Environ. Sci. Technol. 1992, 26, 1792-1798
forms a micelle-like structure which quenches the fluorescence of these compounds. Because of the microheterogeneous nature of humic materials, the interpretation of humic acid binding with synthetic organic fluorophores according to single-exponential decay and SternVolmer theory should be avoided. Equilibrium constants derived from such interpretations should be approached with caution.
Acknowledgments Appreciation is expressed to Karl Topper and David B. Marshall, Department of Chemistry and Biochemistry, Utah State University, Logan, UT, for fluorescence lifetime measurements of difenzoquat, Registry No. Avenge, 43222-48-6; 1-naphthol, 90-15-3.
Literature Cited Gauthier, T. D.; Shane, E. C.; Guerin, W. F.; Seitz, W. R.; Grant, C. L. Environ. Sci. Technol. 1986,20, 1162-1166. Traina, S. J.; Spontak, D. A.; Logan, T. J. J. Enuiron. Qual. 1989, 18, 221-227. Magee, B. R.; Lion, L. W.; Lemley, A. T. Enuiron. Sci. Technol. 1991, 25, 323-331. Morra, M. J.; Corapcioglu, M. 0.;von Wandruszka, R. M. A.; Marshall, D. B.; Topper, K. Soil Sci. Soc. Am. J. 1990, 54, 1283-1289. Puchalski, M. M.; Morra, M. J.; von Wandruszka, R. Fresenius’ Z. Anal. Chem. 1991, 340, 341-344. Chiou, C. T.; Malcolm, R. L.; Brinton, T. I.; Kile, D. E. Enuiron. Sci. Technol. 1986, 20, 502-508. Eftink, M. R.; Ghiron, C. A. J. Phys. Chem. 1976, 80, 486-493. Carraway, E. R.; Demas, J. N.; DeGraff, B. A. Anal. Chem. 1991,63, 332-336.
(9) Nelson, D. W.; Sommers, L. E. In Methods of Soil Analysis, Part 2. Chemical and Microbiological Properties, 2nd ed.; Page, A. L., Miller, R., Keeney, D. R., Eds.; Agronomy Monograph No. 9; ASA and SSSA: Madison, WI, 1982; Chapter 29, pp 539-579. (10) Gee, G. W.; Bauder, J. W. In Methods of Soil Analysis,Part 1. Physical and Mineralogical Methods, 2nd ed.; Klute, A., Ed.; Agronomy Monograph No. 9; ASA and SSSA: Madison, WI, 1986; Chapter 15, pp 383-411. (11) Schnitzer, M. In Methods of Soil Analysis, Part 2. Chemical and Microbiological Properties, 2nd ed.; Page, A. L., MiUer, R., Keeney, D. R., Eds.; Agronomy Monograph No. 9; ASA and S S S A Madison, WI, 1982; Chapter 30, pp 581-594. (12) Kwak, J. C. T.; Nelson, R. W. P.; Gamble, D. S. Geochim. Cosmochim. Acta 1977, 41, 993-996. (13) Parker, C. A. Photoluminescence of Solutions; Elsevier: Amsterdam, 1968; p 222. (14) von Wandruszka, R.; Edwards, W. D.; Puchalski, M. M.; Morra, M. J. Spectrochim. Acta 1990, 46A, 1313-1318. (15) Vaughan, W. M.; Weber, G. Biochemistry 1970,9,464-473. (16) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum Press: New York, 1983; pp 257-295. (17) Stevenson, F. J. Humus Chemistry; John Wiley & Sons: New York, 1982; p 45. (18) Kubota, Y.; Motoda, Y.; Shigemune, Y.; Fujisaki, Y. Photochem. Photobiol. 1979,29, 1099-1106. (19) Ziemiecki, H.; Cherry, W. R. J. Am. Chem. Soc. 1981,103, 4479-4483. (20) Miola, L.; Abakerli, R. B.; Ginani, M. F.; Filho, P. B.; Toscano, V. G.; Quina, F. H. J. Phys. Chem. 1983, 87, 4417-4425.
Received for review July 12,1991. Revised manuscript received May 19,1992. Accepted May 27,1992. Funding for this research was provided by the Idaho State Board of Education and the Western Region Pesticide Impact Assessment Program.
