10080
J . Phys. Chem. 1991, 95, 10080-I0089
that the LDA gives better predictions of capillary condensation from the Kelvin equation results (line 1 ) even at larger pore sizes. pressures than the classical thermodynamic Kelvin e q ~ a t i o n . ~ ~ ~ ~As * ~a second ~ aspect of this study, the effect of the strength of the fluid-solid potential t l on the transition pore radius for argon Results was calculated. As expected, the transition pore radius increases as t l increases (see Figure 6). The linear relationship shown in The coexistence isotherm for argon in a cylindrical pore of solid Figure 6 is also expected since the fluidsolid potential is a linear carbon dioxide was computed using the LDA and the full Lenfunction of e l . This straightforward result is quite useful for nard-Jones 6-1 2 potential for both fluid-fluid and fluid-solid evaluating el based on experimental observations of the condeninteractions. The values used for the LJ parameters are listed sation pressure for two different radii. in Table I. As a comparison, Figure 5 shows the results of this study (line 2) and results for the Kelvin equation (line 1) and Conclusions calculations performed using the LDA and a cutoff and shifted The transition pore radius for capillary condensation in small Lennard-Jones 6-1 2 potential for the fluid-fluid interactions (line cylindrical pores has been calculated using the local density ap3).25 The potential used to generate line 3 was cut off at r = 2 . 5 ~ . proximation and a full Lennard-Jones 6-12 potential for both the The results of this study tend toward the Kelvin equation results fluid-fluid and fluid-solid interactions. Comparison of the prefor large pores and the LDA results using a cutoff and shifted diction of the Kelvin equation and the local density approximation LJ potential for small pores. This comparison demonstrates that with a cutoff and shifted Lennard-Jones potential indicates that the full LJ potential can provide some increased accuracy over the use of the full potential can provide some increased accuracy the cutoff and shifted LJ potential and produce the correct limiting and the proper limiting behavior for small and large pores. behavior. Cutting off the fluid-fluid potential gives it a shorter range than the fluidsolid potential. This causes line 3 to deviate Registry No. Ar, 7440-37-1; C02, 124-38-9.
Identification of Organic Acids and Other Intermediates in Oxidative Degradation of Chlorinated Ethanes on TiO, Surfaces en Route to Mineralization. A Combined Photocatalytic and Radiation Chemical Study Yun Mao, Christian Schoneich, and Klaus-Dieter Asmus* Bereich S, Abteilung Strahlenchemie, Hahn-Meitner Institut Berlin, Postjach 39 01 28, 1000 Berlin 39, Germany (Received: March 12, 1991)
The oxidative degradation of chlorinated ethanes p r d s practically via the same mechanism in 7-radiolysis and photocatalysis at TiOa surfaces, respectively. C-centered radicals generated via hydroxyl radical induced C-H bond cleavage and peroxyl radicals derived therefrom after oxygen addition are the key radical intermediates in these processes. The main molecular products identified and isolated in both the y-radiolytic and photocatalytic experiments are organic (mostly chlorinated) acids, HCI, and C02. Other products formed in minor yields are aldehydes and HCOOH. Photocatalytic degradation of these product acids and nonionic substrates leads eventually to complete mineralization. The results strongly suggest that the photocatalytic degradation is initiated by an oxidation of the chlorinated compounds through Ti02-surface-adsorbed hydroxyl radicals. Only some acids, like trichloroacetic acid and oxalic acid, seem to be oxidized primarily by valence band holes via a photo-Kolbe process. Several rate constants are reported on the oxidation of chlorinated ethanes and acids by free and surface-adsorbed hydroxyl radicals and on the overall photocatalytic degradation of the chlorinated compounds. The paper also includes a discussion of the material balance. The study demonstrates the value of radiation chemical investigations for the understanding of the details in the photocatalytic mineralization process of halogenated organic compounds.
Introduction Halogenated hydrocarbons are still in widespread use as organic solvents, propellants, herbicides, pesticides, and anaesthetics to name the probably most significant fields of their application. They were also, and increasingly, the target of criticism as they can be held responsible for a number of toxic and environmentally hazardous actions. Their chemical behavior in the environment and living beings is consequently of great interest, in particular, for biochemists and ecologists. Photocatalytic degradation of halogenated organic compounds on semiconductor surfaces is one of the possible approaches to convert these substrates into innocuous or less harmful compounds. Product studies, pioneered in - ~ matt hew^,^,^ but also carried out by particular by O l l i ~ ’and (I) (2) (3) 89. (4)
Pruden, A. L.; Ollis, D. F. J . Carol. 1983, 82, 404. Hsiao, C.-Y.; Lee, C.-L.; Ollis, D. F. J . Catal. 1983, 82, 418. Ollis, D. J.; Hsiao, C.-Y.; Budiman, L.; Lee, C.-L. J . Card 1984,88,
Ollis, D. F. Emiron. Sci. Technol. 1985, 19, 480. (5) Matthews, R. W. J . Chem. Soc., Faraday Trans. 1 1984, 80, 457. (6) Matthews, R. W. Water Res. 1986, 20, 569.
many others,7-l8 indicate a more or less complete mineralization of the halocarbons into inorganic acids, HX and C 0 2 (X = halogenj. Yet, despite the numerous and valuable experimental (7) Markham, M. C.; Laidler, K. J. J . Phys. Chem. 1953, 57, 363. (8) Careys, T. H.; Lawrence, J.; Tosine, H. M. Bull. Enuiron. Contam. Toxicol. 1916. 16, 697. (9) Oliver, B. G.;Cosgrove, E. G . ; Carey, J. H. Enoiron. Sei. Techno/. 1979, 13, 1075.
