Radical-driven peroxydisulfate activation by Fe–Mn bimetallic N-doped biochar for fluoxetine defluorination and degradation: Mechanistic insights and toxicity evolution

Fluoxetine (FLX), a widely used antidepressant, has been increasingly detected in aquatic environments due to its high chemical stability and resistance to biodegradation, posing potential ecological risks. In this study, sludge was used as a precursor to synthesize a Fe–Mn bimetallic nitrogen-doped biochar (FeMn/NBC), enabling the resource utilization of solid waste, which was further applied for peroxydisulfate (PDS) activation to achieve efficient degradation and defluorination of FLX. The superior catalytic performance originated from the synergistic coupling between the N-doped carbon framework and Fe–Mn bimetallic active sites. Nitrogen doping enhanced adsorption and PDS activation, while the bimetallic configuration further improved radical generation efficiency, electron transfer, and catalyst stability. Under optimal conditions (FLX = 10 mg/L, PDS = 2 mM, FeMn/NBC-4 = 0.4 g/L), complete removal of FLX was achieved within 60 min. The system exhibited stable performance over a wide pH range, in the presence of coexisting anions, and in real water matrices, with low metal leaching. Quenching experiments indicated that SO₄•⁻ and •OH were the dominant reactive species. The redox cycling of Fe(II)/Fe(III) and Mn(II)/Mn(III) facilitated continuous PDS activation, while the nitrogen-doped carbon framework enhanced electron transfer. DFT calculations combined with LC–MS analysis revealed that SO₄•⁻ first oxidized FLX via electron transfer, followed by •OH weakening the stability of the C–F bond through electrophilic addition and inducing its cleavage. Toxicity evaluation indicated that the degradation intermediates exhibited reduced ecological risk. These findings provide new insights into the degradation and defluorination mechanisms of fluorinated pharmaceuticals.