An EXAFS study of uranium(VI) sorption onto silica gel and ferrihydrite

Reich, Tobias; Moll, Henry; Arnold, Timothy; Denecke, Melissa A.; Hennig, Christoph; Geipel, Gerhard; Bernhard, Gert; Nitsche, H.; Allen, P. G.; Bucher, J. J.; Edelstein, Norman M.; Shuh, David K.

doi:10.1016/s0368-2048(98)00242-4

Cited by 139 publications

(189 citation statements)

References 20 publications

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“…An inner-sphere surface complex is indicated by a U-Fe distance near 3.5 Å ; this distance was taken to indicate a bidentate complex formed by polyhedral edge-sharing by the hydrated uranyl ion, ðUO 2 ÞðH 2 OÞ n 2þ , and a single FeO 6 surface site, (>Fe-O 2 )UO 2 (H 2 O, OH) n . A similar distance was also found by Reich et al (1998). Here, we shall designate the bidentate edge-sharing complex as E2.…”

Section: Introductionsupporting

confidence: 76%

Surface complexation of U(VI) on goethite (α-FeOOH)

Sherman

Peacock

Hubbard

2008

Geochimica et Cosmochimica Acta

199

163

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Sorption of U(VI) to goethite is a fundamental control on the mobility of uranium in soil and groundwater. Here, we investigated the sorption of U on goethite using EXAFS spectroscopy, batch sorption experiments and DFT calculations of the energetics and structures of possible surface complexes. Based on EXAFS spectra, it has previously been proposed that U(VI), as the uranyl cation UO 2 2þ , sorbs to Fe oxide hydroxide phases by forming a bidentate edge-sharing (E2) surface complex, >Fe(OH) 2 UO 2 (H 2 O) n . Here, we argue that this complex alone cannot account for the sorption capacity of goethite (a-FeOOH). Moreover, we show that all of the EXAFS signal attributed to the E2 complex can be accounted for by multiple scattering. We propose that the dominant surface complex in CO 2 -free systems is a bidentate corner-sharing (C2) complex, (>FeOH) 2 UO 2 (H 2 O) 3 which can form on the dominant {101} surface. However, in the presence of CO 2 , we find an enhancement of UO 2 sorption at low pH and attribute this to a (>FeO)CO 2 UO 2 ternary complex. With increasing pH, U(VI) desorbs by the formation of aqueous carbonate and hydroxyl complexes. However, this desorption is preceded by the formation of a second ternary surface complex (>FeOH) 2 UO 2 CO 3 . The three proposed surface complexes, (>FeOH) 2 UO 2 (H 2 O) 3 , >FeOCO 2 UO 2 , and (>FeOH) 2 UO 2 CO 3 are consistent with EXAFS spectra. Using these complexes, we developed a surface complexation model for U on goethite with a 1-pK model for surface protonation, an extended Stern model for surface electrostatics and inclusion of all known UO 2 -OH-CO 3 aqueous complexes in the current thermodynamic database. The model gives an excellent fit to our sorption experiments done in both ambient and reduced CO 2 environments at surface loadings of 0.02-2.0 wt% U.

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Section: Introductionsupporting

confidence: 76%

Surface complexation of U(VI) on goethite (α-FeOOH)

Sherman

Peacock

Hubbard

2008

Geochimica et Cosmochimica Acta

199

163

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“…Upon harvesting, solids were divided into four subsamples based on distance along the flow path: 1.5 to 5.0 cm, 5.0 to 10.5 cm, and 10.5 to 14.5 cm, and 14.5 to 17 cm. Solid phase analysis was conducted as described above, except that they were not extracted with 30 mM KHCO 3 .…”

Section: Column Design and Flow Conditionsmentioning

confidence: 99%

“…Uranyl adsorption onto solids such as Fe (hydr)oxides can be appreciable, but the process subject to changes in aqueous conditions and largely reversible [1][2][3][4][5]. In particular, CO 3 2-, especially in combination with Ca 2+ or, to less extent, Mg 2+ , suppresses adsorption (or enhances desorption) and increases mobility of U(VI) [1,[6][7][8][9][10][11].…”

Section: Introductionmentioning

confidence: 99%

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

Nico

Stewart

Fendorf

2009

Environ. Sci. Technol.

