the changes in bacterial and archaeal communities were enhanced with increasing temperature. Network analysis demonstrated that soil microbes tended to cooccur rather than coexclude. The bacterial and archaeal cooccurrence network became less complex after incubation. The numbers of nodes, edges, and modules declined after 250 days of incubation, indicating that microbial function tends to be simple with prolonged incubation time. These results provide new insights and a scientific basis for the bioremediation of PAH-contaminated sites.