Synthesis of Fe0/Fe3O4@porous carbon through a facile heat treatment of iron-containing candle soots for peroxymonosulfate activation and efficient degradation of sulfamethoxazole

Highlights: • Fe⁰/Fe₃O₄@porous carbon (Fe⁰/Fe₃O₄@PC) was prepared with Fe-containing soots. • Fe⁰/Fe₃O₄@PC can effectively activate peroxymonosulfate to degrade sulfamethoxazole. • Very low level leachable Fe and excellent reusability of Fe⁰/Fe₃O₄@PC were confirmed. • Sulfamethoxazole was degraded through radical and non-radical pathways. Developing highly efficient, reusable, non-toxic and low-cost catalysts is of great importance for persulfate-based advanced oxidation processes (AOPs). In this work, ferrocene was mixed into paraffin to prepare a candle, and the iron-containing candle soots were collected and heated at 500 °C~900 °C under N₂ atmosphere for 1 h to prepare magnetically recyclable Fe⁰/Fe₃O₄@porous carbon (Fe⁰/Fe₃O₄@PC) catalysts. The Fe⁰/Fe₃O₄@PC-700 obtained after pyrolysis at 700 °C exhibited the best catalytic activity for sulfamethoxazole (SMX) degradation. 10 mg/L SMX could be completely degraded within 10 min by 0.2 g/L of Fe⁰/Fe₃O₄@PC-700 and 0.5 mM PMS at pH 5.0. The carbon shell effectively inhibited the Fe leaching of Fe⁰/Fe₃O₄@PC-700, and 99.73% of Fe was retained after five consecutive cycles. In the Fe⁰/Fe₃O₄@PC-700/PMS system, SMX was degraded through the sulfate radical (SO₄·^¯), hydroxyl radical (·OH), superoxide radical (O₂·^¯) dominated radical pathway, and the singlet oxygen (¹O₂) dominated non-radical pathway. The coexisting inorganic ions and natural organic matters (NOM) in actual water inhibited the degradation of SMX. Finally, four possible degradation pathways were proposed based on the degradation intermediates of SMX. This work provides a facile heat treatment of iron-containing candle soots strategy to prepare the metal@carbon catalysts for PMS-based AOP.