Study of low-voltage pulsed plasma discharges inside water using a

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SURE: Shizuoka University REpository
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Author(s)
Study of low-voltage pulsed plasma discharges inside water
using a bubble-generating porous ceramic electrode for
wastewater treatment
Muradia, Sonia
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2013-06
http://doi.org/10.14945/00007921
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DOCTORAL THESIS
Study of low-voltage pulsed plasma discharges
inside water using a bubble-generating porous
ceramic electrode for wastewater treatment
Sonia MURADIA
Graduate School of Science and Technology
Educational Division
Department of Nanovision Technology
Shizuoka University
June 2013
1
博士論文
廃液処理のためのバブル生成多孔質セラミック電極を用
いた低電圧パルス水中プラズマ放電に関する研究
ソニア
ムラディア
静岡大学
大学院自然科学系教育部
ナノビジョン工学専攻
2013 年 6 月
2
PREFACE
The designing of a new geometry for producing multi-bubbles under-water discharge
system and an investigation of its characters and its applicability for water and
wastewater treatment were the topics of the present research. The emphasis has been
mainly laid on the designing of a parallel-plate type electrode set-up that produces
efficient multi-bubbles discharges by consuming low voltage (or energy) and later on
the improvement of the discharge. The work on this study was carried out at the
Nagatsu Laboratory, Department of Nanovision Technology, Faculty of Engineering of
the Shizuoka University, Hamamatsu campus in Japan.
Chapters 1 and 2 present the introduction to the advanced oxidation technologies for
wastewater treatment, fundamentals of the electrical discharges in water and the basic
chemistry of such discharges. Chapter 3 describes the methodology and the experiment
reactors made for the study. Chapter 4 represents the experimental results and
discussion in form of three sections- Section 4.1 Characterization of the discharge,
Section 4.2 Chemical nature of the discharge and Section 4.3 applications of discharge
to decolorize Indigo Carmine. Chapter 5 deals with improvement of the discharge by
changing the physical features of the geometry to make it more efficient and costeffective. Finally chapter 6 summarizes the conclusions.
This thesis is partly based on four internationally published papers and presentations.
Additionally, two papers are in preparation for publishing in international journal,
contain data, which are included in this thesis. The analysis has however almost in all
3
cases been extended and deepened. The thesis also contains new, hitherto unpublished,
material.
The papers and presentations on which this thesis is partly based are:
Sonia Muradia and Masaaki Nagatsu, “Low voltage pulsed plasma discharges inside
water using a bubble self-generating parallel-plate electrode with a porous ceramic”
Applied Physics Letters 102, 144105 (2013)
Sonia Muradia, Yohei Mochizuki, Akihisa Ogino, Masaaki Nagatsu, “Low Voltage
Bubble Discharge in the Water Using Metal Clutched Porous Ceramic Electrode for
Environmental Application” 39th IEEE International Conference on Plasma Science,
Edinburgh, Scotland, UK, July 8-12, 2012, 4E-1 (oral presentation)
Sonia Muradia, Akihisa Ogino, Masaaki Nagatsu, “Characteristics of Bubble
Discharges inside Water Produced Using a Pulsed DBD Configuration with a Metal
Clutched porous Ceramic Electrode for Wastewater Treatment” International Union of
Materials Research Society-ICEM, Yokohama, Japan, September 2012, D-4-O24-003
(oral presentation)
Sonia Muradia, Naoya Okada, Masaaki Nagatsu, “Effects of hole structure of a bubble
self-generating parallel type punched plate electrodes on pulsed discharges inside
water” The 12th Asia Pacific Physics Conference, Chiba, Japan, July 14-19, 2013 (oral
presentation)
4
ACKNOWLEDGEMENTS
I want to express my gratitude to all those who contributed to the accomplishment of
this work, in particular: I wish to thank to my supervisor, Prof. Masaaki Nagatsu for his
valuable guidance and relevant criticism throughout my PhD study.
Special appreciation is given to Associate Prof. Akihisa Ogino for his guidance for all
the instruments and for many helpful discussions that much stimulated my research. I
also thank to Dr. Jun Watanabe for being an inspiration to be more curious in learning
plasma processes and various instruments.
I would also like to pay my thanks to the committee of the reviewers for my thesis that
includes Prof. Tetsu Mieno, Prof. Haruhisa Kinoshita, Associate Prof. Akinori Konno,
and Associate Prof. Akihisa Ogino for their valuable comments to improve my thesis.
Apart from above, I would like to thank my juniors- Shun Tsumura Kun, Yohei
Mochizuki Kun and Okada Naoya Kun for providing me assistance at the needful times.
Little yet important support from all my lab members at various times is well
appreciated.
The financial support from the Japanese Government- Ministry of Education, Culture,
Sports, Science and Technology (MEXT) is gratefully acknowledged.
Last but not the least; I would like to thank my family- old and new both, my friends
and well-wishers for being a constant motivation and support at all my blues during the
whole course.
5
Contents
CHAPTER 1 INTRODUCTION .......................................................................................................... 1
1.1 Wastewater- A global crisis ................................................................................................. 8
1.2 Problem with conventional treatment methods ................................................................ 9
1.3 Scope of the thesis ............................................................................................................ 13
2. LITERATURE REVIEW ............................................................................................................... 15
2.1 What is Plasma? ................................................................................................................ 15
2.1.1 Plasma generation and power sources ...................................................................... 17
2.2 Electric discharge inside liquid .......................................................................................... 19
2.3 Chemistry of plasma discharge in water ........................................................................... 26
2.2.1 OH radical ................................................................................................................... 26
2.2.2 Ozone ......................................................................................................................... 27
2.2.3 Hydrogen peroxide ..................................................................................................... 29
3. EXPERIMENTAL ........................................................................................................................ 31
3.1 Materials used ................................................................................................................... 31
3.1.1 Experimental set-up ................................................................................................... 31
3.1.2 Analytical devices ....................................................................................................... 31
3.1.3 Chemical analysis ....................................................................................................... 32
3.2 Production of electric discharge inside water ................................................................... 33
3.2.1 Electric discharge in the water by using graphite electrode ...................................... 34
3.2.2 Electric discharge in the water by using ceramic electrode ....................................... 43
4. RESULTS AND DISCUSSION ..................................................................................................... 47
4.1 Characterization of the discharge ..................................................................................... 47
4.1.1 Waveforms ................................................................................................................. 47
4.1.2 Location of the discharge ........................................................................................... 49
6
4.1.3 Average power calculation ......................................................................................... 50
4.2 Chemical nature of the discharge ..................................................................................... 52
4.2.1 Optical emission spectra ............................................................................................ 52
4.2.2 H2O2 production ........................................................................................................ 54
4.2.3 Ozone production....................................................................................................... 57
4.3 Applications of the discharge for Indigo Carmine decolorization ..................................... 60
4.4 Efficiency ........................................................................................................................... 66
5. IMPROVEMENT OF DISCHARGE .............................................................................................. 68
5.1 Experimental scheme ........................................................................................................ 70
5.2 Experimental Results ......................................................................................................... 73
5.3 Conclusion and Discussion ................................................................................................ 78
CHAPTER 6 CONCLUSIONS .......................................................................................................... 79
6.1 Experimental ..................................................................................................................... 79
6.2 Outlook .............................................................................................................................. 80
CHAPTER 7 REFERENCES ............................................................................................................. 81
7
CHAPTER 1 INTRODUCTION
This chapter gives a brief introduction about the problem of wastewater across the
world. There are some statistics presented here giving us essential conscience that there
is an utter need to deal with the havoc of wastewater in near future. The problem with
the existing conventional treatment plants and advantages of advance oxidation
processes over them are well discussed. Along with, importance of emerging plasma
technology in the field of advance oxidation processes is also presented. In the second
part, the scope of the thesis to above problem is discussed briefly in which it is
presented that why plasma technology is considered highly efficient and faster sources
for advanced oxidation processes and a cleaner solution to the problem of wastewater.
1.1 Wastewater- A global crisis
A growing world population, unrelenting urbanization, increasing scarcity of good
quality water resources, rising use of fertilizers and reckless disposal of hazardous waste
into the water bodies are the driving forces behind the ever accelerating water pollution
all across the world. Human waste, kitchen waste, sewage treatment discharge,
agricultural waste, oil-spills and various toxic industrial and pharmaceutical wastes
contribute to whole problem of water pollution and are commonly faced by all countries.
The conditions are however severe in developing countries where wastewater treatment
units are either absent or inefficient. According to UN World Water Development
report[1], 2003, about 1500km3 liquid waste is generated everyday all over the world.
Over 80% of this water is not collected or treated and in developing nations, up to 90%
of this water flows untreated into rivers, lakes and highly productive coastal zones
8
threatening health, food security and access to safe drinking and bathing water Corcoran
et. al, 2010 [2]. According to WHO, 2008b[3], each year ~ 3.5million deaths are related
to inadequate water supply, sanitation and hygiene occur predominantly in developing
countries and there are still 884million people who don’t have clean drinking water
source and 2.6billion people in world with no improved sanitation by WHO,UNICEF,
2010 report[4]. By these figures, one can easily conclude that there is an emergent need
to deal with the problem of wastewater treatment.
To one nation, problem of wastewater is now not just limited to health of the citizens
but also is threatening for the economy of the nation. World Bank estimates that
Indonesia lost US $ 6.3billion (2.3% of GDP) in 2006 from poor sanitation and hygiene
(World Bank, 2008c)[5]. Corresponding losses in the Philippines as part of the same
study amounted to US $ 41.1billion or 1.5% of GDP.
1.2 Problem with conventional treatment methods
Treatment methods used so far are not implemented everywhere and also are not so
efficient. These conventional methods used for water and wastewater treatment can be
broadly categorized by the nature of the treatment process operation into biological,
physical and chemical methods. Two kinds of conventional wastewater plants are
shown in fig. 1.1. In both methods, the first stage of the treatment process uses screens
to remove the larger solid inorganic material such as paper and plastics. This is followed
by the removal of particles such as grit and silt which are abrasive to plant equipment
for which wastewater is passed through a primary sedimentation tank where solid
9
particles of organic material are removed from the suspension by gravity settling. The
resultant settled primary sludge is pushed to the centre of the tank where it is
concentrated and pumped away for further treatment. This next stage is a biological
process which breaks down dissolved and suspended organic solids by using naturally
occurring micro-organisms. It is called the activated sludge process. The settled
wastewater enters aeration tanks where air is blown into the liquid to provide oxygen for
mixing and to promote the growth of micro-organisms. The “active biomass” uses the
oxygen and consumes organic pollutants and nutrients in the wastewater to grow and
reproduce. From the aeration tanks, the mixture of wastewater and micro-organisms
passes into a secondary sedimentation tank (also known as a clarifier) where the
biomass settles under gravity to the bottom of the tank and is concentrated as sludge
while the decanted water
is pumped to anaerobic digesters for further treatment.
