1504
Langmuir 1993,9, 1504-1512
Photoinduced Oxidative Reactions of Dioxin and Its Chlorinated Derivative on Laponite Surfaces Yun Mao, Surapol Pankasem, and J. Kerry Thomas* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556 Received August 3,1992. I n Final Form: February 24, 1993 Photochemical reactions of dioxin and its chlorinated derivative on a sodium laponite surface were investigated. Photoirradition of DBOIlaponite (Ta = 20 "C) initiates chemical degradation of DBO, which is confirmed by diffuse reflectance spectral studies. Photoirradiation on DBOIlaponite (Ta = 20 OC) at -196 OC generates DB@+; on warming DBO*+ recombines with electrons or electron adducts and chemiluminescence is observed. Photoirradiation of DBOIlaponite (Ta = 325 "C)at room temperature produces stable DBO*+;the reaction of DBO'+ with water and pyridine can be observed in detail. Timeresolved diffuse reflectance spectral studies give information on the primary processes on the surface. End product analysis by HPLC indicates that the photoproducts of DBO degradation on laponite are the same atall activation temperatures for the solids. This supports the premise that DBO degradation is a radical process in nature, Le., radical cations are initially formed through one-electron oxidation, with subsequent reaction of ions with water or bonded hydroxyl groups on the surface. Reaction of DBO'+ with pyridine is shown to be controlled by pyridine diffusion into the inner layer space. Reaction mechanisms of DBO*+ with water and pyridine are proposed.
Introduction Chlorinated dibenzo-p-dioxins (dioxins, DBO), the structure is shown in Scheme I, are a group of hazardous chemicals, some of which have highly toxic, teratogenic, mutagenic, and potential carcinogenic properties,'" and are the subject of some concern. Large quantities of dioxincontaminated manufacturing wastes are buried at various sites in the USA.4 The hazardous nature of these compounds is compounded by their stability and wide spread presence in the environment. Dioxins are thermally stable and are formed during the thermal combustion of chlorinated organic wastes. Although occurring only sparing in nature, they are dangerous when introduced into the foodwebs, and eventually into the public at large. There has been a growing interest to find efficient and economically feasible methods for treating such pollutants. One important aspect is to establish the reaction mechanism of transformation or decomposition of dioxins into less hazardous forms. On this basis, the chemical processes in the environment are established and possible treatment of these pollutants can be designed. The present report is projected to explore the mechanism of photoinduced chemical reactions of dioxins on clays, as clay and clay minerals possess a unique position in the environment. Heterophotocatalysis on insulators may be a potential method for such treatment, on the basis of the chemical interaction between photoexcited states and active surfaces. The laboratory scale experiments are minimodels for environmental processes and potentially provide valuable information for a solution to the above problem. The thermal formation of aromatic complexes and radical cations on ion-exchanged layered silicates has been
* Author to whom correspondence should be addressed.
(1) Hutzinger, O.,Cru"ett, W.,Karasek,F. W., Merian,E.,Reggiani, G., Reissinger, M., Safe, S., Eds. Chlorinated Dioxins and Relatited Compounds 1985; Proceedings of the Fifth International Symponium. Chemosphere 1986,15, Nos. 9-12. (2) Fellman, A. Dioxin in the Environment: Ita Effect on Human Health; American Council on Science and Health: New York, 1985. (3) (a) Rappe, C., Chowhary, G., Keith, L., Eds. Chlon'nuted Dioxin andDibenzofuran.9 in the TotalEnuironment;LewisPublishers:Chelsea, MI, 1986. (b) Hanson, D. J. Chem. Eng. News, 1991, Aug. 12,7. (4) Esposito, M. P.;. Tiernan, T. 0.; Dryden, F. E. Dixon; U.S. Environmental Protection Agency: Cincinnati, 1980, p 275.
studied.- Formation and polymerization of dioxin radical cations on copper(I1)-exchanged smectite have also been reported by Mortland: as well as the photocatalytic degradation of chlorinated dioxins in aqueous semiconductor suspensions.1° Little if any attention has been paid to photoinduced reaction on clay surfaces. In previous studies, the formation and reactions of pyrene radical cations on alumina, silica-alumina, and laponite surfaces have been reported.11-16 In this report, dioxin (DBO) and 2-chlorodioxin (CDBO) were selected as model compounds, and sodium laponite (henceforth referred to as laponite) was used as a clay model system to study photochemical reactions of DBOs on surfaces. Laponite is a synthetic hectorite, with substitution of Mg2+ by Li+ in its layer structure, and is a suitable organized medium for heterophotochemical reactions. In order to understand the mechanism of photoinduced reactions of DBOs on laponite, a comparison of reactions on nonactivated and activated laponite was performed. This study was carried out in two steps: firstly, photoproducts were observed on the surfaces by steady-state diffuse reflectance, fluorescence and EPR spectroscopy, and timeresolved diffuse reflectance spectroscopy; secondly, separation and assignment of the chemical products were conducted by HPLC and spectral methods after dissolving the sample in aqueous solution. On this basis the chemical reaction mechanism of the surface species was elucidated. The reaction mechanism of DBO and CDBO on insulator surfaces can provide valuable information at the molecular (5) Mortland, M. M.; Pinnavania, T. J. Nature (London),Phys. Sci. 1971, 229, 75. (6)Pinnavaia, T. J.; Mortland, M. M. L. Phys. Chem. 1971, 75,3957. (7) Pinnavaia, T. J.; Hall, P. L.; Cady, S.S.; Mortland, M. M. J.Phys. Chem. 1974, 78,994. (8) Soma, Y.; Soma, M.; Harada, I. J. Phys. Chem. 1986,89, 738. (9) Boyd, S. A.; Mortland, M. M. Nature (London) 1986,316,532. (10) Pelizzetti, E.; Borgarello,M.; Minero,C.; Pramauro, E.; Borgarello, E.; Serpone, N. Chemosphere 1988,17,499. (11) Pankasem, S.; Thomas, J. K. J . Phys. Chem. 1991,95,6990. (12) Pankasem, S.; Thomas, J. K. Langmuir 1992,8, 501. (13) Liu, X.; Thomas, J. K. Langmuir 1991, 7, 2808. (14)Liu, X.; Iu, K.-K.; Thomas, J. K. Langmuir 1992,8, 539. (15) Iu, K.-K.;Liu,X.;Thomas, J. K. Chem.Phys.Lett. 1991,186,198. (16)(a) Mao, Y.; Thomas, J. K. Langmuir 1992,8,2501. (b) Mao, Y.; Thomas, J. K. J. Chem. Soc., Faraday Trans. 1992,88, 3079.
