Spectroscopic study of self-pulsing discharge with liquid electrode

Abstract

The article reports and discusses the results of a thorough spectroscopic investigation of the self-pulsing electrical discharge in contact with aqueous media. The discharge occurs between a bare high voltage electrode positioned over the liquid and a grounded ring submerged by the liquid. It is supplied with DC high voltage, but it operates in pulse mode that is determined by the charging and discharging of a parallelly connected capacitor. This type of discharge has attracted our attention due to its complex physics, as well as to its high efficiency in inducing the degradation of highly inert hazardous pollutants present in the water, such as perfluoroalkyl substances. The generated discharge unites several types of plasma in a single discharge cell. It starts as a high temperature (2500 K), high electron density (1016–17 cm−3) spark-like discharge close to the high voltage electrode and then branches into a large number of cooler leaders characterized by one order of magnitude lower electron density (1015 cm−3) in the middle of the gap and finally touches the liquid electrode through a dense network of low temperature (500 K) and low electron density streamers (1014 cm−3). The paper discusses the results of a parametric study that has provided temperature and electron density data in different regions of the discharge. The measurements were thus performed for discharges in ambient air, in synthetic air (an 80/20 N2/O2 mixture) and in argon with, as liquid electrode, either tap or ultrapure water containing a small amount of perfluorooctanoic acid.

Cold Plasma for Green Advanced Reduction/Oxidation Processes (AROPs) of Organic Pollutants in Water**

“…A few such designs have been developed and are described in the literature, [38,39] including the SPD reactor [34] and the RAP reactor [12] by our group. The remarkable results achieved with both SPD and RAP are attributed to the very extended plasma/liquid interface characterizing these discharges and to the matching surfactant properties of PFOA which accumulates at the liquid surface exposing the perfluorinated alkyl chain directly into the plasma [12,34,37,38] . The prevailing role of plasma electrons in attacking PFOA, thus initiating its degradation in the SPD reactor, is consistent with the results of a recent optical emission spectroscopy study which highlighted a correlation between the efficiency of PFOA degradation in solution and the density of electrons within the plasma at the liquid interface [40] .…”

“…The remarkable results achieved with both SPD and RAP are attributed to the very extended plasma/ liquid interface characterizing these discharges and to the matching surfactant properties of PFOA which accumulates at the liquid surface exposing the perfluorinated alkyl chain directly into the plasma. [12,34,37,38] The prevailing role of plasma electrons in attacking PFOA, thus initiating its degradation in the SPD reactor, is consistent with the results of a recent optical emission spectroscopy study which highlighted a correlation between the efficiency of PFOA degradation in solution and the density of electrons within the plasma at the liquid interface. [40] Hydroxyl radicals, however, do appear to be involved in subsequent steps of the complex process [40] which achieves the fast and complete degradation of PFOA with only trace amounts of organic carbon byproducts remaining in the treated solution.…”

“…We note, incidentally, that failure of PFOA to react in this system is consistent with the conclusion that OH radicals are the major reactive species involved in these processes. Cold plasmas, however, offer more to be exploited than just ROS for activating pollutants degradation, notably gaseous ions and free electrons [14,37] . These are very short‐lived species which can be taken advantage of in reactors designed in such a way as to maximize the contact between plasma and polluted water (Figure 1b).…”

“…Cold plasmas, however, offer more to be exploited than just ROS for activating pollutants degradation, notably gaseous ions and free electrons. [14,37] These are very short-lived species which can be taken advantage of in reactors designed in such a way as to maximize the contact between plasma and polluted water (Figure 1b). A few such designs have been developed and are described in the literature, [38,39] including the SPD reactor [34] and the RAP reactor [12] by our group.…”

Low-temperature plasmas in contact with liquids—a review of recent progress and challenges

“…The use of the noble gas flow enables formation of self-organized patterns on the liquid surface [53,82]. If the interelectrode distance is much longer than the diameter of the electrode the discharge develops in the form of a corona, which may transit to the spark discharge if sufficient energy is stored in the power supply [49,83]. A corona may develop from the Taylor cone also, see figure 1 [78].…”