Article pubs.acs.org/IECR
Effect of Ozone and Sulfur Dioxide on the Photolytic Degradation of Chlorobenzene in Air Lianfeng Zhang and William A. Anderson* Department of Chemical Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada, N2L 3G1 ABSTRACT: The simplest chlorinated aromatic compound, chlorobenzene, was selected as a model organic pollutant to examine the photochemical destruction process in the gas phase, for potential application to flue gas treatment where oxygen, water, and sulfur dioxide may also be present. The degradation products under various reaction conditions were detected and found to include benzene, phenol, chlorophenol, chlorobiphenyls, HOCl, and HCl. The degradation pathways were elucidated based on the detected products and the literature. It was confirmed that dechlorination occurs before the aromatic ring is broken and that biphenyls can be formed. However, in the presence of added ozone, the formation of chlorobiphenyls was suppressed. The presence of sulfur dioxide resulted in attachment onto the aromatic ring, as confirmed by the S−C bond in the FTIR spectrum, and the formation of a benzenesulfonic acid intermediate. gaseous CB at 248 nm has been reported,11 it was of interest to identify the major degradation pathways in the presence of other likely gas-phase components such as ozone, oxygen, nitrogen, sulfur dioxide, and water vapor. The kinetics of such reactions were recently reported,12 but here the focus was on determining the major reaction products and likely pathways.
1. INTRODUCTION Ultraviolet (UV) photochemical technologies have been attracting attention as possible alternatives for the destruction of pollutants in water and air. UV photons have sufficient energy to break some chemical bonds and initiate radical-chain reactions, such that organic pollutants in an air stream can potentially be degraded through illumination with UV. In comparison with water treatment, photodegradation in the gas phase has some advantages. UV light can often be transmitted farther in air than in water, and there are fewer or no inorganic chemicals that consume radicals and reduce the efficiency. Also, there is plentiful oxygen in the air phase for oxidation reactions, and it is easy to introduce ozone into the system to enhance the reaction via the generation of additional radicals.1−3 A series of reactions will occur among these radicals, and the organic pollutants will be decomposed through the combination of direct and indirect photochemical reactions.4−6 It has been reported that UV photooxidation is feasible for the treatment of gaseous effluents contaminated with chlorinated volatile organic compounds (VOCs),7,8 but research on the photolytic decomposition of chlorinated aromatic compounds has not been widely reported. The resonance structures of these compounds increase their chemical stability, and halogenated aromatic compounds have long-term stability in most natural environments. As toxic environmental pollutants,9 they are of significant concern in air emissions such as waste incineration, iron sintering, base metal smelting, and other industries.10 For the treatment of chlorinated aromatic compounds, UV photolysis technology has some additional advantages because of the strong absorbance of 254 nm UV light by aromatic ring structures. To apply this technique effectively in industry, the reaction mechanism must be clarified to understand the products, pathways, and other factors that influence the performance as well as reaction byproduct formation that might have negative environmental impacts. In this study, we chose the simplest chlorinated aromatic compound, chlorobenzene (CB), as a model to investigate the degradation products under different reaction conditions. While the primary photodegradation of © 2013 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Reagents and Analytical Instruments. Chemicals were used as received without further preparation. Methanol was gas chromatography−mass spectrometry (GC−MS) grade (Sigma-Aldrich, Ontario, Canada), chlorobenzene (CB) >99.5% (Sigma-Aldrich, Ontario, Canada), benzene >99% (VWR, Ontario, Canada), and sulfur dioxide (SO2) 100% (Praxair, Ontario, Canada). Ozone was generated as needed by an ozone generator (Ozomax, Québec, Canada). A gas chromatograph−mass spectrometer (CP-3800 with Saturn 200, Varian) was equipped with a VF 5 ms capillary column (30 m, 0.25 mm i.d., 0.25 μm film, Varian) and an electron impact detector. The