Environ. Sci. Technol. 1999, 33, 3171-3176
Photodegradation of 4,4′-Bis(2-sulfostyryl)biphenyl (DSBP) on Metal Oxides Followed by in Situ ATR-FTIR Spectroscopy JANET M. KESSELMAN-TRUTTMANN AND STEPHAN J. HUG* Swiss Federal Institute for Environmental Science and Technology CH-8600 Duebendorf, Switzerland
Photodegradation on soil and mineral surfaces is an important pathway for the transformation of recalcitrant pollutants in the atmosphere and on terrestrial surfaces. Monitoring degradation on surfaces under controlled conditions without intrusive methods, however, remains a challenge. Here, we report in situ measurements of the photodegradation of 4,4′-bis(2-sulfostyryl)biphenyl (DSBP), a fluorescent whitening agent used in laundry detergents, on the surfaces of metal oxides under conditions of controlled humidity using attenuated total reflectance infrared spectroscopy (ATR-FTIR). A ZnSe ATR crystal was coated with 1-5 µm thick layers of high specific surface area hematite, δ-alumina, lepidocrocite, rutile, and anatase. The 0.4 mg oxide layers were spiked with 20 nmol of (E,E)DSBP and illuminated with 365 nm light at 25 °C and 88% relative humidity. Extensive degradation was observed on rutile and anatase due to the photocatalytic action of the TiO2 semiconductor particles. Degradation on alumina, hematite, and lepidocrocite, on the other hand, was slower and occurred primarily through the direct photodegradation of adsorbed DSBP. For hematite and lepidocrocite, light absorption by the oxide layer reduced the degradation rate by reducing the amount of light available to DSBP. Implications for the photodegradation reactions of environmental contaminants on soils are discussed.
Introduction 4,4′-Bis(2-sulfostyryl)biphenyl (DSBP) (structure in Figure 1) is a fluorescent whitening agent used in laundry detergents to make clothes appear whiter. DSBP absorbs UV light and fluoresces in the visible blue region of the spectrum (1), compensating for the yellowish tinge observed in most fabrics due to the presence of compounds that absorb in the near UV-vis blue region. DSBP and other fluorescent whitening agents adsorb to clothes during washing. The unadsorbed fraction is released to sewage treatment plants with the wash water, where 50-98% remain adsorbed on sewage sludge, resulting in concentrations of typically 20-50 mg/kg in dry sludge from Swiss sewage treatment plants (2). The remaining DSBP is discharged to river systems with the treated water. DSBP retained in sewage sludge is of concern because these sludges may be applied to soils as fertilizers. As DSPB and other optical whiteners are recalcitrant to biodegradation, * To whom correspondence should be addressed. E-mail: hug@ eawag.ch; phone: +41-1-823-5454; fax: +41-1-823-5028. 10.1021/es981226t CCC: $18.00 Published on Web 08/11/1999
1999 American Chemical Society
FIGURE 1. Comparison of the spectra of (E,E)-DSBP adsorbed to various oxide layers. The heavy line indicates the spectrum of neat (E,E)-DSBP on the ATR crystal (no oxide). The spectra have been normalized with respect to the peak at 1198 cm-1. photochemical degradation appears to be the most important transformation pathway. Photoisomerization kinetics (3) and photodegradation reactions of DSBP in natural waters (4, 5) and in suspensions (1) have been reported. Photochemical degradation was the main pathway for DSPB removal from a lake (5). In lepidocrocite suspensions at pH 3, light absorbed by lepidocrocite particles might have contributed to the overall photodegradation rate of DSBP (1). In addition to photodegradation in natural waters, photochemical degradation on soils and mineral surfaces is important for many pollutants, including pesticides, herbicides, combustion residues, etc. Although light penetration into soils and minerals is limited to a few micrometers or millimeters (depending on the composition and on the aggregation of soil particles), successive layers may be exposed to sunlight in drying and crumbling clumps of soil. Various oxides that absorb solar energy can potentially act as photocatalysts (6, 7). Titanium dioxide (TiO2), although typically accounting for only about 0.5% of soil matter (8), may contribute to photochemical transformations due to its high reactivity. The photocatalytic properties of semiconducting TiO2 particles have been extensively studied as a promising method for the treatment of organic contaminants in water and air (7, 9, 10). In the presence of oxygen, a series of oxidation reactions eventually leads to the complete mineralization of organic molecules to CO2 and mineral acids. Photochemical transformations catalyzed by iron (hydr)oxides, which are ubiquitous in soils, have also been extensively investigated (11-17). Although also semiconducting, iron(hydr)oxides, unlike TiO2, are not stable in water under illumination. Suspensions of iron oxides are known to release Fe(III) at low pH (12, 13). Dissolved Fe(III) aquo complexes produce hydroxyl radicals in solution when illuminated (14), making it difficult to distinguish between surface reactions and reactions in homogeneous solution. An additional complication is the possible formation of dissolved and/or surface bound iron-organic complexes that are also photoactive. SiO2 and Al2O3, the most prevalent oxides in the earth’s crust with band gaps of 8.6 and 9.0 eV (corresponding to 145 VOL. 33, NO. 18, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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and 138 nm), do not absorb sunlight reaching the earth’s surface (11) and thus cannot act as photocatalysts. In the present study, the reaction of DSBP on irradiated alumina was performed as a control experiment for the direct photochemical degradation of surface-bound DSBP. Studying photodegradation on solid particles is more difficult than in solutions and suspensions for several reasons. First, there is currently no convincing method for the quantification of light absorption in thicker particulate layers. Second, extraction of adsorbed photoproducts could prevent the detection of labile products. Third, it is difficult to maintain controlled conditions (temperature, humidity, composition) in thicker layers of particles. Here, we avoided these difficulties by irradiation of micron thin layers of particles coated on an attenuated total reflection (ATR) element and by monitoring degradation by infrared spectroscopy. ATR-FTIR has been applied previously to study adsorption of organic molecules on oxide layers (18-21). Although FTIR alone is not sufficient for an exact identification of each photoproduct, it can monitor the disappearance of the initial compound and the appearance of a range of functional groups without disturbing the system, and various soil components can be screened quickly for their relative activity as catalysts for photodegradation. In this first study, we worked under conditions of high relative humidity (RH ) 88%), resembling the situation on drying clumps of soils and of mineral surfaces in humid air. Under these conditions, mineral surfaces are typically covered with 5-10 molecular layers of physisorbed water (22) and the surface photochemistry can be compared to the one on surfaces in contact with aqueous solution, with the difference that dissolution and subsequent additional chemistry in solution is not possible. Thus, studies on air-dry oxides help to isolate surface reactions from reactions in the solution phase in suspensions.
