Influence of gaseous atmosphere on corona-induced

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Influence of gaseous atmosphere on corona-induced degradation of aqueous phenol

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J. Phys. D: Appl. Phys. 33 (2000) 2769–2774. Printed in the UK

PII: S0022-3727(00)16037-1

Influence of gaseous atmosphere on corona-induced degradation of aqueous phenol D Hayashi†, W F L M Hoeben, G Dooms, E M van Veldhuizen, W R Rutgers and G M W Kroesen Faculty of Applied Physics, Eindhoven University of Technology, P O Box 513, 5600 MB Eindhoven, The Netherlands Received 2 August 2000 Abstract. The phenol degradation processes by pulsed corona discharges are investigated under three kinds of discharge atmosphere (air, argon and oxygen). The temporal variations of the concentrations of phenol and the intermediate products are monitored by LIF spectroscopy. The species of the intermediate products are identified by spectral analysis. It is clarified that the oxidative gaseous reagents produced from O2 and those from H2 O degrade phenol to intermediate products with comparable degradation rates. The degradation via the reagents from H2 O gives rise to the formation of molecules exhibiting fluorescence at 400–500 nm, in addition to dihydroxybenzene (DHB), while the degradation via the reagents from O2 produces only DHB. The reagents from O2 play an important role in the conversion of phenol to DHB.

1. Introduction

The removal of hazardous organic pollutants from contaminated water is one of the critical and urgent topics in environmental research. Recently, pulsed corona discharges in or above water have been intensively studied for the degradation of phenol (C6 H5 OH) in aqueous solutions [1–11]. Strongly oxidative reagents, such as OH radicals, O atoms and their reaction products (O3 , H2 O2 etc), are produced in pulsed corona streamers in the gaseous phase. They dissolve into water and degrade phenol via oxidation processes. These gaseous reagents produced by pulsed corona discharges have been investigated by in situ optical diagnostics. OH, O and H were detected by optical emission spectroscopy [1, 4, 7–9, 12]. The absolute concentration of O3 (∼1016 cm−3 ) was measured by absorption spectroscopy during the degradation process [4]. The OH concentration (∼1016 cm−3 ) was measured by ultraviolet (UV) absorption spectroscopy in dielectric barrier discharges, of which the discharge scheme is, in principle, identical to our work [12]. With regard to the aqueous phase, off-line methods of chemical analysis, such as high-performance liquid chromatography, have been the main workhorse for diagnostics of phenol and intermediate products in water. Recently, we have applied laser induced fluorescence (LIF) spectroscopy as an in situ diagnostic for phenol † Author to whom correspondence should be addressed.

0022-3727/00/212769+06$30.00

© 2000 IOP Publishing Ltd

and intermediate products in corona-degraded solutions [2, 3]. This enabled us to monitor the in situ chemistry of the phenol degradation process. The temporal variations of the concentrations of phenol and the intermediate products were measured during the degradation process. As the intermediate products, dihydroxybenzene (DHB: C6 H4 (OH)2 ) was observed in the degraded solutions. The reaction pathway of phenol degradation was elucidated experimentally [3]. While progress has been made in understanding the production processes of oxidative reagents in the gaseous phase and the degradation processes of phenol and the intermediate products in the aqueous phase, the correlation between the gaseous reagents and the aqueous degradation processes is not clarified yet. The specification of the gaseous reagents that are decisive in the aqueous phenol degradation and its oxidizing efficiency are of great importance not only in understanding the corona-induced degradation process, but also in developing pulsed corona discharge reactors for industrial implementation. In this paper, we focus on the influence of discharge atmospheres in the gaseous phase to the phenol degradation in the aqueous phase. The pulsed corona discharges are produced under air, oxygen and argon atmospheres. The temporal variations of the concentrations of phenol and the intermediate products are monitored by LIF spectroscopy. The roles of O atoms and OH radicals in the phenol degradation are discussed on the basis of the experimental results. 2769

D Hayashi et al

gas-out

25 kV

Air, Ar, O2 gas-in

Reactor

Nd-YAG laser + 4th harmonics generator

Monochro- ICCD mator unit

PC

Figure 1. A schematic diagram of the experimental set-up.

