ARTICLE pubs.acs.org/IECR
Generation of Reactive Species by Gas-Phase Dielectric Barrier Discharges Qiong Tang,† Wenju Jiang,‡ Ying Cheng,† Song Lin,‡,§ T. M. Lim,§ and Junru Xiong*,† †
Department of Chemistry and Life Science, Leshan Normal University, Leshan, 614004, People's Republic of China Institute of Architecture and Environment, Sichuan University, Chengdu 610065, People's Republic of China § Institute of Environmental Science and Engineering, Nanyang Technology University, Innovation Center, Block 2, Unit 237,18 Nanyang Drive, Singapore 637723 ‡
bS Supporting Information ABSTRACT: This work presents a study of discharge characteristics and generation of reactive species such as OH• radical, hydrogen peroxide, and ozone by a gas-phase dielectric barrier discharge (DBD) process. A series of experiments were performed to investigate the effects of various parameters such as input energy density, feeding gas, gas flow rate, and electrode gap on the formation of OH• radical, hydrogen peroxide, and ozone in solution. The pH and N-containing products (NO2 and NO3) in solution were also determined. The experimental data show that formation rates of OH• radical, hydrogen peroxide, and ozone in solution were found to depend on the input energy density, feeding gas, gas flow rate, and electrode gap. When pure oxygen was used as the feeding gas, O3 was the major reactive species. The OH• radical was observed to be the major reactive species generated and its concentration was approximately 12 times higher than that of O3 when air with 100% relative humidity (RH) was used as the feeding gas. NO3 byproducts formed in the solution were partly responsible for the pH decrease.
1. INTRODUCTION The electrical discharge processes, which have been demonstrated to generate various reactive species such as radicals (H•, O•, OH•, HO2•) and molecular species (H2O2, O3),14 have been primarily utilized for the treatment of environmentally recalcitrant contaminants such as phenol,58 dyes,9,10 and their derivatives in aqueous solution1118 and for the disinfection of drinking water.1921 It has been demonstrated that the principal reactive species involved in degradation of organic compounds are the OH• radical, hydrogen peroxide, and ozone. Moreover, OH• radical has much higher thermodynamic oxidation potential than O3 and hydrogen peroxide and many studies have shown that most pollutants react 1 million1 billion times faster with OH• radical than with ozone.22 Consequently, attempts have been made to produce OH• radical in copious quantity from gasphase electrical discharge reactors. Recently, a few studies have reported that gas-phase pulsed corona discharges could be used to generate OH• radical, in addition to O3, effectively using air containing water vapor as the feeding gas:4,2326 e þ H2 O ¼ e þ H• þ OH•
In our previous studies, therefore, a dielectric barrier discharge (DBD) system was designed to produce reactive species, especially OH• radical, in air containing water vapor. The reactive species were then diffused into an aqueous solution to degrade recalcitrant organics.28 The experimental results suggest that the DBD system is effective in degrading recalcitrant contaminants such as phenol. The discharge process involves a very complex set of chemical reactions where several parameters such as energy density level, gas composition, and gas flow rate play critical roles. It is, therefore, essential to develop a basic understanding of the chemical reaction involved in the discharge process in order to design, scale up, and operate a DBD process on a larger scale and to assess its usefulness for degrading other hazardous wastes. The specific goals of this work are to determine the formation rate of OH• radical, hydrogen peroxide, and ozone in the solution, which will be generated by a DBD process in the gas phase and to study the effect of input power, gas sources, gas flow rate, and electrode gap on the formation of OH• radical, hydrogen peroxide, and ozone. In addition, the pH and the byproducts such as NO2 and NO3 in solution will be also detected.
