Reductive immobilization of U(VI) in Fe(III) oxide-reducing subsurface sediments: Analysis of coupled microbial-geochemical processes in experimental reactive transport systems

摘要

Although the fundamental microbiological and geochemical processes underlying the potential use of dissimilatory metal-reducing bacteria (DMRB) to create subsurface redox barriers for immobilization of uranium and other redox-sensitive metal/radionuclide contaminants are well-understood (Lovley et al., 1991; Gorby and Lovley, 1992; Lovley and Phillips, 1992; Lovley, 1995; Fredrickson et al., 2000; Wielinga et al., 2000; Wielinga et al., 2001), several fundamental scientific questions need to be addressed in order to understand and predict how such treatment procedures would function under in situ conditions in the subsurface. These questions revolve around the dynamic interactions between hydrologic flux and the coupled microbial-geochemical processes which are likely to occur within a redox barrier treatment zone. A brief summary of such questions includes the following: (1) What are the kinetic limitations to the efficiency of microbial U(VI) scavenging in subsurface sediments? (2) Is U(VI) sorbed to Fe(III) oxide and other solid-phase surfaces subject to enzymatic reduction? If so, what are the relative kinetics of aqueous vs. sorbed U(VI) reduction? (3) What are the relative kinetics of direct, enzymatic U(VI) reduction vs. abiotic reduction of U(VI) by surface-bound biogenic Fe(II)? (4) Can coupled Fe(III) oxide/U(VI) reduction be sustained long-term in subsurface environments? What are the kinetic relationships between Fe(III) oxide reduction, DMRB growth, and U(VI) reduction in advectively open sedimentary systems? The overall objective of our research is to address the questions listed above through laboratory-based batch and reactive transport experiments with natural Fe(III) oxide-bearing subsurface materials and a representative pure culture DMRB. A unique feature of our research is that we are using levels of total uranium (ca. 10{sup -6} to 10{sup -4} mol per dm{sup 3} bulk volume) and aqueous/solid-phase ratios ({le} ca. 10{sup -3} mol U per kg sediment) which are much closer to those present in contaminated subsurface environments compared to levels employed in previous experimental studies of microbial U(VI) reduction. The goal is to develop a more realistic picture of the dynamics of U(VI) reduction and its interaction with Fe(III) oxide reduction in subsurface sedimentary environments. In doing so, our studies will provide benchmark information on process dynamics that will be useful for scaling up (e.g. through the use of field-scale reactive transport models) to in situ treatment scenarios. In addition, the experimental methodologies and modeling strategies developed for the project may applicable to the evaluation of in situ remediation technologies for other redox-sensitive metal-radionuclide contaminants such as Cr(VI) and Tc(VII). Numerical simulations are being developed hand-in-hand with the experimental work to aid in the interpretation of the observed dynamics of U(VI) behavior, and to contribute to the development of a predictive framework for assessing in situ metal-radionuclide remediation strategies driven by the activity of DMRB.