(IO) Bideau, M.; Claudel, B.; Otterbein, M. J. Photochem. 1980,14,291. (1 I ) Fox, M. A.; Chen. C. C. J . Am. Chem. SOC.1981, 103,6757; refrahedron Let?. 1983, 24, 547. (12) Herrmann, J.-M.;Mozzanega, M.-N.; Pichat. P. J . Photochem. 1983, 22, 333. (13) Bahnemann, D.; Monig, J.; Chapman, R. J . Phys. Chem. 1987, 91, 3782. (14) Pelizzetti, E.; Borgarello. M.; Minero, C.; Pramauro, E.; Borgarello, E.; Serpone, N. Chemosphere 1988, 17, 499. ( 1 5 ) Hikada, H.; Yamada, S.;Suenaga, S.;Kubota, H.; Serpone, N.; Pelizzetti, E.; Gratzel, M. J . Photochem. Photobiol. 1989, A47, 103. (16) Tanaka, K.; Hisanaga, T.; Harada, K. New J . Chem. 1989, 13, 5. (17) Kormann, C.; Bahnemann, D.; Hoffmann, M. R. J . Photochem. Photobiol. 1989, A48, 161. (18) Chemseddine, A.; Boehm, H. P. J . Mol. Catal. 1990, 60, 295.
0022.365419 112095-10080$02.50/0 0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 24, 1991 10081
Oxidative Degradation of Chlorinated Ethanes data reported so far including serious suggestions about the possible involvement of radicals, conclusive evidence is still missing on two important aspects, namely, what the molecular intermediates are and, based on this information, what the detailed mechanism of the photochemical degradation of these compounds is. The biological toxicity of these xenobiotics has convincingly been associated with the generation of free radicals. Particularly halogenated peroxyl radicals have been considered as key intermediates.Ig3 Strong support for the establishment of this radical mechanism has been provided by radiation chemical studies which revealed that the free-radical chemistry of halogenated hydrocarbons in solution leads to the same products as upon metabolic degradation of these compound^.^'-^^ The main molecular products which originate from halogenated peroxyl radicals are inorganic (HX, C 0 2 ) and organic substrates (acids, aldehydes, and alcohols, with various degrees of halogenation). Considering that the inorganic degradation products from free-radical chemistry and metabolism are the same as the ultimate mineralization products from photocatalysis, it can be suspected that the underlying chemical mechanism in photocatalysis does indeed involve radicals. This, in turn, would suggest that the still missing molecular intermediates in photocatalysis may be the same organic substrates identified in the metabolic and free-radical systems. Since radiation chemistry has proven to be such a powerful tool for the investigation of radical mechanisms a combined photocatalytic and radiation chemical study has now been undertaken in the hope to find an unambiguous answer on the details of the photocatalytic degradation routes. Our study focuses, in particular, on the identification of molecular organic products generated in radiolysis as well as en route to photocatalytic mineralization of chlorine substituted ethanes at T i 0 2 as semiconducting material. Radical formation in radiolysis of aqueous solutions is initiated mainly by hydrated electrons or hydroxyl radicals." For halogenated hydrocarbons (RX) both pathways lead to carbon-centered radicals via dissociative halide ion elimination or H-atom abstraction, respectively: RX *OH
+ eaq-
+ RX
-
-
+ X-
R'
(1)
H2O + RX(-H)'
In photocatalysis, corresponding reactions can be envisaged through reduction by conduction band electrons or oxidation through valence band holes (hVB+)both of which are generated as primary reactive species upon the photoinduced charge separation in light-exposed semiconductor^.^^-^^ RX hVB+
+ RX
-
+ eCB-
-
[RX]'
R'
+
+ X-
RX(-H)'
(3)
+ H+
(4)
Whether or not a photocatalytic reduction or oxidation takes place (19) Slater, T. F. Free Radical Mechanisms in Tissue Injury; Pion: London, 1972. (20) Castro. C. E.; Wade, R.S.; Belser, N. 0.Biochemisrty 1985, 24, 204. (21) Packer, J. E.; Slater, T . F.;Willson, R.L. LifeSci. 1978,23,2617; Naiure 1979, 278, 737. (22) Packer, J. E.;Willson, R. L.; Bahnemann, D.; Asmus, K.-D. J. Chem. SOC.,Perkin Trans. 2 1980, 296. (23) MBnig, J.; Asmus, K.-D.; Schaeffer, M.; Slater, T. F.; Willson, R.L. J . Chem. Soc., Perkin Trans. 2 1983, 1133. (24) Monig, J.; Krischer, K.; Asmus, K.-D. Chem. Biol. Inreracr. 1983,
45, 43. (25) Monig, J.; Bahnemann, D.; Asmus, K.-D. Chem. Biol. Inreracr. 1983, 47. 15. (26) Monig, J.; Asmus, K.-D. J . Chem. Soc., Perkin Trans. 2 1984, 2057. (27) Asmus, K.-D.; Bahnemann, D.; Krischer, K.; Lal, M.; MBnig, J. Life Chem. Rep. 1985, 3, 1. (28) Lal, M.; Monig, J.; Asmus, K.-D. Free Radical Res. Commun. 1986, I , 235. (29) Lal, M.;Monig, J.; Asmus, K.-D. J. Chem. Soc., Perkin Trans. 2 1987, 1639. (30) Lal, M.Radiat. Phys. Chem. 1988, 32, 741. (31) Asmus, K.-D. Methods in Enzymology; Packer, L., Ed.; Academic Press: New York, 1984; Vol. 105, p 167.