Riboflavin Tetraacetate: A Potentially Useful Photosensitizing Agent for the Treatment of Contaminated Waters Richard A. Larson,’ Penney L. Stackhouse, and Thomas 0. Crowley Institute for Environmental Studies and Department of Civil Engineering, University of Illinois, 1101 West Peabody Drive, Urbana, Illinois 6 1801
rn The photosensitizing ability of 2’,3’,4’,5’-tetraacetylriboflavin (RTA) was compared to that of riboflavin in the degradation of various organic compounds and contaminated water samples. RTA was found to be superior to riboflavin under the test conditions. The inductive effects of ring substituents on the photolysis rates between riboflavin and RTA and the anilines (aniline, bromochloroaniline, nitroaniline, p-chloroaniline, p-toluidine, p anisidine, and 4-aminobenzotrifluoride) were studied; electron-donating substituents on the aniline ring enhanced the degradation rate for the RTA-mediated reactions but had little effect on riboflavin-promoted photolyses. Work with actual contaminated water samples further demonstrated RTA’s superior ability to photosensitize the disappearance of a mixture of compounds. The esults suggest that RTA may be a promising agent for t e cleanup of some polluted waters.
fi
Introduction Photochemical reactions, whether initiated by sunlight or by alternative sources of actinic radiation such as artificial lamps, may rapidly degrade potentially hazardous contaminants in wastewater and groundwater. Hexa1792 Environ. Sci. Technoi., Vol. 26, No. 9, 1992
chlorocyclopentadiene, which has a half-life of 4 min when exposed to sunlight, is one such compound ( I ) . However, many pollutants, .because of insufficient absorption of actinic radiation or low quantum yields of photoreaction, are not susceptible to such reactions. In these cases, photosensitizers can be deliberately added to facilitate their degradation. A photosensitizer (S) is a substance that absorbs light energy (hv),transforms it into chemical energy, and transfers that energy under favorable conditions to otherwise photochemically unreactive substrates. Redox processes are possible mechanisms for photoinduced energy transfer. (S + hv S*)
+ - + s*+ x - s++ xs*+ z - s- + z+ S*
M
S+
e-
(A)
(B) (C) A photochemically excited molecule may donate an electron to the medium (M, reaction A) or another molecule acting as an acceptor (X, reaction B), or it may act as an electron acceptor when a suitable electron donor is present (Z, reaction C) (2).
0013-936X/92/0926-1792$03.00/0
0 1992 American Chemical Society
H2COR
I (HCORh I
0
R= H for Riboflavin R= COCH3 for RTA Figure 1. Structures for riboflavln and 2’,3‘,4’,5’-tetraacetylriboflavin.
The photosensitizer riboflavin has been reported to sensitize the degradation of many compounds in aqueous solution (3). However, it is rapidly photodecomposed when irradiated under a medium-pressure mercury arc lamp (4). Thus, its use as a photosensitizer is limited. It has been suggested that the ribose chain of riboflavin is a factor in its photodecomposition (5) as well as in the yield and lifetime of the reactive triplet state (6, 7). Substitution of the hydroxyl groups on the ribityl side chain suppressed photofading (8). Such variation may help to improve the lifetime of the excited flavin and improve its sensitizing capability. Therefore, the ester analogue, 2’,3’,4’,5’-tetraacetylriboflavin (RTA), was chosen for study and synthesized (Figure 1). RTA has been reported to exhibit photochemical redox behavior in the oxidation of various organic substrates such as benzenethiol derivatives (9) and unsaturated fatty acids (10) when irradiated with visible light. RTA has also exhibited catalytic behavior in the cleavage of carbonsilicon bonds in alkoxy- or fluorosilanes (11) and in the photooxidation of benzyl alcohols (12). We report differences in reactivity between RTA and riboflavin in the photosensitized degradation of individual molecules as well as its use in the treatment of contaminated water samples on a laboratory scale. Experimental Section Chemicals. Phenols, anilines, naphthalene, benzoic acid, and riboflavin (Rib) were purchased commercially from Aldrich (Milwaukee, WI) with the exception of 4bromo-3-chloroaniline,which was obtained from Fairfield Chemical Co., Inc. (Blythewood, SC), and all were used without further purification. Water was deionized and glass distilled or purified through Milli-Q reagent grade water system (Millipore, Milford, MA). HPLC-grade acetonitrile, HPLC-grade ammonium acetate, monobasic, potassium phosphate, anhydrous dibasic sodium phosphate, and anhydrous magnesium sulfate were purchased from Fisher Scientific (Fairlawn, NJ). High-purity methanol and methylene chloride were obtained from Baxter, Burdick, and Jackson (Muskegon, MI). Synthesis and Characterization of RTA. RTA was synthesized according to methods previously reported (13, 14). Two grams of riboflavin was suspended in 15 mL of acetic anhydride. In order to initiate reaction, 0.2 mL of perchloric acid was added dropwise prior to heating. The reaction mixture was stirred continuously at a constant temperature of 80 OC for approximately 1h. The progress of the reaction was followed with thin-layer chromatography (TLC) using silica gel plates and a 91 (v/v) EtAc/EtOH mobile phase (product R, = 0.5). The reaction
