Environ. Sci. Technol. 1996, 30, 1660-1666
Environmental Photochemistry on Semiconductor Surfaces: Photosensitized Degradation of a Textile Azo Dye, Acid Orange 7, on TiO2 Particles Using Visible Light
SCHEME 1
Illustration of Photosesitized Degradation of a Colored Polutant That Gets Oxidized on a TiO2 Surface by Injecting Electron from Its Excited State (S*) into the Conduction Band of Semiconductora
K . V I N O D G O P A L * ,† A N D DARREL E. WYNKOOP Department of Chemistry, Indiana University Northwest, Gary, Indiana 46408
P R A S H A N T V . K A M A T * ,‡ Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556
Photosensitized degradation of a textile azo dye, Acid Orange 7, has been carried out on TiO2 particles using visible light. Mechanistic details of the dye degradation have been elucidated using diffuse reflectance absorption and FTIR techniques. Degradation does not occur on Al2O3 surface or in the absence of oxygen. The dependence of the dye degradation rate on the surface coverage shows the participation of excited dye and TiO2 semiconductor in the surface photochemical process. Diffuse reflectance laser flash photolysis confirms the charge injection from the excited dye molecule into the conduction band of the semiconductor as the primary mechanism for producing oxidized dye radical. The surfaceadsorbed oxygen plays an important role in scavenging photogenerated electrons, thus preventing the recombination between the oxidized dye radical and the photoinjected electrons. Diffuse reflectance FTIR was used to make a tentative identification of reaction intermediates and end products of dye degradation. The intermediates, 1,2-naphthoquinone and phthalic acid, have been identified during the course of degradation. Though less explored in photocatalysis, the photosensitization approach could be an excellent choice for the degradation of colored pollutants using visible light.
Introduction Synthetic textile dyes and other industrial dyestuffs constitute the largest group of chemicals produced in the United * Address correspondence to this author. † E-mail address:
[email protected]; telephone: (219) 980-6688; fax: (219) 980-7125. ‡ E-mail address:
[email protected]; telephone: (219) 631-5411; fax: (219) 631-8068.
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a By scavenging the electrons with a redox couple (e.g., O / 2 O2-), one can suppress the back electron transfer between S+ and injected electrons.
States and the world (1-4). Significant losses occur during manufacture and processing and are discharged as effluents into publicly owned water treatment plants (5). These effluents are processed into sludge cake and deposited in landfills (6). Within the overall category of dyestuffs, azo dyes constitute a significant portion and probably have the least desirable consequences in terms of the surrounding ecosystem. They are readily reduced under anaerobic conditions to potentially hazardous aromatic amines (7). Weber et al. (8, 9) have studied the reduction of disperse azo dyes in natural anoxic sediments and have shown that the reduction products are potentially hazardous aromatic amines. Over the past few years, part of our research has focused on methods of photocatalytically degrading textile dyes (1015). Composite SnO2/TiO2 semiconductor photocatalysts show superior degradation rates for azo dyes rather than the single-component (SnO2 or TiO2) semiconductor systems (13, 15). In such cases, the semiconductor particles are irradiated with UV light to induce charge separation. The photogenerated holes then oxidize the pollutant either directly or indirectly via hydroxide radical generation. Recent developments in the area of UV light-induced semiconductor photocatalysis have recently been reviewed by several researchers (16-25). An alternative approach as is used here is to excite the adsorbed organic material and then inject charge from the excited organic into the semiconductor particle (26-30). If not regenerated, the oxidized form of the organic material can then undergo further reactions (10, 31). This process, which is commonly referred to as photosensitization, is illustrated in Scheme 1. In a sensitizer-based photoelectrochemical cell, a redox couple (e.g., I3-/I-) is employed to regenerate the sensitizer (32). In the absence of a regenerative system, the oxidized sensitizer readily undergoes further degradation. The photosensitized oxidation of a variety of colored compounds such as methylene blue (33, 34), phenosafranin (35), fullerene (36), rose bengal (37), diphenylisobenzofuran (38), nitrophenol (39, 40), and ruthenium trisbipyridyl complex (41) has been carried out on semiconductor surfaces. Zepp and others have studied the photosensitized degradation of a commercial dye such as Solvent Red 1 on TiO2 surfaces (42). In all of the cases mentioned above, the colored
0013-936X/96/0930-1660$12.00/0
1996 American Chemical Society
compound adsorbed on the semiconductor surface is completely or partially bleached under steady illumination. We have now extended these studies to explore the feasibility of employing a colored substrate/target pollutant (e.g., textile dyestuff) as a sensitizer so that it initiates its own degradation and additionally we are able to identify tentatively the end products of such degradation. The advantages of this process are (i) the utilization of visible light for degrading colored compounds and (ii) the ability to degrade hazardous colored organics in potentially difficult matrices such as sludge cakes containing dyestuffs from treatment plants. Diffuse reflectance absorption and Fourier transform infrared (DRIFT) spectroscopies have been used to investigate the photosensitized degradation of a representative azo dye, Acid Orange 7 (AO7), on the surface of a semiconductor such as TiO2. All the experiments were performed in the absence of any added solvent. The kinetic and mechanistic aspects of dye photodegradation on semiconductor surfaces have been elucidated by timeresolved transient absorption studies using diffuse reflectance laser flash photolysis.
