Exceptional photo-elimination of antibiotic by a novel Z-scheme heterojunction catalyst composed of nanoscale zero valent iron embedded with carbon quantum dots (CQDs)-black TiO₂

Passivation of nanoscale zero valent iron (nZVI, Fe⁰) impaired its longevity while black TiO₂ (b-TiO₂) suffered from restricted optical properties. Using a facile approach, a novel Z-scheme heterojunction catalyst (Fe⁰@CQDs-TiO₂(b)) of nZVI decorated with carbon quantum dots (CQDs) implanted into b-TiO₂ was designed. Characterization results revealed the optical potential of the passivation coating of nZVI. The incorporation of CQDs stimulated the creation of active •OH during the dark reaction, and led to an accelerated mobility of photo-excited carriers of b-TiO₂ and optimized its band gap (narrowing from 2.36 eV to 2.15 eV) during the light reaction. The photo-elimination capacity of metronidazole (MNZ) on Fe⁰@CQDs-TiO₂(b) (99.36%) was 2.64, 8.25 and 1.34 fold beyond that on nZVI, b-TiO₂ and Fe⁰@b-TiO₂, respectively. The assembled material offered excellent adaptability to environmental substrates, in addition to being virtually unaffected by tap (95.62%) and river water (92.62%). The mechanism of MNZ degradation was elaborated, and the combination of density functional theory (DFT) calculations and LC-MS discerned 12 intermediates and 3 routes. Toxicity assessment of these products was conducted to ensure no inadvertent negative environmental impacts arose. This work proposed an original direction and mechanism for the application of passivation layers in nZVI-based materials for environmental restoration.