Environ. Sci. Technol. 1997, 31, 3126-3131
Photocatalytic Degradation of Carbaryl in Aqueous Solutions Containing TiO2 Suspensions EDMONDO PRAMAURO,* ALESSANDRA BIANCO PREVOT, MARCO VINCENTI, AND GIOVANNA BRIZZOLESI Dipartimento di Chimica Analitica, Universita` di Torino, 10125 Torino, Italy
when the pesticide residue exceeds the admitted concentration limits is, thus, of practical interest. In a work previously reported (7), the continuous flow degradation of carbaryl under UV light irradiation using a pumped TiO2 slurry has been examined, with main attention devoted to the study of the primary process. In the present investigation, attention was focused on the photocatalytic transformation of carbaryl performed in batch solution, in the presence of suspended TiO2 particles and under simulated solar light irradiation. To give insight into the reaction mechanism, the formation and fate of the aromatic intermediates formed before the benzene ring opening was examined in detail.
Experimental Section The photocatalytic degradation of carbaryl in aqueous TiO2 dispersions irradiated with simulated solar light was investigated. Complete mineralization of the pesticide to CO2 was observed, with the formation of nitrate and ammonium ions as nitrogen-containing end products. Various aromatic intermediates, detected during the process, were identified using HPLC and GC-MS techniques, and their kinetic evolution was also investigated. The formation of the transient compound 1,3-indandione, more persistent than carbaryl, was observed. From obtained analytical and kinetic data, a possible reaction scheme was proposed.
Introduction Photocatalysis over irradiated semiconductor dispersions provides a decontamination method that can lead to very effective light-induced redox transformation of pollutants. In particular, TiO2 (anatase form) dispersions have been largely used as efficient catalysts in the photoassisted oxidation of a variety of organic compounds present in water (1-4). The oxidation of such organic molecules is achieved through reactions with the light-induced generated holes at the semiconductor surface or with the radicals coming from water and from the adsorbed oxygen, being CO2 the end product of the transformation of organic carbon in a large number of reported studies. Together with the assessment of the degradation feasibility, which involves the analysis of the substrate decay, careful analytical control of the process is essential in order to give insight into the reaction mechanism and to identify and control any harmful reaction intermediate formed during the treatment. In fact, it is known that heterogeneous photocatalysis may originate a variety of organic molecules as byproducts of a specific target pollutant, if the treatment is not pursued till complete mineralization. The degradation kinetics of such intermediates should be also investigated since in some cases they can be more persistent than the initial pollutants, and thus, the treatment must be conducted until their complete disappearance is observed. Among the organic compounds of environmental concern, carbamates constitute an important class of insecticides, which are widely used against pests on vast forest areas because they have a rapid action and usually exhibit a moderate persistence in the environment. Carbaryl (1naphthyl N-methylcarbamate) is an important member of this family and, although it shows a relatively short residual lifetime (few weeks in soils), its biological half-life is usually larger (e.g., 5-6 months in fish) (5), and some of its toxic metabolites are rather persistent in the environment (until 1-4 months) (6). The treatment of polluted surface waters
