Soil Interactions With Petroleum Hydrocarbons

SOIL TECHNOLOGY

ELSEVIER Soil Technology 10 (1997) 133-153

Soil interactions with petroleum hydrocarbons: Abiotic processes

P. Fine, E.R. Graber * , B. Yaron Institute of Soils and Water, ARO, The Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel

Received 18 January 1995; accepted 22 January 1996

Abstract

Soil and groundwater resources in many parts of the world are threatened by spilled petroleum products, These products generally consist of complex mixtures of volatile hydrocarbons with different vapor pressures. The volatilization of light hydrocarbon fractions leads to changes in viscosity and density of the residual nonaqueous liquid. This may cause changes in the transport characteristics of the porous matrix and of the residual liquid. Differing volatilization and solubility characteristics result in differential distribution of released hydrocarbon components in air, soil, and water environmental compartments. Soil frequently serves as the site of petroleum spills and hence the capacity of the soil to filter, retain, or release hydrocarbons is fundamental in determining the type and extent of environmental contamination. Retention, volatilization, and transport of hydrocarbons as affected by soil physical and chemical properties is reviewed.

Keywords: Petroleum; Soil; Retention; Volatilization; Transport

1. Introduction

Soil and groundwater contamination by leaking petroleum products threaten important resources, jeopardize local and regional ecological systems, and may even pose the threat of explosion in urban areas. Soil frequently serves as the site of petroleum spills and hence the capacity of the soil compartment to filter, retain, or release hydrocarbons will be fundamental in determining the type and extent of environmental contamination.

* Corresponding author. Tel.: + 972-3-9683307; fax: + 972-3-9604017; e-mail: [email protected].

00933-3630/97/$15.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved. PI2 SO933-3630(96)00088-8

134 P. Fine et al. /Soil Technology 10 (1997) 133-153

Extensive environmental contamination by hydrocarbons has underscored the impor- tance of understanding the dynamics of hydrocarbon distribution in different environmental compartments. The fate of petroleum products released into the soil will be particularly varied because these products generally consist of complex mixtures of hydrocarbons with greatly differing vapor pressures and water solubilities. Differences in these physical and chemical properties will lead to differential distribution of released hydrocarbon components in air, soil, and water.

The redistribution and fate of petroleum hydrocarbons spilled on the land surface will depend on both biotic and abiotic processes. The abiotic processes occurring within porous media which are instrumental in controlling hydrocarbon distribution include sorption, volatilization, transformation, and transport. Sorption of hydrocarbon molecules at the solid surface may occur from the nonaqueous liquid, vapor, or aqueous phases. Vaporization from the nonaqueous liquid, or desorption of sorbed molecules to the vapor or aqueous phases may also play an important role in hydrocarbon redistribution in the soil. Ultimately, hydrocarbons are transported either in bulk, as a nonaqueous liquid, in vapor, or in low concentrations in the aqueous phase.

Abiotic aspects of soil-petroleum hydrocarbon interactions were reviewed recently by Calabrese and Kostecki (19881, Yaron (1989), Mercer and Cohen (19901, and Kostecki and Calabrese (1991). The aim of the current work is not a general literature review, rather, a discussion emphasizing the main impacts of soil materials on the fate of petroleum products in the soil compartment. Soil properties including mineralogy, texture, and moisture status, and their effect on sorption, retention, volatilization, and transport will be presented and discussed based on results from our own research group (Acher et al., 1989; Yaron, 1989; Yaron et al., 1989; Galin et al., 1990a; Galin et al., 1990b; Fine and Yaron, 1993; Gerstl et al., 1994; Nye et al., 1994; Jarsjo et al., 1994.

After a petroleum spill on the land surface, petroleum hydrocarbon components will be redistributed in the soil. Retention on the solid phase will occur by entrapment in pores or sorption at mineral and organic matter surfaces. The highly volatile fraction will volatilize into the gas phase of the porous medium. From there it may be lost to the atmosphere, sorbed onto soil solids, or dissolved into soil water or groundwater. Less volatile components will be transported in the nonaqueous phase through the porous medium. Finite but differing aqueous solubilities for the disparate components will also result in differential dissolution and transport as solutes in soil water. If the amount of petroleum in the soil zone is greater than its retention capacity, the hydrocarbon mixture will travel into the vadose zone as a separate, nonaqueous phase. Petroleum-soil interactions will be discussed in the light of this scenario.

2. Materials and methods

The experiments reviewed below were performed on a broad spectrum of soils whose characteristics are summarized in Tables 1 and 2. Five petroleum hydrocarbons (m- xylene, isopropylbenzene (ps-cumene), t-butyl benzene, n-decane and n-dodecane) and a commercial petroleum product ‘kerosene’ characterized by a mixture of more than 100 different hydrocarbons were used in the experiments.

