Environ. Sci. Technol, 1994, 28, 1172-1 179
Role of Partitioning in Biodegradation of Phenanthrene Dissolved in Nonaqueous-Phase Liquids Rebecca A. Efroymsont and Martin Alexander'
Institute of Comparative and Environmental Toxicology and Department of Soil, Crop, and Atmospheric Sciences, Cornell University, Ithaca, New York 14853
A study was conducted to determine whether biodegradation of organic compounds dissolved in nonaqueous-phase liquids (NAPLs) is limited by spontaneous partitioning of the chemical to the aqueous phase. The biodegradation and partitioning rates of phenanthrene from NAPL to water varied with the NAPL and the concentration of the test substrate. The rates of mineralization in 10 of 11 tests with Pseudomonas sp. or an enrichment culture exceeded the rates of spontaneous partitioning with three concentrations of phenanthrene in three NAPLs. However, biodegradation was slow if the partitioning rate was slow. Mineralization by a subsoil slurry was slower than spontaneous partitioning with one phenanthrene concentration in three NAPLs. The maximum rate of biodegradation of pollutants in NAPLs thus cannot be predicted from the rates of their spontaneous partitioning to water.
Introduction Pollutants as diverse as chlorinated hydrocarbon solvents in groundwater, coal tar in soils, and petroleum in seawater exist in or as nonaqueous-phase liquids (NAPLs). Polychlorinated biphenyl oils (1)and residual hydrocarbons from old gasoline (2) are examples of NAPLs that have been shown to sequester hydrophobic organic contaminants. Moreover, the biodegradation of hydrophobic constituents of NAPLs such as those containing di-2-ethylhexyl phthalate (DEHP) as the major component is often quite slow and may proceed at a rate far slower than if the same compounds are not present in a NAPL (3, 4 ) . This reduction in rate could result from a low concentration of the compound in the water and its slow partitioning from the NAPL to the aqueous phase. A relation between the partitioning of a chemical from a NAPL and biodegradation, although assumed (31, has not been established. The importance of partitioning of a chemical to the water phase for its biodegradation is suggested by evidence that the dissolution rates of pure crystals or the rates of desorption of organic compounds sorbed to soil constituents may control their biodegradation. For example, the growth rates of bacteria growing on phenanthrene, naphthalene, or biphenyl are independent of the amount of solid chemical in the medium, apparently because these polycyclic aromatic hydrocarbons are used by some bacteria only in the dissolved state (5, 6). Similarly, a comparison of the rates of dissolution and biodegradation of phenanthrene suggested that the former may have been limiting the latter (7).Moreover, some microorganisms use naphthalene (8) and 2,4-dichlorophenoxyacetate (9) in solution but not when sorbed to soil. More directly related to NAPLs is the observation that the epoxidation ~~
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US.Agency for International Development (G/R&D/ENR), Washington, DC 20523-1812. + Present address:
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Environ. Scl. Technol., Vol. 28, No. 6, 1994
of 1,7-octadiene by Pseudomonas putida depends on the ratio of organic to aqueous phases, presumably because the mass transfer of octadiene to the water controls the reaction (10). Similarly, the finding that increasing volumes of DEHP inhibited the biodegradation of polychlorinated biphenyls was interpreted as resulting from the low partitioning of the biphenyls to water (3). Alternatively, the slow biodegradation of hydrophobic compounds initially present in NAPLs may have other causes than effects of the organic liquid on partitioning. Preferential degradation of NAPLs (11)and toxicity of NAPLs to microorganisms (12) can result in a reduced rate of biodegradation or a lengthened acclimation period prior to biodegradation of components of the NAPL. Also, a depletion of the supply of microbial nutrients in the region of the NAPL can suppress the biodegradation of its constituents (4). Thus, partitioning-limited biodegradation of NAPLs is not always observed. Indeed, given that microorganisms often attach to a NAPL, components of the organic liquid may be degraded directly from the organic phase (13). Nevertheless, because of the high likelihood that the partitioning of constituents of NAPLs to water sometimes limits their biodegradation, a study of the role of partitioning in biodegradation was conducted. Experimental Methods Chemicals. Phenanthrene, DEHP, n-hexadecane, and 2,2,4,4,6,8,8-heptamethylnonane were purchased from Aldrich Chemical Co., Milwaukee, WI. The three NAPLs were chosen because of their low volatility and presumed low toxicity to microorganisms. Benzoic acid was obtained from Mallinckrodt, St. Louis, MO. Two lots of [9-l4C1phenanthrene (13.1 and 8.3 mCi/mmol, both >98% pure) were purchased from Sigma Chemical Co., St. Louis, MO. Subsoil. A sample of subsurface soil (pH 4.6,99.5% sand,