The clarified wastewater is discharged from this clarifier and passes through for final
chemical treatment. This treatment is also called disinfection to reduce pathogens,
which are micro-organisms which can pose a risk to human health.
The fundamental process of treating wastewater in most of conventional methods used
is based on biological methods. One of the main benefits of the biological treatment
processes is their low cost. They are extensively and effectively used for the treatment
of municipal wastewater and some industrial wastes. However, despite advances in
biotechnology, biological systems are unable to remove effectively many classes of
pollutants, including many toxic compounds. Furthermore, these processes tend to be
very large due to the slow rate of the biological reactions [6,7]. Physical treatment
techniques generally only separate the waste from the water either by means of a
10
support system or by transferring them to another phase and do not involve chemical
transformations of pollutants [7]. Chemical oxidation processes are widely used to treat
drinking water, wastewater, and groundwater contaminated with organic compounds.
Direct oxidation of aqueous solutions containing organic contaminants by the chemical
reactants with a high oxidizing potential enables even the destruction of nonbiodegradable substances in water. The most commonly used oxidants are chlorine,
chlorine dioxide, chloramines, ozone, and potassium permanganate. However, their use
for destructive treatment of wastes is limited because such reagents are usually
expensive and the addition of large quantities of oxidizing agents to a waste solution
may result in a new waste treatment problem involving the reduced products of the
oxidizing agents. For example, halogenated organics byproducts can be formed when
chlorine or ozone (in the presence bromide ion) is used, which can cause possible health
effects [8,9]. Therefore, great attention is focused on so-called advanced oxidation
processes (AOP) that are based on generation of highly reactive species, especially
hydroxyl radicals and its secondary species like hydrogen peroxide. This is because the
hydroxyl radical is a very powerful, non-selective oxidant that has the oxidation
potential to completely oxidize organics to carbon dioxide and water. The hydroxyl
radical is so reactive that it will react with virtually any organic in solution so it could
form the basis of a treatment system of general applicability [9]. The comparison of the
oxidation potential of hydroxyl radical with the several chemical reactants is listed in
Table 1.1. There are a number of methods for generating hydroxyl radicals that may be
applied in AOP, such as photochemical and electrochemical oxidation, photolysis of
hydrogen peroxide and ozone, Fenton-type reactions, TiO2 photo catalysis, wet
oxidation, sonolysis, and irradiation of water by high energy electron beams or γ-rays. A
11
Table 1.1 Oxidation potential of several chemical reactants [4]
Oxidant
Oxidation potential [V]
Hydroxyl radical
2.80
Atomic oxygen
2.42
Ozone
2.07
Hydrogen peroxide
1.77
Permanganate
1.67
Chlorine dioxide
1.50
Chlorine
1.36
12
common feature of AOP is that the radical production involves a significant expense of
the energy (either chemical, electrical or radiative).
The need of an energy efficient method for production of highly reactive transient
species has motivated research on the application of high voltage electrical discharges
for water purification. In general, strong electric fields applied to water initiate both
chemical and physical processes as ultraviolet radiation, overpressure shock waves and,
especially, formation of various reactive chemical species such as radicals (OH·, H·, O·,
HO2·) and molecular species (H2O2, H2, O2). These effects have various important roles
in different application regions in the liquid and the magnitude of their contributions
strongly depends upon the energy of the discharge. Now, it has been more than three
decades that electric discharges in the water has been produced and studied for their
mechanisms and chemical nature. Surprisingly, even after so many years of research, no
plasma process has yet to be enough potential to replace traditional methods. It is
because of the two challenges that electric discharge technique face- its complex
production and higher cost.
1.3 Scope of the thesis
As from the above discussion, electric discharge or plasma discharge technology or
plasma technology recently has been found as a potential source for AOPs as a lot of
oxidative species are produced in the medium. This technology is also favored for its
fast and clean processes since rates of the processes and so is the production of reactive
species during these processes are quite fast and depends directly on the energy supplied
to the discharge. Besides, no chemicals or least chemicals are utilized during the process
and almost no byproducts are released by the process that needs extra management.
13
Briefly, plasma is a fourth state of matter formed by breakdown of gaseous molecules
and is basically produced in air or gases particularly noble gases, however, production
of plasma discharge inside liquid: water or dielectric oil has been an interesting field of
research since last three decades. There have already been a lot of methods to produce
electric or plasma discharge inside water but so far none of the process from this field
has been used for commercial or industrial applications. The scope of this thesis is
therefore to produce a plasma discharge in the water which is easy-to-produce, efficient
for the purpose of wastewater treatment and has a potential for commercial and
industrial application i.e. for treating large quantities of wastewater. In this thesis, we
developed a new geometry that produced multi-bubbles pulsed plasma using DBD and
glow-like discharge. We studied physical and chemical characteristics of such discharge
and used it for the decolorization of indigo carmine dye. In the last part, we also tried to
improve the efficiency of the discharge for wastewater treatment by changing the
physical parameters of the geometry of the electrode set-up in order to avoid any
increase in the cost.
14
2. LITERATURE REVIEW
This chapter discusses about the theory of plasma- its sources of production and
classification to provide an idea about the diversity of the subject. Out of which, only a
few kinds of plasma: atmospheric pressure discharges, pulsed discharges, DBD and
glow-like discharges and plasma / electric discharges inside liquid are dealt in this
thesis. More interestingly, the “Electric discharge in the water” is a completely new
chapter in the field of plasma technology and is still growing day by day with its new
phenomenon and mechanisms coming up. We are going to describe some of these in this
chapter.
2.1 What is Plasma?
Plasma is the state of matter heated beyond its gaseous state commonly known as fourth
state of matter, heated to a temperature so high that atoms are stripped off at least one
electron in their outer shells, so that what remains are positive ions in a sea of free
electrons. In this process of ionization, not all the atoms have to be ionized. The extent
of ionization depends on the power source and determines the temperature of the plasma.
Therefore, the plasma produced is mostly classified using these two parameters.
However, plasmas can also be described by many other characteristics like degree of
ionization, their density, and approximations of the model describing them. So, the basic
outline of classification of plasmas is as follows:
Classification of plasmas
15
•
Pseudo-plasmas and real plasmas
•
Cold, warm and hot plasmas
• Hot plasma (thermal plasma)
• Warm plasma
• Cold plasma (non-thermal plasma)
• Ultra cold plasma
•
Plasma ionization
• Degree required to exhibit plasma behaviour
• Fully ionized plasma
• Partially ionized plasma (weakly ionized gas)
•
Collisional plasmas
• Collisional plasma
• Non-collisional plasma
•
Neutral plasmas
• Neutral plasma
• Non-neutral plasma
•
Plasmas densities
• High density plasma
• Medium density plasma
• Low density plasma
•
Magnetic plasmas
• Magnetic plasma
• Non-magnetic plasma
•
Complex plasmas
• Dusty plasmas and grain plasmas
• Colloidal plasmas, Liquid plasmas and Plasma crystals
16
•
Active and passive plasmas
• Passive plasma
• Active plasma
•
Ideal and non-ideal plasmas
•
High Energy Density Plasmas (HED plasmas)
The field of plasma science and technology is vast and diversed. Alan Watts of
Environmental surface technologies in Atlanta, Georgia has suggested for organizing
industrial plasmas with reference to the major revolutions in Energy and Technologies
so far developed in the world. Even the applications of plasma technology are numerous
and cannot be compiled easily in a single thesis.
2.1.1 Plasma generation and power sources
Plasmas are generated by supplying energy to a neutral gas causing the formation of
charge carriers. Electrons and ions are produced in the gas phase when electrons or
photons with sufficient energy collide with the neutral atoms and molecules in the feed
gas. This energy can be supplied either by using electric fields or by using electric or
laser beams. In electric field method, the gas is passed through an electric field during
which it gets ionized to produce its plasma stage. As discussed, the strength of
ionization depends on strength of electric field. One can produce various electric fields
using various electrical power sources like [10]•
DC discharge
•
Pulsed DC discharge
•
DBD discharge
•
High frequency discharge (RF and Microwave)
17
•
Plasmas produced using laser beams
Each of the various plasma sources discussed above has its own peculiarities,
advantages and disadvantages. The choice of the proper source for the specific task
requires the study of the characteristics of the various plasmas. The fig 2.1 presents the
voltagecurrent behavior of a typical gas discharge under low pressure. Variation in
such characteristics can be seen by changing the surrounding pressure, gas species and
of course the power source.
Figure 2.1 Typical DC discharge of a gas
18
2.2 Electric discharge inside liquid
We have so far discussed that the plasma discharge forms the basis of an innovative
advanced oxidation technology for water treatment because these discharges initiate
potential chemical and physical processes useful for its application into various fields
[22-24]. Basically, three categories of plasma treatment technologies exist: remote,
indirect, and direct. Remote plasma technologies involve plasma generation in a
location away from the medium to be treated (e.g. ozone). Indirect plasma technologies
generate plasma near to, but not directly within, the medium to be treated (e.g. UV,
electron beam). More recently, direct plasma technologies (i.e. electrohydraulic
discharge) have been developed that generate plasma directly within the medium to be
treated thereby increasing treatment efficiency. It is quite obvious that it is much easier
to produce discharge in case of remote or indirect discharge as the discharge occurs in
air but for the purpose of wastewater treatment these discharges lack in their efficiency
than the electro-hydraulic discharge or direct discharge in the water[book advance].
Initiated by H. Yamashita et al (1977) and A. H. Sharbaugh et. al (1980) in dielectric
liquid and Clements J et. al.(1987) in water [22,25-26], plasma discharge inside liquid
has attracted many researchers for developing various methods of its production, for
understanding its mechanism and for various applications. There are various types of
electrical discharges generated either directly in water or above the water surface that
have been investigated as possible methods for water treatment. Consequently, a large
variety of reactors and electrode configurations have been used.