software “NIST Mass Spectral Search Program version 1.7” was used to identify MS spectra. Some MS spectra were identified as one of several possible candidates by the software. To confirm the identity of these peaks, all likely candidate chemicals were injected into another gas chromatograph (HP5890 series II equipped with a flame ionization detector and a 30 m × ⌀ 0.53 mm RTX502.2 column), and the retention times were compared. Other analyses were performed using ion chromatography (DX500 Chromatography Systems equipped with a conductivity detector and an Ion Pac AS17 column, Dionex, Ontario, Canada) and Fourier transform infrared (FTIR) spectrometry (Excalibur Series, Bio-Rad, Berkeley, CA). Received: Revised: Accepted: Published: 3315
August 14, 2012 December 18, 2012 January 25, 2013 January 25, 2013 dx.doi.org/10.1021/ie302184p | Ind. Eng. Chem. Res. 2013, 52, 3315−3319
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2.2. Experiments. (1) For detection of the products under various reaction conditions, a 20 mm × ⌀ 18 mm cylindrical quartz cell was used as a reaction cell. It was flushed with the desired gases and then sealed with a rubber septum, after which 2 μL of liquid CB was injected. The cell was illuminated by a low-pressure mercury lamp with a predominant output wavelength of 253.7 nm and no 184.9 nm emission (confirmed by the manufacturer, Philips Lighting). The illumination time was set at 5 min, after which about 2 mL of methanol was injected into the cell to dissolve all of the methanol-soluble chemicals remaining inside. The solution was then analyzed by GS−MS using an oven temperature program as follows: hold at 40 °C for 5 min, then ramp to 300 °C at 20 °C/min, and hold for 12 min. (2) For detection of Cl− and ClO−, 2 mL of water was injected into the cell instead of 2 mL of methanol. After shaking to dissolve all of the water-soluble chemicals in the headspace, the water was analyzed using ion chromatography. Using Iodometric Method I from “Standard Methods for the Examination of Water and Wastewater”,13 the residual chlorine in the sample was qualitatively detected. In the case of CB degradation, the residual chlorine was presumed to be hypochlorous acid (HClO). (3) For detection of SO42‑, the reaction cell was filled with SO2 and illuminated by UV light for 5 min. Then, 2 mL of water was injected, and the cell was shaken to absorb gaseous components into the water phase. The concentration of SO42− in water was analyzed by ion chromatography. (4) For the qualitative detection of other reaction products in the presence of SO2, a photochemical reaction using very high concentrations of reactants was employed. A 22 L reactor containing a 253.7 nm UV lamp (TUV PL-L 18W/4P, Philips, Somerset, NJ) was used in a quartz sleeve. The reactor was first filled with SO2, and then about 10 L of air was flushed through the reactor. Subsequently, 10 mL of liquid CB was added, and the UV lamp was turned on after it had volatilized. After about 18 h of illumination, there was a visible quantity of solid deposits on the surface of the photoreactor quartz sleeve. These deposits were dissolved with methanol and then analyzed by FTIR. A blank test without SO2 was also conducted in a similar manner.
Figure 1. GC chromatogram of a methanol solution of the products from the photolytic decomposition of CB, with peak identifications based on GC−MS and retention time comparisons.
Table 1. Energies for Some Chemical Bonds and the Approximate Wavelengths of Light Corresponding to This Energy bond energy (kJ/mol) 850.8,14 94227 52227 49427 465,28 45927 415,28 41127 360,28 35827 348,28 34627 339,28 32727
wavelength (nm) 141,26 22927 24227 257,28 288,28 332,28 344,28 353,28
12727
26127 29127 33427 34627 36627
It has been reported that some low-pressure mercury lamps have the main output wavelength at 253.7 nm, with a small portion of total irradiation at 184.9 nm.14,15 Water vapor can absorb UV at shorter wavelengths in the 100−200 nm range to dissociate into hydrogen and hydroxyl radicals, as shown in eq 2.3
