I. INTRODUCTION Water pollution is serious problem not only for humans but also for the entire ecological system. Recently, conventional waste water treatment, e.g., biological and chemical methods, has been replaced by a water treatment system using plasma because plasma treatment is environmentally adaptive and chemically reactive. In plasma treatment, organic compounds are generally decomposed by only part of the ozone generated by plasma. This is because ozone has high oxidation potential and is useful for decomposition of organic compounds in water. Although a variety of radicals, such as H 2 O 2 , O･, OH･, O 3 *, N 2 *, e - , etc., are also generated by reactive plasma, the radicals and most of the ozone cannot be used for water purification due to their short lifetime [1]. If the reactive plasma is generated near the solution, the radicals can be utilized efficiently for water purification and the treatment efficiency increases. A water treatment system utilizing the discharge of bubbles, above water and in gas-liquid two phase flow has been developed [1-10]. It has been reported that the method of spraying waste water into reactive plasma shows the highest relative energy efficiency [11]. Furthermore, there are some Persistent Organic Pollutants (POPs) such as dioxin, perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) in water [12-14]. Since the oxidation potential of ozone is insufficient, these POPs cannot be decomposed by ozone. Therefore, attention has been focused on advanced oxidation processes utilizing radicals. In this study, a method for decomposing methylene blue by spraying solution as mist into reactive plasma directly was investigated, using a newly developed mist- flow plasma reactor. Reactive plasma is generated by dielectric barrier discharge (DBD) on the inner wall of a tube. The atomized solution containing micro-sized droplets is introduced into the mist-flow plasma reactor and treated by ozone, free radicals and ultraviolet rays. Dissolved chemical species such as H 2 O 2 , reactive oxidation species (ROS) and O 3 as well as NO 3 - , pH, and conductivity are measured as liquid properties. In addition, the decomposition characteristics of this method were experimentally clarified through decolorization experiments using methylene blue solution. The methylene blue solution is about 100% decomposed by only one treatment under certain operating conditions. II. EXPERIMENTAL APPARATUS AND MEASUREMENT PROCEDURE Fig. 1 shows a schematic of the experimental setup, which mainly consists of electric power supply, ultrasonic atomizer units, a mist-flow plasma reactor, a mist separator and an air pump. Air and Ar are used as carrier gases. The carrier gas flow rate is 9.0 l/min. The applied sinusoidal voltage range is 6-12 kV and the Decomposition of Methylene Blue in Water Using Mist Flow Plasma Reactor T. Shibata 1 and H. Nishiyama 2 1 Graduate School of Engineering, Tohoku University, Japan 2 Institute of Fluid Science, Tohoku University, Japan Abstract—The world is faced with serious problems of water pollution. Recently, conventional chemical treatment has been replaced by a water treatment system using plasma. In this study, using a newly developed mist-flow plasma reactor, a method for decomposing methylene blue by directly spraying solution as mist into reactive plasma was investigated. Reactive plasma is generated by dielectric barrier discharge (DBD) on the inner wall of a tube. An atomized solution containing micro-sized droplets was introduced into the mist-flow plasma reactor and treated by ozone, free radicals and ultraviolet rays. Dissolved chemical species such as H 2 O 2 , reactive oxidation species and O 3 as well as NO 3 - , pH, and conductivities are measured as liquid properties. In addition, the decomposition characteristics of this method were experimentally clarified through decolorization experiments of methylene blue solution. Keywords—Water purification, plasma, mist flow, functional fluid Corresponding author: Tomohiro Shibata e-mail address: [email protected]Presented at the 8th International Symposium on Non- Thermal/Thermal Plasma Pollution Control Technology & Sustainable Energy, in June 2012 Fig. 1. Schematic illustration of experimental setup. 253 International Journal of Plasma Environmental Science & Technology, Vol.6, No.3, DECEMBER 2012