0743-7463/93/2409-1504$04.00/00 1993 American Chemical Society
Photoinduced Oxidative Reactions of Dioxin
level for understanding degradation processes in the environment. Experimental Section Chemicals. DBO and CDBO (Chem Service) were used as received. Sodium laponite (RDS) was supplied by Laporte Industries. The composition of the laponite, as given by the manufacturer, is as follows: SiOz, 55.6%; MgO, 25.1%; Na20, 3.6%; LizO, 0.7%; KzO, 0.2%; TiO2,0.15%; AlzOs, 0.08%;CaO, 0.06%;FezOs, 0.04%. The solid powder was either directly used as received, denoted as nonactivated (Ta = 20 OC; Ta is the activation temperature of the solid), or pretreated with heating activation depending on the experimental goals. The remaining compounds were generally of the highest purity commercially available. Instrumentation. In order to determine absorption spectra of solid samples, steady-state diffuse reflectance spectra were measured using a UV-visible spectrophotometer (Perkin-Elmer 552) equipped with an integrating sphere accessory. The apparatus can be operated in two modes, i.e., giving either the relative diffuse reflectance intensities, RT, or the logarithm of ~ / R T-log , RT. At low absorbances RT and -log RT can be used instead of the Kubelka-Munk function to express the absorption spectra of the solid samples.ll Steady-state emission spectra were measured on a spectrofluorometer (Perkin-Elmer 44B) equipped with a 250-W Xe lamp. EPR spectra were recorded on a Varian Associate, E-lines century series,equipped with a X-band klystron and a rectangular cavity in the TElm operation mode;g-values were determined by comparison with DPPH (diphenylpicrylhydrayl). The cavity has a radiation slit allowing light to excite samples. Samples for EPR measurements were contained in 5 mm 0.d. quartz tubes, which were joined to wider glass tubing to be attached directly to a vacuum line. Time-resolved diffuse reflectance spectra were measured by laser flash photolysis, which was described elsewhere." In brief, a 308-nm laser pulse (energy density, 70 mJ/cm2;pulse width, 10 ns) from a XeCl excimer laser (Lambda Physik, Model EMG 100) was used for excitation of samples. A 450-W xenon lamp was used as the analysis source. The diffusely reflected monitoring light from the sampleswas transferred to a monochromator by an optical fiber and detected by a photomultiplier tube. The output of the photomultiplier was taken to a Tektronix 7912A digitizer, connected to a Zenith data system. Separation and quantitative identificationof chemicalproducts were performed by a Waters' high-performance liquid chromatograph equipped with an Adsorbosphere Phenyl column (Alltech) and UV-absorption detector. The eluent was aqueous acetonitrile. Identification of chloride ion was achieved by using a Waters' high-performance ion chromatograph equipped with an IC-anion column and a conductivity detector (Waters 430). The eluent was borate/gluconate at pH 8.5. The photoirradiation was carried out either by a Xe lamp (150 W) equipped with a 20-cm water filter to remove IR radiation or in achemical photoreactor with a merry-go-roundand equipped with fluorescence lamps RPR 3000 A (Rayonet, Southern New England Co.). The photoirradiation intensities were estimated by using the chemical actinometer KaFe(C2Oda. Sample Preparations. Solid samples for diffuse reflectance spectral and luminescence spectral studies were prepared as follows: 0.5 g of solid powder was heated in a crucible under air at selected temperatures for 20 h. After thermal activation the powder sample was cooled in a desiccator to room temperature. Immediately after cooling the powder was mixed with an aliquot of adsorbate in an aerated cyclohexane solution. In a 1mL 10-9 M DBO cyclohexane solution more than 99% of DBO was adsorbed on laponite (0.50 g) (Ta = 130 "C), which is similar to pyrene.13 Solid samples were dried in air at about 30 OC. The dried samples were evacuated for 1h in a rectangular quartz cell (1mm thickness). For time-resolved measurements, the cell was repositioned prior to each laser excitation in order to expose a fresh surface. Samples for HPLC and ion chromatography were prepared as follows: the irradiated samples were extracted with aqueous methanol solutions under nitrogen- or air-atmosphere; the
Langmuir, Vol. 9, No. 6,1993 1505 Table I. Spectroscopic Data of DBO and CDBO on Laponite DBO CDBO 235 (224,228)b 240 (229,232)b absorption band/nm 306 (290)b 305 (294)b red shift/nm 10-15 -10-15 354 (294,328, 350 (295,330, fluorescence/nm 345)b 340)* absorption bands ofthe 350,420,590 340,420,580 photoinduced species on laponite (Ta = 20 OC)/nm absorption band of radical 640: 664,c685 660: 690,725= (660)d (675)d cations on laponite (Ta = 325 OC)/nm
-
0 Compounds were loaded in air and then degassed to photoirradiate at room temperature. Loading amount: 1.5 X 1W mol/g. The corresponding experimentaldata in solutions,see text. c These peaks appear as shouldes of main peaks. d See ref 20.
