Mechanisms of Ionizing Radiation-Induced Destruction of 2, 6

general can be achieved by using ionizing radiation. The compounds can be irradiated in aqueous solutions or in organic solvents (3). However, due to ...
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Environ. Sci. Technol. 1997, 31, 2473-2477

Mechanisms of Ionizing Radiation-Induced Destruction of 2,6-Dichlorobiphenyl in Aqueous Solutions M. AL-SHEIKHLY* AND J. SILVERMAN Department of Materials and Nuclear Engineering, University of Maryland, College Park, Maryland 20742-2115 P. NETA Physical and Chemical Properties Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 L. KARAM Ionizing Radiation Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899

Polychlorinated biphenyls (PCBs) appear as environmental pollutants in various phases. Aqueous media polluted with such compounds may potentially be detoxified by electron beam irradiation. In the present study, we discuss the possible mechanisms of radiolytic detoxification under various conditions, and we study 2,6-dichlorobiphenyl (DCB) as a model compound for PCBs. Electron beam and γ-irradiation of DCB in aqueous methanol solutions lead to degradation of this compound and formation of Cl- ions, but high doses are required. Solutions of 1 mmol L-1 DCB require a dose of 30 kGy to produce 1 equiv of Cl- (G ) 0.03 µmol J-1) to form the less toxic monochlorobiphenyl (MCB), but 600 kGy is required to achieve full dechlorination. This low yield is due to the concomitant formation of acid, which competes with DCB for the solvated electrons, the main reducing species in this system. The radiolytic yields are improved dramatically by the addition of carbonate; only 20 kGy are required for practically quantitative dechlorination of DCB. For the oxidation process, the dechlorination of DCB by •OH radicals was measured in N2O-saturated solutions and found to be much less effective than the reduction process.

Introduction From 1929 until 1977, polychlorinated biphenyls (PCBs) were widely used in capacitors and transformers, heat transfer and hydraulic fluids, carbonless copy paper, plasticizers, and in other applications (1). Concerns with the toxicity and environmental persistence of PCBs led to regulation and eventual prohibition of their industrial production in the United States in 1977 under the Toxic Substances Control Act of 1976. However, during this period the world production of PCBs was approximately 1.5 million t, with 650 000 produced in the United States. It is estimated that about 20-30% of this amount has entered the environment, and that 150 000 t are still in use in the United States, primarily in transformers, capacitors, and other electrical equipment (1, 2). * Corresponding author telephone: 301-405-5214; fax: 301-3149467; e-mail: [email protected].

