Environ. Sci. Technol. 1906, 20, 934-938 (32) Wilson, M. J. G. Proc. R. SOC.London, Ser. A 1968, A307, 215-22 1. (33) Knutson, E. 0. In Radon and Its Progeny in Indoor Air; Nazaroff, W. W.; Nero, A. V., Eds.; CRC Press: Boca Raton, FL; in press. (34) Young, T. R.; Boris, J. P. J . Phys. Chem. 1977, 81, 2424-2427. (35) Baer, N. S.; Banks, P. N. Int. J . Museum Manage. Curatorship 1985, 4 , 9-20. (36) Mathey, R. G.; Faison, T. K.; Silberstein, S.; Woods, J. E.; Johnson, W. B.; Lull, W. P.; Madson, C. A.; Turk, A.; Westlin, K. L.; Banks, P. N. Air Quality Criteriafor Storage of Paper-Based Archival Records; National Bureau of Standards: Washington, DC, 1983; NBSIR 83-2795. (37) Winer, A. M.; Peters, J. W.; Smith, J. P.; Pitts, J. N., Jr. Environ. Sci. Technol. 1974, 8 , 1118-1121. (38) Hanst, P. L.; Wong, N. W.; Bragin, J. Atmos. Enuiron. 1982, 16, 969-981. (39) Tuazon, E. C.; Winer, A. M.; Pitts, J. N., Jr. Enuiron. Sci. Technol. 1981,15, 1232-1231. (40) Russell, A. G.; Caw, G. R. Atmos. Environ., in press. (41) Grosjean, D.; Fung, K. J . Air Pollut. Control Assoc. 1984, 34, 531-543. (42) Russell, A. G., Carnegie-Mellon University, personal communication, 1985. (43) Pitts, J. N., Jr.; Sweetman, J. A.; Zielinska, B.; Atkinson,
(44) (45) (46) (47) (48)
(49)
(50)
R.; Winer, A. M.; Harger, W. P. Enuiron. Sci. Technol. 1985, 19,1115-1121. Mueller, F. X.; Loeb, L.; Mapes, W. H. Environ. Sci. Technol. 1973, 7, 342-346. Sutton, D. J.; Nodolf, K. M.; Makino, K. K. ASHRAE J . 1976, 18(9), 21-26. Traynor, G. W.; Apte, M. G.; Dillworth, J. F.; Hollowell, C. D.; Sterling, E. M. Environ. Int. 1982, 8, 441-452. Wade, W. A., 111; Cote, W. A,; Yocum, J. E. J . Air Pollut. Control Assoc. 1975,25, 933-939. Miyazaki, T. In Indoor Air: Chemical Characterization and Personal Exposure; Berglund, B.; Lindvall, T.; Sundell, J., Eds.; Swedish Council for Building Research Stockholm, 1984; Vol. 4, pp 103-110. Revzan, K. L. In Indoor Air: Building Ventilation and Thermal Climate; Berglund, B.; Lindvall, T.; Sundell, J., Eds.; Swedish Council for Building Research: Stockholm, 1984; Vol. 5, p p 65-72. Summer, W. Ultraviolet and Infra-Red Engineering; Interscience Publishers: New York, 1962; p 60.
Received for review January 17,1986. Accepted April 21,1986. The research was supported by a contract with the Getty Conservation Institute and by an Earle C. Anthony Graduate Fellowshio.
