Plasma is a gaseous state of matter consisting of ions, atoms, atomic fragments and free electrons. Depending on the energy or temperature of the electrons compared to the temperature of the background gas, plasmas can be classified as thermal or non-thermal. Plasmas of interest for water treatment are typically non-thermal and can be generated by direct current, alternating current, pulsed discharges, and radiofrequency and microwave power supply sources. Non-thermal plasmas are typically formed using less power (compared to that of thermal plasmas) and are characterized by strong non-equilibrium between the electrons whose energy varies from ~1 to ~10 eV (1eV=11,600K) with the background gas temperatures ranging from ambient to approximately one thousand Kelvin. An example of a non-thermal plasma is a dielectric barrier discharge used for commercial ozone generation.
When plasma is formed near or in contact with aqueous media, electrons in the plasma collide with vaporized water molecules resulting in the generation of oxidants (e.g., OH• and H2O2)reductive species such as e-aq, UV light, and heat, among other chemical processes. Compared to traditional advanced oxidative processes (AOPs), plasma electrical discharges can simultaneously oxidize and reduce organic molecules. While the oxidation process is based predominantly on OH• attack, the reduction most likely involves simultaneous reactions of aqueous electrons, hot plasma electrons, and background gas ions. The relative amounts of reactive species can be controlled by adjusting the input power, thereby allowing the plasma system to be optimized for removing a broad variety of organic pollutants. Plasma can be generated directly in a liquid, entirely in a gas phase, or in a gas phase such that the plasma propagates along the interface between the gas and the liquid phases.
In a traditional reactor, plasma is typically generated at the tip of a high voltage point electrode separated from an inert grounded electrode that can be positioned either in the liquid phase (i.e., liquid point-to-plane reactors) or gas phase (i.e., gas point-to-plane reactors) in various configurations within the reactor. Contact between the generated plasma and dissolved constituents in the water is critical to the successful operation of any plasma reactor.
Despite being the most studied bench-scale electrical discharge reactor configuration, liquid point-to-plane reactors (see Figure 1a) have, to date, failed to reach commercial success due to kinetic limitations from the relatively small plasma volume contacting the volume of water being treated. Gas discharge reactors (see Figure 1b) are approximately an order of magnitude more effective than direct-in-liquid electrical discharges and are as effective as commercial advanced oxidation processes in degrading a broad spectrum of trace organic contaminants; however, improvements to the plasma reactor design are needed to enhance plasma-water contact and treatment capacity and efficiency.
The DMAX patent-pending enhanced contact electrical discharge plasma reactor is a next-generation gas-discharge plasma reactor technology that incorporates several innovations to improve large-scale treatment efficiency. The key innovation is the use of a gas diffuser to generate bubbles and foam to concentrate surfactant-like contaminants and enhance contact between plasma and dissolved contaminants (see Figures 2 and 3). Argon gas is pumped through submerged diffusers to produce bubbles and form a layer of foam on the liquid surface. Argon gas is recycled immediately upon leaving the reactor. The enhanced contact electrical discharge plasma reactor simultaneously oxidizes and reduces organics by producing a mixture of hydroxyl radicals, argon ions and electrons. The latter are strong reducing agents and the key species in removing PFASs and other non-oxidizable compounds.
The plasma reactor is modular (reactor dimensions are: L×W×H=30"×20"×0.25"), meaning it can be scaled for a specific treatment rate and/or multiple reactors can be used side-by-side. The reactors are mobile, easy to bring to a contaminated site and place in line with other treatment operations, can readily be moved within a site to operate at different locations at different times, and can be operated either continuously or pulsed on demand.