extracts were centrifuged and filtered thourhg a Millipore filter (0.45 rm) to remove most of the powder. The filtrates were subjected to HPLC.
Results 1. Interaction of DBO and CDBO Molecules with Laponite Surfaces. Details of diffuse reflectance and fluorescence spectra of DBO and CDBO adsorbed on laponite surfaces are summarized in Table I. As an example, DBO will be described in detail subsequently. Diffuse reflectance spectra of DBO adsorbed on laponite surfaces show absorption bands at 235 and 306 nm (Figure 1). Compared with the spectrum of DBO in cyclohexane solution (Figure 1,dotted line), the following features can be observed on the solids: (1)A bathochromic shift of Ca. 10-15 nm and a band broadening. The extent of the spectral shift and the broadening increases with activation temperature of the laponite. The structured bands a t 206, 224, and 228 nm in solution become a single band at 235 nm on the solid surface. (2) A change in the relative intensity of absorption bands. On the solid samples the intensity of the 306-nm band is comparable with the 235nm band, while the corresponding band in solution at 290 nm is much weaker. (3) An extension or tailing of the 306-nm absorption band a t the red edge. Over the temperature range of 20-350 OC, a higher activation temperature results in more tailing at the red edge. The bathochromic shift and the broadening of DBO absorption bands result from interaction of the molecules with the solid surface. It is established that organic compounds form complexes with clay~.~J'Such phenomena were also observed for other organic molecule/solid systems.18 The *-electrons of the phenyl rings in DBO react with Lewis acid sites on the laponite surface to form *-electron c o m p l e ~ e s .In ~~ addition, ~ oxygen in DBO may be considered to be a weak Lewis base, which may also interact with the surface Lewis acid site. These interactions influence the energetics of the adsorbed molecule. The change in the relative band intensities indicates conformational change in the molecular structure due to the surface interaction. DBO molecules in bulk solution adopt a folded conformation at an angle of 165O to the axis connecting the two heteroatoms.19 The interaction of DBO with the surface enhances the transition a t 290 nm, which is prohibited in solution, while the transitions at 224 and (17) Theng, B. K. G. The Chemistry of Clay-OrganicReactions; John Wiley & Sons: New York, 1974. (18) OeUuug,D.; Flemming, W.; Fuellerman,R.; Guenther,R.; Honner, W.; Krabichler, G.; Schafer, M.; Uhl, 5.Pure Appl. Chem. 1986,58,1207. (19) Fraonza, G.; Ragg, E. J. Chem. Soc., Perkin Trans. 2 1982,291.
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1506 Langmuir, Vol. 9,No. 6, 1993 c
-+ d
EL
eo
3
0.15
0.10
0.05
360
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440
Figure 1. Diffuse reflectancespectraof DBO on laponitesurfaces measured at room temperature. The laponite were activated at different temperatures, from bottom to top, 20,210,and 325 O C . DBO loading concentration: 2 X 1o-B moVg. For comparison, the absorption spectrum of DBO in cyclohexane solution is also shown (dotted line).
'OD' Scheme I
(DBO)
'\
410
370
450
490
530
Wavelength, nm
520
Wavelength, m
Diaxln
330
290
250
0
Figure 2. Luminescence spectra of DBO solid line, DBO/ laponite samples, 2 X 1o-B mol/g; dotted line, DBO/isopentane P M. The fluorescencespectra were recorded at sample, 5 X 1 room temperature, with an excitation wavelength of 230 f 10 nm;phosphorescence spectra were recorded at -196 O C , and the excitation source is a 150-Wxenon lamp with UV filter (2-400 nm).
3
1
3 6
[ DBO / lap.]*
5
4
-
-196=C
[ DBO" + e ~ / l a p . ] ~ , 9 6 0 C
228 nm, which are dominant in solution, are diminished on surfaces. Figure 1 shows that the absorption band extends into the region 310-360 nm. This is attributed to formation of surface charge-transfer complexes (CT complex) between DBO molecules and the solid surface (Scheme I). Supporting evidence for this hypothesis is the formation of radical cations under photoirradiation with a photoenergy lower than that of the absorption edge of the parent compound in solution. This will be shown later. The steady-state emission spectra of DBO/laponite (Ta = 130 "C)are shown in Figure 2 and exhibit a broad fluorescence band a t 355 nm, while solutions of DBO show bands at 294,328,and 340 nm (dotted line in Figure 2). On the surface the band corresponding to 328 nm in solution is significantly diminished, and the band corresponding to 340 nm in solution is enhanced. The interaction of the DBO with the surface modifies the observed spectroscopy. The significant effect is the formation of CT complexes that results in a red shift of the DBO absorption spectrum.
