Sonocatalytic degradation of EDTA in the presence of Ti and Ti@TiO₂ nanoparticles

The sonocatalytic degradation of EDTA (C₀ = 5 10^-3 M) in aqueous solutions was studied under 345 kHz (P_ac = 0.25 W mL^-1) ultrasound at 22-51 °C, Ar/20%O₂, Ar or air, and in the presence of metallic titanium (Ti⁰) or core-shell Ti@TiO₂ nanoparticles (NPs). Ti@TiO₂ NPs have been obtained using simultaneous action of hydrothermal conditions (100-214 °C, autogenic pressure P = 1.0-19.0 bar) and 20 kHz ultrasound, called sonohydrothermal (SHT) treatment, on Ti⁰ NPs in pure water. Ti⁰ is composed of quasi-spherical particles (30-150 nm) of metallic titanium coated with a metastable titanium suboxide Ti₃O. SHT treatment at 150-214 °C leads to the oxidation of Ti₃O and partial oxidation of Ti⁰ and formation of nanocrystalline shell (10-20 nm) composed of TiO₂ anatase. It was found that Ti⁰ NPs do not exhibit catalytic activity in the absence of ultrasound. Moreover, Ti⁰ NPs remain inactive under ultrasound in the absence of oxygen. However, significant acceleration of EDTA degradation was achieved during sonication in the presence of Ti⁰ NPs and Ar/20%O₂ gas mixture. Coating of Ti⁰ with TiO₂ nanocrystalline shell reduces sonocatalytic activity. Pristine TiO₂ anatase nanoparticles do not show a sonocatalytic activity in studied system. Suggested mechanism of EDTA sonocatalytic degradation involves two reaction pathways: (i) sonochemical oxidation of EDTA by OH/HO₂ radicals in solution and (ii) EDTA oxidation at the surface of Ti⁰ NPs in the presence of oxygen activated by cavitation event. Ultrasonic activation most probably occurs due to the local heating of Ti⁰/O₂ species at cavitation bubble/solution interface.