Direct discharge or electro-hydraulic discharge or commonly known as underwater
electric discharge being produced inside water provide a higher efficiency due to the
direct interaction of pollutant molecules with the reactive species produced during the
19
discharge. In the field of underwater electric discharge, the question of interest is its
mechanism of production and propagation. Up to now, a lot of researches have been
conducted to understand the mechanism behind plasma discharge inside water. The
results show that the breakdown inside water occurs by two processes namely: streamer
and bubble method shown in figure 2.2 [24,28] . According to first process, discharge
is a consequence of avalanche multiplication of free charge carriers in the liquid similar
to the gas discharge. It forms the basis for streamer or spark discharge as similar to the
gas discharge [25, 29-33]. The second process is via the bubble generation by which the
breakdown inside water is occurred using the pre-breakdown phenomenon occurring in
the gas cavities formed by heating up the liquid. Korobeynikov, et al [34] investigated
such pre-breakdown in micro-bubbles by using pulsed heated wire electrode and
recently Ishijima, et al.[35] and Maehara, et al. [36] produced such discharge by using
microwave and RF power heating, respectively.
During the plasma discharge, the thermal condition of the water is constant. For water
relatively far away from the discharge, it stays in a liquid state with a thermal
conductivity of about 0.68W/mK. When joule heating between the two electrodes is
larger than a threshold value, instability can occur, resulting in instant evaporation and a
subsequent thermal breakdown. On the other hand, when joule heating is smaller than
the threshold value, nothing happens but electrolysis; hence the breakdown never takes
place. Since the joule heating is inversely proportional to the resistance of matter when
a fixed voltage is applied between the two electrodes, the resistance is inversely
proportional to the electric conductivity of the dielectric medium (initially liquid water
and later water vapor).
20
To analyze the thermal instability, it can be assumed that electric conductivity of water
σe can be expressed as an exponential function of temperature T:
=
(2.1)
Where Ea is activation energy, σ0 is the initial electric conductivity, and R is the
universal gas constant. When the temperature of the medium increases, the electric
conductivity of the dielectric medium increases, resulting in the decrease in the
resistance. Thus joule heating increases, increasing the temperature of dielectric
medium. Subsequently, the increased temperature increased the electric conductivity,
further increasing temperature leading to a thermal explosion that can be referred to as
instability and described by linear perturbation analysis of the transient energy
conservation equation:
=
(
− λ∇
)
Where rCp is the thermal mass per unit volume, E (V/cm) is the electric field, and l is
the thermal conductivity of water. The second term on the right side presents heat
conduction; which takes place with a large temperature gradient along the radial
direction. The minus sign in the second term means that it presents heat loss to the
surrounding water. Note that the convection heat loss is not considered because there is
no time for heat to dissipate via convection.
The instability is usually described in terms of its increment W, which is an angular
frequency (rad/s). When Ω is greater than zero, the perturbed temperature exponentially
increases with time, resulting in thermal explosion. When Ω is less than zero, the
perturbed temperature exponentially increases with time, resulting in a steady state
21
condition. When Ω is complex, the perturbed temperature oscillates with time. The
linear perturbation analysis of equation above leads to the following expression for the
increment of the thermal breakdown instability (Fridman, Gutsol, & Cho, 2007):
Ω = 1
−
Where R0 is the radius of the breakdown channel, D ≈ 1.5 x 10 -7 m2/s is the thermal
diffusivity of water, Cp = 4.179 J/Kg.K, and I is 0.6W/m.K. The first term in the right
side is made up of the product of two, represents the frequency of heating as the
numerator is joule heating, whereas the denominator is the heat stored in the water
medium; [Ea/RT0] represents the ratio of the activation energy to temperature, a
sensitivity indicator. The second term in the right hand side represents the ratio of the
thermal diffusivity to the square of the characteristic length for radial heat conduction,
indication how fast heat dissipates along the radial direction. The first term is always
active, even during the period when the pulse power is turned off. Hence, there is a
balance between the joule heat generation by pulse discharges and heat conduction to
the surrounding water. When the heat generation is greater than the conduction lost, the
increment Ω becomes positive, leading to the thermal explosion. Hence the critical
phenomenon leading to the thermal explosion is given as follows:
≥
1
Note that Ω = 0 means the transition from the stabilization to thermal explosion, a
condition that can be defined as the critical phenomenon.
22
Since the electric conductivity s of a dielectric medium is extremely sensitive to
temperature, as shown in equation above one can expect that as the temperature
increases, the breakdown voltage would decrease.
The breakdown voltage V is given by the product of electric strength E and the distance
between two electrodes L. Thus, one can rewrite equation as
(
=
≥
1
⁄
If we introduce a geometry factor, G = L/R0, one can rewrite the above equation as
≥ From this equation, the breakdown voltage V can be obtained as
≥
For the plasma discharge in water, the breakdown voltage can be numerically estimated
as follows:
≥
=
0.613 × 461.5 × 300
≈ 26
0.05 × 700
In a case of L = 1cm, the diameter of the streamer is usually on the order of 10µm,
leadng to the geometry factor G = L/ R0 = 1000, and the breakdown voltage in water
becomes 26000V.
23
Therefore, the bubble discharge process has an advantage over electronic mechanism
for its initiation at low voltage (30 kV/cm), which is significantly less than the former
streamer discharge process (1~2 MV/cm). Considering the low voltage discharge
initiation in bubble discharge process, the idea of producing discharge inside water
using externally supplied gas bubbles introduced the additional advantage by reducing
the energy consumption lost in boiling or heating up the liquid to form bubbles and
avoids increase in the temperature of the liquid where low temperature of the liquid is
prime concern as discussed by Aoki, et al. [37] and Gershman, et al. [38].
Plasma production in single and multi-bubbles system has been reported earlier in
various geometries, some examples are presented in figure 2.3 [31, 33, 37, 39-41]. Most
common method of introducing bubbles between the electrodes is using pin-hole
method as used by Sato, et al. [31] and Sun, et al. [33]. The disadvantage of using such
a method is the need of a large gap between the electrodes to form a space for bubbles
to flow which ultimately increases the breakdown voltage. Another disadvantage is the
damage of discharge electrode which is well dealt by Li, et al. [39] by developing a
reactor where discharge electrode is outside the liquid and discharged gas is introduced
into water through an alumina ceramic container in the form of bubbles.
24
(a) bubble process
(a) streamer process
Fig. 2.2 The main two process of plasma generation inside liquid (a) Streamer process
and (b) bubble process
Fig. 2.2 Various methods of production of bubbles by heating up the liquid to the
boiling temperature or by supplying external gas
25
2.3 Chemistry of plasma discharge in water
The interesting aspect in all above discussed plasma processes for wastewater treatment
is their common chemical nature. The electric discharge occurring over or underwater
produce common reactive species for advanced oxidation processes out of which the
most common reactive species are discussed below.
2.2.1 OH radical
OH· radicals are the key oxidants that control most advanced oxidation processes
currently applied in water treatment technology. It reacts with most organic and many
inorganic compounds with rates that approach diffusion controlled limits. Reactions of
hydroxyl radicals with inorganic or organic compounds may be differentiated by their
mechanisms into three different classes [11-13]:
Abstraction of hydrogen atom
OH· + RH → R· + H2O
(2.1)
Electrophilic addition to double (triplet) bond
OH· + R2C=CR2 →R2 (OH)C-CR2·
(2.2)
OH· + RX → XR·+ + OH-
(2.3)
Electron transfer
Hydroxyl radical will react primarily by hydrogen abstraction with saturated aliphatic
hydrocarbons and alcohols to yield H2O and an organic radical (R·) (see Equation 2.1).
26
On the other hand, in the case of olefins or aromatic hydrocarbons OH· radical adds to
C=C double bonds of organic compound to form a C-centered radical with a hydroxyl
group at the α-C atom (Equation 2.2). Reduction of hydroxyl radicals to hydroxide
anions by an organic substrate (Equation 2.3) is of particular interest in the case where
hydrogen abstraction or electrophilic addition reactions may be disfavored by multiple
halogen substitution or steric hindrance. Organic radicals formed by such reactions may
quickly add an oxygen molecule, if present, to form reactive organoperoxyl radicals
(ROO·) that may become transformed into oxy radicals. Further reactions then often
lead to an abstraction of an H atom by dissolved oxygen and the formation of
hydroperoxyl radicals, hydrogen peroxide, and series of peroxyls and peroxides,
aldehydes, acids, etc. During the process different types of radical species are
simultaneously present. Thereby (most) sequences of reactions of radicals are finally
only terminated by radical-radical combinations and dis-proportionations. Such
reactions between large varieties of different radicals even increase the amount of
different types of product formations and its dependency on many parameters.
2.2.2 Ozone
Ozone is a powerful oxidant, second only to the hydroxyl radical. Therefore, it can
oxidize organic matter in water either directly or through the hydroxyl radicals produced
during the decomposition of ozone. The direct oxidation with molecular ozone is of
primary importance under acidic conditions; however, it is relatively slow compared to
the hydroxyl free radical oxidation [11-14]. In neutral and basic solutions, ozone is
27
unstable and decomposes via a series of chain reactions to produce hydroxyl radicals
(Equation 2. 4) [15].
2 O3+ H2O → OH·+ O2+ HO2·
The added hydrogen peroxide or ultraviolet radiation accelerates the decomposition of
ozone and increases the hydroxyl radical concentration. Also presence of suspensions of
activated carbon might be able to stimulate the production of OH· radicals from ozone
[11].
For the photolysis of ozone in the 200-280 nm region a two step process has been
proposed involving the light induced homolysis of ozone and subsequent production of
OH· radicals by the reaction of excited oxygen form (O(1D)) with water (Equations 2.4
and 2.5) [14].
O3+ hv →O2+ O(1D) (2.4)
O(1D) + H2O →2 OH· (2.5)
The chemistry involved in the generation of OH· radicals by the O3/H2O2 process
(Peroxone) is assumed to be mainly in decomposition of ozone by conjugate base of
hydrogen peroxide (HO2-) to produce ozonate radical (O3·-) that gives hydroxyl radical
by rapid reaction with water [15]:
O3+ HO2- →HO2· + O3·- (2.6)
O3·-+ H2O →OH· + OH-+ O2 (2.7)
This process is further enhanced by the photochemical generation of hydroxyl radicals
in the O3/H2O2/UV process.
