Degradation Mechanism of Methylene Blue by H₂O₂ and Synthesized Carbon Nanodots/Graphitic Carbon Nitride/Fe(II) Composite

In the present study, a ternary combination of graphitic carbon nitride (g-C₃N₄), carbon nanodots (CDs), and ferrous ion (Fe(II)) was proposed. The g-C₃N₄/CDs/Fe(II) composite was prepared through thermal polymerization of urea and citric acid, followed by impregnation with Fe(II). The fabricated composite was characterized using transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and N₂ adsorption/desorption isotherms. The results demonstrated that CDs were embedded in the interlayer of g-C₃N₄ while Fe(II) was grafted onto the surface. The degradation activity of the composite for methylene blue (MB) was systematically investigated. The optimal content of CDs was confirmed to be 3% and a higher Fe(II) content was observed to lead to a greater degradation rate. However, in order to avoid high iron leaching, which may lead to the precipitation of iron, exorbitant Fe(II) content must be deprecated. Besides, the loading of Fe(II) on the g-C₃N₄/CDs composite disrupts the acidic restriction of a typical Fenton reaction, leading to the removal of 57.9%–66.8% MB in 1 h, in the pH range of 3–9, and in the presence of 0.5 g/L catalyst and 1 mM H₂O₂. Interestingly, homogeneous cycled reactions driven by possibly dissolved Fe–C complex showed similar decomposition rates of H₂O₂ in the filtered solution even after 7 cycles with fixed concentration of dissolved Fe. Hydroxyl radical (HO•) and superoxide radical (O₂•–) were identified to be the major reactive species through electron spin resonance (ESR) analysis, which were generated in homogeneous as well as heterogeneous reactions. Therefore, a tentative mechanism based on the above results was proposed. In summary, the ternary g-C₃N₄/CDs/Fe(II) composite was proven to serve as an efficient Fenton-like catalyst, which could be applied in a wide range of pH.