Desorption Kinetics of Trichloroethylene from Powdered Soils

Environ. Sci. Techno/. 1995, 29, 1564- 1568

Desorption Kinetics of Trichloroethylene from Powdered Soils ABDELLATIF FARES, BENJAMIN T. KINDT, PETER LAPUMA, AND GLEN P. PERRAM* Department of Engineering Physics, Air Force Institute of

Environ. Sci. Technol. 1995.29:1564-1568. Downloaded from pubs.acs.org by UNIV OF NEBRASKA-LINCOLN on 09/14/15. For personal use only.

Technology, Wright-Patterson Air Force Base, Ohio 45433- 7765

The short-term desorption of trichloroethylene (TCE) from standard powdered soils including limestone, flint clay, glass sand, plastic clay, and soda feldspar was examined using an infrared absorption technique. The vapor-phase concentration of TCE as a function of desorption time is adequately described by the Langmuir kinetic mechanism. The desorption is rather rapid, with the vapor phase reaching a near steadystate value in several hours. The desorption rates are independent of the resident time for laboratory exposure of the soil to TCE in the range of 191350 h. The rate coefficients for desorption exhibit an Arrhenius temperature dependence with an activation energy of 0.37 f 0.03 eV and are linearly dependent on the surface area to volume ratio for particle sizes up to 10pm. The cross-section for optical absorption at A = 3.2 p m was determined to be CJ = 3.34 f 0.01 x cm2.

Introduction Trichloroethylene (CZHCld is present in 35% of U.S. Superfund sites, making it the most commonly detected volatile organic compound (VOCj(1,2). Air injection and extraction wells have regularly been used for remediation; however, the efficiency of such techniques often decreases with time (3). Desorption of the organic compounds from the soil matrix is likelythe rate-limiting step in this process (4,5). In addition, transport ofVOCs within the soil vadose zone depends strongly on the sorption of these compounds within the soil (6).Desorption may also explain the longterm evolution of contaminants from soils when remediated by incineration (7). While there is considerable information regarding the partitioning of trichloroethylene (TCE) among vapor, aqueous, and sorbed phases in equilibrium (21, considerably less information is available regarding the kinetics of desorption (6, 8-18). The mechanism for desorption apparently involves two distinct time scales: (1) a rapid desorption within about 24 h attributable to surface desorption and (2) a much slower phase extending from days to years (8-10, 17, 18). The slower desorption has been attributed to diffusion from within the interior structure of the soil matrix (17). In dry soils, surface area provides a primary measure of adsorption capacity, and physical adsorption at surface mineral sites dominates partitioning with organic matter (8, 12, 13, 18). Partition coefficients have also been reported as correlated with soil oxygen content (15). Water vapor competes favorablywith the nonpolar TCE for sorption sites (12-141, and the gasphase partition coefficients for TCE are several orders of magnitude larger than the corresponding aqueous-phase partition coefficients (11). While the adsorption kinetics of some organic species have received modest attention, the desorption kinetics oflow molecular weight halocarbons is largely unstudied. The influence of soil properties such as surface area, organic and water content, presence of surface metal oxides, soil type, soil pH, contamination residence time, and temperature on the desorption rates are poorly understood and currently must be evaluated on a system-specific basis (10). In the present study, we seek to determine the shortterm rates for desorption of TCE from powdered soils and to investigate the dependence on temperature, residence time, soil type, and soil size. These studies are limited to short-term laboratory contamination of highly processed soils and, thus, are not directly applicable to field samples. However, the control of individual iniluences on desorption rate and the establishment of a significant kinetic data base should enable the development of predictive models for realistic conditions.

Experimental Section Infrared absorption techniques are widely used to quantitatively assess the concentration of gas-phasespecies using the Beer-Lambert law (19). Apparently, no application of this technique to the desorption of VOCs from soils has previously been reported. In the present study, the * E-mail address: [email protected]; (5 13) - 255- 20 12 (voice); (5 131-255-2921 ( f a ) .

1564 a ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 6, 1995

This article not subject to U.S. copyright. Published 1995 by the American Chemical Society.