ð1Þ
2. EXPERIMENTAL SECTION Oð DÞ þ H2 O ¼ 2OH 1
•
ð2Þ
Most importantly, the concentration of OH• radical generated was found to be several times higher than that of O3.24,27 The lifetime of OH• radical in humid air was found to be a few hundreds of microseconds.27 This makes the direct transfer of OH• radical from air to water with subsequent oxidation of aqueous pollutants possible. r 2011 American Chemical Society
2.1. Experimental Setup. Figure 1 shows the schematic diagram of the experimental setup. It is described in detail in a previous paper;28 here only the most important characteristics Received: January 6, 2011 Accepted: July 15, 2011 Revised: April 22, 2011 Published: July 29, 2011 9839
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Figure 1. Schematic diagram of the experimental setup.
will be mentioned. The inlet gas was passed through a water bubbler by an air pump to increase the humidity and subsequently fed to the DBD reactor. An ac high voltage power supply was used to provide energy to the discharge electrode with a peak-to-peak voltage (Vpeak) in the range 040 kV, a rise time of 25 μs, and a repetition frequency of 525 kHz. The applied voltage (Vpeak) and the discharge current wave of the DBD reactor were recorded using a digital storage oscilloscope (TDS 1001B, Tektronix). The voltage signal was first attenuated using the capacitance (47 pF:47 nF = 1000:1) at the detection exit of the output voltage in series with a Tektronix P2220 probe (10) before being fed to channel 1 of the oscilloscope. The current in the secondary circuit of the voltage transformer was indirectly measured using a resistor of 50 Ω at the detection exit of the output current in series with a Tektronix P2220 (1) probe. This signal was fed to channel 2 of the oscilloscope. Figures 13 of the Supporting Information show the voltage and current waveforms obtained at various experimental conditions. They are typical for a filamentary ac DBD: several current peaks with amplitudes of a few to a dozen amperes and durations of a few microseconds appear on the voltage rise, on both the positive and the negative alternances of the voltage waveform. When the amplitude of the applied voltage (Vpeak) is low, the voltage across the discharge gap is not high enough to ignite plasma, and no discharge current is produced. When the voltage across the discharge gap is high enough to cause the breakdown of the gas, a large number of short-lived microdischarges are observed and the discharge current is formatted. Discharge power consumption in the DBD reactor was calculated by multiplying the corresponding current and voltage values recorded at the front panel of the unit of the ac high voltage power supply instrument, and usually three to five readings of discharge power were taken to get one averaged data point. There were some inefficiencies including the transformer, inductors, and resistances in the ac high voltage power supply.
However, these inefficiencies were negligible compared to discharge power consumption. 2.2. Experimental Procedure. Five hundred milliliters of 0.5 mol L1 dimethyl sulfoxide (DMSO) solution (for trapping OH• radical) or Milli-Q water (for absorbing ozone and hydrogen peroxide) was charged to the reactor vessel for trapping the reactive species generated from the DBD reactor. Samples were taken from the reactor vessel at regular intervals and filtered by a 0.45 μm PTFE membrane for analysis of OH• radical, hydrogen peroxide, and ozone. All experiments were carried out at an electrode gap of 2 mm except for the experiments involving electrode gap variation. All experiments were conducted three times and gave reproducibilities within 5%. 2.3. Analytical Methods. 2.3.1. HPLC-UV for OH• Radical Measurement. The initial OH• radical concentrations in solution were determined by a high-pressure liquid chromatograph (HPLC) (WATERS 2695) with a multiple wavelength UV diode array detector (WATERS 2996), following the dimethyl sulfoxide trapping method developed by Chao.29 The method is based on the reaction between the OH• radical and dimethyl sulfoxide (DMSO) to produce formaldehyde quantitatively, which then reacts with 2,4-dinitrophenylhydrazine (DNPH) to form the corresponding hydrazone (DNPHo) to be analyzed by HPLC-UV. Five milliliter samples taken from the reactor vessel at regular intervals, 1 mL of 0.5 mol L1 phosphate buffer (pH 4.0), and 0.3 mL of 6 mmol L1 DNPH were mixed and diluted to 10 mL. The mixture was allowed to stand for 30 min and then was analyzed by HPLC-UV under the following chromatographic conditions. An XBridge-C18 column (100 4.6 mm, particle size 3.5 μm) was used as the analytical column and the column temperature was set at 35 °C. The mobile phase was a mixture of acetonitrilewater50 mmol L1 HCOOH (50:40:10, v/v) maintained at a flow rate of 1.0 mL min1. An injection volume of 10 μL was used each time. The detection 9840
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The concentration of ozone was calculated with the following equation: ½O3 ¼
Figure 2. Effect of input energy density on formation rate of hydroxyl radical, ozone, and hydrogen peroxide in the solution with 100% RH humid air as feeding gas and gas flow rate of 7 L min1.