Transcript
I. INTRODUCTION
Water pollution is serious problem not only for humans but also for the entire ecological system. Recently, conventional waste water treatment, e.g., biological and chemical methods, has been replaced by a water treatment system using plasma because plasma treatment is environmentally adaptive and chemically reactive. In plasma treatment, organic compounds are generally decomposed by only part of the ozone generated by plasma. This is because ozone has high oxidation potential and is useful for decomposition of organic compounds in water. Although a variety of radicals, such as H2O2, O･, OH･, O3*, N2*, e-, etc., are also generated by reactive plasma, the radicals and most of the ozone cannot be used for water purification due to their short lifetime [1]. If the reactive plasma is generated near the solution, the radicals can be utilized efficiently for water purification and the treatment efficiency increases. A water treatment system utilizing the discharge of bubbles, above water and in gas-liquid two phase flow has been developed [1-10]. It has been reported that the method of spraying waste water into reactive plasma shows the highest relative energy efficiency [11]. Furthermore, there are some Persistent Organic Pollutants (POPs) such as dioxin, perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) in water [12-14]. Since the oxidation potential of ozone is insufficient, these POPs cannot be decomposed by ozone. Therefore, attention has been focused on advanced oxidation processes utilizing radicals. In this study, a method for decomposing methylene blue by spraying solution as mist into reactive plasma directly was investigated, using a newly developed mist-flow plasma reactor. Reactive plasma is generated by
dielectric barrier discharge (DBD) on the inner wall of a tube. The atomized solution containing micro-sized droplets is introduced into the mist-flow plasma reactor and treated by ozone, free radicals and ultraviolet rays. Dissolved chemical species such as H2O2, reactive oxidation species (ROS) and O3 as well as NO3
-, pH, and conductivity are measured as liquid properties. In addition, the decomposition characteristics of this method were experimentally clarified through decolorization experiments using methylene blue solution. The methylene blue solution is about 100% decomposed by only one treatment under certain operating conditions.
II. EXPERIMENTAL APPARATUS AND
MEASUREMENT PROCEDURE
Fig. 1 shows a schematic of the experimental setup, which mainly consists of electric power supply, ultrasonic atomizer units, a mist-flow plasma reactor, a mist separator and an air pump. Air and Ar are used as carrier gases. The carrier gas flow rate is 9.0 l/min. The applied sinusoidal voltage range is 6-12 kV and the
Decomposition of Methylene Blue in Water Using Mist Flow Plasma Reactor
T. Shibata1 and H. Nishiyama2
1Graduate School of Engineering, Tohoku University, Japan 2Institute of Fluid Science, Tohoku University, Japan
Abstract—The world is faced with serious problems of water pollution. Recently, conventional chemical treatment has been replaced by a water treatment system using plasma. In this study, using a newly developed mist-flow plasma reactor, a method for decomposing methylene blue by directly spraying solution as mist into reactive plasma was investigated. Reactive plasma is generated by dielectric barrier discharge (DBD) on the inner wall of a tube. An atomized solution containing micro-sized droplets was introduced into the mist-flow plasma reactor and treated by ozone, free radicals and ultraviolet rays. Dissolved chemical species such as H2O2, reactive oxidation species and O3 as well as NO3
-, pH, and conductivities are measured as liquid properties. In addition, the decomposition characteristics of this method were experimentally clarified through decolorization experiments of methylene blue solution.
Corresponding author: Tomohiro Shibata e-mail address: [email protected] Presented at the 8th International Symposium on Non-Thermal/Thermal Plasma Pollution Control Technology &Sustainable Energy, in June 2012
Fig. 1. Schematic illustration of experimental setup.
