The overlooked fate of piperazine contaminants containing imino groups in the protocatechuic acid/periodate process: Role of ortho-benzoquinones.

Reactive quinone species (RQS)-based mild oxidation processes can selectively remove specific contaminants while resisting interference from water matrices; however, previous studies have primarily focused on anilines and sulfonamides. In this study, we developed a targeted oxidation approach based on periodate and protocatechuic acid (PCA) to eliminate previously overlooked piperazine contaminants (PCs) containing imino groups. Norfloxacin (NOR), as the model contaminant, achieved 98.5% removal in the PCA/periodate process. Based on integrated analysis, carboxy-o-benzoquinone (COOH-o-BQ) was identified as the dominant reactive species and was also found to exhibit long-term stability, sustained oxidative capacity, and regenerability. Density functional theory calculations and electrochemical measurements revealed electron transfer from NOR to COOH-o-BQ, where the imino group acts as the reactive site. The PCA/periodate process demonstrated remarkable resistance to inorganic ions and organic matters while achieving > 94% removal of NOR in real water matrices. Importantly, periodate was predominantly reduced to iodate. Toxicity assessments revealed significant attenuation of the biological toxicity of NOR after treatment. Additionally, tea infusion as a natural source of PCA, also exhibited good performance in NOR elimination. This work establishes a novel and selective oxidation approach dominated by COOH-o-BQ, as well as highlights the overlooked fate of PCs in RQS-based oxidation processes.