Environ. Sci. Technol. 2006, 40, 7668-7674
Oxidation of Aromatic and Aliphatic Hydrocarbons by OH Radicals Photochemically Generated from H2O2 in Ice J I N D Rˇ I Sˇ K A D O L I N O V AÄ , † RADOVAN RU ° Zˇ I C ˇ KA,‡ R O M A N A K U R K O V AÄ , † J A N A K L AÄ N O V AÄ , † A N D P E T R K L AÄ N * , ‡ RECETOX, 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
Oxidation of aromatic and saturated aliphatic hydrocarbons (c ) 10-3-10-5 mol L-1) by the hydroxyl radicals, photochemically produced from hydrogen peroxide (c ) 10-1-10-5 mol L-1), in frozen aqueous solutions was investigated in the temperature range of -20 to -196 °C. While aromatic molecules (benzene, phenol, naphthalene, naphthalen-2-ol, or anthracene) underwent primarily addition-elimination reactions to form the corresponding hydroxy compounds, saturated hydrocarbons (cyclohexane, butane, methane) were oxidized to alcohols or carbonyl compounds via hydrogen abstraction and termination reactions. The results suggest that these photoreactions, taking place in a highly concentrated liquid or solidified layers covering the ice crystals, are qualitatively similar to those known to occur in liquid aqueous solutions; however, that probability of any bimolecular reaction in the environment ultimately depends on organic contaminant concentrations and oxidants availability at specific locations of the ice matrix, temperature, wavelength, and photon flux. They, moreover, support hypotheses that oxidation of organic impurities in the snowpack can produce volatile hydroxy and carbonyl compounds, which may consequently be released to the atmosphere.
Introduction Hydrogen peroxide is an important oxidation agent in the atmosphere (1), but it is also a common trace constituent of natural snow and ice (2). Its concentration in surface snow was found to depend largely on its atmospheric concentration (3). Many field studies at different locations in polar and mid-latitude regions have established that H2O2 (4, 5) and nitrates (6-8) are sources of the hydroxyl (HO•) or hydroperoxyl (HO2•) radical’s emission to the atmosphere. Chu and Anastasio have demonstrated that the majority of HO• produced on polar snow originates from peroxide photolysis, while nitrate photolysis is only a minor contributor, as a result of their different molar absorptivities and concentrations (9, 10). Jacobi and co-workers have just recently pointed to the importance of the photolysis of both H2O2 and nitrate * Corresponding author phone: +420-549494856; fax: +420549492688; e-mail:
[email protected]. † RECETOX. ‡ Department of Organic Chemistry. 7668
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 24, 2006
as the sources of HO• in snow (11). The molar absorption coefficient of hydrogen peroxide at wavelengths over 280 nm is generally low (e.g., 280 ) 4.2 L mol-1 cm-1); the quantum yield of its homolytic dissociation approaches unity in water (12) but decreases with decreasing temperature (10). The photolytic H2O2 lifetime under summer solstice sunlight has been estimated to be ∼140 h at representative sites on Antarctic ice sheets (10). The snow or ice environment is a site of possible photochemical activity, particularly in the surface layers (1315). Chemical reactions of organic pollutants with highly reactive species, such as the hydroxyl radicals (10, 11, 16, 17), or via their direct photoexcitation (18, 19) could result in the production of new, possibly toxic (20) compounds. It was suggested by Sumner, Shepson, and others that photoinitiated HO• could oxidize the snow-phase organic matter to produce volatile carbonyl compounds, such as formaldehyde (21-23). We have shown that photolysis of frozen aqueous solutions of organic aromatic chromophores, such as chlorophenols, in the presence of H2O2 produces a complex mixture of products (17). UV-photolysis of polycyclic aromatic hydrocarbons (PAH) in water ice under astrophysical conditions (T < 50 K; ionizing radiation) was also studied because PAH are believed to be the most abundant class of organic compounds in the universe (24). The aim of the present laboratory work was to evaluate qualitatively and quantitatively photoinduced oxidation reactions that occur on aromatic and saturated hydrocarbons in frozen aqueous solutions in the presence of hydrogen peroxide, being a source of the HO• radicals, under laboratory conditions. Photochemical reactivity, mechanistic considerations, and implications for the environment are discussed.
