Association of Uranium with Iron Oxides Typically Formed on Corroding Steel Surfaces

Abstract

Decontamination of metal surfaces contaminated with low levels of radionuclides is a major concern at Department of Energy facilities. The development of an environmentally friendly and cost-effective decontamination process requires an understanding of their association with the corroding surfaces. We investigated the association of uranium with the amorphous and crystalline forms of iron oxides commonly formed on corroding steel surfaces. Uranium was incorporated with the oxide by addition during the formation of ferrihydrite, goethite, green rust II, lepidocrocite, maghemite, and magnetite. X-ray diffraction confirmed the mineralogical form of the oxide. EXAFS analysis at the U L III edge showed that uranium was present in hexavalent form as a uranyl oxyhydroxide species with goethite, maghemite, and magnetite and as a bidentate innersphere complex with ferrihydrite and lepidocrocite. Iron was present in the ferric form with ferrihydrite, goethite, lepidocrocite, and maghemite; whereas with magnetite and green rust II, both ferrous and ferric forms were present with characteristic ferrous:total iron ratios of 0.65 and 0.73, respectively. In the presence of the uranyl ion, green rust II was converted to magnetite with concomitant reduction of uranium to its tetravalent form. The rate and extent of uranium dissolution in dilute HCl depended on its association with the oxide: uranium present as oxyhydroxide species underwent rapid dissolution followed by a slow dissolution of iron; whereas uranium present as an inner-sphere complex with iron resulted in concomitant dissolution of the uranium and iron.

Mechanism of uranium removal from the aqueous solution by elemental iron

“…U(VI) was therefore enclosed in the matrix of precipitating iron oxides and was not available for desorption with Na 2 CO 3 . The co-precipitation reaction of U(VI) with iron oxides was well described in another context by Dodge et al [53] , Duff et al [54] , and Eng et al [55] The extent of U(VI) co-precipitation by corrosion products of carbon steel was thoroughly characterized by another research group. [53,55] These authors also report about a differential recovery efficiency of surface-sorbed and co-precipitated U(VI).…”

“…The co-precipitation reaction of U(VI) with iron oxides was well described in another context by Dodge et al [53] , Duff et al [54] , and Eng et al [55] The extent of U(VI) co-precipitation by corrosion products of carbon steel was thoroughly characterized by another research group. [53,55] These authors also report about a differential recovery efficiency of surface-sorbed and co-precipitated U(VI). Particularly, for maghemite, magnetite and goethite, U(VI) species were associated as oxyhydroxide species and were readily dissolved in concentrated HCl solution, but for lepidocrocite and ferrihydrite U(VI) species formed a bidentate complex with iron, which resisted acidic dissolution.…”

Investigating the Mechanism of Uranium Removal by Zerovalent Iron

“…[23][24][25][26][27] Furthermore, pollutant coprecipitation with corrosion products has been demonstrated as another removal pathway. [28,29] Therefore there are at least three possible immobilization pathways for several pollutants: reduction by Fe°, by Fe 2+ and coprecipitation with corrosion products. To be able to optimise the functionality of a ZVI wall, the actual main removal pathway for each pollutant has to be identified.…”

Products of abiotic U(VI) reduction by biogenic magnetite and vivianite

“…Magnetite formation has also been reported during biooxidation of Fe(II) coupled to denitrification (Chaudhuri et al, 2001). A number of studies have investigated the role of magnetite in uranium reduction and the findings varied greatly ranging from no observable reduction (Dodge et al, 2002) to clear evidence of reduction (Scott et al, 2005;Aamrani et al, 2007;O'Loughlin et al, 2010) to the formation of a mixed-valence U(IV)-U(VI) phase (Missana et al, 2003;Aamrani et al, 2007;Regenspurg et al, 2009) or the formation of U(V) (Ilton et al, 2010). The variation in findings is presumably linked to variability in morphology, specific surface area and phase stoichiometry Scherer, 2009, Gorski et al, 2010) of the magnetite used as well as differences in experimental conditions.…”

“…Interactions between U(VI) and magnetite have received appreciable attention because magnetite is a ubiquitous, environmentally relevant ferrous-bearing oxide, a metabolic byproduct of bacterial respiration, and a corrosion product of steel with ramifications for nuclear waste repositories (Ishikawa et al, 1998;Dodge et al, 2002;Ilton et al, 2010). Microbial reduction of amorphous ferric oxyhydroxide (Fe(OH) 3 ) has been reported to induce the formation of magnetite (Bell et al, 1987;Lovley et al, 1987;Moskowitz et al, 1989;Zhang et al, 1997;Konhauser, 1998).…”