S0013-936X(96)00741-9 CCC: $14.00

 1997 American Chemical Society

Dechlorination of chlorinated organic compounds in general can be achieved by using ionizing radiation. The compounds can be irradiated in aqueous solutions or in organic solvents (3). However, due to their insolubility in water, the PCBs were dechlorinated by irradiation in organic solvents only. For example, in alkaline 2-propanol solutions, a chain dechlorination of polychlorinated aromatic compounds was reported and was found to be inhibited by the presence of biphenyl, acetone, nitrobenzene, and oxygen (4, 5). Similarly, radiation-induced chain dechlorination of PCB has been achieved by electron beam irradiation in N2saturated alkaline 2-propanol solutions (6). The mechanism of this process is based on the formation of the anionic radical from 2-propanol ((CH3)2C˙ O-), which reacts with PCB and similar molecules to cause dechlorination and form aryl radicals, and then reaction of these aryl radicals with 2-propanol to produce the reducing species again and thus propagate a chain reaction. The primary objective of radiation-induced detoxification of aqueous media containing toxic PCBs is to achieve total dechlorination and to convert the biphenyl molecules into benign low molecular weight compounds. Previous studies on aqueous media concentrated on the radiation-induced oxidation of the toxic chloro-organic compounds (7). This is based on the fast reaction of the hydroxyl radical (•OH) with organic materials, followed by reaction of the resulting radical with oxygen. The present work has taken the approach of attempting to utilize both radiation-induced oxidation and radiation-induced reduction to detoxify aqueous media. The reaction of •OH radicals with chlorinated molecules leads to dechlorination only as a partial reaction pathway. Reactions of reducing radicals lead quantitatively to dechlorination to yield benign aqueous Cl- and carbon-centered radicals. The reaction of •OH radicals with aromatic compounds proceeds predominantly via addition to the ring (3). Addition at the carbon bearing the chlorine atom results in rapid dechlorination, whereas addition at other sites may generally lead to chlorinated organic products (8); eventual dechlorination occurs only after further attack on the first radiation products. Reaction of •OH with chlorinated aliphatic compounds proceeds through H-abstraction and, if unsaturated bonds are present, through addition (9, 10). Both addition and abstraction reactions of •OH with chloro-organic compounds lead to the formation of carbon-centered radicals. These radicals react very rapidly with dissolved molecular oxygen to produce the corresponding peroxyl radicals, which undergo various reactions leading to different products (10). It should be noted that whereas aliphatic carbon-centered radicals react rapidly and irreversibly with O2, the OH adducts of certain aromatic compounds have been shown to react reversibly with O2 (8, 11). This distinction is of considerable importance in the use of advanced oxidation processes for detoxification of water (12). To study the decomposition of chlorinated biphenyls with radicals, it is necessary to dissolve them in water, which is the source of the primary radiation-produced radicals. Because the solubility of PCBs in water is very low, one generally uses an organic cosolvent, such as an alcohol, or a micellar material to solubilize them. Under such conditions, the •OH radicals will react preferentially with the organic cosolvent or the micellar material rather than with the solute PCBs, which are present at much lower concentrations. Hence, the efficiency of the oxidation process to destroy PCB compounds in these systems is expected to be very low. By comparison, the reactions of the hydrated electrons, eaq-, with chloro-organic compounds are highly selective. The reactions are very rapid, and the adducts undergo predomi-

•OH

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nantly C-Cl bond cleavage, resulting in dechlorination and production of C-centered radicals. Certain eaq- adducts may also undergo partial protonation, which will lower the dechlorination efficiency somewhat. One has to take into account, however, that eaq- also reacts very rapidly with dissolved oxygen to produce O2•-. Because H3O+ is also an effective eaq- scavenger, its role must be taken into account as well. In this study, we used 2,6-dichlorobiphenyl (DCB) as a model compound for PCB and examined its radiolytic destruction in water/alcohol solutions.

Experimental Section (13) The DCB was purchased from AccuStandard. Other chemicals used were analytical reagents from various sources, and water was purified by a Millipore Milli-Q-system. Methanol served as a cosolvent. The saturation concentration of DCB in water is only ∼0.01 mmol L-1. With 5-10% methanol (by volume) only ∼0.1 mmol L-1 DCB was dissolved. Most of the experiments were carried out with 50% methanol and 1 mmol L-1 DCB. Solutions were saturated with O2, N2, or N2O by purging with the pure gas for about 30 min. To achieve O2 concentrations lower than the saturation level, the solutions were purged with known mixtures of O2 and Ar. In certain experiments, the pH was adjusted and maintained at ∼10 with 0.01 mol L-1 of a carbonate-bicarbonate (1:1) buffer. For product analysis, the solutions were irradiated in a Gammacell 220 60Co source (dose rate of 1 or 3.3 Gy s-1) or by a 7 MeV electron beam from a linear accelerator. Both of these radiation sources are at NIST, and their dosimetry was provided by NIST staff. The concentration of DCB remaining after irradiation was determined by mass spectrometry. For these determinations, dibromobiphenyl (1 × 10-6 mol) was added as an internal standard after irradiation, and then the solutions were air evaporated and finally frozen and lyophilized. The residue was reconstituted in 1 mL of methanol, and 10-20-µL samples were analyzed by direct insertion probe (using a direct heating sample tip) on a Hewlett Packard 5988 mass spectrometer (100 °C ion source, temperature program: 50 °C for 5 min followed by 50 °C/min ramp to 350 °C and a final time of 10 min at 350 °C; 50 eV, multiplier at 2100). The concentration of Cl- ions released upon radiolysis was determined by ion chromatography. Small volumes of the irradiated solutions were directly injected into a Dionex DX-500 ion chromatograph with an AS11 column. The kinetics and the spectrum of the radiolytically produced free radicals were determined by pulse radiolysis using the NIST Febetron-based pulse radiolysis apparatus (14). The 2-MeV electron pulse supplied an absorbed dose between 2 and 300 Gy per 50 ns pulse. The formation and decay of transient species were monitored by kinetic spectrophotometric detection. Dosimetry for pulse radiolysis was performed in the sample cell using N2O-saturated 0.01 mol L-1 KSCN solutions, assuming G ) 0.61 µmol J-1 for the production of the (SCN)2•- radicals and taking their molar absorptivity at 480 nm as 7600 L mol-1 cm-1. The combined standard uncertainties for the reported radiolytic yields (G values) are estimated to be (5%, and for the reported rate constant are (15%.