is, of course, a question of the respective redox potential of eCB-, hvB+and their redox partners. An interesting question which is still discussed controversially, and which concerns particularly aqueous suspensions of metal oxide semiconductors, is whether oxidations proceed via direct electron transfer between substrate and positive holes (as formulated in reaction 4), or via oxidation of free or surface-bound hydroxyl groups to 'OH (reaction 5), prior to oxidation of any added substrate. Little doubt seems left, however, on the presence (5) of hydroxyl radicals at TiOz surface^.^^-^' Whether they exist entirely as adsorbed or possibly also as freely diffusing species seems, however, also still an open question. In the presence of oxygen the carbon-centered radicals are immediately transformed into the corresponding peroxyl radicals. R'/RX(-H)'
+0 2
-
ROO'/RX(-H)OO'
(6)
These reactions are known to occur with high, close to diffusion-controlled rate constants, in general,48"0 and are the starting points of more or less complex, but with respect to the essential aspects, well-documented decay mechanisms leading to various intermediate and final molecular products. In the present paper it will be demonstrated that the photocatalytic degradation of chlorinated ethanes and other molecular products resulting from this process does indeed proceed via radical mechanisms. It will further be shown that for the majority of the investigated substrates the initiation step is an oxidation by adsorbed, Le., surface-bound hydroxyl radicals. Radiation chemistry is introduced as a powerful and most informative tool to complement photocatalytic studies.
Experimental Section Titanium dioxide (Ti02, P25), a mixture of 80% anatase and 20% rutile with average surface area of 55 m2/g, was supplied from Degussa (Frankfurt/a M) and used without further activation. The data of crystal structure and specific surface area of these powders were kindly provided by Prof. M. Gratzel (EPL Lausanne). Compounds were generally of highest commercially available purity. The chlorinated ethanes I,l-dichloroethane (I,l-DCE), 1,2-dichloroethane (1,2-DCE), 1,l ,I-trichloroethane ( I , 1 , l TriCE), 1,1,2-trichloroethane (1,1,2-TriCE), 1,1,2,2-tetrachloroethane ( 1 , I ,2,2-TetCE), 1,1,1,2-tetrachloroethane ( 1 , I , 1,2-TetCE), and pentachloroethane (PCE) were obtained (32) Stumm, W. Aquaric Surface Chemistry; Wiley Interscience: New York, 1987. (33) Pelizzetti, E.,Serpone, N., Eds. Homogeneous and Heterogeneous Phorocaralysis; NATO-AS1 Series C ; Reidel: Dordrecht, 1988; Vol. 174. (34) Bahnemann. D. In Sulfur-centered Reactive Intermediates in Chemist$ ahdBiology; Chatgilialogh, C., Asmus,K.-D., Eds.; NATO-AS1 Series A; Plenum Press: New York, 1988; Vol. 197, p 103. (35) Volz, H. G.; Kimpf, G.; Fitzky, H. G. Farbe Lock 1972, 78, 1037. (36) Krautler. B.: Bard. A. J. J. Am. Chem. SOC.1978. 100. 5985. i 3 7 j Jaeger. C.D.: Bard. A. J. J. Phvs. Chem. 1979.83. 3746. (38) Izuki, 1.; Dunn, W.'W.; WilboGn, K. 0.;Fan, F. F:; Bard, A. .I. J. Phys. Chem. 1980, 84. 3207. (39) Izumi, 1.; Fan, F. F.; Bard, A. J. J. Phys. Chem. 1981, 85, 218. (40) Nakato, V.; Tsumura, A,; Tsubomura, H. J. Phys. Chem. 1983,87, 2402. (41) Oosawa, Y.; Gratzel, M. J. Chem. SOC.,Chem. Commun. 1984, 1629. (42) Okamoto, K.-I.; Yamamoto, Y.; Tanaka, H.; Tanaka, M.; Itaya, A. Bull. Chem. SOC.Jpn. 1985, 58, 2015. (43) Nakato, V.; Ogawa, H.; Morata, K.; Tsubomura. H. J. Phys. Chem. 1986, 90, 6210. (44) Oosawa, Y.; Gratzel, M. J . Chem. Soc., Faruduy Trans. 1 1988.84, 197. (45) Kobayakawa, K.; Nakazawa, Y.; Ikeda, M.; Sato, Y.; Fujishima, A. Ber. Bunsenges. Phys. Chem. 1990. 94, 1439. (46) Peterson, M.W.; Turner, J. A.; Nozik, A. J. J. Phys. Chem. 1991, 95, 221. (47) Schindler, K.-M. Private communication. (48) Huie, R. E.; Brault, D.; Neta, P. Chem. Biol. Interact. 1987,62, 227. (49) Alfassi, Z. B.; Mosseri, S.; Neta, P. J. Phys. Chem. 1987, 9 / , 3383. (50) Lal, M.; Schoneich, C.; Monig, J.; Asmus, K.-D. Inr. J. Radiaf. Biol. 1988. 54, 773.
