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Ind. Eng. Chem. Res. 2000, 39, 4408-4414
A Pulseless Corona-Discharge Process for the Oxidation of Organic Compounds in Water Won-Tae Shin and Sotira Yiacoumi School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0512
Costas Tsouris* and Sheng Dai Chemical Technology Division, Oak Ridge National Laboratory,† P.O. Box 2008, Oak Ridge, Tennessee 37831-6224
Ozone is widely used to deactivate microorganisms and remove organic contaminants in water industries. However, interest also exists in using radical species, which are stronger oxidants than ozone, in such processes. One means of producing radical species is by corona discharge. This work investigates the use of a novel pulseless corona-discharge system for the removal of organic substances. The method combines corona discharge with electrohydrodynamic spraying of oxygen, forming microbubbles. Experimental results show that pulseless corona discharge effectively removes organics, such as phenol and methylene blue, in deionized water. The coronadischarge method is demonstrated to be comparable to the direct use of ozone at a high applied voltage. The results also show that a minimum applied voltage exists for an effective operation of the corona-discharge method. In this work, the minimum applied voltage is approximately 4-4.5 kV over a 3-cm distance between the electrodes. The kinetic rate of phenol degradation in the reactor is modeled. Modeling results show that the dominant species of the pulseless corona-discharge reactor are hydroxyl radical and aqueous electron. Several radical species produced in the pulseless corona-discharge process are identified experimentally. The major species are hydroxyl radical, atomic hydrogen species, and ozone. Introduction The removal of organic compounds from aqueous solutions has become an increasingly important goal in the treatment of water and wastewater because of the increased industrial production of waste streams containing harmful organic compounds. Among the major sources of such streams are the pulp and paper mill industry, the petrochemical industry, the food-processing industry, and runoff from urban areas. Particularly important among the organic wastes are phenolic compounds.1 The treatment and removal of these organic wastes are among the main goals of the water and wastewater treatment industries, and, as a result, constant advancements are being made to improve the methods of removal. Among other chemicals, the use of ozone as an oxidant and disinfectant for the treatment of water and wastewater is gaining popularity. Ozone, an extremely powerful oxidant, can attack organic materials and convert them to byproducts that are not toxic when consumed. It has the capacity to oxidize bacteria and protozoa in water, as demonstrated in the literature,2,3 as well as a strong ability to oxidize organic substances. However, molecular ozone selectively attacks organic contaminants.4 At a high pH range, molecular ozone decomposes rapidly to form radicals.5,6 These radicals are nonselective oxidants and have a higher oxidation power than molecular ozone. This finding initiated the concept of advanced oxidation processes (AOPs). Common AOPs involve a combination * To whom correspondence may be addressed. Telephone: (865) 241-3246. Fax: (865) 241-4829. E-mail:
[email protected]. †Managed by UT-Battelle, LLC, for the U.S. Department of Energy under Contract DE-AC05-00OR22725.
of O3/UV or O3/H2O2 (Peroxone). Both UV and H2O2 stimulate the decomposition of molecular ozone into radicals, the hydroxyl radical being the most important one. While AOPs have been developed to generate radical species from ozone decomposition, it would be advantageous if the same radical species were formed in the absence of ozone. In this light, several alternative methods were developed for the degradation of organic compounds in aqueous solutions. These methods utilize energy to form radical species in situ and include corona discharge,7 ultrasonic irradiation,8,9 and laser flash radiation.10 Corona discharge is of specific interest in this work. Chang et al.11 discussed the various types of corona discharge. According to their definition, the work presented here is dealing with pulseless corona discharge produced by direct-current (dc) high voltages. The use of pulsed streamer corona discharge for direct production