Journal of Electrostatics 58 (2003) 135–145
Oxidative decoloration of dyes by pulsed discharge plasma in water Anto Tri Sugiartoa, Shunsuke Itoa, Takayuki Ohshimaa, Masayuki Satoa,*, Jan D. Skalnyb a
Department of Biological and Chemical Engineering, Gunma University,1-5-1 Tenpaku-cho, Kiryu-shi, Gunma 376-8515, Japan b Department of Plasma Physics, Comenius University, Bratislava 84215, Slovakia Received 2 February 2002; received in revised form 1 July 2002; accepted 17 November 2002
Abstract Degradation of organic dyes by the pulsed discharge plasma between needle-to-plane electrodes in contaminated water has been investigated in three discharge modes: (i) streamer, (ii) spark, (iii) spark–streamer mixed mode. The process of the decoloration has been found to be most effective if the discharge operates in the spark–streamer mixed mode in dye solutions. The decoloration rate during the pulsed discharge plasma treatment was dependent on the initial pH values. The decoloration rate was increased when more acidic condition was used, especially in the case of streamer discharge mode. The decoloration rate at the pH value of 3.5 was found to be approximately three times higher than that at a pH value of 10.3. A small effect of initial pH during the decoloration process by spark and spark–streamer discharge mode means that the physical effects, such as shock-wave and ultraviolet radiation, may play an important role in the oxidation process. It was found that the decoloration rates in the case of spark and spark–streamer mixed discharge modes, which are characterized by high intensity ultraviolet radiation, were found to be much higher than that in the case of streamer discharge that is characterized by low intensity ultraviolet radiation. In addition, the considerable increase in the decoloration efficiency of H2O2 containing solutions can be attributed to the increase in hydroxyl radicals’ concentration. These are produced by ultraviolet light photodissociation of H2O2 molecules in water surrounding the plasma channel. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Pulsed discharge plasma; Advanced oxidation process; Ultraviolet radiation; Dye decoloration
*Corresponding author. Tel.: +81-277-30-1468; fax: +81-277-30-1469. E-mail address:
[email protected] (M. Sato). 0304-3886/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0304-3886(02)00203-6
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1. Introduction A variety of new synthetic organic dyes are frequently used by modern textile technologies. Therefore, the removal of these dyes from effluents becomes the major environmental problem of the textile industry, not only because of the potential toxicity of certain dyes, but often due to their visibility in wastewater. Although the dye concentration in wastewater is usually lower than that of many other chemical compounds, these dyes are visible even at very low concentrations. In general, dyecontaining wastewater can be treated in two ways: (i) by chemical or physical process and (ii) by biodegradation process. Due to the variety of different organic compounds, containing various substituted aromatic nuclei, there is no universal chemical method for a removal of dye from wastewater. Recent experimental investigations have revealed that reactive dyes can be decolorized by advanced oxidation processes (AOPs). For example, the ultraviolet light induced degradation combined with H2O2/O3 or Fenton process alone was utilized for such processing [1–5]. More recently, among the AOPs, the pulsed discharge plasma in water is considered to be an applicable method for removal of organic pollutants from wastewater [6,7]. Pulsed discharge plasma in water is efficient in the formation of chemically active species such as OH, H, O, O3, H2O2, etc [8,9]. Most of these species are among the strongest oxidizing agents. The major active species involved in the degradation of organic pollutants are hydroxyl radical and hydrogen peroxide [8]. The hydroxyl radicals can directly attack organic pollutants contained in water due to their high oxidation potential, and the hydrogen peroxide can effectively be decomposed by ultraviolet radiation into hydroxyl radical [10]. In addition, depending upon the solution conductivity and the magnitude of the discharge energy, shock-waves and ultraviolet light may also be formed [11]. These effects also play an important role in destroying harmful organic pollutants in wastewater [11]. The pulsed discharge in water has several modes such as streamer, spark, and spark–streamer mixed mode. It has been reported earlier that the pulsed discharge mode can affect the removal efficiency of phenol in water solutions [12]. The differences in removal efficiency were considered to be caused by the differences in the physical and chemical processes of each pulsed discharge mode in the water. The objective of this paper is to present the experimental data on the oxidative decoloration of dyes in water using three pulsed discharge modes. The effect of the initial pH, the emission intensity of ultraviolet light from the pulsed discharge plasma, and the effect of hydrogen peroxide additives on the decoloration efficiency were also examined.
