EXAFS SPECTROSCOPIC STUDY OF NEPTUNIUM(V) SORPTION AT THE ALPHA-FEOOH WATER INTERFACE

The sorption of aqueous Np(V) on crystalline alpha-FeOOH (goethite) has been studied at the molecular level by synchrotron-based X-ray absorption spectroscopy. Sodium nitrate solutions (pH 7.2, ionic strength 0.08 M) having a Np(V) concentration of 1.3 x 10(-5) M were reacted with high surface area goethite (50 m2/g), resulting in approximately 95% uptake of the Np and a sorption density of 10(-5) mol of Np/g of goethite. Np L(III)-edge X-ray absorption spectra were collected in the fluorescence-yield mode on wet pastes after removal of approximately 90% of the solution phase by centrifuging. Comparison of spectra from the sorption sample with Np L(III) spectra from crystalline NpO2, a 1.3 mM Np(V) solution, and U(VI) L(III) spectra from crystalline uranyl nitrate and triacetate and U(VI) sorbed onto ferric hydroxide gel shows that Np(V) sorbs as isolated neptunyl (NpO2+) groups and is not multinuclear under the conditions of our experiment. Least-squares modeling of the EXAFS spectrum of the Np/goethite sorption sample shows two oxygen shells around Np consisting of two oxygens at 1.85 +/- 0.02 angstrom and about five oxygens at 2.51 +/- 0.03 angstrom, consistent with a distorted pentagonal-bipyramid complex. The only significant difference between Fourier transforms of the EXAFS spectra of the Np/goethite sorption sample and the Np solution sample is a weak feature at approximately 3 angstrom in the former. We have tentatively identified this feature as a Np-Fe second-neighbor pair correlation, which suggests that Np(V) sorbs at the goethite/water interface under these solution conditions as an inner-sphere complex. Our results rule out sorption as a simple, ordered neptunium oxide or hydroxide or as a coprecipitate with an iron oxide-hydroxide phase.