Photocatalytic Estrogen Degradation by the Composite of Tin Oxide Fine Particles and Graphene-like Carbon Nitride

This study investigates whether 17β-estradiol (E2), a natural estrogen and one of the endocrine-disrupting chemicals responsible for water pollution, can be oxidatively decomposed under simulated solar light using a composite of tin oxide nanoparticles and graphene-like carbon nitride (g-CN) as a photocatalyst. The composite photocatalyst was prepared by heating a mixture of urea and tin acetate. FT-IR measurements revealed that g-CN possesses structural units similar to g-C₃N₄, a well-studied graphite-like carbon nitride. However, unlike g-C₃N₄, sharp diffraction lines were not observed in the XRD diffraction pattern of g-CN, indicating lower crystallinity. Elemental analysis showed that g-CN is slightly nitrogen-rich compared to g-C₃N₄, and UV-vis measurements indicated that the band gap of g-CN is slightly smaller than that of g-C₃N₄. The presence of tin in the composite of tin oxide and g-CN was clearly confirmed by XPS, although no sharp diffraction peaks were observed in the XRD patterns, suggesting the presence of microcrystals. Furthermore, FE-SEM observations did not reveal large tin oxide crystals, although EDS mapping indicated the presence of tin oxide. It was found that the prepared tin oxide and g-CN composites function effectively as photocatalysts for degrading E2 under simulated solar light. The degradation rate constant was evaluated to be <i>k</i> = 3.34 (0.14) × 10^-2 min^-1. Peroxide ion radicals were detected in ESR measurements from the irradiated solution, suggesting that peroxide ion radicals are generated through oxygen photoreduction as the counter-reaction of the oxidative decomposition of E2.