Effect of DC Bias on the Formation of Long-Lived Species in Microwave Plasma-Treated Water

This study investigates the influence of externally applied DC bias polarity and argon flow rate on the generation of long-lived reactive species, specifically hydrogen peroxide (H ₂ O ₂ ) and nitrite (NO ₂ ^- ), in plasma-treated water (PTW). H ₂ O ₂ ​ concentrations were consistently higher under liquid cathode conditions, supporting a mechanism of Hydroxyl radical (OH) recombination driven by ion bombardment at the liquid cathode. In contrast, NO ₂ ^- formation showed a strong dependence on gas composition. At high air entrainment conditions (low argon flow rate), concentrations were similar under both polarities despite the higher current under liquid anode conditions. On the other hand, at low air entrainment conditions (high argon flow rate), liquid anode bias yielded nearly threefold higher concentrations, reflecting the critical role of discharge current and electron density in NOx precursor generation. These findings demonstrate how bias polarity and gas flow rate jointly regulate plasma–liquid chemistry, offering insights for tailoring PTW composition for specific applications.