Atmospheric-Pressure Ar/Air Plasma Jet-Induced Degradation of Azo Dyes in Aqueous Solutions: Kinetic and Mechanistic Insights

Atmospheric-pressure non-thermal plasmas are promising platforms for advanced oxidation in water treatment, yet quantitative coupling between reactive species delivery, solution chemistry, and molecular fragmentation remains unclear. We investigate degradation of two structurally related azo dyes using an Ar plasma jet in a plasma-liquid discharge configuration with an immersed counter-electrode to enhance interfacial coupling. Plasma exposure generated a reactive oxygen and nitrogen species environment and strong acidification, increasing proton concentration up to 49-fold. UV-Vis analysis showed rapid chromophore decay, achieving 0.88 and 0.94 removal within 40 min. Biphasic kinetics indicated a transition from radical-flux-controlled to transport-influenced regimes. Fluorescence and Raman spectroscopy confirmed transient oxidized intermediates and progressive pi-conjugation breakdown, elucidating plasma-driven oxidative fragmentation mechanisms.