Nature-based solution-inspired electroactive ecological floating bed: a technology for carbon-nitrogen removal and...

The remediation of polluted aquaculture water and sediment, which severely threatens aquatic ecosystems, is of paramount importance. In this study, a Nature-based Solution (NbS) of electroactive ecological floating bed coupled with an oxygen-producing submerged plants system (EEFB-PS) was designed, where a conductive floating-bed matrix functioned as an integrated biocathode embedded within the rhizosphere and was coupled to the anode in the sediment. Under a 12 h light/dark condition, submerged plants induced diel DO oscillations that enabled nitrification in the daytime and electroautotrophic denitrification at night, as supported by distinct nitrate-reduction signals in cyclic voltammetry (CV) and differential cyclic voltammetry (DCV). Compared with the control system, EEFB-PS achieved a low effluent concentration of COD (7.2 mg/L), TN (1.3 mg/L), NH₄⁺-N (0.1 mg/L), and NO₃^--N (1.1 mg/L), and enhanced nitrogen removal dominated by microbially mediated pathways. 16S rRNA and metagenomics revealed enrichment of electroactive and nitrogen-cycling bacteria (e.g., Geobacter and Nitrospira) and functional shifts toward intensified denitrification/DNRA. Notably, the abundance of nosZ gene, which encodes nitrous oxide reductase, was significantly up-regulated (by 13.8-fold), demonstrating the system's high potential to mitigate N₂O emissions. Meanwhile, EEFB-PS significantly mitigated the Global Warming Potential (GWP) to 4.6 mg CO₂-eq m^-2h^-1 by suppressing methanogenesis and promoting methane oxidation modules. Moreover, EEFB-PS rebalanced the aquatic food web by shifting phytoplankton dominance from Chlorophyta to Bacillariophyta, reducing phytoplankton density while increasing zooplankton abundance, thereby mitigating eutrophication risk. In summary, EEFB-PS demonstrates a rhizosphere-integrated, low carbon emission ecotechnology that couples with electron supply, plant-driven process, and microbial functional succession for aquaculture tailwater treatment and ecological restoration.