A critical review on the application of non-thermal plasma bubbles for oxidation in water treatment

Oxidation is a common water treatment operation. Yet, conventional methods face challenges including inefficiency for wide range of pollutants, unsustainability, and high cost. Non-thermal plasma is a promising novel oxidation method with considerable environmental and economic benefits. However, its efficiency is highly dependent on the way the plasma is discharged, with recent research efforts targeting bubble-based delivery methods. Introducing gaseous plasma species through bubbles is proposed to be an effective way to improve mass transfer and lengthen the residence time of reactive species generated inside the liquid solution, thus enhancing efficiency. In this review, different discharge types are compared in the context of water treatment, confirming that plasma bubble discharge methods are the most effective for delivering reactive species as oxidants. The various factors influencing the efficiency of reactive species generation and target pollutant removal efficiency are explored, highlighting the intercorrelation between some factors and variability in the effects depending on the characteristics of target molecules and background matrixes. Furthermore, while several solution properties and bubble size (in macro scale) have been shown to influence plasma oxidation efficiency, there is a lack of literature on the influence of salt type, pH, bubble size (in micro and nano scale), internal pressure, and plasma bubble generation methods on the reactive species generation and pollutant degradation when using plasma bubbles on different types of pollutants, indicating further study in these areas is warranted. Finally, mechanisms and physiochemical interactions associated with plasma bubble delivery are discussed, illustrating the production of center dot OH at the bubble-water interface and the importance of minimizing gas volume and thickening the liquid layer around bubbles to enhance reactive species generation and diffusion.