Singlet Oxygen Reactions on Irradiated Soil Surfaces Kirk Gohre, Roger Scholl, and Glenn C. Miller" Department of Biochemistry, University of Nevada, Reno, Nevada 89557
The photochemical generation of singlet oxygen on a variety of soils was confirmed by using two specific singlet oxygen traps. Geminally deuterated 2,3-dimethyl-2-butene and 1,2-dimethylcyclohexenereact with singlet oxygen to produce characteristic ratios of deuterated alcohols and endocyclic and exocyclic alcohols, respectively. The ability of irradiated soils to photosensitize the formation of singlet oxygen was independent of the amount of organic matter present, and both the organic and inorganic components of the soils appeared to contribute to the sensitizing activity of the soils. Two soils from rice growing areas of California uniquely photosensitized the conversion of the olefins to saturated alcohols. Other reactions observed for the olefins included epoxidation and ketone formation. Free radical oxidations were not observed to significantly contribute to the loss of either of the olefins. Singlet oxygen reactions on soil surfaces may contribute to transformation of several classes of susceptible xenobiotics. Introduction Although significant amounts of agricultural and industrial chemicals released into the environment are deposited on sunlight-exposed soil, photochemical processes occurring on soil surfaces are not well understood. Indirect photochemical processes are of particular interest since sunlight striking the soil surface will be absorbed by both organic and inorganic chromophores, and potentially initiate reactions which can transform pollutants. Previous work has suggested singlet oxygen is produced on sunlight-irradiated surfaces and may contribute to transformation of various susceptible xenobiotics ( I ) . The initial investigations which suggested that singlet oxygen is produced on irradiated soil surfaces utilized two reactive singlet oxygen traps, 2,3-dimethyl-2-butene (1) and 2,5-dimethylfuran (2, 3). Although 2,3-dimethyl-2-butene
* Author to whom correspondence should be addressed. 934
Environ. Sci. Technol., Vol. 20, No. 9, 1986
(1) is believed to be a specific trap for singlet oxygen, yielding 2,3-dimethyl-l-buten-3-01 (2), 2,5-dimethylfuran can react with other oxidants to form the apparent singlet oxygen product, cis-diacetylethylene (4-6). The present investigation utilized two specific singlet oxygen traps, 1,2-dimethylcyclohexene and geminally deuterated 2,3-dirnethyl-2-butene-d6, that, upon photooxidation on the soil surface, would confirm the presence of singlet oxygen. 1,2-Dimethylcyclohexene(3) reacts with singlet oxygen to give peroxides that can be reduced to two isomeric allylic alcohols (6 and 7) (Figure 1) which are readily distinguished from products formed through radical oxidation (3, 7). Yields of the exocyclic alcohol (7) predominate in singlet oxygen reactions. In contrast, radical oxidation of 3 proceeds by abstraction of a hydrogen atom followed by molecular oxygen addition to the allyl radicals finally producing hydroperoxides, which on reduction give 6 and 8 (8). Oxidation of 8 gives the unsaturated ketone 9. The products 8 and 9 are not observed from '02oxidation (Figure 1). Geminally deuterated 2,3-dimethyl-2-butene-d, (12) affords a specific test for singlet oxygen due to a kinetic isotope effect. Singlet oxygen preferentially abstracts a hydrogen vs. a deuterium during the oxidation to give an observed isotope effect ( k H / k D )of 1.45 in the allylic alcohols 13/14 (9).
t
CH
C , D3
/'='\
CH3
CD3
1)
' 0 2
__*
2)p@'S
(4
CH
7
$c-c cH(
d';;CDI
3
+
c y c-c CRb,
p
2
'CO,
MW 106
MW 105
(13)
(141
This selectivity is distinctly different from alkyl, hydroxyl, or peroxyl radical oxidations. Kinetic isotope effects involving peroxyl radicals (ROO') are much larger (3.0-30) ( I O ) . The isotopic ratio (kH/jZD)for hydroxyl
00 13-936X/86/0920-0934$0 1.50/0
0 1986 American Chemical Society
q
(x-a
G
7
OH
IsoH ,4
Gp(Jo* 1
0 0 CH&CH,CH ,CH , C H , ~ C H ~ I1
b]*[a]6-q y
3
k
OOH
E-&-& 8
9
Figure 1. Products of the oxidation of 1,2dimethyicyclohexene. Soil = S;radical oxidation = R; singlet oxygen = IO2.