Figure 3. Effect of input energy density on formation rate of hydroxyl radical, ozone and hydrogen peroxide in the solution with pure oxygen as feeding gas and gas flow rate of 7 L min1.
wavelength was set at 254 nm, and the calibration curve was made with formaldehyde (Sigma, SG). 2.3.2. UVVis for O3 and H2O2 Measurement. The initial concentrations of hydrogen peroxide and ozone in the solution were determined by the potassium titanium(IV) oxalate method and the potassium indigotrisulfonate method, respectively,30,31 using a Jasco V-550 UVvis spectrometer (Japan). In the hydrogen peroxide determination, 5 mL samples taken from the reactor vessel at regular intervals and 2 mL of 0.1 mol L1 potassium titanium(IV) oxalate solution were mixed and diluted to 10 mL. The absorbance of the mixture was measured at 400 nm. Two milliliters of 0.1 mol L1 potassium titanium(IV) oxalate solution was diluted to 10 mL and used as the blank. The calibration curve was made with hydrogen peroxide (Sigma, SG). In the ozone determination, 5 mL samples taken from the reactor vessel at regular intervals and 2 mL of 0.125 mmol L1 potassium indigotrisulfonate solution were mixed and diluted to 10 mL. The absorbance of the mixture was measured at 600 nm. Two milliliters of 0.125 mmol L1 potassium indigotrisulfonate solution was diluted to 10 mL and used as the blank.
ðΔAÞV 20bVx
ð3Þ
where ΔA = Asample Ablank; Ablank is the absorbance of the blank, Asample is the absorbance of the sample, b is the path length of the cell (cm), Vx is the volume of sample (in mL; 5 mL), V is the final volume of solution (in mL; 10 mL), and [O3] is the concentration of ozone (mmol/L). 2.3.3. Ion Chromatography for NO2 and NO3 Measurement. The concentrations of NO2 and NO3 were measured by an ion chromatograph (Dionex ICS-3000, Singapore) coupled with an AS-19 nonsuppressor column (4.6 mm 150 mm) and a CDD-10A VP conductivity detector. A KOH eluent solution (10 mmol L1 for the first 10 min and then raised gradually to 45 mmol L1 in the following 15 min) was pumped at a flow rate of 1.0 mL min1.
3. RESULTS AND DISCUSSION In order to estimate the effects of various process parameters such as input energy density (IED), gas composition, gas flow rate, and electrode gap on formation kinetics of reactive species such as OH• radical, hydrogen peroxide, and ozone, a series of experiments was performed. According to the information in a previous paper,28 a nearly linear increase in concentrations of OH• radical, ozone, and H2O2 in solution was observed with time, indicating that the formation of OH• radical, ozone, and H2O2 in solution was a zero-order reaction kinetics. The zeroorder rate constants k of the three reactive species formation were estimated, and results are presented in Figures 26. 3.1. Effect of Input Power. Due to the discharge characteristics depending on the nature of the gas mixture, the experiment studied the effect of input power at conditions of pure oxygen and air containing water vapor (100% RH) as the feeding gas, respectively. Results from the experiment are shown in Figures 2 and 3. From Figure 2, it can be seen that, for 100% RH air as the feeding gas, the formation rates of OH• radical and ozone increased with increasing IED initially. When the IED was set beyond 0.51 kJ L1, however, the amount of OH• radical and ozone generated began to decrease and became undetectable at the IED of 1.41 kJ L1. Hydrogen peroxide became undetectable in the range 0.261.41 kJ L1. The results in Figure 3 show that, for pure oxygen as the feeding gas, ozone was the dominant species, and a small quantity of OH• radical and hydrogen peroxide was detected. The formation rates of OH• radical, ozone, and hydrogen peroxide increased with increasing IED in the range 0.261.41 kJ L1. When 100% RH air is used, the gas molecules in the discharge region are dissociated or excited by energetic electrons to form gas plasma:32,33
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e þ O2 f O2 þ þ 2e
ð4Þ
e þ O2 f Oþ þ Oð1 DÞ þ 2e
ð5Þ
e þ H2 O f H• þ OH• þ e
ð6Þ
e þ 2H2 O f H2 O2 þ H2 þ e
ð7Þ
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e þ N2 f N2 þ þ 2e
ð8Þ
e þ N2 f 2N þ e
ð9Þ
Among gas molecules, the dissociation energy is around 9.18, 5.12, and 5.11 eV for the nitrogen-to-nitrogen triple bond (NtN), the oxygen-to-oxygen double bond (OdO), and the HOH single bond, respectively.35 At the lower energy, therefore, oxygen and water molecules are dissociated or excited by energetic electrons to produce active oxygen atoms and OH• radical. Then, oxygen atoms react with oxygen and water molecules to produce OH• radical and ozone:32 Oð1 DÞ þ H2 O f 2OH•
