Unveiling the Role of Direct Electron Transfer and Secondary Radicals in Electrooxidation of Tetrabromobisphenol A: Identification of Intermediates and Density Functional Theory Insights

Electrooxidation is commonly used in wastewater treatment but faces challenges with recalcitrant pollutants like tetrabromobisphenol A (TBBPA). Understanding the role of direct electron transfer (DET) and secondary free radicals in forming intermediate products can not only reveal halogenated phenols' electrooxidation mechanism but also aid in electrode design. Coupling triple quadrupole mass spectrometer and quadrupole time-of-flight mass spectrometer, the transformation products (TPs) and their dynamics were investigated during electrooxidation treatments of TBBPA. Furthermore, electrophilic addition and ring-opening mechanisms were simulated by density functional theory (DFT), and toxic changes of TPs were assessed by quantitative structural-activity relationship. The results demonstrate that the steady-state concentration of hydroxyl radicals (•OH) significantly influences the oxidation kinetics before reaching the mass transfer limit. In addition, DET occurs at low potentials (Ep ≈ +0.35 to +0.45 V vs SHE), accompanied with the process of film formation. Six novel intermediates of TBBPA were discovered in electrooxidation process, revealing the ring-opening mechanisms of TBBPA regulated by the steady-state density of hydroxyl radicals. The toxicity of intermediates towards fish and daphnia decreased significantly than that of TBBPA. Our findings offer valuable insights into the electrooxidation process of brominated phenols including their transformation and toxicity changes.