2. Experiments

2.1. Experimental set-up The pulsed corona discharge system [4, 5] and LIF diagnostic [2, 3] in the aqueous phase have previously been described in detail elsewhere. A schematic diagram of the experimental set-up is depicted in figure 1. A pulsed corona discharge in point-to-plate electrode geometry is generated above a solution. An aqueous phenol solution (concentration: 1 × 10−4 mol l−1 ) of 300 ml is contained in a glass vessel of 10 cm in diameter and 10 cm in height. A multi-pin anode is fixed at 1 cm above the surface of the solution. A cathode plate beneath the vessel is grounded to the earth potential. A negative voltage of −25 kV is applied to an energy storage capacitor of 1 nF through a 10 M resistor. A positive voltage of 25 kV, generated by a triggered spark gap, is pulsed between the anode and cathode with the repetition rate of 10 Hz. A pulsed corona current of approximately 10 A at maximum continues for 100 ns. Pulsed corona streamers are formed under atmospheric gaseous media. The discharge atmosphere is controlled by a gas-flow system. The glass vessel is sealed by an O-ring. A couple of Teflon™ tubes are mounted on the top of the vessel. The gas-in tube (see figure 1) is connected to a gas flow controller, which provides 0–200 sccm flows of pure oxygen and argon. The gas-out tube exhausts gases from the vessel to the outside with atmospheric pressure. Three gas flow rates (2, 50 and 100 sccm) are used for the oxygen atmosphere. The flow rate of argon gas is fixed at 200 sccm. For the experiments under oxygen (argon) atmosphere, in order to remove residue air and change the atmosphere to oxygen (argon), the vessel is flushed with an oxygen (argon) gas having a flow rate of 200 sccm for 60 min before the experiments. For these experiments, the main gaseous species is reasonably oxygen (argon). It has been experimentally examined that no degradation of phenol has proceeded while flushing the vessel. For the experiment with air, the gas-out tube is opened to atmospheric air. The UV laser radiation (266 nm) for exciting phenol and the intermediate products is obtained by using the 2770

fourth harmonic of an Nd-YAG laser (Continuum 9030). The pulsewith and linewidth of the laser radiation are approximately 6 ns and 1.0 cm−1 , respectively. The repetition rate of the laser is 10 Hz. The beam waist of the laser at the detection volume is approximately 3 mm. The laserinduced fluorescence radiation, which is emitted in a direction normal to the incident beam, is focused by a quartz lens onto the entrance of a quartz optical fibre. The optical fibre is connected to a spectrometer (JOBIN YVON H25, Instruments S.A.) with a grating of 150 grooves per mm. The broad band spectra of the fluorescence in the wavelength range 250–500 nm are detected by a 2D array of an imageintensified charge-coupled device (Andor ICCD-452). The wavelength-dependent sensitivity of the ICCD is calibrated with a tungsten lamp. The resolution of the detection system is about 3 nm. The LIF signal is accumulated for 50 laser pulses. Phenol has strong absorption bands in the UV region (250–300 nm) [13–15]. Since the distance between neighbouring rotational lines is smaller than the laser linewidth, several rotational lines of an electronic transition from the ground electronic state (S 0 ) absorb the UV laser radiation. Collisions between the molecules induce instantaneous relaxation of the ro-vibrationally excited states of an electronically excited state, such as S 1 , which are excited by the laser irradiation, to the ro-vibrational ground state of the electronic state via vibrational cascade and internal conversion. Then, it emits the fluorescence by radiation when de-exciting to the ground electronic state S 0 . The peak is at approximately 298 nm. As has been mentioned in our previous papers [2, 3], not only phenol but also the intermediate products show fluorescence by laser excitation. Thus, an observed spectrum in a degraded solution consists of the phenol component and others. In accordance with the procedure described in our previous paper [3], the observed spectrum is decomposed to the spectrum component contributed from phenol and that from the intermediate products. In addition, because of strong absorption of the probing laser by phenol and quenching of the laser-excited states by the surrounding media (water,