radical (OH') abstractions are also higher than for the singlet oxygen reactions. For example, hydroxyl radicals react with n-C4Hlo(n-C4Dlo)to give a kinetic isotope effect of 3.83 (11). Alkyl (R') radicals also show a marked preference for abstracting hydrogen over deuterium. Isotopic ratios from 1.9 to 1100 have been reported (12). As the superoxide radical (02-) is very unreactive as a hydrogen atom abstractor, its participation in the observed reactions is unlikely (13). The present work was performed to confirm that singlet oxygen is produced on irradiated soil surfaces. In addition, soils from several locations were examined to establish whether singlet oxygen formation is a general phenomenon of sunlight-irradiated soils. Experimental Section Chemicals. 2,3-Dimethyl-Zbutene,triphenylphosphine P(c$)~,m-chloroperbenzoic acid, and 2,2'-azobisisobutyronitrile (AIBN) were obtained from Aldrich Chemical Co. Methylene blue, 1,2-dimethylcyclohexene,2,3-dimethyl1-buten-3-01,and rose bengal were obtained from ICN-KNK Pharmaceuticals. Deuterated acetone was obtained from Stohler Isotope Chemicals. All organic reagents (excluding solvents) were checked for purity by gas-liquid chromatography (GLC) or high-pressure liquid chromatography (HPLC). Solvents were HPLC grade and obtained from VWR Scientific. Geminally deuterated 2,3-dimethyl-2-butene-&(12) was synthesized as follows. Deuterated acetone was coupled to 2-bromopropane by a Grignard reaction to yield geminally deuterated 2,3-dimethyl-2-butanol. The alcohol was then dehydrated with iodine at 85 OC for 5 h. The reaction progress was monitored by GLC on a 1.2 m X 2 mm i.d. nickel 80-100 mesh Porapak S column at 190 OC and found to be 86% complete. The low boiling olefin was then purified by distillation. Analysis by gas chromatography-mass spectrometry (Finnigan 4023 gas chromatb graph-mass spectrometer with Incos data system) showed the distillate to be 97% pure using a 1.2 m X 2 mm, 80-100 mesh Carbopak B column (Supelco) at 140 "C: mass spectra (electron impact 70 eV), mle (relative intensity) 90 (M, 77), 75 (M - 15, go), 72 (M - 18, 100). 1,2-Dimethylcyclohexane epoxide ( 5 ) and 2,3-dimethyl-2-butene epoxide were prepared from the parent compounds with m-chloroperbenzoic acid (14). 2,7-Octanedione (11) was made by an acid-catalyzed rearrangement of the allylic hydroperoxides of 6 and 7 (15). The allylic hydroperoxides were generated by the photooxidation of 1,2-dimethylcyclohexene with rose bengal. 1,2-Dimethylcyclohexene (500 pL) and rose bengal (100
mg) were coated on the sides of a 500-mL volumetric flask under an O2 atmosphere. The flask was irradiated for 3 h with Westinghouse CW40 fluorescent lamps. The hydroperoxides and unreacted 1,2-dimethylcyclohexenewere separated from the rose bengal by extraction into anhydrous ether. The ether was evaporated, 6 mL of 0.1 N H2S04in methanol was added, and the mixture was allowed to stand overnight. The reaction was neutralized with 6 mL of 0.1 N NaOH in H20, and the mixture was extracted with 5.0 mL of CHC13. The major product was confirmed by GC-MS as 2,7-octanedione. Singlet Oxygen Standards. Photosensitizing dyes known to generate singlet oxygen were used in standard photooxidations to compare soil surface photoreactions to well-documented singlet oxygen systems. The principal dyes used were rose bengal and methylene blue, which absorb visible light at wavelengths between 500 and 600 nm (16, 17). The allylic alcohols (2,3-dimethyl-l-cyclohexen-3-01 (6) and 2-methyl-1-methylenecyclohexan-2-01 (7)) weke made by CW40 photooxidation of 1,2-dimethylcyclohexenewith rose bengal. The irradiations were accomplished in 50-mL volumetric flasks in oxygen-saturated methanolic solutions of rose bengal(1 mg/mL) with 5 pL of 1,2-dimethylcyclohexene.Subsequent