B. Effects of Spill Age

Interaction of Oil Residues in Patagonian Soil



153



FIG. 7 C w , equilibrium aqueous concentration (mg LϪ1 ) as a function of the age of the

e

spill (years).



For the interpretation of the hydrosolubility time dependence, the ratio

(Aliph ϩ Aro)/(Pol ϩ Asph) could be used. The ratio (Aliph ϩ Aro)/(Pol ϩ

Asph) for the case of regional crude oils are 4.59 Ϯ 1.08 and for the degraded

environmental samples are 1.03 Ϯ 0.31 (age, 2–3 years) and 2.31 Ϯ 0.48 (age,

6–57 years). It can be observed that the ratio (Aliph ϩ Aro)/(Pol ϩ Asph) for

crude oils indicates a high content of aliphatic and aromatic components. In the



FIG. 8



TM



K w , distribution coefficients (L kgϪ1 ) as a function of the age of the spill (years).

d



Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved.



154



´

Nudelman and Rıos



case of the degraded environmental samples, Aliph, Aro, Pol, and Asph represent

component fractions (wt%) of the extractable hydrocarbons (EH).

As it is exposed to the environment, the aliphatic fraction decreases due to

loss by volatilization and biodegradation, while the polar fraction decreases, too,

due to loss by solubilization [13,14,17]. But polar compounds could additionally

be formed through aliphatic biodegradation and photooxidative processes of aromatics [2,18]. This is consistent with the ratio (Aliph ϩ Aro)/(Pol ϩ Asph) for

the youngest degraded environmental samples, which contain a high proportion

of polar components and would exhibit important hydrophilic characteristics.

It is known that the chemical extractability and bioavailability of hydrophobic

organic compounds (HOCs) from soil decrease with increasing contact time. The

decrease in extractability may be controlled by physical sequestration of HOCs

and limited mass transfer from soil to solvent or by the action of a soil’s microbial

community. This decrease in extractability and bioavailability has important implications for the risk assessment of HOCs in historically contaminated soil. The

process of HOC sequestration in soil is thought to be driven by partitioning into

the soil organic matter (SOM) and sequestration into soil micropores [19].

For example, the amount of PAHs extractable by butanol and dichloromethane

decreased with compound aging in soil. The decrease in PAH extractability with

aging, and the formation of nonextractable bound residues, increased with compound molecular weight, K ow and K oc . Calculated half-lives for the apparent loss

of PAHs by sequestration were dependent on the method used to extract them

from soil. Sorbed compounds are less available for partitioning and leaching in

groundwater and exhibit reduced bioavailability, toxicity, and genotoxicity compared to dissolved counterparts.

Organic compounds that persist in soil exhibit declining extractability and

bioavailability with increasing contact time, or “aging.” In the past it was assumed

that these observations were due to the degradation of contaminants by microbial

processes in the soil. However, studies utilizing isotopically labeled compounds

have demonstrated that significant amounts of compound are retained in the soil

as nonavailable and nonextractable sequestered residues increase with increasing

soil contact time, or aging. Aging is associated with a continuous diffusion and

retention of compound molecules into remote and inaccessible regions within the

soil matrix, thereby occluding the compounds from abiotic and biotic loss processes [20].

Since the rate of degradation decreases with time, the concentration of the

aliphatic components tends to become constant, while that of the polar ones decreases due to its high solubility. Therefore, an increase in the ratio (Aliph ϩ

Aro)/(Pol ϩ Asph) is expected with age, and the ratio (Aliph ϩ Aro)/(Pol ϩ

Asph) tends to become constant, as observed. This could be interpreted as a

probable indication of EH compositional stabilization. Then the increase in K d values with age not only could be attributed to the loss of the polar components,



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Interaction of Oil Residues in Patagonian Soil



155



but it also suggests that sequestration may be an important process. Because the

index reflects the actual EH composition obtained via organic solvent extraction,

the K d -value gives an idea of the water solubility of the components that are

available to interact with the aqueous phase only.



C. Effects of Soil-Phase and Aqueous-Phase

Ionic Strengths

A factor complicating bioremediation of crude oil spills is salinity. The oil residuals in exploration and production areas are generally accompanied by water spill

that is extracted together with the oil and that frequently has a similar salinity

to seawater. These salts stay on the soils for long times and they become part of

the soil. The changes in ionic strength in the aqueous phase affect the partitioning

of PAHs to surfactant micelles and sorbed surfactants, thus conditioning their

remediation [21]. The aqueous solubility of organic compounds in the presence

of dissolved salts can be expressed by the Setschenow relationship [22]:

log S w, salt ϭ log S w Ϫ K s [salt]



(9)



where S w, salt is the molar solubility in the presence of salts, S w is the molar aqueous

solubility, K s (MϪ1 ) is a function of the hydrophobic surface area of the compound,

and [salt] (molar) is the concentration of dissolved salts. This relationship has

been used, for example, to calculate the aqueous solubility of such organic pollutants as chloroform, lindane, and vinyl chloride in seawater [22].

