Photochemistry of 4-Chlorophenol on Cellulose and Silica

Lisboa, Portugal, and FCT, Universidade do Algarve, Campus de Gambelas, 8005-039 Faro, Portugal. Environ ... Publication Date (Web): September 11,...
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Environ. Sci. Technol. 2003, 37, 4798-4803

Photochemistry of 4-Chlorophenol on Cellulose and Silica J O S EÄ P . D A S I L V A , * , † , ‡ LUIS F. VIEIRA FERREIRA,† A B IÄ L I O M . D A S I L V A , ‡ A N D ANABELA S. OLIVEIRA† Centro de Quı´mica-Fı´sica Molecular, Instituto Superior Te´cnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal, and FCT, Universidade do Algarve, Campus de Gambelas, 8005-039 Faro, Portugal

The photochemistry of 4-chlorophenol (4-CP) was studied on silica and cellulose, using time-resolved diffuse reflectance techniques and product degradation analysis. The results have shown that the photochemistry of 4-CP depends on the support, on the concentration, and also on the sample preparation method. Transient absorption and photoproduct results can be understood by assuming the formation of the carbene 4-oxocyclohexa-2,5dienylidene in both supports. On cellulose, at concentrations lower than 10 µmol g-1, the carbene leads to the unsubstituted phenoxyl radical, and phenol is the main degradation product. At higher concentrations a new transient resulting from phenoxyl radicals coupling was also observed, and dihydroxybiphenyls are also formed. The reaction of the carbene with ground-state 4-CP was also detected through the formation of 5-chloro-2,4′-dihydroxybiphenyl. 4-Chlorophenoxyl radical and degradations products resulting from its coupling were also detected. Oxygen has little effect on the photochemistry of 4-CP on cellulose. On silica the transient benzoquinone O-oxide was formed in the presence of oxygen. Benzoquinone and hydroquinone are the main degradation products. In welldried samples the formation of hydroquinone is reduced. At higher concentrations the same products as detected on cellulose were observed. 4-CP undergoes slow photochemical decomposition under solar radiation in both supports. The same main degradation products were observed in these conditions.

1. Introduction It is now well established that the photochemical transformations of xenobiotics in environmental systems play a major role in determining their behavior and fate (1). Most pollutants, like pesticides, are mainly localized at the solid/ gas interface (2). Therefore, any description of their environmental behavior requires the description of these systems. Diffuse reflectance techniques are among the most important for surface photochemistry studies (3). Diffuse reflectance laser flash photolysis, developed by Wilkinson and co-workers in the 1980s (4-6), is a powerful tool for the identification and characterization of the main photophysical and photochemical processes occurring at the solid/gas interface. Combined with chromatographic analysis, these techniques * Correspondingauthorfax: 351218464455;e-mail: [email protected]. † Instituto Superior Te ´ cnico. ‡ FCT, Universidade do Algarve. 4798

