Interfacial charge transfer-driven UV-activated photocatalytic degradation of metronidazole via δ-MnO2/WO3 heterojunction in aqueous media

Abstract

Pharmaceutical contaminants, particularly recalcitrant antibiotics such as metronidazole (MNZ), are removed from water by engineering an efficient UV-driven δ-MnO₂/WO₃ nano-heterojunction photocatalyst to reduce public health risk and restore ecosystems. In this study, a novel hierarchical nanocomposite photocatalyst was synthesized via a facile two-step hydrothermal route with unique “flower-rod” architectures. Structural and surface analyses confirm the presence of pure δ-MnO₂ and hexagonal WO_3, as well as reveal the coherent interfaces between the flower-shaped δ-MnO₂ and rod-shaped hexagonal WO₃. The nanocomposite exhibited reduced crystallite size, higher micro-strain, and defect-rich surfaces compared to pure phases, which are favorable for charge trapping and adsorption. UV-Vis diffuse reflectance spectroscopy detected broad UV-visible absorption and a slight band gap widening upon coupling, while band-edge calculations indicated a type-I alignment between δ-MnO₂ and WO₃. The photocatalytic performance assessed by the MNZ degradation under UV light irradiation demonstrated that the δ-MnO₂/WO₃ (5%) photocatalyst achieved ∼82% degradation within 100 min at an optimal dosage of 0.5 g L⁻¹ and pH 11, outperforming pristine δ-MnO₂ and WO₃. The corresponding pseudo-first-order rate constant (0.01541 min⁻¹) and electrochemical impedance spectroscopy revealed evidence of fast kinetics, lower charge-transfer resistance, and more efficient separation of photogenerated carriers. These results emphasize δ-MnO₂/WO₃ nano-heterojunctions as a promising and highly efficient photocatalytic treatment for antibiotic-contaminated wastewater.