Degradation of BDE-47 in mangrove sediments under alternating anaerobic-aerobic conditions

Polybrominated diphenyl ethers (PBDEs) resistant to degradation have significant environmental impacts. Anaerobic reductive debromination and aerobic oxidation of PBDEs by microorganisms are main removal mechanisms during natural attenuation, but previous studies often focused on the process under either aerobic or anaerobic condition leading to unsatisfactory removal. The present study aims to remove PBDEs by employing alternating anaerobic-aerobic condition, which is common in inter-tidal mangrove sediments, and elucidate the degradation pathways. During 40-week experiment, BDE-47 reduced with an accumulation of tri-BDEs and di-BDEs as debromination products in all sediments. However, the removal percentages of BDE-47 and the concentrations of debromination congeners varied among flushing regimes. Sediments under less frequent flushing regime (longer duration of aerobic period) had significantly lower concentration and proportion of debromination products, especially BDE-17, than that under more frequent regime (longer anaerobic period). BDE-17 then went through aerobic degradation pathway, as evidenced by the accumulation of its hydroxylation form. Microbial analyses further revealed that less frequent regime favored accumulation of biphenyl dioxygenase gene for aerobic degradation, while more frequent tidal regime promoted growth of dehalogenating bacteria for reductive debromination. This study first time demonstrated that PBDEs in contaminated sediments could be removed under alternating anaerobic-aerobic conditions.