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FIGURE 3. Cross-section for optical absorption at 1 = 3.2 pm, corrected for bandpass of infrared detection system.

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FIGURE 1. Schematic diagram of the experimental apparatus: (a) optical configuration for monitoring TCE gas-phase pressure using infrared absorption technique; (b) desorption cell for handling contaminated soils under vacuum.

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Frequsncy (cm-') FIGURE 2. Absorption spectrum of TCE near 1 ,= 3.2 p m (top) and transmission of bandpass interference filter (bottom).

evolution of TCE from the sorbed phase to the gas phase was monitored by infrared absorption using the experimental apparatus shown schematically in Figure 1. The optical system is presented in Figure la. The hot filament of an Oriel quartz halogen lamp was used as a

source of infrared radiation, approximatelycollimatedwith an f = 500 mm CaF2 lens for transmission through two desorption cells via a pellicle beam splitter and gold-coated mirror. The intensity of the lamp transmitted through the desorption cellswas monitored using liquid nitrogen-cooled photovoltaic indium antimonide detectors. The spectral range of the detection system was limited to v = 30403 140cm-l with BarrAssociates bandpass interference filters. Phase-sensitive detection using a Stanford Model SR540 chopper and Model SR510 lock-in amplifiers was employed to isolate background IR emission and to integrate the detector signals for improved signal-to-noise ratios (20). The incident lamp intensity was also monitored using a Hammamatsu silicon photodiode to correct for temporal variation. The two desorption cells were 12 mm diameter by 43.5 cm long Pyrex tubes with 2.5 cm diameter CaF2windows on the ends, as shown in Figure lb. The cells could be evacuated to 0.1 mTorr and exhibited a leak rate of less than 0.7 mTorrlh. Cajon ultra-Torr vacuum fittings with Kevlar O-rings and Kontes glass valves with glass plugs were used to minimizepossible sorption ofTCE by the apparatus. The cells were always evacuated for at least 12 h between experiments, which was sufficient to eliminate any subsequent desorption from the cell. Cell pressures were monitored with MKS Models 221A and 122A 100 Torr capacitancemanometers, and gas-phasetemperatures were recordedwithK-type thermocouples. The cell temperature was controlled with heat tape in the range T = 25-100 "C to within f 2 "C. All intensity, pressure, and temperature data were recorded as a function of time using a 16-bit analog-to-digital computer acquisition board. Standard reference materials from the National Institute of Standards were used for the soil samples in these experiments: dolomitic limestone (88b1, flint clay (97b), glass sand (81a),soda feldspar (99a),plastic clay (98b),San Joaquin soil (2709),and Montana soil (2710). These were oven-dried, sieved, and blended. Particle sizes were quite uniform as analyzed by inspection using a microscope. Soil samples were saturated with liquid TCE (Fisher, spectrophotometric grade) for 19-1350 h prior to the desorption experiments. A 2.0 f 0.1 g soil sample was transferred to VOL. 29, NO. 6,1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY

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Time (Minutes) FIGURE 4. Vapor-phase pressure of TCE as a function of time for desorption at T = 25 " C ( 0 )Montana soil, ( 0 )dolomitic limestone, and (a)flint clay.

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Time (Minutes) FIGURE 6. Pressure of TCE as a function of time for desorption from plastic clay at: ( 0 )T = 64 "C. (0)T = 42 "C, and la) T = 25 "C. TABLE 1

TCE Sorption Rate Coefficients I

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I 60

120

180

240

Time (Minutes)

FIGURE 5. Dependence of desorption rate on residence time for exposure of flint clay to liquid TCE (MI 55 d and (0)20 h.

the desorption cells and deposited in a thin layer along the full length of the cell. The cells were then evacuated for ahout 30-60 s to remove any TCE vapor from the cell and to dry the soil particles. Thus, vaporization of liquid TCE didnot contribute to theconcentrationofTCEinthevapor phase. The amount of desorption that occurs during this short evacuation process is minimal, as desorption occurs over a period of about 1 h. Other initial conditions for examining adsorption kinetics in this apparatus could include the use of uncontaminated soil (an initial condition of no sorbed phase TCE) and about 50 Torr of TCE in the vapor phase. The vapor pressure ofTCE at T = 20 "Cis 58 Torr (2). The data would then consist of monitoring the decrease in TCE vapor as adsorption to the soil proceeds. Such experiments were not conducted in the present work. 1566 m ENVIRONMENTAL SCIENCE &TECHNOLOGY I VOL. 29. NO. 6.1995

