Plasma Chemistry and Plasma Processing, Vol. 5, No. 2, April 2005 (© 2005) DOI: 10.1007/s11090-004-8839-0
Degradation of Organic Contaminants in Water by Pulsed Corona Discharge Yue Zhong Wen,1,3 Hui Jun Liu,1 Wei Ping Liu,1 and Xuan Zhen Jiang2 Received January 30, 2004; revised July 19, 2004
Degradation of organic contaminants in water by high-voltage pulse discharges was investigated. The effects of gas flow rate and liquid conductivity on the degradation of 4-chlorophenol were studied. With the increase of time, the liquid conductivity increases, which have an important effect on discharge. Meanwhile, with the increase of time, the concentration of H2 O2 increases. Addition of 200 mg/L H2 O2 , the conversion of 4-chlorophenol was greatly enhanced. This may be due to the synergistic effect of high-voltage pulsed discharge and H2 O2 . Also, it was found that the influence of quantity of TiO2 or CuO on degradation of acetophenone is not apparent, maybe the presence of metal oxides hinders the formation of plasma channel due to increase of collusions between metal oxides and oxygen. KEY WORDS: Pulsed corona discharge; degradation; H2 O2 ; organic contaminants in water.
1. INTRODUCTION Over the last several years, the pulsed corona discharge in water has been shown to be effective for the degradation of organic contaminants.(1,2,3) The pulsed corona discharge makes it possible to produce a plasma, which generates various active species (• OH, • O, • H, • HO , O− etc.). The pulsed discharge used in the liquid is usually either 2 2 an arc discharge or a spark discharge when gas is not introduced. The situation changes if gas bubbles are present in the liquid. In that case streamer discharges may appear as well. When a streamer discharge is created, following several effects of the streamer process occur simultaneously: production of various free radicals, aqueous electrons and ultraviolet radiation. These effects play an important role in destroying harmful compounds.(4,5) 1 Institute
of Environmental Science, Zhejiang University, Hangzhou, 310029, Zhejiang, P.R. China. 2 Department of Chemistry, Zhejiang University, Hangzhou, 310027, Zhejiang, P.R. China. 3 To whom correspondence should be addressed. E-mail:
[email protected] 137 0272-4324/05/0400-0137/0 © 2005 Springer Science+Business Media, Inc.
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Recently, advanced oxidation method using pulsed corona discharges combined with ozone has been improved markedly for the remediation of waste water, which results in effective mineralization of aqueous phase wastes.(6,7) However, the efficiency of the mass transfer of gaseous ozone into the aqueous phase was low. An advanced oxidation processes (AOPs) employing pulsed corona discharge combined with hydrogen peroxide has been explored.(8) As compared with ozone, an important advantage of hydrogen peroxide is its homogeneous reactions with organic compounds in water, thus the degradation efficiency increases. The combination of activated carbon particles with the pulsed corona discharge leads to enhanced power efficiency and possible surface-phaseinduced catalytic chemical reactions.(9) Mikula investigated the pulsed discharge in suspensions of TiO2 powder.(10) Prior results(11,12) have shown that the photolysis in pulsed corona discharges plays an important role in degradation of organic contaminants in water. It is well known that photocatalytic oxidation is widely used for eliminating organic pollutants in waters and wastewaters. Photoassisted degradation mediated by semiconductor has been reported to be a progressive alternative to mineralization of organic pollutants. Thus, the combined technique of pulsed corona discharges and hydrogen peroxide, pulsed corona discharges and catalyst need to be explored further. The objective of the present work was to investigate the effect of various reaction parameters on the degradation of organic contaminants in water by high-voltage pulse corona discharge and the study of synergistic effects when the pulsed corona is combined with hydrogen peroxide and catalyst.
2. EXPERIMENTS The 4-chlorophenol, acetophenone, titanium dioxide powder (TiO2 rutile), CuO powder used in this study were reagent grade and all experimental solutions were prepared with deionized water. Oxygen gas was of high purity. Oxygen from gas cylinder was passed through silica gel, activated carbon and molecular sieve columns for purification. On degradation of organic contaminants in water by pulsed corona discharges, a laboratory prototype of the treatment system was designed and constructed as shown in Fig. 1. The degradation system consists of two major components: a high voltage pulse generator and a corona reactor. The reactor is composed of a cylindrical Pryex tube (21.7 mm I.D. and 300 mm length). A stainless steel hypodermic needle (common No.7 injection needle) was inserted into the reactor from bottom, through
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Fig. 1. The schematic diagram of the experimental set-up. R−1 M; L−1 µH;C−0.1 µF; Power 60 W.