reduction of the hydroperoxides was done by the addition of 3 mL of 0.05 M triphenylphosphine in methanol. The isomeric alcohols from the singlet oxygen oxidation of geminally deuterated 2,3-dimethyl-2-butene-d, (13 and 14) were prepared in a similar manner. Analysis of the allylic alcohols was by GC-MS on the Carbopak B column described earlier. The allylic alcohols had identical retention times of 6.4 min. The combined mass spectrum was determined of a singlet oxygen generated mixture of compounds 13 and 14: electron impact 70 eV, mle (relative intensity) 106 (M, 6.4), 105 (M, 4.4), 91 (42), 90 (51), 88 (88), 87 (37), 65 (loo), 60 (62), 59 (43), 46 (76), 43 (58). Radical Reaction Standards. Radical oxidation products of 2,3-dimethylcyclohexene,2,3-dimethylcyclohexen-1-01(S), and 2,3-dimethylcyclohexen-l-one(9) were generated by radical oxidation of 1,2-dimethylcyclohexene (0.114 mol) by heating at 75 "C in the presence of the radical initiator AIBN (1.0 mg) while bubbling oxygen through the solution (18). After 3.5 h a 5-pL aliquot of the solution was reduced by addition to 3.0 mL of 0.05 M triphenylphosphine in methanol to prevent further reactions of the hydroperoxides. In addition to the specific radical products (8 and 9), epoxide ( 5 ) , endocyclic (6), and exocyclic alcohols (7) were produced. Soil Preparation. Seven soils were assayed for their ability to photosensitize the production of singlet oxygen. All soils were air-dried and sieved through a 0.04-cm screen to obtain uniformity of the samples. Except where noted, no further preparation of the soils was undertaken. Each of the soils was partially characterized by using standard techniques (Table I). Irradiation Procedures. In a typical assay 1.75 g of soil was placed in a 50-mL Kimax volumetric flask. This gave a soil depth of 1-2 mm and exposed surface area in the flask of 14.5 cm2. The flask was then sealed with a Teflon-lined septum, and the respective pure olefin (5 pL) was injected into the flask. Because of the high vapor pressure of each of the compounds, they rapidly evaporated following injection into the flasks. No further mixing of the chemical and the soil was performed. Compounds were irradiated in the presence of a sensitizer or on the soil surface with either natural sunlight or by an apparatus containing four broad-band CW40 Westinghouse fluorescent lamps emitting wavelengths between 380 and 740 nm Environ. Sci. Technol., Vol. 20, No. 9, 1986
935
Table I. Soil Characteristics % organic
soil"
matter
Death Valley Oppio Kracaws M.S.F. Chico Durham ricefield Richvale ricefield
0.17 0.53 0.79 1.97 6.31 4.98 5.19
pH 70 clay % silt % sand 7.8 7.3 7.5 7.2 6.5 4.4 4.5
13 19 8 14 14 53 15
39 25 47 32 31 25 35
48
56 48 54 55 22 50
" Soils were collected from the following locations: Death Valley, near Furnace Creek, CA; Oppio, 30 km north of Reno, NV; Kracaws, 40 km north of Winnemucca, NV; M.S.F., University of Nevada research farm; Chico, walnut orchard 4 km west of Chico, CA; Durham ricefield, near Durham, CA; Richvale ricefield, near Richvale, CA. Table 11. Mass Spectra" and Retention Timesbof 1,2-Dimethylcyclohexene Oxidation Products
compound 4
5
6 7
8 9
10 11
R.T., M min (re1 int)
diagnostic ions
4.27 110 (53) 95 (M - 15, loo), 81 (go), 67 (91) 6.64 126 (4) 111 (M - 15, 8), 108 (M 18, 3), 71 (92), 43 (100) 9.48 126 (15) 111 (M - 15, loo), 108 (M - 18, 7), 43 (71) 8.06 126 (17) 111 (M - 15, 28), 108 (M 18, 7), 97 (54), 43 (100) 12.32 126 (23) 111 (M - 15, loo), 108 (M - 18, 8), 43 (46) 16.12 124 (61) 109 (M - 15, 5), 96 (loo), 67 (60) 8.53 128 (10) 113 (M - 15, ll),110 (M 18, 4), 71 (loo), 43 (45) 18.06 84 (26), 58 (16), 43 (100)
"Electron impact (70 eV). *Chromatography on a 30-m SPB-5 (Supelco) capillary column operated in the program mode from 70 to 110 OC at 2 OC/min.
