Insight in sulfadiazine degradation by peroxymonosulfate activated by polydopamine-derived nitrogen-doped carbon supported CoFe2O4: Co leaching inhibition and degradation enhancement

Heterogeneous catalyst-mediated sulfate radical-based advanced oxidation processes (SR-AOPs) showed excellent performance during antibiotics degradation. Spinel was a promising catalyst for SR-AOPs, but the secondary contamination due to metal ions leaching needed to be addressed. And the destruction of catalyst structure could lead to the reduction of catalytic activity and the difficulty of recovery. Thus, a novel nitrogen-doped carbon (NC)-supported CoFe₂O₄ (CoFe₂O₄@NC) was synthesized as the activator of PMS for sulfadiazine (SDZ) degradation under low Co leaching conditions. The consequences indicated that the CoFe₂O₄@NC/PMS system exhibited higher PMS decomposition efficiency and reaction stoichiometry efficiency than the bare CoFe₂O₄/PMS systems (CoFe₂O₄-180 and CoFe₂O₄-800), which in turn demonstrated a better SDZ removal performance. Under the condition of CoFe₂O₄@NC dosage 0.1 g/L, PMS concentration 0.5 mM, solution pH 6.8 and temperature 25°C, SDZ (20 mg/L) was almost completely degraded within 60 min. XPS analysis showed that the NC not only protected and stabilized CoFe₂O₄, but also provided additional active sites for PMS activation. During SDZ degradation, SO₄^•-, HO^•, ^•O₂^- and ¹O₂ were involved in the reaction, among which SO₄^• and HO^• made the main contribution. Meanwhile, CoFe₂O₄@NC could be recovered by magnetic separation, and showed great stability (Co leaching 0.852 mg/L) and reusability. In the fifth cycle experiment, 85.02 % SDZ degradation was obtained. Based on the detected intermediates (12 intermediates were identified) and DFT calculations, possible degradation pathways for SDZ in CoFe₂O₄@NC/PMS were proposed. The condensed dual descriptor indicated that the N7, N11, and C15 atoms on SDZ molecule were the main sites of electrophilic attack, which was consistent with the detected intermediates. The degradation of SDZ involved hydroxylation of NH₂, cleavage of S-N and extrusion of SO₂. This study explored the improvements made in NC support material to catalytic performance and resistance to dissolution of spinel, providing new insights for subsequent researches.