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Multispark Discharge in Water as a Method of Environmental Sustainability Problems Solution

Multispark Discharge in Water as a Method of Environmental Sustainability Problems Solution Hindawi Publishing Corporation Journal of Atomic and Molecular Physics Volume 2013, Article ID 429189, 12 pages http://dx.doi.org/10.1155/2013/429189 Research Article Multispark Discharge in Water as a Method of Environmental Sustainability Problems Solution 1 1 2 1 E. M. Barkhudarov, I. A. Kossyi, Yu. N. Kozlov, S. M. Temchin, 1 3 M. I. Taktakishvili, and Nick Christofi A.M. Prokhorov General Physics Institute of RAS (GPI RAS), Vavilov Street 38, Moscow 119991, Russia Semenov Institute of Chemical Physics of RAS, Kosygin Street 4, Moscow 119991, Russia Edinburgh Napier University, Edinburgh EH9 3JF, UK Correspondence should be addressed to I. A. Kossyi; kossyi@fpl.gpi.ru Received 1 February 2013; Accepted 20 April 2013 Academic Editor: Elena Tatarova Copyright © 2013 E. M. Barkhudarov et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Multispark discharge excited in water is described, and its useful physical and chemical properties are discussed in the light of some environmental issues. Discharge of such a type generates hot and dense plasmoids producing intense biologically active UV radiation and chemically active radicals, atoms, and molecules. Simultaneously, discharge creates strong hydrodynamic perturbations and cavitation bubbles. Particular attention is given to factors influencing on water purity with special reference to discharge application for eeff ctive sterilization of water and its cleaning of harmful chemicals. The gas discharges of this type show considerable promise as a means for solving some actual plasma-chemical problems. eTh above-mentioned discharge properties have been demonstrated in a series of laboratory experiments, which proved the efficiency of disinfection of potable and waste water, water cleaning of pesticide (herbicide) contaminations, and conversion (recovery) of natural methane. 1. Introduction of environmental problems, such as conversion (recovery) of methane (as well as other natural hydrocarbons), and water High voltage electric discharge in water [1, 2]hasbeenconsid- cleaning of pesticide (herbicide) contamination. ered as a potential method of water treatment to kill microor- ganisms and to clean it of harmful contaminations negating the use of chemicals that leads to by-products which may 2. Treatment System additionally compromise human health [3–5]. Factors favor- ing their use include the generation of UV radiation, acous- eTh apparatus used to treat liquids is shown schematically in tic, shock waves, chemically active substances, cavitation Figures 1 and 2. eTh basic components were a chamber filled processes, pyrolysis, and hydrolysis. There are also possible with water, a multielectrode system for exciting of slipping synergetic effects following physical and chemical reactions. surfacedischarge,and high voltagepower supply (Figure 1). Among the different means of in-liquid electric discharge, The multielectrode discharge system ( Figure 2)was similar a novel method involves multielectrode (multispark) slipping in design to that previously described in [6, 8, 9]. The (gliding)discharges(SSDs)[6]which mayhavesomeadvan- discharger consisted of a set of annular electrodes mounted tages over the two-electrode systems generally used at present on a dielectric tube surrounding a back-current conductor. [1, 7]. A gas (air, argon, oxygen, etc.) was injected through a set of The present work describes the construction of a multi- holes into water between the electrodes, producing fine gas spark discharger and discusses results of experimental inves- bubbles. Discharge in each interelectrode gap was produced tigation of SSD-based methods of water disinfection and their throughout the system, including the metal electrodes, a application in plasma-chemical technology for solving some dielectric substrate, a gas bubble, and water. 2 Journal of Atomic and Molecular Physics Clean water Air or O bubbles Microbially contaminated water Air or O Figure 1: Scheme of multispark discharge disinfection of water. (1) Chamber; (2) multispark discharger; (3) generator of high voltage pulses; (4) cleaning water; (5) plasma of gliding discharge. 6 6 Air, O Air, O (a) (b) Figure 2: Multielectrode gliding surface discharge facility. (1) Electrodes; (2, 3) dielectric tube; (4) back-current rod; (5) discharge plasma; (6) gas bubbles. The initial plasma channel may be thought as originating recently adopted terminology) as “microplasma” formations, in ordinary gas discharge in a gas bubble if the electric eld fi involved in various applications [14]. According to the results therein is higher than the gas breakdown threshold [10, 11]. of previously performed experiments, the electron density in 17 −3 But in actual fact, a large (sometimes dominant) part in the plasmoids attains 10 cm ,and thegas temperature4000– interelectrode plasma formation could be played by a gliding 5000 K [15]. According to [16], explosive metallic plasma is a discharge along the dielectric surface with the subsequent source of intensivehardUVradiation. interaction of discharge plasma with electrodes and explosive A typical photograph of the operating system is shown in microplasma production on their surface [12](see Figure 3). Figure 4. There are just these processes that have been considered to eTh principal advantage of the multispark system lies in be operative in the case when multispark discharger works in the following peculiarities of their construction. the gas medium [13]. When the high voltage pulse is applied to the immersed (i) eTh area of the surface of all electrodes contacting water in the multielectrode version can be minimized in the aqueous medium discharger (shown in Figure 2), by introducing insulating dielectric screens ensuring plasma bunches (plasmoids) appear almost simultaneously the SSD operation in high-conducting water (up between electrodes. Reasoning from their characteristics 4 −1 these plasmoids can be classified (in accordance with the to conductivities of 10 𝜇 Scm )without substantial Journal of Atomic and Molecular Physics 3 1 Water 1 Water Gas Gas Gas Gas 3 3 (a) (b) Figure 3: Two consecutive phases of plasma production in each interelectrode gap. (1) Electrodes; (2) dielectric tube; (3) back-current rod; (4) gliding surface discharge; (5) metallic plasma; (6) unipolar arc. the tested metals (Fe, Mo, Cu, Ti, etc.), just stainless steel and titanium have been selected as materials exhibiting the most promise for working as a detail of multispark discharger. Just these two metals have been used in electrodischarge systems applied in the General Physics Institute (GPI RAS) for solution of water purification problems. (v) eTh discharge gaps could be distributed in such a way as to increase the efficiency of the discharge action on liquids, in particular, by focusing the shock waves and UV radiation ux fl [ 17]. The experiments were conducted using the high volt- Figure 4: Typical photograph of multispark discharger operating in age multichannel (5 channels) generator with the following water. parameters: high voltage amplitude, 𝑈≤20 kV; pulse repetition frequency, 𝑓 ≤ 100 Hz;capacitivestorageenergyof one channel,𝑊≤2 J, and pulse duration,𝜏≈5𝜇 s. The circuit of the output stage of each channel is shown in Figure 5. Each reduction of the efficiency of energy supply to the multispark discharger was powered from one channel of a discharge region. multichannel generator. The discharge current and voltage (ii) The discharger has no pointed electrodes; the working were measured with the aid of a Rogowski coil and voltage surface of the electrodes (unprotected by dielectric divider. eTh signals shown on the Figure 6 were recorded with screens) is developed and is either a part of cylindrical an oscilloscope (TDS 3012). es Th e measurements allowed surface of tubular electrodes or the plane surface −3 the determination of the energy density (J cm )releasedin at the exit sections of the tube. us, Th the principal liquid. advantageofSSD system lies in thedecreaseinthe discharge load of each electrode (thereby enhancing 3. Multispark Electric Discharge in Water the erosion resistance on the system as a whole), which ultimately substantially increases the lifetime as a Source of UV Radiation, Ozone, and of the system. Hydrogen Peroxide (iii) The dischargers can aeff ct the aqueous (liquid) Figure 7 shows a schematic of the experiment intended to medium through several simultaneously acting investigate a multispark SSD in water as a source of UV mechanisms, among them the direct influence radiation, ozone, and hydrogen peroxide. Multielectrode of discharge plasma, the action of UV radiation discharger (2) is positioned in a cell (1) with water. A generated by microscopic discharges, the chemical high voltage pulse produces a plasma channel between the action of chemically active radicals, atoms, and electrodes. eTh gas leaving the reactor (as a working gas molecules produced in discharges, and the hydro- air or oxygen has been applied) flows into a quartz cell (3) dynamic action through microscopic cavitation intended for determining the ozone content by the method bubbles. of absorption spectroscopy. In the course of the experiments, (iv) Cleansing action and bactericidal effect of a multi- the production of H O was also measured. UV radiation was 2 2 spark discharge in the water medium unessentially measured in the presence and absence of water in the reactor depend on electrode material. Nevertheless among chamber. 4 Journal of Atomic and Molecular Physics R C 1 3 Air, O + O 2 3 R 2 Air, O + O 7 2 3 Water 1 8 Water Figure 5: Output stage of one channel of the high voltage pulses power supply. (1) Rogowski coil; (2) voltage divider. 𝑅 , 𝑅 -resistors; 1 2 𝐶 -capacitor; 𝐿 -inductor. Air, O Figure 7: Experimental layout. (1) Vessel filled with water; (2) I, U multispark discharger; (3) diagnostic quartz cell; (4) deuterium lamp; (5) discharge plasma; (6) MDR-3 monochromator; (7) gas bubbles; (8) MUM-1 monochromator; and (9) quartz window. 100 A 2 kV 5 𝜇 s Figure 6: Typical oscillograph trace of SSD current and voltage. 240 250 260 270 280 290 𝜆 (nm) The discharge emission spectrum in the region 230 < Figure 8: Spectrum of soft UV radiation from multispark discharge 𝜆 < 300 nm was measured with the help of an MUM-1 in the water. monochromator ((8), Figure 7)and with an FEU-142photo- multiplier. Typical spectra of UV emission from the discharge are shown in Figure 8. In the case of application of air as working gas, the Chemical (actinometric) measurements have been used O content was determined by the chemical method from as well. In this case, the UV intensity was deduced from the reaction between O and potassium iodide in the water photolysis of an irradiated K Fe(C O ) solution with a 3 2 4 3 solution [19]. phenanthroline admixture. Figure 9 shows the ozone density in the diagnostic cell as This technique was described in [ 18] and successfully used a function of the repetition frequency of high voltage pulses in [13] to study the multispark discharge in gaseous (Ar) (𝑓) for a discharge in water (for various oxygen flow rates). medium. Restriction of 𝑓 values by amounts of the order of 100 Hz To measure the O content in the gas flowing from the is not critical and appears explicable only on the basis of reactor, we used both spectroscopic and chemical methods. improper technical equipment of laboratory. The scheme of measurements of the O content in O is 3 2 In the experiments when the oxygen flow rate through shown in Figure 7. From attenuation of the UV radiation the interelectrode gaps and the water-filled reactor was 𝑤≅ passing through the cell, the O density in the gas was 15 L/min, the ozone density in the oxygen flow was equal to determined by the absorption method. eTh spectral interval 15 −3 used to determine the ozone content corresponded to the 𝑛 ≅ (1-2) 10 cm . Hartley absorption band with the maximum near 𝜆 ≅ The H O content in water treated by the electric 𝑑 2 2 255.5 nm. discharge was measured by the iodide-molybdate method I (a.u.) UV Journal of Atomic and Molecular Physics 5 −3 initial number of bacteria in a unit volume (cm ), 𝑃 is 1.8 UV −2 themeanintensity of theflux of bactericidal rays ( 𝜇 Wcm ), 1.6 𝑡 is the irradiation time (s), and 𝑘 = 2500 is the bacterial 𝑎 𝑏 1.4 tolerance factor. 