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Outdoor experiments on enhanced volatilization by venting of kerosene component from soil
Journal of Contaminant Hydrology, 12 (1993) 355-374 Elsevier Science Publishers B,V., Amsterdam
355
Outdoor experiments on enhanced volatilization by venting of kerosene component from soil P. F i n e a n d B. Y a r o n
Department of Soil Organic and Residues Chemistry, Institute of Soils and Water A RO, The Volcani Center, IL-50250 Bet Dagan, Israel (Received April 1, 1992; revised and accepted November 30, 1992)
ABSTRACT Fine, P. and Yaron, B., 1993. Outdoor experiments on enhanced volatilization by venting of kerosene component from soil. J. Contam. Hydrol., 12: 355-374. The effect of soil properties on the retention of kerosene in soils, at equilibrium and under venting, was studied. Eleven soils were studied, which represent a wide range of chemical properties and mechanical composition. The retention of kerosene in dry soils ranges from 3.5 to 18.1 mL/(100 g), and was related linearly to clay, silt and organic matter (OM) contents. A coarsely-aggregated dry vertisol (2-5 mm aggregates) retained half as much kerosene as its finely-aggregated ( < 2 mm) counterpart. Moisture content had a strong inverse effect on kerosene retention. The soil factors that inversely affected kerosene retention also enhanced kerosene stripping by venting. Of these, soil aggregation and porosity were the most important. In addition, kerosene volatilized faster and more completely from an initially moist soil, as compared with an initially dry soil. Differential volatilization of lighter components of kerosene changed the chemical composition of the residue in the soil substantially, as compared with the initial composition.
1. I N T R O D U C T I O N
Petroleum products are mixtures of hydrocarbons with differing vapor pressures. Thus, each petroleum fraction contains compounds with a wide range of volatilization properties. The rate of volatilization of hydrocarbons to the atmosphere from a soil-petroleum mixture will be affected by soil constituents and soil hydration status. Ample information is available on the volatilization of compounds of high and intermediate vapor pressure from soils (e.g., Spencer et al., 1973; Jury et al., 1984, 1987), but less research has been reported on the volatilization from soil of petroleum hydrocarbons. ~Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel. No. 3667-E, 1992 series. 0169-7722/93/$06.00
© 1993 Elsevier Science Publishers B.V. All rights reserved.
356
P. FINE AND B, YARON
Karimi et al. (1987) recently showed that the vapor-phase diffusion of benzene in the soil is greatly reduced by increasing soil bulk density and soil-moisture content. Existing results refer primarily to volatilization of single compounds from soil. However, each petroleum product is a mixture of hydrocarbons, with a range of physico-chemical properties (Yaron, 1989). We recently studied the rate of volatilization of hydrocarbons in simulation mixtures which were applied to soil surfaces. The volatilization of C9 to CI5 hydrocarbons from sandy and loamy sand soils was greater than from a silt loam (Galin et al., 1990a, b). This was ascribed to the influence of differences in the pore-size distributions (or ineffective porosity) on the rates of volatilization of the light carbon fractions (C9-CI2) in the simulation mixtures. In principle, enhanced volatilization is straightforward; the vapor-phase concentration of the volatile hydrocarbons is reduced by accelerating the flow of air through the porous vadose zone. This is done by applying vacuum, pressure or a combination of both to a grid of well-pumping systems (Bowman, 1989). The usefulness of this technique has been proven on a pilot scale in the U.S.A. (J.J. Johnson and Sterrett, 1988; P.C. Johnson et al., 1988; Bowman, 1989; R.L. Johnson, 1989; Wilson, 1989; DiGiulio et al., 1990) whereby polluted porous media were cleaned of organic hydrocarbons. The application of this technique was usually intended to reduce threats to groundwater posed by vadose zone contamination. Modelling of the removal of a mixture of volatile organic compounds from porous media has been widely attempted (Roberts and Dandlliker, 1983; Baehr, 1987; Baehr and Hoag, 1987; Baehr et al., 1989; Gierke et al., 1992). However, not much experimental work has been published on soil criteria for enhanced hydrocarbon volatilization. Most of the research has been sitespecific, related to the cleanup of actual vadose-zone contamination. Recent results (Hoag and Cliff, 1988; Baehr et al., 1989; Donaldson et al., 1992) have demonstrated the successful application of the venting technique to remedy gasoline contaminated soils; they should encourage the adaption of soil venting on a larger scale. More data are required on the effects of soil factors on the remediation technique. The work reported here is intended to provide better understanding of the effects of soil properties and soil moisture on the volatilization of hydrocarbons under enhanced venting, starting from a soil contamination level of residual saturation. The results can be applied to the design of clean-up systems and to the prediction of the removal of kerosene components in the field.
357
ENHANCEDVOLATILIZATIONBY VENTINGOF KEROSENECOMPONENTFROMSOIL TABLE 1 Soil classification and characteristics (in % w/w) Soil series and horizon
6 7 8 9 10 11
Texture clay
Organic matter silt
Moisture content HYG
sand
-33kPa
-100kPa
Golan (Ap): very-fine clayey, montmorillonitic, thermic Typic Chromoxerert 74.4 10.6 15.0 1.56 9.6 49.0
42.0
Carmel (Ap): very-fine clayey, montmorillonitic, thermic Typic Pelloxerert 62.5 30.4 7.1 5.1 8.4 45.4
38.2
Nahsholim (Ap): fine-clayey, montmorillonitic, thermic Typic Chromoxerert 54.1 37.0 8.9 1.9 6.8 30.2
25.8
Nahal Oz (Ap): fine-loamy, mixed, thermic Calcic Haploxeralf 18.1 30.2 51.7 2.0 2.0
18.3
17.8
Hermonit: fine clayey, kaolinitic, mesic Typic Rhodoxeralf (Ap): 29.6 53.6 16.8 2.0 4.3 (Bt2): 56.4 34.4 9.2 0.65 6.2
34.3 34.1
18.5 25.8
Fazael (Bt): fine-loamy, mixed, hyperthermic Typic Haplargid 28.3 44.8 24.0 0.60 4.4
32.3
20.8
Netanya 8 (Bt2): fine-loamy, mixed, thermic Typic Rhodoxeralf 25.3 7.3 67.4 0.34 4.1
17.6
12.8
Netanya 6 (Bt2): fine-loamy, mixed, thermic Palic Rhodoxeralf 35.2 14.5 50.3 0.47 5.9
25.4
21.2
Mitzpe Masua (A): fine-loamy, carbonatic, thermic Lithic Haploxeroll 19.6 59.0 21.4 5.20 2.8 33.1
23.2
Sharon 2 (Bw): sandy, siliceous, thermic Typic Xerochrept 5.1 2.2 92.7 0.11 0.48
3.4
2.8
3.1
2.6
Dune sand 0.3
1.2
98.5
0.01
0.35
2. M A T E R I A L S A N D M E T H O D S
2.1. Materials 2.1.1. Soils Ten soils, representing the main soil types of Israel, and dune sand, were used in the study (Table 1). The soils included sandy, loamy and clayey soils from almost all climatic regions of the country (arid to sub-humid). The A or B horizon, or both, was or were used. Soils were collected in the field, air-dried, crushed to pass a screen with 2-mm openings, and stored in plastic containers until use. An additional sample of Golan vertisol (a Typic
358
P. FINE AND B. YARON
J
i
2
i
I
I
2
3
4
i
I
i
I
3 4 5 6 Peok n u m b e r i
5 Retenlion
I I 7 8 9
i
i
i
6
7
time
Fig. I. T y p i c a l c h r o m a t o g r a m o f the k e r o s e n e used in the e x p e r i m e n t s . P e a k identities are: 1 = m-xylene; 2 = 4 - m e t h y l o c t a n e ; 3 = n - d e c a n e ; 4 = 2,4,6-trimethyl o c t a n e ; 5 = n - d o d e c a n e ; 6 = 2,5,6-trimethyl d e c a n e ; 7 = t e t r a d e c a n e ; 8 = a n u n i d e n t i f i e d C.5 species; 9 = h e x a d e c a n e . R e t e n t i o n t i m e is in minutes.
Chromoxerert) was prepared differently; it was crushed to pass a 5-mm screen, and then separated into two fractions by means of a 2-ram screen. 2.1.2. Kerosene Fig. 1 shows a typical chromatogram of the kerosene used in this study. Nine major components were identified on a carbon number basis by calibrating against the retention times and boiling points of well-defined hydrocarbons. The C8, C,4 and C,6 components are inferred from the calibration method. In addition, the identities of the five major components characterized by carbon numbers 9-13 were obtained by gas chromatography-mass spectrometry (GC-MS) determination (Galin et al., 1990a). 2.2. Experimental procedure 2.2.1. Residual content Kerosene residual content, KRC, in mL/(100 g) was determined, using four soil moisture contents (air-dried, oven-dried at 105°C, and matrix potentials of - 3 3 and - 1 0 0 kPa) as follows: Portions of ~50 g air- dried soil (at
ENHANCED VOLATILIZATION BY VENTING OF KEROSENE COMPONENT FROM SOIL
359
hygroscopic moisture) were packed in screw-top short columns (length = 10cm, ID = 30cm). Oven-dry soils were prepared by heating at 105°C overnight. Kerosene was added to the cooled soil column, which was then sealed, inverted and left for 72 h. The column was then turned upright and left to equilibrate for another 72 h before sampling. The K R C of moist soils was determined after pre-adjusting the soil moisture by means of pressure plates. The soils were packed in 180-mL plastic cylinders which had an inner rim 5 m m wide at the bottom. Cylinders were placed on the pressure plate, filled with air-dry soil, and the plate was submersed under water up to the lower third of the cylinders. After 24 h, the plates were put under pressure for 48 h. Subsequently, the soil containers were removed, and the plate-facing side of the soil was sampled for moisture content. This side of the soil cylinder was then immersed in kerosene for 72 h to a depth of 2 cm. Subsequently, the soil cylinder was turned upside-down, and re-equilibrated for another 72 h. The redistribution of kerosene was performed at room temperature (22 + 3°C) in sealed desiccators over open vessels of kerosene and of water. The kerosene content of the soils was determined in sub-samples removed from the upper portion of the treated soils, just beneath the surface. All kerosene residual content determinations were made in triplicate.
