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(18) Bjeldanes, L. F.; Chew, H. Mutat. Res. 1979,67,367-371. (19) Haworth, S.; Lawlor, T.; Mortelsmans, K.; Speck, W.; Ziegler, E. Environ. Mutagen. Suppl. 1983, 3, 3-142. (20) Kleindienst, T. E.; Shepson, P. B.; Smith, D. F.; Hudgens,
(9) Shepson, P. B.; Kleindienst, T. E.; Edney, E. 0.; Cupitt, L. T.; Claxton, L. D. Environ. Sei. Technol. 1985, 19, 1094-1098. (10) Lewis, C. W.; Baumgardner, R. E.; Stevens, R. K.; Claxton, L. D.; Lewtas, J. Environ. Sei. Technol. 1988,22,968-971. (11) Carter, W. P. L.; Lloyd, A. C.; Sprung, J. L.; Pitts, J. N., Jr. Int. J. Chem. Kinet. 1979, 11, 45-101. (12) Shepson, P. B.; Kleindienst, T. E.; Edney, E. 0.;Pittman,
E. E.; Nero, C. M.; Cupitt, L. T.; Bufalini, J. J.; Claxton, L. D. Environ. Mol. Mutagen. 1990, 16, 70-80. (21) Shepson, P. B.; Kleindienst,T. E.; Edney, E. 0.;Nero, C. M.; Cupitt, L. T.; Claxton, L. D. Enuiron. Sei. Technol. 1986, 20, 1008-1013. (22) Finlayson-Pitts,B. J.; Pitts, J. N., Jr. Atmospheric Chemistry; John Wiley and Sons: New York, 1986; p 350. (23) Shepson, P. B.; Kleindienst, T. E.; Nero, C. M.; Hodges, D. N.; Cupitt, L. T.; Claxton, L. D. Enuiron. Sei. Technol.
J. H.; Namie, G. R.; Cupitt, L. T. Enuiron. Sei. Technol. 1985,19, 849-854. (13) Shiraishi, F.; Hashimoto, S.; Bandow, H. Mutat. Res. 1986, 173, 135-139. (14) Victorin, K.; Skihlberg,M. Environ. Mol. Mutagen. 1988, 11, 79-90. (15) Catching our Breath; Office of Technology Assessment Report. U.S. Government Printing Office: Washington, DC, 1989. (16) Determination of C z to Clz Hydrocarbons in 39 U.S. Cities from 1984 through 1986; EPA/600/3-89/058; U.S. Environmental Protection Agency, Office of Research and De-
velopment, Atmospheric Sciences Research Laboratory, U.S. Government Printing Office: Washington, DC, 1989. (17) Lonneman, W. A.; Seila, R. L.; Bufalini, J. J. Enuiron. Sei. Technol. 1978,12,459-463.
1987,21, 568-513.
Received for review March 13,1991. Revised manuscript received August 8, 1991. Accepted August 22, 1991. The research described in this article has been funded wholly or in part by the U.S. Environmental Protection Agency through Contract 6800-0106 to ManTech Environmental Technology,Znc. Zt has been subjected to the Agency's peer and policy review and approved for publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.