ð10Þ
Oð1 DÞ þ O2 þ M f O3 þ M
ð11Þ
As IED increases, the mean electron energy increases, and electrons with energy exceeding 5.1 eV increase, resulting in increases of dissociated oxygen and water molecules. When the mean electron energy exceeds 9.18 eV, moreover, nitrogen molecules are dissociated or excited by energetic electrons to produce nitrogen atoms and the excited molecules. On the one hand, excitation and dissociation of nitrogen molecules lead to a number of additional reaction paths involving nitrogen atoms and the excited molecular states N2(A3Σ+u ) and N2(B3Πg) that can produce additional oxygen atoms for ozone and OH• radical generation:34 N þ O2 f NO þ Oð1 DÞ
ð12Þ
N þ NO f N2 þ Oð1 DÞ
ð13Þ
N2 ðAÞ þ O2 f N2 O þ Oð DÞ
Figure 4. Effect of gas composition on the formation of hydroxyl radical, ozone, and hydrogen peroxide with input energy density of 0.51 kJ L1 and gas flow rate of 7 L min1. 1, pure oxygen; 2, humid oxygen (70% RH); 3, humid oxygen (100% RH); 4 dry air; 5, humid air (70% RH); 6, humid air (100% RH).
with IED beyond 0.51 kJ L1, and OH• radical and ozone became undetectable at the IED of 1.41 kJ L1. When pure oxygen (∼100% O2) is used, the oxygen molecules in the discharge region are dissociated or excited by energetic electrons to form oxygen atoms and ozone through reactions 4, 5, and 11. The OH• radical and hydrogen peroxide detected in the solution are formatted through reaction 10 and reactions 2124 due to oxygen atoms and ozone molecules being dissolved in solution:36
ð14Þ
1
N2 ðA, BÞ þ O2 f N2 þ 2Oð DÞ
ð15Þ
1
•
Therefore, the formation rates of OH radical and ozone increase with increasing IED. On the other hand, the active nitrogen atoms can react with oxygen atoms and ozone:35 Oð1 DÞ þ NO f NO2
ð16Þ
N• þ O3 f NO þ O2
ð17Þ
NO þ O3 f NO2 þ O2
ð18Þ
NO2 þ O3 f NO3 þ O2
ð19Þ
e þ O 3 f O þ O2
ð20Þ
When the IED is higher than a certain level, rapid NOx reactions can consume oxygen atoms at a faster rate than the OH• radical formation reaction (10) and the ozone formation reaction (11). The result is an accelerated recombination of oxygen atoms, catalyzed by the presence of NO and NO2. Previously formed ozone is also removed in a catalytic ozone destruction process involving NO and NO2. In addition, UV radiation might occur with IED further increasing, which affects the discharge mode of the DBD reactor, and might result in the reduction of microdischarge frequency.34 These are probably the causes for the lowered formation rates of OH• radical and ozone
Oð1 DÞ þ H2 O f H2 O2
ð21Þ
O3 þ H2 O f H2 O 2 þ O 2
ð22Þ
OH þ O3 f O2 • þ HO2 •
ð23Þ
3HO2 • þ O3 f 3OH• þ 3O2
ð24Þ
As the IED increases, the mean electron energy increases, resulting in increasing dissociated oxygen molecules. Therefore, the formation rate of ozone increases, and formation rates of OH• radical and hydrogen peroxide also increase accordingly. In addition, the O3 concentration in solution rapidly increased initially and then tended to increase slowly, and was finally maintained constant (Figure 3). The reason is that the O3 concentration in solution depends on the O3 concentration in the gas phase above the solution surface and the solubility of O3 in solution. According to Henry’s law, the larger the O3 concentration is in the gas phase, the larger the O3 concentration is in solution. Therefore, the O3 concentration in solution is higher at the higher IED due to the higher gas-phase concentration at such an IED. Moreover, the O3 concentration in solution increases slowly due to the poor mass transfer when the O3 concentration in solution is close to the solubility of O3 in solution. 3.2. Effect of Gas Composition. From Figure 4, it can be seen that the gas composition had a significant effect on reactive species generation rates. The water and N2 in the feeding gas significantly accelerated the generation of the OH• radical and had an inhibitory effect on O3 generation. O3 was the major reactive species when pure oxygen was used as the feeding gas. However, when 100% RH air was used as the feeding gas, OH• radical was observed to be the major reactive species generated 9842
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Figure 5. Effect of gas flow rate on the formation rate of hydroxyl radical with input power of 60 W and 100% RH humid air as feeding gas.