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Reagents and Materials. High-purity carbaryl purchased from Union Carbide was used. Standards of the following compounds were employed: 2-hydroxy-1,4-naphthoquinone, 5-hydroxy-1,4-naphthoquinone, 1,4-naphthoquinone, 1,3indandione (from Aldrich), 1,2-dihydroxybenzene, 1,4-dihydroxybenzene, 1-naphthol, and 1,2,4-trihydroxybenzene (from Sigma). TiO2 P25 from Degussa (largely in the anatase form) having a surface area of ca. 55 m2 g-1 was used throughout the work. This oxide was carefully washed with water and irradiated in Solarbox for some hours, in order to decompose any organic impurity present in the starting material. The treated semiconductor was then dried in the oven at 80 °C. Acetonitrile and methanol (Lichrosolv, Merck) were used to prepare the LC eluents. NaOH, HNO3 65%, citrate buffer pH 2, phosphate buffer pH 7, and borate buffer pH 9 (from Merck) were used to adjust the pH. Dichloromethane (Merck) was used in GC-MS analysis. Doubly distilled water was filtered through 0.45-µm HA cellulose acetate membranes (Millipore) before use. Stock solutions of carbaryl (100-200 mg L-1) were prepared in water, protected from light, and stored at 5 °C. Irradiation Experiments. Most experiments were performed in cylindrical glass cells (40 mm i.d. × 25 mm high), previously described (8), on 5 mL of aqueous solutions containing the proper concentration of carbaryl and TiO2. The initial pH of such solutions was adjusted using HNO3 or NaOH. After irradiation for a given time with simulated solar light coming from a 1500-W Xe lamp (Solarbox from CO.FO.MEGRA, Milan), the solution was filtered through a cellulose acetate membrane (HA 0.45 µm, Millipore) and successively analyzed. The temperature within the irradiated cells was 55 °C. A 340-nm wavelength cutoff filter was used in all the experiments. The total photonic flux (determined by actinometry) was 1.3 × 10-5 Einstein min-1. Analysis of the Primary Process. The degradation of carbaryl was followed by HPLC, after injection of 50 µL of the irradiated sample (previously filtered) in the chromatograph. Other experimental conditions used were the following: column RP-C18 (Lichrospher 10 µm, 4 mm i.d. × 125 mm long, from Merck); eluent, acetonitrile/water (40:60 v/v); flow rate, 1 mL min-1; detector wavelength, 280 nm. CO2 Analysis. Complete mineralization of organic samples to CO2 is very often obtained using the photocatalytic method. According to a previously reported procedure (9), the formation of this end product was followed by headspace gas chromatography, by analyzing the gas phase of the cell after acidification of the solution with 5 mL of H2SO4 5 M. Typically, 200 µL of such gaseous phase was injected on a Carlo Erba 4600 apparatus equipped with a Hayesep 80/100 mesh column (2 m long, 6 mm i.d.). Helium was used as carrier (flow rate, 30 mL min-1). The column and injector temperatures were
S0013-936X(97)00072-2 CCC: $14.00
1997 American Chemical Society
FIGURE 1. Effect of initial pH on carbaryl degradation. (a) pH 3; (b) pH 6; (c) pH 9. Co ) 20 mg L-1. TiO2, 500 mg L-1. 110 and 130 °C, respectively. A TCD detector was used (block temperature, 150 °C; filament temperature, 250 °C). A calibration curve was prepared starting from Na2CO3. Blanks obtained after long-term irradiation experiments indicated a very low contribution from the reagents (less than 0.4 mM), which was taken into account to correct the data. Nitrate and Ammonium Determinations. Ammonium and nitrate ions were found as main nitrogen-containing products. A Metrohm 690 IC, equipped with a Bischoff HPLC pump and a Hamilton column (tetraalkylammonium anionic exchanger), was used for the analysis of NO3-. The eluent was composed of 4-hydroxybenzoic acid (5 mM), at pH 8.6. The flow rate was 1.5 mL min-1. The ammonium ion was determined spectrophotometrically at 690 nm using a modified indophenol blue method (10) (reagents Spectroquant, Merck). Linear calibration curves of absorbance vs ammonium concentration were obtained in the range 0.04-3 mg L-1. Analysis of the Aromatic Intermediates. After extraction in dichloromethane, most unknown aromatic intermediates were identified by GC-MS using a Finnigan-MAT 95Q doublefocusing reverse geometry mass spectrometer interfaced to a Varian 3400 gas chromatograph, equipped with a J&W DBMS capillary silica column (30 m long, 0.25 mm i.d.), coated with methylphenylsiloxane. Qualitative analysis were performed in the electron impact (EI) mode at 70 eV potential. The ion-source temperature was 220 °C. The mass range was 60-250 amu. GC conditions were the following: injection volume, 1 µL (splitless injection); injector temperature, 300 °C; carrier, helium. The oven temperature was programmed as follows: isothermal at 40 °C for 3 min, from 40 to 300 °C at 12 °C/min, isothermal at 300 °C for 10 min.