200
280
360
440
520
Mxl
680
Wavelength, nm
Figure 3. Evolution of diffuse reflectance spectra of DBO/ laponite (Ta = 20 " C ) measured at room temperature. Samples were irradiated with RPR 3000 A lamps (-4 X 10-9 einstsin/ (min cm2))at room temperature. Irradiation times from bottom to top, were0,2.0,4.0,8.0, and 12min. DBO loading concentration was 2 X 1o-B mol/g. 2. Photoinduced Chemical Reactions on the Surface. Irradiation of DBO/laponite (Ta = 20 "C) samples through a Pyrex glass filter (A >300 nm) a t room temperature leads to a rapid degradation of the DBO. On irradiation the originally white sample gradually becomes brown as shown in Figure 3. The photochemical products formed during the photolysis show absorption abands at 340, 420,and 580 nm. In addition, the peak at 306 nm gradually increases and broadens during the photoirradiation. The studies show that the product absorption
Photoinduced Oxidative Reactions of Dioxin
Langmuir, Vol. 9, No. 6, 1993 1507
i
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Wavelength, nm
Figure 4. Diffuse reflectance spectra of DBO/laponite (Ta = 20 "C). The samples were irradiated with lamps RPR 3000 A at -196 O C . Solid line was measured at --160 O C . Others were measured at room temperature; see text. Irradiation times, from bottom to top, were 5.0, 9.0, 15.0, and 20.0 min. DBO loading concentration was 4 X lW mol/g.
bands overlap the absorption region of the starting compound. The photoinduced reactions occur in both aerated and degassed samples, and in the presence of oxygen an enhancement of ca. 20% is achieved for the photoinduced absorption band. EPR measurements of these DBO/laponite (Ta = 20 "C)samples exhibit weak radical signals. Photoirradiation of DBO/laponite (Ta = 20 O C ) at -196 "C gives rise to spectra that are different to those at room temperature (Figure 4). After photoirradiation, the samples become blue in color, and diffuse reflectance spectra show a large absorption band at 685 nm with shoulders at 640 and 664 nm, but no observable absorption bands at 340,420, and 580 nm. The 685-nmband is assigned to DBO'+, the documented absorption spectrum of DBO*+ in homogeneous solution exhibits an absorption band at 655-660 nm.20*21The shift of the absorption band on the solid compared to solution is due to interaction of DBO'+ with the surface microenvironment. Such phenomena are observed also for other radical cations on surfaces.16 The blue color fades visually at temperatures above --lo0 "C,and simultaneously a thermal chemiluminescence with ,A at -340 and -430 nm is observed (Figure 5). The spectrum was not recorded under isothermal steady-state conditions, which may lead to some distortion of the spectrum. However, accordingto the band positions, the chemiluminescencebands are assigned to fluorescence and phosphorescence of DBO, respectively. The phosphorescence intensity is much stronger than that of fluorescence. It may be related to the formation mechanism of the thermal chemiluminescence; i.e., the neutralization of the cations leads to triplet states and triplettriplet annihilation leads to the excited singlet. Such results are observed for the neutralization of wurster's blue perchlorate cations.22923 At room temperature the (20)Yang, G. C.; Pohland, A. E. Cation Radicals of Chlorinated Dibenzo-p-Dioxins,In Chlorodioxins-Origin andFaate, 1972; Adv. Chem. Ser., 120; American Chemicai Society: W a e h m n , DC, 1973;pp 33-43. (21) Hammel, K. E.;Kalyanaman, B.; Kirk, T. K. J.Biol. Chem. 1986, 261, 16948.
(22) Weller, A.; Zachariaese, K. J. Chem. Phys. 1967,46,4984.
8
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x l/lCil, Phos$orescence 0
A
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A
0.
250
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350
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500
Wavelength /nm
Figure 5. Spectrum of thermal chemiluminescence,the sample DBO/laponite (Ta = 120 "C) was irradiated under - 4 X 10.8 einstein/(min cm2) for 5 min at -196 O C , then brought to room temperature to record the chemiluminescence. The left side of the figure is the fluorescence, and the right side the phosphorescence (this is reduced 1/10). 5
9
0.3
f I'
I..
:I
0.2
0.1
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600
680
Wavelength, nm
Figure 6. Diffuse reflectance spectra of DBO on laponite (Ta = 325 "C) measured at room temperature. Sampleswere aerated laponite suspensions,DBO loading concentration 2 X 1W mol/g. Irradiation times, from bottom to top, were 0, 1.0, 3.0, and 7.5 min; RPR 3000 A lamp, -4 X 10-9 einstein/(min cm2).
annealed sample exhibits a weak absorption in the region of 340-550 nm. The experimental finding indicates that the luminescence results from recombination of radical cations with electrons (or electron derivatives) and that only a minor part of DBO+ is converted to products (see Scheme I). Repeating this procedure several times, Le., first irradiation at -196 "C, followed by warming to room temperature, shows that the absorption band (340-550 nm) slowly increases after every cycle and the final spectrum is basically consistent with the spectra obtained from the room temperature irradiation (Figure 4, dotted and dashed lines). Photoirradiation of DBO/laponite (Ta = 325 "C)at room temperature gives rise to chemical products whose spectra differ from those on nonactivated surfaces under the same irradiation conditions (Figure 6) but to some extent are similar to those on nonactivated surfaces irradiated at low (23) Weller, A.; Zachariasae, K. J. Chem. Phys. Lett. 1971, 10, 197.
