OPTICAL CHARACTERIZATION AND APPLICATION
OF AN ATMOSPHERIC PRESSURE Ar PLASMA IN CONTACT
WITH LIQUIDS FOR ORGANIC DYES DEGRADATION
D. ZAHARIE-BUTUCEL, S. D. ANGHEL*
Faculty of Physics, Babes-Bolyai University, M. Kogalniceanu 1, Cluj-Napoca 400084, Romania,
* E-mail: [email protected]
Received December 27, 2013
Optical emission, temperatures and electron number densities for two kind of plasma
generated in contact with liquids are presented. Plasma was generated with a single
metallic electrode either in Ar bubbled in liquid or in Ar on the surface of liquid. Both
plasmas were tested for organic dye degradation.
Key words: plasma in/on liquid, radiofrequency, emission spectroscopy, dye degradation.
1. INTRODUCTION
One of the first published methods of generating plasma in contact with
liquids was the electrochemical discharge. In 1887, Gubkin published a systematic
description of this phenomenon later called glow discharge electrolysis [1]. In his
study, he generated a glow discharge over the surface of a liquid electrolyte and he
showed that this method can be used to reduce metallic salts in a liquid electrolyte.
Though steps were taken in the area of discharges in liquids, only for the last two
decades extensive research has been performed trying to explain the physics behind
this kind of discharges [2]. Broadly speaking, they are non-thermal gas discharges
in contact with liquids and were generated with power sources ranging from DC to
RF, using various configurations of electrodes and even introducing gases in the
system.
The case of water discharges has been intensively studied during the last
decades for possible biological, environmental and medical applications. Water
discharges are known to simultaneously produce UV radiation, shock waves and
oxidative species such as OH˙, O, HO2˙ and H2O2 [3, 4]. Their combined effect improves
the efficiency of water decontamination, sterilization and discoloration [5–9].
Rom. Journ. Phys., Vol. 59, Nos. 7–8, P. 757–766, Bucharest, 2014
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D. Zaharie-Butucel, S. D. Anghel
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In this work an alternative method for generating atmospheric pressure
plasma in liquid and on liquid is proposed. The novelty of the plasma generation
system consists in using a single powered electrode and in the modality of
introducing the working gas. The plasma generated in liquid and on liquid is
characterized by spectroscopic methods and is tested as a degradation agent for
organic dyes.
2. EXPERIMENTAL AND METHODOLOGY
The experimental set-up is depicted in figure 1. The power source is a
laboratory made free-running oscillator (10.2 MHz, 1.8 kV sinusoidal waveform,
15 W) [10]. A torch-like device was used to generate a discharge above or inside
liquid (distilled water or MB solution) placed in a glass container. This is
composed of a metal wire electrode (1 mm diameter and 80 mm length) placed via
a holder piece in a quartz tube (5 mm o.d. and 3.8 mm i.d.) through which working
gas (Ar) flows with adjustable flow rate. The electrode is made of kanthal A-1
(Fe 71.02%, Al–5.8%, Cr–22%, Mn–0.4%, Si–0.7% C–0.08%) and its upper end is
connected to the RF generator output. Also the electrode is coated with glass,
except 1 mm in length, to prevent the generation of a corona discharge along it.
The distance between the free end of the electrode and the exit of the quartz tube is
about 0.5–1 mm.
Fig. 1 – Experimental set-up.
