Environ. Sci. Technol. 1997, 31, 2515-2519
Desorption of [14C]Naphthalene from Bioremediated and Nonbioremediated Soils Contaminated with Creosote Compounds P. MICHAEL RUTHERFORD,‡ M U R R A Y R . G R A Y , * ,§ A N D MARVIN J. DUDAS‡ Department of Renewable Resources and Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada
Bioremediation changes the quantity and nature of the contaminant matrix remaining in soil, because some compounds are selectively degraded while others remain undegraded. It was hypothesized that changes to the contaminant matrix may alter the chemical and physical properties of the soil, such that subsequent desorption of a specific PAH compound would be altered. Desorption of [14C]naphthalene from two creosote-contaminated soils was measured before and after bioremediation. Although the bioremediation treatment removed the lower-molecular weight components, increasing the average molecular weight of the residual creosote by 10-36%, partition coefficients based on the mass of organic carbon in the soil were unaffected. Partition coefficients for naphthalene in soil organic matter were 4.6-8.3 times smaller than in the creosote contaminant. When partitioning was modeled as the sum of the contributions of the nonaqueous phase contaminant and the soil organic matter, the partition coefficients for the creosote contaminant were in the range 3500-4040 mL/g of organic carbon, for both soils with and without bioremediation. The insensitivity of partition coefficients to creosote source and to bioremediation suggest that sorption of naphthalene to a residual creosote matrix was relatively insensitive to detailed composition of the creosote.
Introduction Creosote is a complex and variable mixture derived from coal and is used mainly for wood preservation. In the past, soil contamination often occurred at wood preserving facilities. Aromatic hydrocarbons, mainly polycyclic aromatic hydrocarbons (PAHs), can constitute up to 90% of creosote (1). Bioremediation of soils containing creosote and other coal tar-based contaminants often fails to meet target concentrations for residual carcinogenic PAHs (2, 3). One of the limiting steps in bioremediation can be desorption of contaminant compounds from soil components into aqueous solution (4). The sorption of hydrophobic organic compounds (HOC) to pristine soils, or soil components, is roughly proportional to the quantity of soil organic matter present. The composition of soil organic matter also influences the partitioning of the * Author for correspondence: fax: (403) 492-2881; e-mail: murray.
[email protected]. ‡ Department of Renewable Resources. § Department of Chemical and Materials Engineering.
S0013-936X(96)00928-5 CCC: $14.00
1997 American Chemical Society
HOC sorbate between the sorbent and solution phases; for example, the polarity of the soil organic matter sorbent has been shown to be negatively correlated with HOC partition coefficients (5, 6). The soil organic matter is dominated by aromatic humic materials; however, lesser quantities of carbohydrates, nitrogen-containing constituents, and aliphatic compounds are present (7). During bioremediation, a portion of the contaminants are transformed into microbial biomass and metabolic by-products, which in time can become part of soil organic matter. In addition to soil organic matter, a creosote-contaminated soil may contain several hundred types of individual organic compounds, which together form a contaminant matrix, often as a nonaqueous phase liquid (NAPL). This contaminant matrix is a heterogeneous mixture of compounds which collectively impart unique chemical, physical, and biological properties to a soil. The sorption-desorption properties of individual HOC compounds in soil will be determined by the quantity and composition of both the soil organic matter and contaminant matrix of a soil (8). The highly aromatic, nonpolar nature of many NAPLs serves as an effective partitioning medium for individual HOC compounds, limiting the aqueous phase concentrations and subsequent bioavailability of these compounds to active decomposer organisms (9). During bioremediation, contaminant compounds are biodegraded and the overall quantity of the NAPL matrix is reduced. Some compounds are degraded more quickly than others, and some compounds may not be biodegraded at all, therefore, the composition of the remaining NAPL matrix is altered. For example, Rutherford et al. (10) found that biodegradation of GC-elutable compounds in creosote- and petroleum-contaminated soils during slurry phase bioremediation was 1.4-2.7-fold greater than total NAPL biodegradation. Other researchers have found specific biodegradation within the GC-elutable component of creosote-contaminated soils (11, 12). As a result, it is reasonable to assume that the overall chemical and physical properties of the contaminated soil, including sorption and desorption properties, may change as bioremediation proceeds. These changes may alter the subsequent desorption and bioavailablity of the remaining compounds in the contaminated soil. One method for investigating the desorptive behavior of contaminated soils is to use a 14C-labeled HOC probe. Desorption of the probe can be used to study the relative differences in desorptive properties of different soils and to study the impact that bioremediation has on the desorptive properties of an individual soil. A soil which strongly sorbs one HOC will often strongly sorb other structurally similar HOCs relative to other soils (6). Just as sorption varies between soils, so to will the reverse process of desorption vary; therefore, a probe can be used to evaluate the relative desorptive properties of soils. The objective of this study was to determine if a 10-week slurry phase bioremediation treatment (10) altered the desorptive properties of two creosote-contaminated soils. [14C]Naphthalene was first sorbed to bioremediated and nonbioremediated soils, then desorbed in a sequential batch experiment. The contributions of the contaminant organic phase and the soil organic matter to desorption were resolved by experiments on soils with and without NAPL contaminants. Uncontaminated soils (i.e., controls) were not available in this study; therefore, soils without NAPL were obtained by subjecting contaminated soils to extraction with dichloromethane.
