Iron Nodules Scavenging Uranium from Groundwater

Abstract

The scavenging of uranium from groundwater downgradient of the uranium ore deposit at Koongarra, Australia, has been investigated to provide information about the longterm transport of radionuclides. Rock samples collected from diamond-drill cores were examined mainly using scanning and transmission electron microscopy. Here we focus on the U associated with iron oxides and report on (i) the extent to which U has accumulated in the various types of iron oxides (fissure fillings, clay coatings, and nodules) and (ii) the chemical form of U associated with iron-nodules. The iron nodules have a remarkably large capacity for uranium uptake. The uranium enrichment in the nodules reaches approximately 8 wt %, and their uranium contents are greater than those in the other iron forms, such as fissure fillings and clay coatings. The ability of the iron nodules to enrich uranium (to levels 10 6 times higher than the groundwater) is greater than those of any other natural materials in the system. Although the initial step in uranium uptake appears to be adsorption, the uranium in the nodules has been fixed by precipitation of copper uranyl phosphate microcrystals. This precipitation process leads to the long-term retardation of uranium in the system. This result strongly suggests that an understanding of postadsorption processes is necessary for predicting radionuclide retardation over long time scales.

Uranium co-precipitation with iron oxide minerals

“…Uranium sorption and co-precipitation processes with Fe minerals are studied in lieu of repository assessment, nuclear waste treatment and U biogeochemistry (MOYES et al, 2000;OHNUKI et al, 1997;PLOTNIKOV and BANNYKH, 1997;SATO et al, 1997;HOBBS and KARRAKER, 1996;BRUNO et al, 1995;PEARCY et al, 1994;WAITE et al, 1994 and many others). As part of the Poços de Caldas natural analogue study (Brazil), researchers evaluated the co-precipitation and precipitation equilibrium of U(VI) with Fe(III) oxides (BRUNO et al, 1995).…”

“…Uranium is not thought to be incorporated into the α-FeOOH (goethite) structure (GERTH, 1990). However, studies report the uptake of U during the formation of crystalline and amorphous Fe oxides (OHNUKI et al, 1997;PLOTNIKOV and BANNYKH, 1997;SATO et al, 1997;BRUNO et al, 1995).…”

Incorporation of Oxidized Uranium into Fe (Hydr)oxides during Fe(II) Catalyzed Remineralization

“…Alternatively, U has been shown to coprecipitate with Fe in many environments and over a wide range of time scales including: Egyptian Fe deposits, 150-4100 ka Hawaiian soils, Fe nodules down gradient of the Australian Koongarra U deposit, and the DOE Oak Ridge site (where uranium bearing goethite was identified) [23][24][25][26]. Investigations of U(VI) reaction with Fe(0) provide further evidence for the importance of the iron co-precipitation pathway for the uptake of U [27].…”

SHRIMP measurements of U and Pb isotopes in the Koongarra secondary ore deposit, Northern Australia.

“…High uranium contents (40-70 wt.% uranium) were observed within the crystalline iron mineral phases . The "iron nodule" phase has distinctive morphological features, particularly a complex alveolar structure (Sato et al, 1997). Of the various iron-rich phases, the U content is highest in the nodules, i.e., 1.6-7.7 wt.% in minerals, rather than Fe coatings or fissure filling phases (Sato et al, 1997).…”

“…The "iron nodule" phase has distinctive morphological features, particularly a complex alveolar structure (Sato et al, 1997). Of the various iron-rich phases, the U content is highest in the nodules, i.e., 1.6-7.7 wt.% in minerals, rather than Fe coatings or fissure filling phases (Sato et al, 1997). It is therefore thought that the crystalline iron nodules have played a dominant role in the retention of uranium in the dispersion fan, to a greater extent than any other mineral phase.…”