253 International Journal of Plasma Environmental Science & Technology, Vol.6, No.3, DECEMBER 2012
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Fig. 2. Power per unit area with applied voltage w/ and w/o mist flow for Air and Ar.
frequency range is 500-1500 Hz. The oscillating frequency of the ultrasonic atomizer unit is 2.4 MHz and the electric power consumption is 17 W. The atomization rate is 250 ml/h. The mist density is calculated by gas flow rate and amount of treated solution. It is about 100 ppm. The mean diameter of mist is generally given by Eq. (1)
3
1
2
834.0
FD
(1)
where D is the mean droplet diameter, σ is the surface tension coefficient, ρ is the liquid density and F is sound frequency. The calculated mean diameter is about 2 micrometers in this experiment. The mist flow plasma reactor made of Teflon with a thickness of 0.5 mm has an inner mesh electrode made of stainless and an outer grounded electrode made of copper. The inner diameter of this reactor d = 22 and 32 mm and the length of discharge area l = 10, 50 and 100 mm. Sinusoidal voltage is applied to the inner mesh electrode. The mist separator is a T-shaped pipe with stainless mesh at the inlet port. The mist is separated from gas by collision with the stainless mesh. The separated gas is returned to the tank because it includes active species such as ozone.
The electric power consumption is evaluated by the Lissajous figures method. The discharge intensity is calculated by the voltage of an inserted capacitor (1 μF). The amount of generated ozone is measured by an ultraviolet absorption ozone monitor. The ozone concentration is measured near the inner wall and on the central axis at the exit of the reactor and averaged. Reactive species such as OH are detected by spectroscopic measurement. Change of pH, electrical conductivity and the dissolved amount of NO3
- are measured by the glass electrode method, the AC 2-electrode method and the ion electrode method, respectively. Dissolved ozone, hydrogen peroxide (H2O2) and ROS are measured by a water quality meter (MultiDirect, AQUA LYTIC) with N,N-diethyl-p-phenylenediaminesulfate (DPD). In addition, the decomposition efficiency of organic compounds is evaluated by absorptiometry using 5 mg/l of methylene blue solution.
III. EXPERIMENTAL RESULTS
A. Mist-flow Plasma Reactor Characteristics Fig. 2 shows power per unit area using (a) air and (b)
Ar with and without mist flow evaluated by the Lissajous figures method. It is noted that there are no differences of power per unit area for the change of reactor diameter and length. The power per unit area increases as applied voltage and frequency increase. Since the discharge is inhibited by mist and water film on the inner wall, the power with mist flow is lower than that without mist flow. The power using Ar is lower than that using air because Ar is easier to discharge than air.
Fig. 3 shows photos of the mist-flow plasma reactor with and without mist using (a) air and (b) Ar. The mist-flow plasma reactor is made of an acryl pipe and has a transparent conductive sheet as an outer grounded electrode for visualization in these photos. The applied sinusoidal voltage is 14 kV and the frequency is 1000 Hz. Plasma is generated on the inner wall along the mesh electrode using air. In the case with Ar, the plasma is generated on the inner wall more uniformly and brightly in comparison with the air case. With the generation of mist, the mist flow and water film on the inner wall weaken the plasma with both gases.
Fig. 4 shows the emission spectrums observed in air and Ar with and without mist. The N2 2nd positive system bands (315, 337, 357, 380 nm) and the 1st negative system band (391 nm) are observed with air. The Ar bands (700-850 nm) and the OH band at 309 nm are observed with Ar. OH radicals have high oxidization potential (2.8 V). The OH band is observed without mist because of humidity. The emission intensity drops in the case with mist because the mist weakens the plasma and scatters the emission.
Shibata et al. 254
Fig. 5 shows the ozone concentration using air. The ozone concentration is increases with increases in applied voltage, frequency and reactor length in air. In this study, the highest ozone concentration, about 140 ppm, was found for 12 kV applied voltage and 1500 Hz applied frequency using a reactor with a length of 100 mm.
Ozone was not detected using Ar in any electrical conditions.
Fig. 6 shows the ozone generation efficiency calculated by Figs. 2 and 5. The ozone production efficiency decreases as the applied voltage increase because ozone decomposition by electron collision is enhanced in a high electric field. The ozone production efficiency is higher using a short reactor rather than a long reactor. Although ozone is generated with higher efficiency by applying high frequency, the frequency
(a) Air
(b) Ar
Fig. 3. Photos of the plasma flow w/ and w/o mist for Air and Ar.
