Effective nitrogen removal from coastal sediments at 10 °C using sediment microbial fuel cells

Nitrogen pollution in cold coastal sediments has become increasingly prominent. Sediment microbial fuel cells (SMFC) are efficient at preventing ammonia release and hypoxia, but low temperature conditions may significantly limit their efficiency. In this study, two SMFC systems were constructed and evaluated for their performance and potential microbial functions associated with nitrogen removal at 10 °C and hypoxic conditions. The results showed that both systems achieved efficient nitrogen removal at low temperature, with ammonium (NH₄⁺) removal efficiencies of approximately 84% in the water and 30.5% in the sediments relative to initial concentrations. NH₄⁺ removal was not accompanied by increased dissolved nitrous oxide (N₂O) accumulation. Notably, during the weak hypoxia, the charcoal composite anode reactor showed a 10.7% lower dissolved N₂O concentration than its control. The two anode designs showed different NH₄⁺ removal performances, with sediment NH₄⁺ decreasing by 34.6% in the charcoal composite anode reactor, while there was a 26.4% decrease in the stainless-steel anode. In addition, low working potential and limited DO may have inhibited sulfide oxidation at the anode, while NH₄⁺ removal was maintained. Microbial community analysis and functional prediction suggested that nitrogen removal in the system was related to potential ammonia oxidation and denitrification involving the anode microbial taxa SM1A02 and Marinicella. This study confirms that the tested SMFC systems can achieve effective NH₄⁺ removal in the investigated coastal sediment at 10 °C, providing a technical reference for nitrogen pollution management under cold hypoxic coastal conditions.