Superfund Research Program
Improving the Understanding of Contaminant Bioavailability in Soils
The bioavailability of contaminants in soils is a complex process that is influenced by both chemical and biological factors. For example, the chemical process of adsorption is generally thought to decrease the bioavailability of certain contaminants in soils, while some evidence suggests that the presence of bacteria in soils may increase the accessibility of soil contaminants to living organisms. Although these distinct factors appear to influence the fate of contaminants in soils, little is known about their interplay in soil systems.
To provide a more comprehensive understanding of contaminant bioavailability, researchers at Cornell University are investigating how bacterially produced polymers influence the desorption of soil-bound metals and organic compounds. In this interdisciplinary research, the scientists are also developing novel methods, involving microscopy and molecular biology techniques, to observe bacterial polymers, contaminants, and bacterial cells in soils.
Some classes of environmental contaminants, including heavy metals and hydrophobic organic contaminants, become physically unavailable to soil microbes and other organisms when strong adsorption reactions bind these contaminants tightly to the surface of soil particles. However, past laboratory-based studies by the group of researchers at Cornell University have shown that bacterial polymers can desorb cadmium (a toxic metal) and phenanthrene (a model organic compound) from soils, results that suggest bacterial polymers may enhance the bioavailability of metals and organic contaminants in soils. The bacterial polymers may increase the bioavailability of contaminants by providing an adhesive material that attracts contaminants into the aqueous phase of soil systems, where chemicals are more accessible for biological uptake.
Building on the knowledge acquired in their previous studies, the researchers are now in the process of acquiring mechanistic information that will be useful in the development of models for predicting the bioavailability of metal and organic contaminants in soils.
In recent experiments, the researchers have been investigating how bacterial polymers affect the desorption rates or contaminant release kinetics of phenanthrene. Batch and column experiments have been completed with radiolabeled phenanthrene, a test soil, and bacterial polymer produced by a microbe isolated from a coal tar waste site. The desorption kinetics of soil-bound phenanthrene were dramatically enhanced in the presence of the bacterial polymer. A kinetic model was developed to aid in interpreting how additions of bacterial polymer altered the release rate of soil-bound phenanthrene. In describing the mass transfer rates of phenanthrene from soil, where the compound is less bioavailable, to water, where the compound is more bioavailable, this model could aid in hazardous waste site risk assessment and remediation.
Complementing these studies, the scientists are developing methods to evaluate contaminant bioavailability and mobilization at a microscale or small-scale level. This includes developing ways to observe where contaminants become attached in a bacterial polymer matrix, a goal that involves being able to detect specific bacteria, bacterial polymers, and contaminants. The scientists are accomplishing this by applying a combination of microscopic techniques, immunofluorescence, biotechnology, and microautoradiography. For example, they have successfully coupled confocal laser scanning microscopy with microautoradiography and immunofluorescence to produce a three-dimensional view of specific microorganisms, the soil particle matrix, and the sorbed contaminant of a selected soil system.
In developing these methods to view the small-scale movement of contaminants from soil to bacterial polymer, the scientists aim to relate this information to the macroscale methods developed for measuring the kinetics of contaminant binding to polymers. The latter studies reveal mass transfer processes and do not provide the small-scale detail provided by the microscale methods. Coupling these two types of studies together will ultimately provide a more complete understanding of contaminant bioavailability.
Many naturally occurring bacteria produce high molecular weight polymers, which are composed mainly of repeating units of sugars such as glucose, galactose, and mannose. These "extracellular polymers" are thought to help soil bacteria attach to surfaces, offer protection from predators, or help bacteria gain access to soil-bound compounds that can be used for bacterial growth.
Understanding how these naturally produced polymers influence the bioavailability of soil contaminants is important for several reasons. Because bacterial polymers are ubiquitous in the environment, it is important to understand their role in contaminant fate and transport, as they could act to increase the mobility and, ultimately, the bioavailability of contaminants in the environment. Since some bacterial polymers are effective at releasing contaminants from soils and the polymers are easily harvested from bacterial cultures, the potential also exists for using bacterial polymers in the development of new biotechnological approaches to remediate contaminated soils. Thus, the basic scientific information being acquired in these studies may improve our ability for predicting human exposure to environmental contaminants, aid in the risk assessment of hazardous waste sites, and provide information that could be used to design more effective remediation strategies.
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To learn more about this research, please refer to the following sources:
- Czajka DR, Lion LW, Shuler ML, Ghiorse WC. 1997. Evaluation of the utility of bacterial extracellular polymers for treatment of metal contaminated soils: Polymer persistence, mobility and the influence of lead. Water Resour Res 31:2827-2839.
- Ahn I, Lion LW, Shuler ML. 1996. Microscale-based modeling of polynuclear aromatic hydrocarbon transport and biodegradation in soil. Biotechnol Bioeng 51:1-14. PMID:18627082
- Chen J, Czajka DR, Lion LW, Shuler ML, Ghiorse WC. 1995. Trace metal mobilization in soil by bacterial polymers. Environ Health Perspect 103(supp.1):53-58. PMID:7621800
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