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can be used to evaluate the role of photochemical processes of xenobiotics on model and natural surfaces. Diffuse reflectance techniques have been used to study photochemical transformations of several xenobiotics such as dioxins (7, 8), polychlorinated biphenyls (9, 10), 4-chlorophenol (11), pyrene (12), and dyes (13) adsorbed on different supports. The complexity of natural environmental surfaces usually does not allow the use of conventional photochemical techniques. Photochemistry on environmental surfaces will not be successfully modeled until photochemical and sorptive processes on the pure components of natural surfaces are better understood (1). Cellulose and silica have been widely used as solid powdered supports to study the photophysics and the photochemistry of several organic compounds (1419). Cellulose is one of the main structural components of vegetal cells, and silica is one of the major constituents of soil surfaces. Therefore a description of the photochemistry of xenobiotics on silica and cellulose is the starting goal in assessing the photochemical behavior of pollutants on soil and leaf surfaces. Chlorinated phenols are an important class of environmental pollutants known by their general resistance to chemical and biological degradation in environmental conditions (20). The photochemistry of chlorophenols is largely documented in solution (21-24). The photoproduct distribution of 4-CP depends on the dissolved oxygen and also on its concentration. In air-saturated solutions benzoquinone is the main photolysis product. At low concentrations it mainly results from direct photooxidation of 4-CP, while at higher concentrations its formation results from both molecular and bimolecular reactions. In deoxygenated solutions, at lower concentrations, hydroquinone is the main product, while at higher concentrations 5-chloro-2,4′-dihydroxybiphenyl prevails. The mechanism of solution photoreaction of 4-CP (Figure 1) was studied in detail by nanosecond laser flash photolysis (25, 26). Transient absorption spectroscopy gave evidence for the heterolytic character of the process and also revealed the formation of carbene 4-oxocyclohexa-2,5-dienylidene, (transient 1t), by elimination of HCl. The other transients formed in acid aqueous solutions and in alkanols (benzoquinone O-oxide, transient 2t, and unsubstituted phenoxyl radical, transient 3t) and the final transformation products are easily accounted for by the reactions of this carbene. Triplet-triplet absorption was detected in n-hexane, and 4-chlorophenoxyl radical was formed by photoionization. Recently the photophysics and photochemistry of 4chlorophenol adsorbed on silicalite and β-cyclodextrin have been studied (11). 4-Chlorophenoxyl radical formation was detected in both supports, and the unsubstituted phenoxyl radical was also observed in β-cyclodextrin. No evidence was found for the formation of the carbene nor of transient 2t. Photodegradation studies indicated the formation of phenol in β-cyclodextrin and ring cleavage in silicalite. In this paper we report the photochemistry of 4-CP adsorbed on cellulose and silica. The main degradation products were identified, and the main degradation pathways were also established for both supports.

2. Materials and Methods 2.1. Chemicals. 4-Chlorophenol, hydroquinone, 2,3-dichlorophenol, 2,4-dichlorophenol, 2,5-dichlorophenol, 2,6dichlorophenol, 3,4-dichlorophenol, 3,5-dichlorophenol, chlorohydroquinone (Riedel-de Hae¨n, Pestanal); 4,4′-dihydroxybiphenyl, 2,2′-dihydroxybiphenyl (Aldrich); benzo10.1021/es026448c CCC: $25.00

 2003 American Chemical Society Published on Web 09/11/2003

Ground-state absorption spectra of the solid powdered samples were recorded using a Cintra 40 GCB Scientific Equipment spectrophotometer, with a diffuse reflectance attachment. The measured reflectance, R, was used to calculate the remission function F(R) using the KubelkaMunk equation defined by

F(R) )

FIGURE 1. Mechanism of the photodegradation of 4-CP in solution. quinone, cellulose DSO (Fluka); silica (60 Å), phenol (Merck); methanol, acetonitrile, and isooctane (Merck Lichrosolv) were used without further purification. Water was deionized and distilled. 2.2. Methods. Adsorption of 4-CP onto unactivated silica and microcrystalline cellulose (undried and dried for 12 h at 70 °C) was achieved by adding the support to a methanolic solution with the correspondent amount of 4-CP and allowing it to equilibrate for 12 h under stirring. The mixtures were then continuously stirred until all the solvent was evaporated. Final traces of the solvent were removed under reduced pressure (∼10-3 mbar) at room temperature (∼1 h). Samples of 4-CP on silica were also submitted to a 24-h drying in the same conditions. The final concentration of 4-CP was determined by extracting the samples with methanol (a known weight of sample in a known volume of solvent) followed by centrifugation and analysis by HPLC. Photolysis studies were conducted in a system previously used to study pesticides in solution (28, 29), pesticides at the solid/gas interface (30), benzyl in p-tert-butylcalix[n]arenes, (n ) 4, 6, and 8) (31), and 4-CP on silicalite and β-cyclodextrin (11). The samples were irradiated at 254 nm using a 16 W low-pressure mercury lamp (Applied Photophysics) without filters and refrigeration. The samples were placed on Petri dishes and irradiated at a distance of 10 cm from the lamp housing. For sunlight irradiation studies, the samples were placed on crystallizer dishes and sealed with quartz glass. The study was performed in Algarve (south Portugal) in October. After irradiation the residue of 4-CP and its photoproducts were extracted as described for the nonirradiated samples. Photolysis was followed by HPLC using a Merck-Hitachi 655A-11 chromatograph equipped with detectors 655A-22 UV and Shimadzu SPD-M6A Photodiode Array. A column LiChroCART 125 (RP-18, 5 µm) Merck was used, and the runs were performed using a mixture water/ acetonitrile 65%-35% or 75%-25% as the eluent. The extracts were also analyzed by GC-MS using a Hewlett-Packard 5890 Series II gas chromatograph with a 5971 series mass selective detector (E.I. 70 eV). A DB-1 capillary column with 30 m length and 0.25 mm i.d. (J & W Scientific) was used. The initial temperature 70 °C was maintained during 5 min, and then a rate of 5 °C/min was used. Analyses were conducted on irradiated and control samples kept in the dark during irradiation. Controls showed no sign of 4-CP degradation.