Experimental Section Acid Orange 7 was obtained from Aldrich. The dye was purified by column chromatography using a neutral alumina column and ethyl acetate, ethanol, and water as eluents. TiO2 (product name P-25) and Al2O3 powders were a gift sample from Degussa Corporation. TiO2 has a particle size of 30 nm and a surface area 50 m2/g. Al2O3 has a particle diameter of 20 nm and a surface area 100 m2/g. For determining adsorption isotherms of the dye on the oxide surfaces, the suspensions were prepared by mixing 100mL aliquots of aqueous dye solutions of various initial concentrations Ci at natural pH (∼5) with a fixed weight (0.5 g) of Degussa P-25 TiO2. The suspensions were stirred overnight in the dark and then filtered. The absorbance of the filtrate was then measured at 480 nm for AO7 to determine the dye concentrations in it. The extent of equilibrium adsorption was determined from the decrease in the dye concentration (∆C) detected after filtration. All the isotherm measurements were made in the dark. For the diffuse reflectance FTIR and steady-state photolysis studies, samples of dye-coated TiO2 and Al2O3 were prepared in the same manner described above such that the coverage on the samples amounted to 0.10 and 0.02 mmol of AO7/g of TiO2. These concentrations of the dye on TiO2 are referred to as high- and low-coverage samples, respectively. The samples were prepared on the day previous to the measurements and stored in the dark in a vacuum desiccator to prevent degradation. Naphthoquinone (NQ) and phthalic acid (PA) coated TiO2 samples were prepared by adding a known amount of the metal oxide to a solution of either NQ or PA in ethanol. These solutions were stirred for at least 2 h and then evaporated to dryness. The resulting coverage of the NQ and PA on the TiO2 was ∼2 mg of compound/g of TiO2. Airequilibrated samples refer to those exposed to air for at least 2-3 h. Optical Measurements. The diffuse reflectance absorption spectra of azo dye-coated oxide samples were recorded with a Milton Roy 3000 array spectrophotometer. The measurements on degassed samples were carried out in a vacuum-tight 6 × 3 × 40 mm3 rectangular quartz cell (41). The cell design was convenient to degas the powder
samples and carry out the optical measurements. The whole assembly could be inserted into the sample compartment of the respective spectrophotometers without disturbing the vacuum. Steady-state photolysis was executed using a high intensity beam (24-W halogen lamp) from a Fiber-Lite Model 190 fiber optic illuminator. The sample was either pressed between the two microscope slides or filled in a high-vacuum spectrophotometer cell described above. Diffuse Reflectance Laser Flash Photolysis Experiments. Time-resolved diffuse reflectance laser flash photolysis experiments were carried out with the setup described earlier (35, 43). The 532-nm laser pulse (10 mJ, pulse width 6 ns) from a Quanta Ray DCR-1 Nd:YAG laser system was used for the excitation of the sample. A 1000-W xenon lamp was used as the monitoring source. The diffusely reflected monitoring light from the sample was collected and focused onto a monochromator that was fitted to a photomultiplier tube, and the photomultiplier output was input to a Tektronix 7912A digitizer. Before triggering each laser pulse, the cell was shaken to expose a fresh surface for excitation. The principle of diffuse reflectance laser flash photolysis is described by Wilkinson and his collaborators (44-46). Diffuse Reflectance FTIR Experiments. IR absorbance spectra of air-equilibrated samples were measured in the region 4000-400 cm-1, at a resolution of 4 cm-1 using a Bio-Rad FTS 165 FTIR spectrometer. In all cases, the spectra displayed were obtained using the Bio-Rad diffuse reflectance accessory with samples exposed to air. Depending on the particular sample, KBr, TiO2, or alumina was used as the background. Because of strong absorbance by oxides, TiO2, and Al2O3, spectral data below 1200 cm-1 for the adsorbed species were not measurable. Similarly, information above 2500 cm-1 was difficult to obtain because of surface hydroxyl groups on the oxide particles. Diffuse reflectance FTIR spectrum of the pure dye was obtained by mixing it with KBr powder. The diffuse reflectance FTIR spectra of dye coated on TiO2 and Al2O3 were monitored while undergoing steady-state photolysis using a highintensity beam from a Fiber-Lite Model 190 fiber optic illuminator.
