Surface Complexation of Neptunium(V) with Goethite
- Argonne National Laboratory, Argonne, IL, 60439 (United States)
Batch adsorption experiments in which neptunium-bearing solutions were reacted with goethite (alpha-FeOOH) have been performed to study uptake mechanisms in sodium chloride and calcium-bearing sodium silicate solutions. This paper presents results identifying and quantifying the mechanisms by which neptunium is adsorbed as a function of pH and reaction time (aging). Also presented are results from tests in which neptunium is reacted with goethite in the presence of other cations (uranyl and calcium) that may compete with neptunium for sorption sites. The desorption of neptunium from goethite has been studied by re-suspending the neptunium-loaded goethite samples in solutions containing no neptunium. Selected reacted sorbent samples were analyzed by x-ray absorption spectroscopy (XAS) to determine the oxidation state and molecular speciation of the adsorbed neptunium. Results have been used to establish the pH adsorption edge of neptunium on goethite in sodium chloride and calcium-bearing sodium silicate solutions. The results indicate that neptunium uptake on goethite reaches 95% at a pH of approximately 7 and begins to decrease at pH values greater than 8.5. Distribution coefficients for neptunium sorption range from less than 1000 (moles/kg){sub sorbed} / (moles/kg){sub solution} at pH less than 5.0 to greater than 10,000 (moles/kg){sub sorbed} / (moles/kg){sub solution} at pH greater than 7.0. Distribution coefficients as high as 100,000 (moles/kg){sub sorbed} / (moles/kg){sub solution} were recorded for the tests done in calcite equilibrated sodium silicate solutions. XAS results show that neptunium complexes with the goethite surface mainly as Np(V) (although Np(IV) is prevalent in some of the longer-duration sorption tests). The neptunium adsorbed to goethite shows Np-O bond length of approximately 1.8 angstroms which is representative of the Np-O axial bond in the neptunyl(V) complex. This neptunyl(V) ion is coordinated to 5 or 6 equatorial oxygens with Np-O bond lengths of 2.45 angstroms. The absence of a clearly recognizable Np-Fe interaction for the sodium chloride sorption tests suggests that neptunium in these solutions adsorbs as an outer-sphere complex. XAS results from the calcium-bearing sodium silicate sorption tests show evidence for a neptunyl(V) inner sphere surface complex with a Np-Fe interaction at 3.5 angstroms. Desorption tests indicate that samples in which neptunium is bound as inner-sphere complexes show significant sorption hysteresis relative to samples in which neptunium is bound largely as outer-sphere complexes. (authors)
- Research Organization:
- Materials Research Society, 506 Keystone Drive, Warrendale, PA, 15086-7573 (United States)
- OSTI ID:
- 21062434
- Resource Relation:
- Conference: Symposium on Scientific Basis for Nuclear Waste Management, Boston - Massachusetts (United States), 27 Nov - 1 Dec 2006; Other Information: Country of input: France; 3 refs; Related Information: In: Proceedings of the symposium on Scientific Basis for Nuclear Waste Management XXX, by Dunn, Darrell [ed. Southwest Research Inst., San Antonio, Texas (United States)]; Poinssot, Christophe [ed. CEA-Saclay, 91191 Gif-sur-Yvette cedex (France)]; Begg, Bruce [ed. Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW (Australia)], v. 985, 663 pages.
- Country of Publication:
- United States
- Language:
- English
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