Result and Discussion Mechanistic Considerations. Radiolysis of DCB was carried out in aqueous methanol solutions; most experiments were with 50% methanol (12.3 mol L-1) and with 1 mmol L-1 DCB. In this mixture, the radiation is absorbed by the water and the methanol components approximately with a ratio of 55: 45 (3) to produce different primary species. Radiolysis of water generates radicals and stable products (3) H2O

•OH,

H•, eaq–, H2, H2O2, H3O+

(1)

with the following yields in µmol J-1: G(•OH) ) 0.29, G(eaq-)

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FIGURE 1. Decay of the eaq- absorbance at 720 nm monitored by pulse radiolysis of deaerated aqueous methanol (1:1) solutions of DCB at pH 11.3. (a) Typical kinetic trace; (b) decay rate constant as a function of DCB concentration. ) 0.29, G(H•) ) 0.06, G(H2) ) 0.04, and G(H2O2) ) 0.08. Radiolysis of pure methanol also generates solvated electrons, but with a lower yield, G(esol-) ∼ 0.21; the other main species is •CH2OH (3). In the above solvent mixture, this latter radical is formed both by direct radiolysis of methanol and by the reactions of H• and •OH, produced by water radiolysis, with the dissolved methanol

CH3OH + •OH (H•) f •CH2OH + H2O (H2)

(2)

The hydrated electron is the strongest known reducing agent (reduction potential -2.9 V) and reacts very rapidly with halogenated organic compounds through dissociative electron capture (3, 9):

RX + eaq- f R• + X-

(3)

This reaction has been utilized to dehalogenate various aryl halides (15) where the rate constants are in the range of 5 × 109-2 × 1010 L mol-1 s-1 in aqueous solutions (9). The rate constant for the reaction of eaq- with DCB was measured in deoxygenated aqueous methanol solutions containing 0.5-1.0 mmol L-1 DCB at pH 8.4. The measurements were conducted by monitoring the decay of the eaqabsorption at 720 nm (Figure 1a) as a function of DCB concentration. From a plot of the observed rate constant vs [DCB] (Figure 1b), the second-order rate constant was calculated to be k4 ) 3.8 × 109 L mol-1 s-1: Cl •

+ eaq– Cl

+ Cl–

(4)

Cl

This value is somewhat lower than the rate constants for eaqreactions with dichlorobenzene and biphenyl in aqueous solutions (9), probably due to the effect of the added methanol that lowers the polarity of the solvent and thus the rate constant.

estimate that ∼0.1% of the •OH radicals will react with DCB. The reactivity of H• atoms with DCB can be estimated to be about 3 orders of magnitude higher than that with methanol (9), and therefore about 10% of the H• atoms will react with DCB. Although the fraction of H• and •OH radicals reacting with DCB is quite small, their effect cannot be neglected at the high doses used in the present experiments. Both H• and •OH radicals are expected to react with DCB via addition to the phenyl rings to form the various isomers of •DCBH and •DCBOH:

