One bead, triple feats: Alginate-encapsulated Fe/Ni-Citrobacter microsphere (AFC-Syn) unlocks concurrent contaminant elimination, nutrient recovery and self-regeneration in municipal wastewater treatment

Brominated flame retardants (BFRs) resist degradation in conventional wastewater treatments, posing ecological and health risks. Herein, we innovatively developed an alginate-encapsulated Fe/Ni- Citrobacter synergistic system (AFC-Syn) for synergistic BFRs degradation in a continuous stirred-tank reactor (CSTR) treating real municipal wastewater. Remarkably, the system achieved sustained removal efficiencies of >90% for Tetrabromobisphenol A (TBBPA), Decabromodiphenyl ether (BDE-209), and Hexabromocyclododecane (HBCD), over 30 days, with complete degradation within 48 h, while simultaneously controlling COD and nutrient levels (>85% removal). In the system, in addition to direct electron transfer between Fe/Ni and membrane-binding proteins, the carboxyl groups in alginate are proposed to potentially serve as electron-hopping sites to enable long-range electron transport. Meanwhile, the alginate-encapsulated Fe/Ni continuously release Fe 2+ /Ni 2+ , which act as soluble electron carriers to accelerate electron transfer—either to BFRs for debromination or to microorganisms to stimulate their growth. Ultimately, the electrons are captured by dehalogenase, cleaving C Br bonds and driving the stepwise debromination. Metagenomic and metatranscriptomic analyses revealed persistent enrichment of keystone functional genera, collectively accounting for over 30% of the microbial community. These microorganisms activated interconnected gene networks, thereby establishing an integrated functional axis that synergistically links pollutant degradation, electron transfer, and nutrient cycling. This integration prevented toxic intermediate accumulation while achieving complete decontamination.