Fate of the Oil Residuals in Patagonian Soils
Effects of the Environmental Exposure Time

N. S. Nudelman;* S. M. Ríos and O. Katusich

*Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Pab. 2, Ciudad Universitaria, (1428), Argentina. e-mail: nudelman@qo.fcen.uba.ar. Facultad de Ciencias Naturales, Universidad Nacional de la Patagonia San Juan Bosco, Km 4, (9004) Comodoro Rivadavia, Argentina. e-mail: riossm@unpata.edu.ar

ABSTRACT

The distribution coefficient of oil residuals between water and the soil under equilibrium conditions is, in the case of Patagonian soils, strongly dependent on the clay contents and humidity of the soil. Characteristic Patagonian soils have low amounts of organic matter, (usually less than 1%), and high clay contents; therefore, they are convenient media to study the influence of clay in crude oil sorption, avoiding interactions with organic matter. Others variables such as, the environmental exposure time and the soil salinity, affects also the interactions between the phases.

Determinations of the equilibrium aqueous concentration of oil residuals of different ages in soils, were carried out. The distribution coefficients (K_d) vary between 900 (L kg^-1) and 6,600 (L kg-¹) showing a general and marked increase for residues of increasing age. The composition of the oil residue has also an important effect on the sorption. The determined parameters are useful for the modeling of environmental impact on polluted soils and for the design of remediation techniques.

KEY TERMS: Oil, Patagonian soil, sorption, hydrophobic contaminants, petroleum exploitation, soil remediation.

INTRODUCTION

The oil exploration and exploitation is one of the main economic activity in the Patagonian since 80 years ago. This activity has generated, over time, soil contamination during the extraction, storage and transport. When spilled on soil, the environmental behavior of oil components is controlled by a number of processes, such as evaporation, dissolution, degradation and sorption. Recent research is focused on the development of empirical models to predict partition coefficients (Xing, McGill, Dudas, 1994), (Haderlein, Schwarzenbach, 1993) on the application of adsorption and desorption kinetics of organic compounds in sediments (Karimi-Lotfabad, Pickard, Gray, 1996), (Huang and Weber, 1997), (Johnson, Keinath, Weber, 2001), as well as the effects of water on sorption and transport in soils (Lane, Loehr, 1992).

The behavior of the oil components in aqueous phase is of critical importance because solute transport and transformation processes are known to occur predominantly in water. This has driven recent studies of aged polluted systems along the time, with the aim of evaluating the environmental impact on variables such as: the variation of polynuclear aromatic hydrocarbons (PAHs) concentrations in soils and sediments (Hellou, Payne, Hamilton, 1994), (Jackson et al., 1994), (Wild, Jones, 1995) and the changes in oil residuals composition along the environmental exposition time (Short, Heintz, 1997), (Chung, Martin, 1998). Also some physical and chemical properties change during environmental exposure, thus, variations in e.g., hydrocarbons solubility, sorption kinetics and distribution coefficients, can be evaluated (Short, Heintz, 1997), (Chung, Martin, 1998), (Nudelman, Ríos, Katusich, 2000).

Previous studies show that the extractability of PAHs compounds, decreased with increasing contact time and this suggests that sequestration may be an important process resulting in the apparent loss of PAHs in soil and sediments (Macleod C. and Semple K., 2000), (Northcott G., Jones K., 2001). Therefore, the aim of this work was to investigate if the variations observed in the distribution coefficients of the oil residuals, can be interpreted in relation with the age of the oil spill and if, on this basis, any relation with the actual oil residue composition, could be observed. The influence of the clays and salinity soil contents, was also analysed.

MATERIALS AND METHODS

Contaminated soil samples, product of oil spills in ten different locations in the surroundings of Comodoro Rivadavia's city, were obtained. The oil spills are of different ages, crude oil sources and environmental exposure conditions. Table 1 summarizes some properties of the samples.
The contaminated soils were air-dried, ground, and sieved with a 1.7 mm sieve. The total hydrocarbon determination in each sample was carried out by Soxhlet continuous extraction with methylene chloride for 24 h to 48 h, depending on the sample. After distillation of the methylene chloride, the different oil samples were used for the calibration curves (see below).