DCB + H• (•OH) f •DCBH (•DCBOH)

FIGURE 2. Optical absorption spectrum of the peroxyl radical derived from DCB. Monitored by pulse radiolysis of aqueous methanol solutions containing 0.1 mmol L-1 DCB and 0.05 mmol L-1 O2, monitored 10 µs after a 20-Gy pulse. The monochlorobiphenylyl radical (MCB•) produced by reaction 4 was found to exhibit no optical absorbance in the range of 310-740 nm. This is in line with the previous findings that phenyl radicals absorb mainly below 300 nm (16). This range could not be examined in the present case because the starting material (DCB) also absorbs in this range. The other radical produced in irradiated methanol solutions, •CH2OH, is known to be a mild reductant (17), which may be expected to reduce DCB, but with a relatively low rate constant (probably k5 < 1 × 106 L mol-1 s-1, judging from the rate constants reported for CCl4 and C6F6) (17):

CH2OH + ArCl f CH2O + H+ + Ar• + Cl-

•

(5)

When O2 is present in the irradiated DCB solutions, this solute may react with eaq-, •CH2OH, and Ar•:

eaq- + O2 f O2•-

(6)

O •

Cl

Since chlorobenzene reacts about 30% more slowly than benzene (9), it may be expected that addition of •OH to the two rings of DCB will exhibit a slight selectivity toward the non-chlorinated ring. Within each ring, the probability of addition to the various positions may vary by a factor of ∼2-4 (8), but addition at the Cl position is expected to have only a small contribution, by comparison with the results for chlorobenzene (8). Only addition at the Cl site leads to rapid dechlorination. The other OH adducts may undergo partial dechlorination following disproportionation (8). Finally, we have to consider the fate of the •CH2OH radicals under various conditions. In the presence of O2, this radical reacts very rapidly to form the peroxyl radical, HOCH2O2•. In the absence of O2, however, the expected slow reaction of •CH OH with DCB (reaction 5) may have a significant 2 contribution at low dose rates. Under alkaline conditions, the •CH2OH radical dissociates to form the anion radical

CH2OH + OH- a •CH2O- + H2O

(9)

•

(pKa ) 10.7) (19), which is known to be a stronger reductant than the neutral species (17). Therefore, at high pH more effective reduction of DCB is expected.

CH2O- + ArCl f CH2O + Cl- + Ar•

(10)

•

The aryl radicals formed by reactions 5 and 10 are capable of abstracting a hydrogen atom from methanol (16, 18) (in competition with their reaction with O2):

Ar• + CH3OH f ArH + •CH2OH

O•

+ O2

(8)

(7) Cl

The MCB• radical produced by reactions 4 and 5 is expected to react rapidly (15, 18) with O2 to form the peroxyl radical (reaction 7). To observe reaction 7, we minimized the contribution of the competing reaction 6 (k6 ) 2.0 × 1010 L mol-1 s-1) (9) by adjusting [O2] ∼ 0.05 mmol L-1. Figure 2 shows the transient spectrum of the MCBO2•, which exhibits a broad peak around 500 nm, in the same range as those of other arylperoxyl radicals (15, 18). Reaction 7 is not quantitative since a fraction of the aryl radicals are expected to react with methanol via H-abstraction (18). The peroxyl radicals will decay via radical-radical reactions to yield various products, as discussed before for peroxyl radicals in general (10). The H• and •OH radicals produced from water will react predominantly with the alcohol cosolvent to produce •CH OH radicals, since the concentration of DCB is 4 orders 2 of magnitude smaller than that of the methanol. Assuming that the rate constant for reaction of •OH with DCB is ∼5-9 × 109 L mol-1 s-1, in the same range as the values for biphenyl, benzene, and chlorobenzene (9), whereas the rate constant for reaction with methanol is 9.7 × 108 L mol-1 s-1 (9), we can