10082 The Journal of Physical Chemistry. Vol. 95, No. 24, 1991
from Aldrich. Prior to use they were distilled once or twice, with the purity being controlled by gas chromatography. The water was deionized and purified by a Millipore filtration system (218 MQ). The apparatus for illumination was described in detail elsewherea5' Basically it consists of an illumination cell with optically flat quartz entry and exit windows, and a XBO Xe lamp (450 W, Muller, Electronic and Optic, FRG) equipped with a 15-cm water filter to remove IR radiation to the utmost extent. In a typical photocatalytic experiment, the finely ground Ti02powder (10 mg) was suspended in 8 mL of aqueous solution in the illumination cell. The pH of the suspension was adjusted by adding appropriate amounts of 0.1 M NaOH or HCIO,. For better homogenization the suspension was sonicated for 30 s prior to illumination. (This procedure had no effect on the chemistry.) During the illumination the sample was magnetically stirred but always kept sealed. Except for quantum yield measurements polychromatic light penetrating WG 295 filters (cutting off most of the UV light) was admitted. For comparison of product yields photolysis times were kept constant within each set of experiments and were chosen within the linear portion of respective yield vs irradiation time plots in order to minimize secondary reactions. After irradiation the suspensions were centrifuged and the liquid phases subjected to analysis. Quantum yield measurements were carried out in a I-cm quartz cuvette with 3.0 X M solutions of chloroethanes suspended together with 3.7 mg of Ti02. The samples were irradiated by monochromatic (350 nm) light (Chromatography, Kratos Polytec, GM 252) from a 450-W xenon lamp. The incident UV light intensity was measured using Aberchrome 540 as chemical photodosimeters2 (3-mL sample of 5 X IO-' M Aberchrome 540; various irradiation times; quantitative measurement of photoproducts through optical absorption at 494 nm). The y-irradiations were carried out in the field of a ' T o source of the Hahn-Meitner Institute Berlin with a dose rate of 3.6 Gy/min (1 Gy = 1 J kg-I). Total doses absorbed by each sample were generally in the range of 100 Gy. Solutions were prepared in 20-mL glass vessels, which could be sealed by gas tight septa, and did not contain any semiconductor material. Oxidations, in this case, were initiated by free 'OH radicals which in irradiated, N 2 0 saturated aqueous solutions account for about 90% of primary reactive specie^.^' An absorbed dose of 100 Gy corresponds to an 'OH concentration of =6 X M. With chloroethane concentrations from 2 X IO4 to 1 X M this means that the conversion rate was generally kept very low. Identification and quantitative analysis of ionic products were performed by high-performance ion chromatography using a Dionex 2010i machine, which was equipped with a HPIC-AS4 anion-exchange column, fiber suppressor, and conductivity detector. Details and representative chromatograms are to be found el~ewhere.~'The eluant generally consisted of an aqueous solution from a mixture of N a H C 0 3 and Na2C03, 1.4 X IO4 and 1.I x 1O4 M, respectively. Identification of molecular products of ionic species was based on their retention times and comparison with authentic samples. C02was identified also by ion chromatography after conversion into HC03- and analysis with a HPIC-AS1 c ~ l u m n . * ~The , ~ ~aldehydes were identified by reversed-phase HPLC after derivatization with 2,4-dinitrophenylhydrazine (2,4-DNPH) analogous to a published procedure.% The reagent was IOm2 M 2,4-DNPH in 2-6 M aqueous HCI. Mixtures with an aliquot of the irradiated/illuminated sample were stirred for 2 h at 40 "C and another hour while the solution cooled down to room temperature. Extraction of the aldehydes from the aqueous phase was made with cyclohexane (for non-halogenated aldehydes) or chloroform (for the halogenated aldehydes). The chromatograph was a Varian 5000 model equipped with a 5-pm (51) Mao, Y. Ph.D. Thesis, Technical University Berlin, D 83, 1989. (52) Heller, H. G.; Langan, G. R. EPA News Lett. 1981, 71;J . Chem. SOC.,Perkin Trans. 2 1981, 341.
Mao et al. Nucleosil C I SODS column (125 X 4 mm i.d., Macherey and Nagel) and a multichannel UV detector (Philips PU 4021). Analysis of the chlorinated ethanes, chlorinated aldehydes, and chlorinated alcohols was achieved by gas chromatography directly from their aqueous solutions using a RTx-Volatiles (Restek) column. Owing to the sensitivity limits of the FID detector toward water, only small amounts of solution (generally 0.2-1 .OpL) could be injected. If necessary, detection limits could be improved by extraction and concentration into ether or other nonaqueous phases. Applying an injection temperature of 50 "C which was then raised at a rate of 5 "C/min the following retention times were obtained for the oxygen atom containing substrates: CH,CICHO 3.4 min; CCI3CHO.H20 6.2 min; CHC12CH20H 5.8 min; CCI3CH20H 11.7 min. All y-radiolysis and photochemical experiments were carried out at room temperature. Experimental error limits are generally estimated to *IO% for the radiolysis and f 2 0 % for the photocatalytic experiments, unless specifically noted.
Results and Discussion Oxidative Degradation by y-Radiolysis. Products and Rate Constants. A series of chlorinated ethanes dissolved in aqueous solutions, saturated with an N 2 0 / 0 2 (4:l v/v) mixture (pH = 6), were subjected to y-radiolysis. In such systems 'OH radicals and H' atoms are formed as primary reactive species with radiation chemical yields of C = 6.0 and 0.6, re~pectively.'~.~~ (The yields are given in terms of G values, Le., the radiation chemical unit describing the number of molecules generated per 100 eV of absorbed energy.) Rate constants for several examples of the general reaction are recorded in the literature, or have been determined in this study using standard competition methods.50 They are typically of the order of 107-108 M-I s-I (see Table I), indicating that some activation energy is involved in this process. The latter is reflected in the concentration dependence of the product yields from reaction 7, exemplified in Figure 1 for the formation of monochloroacetic acid (MCAA) and chloride ions (CI-) from 1,l-dichloroethane (I,I-DCE). In order to reach the plateau values (at which scavenging of the 'OH radicals is complete) solute concentrations >5 X 10" M are required while for diffusion-controlled reactions around M solute concentration is already sufficient. The possibility of chlorine instead of hydrogen abstraction by hydroxyl radicals can be neglected. Such a reaction has been shown not to occur.2J The small yield of H atoms will react with O2to yield 02'-which in aqueous environment does not react with halocarbon~.~~ Table I gives a list of all acids (organic and inorganic) which are obtained from the 'OH-induced degradation of various halogenated ethanes, their radiation chemical yields, the bimolecular rate constants for the initiation of these processes, and the carbon-centered and peroxyl radicals involved as key intermediates. The underlying experiments were performed by y-radiolysis of aqueous, pH 6 , solutions, saturated with an N 2 0 / 0 2 (4:l v/v) mixture, and containing (3-5) X IO-' M of the respective chloroethanes. Some relevant data are included from a previous study on this subject.29 At this solute concentration a total of about 8045% of the primary *OH radicals, Le., G('0H) = 5, are scavenged by the chloroethane as can be seen in Figure 1. (Higher solute concentrations for complete 'OH scavenging were not feasible in most cases because of increasingly unfavorable dissolution kinetics.) Mechanism. It is noted that the major part of the organic acids still contain the C2skeleton. However, the simultaneous formation of C, products (mainly C 0 2 ) indicates an appreciable probability of C-C bond rupture as well. The present results corroborate on the degradation of halogenated earlier studies of ours29~56
(53) We$, J. Handbuch der ionenchromatographie; Dionex: Germany,
1985. (54) Honer, F.; Schoneich, C.; MGckel, H. J. Fresenius Z . Anal. Chem. 1987, 328, 244.