Results and Discussion Primary Degradation. The mechanism of the light-induced reactions occurring at the surface of the TiO2 particles has been extensively discussed (2, 11-15). As for a large number of previously described organic pollutants, the degradation of carbaryl occurs through a multistep process involving the attack of the substrate by radical species, the •OH radical
being the most powerful oxidant. Under the reported experimental conditions, the complete disappearance of carbaryl was observed after ca. 30 min (see Figure 1). The reaction follows a pseudo-first-order kinetic law, according to
-dCcarb/dt ) kobsCcarb
(1)
where Ccarb is the pesticide concentration and kobs is the observed first-order rate constant. According to eq 1, linear plots of ln C/Co versus time are expected (see the inset in Figure 1) from which slopes kobs can be evaluated. Irradiation of carbaryl solutions in the absence of TiO2 (see Figure 2) shows a negligible decomposition of the pesticide at pH 3 (ca. 0.25% of the initial compound is degraded after 30 min irradiation), whereas the degradation becomes very important at pH 9 (ca. 42%). The change of the absorption spectra in the wavelength range 300-350 nm (see inset in Figure 2) could justify the observed behavior. Experiments performed in the dark indicated that the contribution of hydrolysis is negligible. Analysis of the End Products. Quantitative formation of CO2 after relatively short irradiation cycles (about 40 min) was observed upon treating dilute aqueous solutions of carbaryl (20 mg L-1) in the presence of 500 mg L-1 suspended TiO2, at pH 6 (see Figure 3A). Taking into account that the complete disappearance of the pesticide occurs in a shorter time (ca. 30 min), this provides an indirect evidence of the presence of organic intermediates before the complete mineralization of the substrate. Since aromatic derivatives (with only one exception) were not detected by HPLC after 30 min irradiation, other compounds (e.g., those coming from the ring opening) could presumably contribute to the organic carbon still present after this time. As evidenced in previous investigations, the mineralization of organic nitrogen follows a more complex pattern that largely depends on the oxygen excess present in the system. In particular, complete oxidation to nitrate rarely occurs, being ammonium ions also present in variable amounts in most cases (16-20). In closed cells the formation of ammonium
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FIGURE 2. Contribution of other processes to carbaryl degradation under simulated AM1 light irradiation. Co ) 20 mg L-1. (a) pH 3; (b) pH 6; (c) pH 9. TiO2, absent.
FIGURE 3. (A): Kinetics of CO2 evolution. (B) Formation of nitrogen-containing products (dotted line: stoichiometric nitrogen concentration). Carbaryl, 20 mg L-1; TiO2, 500 mg L-1. Initial pH, 6. predominates, whereas a mixture of ammonium and nitrate was found in long-term experiments (after 8-10 h irradiation) in cells where the air content was renovated by opening the cell at regular time intervals (every hour). Figure 3B shows the evolution of these nitrogen-containing products under the above reported conditions.
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After long-term irradiation, about 60% of the stoichiometric nitrogen was found as ammonium and about 40% as nitrate, whereas lower ammonium and nitrate fractions were found at short irradiation times. For example, after 1 h irradiation the sum of ammonium plus nitrate represents ca. 40% of the total nitrogen, and the existence of other nitrogen-containing
SCHEME 1
FIGURE 4. HPLC pattern of aqueous solutions (initial pH, 3) of carbaryl after 10 min irradiation. Eluent, acetonitrile:water (40:60 v/v); flow rate, 1 mL min-1. Detector wavelength, 220 nm. species, such as hydroxylamine (21, 22), must be hypothesized. The presence of these transient products when no more organics are found can justify the growth of both nitrate and ammonium concentrations at longer irradiation times. It must be underlined that the presence of nitrite, as analyzed using either ionic chromatography and the spectrophotometric Griess method (23), was not detected in our reaction system. Identification of the Organic Intermediates. The HPLC analysis reveals that, after few minutes irradiation, some UVabsorbing intermediates are present together with the unreacted substrate. All these chromatographic peaks exhibit retention times shorter than that of carbaryl and completely disappear within ca. 40 min irradiation. Taking into account that the decomposition rate of the organic intermediates also exhibits direct dependence on pH, all the experiments were performed on solutions irradiated at a fixed initial pH value. At pH 3, in particular, the formation of such compounds reaches a maximum after about 10 min, whereas sampling after irradiation times too short is required when working at higher initial pH values. The observed HPLC pattern is shown in Figure 4. It is important to note that the number of the observed peaks (A-F) and their corresponding retention times do not vary in the pH 3-9 interval suggesting that the reaction intermediates are essentially the same. Two groups of chromatographic peaks were observed after elution of the irradiated samples with acetonitrile/water 40: 60 (v/v). One of them (containing peaks E and F) is close to the dead volume, has been already observed during the photocatalytic degradation of other aromatic derivatives (16, 17, 20, 24, 25) and usually corresponds to polyhydroxybenzenes. In order to improve the peak separation in this region, elution with acetonitrile/water 30:70 (v/v) buffered at pH 3 (0.02 M phosphate buffer) was performed at 1 mL min-1 flow rate, fixing the detector wavelength at 220 nm. The retention times of the following authentic standards were consistent with the corresponding observed HPLC peaks: 1,2-dihydroxybenzene (tR ) 2.6 min); 1,4-dihydroxybenzene (tR ) 1.8 min); 1,3-dihydroxybenzene (tR ) 2.1 min); and 1,2,3-trihydroxybenzene (tR ) 1.6 min). The identification of these intermediates only on HPLC analysis basis is certainly too optimistic, in particular taking into account that they are hydrophilic compounds having retention times near to the column dead time (tm ) 1.2 min). However, two of these hypothesized intermediates (1,2- and 1,4-dihydroxybenzene) have been identified also by GC-MS analysis, confirming that the formation of polyhydroxybenzenes is one of the expected steps.
