Environ. Sci. Technol. 2003, 37, 1568-1574
Environmental Ice Photochemistry: Monochlorophenols J A N A K L AÄ N O V AÄ , † P E T R K L AÄ N , * , ‡ JAN NOSEK,† AND IVAN HOLOUBEK† RECETOX-TOCOEN, Masaryk University, Kamenice 126/3, 625 00 Brno, Czech Republic, and Department of Organic Chemistry, Faculty of Science, Masaryk University, Kotlarska 2, 611 37 Brno, Czech Republic
Photolysis of 2- and 4-chlorophenol samples in water ice of the initial concentrations 10-7 to 10-2 mol L-1 is reported. Major phototransformations appeared to be based on the coupling reactions due to chlorophenol aggregation at the grain boundaries of the polycrystalline state. The main products, chlorobiphenyldiols, belong to the family of phenolic halogenated compounds (such as hydroxylated polychlorobiphenyls) that are known xenobiotics found in nature. No photosolvolysis products, that is products from intermolecular reactions between organic and water molecules, were observed at temperatures below -10 °C. Raising the temperature to -5 °C caused a moderate photosolvolytic activity in the case of 4-chlorophenol (formation of hydroquinone), in contrast to 2-chlorophenol which was almost exclusively transformed into pyrocatechol. It is suggested that photosolvolysis above this temperature occurs in a liquid or quasi-liquid layer that covers the ice crystal surfaces. The results support our model in which significant amounts of some persistent, bioaccumulative, and toxic compounds may be generated by photochemistry of primary pollutants in cold ecosystems and in the upper atmosphere, and may be subsequently released to the environment.
Introduction The current knowledge about snow and ice contamination by organic compounds in cold ecosystems is extensive (1, 2), but there is a relative lack of attention and information about their photochemical fate (3, 4). Solar light is responsible for a large number of reactions in the atmosphere, in natural waters, on soil, and in living organisms, where toxic pollutants can be photomodified into more or less harmful substances by direct irradiation or through sensitization reactions (5). Water ice and snow certainly represent a unique environment for photochemical reactions. The organic solutes in the frozen matrix tend to be segregated from the ice phase and are dispersed at the grain boundaries or interstitial pores (6, 7), where their reactivity might depend on the interactions between the host water molecules and the guest substances (8). The studies of photochemical transformations of organic compounds in ice are sporadic. For example, Hoffmann et al. (9, 10), our laboratory (11-14), and Domine´ and Shepson (4) have reported on environmental as well as mechanistical aspects of ice (snow) photochemistry of organic compounds. In addition, photochemical transformations in ice under * Corresponding author phone: +420-5-41129356; fax: +420-541211214; e-mail:
[email protected]. † RECETOX-TOCOEN. ‡ Department of Organic Chemistry. 1568
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astrophysical conditions may reveal possible interstellar origin of organic compounds delivered to Earth (15-17). We have recently proposed a model according to which persistent, bioaccumulative, and toxic compounds (PBTs) can be generated by photochemistry of primary pollutants in natural ice and snow, and subsequently released to the environment (3). Now we wish to report a laboratory study of ice photochemistry of 2- and 4-chlorophenol, two exemplars of the chlorophenol family which includes the most prevalent chloroorganic compounds in the environment (18). A specific photochemical reactivity, mechanistic considerations, and implications for the environment are discussed.