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1508 Langmuir, Vol. 9, No.6,1993
temperature. These samples also exhibit a blue color which is similar to that observed on the sample irradiated at -196 OC. A large absorption band at 685 nm emerges with shoulders at 640 and 664 nm. This indicates that the formation of DBO'+ is the main reaction on activated laponite. However, there are also absorption bands at 340 and 420 nm. These bands are coincident with the results in Figure 3. 3. EPR Observation of DBO and CDBO Radical Cations on the Surface. After cooling a laponite powder activated at Ta = 325 "C and immediately mixing with an aliquot of DBO solution in cyclohexane, the aerated reaction suspension exhibits a weak EPR absorption due to thermal formation of radical species. Photoirradiation, however, generates much stronger EPR absorption signals. Figure 7a shows a typical first derivative EPR absorption spectrum of DBO/laponite (Ta = 325 "C) sample. The EPR absorption is mainly ascribed to DBO'+, because the EPR signal exists also in aerated samples, whereas neutral and anionic radicals of DBO are expected to react with 0 2 . Comparison to DPPH gives a g value of 2.0034 which is close to that of DBO*+in the HzS04/Hz02 system.% The formation of DBO'+ on an activated surface (Ta = 325 "C) is irreversible. The cessation of irradiation does not diminish the EPR signal, and the DBO'+ in aerated laponite/cyclohexane suspension is stable at room temperature for several days. The photoionization of DBO on nonactivated laponite (Ta = 20 OC) at -196 "C also produces an EPR absorption signal, and increasing the temperature to -120 OC causes the EPR absorption to slowly disappear. This is consistent with the spectra studies (see Figure 4). The EPR spectrum of DBO on laponite shows asymmetrical hyperfine structure with seven lines and a splitting of 0.5 G (Figure 7a). The central line is particularly strong. The hyperfine structure of DBO'+ on surfaces is different from that in solution, where DBO'+ shows a quintet spectrum with intensity ratio of 1:4:6:4:1.'2QN The hyperfine structure is usually the result of interaction of the unpaired electron with protons. For DBO'+ in solution it was proposed that the protons at the 2,3,7, and 8 positions are responsible for the hyperfine splitting, while protons at 1, 4, 6, and 9 positions offer negligible spin densities. This is similar to the assignment for the radical cation thianthrene.= The molecular conformation of adsorbed DBO'+ is different from that in solution and results in a different interaction between the protons and the unpaired electron. CDBO adsorbed on the laponite surface (Ta = 325 "C) also exhibits EPR absorption signals (Figure 7b). Again, the EPR spectrum of CDBO'+ on the surface is different from that in solution. In solution CDBO'+ shows fine structure20 but not on the surface, as the interaction of CDBO'+ with the surface submerges any subtle spectral differences. 4. PhotoionizationProcess of DBO on the Surface. Figure 8 exhibits the formation process of the DBO'+ species on a laponite surface through selective photoexcitation. The sample of DBO/laponite (Ta = 325 "C) was first irradiated with a cutoff filter (Kopp 0-52, X >350 nm), and the DBW+ yield reached saturation after 2 min. The photoenergy was increased by replacing the cutoff filter (Pyrexglass filter, X >300 nm). Saturation of DBO*+ was reached at 7 min when a new cutoff filter (Pyrex glass) was used. Under this irradiation procedure, the DBO species are selectivelyexcited according to their absorption (24) Tozer, T. N.; Tuck, L. D. J. Chem. Phys. 1963,38, 306. (26) Shine, H.J.; Piette, L.J. Am. Chem. SOC. 1964,29, 21.
.I
5 gauss /em 3380 gauss
Figure 7. First derivative EPR abeorptionspectra. (a,top) DBO/ laponite, (b, bottom) CDBO/laponite; laponite Ta = 325 O C , loading concentration 2 x 10-8 moVg; EPR spectrum record conditions,microwave power 10m W microwave frequency,-9.5 GHz; modulation frequency, 100 kHz; modulation amplitude, 1 G; receiver gain factor, -1.25 X 104.
bands. The formation of DBW+ reaches saturation with each irradiation energy. A later experiment will show that in the present system the photoionization is a monophotonic process (Figure 9). It is shown that photoirradiation with an energy lower than that of the DBO absorption edge also generates radical cations. This supports the hypothesis of the formation of CT complexes on the surfaces. Adsorbed DBO molecules react with surface sites to form CT complexes, which are characterized by an additional absorption band (see Figure 1). Formation of such a complex was reported for aromatic hydrocarbons on alumina and silica-alumina.16Vsn Figure 9 shows the effect of the irradiation intensity on the DBO'+ yield at 20 OC. Over a range of low irradiation
Photoinduced Oxidatiue Reactions of Dioxin
Langmuir, Vol. 9, No. 6,1993 1509
h
ci
b
t "
I
i
2
0
4
i
6
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Figure 8. Relative intensity of EPR absorption increases with the photoirradiation, the dotted lines indicate the ranges with different photoirradiation energies,which are adjustedby cutoff filters: I, Kopp &52, A >350 nm;11, Pyrex glass filter, A >300 nm;111,no fiter. The photointensitiesare 1.7 X W,5.3 X W, and 5.9 X 10-8 einetein/(min cml), respectively. Photon source was a RPR 3000 A lamp.
m
I
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4m
Wavclcnpth
m
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nm
Figure 10. Time-resolved diffuse reflectance spectra of DBO/ laponite sample. The spectra were taken 50 pa (O), 100 pa (+), and 1.0 me (01,see text.