2.2.2. Venting experiments Volatilization of kerosene from contaminated soils was tested in an enhanced volatilization system, with an air flow induced through the soil, and compared with a no-enhancement system. Air flow was induced by placing the soil containers in a venting chamber designed for this purpose (Fig. 2). The chamber, which measured 1.2 m x 0.6m x 0.8m, had a metal frame and wooden walls 6 - 1 2 m m thick. Eight 10-L containers, which are described below, were accomodated at one time, with each fitted into an appropriate round hole in the floor of the chamber. An industrial centrifugal fan (model CK250L, manufactured by C.A. Oesberg~' Auesta, Sweden), fitted in the ceiling of the chamber, created a depression of --~350 Pa that forced air to flow through the soil, penetrating from the bottom and emerging at the top. The air flows through the containers, were calculated from the measured air permeability constant and from the depression created by the fan; they ranged from 0.53 to 3.61 m 3 per container per day. These flow rate densities are within the range which was calculated from Bowman's (1989) report on a field case. The venting apparatus and the non-enhanced containers were located outdoors, and all were kept in the shade. The ambient temperature (the same outside and inside the ventilated chamber) ranged from 21 ° to 38°C during the experiment with a maximum diurnal variation of 8°C.
360
P. F I N E A N D
B. Y A R O N
e
d
Y O GO
L
,g ,2o
v
@
Fig. 2. Schematic drawing of the venting chamber used in the outdoor experiments: (a) centrifugal fan; (b) manometer; (c) soil container; (d) detachable wall; and (e) direction of air flow. All dimensions are in centimeters.
The containers used in the venting chamber were round and had a volume of 10 L (24cm deep; 17- and 19-cm internal diameter at the inner bottom and top, respectively). The containers were closed with 4-ply gauze at the bottom, to allow the free passage of air. A 2-cm layer of tuff, with a particle diameters of 0.5-2 cm, was placed at the bottom of the container and covered with gauze to separate it from the soil. The tuff layer at the bottom of the container transmitted an even stream of air to the soil bottom. Soil was packed to a known bulk density. Soils were either left air-dry, or moistened with tap water before kerosene was applied. The addition of each liquid was done after the containers had been placed in their final positions to ensure minimum cracking of the soil. Water or kerosene was applied slowly to the surface of the soil, which had been covered with 3-ply gauze prior to liquid application, to prevent the redistribution of particles. Water and kerosene were added in quantities equal to their respective field capacity or residual capacity. Following the addition to the soil, the liquid (kerosene or water) was allowed to redistribute itself in the soil column for 48 h. During this period, the container was covered with a fitted lid. Soil samples were taken from within the soil container at five depth increments of 5cm each, each sample layer averaging 1 cm in thickness. Sampling involved sacrificing one container at each sampling. Four samples were taken: two were extracted for hydrocarbon content determination, while
ENHANCED VOLATILIZATION BY VENTING OF KEROSENE COMPONENT FROM SOIL
361
the other two were washed with acetone and dried (105°C) to determine the dry weight percentage. Air permeability of soils was determined in situ on soils packed in the 10-L containers. The conical soil container was mounted onto an identical container, with the fitting secured by an O-ring and Silicone~tubricant. CO2 gas was passed through the outer container, and the flow rate and pressure were measured to calculate the permeability coefficient (K,, cm2). The measurement of the air permeability of air-dry soils always preceded the measurements of the soils which were equilibrated with kerosene. Note that equilibration with kerosene was done on air-dry soils or on soils which had previously been moistened to a water content equivalent to field capacity. The soil containers remained mounted on the measuring device while the processes of addition of either liquid and of equilibration took place. All measurements were done on duplicated pairs of containers. Each container was measured at seven different pressures and flow rates. The mean and standard deviation were calculated for all 14 measurements.
2.2.3. Analytical procedure 2.2.3.1. Extraction from soils. In the case of dry soils, kerosene was extracted in a tightly closed 35-mL glass vial b y adding 10 mL carbon tetrachloride to duplicate 0.5-g soil samples and shaking the samples overnight. The soil was allowed to settle and an aliquot of the supernatant was transferred to a 25-mL screw-cap flask with an AI liner. Anhydrous Na2SO4 and A1203 were added, to remove water and humic material. The flasks were stored at 4°C until measurement. The soil residue was washed twice in technical-grade acetone, dried at 105°C (step-wise), and weighed. The procedure was somewhat modified for moist soils; 3 mL of 1% sodium hexametaphosphate solution were added to the carbon tetrachloride to be shaken overnight. After the mixture had settled, ~ 5 m L of the carbon tetrachloride were filtered through 4 Whatman~filter paper, and stored in contact with anhydrous Na2 SO4 and AI2 03. Separate, 1-g duplicate samples were washed three times in 1:1 acetone-carbon tetrachloride mixture and dried to determine the dry weight percentage.
2.2.3.2. Gas chromatographic determination. A DB-l~megabore column, 30 m long, was used to separate the hydrocarbon mixture. The column was mounted in a Varian*~3300 gas chromatograph. Chromatography was performed in a programmed regime: 2 min at 45°C; heating from 45 ° to 200°C at 35°C min-~; and a quick warm-up to 300°C; the last temperature was maintained for 2.5 rain. The carrier gas was nitrogen. Hydrogen and air provided the combustion mixture for the flame ionization detector (FID).
362
P. F I N E A N D B. Y A R O N
Respective flow rates were 120, 30 and 300 mL min -j . The F I D was interfaced with a 2000-d Merck~"integrator to quantify the chromatogram peaks. The carbon tetrachloride extract and the standards were supplemented with an internal standard of propanol, which was added to a known mass of extract. A 1-/~L aliquot was injected into the column. The propanol peak preceded all other peaks. The total hydrocarbon content (total chromatogram area) and nine specific peaks were analyzed for each injection. The total kerosene content in soils was estimated by comparison with kerosene standards. 3. RESULTS AND DISCUSSION 3.1. Fundamental parameters
Both kerosene content and air permeability of soils are basic design parameters that control the removal of volatile contaminants from soils. Therefore, the primary measurements in a soil-venting experiment were devoted to the above parameters. 3.1.1. Kerosene residual content ( K R C ) The degree of soil saturation for a water-immiscible liquid can be expressed as the proportion of pore space for each liquid and air phase (van Dam, 1967; Schwille, 1984) or as the content of the medium in mL (Mercer and Cohen, 1990). These terms are often used interchangeably (e.g., Baehr, 1987). We use the term kerosene residual content (KRC) in mL/(100 g) by analogy with the field capacity of soils. Four soil moisture contents were studied, corresponding to oven-dry, air-dry, 33-kPa tension (field capacity) and 100-kPa tension. The K R C of the oven-dry soils ranged from 3.5 to 18.1 mL/(100 g). It was affected strongly by the clay content of the soil, with the more clayey soils having K R C values 1.5-2 times greater than the less clayey ones. The silt and organic matter (OM) contents also contributed to the KRC. The relationship between these soil components and K R C is described by the following equation:
K R C = 0.08 [% CLAY] + 1.68 [% OM] + 0.06 [% SILT] + 5.29
(1)
with a highly significant (p <0.01) coefficient of determination (R 2) of 0.91. The ranges of values of the three independent variables (X components) were: clay, 0.3-74%; OM, 0-5.2%; and silt, 1.2-59%. The two latter variables seem to cause some of the fluctuations of the K R C at zero moisture content. For example, the calcic, pale Rendzina had a high K R C of 18.1mL/(100 g), corresponding to the combination of a silty-loam texture (19.6% clay and 59% silt) and a high OM content (5.2%). K R C exhibited a linear relationship to the combination of soil texture
E N H A N C E D V O L A T I L I Z A T I O N BY V E N T I N G O F K E R O S E N E C O M P O N E N T F R O M SOIL
363
\
~J
y
t~CD
<..Q
Fig. 3. Kerosene residual content (KRC) of soils as a function of the clay and moisture contents. Soil moisture contents are indicated in Table 1.
(characteristic by clay content) and OM content, and was inversely linearly decreased with the moisture content as follows: K R C -- 0.13 [% CLAY] + 1.48 [% OM] - 0.32 [% MOISTURE] + 4.31
(2) 0.833, and was highly significant ( p < 0 . 0 1 ) (Fig. 3). The moisture content was in the range of 0-49%. The silt content made only a slight contribution to the R 2 coefficient, and was therefore omitted. Water filled up the soil capillaries, and caused a strong reduction of the hydrocarbon sorption on the mineral surfaces (Mingelgrin and Prost, 1989; Yaron, 1990). Increasing the soil moisture content, caused marked reduction of the K R C of the soils to a rather uniform level. The crucial effect of moisture content on K R C can be exemplified by comparing the fine-clayey Chromoxerert with the dune sand. The respective KRC's were 14.8 and 3.2 mL/(100 g) for air-dry soil, and 2.17 and 1.84mL/(100g) for moist soils (at - 3 3 - k P a tension). The reduction in K R C of the soils owing to increasing moisture content was considerably larger than the ~ 30% reduction at "field capacity" reported by Marley and Hoag (1986) for a medium-sized sand. It is interesting to note that the K R C of oven-dry and air-dry soils ( < 2mm) was smaller than their field capacity (WFC). This trend was reversed in the very sandy soils that had extremely low WFC's. For example, the K R C of the more clayey soils was two to three times less than their WFC. Our conjecture is that more kerosene can be adsorbed on external surfaces with low charge density (the vast majority of adsorption sites in sandy soils), as R2
=
364
P. F I N E A N D B. Y A R O N
0.16 -~--
I
INITIAL
0.10
% o
~
, 0.08
0.06 0.04
I
0.00
': GOLAN~2
GOLAN,2
NAHSHOLIM
CARMEL
SAND
NAHAL OZ
Fig. 4. Air permeability coefficient (Kd) of air-dried soils and soils equilibrated with kerosene.
compared with water, perhaps through hydrophobic interactions of this mixture of hydrocarbons. OM may also play a more crucial role in hydrocarbon sorption in such soils (Mingelgrin and Prost, 1989).