(1 - R)2 K ) 2R S

where K and S are the absorption and scattering coefficients, respectively. The Kubelka-Munk equation applies to optically thick samples, i.e., those where any further increase in the thickness does not affect the experimentally determined reflectance. For an ideal diffuser, where the radiation has the same intensity in all directions, K ) 2C, where  is the Naperian absorption coefficient and C is the concentration. Since the support usually absorbs at the excitation wavelength, F(R)probe ) F(R) - F(R)support ) Σi2iCi/S, where F(R)support is the blank obtained with a cell containing only support. Whenever the probe is only in the form of monomer, this equation predicts a linear relationship for the remission function of the probe as function of the concentration (for a constant scattering coefficient). Laser flash photolysis experiments were carried out with the fourth harmonic of a ND:YAG laser (266 nm, 6 ns fwhm, 10-30 mJ/pulse) from B. M. Industries (Thomson-CSF), model Saga 12-10, in the diffuse reflectance mode. A schematic diagram of the system is presented in ref 3. The light arising from the irradiation of the solid samples by the laser pulse is collected by a collimating beam probe coupled to optical fibers (fused silica) and detected by a gated intensified charge coupled device (ICCD, Oriel model Instaspec V) after passing via a compact fixed imaging spectrograph (Oriel, model FICS 77440). The system can be used either by capturing all light emitted by the sample or in time-resolved mode, using a delay box (Stanford Research Systems, Model D6335). The ICCD has high-speed gating electronics (2.2 ns) and intensifier and works in the 200-900 nm wavelength range. Time-resolved absorption and emission spectra are available in the nanosecond to second time range. Transient absorption data are reported as a percentage of absorption (% abs), defined as 100∆Jt /J0 ) (1-Jt /J0)100, where J0 and Jt are the diffuse reflected light before exposure to the laser pulse and at time t after excitation, respectively. In all samples the initial transient absorption increased proportionally with laser intensity. Air equilibrated, deoxygenated and oxygen saturated samples were studied. The removal of oxygen was achieved by purging with argon, and the saturation of O2 was made using a flow of this gas. Samples were kept under gas flow at least during 1 h.