Equation (9) is strictly valid for only a single solute; however, the applicability

of the equation was tested considering the oil residual as only one solute. The

scope of the equation to evaluate the variation of K d with ionic strength was also

examined. The aqueous concentration and the distribution coefficients in this case

are global values, and therefore they account for the interactions among the components in the mixture and for the overall interactions of each of them with the

mineral matrix [22]. The electrical conductivity of the aqueous phase, C, is a

good measure of total ionic strength (the ionic content characteristic of the soil

plus the added salt, calcium chloride in this work), and a relationship like that

of Eq. (9) can be formulated between C and K w :

d

ln



΂ ΃



Kw

d

ϭ a(C Ϫ C 0 )

K w0

d



(10)



where “a” (µSϪ1 cm) is the slope of the straight line, C is the electrical conductivity of the aqueous phase (µS cmϪ1 ), C 0 is the electrical conductivity of the aqueous phase without CaCl 2, K w (L kgϪ1 ) is the distribution coefficient observed with

d

C, and K w0 is the distribution coefficient observed with C 0. The slope of the

d

straight line, “a”, is (1.33 Ϯ 0.05) ϫ 10Ϫ2 µSϪ1 cm for the oldest degraded envi-



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´

Nudelman and Rıos



156



ronmental samples and (1.92 Ϯ 0.26) ϫ 10Ϫ2 µSϪ1 cm for the youngest degraded

environmental samples. The regression values were r 2 Ն 0.923 in both cases.

According to the model, an increase of sorption is observed when the ionic

strength of the aqueous phase increases. The increase of slope “a” for the youngest degraded environmental samples implies a higher salinity effect on K d , in

agreement with the relative increase of polar compounds when age decreases.

A semiempirical model was developed that allows prediction of K d as a function of exposure time, the salinity of the aqueous phase, and the soil’s clay content. The last variable was included because previous studies show an important

dependence of K d on the soil’s clay content [3]. The linear relationship between

the calculated and measured values of K d has a slope equal to 0.994 (r 2 ϭ 0.884);

this value indicates that ln K d can be estimated with an error of less than 6%.

Although the correlation coefficient is relatively poor, it can be considered a good

fit, taking into account the diversity in the environmental conditions and in the

sources and history of the residuals.

To evaluate the sensitivity of the model to variations in the main factors involved in the prediction of K d , Monte Carlo simulation was applied. Data of

soil electrical conductivity C s , soil clay content (wt/wt%), and initial electrical

conductivity of the aqueous phase C i were generated, according to the distributions in Table 8 (five different simulations). C, K 0 , and K d were calculated for

d

oil residuals with spill age equal to 2, 10, and 20 years.

Simulation 1. It is assumed that the aqueous salinity is less than the soil

salinity, a situation that could correspond to rainwater that has increased its salinity during its superficial runoff. Mean values of electrical conductivity have been

assumed for soil salinity, according to regional data. The results are shown in

Figure 9. The values of K d (L kgϪ1 ) are equal to or less than 1000 for 2-year-old

residuals (95%), while only 42% and 15% present these values for 10-year-old



TABLE 8 Assumed Distributions of C i , Soil Clay Content, and C s for

the Monte Carlo Simulations

Simulation



Variable,

distribution

C i , normal

Clay, normal

C s , normal



1

X

σ

X

σ

X

σ



ϭ

ϭ

ϭ

ϭ

ϭ

ϭ



300,

60

50,

15

600,

100



X ϭ mean, σ ϭ standard deviation.



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Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved.



2

X

σ

X

σ

X

σ



ϭ

ϭ

ϭ

ϭ

ϭ

ϭ



300,

60

50,

15

2500,

800



3

X

σ

X

σ

X

σ



ϭ

ϭ

ϭ

ϭ

ϭ

ϭ



300,

60

10,

5

600,

100



4

X

σ

X

σ

X

σ



ϭ

ϭ

ϭ

ϭ

ϭ

ϭ



300,

60

85,

5

600,

100



5

X

σ

X

σ

X

σ



ϭ

ϭ

ϭ

ϭ

ϭ

ϭ



500,

50

50,

15

600,

100