Sonocatalytic-TiO₂ nanotube, Fenton, and CCl₄ reactions for enhanced oxidation, and their applications to acetaminophen and naproxen degradation

Immobilized TiO₂ nanotube (NT) arrays containing anatase phases were synthesized through electrochemical oxidation of TiO₂ foil. At ultrasonic (US) frequencies of 28 and 1000 kHz, the feasibility of the use of TiO₂ NTs was investigated in terms of the production of H₂O₂ and the degradation of acetaminophen (AAP) and naproxen (NPX), which are pharmaceuticals (PhACs) used for pain relief and which persist despite wastewater treatment. The degradation efficiencies of the PhACs, as well as the production of H₂O₂, were enhanced in the presence of TiO₂ NTs. Furthermore, the reduction of TiO₂ slurries after the reaction was achieved at 1000 kHz. The feasibility of the TiO₂ NTs was clearly shown in US/Fenton/TiO₂ NT process, exhibiting the highest degradation (85.3% for AAP and 96.0% for NPX) at low Fe²⁺:H₂O₂ ratio of 20:4 ratio at 1000 kHz. These results could be attributed to the accelerated cycling of Fe²⁺-Fe³⁺, assisted by electrons in the presence of TiO₂ NTs under US irradiation. In the US/CCl₄ process, the degradation rates of both PhACs were improved with an increased concentration of CCl₄, implying that the scavenging H leads to high concentrations of OH and the accumulation of reactive chlorine species, while t-BuOH notably inhibited the degradation of the PhACs. In acidic conditions, high degradation efficiencies were observed because of the hydrophobicity of the PhACs, high OH activity, and high HOCl/OCl^- ratio. Finally, we suggest possible process mechanisms of the US/Fenton/TiO₂ NT and US/CCl₄ processes.