Photoinduced Decomposition of Trichloroethylene on Soil
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Environ. Sci. Technol. 1999, 33, 74-80 extensive research has been reported on the photoassisted Photoinduced Decomposition of decomposition of TCE on TiO2(s) photocatalysts as an alternative environmental remediation method to destroy Trichloroethylene on Soil TCE efficiently (8-17). In the reported studies, the photo- Components catalytic oxidation of TCE in low concentration (ppm level) was investigated at TiO2(s) interfaces with the gas phase (including air or oxygen), or with an aqueous solution, by TING TAO, JANE J. YANG, AND such instrumental methods as IR (11-13, 16), GC (16, 17), GARY E. MACIEL* and MS (13, 14, 16). The ªtotal mineralizationº of TCE to Department of Chemistry, Colorado Sate University, CO2 and HCl has been reported for some conditions (18, 19). Fort Collins, Colorado 80523 Intermediate products have also been observed during the photodegradation of TCE on TiO2(s) catalysts; among them are dichloroacetic acid, dichloroacetyl chloride, dichloro- acetylaldehyde, trichloroacetic acid, trichloroacetylaldehyde, The photoinduced decomposition of trichloroethylene phosgene, and other products (11-17). Various mechanisms adsorbed on Ca-montmorillonite by long-wavelength UV have been proposed for TCE photooxidation on a TiO2(s) irradiation has been studied in a quartz tube open to air or surface. Some researchers have suggested that hydroxyl through which air or oxygen is passed. Solid-sample radicals or hydroperoxide radicals initiate the decomposition and liquid-solution NMR techniques were used to identify of TCE (12, 19). Other researchers have suggested that TCE apparent products or intermediates of the photodecom- is oxidized in chain reactions initiated by chlorine atoms position. Dichloroacetic acid was identified as a major (13, 14, 20) or that TCE is oxidized on TiO2(s) by electronically organic product/intermediate; substantial amounts of activated O2 (11). Recently, Raftery and co-workers (21,22) 13 pentachloroethane and trichloroacetic acid were also reported an in situ solid-state C NMR (nuclear magnetic 13 identified. The formation of CO was characterized resonance) study of the degradation of C-labeled TCE, in 2 which they characterized the reactions of TCE over TiO (s) quantitatively by wet chemical analysis. About 40% and 2 photocatalysts and identified reaction intermediates and 57%, respectively, of the total carbon of trichloroethylene products in the complex surface chemistry. was converted to carbon dioxide in air and O2 environments High-resolution solid-state NMR techniques (23) have over a period of 16 days. Phosgene and HCl were also begun to emerge among the host of investigative methods detected. The photodecomposition of trichloroethylene used in elucidating the interactions of organic pollutants adsorbed on whole soil, on Zn2+-exchanged and Cu2+- with soil or its major components (24-26). The orientation exchanged montmorillonites, on kaolinite, and on silica gel of the study reported here is not the search for improved was also examined in less detail; qualitatively, the remediation methodology but rather to find out what conversion of trichloroethylene to dichloroacetic acid in a photoactivated processes might occur with TCE at the air- 48-h period occurred with the following order of decreasing soil interface due to sunlight. The major focus of the work efficiencies: Zn2+-montmorillonite > silica gel > kaolinite > presented here was to study the chemical behavior of trichloroethylene on Ca-montmorillonite and, to lesser 2+ > > 2+ Ca -montmorillonite whole soil Cu -montmorillonite. extents, whole soil and other adsorbents, induced by ir- These results show that the photoinduced decomposition radiation by long-wavelength UV light under conditions of adsorbed trichloroethylene occurs on a variety of similar to what might be experienced by TCE due to sunlight adsorbents, generating products and intermediates that at the air-soil interface. Because the TCE that was used in 13 are similar to what have been reported previously for TiO2- this study contains C in only its natural abundance (1.1%) based photodecomposition but with much longer time and because of the relatively low inherent sensitivity of NMR, scales. These conversions can, therefore, be expected to this study employed much larger loading levels than one occur in sunlight at the air-soil interface. expects in naturally occurring samples in the environment. One can expect that many, if not most, of the chemical transformations observed in this study will also occur at much lower (e.g., environmentally ªrelevantº) TCE concentrations, Introduction especially since similar product/intermediate species have Trichloroethylene (TCE), an Environmental Protection Agency been reported from previous studies based on other tech- (EPA) priority pollutant (1), has been used in