The desorption protocol was as follows. For each soil sample, water and aliquots of approximately 0.1 g of each soil sample were placed in 15 mL test tubes (at least by duplicate). To each tube, 10 mL of pure water was added, making the soil solution ratio 1:100. Calcium chloride was added to achieve a matrix of 0.01 N CaCl₂ to provide a constant ionic strength and minimize nonsettling particles (Lane, Loehr, 1992). The tubes containing the soil-solvent slurries were periodically shaken during seven days. The soil solution ratio and the total contact time were selected on the basis of previous studies.

Because the samples came from spills more than 2 years old, during which they undergo important degradation processes, it was not considered necessary to add agents that inhibited degradation during the short times of the tests. Although, biodegradation process would take place during this period (7 days) mainly in the youngest samples, the experimental conditions such us the high background salinity, the nutrients absence do not favor them. The tubes were sealed with plastic film and covered by aluminum foil to avoid light exposure and prevent photooxidation.

After the required time had elapsed the supernatant solution was separated from the soil and the solution centrifuged (10 min at 3000 rpm) to exclude any soil particle. The supernatant was immediately withdrawn from each tube after centrifugation and the oil concentration was analyzed using UV-Visible spectrophotometry. The absorbances were measured for each sample at the corresponding wavelengths in the range 200-400nm and the integrated areas were used for the determinations (Nudelman, Ríos, Katusich, 2000). In parallel runs the UV-Visible spectra of the oil samples obtained by Soxhlet extraction were registered and they were used as calibration curves.

Desorption determinations were carried out at ambient temperature (22±3°C). As mentioned previously, the contaminated soils were air-dried. It could be presumed that desorption of pollutants on air-dried soil did not reflect desorption in field, but it has been shown that the air-drying has a minimal effect on the degree of solubilization (Lane, Loehr, 1992).
The oil residues were analyzed by silica gel column chromatography, to separate group components (i.e. Aromatic, aliphatic and polar compounds). The eluent solvents were hexane (30mL), benzene (30mL) and methanol/chloroform (1:1) (30mL). The portion remaining in the column contains the “asphaltenes” fraction (Nudelman, Ríos, Katusich, 2000). The same procedure was utilized for crude oils, to compare results.

RESULTS AND DISCUSSION

The distribution coefficients Kd (L kg^-1) are shown in Figure 1 as a function of the age of the spills. The values are between 900 L kg^-1 and 6,600 L kg^-1. As known, the distribution coefficient of a solute between water and a solid phase (as the contaminated soil) gives the relationship of its concentration between both phases in equilibrium conditions. Their value can be strongly influenced by the characteristics of the soil, such as the content of organic matter, clays and humidity, the composition of the oil residuals and the environmental exposure conditions. As it has been reported in previous works (Nudelman, Ríos, Katusich, 2000), in the case of the regional soils, the content of organic matter and the water are extremely low; the percentage of clays could be then, the only significant factor. However, the observed relationship between the K_d and the clay contents shows that not a simple correlation exits in this case.
The increase of K_d, from samples 1-10, could be attributed, initially, to the loss of the more soluble components of the spilt oil, and subsequently, to the loss of the degradation products. The observed linear relationship between K_d and age is shown in Figure 1; it follows equation (i):

K_d = Kd⁰ + s. t (years) (i)

where the intercept is Kd⁰ = 737 L kg^-1 (±286 L kg^-1), and the slope of the straight line, s = 157 L kg^-1 years^-1 (±15 L kg^-1 years^-1). The calculated regression values are: r2 = 0.928. Extrapolation of this relationship for times smaller than two years is not valid since the more important dispersion and degradation processes of spilled oil components takes from 2-5 years to be completed depending on the oil characteristics and the exposure conditions (Wang, et al., 1998) and (Garrett et al., 1998).

Previous studies report that aged oil residuals show similar characteristics (Pucci, Bak, Peressutti, 1996) and (Wang, et al., 1998). The oil residual could be formed by the recalcitrant original components, particularly the resins and the asphaltenic fraction (Wolfre, et al., 1994), (Venkateswaran, et al., 1995) and by the products of their successive transformations. Numerous examples in the bibliography show the use of different indexes for the evaluation along the time of processes that modify the composition of oil spills in the environment. For example, the differential depletion of n-alkanes to branched alkanes has been used as an indicator of biodegradation (ratios C₁₇/pristane and C₁₈/phytane) (Wang, et al., 1998). The decrease in the Aromatic fraction concomitant with an increase in the resin and polar fractions has been used as an indicator of photooxidation in the degradation of crude oils spilled at the sea (Garret, et al., 1998). Furthermore, it was also found that some ratios of alkylated naphthalenes, phenanthrenes, and chrysenes, can qualitatively asses the extent of weathering an oil has undergone since a spill (Douglas, et al., 1996).