P. Fine et al./Soil Technology 10 (1997) 133-153 135

Table 1 Characteristics of soils used in the experiments (after Gerstl et al., 1994)

Soil series Texture (wt%) SSA” (m’“/g) organic Moisture content (wt%) and

clay silt sand matter

HYG b -33kPaC -1OOkPa horizon (wt%)

1. Golan (Ap): very-fine clayey, montmorillonitic, thermic typic chromoxerert 74.4 - 10.6 -15.0 - ii6 9.6

2. Carmel (Ap): very-fine clayey, montmorillonitic, thermic typic pelloxerert 62.5 30.4 7.1 - 5.1 8.4

3. Nahsholim (Ap): fine-clayey, montmorillonitic, thermic typic chromoxerert 54.1 37.0 8.9 - 1.9 6.8

4. Nahal Oz (Ap): fine-loamy, mixed, thermic calcic haploxeralf 18.1 30.2 51.7 - 2.0 2.0

5. Hermonit: fine clayey, mixed, mesic typic rhodoxeralf (Ad; 29.6 53.6 16.8 - 2.0 4.3 (BQ) 56.4 34.4 9.2 - 0.65 6.2 6. Fazael (Bt): fine-loamy, mixed, hyperthermic typic haplargid

28.3 44.8 24.0 - 0.60 4.4 7. Netanya 8 (Bt2): fine-loamy, mixed, thermic typic rhodoxeralf

25.3 7.3 67.4 - 0.34 4.1 8. Netanya 6 (Bt2): tine-loamy, mixed, thermic palic rhodoxeralf

35.2 14.5 50.3 - 0.47 5.9 9. Mitzpe Masua (A): tine-loamy, carbonatic, thermic lithic haploxeroll

19.6 59.0 21.4 - 5.20 2.8 10. Sharon 2 (Bw): sandy, siliceous, thermic typic xerochrep

5.1 2.2 92.7 - 0.11 0.48 11. Dune sand

0.3 1.2 98.5 - 0.01 0.35 12. Bet Dagan (Ap): loamy, mixed, thermic typic haploxeralf

12 68 0.5 0.8 13. Gilat (Ap): fine-loamy, mixed, thermic calcic haploxeralf

16 52 44 117 0.8 2.9 14. Oxfordshire, UK (Evesham clay) (A)

35 300 7.3 6.5

49.0 42.0

45.4 38.2

30.2 25.8

18.3 17.8

34.3 18.5 34.1 25.8

32.3 20.8

17.6 12.8

25.4 21.2

33.1 23.2

3.4 2.8

3.1 2.6

12.0 -

19 -

35.0 -

a Specific surface area. b Hygroscopic moisture content. ’ Pressure

2.1. Experimental procedures

Kerosene residual capacity (KRC) of moist soils was determined after pre-adjusting soil moisture by means of pressure plates. The moistened soil cylinder was immersed in kerosene to a depth of 2 cm for 72 h. The soil cylinder was then turned upside-down and re-equilibrated for another 72 h. The redistribution of kerosene was performed at room temperature (22 f 3°C) in seated desiccators over open vessels of kerosene and water. The kerosene content of the soils was determined in sub-samples taken immediately below the exposed surface. All kerosene residual content determinations were made in triplicate.

136 P. Fine et d/Soil Technology 10 (1997) 133-153

Table 2 Characteristics of the soils used in transport experiments (after Galin et al., 1990a; Galin et al., 1990b; Gerstl et al., 1994; Jarsjo et al., 1994)

Soil Texture a (%) Org. matter Bulk Total Hygroscopic Water ret.

clay silt sand (%) density b porosity b water capacity (g cmm3) (%I (wt%) WRC)

(wt%) Coarse sand 1 Coarse sand 2 Medium sand Fine sand Sandy loam 1 Sandy loam 2 Loamy mont ’ Clay mont ’ Glacial clay Post gl clay Peat

-

0 - - - - 19 14 15 18 16 36 40 17 51 21 44 25 - -

100 86

100 100

67 67 48 43 28 31 -

0.1 1.82 30 0.35 1.0 0.4 1.80 30 0.28 10 0.1 1.84 29 0.40 2.4 0.1 1.73 33 0.48 2.8 5.4 1.42 44 2.46 47 4.4 1.38 46 2.29 46 0.005 1.47 42 2.5 19 0.012 1.39 54 4.9 28 1.3 1.37 44 3.19 44 2.6 1.44 40 3.36 50

27.8 0.68 63 5.03 74

a According to the USDA system of classification. b In KRC experiments. ’ Montmorillonitic.

Volatilization experiments were performed in glass petri dishes (9.4 cm in diameter and 1.1 cm in height) which were packed with 50 g of air-dry sand in a uniform 4 mm layer. Kerosene was applied to each soil to the pre-determined kerosene retention capacity. The experiments were carried out in a constant temperature room at 27°C for 14 d. Kerosene loss was determined by gravimetric measurements daily, and gas chromatographic identification of residual kerosene was made at 1, 2, 5, 7, and 14 d after application. The experiments were performed in triplicate.