Experimental Section Instrumentation. HPLC analyses were performed on a Hewlett-Packard HP 1100 liquid chromatographic system, equipped with diode-array and fluorescence detectors, and a Polaris C18-A column. Gas chromatographs HewlettPackard HP 5890, or HP 6890, equipped with mass selective detectors (5971 or 5973) and a DB-5MS glass capillary column, were used for identification purposes. Low-temperature experiments (-20 °C) were accomplished in a cryostat MLW MK70 (ethanolic bath), while irradiation at temperatures below -20 °C were carried out in a quartz Dewar bottle using a liquid nitrogen/heptane cooling bath. Chemicals. The following chemicals and analytical standards, acetone (>99.8%), dichloromethane (>99.8%), and phenol (p.a.) from Riedel de Hae¨n, acetonitrile (>99.9%; LabScan), naphthalene (p.a.), hydrogen peroxide (for trace analysis; 30%), naphthalene-1,4-dione (p.a.), naphthalene2,7-diol (technical grade), 5-hydroxy-naphthalene-1,4-dione (purum), and cyclohexane (p.a.) from Fluka, naphthalene1,5-diol (>98%) and naphthalen-2-ol (>99%) from Merck, dimethylsulfoxide (p.a.), hydroquinone (p.a.), benzoquinone (p.a.), pyrocatechol (p.a.), and fumaric acid (p.a.) from Lachema, naphthalen-1-ol (>99%), benzene-1,2,4-triol (99%), benzene-1,2,3-triol (98%), naphthalene-1,3-diol (> 99%), and anthracene-9,10-dione (97%) from Aldrich, anthracene (>99%) from Sigma, and butane (2.5) and methane (3.5) from Linde, were used as received. Water was purified on an Osmonics 2 and a Millipore Simplicity 185. Irradiation Experiments. Aqueous samples, containing H2O2 and an organic compound (benzene, phenol, cyclohexane, or naphthalen-2-ol), were prepared by dilution of the saturated aqueous stock solutions. Because of low naphthalene and anthracene solubilities in water, methanolic 10.1021/es0605974 CCC: $33.50
2006 American Chemical Society Published on Web 11/15/2006
TABLE 1. Photolysis of Aromatic Compounds in Frozen Aqueous Solutions Containing Hydrogen Peroxidea [H2O2] in mol L-1; temperature
starting molecule (concentration) benzene (3 × 10-3 mol L-1)
phenol (3 × 10-3 mol L-1)
naphthalenec (3 × 10-5 mol L-1)
3 × 10-1; -20 °C 3 × 10-2; -20 °C
1.0 7.9
3 × 10-3; -20 °C 3 × 10-1; -20 °C 3 × 10-2; -20 °C
n.d.g 1.6 4.5
3 × 10-3; -20 °C 3 × 10-3; -20 °C
21.3g 0.5
3× -20 °C 3 × 10-5; -20 °C
2.2 6.3g
10-4;
3 × 10-4; -100 °C 3 × 10-4; -196 °C surface photolysis;d 10-1; -15 °C
6.2g 10.4g n.a.g
naphthalene (3 × 10-5 mol L-1)
radiolysis;e -78 °C
n.a.g
naphthalene-2-ol (3 × 10-4 mol L-1)
3 × 10-3; -20 °C 3 × 10-3; -196 °C 3 × 10-3; -20 °Cd
0.3g 2.1g n.d.g
naphthalene (n.d.)