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Optical characterization and application of an atmospheric pressure Ar plasma
759
For generating the plasma in liquid, the quartz tube is vertically immersed
inside at a depth of 15–20 mm. When Ar gas flows, at the immersed exit of the
quartz tube gas bubbles arise with a cadency of 0.5–10 bubbles/second, depending
on the gas flow rate (max. 0.3 l/min). During each bubble life, between the free end
of the electrode and the bubble wall a glow discharge is generated, the bubble wall
representing the secondary electrode with a floating electric potential. When the
torch-like device is vertically placed over the surface of liquid, for an Ar flow-rate
of 0.5 l.min-1 a stable and relatively uniform discharge can be generated for an
electrode-liquid distance of maximum 10 mm. Usually [11], to generate an
atmospheric pressure plasma in contact with liquids two electrodes (a powered one
and a grounded one) are used. As it can be observed, in our experiment a single
metal electrode is used for generating the plasma in or on liquid which represents
an important advantage comparatively with other systems. This is possible by
choosing the most appropriate geometric dimensions of the device in correlation
with the optimum gas flow-rate and the electrical parameters. Moreover, when the
system is used for the liquid treatments the treated solution does not need to be
shaken or recirculated, the homogenization being done by bubbling the working gas.
The discharge dynamics was analyzed based on a video sequence taken with
a commercial camera Nikon Coolpix P500 (12 MPx video resolution, 30 f/s frame
rate). Each video frame of an ignition-extinction cycle was captured by using the
“snapshot” option of the VLC Media Player software. The plasma emission was
monitored using two Ocean Optics HR 4000 spectrometers (wavelength ranges
290–430 nm and 200–1100 nm respectively) controlled by the SpectraSuite
software [12]. For identifying and labeling the atomic and molecular emission lines
and bands and for measuring the relative intensities we used Spectrum Analyzer
version 1.7 [13]. The same software was used to determine the vibrational
temperature of N2 molecules using the N2 molecular bands with the band heads at
371.05 nm, 374.54 nm and 380.49 nm. The temperature of excitation of rotational
states of the OH radicals, TrotOH, was estimated by finding the best fit (chi-square
method) of the measured molecular spectra with the synthetic spectra generated by
the LIFBASE 2.0.64 spectral simulation software [14]. The OH emission band with
a prominent line at 308.9 nm was used. The atomic excitation temperature of Ar
was calculated from the ratio between the relative intensities of the lines at 840.82
nm and 842.47 nm [15]. The Stark broadening of the hydrogen αline was used to
determine electron densities based on the next formula [16]:
The Hα line in the emission spectrum was fitted with a Voigt profile using
OriginPro 8 software. The FHWH of the Voigt profile was corrected with the
instrumental, Doppler and van der Waals broadenings to obtain the Stark broadening.
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D. Zaharie-Butucel, S. D. Anghel
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To determine the degradation capacity of the plasma, 25 ml of methylene
blue solution was treated for 5–50 min with plasma generated in liquid or on liquid.
Concentrations of 100 mg/l and 50 mg/l were used for the two kind of treatment (in
and on liquid respectively). For every treatment time a new solution was employed.
Molecular absorption spectroscopy measurements were performed to analyze both
qualitatively and quantitatively the discoloration of the methylene blue solution.
Absorption spectra were measured using a spectrophotometer V–630 UV-VIS
(wavelength range 190–1100 nm).
3. RESULTS AND DISCUSSION
In order to know the time evolution of the discharge generated in liquid
(distilled water), the argon flow-rate was adjusted so that the frequency of the
bubbles’ formation to be minimal (0.48 bubbles/s, 63 video frames/cycle). Figure 2
shows the most representative video frames from the point of view of the discharge
dynamics. By analyzing these images it can be supposed that a discharge cycle has
three stages. They will be extensively analyzed in an upcoming work. In short they
are: (a) pushing the water towards the end of the quartz tube and the gas bubble
formation, frames 1–37; (b) the discharge initiation at the powered electrode and its
development as streamers with many branches bridging the electrode and the gaswater wall. Simultaneously the gas bubble expands outside the glass tube, frames
38-56; and (c) the extinction of the discharge when the gas bubble detaches from
the border of the quartz tube and slides up on its wall, frames 57–63.
Fig. 2 – Time evolution of the plasma generation process during an ignition-extinction cycle. Each
imprinted number indicates the number of the frame from a total of a 63 frames. The white line was
added for a better visualization of the gas-liquid contact surface.