For the interpretation of the hydrosolubility time dependence, the index (Aliph+Arom)/(Pol+Asph) could be used, because the aliphatic and aromatic groups (Aliph and Arom, respectively) are responsible for the higher global hydrophobic characteristics, while the polar and asphaltene groups (Pol and Asph, respectively) have the higher global hydrophilic characteristic. The Figure 2 shows the ratio (Aliph+Arom)/(Pol+Asph) for the case of regional crude oils and for the degraded environmental samples (see Table 1). It can be observed that the ratio (Aliph+Arom)/(Pol+Asph) for crude oils indicates a high content of aliphatic and aromatic components. As it is exposed to the environment, the aliphatic fraction strongly decrease due to the loss by volatilization in the first stages, while the polar fraction decreases, too, due to loss by solubilization (Pucci, Bak, Peressutti, 1996), (Wang, et al., 1998), (Garrett et al., 1998). Polar compounds could, additionally be formed through the aliphatic biodegradation and photooxidative processes of aromatics (Douglas et al., 1996), (Dutta, Harayama, 1996), (Balmer, Goss, Schwarzenbach, 2000). This is consistent with the observed ratio (Aliph+Arom)/(Pol+Asph) for the samples 1, 2 and 3, these residuals contain a high proportion of polar components and would present important hydrophilic characteristics.

Since the rate of the volatilization and degradation processes decreases along the time the concentration of the aliphatic components tend to become constant while the polar ones decrease due to their high solubility. Therefore, an increase of the index with age is expected, as it is observed for the samples 4-7 in relation with samples 1-3. The Figure 2 shows that the ratio (Aliph+Arom)/(Pol+Asph) tends to become almost constant for the samples 4-10. This plateau could be interpreted as an indication of the probable stabilization of the oil composition.
Therefore, the increase of K_d values with age could be attributed, not only to the loss of the more soluble components, but also to sequestration which may be an important process; in fact, the indexes reflects the actual samples composition determined by organic solvent extraction, while the K_d values give only an idea of the water solubility.
High values of the electric conductivity were measured in soil, as shown in Table 1. The oil residuals, in exploration and production areas, are generally accompanied by water spills that are extracted together with the oil and, that frequently, have salinity similar to the sea water. These salts stay on soils during long times and finally they became part of the soil. Our previous studies show that K_d values increase with increasing soil salinity and decrease when the salinity of the initial aqueous concentration is greater than the soil salinity.

However, more investigations are needed because the interaction of the different kinds of compounds between them and with the solid phase, and the fact that the solubility of each compound is influenced by the presence of the others, makes impossible, at the moment, to develop a more precise model for the complex interactions in this multicomponent system. Anyway, the present determinations of oil residues distribution coefficients give a global idea about the degree of stabilization of residuals in the environment and of the processes that predominantly have affected them along the exposure time, and they can be useful to improve the effectiveness of the remediation technologies.

REFERENCES

1. Balmer, M.E, Goss K. and Schwarzenbach, R.P., 2000. Photolytic Transformation of Organic Pollutants on Soil Surface-An Experimental Approach. Environ. Sci. Technol., 34, 1240-1245.
    
2. Chung, N. and Martin, A., 1998. Differences in Sequestration and Bioavailability of Organic Compounds Aged in Dissimilar Soils. Environ. Sci. Technol. 32, 855-860.
    
3. Douglas, G.S., Bence, A.E., Prince, R.C., Mcmillen, S.J. and Butler, E.L., 1996. Enviromental Stability of Selected Petroleum Hydrocarbon Source and weathering Ratios. Environ. Sci. Technol. 30, 2332-2339.
    
4. Dutta, T. and Harayama, S., 1996. Fate of Crude Oil by the Combination of Photooxidation and Biodegradation. Environ. Sci. Technol. 34, 1500-1505.
    
5. Garrett, R., Pickering, I., Haith, C. and Prince, R., 1998. Photooxidation of Crude Oils. Environ. Sci. Technol. 32, 3719-3723.
    
6. Haderlein, S.B. and Schwarzenbach, R.P., 1993. Adsorption of Substituted Nitrobenzenes and Nitrophenols to Mineral Surfaces. Environ. Sci. Technol. 27, 316- 326.
    