silicone sealing insulator. The needle was connected with high voltage dc source through rotary spark gap switch. The testing gases (O2 , air) were bubbled through the hypodermic needle with a flow of 300 mL min−1 . A stainless steel disk of 1.8 cm diameter, attached to an stainless steel rod in the shape of a piston, was suspended from top through silicone sealing. The point-to-plate distance is 2 cm. The pulsed power supply consists of a HV dc source (30 kV), resistor R (1 M), inductor L (1 µH), rotating spark gap and a HV capacitor C (0.1 µF). The capacitor C was charged through resistor R and inductor L by the high-voltage dc power supply. When the rotating spark gap reached the point of conduction, the electrical energy stored in capacitance C is discharged through the rotating spark gap switch and reactor, generating high-voltage pulse of 8 µs width, 1 µs rise time. The voltage pulse voltage and pulse repetition rate of the source were 30 kV, 50–150 Hz. The input power is 60 W. Positive polarity was used in this study. The output voltages were measured using an oscilloscope (XJ4362A), along with a 310: 1 high-voltage divider. For experiments with high-voltage pulsed discharge, 100 ml solutions were treated in corona reactor at room temperature and oxygen was bubbled into reactor. For the combined experiments, hydrogen peroxide or catalyst was added into reactor. During all experiments the initial pH value of solution was 7. 4-Chlorophenol and acetophenone were analyzed by gas chromatograph (GC: 1102G), equipped with an OV-101 glass capillary column (30 m length), and flame ionization detector. The GC oven temperature was 170◦ C, whereas the injection port and the detector were heated to 180◦ C, 200◦ C, respectively. The concentration of H2 O2 was measured by the triiodide method.(13)
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3. RESULTS AND DISCUSSION 3.1. Effect of Gas Flow Rate As stated before, in a previous work,(3) results of degradation of acetophenone have shown that on the addition of gases, there are obvious increases in conversion of acetophenone. Therefore, the effects of gas flow rate need to be investigated. All other parameters in these experiments were held constant except that gas flow rate was varied over the range of 100–400 mL/min, the testing gas is air and the degradation time was 30 min. The conversion of acetophenone as a function of gas flow rate was shown in Fig. 2. It can be observed that the conversion of acetophenone was greatly influenced by gas flow rate, and had a nonmomotonic dependence to gas flow rate. With the gas flow rate of 300 mL/min, the conversion of acetophenone reaches maximum, then apparently decrease. With a 100 mL/min gas flow rate, a little ultraviolet radiation was observed. Due to high density of water, the electric field strength required for discharges in water is high. Thus electron avalanches cannot form easily, more energy is required for plasma channel formation. With a higher gas flow rate, plasma channel is easy to form, which leads indirectly to photochemical and plasma channel effects and subsequently to a faster degradation of acetophenone. So the conversion of acetophenone will be enhanced. However, when gas flow rate is too high,
Fig. 2. Conversions of acetophenone as a function of gas flowrate.
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the region between inter-electrode is almost full of gas. Like that for gas pulsed corona discharge, the breakdown strength of dischrage become low, thus, the relative small volume of water around plasma channel limits the amount of acetophenone that can be degraded. Therefore, the conversion of acetophenone decreases. 3.2. Variation of Solution Conductivity The conductivity of discharge channel is an important parameter, which depends on plasma pressure, temperature and electric density, also affects resistance between inter-electrodes. If discharge plasma in water is not ideal plasma, then conductivity can be shown as follows:(14) σ = 0.28 × 107
where ν =
1.67×10−14 n
a
+ 3.8 × 10−6 ne T
3 2
ne ν
(1)
T 3/2 7 ln 0.62 × 10 1/2 , ν = collision ne
frequency, na , ne are atom density and Electron density respectively. while the gas flow rate and inter-electrode separation remain constant (200 mL/min, 10 mm, respectively), the acetophenone solution conductivity increase with discharge time as shown in Fig. 3 (initial concentration of acetophenone is 250 mg/L). During discharge process, many active species, such as • OH, • O, • H, • HO2 , and O− 2 , etc., were produced. These active species can react with organic compounds to result in some conductive products which increase the conductivity of water. So the solution conductivity increases with discharge time. Table I also showed liquid conductivity variation of some sample after 30 min discharge time. It can be shown that solution conductivity increases greatly after 30 min discharge time when there is no organic compound in water, i.e., in the case of distilled water and tap water. It is due to produce of active species, such as • OH, • O, • H, • HO , and O− , etc. and maybe the heat from pulse discharge 2 2 affect the conductivity of solution. Water conductivity plays an important role in the generation of the corona discharge.(15) On one hand, at low water conductivity, there is only a very narrow range in the applied voltage to generate a stable discharge without sparking. On the other hand, at high water conductivity, streamers become short and the efficiency of radical production decreases. Clements found that water conductivity had an effect on streamer length.(16) Thus, variation of solution conductivity can affect discharge characteristics and may affect the efficiency of radical production. Finally, the degradation efficiency of organic contaminant decreases.
