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30th ICPIG, August 28th – September 2nd 2011, Belfast, Northern Ireland, UK
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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
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Brno University of Technology, Faculty of Chemistry, Purkynova 118, Brno 612 00, Czech Republic
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Centre for Plasma Physics, Queen's University Belfast, BT7 1NN, Belfast, Northern Ireland, UK
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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
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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
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[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