Electrical discharges in humid air at atmospheric pressure (nonthermal quenched plasma) generate longlived chemical species in water that are efficient for microbial decontamination. The major role of nitrites was evidenced together with a synergistic effect of nitrates and H 2 O 2 and matching acidification. Other possible active compounds are considered, e.g., peroxynitrous acid.Nonthermal plasma gases are currently under study as potential alternatives to conventional sterilization techniques in numerous settings (the food industry, hospitals). Atmospheric nonthermal plasmas of the gliding-arc type (Glidarc) (9, 25) were found to be efficient against microorganisms for treatments performed under burning discharge (12,13,22,26), and the inactivation of cells in water could continue after the discharge had been switched off (13). Microbial cells were also killed by contact with water that had first been activated by discharges (and so-called plasma-activated water [PAW]) without being themselves subjected to the plasma plume (14, 15). Studies performed hitherto using Glidarc in the context of microbial decontamination have aimed to test the influence of biological (i.e., population level, planktonic or adherent state [14]) and physical parameters on decontamination efficiency. Little is known of the mechanisms of action, especially when PAW is used.UV radiation, charged particles, and temperature are some of the principal factors governing microbial inactivation under plasma technology (20), but they are not relevant for PAW decontamination because the burning discharge is switched off during treatment. It is likely that reactive-nitrogen-and -oxygen-based species play an important role in the lethal effect of nonequilibrium atmospheric air-based plasma (10,20). DNA, RNA, proteins, and lipids are the principal targets of these oxidants (4, 8). The main radical species present in the Glidarc plasma plume have been identified as ⅐ OH and NO ⅐ when humid air is the working gas (1). These radicals are precursors of other active species in water, such as nitrates, nitrites, and hydrogen peroxide (3), which endow the medium with high and sustainable reactivity. The efficiency of these long-lived chemical species in removing chemical pollutants was yet evidenced (24), but their implication in microbial inactivation by PAW was demonstrated for the first time here. Chemical species are also responsible for acidification (2) which role in the antimicrobial activity was also considered during the present study.PAW was produced by application of Glidarc (5 min) over 20 ml of sterile distilled water. The design of the device and the procedure for gas discharge have been described previously (23) , and 1.6 Ϯ 0.2 mmol liter Ϫ1 nitrites (Griess reagent; VWR, Fontenay-sous-Bois, France). Its pH value was 3.0 Ϯ 0.1. No major change in PAW characteristics was detected 30 min after the treatment (corresponding to the maximum period of disinfection). The contributions of nitrites, nitrates, and H 2 O 2 to the lethal effect of PAW w...

Environmental applications of electric discharges are being considered increasingly more often: they imply the chemical properties of the activated species generated in and by the discharge. An overview of the resulting chemical effects is presented, based on rationalized classification, i.e., acid-base effects, oxidizing properties, complex forming reactions, and radical reactions. The gliding discharge is considered to be a specifically suitable plasma source for the treatment of liquids for pollutant abatement in the scope of sustainable environment, and this justifies an overview of the chemical properties. Special emphasis is devoted to temporal post-discharge reactions (TPDRs), which occur when the target is no longer exposed to the plasma source, and several typical examples are detailed. These recently evidenced TPDRs seem to present some general character. They are the key parameters to estimating the efficiency of a discharge treatment; they also have major technical and economical importance for the application of the plasma treatment to pollutant and/or micro-organism abatement at atmospheric pressure and quasi-ambient temperature.

Aim:  To evaluate the microbial disinfection efficacy of a plasmachemical solution obtained by the activation of water with gliding electric discharges. Methods and Results:  Distilled water was activated for 5 min by a nonthermal quenched plasma of the glidarc type operating in humid air and at atmospheric pressure. The plasma‐activated water (PAW) was then used to treat planktonic and adherent cells of Staphylococcus epidermidis, Leuconostoc mesenteroides (as models of Gram‐positive bacteria), Hafnia alvei (a Gram‐negative bacteria) and Saccharomyces cerevisiae (as a yeast model). The treatments were less efficient on adherent cells than on planktonic cells in the case of bacteria, but not of S. cerevisiae. Inactivation was more effective for bacteria than for the yeast. Conclusions:  Significant reductions in microbial populations were achieved in all cases, demonstrating the effectiveness of this new approach to treat contaminated media. Significance and Impact of the Study:  PAW is a promising solution with potential application to the decontamination of equipment and surfaces.

