Water Remediation Using Pulsed Power Discharge

Water Remediation Using Pulsed Power Discharge under Water with an Advanced Oxidation Process Katsuyuki Takahashi*, 1, 2, Koichi Takaki2, and Naoya Satta3 1

Shishido Electrostatic, LTD., 4-7-21, Chigasaki-higashi, Tsuzuki-ku, Yokohama, Kanagawa 224-0033, Japan Iwate University, Department of Electrical Engineering and Computer Science, 4-3-5 Ueda, Morioka, Iwate 020-8551, Japan 3 Iwate University, Department of Environmental Sciences for Sustainability, 3-18-8 Ueda, Morioka, Iwate 0208550, Japan 2

Abstract: In the present study, the degradation of organic contaminants by streamer discharge using a pulsed power generator under water is investigated. The experiments are conducted based on the decolorization of two dyes, Acid Red 1 and Acid Blue 74, and the decomposition of 1,4-dioxane. A gas-liquid separated reactor is developed and employed to achieve degradation with high energy efficiency. A tungsten wire electrode is placed in the gas phase, and a grounded 316 stainless steel wire is immersed in the water. The pulsed high voltage is generated by a magnetic pulse compression circuit and is applied to the wire electrode to generate streamer discharges in the gas region, which propagate into the bubble injected into the water. Oxygen and argon gases are injected to identify the dominant reactions of the degradation of organic contaminants. Acid Red 1 and Acid Blue 74 solutions are successfully decolorized by the discharges. The ozone produced by discharges in the gas region primarily decolorizes the dye solutions. The total organic carbon (TOC) of the 1,4-dioxane solution decreases due to discharge when argon is injected. The decrement rate of TOC does not increase through gaseous ozone injection or by discharges in the case of oxygen injection. These results show that the chemical species produced by discharges and by chemical reactions in the solution, such as hydroxyl and hydroperoxyl radicals, primarily decompose 1,4-dioxane. Iron ions dissolved by electrolysis enhanced the TOC decrement rate according to the Fenton reaction in acidic conditions.

Introduction Industrial waste waters are usually treated with conventional methods such as filtration, chemical oxidation, Fenton oxidation and bioremediation. These methods have various limitations (1). Filtration techniques are widely used because of their good performance; however, concentrated waste disposal remains an issue. Chemical oxidation using chlorine and ozone has been shown to produce hazardous byproducts such as trihalomethanes, aldehydes and bromate (2), (3). Ozone is a weaker oxidizing agent than the hydroxyl radical and a more selective oxidant. Ozone oxidation, Fenton oxidation and bioremediation are limited by slow kinetics. A pulsed discharge plasma for treating pollutants in water has attracted a significant amount of attention. The pulsed discharge makes it possible to instantaneously produce a non-thermal plasma in which various chemical species, such as hydroxyl radicals, exist. These species play an important role in degrading organic chemical compounds (4), (5). The production of discharge plasmas with large volume discharges requires pulsed power technologies *Corresponding author; E-mail: [email protected]

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because the discharges in water require a high electric field. The formation mechanism of the discharge in water is considered to be the initial discharge that starts in a small bubble on the electrode surface and propagates into the water. The current in the high electric field region causes joule heating and gasification of the liquid, which forms bubbles (6-8). The injection of gas bubbles into the vicinity of the electrode improves the energy balance to produce chemically active species because of the reduced energy loss caused by the gasification process (9), (10). The energy efficiency for discharge generation and propagation in water is significantly affected by the conductivity of the water (5), (8), (11), (12). In general, the volume of the discharge decreases as the conductivity of the water increases at the same input energy. As a result, the energy efficiency of the water treatment by discharge decreases to a low value when the water is highly conductive. To solve this problem, gas-liquid separated reactors have been proposed and investigated experimentally (13-15). In the proposed reactor, the discharges are generated in the gas region near the water surface and produce various active ISSN 1203-8407 © 2012 Science & Technology Network, Inc.