mixture was cooled and added dropwise to an Erlenmeyer flask containing 100 mL of NaHC03 (0.8 M) to neutralize the remaining acetic acid. The crude product was extracted with CHzClzand washed several times with cold water. The CH2Cl2layer was then dried over anhydrous magnesium sulfate, decanted, and evaporated under reduced pressure. The resulting product was recrystallized from 1 9 5 (v/v) MeOH/H20 (13) to yield bright orange crystals (54%). The purity of the crystals from the starting product was verified using isocratic high-performance liquid chromatography (HPLC) monitoring at 445 nm (Kratos Spectroflow 757 absorbance detector (AB1Analytical, Ramsey, NJ) with a PRP-1 reverse-phase column (250 X 4.1 mm, Hamilton Co., Reno, NV) and a flow rate of 1 mL/min. The mobile phase was 6.5:2.5:1 NH4Ac (0.03 M)/ MeCN/MeOH (v/v) (RTRTA = 15 min). The melting point, corrected for the apparatus, was sharp (within 1“C) at 250 OC and consistent with a literature value (13). The structure was verified using both direct-probe mass spectrometry (MS) (Hewlett-Packard 5985A GC/MS spectrometer) and field desorption ionization MS (Finnigan MAT731), which indicated a molecular ion at 544. WNMR data (Varian XL200) agreed well with literature spectra (15, 16). RTA and riboflavin have virtually identical UV-visible and fluorescence spectra. The UV-visible spectrum of RTA (Perkin-Elmer 552A) showed the characteristic flavin absorption peaks at 370 and 445 nm, and the extinction coefficient (easnrn) of RTA (1.24 X lo4) (10) was virtually identical to that of riboflavin (e445nrn = 1.25 X lo4) (8). In the fluorescence spectra (Perkin-Elmer MPF-44B) of both RTA and riboflavin, the maximum excitation peaks occurred in the visible region at 445 nm, and emissions peaked at 525 nm, typical for flavins (17). Infrared spectra (Perkin-Elmer 727B) also showed the absorption peaks expected (18). The aqueous solubility of RTA was approximately 46 M) at 23 “C as determined by weighing ppm (8.8 X excess undissolved solid from a stirred solution. Riboflavin and RTA photolysis products were confirmed using HPLC [1:1 (v/v) MeCN/H20] coupled with a diode-array detector (Groton PF1 diode array detector system, Groton Technology, Inc., Waltham, MA). Retention times and UVvisible spectra for various molecules were verified with standards (RTRib = 2.5 min; RTRTA = 3.6 min; RTlumiflavin = 2.6 min; RTlumichrorne = 2.8 min) at a flow rate of 1 mL/min. Photoreactions. All photoreactions were performed using a medium-pressure mercury arc lamp (A > 290 nm) supplied with a 200-W power supply (Ace Glass, Vineland, NJ). The lamp was housed in a borosilicate immersion well that was used for water cooling. Prior experience with this lamp arrangement indicates that most photochemical reactions performed in test tubes proceed at rates about twice as fast as they do in full summer sunlight at this latitude (40° N). Kinetic studies were carried out using a “merry-goround” reactor (Ace Glass). Equimolar (20 pM) solutions of substrates and sensitizers (RTA and Rib) were also prepared. The substrate solution was diluted with an equal volume of sensitizer solution to give final concentrations (10 pM) of both substrate and sensitizer with a pH of 7.25 (0.02 M phosphate buffer). Concentrations of sensitizers were verified with a spectrophotometer (Milton Roy Co. Spectronic 20) reading at 445 nm. Aliquots of 2 mL were pipeted into Pyrex disposable screw-cap culture tubes (13 X 100 mm), which were capped, placed into the merryEnviron. Sci. Technol., Vol. 26,No. 9, 1992 1793