Experimental Section Instrumentation. A gas chromatograph HP 6890 equipped with a mass selective detector HP 5972 (Hewlett-Packard) and a gas chromatograph HP 6890 equipped with a FID detector (Hewlett-Packard) were used for identification and quantification of the photoproducts. A modular liquid chromatograph HP 1050 (Hewlett-Packard) equipped with a diode array photometric detector was used for obtaining the LC spectra. UV spectra were obtained on a Shimadzu UV-1601 instrument with matched 1.0-cm quartz cells or on a UV/VIS/NIR spectrometer Lambda 19 (Perkin-Elmer) using both standard absorption arrangement and a 60-mm integrating sphere (absorption measurements in the solid matrix at -50 °C). Low-temperature experiments were accomplished in a cryostat MLW MK70. Chemicals. 2-Chlorophenol (99+%, Lachema Co.), 4-chlorophenol (99+%, Fluka), 2-nitrobenzaldehyde (99+%, Fluka), and hexadecane (99+%, Schuchardt) were used as received. Water was purified on an Osmonics 2 and then a Millipore Simplicity 185. The following chemicals were purchased as the GC standards: pyrocatechol, hydroquinone, benzoquinone (Lachema), floroglucinol, 1,2,4-benzenetriol, chlorohydroquinone, biphenyl-2,2′-diol, biphenyl-4,4′-diol, cyclopentanecarboxylic acid (Aldrich). Syntheses of other analytical standards (biphenyl-2,4′-diol, 3′-chlorobiphenyl-2,4′-diol, 3-chlorobiphenyl-2,2′-diol, and dibenzo[1,4]dioxin) are described in the Supporting Information. Sample Preparation and Irradiation Procedures. Aqueous chlorophenol solutions were prepared by dilution of the stock solutions (the solubilities of 2-chlorophenol and 4-chlorophenol are ∼20 g L-1 and ∼27 g L-1, respectively). Oxygen was removed from the samples either by bubbling the solutions with argon for 15 min or by sonication when necessary. Aqueous solutions (10 mL) in 13 × 100-mm quartz tubes, sealed with septa, were solidified in the cryostat bath at -15 °C. The samples, placed into a merry-go-round apparatus, were irradiated using a 125-W medium-pressure mercury lamp (Teslamp Co., Praha, Czech Republic) in a cryostat box filled with ethanol used as a cooling medium at the temperature of interest. Solutions were extracted with dichloromethane solution (2 mL) containing hexadecane as an internal standard or used directly for the HPLC analysis. The extraction methods with dichloromethane (from both acidified and neutral aqueous solutions) were optimized in order to employ them for the quantitative analysis. Sample Analysis. In GC-MS analyses, dichloromethane extracts were analyzed on a GC-MS instrument supplied with a J&W Scientific fused silica column DB-5MS (60 m × 0.25 mm with 0.25-µm stationary-phase film (5% phenyl/ 95% methyl)polysiloxane). A sample of 1 µL was introduced using the splitless technique under the following temperature program: 80 °C for 1 min, then 15 °C min-1 to 180 °C, 5 °C min-1 to 310 °C, and finally 20 min at 310 °C. Injector and 10.1021/es025875n CCC: $25.00
2003 American Chemical Society Published on Web 03/13/2003
transfer line temperatures were kept at 280 °C. The mass spectra were collected in the scan range of 50-550 m/z for identification purposes. Measured spectra were compared with those of the mass spectral library Wiley 275. The actual compound concentrations were calculated using an internal standard method. In GC-FID analyses, dichloromethane extracts were analyzed on a GC-MS instrument supplied with a fused silica column HP-5 (30 m × 0.32 mm with 0.25-µm stationaryphase film). Temperature program was 80 °C for 2 min, then 10 °C min-1 to 300 °C, and 10 min at 300 °C. Injector and transfer line temperatures were kept at 220 °C. For the HPLC analyses, the thawed samples were sonicated for 5 min to guarantee uniform solutions. Special attention was paid to verification that sonication does not cause any chemical change. A mobile phase of water/ acetonitrile (95:5, w/w) of pH 2.5, acidified with H3PO4, with a flow rate of 0.2 mL min-1 (30-min gradient to 100% acetonitrile; then pure acetonitrile for 10 min), and a Polaris column 3u C18-A (150 × 2.0 mm) were used. Both chlorophenol isomers, 2-nitrobenzaldehyde, phenol, dihydroxybenzenes, benzoquinone, and dihydroxybiphenyls were calibrated on authentic samples. Photoproduct Derivatization. To identify all the reaction products, the compounds with acidic hydrogens (phenols, carboxylic acids) present in irradiated samples were converted into the corresponding pentafluorobenzyl derivatives according to a known procedure (19). An aqueous solution (15 mL) acidified by addition of concentrated H2SO4 (50 µL) was extracted with dichloromethane (1.5 mL) 3 times, then collected organic layers were dried over Na2SO4, filtered, and the solvent was removed by a flow of nitrogen. Acetone (0.5 mL), K2SO4 (10 mg), and a solution of 18-crown-6-ether (4 µg) and pentafluorobenzyl bromide (1 µL) in 2-propanol (12.5 µL) were added to the sample. The mixture was heated at 50 °C for 2.5 h and filtered off through a cellulose filter. The solution was analyzed by GC-MS technique. Quantum Yield Measurements. Frozen water solutions of 2- and 4-chlorophenol (∼10-4 mol L-1) in 13 × 100-mm quartz vessels were irradiated simultaneously at >290 nm (Pyrex filter) using a 125-W Teslamp mercury discharge lamp with 2-nitrobenzaldehyde frozen solutions (∼10-3 mol L-1) used as an actinometer (20) in a merry-go-round apparatus immersed in a methanol bath of the cryostat. The quantum yields were also obtained on an optical bench by irradiation at an isolated wavelength set to 290 ( 10 nm. The samples were degassed by sonication before the containers were sealed with septa. The starting material decomposition and the photoproduct yields were analyzed by HPLC calibrated with the authentic compounds. The reproducibility was (5% in all efficiency measurements.