It is known from previous investigations that hydroxylation of benzene ring does not proceed beyond this point and that ring opening occurs, giving rise to the formation of aliphatic products by oxidation, decarboxylation, and hydrolysis reactions (26, 27). The analysis of such products, which are successively transformed into CO2, was not examined in this work. Our attention was successively focused on the identification of the aromatic intermediates still containing condensed rings, which are present (with one exception) in the second group of HPLC peaks (A-D). These compounds are more hydrophobic than those of the first group, and therefore, their structures are likely to contain the naphthalene moiety. In order to identify them, some reaction mixtures were extracted with methylene chloride, and the extracts were analyzed by GC-MS. Various naphthalene transformation products were identified including the following (the number between parenthesis corresponds to one of the compounds or group of compounds shown in Scheme 1: (2) dihydroxynaphthalenes (MW 160); (3) 1,4-naphthoquinone (MW 158); the isomers (4) 2-hydroxy-1,4-naphthoquinone and (5) 5-hydroxy-1,4-naphthoquinone (MW 174); (6) other hydroxynaphthalendiones (MW 174), less abundant; (7) 1,3-indandione (MW 146). Only traces of the intermediates (2) were detected, according to the expected easy oxidation of aromatic diols to the corresponding quinones. Once identified, the products 3-5 and 7 were purchased as authentic standards and confirmed in both GC-MS and HPLC conditions. In the HPLC profile, three of the products identified by GC-MS proved to yield peaks whose retention time coincided with that of peaks present in the second group. In particular, 3 and 7 correspond to peaks B and C, respectively, whereas the isomer 5 exhibits the same retention time of carbaryl (peak A). The elution of 5, which does not absorb appreciably at 220 nm, was also monitored at a different detector wavelength (208 nm). On the other hand, compound 4 exhibits a very short retention time, near to that of 1,4dihydroxybenzene, and thus belong to the first group. Apart from the coincidence with authentic standards, the identified compounds produced clear, distinguishable, and easy-to-interpret mass spectra. For example, the isomers 4 and 5 yield considerably different mass spectra. Although a common fragmentation process proceeds by consecutive losses of two C units with retention of the charge on the unfragmented benzene ring, cleavage along the quinonic ring produces radically different daughter ions (m/z 105, base peak for 4 and m/z 120 for 5). The mass spectrum of 1,3-indandione (7) is shown in Figure 5.
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FIGURE 5. EI mass spectrum of intermediate (7) (see text).
TABLE 1. Kinetic Parameters of Primary Degradationa compound
t1/2 (min)
kobs × 102 (min-1)
carbaryl 1,4-naphthoquinone 2-hydroxy-1,4-naphthoquinone 5-hydroxy-1,4-naphthoquinone 1,3-indandione 1-naphtholb
5.0 3.8 4.6 5.3 19.1 2.3
14.0 18.1 15.2 13.2 3.6 30.4
b
a Initial substrate concentration, 20 mg L-1; TiO , 500 mg L-1; pH 3. 2 Intermediate hypothesized but not found.