(55) Buxton, G.V. In The Study of Fast Processes and Transient Species by Electron Pulse Radiolysis: Baxendale, J. H., Busi, F., Eds.; NATO-AS1 Series C; Reidel: Dordrecht, 1982; Vol. 86, p 241.
The Journal of Physical Chemistry, Vol. 95, No. 24, 1991 10083
Oxidative Degradation of Chlorinated Ethanes
TABLE I: Identity and Yields of Organic and Inorganic Acids, Relevant Primary C-Centered and Peroxyl Radials, and Rate Constants for 'OH Radical Reaction for Radiation Chemical Oxidation of Chlorinated Ethanes' other compounds &('OH), M-I s-I primary radical C2 acids yield, G responsible peroxyl radical acids yield, G I,I-DCE 1.8 X IOBb CH3-C'CI2 CHS-COOH C CH3-CC1200' HCOOH C
'CHz-CHCI2 CH2CI-C'HCI CH2CI-C'HCI
+
1,2-DCE l,l,l-TriCE
2.0 X loBb 4.0
X
1,1,1,2-TetCE
1.1
X
loBb
1.8 X lo7
1.0 x 107
3.1
CHZCI-COOH CH2CI-COOH CHC12-COOH
2.9 2.5
CC13-C'HCI 'CCI2-CHCIz 'CC12-CH2CI (from e,q- reaction)
CHC12-COOH CH2CI-COOH
3.55
cci3-c*ci2
CCIj-COOH
C
+
PCE
1.1
CC13-C'Hz 'CC12-CHzCI CHZCI-C'H2 'CHC1-CHCIZ 4
I , I ,2-TriCE
CH2CI-COOH CHzCI-COOH
1 .o
50.7
CH2CI-CHCIOO' CH,CI-CHClOO'
CHzClCC1200' CH2C1-CC1200' CHC12-CHC100' CHCl2-CC1200' CH2CI-CC1200'
cc1~-cc1200'
co2
10.8b
HCI HCOOH
C
HCI
1 Sb 6.0
co2
2.3b
co2
HCI HCOOH C02 HCI COZ HCI CO2 HCI
8.3
10.9 C
1.6 10.5
3.1b 15.9
C C
"N20/02(80:20 v/v) saturated, aqueous, pH 6, solutions containing (3-5) X lo-' M chlorinated ethanes; y-radiolysis with ca. 100 Cy. (G refers to number of species formed per 100 eV absorbed radiation energy). bReference 50. CQualitativelyobserved. peroxyl radicals where the latter were generated via initial reductive cleavage of a C-X bond (correspondingto reactions 1 and 3). The degradation of halogenated peroxyl radicals has been established to follow, in principle, the same routes as peroxyl radicals in genera1.29v56,57The major route applying for primary and secondary peroxyl radicals is the so-called Russell m e c h a n i ~ m : ~ ~ R H k, 0'
'CI
CI
I
R-&O
+
CI I R-C-OH
I
+ o2
(8)
H
Hydrolysis of the chlorinated "alcohols" and "aldehydes" formed in reaction 8 yields, of course, the respective aldehydes (ketones) and organic acids CI
I I
-C-OH CI
I
-C=O
H20
H20
>C=O
-COOH
+ HCI + HCI
R-CC120'
+
+ CCI20 CI' + R-CC10
R'
--c
(12a)
For quantitative assessments of the C2 products it is further noted that tertiary R-CCI200' radicals exclusively yield acids (RCOOH) while the secondary R - C H C I W radicals convert into both acids and aldehydes at varying ratio depending on which of the possible decay routes p r e ~ a i I . ~ ~Considering *~* these facts the following C2 acids are mechanistically accounted for: acetic acid from 1,l-DCE (via CH3C'CI2 and CH3CC1200') monochloroacetic acid from 1,2-DCE (via CH2CIC'HC1 and CH2C1CHC100')
(94
monochloroacetic acid from 1,1,2-TriCE (via CH2CIC'C12 and CH2CICCI200') (*)
and thus explains the experimentally observed products for all systems where the precursor peroxyl radicals carries at least one halogen (here chlorine) atom at the peroxyl carbon. The Russell mechanism does not work for tertiary peroxyl radicals which do not provide the necessary hydrogen atom to be shifted between carbon and oxygen. In this case an alternative mechanism (generally valid also for primary and secondary peroxyls) may take place, namely, a bimolecular radical-radical reaction yielding two oxy1 radicals besides the liberation of one 02. 2R-CC1200' 2R-CC120' 02 (10) The oxy1 radicals are known to be easily reduced to the corresponding "alcohol" R-CCI20H by hydrogen abstraction, e.g., from an original halocarbon (initiating a chain process). R-CC120' + RX R-CC120H + RX(-H)' (1 1) +
Alternatively, tertiary oxy1 radicals may undergo fast @-cleavage which would result in either C-C breakage and generation of C, products (eq 12a), or chlorine atom cleavage leaving the C2 skeleton intact (eq 12b). Both R-CC120H and R-CCIO would eventually hydrolyze to the corresponding R-COOH acid.
