Superfund Research Program
Biodegradation of Simple Chemical Mixtures in Soil
Background: Over millions of years, microorganisms have evolved processes to enable them to get energy and nutrients from an astonishing variety of compounds by degrading organic matter into CO2, water, and nutrients. Environmental scientists take advantage of this phenomenon, using bacteria and fungi to degrade pollutants in soil, sediment and water. Biodegradation offers an economical alternative to more intensive methods of hazardous waste remediation such as excavation, soil washing, or incineration. One limitation of biodegradation is that, in some cases, it can increase toxicity and exposure as the intermediate products of microbial oxidation may be more toxic than the parent mixture.
Advances: Researchers in Dr. K.C. Donnelly’s lab at the Texas A&M SBRP conducted a study on the biodegradation of a mixture of model chemicals in soil using three different microbial cultures, quantifying rate of contaminant reduction and changes in genotoxicity of the test systems. As exogenous microorganisms are often used to enhance bioremediation, one goal of this study was to evaluate the efficacy of exogenous organisms and indigenous microorganisms for the degradation of the model compounds.
The test mixture contained three compounds: 1) Benzo[a]pyrene (BaP) - a five-ring polycyclic aromatic hydrocarbon that is a ubiquitous environmental pollutant and a common contaminant of petroleum-based sludges; 2) Pentachlorophenol (PCP) - used as a biocide or fungicide and frequently is detected in soils contaminated with wood-preserving waste; and 3) 2,4,6-trinitrotoluene (TNT) – a nitroaromatic explosive that has been detected in the soil at many munitions sites.
Soil samples were inoculated with one of the following: indigenous microorganisms; Pseudomonas. aeruginso (bacteria); or Phanerochaete sordida (fungi). Solvent extracts were collected from samples on days 0, 90, 180, 360, 540, and 720 to measure model compound concentrations and characterize soil toxicity using the Salmonella/microsome assay to determine the effect of model compound degradation on soil genotoxicity.
Concentrations of the model chemicals in the spiked soils were reduced appreciably over time:
- BaP - Degradation from day 0 to 90 was most rapid in soils amended with either the indigenous microorganisms or P. sordida. By day 360, the mean percent BaP remaining in all three treatments was below 40%. On day 720, the average percent of BaP remaining in indigenous, P. aeruginosa, and P. sordida samples was 0.6, 6.4, and 2.2% of the initial BaP concentrations, respectively.
- PCP - The P. sordida inoculum yielded the most rapid rate of degradation during the first 90 days. All three treatments were capable of reducing PCP to less than 3% of the starting concentration by day 360 and to less than 0.2% of the initial PCP concentration within 720 days.
- TNT - On day 90, TNT concentrations were lowest in soil amended with P. aeruginosa. At day 540, average percent of TNT remaining in indigenous, P. aeruginosa, and P. sordida samples was 3.3, 1.3, and 12.7% ppm of the initial TNT concentrations, respectively.
Although the concentrations of all chemicals in the mixture were consistently were reduced to 30% or less of the starting concentrations by day 540, a similar reduction was not observed when the genotoxicity of soil extracts was measured. All soils induced a positive genotoxic response on day 540. In several cases, genotoxicity of the day 540 extracts was higher than that of the corresponding day 0 extracts. Although TNT is a direct-acting mutagen, the concentration of this chemical was reduced appreciably by day 540 and does not explain the increased genotoxic response. Dr. Donnelly believes that the genotoxicity of the soil extracts at day 540 partly may be due to the formation of genotoxic metabolites. The products of microbial degradation of polycyclic aromatic hydrocarbons (PAHs) such as BaP can include oxygen-containing PAHs; some of these, including certain ortho-quinones, are direct-acting mutagens.
Significance: The results of this study suggest that, under appropriate conditions, enriched cultures of microorganisms may have an increased capacity to degrade individual chemicals, but that the products of degradation in some cases might be more genotoxic than the parent compounds. The data also indicate that often indigenous organisms have the capacity to accomplish significant rates of degradation. This information contributes to our understanding of the fundamental mechanisms of biodegradation – and supports efforts to improve prediction, control and assessment of bioremediation performance.
For More Information Contact:
Kirby C Donnelly
Texas A&M University
Environmental & Occupational Health
The School of Rural Public Health Bldg.
College Station, Texas 77843-1266
To learn more about this research, please refer to the following sources:
- Donnelly KC, Huebner HJ, Claxton LD, Calvin J, Vos GA, Cizmas LH, He L. 2005. Biodegradation of simple chemical mixtures in soil. Environ Toxicol Chem 24(11):2839-2845. PMID:16398121
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