3. Results and Discussion Ground-state absorption spectra of 4-CP on silica and cellulose showed an unstructured absorption band with maximum at 283 nm, in both supports. The bands have similar location on both supports and on β-cyclodextrin (11), but the resolution is better with the latter. This indicates, as expected, that the probe is adsorbed in polar environment. 4-CP absorbs above 290 nm, indicating some overlap with the solar spectrum at ground level. This result suggests that direct photoreaction in natural conditions could be an important dissipation pathway of 4-CP. 3.1. Transient Absorption on Cellulose. Figure 2 shows typical transient absorption spectra of 4-CP on cellulose. Cellulose is a linear arrangement of β-linked glucose units, presenting a uniform distribution of -OH groups on the outside of each chain. When two or more chains make contact VOL. 37, NO. 20, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Transient absorption spectra of a sample of 4-CP on silica (22 µmol g-1, air equilibrated) submitted to a 24-h drying under reduced pressure, at pulse end (1), and 2 µs (2), 1 ms (3), and 500 ms (4) after the laser pulse (∼10 mJ/pulse).

FIGURE 2. Transient absorption spectra of 4-CP on cellulose. A: sample containing 27 µmol g-1, at pulse end (1), and 100 µs (2), 1000 µs (3), and 10 ms (4) after the laser pulse (∼10 mJ/pulse); the inset shows the transient absorption spectra (∼20 mJ/pulse) after 100 µs (1) and 20 ms (2) of a sample (42 µmol g-1) prepared with dry cellulose. B: sample containing 93 µmol g-1, at 1 µs (1), 100 µs (2), and 5 ms (3) after the laser pulse (∼10 mJ/pulse). the hydroxyl groups are ideally situated to “zip” the chains together by forming hydrogen bonds. This gives a highly insoluble, rigid, and fibrous polymer. Microcrystalline cellulose is obtained from purified cellulose after a severe acid hydrolysis in which the amorphous regions are preferentially attacked and transformed into a high crystalline residue (3). By the use of methanol, cellulose undergoes considerable swelling. In this way probe molecules can penetrate into submicroscopic pores and after solvent removal can became rigidly trapped between the chains (14). This way, cellulose provides an environment similar to that supplied by alkanols and also protects probes from molecular oxygen. Therefore, assuming that the first degradation step is the formation of the carbene, one should be able to detect the unsubstituted phenoxyl radical by transient absorption and phenol as the main degradation product. At concentrations about 30 µmol g-1 two bands can be detected, one at 305 nm and another centered at 400 nm. A third band at 600 nm (see inset of Figure 2A) becomes visible at higher excitation energies (∼20 mJ/pulse). This lower energy band can be attributed to the unsubstituted phenoxyl radical (transient 3t) (11, 32). The band centered at 400 nm is therefore due, but only in part, to this radical. The presence of transient 3t is therefore in accordance with the carbene formation on cellulose. The absence of the transient 2t can be explained by the protection of the support to molecular oxygen, in accordance with previous studies for different probes entrapped within cellulose (14-16). The absorption above 400 nm has been observed as resulting from the photochemistry of 4-CP on silicalite and β-cyclodextrin (11) and was assigned to the 4-chlorophenoxyl radical. The transient assignments are also supported by the photoproduct distribution (see photodegradation products section). The band at 305 nm grows with time, has its maximum value around 1 ms, and then decays. A new transient is therefore produced from the initial formed radicals. As we will see from the photoproducts study, this band can be attributed 4800

