Degradation of aqueous 250mg/L 4-chlorophenol (4-CP) by high-ûoltage pulse corona discharges combined with ozone was inûestigated to gain insight into ...
Plasma Chemistry and Plasma Processing, Vol. 22, No. 1, March 2002 ( 2002)
Degradation of 4-Chlorophenol by High-Voltage Pulse Corona Discharges Combined with Ozone Yuezhong Wen,1 Xuanzhen Jiang,2 and Weiping Liu1 Receiûed January 9, 2001; accepted April 9, 2001
Degradation of aqueous 250 mg/L 4-chlorophenol (4-CP) by high-ûoltage pulse corona discharges combined with ozone was inûestigated to gain insight into factors affecting enhancement of the combined system. High-ûoltage pulse corona discharges, ozonation, and a combination of the two were used to facilitate the degradation of aqueous 4-CP. Experimental results indicate that the treatment of 4-CP using a combination of high-ûoltage pulse corona discharges and ozonation within 30 min resulted in the almost degradation (96%) and a 51% reduction of the chemical oxygen demand (COD). This apparent synergistic effect may be attributed to the enhancement of ozone decomposition. The degradation of aqueous 4-CP by high-ûoltage pulse corona discharges combined with ozone was found to be affected by ozone concentration, substrate concentration, and interelectrode separations. The increase of ozone concentration leads to an increase of 4-CP conûersion and COD remoûal. The conûersion of 4-CP decreased with increase in 4-CP concentration and interelectrode separations. KEY WORDS: Advanced oxidation process; 4-chlorophenol; degradation; highvoltage pulsed corona discharges; plasma; ozone.
1. INTRODUCTION Over the last several years, the high-voltage pulse discharge in water has been shown to be effective for the degradation of organic contaminants, the biofouling prevention and debacterialization.(1–6) The high-voltage pulsed discharge makes it possible to instantaneously form a strong electric field and also to produce a plasma, which generates various active species (·OH, ·O, ·H, ·HO2 , O−2 , etc.). The pulsed discharge used in the liquid is usually either an arc discharge or a spark discharge. The situation changes if gas bubbles are in the liquid. In the case partial discharges (streamer) may 1
Institute of Environmental Science, Zhejiang University, Hangzhou 310027, Zhejiang, P.R. China. 2 Department of Chemistry, Zhejiang University, Hangzhou 310027, Zhejiang, P.R. China. 175 0272-4324兾02兾0300-0175兾0 2002 Plenum Publishing Corporation
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appear as well. When a streamer discharge is used, following several effects of the streamer process occur simultaneously, which are production of various free radicals, electrical field, and ultraviolet radiation. These effects play an important role in destroying harmful compounds.(6,7) Ozone is a powerful oxidant with a standard redox potential of 2.07 V, and it can react with organic compounds both directly and indirectly via its aqueous-phase degradation products such as the hydroperoxyl and hydroxyl radicals.(8) However, the oxidation of organic compounds in water by ozone was found to be slow and incomplete because the efficiency of ozonation was significantly limited by the mass transfer of gaseous ozone into the aqueous phase. A variety of advanced oxidation processes (AOPs) employing ozone combined with hydrogen peroxide, ozone combined with ultraviolet irradiation and ozone combined with ultrasonic irradiation have been explored.(9–11) The coupling of corona discharges in water with ozonation provides an alternative AOP. Like that for other AOPs using ozone, ·OH is generated from the decomposition of ozone. Thus, the combined technique of high-voltage pulsed corona discharges and ozone needs to be explored further. The objective of the present work was to investigate the effect of various reaction parameters on the degradation of 4-chlorophenol (4-CP) in water by high-voltage pulse corona discharge combined with ozone and the study of synergistic effects. 2. EXPERIMENTAL The 4-chlorophenol used in this study was 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 4-CP in water by pulsed corona discharges, a laboratory prototype of the treatment system was designed and constructed. 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 silicone sealing insulator. The needle was connected with high voltage dc source through rotary spark gap switch. The testing gases (O2 , O3) were bubbled through the hypodermic needle with a flow of 200 mL min−1. A stainless steel disk of 1.8 cm diameter, attached to a 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 MΩ), inductor L (1 µH) rotating
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Fig. 1. The schematic diagram of the experimental set-up: R,1 MΩ; L,1 µH; C,0.1 µF; power 60 W.