Volatilization of kerosene from contaminated soils was also examined in an outdoors enhanced volatilization system with induced air flow (Fine and Yaron, 1993). A venting chamber was designed to accommodate eight 10 L containers. An industrial centrifugal fan in the ceiling of the chamber created a depression of = 350 Pa which forced air to flow through the soil; air flows were calculated from measured air permeability and the depression created by the fan.

Sorption-desorption isotherms of hydrocarbons from and into the vapor phase were determined at constant temperature. Soil (5 g) was weighed into small vials (4.5 cm2 evaporative surface, 5 cm long) which were placed in glass jars (1 L). A vial containing kerosene (5 mL), providing the vapor source, was placed in the center of each jar which was then closed and sealed with Teflon tape. A rubber septum was fitted to the lids of each jar to enable gas samples to be taken with a gastight syringe. Control jars (without soil) were prepared for analysis of the vapor. The experiment was carried out with five replicates at 27°C for 7 weeks. Adsorption was carried out at 27°C for 3 weeks. During this time the atmospheric composition above the soil samples was checked periodically. Desorption was achieved by leaving the soil samples open to the ambient atmosphere for four weeks. Residual kerosene was analyzed in a time series. These experiments were carried out in triplicate.


Kerosene conductivity experiments were conducted in glass columns with a 51 mm i.d. and a height of 100 mm, plugged at both ends with rubber stoppers covered with Al foil and penetrated fully by glass tubes. The height of the soil column was 70 mm. The columns were packed using a funnel connected to a rubber tube. By gradually raising the funnel, the soil was uniformly packed into the cylinder from the bottom upward, avoiding gravitational fractionation of particles. This procedure resulted in uniform packing densities of 1.84 g/cm3 for the sand and 1.47 and 1.39 g/cm3 for the loam and clay soils, respectively.

Conductivity was determined by the constant-head method as described by Klute (1965). The columns were slowly saturated from below with kerosene to drive out entrapped air. Conductivity measurements (four replicates) were obtained by keeping a constant head of kerosene over the columns and collecting the column eluate.

2.2. Analytical procedures

Kerosene and petroleum hydrocarbons were extracted from the sand and soils in tightly closed 25 cm3 glass flasks by adding 10 mL of Ccl, to 5 g of soil or sand. The flasks were placed on a laboratory shaker for 24 h and then centrifuged (2500 rpm) for 10 min. An aliquot of the Ccl, was transferred to a 3 cm3 screw-cap flask with an Al liner, and Na,SO, and Al,O, were added to remove water and humic material, respectively. The samples were stored at 4°C until analysis.

Extract aliquots (3 pL) were manually injected into a Varian 3300 gas chromato- graph equipped with a flame ionization detector connected to a Merck-Hitachi D-2000 integrator and a 30 m DB-1 megabore column (i.d. 0.53 mm, film thickness 1.2 pm). Chromatographic conditions were: 2 min at 5O”C, ramp of 2”/min to 12O”C, and ramp of SO”C/min to 220°C. The carrier gas was N,. The flow rates for N,, H,, and air were 30, 30, and 300 cm3/min, respectively.

Kerosene viscosity was measured by a ‘Schott Gerate’ type 53001/01 viscometer in a water bath at 27°C. All measurements were replicated five times.

3. Results and discussion

3.1. Retention

Petroleum products can be retained by soil solids either by entrapment in soil capillaries and pores or by sorption at particle surfaces. Physical and chemical properties of the soil solid phase, including hydration status, texture and organic matter content, control the degree of hydrocarbon entrapment and sorption.

3.1.1. Physical entrapment Entrapment is a form of non-adsorption retention of nonaqueous phase liquids in

soils. The immiscible fluids (water and nonaqueous liquid) hinder each other’s transport

138 P. Fine et al/Soil Technology 10 (1997) 133-153

Fig. 1. Kerosene residual content (KFX) of soils as a function of clay and moisture contents (Fine and Yaron, 1993).

through the soil until a minimum saturation is reached. Nonaqueous fluids can then become trapped as ‘blobs’ or ‘ganglia’ in pores in the unsaturated zone for an indefinite time (Schwille, 1994) serving as a source of contamination which decreases in magnitude as the nonaqueous liquid volatilizes, dissolves in water, or degrades. The degree of soil saturation for a water-immiscible liquid can be expressed as the proportion of pore space occluded by a given liquid phase (van Dam, 1967; Schwille, 1981) or as the liquid content in volume per unit weight of soil (Mercer and Cohen, 1990).

The extent of trapping is determined primarily by the physical properties of the vadose zone and the nonaqueous contaminant. Fine and Yaron (1993) and Jarsjo et al. (1994) studied the influence of soil constituents (sand, silt, clay and organic matter) and soil moisture content on entrapment of kerosene, measured as kerosene residual capacity (KRC) in analogy to water residual capacity.