(19). Sunlight irradiations were conducted outdoors with all reaction flasks aligned at the same angle to the sun. Aluminum foil covered dark controls were conducted in all cases. Following irradiation, 6.0 mL of 0.05 M triphenylphosphine in methanol was added to reduce the peroxide photoproducts. Although some reduction of the hydroperoxide occurred on the soil surface during irradiation, use of triphenylphosphine gave higher yields of the alcohols. The flasks were covered to prevent further light reactions, and the reduction was allowed to proceed a minimum of 1 h at 5 "C prior to analysis. The cooled solutions were centrifuged, and the supernatant was analyzed. The analyses were completed within l day, since the epoxide concentrations were observed to decrease over a longer time period. The analysis of 2,3-dimethyl-Z-butenesamples has been described previously (1). Irradiated samples containing 1,2-dimethylcyclohexene were analyzed by gas chromatography on a SPB-5 30-m capillary column (Supelco) using either a flame ionization detector or GC-MS. The photoproducts were identified by their mass spectra (Table 11) and by cochromatography with standards made by reaction of 1,2-dimethylcyclohexene with singlet oxygen or radical oxidation. Geminally deuterated 2,3-dimethyl-2-butene-&samples were analyzed by GC-MS with a 2.6-m Carbopak B column at 180 "C. Identification was performed by comparison of mass spectra to standards made from photooxidations with rose bengal and methylene blue. To determine the kinetic isotope effect, molecular ions at 105 and 106 were integrated over the gas chromatographicpeak 936 Environ. Sci. Technol., Vol. 20, No. 9, 1986
Table 111. Irradiation Yields of 2,3-Dimethyl-l-buten-3-01 (2) on Various Soil Surfaces With Sunlight" and CW40 Lampsb % 2,3-dimethyl-
% conversionc to
2,3-dimethyl1-buten-3-01 22.5 25.9 25.0 28.4 14.2 5.3
soil
light source
2-butene recovered
Death Valley Oppio Kracaws M.S.F. Richvale rice Death Valley Oppio Kracaws M.S.F. Chico Richvale rice Durham rice
CW40 CW40 CW40 CW40 CW40 sunlight sunlight sunlight sunlight sunlight sunlight sunlight
9.5 9.6 tr 4.1 14.9 41.0 35.2 38.0 42.1 51.8 40.0 18.0
9.8
9.6 10.5 7.0 5.0 trd
"A 4-h irradiation (10 a.m.-2 p.m., 9/4/82), average of three samples. bA 24-h irradiation, average of three samples. % yields = moles of 2 formed/moles of 1 starting. dTrace ( Kracaws = Oppio > M.S.F. Trace amounts of acetone were noted on only the low organic soils. Irradiation of 1,2-Dimethylcyclohexene (3) on Soil Surfaces. Results of the photocatalytic oxidation of 1,2-dimethylcyclohexeneindicate that singlet oxygen was produced on all the soils tested (Table IV). Though the product distributions from the soil photooxidations are quite diverse, the singlet oxygen photoproducts 6 and 7 were found on all soils tested. As in the rose bengal photooxidations, the exocyclic alcohol (7)was produced in higher yields than the endocyclic alcohol (6)in the soil
Table IV. Product Distribution for t h e Photosensitized Oxidation of 1,2-Dimethylcyclohexene : photoproduct distributionb
soil or sensitizer Death Valley' Kracaws" M.S.F.' Chico' Richvale riceC Durham riceC rose bengald radical oxid"
aa@qq&&aG
% material starting
recovered" 11
24 23 32 21 34 29
OH
4
ND ND ND ND 11
15 ND ND
6
32 15 tr ND ND ND ND 23
OH
6
7
10 6 tr tr 4
16 37 71 57 13 9 90 8
2
10 29
OH
e
Q
10
ND ND ND ND ND ND ND 32
ND ND ND ND ND ND ND 8
ND ND ND ND 40 47 ND ND
11
9 17 15 17 18 17 ND ND
other 43 25 14 26 14 10
ND ND
Percent recovery of added 1,2-dimethylcyclohexene. Photoproduct distribution for compounds 4-11 is based on total quantity of photoproducts detected; trace < 0.5%. Irradiated 18 h under O2 in closed 50-mL Kimax volumetric flasks. Irradiated with 10 mg coated on the side of the flask for 1h under 02. ePerformed by heating 0.114 mol of 1,2-dimethylcyclohexeneand l.O-mg of AIBN (radical initiator) for 3.5 h at 75 "C while bubbling O2 through the solution.