2. Experimental apparatus and method The schematic diagram of experimental apparatus is shown in Fig. 1. The pulsed electric discharge was generated in the electrode system of the needle-to-plane electrode geometry located in the centre of Plexiglas cylinder (50 mm inner diameter)
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Fig. 1. Schematic diagram of the experimental apparatus.
reactor having the volume 100 cm3. Stainless-steel tube needle (0.5 mm inner and 1.5 outer diameter) protruded 1 mm from silicone insulator and was placed on axis of the reactor opposite to the stainless-steel plane electrode (diameter of 30 mm). Three distances between the needle and plane electrodes were fixed at: 30 mm for the streamer, 15 mm for the spark–streamer mixed, and 7 mm for the spark discharge mode [12], respectively. In the experiment, the spark–streamer mixed mode was in the time course with the mixing degree being 50:50. The pulse power supply with a rotating spark-gap switch was used to generate high voltage pulse. The pulse voltage amplitude, pulse frequency, and the capacitance of the storage capacitor: 20 kV, 25 Hz, and 6 nF, respectively, were kept constant in all experiments. The total volume of 300 ml of solution was circulated through the reactor and temperature controller by peristaltic pump at the solution flow rate of100 ml/min. Rhodamine B (basic dye), Methyl Orange (acid dye), and Chicago Sky Blue (direct dye) were treated in experiments. Their concentrations were 0.01 and 0.05 g/l. The conductivity of the solution was changed by adding KCl and was adjusted to 100 mS/ cm. Since the conductivity greatly affect the formation of plasma discharge in water, the initial conductivity should be kept constant in all experiment. Therefore, the initial solution pH was adjusted by added a little amount of HCl and/or KOH into the solution until the conductivity becomes 100 mS/cm. Spectrophotometer (Shimadzu, UV-1200) was used for measuring dye absorption. The decoloration factor FD was calculated by the following formula: FD ¼
absorption ðinitialÞ absoption ðtreatedÞ : absorption ðinitialÞ
ð1Þ
The temporal development of FD factor was measured. In order to investigate the emission intensity of ultraviolet light produced by pulsed discharge plasma, the light emitted from the discharge was collected by the optical fiber and transmitted to the entrance slit of a monochromator (McPherson
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2035) equipped with a photomultiplier tube (Hamamatsu R764). The end of the optical cable was located 10 mm from the point electrode and 10 mm from the axis of the discharge gap. The temporal variation of the light emission in the wavelength of ultraviolet region (200olo300 nm) was recorded on oscilloscope (Tektronix TDS3032). Ten peak values of successive light pulses were averaged with fluctuation less than 20% of the average value.
3. Results and discussion 3.1. Decoloration of dyes Due to differences in the chemical composition of dyes, there is no universally applicable chemical technology for a removal of dyes from wastewater. Therefore, it is necessary to find a nonselective method in order to simplify the decoloration process. Using the pulsed discharge plasma in water, decoloration of several kinds of dyes was tested. Results obtained in the spark–streamer mixed mode of discharge are shown in Fig. 2. The decoloration factor reached the value of 95% after approximately 100 min of treatment if the initial concentration of all three substances was 0.01 g/l. The rate of decoloration was considerably affected by initial concentration of dye in solution. The decrease in the rate of decoloration for Rhodamine B is evident from Fig. 2. It can be easily surmised from the presented results that the pulsed discharge plasma in water is an effective method for the decoloration of dyes from water solutions. The process of decoloration is complex including more mechanisms that act simultaneously. Among them, the break-up of chromosphores bounds, due to the activity of hydroxyl radicals, is the dominant process in the dye solution.
Fig. 2. Decoloration of aqueous dye solutions by pulsed discharge treatment.
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3.2. Effect of various pulsed discharge modes The efficiency of the decoloration process is affected by the mode of the pulsed discharge as it follows from plots shown in Fig. 3. The differences in decoloration rate of Rhodamine B by various pulsed discharge modes are remarkable. The spark– streamer mixed mode was found to be the most effective for the decoloration of dye in water solutions. The mentioned regime is characterized by great amount of plasma channels distributed bush like in great volume of the discharge gap and also by relatively high discharge currents. Processes acting in the plasma channel determine the differences in the decoloration rate. In the case of the spark discharge, a single plasma channel is formed in the liquid, but the channel has a high peak current of several hundred amperes compared to the streamer discharge characterized by peak currents below 100 A [13]. The concentration of electrons producing the hydroxyl radicals by electron impact dissociation of water molecules and the gas temperature in the plasma channel of the spark mode are higher than that of corresponding parameters in the streamer mode. Therefore, more radicals are formed in the spark mode compared to the streamer mode. For these reasons the decoloration rate becomes higher in the case of the spark mode than in the case of the streamer mode. Moreover, dyes can be decolorized even directly by ultraviolet light. It can be considered that the intense radiation of ultraviolet light emitted from the spark plasma channel is active in radical reaction of the dye decoloration. The spark–streamer mixed discharge mode is characterized by an appearance of many plasma channels [12]. The conditions are favourable for the generation of hydroxyl radicals by electron impact dissociation in the channels distributed over a large water volume that can explain the highest efficiency of such mode in
Fig. 3. Decoloration of aqueous Rhodamine B solution by various pulsed discharge modes.
