Sonocatalytic degradation of EDTA in the presence of Ti and Ti@TiO(2) nanoparticles

The sonocatalytic degradation of EDTA (C(0) = 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%O(2), Ar or air, and in the presence of metallic titanium (Ti(0)) or core-shell Ti@TiO(2) nanoparticles (NPs). Ti@TiO(2) 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 Ti(0) NPs in pure water. Ti(0) is composed of quasi-spherical particles (30–150 nm) of metallic titanium coated with a metastable titanium suboxide Ti(3)O. SHT treatment at 150–214 °C leads to the oxidation of Ti(3)O and partial oxidation of Ti(0) and formation of nanocrystalline shell (10–20 nm) composed of TiO(2) anatase. It was found that Ti(0) NPs do not exhibit catalytic activity in the absence of ultrasound. Moreover, Ti(0) NPs remain inactive under ultrasound in the absence of oxygen. However, significant acceleration of EDTA degradation was achieved during sonication in the presence of Ti(0) NPs and Ar/20%O(2) gas mixture. Coating of Ti(0) with TiO(2) nanocrystalline shell reduces sonocatalytic activity. Pristine TiO(2) 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(•)/HO(2)(•) radicals in solution and (ii) EDTA oxidation at the surface of Ti(0) NPs in the presence of oxygen activated by cavitation event. Ultrasonic activation most probably occurs due to the local heating of Ti(0)/O(2) species at cavitation bubble/solution interface.