80
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Figure 9. Dependence of radical cationyield on photoirradiation intensity. Irradiation was carried out with RPR 3000 A lamps at room temperature; photoirradiation intensity was measured by use chemical actinometer We(CSO#. EPR measurement conditions were the same as in Figure 7.
intensities there is a linear relationship between irradiation intensity and DBO'+yield. With high irradiation intensity the DBO'+ yield reaches a plateau. Most DBO molecules on the surface are ionized under high photoflux leading to saturation. The presence of 02 has a positive effect on formation of DBO'+. This indicates that the photoionization is not a multiphoton excitation involving triplet states as intermediates, as 02effectively quenches triplet states. It is suggested that the photoionization of DBO on an activated surface at room temperature is a monophotonic process. 5. Time-ResolvedLaser Photolysis. Figure 10 shows the time-resolved diffuse reflectance spectra of degassed DBO/laponite (Ta = 20 "C)taken 50 pa, 100 pa, and 1 ms after a laser flash. Absorption bands at 420,650,and 685 nm were observed. The band at 686 nm is ascribed to DBO'+ as in steady-state irradiation. The formation of DBO'+ is rapid and within the resolution time of the system -100 ns. DBO'+ is stable over the time scale of the experiment, i.e., 6ma. However, DBO*+totally disappears -10 min after the laser flash, which indicates subsequent reactions of DBO'+ on the surface. This will be discussed later. As in steady-state photoirradiation experiments the (26) (a) Oelkrug,D.;Erbw,H.;Plnuachinat,M. 2.Phys. Chem.Munich 1976. W3.283. (b) OelJtru~, -. D.: . Plawhinat.. M.:. Keseler. R.W.J. Lumin. lW6;18119,434. (27) O e w ,D.;Krabichler, G.;Honnen, W.;Willdnson, F.;Willeher, C.J. J. Phys. Chem. 1988,92,3589.
~
200
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360
600
520
440
680
Wavelength. nm
Figure 11. Diffuse reflectance spectra of DBO/laponite (Ta = 325 "C), samples were irradiated with RPR 3000A lampa. Solid anddottedlinesarebeforeandafter thecontactwithwatervapor, e mol/g. respectively. DBO loading concentration was 2 X l
presence of 02promotes the electron transfer, the aerated sample has ca. 20% higher yield than that in degassed sample. From 50 ps to 1 me after laser flash the bands at the range of wavelength of 350-450 nm exhibit minor but complex change, while the band of interest at wavelength range of 500-620 nm significantly decreased. This indicates that some intermediate species react during this period. Details about the time-resolved results will be described in another report.% 6. Reaction of DBO*+with Water. The photogenerated DBO'+ is strongly adsorbed to the laponite surface and cannot be removed by dried alkanes and chlo~oalkanes. However, it is extracted by water. The hydration energy of the interlayer cations allows water to enter the space and subsequently to react with DBO'+. Figure 11 shows that after contact with water vapor the DBO'+ absorption band disappears, and a broad band appears over the wavelength range 340-550 nm, while DBO is partly restored, as the blue sample changes back to white. Figure 12 shows HPLC spectra of extracts from the sampleDBO/laponite(Ta = 325 OC), which was irradiated ~
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(28) Pankaeem, 9.; Mao, Y.;Thomas, J. K. Submittadfor publication.
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1510 Langmuir, Vol. 9,No. 6,1993
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b
4
6
8
I 280
300
320
360
340
380
400
Excitvoa wivclCDlth am
\ I
10
12
14
16
18
20
Time, min
Figure 12. HPLC chromatogram of photoinduced chemical products. DBO was adsorbed on laponite (Ta = 325 "C)from cyclohexane solutions, 2 x 1 Pmol/g. The reactantsuepensions were irradiated in the photoreactor with lamps RPR 3000 A, then the reactant mixture was extracted with aqueous methanol (90% vol. methanol), me text. The HPLC conditions were: AdsorboepherePhenyl column (Alltech);eluent, 60%acetonitrile; isocratic; flow rate, 0.5 mL/min. under -4 X 10-8 einsteins/(min cm2) (lamp RPR 300 nm) for 20 min; ca. 2.0% of DBO was converted into products. HPLC chromatograms indicate that the product peaks emerge before elution of DBO. The samples of DBO/ laponite (Ta = 20 "C), which were irradiated at 20 and -196 OC,showsimilarproducts. Theproductsareascribed to quinone-like compounds and will be discussed later. After HPLC separation the chromatographic fractions were collected and spectroscopically studied. Parte a and b of Figure 13 show excitation and fluorescence spectra of fraction 1 (Fl),respectively. The excitation spectrum showsan absorptionband at 330nm, while the fluorescence spectra do not show any pH-dependence. This indicates that after hydrolysis, DBO'+ does not convert to phenolic compounds, as the fluorescence of phenolic compounds exhibits a pH-dependence. 7. Reaction of DBO'+ with Pyridine on a Laponite Surface. Addition of pyridine to a final concentration of 3 X 106 moUg reduces the blue color of photogenerated DBO*+,which slowly changes to a brown color. Figure 14 showsdiffuse reflectancespectra of a DBO/laponite sample before and after addition of pyridine. After the reaction with pyridine a broad band at 340-500 nm appears and DBO is partly restored. As indicated in Figure 16,the reaction of DBO*+with pyridine on laponite is slow, while the second-order rate constant for the reaction of the diphenylanthraceneradical cation with pyridine in solution is reported as 4.5 X lo4
1 354
400
550
500
450
600 Wavclmglh. nm
Figure 13. (a, top) Excitation spectrum obtained from F1.The sample waa concentrated by solvent evacuation and rediesolved in methanol. Emission intensities were measured at 600 f 6 nm. (b, bottom) Fluorescence spectrum of F1 recorded at room temperature. Excitation wavelength was 310 5 nm.