of radical species was extensively studied by Locke and co-workers.7,12-14 Sharma et al.12 explained the fundamental aspects of pulsed streamer corona discharge in the aqueous phase. They reported that corona discharge in aqueous solutions induces reactions that lead to organic breakdown and that hydroxyl radicals, hydrogen radicals, hydrogen, hydrogen peroxide, and aqueous electrons are formed in the aqueous phase by pulsed streamer corona. Joshi et al.7 expanded on the study of Sharma et al.12 and proved the formation of radical species by both experiments and modeling. Sathiamoorthy et al.13 studied the use of a pulsed streamer corona-discharge reactor for the gas-phase contaminants removal. They demonstrated the formation of a wide range of very reactive radicals and chemical species which initiate bulk phase reactions
10.1021/ie0002374 CCC: $19.00 © 2000 American Chemical Society Published on Web 09/23/2000
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that lead to the removal of NO and NO2. Also, Grymonpre´ et al.14 studied an aqueous-phase pulsed streamer corona-discharge reactor for phenol oxidation. Their study was focused on the use of suspended activated carbon in combination with the pulsed streamer corona discharge for a better organic contaminant removal. The formation of radical species by corona discharge in aqueous solutions was also reported by other researchers. Clements et al.15 pioneered a nonthermal plasma AOP in water using a pulsed streamer corona discharge. They showed that pulsed corona discharge in aqueous solutions along with oxygen bubbling into the reactor produces ozone that can lead to the decolorization of dyes. It was also noted that ozone was not produced in the absence of oxygen. Sun et al.16 showed that discharge of a spark in water could produce UV radiation and active radical species. They showed that hydrogen peroxide and hydroxyl radicals were effectively formed by both the streamer corona and spark discharge. Sˇ unka et al.17 also concluded that a pulsed streamer corona discharge produces active radical species. Another type of corona discharge, besides the pulsed streamer, is the pulseless corona, which is the main subject of this work. During pulseless corona discharge, a high flux of electrons in gases or liquids is observed, which may be accompanied by luminescence. Unlike the pulsed streamer corona, pulseless corona discharge is produced by strong dc fields. Similarly to a pulsed streamer corona discharge, the pulseless corona system can provide active radical species under high electric field conditions. The advantage of the pulseless corona discharge process is that it continuously produces radical species for rapid degradation of organics. On the other hand, it may consume more energy than a pulsed corona process because of the continuous operation and is also limited by the conductivity of the water. The advantages of using high electric fields in gasliquid systems were demonstrated by Shin et al.18 and Tsouris et al.,19-21 who studied electrohydrodynamic (EHD) spraying of bubbles in water. They used this method to form microbubbles of oxygen and ozone in water and thereby enhanced their transport rates from the bubbles to the surrounding liquid. The present work expands upon previous studies by combining corona discharge with electrohydrodynamic spraying of microbubbles. The objectives of this study are the following: (1) demonstrate the effectiveness of a pulseless coronadischarge system in combination with oxygen or ozone in removing organics from water, using phenol and methylene blue dye as surrogate contaminants; (2) examine the effects of the input gases (oxygen and ozone) on organic removal under corona-discharge conditions; and (3) identify the radical species formed by the pulseless corona-discharge process. Material and Methods Removal of organic compounds from deionized water was performed in a cylindrical electrostatic ozonation reactor (EOR) shown in Figure 1, which had a 7.5-cm diameter and a 30-cm height. Solutions were prepared with deionized water (conductivity 0.3-0.5 µS/cm and pH 6-7) throughout the experiments. All experiments were performed with 0.01-in.-i.d. conical capillary tubes for the gas inlet to the reactor. The capillary design is similar to that used in our previous studies.18,22 The
Figure 1. Experimental setup for pulseless corona discharge.
