Technical Report

Abstract

PROJECT SUMMARY'Bioimmobilization' of redox-sensitive metals and radionuclides is being investigated as a way to remediate contaminated groundwater and sediments. In this approach, growth-limiting substrates are added to stimulate the activity of targeted groups of indigenous microorganisms and create conditions favorable for the microbially-mediated precipitation ('bioimmobilization') of targeted contaminants. This project investigated a fundamentally new approach for modeling this process that couples thermodynamic descriptions for microbial growth with associated geochemical reactions. In this approach, a synthetic microbial community is defined as a collection of defined microbial groups; each with a growth equation derived from bioenergetic principles. The growth equations and standard-state free energy yields are appended to a thermodynamic database for geochemical reactions and the combined equations are solved simultaneously to predict the effect of added substrates on microbial biomass, community composition, and system geochemistry. This approach, with a single set of thermodynamic parameters (one for each growth equation), was used to predict the results of laboratory and field bioimmobilization experiments at two geochemically diverse research sites. Predicted effects of ethanol or acetate addition on uranium and technetium solubility, major ion geochemistry, mineralogy, microbial biomass and community composition were in general agreement with experimental observations although the available experimental data precluded rigorous model testing. Model simulations provide insight into the long-standing difficulty in transferring experimental results from the laboratory to the field and from one field site to the next, especially if the form, concentration, or delivery of growth substrate is varied from one experiment to the next. Although originally developed for use in better understanding bioimmobilization of uranium and technetium via reductive precipitation, the modeling approach is potentially useful 3 for exploring the coupling of microbial growth and geochemical reactions in a variety of basic and applied biotechnology research settings. 4 INTRODUCTIONThe oxidized and mobile forms of uranium and technetium (such as uranyl carbonates or pertechnetate) have been shown to undergo microbially-mediated reductive transformations that result in the formation of less soluble products that are also less mobile in the environment. For example, certain anaerobic bacteria can enzymatically reduce U (VI) to U (IV) , which is essentially insoluble and, when sorbed, aggregated into large particles, or deposited onto sediment mineral phases, is also immobile (e.g., Lovley et al., 1993). Similarly, by stimulating the bio-reduction of technetium, it is possible to decrease aqueous concentrations of the mobile pertechnetate ion (Tc (VII) O 4 -) by forming insoluble and immobile Tc (IV) -oxides or sulfides (e.g., Wildung et al., 2000). Thus, by stimulating targeted indigenous microbial activities, aqueous con...