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to a transient resulting from the coupling of two unsubstituted phenoxyl radicals. At higher concentrations (see Figure 2B) the mentioned absorption bands are also present and have similar behavior. However new bands are formed, indicating that, as observed in solution, the photochemical reaction is concentration dependent and becomes complex (22-26). Dry samples (prepared after drying the cellulose 12 h at 70 °C) show different spectral behavior (inset of Figure 2A). The band at 305 nm is much less intense. In wet cellulose the chains are already open by the water. Upon sample preparation the probe molecules become entrapped between the polymer chains, and if water in not removed, there is a smaller interaction between the probe and the solid support, which results in greater mobility of the probe and of the formed radicals. This higher mobility in wet cellulose was also reported for samples containing dyes (33). Therefore, the band at 305 nm is weaker in dried cellulose due to the lower mobility of the phenoxyl radicals, which reduces the possibility of radical coupling and therefore of the transient formation. 3.2. Transient Absorption on Silica. Figure 3 shows the transient absorption spectra of a sample of 4-CP on silica (22 µmol g-1, air equilibrated) submitted to a 24-h drying under reduced pressure. Silica gels are porous and granular forms of amorphous silica, formed by a complex net of microscopic pores, which attach and retain water or organic solvents by means of physical adsorption (3). Its specific area is very high (from 20 to 750 m2 g-1) and depends on the pore size. In our case the pore diameter is 60 Å, and the surface area supplied by the manufacture is 480 m2 g-1. Its surface contains both silanol (Si-OH) and siloxane (Si-O-Si) groups, being the former considered to be strong sites for adsorption (3). Unlike cellulose, silica surface allows the diffusion of molecular oxygen to the probes adsorbed on its surface. In this support 4-CP will be thus surrounded by water molecules and silanols, which allow an environment more similar to the one existing in aqueous solution than cellulose. It is thus expected a photochemical behavior different from the one observed on cellulose, and more similar to the one found in aqueous solution, in the presence of oxygen (Figure 1, paths A and B). The transient absorption spectra are markedly different from the observed on cellulose, indicating also different main reaction pathways. A broad band peaking about 400 nm is also present but extends to 600 nm. Since this support does not protect the probe from molecular oxygen, if the pathway that leads to the carbene formation is also present on silica, the absorption band of benzoquinone O-oxide should be observed. In fact a shoulder between 460 and 480 nm is detected in air-equilibrated samples. The band shifts to shorter wavelength at longer times, being the maximum at 400 nm at pulse end and between 385 and 390 nm after 1 ms. The latter is the absorption region of transient

FIGURE 4. Transient absorption spectra of a sample of 4-CP on silica (22 µmol g-1) submitted to a 1-h drying under reduced pressure, after 2 µs, air equilibrated (1), 2 µs, oxygen saturated (2) and 1 ms, oxygen saturated (3) (∼10 mJ/pulse).

FIGURE 6. Chromatogram (HPLC) of an extract of 4-CP irradiated (254 nm) on cellulose (126 µmol g-1). The inset shows the absorption spectra of products V and VI.

FIGURE 5. Chromatogram (GC-MS) of an extract of 4-CP irradiated (254 nm) on cellulose (126 µmol g-1).

FIGURE 7. Chromatogram (GC-MS) of the extract of 4-CP irradiated (254 nm) on silica (sample containing 46 µmol g-1, dried 24 h at room temperature and reduced pressure (∼10-3 mmHg)).

1t (26), suggesting that the same initial degradation step occurs in both supports. As we will see from the photoproduct distribution, the band observed at early times can also be due to the 4-chlorophenoxyl formation. A blue shifted and less resolved band was detected when the samples are dried under low pressure during just 1 h (see Figure 4, spectrum 1). The broadening is due to the unsubstituted phenoxyl radical formation owing to reaction of the carbene with solvent residues (methanol) that were not removed during this 1-h drying treatment. The presence of transient 3t is in agreement with the detection of phenol after the irradiation of these samples. The blue shift is due to slighter formation of the 4-chlorophenoxyl radical in these conditions. As expected, in argon-purged samples the absorption between 385 and 390 nm is also present, and the shoulder at 480 nm was not observed. Upon saturation with oxygen the broad band of transient 2t becomes more visible (Figure 4, spectrum 2). The transient assignments are also supported by the photoproduct distribution (see photodegradation products section). The results confirm that 4-CP on a silica surface has a photochemical behavior similar to the one observed in aqueous solution in the presence of oxygen. 3.3. Photodegradation Products on Cellulose. At concentrations near 10 µmol g-1, phenol (IV) is the main detected photodegradation product on cellulose. This result is in agreement with the transient absorption results since transient 3t leads to phenol by hydrogen abstraction (26). At higher concentrations a degradation product with m/z ) 186 (VI) becomes significant (see Figures 5 and 6). The mass spectra suggested a dihydroxybiphenyl compound. The inset of Figure 6 shows the UV-vis absorption spectra of products V and VI, obtained by HPLC with the diode array detector. By comparison with the spectrum obtained by Ye and Schuler (34) with a similar system we assign product VI to 2,4′-

