Active species formation of pulsed plasma in water
water with reactor with ring-to-cylinder electrode system has been investigated. A large volume of streamer corona discharge was generated around the ring electrode and the streamer corona discharge formations are greatly dependent on the water conductivity. The degradation of phenol red was also tested by this electrode system. It was found that the degradation rate of phenol red was caused by OH radical oxidation and the intermediate products were removed when the treatment time increased.Keywords—Pulsed plasma in water; ring-to-cylinder electrode; active species; OH radical
I. INTRODUCTION
Over the past decade, experimental investigations have revealed that reactive dyes can be decolorized by advanced impact dissociation of water in plasma discharge region.
Development of the oxidation process by electrical discharge in water was carried out by Clements et al. [7] on pulsed positive streamer corona discharge. A basic mechanism of oxidation process by electrical discharge in water is considered to be the formation of a strong electric field and strong non-thermal plasma where collisions of molecules with high-energy electrons produces free radicals and low-energy electrons.
Recently, the development of reactors for generating pulsed streamer corona discharge has been investigated by some researchers, in order to generate a large number of radicals in water [8,9]. However, there is no consensus regarding the most suitable and cost effective reactor for the industrial applications. Therefore, a reactor consisting of ring-to-cylinder electrode system has been proposed by Sugiarto et al. [10,11]. This electrode system is considered to be suitable for industrial applications because of its wide plasma region.
In this study, the formation of active species by pulsed streamer discharge using ring-to-cylinder electrode system was investigated experimentally. The formation of OH radical
and hydrogen peroxide was investigated. The mechanism of dye decoloration was studied too.
II. EXPERIMENTAL AND METHODS
Fig. 1. Schematic diagram of the experiment apparatus
The experimental apparatus consists of a pulsed high voltage power supply and a reactor. The reactor vessel contains a ring-to-cylinder electrode geometry system as shown in Fig. 1. A stainless steel ring (thicknesss 0.5 mm, diameters6 mm) was placed in the center of the Plexiglas cylinder (inner diameters53 mm, lengths 50 mm) for generating a pulsed streamer corona dis- charge in water. The grounded electrode was a stainless steel cylinder (diameters50 mm, lengths30 mm). A positive pulse voltage was applied to the ring electrode.
The 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 were 0–30 kV, 25 Hz, and 6 nF, respectively. The pulse voltage and current were meas- ured with a high-voltage probe (Tektronix P6015A) and a wide band current transducer (Pearson Electronics M411), respectively. The discharge parameters were monitored by a digital oscilloscope (Tektronix TDS360).
A total volume of 300 ml of dye solution was circulated through the reactor by peristaltic pump at a flow rate of 100
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ml/min. Phenol Red were dissolved in distilled water at initial concentration of 0.01 g/l. The conductivity of the solution was changed by adding KCl and was adjusted to 100 mS/cm. The initial pH values of the solutions were adjusted to 3.5, 7.5 and 10.3 by adding HCl and KOH. The concentration of dyes was measured using a spectropho- tometer (Shimadzu, UV-1240).
III. RESULTS AND DISCUSSIONS
A. Streamer discharge formation
The formation and propagation of streamer discharge in water or liquids are greatly dependent on the applied voltage and the water conductivity [7, 12]. In this study, the effect of water conductivity to the streamer formation generated by the ring-to-cylinder electrode system was investigated, using the ring electrode in the case of d=0 mm. Fig. 2 shows photographs of the streamer formation generated by ring-to-cylinder electrode system. As shown in the figure, a uniform streamer discharge is formed in water. For the positive pulse, a filamentary streamer were observed around the tip of the ring at the conductivity of 2.5 µS cm-1, and changed to the magenta colored filamentary streamer with increasing the conductivity. The streamer discharge became stronger and steady when the conductivity increased. When the negative pulse was applied, many white streamer branches were observed around the ring, and changed to magenta streamer branches with increasing the conductivity.
Fig. 2. Schematic diagram of the experiment apparatus
B. Active species formation
Various active species (·OH, ·H, ·O, HO2·, H2O2, etc.) are produced when the streamer discharge injected into water [8]. Among the active species, hydroxyl radicals (·OH) and hydrogen peroxide (H2O2) are the most important for
oxidation processes. A number of papers have been published on pulsed streamer corona discharge generated in water using point-to-plane electrode geometry. Primary reactions initiated by the discharge were assumed to be as follows [6]:
H2O + e à ·H + · OH + e (1)
2 H2O + e à H2O2 + H2 + e (2)
According to Sun et al. [9], in this experiment the production of OH and H radicals by the discharge were detected by optical emission spectroscopy. Fig. 3 shows the typical light emission of OH (309 nm) and Hα (656.3 nm) radicals in water during the streamer discharge generated by ring-to-cylinder electrode at +20 kV with 100 µS cm-1 water conductivity. The oscillation at t=0 in the light signal is noises from the spark gap. As a result, Hα radical emission rose rapidly and had more intense than OH radical. The emissions of OH and Hα radicals were started at about 1 µs after applying the pulse voltage. The emission of radicals is corresponds to the streamer formation in water as shown in Fig. 4. When the high electric field is applied, the water is dissociated and the bubbles begin to form in the water [19, 21]. The delayed time may be due to the process of bubble production, and the streamer discharge is started to form when the electron avalanches occur in the bubbles.
