Chlorella-derived N-doped biochar decorated with Fe₂N/Fe₃C nanocrystals for persulfate-based tetracycline removal

Persulfate (PDS)-based advanced oxidation processes have emerged as promising methods for antibiotic removal, but achieving high catalytic efficiency for tetracycline (TC) degradation remains difficult. The catalytic performance of FeGCB-N in PDS activation depended on Fe content and pyrolysis temperature, with defect density and intermetallic compound formation largely determined by the pyrolysis temperature. The optimized Fe_0.2GCB-N-800 exhibited remarkable PDS activation performance for tetracycline degradation, achieving over 99 % removal within 65 min under conditions of 10 mg L^-1 TC, 1 mM PDS, and pH 3. X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) collectively indicate that surface oxygen-containing functional groups, defect sites, and intermetallic Fe compounds provide active sites for PDS activation by FeGCB. Electron paramagnetic resonance (EPR) and quenching results demonstrated that superoxide radicals (O₂^•-) and sulfate radicals (SO₄^•-) were the primary reactive species in TC degradation. Density functional theory (DFT) calculations indicate that Fe-N is the key active site for PDS activation. Based on DFT results and liquid chromatography-mass spectrometry (LC-MS) analysis, potential degradation pathways for tetracycline (TC) are proposed. ECOSAR toxicity assessment showed that the O₂^•--mediated pathway would generate fewer toxic byproducts. This work offers an effective approach to design environment friendly catalysts from algae biomass for treating wastewater.