anthracene (3 × 10-5 mol L-1)
degradation products at low reaction conversions
τ1/2 (h)b
major: phenol; hydroquinone; pyrocatechol minor: resorcinol; benzene-1,2,3-triol; benzene-1, 2,4-triol; fumaric acid major: hydroquinone; pyrocatechol minor: resorcinol; benzene-1,2,3-triol; benzene-1, 2,4-triol; benzoquinone major: naphthalen-1-ol; naphthalen-2-ol ([1-NpOH]/[2-NpOH] ∼1.5)f minor: 5-hydroxy-naphthalene-1,4-dione; naphthalene-2,7-diol; naphthalene-1,4-dione major: naphthalen-1-ol; naphthalen-2-ol; naphthalene-1,4-dione; 5-hydroxynaphthalene-1,4-dione minor: naphthalene-2,7-diol; naphthalene-1,5-diol major: naphthalen-1-ol; naphthalen-2-ol ([1-NpOH]/[2-NpOH] ∼1.6)f minor: naphthalene-2, 7-diol; 5-hydroxynaphthalene-1,4-dione major: naphthalene-2,7-diol major: anthracene-9,10-dione
Irradiation in Pyrex vessels. The half-lives of an organic compound (τ1/2) in hours; n.d. ) not detected; n.a. ) not applied. c The samples contained below 1% of methanol or DMSO to enhance aqueous solubility of the starting material. Naphthalene photodegraded also in the absence of H2O2; the half-lives are corrected. d Irradiation of the ice (containing H2O2) surface with naphthalene condensed from vapors. e γ-Radiolysis (the total dose was between 15 and 75 kGy). f [1-NpOH]/[2-NpOH] is the concentration ratio of naphthalen-1-ol and naphthalen-2-ol. g A portion of the starting compound remained unreacted after exhaustive irradiation. a
b
(in the case of naphthalene) or DMSO (in the case of naphthalene or anthracene) stock solutions were prepared so that the resulting samples always contained less than 1% of an organic solvent. In experiments with gaseous hydrocarbons (butane, methane), a peroxide solution in sealed headspace Pyrex (>290 nm) vials was bubbled by the corresponding gas for 5 min, solidified in a cryostat, and the residual gas above the ice matrix was removed before irradiation. For bulk irradiation, the samples in 13 × 100 mm Pyrex tubes, sealed with septa, were solidified in a cryostat box filled with ethanol as a cooling medium (-20 °C), and irradiated using a 125-W medium-pressure mercury lamp (Teslamp). For surface irradiation, a thin layer (layer thickness, ∼0.1 mm; surface area, 350 cm2) of a frozen aqueous H2O2 solution (c ) 10-1 mol L-1) was formed on the surface of a cooled quartz finger at -15 °C. Vapors of naphthalene or anthracene (∼0.01 g; heated in a separate flask), carried by a stream of nitrogen at atmospheric pressure, were condensed on a cold surface of ice. The surface was immediately irradiated by a medium-pressure mercury lamp (125 W) through Pyrex (>290 nm) in a 14-cm distance. The steady-state γ-irradiation was carried out by a 60Co source (Bioster, Czech Republic) with a dose rate of ∼1 kGy h-1. The samples were irradiated in an isolated box containing dry ice (-78 °C). Analysis. One aliquot of a thawed ice sample was used for a HPLC/DAD/FLD analysis, and the other was extracted by dichloromethane for GC/MS analysis. The data reported are typical results obtained from at least triplicate measurements. Identification and quantification of the photoproducts was carried out by comparison of their retention times and MS spectra with those of the authentic compounds or the mass spectral library. In the case of experiments with methane, the samples were allowed to melt and equilibrate in tightly sealed headspace vials for 24 h, and the headspace
air concentrations of methanol were measured using GC/ MS.
Results Frozen aqueous solutions of an organic compound (benzene, phenol, naphthalene, naphthalen-2-ol, anthracene, cyclohexane, butane, butan-1-ol, butan-2-ol, butanal, or methane), containing hydrogen peroxide, were irradiated to identify the photoproducts and quantify their concentrations. A multiwavelength UV irradiation filtered by Pyrex (>290 nm) was used to simulate natural solar radiation. Except for naphthalene, anthracene, and naphthalen-2-ol, all other organic compounds used do not absorb significantly above this limit. The total irradiation time (minutes to hours) was adjusted to keep the conversions either low (below 10%) or high to study primary or secondary photoreactions, respectively. The complete mass balance was never achieved in any of the analyses, but it was usually reasonably high (