dihydroxybiphenyl. These authors found that 4,4′-dihydroxybiphenyl, 2,2′-dihydroxybiphenyl, and compound VI result from the second-order combination of phenoxyl radicals, the latter being predominant. Therefore we propose that at higher concentrations the main degradation process is still the one that leads to the phenoxyl radicals, and the new product results from their recombination. The presence of 4,4′-dihydroxybiphenyl (product VII) and the trace quantities of 2,2′-dihydroxybiphenyl detected by GC-MS also supports our proposal. Other sources, namely secondary photoreactions of chlorohydroxybiphenyls, can lead to the hydroxybiphenyl formation, which can also contribute for the majority of 2,4′-dihydroxybiphenyl. Hydroquinone (III) and 2,4′-dihydroxy-5-chlorobiphenyl (V) were also detected (see Figure 6). The identification of the latter was based on its mass spectrum and by comparison of its UV-vis spectrum (inset of Figure 6) with the results published by Sarakha and co-workers (27). The presence of product III, detect by HPLC, indicates that the reaction of the carbene with the water present on cellulose occurs but is a minor pathway. All other identifications were based on the analysis of authentic samples. The results confirm the concentration dependence detected in the transient absorption studies and indicate that, as observed at higher concentrations in solution (26), the reaction of the carbene with a ground-state 4-CP molecule becomes possible. Products with m/z ) 254 were detected in trace amounts at higher concentrations. These were attributed to dimeric products of the 4-chlorophenoxyl radical. This result was expected since these radicals were detected by transient absorption, and its dimerization is an already known reaction in aqueous solution (27). The formation of benzoquinone was not detected on cellulose. This is in agreement with the transient absorption results VOL. 37, NO. 20, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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pressure during 24 h. This procedure does not significantly affect the concentration of 4-CP on the solid powdered samples. Dried samples were then irradiated, and the extracts show that benzoquinone is one of the major degradation products and that only a residual concentration of phenol (see Figure 7) is formed. These results are in agreement with the broadening of the absorption bands in the samples dried 1 h and are in accordance with the carbene formation on silica surfaces. The carbene abstracts a hydrogen atom from the solvent and leads to phenol through the formation of transient 3t or reacts with molecular oxygen and leads to benzoquinone through transient 1t. The presence of hydroquinone indicates that the reaction with water is also an important degradation pathway. The treatment to remove the solvent residues also removed part of the physiosorbed water, therefore decreasing the formation of this product. At higher concentrations, as observed in cellulose, the formation of dimeric products such as product V and also products with m/z ) 254 were detected. Compounds with m/z ) 254, attributable to dimerization of the 4-chlorophenoxyl radicals, were detected in trace amounts suggesting that the formation of the 4-chlorophenoxyl radical is a minor degradation pathway under lamp irradiation conditions on both supports. Dichlorophenols and chlorohydroquinone were also identified. FIGURE 8. Concentration of 4-CP (]), and of its main degradation products, phenol (O), benzoquinone (0), and hydroquinone (4), as function of the irradiation time, on cellulose (A) and silica (B) (254 nm). and is due to the oxygen protection made by the support to the probe and to the formed transients. 3.4. Photodegradation Products on Silica. The analyses of the 1-h dried irradiated samples indicated hydroquinone, benzoquinone, and phenol as the main degradation products. The appearance of phenol was unexpected, since the environment does not supply abstractable hydrogen atoms, and this step is required for the phenol formation (path C, Figure 1) (26). The samples were then dried under reduced

3.5. Photolysis on Cellulose and Silica. The kinetics of 4-CP photodegradation (254 nm) and of its products formation was studied for both supports (concentrations around 20 µmol g-1) in air-equilibrated conditions. The results (see Figure 8) show that in the experimental conditions of the study, photodegradation is faster on cellulose. This result is certainly related to the granulometry of the sample particles, which are about 50 µm for cellulose and 63-200 µm for silica. Thus when spreading the samples on the glass surface more area is exposed to irradiation in the case of cellulose, being the kinetics faster. In both supports the detected quantity of products does not account for the quantity of 4-CP decomposed. This was attributed to degradation of the photoproducts and to the formation of nonextractable compounds.

