Uranium and other trace elements’ distribution in Korean granite: implications for the influence of iron oxides on uranium migration

Abstract

To understand trace radionuclide (uranium) migration occurring in rocks, a granitic batholith located at the Korea Atomic Energy Research Institute (KAERI) site was selected and investigated. The rock samples obtained from this site were examined using mineralogical methods, including scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The changes in the distribution pattern of uranium (U) and small amounts of trace elements, and the mineralogical textures affected by weathering, were examined. Based on the element distribution analyses, it was found that Fe2+ released from fresh biotite is oxidized in short geological time, forming amorphous iron oxides, such as ferrihydrite, around silicate minerals. In that case, the amorphous ferrihydrite does not show distinct adsorption for U. However, as it gradually crystallizes to goethite or hematite, the most U-rich phases were found to be associated with the secondary iron oxides having granular forms. This evidence suggests that the geological subsurface environment is favorable for the crystallized iron oxides to keep their structures more stable for a long time as compared with the amorphous phases. There is a possibility that the long residence of U which is in contact with the stable crystalline phases of iron may finally lead to the partial sequestration of U in their structure. Consequently, it seems that Fe-oxide crystallization can be a dominating mechanism for U uptake and controls long-term U transport in granites with low U contents.

Geochemical Behavior of Uranium and Radon in Groundwater of Jurassic Granite Area, Icheon, Middle Korea

“…Likewise, there is a decrease in the uranium level with increasing oxidizing condition [40]. Mostly uraniferous minerals in Jurassic granites occur as partial replacements in some accessory mineral phases such as monazite and apatite, or in rock-forming minerals such as feldspars and micas [41,42]. In addition, Jurassic granite of Daejeon, middle Korea includes a small amount of uraniferous minerals such as uraninite, coffinite, and uranophane in hydrothermal alteration zone which contains quartz veinlets within a fracture zone or dikes, in association with muscovite, chlorite, epidote, and calcite [24].…”

Reduction of U(VI) by Fe(II) during the Fe(II)-Accelerated Transformation of Ferrihydrite

“…Iron­(III) oxyhydroxide minerals are widespread throughout nature and play an important role in both biotic and abiotic elemental cycles. − Notable within these cycles is the interaction of aqueous Fe­(II) species with solid Fe­(III) minerals, which can enhance the reductive capacity of Fe­(II) and have a profound effect on the fate of redox active species such as radionuclides, metals, and organic contaminants. − Adding to the complexity in these systems is the tendency of aqueous Fe­(II) species to exchange with structural Fe­(III), leading to the renewal of stable Fe­(III) minerals such as goethite or hematite, which has implications for coprecipitated metallic contaminants. − This interaction will also induce the accelerated transformation of thermodynamically unstable Fe­(III) phases such as ferrihydrite into the more crystalline (oxyhyd)­oxide minerals lepidocrocite, goethite, and magnetite. − …”