and its concentration was approximately 12 times higher than that of O3, indicating that the DBD reactor can effectively generate OH• radicals in copious quantity. With increasing water content in the gas mixture, the formation rate of OH• radical and hydrogen peroxide increased, but that of ozone decreased (Figure 4). In addition, when humid air was used as the feeding gas, the hydrogen peroxide became undetectable during the whole process. Water vapor can absorb a substantial part of the electronic energy of the discharge that could otherwise be used in the ozone formation process. The electrolytic dissociation of water can form H• and OH• radicals via collision of water molecules with electrons from the gas-phase discharge to reaction 6. Subsequently, the formed H• and OH• radicals destroy ozone molecules through reactions 25 and 26.3 H• þ O3 f OH• þ O2
ð25Þ
OH• þ O3 f HO2 • þ O2
ð26Þ
In addition, due to the much larger quenching rate of O(1D) by H2O than by N2 and O2 (about 67 times higher),3 oxygen atoms formed by electron impact dissociation of oxygen can also combine with water molecules to produce OH• radical, preventing ozone formation (eq 10). In the presence of nitrogen, the electronically excited nitrogen molecules can dissociate H2O to form OH• and H• radicals through reactions 2729.33 N2 þ þ H2 O f 2NH• þ OH•
ð27Þ
N2 þ ðH2 OÞ þ H2 O f H3 Oþ þ OH•
ð28Þ
N2 ðA 3 ΣÞ þ H2 O f N2 þ OH• þ H•
ð29Þ
Therefore, it is apparent that water vapor and nitrogen molecules significantly accelerated the generation of OH• radical and had an inhibitory effect on O3 generation. In this study, OH• radical in solution can be produced either from the discharge processes reactions 6, 10, and 2529or from the O3 decomposition in water reactions 23 and 24. As observed in Figure 5, a small quantity of OH• radical could be detected when pure oxygen and dry air were used, indicating that O3 decomposition did take place. However, when 100% RH air was used as the feeding gas, OH• radical was the major reactive
Figure 6. Effect of electrode gap on the formation rate of hydroxyl radical with input energy density of 0.51 kJ L1, gas flow rate of 7 L min1, and 100% RH humid air as feeding gas.
