non-thermal plasma technology for degradation of organic

Transcription

J. Environ. Eng. Manage., 17(6), 427-433 (2007)
NON-THERMAL PLASMA TECHNOLOGY FOR DEGRADATION OF ORGANIC
COMPOUNDS IN WASTEWATER CONTROL: A CRITICAL REVIEW
Hsu-Hui Cheng,1 Shiao-Shing Chen,2,* Yu-Chi Wu2 and Din-Lit Ho2
1
Institute of Engineering Technology
National Taipei University of Technology
Taipei, 106 Taiwan, ROC
2
Institute of Environment Engineering and Management
National Taipei University of Technology
Taipei, 106 Taiwan, ROC
Key Words: Non-thermal plasma; organic compound; wastewater treatment
ABSTRACT
Non-thermal plasma is an emerging technique in environmental pollution control technology,
produced by the high-voltage discharge processes and therefore a large amount of high energy
electrons and active species are generated. The degradation of difficult-degraded organic pollutions
will be greatly enhanced by the active species generated from non-thermal plasma process. However,
research on non-thermal plasma technology on organic wastewater cleaning remains scarce. In this
work, recent developments of plasma on wastewater control technology are critically reviewed, and
the operational principles and parameters are outlined with special focus on the degradation of
organic compounds in the wastewater treatment processes.
INTRODUCTION
The degradation of hazardous organic pollutants
in water is one of the critical and urgent topics in
environmental research. However, these hazardous
can not be effectively removed or recovered by conventional biological or chemical method [1,2].
Therefore, considerable attention has been focused on
the so-called advanced oxidation processes (AOPs)
based on the generation of highly reactive species, especially hydroxyl radicals [2,3]. As a very powerful
and non-selective oxidant with sufficient potential to
completely oxidize organic compounds into carbon
dioxide and water [3-5], the hydroxyl radical plays an
important role in pollution control system for organic
degradation [6]. The oxidation potentials of major
chemical reactants, including hydroxyl radical, are
shown in Table 1 [7,8].
There are a number of AOPs to generate hydroxyl radicals. Ultraviolet photolysis, wet oxidation,
and TiO2 photocatalysis [9], for example, have all
reached the stage of practical applications. However,
the hydroxyl radical reactions activated by these
methods are known to be slow [10]. Ozone oxidation,
another AOP technique, takes a time-consuming series
of chain reactions to produce hydroxyl radicals in neu*Corresponding author
Email: [email protected]
tral and basic solutions, but has the unstable nature of
ozone to worry about [10]. Compared to its fellow
AOP approaches, non-thermal plasma can achieve
greater output and better efficiency.
Non-thermal plasma produced in water solutions
forms the basis of an innovative AOPs of water treatment [3,4]. The non-thermal plasma pollution control
process can remove chemical and biological wastes in
all three – gas, liquid, and solid – states. The method
can decompose both high- and low-concentration organic wastewater even in a large flux [2]. Non-thermal
plasma is produced during the discharge processes
where a large amount of high energy electrons and active species (such as active radicals and molecules)
are generated [1-4,11] for direct, and effective oxidation of dissolved organic molecules.
The paper reviews the current researches on
chemical reactions in the aqueous phase under highvoltage electrical discharges with special focus directing onto the degradation of organic compounds in water by non-thermal plasma technology.
PRINCIPLE OF NON-THERMAL PLASMA
TECHNOLOGY IN WATER
Consisting of both positive and negative ions and
428
Table 1. Oxidation potential of major chemical reactants
[7,8]
Oxidant
hydroxyl radical
atomic oxygen
ozone
hydrogen peroxide
permanganate
chlorine dioxide
chlorine
Oxidation potential (V)
2.80
2.42
2.07
1.77
1.67
1.50
1.36
Fig. 1. Phases of matter [12] .
electrons as well as neutral species, plasma is commonly called the fourth state of matter (see Fig. 1) [12]
and exists extensively in nature [13]. Non-thermal
plasma produced by gas discharges is a mixed atmosphere consisting of high-active species, such as electrons, ions, radicals, excited atoms and molecules. Gas
temperatures of these species are much lower than that
of the electrons and even lower than room temperature
[14]. According to related researches, non-thermal
plasma produced in the water tends to exchange energy with the surrounding water medium. Because the
residence time of plasma is very short in water, the
high energy of plasma would be fed into the water.