28
2.2.3 Hydrogen peroxide
The most commonly used ways to generate hydroxyl radicals from hydrogen peroxide
are through the photolysis of H2O2 and Fenton’s reaction. The primary process of H2O2
photolysis in the 200 - 300 nm regions is dissociation of H2O2 to hydroxyl radicals with
a quantum yield of two OH· radicals formed per quantum of radiation absorbed [16-18]:
H2O2+ hv →2 OH· (2.8)
The OH· radicals thus formed enter a radical chain mechanism in which the propagation
cycle gives a high quantum yield of the photolysis of H2O2:
OH· + H2O2 → H2O + HO2· (2.9)
HO2· + H2O2 → O2+ H2O + OH· (2.10)
Fenton-type catalyzed generation of hydroxyl radicals is based on the decomposition of
hydrogen peroxide by ferrous ions [19]:
Fe2++ H2O2 → Fe3++ OH-+ OH· (2.11)
This method is very effective for generating hydroxyl radicals but involves consumption
of one molecule of Fe2+ for each hydroxyl radical produced. The decomposition of
hydrogen peroxide is also catalyzed by ferric ions. In this process, H2O2 is decomposed
to H2O and O2 and a steady-state concentration of ferrous ions is maintained during
peroxide decomposition, according to [20,21]:
Fe3++ H2O2 ⇌ Fe(OOH)2+ ⇌ Fe2+ + HO2
.
Fe3+ + HO2 → Fe2++ H+ + O2
29
.
Ferric system, known as the Fenton-like reagent, is attractive because degradation of
organics can be catalytic in iron. However, the initial rate of destruction of organic
pollutants by the Fe (III)/H2O2 is much slower than that of the Fe(II)/H2O2 due to the
lower reactivity of ferric ion toward hydrogen peroxide.
The oxidizing power of the Fenton-type systems can be greatly enhanced by irradiation
with UV or UV-visible light due to the photoreduction of hydroxylated ferric ion in
aqueous solution [20]:
Fe3++ OH- ⇌Fe(OH) 2+
(2.12)
Fe(OH) 2++ hv →Fe2++ OH· (2.13)
The combined process (Equations 2.11, 2.12 and 2.13), known as the photo-Fenton
reaction, results in increased production of OH· radicals and more importantly, iron is
cycled between the +2 and +3 oxidation states, so Fe(II) is not depleted, and OH·
production is limited only by availability of light and H2O2.
30
3. EXPERIMENTAL
This chapter explains about the procedures of experiments conducted and presents our
experimental schemes for production of electric discharge in the water. Multi-bubbles
discharge reactors are least published and discussed in theory but here, we made a new
geometry using parallel-type electrodes set-up that produces multi-bubbles discharge
system. In this chapter, scheme of the geometry and production of discharges in various
gases are presented.
3.1 Materials used
3.1.1 Experimental set-up
For producing multi-bubbles discharge system, we made a parallel-plate type electrode
set-up by using a hollow metal frame with a gas inlet topped with a porous graphite
electrode or porous ceramic plate and various punched metal plates with different hole
sizes.
3.1.2 Analytical devices
For the measurement of voltage and current waveforms, we used a digital oscilloscope
(Lecroy, Wavepro 7300). We observed the optical emission spectra using the optical
emission spectrometer (Acton, SpectraPro 2300i). In the Indigo Carmine (IC,
C16H8N2Na2O8S2, molar mass M=466.36 g/mol) decolorization experiment using the
present electrodes, we analyzed their concentration from the absorbance at λ=612 nm
using the UV-VIS spectrophotometer (Tecan, Infinite 200Pro). We also measured a
concentration of H2O2 formed in water by plasma discharge using the KI (Potassium
31
Iodide)-method observing the absorbance at λ=351 nm using the same. The high-voltage
bipolar power supply (Haiden Laboratory Inc., SBP-10K-HF) delivering bipolar repetitive
pulses with the voltage of up to roughly 4 kV at a frequency 5 kHz was used. In our
experiments, we used the positive pulses of about 750 ns at an interval of 0.2 ms with a
duty ratio less than 1%.
3.1.3 Chemical analysis
For conducting chemical experiments, we used various reagents shown in the table 2
below. For measuring hydrogen peroxide, we used KI method as follows
•
Make Reagent A by adding 3.3 g KI, 2.5 ml of 1 mol/L NaOH and 0.01 g
ammonium molybdate tetrahydrate into a 50 ml flask and add deionized water till 50 ml
mark.
•
Make Reagent B by adding 1 g potassium hydrogen phosphate into a 50 ml flask
and fill with deionized water till 50 ml mark.
Procedure for measurement of H2O2 in water
Add Reagent A (RA), Reagent B (RB) and the sample/ standard solution in a ratio of
1:1:1, shake or tap the mixture and wait for 5 min. Measure the absorbance of the
mixture at 351 nm.
32
Table 2. Chemicals and reagents used in all experiments
Compound
Formula
MW (g)
Grade(%) Manufacturer
Methylene Blue
C16H18N3SCl
319.85
95.6%
Wako
Indigo Carmine
C16H8N2Na2 O8S2
466.36
97.5%
Wako
Amm. Molybdate
tetrahydrate
(NH4)6Mo7O24
·4H2O
1235.86
99%
Wako
Potassium iodide
KI
166.0028
99.5%
Wako
Sodium hydroxide
NaOH
40
1mol/l
TCI
Potassium hydrogen
phosphate
K2HPO4
174.2
99.5%
Wako
Iron (II) Sulfate
FeSO4
278.015
99-102%
Wako
Hydrogen peroxide
H2O2
34
30%
Wako
Sulphuric acid
H2SO4
98.079
10%
Wako
Hydrochloric acid
HCl
36.46094
35-37%
Wako
Nitric acid
HNO3
63.01
65%
Wako
DI water
H2O
18
18Mῼ
-
Ethyl alcohol
C2H5OH
46.06844
99.5%
Wako
3.2 Production of electric discharge inside water
In our experiments, we chose to produce bubble discharge because it is easy to produce
discharge in them with any desired gas species and also as discussed earlier, bubble
discharge directly inside water has been proved to be chemically effective attributed to
direct kinetic interactions between pollutant molecule and oxidative species. So far,
many researchers have used various geometries to produce bubbles in the water. The
33
most common method to produce bubble discharge is by using hollow pin electrode as
well studied by Sato et. al. and Gershman et. al [31,38]. The other method is by using
external equipments to supply bubbles in the water between two electrodes (Aoki et.
al.)[37]. We introduced a new geometry in the form of capacitative parallel-type
electrodes in which one electrode has porous top producing multi-bubbles and also
functions as ground electrode. In our experiments, initially we used a porous graphite
top for producing discharge. We used this discharge for decolorizing two dyesMethylene Blue and Indigo Carmine. Graphite is a soft material and hence the electrode
was much easily damaged. Later, we replaced the graphite top by porous ceramic top.
The details of discharge production and their characteristics are discussed in the next
sections.
3.2.1 Electric discharge in the water by using graphite electrode
Experimental set-up
We made a complex electrode using a porous graphite plate, diameter= 5cm
(manufactured by Tanken Seal Seiko Co. Ltd.) clutched in a hollow metal frame with a
gas inlet shown. When the gas is injected through this inlet into the frame the gas comes
out through the pores of graphite into the surrounding medium. If the surrounding
medium is liquid/water, the gas comes out in the form of gas bubbles. Figure 3.1(a) and
(b) shows a picture of the complex graphite electrode and the bubbles produce by it. We
used this complex electrode as lower (grounded) electrode. Above this complex
electrode we used a punched metal plate separated by a very thin (170 um) mica sheet.
We kept the upper punched metal plate at higher (positive) voltage. The schematic
diagram of this experimental set up is presented by figure 3.2. The whole experimental
34
set-up is submerged under water. Figure 3.3 has presented the photograph of electrodes
set-up taken using Nikon D90 camera in the laboratory from various angles like top
view (a), side view (b) and the beaker filled with 2000ml of water into which electrodes
are used.
Porous graphite
surface
(a)
Metal
frame
Bubbles
(b)
Metal
frame
Fig 3.1 (a) Complex electrode with porous graphite at the top and (b) bubbles
production at the surface of graphite electrode
35
Fig. 3.2 Schematic diagram of experimental set-up
Gas inlet
Gas inlet of
graphite
electrode
Dielectric
sheet
Gas inlet
Plastic holder
Gas inlet
Dielectric
sheet
Punched metal
sheet
(a) Top view of the electrode set-up
Punched metal
sheet
Graphite
electrode
(b) Lateral view of the electrode set-up
36
(c) Electrodes submerged in water
Fig. 3.3 Laboratory view of the experimental set-up (a) Top view (b) side view and (c)
electrodes sub-merged inside water
Results
1. Production of discharge
In the above experimental set up, applying pulsed voltage across the complex electrode
and punched metal plate, we observed the DBD discharges inside the gas bubbles
produced between the two electrodes. Since the bubbles are blocked by the mica sheet
from top, discharge in the bubbles look like plain and uniform, however, bubbles are
moving from the sides where spacer is used. We used argon, nitrogen and helium gases
for the experiments at the rate of 250sccm. Various gas discharges from the top angle at
5kV are shown in figure 3.3 (a) Argon, (b) Nitrogen and (c) Helium.
37
The discharge was observed at 3-5kV depending on gas species used for discharge.
However, argon gas discharge was most uniform and stable in comparison to other two
gases.
(b) N2 gas discharge
(a) Ar gas discharge
(c)He gas discharge
Fig. 3.3 Top view of various gas discharges at 5kV under water using porous graphite
electrode (a) Ar gas discharge (b) Nitrogen gas discharge and (c) Helium gas discharge
38
graphite electrode_voltage=5kV_pulse=1µs
graphite electrode_Displacement+DBD current
6000
0.4
5000
(a)
(b)
0.2
Current (A)
Voltage (volts)
4000
3000
2000
0.1
Displacement
Current
0
1000
Displacement
Current
-0.1
0
-1000
-6
-7
-1 x 10 -5 x 10
Spiky DBD current
0.3
0
5 x 10
-7
1 x 10
-6
1.5 x 10
-6
2 x 10
-0.2
-6
-7
-1 x 10 -5 x 10
-6
Time (seconds)
0
-7
5 x 10
1 x 10
Time (seconds)
-6
1.5 x 10
-6
2 x 10
-6
Fig 3.4 Voltage-Current graphs of the electric discharge in argon gas bubbles (a) voltage
waveform and (b) Current waveform
2. Wave forms
Figure 3.4 presents the typical voltage-current curves of the electric discharge produced
in the argon gas bubbles. A displacement current was observed corresponding to
increasing voltage just before the DBD occurs. Discharge was however appeared at the
voltage ~3kV but could go far stably up to 5kV, after which we observed very high
current and very unstable discharge. In most of the experiments, we used 5kV voltage.