1.2 For the case of repetitive discharge, expression (1)can be rewritteninthe form 0.8 −𝑃 𝑡 0.6 UV(𝑖) 𝑎 𝑛 ≅𝑛 exp ( ) , (2) 𝑏 𝑏0 0.4 𝑘 0.2 where 𝜏 is pulse duration (s) and 𝑓 is the repetition frequency of high voltage pulses (Hz). 0 20 40 60 80 100 6 −2 It is easy to see that for 𝑃 ∼3⋅10 𝜇 Wcm , 𝜏=5𝜇 s f (Hz) UV(𝑖) and 𝑓 = 100 Hz, the exposure time equal to a few seconds Figure 9: Ozonedensity in thediagnosticcellasafunction of is sucffi ient to decrease the number of bacteria in water by the repetition frequency of multispark discharge in the tap water a factor of ten. This means that the energy cost of treating −1 −1 for various flow rates of O : ◼-𝑤=10 L min ; -15 L min ; 󳵳 - water by bactericidal UV rays is of the order of 𝜉 ≈ (1- −1 UV 20 L min . −4 −1 2) 10 kW h L . Under the experimental arrangement shown in Figure 7, described in [20]andusedin[13] to determine the intensity of ozone generated in the discharge has no time to dissolve in hard UV radiation of the gliding surface discharge in argon. water and is almost completely removed by the air (oxygen) The measurements of hydrogen peroxide production that flow into the space over the water reactor. In principle, it is were carried outinadischargeinwater with injected argon possible to constructareactorsuchthatthe produced ozone showed that a series of discharges for 6-7 minutes in 250 cm will be completely “entrapped” in the water being treated. Let of water produced H O with a mean density of 𝑛 ≅2 ⋅ us estimate how eeff ctive the role of ozone in the sterilization 2 2 H O 2 2 −3 −1 18 −3 action of discharge may be in this case. 10 mol L ≅ 1.2 ⋅ 10 cm .Theenergycostofproduction As follows from the data presented in [19], the eeff ct of of one H O molecule in this case is ℎ ≤ 1.5⋅10 eV/mol. 2 2 H O 2 2 ozone dissolved in water on microorganisms becomes signif- The performed experiments demonstrated that for the icantly stronger when the O content reaches the threshold SSD in the water-gas mixture, at least two factors are real- 3 16 −3 level [𝑛 ] ≅8⋅ 10 cm .Over [𝑛 ] ,the E. coli bacteria ized among the factors that are usually invoked to explain O O 3 th 3 th thesterilization eeff ctofelectricdischarges. eTh se arethe content decreases by more than four orders of magnitude. generation of UV radiation and the production of biologically It is easy to see that the bactericidal treatment capacity of active ozone and hydrogen peroxide. ozone can be as high as It is possible to estimate, using the results of measure- 𝑛 𝑤 O 𝑏 −1 ments, the eeff ctiveness of these two factors in the degra- 3 𝑤 ≅ ≅25 Lh , O (3) dation of microorganisms during operation of the electric- [𝑛 ] th discharge systems under study. Examining the UV radiation from the discharge, we −1 where 𝑤 is the water-treatment rate (L h )and 𝑤 is the O 𝑏 have to take into consideration that according to [21]the −1 air flow rate through the discharge facility (L h ). Then, the strongest bactericidal eeff ct is produced by ultraviolet rays energy cost of water treatment by ozone generated in the with wavelengths from 295 to 220 nm (the “bactericidal” discharge (assuming that it is completely dissolved in water) spectral region). −4 −1 can reach 𝜉 ≅3⋅ 10 kW h L which is comparable with Measurements performed in our work (see [9]) showed 3 the energy cost of sterilization by UV radiation. that the radiation spectrum of the multispark discharge in Finally, we estimate the eeff ctiveness of a possible bac- water contains the biologically active component, and the tericidal action of the multispark discharge in water due to intensity of this component increases substantially as the the production of hydrogen peroxide. Special microbiological pulse energy increases. studies carried out by us showed that an addition of hydrogen Based on the results of absolute measurements of UV 17 −3 peroxide as a level of 𝑛 ∼10 cm to tap water allows radiation by the actinometric method, we estimate the inten- H O 2 2 the number of E. coli bacteria to be reduced by one order of sity of the ux fl of bactericidal rays per pulse discharge as 6 2 magnitude. This means that the experimentally measured rate 𝑃 ≈3⋅10 𝜇 W/cm [9]. UV(𝑖) of H O production ensures the energy cost of water steriliza- 2 2 Given this intensity, in turn, the eeff ctiveness of the action −4 −1 tion at the level 𝜉 ∼10 kW h L , which is close to the of radiation on E. coli bacteria canbeestimated from the H O 2 2 energy cost of sterilization by ozone production in discharge. known relation [21] Hence, the performed direct measurements of UV radia- −𝑃 𝑡 UV 𝑎 𝑛 ≅𝑛 exp ( ), (1) tion and chemically active products evidence that described 𝑏 𝑏0 below multispark slipping surface discharge (SSD) in water where 𝑛 is the number of bacteria in a unit volume that with airasanworking gasispromising forwater sterilization −3 remain living aer ft bactericidal irradiation (cm ), 𝑛 is the since two effects only, examined in our work, can ensure the 𝑏0 15 −3 n ×10 (cm ) 2(O ) 𝜏𝑓 6 Journal of Atomic and Molecular Physics −4 −1 energy cost as low as 𝜉≅ 10 kW h L for reducing the E. 1 coli bacteria content by one order of magnitude (i.e., with a generator with a mean power of 1 kW, it is possible to reach a 0.1 3 −1 water treatment rate of the order of 10 m h ). It should be pointed out that possibility to apply for multispark discharger excitation of practically every gas or 0.01 gaseous mixtures offers great opportunities for action on a microbiological component through the different chemically 1E−3 active atoms and radicals. However, in this work authors have restricted for water sterilization by the application only of air or oxygen taking into account that based on application of 1E−4 these gases discharger will be simplex and cheapest. 1E−5 4. Multispark Electric Discharge Disinfection 0 0.5 1 1.5 2 −3 of Microbially Contaminated Liquids J (cm ) Figure 10: Changes in populations of Escherichia coli and viruses As a step of our activity experimental investigation of effec- (𝑁 ) in treated water relative to the initial populations (𝑁 )asa tiveness of disinfection action of multispark discharge on the −3 function of specific energy release (J cm ) during the treatment. −1 water containing Escherichia coli and its viruses (coliphages) Potable water with a conductivity 𝜎 = 100 𝜇 Scm was used. 𝑓= has been carried out [22]. 10 Hz. eTh initial ( 𝑁 ) concentration of E. coli was ≈10 colony- −1 7 eTh apparatus used to treat liquids is the same as forming units mL and that of coliphages ≈10 plaque-forming −1 shown schematically in Figure 1.Thedischarge device— units mL . (1) E. coli; (2) coliphages. multispark discharger—was situated in the treatment cham- ber through which water contaminated with microorganisms −3 −4 −1 was pumped. Water contaminated with E. coli or viruses of 0.3 J cm (approx. 10 kW h L )toreducethepopulation (somatic coliphages) can be used to test the killing efficiencies by a factor 10 (1 log reduction) while coliphages required an −3 of the discharge system. Samples of water for microbiological energy input of 0.15 J cm for the same result. analyses were taken via a sampling port; triplicate samples in The used multispark discharger regimes are identical with 10 mL sterile bottles being removed for analysis. the regimes previously investigated [9] where an examina- Escherichia coli (NCIMB 86; ATCC 4157) was grown tion was made of the generation of biologically active UV overnight in nutrient broth (oxoid) at 37 C. The cultures were radiation, ozone, hydrogen peroxide, and other active species 6 −1 diluted to population densities of approximately 10 cfu mL (see preceding section of this paper). Measurements carried with tap water and placed in treatment chamber containing out during the present study allowed calculation of energy the multispark discharger. costs of the disinfection action using multispark electric dis- −4 −1 Water samples treated by the electric discharges were chargers, and these were as low as 10 kW h L for bacteria. removedfromthe system at varyingtimeintervals andbacte- These values verified the bacterial action of discharges in the rial killing assessed using spread plate counting methodology. water predicted in the preceding section and confirmed that Escherichia coli was determined by spreading 100 𝜇 L aliquots the main factors aeff cting microbial destruction in the water of diluted samples onto nutrient agar plates. Occasionally, were UV radiation and the production of biologically active MacConcey agar (HMSO 1994) and a spiral platter were chemicals. The latter are not involved in treatment systems utilized. Replicate plates were incubated at 37 Cfor 24h. utilizing UV lamps which would be unable to generate Coliphages were estimated by a plaque assay utilizing E. coli highly reactive chemical species. Acoustic and shock waves C (ATCC 13706) as the host bacterium. Dilutions of treated generated by multispark discharge also played a part in sampleswerespreadontolawns of E. coli C,sensitive to a microbial disinfection but, in addition, they facilitated the broad spectrum of coliphages, and the number of plaques mixing of treated water, delivering reactive chemical species formed aeft r 24 h incubation counted. to all parts of the treatment system. Figure 10 shows the eeff ct of multispark discharges on The possibility that disinfection using electric discharges microorganisms in the water. eTh fraction of surviving bac- mightleadtothe production of toxicby-productswas tested −3 teria and viruses (𝑁/𝑁 ) is plotted versus the energy density 0 by the input of energy as high as ∼1Jcm into water. Water −3 (J cm )releasedinwater.Eachpoint in theplotpresents samples were analyzed for a range of substances and physical the mean of three measurements. Deviation from the mean appearance by the Certicfi ation Control-Analytical Center did not exceed 15%. Numerous experiments were carried out (Moscow State University, Russia). The water was tested for using E. coli, and all showed a similar killing efficiency of color, turbidity, pH, ammonium, Fe, Pb, Cr, u fl orite, chlorite, the multispark discharge system. Data of microbial killing nitrate, and sulphate. eTh quality of the treated water fulfilled in liquids containing tap water-microbe combinations and a the necessary standards of the European Union (Council −1 conductivity of 100 𝜇 Scm arepresented.Itisevidentfrom