3.1.2. Air permeability of kerosene-treated soils Soil permeability to air was tested in the dune sand, in the loessial Nahal Oz soil, in the Nahsholim vertisol, and in the two differently sized fractions (2-5 and < 2 mm) of the Golan vertisol (Fig. 4). The soils were either air-dried or equilibrated to their K R C with kerosene before the measurements were taken. The air permeability coefficient (Ka) of the four air-dried soils ranged from 0.019cm 2 in the loessial soil to 0.145cm z in the coarsely aggregated (2-5 mm) Golan vertisol. It is interesting to note that the dune sand and the < 2-mm-sieve vertisol had similar/<,-values of 0.077 and 0.082 cm 2, respectively. Air permeability of the soils was measured again after their equilibration to their K R C with kerosene (Fig. 4). Kerosene addition decreased air permeability in the dune sand only. The K, was reduced by 9%. Note that the K R C of air-dried sand was 3.2 mL/(100 g), corresponding to an Sr of 0.11 (St being the fraction of the soil voids occupied by kerosene). Hence, the reduction of free soil porosity linearly reduced the air permeability in this non-interacting, siliceous matrix. Kerosene addition did not much affect the air permeability of the wellaggregated soils. For example, the Ka of the kerosene-equilibrated Nahsholim vertisol was 2.4% greater than that of the corrsponding air-dried material, inasmuch as the kerosene Sr was 0.30. This seems to point out that kerosene resides mainly on internal surfaces within the aggregates (micropores). Larger pores, which are involved in air flow, do not hold kerosene to an extent
365
ENHANCED VOLATILIZATIONBY VENTINGOF KEROSENECOMPONENT FROM SOIL TABLE 2
Air permeability coefficient (Ka) of moist and dry Nahsholim vertisol prior to, and after, kerosene addition, and during venting (mean _+standard deviation of 14 measurements) Duration days
K a (cm 2) enhanced venting air-dry
free volatilization moist*
air-dry
moist*
0.131 + 0.009 0.130 +0.012 0.132+0.006
0.080 _+0.005 0.114_+0.008 0.115+0.007
Prior to kerosene addition:
0
0.125 + 0.009
0.052 _+0.002
After kerosene addition:
0
0.128_+0.011
0.036_+0.001
,{~ter volatilization onset:
3 9 18 *Moisture content
0.094-+0.004 0.112-+0.009 0.119-+0.007 at field capacity.
0,130-+0.014 0.123-+0.010 0.118_+0.008
capable of affecting their permeability to air. The slight increase could have resulted from a hydrophobic shrinking of the hygroscopically moist soil aggregates; however, it is more likely that the increase is due to cracking. An extreme example of cracking was found in the loessial Nahal Oz soil. The Sr of the soil was 0.18, yet the K a increased by as much as 70%. After the internal soil surfaces had been blocked with water, only the larger pores could contain kerosene. Hence, addition of kerosene to a moist soil (Nahsholim vertisol; Table 2) reduced its permeability to air.
3.2. Enhanced volatilization of kerosene Volatilization of kerosene from soils is controlled by the saturated vapor density or vapor pressure of each component, and by the rate of transfer away from the surface, that is diffusion controlled. Advective vapor transport can be enhanced by using a venting apparatus capable of inducing air flow through the soil, to carry the kerosene vapor with it. A comparison of the free and enhanced mass transfer of whole kerosene, and of selected kerosene components as affected by the type of soil, its aggregation, and its moisture content, will follow.
3.2.1 Effect of soil characteristics Kerosene volatilization from four soils was tested under two sets of experimental conditions: with kerosene freely volatilizing to the atmosphere, or
366
P. F I N E A N D B. Y A R O N
A
100~
~~
1
I
'°1 t
i::l 17
ILCARMEL HAL OZ 7
8
§
TIME
2
(days) B
100-
~ND ;N
N
17 S 5 2 TIME
(days)
Fig. 5. Volatilization of kerosene from the 5-15 cm soil layer (in % of initial content): (A) free and (B) enhanced venting.
with an advective air stream driven by depression-induced venting. The venting continued for 17 days. Fig. 5 shows the loss of kerosene from the soils. Each value is the mean of duplicate determinations at three depths: 5, 10 and 15 cm. Evidently, the induction of air-phase transport through the soil in the enhanced venting treatment, strongly enhanced the release of light fractions of kerosene from all four soils at all sampling dates. It is also clear that the volatilization of kerosene under both sets of conditions strongly depended on soil characteristics. The latter dependence may be quantified by relating the loss of kerosene (LOSS as a percentage of KRC) to CLAY content, to hygroscopic moisture content (HYG), and to the air permeability coefficient (Ka). The percentage loss after 17 days of enhanced venting is described by the
E N H A N C E D V O L A T I L I Z A T I O N BY V E N T I N G O F K E R O S E N E C O M P O N E N T F R O M SOIL
367
following relation: LOSS = 515[Ka] + 17[HYG] - 2.7[CLAY] + 45
(3)
with an R 2 = 0.982. The contribution of air permeability to the correlation is clear in both the free and the enhanced venting methods. The air flux density (in m 3 per container per day), calculated for enhanced venting as specified above, ranged from 0.53 +0.02 in the loessial soil to 2.23_ 0.25 in the sand. The clay hindered kerosene volatilization, perhaps by limiting kerosene access to open voids. The HYG in itself was not expected to affect volatilization. However, the HYG of soils from Israel correlates well with their specific surface area (R 2 = 0.95; Banin and Amiel, 1969), the internal surfaces also being the site of kerosene binding or release.
3.2.2. Effect of soil aggregation The effect of soil aggregation on kerosene volatilization was studied by comparing the volatilization from the two fractions of Golan vertisol: the < 2-mm grain-size fraction and the 2-5-mm aggregates. The respective KRC of these fractions in the containers were 8.7_+0.25 and 4.1 _+0.11 mL/(100 g). These values were substantially lower than the 14.6 mL/(100 g) obtained after the long, steady-state equilibration procedure (Fig. 3). The total porosities of these two aggregate fractions were very similar: 53% and 55% of total soil volume, respectively. The difference in retention seems to emanate from the effect of pore size distribution on the rate of kerosene flow through the soil column and on the kinetics of kerosene retention in soil micropores. The coarsely aggregated soil had both fine porosity within the aggregates and contrasting coarse inter-aggregate porosity. It seems that the kerosene did not effectively compete with and displace hygroscopic water entrapped within the micropores. The two soil fractions differed also in air permeability, their respective K~-values being 0.082 _+0.009 and 0.145 + 0.011 cm 2. Fig. 6 shows that the rate of kerosene removal from the two soil fractions is related to their permeabilities to air and to the venting treatment. The more permeable to air the soil is, the more readily it releases kerosene under both enhanced and free venting treatments. The most efficient removal of kerosene from soil was obtained with enhanced venting applied to coarse aggregates. This combination facilitated the removal of four to seven times more kerosene than the finelyaggregated soil under free venting, which was the least effective combination. The high ambient temperatures (up to 38°C) during the last 3 days of the experiment enhanced the removal of kerosene; this increased removal of kerosene was more evident in the three least effectively treated specimens which still retained appreciable amounts of kerosene.
368
P. F I N E A N D
B. Y A R O N
100 8o Z
_o <~ N .-I
--
¢, >
60
!
|
40
20o B A < 2-mm
A
7 days ,ys
~5
days
2 days
B
> 2-mm
Fig. 6. Volatilization of kerosene from Golan vertisol as affected by aggregate size and venting method: (A) free and (B) enhanced venting.
3.2.3. Effect of soil moisture content The effect of initial soil moisture content was tested in the Nahsholim vertisol. The soil was either air-dried, or was moistened in the container, prior to kerosene application. The moisture content was 271 mL kg -j , which was somewhat low compared with the field capacity measured by means of pressure plates (Table 1). The K R C of the moist soil was 4.22 mL/(100 g), and that of the air-dried soil was 8.5mL/(100 g). Fig. 7 presents the loss of kerosene (as a percentage of KRC) from the soil container. The enhanced venting treatment was most efficient in removing kerosene from the dry, as well as from the moist soils. Interestingly, the kerosene loss with the moist treatments was faster and more complete than with the dry counterparts. For instance, the enhanced, vented, moist soil emitted more kerosene than did the dry soil, from any one layer and at any one time. Furthermore, the removal of kerosene from the dry-free combination (Fig. 7A-l) after 18 days was no more than 35% of the KRC, whereas the comparable loss from the moist-free (Fig. 7A-2) combination was twice as much, throughout the soil column. However, the relative basis is somewhat misleading. The two free venting treatments, dry (Fig. 7B-l) and moist (Fig. 7B-2), actually removed very similar total amounts of kerosene. They differed on a relative basis, because their initial KRC's were different. The high efficacy of kerosene volatilization with the moist treatments was unexpected, on the basis of the initial low permeability to air of this treatment (Table 2). However, soil cracking during drying restored the permeability of the soil to air to its pre-wetting value, within 3-9 days, at a rate that depended on the rate of evaporation. This is only one feasible explanation, because the
ENHANCED VOLATILIZATION BY VENTING OF KEROSENE COMPONENT FROM SOIL
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~m~IIllll~
369
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Fig. 7. Changes in the kerosene concentration of the Nahsholim vertisol as affected by free (A) and enhanced-venting (B) volatilization from initially dry (1) and moist (2) soils.
K, of the moist-free treatment was still low compared with the dry-free treatment. It seems to us that the movement of water vapor through the soil enhanced the volatilization of kerosene components. We do not have a sound explanation for the K, fluctuations with the dry-enhanced treatment, except for non-homogeneity inherent in even a rather simple system such as this. The coefficient of variation in these measurements was 3-11% (0.00120.0141 cm2). Note also that the Ka-value of the initially air-dried soil in this experiment was greater than the value measured in the experiment reported above (Fig. 4).
3.2.4. Differential volatilization of kerosene components The chemical composition of the kerosene residue in the polluted soils changed with time owing to the differential volatilization of kerosene components. This is shown in Fig. 8 for the four venting treatments of the
370
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P F I N E A N D B. Y A R O N
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Fig. 8. Changes in the relative concentrations of the kerosene components as affected by free (A) and enhanced-venting (B) volatilization from initially dry (1) and moist (2) Nahsholim vertisol.