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decoloration of dyes. Moreover, the direct photo-dissociation of dyes can be also active. 3.3. Effect of pH It is well known that the oxidation processes are very sensitive to the pH of the aqueous solutions. It was reported that the decoloration of dyes using photolysis process is more effective at low pH values than at high pH values [14]. Kang et al. [15] reported that the optimum pH for both the formation of hydroxyl radicals and dye removal in photo-Fenton process ranges from 3 to 5. Using various pulsed discharge plasma modes, the efficiency of decoloration of aqueous Chicago Sky Blue with different initial pH was tested. The obtained results after 30 min treatment are shown in Fig. 4. The initial pH of each plasma discharge mode was 3.5, 7.5 and 10.3. The initial conductivity of each plasma discharge was 100 mS/cm. The decoloration rates of Chicago Sky Blue were different for various modes, and depended on the initial pH of dye solutions. It was found that pH and conductivity after plasma treatment was different for each plasma discharge mode, e.g. in the case of streamer discharge, the pH was changed from 7.5 to 6.2, and conductivity was changed from 100 to 120 mS/cm after 30 min treatment. In the case of spark discharge, the pH was changed from 7.5 to 5.5, and conductivity was changed from 100 to 155 mS/cm after 30 min treatment. These results are in accordance with decoloration of dye using ozonation process [14], which shows that the dye molecules decompose into organic acids, aldehydes, resulting in a decrease in pH values and an increase in water conductivity. The decoloration rates were increased when more acidic conditions were used, especially when the streamer discharge mode was used. The decoloration rate at the initial pH of 3.5 is approximately three times faster than that at the initial pH of 10.3.
Fig. 4. Decoloration efficiency of aqueous Chicago sky blue solution by various pulsed discharge modes as a function of initial pH (applied voltage=20 kV, treatment time=30 min, dye concentration=0.01 g/L).
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Joshi et al. [8] reported that the major active species involved in the degradation of organic pollutants using pulsed streamer corona discharge are hydroxyl radicals and hydrogen peroxide. However, in the case of pH values higher than 7, the hydroxyl radicals are unselective and react readily with the carbonate ions, substantially reducing the efficiency of the oxidation process [16]. Carbonates are generated through the breakdown of the organic materials during the oxidation processes. For these reasons, it can be considered that the decoloration by hydroxyl radical is the dominant process during the streamer discharge mode treatment. On the other hand, in the case of spark and spark–streamer mixed modes, the effect of initial pH on the decoloration rate was small. It means that the other physical effects such as shock-wave and ultraviolet radiation play an important role in the decoloration process during the spark and spark–streamer mixed mode of the discharge treatment. 3.4. Ultraviolet radiation One of the physical effects produced by pulsed discharge plasma in water is ultraviolet radiation. In the case of the pulsed arc discharge reactor, the temperature in the plasma channel, which forms during an electrohydraulic discharge, can reach values of 14.000–15.000 K and thus functions as a blackbody radiation source. A maximum of emittance of such source is in the vacuum ultraviolet (VUV) region of the spectrum (l ¼ 752185 nm) [14]. The vacuum ultraviolet light emitted from the hot plasma is absorbed immediately in the water layer surrounding the plasma channel [11], and the ultraviolet light with l > 185 nm penetrates into the bulk of the solution. The emission intensity of ultraviolet light produced by three pulsed discharge plasma modes in distilled water is shown in Fig. 5. The intensity of ultraviolet light radiation in the case of spark discharge was much higher than that of the other discharge modes. In the case of spark discharge, a single plasma channel is formed in the liquid. The plasma channel has a high peak current of several hundred amperes, and according to Ba( rmann et al. [17] the electron density in the propagated leader channel is high, close to the values typical for a thermal arc. This condition makes the plasma channel that becomes a source of light with high intensity of ultraviolet radiation. However, the intensity of ultraviolet light in the case of streamer discharge was found to be very low. This is because streamer discharge in the distilled water is characterized with the bush like-streamer discharge channel, and a moderate amount of bubbles. Therefore, the plasma channels emit very low intensity of ultraviolet light as shown in Fig. 5. It should be noted that an increase in the solution conductivity might increase the emission intensity of ultraviolet light of the pulsed discharge plasma in water [18]. 3.5. Effect of hydrogen peroxide The use of chemical additives for enhancement of the energy efficiency of the depletion of dyes is one of the recommended methods. In this study, the effect of
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Fig. 5. Spectrum of ultraviolet emission of various pulsed discharge modes in distilled water.