Results and Discussion Adsorption Isotherms. The dye adsorbs strongly on to TiO2 particles from aqueous solution. A measurable decrease in the concentration of the dye solutions was observed upon equilibrating them overnight with TiO2 powder in the dark as per the details mentioned in the Experimental Section (equilibrium 1):
AO7 + TiO2 S (AO7‚‚‚TiO2)
(1)
The adsorption isotherm for Acid Orange 7 in which n2s, the number of dye moles adsorbed per gram of TiO2, is plotted against the equilibrium concentration of the dye solutions (Ceq) is shown in Figure 1. The number of dye moles adsorbed per gram of TiO2 (n2s) can be determined from the adsorption-induced decrease in the molarity (∆C) and V, the volume of dye solution aliquot: n2s ) (V∆C)/W where W is the weight of TiO2 in grams. The behavior of n2s as a function of the equilibrium solute (dye) concentration can be expressed by
n2s ) (nsKCeq)/(1 + KCeq)
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(2)
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A
FIGURE 1. Adsorption isotherm for AO7 on TiO2. Plot of the n2s, the number of mmol of AO7 adsorbed/g of TiO2 versus Ceq, the equilibrium concentration of AO7 remaining in solution.
FIGURE 2. Diffuse reflectance absorption spectra of AO7 on (a) TiO2 and (b) alumina surfaces. (The dye coverage was 0.10 mmol of AO7/g of TiO2. The ordinate scale is expressed in Kubelka-Munk units where R is the reflectivity measured at the corresponding wavelength.) Spectrum c shows the solution spectra of AO7 in water.
where K represents the equilibrium constant for the process of dye adsorption. In the limit of low equilibrium concentrations such that KCeq 380 nm), they readily underwent degradation and the colored titania powders were completely bleached. Figure 3A,B shows the diffuse
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B
FIGURE 3. Steady-state photolysis of air-equilibrated samples of AO7 on TiO2 with visible light at low and high coverages. (Diffuse reflectance spectra were recorded using plain TiO2 as reference. The ordinate scale is expressed in Kubelka-Munk units where R is the reflectivity measured at the corresponding wavelength.) (A) Dye coverage was 0.02 mmol of AO7/g of TiO2, and the spectra were recorded at (a) 0, (b) 5, (c) 10, (d) 30, and (e) 60 min of irradiation. (B) Dye coverage was 0.10 mmol of AO7/g of TiO2, and the spectra were recorded at (a) 0, (b) 5, (c) 30, and (d) 120 min of irradiation.
reflectance spectra of aerated samples of AO7 on TiO2 at low and high coverages, respectively. The spectra were recorded at various time intervals following photolysis with visible light. As seen from a comparison of Figure 3, panels A and B, the low-coverage samples of the dye on TiO2 degrade significantly faster. Almost complete photobleaching of the absorption band at 480 nm is achieved for these low-coverage samples in ∼30 min. The probability of photochemical oxidation events is higher for these lowcoverage samples since the ratio of photons-to-dye molecules is larger. Other aspects of the dependence of degradation rate on the surface coverage is discussed in earlier studies (37, 38). The behavior of degassed samples of AO7 on TiO2 is quite different. The degradation following steady-state photolysis of these degassed samples (Figure 4) did not proceed to completion. Spectrum c in Figure 4, which was obtained after an hour of steady-state photolysis, indicates some bleaching but not to the extent that is observed in the aerated samples. The small amount of degradation observed in the first 10-15 min probably arises from the residual oxygen that is bound to the surface of TiO2. Once this oxygen is consumed, the degradation ceases. This is good evidence of the necessity of oxygen for scavenging photoinjected electrons during the photosensitization
FIGURE 4. Steady-state photolysis of degassed samples of AO7 on TiO2 with visible light. Diffuse reflectance spectra were recorded using plain TiO2 as reference at different photolysis times: (a) 0, (b) 5, (c) 30, and (d) 120 min of irradiation. The ordinate scale is expressed in Kubelka-Munk units where R is the reflectivity measured at the corresponding wavelength. The dye coverage was 0.10 mmol of AO7/g of TiO2.
