Mineral solubility and free energy controls on microbial reaction kinetics: Application to contaminant transport in the subsurface

TITLE: Mineral Solubility and Free Energy Controls on Microbial Reaction Kinetics: Application to Contaminant Transport in the Subsurface ABSTRACT: Recent developments in the theoretical treatment of geomicrobial reaction processes have Recent developments in the theoretical treatment of geomicrobial reaction processes have resulted in the formulation of kinetic models that directly link the rates of microbial respiration and growth to the corresponding thermodynamic driving forces. In this project, these kinetic models for the microbial reduction of uranium(VI) are verified and calibrated. The approach combines laboratory experiments on uranium bioreduction using pure cultures and natural samples, competition between reduction of U(VI) and iron oxides, and field-scale reactive transport modeling. Gibbs free energy yields are manipulated by varying the concentrations of U(VI), type and concentration of iron oxides, electron donor, as well as the carbonate alkalinity, and calcium concentrations. Rates of enzymatic reduction of U(VI) were measured under variable, but controlled, geochemical conditions to determine the effect of pH on bioreduction rates by Shewanella putrefaciens. In the presence of excess carbonates, the pH decreased over time prompting a shift in U(VI) speciation from U(VI)-carbonato complexes toward more readily-reducible U(VI)-hydroxide and increased bioreduction rates. Ca suppressed the formation of labile U(VI) complexes and required a larger decrease in pH to achieve comparable rates. These results indicate that the main reducible fraction of U(VI) consists of hydroxide complexes, despite being the least abundant species in solution, and suggest that the pH decrease associated with U(IV) precipitation is required to promote U(VI) bioreduction in the presence of carbonates and calcium. If the pH is too low, however, it may have a toxic effect on cells. Experiments conducted at pH 7.0 in the presence of different carbonate and calcium concentrations revealed that exposure to uranyl species in these conditions for short periods of time decreased cell viability, though cells grew at the same rate once they recovered. Cell viability decreased with increasing concentration of non-carbonato complexes of U(VI) in solution, suggesting that these complexes are bioavailable yet toxic at high concentrations. A U(VI) bioreduction rate law that accounts for the speciation of U(VI) species is able to reproduce bioreduction rates in all pH, carbonate, and calcium conditions. Overall, these findings suggest that uranyl hydroxide complexes are the main bioavailable species to Shewanella and potentially explain why it does typically not grow efficiently on uranium at pH < 8. Incubations are currently conducted with Geobacter bemidjiensis isolated from Rifle to determine whether a similar effect is observed with other metal-reducing bacteria. In addition, well-mixed retentostat reactor experiments using sterilized native Rifle sediments inoculated with G. bemidjiensis are underway to study microbial reaction kinetics at low to near-zero growth rates and in conditions close to thermodynamic equilibrium.