Decontamination of biological suspensions by pulsed corona discharge: Contribution of UV light to overall bacterial inactivation E. Spetlikova 1 , H. Shejbalova 1 , V. Janda 1 , M. Clupek 2 and P. Lukes 2 1 Institute of Chemical Technology Prague, Technicka 5, Prague 6, 166 28, Czech Republic 2 Institute of Plasma Physics AS CR, v.v.i., Za Slovankou 3, Prague 8, 182 00, Czech Republic Abstract: The contribution of UV light emitted by the pulsed corona discharge generated in water in the plasma induced bacterial inactivation was investigated. The effect was studied in dependence on the solution conductivity (200 and 500 μS.cm -1 ) and the type of microorganism. Role of UV photolysis was evaluated using an UV light transparent spectrometric cell, which was irradiated by underwater discharge generated using a needle to plate geometry of electrodes fully immersed in the water. We investigated the role of UV photolysis in inactivation of several microorganisms of different cell wall structure as Gram-negative bacteria Escherichia coli, Gram-positive bacteria Enterococcus faecalis, vegetative cells and endospores of Bacillus subtilis. Depending on the type of microorganism up to 50% contribution of UV radiation to the overall plasma inactivation induced by the electrical discharge in water was estimated. Keywords: underwater plasma, UV radiation, solution conductivity, bacteria 1. Introduction Previous research has demonstrated that a high voltage pulse electrical discharges generated directly in a liquid phase initiate a variety of chemical and physical processes. These processes include a high electric field, intense ultraviolet radiation, overpressure shock waves and formation of various highly reactive chemical species such as radicals and ions. It was shown that these processes are capable to efficiently destroy or inactivate a number of organic compounds and microorganisms [1, 2]. However, in comparison with destruction of organic compounds, where the degradation mechanism is attributed mainly to the oxidation by OH radicals, detailed mechanism of plasma-induced microbial inactivation in water is still widely not known. In general, it is expected that the inactivation effect of electrical discharges on microorganisms proceeds due to the cell wall damage and by changes in DNA [3, 4]. In addition to the chemical effects induced by reactive oxygen species (e.g. OH radical, atomic oxygen, ozone and hydrogen peroxide) physical effects such as heat, high electric field, UV radiation and shock waves, all commonly present in the electrical discharge plasma generated in water, may contribute in the plasma inactivation process. Concerning the bactericidal effects of UV radiation, UV light (200–300 nm) with dose of several mW.cm −2 is known to cause lethal damage to cells. UV radiation affects the cells of bacteria by inducing the formation of thymine dimmers in DNA. It suppresses replication of DNA that causes lethal effect to bacteria. UV photolysis is generally not considered to participate significantly in the inactivation process in the atmospheric-pressure plasmas due to low intensity of UV radiation emitted by these plasmas (below 50 μW.cm 2 [5]). On the other hand, electrical discharges generated in liquid phase can emit significant intensity of UV light [6]. Research obtained using the emission spectroscopy has showed a radiation from the pulsed corona discharge in liquid phase in a wide range of wavelengths (200–1000 nm), which is dominated by the spectral lines of hydrogen (peaks at 434, 486, 656 nm) and oxygen atom (777 nm) and by emission from OH . radical (309 nm) [7, 8]. Consequently, Lukes et al. [6] have determined that pulse radiant power of the corona discharge in water (190–280 nm) could reach in dependence on the solution conductivity levels of the order of tens to hundreds of watts during the pulse, which corresponds to UV radiation intensity of the order 0.1–10 mW.cm −2 . In this work the role of UV light emitted by the pulsed corona discharge generated in water in the plasma induced microbial inactivation was investigated on several model microorganisms of different cell wall structure as Gram-negative bacteria Escherichia coli, Gram-positive bacteria Enterococcus faecalis, vegetative cells and endospores of Bacillus subtilis. 2. Experimental The reactor for generating pulsed corona discharges in liquid phase was described in detail previously [8]. A needle to plate geometry of electrodes both immersed in a cylindrical glass vessel was used. Needle to plate distance was