FIGURE 6. Time-resolved transient absorption spectra of degassed (O) and air-equilibrated (b) samples of AO7 on TiO2 recorded following 532-nm laser pulse excitation at 65 µs. Inset shows absorption-time profiles recorded at 480 nm of degassed and airequilibrated samples. The dye coverage was 0.10 mmol of AO7/g of TiO2.
undergoes degradation to yield stable products (reactions 3-7):
1
FIGURE 5. Normalized decay traces representing (a) the degradation of AO7 on alumina (air-equilibrated; 0.02 mmol of AO7/g of Al2O3), (b) AO7 on TiO2 (0.02 mmol of AO7/g of TiO2), (c) AO7 on TiO2 at high coverage (0.10 mmol of AO7/g of TiO2), and (d) AO7 on TiO2 at low coverage (0.02 mmol of AO7/g of TiO2). Sample b was degassed while the others were equilibrated in air before the photolysis.
process. In the absence of oxygen, the recombination between injected electrons and the cation radical of the dye results in the regeneration of the sensitizer. To ensure that the visible light-induced degradation of AO7 is a TiO2mediated phenomenon, we have also obtained the diffuse reflectance spectra of degassed and air-equilibrated samples of AO7 adsorbed on an insulator surface such as alumina. The absorption spectrum shows no change following prolonged steady-state illumination with visible light. (Since no changes were observed, spectra of AO7 on alumina before and after photolysis are not shown.) The differing rates of decay of AO7 on oxide surfaces as cited in the experiments above are compared in Figure 5. A first-order exponential fit of the decay for the high- and low-coverage cases gives k values of 2.1 × 10-2 and 3.0 × 10-2 min-1, respectively. The steady-state photolysis results illustrate that degradation occurs rapidly following charge injection from the dye into the semiconductor. The electron from the excited dye molecule is injected into the conduction band of the TiO2, and the cation radical formed at the surface quickly
dye + hν f 1dye* or 3dye*
(3)
dye* or 3dye* + TiO2 f dye•+ + TiO2(e)
(4)
TiO2(e) + O2 f O2•- + TiO2
(5)
dye•+ f products
(6)
dye•+ + O2•- f products
(7)
Diffuse Reflectance Laser Flash Photolysis. The charge injection process was further probed by diffuse reflectance laser flash photolysis of AO7 on TiO2. The principle of diffuse reflectance laser flash photolysis has been established earlier (44-46). If the exciting light is totally absorbed by the sample, the intensity of the diffused monitoring light from the sample can be used to estimate the fractional absorption (Ad) of the transient formed at the surface: Ad ) ln (I°a/It) where I°a and It are the incident and unabsorbed fraction respectively of analyzing light intensities at the solid sample. For small fractional absorption (Ad 600 nm, which we attribute to the cation radical and trapped electrons in TiO2 particles. In an earlier publication, we have reported absorption characteristics of the cation radical in aqueous solution (12) and trapped electrons in TiO2 particles (41). The inset in Figure 6 shows the recovery of the bleaching at 480 nm in the degassed samples as the oxidized dye radical recombines with the injected electrons.
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FIGURE 7. Diffuse reflectance FTIR spectra in the region 1200-2000 cm-1 of AO7 on (a) KBr, (b) TiO2, and (c) Al2O3. (The dye coverage was 0.10 mmol of AO7/g of support.)
