Environ. Sci. Technol. 2000, 34, 1589-1593
Bioavailability of Genotoxic Compounds in Soils RENEE R. ALEXANDER AND MARTIN ALEXANDER* Department of Soil, Crop, and Atmospheric Sciences and Institute of Comparative and Environmental Toxicology, Cornell University, Ithaca, New York 14853
Measurements were made of the bioavailability of mutagens with a bacterial genotoxicity assay specifically designed for use with the solid phase of soil. Short-term sorption of benzo(a)pyrene, 7,12-dimethylbenz(a)anthracene, 9-phenylanthracene, captan, and aldicarb by six markedly different soils resulted in a 28.2 to >99% decline in availability for genotoxicity. This reduced bioavailability varied markedly with the soil and the compound. The reduced genotoxicity was not a result of the destruction of the compounds since, with few exceptions, >90% of each compound was recovered by Soxhlet extraction. The bioavailability of captan was correlated with surface area and organic C content, but the availability of the other mutagens was not correlated with other properties of the six soils. However, if soils with 0.7% organic C (r ) 0.049-0.248).
Discussion The data show that the bioavailability of genotoxic compounds is markedly affected by both soil properties and identity of the toxicant. Among all compounds and soils, the reduction in bioavailability when compared to the nonsorbed compound ranged from 28.2% to >99%. Considering individual chemicals, the bioavailabilities differed by factors of 5.4, 7.1, 4.2, >40, and 3.2 among the tested soils for DMBA, BaP, PA, captan, and aldicarb, respectively. In contrast, the differences in bioavailability for all chemicals in any single soil were more narrow, except for captan in Palouse silty clay loam, and the range in differences in bioavailability of any single compound in all soils did not exceed a value of 2.5. These findings emphasize the key role of soil properties in determining the amount of a substance that will be available for genotoxicity. Such observations are not surprising in view of early studies of acute toxicants, at least those detailed in investigations of the influence of soil type on toxicity of pesticides to plants and insects (6-8). To carry out meaningful studies of the influence of soil type on bioavailability, a reasonable number of soils needs to be tested. In most investigations, however, few soils have been examined. As a result, if bioavailability is low in one soil with a large concentration of one constituent (e.g., organic C or clay) and high in another soil with a low concentration of that constituent, the conclusion may be reached that the property singled out is indeed the one determining bioavailability (20). However, the binding of a chemical in a fashion that reduces availability may be linked with another constituent or property or several acting together, e.g., amount and type of organic matter, amount and type of clay mineral, surface area, nanoporosity, or CEC. The possible relationship between bioavailability of the test compounds and organic C content, percentage clay, surface area, CEC, and volume of micro- and nanopores was assessed. Each of the properties or constituents could reduce bioavailability, as by hydrophobic or ionic sorption, entrapment within clay lattices or nanopores, or providing a large surface on which the chemicals might be retained. The data show that the bioavailability of only captan was related to the soil properties or constituents that were considered, but even in this case, the highest r value was -0.771 (for surface area). However, when the results for the organic matterpoor soils were not considered, higher correlations were 1592
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observed, particularly among the PAHs. Good correlations were also noted for some compounds with CEC, but these correlations may be fortuitous rather than reflecting a mechanism because CEC often increases with organic matter content of soils. Changes in bioavailability associated with the sequestration that occurs as organic compounds persist in soil parallel changes in the quantities of those compounds that are removed from soil by mild extraction with organic solvents. For example, the decline in availability of phenanthrene and atrazine to earthworms parallels the reduced amounts extracted from soil by n-butanol (18), and the diminution in availability of the same two compounds for bacterial biodegradation in 16 soils was accompanied by a reduction in the quantities extracted by 71% or 95% ethanol, respectively (9). Similarly, the uptake by the earthworm Eisenia fetida of anthracene, fluoranthene, and pyrene that had persisted for various periods in soil was highly correlated with the amounts removed by mild extraction with each of three solvents (21). Those studies dealt with compounds that become sequestered during long-term aging. The data presented here, which are from a study of short-term (72 h) aging, similarly show r values of 0.691-0.870 in correlating bioavailability of sorbed PAHs with butanol extractability, but the bioavailability of sorbed captan and aldicarb was not correlated with methanol extractability. The data suggest that a mild extraction may provide the basis for a chemical assay to determine the quantity of sorbed compound that is available to exert a genotoxic effect. Vigorous extractions, as carried out in this study by Soxhlet extraction, are not correlated with availability; the high recoveries, however, confirm that the reduced genotoxicity of the compounds following sorption are not a consequence of the compounds being degraded. That soils affect bioavailability is well appreciated (2225). However, apart from studies of the impact of pesticides on agriculturally important pests, insufficient attention in toxicology has been given to the vast number of soil types and to soil constituents and properties other than organic matter. This investigation dealt only with the mutagencity of a few genotoxic agents that are sorbed for short periods of time in just six soils. Further research is required with a wider range of soils, with compounds that become increasingly sequestered as they persist or age in soil, and with additional chemical classes and types of toxicological responses. The information from such studies will then provide a better basis than exists presently to assess the exposure to toxic organic compounds found in soils.
Acknowledgments The research was supported by research grant ESO5959 from the Natural Institute of Environmental Health Sciences and a grant-in-aid from duPont de Nemours.
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Received for review September 30, 1999. Revised manuscript received January 12, 2000. Accepted January 14, 2000. ES991120D
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