K. Takahashi et al. SI0

PT ( 1:30)

SI1

SI2

SI4

iO

20

C0 = 2.24 uF

C1 = 2 nF

vO C 2 = 2 nF

C3 = 0.7 nF

Figure 1. Schematic diagram of the magnetic pulse compression circuit.

species, such as ozone and hydroxyl radicals. These chemical species dissolve into the liquid and react with organic compounds. The objective of the present work is to identify the chemical species that are produced by the discharges during the degradation of chemical compounds. Two dyes and 1,4-dioxane are employed as chemical compounds to evaluate the reactor performance under various gas injection conditions. The influence of the material of the electrode is also investigated.

Voltage [kV]

Charger ~ 800V

vO

v C1

SI3

10 v C2

0 vC3

No load

-10 0

1

3

4

Figure 2. Typical output voltage waveform of the magnetic pulse compression circuit without a connection to the reactor. Gas outlet 160mm

Upper part (liquid phase)

vO

SUS 316 Wire(GND) 25 mm

Experimental Setup and Procedure A schematic diagram of the magnetic pulse compression circuit (Suematsu Electronics CO., LTD., MPC-3000S SP, Japan) is shown in Figure 1. The capacitor C0 is charged to the charging voltage by the charger. The energy stored in C0 is transferred to C1 through the pulse transformer PT. The pulsed voltage is produced at the secondary side of the pulse transformer. The pulsed voltage is compressed by saturable inductors (SI1, SI2 and SI4) and capacitors (C2 and C3). SI3 is connected in parallel with C3 to shorten the pulse width of the output voltage (vO). Figure 2 shows typical waveforms of the output voltage (vO) and the voltages on capacitors C1 (vC1), C2 (vC2) and C3 (vC3) when the reactor is not connected to the circuit.. The pulse compression procedure is observed clearly. The pulse width of vC1 is 1.19 µs and is compressed to a vO of 130 ns using magnetic compression processes. The output voltage has a peak value of 21 kV, and its rise time is 57 ns. The LC oscillation and the reflection due to the impedance mismatch cause the voltages to oscillate. The pulse repetition rate is maintained at 250 pps in the experiment. The polarity of the applied voltage is fixed as positive. The energy into the reactor per pulse is calculated by integrating the electric power obtained by the output voltage and current over time. A schematic diagram of a gas-liquid separated reactor (14) is shown in Figures 3 (a), (b). Oxygen (O2) and argon (Ar) gases are injected into the lower part of the reactor through the tube with a gas flow rate of 4.5 L/min. The separator consists of a polyethylene film (Figure 3 (b)), which is placed between the lower part and the upper part of the reactor to separate the

2 Time [s]

9mm Separator Sampling port

60 mm

W wire (high-voltaged )

Lower part (gas phase)

Gas injection (4.5 L/min)

330 mm

(a) Front view Polyethylene film (thikness : 0.1mm) Hole diameter: 0.7 mm 15 mm 170 mm Dist ance bet ween holes: 2.5mm

(b) Separator Figure 3. Schematic diagram of the gas-to-liquid discharge reactor: (a) front view and (b) separator.

gas and the liquid regions (Figure 3 (a)). The gas is injected into the liquid region through the holes on the separator. The hole diameter is approximately 0.7 mm, and the distance between center of the holes is 2.5 mm. A 316 stainless steel (SUS 316) wire with a diameter of 0.2 mm is immersed in the solution and used as grounded electrode. A tungsten wire with a diameter of 0.2 mm is placed in the gas region. The gap length between the tungsten wire and the separator is 9 mm. The pulse voltage vO generated by the magnetic pulse compression circuit is applied to the wire electrode in the gas region to generate streamer discharge in the gas region, which propagates into the bubble injected into the water. Figure 4 shows still photograph images of the discharge generated in the gas and liquid regions. The maximum applied voltage, pulse repetition rate and exposure time are 21 kV, 250 pps and 50 ms, respectively. The Ar gas is injected, and 100 mL of purified water is put into the liquid region. From the J. Adv. Oxid. Technol. Vol. 15, No. 2, 2012

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Absorbance [A.U.]