species and its concentration increased significantly compared to the previous scenario. In addition, reactions 23 and 24 indicate that O3 decomposes to form OH• radical only at high pH since O3 mainly exists as molecular ozone at low pH.37 In this study, the pH of the treated solutions was found to be mostly acidic as shown in Figure 9. Therefore, OH• radicals produced from O3 decomposition were likely very limited, and hence the main source of generation was likely from direct production from the DBD process when 100% RH air was used as the gas source. 3.3. Effect of Gas Flow Rate. Various gas flow rates were applied to the DBD reactor to study their effects on the formation of reactive species such as OH• radical. The results are shown in Figure 5. It was observed that increasing the gas flow rate from 3 to 8 L min1 improved the formation of OH• radical and the rate constant rose from 0.80 to 4.22 μmol L1 min1 correspondingly. Increasing the gas flow rate from 3 to 5 L min1 significantly improved the formation of OH• radical, but a further rise from 7 to 8 L min1 was less effective. Gas flow rate significantly affects the quantity of gas molecules present in the DBD reactor and thereby the energy density within. In addition, the energy density affects the number of microdischarges occurring per unit of electrode area. This finally determines the abundance of gas molecules broken down and the quantities of active species generated.34 On the other hand, the gas flow rate changes the residence time of the gas in the reactor and the reaction time of the gas-phase active species (such as OH• radical) with the trapping substance; therefore, the utilization ratio of the active species will vary.10 As the gas flow rate increases, more gas molecules will pass through the DBD reactor and the energy density will decrease within the DBD reactor. Under the condition of input power of 60 W, the energy density decreased from 1.2 to 0.45 kJ L1 with increasing gas flow rate from 3 to 8 L min1. From Figure 2, it can be seen that the formation rate of OH• radical increased with decreasing energy density from 1.2 to 0.51 kJ L1, but decreased when the energy density further decreased from 0.51 to 0.45 kJ L1. On the other hand, with higher gas flow rate, the time required for the gas molecules to reach the reactor vessel is reduced, resulting in more OH• radicals trapped within their lifetime, i.e., a higher utilization ratio of the OH• radical. These likely explain why in the present test the formation rate of the OH• radical increased as the gas flow rate increased from 3 to 7 L 9843
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Figure 7. Effect of input energy density on the formation of NO3 in solution with 100% RH air as feeding gas and gas flow rate of 7 L min1.
Figure 9. Variation of pH in Milli-Q water with various feeding gases with input energy density of 0.51 kJ L1 and gas flow rate of 7 L min1.
solution depended on the IED and the nature of the gas mixture, which increased with increasing IED and decreased with increasing water content in the gas phase. NO2 became undetectable during whole discharge process. As mentioned above, the dissociation energy of the nitrogento-nitrogen triple bond (NtN) (9.18 eV) is larger than that of the oxygen-to-oxygen double bond (OdO) (5.12 eV) and the HOH single bond (5.11 eV). At the lower IED, oxygen and water molecules are the major substances broken down due to the low mean electron energy obtained from the electric field. As the IED increases, the mean electron energy increases, resulting in dissociation of nitrogen molecules to produce the unstable nitrogen atoms through reactions 9 and 10. Then nitrogen oxides can be formed from the well-known gas-phase reactions of dissociated nitrogen and oxygen, reactions 1619, 30 , and 31.38
Figure 8. Effect of water vapor on the formation of NO3 in solution with input energy density of 0.51 kJ L1 and gas flow rate of 7 L min1.
min1, but did not decrease when the flow rate was further raised from 7 to 8 L min1. 3.4. Effect of Electrode Gap. The results in Figure 6 show that increasing the electrode gap from 2 to 6 mm decreased the formation of reactive species such as the OH• radical and the rate constants k reduced from 3.56 to 1.41 μmol L1 min1 correspondingly. Under the same input power condition, the electric field strength decreased with increasing width of the discharge gap due to the increase of applied voltage disproportionately (Figure 2 of Supporting Information) to the weak discharge strength and resulted in the reduction of microdischarge frequency. Therefore, formation rate of reactive species such as OH• radical decreased. 3.5. Formation of NO3 and NO2 in Solution. Milli-Q water was subjected to DBD system treatment for 60 min under different conditions, and then NO2and NO3 were detected with air as the feeding gas by analyzing the discharged Milli-Q water samples using an ICS-3000 ion chromatograph. Figures 7 and 8 show the results. As can be seen, when one of the conditions was varied and others kept constant, a nearly linear increase in concentration of NO3 in solution was observed with time, indicating that the formation of NO3 in solution was zeroorder reaction kinetics. Moreover, the NO3 formation rate in