Evolution of the Intermediates. The formation and fate of the more abundant reaction intermediates has been followed by HPLC, leading to the observation of typical bellshaped profiles in all the cases. An interesting point is the higher stability of 7, which is more persistent of carbaryl itself, whereas the other derivatives show about the same degradation rate of the substrate. In order to give insight into the reaction mechanism, the degradation of the available intermediates 3-5 and 7 was investigated working with the same starting concentrations of the initial substrate (20 mg L-1). Table 1 reports the kobs values calculated using eq 1 and the measured half-lifes (t1/2) of the examined intermediates. The corresponding carbaryl data are also given for comparison purposes. In order to ascertain the origin of 1,3-indandione in the reaction system, the HPLC patterns of the irradiated solutions of compounds 3-5 were carefully examined. It appeared clearly from these experiments that 7 mainly arises from the transformation of 1,4-naphthoquinone (3), whereas no appreciable formation of the long-lived intermediate (as evidenced by the lack of the corresponding peak) is originated from the photocatalytic transformation of hydroxylated naphthoquinones, thus indicating that the hydroxylation of 3 and its rearrangement with a carbon atom loss represent two alternative ways of degradation. Figure 6A shows the kinetic behavior of the related intermediates 3 and 7, formed during the degradation of carbaryl. The neat increase of the chromatographic peak C, observed when higher amounts of 3 are irradiated, is shown in Figure 6B. The formation of 1-naphthol, which is a typical product of the carbaryl hydrolysis in basic media (28), was not found in the reaction system neither by HPLC analysis nor by GC-
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FIGURE 6. (A) Formation and fate of the related intermediates 1,4naphthoquinone (3) and 1,3-indandione (7) during the carbaryl degradation at pH 3. Other conditions as in Figure 1. (B) Observed HPLC peaks of compounds 3 and 7 after 10 min irradiation at pH 3. Co (1,4-naphthoquinone), 20 mg L-1. Other conditions, see text. MS. Irradiation experiments performed on 1-naphthol demonstrated that the degradation kinetics of this compound is faster than that of carbaryl and its intermediates (see Table 1) suggesting that, if 1-naphthol is formed, it could be rapidly transformed without appreciable accumulation in the reaction vessel. Degradation Paths. On the basis of the examined analytical and kinetic data, a degradation scheme can be proposed that accounts for the oxidation processes coming from the attack of generated radical species on the substrate (see Scheme 1). Although this task would be difficult starting only from the identification of the intermediates, some considerations about the process can be done: (i) the N-methylcarbamate residue is rapidly removed from the naphtholic moiety, as demonstrated by the fact that no aromatic compounds containing carbamic fragments were found (reaction a); (ii) hydroxylation of the naphthalene ring is also evident (b) as well as the oxidation of dihydroxy derivatives with the formation of the corresponding quinones (c); (iii) 1,4-naphthoquinone would yield 1,3-indandione by loss of one C atom (hypothesis confirmed from the kinetic data) through path d; (iv) opening of the pentaatomic cycle in 7 (for example, via hydrolysis) followed by decarboxylation of the oxidized aliphatic fragments could be invoked to explain the successive formation of hydroxybenzene derivatives (path e); (v) the opening of one of the condensed benzene rings after oxidation-hydroxylation and/or hydrolysis reactions followed by further decarboxylation steps may be hypothesized to justify the formation of polyhydroxybenzenes from intermediates 4-6 (paths f and g), where adsorbed oxygen plays a major role in the preferential formation of odihydroxybenzenes (4); (vi) it is known from previous studies that photocatalytic degradation of products 8-11 usually proceeds through the ring opening (path h). On the contrary, the presence of coupling reactions between intermediate organic radicals, previously found during the treatment of other aromatic and heteroaromatic pollutants (18, 29), was not observed in this study.
Acknowledgments Financial support from MURST (Rome), CNR (Rome), and Project: Sistema Lagunare Veneziano is gratefully acknowledged.