ground electrode is a rod-shaped bar which is submerged in water near the capillary tip. The electrode distance was maintained at 3 cm in all experiments. Either pure oxygen or a mixture of oxygen and ozone was fed to the reactor using a three-way valve. The oxygen fed to the reactor and ozone generator was extradry, high-purity compressed oxygen (Air Products and Chemicals, Inc., Allentown, PA). A corona-discharge ozone generator (model O3-22, O3 Associates, Kensington, CA) was used to produce ozone. A UV/visible spectrophotometer (diode array spectrophotometer model HP8452, Hewlett-Packard, Palo Alto, CA) was connected online for monitoring of the ozone concentration. For some experiments, the ozone gas was drawn into three 50-mL gas-tight syringes (Popper and Sons, New Hyde Park, NY) and delivered to the line by a syringe pump (model 361, Sage Instruments, Boston, MA) for accurate gas delivery into the reactor. Corona discharge was generated by the high-voltage power supply (model SL1200, Spellman High Voltage Electronics Corp., Plainview, NY; voltage/current range 10 kV/1200 mA). The temperature of the reactor was not controlled. Because of the electric current, an increase in temperature was observed at the end of the experiments. However, the temperature increase was significant only at high applied voltages, e.g., 6 kV. At low applied voltages, experiments lasted up to 30 min without causing a significant variation in the temperature. At high applied voltages, experiments lasted only a few minutes, causing a small increase in the temperature. The solution was assumed to be well-mixed because of strong EHD flows.21 Aqueous samples for concentration measurements were taken from a sampling port, which was connected to a piece of Teflon tubing placed into the reactor, approximately 15 cm from the top. Phenol analyses were performed using solid-phase microextraction.23 The following materials (all from Supelco, Inc., Bellefonte, PA) were required to perform the analyses: a PTE-5 fusedsilica capillary column, a 85-µm polyacrylate-coated fiber for polar semivolatiles, a sampling stand and holder, 4-mL vials with septa, an inlet liner for the gas chromatograph (GC), and stir bars and plates. The GC system was a Varian 3600CX GC with a flame ioniza-
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tion detector (Varian Chromatography Systems, Walnut Creek, CA). Methylene blue analyses were performed using the UV/vis spectrophotometer. Methylene blue concentrations of up to 0.02 mol/m3 obey the BeerLambert law. Light absorbance was measured at a wavelength of 660 nm. Ultraviolet absorption was used to analyze the concentration of ozone in the gas phase entering the reactor. The UV/vis spectrophotometer was placed in line and run continuously in single-cell kinetics mode, using a quartz flow-through cell with a 1-cm path length to give constant readings of the ozone concentration in the gas phase. The measurements were made at 254 nm, and the absorbance readings were converted to concentrations, using molar absorptivity data available in the literature.24 Identification of chemical species produced by corona discharge was achieved by collecting the emission spectra. Emission from chemical species produced by corona discharge was collected by an all-silica fiberoptic probe. The probe was directly placed against the glass wall of the corona discharge apparatus to enhance the collection efficiency. The emission was dispersed with a Spex 500M monochromator with a 150 nm groove/mm grating blazed at 655 nm. The dispersed radiation was detected using a charge-coupled device detector (CCD; Spex System One) with a 1-in. CCD chip. The resolution limit of this system is determined by the dispersion of the grating, the focal length of the monochromator, and the pixel size of the CCD detector and is 1.2 nm. Entrance slit widths of less than 0.8 nm providing this resolution were used in these measurements. The dispersed radiation was detected using a CCD detector with a 1-in. CCD chip. Results and Discussion At least two approaches are possible for the application of electric fields in the removal of organic contaminants. In the first approach, electric fields can be used to form microbubbles of a gas mixture containing ozone in water and thereby increase the surface area for mass transfer (e.g., Shin et al.18,22 and Tsouris et al.19-21). This approach provides higher rates of mass transfer through smaller bubbles and enhanced liquid mixing by EHD flows. In the second approach, stronger electric fields can be used to induce corona discharge. This approach is investigated in the present work. In deionized water systems, using the geometry shown in Figure 1, corona discharge is observed above 4-kV applied voltage. Figure 2 illustrates EHD spraying of oxygen into water by electric fields. This photograph was taken at 7 kV of applied voltage, a level at which intense corona discharge occurs at the tip of the capillary. Effect of Corona Discharge on Methylene Blue Oxidation with Oxygen. Methylene blue dye was selected as one of the two representative organic compounds in this study. The effects of applied voltage and polarity of the corona on oxidation by methylene blue dye are shown in Figure 3. In these experiments, oxygen was supplied to the reactor at a rate of 2 mL/min, using a syringe pump. Both positive and negative electric polarities were tested at the applied voltages of 4, 4.5, and 6 kV. As seen in this figure, applied voltage exerts a strong effect on methylene blue oxidation. For both polarities, the rate of oxidation increases as the level of applied voltage is raised. An applied voltage of 6 kV had the
Figure 2. EHD spraying of oxygen under corona discharge at an applied voltage of 7 kV.
Figure 3. Methylene blue oxidation by oxygen under coronadischarge conditions: oxygen flow rate, 2 mL/min.