O• þ N f NO
ð30Þ
O• þ NO f NO2
ð31Þ
The nitrogen oxides subsequently are dissolved in the solution to form acids and ions in water as in reactions 3235, which arouse the pH decrease. 3NO2 þ H2 O f 2Hþ þ 2NO3 þ NO
ð32Þ
2NO2 ðgÞ f N2 O4 þ H2 OðlÞ f HNO3 ðlÞ þ HNO2 ðlÞ ð33Þ NO2 ðgÞ þ NOðgÞ f 2N2 O3 ðgÞ þ H2 O f 2HNO2 ðlÞ ð34Þ 3HNO2 ðlÞ f HNO3 þ 2NOðgÞ þ H2 OðlÞ
ð35Þ
The NO produced by reactions 32 and 35 may also be oxidized to NO2 when air is used as the feeding gas, thereby increasing the NO3 concentration and with NO2 becoming undetectable in solution. 3.6. Change of the Solutions pH during Discharge Process. The data presented in Figure 9 show a change of the pH for the Milli-Q water with 100% RH air and pure oxygen as the feeding gas, respectively. When 100% RH air was used as the feeding gas, the pH of the solutions dropped rapidly in the early 9844
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Industrial & Engineering Chemistry Research stage of the treatment. After 10 min, the pH decreased from 7.53 to 2.96. The pH further declined gradually toward the end of the treatment. After 60 min, the final pH of the solutions was 2.37. The pH in solution decreased lightly when pure oxygen was used as the feeding gas. The reason is that when 100% RH air is used as the feeding gas, large numbers of NOx formed from the DBD process due to the presence of N2 were dissolved in the aqueous phase and subsequently converted to NO3 and H+ through reactions 3235. The lower pH, therefore, was observed with 100% RH air as the feeding gas. According to the NO3 concentration in solution (Figures 7 and 8) and acid dissociation equilibrium, the theoretical pH was calculated, and the results are shown in Figure 9. It can be seen that the calculated value matched well with the measured value, indicating that a majority of H+ in the solution resulted from dissolution of NOx into the aqueous phase with 100% RH air as the feeding gas. When pure oxygen was used as the feeding gas, ozone was the dominant reactive species produced in the gas phase. Ozone was dissolved in the solution to produce a small quantity of other reactive species (OH• radical, hydrogen peroxide, etc.) through a series of chain reactions, resulting in a light solution pH decrease.
4. CONCLUSIONS In this work, a series of experiments were performed to study the formation of reactive species by a DBD system and the effect of various process parameters such as IED, gas composition, gas flow rate, and electrode gap on formation rates of reactive species such as OH• radical, hydrogen peroxide, and ozone. The pH and N-containing products (NO2 and NO3) in solution were also determined. The conclusions can be drawn as follows: 1. Reactive species such as OH• radical, ozone, and hydrogen peroxide can be generated by the present DBD system, and formation rates of these reactive species were mostly dependent on the IED, composition of the feeding gas, gas flow rate, and electrode gap. When 100% RH air was used as the feeding gas, the OH• radical was the major reactive species and its formation rate was approximately 12 times higher than that of O3. The formation rate of reactive species increased initially and then decreased with increasing IED. Hydrogen peroxide was undetectable during the whole discharge process. When oxygen was used as the feeding gas, ozone was the major one, and the formation rate of reactive species increased at all times with increasing IED. In addition, the formation rate of the OH• radical increased with increasing gas flow rate and decreased with increasing electrode gap 2. When humid air was used as the feeding gas, NO3 byproduct was produced in solution during the discharge process, resulting in a solution pH decrease. NO2 became undetectable during the whole discharge process. In addition, the NO3 formation rate increased with increasing IED and decreased with increasing relative humidity of air. ’ ASSOCIATED CONTENT
bS Supporting Information. Figures showing the voltage and current waveforms obtained at various experimental conditions. This material is available free of charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION Corresponding Author
*Tel.: +86 833 2270785. Fax: +86 833 2270785. E-mail:
[email protected].