The frequency and pulse width of the voltage pulses used were 5kHz and 1µs
respectively.
3. Optical emission spectrum
The emission spectrum of the discharge with various gases is presented in the fig 3.5.
39
graphite electrode_5kV_N2 spectrum
graphite electrode_5kV_Argon spectrum
150000
250000
100000
Intensity (a.u.)
Intensity (a.u.)
200000
150000
100000
50000
50000
0
300
400
500
600
700
Wavelength (nm)
800
0
300
900
400
500
600
700
800
900
Wavelength (nm)
(a)
(b)
graphite electrode_5kV_He spectrum
Intensity (a.u.)
24000
16000
8000
0
300
400
500
600
700
Wavelength (nm)
800
900
(c)
Fig 3.5 Optical emission spectrum of various gases using graphite electrode (a) argon
gas spectrum, (b) Nitrogen gas spectrum and (c) Helium gas spectrum
4. Methylene blue decolorization
Methylene Blue is an organic dye with molecular formula C16H18N3SCl. The
decomposition of Methylene blue by the active species released during the plasma is
well discussed by Fangmin et. al [42] shown in fig xx below. Fig 3.6 shows the
decomposition of Methylene Blue by our discharge. It nearly took 6 hours to decompose
all the 2000ml of Methylene blue. We used 10mg/l of MB for the experiments. 50%
decomposition, however, was attained in less than 1/3rd of the time = 2hours. We used
pulsed voltage = 5kV with frequency= 5kHz and pulse width = 1us.
40
Fig 3.6 Mechanism of MB decomposition
Fig 3.7 Decolorization of MB by porous graphite electrode
41
5. Indigo carmine decolorization
Since, Methylene blue is a high strength dye, it takes more energy to break down the
molecule, we also studied decomposition of 200ml of 10mg/l Indigo carmine using our
discharge. The breakdown of Indigo carmine is well explained by Yamashita et. al more
discussed in later section 4.3. We used same experimental conditions of voltage= 5kV
and used 200ml of 10mg/l of Indigo carmine. We observed complete decolorization of
Indigo carmine in 40min. The Visible absorption spectrum from 450-700nm of Indigo
carmine is presented in fig 3.8. While the Indigo Carmine has absorption spectrum at
612nm.The decolorization is observed gradually with time.
Indigo Carmine decolorization_graphite electrode
0.6
Absorbance at 612nm (a.u.)
0.5
0min
10min
15min
0.4
0.3
20min
0.2
30min
40min
0.1
0
450
500
550
600
650
Wavelength (nm)
Fig 3.8 Decolorization of Indigo carmine by discharge
42
700
6. Efficiency
We calculated the efficiency of the discharge produced by using porous graphite
electrode and the resultant efficiency in case of Methylene blue came out to be in the
range of 0.064mg/kwh which is not so significant decolorization efficiency if
considered other papers mentioned by M. A. Malik [49]. Besides of low efficiency in
applications, there are certain other disadvantages of using current porous graphite
electrode that are dealt in the next section.
Disadvantages of using graphite electrode
Besides giving lower efficiency rate, the graphite electrode exhibits a lot of other
disadvantages
1. Graphite is a soft material which is very prone to damage with the spiky discharge
2. Since graphite is a conductive material, we had to use mica sheet to form the DBD
structure which hinders the movement of bubbles into the solution
3. In order to provide a space for the bubbles between the electrodes, we had to use the
spacer between lower electrode and upper electrode which ultimately increases the
break down voltage
3.2.2 Electric discharge in the water by using ceramic electrode
Advantages of using ceramic over graphite plate
1. Ceramic is a harder substance that is very less prone to damage by the spiky current
43
2. Ceramic is a dielectric material because of which the use mica sheet can be avoided
3. The punched metal plate can be directly put on ceramic which give two benefits- first,
the bubbles are now free-flowing through the holes of the punched metal plate and
second, the break down voltage is reduced from 3kV to 1.7kV.
Experimental Scheme
Figure 3.9 shows the schematic drawing of the experimental set-up.
We used the
electrodes of a parallel plate type configuration where a thin porous ceramic plate with a
diameter of 50 mm and a thickness of 1 mm was used as a dielectric barrier. The porous
ceramic plate is made of mainly Al2O3 with relative permittivity of 4.1 and has 40 %
porosity (Asuzac Fine Ceramics, AZPW40). We sealed this ceramic plate over a punched
metal plate inside a hollow cylindrical metal frame as the grounded electrode. Over the
ceramic plate, we fixed a top punched metal plate with a hole diameter of 4 mm as the top
electrode by inserting a thin dielectric insulator between top punched metal plate and
ceramic plate. When the gas is injected into the electrodes through the gas inlet at the
bottom of the hollow metal frame, the gas comes out through the pores of the ceramic
plate in form of micro-bubbles in water. The diameter of the bubble is roughly several
hundred µm. The distance between the top punched metal plate and the dielectric plate
determined by a thickness of dielectric insulating sheet is nearly 0.5-1 mm which is
roughly comparable to the size of the bubbles.
44
HV probe
CT probe
Water/
Solution
Bubbles
Top punched
metal plate
Insulator
Porous ceramic
Bottom punched
metal plate
Metal frame
Gas
inlet
Fig 3.9 Schematic diagram of experimental set-up with porous ceramic plate as bubble
generator
bubbles
(a)
top
punched
metal
plate
hollow metal frame
bubbles
(b)
top
punched
metal
plate
hollow metal frame
45
Fig 3.10 Production of discharge by ceramic electrode (a) bubbles production and (b)
argon gas discharge in the bubbles
Figure 3.10(a) shows a photograph of the multi-bubbles discharge system with the parallel
plate electrodes. On injecting Ar gas through a gas inlet of the exterior metal frame,
streams of micro-bubbles are uniformly produced over the holes of the top punched metal
plate, as seen in Fig. 3.10(a). Applying pulsed high-voltage across the electrodes, we
clearly observed the discharge in Ar gas bubbles, as shown in Fig. 3.10(b), where the light
was shadowed to make the image clear. The first ignition occurs at 1.5~1.6 kV, although
it is a kind of local discharge at limited places. As the voltage increases to 1.8~ 2.0 kV,
then the uniform discharge is achieved as shown in Fig. 3.10(b). The higher the applied
voltage increases, the brighter and more uniform the bubble discharges become. In the
this chapter, for all the experiments, the voltage for 1.0 to 4.0 kV was used without any
significant damage to the electrodes. The temperature rise during the discharge plays an
important role as it can remarkably impact conductivity- resistivity and other chemical
nature of the water but in our discharge, the maximum temperature rise of the water is
only up to 32-37o C in 8-10minutes of treatment time. Also, the pH of the pure water does
change significantly after the discharge. With 100% argon gas discharge, almost no
change in the pH occurs, however there is slight change in pH from 6.5 (pure water) to
5.8-6.0 was observed when argon with oxygen is discharged.
46
4. RESULTS AND DISCUSSION
In this chapter, we present our experimental results. After we produced a uniform and
stable discharge described in chapter 3, we characterized our discharge using voltagecurrent waveforms and by calculating the yielded average power. We observed an
interesting combination of two kinds of discharges: DBD-like and glow-like occurring
simultaneously inside the water. Later, we studied the chemical nature of the discharge
by measuring emission spectrum of various gases and few other oxidative species using
chemical methods. In the last part of this chapter, we demonstrated the applications of
the discharge in decolorizing an organic dye by using a lab-made pollutant- Indigo
Carmine dye solution.
4.1 Characterization of the discharge
4.1.1 Waveforms
Figure 4.1 indicates the typical waveforms of voltage applied to the electrodes and
flowing current at different source voltages. Here, at the source voltages less than 1.5 kV,
the currents mainly consist of displacement current carrying through the parallel plate
electrode with a ceramic insulator. When the source voltage was about 1.6 kV, we clearly
observed a dielectric barrier discharge (DBD)-like discharge, where spiky currents with a
pulse width of 20~25 ns were superimposed on the displacement current in the rising
period of applied voltage. When we increased the source voltage higher than 1.8 kV,
glow-like discharges characterized by an increase in the current accompanied with a
voltage drop are observed as shown in Fig. 4.1(b). It is considered that these glow-like
47
discharges might be caused by preceding DBD-like discharges in the bubbles staying
between the ceramic and the punched metal electrode.
Voltage vs time Ar:100%/O2:0%_1.6kV_750ns
4
3
3
2
2
1
1
0
0
-1
-1
-1
-0.8 -0.6
-0.4 -0.2
0
0.2
t (μs)
0.4
0.6
4
0.8
1
-1
4
Current vs time Ar:100%/O2:0%_1.6kV_750ns
3
20
0
0.2
t (μs)
0.4
0.6
0.8
1
0.8
1
15
0
-1
-0.1 -0.05
10
-0.4 -0.2
20
2
1
15
-0.8 -0.6
5
0
0.05 0.1 0.15 0.2 0.25
t (μs)
0.3 0.35 0.4
10
5
5
0
0
-5
-5
-1
-0.8 -0.6
-0.4 -0.2
0
0.2
t (μs)
0.4
0.6
0.8
1
-1
-0.8 -0.6
-0.4 -0.2
(a)
3
2
2
1
1
0
0
-1
-1
-0.8 -0.6
-0.4 -0.2
0
0.2
t (μs)
0.4
5
4
0.6
0.8
1
-1
2
1
0
-1
-0.1 -0.05
15
10
-0.8 -0.6
-0.4 -0.2
3
20
0.6
4
3
-1
0.4
(b)
4
0
0.2
t (μs)
0
0.2
t (μs)
0.4
0.6
0.8
1
0.8
1
20
15
0
0.05 0.1 0.15 0.2 0.25
t (μs)
0.3 0.35 0.4
10
5
5
0
0
-5
-5
-1
-0.8 -0.6
-0.4 -0.2
0
0.2
t (μs)
0.4
0.6
0.8
-1
1
(c)
-0.8 -0.6
-0.4 -0.2
0
0.2
t (μs)
0.4
0.6
(d)
Fig. 4.1Typical waveforms of voltage applied to the electrodes and flowing cur-rent at
source voltages of (a) 1.6 kV, (b)1.8 kV, (c) 2.0 kV, and (d) 2.4 kV, respectively
48
When the source voltage was increased to 2.0 kV, the voltage between the electrodes
sharply dropped to roughly 800 V and the current increased to ~8 A, as shown in Fig. 4.1
(c), where we can evaluate the resistivity to be roughly 100 Ω. Furthermore, increasing
the source voltage to 2.4 kV (Fig. 4.1 (d)), the voltage dropped to about 1.0 kV and the
current raised to 12.5 A, where the resistivity is roughly 80Ω At the source voltage of
3.0 kV, the resistivity furthermore reduced to about 33Ω. These streamer-like discharges
are locally generated at the certain spot of the top punched plate electrode and they
visually look distributed uniformly over the entire electrode. Here, we refer to these types
of discharges as the pulsed glow-like discharges.
