Paddy field – A natural sequential anaerobic–aerobic bioreactor for polychlorinated biphenyls transformation

• Paddy fields hold significantly lower level of PCBs than drylands, especially highly-chlorinated PCBs.
• Microbial dechlorination of PCBs is favored under flooded conditions in paddy field.
• Aerobic biodegradation of PCBs is benefited under dry conditions in paddy field.
• PCBs dechlorination rate is accelerated in rice planted paddy field compared to the unplanted one.
• Alternating anoxic–oxic environment in paddy field led to the sequential aerobic–anaerobic transformation of PCBs.

The environmental pollution and health risks caused by the improper disposal of electric and electronic waste (e-waste) have become urgent issues for the developing countries. One of the typical pollutants, polychlorinated biphenyls (PCBs), is commonly found in farmland in Taizhou, a major hotspot of e-waste recycling in China. This study investigated the amount of PCB residue in local farmlands. Biotransformation of PCBs was further studied under different water management conditions in paddy field with or without rice cultivation, with a special focus on the alternating flooded and drying processes. It was found that paddy field improved the attenuation of PCBs, especially for highly chlorinated congeners. In the microcosm experiment, 40% or more of the initial total PCBs was removed after sequential flood–drying treatments, compared to less than 10% in the sterilized control and 20% in the constant-drying system. Variation in the quantity of PCBs degrading and dechlorinating bacterial groups were closely related to the alteration of anaerobic–aerobic conditions. These results suggested that alternating anoxic–oxic environment in paddy field led to the sequential aerobic–anaerobic transformation of PCBs, which provided a favorable environment for natural PCB attenuation.

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