Abstractâ Decomposition of environmental contaminants such as phenol containing in water was investigated using a pulsed high voltage gas phase discharge ...
Aqueous phenol decomposition by pulsed discharge on water surface. Masayuki Sato*, Tsuyoshi Tokutake*, Takayuki Ohshima*, and Anto Tri Sugiarto** *Department of Biological and Chemical Engineering, Gunma University **Research & Development Center, Indonesian Institute of Sciences
Abstract— Decomposition of environmental contaminants such as phenol containing in water was investigated using a pulsed high voltage gas phase discharge on the water surface. Discharge characteristics varied with varying electrode distance between needle tip and water surface, and composition of the surrounding gas. When the electrode distance was decreased, the discharge modes changed from corona to streamer, and then spark discharge in the case of very close to the water surface. The streamer discharge channels spread over the water surface. Argon gas was most effective to decompose phenol in water than the case using oxygen or air for the surrounding gas. Keywords- environmental remediation, organic contaminant, phenol decomposition, water surface plasma, water treatment
I.
INTRODUCTION
Recently, electrical treatment by applying high voltage in the aqueous solution, such as corona streamer discharge [1 - 4], spark discharge [5], has been used to decompose trace
Pulsed discharge in water
contaminants. Most of these methods focused on hydroxyl radical production directly in the aqueous solution. This is because the hydroxyl radical is a very powerful, nonselective oxidant that has the potential to kill bacteria and to oxidize organic compounds. The high voltage discharge method is a plasma process based on pulsed power technology. This method injects energy into an aqueous solution through a plasma channel formed by a high voltage pulsed discharge between two submerged electrodes. Water disinfection and the degradation of organic water pollutants using traditional UV-activated hydrogen peroxide and/or ozone has been reported. However, in recent years, a pulsed high voltage process for the treatment of hazardous chemical wastes in water has been developed [4, 6, 7]. Due to collisions of high-energy electrons with molecules, the intense electrical discharge dissociates water molecules to yield active hydroxyl radicals. These radicals combine with almost any organic chemical compound in a very efficient manner. These
Pulsed discharge in water with gas bubbling
Pulsed streamer discharge in gas on water surface
HV
HV
HV Contaminants
close to the target
HV
Gas Plasma formation
hard
Dc/ac corona discharge in gas on water surface
Radicals / Plasma region
easy far from the target
Fig. 1 Illustration of discharge states of plasma in water ((a) and (b)) and on the water surface ((c) and (d)).
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processes have a synergistic effect in the degradation of organic compounds and also in sterilization. Therefore, this method is considered to be a promising alternative for the treatment of pollutants. Depending on the degradation region, pulsed discharge processes can be conveniently grouped into two categories of effects; localized and extended. Localized effects occur in the immediate vicinity of the plasma channel and include thermal oxidation within the plasma channel, vacuum UV photolysis at the surface of the plasma channel, and supercritical oxidation within the subsequent steam bubbles. On the other hand, extended effects are due to the radical reactions, where active species generated at the vicinity of the plasma channel may expand to the circumference. Some researchers reported on the decomposition of organic contaminants in water with plasma chemical reaction using pulsed discharge in water by submerged electrodes, or dc/ac discharge in gas phase on the water surface. Typical discharge types for the decomposition of organic contaminants in water are illustrated in Fig. 1. Direct discharge in water produces some kinds of active species (mainly hydroxyl radical) that react with organic materials into carbon dioxide and water through the intermediate compounds. On the other hand, corona discharge in air on the water surface also produces active species including ozone, hydroxyl radical, NOx, and so on, which are depending on the components of circumstances. They dissolve into the water through the surface layer to react with organic materials in water, but lifetime of the hydroxyl radical is too short to reach to the water surface. The main cause of the reaction to decompose contaminants could be the action of long life active species. Therefore, it seems to be important that plasma is generated near the water surface. In the present study, high voltage pulsed streamer discharge in gas was formed on the water surface to decompose organic contaminants (such as phenol) contained in water. As illustrated in Fig. 2, pulsed discharge from the needle tip in the gas phase on the water surface generates high speed electrons, UV light, active species, and shock waves. Ozone produced by the discharge may dissolve into water through the water surface, then it was decomposed to some kinds of radicals or hydrogen peroxide. High-speed electrons may dissociate water
UV Photon
⋅H
O3
H2O2 ⋅OH ⋅O
e + O2 → 2O + e O + O2 → O 3 O3 + H2O → O2 + H2O2 H2O2 → 2⋅OH H2O → ⋅H + ⋅OH 2H2O → H2O2 + H2 ⋅OH, O3 + A → [decomposed A], etc... Fig. 2 Illustration of a possible mechanism of water purification using pulsed discharge in gas phase on the water surface.
molecules then produce radicals. Many chemical reactions may occur by water surface plasma, which may decompose organic contaminants in water. The experimental investigation on the observation of the discharge state and phenol reduction was performed to discuss the ability of this newly proposed discharge system. II.