spark gap and a HV capacitor C (0.1 µF) as shown in Fig. 1. 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 and pulse frequency 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. Ozonation experiments without high-voltage pulsed discharge were performed on solutions of 100 ml in the corona reactor at room temperature and dc source was off. The ozone was bubbled through water sample containing 4-CP for various intervals of time. 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, ozone was bubbled into reactor. During all experiments the initial pH value of solution was 7. 4-Chlorophenol 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 and 200°C, respectively. Ozone (O3) concentration in the gas phase was analyzed by iodometric method.(12) The residual ozone in solutions was measured by the indigo
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method.(13) The chemical oxygen demand (COD) was measured by the standard method.(14) Intermediates produced in the 4-CP degradation were analyzed by the following procedure. The sample of treated solution was first extracted for 2 hr using 100 ml of dichloromethane. The organic extract was then concentrated to small volume by flowing N2 . Subsequently the sample was analyzed using FINNIGAN SSKU-700 gas chromatography-mass spectroscopy (GC-MS) with DB-5MS capillary column of 30 m length. The intermediates were identified by matching with standard samples, considering both elution time and mass spectrum. 3. RESULTS AND DISCUSSION 3.1. Comparison of Degradation To investigate the synergistic effects of high-voltage pulse corona discharges and ozonation, 250 mg兾L aqueous 4-chlorophenol solutions were treated by high-voltage pulse corona discharges combined with oxygen flow, ozonation, and high-voltage pulse corona discharges combined with ozone flow, respectively. The concentration of ozone was 8.87 mg兾L in the gas phase and flow rate was 0.018 m3兾hr. The solutions were sampled in intervals of 5 min. The results are shown in Fig. 2. It can be observed that the
Fig. 2. The 4-CP conversion as a function of reaction time at aqueous solutions of 250 mg兾L 4-chlorophenol, pH 7, 8.87 mg兾L O3 .
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Fig. 3. The COD removal as a function of reaction time at aqueous solutions of 250 mg兾L 4-chlorophenol, pH 7, 8.87 mg兾L O3 .
degradation efficiencies of 4-CP can be ranked as follows: (ozoneCpulse corona discharges in water)H(oxygenCpulse corona discharges in water) Hozonation. 4-CP was almost completely degraded (96%) within 30 min by the combined technique. This apparent synergistic effect may be attributed to the enhancement of ozone decomposition under high-voltage pulse corona discharges. The mass transfer of ozone into water is the limiting step in ozonation system where most of the ozone will be lost. Therefore, the conversion of 4-CP is low. Under high-voltage pulse corona discharges, the concentration of residual ozone in aqueous solution could not be detected for our experiments, indicating that high-voltage pulse corona discharges stimulated the decomposition of the dissolved ozone in aqueous solution and the decomposition of ozone mainly occurs in the gas phase (as indicated in Ref. 15). High-energy electrons in high-voltage pulse corona discharges can dissociate ozone to oxygen and O atom. The O atom reacts almost instantaneously with water in its neighborhood, resulting in the production of hydroxyl radicals. Because of the enhancement in ozone decomposition by the pulsed corona discharges in water, the amount of hydroxyl radicals generated in water should increase. The value of COD indicates loss of aromaticity and overall degree of compound degradation. As shown in Fig. 3, degradation rates of COD by
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the combined technique were faster than those with high-voltage pulse corona discharges or ozonation alone. These data suggest that the combined technique favors mineralization of 4-CP.
3.2. Variation of Substrate Concentration and Ozone Concentration The combination of high-voltage pulsed corona discharge and ozonation was carried out at a flow rate of 0.018 m3兾hr and a high-voltage source of 30 kV. The ozone concentration remains to be 7.2 mg兾L. Figure 4 showed the effect of initial concentration of 4-CP on the degradation of 4-CP. As can be seen, at a given time, an increase of initial concentration leads to conversion decrease of 4-CP. However, the degradation rate increases with an increase of 4-CP initial concentration. To determine the effect of ozone concentration on the degradation rate of 4-CP, solutions with initial concentration of 250 mg兾L were treated at various ozone concentrations with the combination of high-voltage pulsed corona discharge and ozone. The residual ozone in solutions was measured. As expected, there was no residual ozone detected after 30 min treatment.
Fig. 4. The effect of initial concentration of 4-CP on degradation rate at an ozone concentration of 7.2 mg兾L.
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Fig. 5. The effect of ozone concentrations on the degradation of 4-CP at aqueous solutions of 250 mg兾L 4-chlorophenol, pH 7.
Figure 5 shows the effect of ozone concentration on the degradation of 4CP in water by the combined technique. As can be observed, the increase of ozone concentration leads to an enhancement of 4-CP conversion at a given time. It may be attributed to the following reasons: (1) the increase of ozone decomposition rate due to photolysis and pryolysis. Thus, the increase of ozone concentration leads to an increase in O atom concentrations, therefore, produces more hydroxyl radicals; (2) the increase of ozone concentration leads to an enhancement of the ozone reaction driving force. Figure 6 also shows the effect of ozone concentrations on the removal of COD. The COD content of the solution decreases with increase of ozone concentrations. This result indicates that ozone accelerates byproduct degradation.