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(2) Ollis, D. F., Al-Ekabi, H., Eds. Photocatalytic Purification and Treatment of Water and Air; Elsevier: Amsterdam, 1993; p 820. (3) Bahnemann, D.; Cunningham, J.; Fox, M. A.; Pelizzetti, E.; Pichat, P.; Serpone, N. In Aquatic and Surface Photochemistry; Helz, G. R., Zepp., R. G., Crosby, D. G., Eds.; Lewis: Boca Raton, FL, 1994; Chapter 21, pp 261-316. (4) Serpone, N.; Khairutdinov, R. F. In Semiconductor Nanoclusters. Physical, Chemical and Catalytic Aspects; Kamat, P. V., Meisel, D., Eds.; Studies in Surface Science and Catalysis, Vol. 103; Elsevier: Amsterdam, 1996; pp 417-444. (5) Miyamoto, J.; Mikami, N.; Takimoto, Y. In Environmental Fate of Pesticides; Hutson, D. H., Roberts, T. R., Eds.; Wiley: Chichester, U.K., 1990; pp 123-147. (6) Khan, S. U. Pesticides in the Soil Environment; Elsevier: Amsterdam, 1980; Chapter 5, pp 164-170. (7) Peris Cardells, E.; Terol, J.; Maurı`, A. R.; de la Guardia, M.; Pramauro, E. J. Environ. Sci. Health B 1993, 28, 431. (8) Pelizzetti, E.; Maurino, V.; Minero,C.; Carlin, V.; Pramauro, E.; Zerbinati, O.; Tosato, M. L. Environ. Sci. Technol. 1990, 24, 1559. (9) Pelizzetti, E.; Minero, C.; Maurino, V.; Sclafani, A.; Hidaka, H.; Serpone, N. Environ. Sci. Technol. 1989, 23, 1380. (10) Greenberg, A. E., Clesceri, L. S., Eaton, A. D., Eds. Standard Methods for the Examination of water and Wastewater, 17th ed.; APHA-AWWA-WPCF: Washington DC, 1989; Chapter 4, p 120. (11) Turchi, C. S.; Ollis, D. F. J. Catal. 1990, 122, 178. (12) Cunningham, J.; Al-Sayyed,G. J. Chem. Soc. Faraday Trans. 1990, 86, 3985. (13) Kohrmann, C.; Bahnemann, D. W.; Hoffmann, M. R. Environ. Sci. Technol. 1991, 25, 494. (14) Minero, C.; Catozzo, F.; Pelizzetti, E. Langmuir 1992, 8, 481. (15) Low, G. K. C.; McEvoy, S. R.; Matthews, R. W. Environ. Sci. Technol. 1991, 25, 460. (16) Peterson, M. W.; Turner, J. A.; Nozik, A. J. J. Phys. Chem. 1991, 95, 221. (17) Pramauro, E.; Vincenti, M.; Augugliaro, V.; Palmisano, L. Environ. Sci. Technol. 1993, 27, 1790.
(18) Maillard-Dupuy, C.; Guillard, C.; Courbon, H.; Pichat, P. Environ. Sci. Technol. 1994, 28, 2176. (19) Doherty, S.; Guillard, C.; Pichat, P. J. Chem. Soc. Faraday Trans. 1995, 91, 1853. (20) Pramauro, E.; Bianco Prevot, A.; Augugliaro, V.; Palmisano, L. Analyst 1995, 120, 237. (21) Mozzanega, H.; Herrmann, J. M.; Pichat, P. J. Phys. Chem. 1979, 83, 2251. (22) Minero, C.; Pelizzetti, E.; Piccinini, P.; Vincenti, M. Chemosphere 1994, 28, 1229. (23) Greenberg, A. E., Clesceri, L. S., Eaton, A. D., Eds. Standard Methods for the Examination of water and Wastewater, 17th ed.; APHA-AWWA-WPCF: Washington DC, 1989; Chapter 4, pp 129131. (24) Augugliaro, V.; Palmisano, L.; Schiavello, M.; Sclafani, A.; Marchese, L.; Martra, G.; Miano, F. J. Appl. Catal. 1991, 69, 323. (25) Barbeni, M.; Morello, M.; Pramauro, E.; Pelizzetti, E.; Vincenti, M.; Borgarello, E.; Serpone, N. Chemosphere 1987, 16, 1165. (26) Hashimoto, K.; Kawai, T.; Sakata, T. J. Phys. Chem. 1984, 88, 4083. (27) D’Oliveira, J. C.; Al-Sayyed, G.; Pichat, P. Environ. Sci. Technol. 1990, 24, 990. (28) Sanceno`n, J.; Carrio`n, J. L.; de la Guardia, M. Fresenius Z. Anal. Chem. 1990, 336, 389. (29) Minero, C.; Pelizzetti, E.; Pichat, P.; Sega, M.; Vincenti, M. Environ. Sci. Technol. 1995, 29, 2226.
Received for review January 28, 1997. Revised manuscript received May 23, 1997. Accepted June 9, 1997.X ES970072Z X
Abstract published in Advance ACS Abstracts, August 1, 1997.
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