go-round cell holder 5 cm from the lamp housing, and irradiated. Samples were taken at various times and analyzed by HPLC. Controls were run with substrates to determine photolysis rates without added sensitizer (direct photolysis). The rate constants were calibrated using a ratio of rate constants for the actinometer valerophenone (10 pM) (23) and subtraction of the direct photolysis contribution, if any, to the degradation of the molecule. Kinetic data were presented as first-order plots, though the order of reaction changed during the photoreactions. Large-volume batch reactions were performed in order to obtain enough material to analyze for reaction products. Buffered (pH 7.25, 0.02 M phosphate buffer), aqueous reaction mixtures were poured into a 1000-mL Pyrex photochemical reaction vessel (Ace Glass) in which the lamp and its immersion well were submerged. Lipophilic reaction products were extracted with C,, cartridges (Bond Elut, 1 mL, Analytichem International) or by liquid/liquid extractions (H20/CH2C12).Products adsorbed onto the packing material were eluted from the column with CHpClzdried over anhydrous MgSO, (0.25 g, Fisher), and evaporated to dryness under a gentle stream of argon. The CH2Clzlayer from the liquid/liquid extraction was also dried over anhydrous MgSO, and evaporated to dryness. Samples were redissolved in CHzClp containing the external standard dibutyl phthalate, and a volume of 2 pL was injected into a GC/MS [5890 Hewlett-Packard gas chromatograph coupled with Finnigan MAT ion trap detector and a DB-1 capillary column, 30m X 0.32 mm i.d. (J&W Scientific, Placerville, CAI. Products were identified and confirmed with standards. Blanks (no photolysis) and controls (to detect direct photolysis products) were performed and analyzed according to the same procedure. The photoproducts, azobenzene, phenazine, and azoxybenzene, were identified from the reactions of riboflavin and RTA with aniline. Photolysis of Contaminated Water. Samples of groundwater contaminated by an abandoned coal gasification plant were obtained from Central Illinois Public Service, Taylorville, IL. Wastewater samples were obtained from an anonymous company in Rhode Island which manufactures phenol-formaldehyde resins. Lipophilic solutes were isolated by passing 400-mL samples of contaminated water through a (218 solid-phase extraction column (Bond Elut, 1 mL, Analytichem International), eluting the adsorbed solutes from the column with highpurity CH,C12, evaporating to dryness, and redissolving in 0.5 mL of CH2C12containing 3.76 X lo4 M dibutyl phthalate as an external standard. The major constituents were identified and confirmed with standards using GC/MS. A liquid/liquid extraction of the water samples showed the same constituents. The Taylorville water was found to contain several aromatic hydrocarbons and related compounds: phenanthrene, fluorene, styrene, a-methylstyrene, benzo[b]thiophene, indene, naphthalene, 2-methylnaphthalene,and a methylphenol. The predominant compounds identified in the dark violet Rhode Island wastewater were phenol, 4-benzoylpyridine, and a phenolic dimer. The concentration of phenol (0.036 M) was back-calculated based upon normalization of HPLC data with a phenol standard and the known dilution factor. Procedures for the photosensitized reactions with the Taylorville and Rhode Island waters were identical except in the initial preparation of the contaminated water samples. Because of the dark violet color of the Rhode Island wastewater, it was diluted 1:50 (v/v) to reduce its optical density (A = 210) to below 0.3 absorbance unit. The 1794
Environ. Sci. Technol., Vol. 28, No. 9, 1992
Table I. Rate Constants for the Disappearance of Various Compounds in the Sensitized Reactionsa compound aniline phenol 2,4-dichlorophenol naphthalene
kRibb kRTAb direct t l j z Rib t l j z RTA tIjz
0.694 0.961 0.059 0.080 0.111 0.167 0.045 0.217
23 h 76 h 21 min 60 h
1min
45 s
11 min
8.7 min
6.3 min 15 min
4.1 min 3.2 min
a Half-lives (tl 2) for the sensitized reactions have been corrected for direct photoiyses. *Units, min-I.
Taylorville sample was not diluted. Samples (100 mL) of the contaminated waters were combined with equal volumes of either distilled, deionized water, riboflavin (lo-, M), or RTA (low4M), depending on whether the direct photolysis (the control) or photosensitized reaction was performed. The reaction mixtures (200 mL) were placed in the 1000-mL Pyrex reaction vessel and irradiated for 45 min. The resultant solution was acidified to a pH of 3.5 with 37% HC1 (Mallinckrodt). Lipophilic solutes were isolated with liquid/liquid extractions and the drying procedure previously described for batch reactions. Two microliters of the final CHzClzsolution was injected into the GC/MS using the following temperature program: 5 rnin at 40 "C, 5 OC/min from 40 to 280 OC, and a 10-min hold at 280 "C, Peak heights of individual compounds were measured and normalized relative to the dibutyl phthalate standard. Final results were reported as the mean of four and three determinations for the Taylorville and Rhode Island water, respectively.