Figure 4. Current profile during a methylene blue oxidation experiment (Figure 3): oxygen flow rate, 2 mL/min.
highest oxidation rate among the three voltages tested, because of increased electron flux. The higher electron flux produces more radical species to react with the solute in the solution. The effect of electric polarity is also shown in Figure 3. For all three applied voltages, the negative polarity is more effective than the positive polarity. The applied voltage of 4.5 kV shows the greatest difference for the two polarities. The current histories for the experiments of Figure 3 are shown in Figure 4. In this figure, it is shown that with positive polarity the current increases more rapidly than with negative polarity. For example, the current increase for the 6-kV positive polarity suggests that the conductivity of the water increases at a higher rate than at other conditions. This result
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Figure 5. Comparison of methylene blue oxidation by oxygen under corona-discharge conditions and ozone: ozone flow rate, 2.17 mL/min; ozone concentration, 3.5 mg/L; oxygen flow rate, 2 mL/ min.
may be due to electrochemical reactions occurring at the electrodes when positive polarity is used. Although the positive polarity is associated with higher electric current, from the oxidation results of Figure 3, it is clear that the negative-polarity voltage leads to a higher oxidation rate. Based on these results, it can be inferred that the unipolar charge injection process is more dominant with the negative-polarity voltage rather than the positive-polarity voltage, as compared to the fielddriven dissociation of molecules. Thus, the negativepolarity voltage has an advantage over the positivepolarity voltage in pulseless corona discharge in water. Effect of Corona Discharge on Methylene Blue Oxidation with Ozone. In these experiments, ozone, instead of oxygen, was introduced into the reactor. Only negative electric polarity was examined because of its better performance. The input ozone concentration was 3.5 mg/L, and the flow rate was maintained at 2.17 mL/ min. The effect of ozone input at the applied voltages of 2, 4, and 6 kV is shown in Figure 5. As previously mentioned, corona discharge is observed above 4 kV. Thus, at an applied voltage of 2 kV, we can assume that the effect on the oxidation of methylene blue derives solely from the presence of ozone. Oxidation results for ozone input at 2 kV are much better than those for oxygen input at 4 kV with corona discharge, which indicates that at 4 kV insufficient quantities of radical species are produced in the corona-discharge process to compete with 3.5 mg/L ozone. However, corona discharge becomes competitive at an applied voltage of 4.5 kV. The rate of methylene blue oxidation at 4.5 kV in the corona process with oxygen input is almost identical with that for ozone input at 4 kV. When the results for 4 and 4.5 kV are compared, it is clear that a sudden enhancement in oxidation occurs at 4.5 kV. On the basis of this observation, we can conclude that the formation of radical species in the corona-discharge process is not linearly related to the applied voltage or that a threshold exists in the applied voltage beyond which the formation of radicals increases significantly. This threshold is between 4 and 4.5 kV in this work. Effect of Corona Discharge on Phenol Oxidation with Oxygen and Ozone. In this experiment, phenol was used as a representative organic molecule for oxidation. The flow rate of oxygen and ozone was maintained at 15 mL/min, and the ozone input concentration was 34 mg/L. The results of phenol oxidation with oxygen and ozone are shown in Figure 6. Enhanced oxidation is observed with oxygen between 4- and 5-kV applied voltage. This result supports the previous as-
Figure 6. Phenol oxidation by ozone and oxygen under coronadischarge conditions: flow rate of ozone or oxygen, 15 mL/min; ozone input concentration, 34 mg/L.
sumption of the threshold voltage of corona discharge, which was found between 4 and 4.5 kV. To compare the effectiveness of corona discharge, we used ozone as the input gas instead of oxygen. The ozone experiment was conducted with a 5-kV applied voltage, a 15 mL/min ozone flow rate, and a 34 mg/L ozone concentration. As shown in Figure 6, the phenol oxidation was completed within 8 min with ozone, while 12 min was required when oxygen was used at the same applied voltage. Thus, at 5-kV applied voltage, better oxidation occurs with ozone than with oxygen. However, oxidation with oxygen becomes comparable when 6-kV voltage is applied. From this experiment, it can be concluded that the corona-discharge process at high applied voltages is comparable to the ozonation process. Selected experimental conditions were also used to test the reproducibility of oxidation results. A variation of up to 10% was observed in the oxidation rate by either corona discharge or ozonation. Modeling of Phenol Oxidation in the Pulseless Corona-Discharge Process. In this section, phenol oxidation