Directives on the quality of water intended for human con- Figure 10 that the viruses were killed using a lower energy sumption 80/778/EEC and the new drinking water Directive input to the liquid. Escherichia coli required an energy input 98/83/EC adopted by the Council on 3 November 1998). The N/N 0 Journal of Atomic and Molecular Physics 7 results for Fe were particularly important as the electrodes more than billion cubic meters of associated gases are used in the study were manufactured from stainless steel. burning down worldwide. Russian oil producing companies Erosion of multispark discharger is small and does not affect forcompensationofanecologicalharmare paying near 500 overall concentrations in water. In addition, incubations rubles for each 1000 m of burning petroleum gas. of multispark discharge treated water with microorganisms Presented work objective is the investigation of possi- were carried out to test whether the killing action con- bility of natural hydrocarbons (namely, CH )recoveryin tinued. This could be due to the persistence of oxidizing plasma-chemical reactor based on the SSD. Traditional for species produced by the discharge but these were rapidly GPI research multispark dischargers have been used with quenched within the system following treatment. eTh re were only one key distinctive feature of their construction: as a no increased eeff cts on E. coli added to system containing discharge formative gas methane (or any other utilizable plasma treated compared with nonplasma-treated tap water. natural hydrocarbons) has been applied. This is contrary to results obtained with two-electrode dis- The diagram of the experiment is shown schematically in charges [1]and couldbeexplained by quitelow levelof Figure 13. A multielectrode discharger is introduced into the operated multispark discharger electrodes sputtering and as reaction chamber in the form of an organic glass vessel filled a result extremely low level (in comparison with the two- with water (volume 𝑉 ∼ 0.25 L). When a high-voltage pulse electrode system) of content of metallic clusters responsible is applied to the discharger, a system of plasma formations (according to [1]) for prolonged action of discharge on a (plasmoids) in which the decomposition of hydrocarbons microbial population. It is of interest to note that a multispark takes place is formed in bubbles of methane or methane- discharge treatment of short duration could sterilize tap water oxygen mixture in the gaps between the electrodes. eTh containing E. coli andcoliphage.Thedurationwas short source of high voltage pulses was a generator producing single enough for the cost-effective treatment of water supplies pulses or operating in the pulse-periodic regime. eTh pulse- (<5 min); contact time being in the region of minutes rather repetition rate was 𝑓≤50 Hz,the pulsedurationwas 𝜏 ≈ than the 30 mins used in chlorination. 1𝜇 s, andthe pulseamplitude was 𝑈 ≈40 kV. This study concentrated on verifying the predictions of We analyzed samples of the gas taken at the outlet of microbial killing made originally in [9]and utilized E. coli the reaction volume. Analysis of the gas passing through the and coliphage as representative organisms. No attempt has discharger was carried out using the following techniques: been made at this stage to examine the eeff ct of multispark (i) special ITT IK/VP test tubes (OOO Impul’s) used for discharge plasma on the other bacteria (Gram-positive or determining the contents of acetylene (C H ), carbon 2 2 -negative types), viruses, or spores (bacterial or fungal). dioxide (CO ), and carbon monoxide (CO); Preliminary experiments have been performed to determine only the effect of multispark plasma on the oocysts of Cryp- (ii) SPECORD IR spectrograph used for determining the tosporidium (a protozoan parasite causing gastrointestinal acetylene content; disorders), which are resistant to chlorination. eTh micro- (iii) gas chromatograph used for determining the concen- scopic examination of cysts after treatment showed cell wall tration of methane (CH ) and hydrogen (H ). 4 2 degradation and an inability to induce excystation in the organism. Figure 14 shows the characteristic spectrograms obtained on the SPECORD IR spectrograph. eTh main absorption lines It is of interest to investigate the possibility of using the multispark system described to treat industrial and domestic of CH ,C H ,and CO canbedistinguished (insubsequent 4 2 2 wastewater. eTh rfi st attempt at such an application has been analysis of the experimental results, CO was disregarded). The lines of the nearest unsaturated hydrocarbon ethylene taken in [8, 23]. Water treatment was carried out using wastewater directly abstracted from final effluent stream at C H are also very weak (at the noise level). In analysis of 2 4 the efficiency of the plasma-chemical conversion of methane, the Livingston Wastewater Treatment Plant in West Lothian, Scotland, UK. The scheme of system for wastewater treatment it is expedient (see [24]) to use such parameters as the is shown in Figure 11. Results of SSD action on a final degree of conversion 𝛼 expressed in fractions (in other words, the fraction of methane fed to the reactor and converted eu ffl ent stream are presented in Figure 12.Itwas shown −3 into a certain product at the output) and the energy value that a specific energy of 1.25–1.5 J cm was required to 𝜀 of conversion (i.e., the energy value of transformation achieve 1 log reduction in bacterial (faecal coliforms/total of methane molecules in eV/molecule). If we disregard for aerobic heterotrophs) content. This study has demonstrated simplicity the small amounts of ethylene formed as a result of the eeff ctiveness of the multispark dischargers in microbial methane treatment, we can assume that mainly two reactions disinfection of wastewater. eTh system can be engineered occur in the plasma-chemical reactor: pyrolysis reaction, to eradicate microbial populations to levels governed by legislation by increasing treatment time or energy input. CH 󳨀→ C +2H (4) 4 2 and the reaction of transformation of methane into acetylene, 5. Plasma-Chemical Converter of Methane on 2CH 󳨀→ C H +3H the Basis of Multielectrode Discharger (5) 4 2 2 2 One from the currently important ecological problem con- It can be seen from simplified reaction formulas ( 4)and sists in utilization of gases accompanying oil recovery. Yearly (5) that the volume of the reaction products exceeds the 8 Journal of Atomic and Molecular Physics High voltage power supply and pulse generator Electrical connections Pump Reaction chamber 2 with SSD electrodes Untreated Reaction wastewater chamber 1 with SSD electrodes Treated wastewater Figure 11: Diagrammatic representation of continuous wastewater treatment using system of multispark dischargers. −0.5 −1 −1.5 Sampling −2 −2.5 −3 −3.5 −4 −3 Specific energy J (cm ) Figure 12: Log bacterial population ( 𝑁/𝑁 ) changes versus specific 0 6 energy released in water during the multispark discharger operation. 󳵳 -Total aerobic heterotrophic bacteria (22 C); ◼-faecal coliforms (37 C). volume of the primary mixture. For this reason, the mea- surements of concentration of methane and decomposition products at the reactor outlet cannot be directly used for CH estimating the degree of conversion. Figure 13: Schematic of the experiment. (1) Dielectric tube; (2) It caneasilybeshown [24, 25] that the degree of con- annular electrodes; (3) working gas (CH )bubbles; (4) water; (5) version 𝛼 of methane into carbon and hydrogen according plasma in the interelectrode gaps; (6) reaction chamber. to reaction (4) and the degree of conversion 𝛼 of methane into acetylene according to reaction (5)are connectedwith experimentally determined concentrations 𝐶 , 𝐶 ,and CH C H 4 2 2 𝐶 by the relations where 𝛼 =𝛼 +𝛼 is the total degree of conversion of 0 1 2 methane over channels (4)and (5), which is determined in 4𝐶 −3(1 −𝐶 ) H CH4 𝛼 = , the given experiment. 1+𝐶 CH 4 The energy value of the reaction of decomposition of a methane molecule (in other words, the value of formation of 4(1 − 𝐶 −𝐶 ) H CH 2 4 products) is defined by the relation (6) 𝛼 = , 1+𝐶 CH 1−𝐶 CH 𝛼 = , 𝜀 = , (7) 0 𝑛 1+𝐶 𝛼 𝑞 CH 𝑛 CH 4 4 log N/N 0 Journal of Atomic and Molecular Physics 9 0.1 0.8 0.08 CO 0.6 0.06 0.4 𝜎 0.04 0.2 0.02 500 1000 1500 2000 2500 3000 3500 4000 −1 𝜆 (cm ) Figure 14: Characteristic adsorption IR spectrum of a working gas q (1/min) sample taken at the reactor outlet. CH Figure 15: Dependence of the total degree of conversion of methane on its flow rate. where 𝑛=0 , 1, 2 is the power supplied to the reactor, 𝑞 is CH the methane flow rate, and 𝑃 is theaverage microwavepower. The dependences of flow rate 𝑞 of methane and of CH the energy value on its decomposition and the formation of products on the degree of conversion of methane are shown in Figures 15 and 16. The dependence of the degree of conversion of methane on its flow rate shown in Figure 15 closely tfi s to the inverse proportionality function 𝛼 = . (8) CH Using iterations, we find that 𝐴 = 0.02809 L/min. The fact that experimental points tfi well to functional dependence 0 0.5 1 1.5 2 2.5 3 3.5 4 (8) suggests that this dependence is preserved in a certain q (1/min) CH interval of 𝑞 beyond the range of the values studied CH experimentally. This in turn raises hopes that if we could Figure 16: Dependence of the energy value of conversion of implement a regime with the methane flow rate on the order methaneonits flowrate. of 0.1 L/min, the degree of conversion would increase to ∼ 28%. The same results could be obtained by increasing the repetition rate of discharge pulses to 1 kHz for a methane flow rate of 1 L/min. By increasing the pulse repetition rate to Analysis made in [26]shows that ahighecffi iencyof 3 kHz for the same methane flow rate, we could reach a degree methane conversion processes characteristic of the described of conversion as high as 84%. eTh implementation of basically technology is due to peculiarities contained just in discharges attainable degrees of conversion involves modernization of localized in an interelectrode gaps. Fast heating (up to the generator of high voltage pulses and the design of 4000–5000 K) of the gas propagating between the electrodes the discharger, which will form the basis of subsequent through the area occupied by microplasma leads to the experimental investigation. It is evident that without special effective decomposition of hydrocarbon. At the same time, justification these increased degrees of conversion are looking fast cooling of the gas penetrating into the surrounding water rather as a wishful thinking. is followed by quenching phenomena, and the level of the It canbeseenfrom Figure 16 that the energy value parent-gas decomposition does not change. of the conversion is almost independent of the methane The low energy value of methane decomposition and flow rate and amounts to approximately 5 eV/molecule. Such the possibility of elevating the degree of conversion justify energy value is close to record-low values for the atmospheric the application of the method of plasma-chemical action for pressure (see, e.g., [25]). solving the topical problem of recovery of natural blowouts of The fact that the energy value of conversion is almost hydrocarbons.Inthisconnection, theroleofpyrolysis in the independent of the methane flow rate in the entire range of methane decomposition is of interest in its