EXPERIMENTAL APPARATUS AND PROCEDURE
Schematic of the experimental apparatus is shown in Fig. 3. Pulsed streamer discharge occurs between the tip of the needle electrode (1 mm diameter, stainless steel) and ground electrode (submerged in water, 1 mm diameter stainless steel wire with various shape), where the positive pulsed high voltage is applied to the needle. Shapes of the submerged ground electrode were ring with the radius of 20 mm, semicircle, and
HV Pulsed Powe r Source
High Voltage probe
HVP
G a s in G as o u t
Oscilloscope
Current probe
Peristaltic pump
Magnetic stirrer
Thermostat
A
Upper view of reactor
Fig. 3 Schematic diagram of experimental apparatus.
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straight line. The electrode distance (distance between needle tip and the water surface) was varied from 5 mm to 20 mm. The reactor was made of Plexiglas having outer diameter, inner diameter, and height are 60 mm, 50 mm, and 57 mm, respectively. The sample liquid was circulated by peristaltic pump with a flow rate of 100 mL/min, in which the water surface level was kept constant at 5 mm depth. High voltage pulse generator with a stationary spark gap was used. The pulse forming capacitor, pulse repetition frequency, and applied voltage were 1 nF, 100 Hz, and 25 kV, respectively. The pulse voltage and current were measured using oscilloscope (Tektronix TDS3032) with a high-voltage probe (Tektronix P6015A) and a wide band current transducer (Pearson Electronics M411), respectively. Average current was measured by installing ammeter with connected parallel capacitor into the ground line of the reactor. Total volume of the sample liquid was 200 mL including 50-ppm phenol dissolved in distilled water. Reduction of the phenol concentration was analyzed using high performance liquid chromatography (Shimadzu, LC-9A) with every 10 minutes. Air, argon, and oxygen were used as surrounding gas to observe discharge characteristics. Consumed electrical energy in the reactor was calculated by following equation:
W = U peak I average t / V where W is consumed energy [J/mL], U is pulse voltage [V], I is average current [A], t is treatment time [s], V is volume of sample liquid [mL], respectively.
III.
RESULTS AND DISCUSSION
A. Effect of surrounding gases Fig. 4 shows the effect of surrounding gas, oxygen, air, and argon, on the phenol decomposition with elapsed treatment time. The decomposition rate increased with increasing treatment time in all cases. In the case of oxygen, the decomposition rate was higher than that in the case of air, because the electrical discharge in oxygen produced much ozone than that in air. Generated ozone may dissolve into the water through the surface layer, then react with phenol by hydroxyl radical or other active species converted from the dissolved ozone. In the case of argon, the decomposition rate was the highest than others. Discharge states in surrounding gases of oxygen and argon are shown in Fig. 5. These photographs were taken at a single pulsed discharge. Discharge in argon showed many streamer paths than the case of oxygen, where the pulse energy was the same in both cases because all of the electrical charges stored in the pulse forming capacitor were discharged. When argon was used as a surrounding gas, the discharge was easy to occur and reached to the water surface even in the case of 10 and 20 mm of electrode distances, then spread widely along the whole surface. On the other hand, bright streamer discharge paths could not be observed in the case of 10 and 20 mm electrode distance in oxygen, which could be weak corona or streamer discharge (cannot be seen in the photos). The discharge path, spread on the water surface having high electron energy, may produce a lot of active species due to
Ar
O2
7.5 mm
10 mm
20 mm
HVP
Ground
Fig. 5 Photographs of pulsed discharge on distilled water surface in the case of different surrounding gases of argon and oxygen with varying electrode distance between needle tip and water surface using ring shape ground electrode, where gas flow rate: 1 L/min, applied voltage: 25 kV.
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B. Effect of electrode distance The effect of distance between the tip of the needle electrode and the surface of water is shown in Fig. 6, where vertical axis shows the decomposition rate of phenol after 60 minutes treatment using three kinds of surrounding gases. In the cases of oxygen and air, there were peaks at around 6 to 7 mm of electrode distance. As shown in figure 5, it was observed that the discharge paths did not reach to the water surface in the cases of oxygen and air when the electrode distance is over 8 mm. Active species were generated far from the phenol molecules in water that resulted in the decrease of the decomposition rate with the distance of electrode over 8 mm. On the other hand, decomposition rate increased and kept in a plateau with increasing electrode distance in the case of argon. It was because the discharge paths reached to the water surface even the electrode distance became wider. The decomposition rate in argon gas was almost 80 %, which was not dependent on the electrode distance in the range from 5 to 20 mm. In argon, the bright streamer paths hit the water surface and the high energy electrons dissociate water molecules into some kinds of active species, which could be the main cause to keep high decomposition rate even in the case of wide electrode distance. C. Effect of ground electrode shape The discharge paths tend to stretch toward the ground electrode, as shown in Fig. 7. In the case of ring electrode, the streamer spread all over the water surface. On the other hand, in the case of ground electrode with a straight-line shape and with a semicircular shape, the discharge paths extended toward the water surface along the ground line. The region of the discharge paths affected to the
Ring
Decomposition ratio after 60 min [-]
direct dissociation of water molecules, which may decompose phenol in water efficiently.