(11)

If reactions 10 and 11 are effective, a chain process may develop that will lead to high G values for dechlorination. An efficient chain reaction based on this mechanism has been found before with alkaline 2-propanol solutions (6). The chain process is expected to be less efficient in aqueous methanol because the limiting step, reaction 10, is known to be slower than the corresponding reaction of the radical derived from 2-propanol. The other step in the propagation, reaction 11, is also slower than the parallel reaction with 2-propanol. The above discussion outlines the mechanisms by which DCB may be dechlorinated under various conditions. The main objectives of this study, however, are to determine the total dose required to achieve destruction of the DCB and complete dechlorination of all aromatic components in an ideal system and to provide guiding principles for achieving the minimum dose for complete mineralization in practical systems. Dechlorination and Destruction of DCB. The degree of radiolytic destruction of 1 mmol L-1 DCB in aqueous methanol solutions was measured by determining the yield of inorganic chloride produced (Figure 3) and the concentration of DCB remaining after irradiation (Figure 4). Radiolytic production of Cl- is relatively efficient in the early stages of irradiation, but it drops significantly with increasing doses. This drop in yield is ascribed to the concomitant formation of H3O+ (reactions 1 and 5), which competes with DCB for the solvated

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FIGURE 5. Yield of Cl- as a function of dose in γ-irradiated aqueous methanol solutions of 1 mmol L-1 DCB and 0.01 mol L-1 carbonate buffer at pH 11.3. The solutions were sealed after saturation with N2 (2), air ((), or N2O (9).

FIGURE 3. Yield of Cl- as a function of dose in irradiated aqueous methanol solutions of 1 mmol L-1 DCB: (a) γ-irradiated, (b) electron beam irradiated, (() solutions were sealed after bubbling N2, (9) solutions were sealed under air (the O2 in these solutions is rapidly depleted upon irradiation).

FIGURE 4. Decrease in DCB concentration as a function of dose in electron beam-irradiated aqueous methanol solution of 1 mmol L-1 DCB. The solutions were initially air saturated but were sealed. electrons. The reaction of eaq- with H3O+ is diffusioncontrolled and leads to the formation of H• atoms, which react predominantly with methanol. Even at low doses, the yields of Cl- correspond to less than half the radiolytic yield of the solvated electrons (∼0.25 µmol J-1 in this solvent mixture). This result suggests that in these unbuffered solutions some of the solvated electrons are scavenged by the acid initially present at low concentrations and also by oxygen if present. The concentrations of DCB remaining after irradiation (Figure 4) are somewhat lower than the concentrations of Cl- produced, indicating that some DCB is destroyed by other reactions that do not lead to dechlorination. It is tempting to speculate that some of the electrons may be adding to DCB to form a radical anion with a finite lifetime, which undergoes only partial dechlorination and partial protonation. Such parallel reactions have been suggested for the radical anion of chlorouracil (20), for example, but no evidence exists for suggesting such a

