Solar-light-driven photocatalytic degradation and detoxification of ciprofloxacin using sodium niobate nanocubes decorated g-C3N4 with built-in electric field

Abstract

Simultaneous degradation and detoxification during pharmaceutical and personal care product removal are important for water treatment. In this study, sodium niobate nanocubes decorated with graphitic carbon nitride (NbNC/g-C₃N₄) were fabricated to achieve the efficient photocatalytic degradation and detoxification of ciprofloxacin (CIP) under simulated solar light. NaNbO₃ nanocubes were in-situ transformed from Na₂Nb₂O₆·H₂O via thermal dehydration at the interface of g-C₃N₄. The optimized NbNC/g-C₃N₄–1 was a type-I heterojunction, which showed a high conduction band (CB) level of −1.68 eV, leading to the efficient transfer of photogenerated electrons to O₂ to produce primary reactive species, ^•O₂⁻. Density functional theory (DFT) calculations of the density of states indicated that C 2p and Nb 3d contributed to the CB, and 0.37 e^– transferred from NaNbO₃ to g-C₃N₄ in NbNC/g-C₃N₄ based on the Mulliken population analysis of the built-in electric field intensity. NbNC/g-C₃N₄–1 had 3.3- and 2.3-fold of CIP degradation rate constants (k₁ = 0.173 min⁻¹) compared with those of pristine g-C₃N₄ and NaNbO₃, respectively. In addition, N24, N19, and C5 in CIP with a high Fukui index were reactive sites for electrophilic attack by ^•O₂⁻, resulting in the defluorination and ring-opening of the piperazine moiety of the dominant degradation pathways. Intermediate/product identification, integrated with computational toxicity evaluation, further indicated a substantial detoxification effect during CIP degradation in the photocatalysis system.