Results and Discussion. Compound Studies. RTA demonstrated a much longer half-life than riboflavin when exposed to mercury arc light; while riboflavin's half-life in distilled water was on the order of 2 min, RTA's was around 80 min. The longer half-life of RTA may result from an increased molecular stabilization gained from the substitution of the ribose hydroxy groups with acetyl groups. Such substitution is thought to inhibit intramolecular photoreduction of the isoalloxazine nucleus by the ribityl side chain (5, 7,8,17, 19). In buffered solutions, both riboflavin and RTA half-lives were shortened to C1.5 and 30 min, respectively, which suggested that the ionic strength of the solution may affect the solvation of the excited state and its chemistry or that interactions may exist between buffer constituents or impurities and the excited states of the flavins. RTA consistently sensitized the photodegradation of structurally diverse aromatic compounds such as aniline, naphthalene, phenol, and 2,4-dichlorophenol at a faster rate than riboflavin (Table I). These results may indicate more efficient complexing between RTA and the aromatic substrates, less competition from unproductive photoprocesses incorporating the side chain, or more efficient electron or energy transfer to the substrates. Functional groups bound to an aromatic molecule inductively alter the electron density surrounding the molecule and affect its behavior in electrophilic reactions. The effects of ring substituents on the photolysis rate constants between riboflavin and RTA and several substituted anilines were studied (Table 11). The Hammett relationship, In @ / I t , ) = pu (20),was used to correlate the data for the photosensitized degradation of anilines, and p was calculated in both the riboflavin- and RTA-sensitized reactions to determine the differences in reactivity due to the inductive and resonance effects (Figure 2). For the riboflavin-sensitized reactions there was little or no correlation between the electron-donating character of the substituent and the reactivity of the aniline. Pre-
Table 11. Substrate Disappearance Rate Constants Calculated for the Sensitized Reactions among the Anilines"
compound
U
R
aniline (1) p-toluidine (2) p-anisidine (3) p-chloroaniline (4) 4-bromo-3-chloroaniline (5) 4-aminobenzotrifluoride (6) p-nitroaniline (7)
0 -0.17 -0.27 +0.227
H CH3 OCHB C1
% direct photolysis Rib/RTA
kRibb
kRTAb
kac?
0.12/0.09 0.21/0.14 1.0/0.83 22/13 12/3.8 0.17/0.09 0.29/0.07
0.694 0.561 0.672 0.531 0.094 0.756 0.512
0.916 0.879 0.81 1.0 0.346 1.38 2.23
0.059 0.058 0.057 0.059 0.063 0.057 0.059
-
CF,
+0.55 +0.778
NOz
K R ~ ~ ~ S ' KRTA'SC
0.682 0.561 0.684 0.522 0.086 0.769 0.503
0.900 0.879 0.824 0.983 0.318 1.4 2.19
"Only the initial rates were considered, and the constants have been corrected for direct photolyses. bNote units for these constants are min-'. Reaction rate constants ( K ) relative to first-order rate constant for the actinometer, valerophenone ((Kkactav)/k:a where kac,B' = 0.058/min) (23). Values omitted because u constants are not calculated for disubstituted compounds in Hammett relationship.
4
0.2 O,?!
,,
y
7 x
...I
../
,...,..."
I
-0 4
i
A QTF
0 2r
1
C R13
R 0
3.1
A
O
r 3:. 3 .[ -0.I
n
I
1
I.....'' A
0
3 41
-04
I
-02
02
00
0.6
0.4
08
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s i grno
vious researchers have suggested that riboflavin's photochemical reactivity is based on the enhanced reduction potential of its excited triplet state (17). Thus, a compound with greater electron availability should accelerate the rate of this reduction, while a compound with a lower electron density should decrease the reaction rate. By this reasoning, p-methoxyaniline and p-methylaniline, with their electron-donating substituents, should have reacted more rapidly with the excited flavins than those having electron-withdrawing substituents; however, this was not observed under our conditions. Similarly, it was expected that the RTA-sensitized reactions would demonstrate a reduced rate constant as aromatic substituents became more electron withdrawing but contrary to expectation, electron-withdrawing substituents increased the reaction rate constant. The positive slope in the Hammett correlation (Figure 2) for RTA suggested that electron transfer was not the sole determinant for the measured rate of reaction. Consider the following scheme for the reaction between the flavin, F, and aniline, AH:
+ AH
- .& + --
[FAH] + hv
[FAH]*
[F*- AH*+]*
AH'+
4
-H+
A'
0
I0
23
30
2
e- transfer
-3
F'-
products
AH'+ (I) (11)
Reaction 2, which represents the rate of electron transfer,
50
40 i
Figure 2. Hammett plot for the photosensltlred reactions of the anilines. The rate constants, K ~ i b(mln-') and KRTA (min-'), are given in Table I1 for the reactlons of rlboflavln and RTA with aniline and various substituted anlllnes, respecthrely. Note the Inverse relationship between electron-wlthdrawlng substituents and rate constant for the RTA (*, ---) sensitized reactions ( p = 0.38; r = 0.95) and the lack of correiatlon for the rlboflavln(0, .-) reactions (p = 0.004; r = 0.22). (Label number indicates type of aniline from Table 11.)