sensitized photooxidations. Many of the compounds in the "other" category were identified as secondary oxidations of primary products by their later appearance in the reaction sequence and the presence of two or more oxygens in their mass spectra (E1 m / e 142, 158, etc.). As in the 2,3-dimethyl-2-butene irradiations, several photoproducts (4,5, and 10) were formed not attributable to singlet oxygen reactions. Irradiation of Geminally Deuterated 2,3-Dimethyl-2-butene-d6(12) on Soil Surfaces. Singlet oxygen reacts with 12 to give allylic alcohols with a characteristic ratio of products (KH/kD = 1.45). Evidence for singlet oxygen involvement is provided when ratios near 1.45 are obtained. Although the isotopic ratios observed on the three soils were higher than with the sensitizers rose bengal and methylene blue, the ratios are within values expected for singlet oxygen reactions (Table V).
Discussion The photochemical generation of singlet oxygen was evaluated on seven soils obtained from different locales. The initial photooxidation studies, conducted with 2,3dimethyl-2-butene and 2,bdimethylfuran, suggested that singlet oxygen is produced on sunlight-irradiated soils (I) by a photosensitized reaction. Additional photooxidation studies conducted with 1,2-dimethylcyclohexene and were used geminally deuterated 2,3-dimethyl-2-butene-d6 to confirm singlet oxygen formation and exclude the participation of radical-initiated reactions. The results suggest that the photogeneration of singlet oxygen is a general characteristic of sunlight-irradiated soil surfaces. Although several products from the soil-sensitized photooxidations of 1,2-dimethylcyclohexenewere observed, the two specific singlet oxygen photoproducts 6 and 7 were found in all soils tested, with the exocyclic alcohol found in higher yields than the endocyclic alcohol. Radical photoreactions were surprisingly not observed. Zepp and co-workers have shown that humic substances are important photoseilsitizers in natural waters (20-22). These studies suggest that the humic fraction of soil may similarly participate in the photoproduction of singlet oxygen on irradiated soil surfaces. Humic substances have been shown to fluoresce and phosphoresce under visible or UV light irradiation (23,24). Under illumination, excited singlet and triplet states of the humic materials can be formed which produce singlet oxygen by a triplet-triplet (type 2) reaction. Chemiluminescence observed at 615-650
nm during the irradiation of humic substances (25) was suggested to be due to an emission from singlet oxygen dimoles. Alternative methods of singlet oxygen production are also possible. Slawinski and co-workers (26) observed inhibition of the chemiluminescence of irradiated humic substances with both free radical inhibitors and singlet oxygen quenchers, suggesting that, in addition to type 2 sensitized singlet oxygen production, a raaical (type 1) mechanism might be involved. This would require lightgenerated free radicals to react with 02,forming a superoxide like complex (HS- - -02-) that decomposes to singlet oxygen. Since there is evidence that superoxide can decompose to form singlet oxygen, this pathway would seem plausible (27). Alternatively, a charge-transfer complex similar to that proposed for polystyrene could be possible (28). However, the exact mechanism of IO2 production from irradiated humic acid remains an open question. If production of singlet oxygen is solely due to the humic fraction of soil, correlation of soil organic content and singlet oxygen reactions would be expected. The rates of loss of 2,3-dimethyl-2-butene and 2,bdimethylfuran and formation of singlet oxygen products on a variety of soils showed no correlation with soil organic content or pH. Concurrent with the investigations of the observed photolysis and organic matter, photooxidation studies on nontransition metal oxide surfaces were also being conducted (29). By use of techniques similar to those described in this work, the production of singlet oxygen was demonstrated on chromatographic silica gel and alumina. The mechanism of formation of a singlet oxygen on these surfaces is probably due to surface defects which result in band gaps of low enough energy to absorb visible light. The inorganic fraction of soil consists predominantly of primary minerals and layered, platelike silicate clay minerals consisting of sheets of silica tetrahedron and aluminum octahedra (30, 31). Since both the clay fraction and the primary mineral fraction of soil are chemically related to silica gel and alumina, it is likely that the inorganic as well as the organic fraction of soil is involved in photochemical production of singlet oxygen. The relative involvement of each fraction is, however, unknown at this time. In addition to the singlet oxygen photoproducts, photooxidation resulted in the formation of several other compounds. Compounds 4, 5, 10, and 11 appear to oriEnviron. Sci. Technol., Vol. 20, No. 9, 1986
937
Table V. Deuterium Isotope Effects in the Photosensitized Oxidation of Geminally Deuterated 2,3-Dimethyl-2-butene (12) sensitizer Kracaws soil" M.S.F. soil4 Chico soil"
kH/kD
1.64 1.60 1.58
sensitizer rose bengalb methylene bluec bare flask
kHlkD
1.45 1.45 ndd
"Irradiated for 12 h with CW40 fluorescent lamps under 02. bIrradiated for 0.5 h in 3 mL of distilled H 2 0 with 10 mg of rose bengal under O2 with CW40 fluorescent lamps. cIrradiated for 2.5 h in 3 mL of distilled H 2 0 with 10 mg of methylene blue with CW40 fluorescent lamps. Products not detected.