1
Gas flow rate: １ L/min
0.8
▲ : Argon ● : Oxygen ■ : Air
0.6 0.4 0.2 0
0
10
20
Electrode distance [mm] Fig. 6 Effect of electrode distance on decomposition ratio at 60 min with three kinds of surrounding gases, where initial phenol concentration: 50-ppm, gas flow rate: 1 L/min. decomposition rate of phenol. As shown in Fig. 8, decomposition rate varied with varying ground electrode shape. Between three kinds of electrodes, the ring shape accomplished higher decomposition rate than the others. It was because the discharge paths went toward the directions of ground electrode, so that they spread widely in the case of ring shape. D. Effect of gas flow rate A mechanism of phenol decomposition in water due to gas phase plasma has been considered that the active species generated in gas phase (such as ozone) by electrical discharge
Semicircle
Straight
Fig. 7 Photographs and illustrations of pulsed discharge on distilled water surface in argon with varying ground electrode shape, where electrode distance: 7.5 mm, applied voltage: 25 kV.
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Decomposition ratio[-]
1
Ground electrode shape ▲ ： Ring ● ： Semicirle ■ ： Strait
Argon gas ： 1 [L/min]
0.8 0.6 0.4 0.2 0
0
10
20
30
40
50 60
Treatment time [min]
(a)
Fig. 8 Influence of ground electrode shape to decomposition ratio of phenol. plasma dissolved into water through the liquid-gas interface, then they decomposed to radicals and reacted with organic materials contained in water. Discharge mode in the present experiment could be somewhat different from the ordinary corona discharge in gas phase. The bright streamer discharge paths reached to the water surface and spread all directions on the surface. Fig. 9 shows the effect of gas flow rate (argon and oxygen) on the phenol decomposition rate. The gas flow rate was large enough to blow off any active species compared with the figures shown previously (1 L/min). In the case of argon, there was almost no change in decomposition rate with increasing gas flow rate. A mechanism of phenol decomposition in argon was considered that directly generated active species in the water phase due to dissociation of water molecules by high speed electrons could contribute to the reaction without active species in gas phase. In the case of oxygen, phenol decomposition decreased with increasing gas flow rate. It was considered that the active species generated in the gas phase by discharge plasma were blown off from the reaction chamber before reaching to the water surface. From the figure, it was estimated in the case of oxygen that about 30 to 40 percent phenol was decomposed by reactions of active species in gas phase plasma and 60 to 70 percent could be by the action of directly generated active species at the liquid-gas interface by hitting the high-speed electrons. IV.
CONCLUSION
Pulsed high voltage discharge on the water surface was investigated to decompose phenol with varying reactor shapes and experimental conditions. When argon was used for surrounding gas, highest decomposition rate was obtained than the cases of oxygen and air. The ground electrode shape influenced to the decomposition rate. A ring shaped electrode was the most effective than strait and semicircular shape
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(b) Fig. 9 Effect of gas flow rate on decomposition rate, where Y axis shows decomposition rate after 60 minutes treatment; pulse voltage, pulse frequency, and electrode distance are 25 kV, 100 Hz, and 7.5 mm, respectively. electrodes. When argon flow rate was increased from 1 to 40 L/min, there was no change in phenol decomposition rate, on the other hand, in the case of oxygen, the decomposition rate decreased with increasing gas flow rate. REFERENCES [1] Sun, B., Sato, M., and Clements, J. S. J. Electrostatics 1997, 39, 189-202. [2] Sato, M., Ohgiyama, T., and Clements, J. S. IEEE Trans. Ind. Appl. 1996, 32, 106-112. [3] Joshi, A. A., Locke, B. R., Arce, P., and Finney, W. C. J. Hazard. Materials 1995, 41, 3-30. [4] Sato, M., Sun, B., Ohshima, T., and Sagi, Y. J. Adv. Oxi. Tech., 1999, 4, 339-342. [5] Sun, B., Sato, M., Harano, A., and Clements, J. S. J. Electrostatics, 1998, 43, 115-126. [6] Willberg, D. M., Lang, P. S., Höchemer, R. H., Kratel, A., and Hoffmann, M. R. Environ. Sci. Technol. 1996, 30, 2526-2534. [7] Sharma, A. K., Locke, B. R., Arce, P., and Finney, W. C. Hazard. Was. Hazard. Mater. 1993, 10, 209-218.
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