Results In this work, 2-chlorophenol (1) and 4-chlorophenol (2) have been photolyzed in water-ice, degassed or aerated, solid matrixes at different temperatures. Samples of different concentrations were irradiated in quartz vessels by a multiwavelength medium-pressure mercury lamp with Simax (Pyrex) glass filter (>290 nm) that allowed efficient photochemistry. The reaction quantum yields were determined in order to compare photochemistry of chlorophenols in ice with that in aqueous solutions. Monochlorophenols in neutral aqueous solutions absorb significantly in the region of 250-290 nm with the absorption maxima at 272 nm ( ) 1920 M-1 cm-1) and 278 nm ( ) 1650 M-1 cm-1) for 2-chlorophenol and 4-chlorophenol, respectively (21, 22). Because both molecules absorb slightly above 290 nm (Figure 1) phototransformations are of environmental interest as depicted by the comparison with a typical solar
FIGURE 1. Absorption spectra of 2-chlorophenol (s) and 4-chlorophenol (- - -): absorbances of approximately 5 × 10-4 mol L-1 aqueous solutions in the spectral region of 260 to 360 nm, compared with the calculated solar spectral actinic flux at the Earth’s surface for cloud-free skies and at solar 45° zenith angle5 (‚‚‚).
FIGURE 2. Normalized absorption spectra of 2-chlorophenol obtained in liquid water at 20 °C (- - -) and ice at -50 °C (s). spectral actinic flux. In addition, it is well-known that absorption behavior of organic chemicals on various surfaces differs from that in solution. Adsorption of phenols on silica gel or alumina shows relatively small red shifts of the absorption maxima, whereas the absorption spectra of chlorinated aromatic hydrocarbons are stretched into the region of above 300 nm, for example (23). Figure 2 shows a comparison of the normalized spectra of 2-chlorophenol obtained in liquid water and in ice. The absorption maxima do not appear to be shifted but the band broadening in ice may cause a more efficient absorption above 290 nm (the same observation was obtained for 4-chlorophenol). Test of Sample Homogeneity. Ice solid solutions of chlorophenols were prepared by freezing the liquid solutions in a cool bath. To find out whether the organic matter is distributed homogeneously over the whole volume, the frozen mass was removed from a test tube by breaking the glass and it was divided into 4 parts: the top, bottom, outside, and inner layers. Analysis of the actual concentrations in all sample segments confirmed that the mass distribution is practically uniform over the whole sample volume at temperature of solidification (-15 °C) and the chlorophenol concentrations between 10-4 and 10-3 M. Photolysis, due to the internal filter effect, proceeded to higher reaction conversions in the outside layers than in those of the middle of the tube. Photoproduct Identification. The photoproducts were identified by HPLC, GC, GC-MS, or NMR techniques using the authentic compounds or a MS spectral library. The authentic compounds were either synthesized or purchased. VOL. 37, NO. 8, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Photoproducts identified from the photochemistry of 2-chlorophenol in ice.
