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30th ICPIG, August 28th – September 2nd 2011, Belfast, Northern Ireland, UK C10 Some of the physical and chemical properties associated with plasma production in aqueous solutions. L. Němcová 1,2 , F. Krčma 1 , C. P. Kelsey2 , W. G. Graham 2 P 1 P P P U UP P P P P P Brno University of Technology, Faculty of Chemistry, Purkynova 118, Brno 612 00, Czech Republic 2 Centre for Plasma Physics, Queen's University Belfast, BT7 1NN, Belfast, Northern Ireland, UK P P Underwater electrical discharges have many potential applications but the study of their creation and physical and chemical properties are challenging. Here a small, multielectrode system is used to explore some of the physical and chemical aspects of plasmas created in NaCl, KCl, BaCl2 and Na2CO3 aqueous solutions. Optical spectroscopy is being used to examine the physical properties of the plasmas while the chemistry induced in the solutions by the plasma is probed by measuring hydrogen peroxide generation and pH. While H2O2 is generated in the first three solutions it is not generated in Na2CO3 aqueous solutions, even though emission spectra shows evidence of OH production in the plasmas created in that solution. 1. Introduction Plasmas produced by electric discharges in liquids have been studied with increasing interest in the last few years. This has been prompted by their potential for use in a number of applications ranging from the everyday to the hitech, for example purification, sterilization, organic synthesis, treatment of surface materials and there is even possibility of application in nanoscience [1-4]. There are many different approaches to such plasma production, the main difference being in the electrode configuration. However in virtually all cases there are highly reactive species generated in the liquid particularly OH, O3, O oxygen radicals and hydrogen peroxide. Two are of particular interest here. The OH radical has a high redox potential (2.80V) and is able to react with any organic substance which is present in solution through non-selective oxidative reactions. Therefore OH radicals are of great importance in influencing the chemistry of liquids exposed to plasmas [5]. Hydrogen peroxide is a versatile chemical and strong oxidant with a redox potential of 1.763 V. While the main industrial applications of H2O2 are bleaching of textiles and paper, there are important environmental applications including the removal of inorganic and organic pollutants from wastewater. The use of H2O2 as an OH generating agent in advanced oxidation processes (AOPs) such as ozonation (O3/H2O2/UV), hydrogen peroxide photolysis (UV/H2O2) and the Fenton processes (Fe2+/H2O2) enhances its effectiveness. Here the early stage of a study of some of the physical and chemical properties of an electrical discharge formed in conducting solutions of NaCl, KCl, BaCl2 and Na2CO3 aqueous solutions are presented. The study is focused on hydrogen peroxide and OH generation and their relationship to the plasma properties. 2. Experimental Technique The apparatus is shown in Figure 1. The plasmas are created using a small (~ 3.5 mm diameter) four electrode circular device with a coaxial earth and with each electrode driven with 100kHz RF bipolar square wave voltage. The time dependent currentvoltage characteristics are measured for one of the electrodes and in the power calculation it is assumed that the characteristics of all four electrodes are similar. The plasma was generated in four different 0.15M distilled water solutions of NaCl, KCl, BaCl2 or Na2CO3. The volume of liquid was 10ml, contained in a 25 mm diameter glass cyclinder. Initially the liquid is at room temperature but quickly heats reaching boiling point within around 2 minutes depending on the solution. The hydrogen peroxide production and pH change were measured using Quantofix and Sigma test strips respectively. The current is determined via the various impedance influencing factors dependent on the environment in which the plasma is formed. Such factors include solution conductivity, which in turn depends on temperature and concentration, as well as on the presence of other dissolved or suspended particles, or nearby physical structures, and on the discharge chemistry in each specific instance. Two Ocean Optics spectrometers are used to acquire spectra over several spectral ranges. One (300 to 900 nm), was used for a plasma generated species overview and in assessing the possibility of using a higher resolution spectrometer to measure electron properties through line ratios and Stark 30th ICPIG, August 28th – September 2nd 2011, Belfast, Northern Ireland, UK C10 Broadening. The other (293 to 393 nm) is being used to determine the gas temperature from the rovibrational spectra of OH emission. The power is calculated from the product of the instantaneous measured current and voltage. the spectroscopy indicates the presence of the precursor to its production, OH. 12000 Sodium D Line NaCl Na I 10000 Intensity (AU) 8000 Hα 6000 OH (A2Σ+-X2Π) 4000 Na II 2000 Na II Na I Hβ Fe I 0 200 300 400 500 600 700 800 900 1000 1100 Wavelength (nm) 16000 Ba I Ba I BaCl2 14000 Figure 1 A schematic diagram of the apparatus. 12000 Intensity (AU) 10000 8000 6000 4000 OH (A2Σ+-X2Π) 2000 0 200 300 400 500 600 700 800 900 1000 1100 Wavelength (nm) 16000 Sodium D Line Na2CO3 14000 12000 10000 Intensity (AU) 3. Results The current drawn is determined by the liquid and vapour environment [5] in which the plasma is formed. Such factors include the solution conductivity which depends on the solute, the concentration and the liquid temperature. The test strips show that in all liquids the pH is unchanged to within one unit even after up to 5 minutes of plasma exposure when the liquid has been boiled. The optical emission spectra of the plasma is dominated by Na, K or Ba emission as illustrated in Figure 2. There is evidence of the dissociation of water molecules reflected in the presence of H, O and OH emission. The OH radical is known to be a H2O2 precursor. Early results of the H2O2 production in the liquids (Table1) demonstrate a dependence on the particular solute. 8000 6000 4000 Table 1: The effect of plasma on pH and H2O2 concentration in various solutions. pH Power [Watt] Exposure time [min.] H2O2 conc. [mg/l] NaCl KCl BaCl2 Na2CO3 8 3.2 8 3.2 8 3.9 11 3.7 2 2 2 1 4±1 7±1 6±1 0 There is evidence of more hydrogen peroxide generation in KCl and BaCl2 solution than NaCl, however the power input is greater in BaCl2. Interestingly while OH radical is observed in the plasma emission spectra from Na2CO3, there are no detectable levels of H2O2 in the liquid even though Hα O II CI OH (A2Σ+-X2Π) 2000 0 200 300 400 500 600 700 800 900 1000 1100 Wavelength (nm) Figure 2 Figure 1 Typical broadband spectrum NaCl, BaCl2 and Na2CO3. 4. References [1] D. Hayashi, V. F. L. M. Hoeben, G. Dooms, E.M. van Veldhuizen, W. R. Rutgers and G. M. W. Kroesen, J. Phys. D: Appl. Phys. 33 (2000) 276974 30th ICPIG, August 28th – September 2nd 2011, Belfast, Northern Ireland, UK C10 [2] M. A. Malik, Ubaid-ur-Rehman, A. Ghaffar and K. Ahmed, Plasma Sources Sci. Technol. 11 (2002) 23640 [3] J. Gao, Y. Liu, W. Yang, L. Pu, J. Yu and Q Lu, Plasma Sources Sci. Technol. 12 (2003) 5338 [4] W. G. Graham and K. R. Stalder J. Phys D (Applied Physics) 44 (2011) 174037 [5] L. Němcová, A. Nikiforov, Ch. Leys and F. Krčma, IEEE: Transactions on Plasma Science. 39 (2011) 865870