1

(a) Liquid region

vOmax = 31 kV Gas:O 2 SUS 316 wire Acid red 1

0.5

0 200

(b) Gas region

images shown in Figure 4, it is clear that the streamer propagates from the wire electrode into the bubble through the tiny holes of the separator. Two dyes and 1,4-dioxane solutions are employed as the organic compounds in the experiments. Acid Red 1 (Azophloxine, CAS No. 3734-67-6) and Acid Blue 74 (Indigo carmine, CAS No. 860-22-0) are dissolved into 100 mL of purified water at a concentration of 20 mg/L. The absorbance of the dye solutions is measured by a spectrometer (Hitachi high-technologies Co., U-1800), and the decolorization rate is obtained from the following equation: (Absorbance (initial) - Absorbance (treated))  100 Absorbance (initial) (1)

Here, the absorbance is obtained at a wavelength of 531 nm in the case of Acid Red 1 and at a wavelength of 610 nm in the case of Acid Blue 74. 1,4Dioxane (CAS No. 123-91-1) is dissolved into 100 mL of purified water at a concentration of 10 mg/L. The concentration of total organic carbon (TOC) is measured by a TOC analyzer (Shimadzu Co., TOC-V CSH), and the TOC decrement rate is obtained from the following equation: TOC decreament rate [%] 

(2)

The solutions are at room temperature, and the temperature does not increase during the experiments. The concentrations of hydrogen peroxide (H2O2) and total iron are measured using PACKTEST (KYORITSU CHEMICAL-CHECK Lab., Co. WAK-H2O2) and an atomic absorption flame emission spectrophotometer (Shimadzu Co., AA-6200), respectively. 367

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800

1 vOmax= 16 kV Gas:Ar SUS 316 wire Acid red 1

0.5

0 200 (b) Ar injection

Decolorization rate [%] 

(TOC (initial) - TOC (treated))  100 TOC (initial)

400 600 Wavelength [nm]

(a) O2 injection

Absorbance [A.U.]

Figure 4. Typical photograph of streamer discharges: (a) in liquid and (b) in gas regions.

Treatment time [s] 0 20 40 60 80 120 180

Treatment time [min] 0 5 10 15 20 30 40

400 600 Wavelength [nm]

800

Figure 5. Light absorption spectra of Acid Red 1 solution at various treatment times: (a) in the case of O2 injection and (b) in the case of Ar injection.

Results and Discussions Decolorization of the Dye Solutions To evaluate the energy efficiency for decolorization, the input energy into the reactor per pulse is controlled by the maximum applied voltage (vOmax) for two injected gas species and is fixed to 13.5 mJ. The values of vOmax in the case of O2 injection and Ar injection are 31 and 16 kV, respectively. Figure 5(a) shows the absorbance spectra of the Acid Red 1 solution after various treatment times in the case of O2 injection. Acid Red 1 has a double peak with wavelengths of 531 and 505.5 nm because of the azo chromophore and the conjugated electron structure, which is responsible for the red color of the solution. These peaks decrease as the treatment time increases. Figure 6 shows the decolorization rate as a function of treatment time for two different injected gas species. The values of the decolorization rate are determined by equation (1) with the absorbance at wavelength 531 nm obtained by the light absorption spectra of the dye solution, as shown in Figure 5. The decolorization rate reaches 100%, and the solution is decolorized

K. Takahashi et al.

100

80 60 40 20 0 0

Gas vOmax [kV] Ar 16 O2 31 SUS 316 wire Acid red 1

10 20 30 Treatment time [min]

40

Figure 6. Decolorization rate of Acid Red 1 solution as a function of treatment time with two different injected gas species.