a wide range niques and much lower TCE loading levels; however, the of industrial processes, e.g., for degreasing metals and for results of this paper cannot be extrapolated quantitatively dry cleaning (2). As a result of its leaking from underground to lower TCE levels until the relevant kinetic orders are storage tanks and improper disposal practices, TCE has been determined. We believe that the results reported here can identified in at least half of the sites of the National Priorities provide useful information on important aspects of the List (Superfund sites) (3, 4) and represents one of the most qualitative chemical behavior of a contaminated site. This frequently detected contaminants of national concern in study shows that the same kind of photoinduced products hazardous waste sites (5). Consequently, it is important to that have been reported previously from TiO2-based pho- know the chemical behavior and fate of TCE in the environ- todecompositions can also be generated photochemically ment. One aspect of this complex issue is photochemical from trichloroethylene adsorbed on soil components present behavior of TCE at the air-soil interface. at the soil-air interface, albeit on a slower time scale. In addition to the development of bioremediation tech- niques aimed at degrading TCE in groundwater (6, 7), Experimental Section Materials. Trichloroethylene (ACS reagent grade) was * Corresponding author phone: (970)491-6480; fax: (970)491-1801; purchased from Fisher Chemical Company and used as e-mail: [email protected]. received without further purification. Ca-montmorillonite 74 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 1, 1999 10.1021/es980435a CCC: $18.00 1998 American Chemical Society Published on Web 11/04/1998 (STx-1) and kaolinite (KGa-1b) were purchased from the Clay and the exit gas was analyzed. In the air case, before the air Minerals Repository of the University of Missouri at Columbia entered the quartz reaction vessel, it was bubbled through and used as received. The chemical composition (wt %) of a trap containing a saturated aqueous Ba(OH)2 solution to the Ca-montmorillonite is SiO2, 70.1; Al2O3, 16.0; TiO2, 0.22; remove CO2 in the air and then passed through a tower of Fe2O3, 0.65; FeO, 0.15; MnO, 0.009; MgO, 3.69; CaO, 1.59; sodium hydroxide pellets to remove any remaining CO2 and Na2O, 0.27; K2O, 0.078; P2O5, 0.026; S, 0.04; and F, 0.084. The to dry the air before being introduced via an adjustable flow chemical composition of the kaolinite is SiO2, 44.2; Al2O3, valve into the reaction vessel. On the exit side of the quartz 39.7; TiO2, 1.39; Fe2O3, 0.13; FeO, 0.08; MnO, 0.002; MgO, reactor tube, the gas (air or O2, plus volatile reaction products) 0.03; Na2O, 0.013; K2O, 0.050; P2O5, 0.034; and F, 0.013. The was passed through a series of traps in the following water contents of the Ca-montmorillonite (STx-1) and sequence: (a) aqueous 1.0 M AgNO3 solution (pH ) 1) to kaolinite (KGa-1b) employed were 15% and 10%, respectively, detect HCl via AgCl(s) formation; (b) two successive traps determined by weight loss upon dehydration at 150 °C under with saturated aqueous Ba(OH)2 solution to detect CO2 via -3 vacuum (10 Torr). The Clay Minerals Repository provided formation of BaCO3(s), and (c) a final trap containing silicone the following information on the clays: The Ca-mont- oil to minimize diffusion of atmospheric CO2 back into the morillonite has a surface area of 83.8 m2/g with 200-300 system. In a separate set of experiments, two successive mesh particle size, and the kaolinite has a surface area of traps with aqueous aniline solution (0.02 M) were used to 10.1 m2/g with 200-300 mesh particle size. The interlayer detect phosgene via the formation of 1,3-diphenylurea. space is 15.17 Å for Ca-montmorillonite and 7.11 Å for NMR Spectroscopy. Solid-state 13C NMR spectra were kaolinite, based on packed powder XRD results. The carbon obtained at 25.27 MHz by the DP-MAS technique (direct contents are less than 0.3% (∼0.2% CO2) in both the Ca- polarization-magic angle spinning, i.e., no cross polarization) montmorillonite and kaolinite. Zn2+-exchanged and Cu2+- with high-power 1H decoupling, on a home-built 100 MHz exchanged Ca-montmorillonites were prepared by stirring (1H frequency) spectrometer, using a Chemagnetics Phoenix 500 mL of 1.0 M ZnCl2(aq) and 1.0 M CuCl2(aq) solutions, data system, with a 90° pulse length of 6.5 µs and a recycle respectively, with 40.0 g Ca-montmorillonite (as received) at delay of 3 s. A Kel-F sample cell with an O-ring sealed cap 25 °C for 1 day. In each experiment, the resulting suspension was inserted into a Chemagnetics 14 mm PENCIL rotor and was divided into six portions; each portion was washed with was used at a spinning speed of about 3 kHz. Liquid-solution 250 mL of water and then centrifuged. The solids were dried 1H and 13C NMR spectra were obtained at 300.13 and 75.47 at 25 °C under vacuum (10-3 Torr).