Nahsholim vertisol. The analyses were done in extracts of soil samples which were taken at a depth of 10cm beneath the surface. Nine kerosene components were determined. The data points depict the percentage contribution of each component to the total K R C of the soil. The nine components appear in Fig. 8 in the order of their retention time. Each value is the mean of duplicate samples. It is evident from Fig. 8 that the contribution of each of the nine components to the kerosene residue in the soil depends on its relative volatility and on the experimental conditions: the mode of volatilization from soil (enhanced or free venting) and the initial soil moisture content. The composition of kerosene in the soil samples exposed to the experimental conditions least conducive to volatilization (air dried; free venting), was rather constant throughout the entire experiment (Fig. 8A-I). In treatments where volatilization was more intense, the less volatile, aliphatic, and other larger molecules (characterized by longer retention time), tended to dominate the spectrum of compounds within the residue. This was most prominent for enhanced venting of air-dried soil, which was the combination of conditions most conducive to kerosene volatilization (Fig. 8B-l). The heavier components already dominated the chromatogram after 9 days of venting, and became overwhelming after 18 days.
371
E N H A N C E D V O L A T I L I Z A T I O N BY V E N T I N G O F K E R O S E N E C O M P O N E N T F R O M SOIL
(A)
m-XYLENE
(A) n-DECANE
(A)
HEXADECANE
(8)
HEXADECANE
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.-+, +.,,
(B) m-XYLENE
(B) n-DECANE
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Fig. 9. Concentration of m-xylene, n-decane and hexadecane (mL/(100 g soil) in the kerosene-polluted, air-dry Nahsholim vertisol as a result of free (A) and enhanced-venting (B) volatilization. The Z-axis (content) scale differs between compounds. Note that the X-axis (soil depth) and the Y-axis (time) for hexadecane are effectively interchanged, relative to those for the other compounds.
As mentioned previously, almost all the redistribution of kerosene components shown in Fig. 8 was due to differential volatilization into the atmosphere. There was but a small net enrichment of any component in any one layer or at any particular time. This is illustrated in Fig. 9, which shows the actual content of three representative kerosene components in the soil column (m-xylene (C8), n-decane (C10) and hexadecane (C16)) during the free and enhanced venting treatments of the air-dried Nahsholim vertisol. The Z-axis (content in mL/(100 g soil) in Fig. 9 is presented as a function of soil depth (X-axis), and vs. time (Y-axis). Note that the X-axis (soil depth) and the Y-axis (time) for hexadecane interchanged relative to those for the other components. It is evident that m-xylene, a relatively volatile component of kerosene, is eliminated quickly from the polluted soil. Under the enhanced venting treatment, it virtually disappeared from the entire soil column after the third day from the onset of venting. Free volatilization was less effective, and 30% of the initial content of m-xylene still remained in the soil after day 18. Furthermore, the content of m-xylene seemed to increase somewhat after the
372
P. FINE AND B. YARON
third day at the layer 5 cm beneath the surface. Under these conditions, trends in the distribution of n-decane are similar; its content in the 5- and 10-cm layers increased by the 3rd and 9th day from onset, respectively. The loss of n-decane under enhanced venting was almost complete by the 18th day. Hexadecane had a different mode of redistribution; its content did not decrease appreciably throughout the 18-day period. Hexadecane was somewhat removed from deeper layers, and was redeposited at the soil surface. This was reflected by an increase in hexadecane content at the soil surface of up to five times the initial content. The increase was more marked under free volatilization than under enhanced venting. As venting continued, the more effective stripping of the enhanced venting removed this redeposited hexadecane. The removal occurred in the unenhanced treatment too, but to a lesser extent. It should be mentioned that changing the experimental conditions may strongly alter the pattern of differential volatilization. 4. C O N C L U S I O N S
The kerosene residual content exhibits a strong positive linear relationship to the combination of clay and organic matter contents and a negative linear relationship to the soil moisture content. Kerosene content reduces the permeability of sand to air; however, it does not significantly affect the permeability of the other soils. Two factors are shown to control the rate of volatilization of petroleum hydrocarbons from soils. One is their vapor pressure and the other is their transfer to and away from the surface. The induction of air-phase transport in the soil under enhanced-venting treatment strongly enhances the removal of the lighter fractions of kerosene from all the porous materials studied. The volatilization causes a substantial shift in the composition of the kerosene residue in soil to dominance of heavier fractions. The rate of loss of hydrocarbons from soil increases with coarser aggregation and with the cracking of initially moist soils. Soil constituents and the initial moisture content need to be considered, in conjunction with soil residual capacity for kerosene and with soil permeability to air, as fundamental design parameters for an enhanced venting decontamination procedure. ACKNOWLEDGEMENTS This study was supported by the research grant WT 8951/913 of the Israel Ministry of Science and Development and the German Bundesministerium ffir Forschung und Technologic and by grant 308-130 from the Chief Scientist of the Ministry of Agriculture, Israel.
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373
REFERENCES Agrelot, J.C., Malot, J.S. and Visser, M.J., 1985. Vacuum: Defence system for ground water VOC contamination. Proc. 5th Natl. Symp. of Aquifer Restoration and Groundwater Monitoring, Columbus, OH, May 21-24, 1985. API (American Petroleum Institute), 1980. Underground spill cleanup manual. Am. Pet. Inst., Washington, DC, Publ. No. 1628. Baehr, A.L., 1987. Selective transport of hydrocarbons in the unsaturated zone due to aqueous and vapor phase partitioning. Water Resour. Res., 23: 1926-1938. Baehr, A.L. and Hoag, G.F., 1987. A modeling and experimental investigation of induced venting of gasoline contaminated soils. Proc. Conf. on Environmental and Public Health Effects of Soils Contaminated with Petroleum Products, Univ. of Massachusetts, Amherst, MA, Oct. 30-31, 1985. Baehr, A.L., Hoag, G.E. and Marley, M.C., 1989. Removing volatile contaminants from the unsaturated zone by inducing advective air-phase transport. J. Contam. Hydrol., 4: 1-26. Banin, A. and Amiel, A., 1969. A correlative study of the chemical and physical properties of a group of natural soils of Israel. Geoderma, 3: 185-198. Bowman, R.S., 1989. Manipulation of the vadose zone to enhance toxic organic chemical removal. In" Z. Gerstl, Y. Chen, U. Mingelgrin and B. Yaron (Editors), Toxic Organic Chemicals in Porous Media. Springer, Berlin, pp. 275-286. DiGiulio, D.C., Cho, J.S., Dupont, R.R. and Kemblowski, M.W., 1990. Conducting field tests for evaluation of soil vacuum extraction application. Proc. 4th Natl. Outdoor Action Conf. on Aquifer Restoration, Ground Water Monitoring and Geophysical Methods. Natl. Water Well Assoc., Dublin, OH, pp. 587-601. Donaldson, S.G., Miller, G.C. and Miller, W.W., 1992. Remediation of gasoline-contaminated soil by passive volatilization. J. Environ. Qual., 21: 94-102. Fleischer, E.J., Noss, P.R., Kostecki, P.T. and Calabrese, E.J., 1986. Evaluating the subsurface fate of organic chemicals of concern using the SESOIL environmental fate model. Proc. 3rd Eastern Reg. Groundwater Conf., Natl. Water Well Assoc., Springfield, MA. Galin, T., Gerstl, Z. and Yaron, B., 1990a. Soil pollution by petroleum products, III. Kerosene stability in soil columns as affected by volatilization. J. Contam. Hydrol., 5: 375-385. Galin, T., McDowell, C. and Yaron, B., 1990b. The effect of volatilization on the mass flow of a non-aqueous pollutant liquid in an inert porous media: experiments with kerosene. J. Soil Sci., 41: 631-641. Gierke, J.S., Hutzler, N.J. and McKenzie, D.B., 1992. Vapor transport in unsaturated soil columns: implication for vapor extraction. Water Resour. Res., 28: 323-335. Hoag, G.E. and Cliff, B., 1988. The use of soil venting technique for the remediation of petroleum-contaminated soils. In: E.J. Calabrase and P.T. Kostecki (Editors), Soils Contaminated by Petroleum, Environmental and Public Health Effects. Wiley, New York, NY, pp. 301-315. Johnson, J.J. and Sterrett, R.J., 1988. Analysis of in situ air stripping data. Proc. 5th Natl. Conf. on Hazardous Wastes and Hazardous Materials, HMCRI (Hazard. Mater. Contain. Res. Inst.), Las Vegas, NV, Apr. 19-21, 1988, pp. 451-455. Johnson, P.C., Kemblowski, M.W. and Colthart, J.D., 1988. Practical screening models for soil venting applications. Natl. Water Well Assoc.-Am. Pet. Inst. Conf. on Petroleum Hydrocarbons and Organic Chemicals in Groundwater, Houston, TX. Johnson, R.L., 1989. Soil vacuum extraction: laboratory and physical model studies. Proc. of the Workshop on Soil Vacuum Extraction, R.S. Kerr Environ. Res. Lab., Ada, OK, Apr. 27-28, 1989.
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Jury, W.A., Spencer, W.F. and Farmer, W.J., 1984. Behavior assessment model for trace organics in soils, IV. Review of experimental evidence. J. Environ. Qual., 13: 580-586. Jury, W.A., Winer, A.M., Spencer, W.F. and Fochd, D.D., 1987. Transport and transformation of organic chemicals in the soil-air-water ecosystem. Rev. Environ. Contam. Toxicol., 99: 119-163. Karimi, A.A., Farmer, J.J.H. and Cliath, M.M., 1987. Vapor phase diffusion of benzene in soil. J. Environ. Qual., 16: 38-44. Marley, M.C, and Hoag, G.E., 1986. Induced soil venting for recovery/restoration of gasoline hydrocarbons in the vadose zone. Proc. Water Well Assoc.-Am. Pet. Inst. Conf. on Petroleum Hydrocarbons and Organic Chemicals in Ground Water - - Prevention, Detection, Restoration. Natl. Water Well Assoc., Houston, TX, 1984. Mercer, J.W. and Cohen, R.M., 1990. A review of immiscible fluids in the subsurface: properties, models, characterization and remediation. J. Contam. Hydrol., 6: 107-163. Mingelgrin, U. and Prost, R., 1989. Surface interactions of toxic organic chemicals with minerals. In: Z. Gerstl, Y. Chen, U. Mingelgrin and B. Yaron (Editors), Toxic Organic Chemicals in Porous Media. Springer, Berlin, pp. 91-135. Roberts, P.V. and Dandlliker, G.P., 1983. Mass transfer of volatile organic contaminants non-aqueous solution to the atmosphere during surface aeration. Environ. Sci. Technol., 13: 484-489. Schwille, F., 1984. Migration of organic fluids immiscible with water in the unsaturated zone. In: B. Yaron, G. Dagan and J. Goldschmid (Editors), Pollutants in Porous Media. Springer, Berlin, pp. 27-48. Spencer, W.F., Farmer, W.J. and Cliath, M.M., 1973. Pesticide volatilization. Residue Rev., 49: 1-47. van Dam, J., 1967. The migration of hydrocarbons in a water bearing stratum. In: P. Hepple (Editor), The Joint Problems of the Oil and Water Industries. Institute of Petroleum, London, pp. 55-96. Wilson, D.J., 1989. Modeling of soil vapor stripping. Proc. Workshop on Soil Vacuum Extraction, R.S. Kerr Environ. Res. Lab., Ada, OK, Apr. 27-28, 1989. Yaron, B., 1989. On the behavior of petroleum hydrocarbons in the unsaturated zone: abiotic aspects. In: Z. Gerstl, Y. Chen, U. Mingelgrin and B. Yaron (Editors), Toxic Organic Chemicals in Porous Media. Springer, Berlin, pp. 211-230.. Yaron, B., 1990. Behaviour of exogenous organic chemicals in soils as controlled by abiotic processes. Proc. Int. Congr. Soil Sci., Kyoto, 1: 30-42.