3. RESULTS AND DISCUSSION 3.1. Identification of Products. The reaction products in the gas-phase experiments were identified by GS−MS, with a typical result shown in Figure 1. The peak at about the third minute was identified to be benzene and/or cyclohexane by the MS software. For confirmation, GC retention times were compared between the sample and methanol solutions of benzene and cyclohexane, and it was concluded that the photolytic product was indeed benzene. 3.2. Pathways. There were four predominant chemicals identified inside the reaction cell: water, CB, O2, and N2. When a photon attacks chemicals to break a chemical bond, it must be absorbed and possess energy equal to or greater than the chemical bond. Table 1 lists some chemical bond energies, where it is evident that a photon with 253.7 nm wavelength cannot break O2 and N2 (and neither have substantial absorbance at 253.7 nm either). However, the Cl−C bond in the CB molecule can be broken according to eq 1:5,6 PhCl + hν → Ph•+Cl•
bond NN SO OO O−H C−H C−O C−C C−Cl
H 2O + hν → OH•+H•
(2)
A 184.9 nm UV photon can also promote dissociation of the oxygen molecules O2 + hν → O• +O•
(3)
•
and the O radical will attack H2O to generate the OH• radical2,3,5,16−18. Consequently, the following reaction scheme could be postulated: Ph•+OH• → PhOH
(4)
Ph•+H• → PhH
(5)
•
•
Cl +OH → HOCl
(6)
Cl•+H• → HCl
(7)
However, with the low-pressure mercury lamp (PL-L18W/ TUV, Philips) used in this work, there is no 184.9 nm wavelength output, and therefore the reaction sequence initiated by eq 3 cannot occur. Because a UV photon at 253.7 nm cannot dissociate a water molecule,18 it is therefore
(1)
where Ph• represents a phenyl radical. 3316
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HO• +HO• → H 2O2
proposed that the pathways under the conditions in this work are as follows: Ph•+H 2O → OH•+PhH •
(ref 5)
(8)
Ph +H 2O → PhOH + H
•
H 2O2 + hν → 2HO•
(9)
HO• +O3 → HO2 + O2
•
Cl + H 2O → HOCl + H
(10) •
HO2 + O3 → HO• +2O2 Cl•+O3 → O2 + ClO•
(ref 5) As a radical species, Ph• may also attack a CB molecule, as has also been reported in the aqueous phase5 and indeed o-, m-, p-chlorophynyl were detected (Figure 1). (13)
The role of the reactant O2 appears to be as follows: O2 + H• → HO2•
(14)
HO2• +H• → H 2O2
(15)
H 2O2 + hν → 2HO•
(16)
(refs 5, 20) HOO• +HOO• → H 2O2 + O2
(17)
(18)
PhO−O• +2H• → Ph−OH + HO•
(19)
•
The radical O can react with a water molecule. O• +H 2O → 2HO•
(27)
SO2 * + SO2 → SO3 + SO
(28)
SO + O2 → SO3
(29)
SO2 * + O2 → SO2 + O2 *
(30)
O2 * + SO2 → SO3 + O
(31)
SO2 + O → SO3
(32)
SO3 + H 2O → H 2SO4
(33)
Figure 2. Molecular structures of SO2, SO3, and SO4−.
Therefore, the presence of O2 should enhance production of the radicals, as shown in eqs 14−19. Conversely, in previous work,10,12 it has been shown that an anoxic environment inhibits the photochemical degradation of CB. 3.3. Enhancement by Ozone. When ozone is illuminated by 253.7 nm UV light, it will dissociate to generate the O• radical.2,16−18
O3 + hν → O2 + O•
SO2 + hν → SO2 *
where * represents an activated species. Figure 2 depicts the molecular structures of SO2, SO3, and SO4−. Observing the electron orbitals of the oxygen (1s22s22p4)
(refs 17, 20) The literature indicates that O2 reacts with both aromatic ring radicals and nonaromatic ring radicals.2,6 It therefore appears that phenol was also generated as follows: Ph•+O2 → PhO−O•
(26)
(ref 1) Chlorobiphenyls were not detected in the UV/O3 system likely because there are more available HO• radicals than in the UV/air system. These consume Ph• (eq 4), and thus there is less opportunity for a Ph• radical to combine with CB and form chlorobiphenyls. 3.4. Influence of SO2. For photooxidation of SO2, an interesting phenomenon was observed. According to the experimental procedure, the reaction cell was filled with SO2 and illuminated with UV light for 5 min, after which a small amount of water was injected. As soon as water was injected, a great amount of mist appeared. The mist was collected/ dissolved in water, and it was analyzed by ion chromatography, which indicated that 82.3% of SO2 was transformed to SO42−. The reaction pathways are likely the following:21,22
(12)
PhCl + Ph → Ph − PhCl
(25)
(ref 18)
In GS−MS analysis, phenol and benzene were detected (Figure 1) and analyses of inorganic ions indicated the existence of Cl− and ClO−. The ion chromatography results indicated that 18.3% of the chlorine atoms in CB were transformed to Cl−. Therefore, the products PhOH, PhH, HOCl, and HCl were experimentally identified, confirming the likelihood of the proposed pathway. The subsequent reaction must be that the generated OH• radical (from eqs 8 and 10) attacks the CB molecule. Because C−Cl bonds are relatively inert toward potential radical substitution by OH•,10,19 the product is more likely to be chlorophenol through eq 12. Chlorophynyl was detected by GC-MS (Figure 1).