’ ACKNOWLEDGMENT The project is supported by the Key Project Fund of Science and Technology Agency, Leshan (10GZD039), and by the Key Project Fund of Department of Education, Sichuan province (10ZA031). ’ REFERENCES (1) Sunka, P.; Babicky, V.; Clupek, M.; Lukes, P.; Simek, M.; Schmidt, J.; Cernak, M. Generation of Chemically Active Species by Electrical Discharges in Water. Plasma Sources Sci. Technol. 1998, 8, 258. (2) Kirkpatrick, M.; Locke, B. R. Hydrogen, Oxygen, and Hydrogen Peroxide Formation in Electrohydraulic Discharge. Ind. Eng. Chem. Res. 2005, 44, 4243. (3) Lukes, P.; Clupek, M.; Babicky, V.; Janda, V.; Sunka, P. Generation of Ozone by Pulsed Corona Discharge Over Water Surface in Hybrid Gas-Liquid Electrical Discharge Reactor. J. Phys. D: Appl. Phys. 2005, 38, 409. (4) Kornev, J.; Yavorovsky, N.; Preis, S.; Khaskelberg, M.; Isaev, U.; Chen, B.-N. Generation of Active Oxidant Species by Pulsed Dielectric Barrier Discharge in Water-Air Mixtures. Ozone Sci. Eng. 2006, 28, 207. (5) Gao, J. Z.; Liu, Y. J.; Yang, W.; Pu, L. M.; Yu, J.; Lu, Q. F. Oxidative degradation of phenol in aqueous electrolyte induced by plasma from a direct glow discharge. Plasma Sources Sci. Technol. 2003, 12, 533. (6) Sun, B.; Sato, M.; Clements, J. S. Oxidative Processes Occurring When Pulsed High Voltage Discharges Degrade Phenol in Aqueous Solution. Environ. Sci. Technol. 2000, 34, 509. (7) Hoeben, W.-M.; Van Veldhuizen, E. M.; Rutgers, W. R.; Kroesen, G.-W. Gas-Phase Corona Discharges for Oxidation of Phenol in an Aqueous Solution. J. Phys. D: Appl. Phys. 1999, 32, L133. (8) Lukes, P.; Locke, B. R. Plasma chemical oxidation processes in a hybrid gasliquid electrical discharge reactor. J. Phys. D: Appl. Phys. 2005, 38, 4074. (9) Tezuka, M.; Iwasaki, M. Plasma-Induced Degradation of Aniline in Aqueous Solution. Thin Solid Films 2001, 386, 204. (10) Zhang, R. B.; Zhang, C.; Cheng, X. X.; Wang, L. M.; Wu, Y.; Guan, Z. C. Kinetics of decolorization of azo dye by bipolar pulsed barrier discharge in a three-phase discharge plasma reactor. J. Hazard. Mater. 2007, 142, 105. (11) Sharma, A. K.; Josephson, G. B.; Camaioni, D. M.; Goheen, S. C. Destruction of Pentachlorophenol using Glow Discharge Plasma Process. Environ. Sci. Technol. 2000, 34, 2267. (12) Willberg, D. M.; Lang, P. S.; Hochemer, R. H.; Kratel, A.; Hoffmann, M. R. Degradation of 4-chlorophenol, 3,4-dichloroaniline, and 2,4,6-trinitrotoluene in an electrohydraulic discharge. Environ. Sci. Technol. 1996, 30, 2526. (13) Johnson, D. C.; Shamamian, V. A.; Callahan, J. H.; Denes, F. S.; Manolache, S. O.; Dandy, A. S. Treatment of Methyl tert-Butyl Ether Contaminated Water Using a Dense Medium Plasma Reactor: A Mechanistic and Kinetic Investigation. Environ. Sci. Technol. 2003, 37, 4804. (14) Locke, B. R.; Sato, M.; Sunka, P.; Hoffmann, M. R.; Chang, J. S. Electrohydraulic discharge and nonthermal plasma for water treatment. Ind. Eng. Chem. Res. 2006, 45, 882. (15) Zhang, R. B.; Wu, Y.; Li, J.; Li, G. F.; Li, T. F.; Zhou, Z. G. Water treatment by the bipolar pulsed dielectric barrier discharge (DBD) in waterair mixture. J. Adv. Oxid. Technol. 2004, 7, 172. (16) Shin, W.-T.; Yiacoumi, S.; Tsouris, C.; Dai, S. A Pulseless Corona-Discharge Process for the Oxidation of Organic Compounds in Water. Ind. Eng. Chem. Res. 2000, 39, 4408. 9845
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