(a) Air
(b) Ar Fig. 4. Spectroscopic measurement of plasma w/ and w/o mist for Air and Ar.
Fig. 7. Ozone production efficiency with ozone concentration.
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dependence of ozone production efficiency decreases as applied voltage increase.
Fig. 7 shows the ozone production efficiency as a function of ozone concentration. In this study, a reactor with a length of 10 mm was found to be most efficiency for ozone generation within 30 ppm, and a reactor with a length of 50 mm was most efficient for 30-100 ppm ozone generation.
B. Liquid Property Evaluation Fig. 8 shows the dissolved amounts and energy yields of chemical species such as H2O2, ROS and ozone with applied voltage of 10 kV and applied frequency of 1000 Hz after one treatment. The solution is purified water and the solution pH is adjusted by HCl and NaOH. In this operating condition, the energy consumption is about 1.9 W. The amount of dissolved H2O2 increases as pH increases. The ROS is effectively dissolved, especially in acid solution. The dissolved ozone concentration is almost the same at any pH. This is because ozone, free radicals and UV generated by air plasma react with water droplet. Ozone decomposes by itself in water [15, 16]. Ozone self- decomposition reaction starts with (2) and (3).
O3 + OH- → ·HO2-· (2)
·HO2-· ⇄ H+ + ·O2
- (3) O2
- generated in (2) reacts with O3 and the radical chain reaction starts. Reactive oxidation species such as OH are
generated in this reaction. Finally, H2O2 is generated as the final product of this reaction. As shown in (1), the ozone self-decomposition reaction depends on pH and is enhanced in alkaline solution. Therefore, the H2O2 concentration increases especially in alkaline solution. On the other hand, the ROS increases especially in acid solution because HO2 radicals are generated by (3) in H+ rich solution. Furthermore, ozone concentration is low compared with the concentration of other chemical species because of its low solubility. Fig. 9 shows the amount of dissolved H2O2 in the treated solution after one treatment. The solution is purified water. H2O2 is generated mainly by the recombination of OH radicals detected in Fig. 4 using Ar.
OH + OH → H2O2 (4)
Fig. 8. Concentrations of dissolved species and energy yields in the treated solutions for air.
Fig. 9. Concentration of dissolved H2O2 in the treated solutions for air and Ar.
(a) pH
(b) NO3- concentration
(c) Electrical conductivity
Fig. 10. Change of solution pH, NO3- concentration and electrical
conductivity for air.
Shibata et al. 256
The concentration of H2O2 using air is higher than that using Ar. Fig. 10 shows the change of solution (a) pH, (b) NO3
- concentration and (c) electrical conductivity before treatment and after treatment without plasma (only sprayed solution) and with plasma. The solution pH decreases by spraying in neutral and alkaline solutions because mist has a large specific surface area and CO2 in air dissolves effectively. The solution pH increases more than that without plasma. This is because NOx generated plasma reacts with mist and nitric acid is generated. Generation of nitric acid is detected by the increase of NO3
- concentration shown in Fig. 10 (b). Electric conductivity increases after treatment in acid and neutral solution due to the dissolution of ions such as NO3
-. On the other hand, the electric conductivity decreases in alkaline solution because of neutralization. C. Methylene Blue Decomposition Fig. 11 shows the photos of 5 mg/l methylene blue solution before treatment and treated solution using 100 mm length reactor with 10 kV 1000 Hz for air. In this condition, the methylene blue solution is decolorized completely at one time treatment. Fig. 12 shows the methylene blue conversion measured by absorptiometry. Using air, methylene blue conversion increases with applied voltage and frequency. It obtains about 100% in the case of short reactor length. Methylene blue is completely decolorized after one pass in the case of reactor length l = 100 mm for any applied frequency and voltage. Using Ar, methylene blue conversion is lower than that using air and decreases as the applied voltage increases. This is because methylene blue is mainly decomposed by ozone generated by air plasma. Since the lifetime of OH radicals are