The fate and behavior of pollutants is often dictated by the coupling of chemical and biologically-mediated reactions. Both hydrophobic organic pollutants and toxic transition metals are strongly sorbed to soil surfaces at Superfund sites. Sorption reactions together with the hindered diffusion of pollutants. through small pores appear to control the concentrations to which biological populations (including indigenous soil microorganisms) are exposed. Reactions which act to enhance pollutant release kinetics can both exacerbate exposure and can be deliberately employed to enhance aquifer clean-up procedures. Bacterial polymers naturally occur in soil solution and have documented binding.properties for both transition metals and hydrophobic organic contaminants. Project research to date has demonstrated that bacterial polymers will act to enhance metal desorption and mobility in porous media, and related research has similarly shown that the bacterial polymers will act to enhanced sorption and mobility of hydrophobic organic pollutants in porous media. Therefore, bacterial polymers may enhance the bioavailability of both toxic transition metals and organic contaminants in soil. The proposed continuation of this research will quantify the desorption kinetics of a radio-labeled transition metal (109Cd) and a polynuclear aromatic hydrocarbon (PAH), 14C.phenanthrene, in the presence and absence of selected extracellular polymers, including several produced by isolates of indigenous soil bacteria. The research will use a novel kinetic model that employs a statistical distribution of release rates to reflect the multiplicity of binding site types and pore sizes that collectively act to control contaminant release. Techniques using fluorescent antibody (Fab) staining and microautoradiography (MARG), developed during prior phases of this NIEHS- sponsored research, will be employed to view the distribution of radiolabeled organic compounds, polymers, and bacterial cells at the microscale and will provide an improved understanding of polymer-mediated contaminant release mechanisms. MARG methods will also be adapted for use in assessing the bioavailability of sorbed metals and PAH to soil bacteria that adhere to soil surfaces through production of extracellular polymers. The project results will yield basic information on the micro- and macro-scale influence of extracellular polymers on the bioavailability of contaminants. in porous media. A range of experimental scales will be considered from the macroscopic behavior of pollutant sorbates in suspensions or columns of sorbent material to direct microscopic measurements at the scale of bacterial cells in their microhabitats. This range in experimental scale will facilitate construction and verification of environmentally relevant models that employ mechanistic descriptions of reaction and transport at the pore scale to make predictions of macroscopic behavior.
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