using the pulseless corona-discharge process is modeled based on a phenol-oxidation model for a pulsed streamer corona-discharge reactor.14 Grymonpre´ et al.14 developed the model based on the fact that hydroxyl radicals formed from a pulsed streamer corona react with phenol to produce oxidation products. The assumptions in developing the model presented here include the following: (1) the reactor is isothermal and well-mixed; (2) the volume of the reactor is approximately constant; and (3) the initiation rates are constant over the period of the process and are functions of voltage input. On the basis of these assumptions, a material balance was written for each molecular species, as well as the radicals and ions, involved in the chemical reactions shown in Table 1. Because 14 chemical species were involved in the reaction process, a total of 14 ordinary differential equations were obtained. The rate constants for reactions involved in the process were taken from the literature (see work by Grymonpre´ et al.14) except for the initiation rate constants k1, k2, and k3, which were obtained through model fitting of experimental data. The initiation reactions (eqs 1-3 in Table 1) were assumed to be directly associated with the corona-discharge process, and the reaction constants were assumed to be functions of applied voltage. The rest of the reactions were assumed to be independent of the corona-discharge process. This model was employed in our work to permit comparison of the pulsed and pulseless corona-discharge methods. The ordinary differential equations obtained from chemical reaction systems are known as stiff equations.25 Thus, the 14 equations obtained here were
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Table 1. Chemical Reactions in the Aqueous Solution of the Pulseless Corona Reactor chemical reaction
rate constant
eq
initiation reactions see Table 2
(1)
k2
see Table 2
(2)
k3
see Table 2
(3)
k4 ) 1.0 × 1010 M-1 s-1
(4)
k5 ) 1.0 × 1010 M-1 s-1
(5)
k6 ) 5.0 × 107 M-1 s-1
(6)
k7 ) 3.0 × 1010 M-1 s-1
(7)
k8 ) 2.5 × 1010 M- 1s-1
(8)
k9 ) 1.2 × 1010 M-1 s-1
(9)
k10 ) 2.4 × 1010 M-1 s-1
(10)
k11 ) 4.0 × 109 M-1 s-1
(11)
k12 ) 2.0 × 106 M-1 s-1
(12)
k13 ) 1.0 × 1010 M-1 s-1
(13)
k14 ) 1.0 × 1010 M-1 s-1
(14)
k15 ) 1.0 × 1010 M-1 s-1
(15)
k16 ) 3.0 × 1010 M-1 s-1
(16)
k17
k17 ) 6.5 × 109 M-1 s-1
(17)
k18
k18 ) 8.0 × 109 M-1 s-1
(18)
k19
k19 ) 1.0 × 109 M-1 s-1
(19)
k20 ) 1.0 × 1011 M-1 s-1
(20)
k21 ) 1.0 × 1010 M-1 s-1
(21)
k22 ) 1.0 × 1010 M-1 s -1
(22)
k1
H2O 98 H• + •OH 2H2O 98 H2O2 + H2 2H2O 98 H3O+ + eaq- + •OH propagation reactions k4
H• + O2 98 HO2• k5
H• + H2O2 98 H2O + •OH •
k6
OH + H2O2 98 H2O + HO2• k7
eaq- + •OH 98 OHk8
eaq- + H• + H2O 98 OH- + H2 k9
eaq- + H2O2 98 •OH- + OHtermination reactions k10
H• + •OH 98 H2O k11
2•OH 98 H2O2 k12
2HO2• 98 H2O2 + O2 k13
H• + HO2• 98 H2O2 k14
2•H 98 H2 k15
HO2• + •OH 98 H2O + O2 k16
H3O+ + OH- 98 2H2O oxidation reactions
phenol + •OH 98 hydroquinone + H• phenol + •OH 98 catechol + H• phenol + •OH 98 resorcinol + H• k20
hydroquinone + •OH 98 products k21
catechol + •OH 98 products k22
resorcinol + •OH 98 products
simultaneously solved using the computer program STIFF,26 which is suitable for stiff initial-value problems. The initiation rate constants (k1, k2, and k3) were optimized to fit the experimental data of this work because no experimental data are available in the literature for the same experimental conditions. Modeling and experimental results of the phenol concentration with time are shown in Figure 7. As shown in this figure, the model provides a good fit to the experimental data for all cases. The resulting initiation reaction constants (k1, k2, and k3) are shown in Table 2 for each applied voltage. In this table, we can see that k3 is about 1 order of magnitude greater
than k1 and k2 for all applied voltages. This result implies that the reaction described by eq 3 is the dominant mechanism of the pulseless corona-discharge process, producing a hydroxyl radical and an aqueous electron as the major species. Although several species are produced, these two dominant species are assumed to be responsible for the complete degradation of phenol in this work, either with oxygen at 5- and 6-kV applied voltage or with ozone at 5 kV. If we compare 5 kV with oxygen input and 5 kV with ozone input, we see that the value of k3 is higher with ozone than with oxygen, as shown in Table 2. This result is expected because the experimental results showed a
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Figure 7. Comparison of phenol degradation modeling results with experimental data; lines represent modeling results. Table 2. Initiation Rate Constants Obtained from the Model (Units are M-1 s-1) initial rate constant k1 k2 k3
applied voltage 4 kV
5 kV
6 kV
PSCa 5 kV/ozone
at 57 kV
3.0 × 10-9 7.0 × 10-9 7.7 × 10-9 8.0 × 10-9 9.25 × 10-10 3.0 × 10-9 7.0 × 10-9 7.7 × 10-9 8.0 × 10-9 1.2 × 10-6 1.3 × 10-8 5.5 × 10-8 7.0 × 10-8 1.0 × 10-7 2.35 × 10-9
a Values obtained from Grymonpre ´ et al.14 for a pulsed streamer corona discharge reactor.
