Catalyst-Assisted High-Voltage DC Discharge for the Degradation of Methylene Blue in Wastewater

The high color intensity, chemical stability, and toxic persistence of textile dye wastewater form a serious environmental problem. In this work, the degradation of methylene blue (MB) using a high-voltage discharge plasma (HVDP) system has been studied, with a focus on plasma–catalyst interactions. A high-voltage DC surface-discharge reactor with a point-to-plane electrode configuration was used. The positive electrode was placed in the air, and the negative electrode was placed in the dye solution. A DC voltage was applied to generate plasma discharge at the air–liquid interface. The influence of solution pH and conductivity, as well as the use of a magnetite (Fe3O4) catalyst, were systematically studied. These discharges generate a strong electric field, thereby contributing to the dye decomposition. Plasma propagation and degradation efficiency were significantly improved under the acidic conditions (pH 3). As the solution conductivity increased, the current flow also increased, but plasma chemical activity of streamer discharge decreased at higher conductivity levels. Incorporation of a magnetite catalyst led to a further enhanced decolorization performance by amplifying local electric fields and surface interactions. The highest color removal efficiency of 98.14% was obtained at optimized conditions, i.e., pH of 3, Fe3O4 dosage of 7.5 g/L, and DC voltage of 15.9 kV after 60 min. Results show that plasma-assisted degradation with magnetite catalysis is an efficient and sustainable method for treating dye-contaminated wastewater. Operational parameters, such as pH, conductivity, and catalyst use, should be optimized to maximize treatment efficiency.