(224d) Chemical Reduction in Gas-Liquid Plasma Reactors

Plasma generated by electrical discharges in gas-liquid (water) systems is of significant fundamental and practical interest in biomedical, environmental, materials, agricultural and chemical applications due to the high efficiency of production of both oxidative and reductive species. While much attention has focussed on the formation of reactive oxygen and, in the case of air or nitrogen carrier gases, reactive nitrogen species that can react with many organic and inorganic compounds to either degrade pollutants or form desired species, less attention has been devoted to the formation of chemically reductive species, particularly hydrated, or solvated, electrons. The key elements of the gas-liquid plasma reactor system that determine the formation and efficiency of the various chemical species include the electrical power supply (e.g., pulsed, DC, AC, microwave), the electrode configuration (e.g., over the liquid, in the liquid, or both as well as polarity), the reactor configuration (batch, semi-batch, continuous), the gas-liquid hydrodynamic flow patterns, the nature of the plasma (e.g, free electron density and plasma gas temperature) and the way the plasma contacts the liquid phase.

We have developed a plasma chemical reactor whereby small capillary tubes are used as the inlet and outlet flow nozzles to a central reactor body (3 mm diameter) and these capillaries also function as the electrodes for various nano-second and micro-second pulsed power supplies. The gas-liquid two-phase hydrodynamics in this system follows an annular flow pattern whereby a small liquid film occurs on the walls of the capillary tube and a central core of gas flows through the reactor. Filamentary plasma channels are generated and propagate along the interface between the flowing gas and flowing liquid. The reactor has continuous flows of both gas (e.g., argon, helium, air, oxygen, nitrogen, and various combinations of these gases) and liquid (deionized water, or water with various chemical additives including salts and organic compounds) with residence times of 2 to 5 ms and 100 to 200 ms, respectively. The filamentary plasma channels propagate along the gas-liquid interface across the full width of the 4 mm electrode gap. In previous work, this reactor has been characterized with respect to hydrocarbon oxidation, hydroxyl radical formation, plasma properties and the effects of solution conductivity and gas composition.

The present work focuses on analysis of the chemical reduction pathways involving hydrated electrons using the reaction of ferricyanide over a range of pH values and electrical field conditions using our plasma reactor. Such reduction reactions are important for the degradation and removal from water of fluorinated carbons. Initial work has demonstrated high efficiency of hydrated electron formation, 1 molecule/100 eV, which is comparable to the formation of hydroxyl radicals (2 to 5 molecules/100 eV) determined in previous work in the same reactor and these efficiencies are significantly higher than found in other methods such as ultrasonication.

This work was supported by the National Science Foundation (CBET 1702166) and Florida State University.