The physical retention of liquid kerosene in soil pores, or kerosene residual capacity, was evaluated for eleven different soils under four soil moisture regimes (oven-dry, air-dry, 33 kPa tension and 100 kPa tension) (Fine and Yaron, 1993). KRC of the oven-dry soils ranged from 3.5 to 18.1 mL/lOO g, and was in most cases significantly lower than the corresponding water retention capacity (33 kPa tension). KRC was affected strongly by the clay content of the soil, such that heavy clay-rich soils had KRC values 1.5-2 times greater than light sandy soils (Fig. 1). KRC was also directly related to silt and organic matter (OM) contents. Using multivariant analysis, a highly significant relationship between silt, clay, and organic matter components and KRC was obtained. In a similar study of glacial soils, KRC was also found to vary as a function of soil texture and organic matter content (Jarsjo et al., 1994).

When moisture content was varied, both Fine and Yaron (1993) and Jarsjo et al. (1994) found that KRC was inversely linearly related to the moisture content and was linearly related to soil texture (characterized by clay content) and OM content (Fig. 1). As soil moisture content increased, the KRC decreased markedly and attained a

P. Fine et al. / Soil Technology IO (1997) 133-153 139

relatively uniform, low value in all soils. It was postulated that water was strongly retained in small soil capillaries resulting in a reduction in soil effective porosity. This would lead to a significant reduction in both the physical entrapment of liquid hydrocarbon in small pores and capillaries and a reduction in hydrocarbon sorption at mineral and organic matter surfaces (e.g., Mills and Biggar, 1969; Mortland, 1970; Chiou and Shoup, 1985; Pignatello, 1989; Mingelgrin and Prost, 1989).

3.1.2. Sorption from the vapor phase Sorption of petroleum hydrocarbons by the soil solid phase and on different soil

constituents has been the subject of numerous studies over the years. Nathwani and Philips (1977) studied the sorption of selected hydrocarbons from the vapor phase (benzene, o-xylene, toluene, and n-hexadodecane) on a number of Canadian soils and found that for concentrations between 1 and 100 ppm, hydrocarbon sorption was well described by the Freundlich isotherm. Soils with a higher organic matter content exhibited higher values of the distribution coefficient. Many other studies reported that sorption of organic compounds from vapor, aqueous, and organic solvent phases onto clay surfaces could be described by the Langmuir isotherm or more complex, composite isotherms (Greene-Kelly, 1954a; German and Harding, 1969; Olejnik et al., 1974). The intercalation of hydrocarbon species into expanding layer silicates is considered a process by which some part of the interlayer water associated with the exchangeable cations is replaced by the organic species (Theng, 1974). In dehydrated systems where the silicate layers of a clay crystal are fully collapsed, intercalation of hydrocarbons is either absent or proceeds only with difficulty (Greene-Kelly, 1954b). The studies of Clementz (1976) on adsorption of heavy petroleum fractions (asphaltenes) from solution at montmorillonite surfaces confirm these findings.

Nye et al. (1994) studied the kinetics of vapor sorption of m-xylene and n-dodecane on oven- and air-dried Evesham clay soil and on a medium sand. Sorption equilibrium for both hydrocarbons was reached in less than 2 h on the sand and in 40-80 h on the clay soil (Fig. 2). m-xylene reached sorption equilibrium faster on the oven-dry Evesham clay soil than on the air-dried clay soil. The total sorption capacity of the air-dried clay was nearly twice that of the oven-dried clay while the sorption capacity of the sand was relatively unaffected by its moisture content. The total sorption capacity of the clay for m-xylene was 20 to 40 times that of the sand.

The sorption capacity of the clay soil for n-dodecane was approximately 15 times that of the sand. The rate and total uptake of n-dodecane were equal in both the air-dried and oven-dried sand, but the total sorbed amount in the air-dried clay was less than in the oven-dried clay. In contrast, m-xylene sorption was greater in oven-dried sand than air-dried sand, and far higher in air-dried clay than in oven-dried clay (Fig. 2). This behavior can be partly explained on the basis of structurally-related size exclusion phenomena. The n-dodecane molecule is too large to penetrate clay interlayer spaces and can sorb only to external surface area. In the air-dried clay, water successfully competes for sorption sites resulting in a reduction of n-dodecane sorption compared with oven-dried clay. In the sand, the moisture content for air-dried and oven-dried sands are so similar that there is little or no competition for external sorption sites between water and n-dodecane. The smaller, planar m-xylene molecule in contrast, can

140 P, Fine et al./Soil Technology 10 (1997) 133-153

m - Xylene n - Dodecone

0 2 4 6 8 0 4 8 12 16

Time (days 1 Time (days)

Fig. 2. Kinetics of m-xylene and n-dodecane vapor sorption by medium sand and Evesham clay soil (Nye et al., 1994).

penetrate clay interlayers in the expanded air-dried form, while the oven-dried clay interlayer spaces are not accessible. In dehydrated systems the silicate layers of clay minerals are fully collapsed and the intercalation of non-polar organic liquids and gases is either absent or proceeds only with difficulty, with sorption occurring predominantly on external surfaces (Theng, 1974). In partly dehydrated or air-dry systems where clay layers are slightly separated, intercalation of nonpolar organics is favored. Sorption of m-xylene sorption in the air-dried and oven-dried sand is similar, with a small reduction in total sorption on the air-dried sand. In this case, water may have successfully competed for surface sorption sites with the m-xylene molecule.