F
I
-- ' . 0. 6 .4
7
me
(
63
70
SC
90
100
seconas)
Figure 3. Illustration of the devlation from firstorder kinetics exhibited in the sensitized reactions of the anilines.
should be directly affected by the electron densities of the reactants. However, the other steps (1,3,4) may be more important for the overall reaction rate. For example, RTA might form a more stable complex than riboflavin, with the more electron-poor anilines, which would affect the equilibria of the formation of the flavin-aniline complex (involving steps 1and -1) and of the release of the radical cation (AH*+)from the exciplex (steps 3 and -3). The relative rates of steps 1 and -1 and 3 and -3 might be different, leading to differences in reactivity between riboflavin and RTA. UV-visible absorption studies on separate reaction mixtures, one containing riboflavin and aniline and the other containing RTA and aniline, showed the presence of absorption peaks (A = 404,494 nm) redshifted from a flavin's characteristic absorption spectrum (A = 370, 444 nm) and strongly suggest the presence of such charge-transfer complexes. Additionally, photodegradation rates were first order only in the few first seconds or minutes of the reaction (Figure 3). Generally, photochemical reactions, especially direct photolyses, exhibit first-order kinetics (21). For a sensitized reaction to be first order, the sensitizer concentration must remain constane however, it is known that riboflavin undergoes photolytic degradation and breaks down into the less efficient photosensitizers lumiflavin and lumichrome, depending on the pH of the reaction mixture (19). RTA was also photolyzed to form the same photoproducts, although more slowly. Therefore, as a reaction proceeds, there is less efficient sensitizer available for energy transfer, reducing its capacity for photosensitization of a substrate with time. Experiments were conducted to determine if such factors were contributing to the first-order kinetic deviation (Figures 4 and 5). Initially, an attempt to eliminate the observed tailing was carried out by adding more riboflavin Environ. Scl. Technoi., Voi. 26, No. 9, 1992
1795
I
p
c 4 1
x
I
i
I
1 05
00
15
10
2 0
25
35
3 0
T i m (minktes:
Flgure 4. Inhibition of the ribofiavln-sensitized reaction due to added reaction products (", .-, riboflavin/aniline; +, -, riboflavin/aniiine/ azobenzene; A, --, rlboflavln/anillne/photoproducts).
--
-
I i
xw
1
x
0
7
..
1.21
I
1.41
0
IO
20
30
40
50
T I me
(
60
7G
80
63
IO0
110
seconds)
Flgure 5. Inhlbltion of the RTA-sensitized reaction due to added reaction products (0, .-,RTA/anillne; *, - - -, RTA/aniline/azobenzene; +, - a -, RTA/aniline/photoproducts).
