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Gerdien Arc as a Tool for Decomposition of Water-solubleOrganic CompoundsM. Hlina 1 , J. Domlatil 2 , V. Brozek 1,2 , M. Hrabovsky 11 Institute of Plasma Physics, ASCR, Za Slovankou 3,182 00 Prague 8, Czech Republic2 Institute of Chemical Technology Prague, Technicka 5, 166 05 Prague 6, Czech RepublicAbstract: Plasma torch with Gerdien arc was successfully used for decomposition of organiccompounds dissolved in water that is forced to swirl around the arc and helps to stabilizeit. Therefore treated compounds get to the extreme vicinity of the source of intensiveUV radiation. The rapid decrease of the concentrations of treated compounds was measured.Keywords: Gerdien arc, decomposition, degradation, water treatment, radiation1. IntroductionWater pollution is a major problem in the global contextand its treatment by processes stimulated by various typesof electric discharges has been frequently investigated inrecent years. This paper presents the study of capacity ofGerdien arc for degradation of the organic compounds inwater. The plasma torch with high power Gerdien arc wasused for the experiments. In this type of the arc the stabilizingwater is in direct contact with plasma of arc column.The main features of Gerdien arc are very high plasmatemperatures and thus high intensity of short-wave ultravioletradiation. Water flows with high flow rate in a thinlayer around the arc column. This configuration results inhigh level of ultraviolet radiation emitted to water flowingthrough the torch.enters the chamber with Gerdien arc (the first idea of thearc stabilization by water put out back in twenties byGerdien and Lotz [2, 3]), where the arc column is stabilizedby a direct contact with water vortex, which surroundsthe arc. The schematic of the torch is shown in Fig.1.Fig. 1 Schematic of hybrid stabilized plasma torch2. ExperimentsHybrid stabilized DC plasma torch WSP ® H [1] with thearc power adjustable from 60 to 150 kW was used forexperiments. The cathode part of the arc column iscreated by an arc stabilized by argon flow, argon plasmaArc power was set up to 100 kW during the experimentsand argon flow rate was 22.5 slm. Water with theflow rate of 20 l.min -1 circulates around the arc in thelayer depth of 1 mm at the distance of 3 mm from thecentreline of high temperature arc column. The centreline

plasma temperature that was measured by optical emissionspectroscopy is 19 000 K [1]. Water is injected tangentiallyinto the arc chamber under constant pressure andflow rate and evaporates at the rate of 15 g.min -1 . Thisvalue is possible to neglect with regard to the volume of awater tank (40 l) and also to the water mass flow rate (20kg.min -1 ). Water-soluble organic compounds such as dyeOrange II, nicotine, oxalic acid and other were placed intothe water tank. Water is forced by a pump to flow throughthe system and swirl around the arc (Fig. 2).Fig. 3 Decrease of Orange II concentration during experiment.Fig. 2 Simplified scheme of plasma torch water systemPresented results show decomposition of dye Orange II(1.4 g ~ 10 -4 mol.l -1 ) and nicotine (0.53 g ~ 8.2.10 -5mol.l -1 ). The changes of concentrations were measured bymeans of UV/Vis spectroscopy. Intermediates (and partiallyalso final products) produced during degradation [4]were analyzed by total carbon (TC), total organic carbon(TOC), and total inorganic carbon (TIC) measurements.3. Results and discussionAbsorbance is directly proportional to concentration:A lc(1)where A is absorbance, ε molar absorptivity, l path lengthand c concentration. Absorbance maxima in the spectra oftreated water that were measured at different times ofexperiment showed rapid decrease of the concentrationsof dye Orange II (Fig. 3 and 4).Fig. 4 Comparison of the dye concentration in water inflowingthe torch and out-flowing the torch.It is possible to see that decomposition proceeds withhigh effectiveness. All treated dye Orange II passing thetorch is decomposed, however, some organic residualswithstood as analyses of total organic carbon revealed.Seemingly slow decomposition rate in Fig. 3 is caused bysampling from the place after the water tank (sampling A)where treated and untreated water is mixed so the resultantconcentration can be described by this equation:dC R(2) Cdt Vwhere R is water flow rate (20 l.min -1 ), C concentration, ttime of experiment and V the volume of the watertank (40 l).

