Mechanistic insights and application of SS/MS/C/CaO₂ composite for the simultaneous removal of ammonia nitrogen and multiple antibiotics

Ammonia nitrogen (NH₄⁺-N) and antibiotics often co-exist in aquaculture effluents, exacerbating ecological toxicity and complicating their removal. A novel multiphase composite (SS/MS/C/CaO₂) was developed, in which steel slag (SS) and manganese slag (MS) served as metal sources, activated carbon functioned both as an electron mediator and adsorbent, and calcium peroxide (CaO₂) functioned as an oxidant precursor, enabling the simultaneous removal of NH₄⁺-N and four typical antibiotics (sulfamethoxazole (SMX), enrofloxacin (ENR), ofloxacin (OFX), and florfenicol (FLO)). Coupling this material with a microbial system further revealed a "material-microbe" synergy and its potential for pollution control. The optimized composite was formulated with SS:MS = 1:1 and SS+MS:CaO₂ = 3:2, supplemented by 30 % cement and 5 % activated carbon. Batch experiments demonstrated efficient simultaneous removal of NH₄⁺-N and antibiotics under both acidic and neutral conditions, with removal efficiencies exceeding 70 % and 64 %, respectively. The system also exhibited strong tolerance to common anions and humic acid, with maximum inhibition of only ∼20 %. Mechanistic analysis revealed that the slow release of CaO₂ generated H₂O₂, which served as a continuous oxidant precursor. The redox cycle of multivalent Fe/Mn states in SS and MS sustained an efficient Fenton-like reaction, facilitating the generation of •OH and ¹O₂ via synergistic interactions with the surface functional groups of activated carbon. ¹O₂ was identified as the dominant reactive species, selectively facilitating the conversion of NH₄⁺ to N₂ and effectively degrading key antibiotic structures, thereby mitigating ecotoxicity (OD₆₀₀ of Escherichia coli remained between 1.2 and 1.4 during 24 h of treatment). Continuous flow column experiments further demonstrated that the system consistently achieved 80-99 % NH₄⁺-N removal efficiency and 70-99 % antibiotic removal efficiency in both simulated and actual aquaculture tailwater. It also enriched dominant bacterial genera (such as Silanimonas, Fontibacter, and Hydrogenophaga) capable of denitrification and antibiotic degradation. Notably, Fe/Mn levels stayed below detection limits, with operating costs 20-35 % lower than conventional Fenton-like systems. In conclusion, the composite shows excellent catalytic activity, ecological adaptability, and engineering potential for treating complex pollutants.