to the reaction mixture during the course of the reaction. When riboflavin (1 mL, 20 pM)was added to the reaction mixture (initially containing 10 pM of both riboflavin and aniline) after 2 min of irradiation, an increase in the rate of photolysis and a return to first-order-type kinetics were observed. However, after 3-4 min, once again the rate dropped off. RTA also showed similar trends. In the RTA-sensitized oxidations of various unsaturated fatty acids, earlier investigators noted cessation of the reaction after a certain time due to the photodegradation of the flavin (IO). It is also possible that photoproducts may compete photochemically with riboflavin and RTA. Lumiflavin, for example, has an absorption spectrum similar to riboflavin and RTA and therefore may screen out some of the wavelengths that induce the reactive excited triplet states in the flavins (A = 445 nm). Riboflavin photoproducts have been observed to quench fluorescence (8) and may affect the overall quantum yield of the degradation reaction. In addition to fluorescence quenching, it has been reported that flavin photoproducts quench the production of the triplet state (8,22), which is the reactive excited state of the molecule. Therefore, this quenching would cause less triplet availability and may result in a decrease in the rate of substrate disappearance. The inhibiting effect of the flavin degradation products on the reaction with aniline was tested (Figures 4 and 5). A riboflavin solution was photolyzed "completely" (i.e., 1796
Environ. Sci. Technoi., Voi. 26, No. 9, 1992
until there was no observable color) and added to a solution containing 20 pM of both riboflavin and aniline such that final concentrations were 10 pM riboflavin, aniline, and photoproducts. RTA was also tested in a similar manner. A decrease in the rate constant for aniline photolysis for both of the sensitizers was observed. As more of the photoproducts were produced, this quenching effect should be enhanced. At some point, there should be a leveling out of the substrate concentration such that no more substrate will be degraded. Larson et al. developed a kinetic model that accounted for some of these effects in the reactions of riboflavin with various anilines and phenols (4). Such effects also appear to be a factor in the RTA-sensitized reactions. Competition effects may not be limited to sensitizer photoproducts (Figures 4 and 5). Substrate photoproducts may also compete with the substrate for reaction with the photosensitizer and reduce the probability of substrate reaction with the sensitizer. Azobenzene was isolated as a reaction product in the photosensitization of aniline by riboflavin and RTA. To test the inhibition of the reaction by the presence of such products, an equimolar amount of azobenzene was added to the initial reaction mixture. Slight decreases were observed for the RTA- and riboflavin-sensitized reactions. The role of oxygen in these reactions was also investigated. It is well-known that dissolved O2 is a prime quencher for excited triplet states and such quenching could lead to the formation of singlet oxygen (IO,)( 4 , I O ) . Also, superoxide (02'-), a reactive chemical species resulting from the reduction of oxygen, has been implicated in many sensitized reactions, including those of flavins (4). Two possible mechanisms for 02'formation are the oxidation by O2of a reduced flavin (a two-step process) or the elimination of HOz' from a flavin peroxy radical. Riboflavin generated greater quantities of 0;- than RTA, and it is expected that both mechanisms may contribute formation by riboflavin because of its ability to to 02'autoreduce itself while RTA could only produce 02'through the peroxy-adduct mechanism. If 0;- had a role in degrading aniline, then the riboflavin-sensitizedreaction should be more efficient than those sensitized by RTA. However, as demonstrated, RTA consistently degraded pollutants and related molecules at a faster rate than ridoes not play a boflavin; we therefore conclude that 02'role in the degradation of the anilines. Furthermore, irradiation of both oxygen-containing and argon-sparged reaction mixtures containing equimolar concentrations of sensitizer and aniline showed that the loss of aniline proceeded faster without oxygen than with oxygen. Therefore, in addition to the production of 02'-, we conclude that O2 may also play a competitive role in the formation of lo2or some other photochemically unproductive species and inactivate the flavin for reaction. Overall, the degradation of the photosensitizer primarily inhibited the aniline reaction. Other factors, such as the quenching of the flavin excited state by its own and aniline's decomposition products and oxygen, appeared to inhibit the reaction as well. Contaminated Water Sample Studies. The ability of RTA and riboflavin to degrade certain compounds in the presence of near-visible-W and visible light may make these compounds an alternative for the treatment of organically contaminated waters. Therefore, the photosensitizing effects of riboflavin and RTA on two actual samples of contaminated waters were investigated. Three types of reactions were carried out on each water sample. First, a direct photolysis was performed to de-
Table 111. Comparison between Riboflavin and RTA in the Photosensitized Degradation of the Taylorville Water Constituents compound
blank" control" Rib4 RTA"
naphthalene
0.548
0.502
0.342 0.243
benzo[b]thiophene
0.429
0.292
0.135 0.074
phenanthrene
0.345
0.235
0.183 0.094
fluorene
0.214
0.134
0.096 0.056
methylnaphthalene
0.154
0.136
0.104 0.059
methylphenol
0.149
0.105
0.039 0.005
indene
0.062
0.06
0.021 0.005
styrene
0.049
0.036
0.012 0.007
methylstyrene
0.028
0.022
0.007 0.005
Table IV. Comparison between Riboflavin and RTA in the Photosensitized Degradation of the Rhode Island Wastewater Constituents compound
structure
6
phenol
4-benzoylpyridine
31.7
&
OOQ
phenoxybenzyl alcohol
blank"
Rib" 14.7
RTA" 10.9
0.045
0.021
0.015
3.7
1.97
0.4
CHzOH
" These numbers represent concentrations relative to an external standard (%) and are the mean of three determinations.