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Fig. 3. Liquid conductivity as a function of discharge time. Table I. Variation of Liquid Conductivity (µS/cm)
Before discharge After discharge
Distilled water
Tap water
Acetophenone (I)
Acetophenone (II)
7.5 200
225 400
9.5 170
23.5 170
3.3. Effect of H2 O2 Addition Active oxygen species like OH and O are generated in the discharge. Hydrogen peroxide could be produced from OH radicals.(16) In order to distinguish H2 O2 from O3 or other oxidizing agents, Sato made a series of measurements to prove that this oxidizing is H2 O2 .(17) Figure 4 shows the concentration of H2 O2 in pure water as a function of discharge time. As can be seen in the Fig. 4, the H2 O2 concentration increases approximately linearly with increasing discharge time. Similar result was also obtained while initial concentration of H2 O2 in pure water reached 100 mg/L. Pulsed corona discharges in the aqueous phase may directly produce hydroxyl radicals. In addition, when molecular oxygen is present, it readily scavenges electrons, forming an O (1D) atom and hydroxyl radicals.(12) O(1D) + H2 O → 2 · OH ·
(2)
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Fig. 4. Concentration of H2 O2 as a function of discharge time.
The OH radicals can recombine and form H2 O2 through the following reaction pathway: OH + OH → H2 O2
(3)
The rate constant (k) for this reaction is k = 5 × 109 1/ms.(18) Thus, the H2 O2 concentration in pure water increased with increasing discharge time. However, the decomposition of H2 O2 is also present during discharges. Discharge plasma is a very powerful source of ultraviolet (UV) (5) radiation, which can cause photolytic decomposition of H2 O2 via reaction (4). H2 O2 + hν → 2OH
(4)
But, the recombination of OH is a main reaction in pure water during discharges, so the H2 O2 concentration increased with increasing discharge time although initial concentration of H2 O2 reached 100 mg/L. H2 O2 is a powerful oxidant, and it can react with organic compounds both directly and indirectly via its aqueous-phase degradation products, i.e, the hydroxyl radicals. Thus, the degradation of 4-chlorophenol was studied. Figure 5 shows the conversions of 4-chlorophenol as a function of concentration of H2 O2 while the initial concentration of 4-chlorophenol is 250 mg/L. Approximately 40% conversion was reached after 14 min
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Fig. 5. The conversions of 4-chlorophenol as a function of concentration of H2 O2 .
of discharge time while H2 O2 was not added into water. On the addition of 100 mg/L H2 O2 , the conversion increased to 60%. In evidence, it can be observed that, on the addition of 200 mg/L H2 O2 , there are obvious increases in conversion of 4-chlorophenol, which are up to 90% after 14 min and the conversion of 4-chlorophenol reached 46% after 2 min. These results show that the addition of H2 O2 clearly enhances the degradation efficiency. This may be due to the synergistic effect of high-voltage pulsed discharge and H2 O2 . In order to verify the existence of the synergistic effect, the degradation of 4-chlorophenol was studied while no pulse corona discharges were applied to water. The experimental result showed that the 4-chlorophenol of 250 mg/L was not degraded after 14 min under oxygen-sparged condition. This suggests that H2 O2 is difficult to degrade 4-chlorophenol in this condition, and also the decomposition of H2 O2 is enhanced by the pulsed corona discharges. Therefore, the conversion of 4-chlorophenol increases. 3.4. Effect of TiO2 or CuO Addition There are several reasons to add a metal oxide to the water discharge. Firstly, metal oxides can catalyze the decomposition of The decomposition of H2 O2 catalyzed by iron oxides and copper was found to be first order with respect to the concentration of
during H 2 O2 . oxides H2 O2 .
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The first order decomposition rate was a function of the quantity of the catalyst added.(19) Furthermore, many studies appeared on accelerating effects of TiO2 assisted photocatalytic degradation. However, it can be seen from Fig. 6 that the influence of quantity of TiO2 on degradation of acetophenone is not apparent. Similar result was also obtained while CuO was added into water during discharges. Also it can be observed from experiment that the discharge become unstable when TiO2 was added into water. Maybe the presence of metal oxides hinders the formation of plasma channel due to increase of collusions between metal oxides and oxygen. 4. CONCLUSIONS Experimental results indicated that gas flow rates, liquid conductivity and H2 O2 addition play important role in degradation of organic contaminants in water. With the increase of time, the liquid conductivity increases, which have an important effect on discharge. Meanwhile, the concentration of H2 O2 increases. Addition of 200 mg/L H2 O2 , the conversion of 4-CP was greatly enhanced. This may be due to the synergistic effect of high-voltage pulsed discharge and H2 O2 . However, on addition of TiO2 or CuO, influence of quantity of TiO2 on degradation of acetophenone is not apparent. Future studies are certainly needed to explore good catalyst to improve the degradation efficiency of this process.
Fig. 6. Effect of TiO2 on conversion of acetophenone .
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