•
(24)
(ref 18)
(11)
PhCl + HO• → HOPhCl
(23)
(ref 5)
(ref 5) Cl•+ H 2O → HO• +HCl
(22)
and sulfur (1s22s22p63s23p4) atoms, it is found that the sulfur atom in SO3 has one empty sp hybrid orbital. When SO2 and CB coexist, though SO2* and SO3 will also attack the benzene ring, the radical Ph• has only one free electron, which is not enough to fill the empty sp hybrid orbitals of SO2* and SO3. Therefore, it is believed that SO2* and SO3 do not combine with Ph• and attach onto the aromatic ring. For similar reasons, it can be postulated that SO2* and SO3 cannot substitute for a hydrogen atom on the aromatic ring. However, possibly one bond of the double bond SO is broken, and the sulfur atom connects to the aromatic ring while the oxygen atom connects
(20) 2,3,5,16−18
(21)
Then a series of radical reactions will occur. 3317
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hydrogen bonds increases with increasing acidity of the hydrogen donor.25 The hydrogen bond slightly weakens the O−H bond in a water molecule because some energy may move from the O−H bond inside the water molecule to the hydrogen bond. It is evident that such an effect may enable the water molecule to react with radicals more readily, leading to a positive effect on the overall rates. On the other hand, SO2 may have a negative influence because it competes with the CB target molecule for UV photons. In other work12 in a 22 L batch reactor, the positive influence of SO2 on the rates was experimentally observed when the SO2 concentration was equal to or less than 2000 ppm in humidified air.
to a hydrogen atom. The bond energy of SO is very high (Table 1), and it is difficult to break it by photolytic action. However, when it collides with Ph•, the π bond in SO may be affected by the large π bond on the aromatic ring of Ph•; i.e., the π bond in SO breaks and the electron in the sp hybrid orbital on the sulfur side combines with Ph• and that on the oxygen side combines with hydrogen. The resulting product will therefore be benzenesulfonic acid. Ph•+SO3 + H• → PhSO2 OH
(34)
To experimentally test for the existence of benzenesulfonic acid, FTIR spectrometry was used to find the S−C bond in the decomposition products. Aguilar-Hernández et al.23 reported that, in the FTIR spectrum, the S−C bond of 4hydroxybenzenesulfonic acid sodium salt showed peaks near 694−698 cm−1. It is further known that the S−C bond peak lies in the range of 590−735 cm−1.24 In Figure 3, it can be seen that
4. CONCLUSIONS In the photochemical degradation of CB, the products phenol, benzene, chloride ion, and hypochlorous acid were detected, indicating that that the Cl−C bond in the CB molecule was broken by UV photons with 253.7 nm wavelength. Production of chlorinated biphenyls was also detected, which is a negative factor from a regulatory and environmental impact perspective. The experimental results also indicated that, under UV illumination, SO2 will be oxidized to SO3. Analysis of the electronic configuration of a SO3 molecule, and related experimental data, revealed that the sulfur groups likely attach to the aromatic ring, while SO3 also react with water to become H2SO4. The hydrogen ions form hydrogen bonds with water molecules to positively affect the photolysis of water molecules and the reaction between the water molecules and radicals, which has a beneficial effect on the degradation of CB. The introduction of ozone in a UV/O3 system can enhance the photolytic degradation process via an increase in the radical production, and this suppresses the direct attack of Ph• on CB and the formation of chlorobiphenyl intermediates.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: (519)888-4567. Fax: (519)888-4347. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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Figure 3. FTIR spectra of methanol solutions of the products of photolytic decomposition of CB, with and without SO2 present. The spectra of three standards are shown for comparison.
ACKNOWLEDGMENTS The authors are grateful to Ralph Dickhout and Joe Clifford for their valuable technical assistance with ion chromatography and GC−MS analyses. Portions of the work were funded by the Natural Sciences and Engineering Research Council of Canada.
4-chlorobenzenesulfonic acid has no peak near 694−698 cm−1, but there are three peaks at 644, 652, and 711 cm−1 within the 735−590 cm−1 range, and one of them should be the S−C bond. This indicates that, even with a similar aromatic structure, S−C bond peaks can lie at different wavenumbers, and it is reasonable to consider that the S−C bond peak of benzenesulfonic acid could be at a wavenumber other than 694−698 cm−1. In Figure 3, when the spectra of the decomposition products are compared, it is seen that there is an extra peak at 735 cm−1 when SO2 was present in the reaction. Therefore, we can hypothesize that the peak at 735 cm−1 is the S−C bond. Furthermore, when the spectra of the reaction products are compared with those of pure benzenesulfonic acid (Figure 3), it appears that benzenesulfonic acid is in the product mixture. Under UV illumination in the presence of water, SO2 will be converted to H 2 SO 4 . In this case, the hydrogen ion concentration (H+) increases, and this can form hydrogen bonds with other water molecules. The strength of the
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