Plasma consists of a lot of particles that need to intertwine with each other during the birth, existence, and
extinction of plasma. So the collision, stimulation,
ionization, and transitions of energy levels between
particles are very common in the water [11]. Besides
these features of plasma itself, there are several special related physical functions to be noted. First, when
plasma is produced, the temperature of water reports a
steep increase (dT/dt ≈ 109 K s-1). Second, due to the
limited space in the water, plasma cannot spread
freely, and a pressure as high as 105-107 MPa is generated. Because of the incompressibility of the water,
the high pressure would transform into impact shock
wave in water. Finally, plasma is generated as the result of high-voltage discharge that creates the instantaneous electric field and magnetic field. The electromagnetic field is capable of sterilizing wastewater,
and plasma is often integrated with other physical
functions to clean wastewater [1,3,4,9,11]. Therefore,
plasma technology is considered to be more effective
in treating wastewater.
DISCUSSION OF EXPERIMENTAL
PARAMETERS
1. Effects of Voltage
Ruan et al. [15] examined the removal efficiencies of three organic compounds that escalated with
the increase in peak voltage. At higher voltage, electrons are produced more easily, and the formation of
electron avalanches is accelerated [16]. The energy of
electrons at a higher peak voltage is greater than the
activation energy of the organic compounds, thus, the
bond of the organic compounds can be broken and
converted into different substances. The measured removal efficiencies of ethanethiol, trimethylamine and
ammonia read 95, 86 and 87%, respectively [15]. In
addition, Chen and co-workers [1] reported that phenol removal rate increased with longer discharge time,
and raising the peak voltage was beneficial to phenol
degradation. Yan et al. [17] also discovered that phenol degradation increases with rising voltage. It was
found that growth in voltage could increase the solution’s electric field intensity, generating more active
species, such as •O, •OH, O3 and H2O2, capable of
oxidizing more phenol into lower molecular weight
compounds like carboxylic acid or carbon dioxide.
Table 2 shows the effects of supply voltage on removal efficiency of organic compounds.
2. Effects of Electrodes Distance
As indicated in Fig. 2, the smaller the gap between electrodes, the better the COD removal efficiency. Compared to electrodes with a greater distance
between each other, those with a smaller distance can
be expected to achieve the same level of efficiency in
less than half of the time [18].
The change of electrode distance may alter the
intensity of the electric field. This effect is similar to
that of the change of applied voltage for the variation
of the intensity of electric field, and leads to different
electrical discharge types. According to Sugiarto and
Sato [9], it is possible for each type of electrical discharge to degrade phenol from water solution. The
discharge types are explained and illustrated in Fig. 3.
As shown in Fig. 4, the spark discharge shows high
degradation rate at the beginning, but the spark with
streamer discharge is found to be effective for the
complete degradation of phenol [9].
Chen et al. [1] also reported the effects of two
different electrode distances on phenol removal from
aqueous solution. Thus, we can understand that with a
shorter electrode distance, it is easier to produce the
plasma, and more energy can be deposited into the solution, leading to some photochemical and plasma
channel effects and to a subsequently higher removal
rate of phenol existing in the solution.
Cheng et al.: Review for Non-thermal Plasma
429
Table 2. Effects of supply voltage on removal efficiency of organic compounds
Organic compounds
Phenol (1)
Supply voltage (kV)
7
8
9
10
12
24
36
40
Phenol (2)
Ethanethiol
Trimethyamine
Ammonia
Ethanethiol
Trimethyamine
Ammonia
Ethanethiol
Trimethyamine
Removal efficiency (%)
55
65
76
86
5
19
23
19
38
40
31
55
78
45
82
52
62
Ammonia
Ethanethiol
Trimethyamine
Ammonia
Ethanethiol
Trimethyamine
Ammonia
74
Ref.
[1]
–
[17]
air
[15]
air
air
92
68
84
97
87
86
95
70
Gas
air
air
air
30
100.0
Phenol Concentration (%)
COD Removal (%)
25
20
15
1cm
2cm
3cm
10
5
60.0
40.0
20.0
0.0
0
0
5
10
Time (min)
15
20
Fig. 2. Effects of electrodes distance [18].
Ground
(A)
HV
(B)
10
20
30
40
Treatment time (min)
50
60
3. Effects of Gas Source
Ground
HV
0
Fig. 4. Degradation rates of phenol with various
electrical discharge types liquid conductivity =
200 μS cm-1 [9].
