Optimization of the CoFe2O4 electronic structure using reticulated expanded graphite to accelerate non-radical activation of peroxymonosulfate for efficient antibiotic degradation in wastewater

Abstract

To tackle the persistent challenge of antibiotic pollutants in wastewater, an expanded graphite-supported CoFe₂O₄ catalyst (CoFe₂O₄-EG) was synthesized via microwave expansion and co-precipitation methods. The efficiency of CoFe₂O₄-EG to activate peroxymonosulfate (PMS) for tetracycline (TC) degradation was systematically investigated. At 0.060 g L⁻¹ CoFe₂O₄-EG, 0.400 mM PMS, and pH 6.0 ± 0.3, 10 mg per L TC was completely oxidized within 30 min, with a mineralization rate of 34%. Trace metal leaching (0.04 mg per L Co ions with a leaching rate of 0.59% and 0.08 mg per L Fe ions with a leaching rate of 0.67%) and excellent reusability were simultaneously obtained. Mechanistic studies revealed that singlet oxygen (¹O₂) was the primary reactive species, while sulfate radicals (SO₄˙^–) and hydroxyl radicals (HO˙) played auxiliary roles. Reticulate channels of EG significantly shortened the mass transfer distance between pollutants and reactive oxygen species. Density functional theory (DFT) and electrochemical analyses demonstrated that the enhanced electron transfer efficiency endowed the CoFe₂O₄-EG composite with a superior catalytic activity compared to CoFe₂O₄. The potential reason was that EG shifted the Fe 3d (spin-up) states further from the Fermi level while moving the Co 3d (spin-down) states closer to the Fermi level. This modification of the electronic structure reduced the oxidation states of the metals, enhanced the electron-donating capacity of Fe ions, and facilitated anion co-catalysis of oxygen species, thereby significantly improving the catalytic efficiency. In conclusion, this study highlighted the superior performance of CoFe₂O₄-EG for TC removal and provided valuable insights into the key mechanisms of the CoFe₂O₄-EG/PMS system, offering promising potential for practical wastewater treatment applications.