FIGURE 4. Photoproducts identified from the photochemistry of 4-chlorophenol in ice. The main products were isolated by a column chromatography. No photosolvolysis products (e.g., quinones) were found in frozen solutions below -10 °C but they were dominant in liquid or quasi-liquid samples (at temperatures above -5 °C). Figures 3 and 4 list all compounds identified from the irradiation experiments in frozen solutions. The major products of 2-chlorophenol photochemistry in ice, 3′-chlorobiphenyl-2,4′-diol (3) and 3-chlorobiphenyl2,2′-diol (4) were synthesized (see the Supporting Information) in order to carry out the HPLC and GC (GC-MS) comparisons. Because the mass spectra of both photoproducts were very similar, an independent evidence was necessary. Three and two monochlorobiphenyldiols were obtained from the reaction of 2,4′- and 2,2′-biphenyldiol, respectively. Biphenyl-2,4′-diol was synthesized from anisidine and 4-anisole, and two other biphenyldiols (7 and 11) were purchased. 5-Chlorobiphenyl-2,4′-diol (9) was identified in the irradiated 4-chlorophenol in aqueous solution according to Boule et al. (22, 24). An evaluation of the structure of dibenzofuranol (8) was based on comparison of the mass spectrum with a nearly identical one of dibenzofuran-1-ol, found in the MS spectral library. The regioisomer dibenzofuran-4-ol would be a logical structure obtained by a direct coupling condensation reaction of two 2-chlorophenol molecules. An expected photoproduct, dibenzo[1,4]dioxin (the authentic compound was synthesized independently), from the irradiation of 2-chlorophenol in ice was not found even at high reaction conversions. The identification of the ethers 5 and 12 and the remaining (trace) compounds was based on the analysis of their mass spectra. The mass spectra of the compounds 3, 4, 5, 8, 9, and 12, as well as two typical GC chromatograms, are included in the Supporting Information for this article. The reaction with pentafluorobenzyl bromide was done to transform possible carboxylic acids (such as cyclopentadiene carboxylic acid known from the 2-chlorophenolate photochemistry (25)) into the corresponding esters, which should subsequently be detectable by GC-MS. Phenolic compounds in crude irradiated mixtures were transformed into pentafluorobenzyl ethers and their structures were analyzed by GC-MS. This method, however, did not elucidate 1570
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TABLE 1. Photolysis of 2-Chlorophenol in Icea photoproduct
typical relative photoproduct concentration [%]
3′-chlorobiphenyl-2,4′-diol (3) 3-chlorobiphenyl-2,2′-diol (4) 2-(2-chlorophenoxy)phenol (5) phenol (6) biphenyl-2,2′-diol (7) dibenzofuranol (8) other chlorobiphenyldiols other biphenyldiols chloroterphenyltriols
55-65 20-30 3-4 ∼1 3-4 1-2 2-3 ∼1 ∼1
a The samples of 10-2 to 10-4 mol L-1 concentration were irradiated at -15 °C to 5-70% conversion. The reproducibility was (7%. Calculated by a method of the internal normalization.
TABLE 2. Photolysis of 4-Chlorophenol in Icea photoproduct
typical relative photoproduct concentration [%]
5-chlorobiphenyl-2,4′-diol (9) biphenyl-2,4′-diol (10) biphenyl-4,4′-diol (11) phenol (6) 4-(4-chlorophenoxy)phenol (12) other chlorobiphenyldiols chloroterphenyltriols
85-95 ∼1 1-2 ∼1 2-3 ∼1 1-2
a The samples of 10-2 to 10-4 mol L-1 concentration were irradiated at -15 °C to 5-70% conversion. The reproducibility was (7%. Calculated by a method of the internal normalization.
any products other than those found by direct GC-MS or HPLC techniques. Photoproduct Distribution. Tables 1 and 2 show a typical distribution of photoproducts obtained from the laboratory ice photochemical experiments. The samples were irradiated through a Pyrex glass filter allowing irradiation at wavelengths longer than 290 nm. Irradiation of 2-chlorophenol (1) provided two major coupling chlorobiphenyldiol products
TABLE 3. Apparent Quantum Yields (O) of Chlorophenol Photodegradation at 290 ( 10 nm compound
O (in ice)a
Ob (in aqueous solution)
2-chlorophenol 4-chlorophenol
0.03 0.04
0.03 (0.30c) 0.4
a 2-Nitrobenzaldehyde used as an actinometer; the reproducibility was (5%. b The quantum yields from the literature.25 c The quantum yield of the anionic form.21
FIGURE 5. Photolysis of 2-chlorophenol (10-2 mol L-1) as a function of temperature.
always kept below 10% to avoid the photoproduct interference and the yields of 2-nitrosobenzoic acid were uniform over the whole sample volume. Low reaction conversions and identical irradiation conditions for both chlorophenols and the actinometer guaranteed that the magnitude of light scattering on ice and the uncertainties associated with the path length of light propagation through the ice were practically the same in all samples. The quantum efficiencies were corrected on the molar absorption coefficients at 290 nm in liquid water. Because oxygen saturation somewhat lowered the photochemical efficiency of the coupling reaction, the samples were degassed by sonication before the irradiation.