+
+
(56) Asmus, K.-D.; Lal, M.;MBnig, J.; SchBneich, C. In Oxygen Radicals in Biology and Medicine; Simic, M.G., Taylor, K. A., Ward, J. F., Sonntag, C. v.. Eds.; Plenum: New York, 1989; p 67. (57) Sonntag, C. v. The Chemical Basis of Radiation Biology; Taylor & Francis: London, 1987. (58) Russell, G. A. J . Am. Chem. SOC.1957, 79, 3871.
dichloroacetic acid from 1,1,2-TriCE (via CHC1,C'HCI and CHCI2CHC100') trichloroacetic acid from PCE (via CCI3C'CI2 and cc13cc1200') Not explained on this basis, however, are the formation of monochloroacetic acid (MCAA) from 1,l -DCE, 1,I ,1 -TriCE, and I , I , I ,2-TetCE, nor the formation of dichloroacetic acid (DCAA) from I , I , 1,2-TetCE. Neither is the lack of trichloroacetic acid (TriCAA) (and chloral hydrate) from 1,1,1,2-TetCE understandable. A simple explanation can only be given for the MCAA formation from 1,1,1,2-TetCE. Under the experimental conditions a small fraction of hydrated electrons reacts directly with this compound rather than with N 2 0 (rate constants 1 X 1Olo and 8 X 1 O9 M-l s-I , respectively) to yield predominantly CH2CIC'C12 as primary radical via dissociative electron attachment. This then adds oxygen in a fast reaction50 leading to CH2CICCI200' as precursor of CH2C1COOH. The calculated yield of this reductive route amounts to G = 0.6 in very good agreement with the experimental MCAA yield of G I0.7.
10084 The Journal of Physical Chemistry, Vol. 95, No. 24, 1991 1
147
Mao et al. and hydrolysis (all well-known reaction^),^^^^^^^^ as exemplified for R(CI)'='CHCI2:
,0-
H Cl-A*
I
+ 02
CI CI-C-OH
I
CI
0
2
4
6
8
1
0
1
2
[I,I.DCEI, 10.'~
Figure 1. Yield of monochloroacetic acid ( M C A A ) (0)and HCI (0) from y-radiolysis of N 2 0 / 0 2 (80:20 v/v) saturated aqueous, pH 6, solutions as function of l,l-dichloroethane (1,I-DCE) concentration. Radiation dose: 108 Gy (J kg-I).
For the other acids, where even a minor reductive route does not take place and where, in particular, the degree of chlorination in the acid differs totally from the substitution pattern in the original chloroethane a rearrangement must be considered. This process constitutes a 1,2-shift of a chlorine atom from a higher chlorinated carbon to a less chlorinated radical carbon thereby increasing the chlorine substitution at the radical carbon. This is well documented, for example, in organic synthesis and theoretical studies; and with respect to the present study it is formulated in reactions 1 3a-cSM7 Peroxidation of the rearranged C-cenCHC12-C'H2 a 'CHCI-CH2CI
CCI3-C'HCI
4
'CCIZ-CHC12
( 13c)
tered radicals provides then a direct route to the respective acids observed. The respective rearranged species are also listed in Table I . Such rearrangements have so far not been studied in the presence of oxygen. Our results now show that this process successfully competes with the oxygen addition. Details on this complex aspect will be presented in a separate publication.68 The major component of the C, products is C02while HCOOH and HCHO seem to be formed at lesser extent as indicated from the (as yet only qualitative) measurements. They all result from C-C bond rupture (e.g., eq 12a). The formation of C 0 2 would in this case be accounted for by hydrolysis of the phosgene, but also by degradation of the liberated carbon-centered radical. One feasible mechanism for the latter would include peroxidations, oxy1 radical formation, possible 1,2-hydrogen shift, H02' cleavage, (59) Freidlina, R. Kh. In Aduances in Radical Cfiemisfry;Williams, G. H., ed.; Academic Press: New York, 1965; p 21 I . (60) Wilt, J . W. In Free Radicals; Kochi, J. K., Ed.; Wiley: New York, 1973; p 333. (61) Skell, P. S.;Pavlis, R. R.; Lewis, D. C.; Shea, K. J . J . Am. Cfiem. SOC.1973. 95. 6732. (62) Chen,'K. S.;Tang, D. Y. H.; Montgomery, L. K.; Kochi, J. K. J . Am. Cfiem. Soc. 1974. 96. 2201: - . ~ ., ~ (63) Gasanov,'R. G.; Ivanova, L. V.; Freidlina, R. Kh. Bull Acad. USSR. (Engl. Trans/.) 1979, 28, 2618. (64) Beckwith, A. L. J.; Ingold, K. U. In Reurrungements in Ground and Excifed States; de Mayo, P., Ed.; Academic Press: New York, 1980; Vol. I , Chapter 4. (65) Skell, P. S.;Traynham, J . G. Acc. Chem. Res. 1984, 17, 160. (66) Golding, B. T.; Radom, L. J . Chem. Sor., Chem. Commun. 1973,939. (67) (a) Clark, T.; Symons, M. C. R. J . Cfiem. SOC.,Cfiem. Commun. 1986,96. (b) Onciul, A. v.; Clark, T. J . Chem. Soc., Cfiem. Commun. 1989, ~
~
~
-
-
H
I
Cl-c-00.
