Supporting of pristine TiO₂ with noble metals to enhance the oxidation and mineralization of paracetamol by sonolysis and sonophotolysis

Pharmaceutical residues in fresh water bodies are of significant environmental concern due to their undesirable effects on aquatic organisms at very low concentrations. The present study is about the elimination and overall degradation of a common anti-inflammatory pharmaceutical-paracetamol (PCT) in water by catalytic oxidation using high-frequency ultrasound, UV-irradiation (254nm) and both. The catalysts were synthesized sonolytically by the reduction of Pd and Au to the zero-valent state followed by immobilization of the nanoparticles on the surface of commercial TiO₂ (P-25) to improve the activity of the semiconductor under ultrasonic and UV irradiation. It was found that sonolytic oxidation of PCT during catalysis with Pd-TiO₂ and Pd/Au-TiO₂ was very effective owing to the significantly smaller size of these particles (than that of Au-TiO₂), which promoted the formation and collapse of cavitation bubbles on their surface and the incidence of solute contact with the active sites. The addition of persulfate further enhanced the rate of PCT decay, signifying the role of ultrasound in activating the reagent and that of radical-initiated oxidation reactions in the decomposition of the parent compound. The activity of P-25 was low and almost equal under sonolysis and photolysis, but considerably higher under sono-photolysis, where the unfavorable properties of the semiconductor were minimized via combined effects of the two processes. Hybridization also induced a considerable synergy in carbon-mineralization, which was relatively low with single processes. The results were attributed to minimization of electron-hole pair recombination, surface corrosion and competition for photons and radical species. Au-supported particles exhibited higher activity in the presence of UV-irradiation than ultrasound, which was attributed to the unique activity of gold in the UV-vis region. Finally, Pd-TiO₂ was found to be the most stable nanocomposite in terms of the ease of recovery and the efficiency of reuse.