own right. If the its variation in the experiment is an additional argument in contribution from reaction (4) is significant, it is expedient favorofthe possibilityofasubstantialincreaseinthe degree to determine the form and efficiency of the production of of conversion due to passage to small values of 𝑞 . carbon accompanying the decomposition of methane. CH Transmittance 732 C H 618 2 2 942 C H 2 4 1304 CH 3016 CH 3260 C H 2 2 3316 C H 2 2 Energy cost (eV/molecule) 0 10 Journal of Atomic and Molecular Physics The experiments performed in accordance with the dia- gram in Figure 13 have shown that if the multispark discharge is initiated in water using CH as the bubble-forming gas, most carbon particles appearing in water as a result of plasma-chemical decomposition of methane precipitate. Analysis of the precipitate shows that its main part is nanosize carbon. Figure 17 shows the characteristic size distribution of carbon particles as a function of the time of electric-discharge treatment of water, which was deter- mined using Fotokor dynamic scattering spectrometer. eTh typical photograph of nanocarbon produced in the course of methane recovery by multispark discharge in the water is shown on the Figure 18. −100 eTh rate of production of nanosized particles in the 10 15 20 25 30 discharge, which was determined by evaporation of the SSD- t (min) processed liquid and weighing the precipitate, was about Figure 17: Dependence of the average size of carbon particles 35 mg/h. This means that the energy value of production produced in the reactor on the time of electric discharge processing of nanocarbon upon decomposition of methane in the SSD of methane. is 0.3 kW h/g. eTh measured value is close to that obtained for arc discharges with carbon electrodes in water, in which carbon is formed in the liquid as a result of destruction of the electrodes [27, 28]. The structure of the precipitate was determined using a LAB RAM HR 800 Raman spectrometer from the Raman shift. Fractions of disordered graphite and carbon were detected. 6. Water Cleaning of 2,4-Dichlorophenoxyacetic Additive Polychlorinated biphenyls (PCBs), among man-made pollu- tions, deserve particular attention. These compounds were Figure 18: eTh typical photograph of nanocarbon produced in the synthesized in 1920 s, and with their advent new materials course of methane recovery by multispark discharge in the water. with unique thermophysical and electrical insulating prop- erties became available. −3 concentration in the container was estimated at ∼8 𝜇 gcm , However, in spite of the presence of a number of unique that is, about 300 times larger than the maximum allowable properties, these compounds were withdrawn from industrial (“permissible”) concentration. processes already in 1970 s. This is due to the fact that PCBs Decomposition of the acidic additive was accomplished were implicated in a number of incidents in different coun- using a multispark discharger mounted in a plexiglass reactor tries by causing mass intoxication and exerting a detrimental chamber of volume 𝑉 = 15×6×4.5 cm .Themultispark effect on the health of humans on a large scale. discharger, which was placed on inside of the reactor cover, eTh PCBs are no longer manufactured but remain in the produced a discharge in water. The working gas passed environment, so that the search for ways of their destruction through the discharger was oxygen. is one of the urgent problems of the day. At the General Analysis of the SSD-processed solutions was conducted in Physics Institute of RAS, experiments were carried out to the Laboratory of Analytic Environmental Toxicology at the examine possibility of electric discharge (SSD) in water Severtsov Institute of Ecology and Evolution of the Russian as an efficient and inexpensive method for cleaning the Academy of Sciences. manufacturing water of PCBs. Instead of a toxic PCB in GC/MS (Gas chromatography/Mass spectrometry) anal- our experiments, we used a 2,4-D dichlorophenoxyacetic ysis of solutions was performed by using a Finnigan TRACE acid (2,4-D). This material was chosen for plasmachemi- GC Ultra gas chromatograph coupled with a Finnigan Polaris cal decomposition, because the congfi uration of the 2,4- Q mass spectrometer (ion trap). This GC/MS system possess- D molecule somewhat resembles PCB. More exactly, the ing ultra-high sensitivity allows detection of 2,4-D compound 2,4-D molecule, like the PCB congeners, contains a doubly and its possible organic products of fragmentation with chlorine-substituted benzene ring with attached acetic acid. −9 −3 sensitivity ∼10 gcm . The experimental procedure was as follows. Two solid All our experiments were conducted at xfi ed values of particles of 2,4-D (97%) of weight 40 mg were preliminarily −3 the initial 2,4-D concentration 𝑁=8𝜇 gcm and solution dissolved in 10 mL of alcohol. eTh solution was poured into a polyethylene container with 5 L of distilled water. eTh acid volume 𝑉 = 250 cm . In all experiments, a sample of solution Diameter of particle (nm) Journal of Atomic and Molecular Physics 11 was taken from the reactor before processing in order that the 9 initial 2,4-D concentration will be accurately known. Data of GC/MS measurements ensure complete decom- −9 −3 position of 2,4-D (at the level of sensitivity 10 gcm )inall of the experiments when the processing time was longer than 150 s and the mean power of the high-voltage generator was ∼20 W. These experiments give a conservative estimate of the efficiency of plasmachemical decomposition of the organic 2,4-D compound by the use of a multielectrode system excited electric discharge (SSD) in water. A characteristic dependence of the 2,4-D concentration on the duration of SSD processing is presented in Figure 19. 0 150 300 450 600 Almost complete (∼100%) decomposition of 2,4-D a Time (s) high-concentrated solution shows that the SSD processing will outperform the traditional reactors. From the experi- Figure 19: 2,4-D content as a function of time of water treatment by ments,itmightbeinferredthatSSDworkinginthewatercon- means of multispark discharge. taining about 300 maximum allowable concentration of 2,4- D provides almost complete decomposition of liquid solution −3 with expenditure of energy as low as ∼ 2⋅10 kW h/L. For each concrete application, the electric-discharge sys- Accordingly, with a power source ∼1kW it is possible to clean temmay be modiefi dindesignsoastoincreaseone or the more than 0.5 m of water per hour. other of these factors. We do not have a clear notion of what mechanism is The experiments demonstrated high efficiency of multi- dominant in the technological process of water cleaning of the spark discharge in water for solving diversified environmental 2,4-D additive. Special experiments have yet to be performed problems listed above. Note that the dominant mechanism in to construct a physicochemical model for electric-discharge sterilization of potable and waste water was the biologically destruction of the acid (and its decomposition fragments). active UV radiation and generation of chemically active However, we have good reason to believe that a leading part in molecules (ozone, hydrogen peroxide). eTh achievement of destruction is played by plasma-chemical reactions occurring encouraging results in conversion of natural hydrocarbons is in SSD with the resulting formation of chemically active credited to the immediate action of microplasma formation radicals and molecules. on the gas being treated. eTh success in the accomplishment of water cleaning of 2,4-D is attributed to plasmochemical mechanismofgeneratingchemicallyactivesubstances. 7. Conclusion In conclusion, the multispark discharge in water is being A new electric-discharge system, which has been developed used more andmore. uTh s, theactionofSSD on theorganic and tested at the GPI RAS, has a multitude of potential pollutions has been investigated in [29]. Decomposition of uses. Examples can be found in the present paper. A plasma- dissolved pentachlorophenol and parachlorophenol under chemical reactor of simple design using a multielectrode multispark discharge action has been measured. Efficiency of (multispark) discharger operating in aqueous medium may reforming these phenols was as good as 1-2 kJ/mg. serve for efficient disinfection of microbially contaminated potableand wastewater,conversion(recovery)ofmethane, References destruction of acidic 2,4-D pollutant. The SSD-based electrode system is capable of produc- [1] V. L. Goryachev, F. G. Rutberg, and V. N. Fedyukovich, ing multiple microplasma formations in liquid medium “Electric-discharge method of water treatment. Status of the at relatively low electrode voltages. Physical and chemical problem and prospects,” Applied Energy, vol. 36, pp. 35–49, 1998. properties peculiar to this type of discharges have been [2] L. A. Yutkin, Electrohydroulic Eec ff t and Industrial Application , studied experimentally. It is shown that these properties are Mashinostroenie, Leningrad, Russia, 1986. controlled by the following four factors simultaneously acting [3] J. Sketchell, H.-G. Peterson, and N. Christofi, “Disinfection by- upon the liquid (aqueous) medium: product formation after biologically assisted GAC treatment of water supplies with different bromide and DOC content,” Water (i) directinufl enceofelectric-dischargeplasmapos- Research, vol. 29, no. 12, pp. 2635–2642, 1995. sessing a high electron density and relatively high [4] F. X. R. Van Leeuwen, “Safe drinking water: the toxicologist’s temperatures of the gas and electron component, approach,” Food and Chemical Toxicology,vol.38, pp.851–858, (ii) exposure to intense UV radiation emitted by [5] U.Von Gunten,A.Driedger, H. Gallard, andE.Salhi,“By- microplasma formations, products formation during drinking water disinfection: a tool (iii) chemical action of chemically active radicals, atoms, to assess disinfection efficiency?” Water Research,vol.35, no.8, and molecules produced in discharges and penetrated pp. 2095–2099, 2001. the water, [6] PCT, Treatment of Liquid International Patent Application no (iv) hydrodynamic action through cavitation bubbles. PCT/GB99/00755, 1999. 2,4-D content ( g/cm ) 12 Journal of Atomic and Molecular Physics [7] L. A. Kul’skii, O. S. Savchuk, and E. Yu. Deinega, Inu fl ence of [24] A. I. Babaritskii, S. A. Demkin, V. K. Zhivotov et al., Plasma- Electron Field on Process of Water Sterilization,Nauk. Dumka, chemistry-91 (INKhS AN SSSRv),vol.2,pp. 286–303, 1991. Kiev, Ukraine, 1980. [25] S. I. Gritsinin, P. A. Gushchin, A. M. Davydov, E. V. Ivanov, I. [8] E.M.Barkhudarov,I.A.Kossyi,M.I.Taktakishvili,N.Christo,fi A. Kossyi, and M. A. Misakyan, “Conversion of methane in a and V. Zadiraka Yu, “Multispark generation of plasma in liquids coaxial microwave torch,” Plasma Physics Reports,vol.35, no. and its utilization in waste water treatment,” in Proceedings 11, pp.933–940,2009. of the 13th International Conference on Gas Discharges and [26] A. M. Anpilov, E. M. Barkhudarov, N. K. Berezhetskaya et al., their Applications,vol.2,pp. 680–683, StrathclydeUniversity, “Methane conversion in a multielectrode slipping surface dis- Glasgow, UK, 2000. charge in the two-phase water-gas medium,” Technical Physics, [9] A.M.Anpilov,E.M.Barkhudarov,Y.B.Barketal.,“Electric vol. 56, no. 11, pp. 1588–1592, 2011. discharge in water as a source of UV radiation, ozone and [27] N. Parkansky, O. Goldstein, B. Alterkop et al., “Features of hydrogen peroxide,” Journal of Physics D,vol.34, no.6,pp. 993– micro andnano-particlesproducedbypulsedarc submergedin 999, 2001. ethanol,” Powder Technology,vol.161,no. 3, pp.215–219,2006. [10] S. M. Korobeinikov and E. V. Yashin, “Bubble model for [28] N. Sano, “Low-cost synthesis of single-walled carbon nano- breakdown in water at pulsed voltage. Electric discharge in horns using the arc in water method with gas injection,” Journal liquid and its industrial application, part 1, Nikolaev, Russia,” of Physics D,vol.37, no.8,p.L17,2004. [29] V. M. Shmelev, N. V. Evtyukhin, Y. N. Kozlov, and E. M. [11] V. L. Goryachev, A. A. Ufimtsev, and A. M. Khodakovskii, Barkhudarov, “Action of pulsed surface discharge on organic “Mechanism of electrode erosion in pulsed discharges in water contaminants in water,” Khimicheskaya Fizika,vol.23, no.9,pp. with a pulse energy of∼1J,” Technical Physics Letters,vol.23, no. 77–85, 2004 (Russian). 