Sorption isotherms for m-xylene and n-dodecane are plotted in Fig. 3 as micromoles of compound per gram soil versus the relative vapor concentration of the compound, P/P,. The use of P/P,, in isotherm plots to normalize the activity of each compound with respect to its own pure state permits a direct comparison between the uptake of different compounds. P values were measured and P, values for the compounds studied were taken from the literature.

Slopes of the isotherms for both m-xylene and n-dodecane in the clay soil increase with increasing P/P,,. Uptake of both compounds is strongly affected by moisture content and by soil texture. The maximum sorption on oven-dried sand was about 10 pmol/g for m-xylene and 5 p,mol/g for n-dodecane, while on the Evesham clay soil uptake was nearly 300 p,mol/g for m-xylene and over 100 p,mol/g for n-dodecane. Partial hydration of the Evesham clay soil (air-dried) resulted in a reduction of n-dodecane sorption for every P/P,. In contrast, sorption of m-xylene by the air-dried clay soil was either equal (P/P, < 0.5) or greater (P/P, > 0.5) than for oven-dried soil.

These studies considered interactions between individual hydrocarbon species and the

P. Fine et al./Soil Technology IO (1997) 133-153 141

Evesham clay Sand

8

P/Pa P/PO

Fig. 3. Sorption of m-xylene and n-dodecane vapors by medium sand and Evesham clay soils. Lines are best-fit third-order regressions through the origin (Nye et al., 1994).

bulk soil or soil constituents. Petroleum products, however, are comprised of complex mixtures of many hydrocarbons with individual components competing for sorption sites on porous media. Only recently has the adsorption of petroleum products in the vadose zone been studied as a function interactions and competition between individual components (e.g., Baehr and Caropcioglu, 1987; Acher et al., 1989; Yaron et al., 1989).

Soil texture has a strong impact on the kinetics of hydrocarbon sorption from gaseous hydrocarbon mixtures (Yaron et al., 1989; Nye et al., 1994). Vapor phase sorption of five compounds (m-xylene, ps-cumene, n-decane, n-but.-benzene, and n-dodecane) from a mixture by three soils with different clay and organic matter contents (Bet Dagan: clay 12%, OM 0.5%; Gilat: clay 16%, OM 0.6%; Oxford: clay 35%, OM 7.2%) is shown in Fig. 4 (Yaron et al., 1989). For oven-dry soils, sorption of the studied compounds was greatest on the Gilat soil and was essentially equal in the Bet Dagan and Oxford soils. Despite the fact that the Oxford soil has twice the clay content of the Gilat soil, its sorptive capacity was significantly lower. This may be because sorption sites on mineral surfaces are occluded by soil organic matter. Complementary studies of sorption from the same hydrocarbon mixture on peat showed very low sorption capacity. In sorption from the vapor phase, free mineral surfaces rather than organic matter control hydrocarbon sorption. This is in contrast to sorption from the aqueous phase, where soil organic matter content is strongly correlated with sorptive capacity (Karickhoff, 1981; Chiou, 1989; Chin et al., 1991). In a related study it was shown that the sorption order from the mixture (m-xylene > n-decane > ps-cumene > t-butylbenzene > n-dodecane) corresponds to increasing boiling point and molar weight, rather than to vapor pressure (Acher et al., 1989).

An increase in soil moisture content led to a decrease in soil sorptive capacity for all the hydrocarbons studied (Fig. 5; Yaron et al., 1989). At a soil moisture content

142 P. Fine et al./Soil Technology 10 (1997) 133-153

- Bet Dagon

---- Oxford

.-.-. Gilot

zocyy;* ;-l,y 1 J&L-&y 1 0 IO 20 30 40 0 IO 20 30 40

Time (days) Time (days 1

Fig. 4. Sorption kinetics for hydrocarbons from a surrounding atmosphere of a simulated ‘kerosene’ by three different soils: Bet Dagan (clay 12%, OM 0.5%); Gilat (clay 16%, OM 0.6%); and Oxford (clay 35%, OM 7.2%) (Yaron et al., 1989).

BET ‘XW MOISTURE (76)

-.-.-.- 0.0

0.0 _______________-- II.2 + STANDARD DEVIATION

TIME (days)

Fig. 5. Effect of initial soil moisture content on the kinetics of hydrocarbon adsorption from a surrounding atmosphere of a simulated ‘kerosene’ by Gilat soil (Yaron et al., 1989).