-
I
H
I
Cl-c-0'
CI
I
CI
00.
I I
CI-C-OH CI
--+
CI-C=O
I
+
HO;
Material Balance. A quantitative material balance must take into account all C2 and C , products plus the chloride yield. In this study we shall exemplify this only for one compound, namely, the y-radiolysis of 1, I , I ,2-TetCE at pH 6 . In this case the acids DCAA (G = 3.39, MCAA (G 5 0.7), C02 (G = 3.7), and CI(G = 15.9) account practically quantitatively for the degradation products (no formation of C,-aldehydes). The total destruction of I , I , 1,2-TetCE given by the sum of C2 products plus half the C 0 2 yield amounts to G = 5.9. This is practically identical with the initially available 'OH and eap yield for the radical production. Calculation of the CI- yield takes into account that formation of each DCAA is accompanied by liberation of two CI-, each MCAA by three CI-, and each C 0 2 by two CI-. The theoretical chloride yield thus evaluated amounts to G = 16.2, again a value in very good agreement with the experimental one of G = 15.9. (Note that experimental error limits are much higher than the deviations between actual and calculated yields.) For most of the other compounds the situation appears to be somewhat more complex as aldehydes, formic acid, and possibly some other C2 and C1 products are also formed. The organic C2 acids, C 0 2 plus the associated CI- account, however, always for at least 70-8095 of the chloroethanes destroyed. It seems though that no significant chain processes are involved, suggesting that the decay of the tertiary oxy1 radicals proceeds mainly via pcleavage (eqs 12a,b) rather than "alcohol" formation (eq 11). A complete quantitative product study is underway and will be reported separately. Photocatalytic Degradation at Ti02Surfaces. Acid Formation. Table 11 surveys the stable products identified from photolyzed aqueous, air-saturated, pH 6, suspensions of 3 X M chloroethanes and 1.25 mg/mL of Ti0,. The main ionic products are the same as in y-radiolysis, namely, acetic and/or halogenated acetic acids and HCI. The respective photochemical yields of these products obtained after 10 min of polychromatic (A > 295 nm) irradiation in terms of micromolar concentrations, the relevant peroxyl radicals, the identity of other products which could be detected (C2-aldehydesand CO,, no quantitative measurements possible at present), and some relevant literature information on the products from oxidative degradation of the non-halogenated ethane itselP9 are also listed. The possible C1 compounds (C02, CO, HCHO, HCOOH) were not specifically analyzed for at this stage since their origin is not necessarily the respective original chlorinated ethane but possibly also secondary products generated therefrom. There is no doubt, however, on the formation of at least CO, from any of the investigated chloroorganic compounds. From the qualitative point of view it is noted that organic C2 acids are formed from all chloroethanes but not from ethane itself. As has been found in y-radiolysis, the chlorine substitution pattern in the product acids is not necessarily identical with that of the parent compounds. For example, the only organic acid formed from l,l,l-TriCE is monochloroacetic acid (MCAA). Comparison of the quantitative C2 acid and HCI yields shows that the HCI yield always significantly exceeds that of the organic C2 acids (by factors up to IO). The smallest deviation is obtained for the degradation of PCE. For this compound the formation of each trichloroacetic acid (TCAA) is accompanied by the overall release of two chloride ions, resulting in a theoretical [Cl-]/ [TCAA] ratio of 2.0. The actual ratio of [Cl-]/[TCAA] = 4.17 indicates, however, that about twice as many chloride ions are liberated as calculable on the basis of the most prominent C2 acid.
1082.
(68) Mao, Y.; Schoneich, C.; Asmus, K.-D. To be published.
(14)
CI
(69) Djeghri, N . Faraday Discuss. Chem. SOC.1974, 58, 185.
The Journal of Physical Chemistry, Vol. 95, No. 24, 1991 10085
Oxidative Degradation of Chlorinated Ethanes
TABLE 11: Formation of Organic and Inorganic Products Including Relevant Peroxyl Radicals from Photocatalytic Degradation of Chlorinated Ethanes in Aqueous TiO? Suswnsions'
compounds ethane
yield, jtM
C2 acids
1,l-DCE
CHZC1-COOH CH3-COOH CHZCI-COOH
I,2-DCE I , I , I-TriCE I , 1,2-TriCE
14.0 C
43.3
CHzC1-COOH CHZCI-COOH CHCIZ-COOH CHCIZ-COOH
1,I , 1,2-TetCE
50.0
32.5 42.0 70.0
other productsc CH3CHOb CH3CHz0Hb
responsible peroxyl radical CHj-CHzOO'
HCI, jtM
CH2CI-CHC100' CH,-CCIz00' CHzCI-CHC1OO'
350
co2
250
CHZCICHO
CHzC1-CC1200' CH2CI-CC1200' CHCIz-CHC100' CHC12-CC1200'
400 390
co2 co2 co2
680
CCIjCHO
co,
CHC12-COOH 84.9 CHCI~-CCl~OO' 800 co; CCI3-COOH 236.0 CCI3-CC1200' 980 CO2 'Air-saturated suspensions containing 3 X IO-' M chlorinated ethanes, and 1.25 mg/mL TiOz. IO min illumination with polychromatic,WG 295 filtered light. (For primary C-centered radicals see Table I.) bReference 35. 'Only qualitatively observed. dResult of rearrangment. I , 1,2,2-TetCE PC E
J"
I
E
5
3.54
,
3.0i
tm 3
#'
0.5 "
",
I
340
360
380
400
420
440
0
k,nm
I
5
IO
15
20
-
25
time, min
Figure 2. Action diagram for the formation of monochloroacetic acid (MCAA) upon photocatalytic degradation of 3 X lo-' M, l,l,l-trichloroethane (l,l,l-TriCE) in air-saturated aqueous T i 4 (1.25 mg/mL) suspensions.
