Plasma microbubbles vs gas-liquid DBD energized by low-frequency high voltage nanopulses for pollutants degradation in water: Destruction mechanisms, composition of plasma-activated water and energy assessment

The main scope of this study was the direct comparison of gas-liquid DBD and plasma microbubbles (PMB) driven by high voltage nanopulses in terms of pollutant destruction mechanisms, plasma-activated water composition and cost-effectiveness. The comparison between gas-liquid DBD and PMB was evaluated in terms of water physicochemical properties and plasma species concentrations, performance towards the degradation of dyes with different structure, energy yield and electrical energy per order (EEO) whereas the role of various critical parameters including feeding gas, gas flow rate, applied voltage, water matrix and pollutant concentration was investigated. Different plasma-liquid interactions between the two systems led to high concentrations of longlived species (NO3-, NO2-, H2O2, O3) and a significant decrease in solution pH in gas-liquid DBD compared to low concentrations of long-lived species and near-neutral pH in PMB. PMB bears important advantages, including higher .OH concentrations, faster degradation kinetics and lower energy requirements compared to gas-liquid DBD whereas the decisive species were found to depend on the target pollutant. The energy yield of the current nanopulsed-driven PMB system was extremely high (82.1 g/kWh) whereas its EEO (0.25 kWh/m3) was one of the lower ever reported. However, the low concentrations of long-lived species and almost neutral pH may act negatively for the removal of contaminants whose degradation is favored at lower pH and/or high long-lived species concentrations. This study provides insights into both gas-liquid DBD and PMB systems contributing on the further exploitation of the plasma-based wastewater treatment.