phenol itself and the intermediate products), the LIF intensity of the phenol component is not exactly proportional to phenol concentration. It is necessary to calibrate the intensity by taking this into account. The LIF intensity is adjusted to be proportional to the phenol concentration by the calibration method, as has been detailed in our previous paper [3]. Hereafter, we refer to the peak intensity (at 298 nm) of the spectrum component of phenol as phenol LIF intensity. It is noted that the LIF intensities of the intermediate product components are not calibrated. 2.2. Discharge atmosphere The dominant and minor gaseous species of air, argon and oxygen atmospheres for pulsed corona discharges are tabulated in table 1, together with gaseous oxidative reagents produced under these atmospheres. The concentration of H2 O is roughly evaluated from the vapour pressure (∼ 3.4 × 10–2 atm) at room temperature (293 K) [16]. The vapour pressure of H2 O in the vessel is likely to be the same for air, argon and oxygen atmospheres because water temperature, gas temperature and gas pressure are constant for all experiments. The percentages in the brackets are the relative concentrations of the species in those atmospheres. VRP is the volatile reaction product produced by the degradation process. As an example of VRP, the concentration of CO2 has been measured by Fourier transform infrared spectroscopy in a pulsed corona discharge system identical to that of the present work [4]. The CO2 concentration, after three hours exposure to pulsed corona discharges, was approximately 2.4 × 1013 cm−3 for a phenol solution having a concentration ten times higher than that in the present work. VRP can therefore be recognized as a minor species. As the oxidative reagents, we consider the dominant and strongly oxidative species in the discharge atmospheres: oxygen atom (O) and hydroxyl radical (OH). Nitrogen and nitric products are omitted because their oxidization rates of phenol and the intermediate products are smaller than those of O, OH and their reaction products. OH radicals and O atoms are mainly produced by electron impact dissociation of H2 O and O2 , respectively. The reaction products among them are not explicitly mentioned for simplicity. For instance, OH radicals react each other to form H2 O2 , and O atoms react with O2 to form O3 . Oxidative molecular reagents (e.g., H2 O2 , O3 ) produced by the reactions with OH and O are conventionally included in their parent species, OH and O, which are precursors to produce these reaction products. The O atoms, O2 itself and their reaction products, such as O3 , are produced only under the atmospheres containing oxygen (not under the argon atmosphere), while OH radicals and their reaction products (H2 O2 ) are produced under the humid atmospheres. Here, we refer to the former reagents as O-reagents and the latter as OH-reagents. The concentration of the O-reagents is expected to increase with the increase of the O2 gas flow rate for the oxygen atmospheres, and it is plausible that the concentration of the O-reagents with the oxygen atmosphere is much larger than that with the air atmosphere. In order to degrade phenol and the intermediate products via the oxidation process, the gaseous oxidative reagents must

LIF intensity (Arb. Units)


Air Ar O2 2 sccm O2 50 sccm O2 100 sccm

1

0.1

0.01 0

20 40 60 80 100 120 140 160 180

time (min.) Figure 2. The temporal variations of phenol LIF intensities as a function of the time after starting the discharge. The discharge atmospheres are air, argon (200 sccm), oxygen (2, 50, 100 sccm).

dissolve into the water and form aqueous oxidative reagents. Hence, the solubilities of the gaseous reagents into water are also important. The concentrations of typical O- and OH-reagents dissolved in water for the air atmosphere (O, OH, O2 and O3 ) are roughly estimated using Henry’s law. Henry’s law constants for O, OH [17], O2 [18], O3 [19] are ∼0.0, 30.0, 1.3 × 10−3 and 1.1 × 10−2 l.atm mol−1 , respectively. The gaseous concentrations of O3 and OH under the air atmosphere are assumed to be 1 × 1016 cm−3 and 1 × 1016 cm−3 ; the same orders as the experimental values in [4] and [12], respectively. The concentrations of O, OH, O2 and O3 dissolved into the aqueous phase under air atmosphere are approximately ∼0.0, 1 × 10−1 , 3 × 10−4 and 5 × 10−6 mol l−1 , respectively. The dissolved concentration of OH is correspondingly about three and four orders larger than that of O2 and O3 . Since O3 concentration in the gaseous phase is expected to increase with O2 concentration, that dissolved into the aqueous phase also plausibly increases in the order of air, 10-sccm-flow oxygen, 50-sccm oxygen and 100-sccm oxygen atmospheres. Because of the same H2 O concentrations in the gaseous phases, the OH concentration dissolved into the aqueous phase is likely to be of the same order for all atmospheres. Hence, in the aqueous phase, the O-reagents are negligible for the argon atmosphere, and the concentration of the O-reagents increases in the order of air, 20-sccm oxygen, 50-sccm oxygen and 100-sccm oxygen atmospheres, while the concentrations of the OH-reagents are of the same order for all atmospheres. By comparing the degradation rates of phenol and the intermediate products under the oxygen atmosphere with that under the argon atmosphere, we can see the influence of the O-reagents to the degradation processes. The degradation rates of phenol and the intermediate products under the argon atmosphere plausibly correspond to the degradation rate by the OHreagents. 3. Results and discussion