P. Fine et al/Soil Technology 10 (1997) 133-153

100 m- Xylene - Bet Dogon ---- Oxford .-.-. Gilot

n - But. Benzene

143

Time(h) Time (h)

Fig. 6. Desorption of kerosene components by volatilization from the surfaces of three oven-dried soils as a function of time: Bet Dagan (clay 12%. OM 0.5%); Gilat (clay 16%, OM 0.6%); and Oxford (clay 35%, OM 7.2%) (Yaron et al., 1989).

equivalent to 70% of field capacity (11%) the adsorption of hydrocarbons was almost negligible. This behavior was also observed for the same hydrocarbon mixture by Acher et al. (1989) and by Chiou and Shoup (1985) and Barbee and Brown (1986) for benzene and xylene, respectively.

3.2. Volatilization

Petroleum products typically contain components with vapor pressures ranging over several orders of magnitude. Rates of volatilization for each component will differ such that more volatile components will diffuse out of the porous framework more rapidly than less volatile compounds. The residual hydrocarbon mixture will thus be enriched in less volatile components.

Desorption of kerosene components from soil surfaces to the vapor phase depends upon the vapor pressure of each component and is affected by soil properties (Fig. 6; Yaron et al., 1989). The highest rate of desorption was exhibited by m-xylene and the slowest by n-dodecane. The Oxford and Gilat soils had the highest, essentially co-equal retention capacities while the Bet Dagan soil had a significantly lower retention capacity. Residual hydrocarbon after 7 days in the Bet Dagan soil was n-dodecane > n- butylbenzene > ps-cumene > n-decane > m-xylene. Significant hydrocarbon residues were found in both the Oxford soil and Gilat soils.

144 P. Fine et al. / Soil Technology 10 (1997) 133-1.53

17 8 5 2

TIME (dayr)

I ’

I II

I SAND

-5OLAN

17 8 6 2

TIME (days)

Fig. 7. Volatilization of kerosene from the 5-15 cm soil layer (in % of initial content): (A) free and (B) enhanced venting. (Fine and Yaron, 1993.)

Fine and Yaron (1993) reported the results of a study in which the volatilization of kerosene from four soils was examined as a function of natural or enhanced venting. The soils were characterized by clay, moisture and organic matter content: dune sand, very fine montmorillonitic clay soil with 1.6% OM (Golan), very fine montmorillonitic clay soil with 5.1% OM (Carmel), and fine loamy soil with 2% OM (Nahal Oz) (Table 1). Induction of an-phase transport via enhanced venting significantly increased the loss of kerosene through volatilization from all four soils (Fig. 7). Volatilization losses were also dependent on soil clay content, moisture status, and air permeability. This dependence was quantified by relating the loss of kerosene (LOSS as a percentage of KRC) to clay content, hygroscopic moisture content (HYG), and air permeability coefficient (K,). The percent loss after 17 days of enhanced venting was strongly dependent on the air permeability:

with an R2 = 0.98. The contribution of air permeability to the correlation was clear in

P. Fine et al. /Soit Technology 10 (1997) 133-153 145

o.*A :p 0.6 -&

I 0.4 3

--------- \A ‘--l-i il

------. ----__

0.2 '!.\.A

0 \ N-ep.-. .*.- , . . ,

Time(h)

. - . - . - . - .

0 40 60 120 160 200

Time (hl

Fig. 8. Evaporation of m-xylene (solid squares), n-dodecane (solid circles), and m-xylene:n-dodecane mixtures (32:68 open squares; 54:46 solid triangles; 84:16 open triangles). Lines are model predictions, symbols are actual data. The results are plotted as fraction of initial mass remaining after time dependent evaporation from (A) liquid, (B) medium sand, (C) Evesham oven-dry clay, (D) Evesham air-dry clay (Nye et al., 1994).

both the free and the enhanced venting methods. The air flux density (in m3 per container per day) calculated for enhanced venting ranged from 0.53 & 0.02 in the loessial soil to 2.23 k 0.25 in the sand. Increased clay content resulted in decreased kerosene loss by volatilization, perhaps by blocking kerosene access to open voids or because the voids themselves are smaller.

Nye et al. (1994) studied the relative volatilization of m-xylene and n-dodecane separately and as mixtures from solution and soil (Fig. 8). The rate of volatilization of the mixture is controlled by the ratio between its two components. As volatilization proceeds, the decrease in m-xylene:n-dodecane ratio in the mixture led to a decrease in the rate of evaporation of the hydrocarbon mixture. Interactions between the two components and sorption at soil surfaces temporarily reduced the vapor phase concentration and diffusion into the gas phase. Once the system reached a steady state, however, the final apparent steady state vapor diffusion coefficient was a function of the air-filled porosity and was not related to properties of the soil matrix. As in the case of individual petroleum hydrocarbon mass transfer into the atmosphere, the greatest remaining portion of initial mass for the mixtures occurred in oven-dry Evesham clay soil, and the highest volatilization rate occurred in a coarse sand. These results agreed with those of Mackay and Yeun (1983), such that volatilization from a mixture of components is a function of the vapor pressures of individual components.