ginate from other photoreactions not related to the singlet oxygen reaction. However, 2,7-octanedione (11) is believed to arise from a rearrangement of the hydroperoxides of 6 and 7 on the soil surface. The initial attack of singlet oxygen on 3 results in the formation of the hydroperoxides of 6 and 7,which can either undergo reduction to 6 and 7 or rearrangement to other compounds. One identifiable rearrangement is to 2,7-octanedione (11) by a Hock cleavage (32). While the rearrangement is generally acid catalyzed, cases of thermal cleavage in the absence of acid have been reported (32). It was determined that substances in the soil catalyzed this rearrangement by applying the isolated hydroperoxides of 6 and 7 to the soil and allowing them to decompose in the dark or under CW40 irradiation for 24 h. Rearrangement of the hydroperoxides to the 2,7-octanedionewas observed on soils from a Richvale ricefield, M.S.F., and Aldrich SiOz surfaces but not in bare flasks. Therefore, the observed yields of the singlet oxygen products 6 and 7 indicated in Table IV are minimum levels and may be higher if other degradative pathways of the hydroperoxides are considered. The ricefield soils, due to their low pHs, are expected to be quite reactive in the rearrangement, which explains in part the lower yields of the allylic alcohols observed. The photoreactivity of the ricefield soils was quite distinct from the other soils since other soil-sensitized photoproducts of 3, including a photoisomerization product (4) and reduced alcohol (lo),were uniquely observed. Two practices in rice agriculture are noteworthy: ricefields are flooded for considerable amounts of time during the year, and rice growers burn rice plant residues after harvest. The pH of these soils was much lower than the other soils examined and suggests the presence of reactive aluminum, manganese, and iron, which might have contributed to the different photoreactivity of the ricefield soils. However, a clear rationale for the different photoreactivity of these soils is not available. The photosensitized production of the largest quantities of epoxides occurred on soils with the lower organic content, suggesting that semiconductor type oxidations may be promoted by the inorganic fraction of soils. Since photoinitiated oxidations, including electron donation, have been demonstrated to occur on many pure oxide surfaces, it is likely that photohole-electron pairs (exitons) can be produced on soil surfaces. The photohole produced must only be greater in energy than the ionization potential of the compound to facilitate an electron-transfer oxidation (33). These studies confirm the generation of singlet oxygen on irradiated soil surfaces at environmentally significant rates and suggest a previously unrecognized degradative pathway for xenobiotics deposited on soil surfaces. Although singlet oxygen exhibits substantial differences in reactivities with classes of organic compounds, it is known to react with several types of olefins, furans, sulfides, 938
Environ. Sci. Technol., Vol. 20, No. 9, 1986
amines, polynuclear aromatic hydrocarbons, and other electron-rich compounds at significant rates (34). These indirect oxidations thus are potentially a major transformation pathway for terrestrially deposited pesticides, industrial pollutants, and combustion products. Registry No. 1, 563-79-1; 2, 10473-13-9; 3, 1674-10-8; 4, 1759-64-4;5,17612-36-1;6,51036-24-9;7,52134-08-4;8,52134-09-5; 9, 1122-20-9; 10, 5402-29-9; 11, 1626-09-1; 12, 38132-23-9; 13, 102851-31-0;14,102831-05-0;02, 7782-44-7;2,3-dimethyl-2-butene epoxide, 5076-20-0; 2,3-dimethyl-2-butanol, 594-60-5; acetone, 67-64-1.
