Morphological tunable three-dimensional flower-like zinc oxides with high photoactivity for targeted environmental remediation: Degradation of emerging micropollutant and radicals trapping experiments

Zinc oxide (ZnO) with a high potentiality for tunable morphology is gaining immense research interests for targeted environmental remediation of emerging micropollutants in source waters. Various three-dimensional (3D) flower-like nanostructures of ZnO were synthesized by following a facile and low-temperature hydrothermal synthesis approach, using Zn(CH₃COO)₂∙2H₂O as the starting precursor reagent, in the presence of different bases at varying concentrations as the structure-directing reagents. BET studies indicated that the ZnO-flowers 1 were more superior than ZnO-flowers 2 and 3, as it possessed the highest specific surface area (10.83 m²/g), pore volume (0.054 cm³/g) and pore size (23.52 nm). Subsequently, the photoactivities of ZnO-flowers on the degradation of emerging salicylic acid micropollutant under UV-A illumination were examined. Batch studies suggested that the ZnO-flowers were photoactive and photocatalysed optimally under neutral pH condition that are aligned with the cost-effective and practical implementation for water/wastewater treatment. ZnO-flowers 1 demonstrated the highest photoactivity according to its reaction rate constant, where the kinetics data was well-fitted using the pseudo-first order Langmuir-Hinshelwood rate equation. This finding was well correlated to the BET studies, where the high photoactivity in ZnO-flowers 1 could be ascribed to its exceptional surface morphology that maximises the surface contact area with salicylic acid during photodegradation. Finally, the radicals trapping experiments were carried out in order to validate the role of primary photogenerated holes (h⁺) and various secondary reactive oxygen species (ROS) in the photodegradation of salicylic acid using ZnO-flowers photocatalysts. It was confirmed that the active species of photogenerated h⁺, •OH, and •O₂⁻ were responsible for the photodegradation process. The plausible mechanistic pathways for the reaction between scavenging reagents and the reactive transitory species to form by-products and intermediates were also proposed.