355
Outdoor experiments on enhanced volatilization by venting of kerosene component from soil P. F i n e a n d B. Y a r o n
Department of Soil Organic and Residues Chemistry, Institute of Soils and Water A RO, The Volcani Center, IL-50250 Bet Dagan, Israel (Received April 1, 1992; revised and accepted November 30, 1992)
ABSTRACT Fine, P. and Yaron, B., 1993. Outdoor experiments on enhanced volatilization by venting of kerosene component from soil. J. Contam. Hydrol., 12: 355-374. The effect of soil properties on the retention of kerosene in soils, at equilibrium and under venting, was studied. Eleven soils were studied, which represent a wide range of chemical properties and mechanical composition. The retention of kerosene in dry soils ranges from 3.5 to 18.1 mL/(100 g), and was related linearly to clay, silt and organic matter (OM) contents. A coarsely-aggregated dry vertisol (2-5 mm aggregates) retained half as much kerosene as its finely-aggregated ( < 2 mm) counterpart. Moisture content had a strong inverse effect on kerosene retention. The soil factors that inversely affected kerosene retention also enhanced kerosene stripping by venting. Of these, soil aggregation and porosity were the most important. In addition, kerosene volatilized faster and more completely from an initially moist soil, as compared with an initially dry soil. Differential volatilization of lighter components of kerosene changed the chemical composition of the residue in the soil substantially, as compared with the initial composition.
1. I N T R O D U C T I O N
Petroleum products are mixtures of hydrocarbons with differing vapor pressures. Thus, each petroleum fraction contains compounds with a wide range of volatilization properties. The rate of volatilization of hydrocarbons to the atmosphere from a soil-petroleum mixture will be affected by soil constituents and soil hydration status. Ample information is available on the volatilization of compounds of high and intermediate vapor pressure from soils (e.g., Spencer et al., 1973; Jury et al., 1984, 1987), but less research has been reported on the volatilization from soil of petroleum hydrocarbons. ~Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel. No. 3667-E, 1992 series. 0169-7722/93/$06.00
© 1993 Elsevier Science Publishers B.V. All rights reserved.
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P. FINE AND B, YARON
Karimi et al. (1987) recently showed that the vapor-phase diffusion of benzene in the soil is greatly reduced by increasing soil bulk density and soil-moisture content. Existing results refer primarily to volatilization of single compounds from soil. However, each petroleum product is a mixture of hydrocarbons, with a range of physico-chemical properties (Yaron, 1989). We recently studied the rate of volatilization of hydrocarbons in simulation mixtures which were applied to soil surfaces. The volatilization of C9 to CI5 hydrocarbons from sandy and loamy sand soils was greater than from a silt loam (Galin et al., 1990a, b). This was ascribed to the influence of differences in the pore-size distributions (or ineffective porosity) on the rates of volatilization of the light carbon fractions (C9-CI2) in the simulation mixtures. In principle, enhanced volatilization is straightforward; the vapor-phase concentration of the volatile hydrocarbons is reduced by accelerating the flow of air through the porous vadose zone. This is done by applying vacuum, pressure or a combination of both to a grid of well-pumping systems (Bowman, 1989). The usefulness of this technique has been proven on a pilot scale in the U.S.A. (J.J. Johnson and Sterrett, 1988; P.C. Johnson et al., 1988; Bowman, 1989; R.L. Johnson, 1989; Wilson, 1989; DiGiulio et al., 1990) whereby polluted porous media were cleaned of organic hydrocarbons. The application of this technique was usually intended to reduce threats to groundwater posed by vadose zone contamination. Modelling of the removal of a mixture of volatile organic compounds from porous media has been widely attempted (Roberts and Dandlliker, 1983; Baehr, 1987; Baehr and Hoag, 1987; Baehr et al., 1989; Gierke et al., 1992). However, not much experimental work has been published on soil criteria for enhanced hydrocarbon volatilization. Most of the research has been sitespecific, related to the cleanup of actual vadose-zone contamination. Recent results (Hoag and Cliff, 1988; Baehr et al., 1989; Donaldson et al., 1992) have demonstrated the successful application of the venting technique to remedy gasoline contaminated soils; they should encourage the adaption of soil venting on a larger scale. More data are required on the effects of soil factors on the remediation technique. The work reported here is intended to provide better understanding of the effects of soil properties and soil moisture on the volatilization of hydrocarbons under enhanced venting, starting from a soil contamination level of residual saturation. The results can be applied to the design of clean-up systems and to the prediction of the removal of kerosene components in the field.
357
ENHANCEDVOLATILIZATIONBY VENTINGOF KEROSENECOMPONENTFROMSOIL TABLE 1 Soil classification and characteristics (in % w/w) Soil series and horizon
6 7 8 9 10 11
Texture clay
Organic matter silt
Moisture content HYG
sand
-33kPa
-100kPa
Golan (Ap): very-fine clayey, montmorillonitic, thermic Typic Chromoxerert 74.4 10.6 15.0 1.56 9.6 49.0
42.0
Carmel (Ap): very-fine clayey, montmorillonitic, thermic Typic Pelloxerert 62.5 30.4 7.1 5.1 8.4 45.4
38.2
Nahsholim (Ap): fine-clayey, montmorillonitic, thermic Typic Chromoxerert 54.1 37.0 8.9 1.9 6.8 30.2
25.8
Nahal Oz (Ap): fine-loamy, mixed, thermic Calcic Haploxeralf 18.1 30.2 51.7 2.0 2.0
18.3
17.8
Hermonit: fine clayey, kaolinitic, mesic Typic Rhodoxeralf (Ap): 29.6 53.6 16.8 2.0 4.3 (Bt2): 56.4 34.4 9.2 0.65 6.2
34.3 34.1
18.5 25.8
Fazael (Bt): fine-loamy, mixed, hyperthermic Typic Haplargid 28.3 44.8 24.0 0.60 4.4
32.3
20.8
Netanya 8 (Bt2): fine-loamy, mixed, thermic Typic Rhodoxeralf 25.3 7.3 67.4 0.34 4.1
17.6
12.8
Netanya 6 (Bt2): fine-loamy, mixed, thermic Palic Rhodoxeralf 35.2 14.5 50.3 0.47 5.9
25.4
21.2
Mitzpe Masua (A): fine-loamy, carbonatic, thermic Lithic Haploxeroll 19.6 59.0 21.4 5.20 2.8 33.1
23.2
Sharon 2 (Bw): sandy, siliceous, thermic Typic Xerochrept 5.1 2.2 92.7 0.11 0.48
3.4
2.8
3.1
2.6
Dune sand 0.3
1.2
98.5
0.01
0.35
2. M A T E R I A L S A N D M E T H O D S
2.1. Materials 2.1.1. Soils Ten soils, representing the main soil types of Israel, and dune sand, were used in the study (Table 1). The soils included sandy, loamy and clayey soils from almost all climatic regions of the country (arid to sub-humid). The A or B horizon, or both, was or were used. Soils were collected in the field, air-dried, crushed to pass a screen with 2-mm openings, and stored in plastic containers until use. An additional sample of Golan vertisol (a Typic
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P. FINE AND B. YARON
J
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i
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4
i
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i
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5 Retenlion
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time
Fig. I. T y p i c a l c h r o m a t o g r a m o f the k e r o s e n e used in the e x p e r i m e n t s . P e a k identities are: 1 = m-xylene; 2 = 4 - m e t h y l o c t a n e ; 3 = n - d e c a n e ; 4 = 2,4,6-trimethyl o c t a n e ; 5 = n - d o d e c a n e ; 6 = 2,5,6-trimethyl d e c a n e ; 7 = t e t r a d e c a n e ; 8 = a n u n i d e n t i f i e d C.5 species; 9 = h e x a d e c a n e . R e t e n t i o n t i m e is in minutes.
Chromoxerert) was prepared differently; it was crushed to pass a 5-mm screen, and then separated into two fractions by means of a 2-ram screen. 2.1.2. Kerosene Fig. 1 shows a typical chromatogram of the kerosene used in this study. Nine major components were identified on a carbon number basis by calibrating against the retention times and boiling points of well-defined hydrocarbons. The C8, C,4 and C,6 components are inferred from the calibration method. In addition, the identities of the five major components characterized by carbon numbers 9-13 were obtained by gas chromatography-mass spectrometry (GC-MS) determination (Galin et al., 1990a). 2.2. Experimental procedure 2.2.1. Residual content Kerosene residual content, KRC, in mL/(100 g) was determined, using four soil moisture contents (air-dried, oven-dried at 105°C, and matrix potentials of - 3 3 and - 1 0 0 kPa) as follows: Portions of ~50 g air- dried soil (at
ENHANCED VOLATILIZATION BY VENTING OF KEROSENE COMPONENT FROM SOIL
359
hygroscopic moisture) were packed in screw-top short columns (length = 10cm, ID = 30cm). Oven-dry soils were prepared by heating at 105°C overnight. Kerosene was added to the cooled soil column, which was then sealed, inverted and left for 72 h. The column was then turned upright and left to equilibrate for another 72 h before sampling. The K R C of moist soils was determined after pre-adjusting the soil moisture by means of pressure plates. The soils were packed in 180-mL plastic cylinders which had an inner rim 5 m m wide at the bottom. Cylinders were placed on the pressure plate, filled with air-dry soil, and the plate was submersed under water up to the lower third of the cylinders. After 24 h, the plates were put under pressure for 48 h. Subsequently, the soil containers were removed, and the plate-facing side of the soil was sampled for moisture content. This side of the soil cylinder was then immersed in kerosene for 72 h to a depth of 2 cm. Subsequently, the soil cylinder was turned upside-down, and re-equilibrated for another 72 h. The redistribution of kerosene was performed at room temperature (22 + 3°C) in sealed desiccators over open vessels of kerosene and of water. The kerosene content of the soils was determined in sub-samples removed from the upper portion of the treated soils, just beneath the surface. All kerosene residual content determinations were made in triplicate.