5, pp. 386–387, 1997. [12] A. M. Anpilov, E. M. Barkhudarov, N. K. Berezhetskaya et al., “Sourceofadensemetal plasma,” Plasma Sources Science and Technology,vol.7,no. 2, pp.141–148,1998. [13] Y. B. Bark, E. M. Barkhudarov, Y. N. Kozlov et al., “Slipping surface discharge as a source of hard UV radiation,” Journal of Physics D,vol.33, no.7,pp. 859–863, 2000. [14] K. H. Becker, K. H. Schoenbach, and J. G. Eden, “Microplasmas and applications,” Journal of Physics D,vol.39, no.3,pp. R55– R70, 2006. [15] A.M.Anpilov,N.K.Berezhetskaya,V.A.Kop’evetal., “Explosive-emissive source of a carbon plasma,” Plasma Physics Reports,vol.23, no.5,pp. 422–428, 1997. [16] N. K. Berezhetskaya, V. A. Kop’ev, I. A. Kossyi, I. I. Kutuzov, and B. M. Tiit, “Explosive emission phenomena on a metal-hot plasma interface,” Zhurnal Tekhnicheskoi Fizikiv,vol.61, no.2, pp. 179–184, 1991 (Russian). [17] E. M. Barkhudarov, I. A. Kossyi, and M. I. Taktakishvili, “Distributed plasma generation in liquids,” in Proceedings of 13th International Conference on Gas Discharges and their Appli- cations, vol. 2, pp. 340–342, Strathclyde University, Glasgow, UK, 2000. [18] C. G. Hatchard and C. A. Parker, “A new sensitive chemical actinometer. II. Potassium ferrioxalate as a standard chemical actinometer,” Proceedings of the Royal Society A,vol.235,no. 1203, pp. 518–536, 1956. [19] V. V. Lunin, M. P. Popovich, and S. N. Tkachenko, Physical Chemistry of Ozone, Moscow State University Press, Moscow, Russia, 1998. [20] J. H. Baxeudale, “eTh flash photolysis of water and aqueous solutions,” Radiation Research,vol.17, no.3,pp. 312–326, 1962. [21] B. N. Frog and A. P. Levchenko, Preparation of Water,Moscow State University Press, Moscow, Russia, 1996. [22] A. M. Anpilov, E. M. Barkhudarov, N. 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Multispark Discharge in Water as a Method of Environmental Sustainability Problems Solution

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Copyright © 2013 E. M. Barkhudarov et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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10.1155/2013/429189
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Hindawi Publishing Corporation Journal of Atomic and Molecular Physics Volume 2013, Article ID 429189, 12 pages http://dx.doi.org/10.1155/2013/429189 Research Article Multispark Discharge in Water as a Method of Environmental Sustainability Problems Solution 1 1 2 1 E. M. Barkhudarov, I. A. Kossyi, Yu. N. Kozlov, S. M. Temchin, 1 3 M. I. Taktakishvili, and Nick Christofi A.M. Prokhorov General Physics Institute of RAS (GPI RAS), Vavilov Street 38, Moscow 119991, Russia Semenov Institute of Chemical Physics of RAS, Kosygin Street 4, Moscow 119991, Russia Edinburgh Napier University, Edinburgh EH9 3JF, UK Correspondence should be addressed to I. A. Kossyi; kossyi@fpl.gpi.ru Received 1 February 2013; Accepted 20 April 2013 Academic Editor: Elena Tatarova Copyright © 2013 E. M. Barkhudarov et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Multispark discharge excited in water is described, and its useful physical and chemical properties are discussed in the light of some environmental issues. Discharge of such a type generates hot and dense plasmoids producing intense biologically active UV radiation and chemically active radicals, atoms, and molecules. Simultaneously, discharge creates strong hydrodynamic perturbations and cavitation bubbles. Particular attention is given to factors influencing on water purity with special reference to discharge application for eeff ctive sterilization of water and its cleaning of harmful chemicals. The gas discharges of this type show considerable promise as a means for solving some actual plasma-chemical problems. eTh above-mentioned discharge properties have been demonstrated in a series of laboratory experiments, which proved the efficiency of disinfection of potable and waste water, water cleaning of pesticide (herbicide) contaminations, and conversion (recovery) of natural methane. 1. Introduction of environmental problems, such as conversion (recovery) of methane (as well as other natural hydrocarbons), and water High voltage electric discharge in water [1, 2]hasbeenconsid- cleaning of pesticide (herbicide) contamination. ered as a potential method of water treatment to kill microor- ganisms and to clean it of harmful contaminations negating the use of chemicals that leads to by-products which may 2. Treatment System additionally compromise human health [3–5]. Factors favor- ing their use include the generation of UV radiation, acous- eTh apparatus used to treat liquids is shown schematically in tic, shock waves, chemically active substances, cavitation Figures 1 and 2. eTh basic components were a chamber filled processes, pyrolysis, and hydrolysis. There are also possible with water, a multielectrode system for exciting of slipping synergetic effects following physical and chemical reactions. surfacedischarge,and high voltagepower supply (Figure 1). Among the different means of in-liquid electric discharge, The multielectrode discharge system ( Figure 2)was similar a novel method involves multielectrode (multispark) slipping in design to that previously described in [6, 8, 9]. The (gliding)discharges(SSDs)[6]which mayhavesomeadvan- discharger consisted of a set of annular electrodes mounted tages over the two-electrode systems generally used at present on a dielectric tube surrounding a back-current conductor. [1, 7]. A gas (air, argon, oxygen, etc.) was injected through a set of The present work describes the construction of a multi- holes into water between the electrodes, producing fine gas spark discharger and discusses results of experimental inves- bubbles. Discharge in each interelectrode gap was produced tigation of SSD-based methods of water disinfection and their throughout the system, including the metal electrodes, a application in plasma-chemical technology for solving some dielectric substrate, a gas bubble, and water. 2 Journal of Atomic and Molecular Physics Clean water Air or O bubbles Microbially contaminated water Air or O Figure 1: Scheme of multispark discharge disinfection of water. (1) Chamber; (2) multispark discharger; (3) generator of high voltage pulses; (4) cleaning water; (5) plasma of gliding discharge. 6 6 Air, O Air, O (a) (b) Figure 2: Multielectrode gliding surface discharge facility. (1) Electrodes; (2, 3) dielectric tube; (4) back-current rod; (5) discharge plasma; (6) gas bubbles. The initial plasma channel may be thought as originating recently adopted terminology) as “microplasma” formations, in ordinary gas discharge in a gas bubble if the electric eld fi involved in various applications [14]. According to the results therein is higher than the gas breakdown threshold [10, 11]. of previously performed experiments, the electron density in 17 −3 But in actual fact, a large (sometimes dominant) part in the plasmoids attains 10 cm ,and thegas temperature4000– interelectrode plasma formation could be played by a gliding 5000 K [15]. According to [16], explosive metallic plasma is a discharge along the dielectric surface with the subsequent source of intensivehardUVradiation. interaction of discharge plasma with electrodes and explosive A typical photograph of the operating system is shown in microplasma production on their surface [12](see Figure 3). Figure 4. There are just these processes that have been considered to eTh principal advantage of the multispark system lies in be operative in the case when multispark discharger works in the following peculiarities of their construction. the gas medium [13]. When the high voltage pulse is applied to the immersed (i) eTh area of the surface of all electrodes contacting water in the multielectrode version can be minimized in the aqueous medium discharger (shown in Figure 2), by introducing insulating dielectric screens ensuring plasma bunches (plasmoids) appear almost simultaneously the SSD operation in high-conducting water (up between electrodes. Reasoning from their characteristics 4 −1 these plasmoids can be classified (in accordance with the to conductivities of 10 𝜇 Scm )without substantial Journal of Atomic and Molecular Physics 3 1 Water 1 Water Gas Gas Gas Gas 3 3 (a) (b) Figure 3: Two consecutive phases of plasma production in each interelectrode gap. (1) Electrodes; (2) dielectric tube; (3) back-current rod; (4) gliding surface discharge; (5) metallic plasma; (6) unipolar arc. the tested metals (Fe, Mo, Cu, Ti, etc.), just stainless steel and titanium have been selected as materials exhibiting the most promise for working as a detail of multispark discharger. Just these two metals have been used in electrodischarge systems applied in the General Physics Institute (GPI RAS) for solution of water purification problems. (v) eTh discharge gaps could be distributed in such a way as to increase the efficiency of the discharge action on liquids, in particular, by focusing the shock waves and UV radiation ux fl [ 17]. The experiments were conducted using the high volt- Figure 4: Typical photograph of multispark discharger operating in age multichannel (5 channels) generator with the following water. parameters: high voltage amplitude, 𝑈≤20 kV; pulse repetition frequency, 𝑓 ≤ 100 Hz;capacitivestorageenergyof one channel,𝑊≤2 J, and pulse duration,𝜏≈5𝜇 s. The circuit of the output stage of each channel is shown in Figure 5. Each reduction of the efficiency of energy supply to the multispark discharger was powered from one channel of a discharge region. multichannel generator. The discharge current and voltage (ii) The discharger has no pointed electrodes; the working were measured with the aid of a Rogowski coil and voltage surface of the electrodes (unprotected by dielectric divider. eTh signals shown on the Figure 6 were recorded with screens) is developed and is either a part of cylindrical an oscilloscope (TDS 3012). es Th e measurements allowed surface of tubular electrodes or the plane surface −3 the determination of the energy density (J cm )releasedin at the exit sections of the tube. us, Th the principal liquid. advantageofSSD system lies in thedecreaseinthe discharge load of each electrode (thereby enhancing 3. Multispark Electric Discharge in Water the erosion resistance on the system as a whole), which ultimately substantially increases the lifetime as a Source of UV Radiation, Ozone, and of the system. Hydrogen Peroxide (iii) The dischargers can aeff ct the aqueous (liquid) Figure 7 shows a schematic of the experiment intended to medium through several simultaneously acting investigate a multispark SSD in water as a source of UV mechanisms, among them the direct influence radiation, ozone, and hydrogen peroxide. Multielectrode of discharge plasma, the action of UV radiation discharger (2) is positioned in a cell (1) with water. A generated by microscopic discharges, the chemical high voltage pulse produces a plasma channel between the action of chemically active radicals, atoms, and electrodes. eTh gas leaving the reactor (as a working gas molecules produced in discharges, and the hydro- air or oxygen has been applied) flows into a quartz cell (3) dynamic action through microscopic cavitation intended for determining the ozone content by the method bubbles. of absorption spectroscopy. In the course of the experiments, (iv) Cleansing action and bactericidal effect of a multi- the production of H O was also measured. UV radiation was 2 2 spark discharge in the water medium unessentially measured in the presence and absence of water in the reactor depend on electrode material. Nevertheless among chamber. 