Absorbance [A.U.]

1 0.5 0 200

80 60 Gas vOmax [kV] Ar 16 O2 31 SUS 316 wire Acid blue 74

40 20 0 0

10 20 30 Treatment time [min]

40

Figure 8. Decolorization rate of Acid Blue 74 solution as a function of treatment time with two different injected gas species.

E50 [J] = t50 × 60 × NR × JP

2 1.5

Decolorization rate [%]

Decolorization rate [%]

100

vOmax= 31 kV Treatment time [s] Gas:O 2 0 SUS 316 wire 2 Acid blue 74 4 8 12 16 20

400 600 Wavelength [nm]

800

Figure 7. Light absorption spectra of Acid Blue 74 solution at various treatment times in the case of O2 injection.

completely, after 3 min of treatment in the case of O2 injection. The solution is also fully decolorized after 40 min of treatment in the case of Ar injection. Figure 7 shows the absorbance spectra of the Acid Blue 74 solution for the various treatment times. The absorbance peak at a wavelength of 610 nm is due to the carbon-carbon double bond in the H-chromophore of Acid Blue 74, which is responsible for the blue color of the solution. The absorbance decreases as the treatment time increases. Figure 8 shows the decolorization rate as a function of treatment time for the two different injected gas species. The solution is fully decolorized after 0.25 min of treatment in the case of O2 injection and after 40 min of treatment in the case of Ar injection. Table 1 shows the treatment time (t50), total input energy into the reactor (E50) and energy efficiency for decolorization (G50 and η50) needed for 50% decolorization of the two dyes for the two injected gas species. The values of E50 and η50 are obtained from the following equations (16):

-3

(3), 3

G50 [g/kWh] = 5 × 10 / (E50/3.6 × 10 )

(4),

η50 [mmol/kWh] = G50 × 103/MW

(5),

where NR is the pulse repetition rate per a second (250 pps), JP is the input energy into the reactor per pulse (13.5 mJ) and MW is the molecular weight of the dyes (Acid Red 1 has a MW of 509.42 and Acid Blue 74 has a MW of 466.35). The value of η50 in the case of O2 injection is one or two orders of magnitude than that of Ar injection. The discharges inside the bubble generate various active species, such as ozone (O 3) and hydroxyl radicals (•OH), through the reactions shown in Table 2 (17-27), where M is the third collision partner; it takes part in energy absorption, but does not react chemically. Equations (1)-(9) and Equations (28), (31) and (33) in Table 2 are major reactions that produce O3 and •OH, respectively. These species are produced by discharges, dissolved into the solution from the bubble surface, and produce various active species in the solution through the reactions shown in Table 3 (12), (28-38). The reactivity of hydroperoxyl radicals (HO2•), superoxide anion radicals (O2-•) and hydrogen radicals (•H) with organic compounds is very low (28), (40). These species produce •OH and O3 through reactions (40), (70), (88), (93) and (95), shown in Table 3, which also react with dyes. However, O 3 and •OH are consumed by various reactions in water because of their high reactivity, as shown in Table 3. Therefore, it is possible that highly reactive chemical species such as O3 and •OH produced by the discharge over the solution surface and transported into the solution decolorize dye solutions. The dye solutions are decomposed into major intermediate products, such as isatin sulfonic acid in the case of indigo carmine (37), and CO2 by chemical species such as O3 and •OH. CO2 is ejected from the solution through gas bubbling. J. Adv. Oxid. Technol. Vol. 15, No. 2, 2012

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K. Takahashi et al. Table 1. Treatment time (t50), total input energy into the reactor (E50) and energy efficiency (G50 and η50) at 50% decolorization of two dyes with two injected gas species.

Dyes Acid Red 1 Acid Blue 74

Injected Gas O2 Ar O2 Ar

t50 [min] 0.45 7.16 0.03 6.21

E50 [J] 91.1 1450 6.08 1258

G50 [g/kWh] 39.5 2.48 593 2.86

η50 [mmol/kWh] 77.0 4.84 1271 6.14

Table 2. Chemical reactions in plasma.