I n Situ Investigation of 1,2-Dibromoethane Sorption/Desorption Processes on Clay Mineral Surfaces by Diffuse Reflectance Infrared Spectroscopy Yukiko 0. Aochi' and Walter J. Farmer
Department of Soil and Environmental Sciences, University of California, Riverside, California 92521 Brlj L. Sawhney The Connecticut Agricultural Experiment Station, New Haven, Connecticut 06504-1 106
SorPtion/deSorPtion Processes for ,2-dibromoethane (EDB) at clay surfaces were investigated in situ by of diffuse reflectance infrared spectroscopy* Spectra were during the sorption phase while Nz saturated with EDB flowed through the clay samples. Desorption processes Were similarly followed by flowing N2 alone through the samples after the sorption period was completed. The spectral results indicated the presence of EDB in two conformations, anti and gauche, within a liquid phase. The relative abundance of the two conformers was a function of sorption or desorption time as well as the particular clay used as the sorbent. The abundance of the gauche conformer, however, was consistently higher &ring both phases of the experiment, indicating preferential retention of this conformer. A conceptual model of a clay as a dielectric medium is presented to explain these results. The chemical and physical properties of the soil fumigant 1,2-dibromoethane (EDB), sorption coefficients determined by batch techniques, and microbial degradation studies are all consistent with a prediction of very low persistence for this chemical in the field. For this reason, the significant levels of EDB detected in Connecticut soils by Steinberg et al. (I) up to 19 years after the last known application were cause for considerable concern. The inadequacy of standard EPA methods for the extraction and determination of volatile organic compounds when applied to EDB residues in field samples as reported by these same investigators (2) further indicated that EDB was retained by soils much more strongly than would be predicted from its physicochemical properties. Their studies of EDB sorption/desorption behavior on soils from the fields where 0013-936X/92/0926-0329$03.00/0
contamination had been discovered led to the proposal that the unexpected persistence of EDB was caused by its entrapment in intraparticle micropores. Results of experiments conducted by these authors with different size fractions of the soils suggested that such intraparticle micropores are Present in all Particle sizes. In order to better understand the role of mineral fractions in Such an entrapment process, Sawhney and Gent (3) passed a Nz stream saturated with EDB vapor through glass columns filled with clay minerals of different surface and charge characteristics for different periods of time. Quantities of EDB sorbed as a function of time were determined by extraction of the EDB from the clay. All the clays were found to sorb significant quantities of EDB 3% by weight for pyrophyllite, 5% for kaolinite, 6% for illite, and 9% for smectite in a 16-h period. Similar results were obtained for trichloroethene. Sorbed quantities of either chemical were not well correlated with BET surface areas determined for the clays. Desorption data were obtained by monitoring EDB concentrations as nitrogen flowed through the columns for time intervals up to 24 h after a period of sorption. The data thus obtained were fit to a two-compartment efflux model developed by Sawhney and Gent (4). The results suggested that both sorption and desorption were comprised of a rapid and a slow component; the former was proposed to arise from EDB sorbed on external surfaces and the latter from EDB trapped in the intraaggregate micropores. The present investigation was undertaken to provide evidence a t the molecular level for the mechanisms by which EDB is retained on mineral surfaces and explore possible causes for the multiple stages in the sorption/
0 1992 American Chemical Society
Environ. Sci. Technol., Vol. 26,No. 2, 1992 329
desorption processes. Through the use of a diffuse reflectance accessory with a controlled environment chamber designed for use in a Fourier transform infrared spectrometer (FTIR), it was possible to provide a physical environment similar to that of the mineral columns of Sawhney and Gent ( 3 ) . The sample held within the accessory formed a small column of powdered material through which EBD-saturated vapor could flow from the bottom. Flow rates through the sample were controlled at the level used in the earlier experiments and activity at the clay surfaces could be followed in situ by collecting spectra at selected time intervals during the course of the experiment. If comparable sorption and desorption processes were indeed occurring, correlation of the spectral results with the macroscopic results of the column experiments could result in the identification of molecular species responsible for different rates of EDB sorption and desorption. The relative importance of organic and mineral fractions of the soil to sorption processes has been a matter of considerable controversy in recent years (5, 6). A series of investigations were conducted by Chiou and co-workers (7-11) to evaluate the contributions of these soil components to the retention and transport of nonionic organic molecules in the soil environment. These investigators concluded that both constituents can play an important role in sorption and retention. Under conditions of water saturation, sorption is dominated by a partitioning of the organic compound into soil organic matter. In dry and subsaturated soils, however, sorption of the organic molecules from the vapor phase onto mineral surfaces was shown to be more important than partitioning into organic matter (6). Materials and Methods