The air-equilibrated sample shows increased bleaching in the 480-nm band and decreased absorption in the red region. The reverse electron transfer between injected electron and oxidized dye radical is suppressed as the surface-adsorbed oxygen scavenges away the trapped electrons (reaction 5). The absorption-time profiles recorded at 480 nm (inset in Figure 6) show the difference in the bleaching recovery of the degassed and airequilibrated samples. The depletion of ground-state dye in air-equilibrated sample indicates permanent changes following charge injection into the semiconductor and irreversible degradation of AO7. The reduced-oxygen species could further promote degradation of the dye on the semiconductor surface. Diffuse Reflectance FTIR. In order to probe the surface photochemical events of AO7 adsorbed on TiO2 powder and to identify the products rising from such a selfsensitized degradation, a diffuse reflectance FTIR study of the powders was carried out at various irradiation times. The FTIR spectra of AO7 on KBr, TiO2, and Al2O3 are shown in Figure 7 as spectrum a-c, respectively. In all three cases, a characteristic vibration at 1500 cm-1 is observed. For symmetry reasons, the intensity of an azo bond vibration is expected to be weak in the infrared spectrum, but para substituents to the azo group enhance the intensity of the latter considerably. We observe a strong band at this frequency for a variety of azo dyes adsorbed on TiO2 including 4-phenylazophenol, Naphthol Blue Black, Chicago Sky Blue, etc. Given the reproducibility of this band in all of these azo dyes, we believe that it is an azo bond vibration or an aromatic ring (CdC) vibration that is sensitive to the azo bond. At least five other vibrations observed in the same region at 1620, 1596, 1568, 1555, and 1450 cm-1 can be attributed to aromatic skeletal vibrations. Similarly, by comparison with the FTIR spectra of other phenols adsorbed on TiO2 and available in the literature, the peaks at 1405 and 1318 cm-1, which are relatively weak on the metal oxide surfaces, can be assigned to O-H bending vibrations, while the band at 1250 cm-1 is a C-O-H stretching vibration. The IR spectra of AO7 on both TiO2 and alumina were recorded following steady-state photolysis at various time intervals. Substantial changes are observed upon photolysis of the TiO2 sample. No spectral changes are observed with the alumina sample since charge injection from excited dye can occur only into the TiO2. Only the FTIR spectrum
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FIGURE 8. In situ diffuse reflectance FTIR spectra of AO7 on TiO2 surface (0.10 mmol of AO7/g of TiO2) during steady-state photolysis. The spectra were recorded at time intervals of (a) 0 min, (b) 60 min, and (c) 24 h of irradiation with visible light.
FIGURE 9. Diffuse reflectance FTIR spectra of 1,2-naphthoquinone on TiO2 surface during steady-state photolysis. The spectra were recorded at time intervals of (a) 0 and (c) 90 min of irradiation with visible light. Also shown is the spectrum of AO7 on TiO2 (b) after photolysis for 120 min.
of AO7 on TiO2 is shown in Figure 8. The biggest decrease is observed in the azo bond-sensitive vibration at 1500 cm-1, suggesting that the molecule is being cleaved at the azo bond. Nearly all of the aromatic skeletal vibrations also disappear, suggesting that the molecule is undergoing irreversible chemical changes. The spectra in Figure 8 also show a new band appearing at 1700 cm-1 that is characteristic of a carboxylic acid functional group. Spectrum c was obtained after 24 h of irradiation and shows only two peaks at ∼1700 and 1400 cm-1. Literature references on the ozonation of AO7 indicate that the molecule in such an oxidative degradation is cleaved at the point where the azo bond is attached to the naphthalene moiety giving rise to the intermediate, 1,2naphthoquinone (NQ) (49, 50). This intermediate is oxidized subsequently to a phthalic acid derivative. With the assumption that the oxidative pathway in our photosensitized degradation is similar to the ozonation process, we prepared samples of naphthoquinone and phthalic acid on TiO2. NQ on TiO2 forms a gray-colored powder, which undergoes a sensitized decay similar to the parent dye on TiO2. The FTIR spectra of NQ on the TiO2 surface before and after 90 min of irradiation are shown in Figure 9 (spectra a and c, respectively). Also included in Figure 9 is a spectrum of the AO7 on TiO2 after 120 min of irradiation (spectrum b). The spectrum of the photolyzed AO7 is
conduction band of TiO2. The laser flash photolysis data also supports a charge injection mechanism. The faster decay rates obtained with the low-coverage samples is due to higher ratio of photon-to-dye concentrations. The different degradation rates obtained with different supports (Al2O3 and TiO2) show the necessity of semiconductor surface for initiating charge injection.
