Unraveling the Reciprocal Enhancement Mechanism in a Pd–Gr + BDD Electrochemical System for High-Efficiency Degradation of Tetrabromobisphenol A

Electrochemical wastewater treatment technologies are constrained by the unclear degradation mechanisms of pollutants, as these mechanisms involve multiphase reactions and a variety of active species. This study investigates the control of active species ratios and introduces the concept of reciprocal enhancement between electrodes in electrochemical system (REES). Comparative analysis of cathodes shows that the oxidizing and reducing species balance is pivotal for REES operation. A palladium–graphene (Pd–Gr) cathode was synthesized. When coupled with a boron-doped diamond (BDD) anode, the system achieved 98.01% mineralization of tetrabromobisphenol A (TBBPA) within 3 h. The mineralization ratio increased by 95.46% compared to the process using only the Pd–Gr cathode, and by 75.43% compared to that using only the BDD anode. Quantitative analysis of reactive species revealed that singlet oxygen (1O2) contributed most significantly to TBBPA degradation. In situ electrochemical tests and quantum chemical calculations revealed that atomic hydrogen (H*) from the cathode promotes the formation of *OOH intermediates, which further generate hydroxyl radical (•OH) and 1O2 to drive the REES operation. TBBPA degradation proceeds via a three-stage mechanism: initial hydrodehalogenation and •OH oxidation at the cathode interface, followed by 1O2-mediated degradation in the bulk solution. The resultant intermediates subsequently migrate to the anode interface, where the anode continuously drives the mineralization process. This work offers new insights into the degradation of halogenated flame retardants in complex electrochemical systems and provides a theoretical foundation for REES.