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The infrared absorption spectrum ofTCE was recorded at 0.02 cm-I resolution using a Bomem DA-8 Fourier transform spectrometer. The spectrum is compared to the bandwidth of the interference filter in Figure 2. Note that the filter bandwidth is slightly larger than the TCE ahsorption feature, which complicates the determination of absolute TCE vapor concentrations. The cross-section for optical absorption at . I= 3.2 pm was determined by recording the InSh detector intensity as a function of total pressure for pure TCE vapor, as shown in Figure 3. Beer's law for absorption must he modified to account for the bandwidth of the detection system:

where a is the cross-section for absorption at I, = 3.2 pm; 1 is the length of desorption cell = 43.5 cm; [TCEI is the gas-phase trichloroethylene concentration; I is the transmitted intensity at InSb detector; and a is the fractional intensitytransmitted as [TCEI Aleast-square fit ofeq 1 to the data of Figure 3 provides u = 3.34 f 0.01 x cm2 and a = 0.36 % 0.02. This agrees favorably with the comparison of the TCE absorption feature and the handwidth of the interference filter as shown in Figure 2, which

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FIGURE 7. Arrhenius plot for the temperature dependence of the desorption rate from plastic clay, yielding activation energy of 0.37 i 0.03 eV. would suggest a = 0.33, assuming rectangular transmission

functions. The TCE detection limit in these experiments is 0.02 Torr. Further detail on the experimental apparatus may be found in refs 21-23.

Results and Discussion Severaltemporal profiles for the evolution of TCE from the sorbed phase to the gas phase are shown in Figure 4. The desorption is relatively rapid during the first hour and reaches a near steady-state value substantially below the TCE vapor pressure. Furthermore, significant concentrations of TCE remain in the soil, as evidenced by further rapid desorption after a second or third evacuation of the cell. Thus, we interpret the limiting TCE pressure of 2-10 Torr in Figure 4 to result from a steady-state condition established between desorption and re-adsorption. The previously reported biphasic desorption pattern (10, 18) is not evident in the relatively short period, less than 10h, of the current data. This is consistentwith the previous studies (10)where long-term desorption ofTCE from fieldcontaminated soils persisted for at least 168 h. Even then, 49-94% of the contaminant remained in the sorbed phase. Of the total TCE desorbed, 80% was accumulated in the first 24 h. No significant variation in the desorption data was observed as the residence time for the soil sample exposure to liquid TCE was increased from 19 h to 56 d. A comparison of the temporal profiles for desorption from flint clay at room temperature for several exposure times is shown in Figure 5. In no case did the observed TCE pressure vary by more than 0.5 Torr. The error bound in Figure 5 is an estimate of the effects of temporal variations in detector sensitivity. Apparently, the sites for bonding of TCE to the soil surface are rapidly saturated. To extract desorption rate coefficients from data similar to that presented in Figures 4 and 6, we use the Langmuir kinetic mechanism (24): TCE

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TCE

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FIGURE 8. Linear correlation of desorption rate at T = 25 "C with surface area to volume ratio or inverse of soil particle diameter.

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-S- represents a surface site, either occupied or unoccupied by aTCE molecule. The desorption and adsorption rate coefficients are denoted k d and k,, respectively. This mechanism predicts the time dependence for the gas-phase TCE concentration, assuming no gas-phase TCE at t = 0 and complete saturation of the available binding sites as

