Elucidation of Radical Species in an Electrolytic Non-Equilibrium Plasma-Water System

Plasma water treatment (PWT) is an emerging water treatment technology that utilizes electrical discharges in gases to degrade aqueous contaminants. Prior work has demonstrated that PWT is capable of degrading perfluorooctanoic acid, a widespread contaminant that is unable to be degraded by denitrification or UV processes that wastewater facilities currently employ. In addition, unlike other approaches, PWT forms energetic species directly from water, eliminating the need for additional chemicals or catalysts. However, internal mechanisms within PWT treatment process, such as the production of different radical species are not well understood. Characterization of the species and their role in degradation is necessary to optimize the process. A challenge is that PWTs generate ultraviolet radiation, electrons, ions, and other non-equilibrium species, and these species are extremely short-lived and recombine or react within nanoseconds of formation. As a result, these species cannot be detected using analytical chemical techniques alone.

In this work, we generated an argon-based, non-equilibrium plasma at the gas-liquid interface of aqueous solutions. Using PFOA as a model contaminant at environmentally-relevant concentrations from 0.02 mM to 1 mM, we demonstrate degradation with a maximum of 29% after 2 hours of treatment. To understand the degradation mechanism, we applied a chemical probe, terephthalic acid (TPA), which can specifically react with hydroxyl radicals (OH) and form a fluorescent compound, 2-hydroxyterephthalate (HTPA), that can be optically detected. While we observed the formation of OH, as the exposure to the plasma increased from 10 minutes to 4 hours, the intensity of the fluorescent peak was found to rapidly decrease. Using a control experiment, we show that the plasma is capable of degrading HTPA. These results indicate that chemical probes and their products are susceptible to species produced by the plasma which can compromise this technique. In the future, stable chemical probes or process conditions which avoid their degradation are required.