Successful decomposition can be explained by very intensiveUV radiation that is emitted by the arc plasma dueto its high temperature because centre line temperature is19000 K at 100 kW [1]. The wavelength (λ max ) correspondingto the maximum intensity of radiation and otherradiation properties might be calculated [5, 6]. λ max for theconditions of the experiments (at power of 100 kW)equals approximately to 150 nm so the emitted UV fallsinto the far ultraviolet radiation range (FUV) with extremelyhigh energy per photon.It is necessary to mention that also some reactive species(such as H· and OH· radicals, H 2 O 2 etc.) are formedthrough the excitation and/or ionization of the water moleculesby the energetic electrons [7, 8] and the speciesreact with treated substance. The degradation process willbe probably caused by the joint influence of the both effects(UV and reactive species).Analyses of total carbon (TC), total organic carbon(TOC), and total inorganic carbon (TIC) showed fast decreaseof organically bound carbon, however, not as fastas the absorbance spectra revealed (see Table 1.).Table 1. Total carbon (TC), total organic carbon (TOC),and total inorganic carbon (TIC).It is comprehensible when assuming the complex systemof chain reactions that lead to the utter decompositionof molecules with high molecular weight (Fig. 5). Organicfragments (intermediates) are formed and analyzed astotal organic carbon (TOC). The slight increase of totalinorganic carbon is caused by the formation of carbonateswhich are formed by the oxidation of carbon present atthe organic fragments.Fig. 5 Nicotine (mol. weight ~ 162 g.mol -1 ) andOrange II (mol. weight ~ 350 g.mol -1 )Also other compounds such as oxalic acid and chloralhydrate (trichloroacetaldehyde monohydrate) were decomposedby the flow through the torch.4. ConclusionsIt has been shown that Orange II and nicotine dissolvedin water were decomposed successfully in hybrid stabilizedtorch WSP ® H [1], where the treated water flowsthrough the torch and is in a direct contact with the arc sowith the very intensive source of far ultraviolet radiationthat is probably the main cause of the degradation. However,other not fully understood mechanisms supposedlyplay a role in degradation process as well. The effectivenessof the process was determined by the measurementsof absorbance spectra and analyses of total carbon (TC),total organic carbon (TOC), and total inorganic carbon(TIC).The direct placing of a treated substance into the part ofthe plasma torch equipment – into the water circuit thatstabilizes the arc column revealed the principal possibilityof such an application. However, the torch and the watersystem have not been optimized for such experiments and

the presented experiments concern just principal testingfor several chemical compounds. That leads to the factthat the ratio energy to mass of decomposed compound isnot profitable at all. The optimization of the system wouldhave to include the decrease of the torch power and/or theincrease the flow rate through the torch. Also mixing oftreated and untreated water is not convenient and shouldbe avoided in the optimized system. Higher concentrationof treated compounds would also increase the effectivenessof the process.[7] W. J. Bian, M. H. Zhou and L. Ch. Lei, Formationsof active species and by-products in water by pulsedhigh-voltage discharge, Plasma Chem. Plasma Proc.27 (2007) 337.[8] P. Lukes and B. R. Locke, Plasmachemical oxidationprocesses in a hybrid gas–liquid electrical dischargereactor, J. Phys. D: Appl. Phys. 38 (2005)4074–4081.The authors gratefully acknowledge the support of thiswork by the Grant Agency of the Czech Republic under theproject No. 202/08/1084.References[1] M. Hrabovsky, V. Kopecky, V. Sember, T. Kavka, O.Chumak and M. Konrad, Properties of hybrid water/gasDC arc plasma torch, IEEE Trans. on PlasmaScience 34 (2006) 1566.[2] H. Gerdien and A.Lotz; Wiss. VeroffentlichungenSiemenswerk 2 (1920) 489.[3] H. Gerdien and A. Lotz, On a Light Source of VeryHigh Intensity, Z. Tech. Phys. 4 (1923) 157.[4] D. Moussa, A. Doubla, G. Karagang-Youbi and J. L.Brisset, Postdischarge long life reactive intermediatesinvolved in the plasma chemical degradation of anazoic dye, IEEE Trans. On Plasma Sci. 35 (2007)444.[5] J. Jenista, M. Bartlova and V. Aubrecht, Czech. J.Phys., Performance of water and hybrid stabilizedelectric arcs: the impact of dependence of radiationlosses and plasma density on pressure, vol. 56 (Suppl.B) (2006) B1224.[6] V. Aubrecht and M. Bartlova, Radiation absorptioncoefficients in arc plasmas, Czech. J. Phys., vol. 54(2004) 759.