*
01 200
100
400
5w
m
700
Wavelength. IUII
Figure 14. Diffuse reflectance spectra of DBO/laponite (Ta = 325 "C). DBO was generated by irradiation with RPR 3000 A lamps. Solid and dotted line are before and after contact with pyridine vapor, respectively. DBO loading concentration waa 2 x 1od mol/g. A similar reactivity is expected for DBO'+. The reason for the slow decay of DBO'+ is due to the diffusion of pyridine molecules into the interlayer space of laponite before it reacts with DBO*+. This hypothesis is supported by the linear relationship between the DBO+ absorption band intensity and the square root of time (see inset in Figure 15). According to a model of diffusion in planar M-1 a-1.29
(29) Svanholm, U.;Parker, V. D.Acta Chem. Scand. 1979,27,1454.
Photoinduced Oxidative Reactions of Dioxin
Langmuir, Vol. 9, No. 6,1993 1511
0.90,
4
Q
080
Q
+
L I2
8
0.7 0 0
Square rooi of lime (mm)
0
Q 0 Q Q
n ?n "
1
0
50
100
150
200 t. min
Figure 15. Monitoring of DBO+ absorption at 685 nm. After addition of pyridine/cyclohexanethe absorption decreases with time. The inset shows a relationshipbetween absorptionintensity and square root of time.
geometry,g0an approximaterelationshipbetween diffusate concentration qt and time t is given by 0
2
4
6
8
10
12
14
16
18
20
22 ~
and a plot of qt/q, against t1/2is initially linear and with a slope related to D112. The rate of decrease of DBO*+is proportional to the amount of pyridine which diffuses into the interlayer space. Therefore the decay of the absorption band of DBO'+ can be used as an indicator of the pyridine molecule diffusion into interlayer spaces. The data also indicate that DBO molecules adsorb mostly in the interlayer spaces. The products of reaction with pyridine which are adsorbed on the surface are only removed by aqueous solution. Pyridine is a weak base and interacts with the silica gel column material. Due to this interaction pyridine exhibits a peak with long tail which occurs in the front of DBO in the chromatogram. Figure 16 shows an HPLC separation spectrum of the chemical products of DBO'+ with pyridine (see solid line). After elution of DBO a peak having the character of pyridine emerges, which is ascribed to the product of DBO'+ with pyridine.% The product is not stable in aqueous solution and after several hours decomposesto pyridine and hydrolyzed products (see dotted line in Figure 16). 8. Photochemical Reaction of CDBO on a Laponite Surface. The absorption spectra of adsorbed CDBO on laponite shows a bathochromic shift, and photoirradiation generates CDBO'+ on activated laponite surfaces and chemical products on nonactivated laponite surfaces (see Table 1). Figure 17 shows an HPLC spectrum of an aqueous methanol extract of irradiated CDBO/laponite. As in the case of DBO the hydrolysis products have shorter retention times on the reversed-phase column and emerge in front of CDBO. In addition, DBO and C1- were also observed as photochemical products of CDBO; C1- was identified by ion chromatography. DBO and C1- may be generated through a photoinduced dechlorination, Le., direct C-Cl bond cleavage, as the photon energy used (-300 nm) is also comparablewith the (Ph)C-C1 bond energy, 94.5 kcal m01-l.~lJ~The two radicals from the photolysis, C1' and (30) Meares, P. Polymers Structure and Bulk Properties; D. Van Noetrand Co., Ltd.: London, 1966; p 320. (31) Egger, K.W.;Cocke, A. T. Helv. Chim. Acta 1976,66, 1616.
Time, min
Figure 16. HPLC chromatographic spectrum of the chemical products of DBO'+ with pyridine on laponite. Experimental conditions were similar to Figure 12. The dotted line indicates the change of the sample after 6 h of aging.
DBO', react further via hydrogen abstraction with solvent. Such a process is well documented for chlorohydrocarbons.ms CDBO
-
hrlswpemion
CDBO*
-
Cl'
-
+ DBO' RH HC1+ DBO (2)
The photoinduced dechlorination of CDBO on laponite surfaces gives similar chemical yields to that in homogeneous solution,while under the same irradiation conditions no enhancement of dechlorination on the surface is observed in the present system.
Discussion Assignment of Chemical Products and Proposal of Reaction Mechanism. On the basis of the experimental data and previous workMa7the hydrolysis products of DBO*+on laponite surfaces are assigned to quinone-like compounds. This assignment is supported by the following: The mode of HPLC separation on the reversed-phase column predicts that more polar samples will exhibit (32) Davideon, €2. S.;Goodin, J. W.; Kemp, G. Adv. Phys. Org. Chem.
1984,20, 191.
