Abstract
Superoxide is one of the reactive oxygen species (ROS) in non-thermal plasmas generated by electrical discharges in air at room temperature and atmospheric pressure. One important application of such plasmas is the activation of advanced oxidation processes for air and water decontaminating treatments. When in contact with aqueous media, ROS and notably superoxide can react at the plasma/liquid interface or transfer and react into the liquid. While the detection of superoxide in plasma-treated water has been reported in the literature, to the best of our knowledge, quantitative determinations are lacking. We report here the determination of superoxide rate of formation and steady-state concentration in water subjected to air non-thermal plasma in a streamer discharge reactor used previously to treat various organic contaminants. After detecting the presence of superoxide by spin-trapping and electron paramagnetic resonance analyses, we applied superoxide-selective fluorescent probes to carry out quantitative determinations. The first probe tested, 3′,6′-bis(diphenylphosphinyl) fluorescein (PF-1), was not sufficiently soluble, but the second one, fluorescein-bis-[(N-methylpyridinium-3-yl)sulfonate iodide] (FMSI), was applied successfully. Under typical plasma operating conditions, the rate of superoxide formation and its steady-state concentration were (0.27 ± 0.15) μM s–1 and (0.007 ± 0.004) nM, respectively. The procedure outlined here can be usefully applied to detect and quantify superoxide in water treated by different plasma sources in various types of plasma reactors.
Highlights
Superoxide is one of the reactive oxygen species (ROS) in non-thermal plasmas generated by electrical discharges in air at room temperature and atmospheric pressure
As mentioned above, OH radicals are very reactive and are detected and determined by indirect methods based on trapping by suitable molecular probes to produce either stable radicals which can be analyzed by electron paramagnetic resonance (EPR) spectroscopy (e.g., 5,5-dimethyl-1-pyrroline N-oxide (DMPO) forms a radical adduct that is more stable than the parent radical)[3] or fluorescent products, which can be quantified by fluorimetric determinations (e.g., coumarin-3carboxylic acid (CCA), giving the fluorescent product 7hydroxycoumarin-3-carboxylic acid (7-OH-CCA),[4,5] or terephthalate (TPA), giving the fluorescent product 2-hydroxyterephtalate).[6]
In plasma/ liquid research, the presence of superoxide in the aqueous phase has been inferred by EPR spectroscopy using suitable spin traps.[3,8,15−19] In most of these studies, DMPO has been employed, which reacts with superoxide to form a relatively stable adduct radical, DMPO−OOH, with characteristic spectral properties.[20]
Summary
Superoxide is one of the reactive oxygen species (ROS) in non-thermal plasmas generated by electrical discharges in air at room temperature and atmospheric pressure. While the detection of superoxide in plasma-treated water has been reported in the literature, to the best of our knowledge, quantitative determinations are lacking. We report here the determination of superoxide rate of formation and steady-state concentration in water subjected to air non-thermal plasma in a streamer discharge reactor used previously to treat various organic contaminants. Under typical plasma operating conditions, the rate of superoxide formation and its steady-state concentration were (0.27 ± 0.15) μM s−1 and (0.007 ± 0.004) nM, respectively. The procedure outlined here can be usefully applied to detect and quantify superoxide in water treated by different plasma sources in various types of plasma reactors. The widespread use of these spin traps in many studies was prevented by their high costs and relatively low availability
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