Discussion FIGURE 6. Photolysis of 4-chlorophenol (10-2 mol L-1) as a function of temperature. 3 and 4 (Table 1), whereas irradiation of 4-chlorophenol led to formation of a single one (9) (Table 2). The remaining photoproduct concentrations were not significant. The photoproduct distribution slightly varied at the conversions of 5-70% and concentrations from 10-2 to 10-4 mol L-1. The reaction conversions close to 90% were obtained only for diluted solutions (10-4 up to 10-7 mol L-1), in which the coupling products still dominated. Concentrated samples quickly changed their color to dark brown upon irradiation. Formation of strongly absorbing products obviously caused slowing of the reaction efficiency due to the internal filter effect. In addition, we compared the photoproduct distribution and the reaction efficiency in terms of the wavelength applied. Photochemistry in quartz vessels was about 3 times as efficient as that in Pyrex vessels due a stronger absorption in the range of 250-280 nm, but the distribution of the products was almost identical. Temperature Studies. Ice formed from 10-2 mol L-1 aqueous solutions of 1 or 2 at the lowest temperatures studied (-15 and -10 °C) was visually white polycrystalline solid that quickly turned to yellow when irradiated. In contrast, ice crystals were visually slightly melted in samples cooled at -5 °C. Figures 5 and 6 show relative photoproduct distributions at 4 different temperatures, including liquid water solutions at 20 °C. High starting concentrations of materials have been chosen to ensure that the coupling products in the 4-chlorophenol samples are still formed in liquid water solutions, because irradiation of lower-concentration solutions leads to the production of simple photosolvolytic products only, which is well-known from the literature (24). Apparent Quantum Yields. The apparent quantum yields (φ) for 2-chlorophenol and 4-chlorophenol degradation (Table 3) were obtained by the actinometric measurements using 2-nitrobenzaldehyde photoisomeration into 2-nitrosobenzoic acid in ice. Hoffmann et al. reported that this reaction proceeds in ice with a quantum yield of φ ) 0.5 (20) because the isomerization quantum yield is phase- and temperature-independent (26, 27). The conversions were
Chlorophenols belong among the most widely studied organic micropollutants. 2-Chlorophenol and 4-chlorophenol are used in the paper, herbicide, and pesticide industries, and they have been identified in aquatic and soil environments (28, 29). Since the beginning of the 1970s many research groups have focused on their photochemical behavior. Chlorophenols in aqueous solution can be phototransformed by sunlight, and their photoproducts are well described (21, 24, 30-36). Photosolvolysis (nucleophilic aromatic displacement) to pyrocatechol and a ring-contraction mechanism involving a Wolff rearrangement to cyclopentadienecarboxylic acid (13) are known major pathways of 2-chlorophenol (1) degradation in aquatic environments, as thoroughly described by Boule and co-workers, whereas photolysis of the phenolate form results in 13 only (21, 25, 32, 37). The quantum yield of the photodepletion of neutral aqueous solutions was reported to be 0.03 but 10 times higher for the anionic form degradation (Table 3) (25). A typical irradiated reaction mixture contained 90% of pyrocatechol and less than 10% of the acid 13 (38). In this work, neither pyrocatechol nor cyclopentadienecarboxylic acid were found in (aerated or degassed) irradiated solid-ice solutions of 1. The main products were two coupling compounds 3 and 4 in a ∼2:1 ratio (Table 1), which was never observed in liquid solutions before. This clearly suggests a reaction pathway totally different from that occurring in water. The same quantum yield values (φ ) 0.03) in liquid and solid solutions suggest that photosolvolysis and coupling reactions are of similar efficiency. Ice presents a polycrystalline material (6) possessing free volume (reaction cavity (8)) that is essentially stationary and constant. A lack of the solvolysis product (pyrocatechol) indicates that there are no reactions of guest molecules with the cavity walls. Similar observations were already accomplished in the studies of chlorobenzene, dichlorobenzene, and valerophenone photochemistry in ice by us (11-13). Scheme 1 shows an overall mechanism of the 2-chlorophenol photolysis in liquid water based on Boule’s works as compared to the proposed reaction pathways occurring in the ice cavity. Intramolecular Wolff rearrangement of an R-ketocarbene to a ketene has been suggested for liquid solutions, and there was no reason to believe that it should not occur in ice. When oxygen-saturated 2-chlorophenol samples were irradiated no new photoproducts were identified, including quinoid compounds, given VOL. 37, NO. 8, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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SCHEME 1. Reaction Schemes of 2-chlorophenol Photolysis in Liquid Water (a) and in Ice (b)