2.2.2. Venting experiments Volatilization of kerosene from contaminated soils was tested in an enhanced volatilization system, with an air flow induced through the soil, and compared with a no-enhancement system. Air flow was induced by placing the soil containers in a venting chamber designed for this purpose (Fig. 2). The chamber, which measured 1.2 m x 0.6m x 0.8m, had a metal frame and wooden walls 6 - 1 2 m m thick. Eight 10-L containers, which are described below, were accomodated at one time, with each fitted into an appropriate round hole in the floor of the chamber. An industrial centrifugal fan (model CK250L, manufactured by C.A. Oesberg~' Auesta, Sweden), fitted in the ceiling of the chamber, created a depression of --~350 Pa that forced air to flow through the soil, penetrating from the bottom and emerging at the top. The air flows through the containers, were calculated from the measured air permeability constant and from the depression created by the fan; they ranged from 0.53 to 3.61 m 3 per container per day. These flow rate densities are within the range which was calculated from Bowman's (1989) report on a field case. The venting apparatus and the non-enhanced containers were located outdoors, and all were kept in the shade. The ambient temperature (the same outside and inside the ventilated chamber) ranged from 21 ° to 38°C during the experiment with a maximum diurnal variation of 8°C.
360
P. F I N E A N D
B. Y A R O N
e
d
Y O GO
L
,g ,2o
v
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Fig. 2. Schematic drawing of the venting chamber used in the outdoor experiments: (a) centrifugal fan; (b) manometer; (c) soil container; (d) detachable wall; and (e) direction of air flow. All dimensions are in centimeters.
The containers used in the venting chamber were round and had a volume of 10 L (24cm deep; 17- and 19-cm internal diameter at the inner bottom and top, respectively). The containers were closed with 4-ply gauze at the bottom, to allow the free passage of air. A 2-cm layer of tuff, with a particle diameters of 0.5-2 cm, was placed at the bottom of the container and covered with gauze to separate it from the soil. The tuff layer at the bottom of the container transmitted an even stream of air to the soil bottom. Soil was packed to a known bulk density. Soils were either left air-dry, or moistened with tap water before kerosene was applied. The addition of each liquid was done after the containers had been placed in their final positions to ensure minimum cracking of the soil. Water or kerosene was applied slowly to the surface of the soil, which had been covered with 3-ply gauze prior to liquid application, to prevent the redistribution of particles. Water and kerosene were added in quantities equal to their respective field capacity or residual capacity. Following the addition to the soil, the liquid (kerosene or water) was allowed to redistribute itself in the soil column for 48 h. During this period, the container was covered with a fitted lid. Soil samples were taken from within the soil container at five depth increments of 5cm each, each sample layer averaging 1 cm in thickness. Sampling involved sacrificing one container at each sampling. Four samples were taken: two were extracted for hydrocarbon content determination, while
ENHANCED VOLATILIZATION BY VENTING OF KEROSENE COMPONENT FROM SOIL
361
the other two were washed with acetone and dried (105°C) to determine the dry weight percentage. Air permeability of soils was determined in situ on soils packed in the 10-L containers. The conical soil container was mounted onto an identical container, with the fitting secured by an O-ring and Silicone~tubricant. CO2 gas was passed through the outer container, and the flow rate and pressure were measured to calculate the permeability coefficient (K,, cm2). The measurement of the air permeability of air-dry soils always preceded the measurements of the soils which were equilibrated with kerosene. Note that equilibration with kerosene was done on air-dry soils or on soils which had previously been moistened to a water content equivalent to field capacity. The soil containers remained mounted on the measuring device while the processes of addition of either liquid and of equilibration took place. All measurements were done on duplicated pairs of containers. Each container was measured at seven different pressures and flow rates. The mean and standard deviation were calculated for all 14 measurements.
2.2.3. Analytical procedure 2.2.3.1. Extraction from soils. In the case of dry soils, kerosene was extracted in a tightly closed 35-mL glass vial b y adding 10 mL carbon tetrachloride to duplicate 0.5-g soil samples and shaking the samples overnight. The soil was allowed to settle and an aliquot of the supernatant was transferred to a 25-mL screw-cap flask with an AI liner. Anhydrous Na2SO4 and A1203 were added, to remove water and humic material. The flasks were stored at 4°C until measurement. The soil residue was washed twice in technical-grade acetone, dried at 105°C (step-wise), and weighed. The procedure was somewhat modified for moist soils; 3 mL of 1% sodium hexametaphosphate solution were added to the carbon tetrachloride to be shaken overnight. After the mixture had settled, ~ 5 m L of the carbon tetrachloride were filtered through 4 Whatman~filter paper, and stored in contact with anhydrous Na2 SO4 and AI2 03. Separate, 1-g duplicate samples were washed three times in 1:1 acetone-carbon tetrachloride mixture and dried to determine the dry weight percentage.
2.2.3.2. Gas chromatographic determination. A DB-l~megabore column, 30 m long, was used to separate the hydrocarbon mixture. The column was mounted in a Varian*~3300 gas chromatograph. Chromatography was performed in a programmed regime: 2 min at 45°C; heating from 45 ° to 200°C at 35°C min-~; and a quick warm-up to 300°C; the last temperature was maintained for 2.5 rain. The carrier gas was nitrogen. Hydrogen and air provided the combustion mixture for the flame ionization detector (FID).
362
P. F I N E A N D B. Y A R O N
Respective flow rates were 120, 30 and 300 mL min -j . The F I D was interfaced with a 2000-d Merck~"integrator to quantify the chromatogram peaks. The carbon tetrachloride extract and the standards were supplemented with an internal standard of propanol, which was added to a known mass of extract. A 1-/~L aliquot was injected into the column. The propanol peak preceded all other peaks. The total hydrocarbon content (total chromatogram area) and nine specific peaks were analyzed for each injection. The total kerosene content in soils was estimated by comparison with kerosene standards. 3. RESULTS AND DISCUSSION 3.1. Fundamental parameters
Both kerosene content and air permeability of soils are basic design parameters that control the removal of volatile contaminants from soils. Therefore, the primary measurements in a soil-venting experiment were devoted to the above parameters. 3.1.1. Kerosene residual content ( K R C ) The degree of soil saturation for a water-immiscible liquid can be expressed as the proportion of pore space for each liquid and air phase (van Dam, 1967; Schwille, 1984) or as the content of the medium in mL (Mercer and Cohen, 1990). These terms are often used interchangeably (e.g., Baehr, 1987). We use the term kerosene residual content (KRC) in mL/(100 g) by analogy with the field capacity of soils. Four soil moisture contents were studied, corresponding to oven-dry, air-dry, 33-kPa tension (field capacity) and 100-kPa tension. The K R C of the oven-dry soils ranged from 3.5 to 18.1 mL/(100 g). It was affected strongly by the clay content of the soil, with the more clayey soils having K R C values 1.5-2 times greater than the less clayey ones. The silt and organic matter (OM) contents also contributed to the KRC. The relationship between these soil components and K R C is described by the following equation:
K R C = 0.08 [% CLAY] + 1.68 [% OM] + 0.06 [% SILT] + 5.29
(1)
with a highly significant (p <0.01) coefficient of determination (R 2) of 0.91. The ranges of values of the three independent variables (X components) were: clay, 0.3-74%; OM, 0-5.2%; and silt, 1.2-59%. The two latter variables seem to cause some of the fluctuations of the K R C at zero moisture content. For example, the calcic, pale Rendzina had a high K R C of 18.1mL/(100 g), corresponding to the combination of a silty-loam texture (19.6% clay and 59% silt) and a high OM content (5.2%). K R C exhibited a linear relationship to the combination of soil texture
E N H A N C E D V O L A T I L I Z A T I O N BY V E N T I N G O F K E R O S E N E C O M P O N E N T F R O M SOIL
363
\
~J
y
t~CD
<..Q
Fig. 3. Kerosene residual content (KRC) of soils as a function of the clay and moisture contents. Soil moisture contents are indicated in Table 1.
(characteristic by clay content) and OM content, and was inversely linearly decreased with the moisture content as follows: K R C -- 0.13 [% CLAY] + 1.48 [% OM] - 0.32 [% MOISTURE] + 4.31
(2) 0.833, and was highly significant ( p < 0 . 0 1 ) (Fig. 3). The moisture content was in the range of 0-49%. The silt content made only a slight contribution to the R 2 coefficient, and was therefore omitted. Water filled up the soil capillaries, and caused a strong reduction of the hydrocarbon sorption on the mineral surfaces (Mingelgrin and Prost, 1989; Yaron, 1990). Increasing the soil moisture content, caused marked reduction of the K R C of the soils to a rather uniform level. The crucial effect of moisture content on K R C can be exemplified by comparing the fine-clayey Chromoxerert with the dune sand. The respective KRC's were 14.8 and 3.2 mL/(100 g) for air-dry soil, and 2.17 and 1.84mL/(100g) for moist soils (at - 3 3 - k P a tension). The reduction in K R C of the soils owing to increasing moisture content was considerably larger than the ~ 30% reduction at "field capacity" reported by Marley and Hoag (1986) for a medium-sized sand. It is interesting to note that the K R C of oven-dry and air-dry soils ( < 2mm) was smaller than their field capacity (WFC). This trend was reversed in the very sandy soils that had extremely low WFC's. For example, the K R C of the more clayey soils was two to three times less than their WFC. Our conjecture is that more kerosene can be adsorbed on external surfaces with low charge density (the vast majority of adsorption sites in sandy soils), as R2
=
364
P. F I N E A N D B. Y A R O N
0.16 -~--
I
INITIAL
0.10
% o
~
, 0.08
0.06 0.04
I
0.00
': GOLAN~2
GOLAN,2
NAHSHOLIM
CARMEL
SAND
NAHAL OZ
Fig. 4. Air permeability coefficient (Kd) of air-dried soils and soils equilibrated with kerosene.
compared with water, perhaps through hydrophobic interactions of this mixture of hydrocarbons. OM may also play a more crucial role in hydrocarbon sorption in such soils (Mingelgrin and Prost, 1989).