Ground
HV
streamer
spark
spark with streamer
80.0
(C)
Fig. 3. Illustration of three electrical discharge types on
phenol removal (A) streamer discharge with an
electrode gap of 45-mm; (B) spark with streamer
discharge with an electrode gap of 15-mm; and
(C) spark discharge with an electrode gap of 6
mm [9].
Table 3 presents the effect of gas type on organic
pollutant degradation when oxygen, air and argon are
bubbled through the discharge. According to the results of the study conducted by Sung et al. [19] compared to other gases, oxygen can facilitate higher efficiency. In other studies, the organic pollutant degradation increases with the number of gas bubbles in reactors, because •OH, •H and •O are produced by gas
bubbling, and their intensity increases with the growth
in the gas bubbling rate [20,21]. Besides, Clements et
al. [22] and Šunka et al. [4] suggested that H2O2 and
O3 were produced with air or oxygen bubbles in the
reactors, indicating the important role of active spe-
430
Table 3. Efficiency of pollutant degradation at various gases bubbling type
Pollutants
Phenol, 50 mg L-1
Acetophenone (AP),
100 mL
Phenol, 100 mg L-1
Pulsed corona
discharge
Parachlorophenol
(4-CP), 100 mg L-1
Pulsed streamer
discharge
Chicago Sky Blue, 10
mg L-1
Gas type
O2, 0 mL min-1
Results
Can not 100% removal in 60 min
O2, 30 mL min-1
O2
N2
–
N2, 200 mL min-1
O2, 200 mL min-1
N2, 100 L h-1
Air, 100 L h-1
O2, 100 L h-1
O2
Air
Ar
cies in the pollutant degradation reaction.
In the Sugiarto-Sato study [9], oxygen gas was
bubbled into the reactor at 30 mL min-1 for all discharge plasma types; the degradation rate of phenol
became higher when oxygen was bubbled into the reactor. The ozone was decomposed by UV light or energetic electrons to form •O and •OH through a series
of chain reactions, which could attack and degrade organic compounds. Oxygen bubbling into the reactor
greatly affects the degradation of phenol. The addition
of oxygen enhances the phenol degradation rate [9].
4. Effects of Solution pH and Conductivity
In the previous studies, gas phase discharges
generate over a water surface to produce in the gas or
at the gas-liquid interface strongly oxidative species,
such as •OH, •O and their reaction products (O3,
H2O2), that can dissolve into the water to initiate oxidation process. Chen et al. [1] reported the formation
of ozone by bubbled oxygen under high-voltage discharge. In the aqueous solution, ozone also reacts with
water molecules to form •OH. Ozone tends to be vulnerable to decomposition at high pH but remains
fairly stable at low pH [1]. The •OH density is higher
in neutral or alkaline media than in an acidic medium
under the same experimental conditions [20]. Therefore, more •OH are formed at higher pH. That explains the faster phenol breakdown rate in Fig. 5.
The research of Yan et al. [17] indicates that
higher phenol degradation efficiency can be achieved
in an environment of high acidity or alkalinity. Sun et
al. [20] reported similar findings that the oxidative
ability of plasma tends to be stronger in acidic conditions, and the gliding arc discharge would produce
stronger UV radiation when the solution is alkaline.
Since ozone is generated during the plasma process,
the UV radiation can enhance the degradation of organic compounds by O3 and the formation of •OH accords with the following reactions [17,23].
O2 + •O + → O3
(1)
Ref.
[9]
100% removal in 50 min
AP has 72% loss after 30 min of treatment
AP has 53% loss after 30 min of treatment
Phenol removal less than 5%
Phenol removal is 15~16%
Phenol removal is 40%
75~84% of 4-CP removal
Decoloration ratio is 70%
Decoloration ratio is 32~36%
Decoloration ratio is 11~15%
[34]
[1]
[26]
[19]
40
Phenol Removal (%)
Discharge type
Pulsed spark
discharge
Pulsed corona
discharge
Pulsed high voltage
discharge
30
pH=3.0
pH=7.0
pH=10.0
20
10
0
0
60
120
180
Time (min)
Fig. 5. Effects of pH [1].