3.3. Variation of Inter-Electrode Separations A series of experiments were conducted to investigate the dependence of the 4-CP conversion on the inter-electrode separations in the combined process (Fig. 7). The ozone concentration remains to be 5.92 mg兾L. All other parameters in these experiments were held constant at standard values cited above except that inter-electrode separation was varied over the range
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Fig. 6. The effect of ozone concentrations on the removal of COD at aqueous solutions of 250 mg兾L 4-chlorophenol, pH 7.
Fig. 7. The effect of inter-electrode separations on the degradation of 4-CP at aqueous solutions of 250 mg兾L 4-chlorophenol, pH 7, 5.92 mg兾L O3 .
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of 10–20 mm and the degradation time was 15 min. The data of Fig. 7 indicate that the conversion of 4-CP decreases as the inter-electrode separation increases. Approximately 78% of the initial 4-CP was degraded when degradation experiments were performed with 10 mm inter-electrode separation. However, when the inter-electrode separation was 20 mm, only 45% of 4CP was degraded after 15 min. With a relative larger inter-electrode separation, more energy is required for plasma channel formation. With a shorter inter-electrode separation, plasma channel is easy to form, which leads indirectly to photochemical and plasma channel effects and subsequently to a faster degradation of 4-CP.(2) On the other hand, the decomposition rate of ozone increases with an increase of plasma channels. So the conversion of 4-CP will be enhanced.
3.4. Intermediates and Possible Pathways Ozone can react in aqueous solutions either directly with target substrates of indirectly via reactions of its free radical decomposition products.(6) The direct reactions are often rather slow but highly selective. On the other hand, the indirect reactions by free hydroxyl radicals are highly reactive but less selective. In the combined process, however, ozone most likely decomposes in the plasma channel by electron collisions, pyrolysis and photolysis process that yield oxygen atoms and oxygen as following steps: e·
O3 → O2CO hν
(1)
O3 → O2CO
(2)
O3 → O2CO
(3)
pyrolysis
k3
OCH2O → 2 · OH
(4)
Since hydrogen peroxide has been shown to form during the high-voltage pulse corona discharges.(1) Its reactions will also contribute to overall activity with organic compounds. hû
H2O2 → 2 · OH
(5)
Generally, the effectiveness of oxidation process is associated with the very reactive species, hydroxyl (·OH) radicals. The hydroxyl radicals attack organic compounds relatively non-selectively with rate constants ranging from 106 to 1010 M −1 s−1, oxidizing them by hydrogen atom abstraction or by addition to double bonds.(17)
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To identify intermediates of degradation reactions, 250 mg兾L aqueous 4-chlorophenol (4-CP) solutions were treated by high-voltage pulse corona discharges, ozonation, and high-voltage pulse discharges combined with ozone, respectively, for 30 min, and then were analyzed by GC兾MS to identify intermediate products. In these analyses, intermediates identified in three cases were p-benzoquinone and hydroquinone. This result showed that all the three processes proceed through similar degradation reaction pathways. The reaction pathway which leads to degradation of 4-CP is not known, however, according to determination of degradation intermediates, 4-CP seems to undergo hydroxyl radical insertion as an initial step and a possible degradation pathway of 4-CP may be expressed as follows:
4. CONCLUSION The use of high-voltage pulse corona discharges combined with ozone leads to high degradation rate of 4-CP and the COD removal. 4-CP could be almost completely (96%) degraded by the combined technique within 30 min. This apparent synergistic effect may be attributed to the enhancement of ozone decomposition in high-voltage pulsed corona discharges. The degradation of 4-CP in water by the combined technique was found to be
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affected by ozone concentration, substrate concentration, and inter-electrode separation. The increase of ozone concentration leads to an increase of 4-CP conversion and COD removal. The conversion of 4-CP decreased as 4-CP concentration and the inter-electrode separation were increased. The results of this study demonstrate that the combined technique can effectively degrade 4-CP. Although the reaction mechanisms of high-voltage pulse corona discharge induced degradation of chlorophenols are rather complicated, the combined technique may provide a suitable alternative in future for the elimination of chlorinated, aromatic substance, which may be present as pollutants in water. In addition, due to a very poor transfer coefficient of hypodermic needle, the degradation rates of 4-CP are generally low, thus, further work need to design appropriate reactor for ozone transfer into water, build higher power supply with shorter pulses and to optimize the plasma parameters to improve the energy efficiency of this process.
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