Hanford invests in redox technology

Where are they now? Hanford invests in redox technology

It proved its worth during two years of field testing at two sites at Hanford. It was a winner of a 1998 R&D 100 Award given by R&D Magazine (see Initiatives, Winter 1998). And regulators and stakeholders in Washington State have recently endorsed its full-scale use. Now, that the barriers to its deployment have been successfully dealt with, In Situ Redox Manipulation (OST/ TMS ID 15) is being installed at Hanford’s 100-D Area Bechtel Hanford installed a permeable sodium dithionite barrier at Hanford. near two of the site’s old reactors to reduce the toxicity of an underground chromium plume. Its deployment as an Accelerated Site Technology Deployment project represents another successful partnership between a DOE site and the DOE organization dedicated to developing and promoting innovative solutions to DOE’s cleanup problems—the Office of Science and Technology. Support through OST’s Subsurface Contaminants Focus Area has helped fund ISRM’s development by Pacific Northwest National Laboratory, resolve stakeholder and regulator concerns that threatened to delay the technology’s use, and finance its first full-scale deployment at Hanford.

In February, Bechtel Hanford, the M&I contractor at Hanford, began installing a permeable sodium dithionite barrier along the banks of the Columbia River to intercept and neutralize a hexavalent chromium plume that has the potential to foul the river’s salmon spawning beds. As applied at Hanford’s 100-D Area, ISRM is the injection of sodium dithionite into an underground aquifer to create a permeable, in situ treatment zone, through which groundwater passes as it feeds into the Columbia River. As the groundwater passes through this chemically altered zone, each chromium ion gains three electrons and is reduced from Cr(VI) to a less mobile and less toxic state, Cr(III). By controlling the redox state of chromium, sodium dithionite is lessening the mobility of chromium ions by causing them to sorb to neighboring soils and sediments, instead of being transported along with the groundwater. While the reagent dithionite is being used at Hanford as the redox agent, DOE is also investigating, through an Environmental Management Science Program project, the use of microbes in reducing the toxicity and mobility of contaminants (see Initiatives, “Building a bacteria wall,” Summer 1999).

So, why is it called
“redox manipulation”?
When chromium and other contaminants such as uranium and technetium emerge as wastes from various industrial processes, they are in an oxidized state. Oxidized ions have a more positive valence resulting from the loss of electrons. In the case of chromium, the oxidized species has a plus six valence—Cr(VI). These ions with higher valences are dangerous because they are more toxic and more readily dissolved in and spread by migrating groundwater. At Hanford’s 100-D Area, the introduction of the chemical sodium dithionite causes iron that is indigenous to the aquifer to become reduced, thereby producing more valence electrons. When Cr(VI) ions interact with the reduced iron, they gain electrons, which lowers their positive valence to three—Cr(III). These ions with lower valences are much less toxic and sorb to nearby soils and are therefore less available to pollute the groundwater. At Hanford, sodium dithionite is the reagent that’s being used to manipulate reduction-oxidation (redox) reactions.

During initial field testing of the technology at Hanford in 1997 and 1998, PNNL researchers placed five groundwater wells about 500 feet from the Columbia River to inject sodium dithionite into a well for approximately 10 hours. On reaching groundwater, the sodium dithionite reacted with iron in the ground and spread into a circle approximately 50 feet in diameter. Beginning 30 hours later, a nontoxic by-product, sodium sulfate, was removed over the span of a week. The five wells formed a 150-foot-wide barrier for the groundwater to filter through when flowing toward the river. The ISRM process was a technical success during its testing at Hanford: it reduced chromium concentrations in groundwater by 99 percent from more than 700 parts per billion to less than 7 parts per billion over the 150-foot testing area.

Despite its superior performance during testing at Hanford, ISRM’s full-scale deployment was not assured. At the request of the Nez Perce Tribe, DOE accelerated an American Society of Mechanical Engineers peer review that resolved a number of issues but also identified three that required further evaluation. SCFA provided funding to resolve these last barriers to deployment, which enabled the technology to be supported by the Nez Perce Tribe. The Hanford Advisory Board also endorsed the technology, and in October 1999, federal and state regulators approved an amendment to an Interim Record of Decision authorizing the use of ISRM to treat Hanford’s chromium plume at the 100-D Area.

An independent study has shown that ISRM deployment at Hanford’s 100-D Area can bring a 60 percent cost avoidance compared to conventional pump-and-treat technologies. ISRM is estimated to cost about $4.5 million to install and maintain for 20 years, compared to an estimated $11 million for a new pump-and-treat system to replace the existing pump-and-treat program that Hanford’s contractors have relied on thus far to control the spread of the chromium plume. ISRM’s cost effectiveness stems from the technology’s reliance on standard 6-inch groundwater wells, as opposed to the costs involved in pumping water to the surface, building an aboveground treatment facility, treating water, and returning it to its source. ISRM is expected to be a permanent solution since chromate ions stay immobilized once they are reduced to Cr(III). While the barrier is expected to last 30 years without additional maintenance, it can be replenished if groundwater monitoring indicates that hexavalent chromium levels are building. And since ISRM works in situ underground, it will lower the risk of worker exposure as compared to pump and treat.

ISRM is undergoing more field testing to determine its efficacy in treating chlorinated solvents such as trichloroethylene at Fort Lewis, Washington as well as perchloroethylene at Moffett Federal Airfield in California. Deployment at a commercial chrome plating plant in Vancouver, Washington is in negotiations. And Hanford is considering using the technology at two other chromium-contaminated sites.

For more information, contact Jonathan Fruchter, principal investigator, at Pacific Northwest National Laboratory at (509) 376-3939, john.fruchter@pnl.gov.