Removal of wastewater-borne polybrominated diphenyl ethers by mangrove wetland microcosms

Polybrominated diphenyl ethers (PBDEs) are persistent and toxic organic
pollutants in environments. They are difficult to remove by conventional wastewater treatment processes, and alternatives are needed. A greenhouse experiment was conducted to investigate the removal of wastewater-borne PBDEs by constructed mangrove wetland planted with one-year old seedlings of Kandelia obovata (Ko) under different tidal regimes, that is, everyday (Te), every-two-day (To), every-two-week (Tt) and no tidal flushing (Tn). During the 8-month experiment, all six congeners of PBDEs in wastewater, including
BDE-47, -100, -99, -154, -153 and -209, were not detected in effluents, suggesting the constructed mangrove wetland was effective in removing PBDEs under all tidal regimes. Around 10 to 42% of the wastewater-borne PBDEs were retained in sediments, depending on types of congeners and duration of tidal regimes. Significantly higher concentrations of PBDEs were found in upper than bulk sediments, and rhizosphere sediment had the lowest retention. Tidal regimes also affected the accumulation of PBDEs in sediment. The lowest
concentration was found in Te regime with frequent tidal flushing creating the most anaerobic sediment, suggesting that PBDEs retained in sediment were then removed by reductive debromination. This study revealed that mangrove wetland could be constructed to remove PBDEs, and the immobilization of PBDEs congeners in sediment depended on sediment locations, types of congeners and tidal regimes.