VOL. 31, NO. 9, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2515
TABLE 1. Total Dichloromethane Extractable Organics (TEO), Organic C Content of TEO, and Total Organic C Content in Nonextracted and Extracted Soilsa total organic C in soil soil
TEO content in soilb
organic C content in TEO
molecular weight of TEOc
nonextracted soilb
extracted soild
EDM PAA
1.45 (0.02) 1.08 (0.04)
85.9 (0.2) 88.2 (0.2)
Before Bioremediation 248 (3) 237 (3)
2.83 (0.08) 1.26 (0.03)
1.50 (0.04) 0.32 (0.02)
EDM PAA
0.84 (0.02)*** 0.75 (0.01)**
85.2 (0.4)NS 86.9 (0.3)*
After Bioremediatione 338 (1)*** 264 (2)**
2.40 (0.02)* 0.82 (0.02)***
1.69 (0.02)*** 0.20 (0.01)**
a All values are expressed as grams (100 g)-1. Standard errors shown in brackets. b Contents are expressed on an oven-dry nonextracted soil mass basis. c Molecular weight determined by vapor pressure osmometry in toluene (10). d Contents are expressed on an oven-dry extracted soil mass basis. e Superscripts present level of significance between nonbioremediated and bioremediated treatments within each soil; *, **, *** designate significance at 0.05, 0.01, and 0.001 probability levels; NS, not significantly different.
Theory The equilibrium partitioning of HOC in moist contaminated soil is dominated by three phases: the aqueous phase, the soil organic matter, and the contaminant matrix or NAPL. Equilibrium implies that the rate of sorption is equal to the rate of desorption. Sorption of HOC to mineral surfaces becomes important only in dry soils (13). In this study, NAPL is defined as the organic fraction which can be removed by extraction with nonaqueous solvents, while the soil organic matter is the organic fraction that remains in the soil. The total amount of HOC sorbed to soil from solution, q [µg (g of soil)-1], normally follows a linear relationship:
q ) KdC
(1)
where Kd is the overall partition coefficient [mL (g of soil)-1] and C is the concentration of HOC in the aqueous phase (µg mL-1). In some cases, a portion of the HOC may be irreversibly sorbed (14). Equation 1 can be modified by adding a constant, q′, to represent the irreversibly sorbed HOC:
q ) KdC + q′
(2)
If the HOC is partitioned linearly between the soil and the aqueous phase, then from eq 2, the value of Kd can be determined from experimental data by plotting C versus q and calculating the slope. Since only a portion of the soil is active in sorbing the HOC, the partition coefficient is often calculated relative to the content of organic carbon in the soil:
Koc ) Kd/Foc
(3)
where Koc has units of milliliters per gram of total organic carbon. Foc is the mass fraction of total organic carbon in soil (grams per gram of soil), due to the sum of the contributions from organic material (Fsom-c) and NAPL (FNAPL-c):
Foc ) Fsom-c + FNAPL-c
(4)
In the context of a contaminated soil, the soil organic matter carbon is defined empirically as all organic carbon in soil which is not extracted by a solvent, such as dichloromethane. Assuming that the organic materials contribute additively to sorption (8), the overall partition coefficient, Kd (milliliters per gram of soil) for an HOC can be separated into two components:
Kd ) Fsom-cKsom-c + FNAPL-cKNAPL-c
(5)
where Ksom-c and KNAPL-c are the partition coefficients for HOC between the soil organic carbon and the NAPL, respectively (both with units of milliliters per gram of organic carbon). The assumption of additivity of NAPL and soil
2516
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 9, 1997
organic matter to sorption is reasonable when contaminated soil contains a relatively large amount of nonpolar NAPL, relative to the quantity of soil organic matter present in the soil. For this reason, partitioning of NAPL into soil organic matter was not considered to be significant. Substitution of eq 5 into eq 2 gives the overall sorption as a function of the contributions of soil organic matter and NAPL:
q ) q′ + (Fsom-cKsom-c + FNAPL-cKNAPL-c)C
(6)
Materials and Methods Contaminated Soils. Soil samples were obtained from inactive wood preserving sites at Edmonton, Alberta (EDM) and Prince Albert, Saskatchewan (PAA); soils were contaminated mainly with creosote compounds. The EDM soil was fine-textured soil (41% clay; 16% sand) and was sampled at the soil surface (0-0.30 m) from a disturbed area where subsoil and topsoil had apparently been mixed. The PAA soil was coarse-textured (4% clay, 90% sand) and was sampled at a 2-4 m depth. The EDM site was in operation between 1924 and 1988, and the PAA site was in operation between 1932 and ∼1972; therefore, the soils received contaminants over an extended period. Numerous 20 L buckets of contaminated soil were taken from each site, at various sampling depths. Selected subsamples (i.e., bucket samples) from each site were combined to form composite samples. Soils selected for homogenization (