3.1.2. Air permeability of kerosene-treated soils Soil permeability to air was tested in the dune sand, in the loessial Nahal Oz soil, in the Nahsholim vertisol, and in the two differently sized fractions (2-5 and < 2 mm) of the Golan vertisol (Fig. 4). The soils were either air-dried or equilibrated to their K R C with kerosene before the measurements were taken. The air permeability coefficient (Ka) of the four air-dried soils ranged from 0.019cm 2 in the loessial soil to 0.145cm z in the coarsely aggregated (2-5 mm) Golan vertisol. It is interesting to note that the dune sand and the < 2-mm-sieve vertisol had similar/<,-values of 0.077 and 0.082 cm 2, respectively. Air permeability of the soils was measured again after their equilibration to their K R C with kerosene (Fig. 4). Kerosene addition decreased air permeability in the dune sand only. The K, was reduced by 9%. Note that the K R C of air-dried sand was 3.2 mL/(100 g), corresponding to an Sr of 0.11 (St being the fraction of the soil voids occupied by kerosene). Hence, the reduction of free soil porosity linearly reduced the air permeability in this non-interacting, siliceous matrix. Kerosene addition did not much affect the air permeability of the wellaggregated soils. For example, the Ka of the kerosene-equilibrated Nahsholim vertisol was 2.4% greater than that of the corrsponding air-dried material, inasmuch as the kerosene Sr was 0.30. This seems to point out that kerosene resides mainly on internal surfaces within the aggregates (micropores). Larger pores, which are involved in air flow, do not hold kerosene to an extent
365
ENHANCED VOLATILIZATIONBY VENTINGOF KEROSENECOMPONENT FROM SOIL TABLE 2
Air permeability coefficient (Ka) of moist and dry Nahsholim vertisol prior to, and after, kerosene addition, and during venting (mean _+standard deviation of 14 measurements) Duration days
K a (cm 2) enhanced venting air-dry
free volatilization moist*
air-dry
moist*
0.131 + 0.009 0.130 +0.012 0.132+0.006
0.080 _+0.005 0.114_+0.008 0.115+0.007
Prior to kerosene addition:
0
0.125 + 0.009
0.052 _+0.002
After kerosene addition:
0
0.128_+0.011
0.036_+0.001
,{~ter volatilization onset:
3 9 18 *Moisture content
0.094-+0.004 0.112-+0.009 0.119-+0.007 at field capacity.
0,130-+0.014 0.123-+0.010 0.118_+0.008
capable of affecting their permeability to air. The slight increase could have resulted from a hydrophobic shrinking of the hygroscopically moist soil aggregates; however, it is more likely that the increase is due to cracking. An extreme example of cracking was found in the loessial Nahal Oz soil. The Sr of the soil was 0.18, yet the K a increased by as much as 70%. After the internal soil surfaces had been blocked with water, only the larger pores could contain kerosene. Hence, addition of kerosene to a moist soil (Nahsholim vertisol; Table 2) reduced its permeability to air.
3.2. Enhanced volatilization of kerosene Volatilization of kerosene from soils is controlled by the saturated vapor density or vapor pressure of each component, and by the rate of transfer away from the surface, that is diffusion controlled. Advective vapor transport can be enhanced by using a venting apparatus capable of inducing air flow through the soil, to carry the kerosene vapor with it. A comparison of the free and enhanced mass transfer of whole kerosene, and of selected kerosene components as affected by the type of soil, its aggregation, and its moisture content, will follow.
3.2.1 Effect of soil characteristics Kerosene volatilization from four soils was tested under two sets of experimental conditions: with kerosene freely volatilizing to the atmosphere, or
366
P. F I N E A N D B. Y A R O N
A
100~
~~
1
I
'°1 t
i::l 17
ILCARMEL HAL OZ 7
8
§
TIME
2
(days) B
100-
~ND ;N
N
17 S 5 2 TIME
(days)
Fig. 5. Volatilization of kerosene from the 5-15 cm soil layer (in % of initial content): (A) free and (B) enhanced venting.
with an advective air stream driven by depression-induced venting. The venting continued for 17 days. Fig. 5 shows the loss of kerosene from the soils. Each value is the mean of duplicate determinations at three depths: 5, 10 and 15 cm. Evidently, the induction of air-phase transport through the soil in the enhanced venting treatment, strongly enhanced the release of light fractions of kerosene from all four soils at all sampling dates. It is also clear that the volatilization of kerosene under both sets of conditions strongly depended on soil characteristics. The latter dependence may be quantified by relating the loss of kerosene (LOSS as a percentage of KRC) to CLAY content, to hygroscopic moisture content (HYG), and to the air permeability coefficient (Ka). The percentage loss after 17 days of enhanced venting is described by the
E N H A N C E D V O L A T I L I Z A T I O N BY V E N T I N G O F K E R O S E N E C O M P O N E N T F R O M SOIL
367
following relation: LOSS = 515[Ka] + 17[HYG] - 2.7[CLAY] + 45
(3)
with an R 2 = 0.982. The contribution of air permeability to the correlation is clear in both the free and the enhanced venting methods. The air flux density (in m 3 per container per day), calculated for enhanced venting as specified above, ranged from 0.53 +0.02 in the loessial soil to 2.23_ 0.25 in the sand. The clay hindered kerosene volatilization, perhaps by limiting kerosene access to open voids. The HYG in itself was not expected to affect volatilization. However, the HYG of soils from Israel correlates well with their specific surface area (R 2 = 0.95; Banin and Amiel, 1969), the internal surfaces also being the site of kerosene binding or release.
3.2.2. Effect of soil aggregation The effect of soil aggregation on kerosene volatilization was studied by comparing the volatilization from the two fractions of Golan vertisol: the < 2-mm grain-size fraction and the 2-5-mm aggregates. The respective KRC of these fractions in the containers were 8.7_+0.25 and 4.1 _+0.11 mL/(100 g). These values were substantially lower than the 14.6 mL/(100 g) obtained after the long, steady-state equilibration procedure (Fig. 3). The total porosities of these two aggregate fractions were very similar: 53% and 55% of total soil volume, respectively. The difference in retention seems to emanate from the effect of pore size distribution on the rate of kerosene flow through the soil column and on the kinetics of kerosene retention in soil micropores. The coarsely aggregated soil had both fine porosity within the aggregates and contrasting coarse inter-aggregate porosity. It seems that the kerosene did not effectively compete with and displace hygroscopic water entrapped within the micropores. The two soil fractions differed also in air permeability, their respective K~-values being 0.082 _+0.009 and 0.145 + 0.011 cm 2. Fig. 6 shows that the rate of kerosene removal from the two soil fractions is related to their permeabilities to air and to the venting treatment. The more permeable to air the soil is, the more readily it releases kerosene under both enhanced and free venting treatments. The most efficient removal of kerosene from soil was obtained with enhanced venting applied to coarse aggregates. This combination facilitated the removal of four to seven times more kerosene than the finelyaggregated soil under free venting, which was the least effective combination. The high ambient temperatures (up to 38°C) during the last 3 days of the experiment enhanced the removal of kerosene; this increased removal of kerosene was more evident in the three least effectively treated specimens which still retained appreciable amounts of kerosene.
368
P. F I N E A N D
B. Y A R O N
100 8o Z
_o <~ N .-I
--
¢, >
60
!
|
40
20o B A < 2-mm
A
7 days ,ys
~5
days
2 days
B
> 2-mm
Fig. 6. Volatilization of kerosene from Golan vertisol as affected by aggregate size and venting method: (A) free and (B) enhanced venting.
3.2.3. Effect of soil moisture content The effect of initial soil moisture content was tested in the Nahsholim vertisol. The soil was either air-dried, or was moistened in the container, prior to kerosene application. The moisture content was 271 mL kg -j , which was somewhat low compared with the field capacity measured by means of pressure plates (Table 1). The K R C of the moist soil was 4.22 mL/(100 g), and that of the air-dried soil was 8.5mL/(100 g). Fig. 7 presents the loss of kerosene (as a percentage of KRC) from the soil container. The enhanced venting treatment was most efficient in removing kerosene from the dry, as well as from the moist soils. Interestingly, the kerosene loss with the moist treatments was faster and more complete than with the dry counterparts. For instance, the enhanced, vented, moist soil emitted more kerosene than did the dry soil, from any one layer and at any one time. Furthermore, the removal of kerosene from the dry-free combination (Fig. 7A-l) after 18 days was no more than 35% of the KRC, whereas the comparable loss from the moist-free (Fig. 7A-2) combination was twice as much, throughout the soil column. However, the relative basis is somewhat misleading. The two free venting treatments, dry (Fig. 7B-l) and moist (Fig. 7B-2), actually removed very similar total amounts of kerosene. They differed on a relative basis, because their initial KRC's were different. The high efficacy of kerosene volatilization with the moist treatments was unexpected, on the basis of the initial low permeability to air of this treatment (Table 2). However, soil cracking during drying restored the permeability of the soil to air to its pre-wetting value, within 3-9 days, at a rate that depended on the rate of evaporation. This is only one feasible explanation, because the
ENHANCED VOLATILIZATION BY VENTING OF KEROSENE COMPONENT FROM SOIL
A-I
A--~
B-1
B-2
411111 411111111111 111111111111111 ",s,
~m~IIllll~
369
o
--'~-~
Fig. 7. Changes in the kerosene concentration of the Nahsholim vertisol as affected by free (A) and enhanced-venting (B) volatilization from initially dry (1) and moist (2) soils.
