Transformation products and reaction kinetics in simulated solar light photocatalytic degradation of propranolol using Ce-doped TiO2

Abstract

The visible light photocatalytic degradation of propranolol was studied using cerium doped titanium dioxide, a catalyst that showed an increased light absorption in the visible region. The experiments were carried out in ultrapure water and in a biologically treated wastewater from the secondary effluent of a treatment plant spiked with propranolol. The best results were obtained for a cerium loading of 0.5wt.% and a bulk catalyst concentration of 0.14g/L, with which propranolol became essentially depleted after 1.5h of irradiation. The extent of mineralization reached 17.4% after 6h on stream under the same conditions. Both oxidation on catalyst holes and reaction with hydroxyl radicals contribute to propranolol depletion, but for the reaction conditions tested in this study, the first mechanism predominated and accounted for 60% of the propranolol rate constant. The runs performed in wastewater matrix led to a very low photocatalytic rate compared with pure water, which was attributed to the presence of radical scavengers and competing substances. Over thirty reaction intermediates were detected by means of exact mass measurements performed by liquid chromatography coupled to quadrupole-time-of-flight mass spectrometry (LC-ESI-QTOF–MS/MS) based on the characteristic fragmentation of oxidation by-products. Their relative abundance was also assessed in catalytic and non-catalytic runs. The most abundant transformation products could be attributed to the cleavage of the ether bond of propranolol. Other compounds detected derived from the addition of hydroxyl groups to the aromatic nuclei or to the ring-opening attack of hydroxyl radicals on the naphthol moiety. Finally, the toxicity of oxidized mixtures was determined using the green algae Pseudokirchneriella subcapitata and the bioluminescent marine bacterium Vibrio fischeri. Although the toxicity of treated mixtures tended to decrease as propranolol was depleted, mixtures treated in pure water may lead to the accumulation of toxic transformation products.