short, OH radicals generated Ar plasma cannot react with methylene blue. Fig. 13 shows methylene blue decomposition efficiency calculated based on Figs. 2 and 10. The decomposition efficiencies using a short reactor show higher efficiency, especially for l = 10 mm and decrease as applied voltage increases in all cases. The decomposition efficiency with Ar is lower than that with air. These tendencies of methylene blue conversion and decomposition efficiency are same as those of ozone. Therefore, it is suggested that the dominant reaction of methylene blue decomposition is ozonation. The Relative Energy Efficiency (REE) determined by Malik [11] is about 120 in our experiment with 10 mm reactor for applying voltage of 6 kV, 1000 Hz. This data is not as good as the best reported in the paper above by Malik (The REE is 1000 to 2000). There are two reasons for lowness of REE. The first is the deference of power supply. The all data showing higher REE than 120 in Malik’s paper use pulse power supply compared with the AC power supply in this paper. In general, the pulsed discharge can generate radical species more effectively. The second is the amount of the treated solution. The throughput is low (about 10 ml/min) in this paper. So the decomposition efficiency can be enhanced
Fig. 11. Photos of methylene blue solution before and after treatment.
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Fig. 12. Methylene blue conversion with applied voltage for air and Ar.
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Fig. 13. Methylene blue decomposition efficiency with applied voltage for air and Ar.
Fig. 14. Metylene blue conversion for any pH solutions.
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by using pulse power supply and optimization of the mist density. Although the plasma source is different, the methylene blue decomposition efficiency is about 0.1 mg/kJ (3×10-10 moles/J) in our previous research on direct ozone and UV treatment using microbubble [17]. Present research shows higher efficiency compared with previous study. Fig. 14 shows the methylene blue conversion for any pH solution using air. The methylene blue conversion increases with solution pH. There are two ways to decompose methylene blue using ozone. The First is direct reaction with substrates. The second is reaction of radicals which is the decomposition products of ozone. It is generally recognized that the decomposition of O3 leads to free radicals such as OH radical. Furthermore, ozone decomposition is the result of a chain reaction in which hydroxide ions act as initiator. Discussed in chapter III-B, ozone decomposition is enhanced in alkaline solution. Therefore, OH radical which has high oxidation potential is generated as intermediate of ozone decomposition and act as oxidative species in alkaline solution.
IV. CONCLUSION
The results obtained in this study are summarized as
follows. (1) A mist-flow plasma reactor, which has an inner mesh
electrode and an outer grounded electrode, is newly developed. Reactive plasma is generated on the inner wall of a tube at a few watts with and without mist.
(2) OH radicals are detected from emission spectrum for Ar. Ozone is generated at a few watts for air. The ozone production efficiency increases for short reactor length, low applied voltage and high frequency.
(3) Reactive species such as H2O2, ROS and O3 are dissolved in the droplets effectively. The amount of dissolved H2O2 increases with pH. ROS is dissolved especially in acid solution. The dissolved ozone concentration is almost the same at any pH.
(4) The solution pH decreases in neutral and alkaline solutions after plasma treatment. The NO3
- concentration increases in any solution. The electrical conductivity of solution increases in acid and neutral solutions, and decreases in alkaline solution after treatment.
(5) The methylene blue solutions can be decomposed about 100% at one treatment for air.
ACKNOWLEDGMENT
This work was partly supported by Grant-in-Aid for Challenging Exploratory Research (24656117) in JSPS and a Grant-in-Aid for JSPS Fellows (24･9008). The authors would like to thank Assoc. Prof. H. Takana for
valuable discussion, Mr. T. Nakajima and Mr. K. Katagiri for technical supports with IFS, Tohoku University.
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