that 2-benzoquinone, a possible product of R-ketocarbene reaction with oxygen (39), is unstable. In addition, neither cyclopentadienecarboxylic acid nor Diels-Alder dimers (21, 25) were detected. Thus, we cannot exclude the existence of an R-ketocarbene, but, due to the compounds identified in irradiated samples, our suggested mechanism presents rather classical coupling reactions involving a homolytic C-Cl bond cleavage to form the free phenolic radical and the chlorine atom. The radical can either arylate a suitable aromatic reaction partner in a coupling reaction or abstract hydrogen from a hydrogen donor (phenol formation). Presence of phenol may consequently lead to sensitization reactions by the energy transfer (22). Radicals may also undergo the electron transfer forming ion pairs and the aryl cations may further react with the other aromatic compound in a FriedelCrafts fashion, providing essentially the same products as those povided by the free radicals. The reaction regioselectivity clearly corresponds to the well-known orientation rules for the electrophilic aromatic substitution. No further information about the transients formed in ice matrix is available to date, thus other mechanisms, such as a coupling between two free radicals, one formed by homolytical cleavage described above and a phenoxy radical formed by the abstraction of the hydrogen atom from the OH group as was suggested elsewhere (25), or the involvement of an R-ketocarbene in the coupling reaction, should not be overruled. Further work, such as a laser flash photolysis study, is undoubtedly needed. Photolysis of 4-chlorophenol (2) in aqueous solution is known to give a mixture of products, the composition of which depends on the presence of oxygen and the initial concentration. The major products in degassed solutions 1572
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were found to be hydroquinone (through the photosolvolysis mechanism) and 5-chlorobiphenyl-2,4′-diol (9) (24, 25, 32, 40-42). It was reported that although hydroquinone (and consequently benzoquinone in air-saturated solutions) was the major product at lower concentrations (∼10-4 mol L-1), the production of 9 increased with increasing concentration. Secondary photosolvolysis reactions became significant when solutions were irradiated to higher conversions. The quantum yield (φ ) 0.4) of 4-chlorophenol degradation in water was found to be independent of wavelength of the absorbed quantum (25). Photolysis of 2 in fully frozen water ice (below -10 °C) gave the only major product, 5-chlorobiphenyl-2,4′-diol (9), identical to that found in liquid water, with the apparent quantum yield of 0.04 (Table 3). The reaction regioselectivity corresponds well to the orientation rules for electrophilic aromatic substitutions again. The quantum yield was comparable to that of 2-chlorophenol photolysis (similar reaction pathways are expected) but was 1 order of magnitude lower than that found in water. This must be partially due to the fact that the efficient reaction solvolysis pathway in ice is absent. Grabner et al. examined the details of 4-chlorophenol photolysis by laser flash photolysis assuming that elimination of HCl gives a carbene, 4-oxocyclohexa-2,5-dienylidene, which may further react with oxygen to form benzoquinone (43). Neither benzoquinone nor hydroquinone were identified in oxygen-saturated ice samples of 2 upon irradiation below -10 °C but this, again, should not preclude the carbene formation. The absolute concentration of oxygen in the ice matrix is significantly lower than that of the liquid solution due to a degassing process of the solidification, but we assume that its actual concentration in the ice cavity should be higher.
