Electrochemical Oxidation Mechanism of Tetracycline in Various Supporting Electrolytes Based on a Boron-Doped Diamond Anode

The active free radical species vary with supporting electrolyte during electrochemical advanced oxidation processes, thus, resulting into the significant difference in removal rate and degradation pathways of organic compounds. Herein, the electrochemical oxidation behavior of tetracycline (TC) is studied based on the boron-doped diamond anode in Na2SO4, NaCl, and NaNO3 systems. It is revealed that the SO4 center dot-$\text{SO}_{4}<^>{\cdot -}$ and ClO- initialize the oxidation reaction of TC followed by the gradual degradation through center dot OH. Notably, the enhanced lifetime of ClO- compared to SO4 center dot-$\text{SO}_{4}<^>{\cdot -}$ ensures a more effective and prolonged degradation of TC. Thus, the TC removal rate in the NaCl system is remarkably larger than that in the Na2SO4 and NaNO3 systems. Despite different degradation pathways across each system, consistent oxidation mechanism is identified: initial hydroxylation occurs at arbitrary positions on TC followed by the further oxidation of the hydroxyl group to form aldehyde or ketone. Ketones undergo direct oxidation to facilitate ring-opening reaction; whereas aldehydes are further oxidized to form carboxylic acid and eventually transform into H2O and CO2. This work sheds deep insights into the oxidation mechanism of organic compounds across various supporting electrolytes, which benefits the efficient degradation of TC in practical applications.