Pore-Scale Evaluation of the Effect of Bioavailable Iron on U(Vi) Reduction Kinetics and U(Iv) Re-Oxidation of Relevance to Bioremediation Efforts at the Hanford Site
EMSL Project ID
39945
Abstract
Scores of hazardous waste sites are contaminated with uranium, primarily due to uranium mining and processing by the Department of Energy over a 40-year period between 1940 and 1980 (Wolbarst et al., 1999). Uranium is present in subsurface environments as primarily U(VI) or U(IV). The former is soluble and highly toxic, while the latter is insoluble and not bioavailable. Bioremediation is a very promising approach for transferring U(VI) in groundwater to U(IV). This typically involves amending groundwater with an electron donor (e.g., acetate or lactate) in order to promote the biologically mediated precipitation of U(IV) from U(VI). While initial field results are promising, the long-term validity of this approach is not clear. Specifically, it is not clear how precipitation of U(IV) affects mixing between the electron donor and the remaining fraction of persistent U(VI) sources, and it is not clear what conditions limit and promote the reoxidation of U(IV) to soluble and mobile U(IV) over long time periods. The primary objective of this work is to characterize the combined effects of hydrology, geochemistry, and biology on the bioremediation of U(VI). Our underlying hypotheses are that bioremediation of U(VI) in groundwater is controlled by transverse mixing with an electron donor along plume margins, and that iron bioavailability in these zones critically affects U(VI) reduction kinetics and U(IV) re-oxidation. Our specific objectives are to a) understand the influence of bioavailable iron on U(VI) reduction and U(IV) re-oxidation along transverse mixing zones, b) determine how transverse mixing limitations and the presence of biomass in pores affects these reactions, and c) identify microbial populations that develop along transverse mixing zones and how the community development is influenced by the presence of iron and the concentration of electron donor.
The experimental tasks will use etched silicon etched microfluidic pore networks (micromodels) to simulate micro-scale hydraulic mixing zones within model aquifer material. We will use micromodels that contain Fe(III) oxides versus micromodels that are devoid of all iron, to assess the critical role that total iron plays in uranium bioremediation. We will introduce electron donors, U(VI), and Fe(III)-reducing cells to the micromodels in a controlled manner to characterize the mechanisms and kinetics of both U(VI) reduction and U(IV) reoxidation, and to correlate this with biofilm structure along the transverse mixing zone(s). Reflected light differential interference contrast (DIC) microscopy will be used to image the location of total biomass in the pore structures. The effluent U(VI) concentration will be used to determine overall reaction kinetics as a function of other system parameters (e.g., biomass and U(IV) precipitation in pores). Raman backscattering spectroscopy will be used to determine the mineral phases that are forming. Laser confocal microscopy will be used to image the biomass with depth. All experiments will continue until pseudo steady state growth is achieved (i.e., until biomass growth at new locations is no longer observed.).
The EMSL facility has several unique facilities that will make the proposed work possible. It has a new microfluidics fabrication laboratory to make the required micromodels. It has a new microfluidics visualization laboratory and expert scientists that will allow us to run the experiments without interruption. It has unique Raman backscattering spectroscopy capabilities that will allow us to determine mineral phases in the micromodels.
Previously reported data on uranium reduction and oxidation have been generated using laboratory incubations with multiple variables changing at one time, or batch systems where hydrodynamics are not a contributing factor. The experimental procedure outlined above will not only identify the mechanisms for reduction of U(VI) and oxidation of U(IV) in flowing pore scale models, but it will also determine if the mechanisms are relevant in the presence and absence of Fe(III) by changing one variable at a time. The data regarding electron donor concentration and mixing will be critical to future remediation efforts; this study will allow practitioners to optimize the type and concentration of electron donor based on specific geochemical conditions.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2010-10-01
End Date
2013-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
Fanizza, M.F., H. Yoon, C. Zhang, M. Oostrom, T.W. Wietsma, N.J. Hess, M.E. Bowden, T.J. Strathmann, K.T. Finneran, C.J. Werth, Pore Scale Evaluation of Uranyl Phosphate Precipitation in a Model Groundwater System, Water Resour. Res., DOI: 10.1002/wrcr.20088, Feb, 2013.