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Optical characterization and application of an atmospheric pressure Ar plasma
761
Fig. 3 – Emission spectrum of plasma generated in Ar bubbled in water.
Figure 3 shows the emission spectrum of Ar plasma generated in water. The
plot is the result of average of 11 scans (1 s integration time) in order to
compensate the light fluctuation during an ignition-extinction discharge cycle.
Except for the argon emission lines, the emission lines of hydrogen (Hα) and
oxygen, and the emission bands of OH˙ radical as a result of water molecules
dissociation are present. In figure 4 the emission spectrum at the plasma-water
interface and plasma-MB solution interface are presented. The similarities between
the two spectra lead us to conclude that the discharge is not affected substantially
when methylene blue is added in water. The difference is that, in the emission
spectrum of Ar plasma generated on water the emission bands of molecular
nitrogen are also present. This is because the plasma is generated in open air.
Fig. 4 – Emission spectrum of the Ar plasma generated on water and on a methylene blue solution.
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D. Zaharie-Butucel, S. D. Anghel
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The estimated parameters of the plasma generated in Ar bubbled in water and
at the water surface are presented in Table 1.
Table 1
Characteristics of Ar plasmas generated in and on water
TexcAr [K]
TrotOH [K]
TvibN2 [K]
ne [cm-3]
In water
6080
1990
–
3.79 1015
On water
7510
2600
2970
2.73 1015
As it was expected, both plasmas are not in the thermodynamic equilibrium
state and the generally accepted relationship Texc > Tvibr > Trot is satisfied. On the
other hand, the high excitation temperature of the electrons in Ar atoms indicates
the presence of energetic electrons capable of generating, by inelastic collisions,
molecular species able to degrade organic dyes. Both the electron number densities
and the rotational temperatures (which for atmospheric pressure plasmas can
approximate the gas temperature) are in accordance with the general characteristics
of Ar plasmas generated in or on liquids [2]. The plasma temperature is higher
when plasma is generated in open air because more power is transferred from the rf
generator to the molecular species. At the same time, the electron number density
is higher when the plasma is generated in Ar bubbled in liquid because the plasma
dimensions are lower and the plasma power density is higher than in the case of
plasma generated on liquid.
Fig. 5 – Methylene blue solution before and after treatment with plasma generated in liquid.
To study the organic dyes degradation capability of the plasma generated in
liquid, methylene blue solutions (100 mg/l) were treated for 5, 15, 20 and 50
minutes. The Ar flow rate was 0.3 l/min. At a visual observation, after 50 minutes
of plasma treatment the solution was completely discolored (figure 5).
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Optical characterization and application of an atmospheric pressure Ar plasma
(a)
763
(b)
Fig. 6 – Absorption spectra of the methylene blue solutions treated with plasma generated in liquid:
(a) after treatment and (b) after two months.
The absorption spectra (figure 6a) presents the typical methylene blue
absorption bands at 665 nm, 613 nm, 293 nm and 245 nm as well as an absorption
band in the UV region around 200 nm corresponding to smaller products coming
from the decomposition of methylene blue. As the treatment time increases the
methylene blue bands decrease in intensity and the UV band increases indicating
that a greater quantity of methylene blue is decomposed. The degradation has
continued following the ending of the exposure to plasma. The absorption band at
664 nm which after an exposure of 5 minutes to plasma still has a significant value,
becomes flat after two months.
Based on the absorption measurements, the energy efficiency calculated as
the amount of substance (MB) processed by unit of energy consumption is 4.32 x
10-10 mol J-1. It is comparable with that reported in [8] and is about 18 times higher
than that reported in [9] for the working frequencies very close to 10 MHz. In the
first case an argon plasma jet in the presence of oxygen (3 l/min and 0.05 l/min
flow-rates, respectively) was used and the MB solution was recirculated
continuously. In the second case, a system with two electrodes was used.