Fig. 3. Time characteristics of OH and Hα radicals in water
Fig. 5. Time characteristics of OH radical emission with various water conductivity
Fig. 6. Effect of water conductivity on production of hydrogen peroxide in water (a) positive; (b) negative
Fig. 5 shows the effect of water conductivity on the light emission of OH radical at 20 kV. It was found that the radicals emission intensity and half width of emission light was dependent of the water conductivity. The light emission of OH radical is stronger in more conductive water. This is because, at high conductive water the streamers channels are more intense and stable (see Fig. 2). On the other hand, the developing of streamer is slow when the water conductivity
decreased. Therefore, the half width of OH radical emission light increased with decreasing the water conductivity.
The formation of hydrogen peroxide by the streamer discharge at the various water conductivity for different electrode polarity was shown in Fig. 6(a) and (b). The results showed that the concentration of hydrogen peroxide was higher when the conductivity was low in both positive and negative pulses. This is because the streamer formation is more longer, and filamentary at low conductive water than that at high conductive water. Moreover, the ultraviolet emission by the discharge is higher in the high conductive water, which can lead to the high decomposition of hydrogen peroxide during the discharge.
C. Dye decoloration
The mechanism of dye decoloration was complex due to various physical and chemical effects occur during the injection of pulsed electrical discharge in water. In this study, preliminary analysis of dye decoloration was carried out using liquid chromatograph. Based on the literature data [7, 16] on phenol oxidation, phenol red was selected as model of dye in this investigation since phenol red has one phenol molecule in their structure. Several important phenol oxidation products were selected for quantitative analysis. The retention time of standard compounds used for examination of phenol red oxidation product are listed in Table 1
Fig. 7. Liquid chromatograms of the solution treated by pulsed discharge plasma
products produced with pulsed discharge plasma during the treatment process were hydrogen peroxide, hydroquinone,
p-benzoquinone, and X (unknown product). However, phenol was not detected during the plasma treatment. The intermediate products were found to be hydroquinone,
p-benzoquinone, and X, which were disappeared when the treatment time increased. Fig. 8. shows the variations of concentration of phenol and intermediate products. The concentration of hydroquinone and p-benzoquinone were increased with time up to 35 minutes and then decrease, and disappears after 60 minutes treatment. The concentration of X was found to be high during degradation of phenol red, but it was also disappeared when treatment time increased.
Fig. 8. Various by products at degradation of phenol red aqueous solution
Fig. 9. A possible mechanism of the degradation of phenol red
The possible mechanism of degradation of phenol red is shown in Fig. 9. From the results of analysis of intermediates indicated that the degradation of the dye possibly occurred by the attack of hydroxyl radicals. The central carbon is attacked
by hydroxyl radical to form hydroquinone and other intermediates. According to Adams [25] the oxidation of hydroquinone by hydroxyl radical proceeds to form
p-benzoquinone. The intermediate products formed are easily oxidized further under action of the radicals and oxygen. The ring opening lead to the formation of low molecular weight compounds, mainly organic acids. On the other hand, organic acids also oxidized through hydroxylation and hydration, finally forms carbon dioxide [26-27]
TABLE I. RETENTION TIME OF STANDARD COMPOUNDS IN THE PHENOL RED OXIDATION
Active species formation of pulsed plasma in water using ring-to-cylinder electrode was investigated. The results obtain in this study led to the following conclusions.
1. The streamer length was strongly dependent on the water conductivity and the voltage polarity. The maximum streamer length was obtained at the water conductivity of 10 mS/cm. The streamer length increased with increasing applied voltage for positive pulse than for negative pulse. 2. The decoloration rate of dyes by using positive pulse
streamer discharge in the present electrode system was high in compared with that by using positive pulse in the needle-to-plane electrode system.
3. The results results of analysis of intermediates indicated that the degradation of the dye possibly occurred by the attack of hydroxyl radicals.
4. The ring-to-cylinder system is considered to be suitable for the commercial application by scaling up the reactor with stacking rings and making the cylinder length longer
Acknowledgment
The author would like to thank Professor Takayuki Ohshima for his kind fruitful discussion and assistance. Thanks also due to all members of Sato Laboratory’s for their friendship, help and cooperation
.
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