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mechanism for chloroaromatic compounds not containing other electron-withdrawing groups. It is more likely that some DCB is destroyed by reaction with other radicals, such as H• atoms and •CH2OH radicals reacting via addition. In fact, the mass spectral measurements indicate that several products are formed upon destruction of DCB, not only MCB but also hydroxylated and methylated DCB as well as other products that were not identified. Further irradiation leads to eventual dechlorination of all these products. The overall yields of Cl- in the above experiments show that a dose of 20-30 kGy is required to form 1 equiv of Clfrom 1 mmol L-1 DCB. Elimination of one chloride converts a large fraction of the DCB into the monochlorobiphenyl, with a small fraction of DCB presumably still remaining while some nonchlorinated biphenyl was also produced. Continued irradiation proceeds with a much lower efficiency. To achieve ∼100% dechlorination, i.e., to produce ∼2 mmol L-1 inorganic Cl- from 1 mmol L-1 DCB, doses of ∼500-600 kGy are required. These high doses required for dechlorination in unbuffered solutions make this process quite costly and impractical. To counteract the strong effect of H3O+, 0.01 mol L-1 carbonate/bicarbonate (1:1) buffer was introduced into the system to maintain the pH at ∼10. This particular buffer was chosen as an additive with benign environmental impact that can provide a mildly alkaline medium. Furthermore, although the CO32- ions can react with •OH radicals rapidly (k ) 3.5 × 108 L mol-1 s-1) (9), they do not compete with methanol for the •OH radicals to any significant extent. The addition of carbonate was found to increase the efficiency of dechlorination by a large factor. As shown in Figure 5, by preventing the solution from becoming acidic, irradiation with a dose of only ∼20 kGy leads to total dechlorination, i.e., the radiolytic efficiency of dechlorination is increased by a factor of ∼30. Furthermore, at this pH, the radical derived from methanol is partially ionized, and the anionic form is expected to reduce and dechlorinate DCB (reaction 10) more effectively than the neutral form. However, the yield of Cl- in the initial stages of irradiation is ∼0.23 µmol J-1, close to the yield of solvated electrons in this mixture, which indicates that no significant chain reaction develops under such conditions and that the contribution of reactions 5 and 10 are very low at the low concentrations used in these experiments. Figure 5 also shows that when the solutions were saturated with N2O to convert eaq- into •OH, the degree of dechlorination decreased drastically. The small yield of Cl- observed under N2O can be ascribed to the small fraction of the eaq- that do react with DCB (estimated from the known concentrations and rate constants). Most eaq- are converted into •OH, however, and the latter species reacts predominantly with methanol. Only a small fraction of the •OH radicals attacks the DCB (∼0.1%, as estimated above). The low yield of Cl-

under N2O further indicates that reaction 10 does not have a significant contribution under the conditions of Figure 5. In fact, from this low yield and the known rates of radical production and radical decay (21), we estimate the rate constant for reaction 10 to be e1 × 103 L mol-1 s-1. In conclusion, the oxidizing powers normally ascribed to ionizing radiation are of little value in electron beam processing of PCB solutions. This is due to the fact that the solubility of PCBs in water is so low that organic cosolvents are required to bring even small concentrations of PCBs into true aqueous solution, and this organic cosolvent, in all practical cases, effectively scavenges all •OH radicals. On the other hand, even in the presence of many organic cosolvents, cleavage of the C-Cl bond of PCBs by eaq- may be achieved with high efficiency, giving rise to chlorinated organic residue and chloride ions. The removal of additional Cl atoms from the residue by subsequent reduction reactions is less efficient in unbuffered solutions, due to acid production upon radiolysis, and requires much higher doses. This make the process too costly. If buffering is impractical, the optimal approach may be to carry out radiolytic dechlorination to an extent that cleaves only that fraction of the Cl atoms in each molecule that would be regarded sufficient to convert the PCB to much less toxic products, which can be then removed from the effluent by classical sorption columns. The better choice, however, is to increase the efficiency of the dechlorination process (by a factor of 30 in the present example) by introducing into the system a carbonate/bicarbonate buffer as a nontoxic additive. This additive neutralizes the acid formed upon radiolysis and makes the dechlorination reaction by eaq- more complete until full dechlorination is achieved.

Acknowledgments This research was supported in part by the National Science Foundation through Grant BES-9320339 with the University of Maryland. We are grateful to Dr. Charles E. Dick and Mr. Melvin R. McClelland for operating the electron beam accelerator at NIST and to Mr. Steven Petras and Mr. Siddarth Chabria for valuable technical assistance.