4 Journal of Atomic and Molecular Physics R C 1 3 Air, O + O 2 3 R 2 Air, O + O 7 2 3 Water 1 8 Water Figure 5: Output stage of one channel of the high voltage pulses power supply. (1) Rogowski coil; (2) voltage divider. 𝑅 , 𝑅 -resistors; 1 2 𝐶 -capacitor; 𝐿 -inductor. Air, O Figure 7: Experimental layout. (1) Vessel filled with water; (2) I, U multispark discharger; (3) diagnostic quartz cell; (4) deuterium lamp; (5) discharge plasma; (6) MDR-3 monochromator; (7) gas bubbles; (8) MUM-1 monochromator; and (9) quartz window. 100 A 2 kV 5 𝜇 s Figure 6: Typical oscillograph trace of SSD current and voltage. 240 250 260 270 280 290 𝜆 (nm) The discharge emission spectrum in the region 230 < Figure 8: Spectrum of soft UV radiation from multispark discharge 𝜆 < 300 nm was measured with the help of an MUM-1 in the water. monochromator ((8), Figure 7)and with an FEU-142photo- multiplier. Typical spectra of UV emission from the discharge are shown in Figure 8. In the case of application of air as working gas, the Chemical (actinometric) measurements have been used O content was determined by the chemical method from as well. In this case, the UV intensity was deduced from the reaction between O and potassium iodide in the water photolysis of an irradiated K Fe(C O ) solution with a 3 2 4 3 solution [19]. phenanthroline admixture. Figure 9 shows the ozone density in the diagnostic cell as This technique was described in [ 18] and successfully used a function of the repetition frequency of high voltage pulses in [13] to study the multispark discharge in gaseous (Ar) (𝑓) for a discharge in water (for various oxygen flow rates). medium. Restriction of 𝑓 values by amounts of the order of 100 Hz To measure the O content in the gas flowing from the is not critical and appears explicable only on the basis of reactor, we used both spectroscopic and chemical methods. improper technical equipment of laboratory. The scheme of measurements of the O content in O is 3 2 In the experiments when the oxygen flow rate through shown in Figure 7. From attenuation of the UV radiation the interelectrode gaps and the water-filled reactor was 𝑤≅ passing through the cell, the O density in the gas was 15 L/min, the ozone density in the oxygen flow was equal to determined by the absorption method. eTh spectral interval 15 −3 used to determine the ozone content corresponded to the 𝑛 ≅ (1-2) 10 cm . Hartley absorption band with the maximum near 𝜆 ≅ The H O content in water treated by the electric 𝑑 2 2 255.5 nm. discharge was measured by the iodide-molybdate method I (a.u.) UV Journal of Atomic and Molecular Physics 5 −3 initial number of bacteria in a unit volume (cm ), 𝑃 is 1.8 UV −2 themeanintensity of theflux of bactericidal rays ( 𝜇 Wcm ), 1.6 𝑡 is the irradiation time (s), and 𝑘 = 2500 is the bacterial 𝑎 𝑏 1.4 tolerance factor. 1.2 For the case of repetitive discharge, expression (1)can be rewritteninthe form 0.8 −𝑃 𝑡 0.6 UV(𝑖) 𝑎 𝑛 ≅𝑛 exp ( ) , (2) 𝑏 𝑏0 0.4 𝑘 0.2 where 𝜏 is pulse duration (s) and 𝑓 is the repetition frequency of high voltage pulses (Hz). 0 20 40 60 80 100 6 −2 It is easy to see that for 𝑃 ∼3⋅10 𝜇 Wcm , 𝜏=5𝜇 s f (Hz) UV(𝑖) and 𝑓 = 100 Hz, the exposure time equal to a few seconds Figure 9: Ozonedensity in thediagnosticcellasafunction of is sucffi ient to decrease the number of bacteria in water by the repetition frequency of multispark discharge in the tap water a factor of ten. This means that the energy cost of treating −1 −1 for various flow rates of O : ◼-𝑤=10 L min ; -15 L min ; 󳵳 - water by bactericidal UV rays is of the order of 𝜉 ≈ (1- −1 UV 20 L min . −4 −1 2) 10 kW h L . Under the experimental arrangement shown in Figure 7, described in [20]andusedin[13] to determine the intensity of ozone generated in the discharge has no time to dissolve in hard UV radiation of the gliding surface discharge in argon. water and is almost completely removed by the air (oxygen) The measurements of hydrogen peroxide production that flow into the space over the water reactor. In principle, it is were carried outinadischargeinwater with injected argon possible to constructareactorsuchthatthe produced ozone showed that a series of discharges for 6-7 minutes in 250 cm will be completely “entrapped” in the water being treated. Let of water produced H O with a mean density of 𝑛 ≅2 ⋅ us estimate how eeff ctive the role of ozone in the sterilization 2 2 H O 2 2 −3 −1 18 −3 action of discharge may be in this case. 10 mol L ≅ 1.2 ⋅ 10 cm .Theenergycostofproduction As follows from the data presented in [19], the eeff ct of of one H O molecule in this case is ℎ ≤ 1.5⋅10 eV/mol. 2 2 H O 2 2 ozone dissolved in water on microorganisms becomes signif- The performed experiments demonstrated that for the icantly stronger when the O content reaches the threshold SSD in the water-gas mixture, at least two factors are real- 3 16 −3 level [𝑛 ] ≅8⋅ 10 cm .Over [𝑛 ] ,the E. coli bacteria ized among the factors that are usually invoked to explain O O 3 th 3 th thesterilization eeff ctofelectricdischarges. eTh se arethe content decreases by more than four orders of magnitude. generation of UV radiation and the production of biologically It is easy to see that the bactericidal treatment capacity of active ozone and hydrogen peroxide. ozone can be as high as It is possible to estimate, using the results of measure- 𝑛 𝑤 O 𝑏 −1 ments, the eeff ctiveness of these two factors in the degra- 3 𝑤 ≅ ≅25 Lh , O (3) dation of microorganisms during operation of the electric- [𝑛 ] th discharge systems under study. Examining the UV radiation from the discharge, we −1 where 𝑤 is the water-treatment rate (L h )and 𝑤 is the O 𝑏 have to take into consideration that according to [21]the −1 air flow rate through the discharge facility (L h ). Then, the strongest bactericidal eeff ct is produced by ultraviolet rays energy cost of water treatment by ozone generated in the with wavelengths from 295 to 220 nm (the “bactericidal” discharge (assuming that it is completely dissolved in water) spectral region). −4 −1 can reach 𝜉 ≅3⋅ 10 kW h L which is comparable with Measurements performed in our work (see [9]) showed 3 the energy cost of sterilization by UV radiation. that the radiation spectrum of the multispark discharge in Finally, we estimate the eeff ctiveness of a possible bac- water contains the biologically active component, and the tericidal action of the multispark discharge in water due to intensity of this component increases substantially as the the production of hydrogen peroxide. Special microbiological pulse energy increases. studies carried out by us showed that an addition of hydrogen Based on the results of absolute measurements of UV 17 −3 peroxide as a level of 𝑛 ∼10 cm to tap water allows radiation by the actinometric method, we estimate the inten- H O 2 2 the number of E. coli bacteria to be reduced by one order of sity of the ux fl of bactericidal rays per pulse discharge as 6 2 magnitude. This means that the experimentally measured rate 𝑃 ≈3⋅10 𝜇 W/cm [9]. UV(𝑖) of H O production ensures the energy cost of water steriliza- 2 2 Given this intensity, in turn, the eeff ctiveness of the action −4 −1 tion at the level 𝜉 ∼10 kW h L , which is close to the of radiation on E. coli bacteria canbeestimated from the H O 2 2 energy cost of sterilization by ozone production in discharge. known relation [21] Hence, the performed direct measurements of UV radia- −𝑃 𝑡 UV 𝑎 𝑛 ≅𝑛 exp ( ), (1) tion and chemically active products evidence that described 𝑏 𝑏0 below multispark slipping surface discharge (SSD) in water where 𝑛 is the number of bacteria in a unit volume that with airasanworking gasispromising forwater sterilization −3 remain living aer ft bactericidal irradiation (cm ), 𝑛 is the since two effects only, examined in our work, can ensure the 𝑏0 15 −3 n ×10 (cm ) 2(O ) 𝜏𝑓 6 Journal of Atomic and Molecular Physics −4 −1 energy cost as low as 𝜉≅ 10 kW h L for reducing the E. 1 coli bacteria content by one order of magnitude (i.e., with a generator with a mean power of 1 kW, it is possible to reach a 0.1 3 −1 water treatment rate of the order of 10 m h ). It should be pointed out that possibility to apply for multispark discharger excitation of practically every gas or 0.01 gaseous mixtures offers great opportunities for action on a microbiological component through the different chemically 1E−3 active atoms and radicals. However, in this work authors have restricted for water sterilization by the application only of air or oxygen taking into account that based on application of 1E−4 these gases discharger will be simplex and cheapest. 1E−5 4. Multispark Electric Discharge Disinfection 0 0.5 1 1.5 2 −3 of Microbially Contaminated Liquids J (cm ) Figure 10: Changes in populations of Escherichia coli and viruses As a step of our activity experimental investigation of effec- (𝑁 ) in treated water relative to the initial populations (𝑁 )asa tiveness of disinfection action of multispark discharge on the −3 function of specific energy release (J cm ) during the treatment. −1 water containing Escherichia coli and its viruses (coliphages) Potable water with a conductivity 𝜎 = 100 𝜇 Scm was used. 𝑓= has been carried out [22]. 10 Hz. eTh initial ( 𝑁 ) concentration of E. coli was ≈10 colony- −1 7 eTh apparatus used to treat liquids is the same as forming units mL and that of coliphages ≈10 plaque-forming −1 shown schematically in Figure 1.Thedischarge device— units mL . (1) E. coli; (2) coliphages. multispark discharger—was situated in the treatment cham- ber through which water contaminated with microorganisms −3 −4 −1 was pumped. Water contaminated with E. coli or viruses of 0.3 J cm (approx. 