Metal adsorption onto bacteria not only affects the mobility and transport of metals in geologic systems, but also may control the bioavailability of metal ions to bacteria. Surface complexation modeling (SCM) represents a flexible approach for quantifying the extent of metal adsorption onto bacterial cell walls, and hence may be used to predict the fate and transport of metals in water-rock systems and even the bacterial bioavailability of metals. The goal of this dissertation research is to test if metal adsorption onto bacteria controls the bioavailability of metals to bacteria and whether it is possible to use a SCM approach to predict metal bioavailability. Hence, the studies described in this dissertation examine possible correlations between U(VI) adsorption onto bacteria and the kinetics of enzymatic reduction of U(VI) by bacteria, and also put rigorous constraints on the uranyl bacterial surface complexation reactions in order to better understand the bioavailability of uranyl to bacteria, and the fate of uranium in natural and engineered environments.
Three studies are presented in this dissertation. The first study (Chapter 2) examines U(VI) reduction by bacteria in the presence of variable concentrations of EDTA or dissolved Ca. The results of this study demonstrate that EDTA increases the U(VI) reduction rate by forming U4+-EDTA aqueous complexes which remove U(IV) from the cell surface after reduction, and prevent UO2 precipitation on the cell wall, thereby preventing blockage of U(VI) binding sites. Furthermore, the data demonstrate that dissolved Ca decreases the U(VI) bioreduction rate by forming Ca-uranyl-surface complexes, and that the U(VI) in these surface complexes is not easily reducible. Therefore, the concentration of Ca-free uranyl surface complexes controls the U(VI) bioreduction rate in the presence of dissolved Ca. In summary, the results of this study indicate that U speciation, both of U(VI) before reduction and of U(IV) after reduction, affects the reduction kinetics, and that thermodynamic modeling of the U speciation may be useful in the prediction of reduction kinetics in realistic geologic settings.
In the second study (Chapter 3), I measured U(VI) adsorption onto bacteria as a function of dissolved NaHCO3 concentration. The data provide unequivocal evidence for the formation of a series of uranyl-, uranyl-hydroxide-, uranyl-carbonate-hydroxide-, and uranyl-carbonate-bacterial surface complexes. The calculated stability constants for the uranyl-bacterial complexes from this study provide a framework for estimating the adsorption and speciation of U(VI) on bacterial cell walls in complex environments. Based on the results of the first two studies, the third study (Chapter 4) focused on the enzymatic reduction of U(VI) by bacteria in systems with elevated concentrations of NaHCO3, revealing a strong positive correlation between the U(VI) reduction rate and the total concentration of adsorbed U(VI). This positive correlation indicates that the speciation and adsorption of U(VI) on the bacterial cell wall control the kinetics of enzymatic reduction of U(VI) by bacteria. The successful use of surface complexation modeling to relate U speciation to enzymatic reduction rates in this study may enable predictions of enzymatic U(VI) reduction kinetics or bioavailability of uranium to bacteria in complex geologic settings.