Clay minerals used in this study were obtained from Ward's Natural Science Establishment. They were illite from Fithian, IL (No. 35), smectite from Clay Spur, WY (No. 26), kaolinite from the Oneal Pit, Macon, GA (No. 4), and pyrophyllite from Robbins, NC (No. 49). Numbers in parentheses indicate American Petroleum Institute designations. A Model 8000 Spex mixer mill was used to grind the clays contained in an agate vial and ball assembly, after which they were sieved through a 45-wm stainless steel sieve. Samples of the same clay minerals had been used by Sawhney and Gent (3) to represent layer silicates with a range of surface characteristics. BET surface areas determined by those investigators for these clays were 43.4, 24.9,26.8, and 12.3 m2/g, respectively. EDB was obtained from Aldrich Chemical Co., Milwaukee, WI, and was greater than 99 % pure, Spectrophotometric grade KBr was obtained from Mallinckrodt Chemicals, St. Louis, MO. Spectra were obtained on a Mattson Cygnus 100 FTIR equipped with a mercury cadmium telluride detector (Mattson Instruments, Madison, WI). Single-beam spectra obtained by averaging 200 scans at 4-cm-l resolution were referenced against a KBr background. The diffuse reflectance accessory, Model DRA-2D, was obtained from Harrick Scientific, Ossining, NY, along with a controlled environment chamber, HVC-DRP. The chamber is designed with KBr windows that allow spectra to be taken at any time while a continuous flow of gases through the sample is maintained. Samples of sieved clay were prepared for diffuse reflectance by drying overnight at 105-110 "C. A weighed amount of the dried clay was combined with KBr to make up a 2% or 5% sample in the agate vial and the resultant mix was ground for 2 min in the mixer mill. Samples of wet clay were prepared by equilibration at 100% RH prior to mixing with the KBr. 330
Environ. Sci. Technol., Vol. 26,No. 2, 1992
The sampling cup of the accessory was overfilled with this mixture and then leveled using a razor edge. The chamber was purged with N2 at approximately 20 mL/min before the flow was adjusted to 1mL/min for the EDB treatment. All of the EDB/clay spectra presented here were obtained through the use of spectral subtraction. The spectrum of the sample taken immediately before EDB flow was initiated was subtracted from each subsequent spectrum in order to reduce interference from structural absorption bands of the clay. A subtraction factor of 1.0 was used in all cases. Similarly, compensation for contributions from vapor-phase EDB and EDB/ KBr interactions was achieved by subtracting the spectrum obtained when EDB flowed through KBr alone from the clay spectrum for the same time period. The same series of KBr spectra was subtracted from each clay series using a subtraction factor equal to the percent KBr. The gas-phase spectrum of EDB was obtained in a 10-cm A1 gas cell with KBr windows. A liquid transmission cell with 0.025-mm spacing and KBr windows was used for the liquid-phase spectrum. Intensities in diffuse reflectance spectra are most commonly described by using the Kubelka-Munk or remission function, f(R/Ro) = (1- R/Ro)2(2R/R,)-1, where R is the sample reflectance and Ro is the reflectance of a nonabsorbing reference at infinite depth. Under appropriate conditions this function can be related to concentration in the same way as absorbance in transmission spectra. The advantages of an alternative mode of presentation in terms of diffuse absorbance, -log (RIR,), are discussed by Smyrl et al. (12). In this mode, diffusely reflected radiation is detected as transmittance by the instrument and treated accordingly. This mode was selected for use in the present investigation because the spectral subtraction was better and the qualitative changes in the spectra were much more easily observed. As the intent of this paper is to investigate mechanisms rather than report measured quantities, use of the alternative mode was preferred. Thus, band intensities shown here are all in absorbance and should be considered only in relative rather than in absolute terms. Measurement of band intensities was facilitated by the use of spectral deconvolution. Deconvolution is a resolution-enhancement technique that involves the mathematical manipulation of the interferogram data, the raw spectral