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

Abstract

Immobilized TiO2 nanotube (NT) arrays containing anatase phases were synthesized through electrochemical oxidation of TiO2 foil. At ultrasonic (US) frequencies of 28 and 1000kHz, 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 1000kHz. 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 1000kHz. 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.