Photocatalytic ozonation of phenazopyridine using TiO2 nanoparticles coated on ceramic plates: mechanistic studies, degradation intermediates and ecotoxicological assessments

• Photocatalytic ozonation of PhP was studied using immobilized TiO2 NPs.
• The surface morphology of TiO2-coated ceramic plates was analyzed by SEM and AFM.
• The presence of various oxidizing species was identified using selective scavengers.
• Phytotoxicity of PhP during oxidation processes was evaluated using Spirodela polyrrhiza.
• Photocatalytic ozonation using immobilized TiO2 is more efficient than ozonation.

Removal of phenazopyridine (PhP), an analgesic drug, from water with different oxidation processes including photocatalysis, ozonation and photocatalytic ozonation was studied. The experiments were conducted using a semi-batch reactor where TiO2 nanoparticles were immobilized on ceramic plates irradiated by UV-A light in the proximity of oxygen and/or ozone. The surface morphology, topography and roughness of the bare and TiO2-coated ceramic plates were analyzed using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Comparing the processes of photocatalysis, ozonation and photocatalytic ozonation revealed that using photocatalytic ozonation led to the highest efficiency (85% at 35 min) in PhP removal. The influence of pH, PhP initial concentration and ozone gas flow rate on the removal of PhP was investigated in both individual and combined processes. The presence of synergy was investigated under various operational parameters. The mechanism of photocatalytic ozonation process was investigated in the presence of various reactive oxygen species (ROS) scavengers. The main intermediate products of PhP produced in ozonation and photocatalytic ozonation processes were verified by the GC–MS technique. The chronic phytotoxicity of PhP and its intermediate compounds formed in ozonation and photocatalytic ozonation was evaluated using aquatic species Spirodela polyrrhiza (S. polyrrhiza). It was found that the photocatalytic ozonation process, in contrast to the single ozonation process, can efficiently reduce the phytotoxicity of the PhP from aqueous solutions.

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