O3 + hv + H2O → H2O2 + O2
(2)
H2O2 + hv → 2OH
(3)
2O3 + H2O2 → 2OH + 3O2
(4)
The pH affects the chemical composition of the
O3. For example •OH are formed by decomposing O3
at high pH [24], but the O3 content remained unchanged at a low pH. In Table 1, O3 has an oxidation
potential of 2.07 V, and the •OH has an oxidation potential of 2.80 V. Hence, the direct oxidation by •OH
is faster than O3 oxidation. And the organic compound
degradation increased with the pH under the alkaline
solution condition.
In addition to the solution pH, solution conductivity is another important parameter that affects the
degradation of organic compound. The high liquid
conductivity may inhabit the formation of streamer
and plasma channels and the production of •OH
[20,21]. More ions present in the solution at higher solution conductivity will cause the establishment of the
strong electric field more difficult. Furthermore, the
quantities of ions present in the solution can lead to
the decrease in the generation of chemically active
species, such as •OH [25]. Therefore, low electric
conductivity of the solution is beneficial to the phenol
removal.
5. Effects of Chemical Additives
With the applications of the oxidants such as
431
Table 4. Compare efficiency of pollutant degradation by adding various chemical additives in plasma system
Discharge type
Pulsed corona discharge
Pollutant
Phenol, 50 mg L-1
Chemical additives
H2O2, 0 M
H2O2, 4.4× 10-5 M
Pulsed streamer and spark
dischar
Phenol, 50 mg L-1
Pulsed streamer corona
Phenol, 100 mg L-1
Phenol, 100 mg L-1
Gas-liquid gliding arc
discharges
Phenol, 200 mg L-1
2-chlorophenol, 500
μM
para-chlorophenol,
100 mg L-1
para-chlorophenol,
100 mg L-1
Phenol, l mM
H2O2, 0 mg L-1
H2O2, 100 mg L-1
H2O2, 1000 mg L-1
Activated carbon, 0 g L-1
Activated carbon, 1 g L-1
No inorganic salt
Ferrous sulfate, 67.8 mg
L-1
Without Fe2+
With Fe2+, 40 mg L-1
FeSO4, 250 μM
H2SO4, 100 μM
No chemical additives
Fe2+, 0.2 mM
Fe3+, 0.2 mM
TiO2, 50 mg L-1
TiO2, 50 mg L-1; Fe2+, 0.2
mM
TiO2, 50 mg L-1; Fe3+ 0.2
mM
TiO2, 0 mg L-1
TiO2, 50 mg L-1
TiO2, 100 mg L-1
TiO2, 0 g L-1
TiO2, 1 g L-1
TiO2 [10,24-28], ferrous ion [3,5,17,27], and hydrogen
peroxide [5,9,29,30], recent, researches have been
committed to enhance energy efficiency and degradation efficiency of organic contaminants. However,
these reports seldom focus on the pulsed discharge
plasma process which combines with chemical additives for the decomposition of organic pollutants in
wastewater [26]. Table 4 is the comparison efficiency
of pollutant degradation by adding various chemical
additives in plasma system. The efficiency of the
chemical additives on non-thermal plasma are introduce in the followings.
5.1 TiO2
The anatase form TiO2 irradiation of the photocatalytic activity with light of wavelength λ < 390 nm
leads to the formation of excited-state conduction
band electron and valence band hole pairs, which are
capable of initiating a large variety of oxidation and
reduction reactions. The increased contaminant degradation by the corona discharge in the presence of TiO2
is most likely related to the photocatalytic processes
on the surface of TiO2 induced by the UV radiation
from the plasma [3,24].
Results
100% removal with 300 J mL-1 of input
energy
100% removal with < 100 J mL-1 of
input energy
Completely degradation in 30 min
Completely degradation in 20 min
Completely degradation less than 20 min
Phenol removal 40%
Phenol removal 89%
Phenol removal 40%
Phenol removal 46-47%
Ref.
[29]
Degradation efficiency is 85%
Degradation efficiency is 95%
Removal efficiency 3.5 × 10-3 μmol J-1
Removal efficiency 1.9 × 10-4 μmol J-1
Rate constant k = 1.6 × 10-3 s-1
[17]
[9]
[32]
[1]
[3]
[27]
[26]
[28]
Hao et al. [26] reported that the removal efficiency of 4-chlorophenol (4-CP) was greatly promoted with the increase of TiO2 concentration. As
shown in Table 4, the rate constants of 4-CP degradation were the highest with TiO2 concentration at 50
mg L-1, but the efficiency with TiO2 concentration of
100 mg L-1 was less than that of without TiO2 [26].
