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Acidity of plasma activated water from non-equilibrium plasma discharges and identification of superoxide and ozonide as the conjugate bases of metastable plasma acid
Dissertation   Open access

Acidity of plasma activated water from non-equilibrium plasma discharges and identification of superoxide and ozonide as the conjugate bases of metastable plasma acid

Ryan D. Robinson
Doctor of Philosophy (Ph.D.), Drexel University
Dec 2018
DOI:
https://doi.org/10.17918/r0jg-8a55
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Abstract

Dielectrophoresis Nonequilibrium plasmas Active oxygen Mechanical Engineering
When liquid water is treated with a non-thermal plasma discharge, a solution of reactive species is generated, also referred to as plasma activated water (PAW). For some time now, interest in non-thermal plasma treatment of liquids has been increasing in the biomedical, environmental, and agricultural fields as well as food processing. PAW solutions are sought after as advanced oxidative procedures for their oxidizing potential, ease of implementation, and wide range of reactive species produced (e.g., ·OH, ·O, H₂O₂, NO3-, ONOO-). These discharges can be applied near room temperature and atmospheric pressure with a variety of gases including atmospheric air. However, applying non-equilibrium discharges in oxygen or oxygen/noble gas mixtures has been shown to produce acidic solutions. The resulting acidic water solution, sometimes referred as "plasma acid", was shown to have strong but temporary oxidizing properties. The conjugate base of this oxygen plasma-produced acid remained unidentified, while existence of strong temporal water-based oxidizer offers a number of exciting potential applications. At the same time, the applicability of chemical processes observed in treatments with low power discharges used in laboratory setting non-equilibrium plasmas to higher powered discharges for upscaling to industrial needs is still not fully understood. The occurrence of acidic solutions produced from discharges in both air and oxygen environments as well the temperature dependence on the production of reactive nitrogen and oxygen species (RONS) in water were investigated in this study. The results obtained for the water treated with oxygen plasmas suggest that "plasma acid" contains unstable O2- and O3- as possible sources of the "plasma acid" anion which explains the nature and the temporary oxidizing properties of "plasma acid". Production of PAW from a gliding arc plasmatron (GAP) and with nanosecond-pulsed dielectric barrier discharge (DBD) batch treatments are used to investigate the viability of increasing power for the upscaling from laboratory to industrial settings. GAP was a source of "warm" plasma for continuous production useful to an industrial setting, while DBD was a source of cold plasma typically used in laboratory settings and direct biomedical applications. The GAP was shown to produce comparable PAW solution compositions to DBD (specifically HNO3, HNO2, H₂O₂, and pH), despite the increased power and elevated gas temperatures, largely attributed to the rapid cooling of the solution post treatment.

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