Metronidazole (MNZ) is a brand of nitroimidazole antibiotic, which is generally used in clinical applications and extensively used for the treatment of infectious diseases caused by anaerobic bacteria and protozoans. The aim of this investigation was to degrade MNZ with illuminated TiO₂ nanoparticles at different catalyst dosage, contact time, pH, initial MNZ concentration and lamp intensity. Maximum removal of MNZ was observed at near neutral pH. Removal efficiency was decreased by increasing dosage and initial MNZ concentration. The reaction rate constant (k _obs ) was decreased from 0.0513 to 0.0072 min⁻¹ and the value of electrical energy per order (E_Eo) was increased from 93.57 to 666.67 (kWh/m³) with increasing initial MNZ concentration from 40 to 120 mg/L, respectively. The biodegradability estimated from the BOD5/COD ratio was increased from 0 to 0.098. The photocatalyst demonstrated proper photocatalytic activity even after five successive cycles. Finally, UV/TiO₂ is identified as a promising technique for the removal of antibiotic with high efficiency in a relatively short reaction time.

Photocatalytic removal of Cr(VI) and Ni(II) from aqueous solution using synthesized nanoparticle ZnO under ultraviolet (UV) light irradiation was studied in this work. Firstly, nanoparticle ZnO was prepared by the chemical method with an organic chemical inhibitor. Then removal efficiency of Cr(VI) and Ni(II) by nanoparticle ZnO was investigated with variation of the solution pH, ZnO dosage, contact time and initial Cr(VI) and Ni(II) concentration. Maximum removal of Cr(VI) and Ni(II) was observed at near‐neutral pH because the reduced photocatalytic activity of ZnO at exceedingly low and high pH values originates from either acidic/photochemical corrosion of the catalyst and/or surface passivation with Zn(OH)₂. As the ZnO dosage increased, the removal efficiency of Cr(VI) and Ni(II) was continuously enhanced, but was gradually decreased above 1.25 g/l due to the increased blockage of the incident UV light used for the photocatalytic reaction. The optimum ZnO dosage was determined as 1 g/l. Removal efficiencies of Cr(VI) and Ni(II) decreased as initial Cr(VI) and Ni(II) concentration increased, due to an increased inhibition effect on the surface of ZnO resulting from the decreased reaction sites on the surface of ZnO required for the further photocatalytic reaction.

The effect of dissolved oxygen on the photocatalytic reduction of Cr(VI) with illuminated TiO₂ was studied in this work with variation of the solution pH, contact time, and initial Cr(VI) concentration. Oxygen or nitrogen gas was used as a purging gas. Overall, the removal efficiency of Cr(VI) decreased as the solution pH increased. The removal of Cr(VI) by UV/TiO₂ increased by decreasing the solution pH because of the increased potential difference between the conduction band of TiO₂ and Cr(VI)/Cr(III) as well as the anionic-type adsorption of Cr(VI) onto the TiO₂ surface. The removal efficiency of Cr(VI) increased in purging of nitrogen gas compared to that of oxygen gas because of less competition between dissolved oxygen and Cr(VI) in the reaction with the electron in the conduction band of TiO₂. The reduction pattern was better described by the first-order kinetic model.