FIGURE 9. Mechanism of the photodegradation of 4-CP on cellulose and silica. 4802

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One-hour irradiation cleans the cellulose from 4-CP and from phenol. In silica only hydroquinone persists after this irradiation time. These results clearly show that photolysis at the solid/gas interface can be used to decompose and eventually mineralize organic pollutants. Under solar irradiation, an exposure between 9 a.m. and 18 p.m. with clear sky in October (Algarve, south Portugal), causes little degradation of 4-CP (within the experimental error). However phenol was detected on cellulose and hydroquinone and benzoquinone were detected on silica, suggesting that the same photochemical behavior is expected to occur in environmental conditions. The result also indicates that 4-CP can undergo direct photolysis on solid/ gas interfaces in natural conditions and that dechlorination is the main degradation pathway. 3.6. Photodegradation Mechanism. Figure 9 shows the reaction mechanism proposed for 4-CP adsorbed on cellulose and silica. The degradation at the solid/gas interface follows essentially the main degradation pathways observed for this compound in solution (25, 26). However some important differences were detected for cellulose and silica as adsorbents. The formation of the carbene 4-oxocyclohexa-2,5dienylidene is the main primary degradation step. 4-Chlorophenoxyl radical formation is expected to be a minor degradation pathway in natural conditions. Except for the 4-chlorophenoxyl radical and the compounds with m/z ) 254, all other transients and degradation products can be accounted for by reactions of this carbene. On cellulose, at lower loadings of 4-CP, as observed for alkanols, the carbene leads to phenol through hydrogen abstraction and phenoxyl radical formation. For higher concentration samples phenoxyl radicals can couple and form dihydroxybiphenyls, and 5-chloro-2,4′-dihydroxybiphenyl is formed due to reaction of the carbene with ground-state 4-CP molecules. Products resulting from the 4-chlorophenoxyl radical coupling were also observed at higher concentrations. On silica, the main degradation pathways available to the carbene are the same as those observed in aqueous solution: reaction with molecular oxygen to form benzoquinone through the formation of transient 3t, reaction with water to form hydroquinone, and, at higher concentrations, reaction with ground-state 4-CP to form 5-chloro-2,4′-dihydroxybiphenyl. Products resulting from the 4-chlorophenoxyl radical coupling were also observed on silica at higher concentrations. In silica the oxygen effect is less important than in aqueous solution. In fact and unlike the water behavior in the presence of oxygen, the formation of hydroquinone is an important degradation pathway, and the observation of transient 2t is only clear under oxygen atmosphere. The observation of a strong absorption band even after 20 ms indicates a slow decomposition of the formed transients. In solution the solvent cage ensures that many collisions of the carbene with molecular oxygen occur in one encounter of the two reactants. On a solid surface no such cage exists, and simple collisions with the surface resulting from bombardment from the gas phase occur; this is the main process for oxygen to reach the carbene (17). Therefore the reaction rate is reduced compared to that in aqueous solution, and the reaction of the carbene with water efficiently competes with the formation of transient 2t.

Acknowledgments Post-doc grants SFRH/BPD/3650/2000 and SFRH/BPD/5589/ 2001, supported by Fundac¸ a˜o para a Cieˆncia e a Tecnologia, are gratefully acknowledged.

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Received for review December 19, 2002. Revised manuscript received July 5, 2003. Accepted July 30, 2003. ES026448C VOL. 37, NO. 20, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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