4.1.2 Location of the discharge
Here, we discuss the discharge characteristics of the present bubble self-generating
parallel plate type electrodes. In the present experiment, the bubbles emerged out from the
surface of ceramic plate just beneath the top punched metal plate when Ar gas in injected
into the bottom metal frame. When the bubbles are between the top punched metal
electrode and the porous ceramic plate as shown in Fig. 5(a)), it is expected that DBD-like
discharges can be ignited inside the bubbles at relatively low voltage of 1.5-1.6 kV, in a
similar way to the normal DBD in atmosphere.
This situation corresponds to the
waveforms of voltage and current shown in Fig. 4.1(a). At the source voltage higher than
1.8 kV, in the Ar gas bubbles that are in contact with both punched metal electrodes (top
and bottom) through the micro-pores of the ceramic plate, the glow-like discharges occur
with a current flowing through these micro-pores as illustrated in Fig. 4.2 (b). Transition
from the DBD-like discharge to glow-like discharge occurred after about 0.2 µs, as shown
49
in Fig. 4.1(b). When the source voltage increases to 2.0 kV or higher, the glow-like
discharges with a higher current are observed about 0.15 µs after the initiation of DBDlike discharge. So, it is interpreted that the DBD-like discharge plays a role as a trigger to
glow-like discharges between two metal electrodes through the porous thin ceramic plate.
So, in our experimental set-up, we observed two different types of discharges occurring
simultaneously. It can be clearly seen that the upper punched metal plays a significant role
in determining the kind of discharge occurring and the corresponding characteristics. The
metal-hole ratio of punched metal plates and their shape can affect the discharge
effectively. Such kind of study should be more elaborated and deepened that we covered
in the chapter 5 in which we will discuss how can we improve the discharge
characteristics by changing the geometry of the electrodes set-up.
4.1.3 Average power calculation
The maximum instantaneous power dissipated by each pulse was evaluated as roughly 30
kW (at an inter-electrode voltage of ~2 kV and a carrying current of ~15 A) at the source
voltage of 4.0 kV. Here, we calculated the time average dissipated power Pave defined by
Pave ≡
1
Tobs

T0 bs
0
V (t ) I (t )dt
where we took the integration time of Tobs= 10 ms in order to take enough pulses to
reduce the statistical error. In Fig. 4.3, we plot a relation of Pave versus the source voltage
ranging from 1.0 to 4.0 kV in the case of Ar gas.
It is seen that the Pave increases
monotonically with the applied voltage and tends to saturate at roughly 30 W when the
source voltage is 4.0 kV.
50
DBD in bubble
Top punched
metal electrode
Water
Porous
ceramic
Metal frame
Bubbles
Bottom punched
metal electrode
Gs inlet
Gs inlet
(a)
(b)
Glow-like
discharge
through porous
ceramic
Fig 4.2 location of the discharge (a) DBD type discharge (b) Glow like discharge
Voltage vs Averaged Power at Ar:100%
(10ms obserbation, pulse width 750 ns)
40
35
30
25
20
15
10
5
0
0
1
2
3
4
V (kV)
Fig 4.3 Average Power during the discharge
51
5
4.2 Chemical nature of the discharge
4.2.1 Optical emission spectra
Optical emission spectrum in the case of the Ar gas bubble discharge at 3.0 kV is shown
in Fig. 4.4(a). We did not directly observed OH radicals in the discharges probably
because of two reasons: the first reason could be a direct breakdown of H2O molecule into
H and O atoms by the frequent impact of electrons produced during µs pulses, which are
clearly observed as Hα at 656.3 nm, Hβ at 486.1 nm, Hϒ at 434.1 nm, and O I at 777.2 nm
in Fig. 4.4(a); the other reason could be the quick self-quenching properties of OH
radicals to form H2O2 as also observed and explained by Dang et al. and Maehara et al [43,
44]. Along with the spectral lines of Hydrogen and Oxygen, we also observed the spectral
lines of reactive species NO at 358.4nm. The other spectral lines are probably a mixture
of argon and nitrogen. NO and N2 lines are attributed to the atmospheric discharge where
air is well in contact with discharge during the experiments.
The effects of adding oxygen with argon as discharge gas on the emission spectrum is
shown in the Fig. 4.4 (b). The intensity of the discharge is significantly improved on
addition of oxygen. Along with few new spectral lines of reactive species NO and
Nitrogen is observed.
However, The spectral line O I 777.2 is significantly reduced which can be attributed to
the combination of free O with the other oxygen molecules to form more reactive species
like Ozone. The spectral line between 530- 560nm could not be identified but can
represent the ozone as also observed be J S Clements et. al.
52
Fig 4.4 (a) Emission spectrum of Argon gas (100%)
Fig 4.4 (b) Emission spectrum of Ar + O2 gas (80%:20%)
53
4.2.2 H2O2 production
As we discussed in previous section that the OH radical peak is not well clearly
observed in the emission spectrum of the discharge. We measured most expected
molecule H2O2 which is the secondary product commonly formed of the self-quenching
of OH radicals as discussed by Dang et al. and Maehara et al. [43,44]. We used KI
method- a chemical colorimetric method to measure H2O2 using 351nm in UV-VIS
spectrophotometer. The details of the chemical method are discussed in the section 3.1.
We used 3kV of pulsed voltage with 5kHz frequency and we measured H2O2 at
different times of the discharge. Fig 4.5 shows the linearly increased H2O2 concentration
with increasing discharge time from 0min to 5min. Nearly 15mg/l of H2O2 is produced
in the pure water by 5 minutes of discharge produced by our electrode set-up.
As we did not observe OH in our discharge, the obvious reactive species in our
discharge is hydrogen peroxide. In our experiments with Indigo Carmine (IC)
decolorization using our discharge with 100% argon, we observed that hardly 30% of
50mg/l IC was decolorized within 4min of discharge presented in Fig. 4.10 but if we
give a standing time of 24hours to the same discharged IC solution, the same IC gets
decolorized completely 100% within few hours (figure 4.9). This kind of behaviour is
attributed to the long life span of H2O2. According to literature, The H2O2 has a life in
minutes in its gaseous form, in hours when it is in soil but can remain 8-10 days
dissolved in the water. We studied the behaviour of H2O2 produced during the electric
discharge in the water in our experimental set-up which is presented in Fig 4.6. The
increased production of H2O2 in the form of absorbance during the discharge is
54
presented at 0th hour with different treatment time of 0min to 5min which increases from
0 to 0.5 in 1min treatment time and then more than 3.5 in just 5 minutes. After the
discharge, we gave the water/solution a standing time of more than 5 days (> 120hours).
We can clearly observe the presence of highly active H2O2 remaining in the water for
more than 5 days. This kind of phenomenon is also observed by many other researchers
but not yet published and recently, such kind of chemically active water is termed as
“Plasma water” or “Plasma activated water”.
We also studied the effectiveness of plasma water for decolorizing IC solutions of
various concentrations. Our results showed significant importance of this plasma
activated water as it is as effective as the direct discharge treatment of Indigo carmine.
This plasma water decolorizes IC completely 100% with a little more time.
By this study, one can clearly say that the species produced during electric discharge in
the water are of great importance in the field of wastewater treatment. One thing is also
observed that, during plasma- the shockwaves, radiations- UV or gamma etc are also of
significant importance.
We studied the effects of addition of oxygen gas on the production H2O2 by the
discharge. Fig 4.8 shows that on addition of oxygen, H2O2 is gradually decreased by
small amounts. This behaviour corresponds to gradual increase in production of ozone
which is more discussed in later section.
55
Conecntration of H2O2 produced (mg/l)
Production of H2O2 at 3kV
15
10
5
0
0
1
2
3
4
5
6
Treatment Time (minutes)
C/ C0 H2O2 produced during discharge
Fig 4.5 Production of hydrogen peroxide with discharge
H2O2 self-decomposition with time
1.2
1
0.8
4min
0.6
3min
0.4
1min
0.2
2min
0
0
20
40
60
80
100
120
140
Observation/ Standing Time (hours)
Fig 4.6 Hourly self-decomposition of hydrogen peroxide
56
4.2.3 Ozone production
In order to improve the chemical characteristics of our plasma discharge, we studied the
effects of adding oxygen in various combinations with background Argon gas. We have
already mentioned some of the effects of adding oxygen on emission spectrum and
production of hydrogen peroxide. Here, we will discuss those results in details. On
addition of oxygen from 0 to 20% ratio to the background gas-argon, we observed the
decrease in H2O2 production but at the same time, Indigo carmine decolorization was
significantly enhanced. This result is because of the production of ozone. As we increase
the ratio of oxygen gas in the gas mixture, ozone is produced logarithmically as shown in
Fig 4.7. At the addition of 20% of oxygen, we observed the production of more than
140mg/l which is highly toxic and was unbearable in the lab too.
We studied the phenomenon of increase in ozone production and decrease in H2O2
production and found in the literature that ozone and hydrogen peroxide are
complementary to each other for enhancing their oxidation mechanisms. The literature is
well explained by Duguet et al. [45] He explained peroxone process that requires an
ozone generation system as described in and a hydrogen peroxide feed system. The
process involves two essential steps: ozone dissolution and hydrogen peroxide addition.
Hydrogen peroxide can be added after ozone (thus allowing ozone oxidation and
disinfection to occur first) or before ozone (i.e., using peroxide as a pre-oxidant,
followed by hydroxyl radical reactions) or simultaneously. There are two major effects
from the coupling of ozone with hydrogen peroxide
57
•
Oxidation efficiency is increased by conversion of ozone molecules to hydroxyl
radicals; and
• Ozone transfer from the gas phase to the liquid is improved due to an increase in
ozone reaction rates
Our observation at coupling of ozone and hydrogen peroxide in order to enhance their
oxidation effects are presented in Fig 4.8. In our results, it is clearly seen that ozone
production increases with the increase in oxygen content in the gas that corresponds with
the reduction in H2O2 production based on the basic chemistry discussed above.