faster degradation of phenol in the case of the ozone input. Because the model employed here does not include ozone in the reactions, the k3 value is treated as a lumped constant having both the effect of corona discharge and the effect of ozone. Upon entering the reactor, the ozone in the input gas is transferred to water via reactions listed in Shin et al.,22 producing hydroxyl radical, which disintegrates phenol. Thus, ozone in the input gas independently disintegrates phenol in the solution along with the pulseless coronadischarge process. Therefore, the value of k3 in the 5-kV experiment with ozone includes the effect of ozone in the input gas as well as the effect of pulseless corona discharge on phenol degradation. For comparison, the initiation rate constants for pulseless corona discharge and pulsed streamer corona are shown in Table 2 for each applied voltage. As shown in this table, these constants are different. Grymonpre´ et al.14 reported a higher value of k2 than k3, while here we report a higher value for k3. The reason for this discrepancy may be the different conditions of the two reactors. Note that the experiments of the pulsed streamer corona were conducted without gas flow. Based on the initiation reaction constants, it can be inferred that hydrogen peroxide produced by reaction (2) is the dominant species in the pulsed streamer coronadischarge system, while the aqueous electron and hydroxyl radical produced by reaction (3) are dominant for the pulseless corona-discharge system. Although the model presented in this work needs to be extended with additional reactions, the values of rate constants presented in Table 2 show a good comparison of the two different corona-discharge systems. Identification of Chemical Species Formed under Corona Discharge. The chemical species produced in the corona-discharge process were qualitatively identified using emission spectra (see also Clements et al.15 and Sato et al.27). These chemical species are responsible for the destruction of bacteria and organic compounds, such as methylene blue and phenol. Figure 8 shows the emission spectra of corona discharge at 7 kV of applied voltage with the oxygen input. The emission spectra
Figure 8. Emission spectra for the identification of the chemical species formed under corona discharge: applied voltage, 7 kV; input gas, oxygen.
were obtained between 200 and 600 nm using a monochromator. Several species were identified in this experiment. The major species responsible for the oxidation of methylene blue and phenol are •OH radical, ozone (O3), OH+ ion, and atomic hydrogen (HR and Hβ). The emission spectra for all species identified appear in the following wavelength ranges: hydroxyl radical, 280-340 nm; ozone, 310-340 nm; hydroxyl ion, 350360 nm; R atomic hydrogen (HR), 434 nm; β atomic hydrogen (Hβ), 486.1 nm.27,28 Although at the upper end of the spectra there are some unidentified species, the emission spectra verify that H and OH radicals are produced by pulseless corona discharge in the presence of oxygen and water. They also suggest that more chemical/electrochemical reactions need to be considered in the modeling. Summary and Conclusions This paper describes the application of electric fields to simultaneously generate radical species and microbubbles of oxygen or ozone in water for the oxidation of methylene blue dye and phenol as representative organic contaminants in water and wastewater treatment. The objectives of using dc electric fields in this process are to (1) form radical species from oxygen and water and (2) generate microbubbles of oxygen, thus enhancing the interfacial area for mass transfer. This work can readily be applied to the ultrapure-water production29 because of the low electrical conductivity of the water that allows a higher efficiency of radical formation. The oxidation of methylene blue dye was successfully performed at 5- and 6-kV applied voltage. The effect of the polarity of the input power was investigated. Negative-polarity voltage showed approximately 20% better performance than positive-polarity voltage in methylene blue destruction. Two