Abstract's Details

Microprobe Studies of Permeable Reactive Barriers for Uranium Remediation
Abstract ID	W:MICRO-06
Presenter	Chris Fuller
Presentation Type	Microfocusing Workshop
Full Author List	C. C. Fuller (1), S. M. Webb (2), J. R. Bargar (2), D. L. Naftz (3)
Affiliations	(1) US Geological Survey, Menlo Park, CA (2) Stanford Synchrotron Radiation Laboratory, Stanford Linear Accelerator Center, Menlo Park, CA (3) US Geological Survey, Salt Lake City, UT
Category	Environmental Science
Abstract	The SSRL BL 2-3 X-ray microprobe is being used in conjunction with bulk EXAFS and SR-XRD to characterize uranium sequestration and biogeochemical reactions that affect the performance of permeable reactive barriers (PRB) for remediation of uranium contaminated ground water. The PRB reactive materials (zero valent iron, ZVI and bone char apatite, PO4) being investigated were recovered after a 9-year field demonstration of PRBs at Fry Canyon, UT, an abandoned uranium ore upgrader facility. The ZVI PRB was intended to remove dissolved uranium by reduction of soluble U(VI) to insoluble U(IV) in response to extreme reducing conditions produced by ZVI corrosion, followed by precipitation of insoluble UO₂. Despite near-complete uranium removal by ZVI over the 9-year deployment, a five-fold decrease in PRB permeability was observed that limits flow through the PRB. Micron-scale Fe-XANES mapping of thin sections of the ZVI PRB grains (0.5 to 4 mm) indicates extensive precipitation of carbonate green rust, mackinawite, and siderite in the ZVI PRB resulting from corrosion coupled with microbial sulfate reduction. This precipitation has cemented grains and filled internal porosity resulting in decreased PRB permeability. U-XANES mapping indicates that U removal occurs by reduction to +4 oxidation state with coatings of U(IV) on ZVI surfaces that often are buried by the secondary Fe precipitates. The observed distribution of corrosion products and sequestered U suggests that the ongoing precipitation of iron and Ca phases has decreased both PRB permeability and U reaction on the ZVI surface. Although U sequestration by PO4 previously was shown to occur by surface complexation of U(VI), uptake of up 20-times the U-sorption maximum were measured on PO4 PRB material. Bulk and micro-XANES indicates that U(IV) dominates U speciation consistent with enhanced U removal by bioreduction occurring within the PRB.
Footnotes
Funding Acknowledgement	This research is funded in part by the Bureau of Land Management and by the USGS Water Resources National Research Program.