Polarity-regulated reactive species generation and energy-efficient mineralization of methylene blue by liquid electrode glow discharge

Synthetic organic dyes in water bodies are recalcitrant and highly toxic, posing serious threats to aquatic ecosystems and human health. This study investigates the degradation of methylene blue (MB) under three operating modes: liquid cathode glow discharge (LCGD), alternating current glow discharge (ACGD), and liquid anode glow discharge (LAGD). By integrating electrical diagnostics, optical emission spectroscopy, physicochemical analysis of the liquid phase, and radical scavenging experiments, the relationships among voltage polarity, reactive oxygen and nitrogen species (RONS) in the liquid phase, mineralization rate, and energy yield were established. The results demonstrated that, compared with the other modes, LCGD promoted stronger OH(A–X), H_α, and O I emissions and generated higher concentrations of RONS. Consequently, LCGD achieved 99.1% MB degradation within 20 min, along with 56.3% total organic carbon (TOC) mineralization and an energy yield of 0.38 g/(kW•h). In contrast, ACGD required 40 min to reach 93.1% degradation, while LAGD removed only 57.5% after 60 min. The scavenger experiments revealed that the bulk-phase oxidation processes mediated by hydroxyl radicals (•OH) and superoxide radicals (•O₂⁻) were the primary drivers of MB degradation in the LCGD and ACGD modes, whereas electron-mediated plasma–liquid interactions played a significant role in MB degradation in the LAGD mode. These findings indicate that voltage polarity modulation can govern RONS partitioning, thereby enhancing the degradation efficiency of dye wastewater. This provides a mechanistic basis and design strategy for optimizing and extending liquid electrode glow discharge (LEGD)-based wastewater treatment technologies.