7. Hellou, J., Payne, J.F. and Hamilton, C., 1994. Polyciclic Aromatic Compounds in Northwest Atlantic COD. Environmental Pollution 84, 197-202.
    
8. Huang, W. and Weber, W., 1997. A Distributed Reactivity Model for Sorption by Soils and Sediments. 10. Relationships between Desorption, Hysteresis, and the Chemical Characteristics of Organic Domains. Environ. Sci. Technol. 31, 2562-2569
    
9. Jackson, T.J., Wade, T.L., McDonald, T.J., Wilkinson, D.L. and Brooks, J.M., 1994. Polynuclear Aromatic Hydrocarbon Contaminants in Oysters from the Gulf of Mexico. Environmental Pollution 83, 291-298.
    
10. Johnson, M., Keinath, M. and Weber, W. Jr., 2001. A Distributed Reactivity Model for Sorption by Soils and Sediments. 14. Caracterization and Modeling of Phenanthrene Desorption Rates. Environ. Sci. Technol. 35, 1688-1695.
     
11. Karimi-Lotfabad, S., Pickard, M. and Gray, M., 1996. Reactions of Polynuclear Aromatic Hydrocarbons on Soil. Environ. Sci. Technol. 30, 1145-1151.
     
12. Lane, W. and Loehr, R., 1992. Estimating the Equilibrium Aqueous Concentrations of Polynuclear Aromatic Hydrocarbons in Complex Mixtures. Environ. Sci. Technol. 26, 983-990.
     
13. Macleod, C. and Semple K., 2000. Influence of Contact Time on Extractability and Degradation of Pyrene in Soils. Environ. Sci. Technol., 34, 4952-4957.
     
14. Nudelman, N., Ríos, S.M. and Katusich, O., 2000. Interactions between Crude Oil and Patagonian Soil as a function of the Soil clay-water contents. Environ. Technol. 21, 437-445.
     
15. Northcott G. and Jones K., 2001. Partitioning, Extractability, and Formation of Nonextractable PAH Residues in Soil. 1. Compound Differences in Aging and Sequestration. Environ. Sci. Technol. 35, 1103-1110.
     
16. Pucci, O., Bak, M. and Peressutti, S., 1996. 2das Jornadas de Preservación de Agua, Aire y Suelo en la Industria Petrolera, Instituto Argentino del Petróleo y del Gas, San Martín de los Andes, Argentina, 291-297.
     
17. Short, J.W. and Heintz, R.A., 1997. Identification of the Exxon Valdez Oil in Sediments and Tissues from Prince William Sound and the Northwestern Gulf of Alaska Based on a PAH Weathering Model. Environ. Sci. Technol. 31, 2375-2384.
     
18. Venkateswaran, K., Hoaki, T., Kato, M. and Maruyama, T., 1995. Microbial degradation of resins fractionated from Arabian light crude oil. Can. J. Microbiol. 41, 418-424.
     
19. Wang, Z.., Fingas, M. , Blenkinsopp, S., Sergy, G., Landriault, M., Sigouin, L. and Lambert, P., 1998. Study of the 25-Year-Old Nipisi Oil Spill: Persistence of Oil Residues and Comparisons Between Surface and Subsurface Sediments. Environ. Sci. Technol. 32, 2222-2232.
     
20. Wild, S.R. and Jones, K.C., 1995. Polynuclear Aromatic Hydrocarbons in the United Kingdon Environment: A preliminary Source Inventory and Budget. Environmental Pollution 88, 91-108.
     
21. Wolfe, D.A., Hameedi, M.J., Galt, J.A., Watabayashi, G., Short J., O´ Claire , C., Rice, S., Michel, J., Payne, J.R., Braddock, J., Hanna, S., Sale, D., 1994. The Fate of the Oil Spilled from the Exxon Valdez. Environ. Sci. Technol., 28, 561-569.
     
22. Xing, B., McGill, W.B.and Dudas, M.J., 1994. Cross-Correlation of Polarity Curves To Predict Partition Coefficients of Nonionic Organic Contaminants. Environ. Sci. Technol. 28, 1929-1933 .

Table 1.Description of oil contaminated soils samples

Figure 1. Kdw (L kg-1) of oil contaminated soil samples as a function of the age (in years).

Figure 2. Indexes (Aliph+Arom)/(Pol+Asph) as a function of the age (in years), for environmentally degraded oil spills in patagonian soils (circles). The indexes for regional crude oils (triangles) are also plotted (age=0) for comparison.
 

Top