These results can be explained starting from the molecular structure of the
dye (figure 7). The thiazine group (the three aromatic rings) is responsible for the
blue color of the compound and makes it difficult to degrade. To degrade the dye it
is necessary to break the double bond of N separating the aromatic rings and
obtaining smaller products.
Fig. 7 – The molecular structure of methylene blue [7].
764
D. Zaharie-Butucel, S. D. Anghel
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Three main degradation mechanisms were identified. The first is a highenergy electron mechanism which implies that the methylene blue molecule is
decomposed by high-energy electrons [17]. In our experiment they are the result of
the ionization processes and of their acceleration in the very intense radiofrequency
field. The energetic electrons interfere with the electronic distribution of the double
bond between N and the aromatic ring causing the bond to break. A second
mechanism is based on the strong oxidation character of the hydroxyl radical in the
plasma. The reaction of total decomposition is [18]:
(1)
The hydroxyl radical is produced in plasma by water molecule dissociation
under the action of electrons [7, 17, 19]:
H2O + e-
OH● + H + e-
(2)
It has small lifetime making its oxidation action local [4]. The degradation in
the bulk region could be the consequence of the action of another oxidant
compound, hydrogen peroxide, which is the third degradation mechanism. The
hydrogen peroxide is produced in the plasma through the next reaction mechanism
[11]:
2OH●
H2O2
(3)
Being more chemically stable than the hydroxyl, the hydrogen peroxide can
diffuse in the entire volume of the solution thus having a global effect [4]. Its
degradation effect is not as strong as that of the hydroxyl because its oxidation
potential (1.78 V) is lower than the oxidation potential of hydroxyl (2.8 V).
The action manner of degradation mechanisms can be clarified by visual
observation of the discoloration of a MB solution (50 ml, 50 mg/l) under the action
of plasma generated on liquid (0.5 l/min Ar flow-rate).
Fig. 8 – Time evolution of MB degradation by treatment with plasma generated on liquid.
As it can be seen in figure 8, the plasma degrades the solution in a manner
that leads to a non-uniform color gradient which means that the degradation
process is not the consequence of a single mechanism. A possible explanation for
the formation of two layers with different degradation level is given by the two
decomposing capabilities of the hydroxyl and hydrogen peroxide species. We
suppose that at the top surface the discoloration is given mainly by the hydroxyl
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Optical characterization and application of an atmospheric pressure Ar plasma
765
and at the bottom in the volume by the hydrogen peroxide as the hydroxyl has a
shorter lifetime. Since the hydroxyl has a greater oxidation potential than the
hydrogen peroxide, it will decompose more methylene blue and the upper layer
will be more discolored than the lower one. In addition to that, when the plasma is
generated on the liquid in open air, the decomposition rate could be enhanced by
the production of ozone [17]. The layered pattern did not occur in the previous type
of treatment because the solution was homogenized by bubbling the Ar gas.
Another observation is the decrease of the liquid volume during plasma treatment.
This is because at the plasma-liquid contact surface the local temperature is high
and the liquid is gradually vaporized. It is also evident that for the same volume of
liquid and for a lower concentration of MB, after the same treatment time the
degradation is not complete as in the case of treatment with plasma generated in
liquid. The main cause is the absence of a homogenization mechanism during the
plasma treatment.
4. CONCLUSIONS
An alternative method for generation of plasma in liquid and on liquid is
proposed. Based on the analysis of dynamics of plasma images it was concluded
that, after its ignition, the plasma generated in Ar bubbled in liquid has two
developing stages. The spectroscopic measurements indicate the non-equilibrium
state of plasmas and the presence of energetic electrons able to generate chemically
active species which degrade the organic dyes. By monitoring and comparing the
time evolution of the methylene blue degradation process the different action mode
of hydroxyl radical and oxygen peroxide was highlighted.
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