Literature Cited (1) Webster, T.; Commoner, B. In Overview of the Dioxin Debate; Schecter, A., Ed.; Plenum: New York, 1994; pp 1-50. (2) Amend, L. J.; Lederman, P. B. Environ. Prog. 1992, 11, 173. (3) Spinks, J. W. T.; Woods, R. J. Introduction to Radiation Chemistry, 3rd ed.; Wiley: New York, 1990.

(4) Sawai, T.; Shinozaki, Y. Chem. Lett. 1972, 10, 865. (5) Shinozaki, Y. Presented at the 13th Japan Conference on Radioisotopes, Tokyo, 1977. (6) Singh, A.; Kremers, W.; Smalley, P.; Bennett, G. Radiat. Phys. Chem. 1985, 25, 11. (7) Gehringer, P.; Proksch, E.; Eschweller, H.; Szinovatz, W. Appl. Radiat. Isot. 1992, 43, 1107. (8) Merga, G.; Schuchmann, H.-P.; Rao, B. S. M.; von Sonntag, C. J. Chem. Soc., Perkin Trans. 2 1996, 551 and 1097. (9) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J. Phys. Chem. Ref. Data 1988, 17, 513. (10) (a) von Sonntag, C. The Chemical Basis of Radiation Biology; Taylor and Francis: New York, 1987; pp 57-93. (b) von Sonntag, C.; Schuchmann, H.-P. Angew. Chem., Int. Ed. Engl. 1991, 30, 1229. (c) von Sonntag, C.; Schuchmann, H.-P. In Peroxyl Radicals; Alfassi, Z., Ed.; Wiley: New York, 1997; pp 173-234. (11) (a) Pan, X.-M.; von Sonntag, C. Z. Naturforsch. 1990, 45B, 1337. (b) Pan, X.-M.; Schuchmann, M. N.; von Sonntag, C. J. Chem. Soc., Perkin Trans. 2 1993, 289. (c) Fang, X.; Pan, X.-M.; Rahmann, A.; Schuchmann, H.-P.; von Sonntag, C. Chem. Eur. J. 1995, 1, 423. (12) von Sonntag, C. J. Water SRTsAqua 1996, 45, 84. (13) The mention of commercial equipment or material does not imply recognition or endorsement by the National Institute of Standards and Technology, nor does it imply that the material or equipment identified is necessarily the best available for the purpose. (14) Hunter, E. P.; Simic, M. G.; Michael, B. D. Rev. Sci. Instrum. 1985, 56, 2199. (15) (a) Alfassi, Z. B.; Marguet, S.; Neta, P. J. Phys. Chem. 1994, 98, 8019. (b) Khaikin, G. I.; Alfassi, Z. B.; Neta, P. J. Phys. Chem. 1995, 99, 11447. (16) Madhavan, V.; Schuler, R. H.; Fessenden, R. W. J. Am. Chem. Soc. 1978, 100, 888. (17) Neta, P.; Grodkowski, J.; Ross, A. B. J. Phys. Chem. Ref. Data 1996, 25, 709. (18) (a) Mertens, R.; von Sonntag, C. Angew. Chem. Int. Ed. Engl. 1994, 33, 1262. (b) Fang, X.; Mertens, R.; von Sonntag, C. J. Chem. Soc., Perkin Trans. 2 1995, 1033. (19) (a) Asmus, K.-D.; Henglein, A.; Wigger, A.; Beck, G. Ber. BunsenGes. Phys. Chem. 1966, 70, 756. (b) G. P. Laroff, G. P.; Fessenden, R. W. J. Phys. Chem. 1973, 77, 1283. (20) Rivera, E.; Schuler, R. H. J. Phys. Chem. 1983, 87, 3966. (21) Wang, W.-F.; Schuchmann, M. N.; Bachler, V.; Schuchmann, H.-P.; von Sonntag, C. J. Phys. Chem. 1996, 100, 15843.

Received for review August 28, 1996. Revised manuscript received April 9, 1997. Accepted April 14, 1997.X ES960741T X

Abstract published in Advance ACS Abstracts, June 15, 1997.

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