Photocatalytic H₂O-to-H₂O₂ synchronized oxidation of an organic pollutant by carbon dot/g-C₃N₄ composites

Photocatalytic oxidation of organic pollutants using the solar energy is environmentally and economically attractive for developing an energy sustainable organic wastewater treatment process. Here, we employed the impregnation-thermal method to prepare novel carbon dot (CD) dispersed graphitic carbon nitride (g-C₃N₄) (GCN)/pyromellitic dianhydride (PDI) (CD/GCN/PDI) composites for photocatalytic H₂O-to-H₂O₂ and on-site oxidation of organic pollutants in contaminated or waste water. After a 5-h visible-light irradiation, 252 μM of H₂O₂ can be yielded by the CD/GCN/PDI composites. The CD, serving as an electron reservoir, dispersed on the GCN/PDI composite (i.e., CD_0.003/GCN/PDI) facilitates the separation of photo-excited electrons to promote the two-electron reduction O₂-to-H₂O₂ (O₂→˙O₂⁻→H₂O₂). Most importantly, it also on-site activates H₂O₂ to form ·OH radicals (H₂O₂→·OH) to enhance oxidation of organic pollutants (e.g., methylene blue and bisphenol A). The ·OH, ∙O₂⁻, and photogenerated h⁺ account for 4–5%, 14–18%, and 20–25% oxidation of organic pollutants under visible-light irradiation for 3 h, respectively. The CD_0.003/GCN/PDI composite was also tested for photocatalytic oxidation of the organic pollutant under visible-light irradiation for at least 12 h to demonstrate its photostability and reusability. This visible-light photocatalytic H₂O-to-H₂O₂ for the enhanced oxidation of organic pollutants by the low-cost and metal-free CD/GCN/PDI composites using solar energy was developed to demonstrate the feasibility of an energy self-sufficient organic wastewater treatment process.