The phenol degradation is investigated in the solutions exposed by pulsed corona discharges under air, oxygen and 2771

D Hayashi et al

Atmosphere

Dominant species

Minorities

Oxidative reagents

Air Argon Oxygen

N2 (∼78%), O2 (∼19%), H2 O (∼3%) Ar (∼97%), H2 O (∼3%) O2 (∼97%), H2 O (∼3%)

VRP Air, VRP Air, VRP

OH, O OH OH, O

1.0 0.8 Ar 0.6

Air

0.4 0.2 0.0

O2 (100sccm) 0

20

40

60

80

100

120

Time (min.) Figure 3. The temporal variations of the peak intensities (at 310 nm) of the LIF spectra of the intermediate products. The temporal variations of the LIF intensity qualitatively reflect the variations in the concentrations mainly of DHB.

LIF intensity (arb. Unit)

LIF intensity (Arb. Units)

Table 1. The dominant and minor gaseous species of pulsed corona discharges under air, argon and oxygen atmospheres, together with the gaseous oxidative reagents.

40 min. 1

100 min. 0 250

300

350

400

450

500

Wavelength (nm) argon atmospheres. Figure 2 shows the temporal variations of phenol LIF intensities as a function of the time after starting the discharge. The decay rate under the oxygen atmosphere is much faster than the rates under the air and argon atmospheres. For the first 30 min, the degradation rates of phenol under the air, argon and oxygen (2, 50 and 100 sccm) atmospheres are correspondingly 4.8 × 10−2 , 6.7 × 10−2 , 9.1 × 10−2 , 1.0 × 10−1 and 1.3 × 10−1 s−1 . The degradation rate under the argon atmosphere is slightly larger than that under the air atmosphere, and smaller than that under the oxygen atmosphere by 1.5–2 times. From the result of the argon atmosphere, it is evident that the phenol can be degraded without the O-reagents. Under the air atmosphere, a part of the input energy of pulsed corona discharges is dissipated to the dissociation of the main gaseous species of N2 , while the input energy under the argon, and oxygen atmospheres is mainly consumed to produce the OH-reagents, and the O- and OH-reagents, respectively. Both concentrations of the produced reagents under the argon and oxygen atmospheres are plausibly larger than the concentration under the air atmosphere. This may result in the larger degradation rates under the argon and oxygen atmospheres than the rate under the air atmosphere. The degradation rate under the argon atmosphere roughly reflects the degradation rate via the OH-reagents, while those under the oxygen atmospheres reflect that in combination with the O- and OH-reagents. Since the degradation rate under the oxygen atmosphere is 1.5–2 times larger than that under the argon atmosphere, it is considered that the Oreagents have the degradation rate comparable to that of the OH-reagents. 2772

Figure 4. The LIF spectra of the intermediate products before (at 40 min) and after (at 100 min) the peak intensities from the intermediate products under the air atmosphere.

After 30 min, the LIF intensities under the oxygen atmospheres decrease drastically with the decay time constants of the order of 10−3 min, while the rates under the air and argon atmospheres decrease with the decay time constants as large as 41–44 min. In order to examine the influence of the intermediate products produced in the first 30 min to the phenol degradation process, the LIF intensity from the intermediate products are measured. Figure 3 shows the temporal variations of the peak intensities (at 310 nm) of the LIF spectra of the intermediate products. As mentioned in the previous paper [4], the wavelength of 310 nm corresponds to the peak wavelength of the spectrum of resorcinol. The temporal variations of the LIF intensity qualitatively reflect the variations in the concentrations mainly of DHB. The LIF intensity under the oxygen atmosphere (with a 100 sccm flow) peaks at 10 min and then rapidly decreases to zero at 35 min. On the other hand, both the LIF intensities under the air and argon atmospheres peak at approximately 30 min and decrease slower than the intensity under the oxygen atmosphere. The degradation rate of the intermediate products under the oxygen atmosphere is obviously much larger than the rates under the air and argon atmospheres. By comparing the degradation rates under the oxygen and argon atmospheres, it is considered that the O-reagents are much more effective for the degradation of the intermediate products than the OH-reagents.