(33) Ruzo, L.0.; Zabik, M. J.; Scbuetz, R.D. J. Am. Chem. SOC.1974, 96,3809. (34)Ruzo, L. 0.; Safe, S.;Zabik, M.J. J. Agric. Food. Chem.1976,29, 594. (35) Bunce, N. J.; Kumar, Y.; Ravanal, L.; Safe, S. J. Chem. SOC., Perkin Tram. 2 1978,880. (36) Bard, A. J.; Ledwith, A. and Shine,H. J. Formation, Properties and Reactions of Cation Radicals in Solution,in "Advances in Physical Organic Chemistry; Gold,V., Bethell,D., W.;Academic Press: London, 1978; Vol. 13. (37) Harmrich, 0.; Parker, V. D. Kineticsand Mechanism of Reactions of Organic Cation Radicals in Solution. In Advance in Physical Organic Chemistry; Gold, V., Bethell,D., Eds.;Academic Press: London, 19&1; VOl. 20, p 66.
Ma0 et al.
1512 Langmuir, Vol. 9, No.6, 1993
-3 -'a
2
rapidly deprotonates resulting in a conjugatedDBO(OH)* radical. DBO"
8.0
a
2
--
7.0
0
.-
'
-
+ H,O
6.0
DBO(0H)' + DBO"
4.0
3.0
-
2.0
CSHSN
0
(3)
DBO + DBO(=O)
+ H+
(5) This agrees with the half-regeneration m e c h a n i ~ m . ~ ~ In electrochemistry, reactions of pyridine with radical cations of diphenylanthracene, perylene, and thianthracene were r e p ~ r t e d , ~ ~and * ~DBO'+ l t ~ may undergo similar processes. Compared to the reaction of perylene radical cations with pyridine,%the reaction of DBO'+ with pyridine may be considered as follows, pyridine molecules first diffuse into the interlayer space of laponite, then add to DBO*+to form an intermediate complex
5.0
1.o
DBO(OH2)'+
DBO(OH,)*+ DBO(0H)' + H+ (4) DBO(0H)' radicals react further with DBO'+ through electron transfer resulting in formation of dioxinquinone and DBO.
.-E
L 0
2
4
--
(6)
--
(8)
CSHSNad
CsHSNad DBO'+,d [CsHsN+-DBO'],d (7) The intermediate complex reacts further, either through deprotonation to form a product% or through electron transfer leading to pyridine ion and DBO. 6
8
10
12
14
16
18
[CsHsN+*DBO'],d
20
[CsH,N*DBOl + H+
Figure 17. HPLC chromatographic spectrum of the photoinduced chemical products of CDBO on laponite. Experimental conditions were similar to Figure 12.
[CsHsN+*DBO'],d CsHsN+ + DBO (9) The experimental data confirm that DBO is partly restored and the HPLC spectrum shows that an unstable product of DBO'+ and pyridine is formed.
shorter retention times. The retention times (Figure 12) show that the products are more polar than the parent compound. These separated peaks, F1, F2, and F3,are assigned to different isomeric compounds, where the functional group is located at various positions on the DBO molecule. Considering previous work, i.e., reaction of perylene radical cations with water in homogeneous solution produces 3,lO-perylenequinone and perylene,s8 the reaction of the thianthrene radical cation with water in acetonitrile gives equal amounts of thianthrene 5-oxide and t h i a ~ ~ t h r e nand e , ~a~comparable ~ reaction mechanism may be expected for DBO*+ on laponite surfaces. The possible product for hydrolysis of DBO'+ was discussed by Cauquis and Shine.42*43 Considering the nuclephilic nature of water molecules, the reaction of DBO'+ with water may be rationalized as a nucleophilic addition at the cation centre to form an adduct, and may be a rate-determining step. The adduct
Conclusion The photochemical reactions of the dioxin model compounds, DBO and CDBO, on laponite surfaces have been studied. Observation by diffuse reflectance, fluorescence, and EPR spectroscopy indicates that the adsorbed DBO and CDBO undergo physical and chemical interactions with solid surfaces. The chemical interactions lead to conformational changes and surface complexes with charge-transfer character are observed. Photoirradiation of the latter results in the formation of radical cations, DBO'+ and CDBO'+, these may be also generated thermally. End product analysis confirms that the photochemical degradation of DBO on nonactivate laponite is a one electron radical process. The important reaction of radical cations is hydrolysis resulting in dioxinquinone, while the radical DBO(0H)' is suggested as a key intermediate in hydrolysis. The reaction of DBO'+ with pyridine may be used as an indicator of diffusion of pyridine into the interlayer space of the clay.
Time, mtn
(38) (a) Rochlitz, J. Tetrahedron 1967,23,3043. (b)Rietagno, C. V.; Shine, H.J. J. Org. Chem. 1971,36,4060. (c) Svanholm, U.; Parker, V. D. Acta Chem. Scand. 1973,327, 1454. (39) Shine, H. J.; Murate, Y. J. Am. Chem. SOC.1969,91,1872. (40)Murata, Y.; Shine, H. J. J. Org. Chem. 1969,34,3368. (41) Evane, J. F.;Huryez, L.F. Tetrahedron Lett. 1977, 3103. (42) Cauquis, G.; Maurey-Mey, M. Bull. SOC.Chim. Fr. 1972, 3688. (43) Shine, H.J.; Shade,L. J. Heterocycl. Chem. 1974,11, 139.
Acknowledgment. We acknowledge research support by the EnvironmentalProtection Agency (EPA-R-81595301-0). Professor A. Trozzolo is thanked for a helpful discussion. (44)Manning, G.; Parker, V. D.;Adams, R. N.J. Am. Chem. SOC.1969, 92,4584.