FIGURE 10. Diffuse reflectance FTIR spectra of (a) phthalic acid on TiO2 and (b) AO7 on TiO2 after steady-state photolysis for 24 h.
relatively similar, especially in the area of 1400-1800 cm-1, to the spectra of the photolyzed naphthoquinone samples. It is reasonable to conclude therefore that naphthoquinone is an intermediate from the photosensitized degradation of AO7 on TiO2. The naphthoquinone then undergoes further degradation to form a phthalic acid derivative. Spectra of the photolyzed AO7 and phthalic acid on TiO2 are shown for comparison in Figure 10. Spectrum a is the phthalic acid on TiO2, while spectrum b is the photolyzed AO7. The obvious similarity between the two spectra is the peak in the 1700 cm-1 region, which suggests that the end product of oxidation of AO7 is an aromatic carboxylic acid.
Discussion It is clear from the experimental results presented above that the AO7 dye from its excited state can inject electrons into the semiconductor particle and that the cation radical of the dye formed at the TiO2 surface quickly recombines with trapped electrons (degassed samples) or in airequilibrated samples decays to products such as phthalic acid. The oxidation potential of AO7 has been reported by us earlier to be 1.0 V vs SCE (13) or 0.76 V vs NHE, and for the excited state this potential corresponds to -1.24 V vs NHE, given Es ≈ 2 eV/molecule. Thus, the energy gained from the optical excitation provides the necessary driving force to inject electrons from dye molecules into the
Resolving the decay mechanism of the oxidized dye radical generated at the surface subsequent to charge injection is a little more challenging. It is clear from the experimental evidence that that decay occurs only in the presence of oxygen. This is likely due to molecular oxygen’s capability to scavenge electrons and thus suppress the recombination between AO7•+ and the trapped electron on the TiO2 surface. The absorption-time profiles recorded in Figure 6 support such an argument. It is likely that the reduced-oxygen species produced at the surface such as O2•- also participate in the oxidation. Our tentative mechanism for the decay of the AO7•+ to produce NQ and benzene sulfonic acid is shown in Scheme 2. The FTIR results lend credence to this proposed mechanism. More intriguing however is the degradation pathway for naphthoquinone to phthalic acid. NQ when adsorbed on TiO2 forms a gray-colored powder so that selfsensitized decay of NQ by charge injection into the semiconductor is quite possible, and our FTIR studies of NQ on TiO2 provide evidence for such decay. References are available in organic literature of the oxidation of naphthoquinone to phthalic acid by superoxide (51). Within the limitations of our experimental data, it is hard to distinguish between the two oxidation processes, especially since the end products are likely to be the same. Our FTIR studies give us some insight into the nature of the products in so far as we are able to pinpoint the ultimate product to be a carboxylic acid. It is difficult to believe that the decay on the surface could proceed to such an end product without the participation of oxygen in some form. There is also another possibility suggested in our degradation scheme that water in the form of adsorbed moisture participates in the degradation. Degradation of the degassed samples of AO7/TiO2 would be inhibited due to the removal of such adsorbed water. We are currently in the process of designing experiments for a dye-sensitized photoelectrochemical cell, which would help us resolve
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the question of the role of oxygen in the degradation scheme. The diffuse reflectance measurements carried out in this study confirm the visible light-induced degradation of azo dyes to proceed via photosensitization pathway. Though less explored in photocatalysis, this approach could be an excellent choice for the degradation of colored pollutants in sludge cakes. While complete mineralization of the dye has not been achieved on the surface, it is likely that such a degradation can be achieved by using broad band illumination, which would combine both the conventional photocatalytic and the photosensitized degradations.
Acknowledgments We would like to thank Prof. M. V. George for helpful discussions and Degussa Corporation for supplying us the gift sample of TiO2 and Al2O3 powders. K.V. acknowledges the support of Indiana University Northwest through a Grant-In-Aid. P.V.K. acknowledges the support of the Office of Basic Energy Sciences of the U.S. Department of Energy. This is Contribution NDRL-3868 from the Notre Dame Radiation Laboratory.
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Received for review September 6, 1995. Revised manuscript received December 13, 1995. Accepted December 20, 1995.X ES950655D X
Abstract published in Advance ACS Abstracts, March 1, 1996.