Insufficient TiO2 addition (< 50 mg L-1) cannot effectively utilize the UV radiation to enhance the 4-CP
removal while too much TiO2 concentration added
into the system may impose adverse effects on 4-CP
removal. The main purpose of TiO2 addition is to utilize the ultraviolet radiation from plasma to generate
photocatalytical formation of •OH on the surface of
TiO2 particles and to increase the yield of OH· for
phenol degradation.
5.2 Fe2+
Non-thermal plasma reacts with additional ferrous ion to form Fenton’s reaction when hydrogen
peroxide produced by the discharge reacts with ferrous ion to produce hydroxyl radicals. The removal efficiency of the discharge can be significantly enhanced in the presence of iron. The 4-CP and phenol
432
of the degradation were found to be attributed mainly
to the oxidation by hydroxyl radicals produced directly by the discharge [10,31].
To enhance the degradation efficiency of organic
contaminants and energy efficiency of power source,
Fe2+ is introduced into the pulsed discharge plasma
system [21]. Chen et al [1] showed the rapid increase
in the reaction rate upon the addition of FeSO4, which
was attributed to the presence of the Fenton’s reaction.
The addition of Fe2+ can enhance the phenol degradation efficiency under plasma conditions. This addition
of iron increases the OH· concentration by decomposing H2O2 in Fenton or Fenton-like reactions [31].
Fe2+ + H2O2 → •OH + OH− + Fe3+
(5)
Fe2+ + •OH → Fe3+ + OH−
3+
2+
(6)
+
Fe + H2O2 → Fe + HO2 + H
(7)
(8)
HO2 + H2O2→ O2 + H2O + OH
The study of Lukeš et al. [28] showed the same
situation; 2-CP was treated by pulsed corona discharge plasma (PCDP), and (a) H2SO4 100 μM (b)
FeSO4 250 μM were added in the 2-CP solution. The
degradation of 2-CP was notably accelerated in the
presence of ferrous ions in the solution. The removal
efficiencies were ranked as follows: PCDP/FeSO4 (3.5
× 10-3 μmol J-1) > PCDP / H2SO4 (1.9 × 10-4 μmol J-1)
[3]. The same effect of iron on the efficiency of the
pulsed corona discharge process was observed previously during the degradation of phenol [32].
5.3 H2O2
Hydrogen peroxide could be produced by plasma
system with photocatalysis in water. Under photocatalysis, H2O2 is demonstrated to be formed by the reduction of adsorbed O2 by conduction band electrons
while corona discharge in water produces H2O2 as a
result of dissociation and excitation of water molecules by propagating plasma channels through the
aqueous solution [21].
The Sugiarto-Sato study [9] showed the degradation of phenol by various discharge plasma types at
the presence of hydrogen peroxide (100 and 1000 mg
L-1). As shown in Table 4, when H2O2 was added in
the system, •OH concentration went up and triggered
thereby an increase in the degradation rate of phenol
[9]. An additive like H2O2 shows during electrical discharges a synergistic effect on the destruction of organic compounds [33]. The degradation process may
be due to the reaction of •OH formed by photolysis of
H2O2 caused by UV radiation from electrical discharge plasma [29,32]. This reaction process is based
on the following reaction.
H2O2 + UV → •OH + OH
(9)
Adding chemical additives H2O2 into the reactor
leads to a dramatic increase in phenol degradation rate.
The mechanism prompting the increase of the degra-
dation rate relies on the photolysis of additives.
CONCLUDING REMARKS
Critically reviewed, non-thermal plasma technology for wastewater organic compounds reports the
following advantages: greater efficiency, more costeffective equipment, and easier operation. Moreover,
no second pollutant is generated since the technology
follows the green chemistry guidelines in the whole
degradation process. As a novel and promising technology, non-thermal plasma for wastewater treatment
deserves further investigations for extensive and effective applications in the future.
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Discussions of this paper may appear in the discussion section of a future issue. All discussions should
be submitted to the Editor-in-Chief within six months
of publication.
Manuscript Received: April 30, 2007
Revision Received: September 15, 2007
and Accepted: September 19, 2007

non-thermal plasma technology for degradation of organic

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