10 kW h L )toreducethepopulation (somatic coliphages) can be used to test the killing efficiencies by a factor 10 (1 log reduction) while coliphages required an −3 of the discharge system. Samples of water for microbiological energy input of 0.15 J cm for the same result. analyses were taken via a sampling port; triplicate samples in The used multispark discharger regimes are identical with 10 mL sterile bottles being removed for analysis. the regimes previously investigated [9] where an examina- Escherichia coli (NCIMB 86; ATCC 4157) was grown tion was made of the generation of biologically active UV overnight in nutrient broth (oxoid) at 37 C. The cultures were radiation, ozone, hydrogen peroxide, and other active species 6 −1 diluted to population densities of approximately 10 cfu mL (see preceding section of this paper). Measurements carried with tap water and placed in treatment chamber containing out during the present study allowed calculation of energy the multispark discharger. costs of the disinfection action using multispark electric dis- −4 −1 Water samples treated by the electric discharges were chargers, and these were as low as 10 kW h L for bacteria. removedfromthe system at varyingtimeintervals andbacte- These values verified the bacterial action of discharges in the rial killing assessed using spread plate counting methodology. water predicted in the preceding section and confirmed that Escherichia coli was determined by spreading 100 𝜇 L aliquots the main factors aeff cting microbial destruction in the water of diluted samples onto nutrient agar plates. Occasionally, were UV radiation and the production of biologically active MacConcey agar (HMSO 1994) and a spiral platter were chemicals. The latter are not involved in treatment systems utilized. Replicate plates were incubated at 37 Cfor 24h. utilizing UV lamps which would be unable to generate Coliphages were estimated by a plaque assay utilizing E. coli highly reactive chemical species. Acoustic and shock waves C (ATCC 13706) as the host bacterium. Dilutions of treated generated by multispark discharge also played a part in sampleswerespreadontolawns of E. coli C,sensitive to a microbial disinfection but, in addition, they facilitated the broad spectrum of coliphages, and the number of plaques mixing of treated water, delivering reactive chemical species formed aeft r 24 h incubation counted. to all parts of the treatment system. Figure 10 shows the eeff ct of multispark discharges on The possibility that disinfection using electric discharges microorganisms in the water. eTh fraction of surviving bac- mightleadtothe production of toxicby-productswas tested −3 teria and viruses (𝑁/𝑁 ) is plotted versus the energy density 0 by the input of energy as high as ∼1Jcm into water. Water −3 (J cm )releasedinwater.Eachpoint in theplotpresents samples were analyzed for a range of substances and physical the mean of three measurements. Deviation from the mean appearance by the Certicfi ation Control-Analytical Center did not exceed 15%. Numerous experiments were carried out (Moscow State University, Russia). The water was tested for using E. coli, and all showed a similar killing efficiency of color, turbidity, pH, ammonium, Fe, Pb, Cr, u fl orite, chlorite, the multispark discharge system. Data of microbial killing nitrate, and sulphate. eTh quality of the treated water fulfilled in liquids containing tap water-microbe combinations and a the necessary standards of the European Union (Council −1 conductivity of 100 𝜇 Scm arepresented.Itisevidentfrom Directives on the quality of water intended for human con- Figure 10 that the viruses were killed using a lower energy sumption 80/778/EEC and the new drinking water Directive input to the liquid. Escherichia coli required an energy input 98/83/EC adopted by the Council on 3 November 1998). The N/N 0 Journal of Atomic and Molecular Physics 7 results for Fe were particularly important as the electrodes more than billion cubic meters of associated gases are used in the study were manufactured from stainless steel. burning down worldwide. Russian oil producing companies Erosion of multispark discharger is small and does not affect forcompensationofanecologicalharmare paying near 500 overall concentrations in water. In addition, incubations rubles for each 1000 m of burning petroleum gas. of multispark discharge treated water with microorganisms Presented work objective is the investigation of possi- were carried out to test whether the killing action con- bility of natural hydrocarbons (namely, CH )recoveryin tinued. This could be due to the persistence of oxidizing plasma-chemical reactor based on the SSD. Traditional for species produced by the discharge but these were rapidly GPI research multispark dischargers have been used with quenched within the system following treatment. eTh re were only one key distinctive feature of their construction: as a no increased eeff cts on E. coli added to system containing discharge formative gas methane (or any other utilizable plasma treated compared with nonplasma-treated tap water. natural hydrocarbons) has been applied. This is contrary to results obtained with two-electrode dis- The diagram of the experiment is shown schematically in charges [1]and couldbeexplained by quitelow levelof Figure 13. A multielectrode discharger is introduced into the operated multispark discharger electrodes sputtering and as reaction chamber in the form of an organic glass vessel filled a result extremely low level (in comparison with the two- with water (volume 𝑉 ∼ 0.25 L). When a high-voltage pulse electrode system) of content of metallic clusters responsible is applied to the discharger, a system of plasma formations (according to [1]) for prolonged action of discharge on a (plasmoids) in which the decomposition of hydrocarbons microbial population. It is of interest to note that a multispark takes place is formed in bubbles of methane or methane- discharge treatment of short duration could sterilize tap water oxygen mixture in the gaps between the electrodes. eTh containing E. coli andcoliphage.Thedurationwas short source of high voltage pulses was a generator producing single enough for the cost-effective treatment of water supplies pulses or operating in the pulse-periodic regime. eTh pulse- (<5 min); contact time being in the region of minutes rather repetition rate was 𝑓≤50 Hz,the pulsedurationwas 𝜏 ≈ than the 30 mins used in chlorination. 1𝜇 s, andthe pulseamplitude was 𝑈 ≈40 kV. This study concentrated on verifying the predictions of We analyzed samples of the gas taken at the outlet of microbial killing made originally in [9]and utilized E. coli the reaction volume. Analysis of the gas passing through the and coliphage as representative organisms. No attempt has discharger was carried out using the following techniques: been made at this stage to examine the eeff ct of multispark (i) special ITT IK/VP test tubes (OOO Impul’s) used for discharge plasma on the other bacteria (Gram-positive or determining the contents of acetylene (C H ), carbon 2 2 -negative types), viruses, or spores (bacterial or fungal). dioxide (CO ), and carbon monoxide (CO); Preliminary experiments have been performed to determine only the effect of multispark plasma on the oocysts of Cryp- (ii) SPECORD IR spectrograph used for determining the tosporidium (a protozoan parasite causing gastrointestinal acetylene content; disorders), which are resistant to chlorination. eTh micro- (iii) gas chromatograph used for determining the concen- scopic examination of cysts after treatment showed cell wall tration of methane (CH ) and hydrogen (H ). 4 2 degradation and an inability to induce excystation in the organism. Figure 14 shows the characteristic spectrograms obtained on the SPECORD IR spectrograph. eTh main absorption lines It is of interest to investigate the possibility of using the multispark system described to treat industrial and domestic of CH ,C H ,and CO canbedistinguished (insubsequent 4 2 2 wastewater. eTh rfi st attempt at such an application has been analysis of the experimental results, CO was disregarded). The lines of the nearest unsaturated hydrocarbon ethylene taken in [8, 23]. Water treatment was carried out using wastewater directly abstracted from final effluent stream at C H are also very weak (at the noise level). In analysis of 2 4 the efficiency of the plasma-chemical conversion of methane, the Livingston Wastewater Treatment Plant in West Lothian, Scotland, UK. The scheme of system for wastewater treatment it is expedient (see [24]) to use such parameters as the is shown in Figure 11. Results of SSD action on a final degree of conversion 𝛼 expressed in fractions (in other words, the fraction of methane fed to the reactor and converted eu ffl ent stream are presented in Figure 12.Itwas shown −3 into a certain product at the output) and the energy value that a specific energy of 1.25–1.5 J cm was required to 𝜀 of conversion (i.e., the energy value of transformation achieve 1 log reduction in bacterial (faecal coliforms/total of methane molecules in eV/molecule). If we disregard for aerobic heterotrophs) content. This study has demonstrated simplicity the small amounts of ethylene formed as a result of the eeff ctiveness of the multispark dischargers in microbial methane treatment, we can assume that mainly two reactions disinfection of wastewater. eTh system can be engineered occur in the plasma-chemical reactor: pyrolysis reaction, to eradicate microbial populations to levels governed by legislation by increasing treatment time or energy input. CH 󳨀→ C +2H (4) 4 2 and the reaction of transformation of methane into acetylene, 5. Plasma-Chemical Converter of Methane on 2CH 󳨀→ C H +3H the Basis of Multielectrode Discharger (5) 4 2 2 2 One from the currently important ecological problem con- It can be seen from simplified reaction formulas ( 4)and sists in utilization of gases accompanying oil recovery. Yearly (5) that the volume of the reaction products exceeds the 8 Journal of Atomic and Molecular Physics High voltage power supply and pulse generator Electrical connections Pump Reaction chamber 2 with SSD electrodes Untreated Reaction wastewater chamber 1 with SSD electrodes Treated wastewater Figure 11: Diagrammatic representation of continuous wastewater treatment using system of multispark dischargers. −0.5 −1 −1.5 Sampling −2 −2.5 −3 −3.5 −4 −3 Specific energy J (cm ) Figure 12: Log bacterial population ( 𝑁/𝑁 ) changes versus specific 0 6 energy released in water during the multispark discharger operation. 󳵳 -Total aerobic heterotrophic bacteria (22 C); ◼-faecal coliforms (37 C). volume of the primary mixture. For this reason, the mea- surements of concentration of methane and decomposition products at the reactor outlet cannot be directly used for CH estimating the degree of conversion. Figure 13: Schematic of the experiment. (1) Dielectric tube; (2) It caneasilybeshown [24, 25] that the degree of con- annular electrodes; (3) working gas (CH )bubbles; (4) water; (5) version 𝛼 of methane into carbon and hydrogen according plasma in the interelectrode gaps; (6) reaction chamber. to reaction (4) and the degree of conversion 𝛼 of methane into acetylene according to reaction (5)are connectedwith experimentally determined concentrations 𝐶 , 𝐶 ,and CH C H 4 2 2 𝐶 by the relations where 𝛼 =𝛼 +𝛼 is the total degree of conversion of 0 1 2 methane over channels (4)and (5), which is determined in 4𝐶 −3(1 −𝐶 ) H CH4 𝛼 = , the given experiment. 