+

where y z = kd2B2 4kdkd; ,8 is the cell volumelnumber of surface binding sites = a,/(e$; a,is the TCE cross-sectional area = 30.6 A2 (ref 13);e is the soil mass density; and S is the soil surface area per unit mass. The TCE temporal profiles exhibit two dominant features: (1) the linear increase in TCE pressure at early times, [TCE], = kdt, and (2)the limiting steady-state TCE pressure for desorption times greater than 1 h, [TCEI, zz 2kd/(y f ,8kd). As a result, the temporal profiles do not provide sufficient detail to precisely define all three fit parameters, k,, kd, and ,8. By noting that k, >> k&, eq 3 approximates to

which exhibits onlytwo independent fit parameters, k d and Ska. Nonlinear least squares fits of eq 4 to the TCE temporal profiles are shown in Figures 4 and 6. The Langmuir mechanism appears to provide an adequate representation of the data. Similar results were obtained for each of the soil types and for soil temperatures in the range 25- 100 "C. The resulting desorption rate coefficients, kd,and adsorption parameters, Bka, are summarized in Table 1 and Figures 7 and 8. The reported error bounds are based on the 95% confidence limits in the fit parameter. The rate coefficients reported in Table 1agree favorably with previously reported partition coefficients for vapor VOL. 29, NO. 6 . 1 9 9 5 / ENVIRONMENTAL SCIENCE &TECHNOLOGY

1

1667

sorption of TCE to soils (12,131. The linear portion of the Langmuir isotherm may be expressed as 8(Slus)= K,[TCEI,

(5)

where Kpis the partition coefficient(mL/g). For our limiting case, k, >> k d , the relationship between sorption rate coefficients and the partition coefficient is

For example, the vapor partition coefficient for TCE with kaolinite clay is 240 88 mL/g (12). The BET surface area for this soil is 8.47 m2/g, which provides the ratio of adsorption and desorption rate coefficients @ka/kd)”2 = 0.27. This agrees favorably with the current result for the flint clay of @ka/kd)”’ = 0.22. The rate coefficient for desorption of TCE from plastic clay, kd, exhibitsa strong temperature dependence as shown in Figure 6 . At higher temperatures, the TCE pressure rises more rapidly and reaches a higher steady-state value. A fit of eq 4 to this and similar data yield the desorption rate coefficients as a function of temperature shown in Figure 7. To estimate the binding energy of TCE to the surface of the plastic clay, an Arrhenius temperature dependence is assumed:


*

(7) where kis the Boltzmann’s constant;A is the rate coefficient amplitude; and Eais the activation energy. A fit of the data in Figure 7 to eq 5 provides an activation energy for desorption of E, = 0.37 k 0.03 eV and rate coefficient amplitude ofA = 1.1f 0.8 x lo3Torr/% This result agrees favorably with previous studies of the temperature dependence of the partition coefficient for sorption of TCE to quartz sand (25)and several oven-dried soils (12,13)that were analyzed using the van’t Hoff equation to yield heat of sorption in the range 0.3-0.8 eV. The temperature dependence of the desorption rate coefficientsfor the other soils used in this study were not investigated. The adsorption rate coefficient exhibited no significant dependence on temperature, and an Arrhenius plot for @ yields an activation energy of -0.13 i 0.16 eV. This is consistent with adsorption to external surface sites where one would expect little or no barrier to adsorption. The observed desorption rate coefficients also exhibit a linear correlation with the ratio of particle surface area to volume or inverse of particle diameter as shown in Figure 8. This result is indicative of desorption from the surface of the soil particles rather than diffusion from interior sorption sites. Note that the extrapolation to large particle diameters, typical of natural soils,cannot continue the linear trend as Figure 8 indicates a negative intercept. The linear correlation is particularly good considering the variation in mineral content for the different soil types. This result is consistent with previous studies of the partition coefficients for TCE vapor on oven-dried soils that exhibited a linear dependence on BET surface area, with a correlation coefficient of 3 = 0.94 (12).These results contrast with an extensive study of the long-term desorption of TCE from field-contaminatedsoils,which found no correlation of rate or extent of desorption with soil type or surface area (10). This optical approach for continuous monitoring ofVOC desorption from soils appears quite promising. Application of the technique to examine the long-term (days)desorption 1568

kinetics, the presence of water vapor in competition for sorption sites, and the influence of mineral content on the nature of binding sites is in progress.