3.3. Transport

Multiphase transport of petroleum hydrocarbons in porous media is affected by the hydraulic and physical properties of the soil as well as its hydration status. Acher et al. (1989) showed that during liquid kerosene infiltration into a soil, there is simultaneous but faster vapor transport creating a penetration front in advance of the liquid front. The penetration of the hydrocarbon vapor may be two to three times greater than that of the immiscible liquid. As the composition of the vapor phase is different from that of the original hydrocarbon mixture (Fig. 91, soil contamination by the vapor phase will be different from that caused by the immiscible liquid phase.

The initial soil moisture content affects the penetration of hydrocarbons both as an immiscible liquid and as vapor in the gas phase. Liquid hydrocarbon infiltration rate and extent in a sandy loam increased with increasing soil moisture content (Fig. 10; Acher et al., 1989). Infiltration rate and extent for the vapor, in contrast, was inversely related to soil moisture content. The highest rate and greatest depth of m-xylene vapor penetration was observed in oven-dry soil (moisture content O%), with decreasing penetration rate and depth with increasing moisture content (Fig. 11). The distribution of the less volatile n-dodecane showed no dependence on soil moisture content (Fig. 11). The redistribution of n-decane was very similar to that of m-xylene, while that of r-butylbenzene and of pseudo-cumene was between that depicted by (a) and (b) in Fig. 11. In another series of column experiments, a soil moisture content over 40% prevented the development of a separate vapor phase. These results are similar to those of Barbee and Brown (198~3

IOO-

80 -

60 -

40-

20 -

% 0 -

20 -

40-

Initial I Day 50 Days

Vapour phase

i

Liquid phase 60

p m-Xylenc

80 1 ps- Cumcna

100 tJ n-But.- Benzen

a n-oecane

n n - Oodscane

Fig. 9. Relative compositions of simulated ‘kerosene’ and surrounding gaseous phase (after Yaron et al., 1989).


Y

Moisture content (%)

I - 0.0 2 - 0.8 3 - 4.0 4 - 12.0

of ’ ’ ’ ’ ’ ’ ’ ’ 0 2 4 6 8 IO 12 I4 I6

Time (days)

Fig. 10. Penetration depth as a function of time of simulated ‘kerosene’ in a Bet Dagan soil column as affected by initial soil moisture content: oven-dry, 0.0%; air-dry, 0.8% (Acher et al., 1989).

who reported that xylenes applied to a sandy loam with a moisture content close to field capacity were retained by the soil and their movement significantly attenuated by the presence of water in soil pores.

In additional column studies, it was shown that kerosene components moved faster and deeper in air-dry than in oven-dry soil columns (Acher et al., 1989). This behavior agrees with the adsorption pattern described above. The extent of redistribution with depth was affected by the hydrocarbon properties. In the case of oven-dry soils, the penetration of the kerosene products was in order of increasing vapor pressure, with m-xylene exhibiting the greatest depth of penetration, and n-dodecane the least. In the

Fig. 11. Redistribution of (a) m-xylene and (b) n-dodecane from a simulated liquid ‘kerosene’ in Bet Dagan soil columns with different initial moisture contents: 1, 0.0%; 2, 0.8%; 3, 12.0%. Numbers on curves represent days after ‘kerosene’ application (Yaron et al., 1989).


I 1 I .t” E 2

Cl4 Cl3 Cl4

C (b) Sandy

clay

Retention time (min 1

Fig. 12. Composition of a kerosene in soils (a) immediately, and (b) 14 days after addition.

case of air-dried soil columns, the redistribution of kerosene components was only slightly affected by their volatility.

Interactions between soil and nonaqueous phase liquids are affected not only by soil texture and moisture content but by the nature of the nonaqueous liquid. The composition of a volatile organic liquid mixture (kerosene) in different soils undergoes preferen- tial volatilization of lighter fractions as a function of clay content (Fig. 12). As the composition of the mixture changes, the physical properties of the residual liquid also change. The relationship between the viscosity of kerosene after volatilization and the relative concentrations of its major components is depicted in Fig. 13 (Gerstl et al., 1994). An increase in kerosene viscosity was accompanied by a relative increase in heavy fractions (C13-C15) and decrease in light fractions (C9-Cl 1). Galin et al.

viscosity (Pa 0 10-3)

0 c9 Cl0 CII Cl2 Cl3 Cl4 Cl5

Carbon number

Fig. 13. Composition of kerosene as a function of viscosity (Gerstl et al., 1994).


1.2 1.4 1.6 I.8 2.0

Viscosity (Pa s 10m3)

Fig. 14. Kerosene conductivity as a function of viscosity (Gerstl et al., 1994).