1+𝐶 CH 4 The energy value of the reaction of decomposition of a methane molecule (in other words, the value of formation of 4(1 − 𝐶 −𝐶 ) H CH 2 4 products) is defined by the relation (6) 𝛼 = , 1+𝐶 CH 1−𝐶 CH 𝛼 = , 𝜀 = , (7) 0 𝑛 1+𝐶 𝛼 𝑞 CH 𝑛 CH 4 4 log N/N 0 Journal of Atomic and Molecular Physics 9 0.1 0.8 0.08 CO 0.6 0.06 0.4 𝜎 0.04 0.2 0.02 500 1000 1500 2000 2500 3000 3500 4000 −1 𝜆 (cm ) Figure 14: Characteristic adsorption IR spectrum of a working gas q (1/min) sample taken at the reactor outlet. CH Figure 15: Dependence of the total degree of conversion of methane on its flow rate. where 𝑛=0 , 1, 2 is the power supplied to the reactor, 𝑞 is CH the methane flow rate, and 𝑃 is theaverage microwavepower. The dependences of flow rate 𝑞 of methane and of CH the energy value on its decomposition and the formation of products on the degree of conversion of methane are shown in Figures 15 and 16. The dependence of the degree of conversion of methane on its flow rate shown in Figure 15 closely tfi s to the inverse proportionality function 𝛼 = . (8) CH Using iterations, we find that 𝐴 = 0.02809 L/min. The fact that experimental points tfi well to functional dependence 0 0.5 1 1.5 2 2.5 3 3.5 4 (8) suggests that this dependence is preserved in a certain q (1/min) CH interval of 𝑞 beyond the range of the values studied CH experimentally. This in turn raises hopes that if we could Figure 16: Dependence of the energy value of conversion of implement a regime with the methane flow rate on the order methaneonits flowrate. of 0.1 L/min, the degree of conversion would increase to ∼ 28%. The same results could be obtained by increasing the repetition rate of discharge pulses to 1 kHz for a methane flow rate of 1 L/min. By increasing the pulse repetition rate to Analysis made in [26]shows that ahighecffi iencyof 3 kHz for the same methane flow rate, we could reach a degree methane conversion processes characteristic of the described of conversion as high as 84%. eTh implementation of basically technology is due to peculiarities contained just in discharges attainable degrees of conversion involves modernization of localized in an interelectrode gaps. Fast heating (up to the generator of high voltage pulses and the design of 4000–5000 K) of the gas propagating between the electrodes the discharger, which will form the basis of subsequent through the area occupied by microplasma leads to the experimental investigation. It is evident that without special effective decomposition of hydrocarbon. At the same time, justification these increased degrees of conversion are looking fast cooling of the gas penetrating into the surrounding water rather as a wishful thinking. is followed by quenching phenomena, and the level of the It canbeseenfrom Figure 16 that the energy value parent-gas decomposition does not change. of the conversion is almost independent of the methane The low energy value of methane decomposition and flow rate and amounts to approximately 5 eV/molecule. Such the possibility of elevating the degree of conversion justify energy value is close to record-low values for the atmospheric the application of the method of plasma-chemical action for pressure (see, e.g., [25]). solving the topical problem of recovery of natural blowouts of The fact that the energy value of conversion is almost hydrocarbons.Inthisconnection, theroleofpyrolysis in the independent of the methane flow rate in the entire range of methane decomposition is of interest in its own right. If the its variation in the experiment is an additional argument in contribution from reaction (4) is significant, it is expedient favorofthe possibilityofasubstantialincreaseinthe degree to determine the form and efficiency of the production of of conversion due to passage to small values of 𝑞 . carbon accompanying the decomposition of methane. CH Transmittance 732 C H 618 2 2 942 C H 2 4 1304 CH 3016 CH 3260 C H 2 2 3316 C H 2 2 Energy cost (eV/molecule) 0 10 Journal of Atomic and Molecular Physics The experiments performed in accordance with the dia- gram in Figure 13 have shown that if the multispark discharge is initiated in water using CH as the bubble-forming gas, most carbon particles appearing in water as a result of plasma-chemical decomposition of methane precipitate. Analysis of the precipitate shows that its main part is nanosize carbon. Figure 17 shows the characteristic size distribution of carbon particles as a function of the time of electric-discharge treatment of water, which was deter- mined using Fotokor dynamic scattering spectrometer. eTh typical photograph of nanocarbon produced in the course of methane recovery by multispark discharge in the water is shown on the Figure 18. −100 eTh rate of production of nanosized particles in the 10 15 20 25 30 discharge, which was determined by evaporation of the SSD- t (min) processed liquid and weighing the precipitate, was about Figure 17: Dependence of the average size of carbon particles 35 mg/h. This means that the energy value of production produced in the reactor on the time of electric discharge processing of nanocarbon upon decomposition of methane in the SSD of methane. is 0.3 kW h/g. eTh measured value is close to that obtained for arc discharges with carbon electrodes in water, in which carbon is formed in the liquid as a result of destruction of the electrodes [27, 28]. The structure of the precipitate was determined using a LAB RAM HR 800 Raman spectrometer from the Raman shift. Fractions of disordered graphite and carbon were detected. 6. Water Cleaning of 2,4-Dichlorophenoxyacetic Additive Polychlorinated biphenyls (PCBs), among man-made pollu- tions, deserve particular attention. These compounds were Figure 18: eTh typical photograph of nanocarbon produced in the synthesized in 1920 s, and with their advent new materials course of methane recovery by multispark discharge in the water. with unique thermophysical and electrical insulating prop- erties became available. −3 concentration in the container was estimated at ∼8 𝜇 gcm , However, in spite of the presence of a number of unique that is, about 300 times larger than the maximum allowable properties, these compounds were withdrawn from industrial (“permissible”) concentration. processes already in 1970 s. This is due to the fact that PCBs Decomposition of the acidic additive was accomplished were implicated in a number of incidents in different coun- using a multispark discharger mounted in a plexiglass reactor tries by causing mass intoxication and exerting a detrimental chamber of volume 𝑉 = 15×6×4.5 cm .Themultispark effect on the health of humans on a large scale. discharger, which was placed on inside of the reactor cover, eTh PCBs are no longer manufactured but remain in the produced a discharge in water. The working gas passed environment, so that the search for ways of their destruction through the discharger was oxygen. is one of the urgent problems of the day. At the General Analysis of the SSD-processed solutions was conducted in Physics Institute of RAS, experiments were carried out to the Laboratory of Analytic Environmental Toxicology at the examine possibility of electric discharge (SSD) in water Severtsov Institute of Ecology and Evolution of the Russian as an efficient and inexpensive method for cleaning the Academy of Sciences. manufacturing water of PCBs. Instead of a toxic PCB in GC/MS (Gas chromatography/Mass spectrometry) anal- our experiments, we used a 2,4-D dichlorophenoxyacetic ysis of solutions was performed by using a Finnigan TRACE acid (2,4-D). This material was chosen for plasmachemi- GC Ultra gas chromatograph coupled with a Finnigan Polaris cal decomposition, because the congfi uration of the 2,4- Q mass spectrometer (ion trap). This GC/MS system possess- D molecule somewhat resembles PCB. More exactly, the ing ultra-high sensitivity allows detection of 2,4-D compound 2,4-D molecule, like the PCB congeners, contains a doubly and its possible organic products of fragmentation with chlorine-substituted benzene ring with attached acetic acid. −9 −3 sensitivity ∼10 gcm . The experimental procedure was as follows. Two solid All our experiments were conducted at xfi ed values of particles of 2,4-D (97%) of weight 40 mg were preliminarily −3 the initial 2,4-D concentration 𝑁=8𝜇 gcm and solution dissolved in 10 mL of alcohol. eTh solution was poured into a polyethylene container with 5 L of distilled water. eTh acid volume 𝑉 = 250 cm . In all experiments, a sample of solution Diameter of particle (nm) Journal of Atomic and Molecular Physics 11 was taken from the reactor before processing in order that the 9 initial 2,4-D concentration will be accurately known. Data of GC/MS measurements ensure complete decom- −9 −3 position of 2,4-D (at the level of sensitivity 10 gcm )inall of the experiments when the processing time was longer than 150 s and the mean power of the high-voltage generator was ∼20 W. These experiments give a conservative estimate of the efficiency of plasmachemical decomposition of the organic 2,4-D compound by the use of a multielectrode system excited electric discharge (SSD) in water. A characteristic dependence of the 2,4-D concentration on the duration of SSD processing is presented in Figure 19. 0 150 300 450 600 Almost complete (∼100%) decomposition of 2,4-D a Time (s) high-concentrated solution shows that the SSD processing will outperform the traditional reactors. From the experi- Figure 19: 2,4-D content as a function of time of water treatment by ments,itmightbeinferredthatSSDworkinginthewatercon- means of multispark discharge. taining about 300 maximum allowable concentration of 2,4- D provides almost complete decomposition of liquid solution −3 with expenditure of energy as low as ∼ 2⋅10 kW h/L. For each concrete application, the electric-discharge sys- Accordingly, with a power source ∼1kW it is possible to clean temmay be modiefi dindesignsoastoincreaseone or the more than 0.5 m of water per hour. other of these factors. We do not have a clear notion of what mechanism is The experiments demonstrated high efficiency of multi- dominant in the technological process of water cleaning of the spark discharge in water for solving diversified environmental 2,4-D additive. Special experiments have yet to be performed problems listed above. Note that the dominant mechanism in to construct a physicochemical model for electric-discharge sterilization of potable and waste water was the biologically destruction of the acid (and its decomposition fragments). active UV radiation and generation of chemically active However, we have good reason to believe that a leading part in molecules (ozone, hydrogen peroxide). eTh achievement of destruction is played by plasma-chemical reactions occurring encouraging results in conversion of natural hydrocarbons is in SSD with the resulting formation of chemically active credited to the immediate action of microplasma formation radicals and molecules. on the gas being treated. eTh success in the accomplishment of water cleaning of 2,4-D is attributed to plasmochemical mechanismofgeneratingchemicallyactivesubstances. 7. Conclusion In conclusion, the multispark discharge in water is being A new electric-discharge system, which has been developed used more andmore. uTh s, theactionofSSD on theorganic and tested at the GPI RAS, has a multitude of potential pollutions has been investigated in [29]. Decomposition of uses. Examples can be found in the present paper. A plasma- dissolved pentachlorophenol and parachlorophenol under chemical reactor of simple design using a multielectrode multispark discharge action has been measured. Efficiency of (multispark) discharger operating in aqueous medium may reforming these phenols was as good as 1-2 kJ/mg. serve for efficient disinfection of microbially contaminated potableand wastewater,conversion(recovery)ofmethane, References destruction of acidic 2,4-D pollutant. The SSD-based electrode system is capable of produc- [1] V. L. Goryachev, F. G. Rutberg, and V. N. 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