K, of the moist-free treatment was still low compared with the dry-free treatment. It seems to us that the movement of water vapor through the soil enhanced the volatilization of kerosene components. We do not have a sound explanation for the K, fluctuations with the dry-enhanced treatment, except for non-homogeneity inherent in even a rather simple system such as this. The coefficient of variation in these measurements was 3-11% (0.00120.0141 cm2). Note also that the Ka-value of the initially air-dried soil in this experiment was greater than the value measured in the experiment reported above (Fig. 4).
3.2.4. Differential volatilization of kerosene components The chemical composition of the kerosene residue in the polluted soils changed with time owing to the differential volatilization of kerosene components. This is shown in Fig. 8 for the four venting treatments of the
370
E
P F I N E A N D B. Y A R O N
16 12
A-I
A-2
0,', y.~l.s -.-t- -
0
""o--3
E8 0 0
0
~0
I
"10
~ 12
I
I
I
I
!
I
I
B-I
o 8
B-2 / ~
,,7!
._,y
4
"%
o
X
i
E
o
x°
®
3
®
~I
® Io
c
~
:i:
Fig. 8. Changes in the relative concentrations of the kerosene components as affected by free (A) and enhanced-venting (B) volatilization from initially dry (1) and moist (2) Nahsholim vertisol.
Nahsholim vertisol. The analyses were done in extracts of soil samples which were taken at a depth of 10cm beneath the surface. Nine kerosene components were determined. The data points depict the percentage contribution of each component to the total K R C of the soil. The nine components appear in Fig. 8 in the order of their retention time. Each value is the mean of duplicate samples. It is evident from Fig. 8 that the contribution of each of the nine components to the kerosene residue in the soil depends on its relative volatility and on the experimental conditions: the mode of volatilization from soil (enhanced or free venting) and the initial soil moisture content. The composition of kerosene in the soil samples exposed to the experimental conditions least conducive to volatilization (air dried; free venting), was rather constant throughout the entire experiment (Fig. 8A-I). In treatments where volatilization was more intense, the less volatile, aliphatic, and other larger molecules (characterized by longer retention time), tended to dominate the spectrum of compounds within the residue. This was most prominent for enhanced venting of air-dried soil, which was the combination of conditions most conducive to kerosene volatilization (Fig. 8B-l). The heavier components already dominated the chromatogram after 9 days of venting, and became overwhelming after 18 days.
371
E N H A N C E D V O L A T I L I Z A T I O N BY V E N T I N G O F K E R O S E N E C O M P O N E N T F R O M SOIL
(A)
m-XYLENE
(A) n-DECANE
(A)
HEXADECANE
(8)
HEXADECANE
+++. ' ,,Ii JlMI IIIIIIIIIP, .+.;~ ""m£Jllllllllllllt~lllllllltlr
•
.-+, +.,,
(B) m-XYLENE
(B) n-DECANE
"~.e-.,~o~o-Qa-
Fig. 9. Concentration of m-xylene, n-decane and hexadecane (mL/(100 g soil) in the kerosene-polluted, air-dry Nahsholim vertisol as a result of free (A) and enhanced-venting (B) volatilization. The Z-axis (content) scale differs between compounds. Note that the X-axis (soil depth) and the Y-axis (time) for hexadecane are effectively interchanged, relative to those for the other compounds.
As mentioned previously, almost all the redistribution of kerosene components shown in Fig. 8 was due to differential volatilization into the atmosphere. There was but a small net enrichment of any component in any one layer or at any particular time. This is illustrated in Fig. 9, which shows the actual content of three representative kerosene components in the soil column (m-xylene (C8), n-decane (C10) and hexadecane (C16)) during the free and enhanced venting treatments of the air-dried Nahsholim vertisol. The Z-axis (content in mL/(100 g soil) in Fig. 9 is presented as a function of soil depth (X-axis), and vs. time (Y-axis). Note that the X-axis (soil depth) and the Y-axis (time) for hexadecane interchanged relative to those for the other components. It is evident that m-xylene, a relatively volatile component of kerosene, is eliminated quickly from the polluted soil. Under the enhanced venting treatment, it virtually disappeared from the entire soil column after the third day from the onset of venting. Free volatilization was less effective, and 30% of the initial content of m-xylene still remained in the soil after day 18. Furthermore, the content of m-xylene seemed to increase somewhat after the
372
P. FINE AND B. YARON
third day at the layer 5 cm beneath the surface. Under these conditions, trends in the distribution of n-decane are similar; its content in the 5- and 10-cm layers increased by the 3rd and 9th day from onset, respectively. The loss of n-decane under enhanced venting was almost complete by the 18th day. Hexadecane had a different mode of redistribution; its content did not decrease appreciably throughout the 18-day period. Hexadecane was somewhat removed from deeper layers, and was redeposited at the soil surface. This was reflected by an increase in hexadecane content at the soil surface of up to five times the initial content. The increase was more marked under free volatilization than under enhanced venting. As venting continued, the more effective stripping of the enhanced venting removed this redeposited hexadecane. The removal occurred in the unenhanced treatment too, but to a lesser extent. It should be mentioned that changing the experimental conditions may strongly alter the pattern of differential volatilization. 4. C O N C L U S I O N S
The kerosene residual content exhibits a strong positive linear relationship to the combination of clay and organic matter contents and a negative linear relationship to the soil moisture content. Kerosene content reduces the permeability of sand to air; however, it does not significantly affect the permeability of the other soils. Two factors are shown to control the rate of volatilization of petroleum hydrocarbons from soils. One is their vapor pressure and the other is their transfer to and away from the surface. The induction of air-phase transport in the soil under enhanced-venting treatment strongly enhances the removal of the lighter fractions of kerosene from all the porous materials studied. The volatilization causes a substantial shift in the composition of the kerosene residue in soil to dominance of heavier fractions. The rate of loss of hydrocarbons from soil increases with coarser aggregation and with the cracking of initially moist soils. Soil constituents and the initial moisture content need to be considered, in conjunction with soil residual capacity for kerosene and with soil permeability to air, as fundamental design parameters for an enhanced venting decontamination procedure. ACKNOWLEDGEMENTS This study was supported by the research grant WT 8951/913 of the Israel Ministry of Science and Development and the German Bundesministerium ffir Forschung und Technologic and by grant 308-130 from the Chief Scientist of the Ministry of Agriculture, Israel.
ENHANCED VOLATILIZATION BY VENTING OF KEROSENE COMPONENT FROM SOIL
373
REFERENCES Agrelot, J.C., Malot, J.S. and Visser, M.J., 1985. Vacuum: Defence system for ground water VOC contamination. Proc. 5th Natl. Symp. of Aquifer Restoration and Groundwater Monitoring, Columbus, OH, May 21-24, 1985. API (American Petroleum Institute), 1980. Underground spill cleanup manual. Am. Pet. Inst., Washington, DC, Publ. No. 1628. Baehr, A.L., 1987. Selective transport of hydrocarbons in the unsaturated zone due to aqueous and vapor phase partitioning. Water Resour. Res., 23: 1926-1938. Baehr, A.L. and Hoag, G.F., 1987. A modeling and experimental investigation of induced venting of gasoline contaminated soils. Proc. Conf. on Environmental and Public Health Effects of Soils Contaminated with Petroleum Products, Univ. of Massachusetts, Amherst, MA, Oct. 30-31, 1985. Baehr, A.L., Hoag, G.E. and Marley, M.C., 1989. Removing volatile contaminants from the unsaturated zone by inducing advective air-phase transport. J. Contam. Hydrol., 4: 1-26. Banin, A. and Amiel, A., 1969. A correlative study of the chemical and physical properties of a group of natural soils of Israel. Geoderma, 3: 185-198. Bowman, R.S., 1989. Manipulation of the vadose zone to enhance toxic organic chemical removal. In" Z. Gerstl, Y. Chen, U. Mingelgrin and B. Yaron (Editors), Toxic Organic Chemicals in Porous Media. Springer, Berlin, pp. 275-286. DiGiulio, D.C., Cho, J.S., Dupont, R.R. and Kemblowski, M.W., 1990. Conducting field tests for evaluation of soil vacuum extraction application. Proc. 4th Natl. Outdoor Action Conf. on Aquifer Restoration, Ground Water Monitoring and Geophysical Methods. Natl. Water Well Assoc., Dublin, OH, pp. 587-601. Donaldson, S.G., Miller, G.C. and Miller, W.W., 1992. Remediation of gasoline-contaminated soil by passive volatilization. J. Environ. Qual., 21: 94-102. Fleischer, E.J., Noss, P.R., Kostecki, P.T. and Calabrese, E.J., 1986. Evaluating the subsurface fate of organic chemicals of concern using the SESOIL environmental fate model. Proc. 3rd Eastern Reg. Groundwater Conf., Natl. Water Well Assoc., Springfield, MA. Galin, T., Gerstl, Z. and Yaron, B., 1990a. Soil pollution by petroleum products, III. Kerosene stability in soil columns as affected by volatilization. J. Contam. Hydrol., 5: 375-385. Galin, T., McDowell, C. and Yaron, B., 1990b. The effect of volatilization on the mass flow of a non-aqueous pollutant liquid in an inert porous media: experiments with kerosene. J. Soil Sci., 41: 631-641. Gierke, J.S., Hutzler, N.J. and McKenzie, D.B., 1992. Vapor transport in unsaturated soil columns: implication for vapor extraction. Water Resour. Res., 28: 323-335. Hoag, G.E. and Cliff, B., 1988. The use of soil venting technique for the remediation of petroleum-contaminated soils. In: E.J. Calabrase and P.T. Kostecki (Editors), Soils Contaminated by Petroleum, Environmental and Public Health Effects. Wiley, New York, NY, pp. 301-315. Johnson, J.J. and Sterrett, R.J., 1988. Analysis of in situ air stripping data. Proc. 5th Natl. Conf. on Hazardous Wastes and Hazardous Materials, HMCRI (Hazard. Mater. Contain. Res. Inst.), Las Vegas, NV, Apr. 19-21, 1988, pp. 451-455. Johnson, P.C., Kemblowski, M.W. and Colthart, J.D., 1988. Practical screening models for soil venting applications. Natl. Water Well Assoc.-Am. Pet. Inst. Conf. on Petroleum Hydrocarbons and Organic Chemicals in Groundwater, Houston, TX. Johnson, R.L., 1989. Soil vacuum extraction: laboratory and physical model studies. Proc. of the Workshop on Soil Vacuum Extraction, R.S. Kerr Environ. Res. Lab., Ada, OK, Apr. 27-28, 1989.
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