Literature Cited Gohre, K.; Miller, G. C. J . Agric. Food Chem. 1983, 31, 1104-1108. Gleason, W. S.; Broadbent, A. D.; Whittle, E.; Pitts, J. N., Jr. J . Am. Chem. SOC.1970, 92, 2068-2075. Foote, C. S. Photochem. Photobiol. 1978, 28, 922. Noguchi, T.; Takayama, K.; Nakano, M. Biochem. Biophys. Res. Commun. 1977, 78, 418-423. Foote, C. S. Acc. Chem. Res. 1968, 1 , 104-110. Held, A. M.; Hurst, J. K. Biochem. Biophys. Res. Commun. 1978,81, 878-885. 1978,100, Danen, W. C.; Arudi, R. L. J . Am. Chem. SOC. 3944-3945. Foote, C. S., Biochemical and Clinical Aspects of Oxygen; Academic: New York, 1979; pp 603-626. Stephenson, L. M.; Grdina, M. J.; Orfanopoulos, M. Acc. Chem. Res. 1980,13, 419-425. Ingold, K. U. Acc. Chem. Res. 1969, 2, 1-9. Paraskevopoulos, G.; Nip, W. S. Can. J . Chem. 1980,58, 2146-2149. Engel, P. S.; Chae, W.; Baughman, S. A.; Marschke, G. E.; Lewis, E. S.; Timberlake, J. W.; Leudtke, A. E. J . Am. Chem. SOC. 1983,105, 5030-5034. Liebman, J. F.; Valentine, J. S. Isr. J. Chem. 1983, 23, 439-441. 1966, 88, Price, C. C.; Carmelite, D. D. J. Am. Chem. SOC. 4039-4044. March, J. Advanced Organic Chemistry; McGraw-Hill: New York, 1977; p 1012. Schaap, P. A.; Thayer, A. L.; Blossey, E. C.; Neckers, D. C. J . Am. Chem. SOC. 1975,97, 3741-3745. Wasserman, H. H.; Murray, R. W. Singlet Oxygen; Academic: New York, 1979; p 586. Griesbaum, K.; Oswald, A. A.; Quiram, E. R.; Butler, P. E. J. Org. Chem. 1965,30, 261-266. Westinghouse Lighting Handbook; Westinghouse Electric Corporation: Bloomfield, NJ, 1974; pp 3-46. Zepp, R. G.; Baughman, G. L.; Schlotzhauer, B. F. Chemosphere 1981, 10, 109-117. Zepp, R. G.; Baughman, G. L.; Schlotzhauer, B. F. Chemosphere 1981, 10, 119-126. Zepp, R. G.; Wolfe, N. L.; Baughman, G. L.; Hollis, R. C. Nature (London) 1977,267, 421-423. Almgren, T.; Josefsson, B.; Nyquist, G. Anal. Chim. Acta 1975, 78, 411-422. Choudhry, G. G. Toxicol. Environ. Chem. 1981,4,261-295. Slawinski, J.; Puzyna, W.; Slawinski, D. Photochem. Photobiol. 1978, 28, 75-81. Slawinski, J.; Puzyna, W.; Slawinski, D. Photochem. Photobiol. 1978, 28, 459-464. Mayeda, E. A.; Bard, A. J. J . Am. Chem. SOC.1975, 95, 6223-6226. Rabek, J. F.; Ranby, B. Polym. Eng. Sci. 1975,15, 40-43. Gohre, K.; Miller, G. C. J . Chem. SOC., Faraday Trans. 1 1985,81, 793-800. Gieseking, J. E. Adv. Agron. 1949, I, 159-204. Rich, C. I.; Thomas, G. W. Adv. Agron. 1960, 12, 1-39. Frimer, A. A. Chem. Rev. 1979, 79, 359-386. Fox, M. A. Acc. Chem. Res. 1983,16, 314-321. Wilkenson, F.; Brummer, J. G. J . Phys. Chem. Ref. Data 1981,10, 809-998.
Received for review August 19, 1985. Accepted March 26, 1986.