(1)

e* (>8.4 eV) + O2 → •O(1D) + •O(3P) + e

(2)

e* (>6.1 eV) + O2 → •O(3P) + •O(3P) + e

(3) (4)

e* + O2 → O2(a1g) + e e* + O2 → O2(b1g) + e

(5)

e* + O2 → •O(3P) + O• + e

(6)

e* + O2 → O2+ + 2e

(7) (8) (9) (10) (11) (12) (13) (14) (15)

O(1D) + O2 → •O(3P) + O2(A1g) • O(1D) + O2 → •O(3P) + O2(B1g) • O(1D) + O2 + M → O(3P) + M • O(3P) + O2 + M → O3 + M • O(3P) + •O(3P) + M → O2 + M e* + O3 → •O(3P) + O2 + e O3 + •O(1D) → O2 + O2 O3 + •O(1D) → •O(3P) + •O(3P) + O2 O3 + •O(1D) → O2 + O2(a1g) •

(16) O3 + •O(3P) → O2 + O2(a1g) (17) O3 + •O(3P) → O2 + O2 (18) O3 + O2(b1g-) → •O(3P) + O2 + O2 (19) O3 + O2(a1g) → •O(3P) + O2 + O2 (20) O2(a1g) + M → O2 + M (21) O2(b1g-) + M → O2 + M

The decolorization of Acid Red 1 and Acid Blue 74 is caused by the cleavages of the N-N double bond of the azo chromophore and the C-C double bond of the H-chromophore, respectively. O3 is generated in the case of O2 injection according to equations (1)(10) in Table 2. The O3 concentration of 320 ppm is measured with an ozone analyzer (Ebara Jitsugyo CO., LTD., PG-320L) at the gas outlet of the reactor. It is well known that ozone reacts with C-C and N-N double bonds rapidly. However, the reaction of •OH with dyes is at least four orders of magnitude faster than that of O3. The solubility of •OH in water is much higher than that of O3 because the value of Henry’s law constants for •OH (30 M/atm at 298 K) is approximately 2700 times higher than that for O3 (34). The radical attack takes places not only on the azo and H-chromophores but also directly on the aromatic rings in different 369

J. Adv. Oxid. Technol. Vol. 15, No. 2, 2012

(22) •O(1D) + H2O → •OH + •OH (23) •O(1S) + H2O → •OH + •OH (24) •H + O2 + M → HO2 + M (25) O3 + •OH → HO2 + O2• (26) (27) (28) (29)

O3 + •H → •OH + O2• O3 + e → O3• O3 + •H → HO•3 H2O + e* (>6.4 eV) → •OH + •H + e

(30) H2O + e* (>12.62 eV) → H2O+ + 2e (31) H2O+ + H2O → H3O+ + •OH (32) e* (>11.5 eV) + Ar → Ar(3P2) + e (33) e* (>15.5 eV) + Ar → Ar+ + 2e (34) Ar(3P2) + H2O → Ar + •OH(A2+) + •H (35) (36) (37) (38) (39) (40) (41) (42)

OH(A2+) + H2O → •OH(X2Π) + H2O Ar+ + H2O → Ar + H2O+ 2HO2• + Ar → H2O2 + O2 + Ar • H + •OH + M → H2O + M • H + HO2• → H2O + •O • H + HO2• → H2 + O2 • OH + •OH → H2O2 • OH + H2O2 → H2O + HO2• •

positions (41). Figure 5(b) shows the absorbance spectra of the Acid Red 1 solution at various treatment times in the case of Ar injection. The spectrum in the visible range shifts to red as the treatment time increases. This shift is caused by intermediates with an extra OH-group attached to one of the aromatic rings of the dye molecule (41), (42). Moreover, the lifetime of •OH in water is on the order of nanoseconds (43). These results show that the ozone produced by the discharges inside the bubbles primarily decolorizes the dye solutions. The value of η50 for the decolorization of Acid Red 1 is approximately 16 times higher than that of Acid Blue 74 in the case of O 2 injection, as shown in Table 1. The reactivity of an azo group with ozone is very low compared to that of olefins (44), which is one of the reasons that Acid Blue 74 is more readily decolorized than Acid Red 1.