data from an FTIR, so that narrower bands result in the spectrum upon subsequent Fourier transformation. The theory behind deconvolution and the concepts used in the implementation of this technique in current instrument software was developed by Kauppinen et al. (13). A simplified conceptual description of the technique has been presented by Y.O.A. and W.J.F. in a previous publication (14). Since deconvolution is an interactive, empirical technique in which parameters are selected by the user based on the appearance of the spectrum, caution must be exercized in its application. In particular, the appearance of negative side lobes adjacent to absorption bands and spectral features for which there is no evidence in the original spectrum must be avoided. These guidelines were observed in selecting parameters for the current spectra. Results and Discussion EDB is a relatively simple molecule, yet its infrared spectra are surprisingly complex. The presence of a greater number of absorption bands in the spectra of EDB as well as other 1,2-dihaloethanes than would be predicted from molecular structure considerations prompted investigations by several research groups, primarily by Mizushima and co-workers in Japan (summarized in ref 15). From measurements of dielectric constants, band intensity ratios in
H
Table I. Vibrational Frequency (cm-') Assignments
Br
symm ref 20n liq type
liq
ref 17b Ar matrix
present work liq
assignmenta,*
3040 1086 750 2980 1438 1189
C-H stretch CH2 twist CH2 rock C-H stretch CH2 scissor CH2 wag C-Br stretch
3004 2954 1420 1277 1017 899
C-H stretch C-H stretch CH2 scissor CH2 wag C-C stretch CH2 rock C-Br stretch C-H stretch C-H stretch CH2 scissor CH2 wag CH2 twist CH2 rock C-Br stretch
Anti A, H \ J /
H
Fir
gauche
Er anti
B,
H
I'
3039 1086 751 2981 1438 1185 588
3037 2948 1419 1276 1017 898 552 3012 2963 1419 1245 1104 835 588
3003 2953 1419 1277 1018 899 553 3017 2953 1419 1246 1103 836 588
1080 751 2989 1445 1184 Gauche
A
SY n
Flgure 1. Rotational conformers of EDB.
Raman and infrared spectra, and electron diffraction measurements, these investigators concluded that 1,2-dihaloethanes exist predominantly in two isomeric conformations, anti and gauche, resulting from internal rotation about the C-C bond. These two conformers, along with the syn conformation to distinguish it from the two staggered conformations, are presented in Figure 1. The relative abundance of the two stable conformers was found to be highly dependent on the temperature and physical state of the molecule. For example, the relative abundance of gauche to anti forms was 0.34:l for 1,2-dichloroethane vapor at its boiling point and 1.3:l for its liquid at 25 "C. Because the energy difference between the anti and gauche forms was calculated to be larger for EDB compared to its chloro analogue, the relative abundance of the anti conformer was expected to be greater for this molecule at all temperatures (16). The existence of 1,2-dihaloethanes in anti and gauche forms that vary in relative abundance continues to be widely accepted (17-19). Much of the physicochemical behavior of this group of chemicals could be explained by the coexistence of the rotational conformations, and the results reported here can be similarly understood. Band assignments for 1,2-dihaloethanes to specific vibrational modes has continued to be the object of investigations since this early work. More current studies have utilized isotopic substitution, phase/ temperature dependence of conformer populations, and theoretical normalmode analysis as primary tools. Vibrational assignments resulting from such an investigation were made by Tanabe et al. (20),and their values are tabulated in Table I. These assignments are in close agreement with other reported values (21,22) with respect to both the band frequencies and the vibrational mode assignments. Recently, Bose et al. (17) confirmed these assignments with few modifications through the use of ab initio vibrational analyses in conjunction with matrix isolation spectra. Matrix-isolated spectra result from vapor-phase molecules isolated in an inert low-temperature matrix. Molecules thus isolated are expected to exist in the same conformation as in the vapor phase (17) but are prevented from interacting with like molecules. Observed band widths are thus reduced dramatically. Values reported in this latter study for both liquid and matrix-isolated phases are also included in Table I. Spectra are presented in Figure 2 for liquid- and vapor-phase EDB at room temperature in the region from 1800 to 1100 cm-l. On the basis of the literature values presented in Table I, bands located at 1438 and 1189 cm-l in the liquid-phase spectrum arising from CH2scissor and wag modes, respectively, are assigned to the anti form. The remainder of the bands at 1420,1277, and 1246 cm-l are assigned to vibrations of the gauche form. These three
3037 1086 748 2974 1437 1187 588
B
3018 2954 1420 1247 1103 836
1251
"Tanabe et al. (20). *Base et al. (17).
w u Z a m CY D Ln
m