Fig. 6. Decoloration of aqueous Rhodamine B solution by various pulsed discharge modes in the presence of hydrogen peroxide.
hydrogen peroxide on the removal of Rhodamine B by various pulsed discharge modes has been investigated. Results are shown in Fig. 6. The initial concentration of hydrogen peroxide of 8.8 103 mol/l was used for all experiments. The initial pH of each plasma discharge mode was 7.5. The increase, by addition of hydrogen peroxide, in the rate and in FD values is apparent for all three modes of the discharge.
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The dramatic increase of decoloration rate most likely appears due to the reactions of dyes with hydroxyl radicals formed by photo-dissociation of hydrogen peroxide in the gap surrounding discharge channels. The low threshold energy for photodissociation of H2O2 in comparison with water makes this process strictly dependent on the light intensity. The spark discharge mode is characterized with high light intensity as shown in Fig. 5. Therefore, the process of dye removal from water is most effective in the case of spark discharge mode. The production of hydroxyl radicals by electron impact dissociation, which normally dominates in streamer or spark–streamer mixed modes, becomes less important. The changes in the dominant mechanism of hydroxyl radical production can be explained by the high efficiency of the single spark mode of the discharge. The intensity of UV radiation from spark– streamer mixed mode of discharge and especially pure streamer mode are evidently lower. 3.6. Mechanism and intermediates Decoloration process using plasma discharge in water is complex including more mechanisms that act simultaneously. Our experimental results showed that hydroxyl radical and ultraviolet radiation were dominant on decoloration process during plasma discharge. This process is similar like in photooxidation of dye using combination of ultraviolet radiation and hydrogen peroxide. As reported by several authors, the schematic mechanism of photooxidation on decoloration process are proposed as follows [14, 19]: RH þ hn-Rd þ H;
ð2Þ
H2 O2 þ hn- OH þ OH;
ð3Þ
OH þ RH-Rd þ H2 O;
ð4Þ
Rd þ O2 -ROd2 :
ð5Þ
A direct photooxidation of dye with ultraviolet light alone can lead to the decoloration of dye. However, direct photooxidation of dye in water is very limited since water absorbs significantly in the vacuum UV region. Therefore, ultraviolet light is practically used in combination with oxidant (e.g. hydrogen peroxide) or catalysts (e.g. titanium oxides). The combination of ultraviolet and hydrogen peroxide can form hydroxyl radicals. The hydroxyl radicals react with dye (RH) to form dye+d(Rd), and then dye+d could hydrolize or react with some oxidizer such as dissolved oxygen (19). If the decoloration process by plasma discharge is as same as the photooxidation process by ultraviolet with hydrogen peroxide, the decoloration of methyl orange occurs due to the break up of the azo bond (N=N) by hydroxyl radicals, generating 4-methylamino aniline, and then underwent a further opening of phenyl-rings to
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form small molecular compounds such as oxalic acid and butenedioic acid [20]. In the case of plasma discharge with strong ultraviolet radiation (i.e. spark and/or spark–streamer mixed discharges), ultraviolet radiation will take part in the break up of the azo bond. 3.7. Energy efficiency The above investigation was focused on the reaction mechanisms and improvement of the removal rate. However, in practice, energy efficiency is an important factor. The energy efficiencies of three pulsed discharge modes for Rhodamine B (see Fig. 6) were 25 mg/kWh for streamer discharge, 80 mg/kWh for spark discharge, and 160 mg/kWh for spark–streamer mixed discharge modes. When the additive (8.8 103 mol/l hydrogen peroxide) was injected into the reactor, the energy efficiency became 320 mg/kWh for spark–streamer mixed discharge.
4. Conclusions The process of dye decoloration from water solutions has been investigated using three pulsed discharge modes in water. The decoloration process was complex and active oxidation mechanisms were most likely hydroxyl radicals and ultraviolet light radiation. In the case of streamer discharge, the decoloration rate was dependent on the initial pH solution. Hence, the oxidation process by hydroxyl radicals is considered to be the dominant dye removal process. However, a small effect of the initial pH solution on the decoloration rates by spark discharge and spark–streamer mixed discharge mode mean that the other physical effects such as shock-waves and ultraviolet radiation play an important role in the decoloration process. In water solution hydroxyl radical generated mostly in the discharge channel by the electron impact dissociation of water molecules. Therefore, the spark–streamer mixed discharge mode was the most effective. In addition, the intensity of ultraviolet light radiation generated by the plasma channels was high enough for direct photodissociation of dyes. In solutions containing hydrogen peroxide, hydroxyl radicals formed in the water by ultraviolet light stimulated photolysis of hydrogen peroxide. Therefore, the highest efficiency of spark mode characterized by the highest intensity of emitted light has been found in experiments.
Acknowledgements Part of this work was supported by a Grant-in Aid for Scientific Research of the Ministry of Education, Science, Sport and Culture, Japan, #11555205 and 12895021.
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