nm at the equator at noontime is about 10 mW/cm2, assuming that 10% of the incident solar energy is in this wavelength range. FTIR. Spectra were recorded on a Bio-Rad FTS45 instrument equipped with a mercury cadmium telluride (MCT) detector and a Specac horizontal ATR unit with a horizontal 45° ZnSe ATR crystal: larger horizontal upper probe face 10 × 72 mm; thickness 6 mm; 5 internal reflections. Further characterization of the optical properties of the ATR setup are reported in (18). Spectra are the average of 200 scans taken at 4 cm-1 resolution vs the appropriate single-beam background spectrum. The ATR-crystal was sealed in the bottom of 150 mL volume cell made of polypropylene with a cover containing a glass window for transmission of the 365 nm light. Humidity was maintained by a saturated solution of BaCl2 (88% RH at 24.5 °C) (25). The cell was thermostated to 25 °C by a circulating bath. Background spectra of the oxide layer were obtained after equilibration with the cell humidity. After application of the DSBP the cell was resealed to reestablish the desired humidity. Before illumination, the humidity equilibration could be observed at 1630 cm-1 by the bands of adsorbed water and typically required 20-45 min depending on the oxide layer. The FTIR instrument was controlled by a SPC3200 data station and final data analysis was performed on a PC using Matlab software (The Mathworks, Inc). UV-vis. For some photodegradation experiments, the oxide layers were extracted with 5 mL of a pH 12 solution after irradiation. This procedure was shown to remove more than 90% of adsorbed DSBP. The extract solutions were then characterized by UV-vis spectroscopy in order to provide an additional measure of DSBP degradation.
Experimental Section
Adsorption of DSBP on Various Oxides. Figure 1 shows the infrared spectra of an air-dry film of DSBP on an uncoated ATR crystal and adsorbed on various air-dried oxide layers. It has been shown that dissolved DSPB adsorbs strongly to metal oxides with a typical Langmuir-type saturation behavior and increasing adsorption at lower pH. On lepidocrocite (specific surface area of 130m2/g), saturation at pH 3 was reached at an aqueous concentration of 4 µM (E,E)-DSPB and a coverage of 60 nmol/g, corresponding to approximately 0.3 DSPB molecules/nm2 (1). Thus, by applying 20 µL of a 1 mM DSPB solution at pH 3 to 0.4 mg oxide and subsequent air-drying, most DSPB was initially adsorbed from solution. After drying, the surface concentration of DSBP was between 0.3 and 0.45 molecules/nm2 on Al2O3, lepicrocite and hematite, and between 0.08 and 0.15 molecules/nm2 on rutile and anatase. The spectra in Figure 1 have been normalized with respect to the peak at 1198 cm-1. The spectral region shown (9501350 cm-1) is dominated by absorbances due to the symmetric and asymmetric S-O sulfonate stretching vibrations, the C-S stretching vibration, and coupled ring vibrations. On the basis of the assignments given by Varsa´nyi (26) for p-toluenesulfonic acid as well as Hyperchem PM3 (Hypercube, Inc.) calculations for benzenesulfonate, the peaks at 1198 and 1182 cm-1 are assigned to the asymmetric stretching vibrations of the sulfonate group. PM3 calculations for the five most intense vibrations between 1000 and 1300 cm-1 gave the following order in energy: two νa(S-O) > [νring + νa(S-O)] > ν(C-S) > νs(S-O). Figure 1 shows that interaction of DSBP molecules with the oxide layers causes only minor shifts in the vibrational energies, although the relative intensities of the various peaks are strongly affected. Upon contact with oxide layers, the peaks at 1198 and 1182 cm-1 broaden and shift to slightly (70:1). Thus, adsorption to ZnSe and possibly ZnSe-mediated photodegradation did not contribute significantly to the observed signal. Irradiation Setup. A monochromator (AMKO) and a UG1 UV filter (Schott) were used to select the 365 nm wavelength light from a 100 W mercury-xenon compact arc lamp (HBO100W, Osram). The light beam was homogenized by diffuse reflection off an aluminum foil and refocused with a semicylindrical BK7 glass lens. The light intensity on the 4 cm2 of irradiated surface area was between 1 and 2 mW/ cm2, as measured by potassium ferrioxalate actinometry (24). For comparison, the flux of incident solar energy below 400 3172
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Results and Discussion
going from the dry disodium salts of DSPB to aqueous solutions. The other peaks display only small shifts (