58
Ceramic electrode_Ozone production
140
Ozone concentration (mg/l)
120
20% O2
100
15% O2
80
60
10% O2
40
20
5% O2
0
0
30
60
90
Treatment Time (seconds)
120
150
Fig 4.7 Production of Ozone on addition of Oxygen gas (%)
Effects of adding Oxygen to the gas
17
160
140
H2O2
15
120
14
100
13
80
12
60
O3
11
40
10
Ozone Concentration (mg/l)
H2O2 Concentration (mg/l)
16
20
9
0
0
5
10
15
20
25
% of Oxygen added to Argon gas
Fig 4.8 Relation between ozone production and hydrogen peroxide reduction
59
4.3 Applications of the discharge for Indigo Carmine
decolorization
Indigo Carmine
We used an organic dye- Indigo Carmine- C16H8N2Na2O8S2 (MW= 466.36g)
solution in pure water in various concentrations to conduct experiments on applications of
the discharge. The basic mechanism of decolorization or complete decomposition of
Indigo Carmine is well explained by Minamitani et al . [46]. According to his study, the
blue color of IC is due to the double bond between the two aromatic sulfonic molecules
which is easy to get broken. This process decolorizes the IC from blue to pale yellow in
color. However, it is quite difficult to break the aromatic sulfonic molecules that may take
longer hours to bring out the results. In our study we did not go into the details of such
mechanisms since our focus is on the production of discharge which is more efficient and
cost-effective.
Experimental conditions
For conducting various experiments with IC, we used following experimental conditions
 Pulsed Voltage- 4kV
 Pulse frequency- 5kHz
 Pulsed width- 750ns
 IC solution- 200ml
60
 IC concentration- 10- 50mg/l
 Gas species- 100% Ar, Ar + O2 (various ratios)
Using 100% Argon
We conducted experiments using 200 ml of 50 mg/l Indigo Carmine solution. We used
100% Argon gas at the flow rate of 250 sccm. We used voltage of 4 kv at the pulse rate of
5 kHz. We observed decolorization of IC upto only 30% after 4 minutes of discharge but
when this solution was given a standing time of 24 hours, we observed a complete
decolorization of IC due to the activity of H2O2 produced during the discharge as
discussed in above sections. The hourly decolorization of IC is presented in figure 4.9.
The decolorization of IC during the discharge is presented at 0th hour. The initial
absorbance of 50mg/l IC was ~0.8333 and after 4 min of treatment by the electric
discharge, the absorbance is 0.64 that yields an efficiency of only 23% by using the
formula
[(Initial absorbance – final absorbance) / Initial absorbance] X 100 %
But, gradually in the same IC solution, even after the discharge chemical activities take
place and we observed a complete decolorization of IC after 24hour in the solution with 3
and 4min of treatment time.
Effect of adding O2
Figure 4.10 shows the temporal behaviors of absorbance at 612 nm of 50 mg/l IC aqueous
solution for different gas mixture ratios of Ar/O2=100/0, 90/10, 80/20 and 70%/30% at a
fixed total gas flow rate of 250 sccm and a source voltage of 4.0 kV. By adding oxygen
gas into Ar gas, the decolorization rate was significantly improved as shown in Fig. 7.
61
Absorbance at 612nm (4kV)
IC Absorbance at 612nm
0.8
0.7
0.6
1min
0.5
0.4
0.3
2min
3min
0.2
0.1
4min
0
0
10
20
30
40
50
60
70
Observation Time after discharge (hours)
Fig 4.9 Hourly observation of IC solution after 4min of treatment time
Absorbance at 612 nm ( at 4 kV)
1
Ar 100%
0.8
0.6
Ar 90%/O2 10%
0.4
Ar 70%/O2 30%
Ar 80%/O2 20%
0.2
0
0
1
2
3
4
5
6
7
8
Treatment time (min)
Fig 4.10 Indigo Carmine decolorization with various gas ratios
62
This result is supposedly explained by taking into account the generation of ozone by
the Ar and oxygen gas bubble discharges as explained in section 4.2.
Effects of Voltage and power on discharge characteristics
Fig 4.11(a) and (b) show the effects of voltage on IC decolorization. It can be observed as
we increase the voltage, the decolorization is enhanced exponentially. Fig b represents the
fig a in linear form by calculating the slope of curves- α by the following equation:
∆
=
Where, NIC represents the concentration at any time t , N0 is the initial concentration of
IC, α is the rate constant and ∆t is the time from 0min to time t.
Figure 4.12 shows the power dependence of decolorization rate constant. It provides us
a linear dependence of decolorization on to the power after 2kV or 15W. As we increase
the power, the decolorization rate increases. Though, at lower voltage or power,
decolorization is very small to be considered.
Effects of concentration of IC on its decolorization
For this experiment, we used various concentration of Indigo Carmine from 10-50mg/l
in order to understand the interactions between the chemical species produced during
the discharge and the contaminant molecule. According to our results, interaction
between reactive species and IC molecule follow 2nd order reaction with reaction rate
being dependent
63
Indigo Cramine decomposition at various voltage
Normalized absorbance of IC at 612nm
1.2
1kV
1
0.8
2kV
0.6
0.4
3kV
0.2
4kV
0
0
1
2
3
4
5
Treatment Time (min)
6
7
Fig 4.11(a) Variation of IC decolorization with different voltage
Absorbance at 612 nm (Ar:80%/O :20%)
2
1
N I .C , = N 0 exp(−α t　)
3 kV
3.5 kV
0.1
4 kV
0
1
2
3
4
Treatment time (min)
5
6
Fig 4.11(b) IC decolorization with different voltage by decolorization rate
64
Power vs decolorization rate
decolorization rate α(per min)
1.8
N I .C , = N 0 exp(−α t　)
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0
5
10
15
20
25
30
35
Dissipated Power (W)
Fig 4.12 Decolorization rate versus dissipated power
on both of them which can also be judged well by above discussion from Fig b that as
we increase the discharge time, more reactive species like H2O2 and Ozone are
produced and more is IC decolorization (50mg/l) observed.
Here, we used various concentration of IC- 10, 20, 30, 40 and 50mg/l and used our
discharge to decolorize them. We observed more is the concentration faster is the rate of
the reaction. We observed ~80% decomposition of 10mg/l at around 6.5min while
50mg/l solution was 90% decolorized in just 4min. Not only this, but we also observed
that increasing concentration of IC also increases efficiency (g/kWh) of the discharge in
decolorization of IC as shown in Fig 4.13.
65
Effect of changing concentration of IC solution
IC decolorization efficiency (g/kWh)
8
7
6
5
4
3
2
1
0
0
10
20
30
40
50
60
Concentration of IC solution (mg/l)
Fig 4.13
4.4 Efficiency
With the results of power dissipation shown in Fig. 4, the present results of 99 %
decolorization of 50 mg/l IC aqueous solution(200 ml) after about 3.2 min discharge at
4.0 kV lead to the energy efficiency for IC decolorization of roughly 3.7 µmol/kJ or 6.3
g/kWh.
M. A. Malik (2010) has brought all the plasma processes so far together to compare with
each other and to find out the reactors that are best for the purpose of wastewater
treatment. He has compared the efficiency by using G50 values which is the yield value at
50% decomposition of the dye. To our opinion, it is very difficult to compare between two
reactors with different parameters. The most common parameter used for the comparison
66
is mass of the pollutant or the dye decomposed per unit energy but the parameters like
waste produced, type of gases uses, amount of gases used and most important the
complete decomposition rate are usually not considered in calculating total efficiencies
which yield an incomplete information. In the Table 2 below- we compared few reactors
that have used only Indigo Carmine in the experiments.
References
Discharge
Material
Efficiency
Maehara [36]
RF
MB
96mg/kWh
Aoki [37]
RF
MB
28.125mg/kWh
Ishijima [35]
MW
MB
2500mg/kWh
Yano [47]
Pulsed DC Spark
IC
18720mg/kWh
Minamitani [46]
Pulsed Corona
IC
13700mg/kWh
Takahashi [48]
Pulsed Streamer
IC
60000mg/kWh
our discharge [50]
Pulsed DBD/Glow
IC
6300mg/kWh
To our knowledge, the ignition voltage of bubble discharges in the present technique is
much lower than those reported in the other works using pulsed power sources. Although
it is hard to compare the energy efficiency with the others because of different
concentration of IC aqueous solution ranging from 10 to 50 mg/L, the type and amount of
gases used. For example, Takahashi et. al has used almost 8L/min of gas in his
experiments which is more than 30 times of what is used in our experiments. The size of
the reactor too plays an important role. Therefore, a larger electrode-set-up for a larger
discharge area or the treatment area is the next goal in our research. However, the present
value obtained at 50 mg/L IC concentration is promising as the future energy-saving
waste water treatment technology.
67
5. IMPROVEMENT OF DISCHARGE
This chapter introduces a new approach towards the improvement of the discharge.
Instead of introducing external efforts in form of reactive gases or chemicals like
Fenton’s reagent or TiO2, we planned to study the physical parameters like shape and
structure of the electrodes to bring out improvement in the discharge. An introduction
about the scheme of experiments and few results in form of average power and increase
in efficiency by the change in structure of electrodes are presented here and there are
more results to be presented on this work that are under progress which we look
forward to include into the final thesis.
The efficiency of the plasma discharge for the purpose of the wastewater treatment can
be improved by various methods. The plasma discharge efficiency can be easily
enhanced by supplying higher power, using more reactive gases or usage of chemicals
supplied from outside. The improvement of the reactive species of the discharge using
chemicals like TiO2 and iron (II) and (III) oxides- Fenton’s reagent are very common
and well discussed by Zhang et. and Maehara et. al. respectively. The effect of amount
of gases and types of the gas species used for the discharge also play an important role.
Many researchers Clements et. al., Sato et. al. has discussed the effect of changing gas
species on the improvement of the discharge while Yamashita et. al. in his paper has
covered the effects of amount of gas on the discharge. It can be well noticed that
increasing amount of gas supplied to the discharge increases the total effects by
increasing total reactive species which is very much obvious with mass conservation
rule.
68
Plasma discharge is used for wastewater treatment using its physico-chemical properties
that it gains by consuming the electrical power and these properties are mainly dependent
on the type of power supply but the experimental geometry plays much more important
role in delivering that electrical energy into the purpose. As not all energy is utilized in
production of discharge, some of the energy gets used up in heating up the liquid as well
[11, 13]. In simpler words, power supply and geometry are complementary to each other.
