Non-Thermodynamic Equilibrium Plasma, an Oxidation Process for Environmental Protection: Principles, Mechanisms, and Prospects

Plasma is the fourth state of matter, and non-thermal plasma (NTP), also called cold plasma, is partially ionized gas with electron temperatures considerably higher than the other species. This peculiarity has enabled its wide application in medicine, chemistry, agriculture, environmental remediation, and metallurgy. In this review, we discussed the basic principles of NTP generation and the various NTP generation methods, as well as factors affecting NTP performance. We also highlighted its environmental applications. Typical requirements for NTP generation include two electrodes, a high voltage, and a gas (working gas) or a mixture of gases that flows through the gap between two electrodes (i.e., discharge gap). The main species generated in NTPs are radicals, charged species, reactive oxygen species (ROS-O, O2*, O3, OH-), reactive nitrogen species (RNS-NO, NO2), ultraviolet light, and the electric field, and together they produced a combined effect. Although the modes of action of NTP are known, the specific details of the mechanisms are generally poorly understood. NTP has shown effectiveness against microorganisms from available literature; however, its efficiency is affected by operational, biological, and external environmental parameters that must be of utmost consideration for optimal delivery. While NTP is a promising tool with great potential and versatility, efficiency, scalability, and safety issues must be adequately tackled before full implementation and widespread adoption can be realized.