Figure 3. Time profile of l,l,l-TriCE degradation (0)and formation of monochloroacetic acid (MCAA) ( 0 )upon polychromatic illumination (WG 295 filter) of air-saturated,aqueous 3 X IO-' M l,l,l-TriCE, Ti02 (1.25 mg/mL) suspensions.
Action Diagram for Acid Formation. Figure 2 shows the wavelength dependence, Le., the action diagram, for the acid formation (here MCAA from TriCE). Samples of aqueous, air-saturated suspensions of 3 X IC3M TriCE and 1.25 mg/mL of Ti02 were illuminated with light of various wavelengths (selected by filters). It can be seen that appreciable acid formation occurs only at wavelengths below 390 nm. This coincides with the band gap of 3.2 eV of Ti02and clearly demonstrates that the observed chemistry is a direct function of the charge separation in the illuminated semiconductor. Quantum Yield. The quantum yield of MCAA from l , l , l TriCE has been measured with monochromatic light at 360 nm and a photon flux of 1.77 pmol s-'. For the steepest (initial) part of the yield vs illumination time curve a quantum yield of = 0.3% has been measured. The actual MCAA concentration per 10 min illumination time amounted to 1.55 pM which has to be compared with the much higher yield of 50 pM generated in a IO-min illumination period with the polychromatic light. Time Profile of Acid Formation. Figure 3 shows a representative example for the time profile of acid formation and overall degradation of a chloroethane (here MCAA from TriCE) upon illumination of Ti02suspensions. The latter has been measured by means of gas chromatography. Qualitatively the curves are seen to complement each other, both leveling off after about 10 min. At this time ca.70 pM MCAA is formed while 2 mM TriCE have been degraded. This means that the maximum measurable MCAA concentration accounts only for a very small fraction (ca. 3.5%) of the converted chloroethane. The same holds, in principle, for the chloride ion yield where the 8 times higher concentration must be related to the fact that at most only three CI- could be liberated from each l,l,l-TriCE molecule. As will be discussed later, the main reason for this mismatch between MCAA formation and TriCE degradation is the limited availability of oxygen. TWOfurther reasons which probably contribute are onset of degradation of MCAA (and other primary products) as evidenced by the nonlinearity in Figure 3, and possible adsorption of products,
particularly acids, at the amphoteric Ti02 surface. The same holds, in principle, for all the other compounds investigated in this study. Eflect of Electron Scuuengers. Table 111 provides information on the formation of monochloroacetic acid (MCAA) and Cl- from the photocatalytic degradation of l , l , 1 -TriCE in Ti02 suspensions and TiO2-free solutions in the presence and absence of electron scavengers. The results stand as a representative example for all the other chloroethanes investigated in this study. N1-or airsaturated aqueous solutions without T i 0 2 did not result in any photocatalytic degradation of the chloroethane, confirming the semiconductor as an essential prerequisite for the initiation of this process. In the presence of Ti02appreciable yields of photocatalytically generated acids were obtained only in oxygen-containing suspensions. These could be suppressed by addition of methanol which is an efficient scavenger of the oxidizing equivalents, (hvB+ or *OHlree/ads,)r indicating that the primary step is indeed an oxidation of the halocarbon. At the same time this result unambiguously excludes a reductive process being significant since scavenging of the holes would enhance the lifetime of the conduction band electrons and thus favor reduction even further. Obviously, the redox potential of ecB- is not sufficiently negative (ca. -0.2 V)'O for inducing a dissociative electron uptake by the halocarbon according to reaction 1. Substitution of oxygen by a different substrate which (other than the chloroethane) is able to scavenge ecB-, e.g., methyl M, in N2-saturated, viologen (MV2+), Fe3+, or Cu2+ at methanol-free Ti02suspensions gave again only negligible yields of acids, demonstrating that oxygen serves, in fact, two separate functions. Firstly, any scavenging of eCB-prolongs the lifetime of the holes (relative to electron-hole recombination) and thus facilitates oxidation reactions. The other and more important function of oxygen is, however, that of a necessary chemical (70) Gerischer, H. Solar Energy Conversion; Springer: Berlin, 1979; p 1 IS.
10086 The Journal of Physical Chemistry, Vol. 95, No. 24, 1991 TABLE HI: Photocatalytic Formation of MoaoehloroaceticAcid and HCI from l,l,l-Trichloroethaneo
CH2ClCOOH and HCI formation
conditions
TABLE I V Rate Constants for the Reaction of Adsorbed and Free *OH Radicals with Methanol and Some Selected Cbloroetbanes' rate constants, M-' s-I
compounds HO-CHI CCI3-CH3 CCI,-CH2CI CCIj-CHCI,
-
N2O/H2O Oz(air)/H20 N;/H20/Ti02 02(air)/H20/Ti02 02/H 20/Ti02/MV2+,Fell', cu2+ N2/H20/Ti02/MV2*,Fe"', cu2+ 02(air)/H20/Ti02/CH30H N2/H20/Ti02/CH30H
Mao et al.
-
adsorbed *OH 4.1 4.9 1.1 3.7
free 'OH 9.2 X I O n b 4.0 x 107~ 1.8 x 107 1.0 x 107
IO8" x 107 x 108 X
x 107
OReference 5 . *Reference72. CReference50.
Om-' means no formation at all or only traces of