This study aimed to characterize the bacterium-destroying properties of a gliding arc plasma device during electric discharges and also under temporal postdischarge conditions (i.e., when the discharge was switched off). This phenomenon was reported for the first time in the literature in the case of the plasma destruction of microorganisms. When cells of a model bacterium, Hafnia alvei, were exposed to electric discharges, followed or not followed by temporal postdischarges, the survival curves exhibited a shoulder and then log-linear decay. These destruction kinetics were modeled using GinaFiT, a freeware tool to assess microbial survival curves, and adjustment parameters were determined. The efficiency of postdischarge treatments was clearly affected by the discharge time (t*); both the shoulder length and the inactivation rate k max were linearly modified as a function of t*. Nevertheless, all conditions tested (t* ranging from 2 to 5 min) made it possible to achieve an abatement of at least 7 decimal logarithm units. Postdischarge treatment was also efficient against bacteria not subjected to direct discharge, and the disinfecting properties of "plasma-activated water" were dependent on the treatment time for the solution. Water treated with plasma for 2 min achieved a 3.7-decimal-logarithm-unit reduction in 20 min after application to cells, and abatement greater than 7 decimal logarithm units resulted from the same contact time with water activated with plasma for 10 min. These disinfecting properties were maintained during storage of activated water for 30 min. After that, they declined as the storage time increased.

This paper describes the effects of initial microbial concentration and planktonic/adherent/detached states on the efficiency of plasma-activated water. This disinfecting solution was obtained by treating distilled water with an atmospheric pressure plasma produced by gliding electric discharges in humid air. The inactivation kinetics of planktonic cells of Hafnia alvei (selected as a bacterial model) were found to be of the first order. They were influenced by the initial microbial concentration. Efficiency decreased when the initial viable population N(0) increased, and the inactivation rate k(max) was linearly modified as a function of Log(10) (N(0)). This relation was used to compare planktonic, adherent, and detached cells independently from the level of population. Bacteria adhering to stainless steel and high-density polyethylene were also sensitive to treatment, but at a lower rate than their free-living counterparts. Moreover, cells detached from these solid substrates exhibited an inactivation rate lower than that of planktonic cells but similar to adherent bacteria. This strongly suggests the induction of a physiological modification to bacteria during the adhesion step, rendering adherent--and further detached--bacteria less susceptible to the treatment, when compared to planktonic bacteria.

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The gliding arc discharge, which is a source of nonthermal plasma, was used to enhance the biodegradation of crystal violet (CV), a triphenylmethane nonbiodegradable organic dye. The determination of the biodegradability index, i.e., biochemical oxygen demand (BOD 5 )/chemical oxygen demand (COD) ratio, and the total organic carbon measurement were used to assess the biodegradability. For the biological treatment alone, a bacterial strain of Aeromonas hydrophila (8 9 10 8 -CFU mL -1 ) bleached 42 % of CV solution (50 mg L -1 ) after 12-h incubation. The bleaching rate was enhanced by increasing the initial bacterial concentration; however, a drop in the bleaching rate was noted when CV concentration was increased. For the plasma process alone, a 15-min treatment resulted in a color removal of 49.7 %, at a mineralization rate of 12.2 %, thereby increasing the BOD 5 /COD ratio from 0.11 to 0.23. There was an increase in the bleaching rate in temporal post-discharge conditions (i.e., self-continuity of reaction after the discharge was switched off): For 2 h of temporal post-discharge reaction, the color removal of the 15-min plasma-pre-treated CV increased to 55 %. The disappearance of color during each treatment method followed the first-order kinetics. With regard to the combined plasma/biological treatment process, the 15-min plasma-pre-treated sample was bleached at 92 % by A. hydrophila after 2-h incubation and completely bleached for 6 h. Therefore, there is a positive synergism of bacterial and plasma treatments. This combined treatment is useful in reducing the energy involved in complete mineralization of wastewater containing nonbiodegradable dyes.