(1990b) showed that the increase in viscosity due to the change in kerosene composition was not accompanied by changes in its surface tension and by only small changes in its density (from 0.805 to 0.819 g ml-‘). An increase in viscosity from 1.32 to 1.96 lop3 Pa s led to a 28% decrease in kerosene conductivity in sand, a 76% decrease in loam soil, and an 83% decrease in clay soil (Fig. 14). Intrinsic permeability for the three soils was calculated by:

K= kpgp-’

where p is fluid density (g ml-‘), g is acceleration due to gravity (m’/s) and p, is dynamic viscosity of the fluid (Pa s- ’ 1. The measured kerosene intrinsic permeability of the sand was unaffected by changes in viscosity, while in the loam and clay soils, intrinsic permeability decreased as viscosity increased (Fig. 15)

The rate of flow of a fluid through a pore is proportional to the fourth power of its diameter (Poiseuille’s law), so that even a small change in the pores’ effective diameter may have a significant effect on permeability. Effective porosity may have been reduced by the presence of hydrocarbon films on soil surfaces forming an immobile region analogous to that of immobile water in natural soil-water systems.

A second mechanism which can reduce the intrinsic permeability of a medium is the swelling of expandable components such as clays. No changes in intrinsic permeability were detected in a non-swelling medium (sand) upon volatilization of lighter components (Galin et al., 1990a; Galin et al., 1990b; Gerstl et al., 19941, while intrinsic permeability decreased in swelling media (loam and clay) (Gerstl et al., 1994). Thus, swelling or dispersion of clay in the changing solvent mixture may have been responsi- ble for decreases in intrinsic permeability.

Swelling and contraction of expandable clays in contact with solvents are sometimes explained by the diffuse double layer (DDL) theory which predicts an increase in the

150 P. Fine et aL/Soil Technology 10 (1997) 133-153

u .,z is

- - Theorctkol

2 0.2- --.-- Sand

--o-- Loan

--m-- Cloy

0 I I I I I I I 1.3 1.4 1.5 1.6 I.? 1.8 1.9 2.0

Viscosity (Po s (OS31

Fig. 15. Kerosene conductivity (relative to that of the original kerosene) as a function of viscosity. The solid line was calculated with the experimentally determined values of the viscosity and density of the kerosene (Gerstl et al., 1994).

equilibrium distance between clay plates as pore fluid dielectric constant (E) increases (Barshad, 1952; Norrish, 1954; Olejnik et al., 1974; Murray and Quirk, 1982). However, Graber and Mingelgrin (1994) showed that regular solution theory is more successful in describing clay swelling in solvents than is the DDL theory, in accordance with the suggestion that swelling is controlled by solvation rather than changes in the DDL (MacEwan, 1948; Low and Margheim, 1979; Low, 1980; Low, 1981). The regular solution theory is a thermodynamic model originally developed to describe miscibility in binary, small molecule, non-polar systems (Hildebrand et al., 1970). It is also successfully applied to swelling or dissolution of polymers in solvents. Graber and Mingelgrin (1994) extended the regular solution theory to clay swelling in solvents by way of analogy to polymer swelling.

Permeability of a porous medium can either decrease or increase as a result of swelling depending on the composition and structure of the porous medium. In a system of uniform spheres of radius R, intrinsic permeability is proportional to R2. As R increases, intrinsic permeability also increases. If interstices between the spheres are filled with cuboids and the volume of each cuboid increases (for example, sand and clay), the intrinsic permeability of the system will decrease.

Gerstl et al. (1994) reported that in a clay soil, volatilization of kerosene was accompanied by a decrease in intrinsic permeability. During volatilization, the dielectric constant of the mixture remained unchanged. Thus, by the DDL theory, the observed decrease in intrinsic permeability cannot be attributed to swelling. However, the solubility parameter of the mixture did increase. This, according to the work of Graber and Mingelgrin (19941, can result in clay swelling and the observed reduction in intrinsic permeability.

P. Fine et al/Soil Technology 10 (1997) 133-153 151

4. Conclusions

Petroleum products commonly consist of complex mixtures of volatile hydrocarbons with different vapor pressures. In soils contaminated by petroleum products, the volatilization of light hydrocarbon fractions will lead to changes in the viscosity and density of the residual nonaqueous liquid. These changes affect transport characteristics of the residual liquid. Changes in residual hydrocarbon composition that occur upon volatilization may also result in irreversible changes in the transport characteristics of the porous media by virtue of interactions between the liquid and the matrix. Interactions resulting in swelling or shrinking of clays, for example, can alter the intrinsic permeability of the porous medium. Both vapor phase and nonaqueous liquid phase transport can contribute to soil contamination. As soil moisture content decreases, vapor phase transport increases and soil retention of hydrocarbons increases. With increasing moisture content, vapor phase transport and soil retention by entrapment or sorption decreases, resulting in an increase in transport of nonaqueous phase liquid. Contami- nants transported in the vapor phase will differ from those transported in the liquid phase.

Acknowledgements

The reported results were obtained with the support of the Ministry of Science and Technology (MOST), Israel, and the Bundeministerium fur Forschung und Technologie (BMFT), Germany.

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