Electrochemical degradation of the β-blocker metoprolol by Ti/Ru0.7Ir0.3O2 and Ti/SnO2-Sb electrodes

Electrochemical oxidation has been proposed for the elimination of pesticides, pharmaceuticals and other organic micropollutants from complex waste streams. However, the detrimental effect of halide ion mediators and the generation of halogenated by-products in this process have largely been neglected thus far. In this study, we investigated the electrochemical oxidation pathways of the β-blocker metoprolol in reverse osmosis concentrate (ROC) from a water reclamation plant using titanium anodes coated with Ru0.7Ir0.3O2 or SnO2-Sb metal oxide layers. The results of liquid chromatography-mass spectrometry analysis indicated that irrespective of the electrode coating the same oxidant species participated in electrochemical transformation of metoprolol in ROC. Although Ti/SnO2-Sb exhibited higher oxidizing power for the same applied specific electrical charge, the generation of large fractions of chloro-, chloro-bromo- and bromo derivatives was observed for both electrode coatings. However, degradation rates of metoprolol and its degradation products were generally higher for the Ti/SnO2-Sb anode. Chemical analyses of metoprolol and its by-products were complemented with bioanalytical tools in order to investigate their toxicity relative to the parent compound. Results of the bioluminescence inhibition test with Vibrio fischeri and the combined algae test with Pseudokirchneriella subcapitata indicated a substantial increase in non-specific toxicity of the reaction mixture due to the formed halogenated by-products, while the specific toxicity (inhibition of photosynthesis) remained unchanged.

► Electro-oxidation products of metoprolol in concentrate stream were identified.
► Transformation pathways were elucidated on the low and high oxidation power anode.
► The influence of current density on electrochemical transformation was investigated.