All of above discussed methods are associated with very high costs and addition of extra
efforts to handle chemicals and their waste and do not go well with the approach
adopted in this thesis. To approach a cleaner and cost-effective method of improvement
of the discharge, there was need to study the physical parameters of the experimental set
up geometry. For this purpose, in this section we studied the effects of changing the
shape and geometry of upper punched metal plate. The reason behind this approach is
well discussed in the previous section 3.2.2 where the spatial location of the discharge
in the gas bubbles between the punched metal plates and the pores of the ceramic was
described. To recall the phenomenon once again, here we described the location of
discharge in our experimental set-up. At lower voltages, the discharge usually occurs
between the metal part of upper punched metal plate and dielectric porous ceramic
surface. When the voltage is increased, discharge become streamer like approaching the
lower metal electrode through the pores of the ceramic giving rise to glow like
discharge characterized by very high current. In this section, we are going to discuss and
understand more of this behaviour by using various punched metal plates- varying in
hole-metal ratios and hole sizes. In other words, we will study whether increase in DBD
discharge which also is expected to be the precursor of the glow discharge or the direct
69
increase in the area of glow discharge impacts the quality and characteristics of the
discharge.
5.1 Experimental scheme
Figure 5.1 shows the parallel electrodes set up. The spatial position of the bubbles can
be clearly seen between the porous ceramic plate and upper punched metal plate. Many
of the bubbles are hindered or struck between the metal part of the punched metal plate
and porous ceramic plate. In the Figure, the upper punched metal plate is highlighted in
red. In this section, the effect of upper punched metal plate will be studied and
discussed by changing its shape and geometry.
Fig 5.1 Parallel-plate electrodes highlighting (red) upper punched metal plate
Experimental conditions
 Upper punched plates (Stainless steel-SUS)
a) Hole size (diameter): 1mm, 2mm, 3mm and 4mm
70
b) Hole-metal ratio: 51%, 40.3%, 32.6% and 22.7%
 Voltage- 5kV
 Pulsed width- 750ns (5kHz)
 IC solution- 200ml (50mg/l)

Gas species- 80%Ar + 20%O2

Treatment time- 4minutes
Fig 5.2(a) Scheme of electrodes with various hole sizes
71
Fig 5.2(b) Scheme of electrodes with various hole-metal ratio
Figure 5.2 (a) and (b) below shows the images of various punched metal plates that are
used for conducting these experiments. Two different parameters- i) different hole sizes
of punched metal plates and ii) the hole metal ratio by keeping the hole size of punched
metal plate constant were chosen for conducting these experiments. To understand why
the perforation (hole variation) in a punched metal plate can affect the characteristics of
the discharge, let us recall the spatial locations of the bubbles and see once again how
72
was the discharge occurring between two parallel electrodes. It is clearly observed in the
following picture (fig. 4.2) that few bubbles are struck under the metal of upper pinched
metal plate and lower ceramic layer while other bubbles are free to move. It is
understandable that the discharge under metal part is prolonged glow-like discharge and
stable and different than that is in a moving bubble.
DBD in bubble
Water
Porous
ceramic
Metal frame
Top punched
metal electrode
Bubbles
Bottom punched
metal electrode
Gs inlet
Gs inlet
(a)
(b)
Glow-like
discharge
through porous
ceramic
5.2 Experimental Results
Average Power
The average power dissipated during the discharge with various electrodes is presented
in the figure 5. 3 with different hole sized electrodes and figure 5.4 with electrodes with
different hole-metal ratio. It both results, we can easily observe that hole structure and
so is the discharge type has significant role in determining the characteristics of a
discharge. We observed that as the hole size increases, the average dissipated power
decreases with minimum at 3mm. While, in case of hole-metal ratio, as we increase the
hole ratio more than the metal, average power significantly increases by 5 W (40%) but
then decreases again for 51%.
73
Power and decolorization rate vs hole size of electrode
100
Decol. rate %
40
30
80
60
20
40
Power (W)
10
20
0
IC Decolorization rate (%)
50
0
1mm
2mm
3mm
4mm
5mm
Hole size of electrode (mm)
Fig 5.3 Dissipated power and decolorization rate (%) of different hole-sized electrodes
Power and Decolorization rate (%) vs hole-metal ratio
100
decol. rate %
40
30
80
60
Power
20
10
40
Decolorization rate (%)
50
20
0
0
51%
40%
33%
23%
Hole-metal ratio
Fig 5.4 Dissipated Power and decolorization rate (%) with different hole-metal ratio
74
IC decolorization vs different hole sized electrode
1.2
1mm
2mm
3mm
4mm
C/C0 Indigo Carmine
1
0.8
1mm
0.6
0.4
4mm
2mm
0.2
3mm
0
0
1
2
3
4
5
6
7
8
Treatment Time (minutes)
Fig 5.5 Indigo Carmine decolorization with different hole sized electrodes
IC decolorization vs different hole-metal ratio electrode
C/C0 Indigo Carmine
1.2
51.1%
40.3%
33%
23%
1
0.8
33%
0.6
51.1%
0.4
23%
0.2
40.3%
0
0
1
2
3
4
5
6
7
8
Treatment Time (min)
Fig 5.6 Indigo Carmine decolorization with different hole-metal ratio electrodes
75
Indigo Carmine decolorization
Fig 5.5 and 5.6 presents our result of IC decolorization with various electrodes. As can
be seen, hole size of the electrodes shows remarkable effects. For experiments, we used
200ml of 50mg/l solution of Indigo carmine and used 5kV of pulsed discharge for 4 to
8min. The decolorization rate with 3mm electrode at 3 min is more than 95% while at
same time, 1mm electrode yields not even 70% of decolorization. There is significant
increase in 25% of decolorization using 3mm electrode. While the other case, holemetal ratio does not show remarkable difference bur 40.3 % is resulting into maximum
yield which also may be corresponding to higher average dissipated power.The
maximum of decolorization rate is presented in fig. 5.3 and 5.4 to compare it with the
average power. Here too, we observed that there is slight difference in the
decolorization rate of various electrodes.
Efficiency
We calculated efficiency of these electrodes using the same method as we used in
chapter 4. Our results are shown in figure 5.7 and 5.8 in which we compared the
average power dissipated between electrodes and their efficiency in decolorizing the
Indigo carmine by 5kV pulsed discharge in 4minutes. From the fig 5.7, it is observed
that hole size (d)= 3mm proves to be most efficient. The efficiency of 3mm electrode is
nearly the double of what is yielded by 1mm electrode while it is 15-20% more than the
other 4mm and 5mm electrode respectively. In case of different hole-metal ratio, not
much difference but not negligible difference is observed among various electrodes. A
rise of nearly 25% was observed in efficiency as we decrease the hole part of electrode.
76
60
6
50
5
Efficiency (g/kWh)
40
4
30
3
2
20
Efficiency (g/kWh)
Power vs efficiency with different hole sized electrode
Power(W)
1
10
0
0
1mm
2mm
3mm
4mm
5mm
Hole size of electrode (mm)
Fig 5.7 Efficiency and power comparison of different hole-sized electrodes
Power vs efficiency with different hole-metal ratio electrodes
7
6
30
Power (W)
5
25
4
20
Efficiency (g/kWh)
15
3
10
2
5
1
Efficiency (g/kWh)
35
0
0
51%
40%
33%
23%
Hole-metal ratio of electrodes (%)
Fig 5.8 Efficiency and power comparison of different hole-metal ratio electrodes
77
5.3 Conclusion and Discussion
From the above study, we observed that electrode structure can of course play a
significant role in determining the discharge characteristics. We observed from small
change (10%) to a remarkable change (max. 50%) in the efficiency of the electrodes
with various structures. However, it was just a preliminary study for observing these
results, a thorough study of the impacts of electrode structure on the chemical nature of
the discharge is yet to be studied for a complete understanding of the mechanism by
which upper punched metal play a role in determining the discharge nature. We also
look forward to study, the physical behaviour of the discharge in form of voltagecurrent waveforms with these electrodes which further light up the physics behind this
phenomenon.
78
CHAPTER 6 CONCLUSIONS
6.1 Experimental
In this thesis, an introduction of a novel geometry to produce multi-bubbles discharge
system is presented. The idea of producing bubbles right beneath the high voltage
electrode has shown potential advantages. A low voltage (~1.5kV) DBD followed by
glow discharge under the water is produced. It has been demonstrated that pulsed
bubble discharge in water can effectively degrade organic pollutants dissolved in the
aqueous solution, utilizing combined physical and chemical effects initiated by the
discharge in the water. It was observed that the principle reactive species involved in the
breakdown process are Hydrogen and Oxygen and the secondary reactive species
hydrogen peroxide forming hydroxyl radicals during the reactions. Ultraviolet light and
the shock waves both produced by the discharge play an important role in the oxidation
processes of the discharge as well.
The measurement and study of the behaviour of hydrogen peroxide in the pure water
was of great interest. The transformation of plain deionized water into highly active
water by the electric discharge brings out a new chapter in plasma chemistry. The
production of Ozone strongly enhanced the decolorization process. We observed that
the coupling reactions of ozone and hydrogen peroxide moreover enhanced the
oxidative properties of the discharge.
In the later section of the thesis, we mentioned to improve the discharge cost-effectively
by changing the physical parameter of the electrodes set-up rather than changing the
input energy or external chemicals. The experiments are still under process to bring out
79
some clear result but even the current results show that the physical geometry plays a
significant role in determining the total characteristics of the discharge.
We also measured and compared the efficiency of our discharge with the other papers
that represents most efficient reactors among other. Our efficiency is comparable to
them but still 10times lesser than the maximum but at the same time, we feel that there
must be more parameters taken into account for the justified comparison. We plan
present this comparison in the next manuscript.
6.2 Outlook
Keeping the current status of wastewater produced per day all across the world, we
understand the immediate need of its solution. Research on plasma technology in the
last three decades has proved that it is a fast and clean technology working at the
expense of electricity. So, there is utter need to bring out a plasma process which is
efficient and cost-effective which was whole purpose of this thesis and we have
considered these factors all through the study. Our study was on a minute scale plasma
reactor, but the stability and longetivity of the experimental set-up instigate us to
implement it on a larger scale or pilot scale projects.
80
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84

Study of low-voltage pulsed plasma discharges inside water using a

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