The mechanism of the coupling reactions could be, nevertheless, analogous to that shown in Scheme 1. It was believed that physical (and consequently chemical) properties of the solid matrixes containing chlorobenzene, which was studied before (11, 12), or monochlorophenols should be different, because the former compound is poorly soluble in water but the latter forms fairly concentrated solutions. A more soluble organic solute was anticipated to be more dispersed and separated in the frozen matrix, that is having a longer average mutual distance among the molecules. However, the existence of the coupling products in >85% relative yields at -15 °C (Tables 1 and 2) and in high chemical conversions clearly demonstrated that the solutes were fully aggregated in ice cavities, having at least 2 molecules in a close proximity at up to 10-7 mol L-1 concentrations. The monochlorophenol photochemistry thus parallels that of chlorobenzene, in which the coupling compounds (chlorinated biphenyls) were the major products (11, 12). The ice cavity became reactive (8) only when the temperature of the matrix increased and ice crystals started to melt, most probably initially at the sites of segregated organic molecules (grain boundaries (6, 7)). Although irradiation of the samples at temperatures below -10 °C did not result in the photosolvolysis products, raising the temperature to -5 °C caused a moderate photosolvolytic activity in the case of 4-chloropenol (formation of hydroquinone in addition to a dehalogenation product, phenol), and 2-chlorophenol was almost exclusively transformed into pyrocatechol (Figures 5 and 6). This tendency continued at higher temperatures when the samples were completely liquid. We may expect that photosolvolysis at -5 °C occurred in a liquid or quasi-liquid layer that covers the ice crystal surfaces where the organic molecules are segregated from the ice medium and form relatively high-concentration melted solutions. The fact that 2-chlorophenol undergoes more efficient solvolysis at -5 °C may be connected to its lower melting point (9 °C) relative to that of 4-chlorophenol (43 °C). It is clear that in natural ice or snow samples, that is having very low concentrations of organic hydrophobic molecules, the temperature of a liquid layer formation will be very close to 0 °C. Because of their absorption characteristics, natural solar irradiation can initiate a photodegradation process in monochlorophenols in the atmosphere, aqueous, or ice environment (Figures 1 and 2). Our recent field experiments in Ny-Ålesund, Svalbard (79° N, 12° E), with various samples containing different organic molecules frozen in the ice matrix, proved that a 3-day irradiation of monochlorophenols by natural sunlight at -15 °C caused a moderate photochemical activity (44). The photoproducts were found to be identical to those obtained in the laboratory experiments. We must realize that the concentration range of the compounds studied here was more than 3 orders of magnitude higher than that of the naturally occurring pollutants in polar ice and snow, the molecular aggregation of which (and consequently photochemistry) could be significantly different. Thus, additional studies of photochemical processes of naturally occurring organic pollutants in ice and snowpack are urgently needed. The major products of chlorophenol photolysis (3, 4, and 9) belong to the family of phenolic halogenated compounds (such as hydroxylated chlorobiphenyls). A growing number of studies have reported that those compounds are retained in the blood of humans or wildlife. They may be industrial chemicals or their metabolites, as in the case of polychlorobiphenylols (45-54). On the basis of Hoffmann’s model (10) and our model which propose a possibility of a secondary-pollutant formation in (on) ice or snow matrix (3), we suggest a potentially new aspect of the fate of chlorophenols in the polar environment or on ice crystals in
the upper atmosphere. Provided that these molecules are adsorbed or segregated into molecular clusters (aggregations), which unquestionably implies a deposition of a higher amount of the molecules, they will undergo relatively efficient phototransformations. The products of a potentially high environmental risk then might be introduced into the environment via ice melting or evaporation.
Acknowledgments We gratefully acknowledge financial support from the Grant Agency of the Czech Republic (205/02/0896). We also thank Dominik Heger, Jaromir Jirkovsky, Jindriska Dolinova, and Radovan Ruzicka for a fruitful collaboration.
Supporting Information Available Mass spectra of the compounds 3, 4, 5, 8, 9, and 12, two typical GC chromatograms, and the synthetic procedures. This material is available free of charge via the Internet at http://pubs.acs.org.
Note Added after ASAP This paper was released ASAP on 03/13/2003 with an error in ref 44. The correct version was posted on 03/19/2003.
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Received for review June 13, 2002. Revised manuscript received January 3, 2003. Accepted February 9, 2003. ES025875N