120

132

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The form of solid phase U after Fe(II) induced anaerobic remineralization of ferrihydrite in the presence of aqueous and absorbed U(VI) was investigated under both abiotic batch and biotic flow conditions. Experiments were conducted with synthetic ground waters containing 0.168 mM U(VI), 3.8 mM carbonate, and 3.0 mM Ca 2+ . In spite of the high solubility of U(VI) under these conditions, appreciable removal of U(VI) from solution was observed in both the abiotic and biotic systems. The majority of the removed U was determined to be substituted as oxidized U (U(VI) or U(V)) into the octahedral position of the goethite and magnetite formed during ferrihydrite remineralization. It is estimated that between 3% and 6% of octahedral Fe(III) centers in the new Fe minerals were occupied by U(VI). This site specific substitution is distinct from the non-specific U co-precipitation processes in which uranyl compounds, e.g. uranyl hydroxide or carbonate, are entrapped with newly formed Fe oxides. The prevalence of site specific U incorporation under both abiotic and biotic conditions and the fact that the produced solids were shown to be resistant to both extraction (30 mM KHCO 3 ) and oxidation (air 2 for 5 days) suggest the potential importance of sequestration in Fe oxides as a stable and immobile form of U in the environment.

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“…Single and multiple scattering paths included in the four models, along with the mathematical constraints placed on the coordination numbers (CN) and Debye Waller factors ( 2 ), are presented in Table S1 (Table S1, Supporting Information). The CNs were constrained to ideal values based on either known crystal structures (3,5) or previously published models (1,(6)(7)(8)(9). The  2 s were grouped roughly by distance from the central atom and identity of the scattering atom.…”

Section: Uranium Exafs Fitting Detailsmentioning

confidence: 99%

“…Under anaerobic conditions, U(VI) may be reduced to U(IV), which subsequently forms the sparingly soluble UO 2 phase, by dissimilatory metal reducing bacteria (DMRB) that couple uranium reduction with the oxidation of organic carbon or H 2 (1)(2)(3). In fact, stimulated (through addition of a carbon source) reductive immobilization has been studied and implemented in field-scale contaminated zones as a potential means of uranium remediation (4)(5)(6)(7)(8). However, the presence of dissolved Ca and carbonate promotes the formation of ternary uranyl-calcium-carbonato complexes (9-12) that diminish both enzymatic and chemical reduction rates (13)(14)(15).…”

Section: Introductionmentioning

confidence: 99%

Stability of Uranium Incorporated into Fe (Hydr)oxides under Fluctuating Redox Conditions

Stewart

Nico

Fendorf

2009

Environ. Sci. Technol.

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Reaction pathways resulting in uranium bearing solids that are stable (i.e., having limited solubility) under both aerobic and anaerobic conditions will limit dissolved concentrations and migration of this toxin. Here we examine the sorption mechanism and propensity for release of uranium reacted with Fe (hydr)oxides under cyclic oxidizing and reducing conditions. Upon reaction of ferrihydrite with Fe(II) under conditions where aqueous Ca-UO 2 -CO 3 species predominate (3 mM Ca and 3.8 mM CO 3 -total), dissolved uranium concentrations decrease from 0.16 mM to below detection limit (BDL) after 5 to 15 d, depending on the Fe(II) concentration.In systems undergoing 3 successive redox cycles (15 d of reduction followed by 5 d of oxidation) and a pulsed decrease to 0.15 mM CO 3 -total, dissolved uranium concentrations varied depending on the Fe(II) concentration during the initial and subsequent reduction phases-U concentrations resulting during the oxic 'rebound' varied inversely with the Fe(II) concentration during the reduction cycle. Uranium removed from solution remains in the oxidized form and is found both adsorbed on and incorporated into the structure of newly formed goethite and magnetite. Our results reveal that the fate of uranium is dependent on anaerobic/aerobic conditions, aqueous uranium speciation, and the fate of iron.2

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An EXAFS study of uranium(VI) sorption onto silica gel and ferrihydrite

Cited by 139 publications

References 20 publications

Surface complexation of U(VI) on goethite (α-FeOOH)

Surface complexation of U(VI) on goethite (α-FeOOH)

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

Stability of Uranium Incorporated into Fe (Hydr)oxides under Fluctuating Redox Conditions

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