Degradation of Methylene Blue by Using an Argon Microwave Plasma Jet in Humid Environment

Plasma-liquid interactions yield numerous physicochemical phenomena, rendering them promising for various applications. Plasma-based technology is proposed for water treatment due to its high efficiency in removing contaminants unattainable by conventional techniques. In this study, we employ an argon microwave plasma jet (MWPJ) to investigate methylene blue (MB) degradation. We observe a significant enhancement in the MB degradation rate in a covered system, attributed to increased air humidity promoting hydroxyl radicals (OH) production, which degrade approximately 95% of MB. Furthermore, the injection of O₂ gas into the solution under the plasma generates more hydrogen peroxide (H₂O₂), around 30 mg/L compared to approximately 20 mg/L without injection, although the MB degradation efficiency is reduced. We evaluate MB degradation under various solution properties, revealing that increasing electrical conductivity decreases the MB degradation rate until it becomes independent for conductivities > 10,000 µS/cm. In these latter conditions, a non-conventional temporal evolution of solution conductivity was observed: a decrease during the first tens of minutes followed by a continuous increase for longer treatment time. Conversely, solution acidity minimally affects the MB degradation rate. The MWPJ is characterized by optical emission spectroscopy, showing stability over time and under various solution properties. The energy yield (Y_50%) consistently demonstrates superior performance of the MWPJ in a closed environment compared to an open-to-air environment. Although its efficiency is relatively low compared to other systems, we anticipate improvements through parameter adjustments.