" These numbers represent concentrations relative to an external standard (%) and are the mean of four determinations. termine the susceptibility of the contaminants in the samples to undergo photolysis. Samples were then subjected to photosensitized reactions with riboflavin and RTA, and the results were compared. (a) Taylorville, IL. GC/MS analysis of the contaminated groundwater obtained from Taylorville, IL, showed that the water contained several aromatic hydrocarbons and related compounds: phenanthrene, fluorene, styrene, a-methylstyrene, benzo[b]thiophene, indene, naphthalene, 2-methylnaphthalene, and a methylphenol. The average extent of photodegradation (after a 45-min exposure period) of these compounds relative to a dibutyl phthalate standard is shown in Table 111. With every compound, RTA was the more efficient sensitizer. Several products were identified from the riboflavin and RTA reactions with the Taylorville water. Products identified from both sensitized reactions were coumarin and a methylbenzofuran. The production of 9,lOphenanthrenequinone was unique to the RTA reaction. The mechanisms of formation of these products remain to be determined. These data provide evidence for increased reactivity of RTA toward hydrophobic compounds. For the polycyclic aromatic hydrocarbons, the difference in reactivity appeared to increase with the hydrophobicity of the substrate. The increased extent of reaction of RTA versus riboflavin differed by a factor of 1.4, 1.75, and 1.95 for naphthalene, methylnaphthalene, and phenanthrene, respectively. However, more research is needed to determine why and how extensively a substrate's hydrophobicity affects the photosensitization reaction.
(b) Rhode Island. Experiments were also conducted on a sample of wastewater from a holding pond of a plastics manufacturer in Rhode Island. The predominant compounds identified in this water were phenol, 4benzoylpyridine, and a phenolic dimer. This wastewater was expected to demonstrate reductions in the identified constituents since phenols, similar to the anilines, are powerful electron donors. The results showed roughly 50% decreases in concentrations of the identified constituents, and once again, RTA proved to be the more efficient photosensitizer (Table IV). Conclusion RTA is more photochemically stable as well as a more efficient photosensitizer than riboflavin. The electron-rich compound aniline is degraded by RTA at a faster rate than riboflavin, as was expected. However, when tested with substituted aniline derivatives, experiments indicated an inverse relationship between electron-withdrawing substituents and reactivity with RTA. Additionally, photoproducts inhibited the sensitized reaction in its later stages. Contaminated water samples were subjected to the photosensitized reactions. Again, RTA showed superior photosensitizing activity relative to riboflavin. Such work may lead to application in the treatment of waters contaminated with organic compounds. However, more information is needed on the kinetics, mechanisms, and products of the reactions. Acknowledgments We thank Karen Marley, John Antonoglu, and Wayne Chan for helpful discussions and assistance. Registry No. RTA, 752-13-6; riboflavin, 83-88-5; aniline, 62-53-3; bromochloroaniline, 118804-39-0; nitroaniline, 29757-24-2; p-anisidine, 104-94-9; p-chloroaniline, 106-47-8; p-toluidine, 106-49-0; 4-aminobenzotrifluoride, 455-14-1.
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Received for review October 23, 1991. Revised manuscript received April 7, 1992. Accepted May 21, 1992. We thank the Illinois Department of Energy and Natural Resources, Hazardous Waste Research and Information Center, for financial support and the Molecular Spectroscopy and Mass Spectrometry Laboratories, School of Chemical Sciences, University of Illinois, for use of equipment and assistance.
Gas-Phase Atmospheric Chemistry of Selected Thiocarbamates Erlc S. C. Kwok,? Roger Atklnson,*,$ and Janet Arey*s$
Statewide Air Pollution Research Center, University of California, Riverslde, California 9252 1 The kinetics and products of the gas-phase reactions of OH radicals, NO, radicals, and 0,with the thiocarbamates S-methyl N,N-dimethylthiocarbamate (MDTC), S-ethyl N,N-dipropylthiocarbamate(EPTC), and S-ethyl N-cyclohexyl-N-ethylthiocarbamate(cycloate) have been studied at 298 f 2 K and -735 Torr air. By use of relative rate techniques, all three thiocarbamates were observed to react with OH and NO, radicals, with the following respective reaction rate constants (in cm3 moland 7.3 X EPTC, ecule-l &): MDTC, 1.33 X 3.18 X and 9.2 X cycloate, 3.54 X and 3.29 X 10-14. No reactions with 0,were observed, and upper limits to the rate constants (cm3molecule-l s-l) were determined: MDTC,
