Dysprosium-doped carbon nitride activates sulphites for synergistic photocatalytic degradation of methylene blue

Abstract

The sulfate radical-based advanced oxidation process (SR-AOP) combined with photocatalysis has attracted increasing attention for enhancing the efficiency of pollutant treatment. Improving photocatalytic efficacy and exploring novel SO₄˙⁻ generation methods remain critical for boosting the integrated system's catalytic capability. Herein, the degradation of methylene blue (MB) was investigated by preparing dysprosium-doped modified g-C₃N₄ materials (xDy-CN) to activate sulphites, in combination with visible light. xDy-CN was synthesized via thermal polymerization and characterized using techniques such as SEM, XRD, FTIR spectroscopy, XPS, and BET analysis. The influences of pH, xDy-CN dosage, sulphite dosage, and coexisting anions on the removal of MB were examined, and a degradation pathway for MB was also proposed. The results demonstrated that with a catalyst dosage of 0.4 g L⁻¹ and a sulphite concentration of 0.5 mM, 95% of MB was degraded within 40 min. The reaction rate constant of 0.3Dy-CN in the SO₃²⁻/vis synergistic system (0.055 min⁻¹) was approximately 2.8 times higher than that of g-C₃N₄ (0.0199 min⁻¹). Dy doping can reduce the bandgap width of g-C₃N₄, increase the material's specific surface area, and enhance the degradation efficiency of MB. Free radical quenching experiments and EPR analysis revealed that O₂˙⁻ played a dominant role in MB degradation, while ˙OH, SO₄˙⁻ and ¹O₂ made secondary contributions, and h⁺ had no direct effect. Ultimately, MB was degraded by these abundant active substances into CO₂, H₂O and small molecule intermediates.