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 6 . 1 9 9 5

Conclusions The evolution of VOCs from the sorbed to gas phase is readily monitored using infrared absorption techniques. The short-term kinetics, less than 24 h, are apparently dominated by TCE desorption from surface sites. These surface sites become saturated when exposed to liquid TCE in less than 19 h, and further residence time for laboratory contamination does affect the desorption kinetics. The binding of TCE to these surface sites for plastic clay is relativelyweak, about 7.7 kcal/mol. The Langmuir kinetic mechanism, including desorption and subsequent readsorption, adequately represents this data. The desorption rate is strongly dependent on soil properties, with a linear dependence on inverse particle size.

Literature Cited (1) Travis, C. C.; MaCinnis, J. M. Environ. Sci. Technol. 1992, 26,

1185. (2) Siegrist, J. M. J. Hdzard. Mater. 1992, 29, 3. (3) Goltz, M. N.; Oxley, M. F. Water Resour. Res. 1991, 27, 547. (4) Mackay, D. M.; Cherry, J. A. Enuiron. Sci. Technol. 1989,23,630. (5) Ball, W. P.; Roberts, P. V. Environ. Sci. Technol. 1991, 25, 1237. (6) Steinberg, S. M.; Kreamer, D. K. Environ. Sci. Technol. 1993,27, 883. (7) Tognotti, L.; Flytzani-Stephanopoulos, M.; Sarofim, A. F.; Kopsinis, H.; Stoukides, M. Enuiron. Sci. Technol. 1991, 25, 104. (8) Estes, T. J.; Shah, R. V.; Vilker, V. L. Environ. Sci. Technol. 1988, 22, 377. (9) Pavlostathis, S. G.; Jaglal, K. Environ. Sci. Technol. 1991,25,274. (10) Pavlostathis, S. G.; Mathavan, G. M. Environ. Sci. Technol. 1992, 26, 532. (11) Peterson, M. S.; Lion, L. W.; Shoemaker, C. A. Environ. Sci. Technol. 1988, 22, 571. (12) Ong, S. K.; Lion, L. W. J. Environ. Qual. 1991, 20, 180. (13) Ong, S. K.; Lion, L. W. Soil Sci. SOC.Am. J. 1991, 55, 1559. (14) Thibaud, C.; Erkey, C.; Akgerman, A. Enuiron.Sci. Technol. 1993, 27, 2373. (15) Garbarini, D. R.; Lion, L. W. Enuiron. Sci. Technol. 1986, 20, 1263. (16) Chiou, C. T.; Shoup, T. D. Enuiron. Sci. Technol. 1985,19, 1196. (17) Grathwohl, P.; Reinhard, M. Enuiron. Sci. Technol. 1993,27,2360. (18) Sawhey, B. L.; Gent, M. P. N. Clays Clay Miner. 1990, 38, 14. (19) Svanberg, S. Atomic and Molecular Spectroscopy, 1st ed.; Springer-Verlag: New York, 1991; p p 131-144. (20) Demtroder, W. Laser Spectroscopy,2nd ed.; Springer-Verlag: New York, 1981; pp 380-382. (21) Fares, A. Air Force Institute of Technology Thesis, AFITIGEEI ENPI94S-01, Wright-Patterson AFB, OH, 1994. (22) Kindt, B. T. Air Force Institute ofTechnology Thesis, AFITlGEEI ENPi94S-02, Wright-Patterson AFB, OH, 1994. (23) LaPuma, P.Air Force Institute ofTechnology Thesis, AFITIGEEI ENPI94S-03, Wright-Patterson AFB, OH, 1994. (24) Laidler, K. J. ChemicalKinetics,2nd ed.; McGraw-Hill: NewYork, 1965; pp 256-265. (25) Kreamer, D. K.; Oja, K. J.; Steinberg, S. M.; Phillips, H. J. Environ. Eng. 1994, 120, 348.

Received for review September 23, 1994. Revised manuscript received March 2, 1995. Accepted March 7, 1995.’ ES940597G Abstract published in Advance ACS Abstracts, April 15, 1995.

Desorption Kinetics of Trichloroethylene from Powdered Soils

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