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Final Progress Reports: Cornell University: Effect of Aging of Pollutants in Soil on Bioavailability, Extractability and Toxicity

Superfund Research Program

Effect of Aging of Pollutants in Soil on Bioavailability, Extractability and Toxicity

Project Leader: Martin Alexander
Grant Number: P42ES005950
Funding Period: 1995 - 2000

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Final Progress Reports

Year:   1999 

This project has addressed the numerous factors and potential mechanisms by which organic chemicals that persist in soils – sometimes for decades – remain extractable by vigorous processes but otherwise not available for biodegradation or toxicity, even when effective microbial degraders or sensitive potential targets (earthworms, insects, plants, genotoxicity tests) are added. Bioavailability may be accurately estimated by less vigorous extraction (e.g., n-butanol), even in soils contaminated decades earlier. A solid-phase genotoxicity assay, based on mutant reversal of rifampicin-resistant Pseudomonas putida, demonstrated that in some instances, bioavailability could be predicted by soil properties, even though results vary greatly with soil type. Aging and subsequent bioremediation also reduce but do not eliminate bioavailability. A study of 16 soils in detail regarding potential characteristics expected to affect bioavailability or sequestration showed appreciable differences, with only organic carbon content and nanoporosity providing simple correlations for some measures varying between chemicals. Destruction of organic carbon in soils by H2O2 confirmed this factor for polycyclic aromatic hydrocarbons (PAHs). Detailed study of surface area and pore volume showed correlations of sequestration with organic matter content and 0.1-10 µm pores and with clay content and volume of pores <100nm. In other studies, the initial biodegradation rate was found to be a major factor in determining sequestered residual PAHs and organochlorine pesticides. Comparison of the properties of 22 chemicals to the residues not available via mild extraction suggests that sequestration in 28-d tests may be partly predicted by dipole moment or molecular length and molecular connectivity index. Bioavailability was enhanced by soil slurrying, use of specialized microbes, addition of some (but not all) surfactants, supplementation with unaged PAHs, and cyclic wetting and drying of the soil. Attempts to clarify whether entrapment in nanopores or sorption into the solid phase of soil organic matter was the dominant mechanism of sequestration resulted in both proposed mechanisms being considered plausible.

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