K. Takahashi et al. Table 3. Chemical reactions in water.

Reaction (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34) (35) (36) (37) (38) (39) (40) (41) (42) (43) (44) (45) (46) (47)

k [1/MS] 

2H2O → H3O + OH H3O+ + OH → 2H2O H2O → •OH + •H 2H2O → H2O2 + H2 H2O → H+ + eaq + •OH H2O + e → •H + •OH 2H2O + 2e → 2H2 + OH H+ + OH ↔ H2O H+ + OH → H2O H+ + HO2 → H2O2 H2O2 → H+ + HO2 H2O2 ↔ H+ + HO2 O3 → •O(3P) + O2 • O(3P) + O2 → O3 • O(3P) + OH → HO2 • O(3P) + H2O2 → •OH + HO •2 • O(3P) + HO2 → •OH + O2• 3/2O2 → O3 O3 + HO2 → HO•2 + O3• O3 + O2• → O2 + O3• O3 + •OH → O2 + HO•2 O3 + OH → HO•2 + O•2 O3 + OH → •OH + O3• O3 + HO2 → HO•3 + O•2 O3 + H2O → HO+3 + OH ↔ 2HO•2 O3• + H+ → HO•3 O3 + OH → HO2 + O2 O3 + •OH → HO•4 HO•3 → O3• + H+ HO•3 → O3• + H+ HO•3 ↔ •OH + O2 HO•4 → HO•2 + O2 2HO•4 → H2O2 + 2O3 HO•4 + HO •3 → H2O2 + O3 + O2 O3• + H2O → •OH + O2 + OH O3• + •OH → O2• + HO•2 O3• + •OH → O3 + OH H2O2 + O3 →•OH + OH H2O2 + eaq →•OH + OH H2O2 + •H →•OH + H2O O• + H2O →•OH + OH O• + HO2 → O2• + OH O• + H2 → •H + OH O• + H2O2 → O2• + H2O O• + O2• → 2OH + O2 O• + O• + H2O → HO2 + OH O• + O3 → O2• + O2 +

-9

2.35*10 3*109 9.25*10-10 8.0*10-8 2.35*10-9

(48) (49) (50) (51) (52) (53) (54) 1.4*1011 (55) pKa = 13.77 (56) 2.6*1010 (57) 3.7*10-2 (58) pKa = 11.6 (59) 3.0*10-6 (60) 4.0*109 (61) 8 4.2*10 (62) 1.6*109 (63) 5.3*109 (64) -7 1.69*10 (65) 5.5*106 (66) 1.6*109 (67) 8 1.1*10 (68) 7.0*10 (69) (70) 5.5*106 (71) (72) 10 5.2*10 (73) 4.0*101 (74) 2.0*109 (75) 2 2.3*10 /s (76) pka = 8.2 (77) 1.1*105 /s (78) 2.8*104 /s (79) 5*109 (80) 9 5*10 (81) 2.5*101 (82) 6.0*1010 (83) 9 2.5*10 (84) (85) 10 1.3*10 (86) 5.0*107 (87) 9.4*107 (88) 8 4.0*10 (89) 8.0*107 (90) 4.0*108 (91) 8 6.0*10 (92) 1.0*109 (93) 8 1.0*10 (94)

Reaction

k [1/MS]

O + O3 → 2O2 7.0*108 O2• + H+ → HO•2 5.0*105 • + 2O2 + 2H → H2O2 O2• + O2• + 2H2O → H2O2 + 2OH + O2