LIF intensity (Arb. Unit)

LIF intensity (arb. Unit)


40 min. 1

100 min. 0 250

300

350

400

450

1

10 min.

30 min. 0 250

Figure 5. The LIF spectra of the intermediate products before (at 40 min) and after (at 100 min) the peak intensities from the intermediate products under the argon atmosphere.

350

400

450

500

Wavelength (nm)

500

Wavelength (nm)

300

Figure 6. The LIF spectra of the intermediate products before (at 10 min) and after (at 30 min) the peak intensities from the intermediate products under the oxygen atmosphere (100 sccm).

4. Conclusions

In order to investigate the changes in the intermediate products, the LIF spectra of the intermediate products before and after the peak intensities from the intermediate products (see figure 3) are examined. The spectra of the intermediate products under the air atmosphere are shown in figure 4. The spectra at 40 and 100 min correspond to those before and after the peak, respectively. The LIF spectrum from a pure resorcinol solution is also plotted as an example of the spectrum of DHB. The spectra before and after the peak are both identical to the spectrum of resorcinol. The spectra of the intermediate products under the argon atmosphere are shown in figure 5. The spectra before the peak (at 40 min) and after the peak (at 100 min) are not identical to the spectrum of resorcinol. They have a weak additional spectral component in the wavelength region of 400–500 nm. The spectra of the intermediate products under the oxygen atmosphere (with a 100-sccm flow) are shown in figure 6. The spectrum before the peak (at 10 min) is identical to that of resorcinol. Though the data points of the spectrum after the peak (at 30 min) are scattered because of low emission intensity, the shape of the spectrum at 30 min is likely to be similar to that of resorcinol. Both the spectra show no other spectral components in the wavelength region of 400–500 nm. From figures 4 and 6, phenols under the air and oxygen atmospheres are mainly converted to DHB by the addition of OH to benzene ring. On the other hand, under the argon atmosphere (see figure 5), a part of phenol is converted to unknown molecules (denoted as UM), which exhibit the LIF emission around 400–500 nm. Hence, the degradation of phenol with the O-reagents results in the conversion to DHB, while the degradation with the OH-reagents gives rise of the formation not only of DHB but also of UM. From figure 3, it is concluded that the conversion rate of phenol to DHB via the O-reagents is much larger than that via the OH-reagents.

The phenol degradation processes by pulsed corona discharges are investigated under three kinds of discharge atmosphere (air, argon and oxygen). The temporal variations of the concentrations of phenol and the intermediate products are monitored by LIF spectroscopy. The species of the intermediate products are identified by the spectral analysis. It is clarified that both the O- and OH-reagents degrade phenol to the intermediate products with the comparable degradation rates. The degradation via the OH-reagents attack gives rise to the formation of UM in addition to DHB, while the degradation via the O-reagents attack produces only DHB. The O-reagents play a decisive role in the conversion of phenol to DHB. Acknowledgments

The authors acknowledge A H F M Baede and J M Freriks for their technical support. References [1] Clements J S, Sato M, and Davis R H 1987 IEEE Trans. Ind. Appl. IA-23 224 [2] Hayashi D, Hoeben W F L M, Dooms G, van Veldhuizen E M, Rutgers W R and Kroesen G M W 2000 J. Phys. D: Appl. Phys. 33 1484 [3] Hayashi D, Hoeben W F L M, Dooms G, van Veldhuizen E M, Rutgers W R and Kroesen G M W 2000 Appl. Opt. submitted [4] Hoeben W F L M, van Veldhuizen E M, Rutgers W R and Kroesen G M W 1999 J. Phys. D: Appl. Phys. 32 L137 [5] Hoeben W F L M 2000 Pulsed-corona induced degradation of organic materials in water PhD Thesis Faculteit Natuurkunde, Technische Universiteit Eindhoven, The Netherlands ISBN 90-386-1549-3 [6] Joshi A A, Locke B R, Arce P and Finney W C 1995 J. Hazard. Mater. 41 3 2773

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