Through the Environmental Management Science Program (EMSP), DOE’s Office of Environmental Management (EM) and Office of Science (SC) collaborate to fund basic research to solve intractable problems that threaten the successful closure of DOE sites. As one of the programs within the Office of Science and Technology, EMSP ensures that OST’s projects cover the full spectrum of R&D.

Containment barriers can significantly shorten the schedule and reduce the cost of subsurface remediation by slowing or stopping the movement of contaminants through soil. To optimize the ability of active biowalls to contain priority contaminants on DOE sites, scientists need a greater understanding of microbial, geochemical, and hydrogeological processes that interact and often compete.

An EMSP project titled “Containment of Toxic Metals and Radionuclides in Porous and Fractured Media: Optimizing Biogeochemical Reduction versus Geochemical Oxidation” is providing basic knowledge about the optimal conditions for bacteria to immobilize certain contaminants. The study, led by Oak Ridge National Laboratory’s Phil Jardine and Scott Brooks, is motivated by the likelihood that subsurface microbial activity can alter the state of toxic metals and radionuclides so that they are immobilized and contained for the long term.

The project’s overall goal is to understand and model the mechanisms whereby metal-reducing bacteria aid the stabilization of these contaminants in porous soil. Results could lead directly to cost-effective strategies for active biowalls in ongoing or planned remediation projects at Hanford’s In Situ Redox Manipulation site, Savannah River’s Old Burial Ground, and sites at Oak Ridge’s Y-12 Plant.

The study’s three multidisciplinary tasks build on collaborations established years ago within DOE’s Subsurface Science Program:

• Jardine, Brooks, and their ORNL associates are using a dynamic flow technique to quantify rates of oxidation and reduction and mechanisms controlling the mobility of uranium, chromium, and cobalt-EDTA (ethylenediaminetetraacetic acid).

• Stanford University’s Scott Fendorf uses X-ray absorption spectroscopy (XAS) at the Stanford Synchrotron Radiation Laboratory to measure the redox transformation and immobilization of these contaminants by subsurface media.

• Finally, James Saiers, formerly of Florida International University and now at Yale, is using the experimental data to develop computer models that simulate hydrologic-biogeochemical transport processes.

The electron tug-of-war
When the contaminants of concern contact metal oxides indigenous to the soil, the contaminants undergo geochemical oxidation (lose electrons). The resulting ions with higher valences—
U(VI), Cr(VI), and
⁶⁰Co(III)EDTA—are dangerous for a variety of reasons. The chromium is more toxic, and all three are more readily dissolved in and spread by migrating groundwater. But scientists have discovered a number of bacteria whose metabolic processes affect the redox state of toxic metals and radionuclides. When the contaminants are reduced (gain electrons, so that their positive charges decrease), the resulting ions—U(IV), Cr(III), and ⁶⁰Co(II)EDTA—are less soluble. Instead of being transported by groundwater, they sorb to neighboring soils and sediments. This project has tackled the challenges of sustaining microbial reduction processes in situ for long periods of time and optimizing the processes in spite of competing geochemical oxidation and sorption reactions.

The dynamic flow experiments are performed by sending solutions up through columns of simulated soils containing pure mineral oxides and of heterogeneous soils and sediments from Oak Ridge, Savannah River, and Hanford sites. Jardine reports that Brooks was the first to demonstrate the sustained microbial reduction of ⁶⁰Co(III)EDTA to ⁶⁰Co(II)EDTA under dynamic flow conditions. “After discovering a way to keep the bacteria healthy and growing, we were able to effectively stabilize ⁶⁰Co(II)EDTA in a flowing system, even in the presence of strong mineral oxidants like manganese and iron oxides commonly found in the subsurface.”

Experiments with uranium are focused on the effects of geochemical oxidation and interfacial sorption reactions and the effects of biological reduction processes on mobility rates of uranium in the actual site sediments. Because XAS is sensitive to metal redox shifts and interfacial surface reactions, the researchers use it to quantify the time-dependent bacterial reduction of U(VI) to U(IV) for a variety of environmental conditions. The transformation has proven generally quite rapid, with time scales of hours or even minutes.

Problem solving, number crunching, and beyond
Toxicity presents problems for bacterial reduction of chromium to the less toxic and mobile species, but project personnel have discovered two alternative methods. The first is a direct, abiotic process in which natural organic matter (NOM) reduces Cr(VI) to Cr(III), slowing its subsurface mobility by many orders of magnitude. NOM’s ready sorbancy to soils and sediments makes this a promising technique for efficient, cost-effective geochemical barriers. The second method is indirect: bacteria reduce iron oxides to Fe(II), which in turn quickly reduces Cr(VI) to Cr(III). This catalytic process, in which the only reactant lost is Cr(VI), shows promise for use in active biowalls for the in situ containment of chromium. Both techniques have been quantified using time-resolved XAS.

Jardine reports that efforts in this third and final year of the project focus on troubleshooting the uranium system and computer modeling. The bacteria ( Shewanella alga) are forming U(IV), but sustained growth during dynamic flow has been problematic.

Experimental data already derived are being crunched by the team’s computer specialist into a biohydrogeochemical model whose calibrated code can assist with various DOE site needs. The team hopes the next step will be a field-scale bioreduction demonstration at a field facility developed at ORNL to investigate groundwater flow and transport processes in fractured shale bedrock. For information on that facility, see www.esd.ornl.gov/facilities/hydrology/WAG5.

For further information, contact Phil Jardine, ORNL, (423) 576-8085, ipj@ornl.gov.