Effect of pH on the Catalytic Degradation of Rhodamine B by Synthesized CDs/g-C3N4/Cu x O Composites.

Abstract

The narrow pH range of Fenton oxidation restricts its applicability in water pollution treatment. In this work, a CDs/g-C3N4/CuxO composite was synthesized via a stepwise thermal polymerization method using melamine, citric acid, and Cu2O. Adding H2O2 to form a heterogeneous Fenton system can degrade Rhodamine B (Rh B) under dark conditions. The synthesized composite was characterized by Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and N2 adsorption/desorption isotherms. The results showed that CDs, Cu2O, and CuO were successfully loaded on the surface of g-C3N4. By evaluating the catalytic activity on Rh B degradation in the presence of H2O2, the optimal contents of citric acid and Cu2O were 3 and 2.8%, respectively. In contrast to a typical Fenton reaction, which is favored in acidic conditions, the catalytic degradation of Rh B showed a strong pH-dependent relation when the pH is raised from 3 to 11, with the removal from 45 to 96%. Moreover, the recyclability of the composite was evaluated by the removal ratio of Rhodamine B (Rh B) after each cycle. Interestingly, recyclability is also favored in alkaline conditions and shows the best performance at pH 10, with the removal ratio of Rh B kept at 95% even after eight cycles. Through free radical trapping experiments and electron spin resonance (ESR) analysis, the hydroxyl radical (•OH) and the superoxide radical (•O2–) were identified as the main reactive species. Overall, a mechanism is proposed, explaining that the higher catalytic performance in the basic solution is due to the dominating surface reaction and favored in alkaline conditions.