corona-discharge processes with either oxygen input or ozone input were compared. At 4 kV, ozone performed better than oxygen. However, at applied voltages of 4.5 and 6 kV, oxidation by oxygen and corona discharge became comparable to that by ozone. The results of phenol oxidation showed a behavior similar to that of methylene blue oxidation. Phenol was completely oxidized within approximately 10 min with the oxygen input at applied voltages of 5 and 6 kV. These results prove the effectiveness of the pulseless corona-discharge process in the removal of organic compounds and suggest more studies to investigate bacterial sterilization. From the results of this work, we can conclude that a minimum applied voltage exists, above which the oxidation rate in the corona-discharge
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process increases significantly. The threshold voltage was found to be approximately 4-4.5 kV. Thus, it is recommended that the corona-discharge process be operated above 4.5 kV. The phenol-degradation model developed for a pulsed streamer corona by Grymonpre´ et al.14 was employed in this work to provide the initiation rate constants. Modeling results showed that the dominant species are aqueous electrons and hydroxyl radicals for pulseless corona discharge as opposed to hydrogen peroxide for the pulsed streamer corona system. These two species, aqueous electron and hydroxyl radical, are responsible for the complete degradation of phenol in the pulseless corona-discharge system. The radical species produced from the corona-discharge process were identified using emission spectra. The results showed that the major radical species produced from the process were hydroxyl radicals, ozone, and atomic hydrogen. Emission spectra results also suggest that more chemical/electrochemical reactions need to be included in the modeling. In addition, further studies are needed to examine the energy efficiency of the pulseless corona-discharge process and compare it with that of commercial oxidation methods. Acknowledgment This research was supported by the Electric Power Research Institute (EPRI). Partial support was provided by NSF (BES-9702356) and DOE (Environmental Management Science Program, Office of Environmental Management, and Basic Energy Sciences) under Contract DE-AC05-00OR22725. The authors are thankful to Dr. Marsha Savage for editing the manuscript. Literature Cited (1) Hawash, S.; El-Ibiari, N.; El-Diwani, G. Study of ozonation reaction in phenolic effluents. Waste Manage. 1990, 10, 269. (2) Parker, J. F. W.; Greaves, G. F.; Smith, H. V. The Effect of Ozone on The Viability of Cryptosporidium Parvum Oocysts and A Comparison of Experimental Methods. Water Sci. Technol. 1993, 27, 93. (3) Korich, D. G.; Mead, J. R.; Madore, M. S.; Sinclair, N. A.; Sterling, C. R. Effects of Ozone, Chlorine Dioxide, Chlorine, and Monochloroamine on Cryptosporidium Parvum Oocyst Viability. Appl. Environ. Microbiol. 1990, 56, 1423. (4) Gurol, M. D.; Singer, P. C. Dynamics of the Ozonation of Phenol. Water Res. 1983, 17, 1163. (5) Hoigne´, J.; Bader, H. Rate Constants of Reactions of Ozone with Organic and Inorganic Compounds in WatersII: Dissociation Organic Compounds. Water Res. 1983, 17, 185. (6) Yurteri, C.; Gurol, M. D. Removal of Dissolved Organic Contaminants by Ozonation. Environ. Prog. 1987, 6, 240. (7) Joshi, A. A.; Locke, B. R.; Arce, P.; Finney, W. C. Formation of Hydroxyl Radicals, Hydrogen Peroxide and Aqueous Electrons by Pulsed Streamer Corona Discharge in Aqueous Solution. J. Hazard. Mater. 1995, 41, 3. (8) Hung, H.-M.; Hoffmann, M. R. Kinetics and Mechanism of the Enhanced Reductive Degradation of CCl4 by Elemental Iron in the Presence of Ultrasound. Environ. Sci. Technol. 1998, 32, 3011. (9) Petrier, C.; Micolle, M.; Merlin, G.; Luche, J. L.; Reverdy, G. Characteristics of Pentachlorophenate Degradation in AqueousSolution by Means of Ultrasound. Environ. Sci. Technol. 1992, 26, 1639.
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Received for review February 14, 2000 Revised manuscript received August 1, 2000 Accepted August 6, 2000 IE0002374
